Vahid Safarifard

Sabina Rodríguez-Hermida

Vincent Guillerm

Inhar Imaz

Mina Bigdeli

Alireza Azhdari Tehrani

Jordi Juanhuix

Ali Morsali

Mirian E Casco

Joaquín Silvestre-Albero

Joaquin Silvestre-Albero

Enrique V Ramos-Fernandez

Daniel Maspoch

Joaquín Silvestre-Albero

The Influence of the Amide Groups in the CO2/N2 Selectivity of a Series of Isoreticular, Interpenetrated Metal-Organic Frameworks

Keywords: Metal-Organic Frameworks, Amide, Adsorption, Separation, Breakthrough

d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Microporous metal-organic frameworks (MOFs), constructed from metal-based nodes and polytopic organic ligands, are a promising class of porous materials for carbon capture, separation and storage owing to their very high surface areas, tuneable pore sizes and shapes, and adjustable pore surface functionality. [START_REF] Zhou | Themed issue: Metal-Organic Frameworks[END_REF][START_REF] Long | Themed issue: Metal-Organic Frameworks[END_REF][START_REF] Zhou | Themed issue: Metal-Organic Frameworks[END_REF] For these specific applications, it is widely accepted that certain chemical functionalities located at the pore surfaces of MOFs can increase their CO 2 uptake and selectivity. [4][5][6][7][8][9][10] Among these functionalities, CO 2 has been found to bind at unsaturated metal centres via the electron rich terminal oxygen atom [11][12][13] . Also, CO 2 can interact with electron rich Lewis basic groups via electron deficient carbon atom. 14 For this purpose, most efforts have been made on the direct and post-synthetic functionalization of MOFs with amine groups. [15][16][17][18] More recently, another approach based on the combined optimization of pore size and high charge density has anticipated very promising results. 4,19,20 Besides the use of amine groups, the intrinsic characteristics of the amide groups make them also very attractive to establish cooperative intermolecular interactions with CO 2 . 21 Amide groups possess two types of donor-acceptor affinity sites: the -NH moiety that can act as an electron acceptor (Lewis acid), and the -C=O group that can act as an electron donor (Lewis base); both allowing to establish NH•••OCO and NC=O•••CO 2 interactions. [22][23][24] In the last few years, the study of some amide-functionalized MOFs has confirmed their potential for selectively adsorb CO 2 . 5,22,[24][25][26][27][28][29][30] For example, Chen, Wang et al. have reported that pores decorated with acylamide groups in the Cu(II)-based UTSA-48 MOF promote strong interactions with CO 2 and high selectivity for CO 2 over CH 4 . 5 Recently, Wang et al. reported a 3-fold interpenetrated acylamide-MOF that shows higher selective adsorption of CO 2 rather than other gases with a significant isosteric heat. 31 Similarly, Zaworotko et al. found that the decoration of a rhttype MOF also with acylamide functions can significantly enhance the CO 2 uptake and the selective adsorption of CO 2 over N 2 in comparison to PCN-61; an isoreticular analogue with acetylene moieties instead of amide groups. 24 Remarkably, comparable trends of selectivity have also been observed in other MOFs decorated with acylamide [25][26][27][28] and oxamide 22 groups as well as in some recent porous covalent organic frameworks also containing acylamide groups. 29,30 Recent studies reported by us have shown that a series of porous interpenetrated MOFs can be designed by pillaring 2D layers, comprising Zn(II) ions and the V-shaped dicarboxylate ligand 4,4ʹ-oxybisbenzoic acid (H 2 oba), with linear dipyridylbased ligands (Fig. 1). [32][33][34] Following this approach, we found that the pores of the synthesized MOFs can be easily functionalized with the two groups incorporated in the pillar ligands since the pores of these MOFs, which run perpendicular to the {Zn(oba)} layers, are also delimited by these ligands. 29,30,35 For example, the use of the pillar ligands N,Nʹ-bis-(4-pyridylmethylene)-1,4-benzenediamine (bpmb) and its naphthalene analogue N,Nʹ-bis-(4-pyridylmethylene)- (TMU-24), in which the use of the naphthalene analogue also induces that the acylamide groups are more directed towards the pores than in TMU-22 and TMU-23 (Figure 2). Importantly, the resulting 3-fold interpenetrated TMUs-22/-23/-24 are isoreticular to TMU-6 and TMU-21. The similarities (structure and stability) and differences (functional group and accessibility) of these MOFs allow studying the influence of the amide and the imine groups on their N 2 and CO 2 sorption properties as well as on their selective sorption of CO 2 over N 2 . More specifically, this latter selectivity was evaluated by performing kinetics and breakthrough experiments for a CO 2 /N 2 gas mixture.

EXPERIMENTAL SECTION

Materials and Characterization

The commercially available reagents were used without further purification. Ligands, TMU-6 and TMU-21, were synthesized following reported procedures. 33,34 See Supporting Information and Figure S1 for synthetic details. Elemental analyses were collected on a CHNS Thermo Scientific Flash 2000 elemental analyser. Powder X-ray diffraction (PXRD) measurements were collected on a Philips X'pert diffractometer with monochromatic Cu-Kα radiation (λ Cu = 1.5406 Å). Thermogravimetric analysis (TGA) were carried out in a Perkin Elmer Pyris 1 under N 2 atmosphere and heating rate of 10 °C/min. Fourier transform infra-red (FT-IR) spectra were recorded on a Bruker Tensor 27FTIR spectrometer equipped with a Golden Gate diamond attenuated total reflection (ATR) cell, in absorption mode at room temperature.

General synthesis of TMU-22, TMU-23 and TMU-24

A mixture of Zn(NO 3 ) 2 .6H 2 O (0.297 g, 1 mmol), H 2 oba (0.258 g, 1 mmol), and the corresponding amide ligand (0.5 mmol) and DMF (50 mL) was sonicated until all solids were uniformly dispersed (≈ 3 minutes) and divided into 7 glass vials. The vials were heated at 120 °C for 3 days and then cooled to room temperature. Colourless (TMU-22) and redbrown (TMU-23 and TMU-24) crystals were obtained as pure phases, washed by DMF, and dried at room temperature.

Crystallography

Crystallographic data for TMU-22 and TMU-23 were collected at 100 K at XALOC beamline at ALBA synchrotron 36 (λ = 0.79474 Å and 0.82653 Å, respectively). Data were indexed, integrated and scaled using the XDS 37 and IMOSFLM 38 programs. Absorption correction was not applied. SCXRD for TMU-24 was collected at 293 K on a Bruker AXS SMART Apex diffractometer using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) and were corrected for Lorentz and polarization effects. The frames were integrated with the Bruker SAINT 39 software package. Absorption corrections were applied using the program SADABS [START_REF] Krause | [END_REF] giving max./min. transmission factors of 1.000/0.270. The structures were solved by direct methods and subsequently refined by correction of F 2 against all reflections, using SHELXS2013 41 and SHELXL2013 42 within the WinGX package. 43 All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F 2 using the program SHELXL2013. Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. In TMU-24, the carbonyl group of the N,N-ligand is distorted in two positions with 55 % and 45 % of occupancy. The structures contain some disorder molecules. Attempts to adequately model of disordered DMF molecules (40 electrons) were unsatisfactory; therefore the PLATON/SQUEEZE [START_REF] Spek | PLATON A Multipurpose Crystallographic Tool[END_REF] routine has been applied to mask out the disordered electron density. The residual electron density was assigned to 3, 5 and 1 molecules of DMF in TMU-22 (509 electrons per unit cell), TMU-23 (876 electrons located per unit cell) and in TMU-24 (384 electrons per unit cell). Crystallographic and refinement data, and the main bond distances and angles are listed in Table 1 andTables S1-3.

Low-Pressure Sorption Measurements

Volumetric N 2 and CO 2 sorption isotherms were collected at 77 K (N 2 ) and 203 K, 258 K, 273 K, 288 K and 298 K (CO 2 ) using an ASAP 2020 HD (Micromeritics). Temperature was controlled by using a liquid nitrogen bath (77 K) or a Lauda Proline RP 890 chiller (203 K -298 K). The total pore volumes (Vt) were calculated at P/P0 = 0.95 (N 2 ) and at 760 torr (CO 2 ).

Kinetics of adsorption

Kinetic evaluation of the sorbents was performed at 298 K in a home-made glass manometric adsorption equipment using pure gas components (CO 2 and N 2 ). This equipment is constituted by a manifold of a well-known volume (to define the initial gas dose), a sample holder containing the adsorbent and a pressure transducer. Prior to the adsorption experiment, the samples were degassed under vacuum at the activation temperature used for single gas sorption experiments. The initial pressure in the manifold was defined at 500 mbar for all adsorbates. Once equilibrated, the manifold was expanded to the sample holder, pressure readings being recorded every second and lasting for ten minutes. Moreover, the sample holder was immersed in a temperature-controlled water bath at 298 K to minimize any heat effect due to the adsorption process itself.

Breakthrough measurements

Breakthrough curve experiments for different mixture of gases were carried out using a column. The samples powders were packed in the middle part of the column. Here, the samples masses used were 0.30 g. The flow rates of all gases were controlled by mass flow controllers. Before the measurement, the samples were activated at the desired temperature for 2 h using a He total flow of 50 mL.min -1 .

The gas stream from the outlet of the column was analysed online with a mass spectrometer.

The CO 2 /N 2 selectivities (σ) of samples were evaluated by the following equation: where q i is the adsorption capacity of i component, p i is the partial pressure of i component.

                =
The amount adsorbed for each gas was calculated by the integration of the breakthrough curves. For this purpose, breakthrough experiments with H 2 (not adsorbed in these materials), have been performed using the same bed and the same flows. Then, the H 2 breakthrough curve has been used to calculate the time zero and more importantly to consider the shape of the breakthrough in the integration. The amount of gas adsorbed was the different between the shape of the CO 2 or N 2 breakthrough and the H 2 breakthrough. Thus, the dispersion and the very small pressure drop can be considered.

Results and discussion

Synthesis and crystal structures

TMUs-22/-23/-24 were synthesized following the same pillaring strategy reported for TMU-6 and TMU-21. 33,34 Solvothermal reactions between Zn(NO 3 ) 2 .6H 2 O, H 2 oba and the correspondent pillar ligand in DMF at 120 o C for 3 days produced prismatic crystals of TMUs-22/-23/-24 suitable for single-crystal X-ray diffraction analyses. The simulated (derived from the single crystal structures) and experimental powder X-ray diffraction (PXRD) patterns were consistent (Figure S2), confirming that all synthesized MOFs can be obtained as pure phases; as also corroborated by Fourier transformed infrared spectroscopy and elemental analysis (performed on the activated samples, vide infra).

TMU-22 and TMU-23 crystallize in the monoclinic P2 1 /n and P2 1 /c space groups, respectively (Table 1). They are neutral 3-fold interpenetrated MOFs formed by {Zn(oba)} layers connected through the N,Nʹ-pillar ligands. As in TMU-6, their basic unit is a dinuclear zinc cluster, in which both Zn(II) centres are penta-coordinated to four carboxylate O atoms from three fully deprotonated oba ligands and to one N atom from the N,Nʹ-donor ligand (Figure 1). The geometry around the metal centres can be described as a distorted square pyramid, with Addison parameter 45 τ values in the range of 0.35-0.49. In these structures, each nonlinear dicarboxylate oba ligand is coordinated to three Zn(II) centres forming the {Zn(oba)} layers along the ab plane. Because of the dinuclear nature of the metal cluster, these layers are formed by two sheets of parallel oba ligands that form π-π stacking interactions between them (Figure 1; Tables S4,S5). These {Zn(oba)} layers are then pillared by the linear acylamidefunctionalized ligands, extending the structure in three dimensions. It is important to mention here that, due to the conformational flexibility of both oba and bpta ligands, TMU-22 is a supramolecular conformational isomer of a reported MOF. 31 Although both compounds are 3-fold interpenetrated, they still possess apparent 1D pore channels (≈ 7.0 × 6.5 Å and ≈ 6.8 × 6.4 Å -including van der Waals (vdW) radii-for TMU-22 and TMU-23, respectively) running along the <101> direction and noticeable free space (33 % for TMU-22 and 32 % for TMU-23), comparable to the values found for TMU-6 (≈ 8.1 × 7.4 Å; and 33 % free space) 46 (Figures 1 andS3-5). The channels are filled with disordered guest solvent molecules, squeezed due to thermal disorder. TMU-24 crystallizes in the monoclinic C2/c space group (Table 1), showing a crystal structure isoreticular to those of TMUs-6/-21/-22/-23. Thus, it also consists on a 3-fold interpenetrated network that exhibits 1D pore channels of ≈ 6.7 × 5.5 Å (free space = 30 %) functionalized with acylamide groups. These values are similar to those exhibited by its imine-functionalized TMU-21 analogue, which shows a pore size of ≈ 8.2 × 7.0 Å and a free space of 30 %. 46 (Figures S6-7).

Here it is very important to note that, even though TMUs-22/-23/-24 are isoreticular, their acylamide groups do not show the same orientation on the pore surfaces (Figure 2b,c,e) as well as do not participate in the same number of supramolecular interactions between the interpenetrated networks (Tables S4-6). In TMU-23, all donor -N-H and acceptor -C=O groups of the two acylamide groups per pillar ligand establish H-bonds and N-H•••π interactions with the other two networks forming the 3-fold interpenetrated structure. In TMU-22, each donor -NH moiety of the two acylamide groups per pillar ligand establishes H-bonds with carboxylate groups of the other networks, whereas the two acceptor -C=O groups do not participate in H-bonds. In TMU-24, only one of the two acylamide groups forms H-bonds. Interestingly, the other acylamide group is pointing towards the pores so that it does not participate in direct H-bonds with the other networks (Figure 2e). Altogether makes this acylamide group easily accessible to potential interactions with CO 2 guest molecules. Contrariwise, even though all the other H-bonded acylamide groups of TMUs-22/-23/-24 are located on the surfaces of the pores, they are not directly pointing towards the channels, and therefore they are in principle less accessible to guest molecules.

Topology

Topological analysis performed with TOPOS 47 revealed that all the studied MOFs show the same 3-fold interpenetrated, 6connected net with pcu topology (Figure 1). The pcu topology is ubiquitous in MOFs, including the iconic MOF-5. 48 Here, it is worth to mention that the pcu net is edge-transitive with cubic symmetry and Pm-3m space group. However, in the present case, the overall symmetry of TMUs-22/-23/-24 decreased to monoclinic (Table 1). This symmetry decrease is attributed to the fact that the points of extension of the Zn(II) dimer do not match with the vertices of the perfect octahedron required to produce highly symmetrical pcu-MOFs (Figure 1). In addition, the use of a pillaring strategy with mixed ligands (including the flexible oba and pillar N,Nʹ-ligands) also forces the symmetry to differ from that of MOF-5.

Thermal stability

The TGA diagrams of all as-synthesized materials revealed a first weight loss of 16.08 % (room temperature (r.t.)-300 °C) for TMU-6, 13.71 % (80-325 °C) for TMU-21, 9.93 % (100-240 °C) for TMU-22, 15.16 % (r.t.-300 °C) for TMU-23, and 15.46 % (r.t.-325 °C) for TMU-24. In all cases, this weight loss was attributed to the removal of all guest solvent molecules. Decomposition of all materials starts at around 400 o C, confirming their high thermal stability (Figure S8).

Activation process

All compounds were activated using a very similar protocol that started with a solvent-exchange step. TMUs-6/-21 and TMUs-22/-23/-24 were immersed in acetonitrile and dichloromethane, respectively, for 3 days, during which the solvent was exchanged once daily. Note here that previous stability tests showed that all synthesized MOFs were stable in these solvents (Figure S9) for at least 24 h at r.t. Then, each MOF was evacuated for 12 h under vacuum at 150 °C prior to gas sorption measurements. It is noteworthy that PXRD diagrams collected after the sorption studies confirm that the frameworks retain their crystallinity and remained unaltered after both processes (Figure S10). These observations were further corroborated by elemental analysis and TGA, in which only the weight loses around 400 o C corresponding to their decomposition were observed (Figure S8).

Nitrogen sorption at 77 K

N 2 isotherm at 77 K on TMU-22 showed a characteristic type I behaviour of microporous materials, as expected from its crystal structure (BET area (A BET ) = 680 m 2 .g -1 ; pore volume (V t ) = 0.28 cm 3 .g -1 ). Contrariwise, TMU-21, TMU-23 and TMU-24 were found to be non-porous materials (negligible N 2 uptake). In the case of TMU-6, it showed much lower porosity than that expected from its crystal structure (A BET = 150 m 2 .g -1 ; V t = 0.08 cm 3 .g -1 ) (Table 2, Figure S11). Thus, all activated compounds, except TMU-22, reveal a "N 2phobic" behavior. [49][50][51][52] We reasoned that this behaviour could be due to the existence of different structural rearrangements depending on the MOF during sorption or when exposed to cryogenic temperatures and/or under vacuum, reducing/preventing access to the porosity. Indeed, similar temperature dependent behaviours are already well-known in soft materials such as in MIL-53 (Al). 53 However, other factors cannot be fully excluded, such as the existence of strong interactions between N 2 and the channel walls at 77 K that hinder diffusion into the material. 54

CO 2 sorption at various temperatures and heats of adsorption

Contrariwise to the adsorption of N 2 , the CO 2 sorption isotherms at various temperatures revealed that all of them are porous (Figures 3a andS12a-16a). CO 2 uptake at 203 K were calculated to be 9.0 mmol.g -1 , 4.5 mmol.g -1 , 7.2 mmol.g -1 , 7.2 mmol.g -1 and 6.3 mmol.g -1 for TMU-6, TMU-21, TMU-22, TMU-23 and TMU-24, respectively (Table 2). The observed pore volumes (at 760 torr) were: 0.32 cm 3 .g -1 (TMU-6), 0.16 cm 3 .g -1 (TMU-21), 0.25 cm 3 g -1 (TMU-22), 0.27 cm 3 .g -1 (TMU-23) and 0.22 cm 3 .g -1 (TMU-24). These pore volumes calculated from the CO 2 adsorption data are in agreement with the theoretical values extracted from the crystal structures. The variations observed can be attributed to small structural changes widely observed in such interpenetrated sorbents. 35 Isosteric heats of adsorption of CO 2 (Q st ) were calculated from the isotherms collected at different temperatures (Table 2 and Figures S12-16). The two acylamide-functionalized TMU-24 and TMU-22 exhibited the highest Q st for CO 2 , from 24 to 26 kJ•mol -1 and 23 to 24 kJ•mol -1 , respectively. The different Q st for CO 2 between TMU-24, TMU-22, and TMU-23 can be attributable to the different accessibility of their acylamide groups and to the different number of supramolecular interactions in which they are involved (vide supra).

CO 2 /N 2 separation properties

In light of the porosity of the materials for CO 2 and their apparent low affinity for N 2 , we further investigated their potential for CO 2 /N 2 separation by performing kinetics experiments (Figures 4 andS17). As expected, the highest selectivity (σ) for CO 2 over N 2 was observed for TMU-24 (σ = 14.1, Table 3); which is the material exhibiting the highest Q st of CO 2 and with the most accessible acylamide groups pointing towards the porous channels. Overall, the CO 2 /N 2 selectivity of TMU-24 estimated from kinetics experiment is ca. 70 % higher than for the other materials of the family. These results were further confirmed by breakthrough experiments with CO 2 /N 2 gas mixture (Figures 5 andS18). Once again, TMU-24 performed much better than its isoreticular variants, with a CO 2 /N 2 selectivity of 10.0 at r.t. for an equimolar CO 2 /N 2 gas mixture (Figure 5). This value represents 700 % of the selectivity found in similar conditions for its imine analogue TMU-21 (σ = 1.4). It also performs better than TMU-22 and TMU-23, the other acylamide containing MOFs with less accessible functional groups. Indeed, the selectivity of TMU-24 is 500 % and 580 % the selectivity of TMU-22 and TMU-23, respectively. Therefore, not only the incorporation of amide functional groups in the frameworks but also their accessibility for the CO 2 guest molecules is crucial to enhance the CO 2 /N 2 selectivity. This observation was further confirmed in more realistic gas mixture conditions (CO 2 :N 2 = 1:9), in which TMU-24 was still found to perform well with a selectivity of 4.6 (Figure 5). It is important to remark that the CO 2 adsorption capacity found in the breakthrough experiment at r.t. (≈ 0.65 mmol.g -1 ) for TMU-24 is slightly lower than the one extracted from the isotherm at 500 mbar and r.t. (≈ 0.85 mmol.g -1 ). This feature can be attributed to a small amount of N 2 adsorbed in these conditions (298 K and gas mixture).

CONCLUSIONS

We successfully synthesized three novel, 3-fold interpenetrated pcu-MOFs containing acylamide functional groups (TMU-22, TMU-23 and TMU-24), isoreticular to the recently reported imine-containing MOFs, TMU-6 and TMU-21. The study of their sorption properties revealed that the insertion of amide functional groups in MOFs does not necessarily induce a noticeable improvement of the CO 2 /N 2 separation properties. Indeed, another aspect that needs to be addressed is the accessibility of the functional groups, especially in interpenetrated frameworks. If neglected, it could jeopardize the efficiency of MOFs functionalization, as illustrated by the performance of TMU-22 and TMU-23, two acylamide-functionalized analogues of the parent iminefunctionalized TMU-6; that all these three MOFs show negligible difference of the selectivity. This demonstrates the needs for smart materials design approaches, further confirmed by the deliberate alteration of the pore shape of TMU-24 using a bulky, central naphthalene core. This resulted in better accessibility of the acylamide groups,

Figure 1 .

 1 Figure 1. 2D layers formed by the association between Zn(II) ions and oba linkers are further pillared by amide/imine-functionalized dipyridyl-based ligands yielding to 3D-interpenetrated porous pcu-MOFs.

Figure 2 .

 2 Figure 2. View of the pore channels of a) TMU-6, b) TMU-22, c) TMU-23, d) TMU-21 and e) TMU-24, highlighting the acylamide groups. Colour code: N (green), O (purple), the more accessible O atoms in TMU-24 (lavender).

, 55- 57 Figure 3 .

 573 Figure 3. (a) Heats of adsorption of CO 2 and (b) CO 2 sorption isotherms at 298 K for TMUs-6/-21-24.

Figure 4 .

 4 Figure 4. Adsorption kinetics of CO 2 and N 2 for TMU-24 at 298 K (initial manifold pressure 500 mbar).

Figure 5 .

 5 Figure 5. CO 2 /N 2 Breakthrough separation curves at 298 K for TMU-24 with total flow of 5 mL.min -1 for CO 2 :N 2 = 1:1 (a) and 25 mL.min -1 for 1:9 (b).

Table 1 . Crystal and structure refinement data.

 1

	Compound	TMU-22	TMU-23	TMU-24
	Emp. formula		C 55 H 51 N 7 O 15 Z n 2	C 61 H 65 N 9 O 17 Z n 2	C 28 H 23 N 3 O 7 Zn
	Formula		1180.76	1326.96	578.86
	weight
	Crystal		Monoclinic	Monoclinic	Monoclinic
	system
	Space group P2 1 /n	P2 1 /c	C2/c
	CCDC ref.		1480227	1480228	1480229
	Unit cell dimensions
	a (Å)		14.680(4)	15.110(3)	14.881(4)
	b (Å)		24.780(5)	24.110(5)	24.993(6)
	c (Å)		16.270(5)	15.910(5)	16.013(4)
	β (deg)		113.27(5)	112.10(6)	111.422(5)
	V (Å 3 )		5437.0(2)	5370.2(2)	5544.0(2)
	Z		4	4	8
	F (000)		2440	2760	2384
	θ range (°)		1.850-33.822	1.626-33.681	1.630-
					28.396
	Ind. Refln.	11071	12947	6462
	(R int )		(0.1528)	(0.1164)	(0.0637)
	Goodness-of-fit on F 2	1.331	1.090	1.018
	Final	R	R1 = 0.1402	R1 = 0.0939	R1 = 0.0717
	indices		wR2 = 0.3805	wR2 = 0.2444	wR2	=
	[I > 2σ(I)]				0.1819

Table 2 . Summary of BET areas, total pore volumes and CO 2 uptakes observed for TMUs-6/-21-24.

 2

	MOF	A BET (m 2 .g -1 )	Total (cm 3 .g -1 )	V t	Theo V t (cm 3 .g -	CO 2 uptake b
		(N 2 , 77 K)	a CO 2 (N 2	b )	/	1 ) c	(mmol.g -1 ) (203 K)
	TMU-6	150	0.08 / 0.32	0.22	9.0
	TMU-21	≈ 0	≈ 0 / 0.16		0.25	4.5
	TMU-22	680	0.28 / 0.25	0.25	7.2
	TMU-23	≈ 0	≈ 0 / 0.27		0.27	7.2
	TMU-24	≈ 0	≈ 0 / 0.22		0.25	6.3
	a calculated at P/P 0 = 0.95. b Calculated at P = 760 torr. c Calculated with PLATON.

Table 3 Summary of CO 2 separation properties in TMUs- 6/-21/-22/-23/-24.

 3

	MOF	Q st of CO 2 (kJ.mol -1 )	Selectivity (Kinetics)	Selectivity (Breakthroug
				h,	CO 2 :N 2
				1:1)
	TMU-6	17-14	8.2	1.8
	TMU-21	18-20	8.2	1.4
	TMU-22	23-24	8.6	2.0
	TMU-23	12-18	8.2	1.7
	TMU-24	24-26	14.1	10.0

  leading into a 70 % improvement of CO 2 /N 2 selectivity according to kinetics studies (from 8.2 to 14.1 for TMU-21 and TMU-24, respectively). Moreover, a dramatic increase of the selectivity was obtained from breakthrough experiments. The CO 2 /N 2 selectivity for TMU-24 corresponds to ca. 700 % of this imine-containing analogue, TMU-21. Moreover, its selectivity is ca. 500 % and 580 % of TMU-22 and TMU-23, two isoreticular, amide containing MOFs with less accessible functional groups.

	a	0.004 0.005	CO 2 N 2			CO 2 :N 2 1:1
	2 or CO 2 /s	0.003
	mmol N	0.001 0.002
		0.000
		0	50	100	150	200	250	300
					time (s)
	b		CO 2 N 2			CO 2 :N 2 1:9
	2 /s
	mmol N 2 or CO
					time (s)

ACS Paragon Plus EnvironmentCrystal Growth & Design

ACKNOWLEDGMENTS

This work was supported by the MINECO-Spain through projects PN MAT2015-65354-C2-1-R, 2014-SGR-80, MAT2013-45008-P and EU FP7 ERC-Co 615954. I.I. and E.V.R.F. thank the MINECO for the RyC fellowship RYC-2010-06530 and RyC-2012-11427 and V.G. is grateful to the Generalitat de Catalunya for a Beatriu de Pinós Fellowship. ICN2 acknowledges the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2013-0295. E.V.R.F. and J.S.A. acknowledge the Generalitat Valenciana for PROMETEOII/2014/004. Support of this investigation by Tarbiat Modares University is gratefully acknowledged.

ASSOCIATED CONTENT

Supporting Information. Synthetic procedure for ligand synthesis, crystallographic data, PXRD diagrams, TGA plots, gas sorption and separation data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

* E-mails: daniel.maspoch@icn2.cat, morsali_a@modares.ac.ir

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

For Table of Contents Use Only

The influence of the amide groups in the CO 2 /N 2 selectivity of a series of isoreticular, interpenetrated metal-organic frameworks.

Vahid Safarifard, Sabina Rodríguez-Hermida, Vincent Guillerm, Inhar Imaz, Mina Bigdeli, Alireza A. Tehrani, Jordi Juanhuix, Ali Morsali,* Mirian E. Casco, Joaquín Silvestre-Albero, Enrique V. Ramos-Fernandez and Daniel Maspoch* The accessibility of amide functional groups within MOF structures is crucial to obtain enhanced CO 2 sorption and CO 2 /N 2 selectivity.

ACS Paragon Plus Environment Crystal Growth & Design


Vincent Guillerm

Luis Garzón-Tovar

Amirali Yazdi

Inhar Imaz
email: inhar.imaz@icn2.cat

Jordi Juanhuix

Daniel Maspoch
email: daniel.maspoch@icn2.cat[

Continuous One-Step Synthesis of Porous M-XF6based Metal-Organic and Hydrogen-Bonded Frameworks

Keywords: Metal-Organic Frameworks, Spray-Drying, CO2 sorption, Stability, Fluorinated Materials

Metal-organic frameworks (MOFs) built up from connecting M-XF6 pillars through N-donor ligands are among the most attractive adsorbents and separating agents for CO2 and hydrocarbons today. Here, we show the continuous, one-step spray-drying synthesis of several members of this isoreticular MOF family varying the anionic pillar (X = [SiF6] 2-and [TiF6] 2-), the N-donor organic "This is the peer reviewed version of the following article: V.

INTRODUCTION

The environmental impact associated to the energy demand is a major problem worldwide. [1] For example, CO2 emission caused by humanity -CO2 concentration at the South Pole recently passed the milestone of 400 ppm for the first time in the last 4 million years - [START_REF]South Pole is the last place on Earth to pass a global warming milestone[END_REF] highly contributes to the climate change. In this sense, 2016 has also been the first year that the weekly CO2 concentration average monitored in the Mona Loa observatory did not go below this key value, meaning that the average global temperature is likely to increase more than the 1.5 oC warming threshold. [START_REF]Up-to-date weekly average CO2 at Mauna Loa[END_REF] To address the current and future energy needs while mitigating the environmental impact, one of the strategies has been the development of efficient CO2 capture, storage and separation materials for achieving cleaner combustible supplies. [4] These materials mainly include zeolites, activated carbons, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs).[5] Among these innovative materials, an old-fashioned class of fluorinated materials [6] have recently been brought back to the spotlights by Eddaoudi and Zaworotko groups thanks to their exceptional uptake and selectivity towards CO2 and hydrocarbons. [7] These MOFs (Figure 1) are constructed from the assembly of pre-made M-XF6 pillars (M = Co, Ni, Cu, Zn; X = Si, Ti, Sn, Zr, Ge, V, Ga) with N-donor type ligands (e.g. pyrazine (pyz), pyridil-based ligands etc.). However, despite these great developments, scientific community and industrials still need to join their efforts for transferring these materials from the laboratory to industry. A very important step here is the optimization of their fabrication. [8] This fabrication must always envision fast and scalable one-step processes that produce ready-to-use products without the need of additional purification and drying steps. Here we report a synthetic method that allows producing several of the isoreticular M-XF6-based CO2 sorbents fulfilling all these requirements.

Our team recently introduced an industrially well-established spray-drying (SD) technique as a new synthetic way to prepare various MOFs (Scheme S1).[8l, 9] The SD technique is a scalable and fast method allowing continuous synthesis of MOFs in the form of spherical superstructures or beads based on the assembly of nanosized crystals.[9c] The strong expertise acquired from these previous studies, among several reports suggesting the capital importance of the formation of the inorganic secondary building unit (SBU) for the nucleation and growth of MOFs,[10] convinced us that premade pillars of the M-XF6 MOF platform would be ideal candidates for SD synthesis.

We therefore successfully embarked in the SD-based synthesis of several M-XF6 based materials, showing that this method is also compatible for reticular synthesis and metal tuning, ligand elongation and pillar substitution. Importantly, their rapid synthesis (few minutes versus few hours up to few days) does not negatively affect their sorption properties, demonstrating in most cases its ready-to-use character without the need of additional purification steps or time consuming, repeated solvent exchange procedures. Moreover, we also demonstrate that SD can be used not only to synthesize porous materials based on coordination bonds but also based on hydrogen bonds.

RESULTS AND DISCUSSION

SIFSIX-3-M Materials

Synthesis and characterization

The one-step SD synthesis consisted on the combined atomisation of two methanolic solutions containing (i) M-SiF6 (M = Co, Cu, Zn) and (ii) pyz at 85 oC, which produced fine powders that were collected with a minimum amount of methanol (MeOH). Collection of these powders in methanol was a crucial protection step as we found that they were air-sensitive (vide infra). To assess the quality of the as-made SIFSIX-3-M MOFs, their CO2 sorption properties were compared with those of their bulk analogues. For this, the SD-synthesized SIFSIX-3-M collected in methanol were directly transferred from the spray drier collector to sorption cells, and dried and evacuated for 12 h at 65 oC. Then, their CO2 uptake at 298 K was measured. Notably, we confirmed that spray-dried SIFSIX-3-M did not require additional solvent exchange or purification step to exhibit remarkable CO2 capacities at low pressure and 298 K (Figure 2h, Table 1), with an uptake less than 10 % lower than the reported bulk materials. SD-synthesized SIFSIX-3-M were further characterised by powder X-ray diffraction (PXRD), from which experimental patterns were in excellent agreement with the theoretical diagrams calculated from the corresponding structures (Figure 2g), demonstrating both the high crystallinity and purity of the MOFs.

The morphology of the materials was also investigated by field-emission scanning electron microscopy (FE-SEM), showing in all cases the occurrence of nanosized SIFSIX-3-M crystals assembled into spherical superstructures or beads; a shape that is typical from MOFs assembled by the SD method. The sizes of these superstructures were 7.9 ± 4.8 µm for SIFSIX-3-Co, 6.9 ± 3.4 µm for SIFSIX-3-Cu, and 3.5 ± 2.7 µm for SIFSIX-3-Zn (Figure 2 a-c and Figure S14a).[9, "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

6 11] Moreover, FE-SEM images confirmed the homogeneity of the materials, as already suggested by the absence of crystalline impurity peaks in the PXRD diagrams.

To finally determine the size of crystals composing the superstructures, they were disassembled by sonication and immediately transferred to a transmission electron microscopy (TEM) grid.

TEM images confirmed the formation of nanocrystals with a size of 32 ± 13 nm for SIFSIX-3-Co, 80 ± 12 nm for SIFSIX-3-Cu, and 28 ± 9 nm for SIFSIX-3-Zn (Figure 2d-f and Figure S14b).

Table 1 Comparison of the CO2 uptakes (760 torr, 298 K) in spray-dried and bulk SIFSIX-3-M MOFs.

CO2 uptake at 760 torr and 298 K (mmol.g -1 )

MOF

Bulk Sprayed (% of bulk) SIFSIX-3-Co ≈ 2.79 [7k] 2.56 (92 %) SIFSIX-3-Cu ≈ 2.40 [7b] 2.23 (93 %) SIFSIX-3-Zn ≈ 2.46 [7a] 2.27 (91 %)

Stability in different media

"This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. 7

The use of methanol to collect the fine powders produced with the SD method is capital to protect and use the as-made SIFSIX-3-M MOFs. [12] Without this precaution, all nanosized SIFSIX-3-M MOFs suffered a fast degradation and a loss of their CO2 sorption properties when they were exposed to ambient conditions. In addition, following the optimized washing procedure reported by Nugent et al.,[7a] we observed that the SD-synthesized SIFSIX-3-Zn was instantaneously solubilized upon addition of N,N-dimethylformamide (DMF), leading to a clear solution. Re-spray drying attempts of this clear solution did not allow re-assembling the SIFSIX-3-Zn framework.

Instead, colourless single crystals (versus the yellow colour of SIFSIX-3-Zn crystals) appeared in the DMF solution after a period of ca. a month. This unknown structure, which crystallizes in P-1 space group, was solved by single crystal X-ray diffraction and appeared to be a cationic 1D It is important to mention here that a similar behaviour was found when immersing SIFSIX-3-Zn in water, also leading to a complete solubilisation of the crystals. The incubation of nanosized SIFSIX-3-Zn in other organic solvents, including acetonitrile, hexane, dichloromethane, chloroform, toluene, tetrahydrofuran and acetone, did not result in the solubilisation of the crystals but a fast phase transition into unidentified crystalline powders (Figure S20).

The only tested solvent in which SIFSIX-3-Zn showed certain stability was MeOH. Initial incubation studies using microcrystals instead of nanocrystals -to easily follow the evolution by FE-SEM-showed that SIFSIX-3-Zn is also etched and finally solubilized in MeOH (Figure S13).

However, we found that the minimum amount of MeOH needed for the complete solubilisation of the nanosized SIFSIX-3-Zn without stirring was around 0.14 mLMeOH per mg SIFSIX-3-Zn. In addition, below this amount of MeOH, SIFSIX-3-Zn remained stable and did not suffer any phase transition, as confirmed by PXRD. It is also worth to mention that a similar behaviour occurred for the Cu and Co analogues. To this end, altogether these observations allowed defining MeOH as the best solvent to collect these materials, which were in all cases collected using no more than 0.015 mLMeOH per mg of MOF. nm (e).

Reticular chemistry: organic ligand and anionic pillar tuning

Synthesis and characterization of SIFSIX-1-Zn. In line with global efforts of the "MOF community" to use some network topologies as design platforms to rationally synthesize isoreticular MOFs,[13] we intended to take advantage of the already reported versatility of the M-XF6 platform[6c, 6e, 7a, 7b, 7e-g, 7k] to demonstrate the suitability of the SD method for reticular chemistry. Following the successful synthesis of SIFSIX-3-M (M = Co, Cu and Zn), we successfully achieved the synthesis of an expanded analogue, SIFSIX-1-Zn,[6e] by replacing the pyz ligand by the longer 4,4'-bipyridine (bpy). Here, a methanolic solution of ZnSiF6 was spraydried along with a methanolic solution of bpy using a 3-fluid nozzle at 85 oC. The resulting yellow spherical superstructures (size = 7.9 ± 3.6 µm; Figure 3c and Figure S14a) were collected and washed with MeOH. The material was obtained as a pure phase, as confirmed by PXRD (Figure 3b). Also, it was found to be porous to N2 at 77 K, exhibiting an apparent BET area of 1300 m2.g-1 (Table 2, Figure S21). Again, the nanosized nature of the SIFSIX-1-Zn crystals was confirmed by TEM performed after disassembling the superstructures by sonication. The size of these nanocrystals was 20 ± 5 nm (Figures S14b,S24).

Synthesis and characterization of TIFSIX-1-Cu

Recently, Nugent et al. [7f] reported the possibility to not only vary the metal in this type of pcu-MOFs, by achieving the replacement of the pillaring [SiF6] 11 Cu(NO3)2•2.5(H2O) and TiF6(NH4)2 was spray-dried along with a methanolic solution of bpy using a 3-fluid nozzle at 150 o C. The resulting grey/purple spherical superstructures (size = 5.3 ± 3.1 µm; Figure 3d and Figure S14a) were collected with MeOH. Their PXRD diagram was found to be in excellent agreement with the theoretical diagram (Figure 3b). N2 sorption performed at 77 K revealed an apparent BET area of 1650 m 2 .g -1 . This value is very similar to that reported for the bulk material (1690 m 2 .g -1 ) [7f] demonstrating the high quality of the SD-synthesized TIFSIX-1-Cu MOF (Table 2, Figure S22). The nanosized nature of the TIFSIX-1-Cu crystals (size = 43 ± 9 nm, Figures S14b,S25) was again confirmed by TEM performed after disassembling the superstructures by sonication.

Synthesis and characterization of supramolecular open material

Finally, due to the apparent ease of synthesis of these M-XF6 based MOFs by SD technology, we selected another porous material, MPM-1-TIFSIX,[7d] based on the supramolecular assembly of [Cu2(ade)4(TiF6)2] (ade = adenine) paddlewheels (Figure 3a). It is worth to mention that, in the present case, the selected material to be synthesized by SD was not a MOF but a supramolecular hydrogen-bonded network. To our knowledge, the aerosol synthesis of such kind of open materials based on weak bonding has not been reported yet.

Table 2 BET areas and pore volumes for SIFSIX-1-Zn, TIFSIX-1-Cu and MPM-1-TIFSIX (N2, 77K

).

An aqueous solution of Cu(NO3)2•2.5(H2O) and TiF6(NH4)2 was spray-dried along with a solution of adenine in water/acetonitrile mixture using a 2-fluid nozzle (Scheme S3) at 150 o C. The resulting grey/purple spherical superstructures (size = 6.1 ± 3.1 µm; Figure 3e and Figure S14a) were collected with MeOH. Remarkably, the PXRD diagram was found to be in excellent agreement with the theoretical one (Figure 3b). N2 sorption performed at 77 K after methanol solvent exchange procedure revealed an apparent BET area of 805 m 2 .g -1 (996 m 2 .g -1 for bulk), [7d] demonstrating the porosity of the SD-synthesized MPM-1-TIFSIX supramolecular material (Table 2, Figure S3). In this case, TEM experiments performed after disassembling the superstructures by sonication showed MPM-1-TIFSIX crystals with a size of 201 ± 65 nm (Figures S14b,S26).

CONCLUSION

We reported here the one-step and continuous synthesis of various M-XF6 based MOFs using the SD technique. Using this method, these M-XF6 based MOFs could be synthesized at the nanoscale. There is no doubt that the possibility of obtaining and stabilizing nanosized M-XF6 and 298 K (CO2) using an ASAP 2020 HD (Micromeritics). Temperature was controlled by using a liquid nitrogen bath (77 K) or a Lauda Proline RP 890 chiller (298 K). For N2 sorption at 77 K, micropore volumes (Vmicro) were calculated at P/P0 = 0.3, whereas the total pore volumes (Vt)

were calculated at P/P0 = 0.95. Field-emission scanning electron microscopy (FE-SEM) images were collected on scanning electron microscopes (FEI Magellan 400L XHR and Quanta 650 FEG) at acceleration voltage of 1.0 kV, and using dry powder on carbon as support. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM 1400 at 100 kV. Prior to TEM, all superstructures immersed in MeOH were disassembled by sonication for 10 s in a Fisher Scientific FB15051 sonicator and then, directly transferred on the TEM grids. Average size range of superstructures and crystals has been calculated on 100 crystals/superstructures using the Image J software.

Stability study of SIFSIX-3-Zn A 6 mL methanolic solution of 300 mg (1.45 mmol, anhydrous based) of ZnSiF6•xH2O was injected to a 6 mL methanolic solution of 325 mg (4.05 mmol) of pyz at room temperature, without stirring. After 1 h, 1 mL of the solution containing the resulting microcrystals was pipetted from the middle of the vial and mixed with 2 mL of MeOH. Aliquots of the sample were transferred immediately to the microscope to avoid contact with air, and the morphology of the crystals was studied by FE-SEM after 15 min (Figure S13).

Crystallography

Crystallographic data for 1 were collected at 100 K at XALOC beamline at ALBA synchrotron [14] (λ = 0.79472 Å). Data were indexed, integrated and scaled using the XDS program. [15] Absorption correction was not applied. The structure was solved by direct methods and subsequently refined by correction of F2 against all reflections, using SHELXS2013 [16] and SHELXL2013 [17] within the WinGX package. [18] All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using the program SHELXL2013.

Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters.

ASSOCIATED CONTENT Supporting Information.

Figure 1 .

 1 Figure 1. Schematic representation of the different isoreticular pcu M-XF6-based MOFs

Figure 2 .

 2 Figure 2. Representative FE-SEM images of (a) SIFSIX-3-Co, (b) SIFSIX-3-Cu, and (c) SIFSIX-

Figure 3 .

 3 Figure 3. Schematic for the synthesis and structure of MPM-TIFSIX-1 (a); comparison of

  Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

"

  This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J.Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

  This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J.Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

	2-anion by [TiF6] 2-. Bulk TIFSIX-1-
	Cu is commonly synthesized using a layering of methanol and ethylene glycol at room
	temperature. However, this synthesis could not be reproduced in the spray-drier due to the high
	boiling point of ethylene glycol (197.3

o C). For this reason, we successfully replaced ethylene glycol by water, adjusting the SD temperature to 150 o C. Thus, an aqueous solution of "

  This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J.Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 13 based MOFs opens new avenues on the re-exploration of this old-fashioned sub-class of MOFs for emerging applications. In addition, we demonstrated the suitability of the SD method to perform fine structural tuning via ligand size variation (pyz versus bpy), anionic pillar substitution ([SiF6]2-versus [TiF6]2-), and variation of metal ion (Co, Cu, and Zn). Importantly, this novel way to synthesize these materials does not jeopardize their sorption properties. This fact together with the short synthesis times and the absence of time consuming purification and solvent exchange procedures demonstrates the ability and competitiveness of SD methods versus conventional ones for the fast production of ready-to-use sorbents. Finally, we anticipate the first aerosol synthesis of a supramolecular, hydrogen bonded porous network, MPM-1-TIFSIX, which opens new avenues for the synthesis of this class of porous materials using SD.

	MOF SIFSIX-1-Zn TIFSIX-1-Cu MPM-1-TIFSIX IR bands (cm-1): 715 (m), 789 (m), 827 (w), 806 (s), 889 (m), 938 (m), 1037 (w), 1107 (w), 1149 ABET (m 2 .g -1 ) Vmicro (cm 3 .g -1 ) at P/P0 = 0.3 Vt (cm 3 .g -1 ) at P/P0 = 0.95 Theo Vt (cm 3 .g -1 ) 1300 0.53 0.58 0.68 1650 0.66 0.88 0.70 805 0.32 0.32 0.39 (w), 1231 (s), 1314 (s), 1409 (s), 1607 (s), 1652 (s), 1702 (w), 3089 (w), 3200 (w), 3419 (w) (Figure S6). Characterization PXRD diagrams were collected on a Panalytical X'pert diffractometer with monochromatic Cu-Kα radiation (λCu = 1.5406 Å) under a Kapton film. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27FTIR spectrometer equipped with a Golden Gate diamond attenuated total reflection (ATR) cell, in transmittance mode at room temperature. Main IR bands (Figures S1-6) are reported as follow: strong (s), medium (m), weak (w) and broad (br). TGA "EXPERIMENTAL SECTION curves (Figures S7-12) were carried out in a Perkin Elmer Pyris 1 under O2 atmosphere and a
	heating rate of 10
	Materials
	All the materials were synthesized using a Mini Spray Dryer B-290 (BÜCHI Labortechnik). All
	solvents and reagents were purchased from Sigma-Aldrich, City Chemicals and Scharlab and used
	as received.

o C.min -1 . Volumetric N2 and CO2 sorption isotherms were collected at 77 K (N2)

"This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." ACKNOWLEDGMENT This work was supported by the MINECO-Spain through projects PN MAT2012-30994, 2014-SGR-80, EU FP7 ERC-Co 615954, and European Union's Horizon 2020 research and innovation programme under grant agreement No 685727. V.G. is grateful to the Generalitat de Catalunya for a Beatriu de Pinós fellowship (2014 BP-B 00155) and I.I. thanks the MINECO for a Ramón y Cajal fellowship (RYC-2010-06530). A.Y. and ICN2 acknowledge the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2013-0295.

Synthesis of SIFSIX-3-Co

A 6 mL methanolic solution of 300 mg (1.49 mmol) of CoSiF6 and a 6 mL methanolic solution of 325 mg (4.05 mmol) of pyz were simultaneously spray-dried using a 3-fluid nozzle (Scheme S2), a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature of 85 oC, and a spray cap with a 0.5 mm hole. The light pink powder of SIFSIX-3-Co was recovered with a minimum amount of methanol (MeOH) (235 mg; 44% yield based on Co). IR bands (cm-1): 690 "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 14 (s), 1062 (s), 1124 (w), 1153 (m), 1421 (s), 1628 (m), 1660 (m), 3196 (br), 3403 (br), 3522 (w) (Figure S1).

Synthesis of SIFSIX-3-Cu

A 6 mL methanolic solution of 300 mg (1.34 mmol) of CuSiF6•H2O and a 6 mL methanolic solution of 325 mg (4.05 mmol) of pyz were simultaneously spray-dried using a 3-fluid nozzle, a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature of 85 oC, and a spray cap with a 0.5 mm hole. The blue powder of SIFSIX-3-Cu was recovered with a minimum amount of MeOH (272 mg; 55% yield based on Cu). IR bands (cm-1): 686 (s), 822 (s) 1078 (s), 1132 (s), 1161 (s), 1429 (s), 1652 (m), 1660 (m), 3147 (m), 3325 (br) (Figure S2).

Synthesis of SIFSIX-3-Zn

A 6 mL methanolic solution of 300 mg (1.45 mmol, anhydrous based) of ZnSiF6•xH2O and a 6 mL methanolic solution of 325 mg (4.05 mmol) of pyz were simultaneously spray-dried using a 3-fluid nozzle, a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature of 85 oC, and a spray cap with a 0.5 mm hole. The yellow powder of SIFSIX-3-Zn was recovered with a minimum amount of MeOH (305 mg; 57% yield based on Zn). IR bands (cm-1): 686 (s), 822 (w), 1062 (s), 1099 (w), 1128 (w), 1161 (w), 1425 (s), 1635 (s), 1660 (m), 3432 (br) (Figure S3).

Synthesis of SIFSIX-1-Zn

A 6 mL methanolic solution of 300 mg (1.45 mmol, anhydrous based) of ZnSiF6•xH2O and a 6 mL methanolic solution of 650 mg (4.16 mmol) of bpy were simultaneously spray-dried using a 3-fluid nozzle, a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 15 of 85 o C, and a spray cap with a 0.5 mm hole. The yellow powder of SIFSIX-1-Zn was recovered with a minimum amount of MeOH (302 mg; 40 % yield based on Zn). Sample was then washed with 5 mL of MeOH to remove potential contamination with the highly soluble, unreacted precursors. IR bands (cm-1): 632 (m), 649 (s), 764 (m), 806 (s), 851 (s), 1008 (m), 1062 (s), 1223 (m), 1317 (w), 1413 (s), 1491 (m), 1536 (m), 1611 (s), 1644 (w), 3230 (w), 3345 (br) (Figure S4).

Synthesis of TIFSIX-1-Cu

A 6 mL aqueous solution of 30 mg (0.15 mmol) of TiF6•(NH4)2 and 35 mg (0.15 mmol) of Cu(NO3)2•2.5(H2O) and a 6 mL methanolic solution of 46.8 mg (0.30 mmol) of bpy were simultaneously spray-dried using a 3-fluid nozzle, a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature of 130 oC, and a spray cap with a 0.5 mm hole. The blue powder of TIFSIX-1-Cu was recovered with a minimum amount of MeOH (64 mg; 79 % yield based on Cu). IR bands (cm-1): 632 (s), 678 (w), 728 (m), 810 (s), 851 (w), 921 (w), 1012 (s), 1066 (s), 1223 (m), 1280 (s), 1326 (w), 1409 (s), 1471 (m), 1491 (m), 1536 (m), 1607 (s), 1644 (w), 3101 (w), 3283 (w), 3357 (w) (Figure S5).

Synthesis of MPM-1-TIFSIX

A 12 mL aqueous solution of 30 mg (0.15 mmol) of TiF6•(NH4)2 and 35 mg (0.15 mmol) of Cu(NO3)2•2.5(H2O) and a 12 mL H2O:CH3CN (1:1 vol.) solution of 82 mg (0.60 mmol) of adenine were simultaneously spray-dried using a 2-fluid nozzle and T-shaped system (Scheme S3), a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature of 150 oC, and a spray cap with a 0.5 mm hole. The purple/grey powder of MPM-1-TIFSIX was recovered with a minimum amount of MeOH (110 mg; 74% yield based on Zn). Prior to sorption measurements, this powder was immersed in MeOH for 3 days, refreshing the MeOH twice a day.

"This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

AUTHOR INFORMATION

Corresponding Author * E-mail: inhar.imaz@icn2.cat, daniel.maspoch@icn2.cat

Notes

There are no conflicts to declare.


David Rodríguez-San-Miguel

Amirali Yazdi

Vincent Guillerm

Javier Pérez-Carvajal

Víctor Puntes

Daniel Maspoch

Félix Zamora

Confining functional nanoparticles into colloidal iminebased COF spheres by a sequential encapsulationcrystallization method

Here we report a two-step method that enables imparting new functionalities to covalent-organic frameworks by nanoparticle confinement.

The direct reaction between 1,3,5-tris(4aminophenyl)benzene and 1,3,5-benzenetricarbaldehyde in presence of a variety of metallic/metal oxide nanoparticles results in the nanoparticles being embedded in amorphous and non-porous imine-linked polymer organic spheres, NP@a-1. Post-treatment reactions of NP@a-1 with acetic acid under reflux lead to crystalline and porous imine-based COF hybrid spheres, NP@c-1. Interestingly, Au@c-1 and Pd@c-1 have been found to be catalytically active.

The confinement of metallic and metal oxide nanoparticles (NPs) in porous materials is a fruitful strategy to develop composites in which the porous matrix acts both as a dispersive and protective media for the NPs while providing effective and selective accessibility to them. Thanks to these properties, this class of composites shows promises for myriad applications, including heterogeneous catalysis, [1] sensing, [2] gas storage, [3] plasmonics, [4] remediation, [5] batteries, [6] biocides, [7] magnetic refrigeration, [8] and controlled guest release. [9] To date, different porous materials have been explored for producing these composites, including activated carbon, [10] silica, [11] microporous zeolites, [12] porous organic polymers [13] and, more recently, metal-organic frameworks (MOFs). [14] Among the new porous materials available today, their covalent counterparts covalentorganic frameworks (COFs) are also emerging as attractive candidates for the elaboration of these composites. As MOFs, COFs are attainable in various pore sizes and shapes, with large surface areas and tailored pore surfaces. Despite their challenging crystallization, [15] COFs show better thermal and chemical (e.g. aqueous, alkali and acidic) stability, which can be a crucial feature for certain applications, [16] such as in heterogeneous catalysis, remediation and biomedicine. Recently, NPs have begun to be incorporated in COFs by infiltration of the NP precursors and subsequent growth of the NPs inside the pores. [17] In MOFs, an alternative approach that has proved very productive for making a wide variety of MOF/NP composites is the encapsulation of pre-synthesized NPs into the MOF crystals. [18] This approach allows controlling the size, shape and composition of the NPs and therefore, their inherent properties. [14a] It also allows improving their dispersion and controlling the number and localization of these NPs inside the MOF crystals. [19] Thus far, however, no one has demonstrated yet the ability to apply this approach in COF particles. This is in part because of the incompatibility of the NP chemistry and that used to synthesize and crystallize COFs.

Here, we report a two-step strategy that enables the encapsulation of several types of NPs into imine-based COF spheres (Scheme 1). The first step is based on the encapsulation of NPs into amorphous imine-based spheres under mild conditions. This process resembles the traditional encapsulation technologies widely used for the entrapment of species into colloidal amorphous purely organic or metal-organic polymer spheres. [20] Scheme 1. Schematic representation of the two-step encapsulationcrystallization process used to incorporate functional NPs into porous and crystalline COF spheres. (d) spheres. e-h) Evolution of the roughness of the COF spheres at t = 0 h (e), 2 h (f), 24 h (g) and 168 h (h). i-l) Evolution of the crystallinity (i), porosity (j, k), and water sorption uptake (l) during the amorphous  crystalline transformation. In k, ABET (red), micropore volume (blue) and total pore volume (black). In l, water sorption isotherms of a-1 (pink), a-1 treated for 24 h (magenta), and c-1 (blue).

The second step is based on the possibility to transform solid amorphous imine-like polymers to their crystalline COF analogues by exposing them to acidic conditions at 70 ºC. Initially reported by Dichtel et al., [21] a variation of this was recently used by Guo et al. to coat single  200-300 nm-indiameter iron oxide spheres with a layer of an imine-based COF for photothermal therapy. [17] Remarkably, in the present study we show that this second crystallization step preserves the initial colloidal property of the amorphous imine-based spheres and the dispersion, size, shape and properties of the embedded NPs.

The design of our method began with the study of the formation of the amorphous imine-based polymer spheres and their subsequent transformation into their analogue porous COF spheres (see details in ref. [22] ). To this end, two trigonal building blocks, 1,3,5-tris(4-aminophenyl)benzene and 1,3,5benzenetricarbaldehyde, were dissolved in an acetone/acetic acid mixture and stirred for 1 h at room temperature. [22] The resulting yellow suspension was centrifuged (18407 rcf) for 2 min, and the collected solid was finally washed twice with acetone and tetrahydrofuran, and finally dried for 48 h at room temperature and for 24 h at 150 ºC under vacuum. Fieldemission scanning (FESEM) and transmission (TEM) electron microscopy images and X-ray powder diffraction (XRPD) studies revealed that this fast condensation reaction produced the kinetically favorable, amorphous spheres with diameter of ca. 600 nm (hereafter called amorphous-1 or a-1; Figure 1a,c andi). The spectroscopic and analytical characterization of a-1 agrees with those data already reported (Figures S1-S3). [22] The formation of the more thermodynamically stable longrange ordered structure was then achieved under reversible bond formation conditions. For this, a-1 spheres were treated with acetic acid containing a controlled amount of water under reflux in a mixture of mesitylene/dioxane for 2, 12, 24, 48 and 168 h. Then, each one of the synthesized solids was collected by centrifugation (2348 rcf), washed with toluene, and dried for 48 h at room temperature and for 24 h at 150 ºC under vacuum. The spectroscopic and analytical characterization of c-1 does not show any significant change to that observed for its amorphous precursor a-1 (Figures S1-S4). The evolution of the amorphous  crystalline transformation was then followed by analyzing these solids by TEM, FESEM, XRPD, and gas and water sorption studies (Figure 1). TEM images showed submicrometric spheres (ca. 600 nm diameter) for all samples (Figure 1c-h), thereby confirming that this treatment retains the shape and size of the initial a-1 spheres. However, a closer look at the texture revealed a gradual increase in the roughness of the material for longer treatment times. XRPD clearly showed a gradual evolution of the crystallinity as a function of time (Figure 1i), with the appearance of peaks that properly match those calculated for the expected eclipsed COF structure (rather than the staggered) (Figure S6). [23] Thus, we assumed that the increase in roughness detected during the transition from a-1 to the spheres treated for 168 h (hereafter called crystalline-1 or c-1) is due to the formation of crystalline domains based on COF nanolayers that are confined within the spheres (Figure 1a-f).

This phenomenon was further confirmed by the calculation of the apparent Brunauer Emmet Teller (BET) area from N2 sorption experiments performed at 77 K (Figures S7-12), showing a dramatic, gradual increase (Figure 1j,k) from nonporous (ABET = 25 m 2 .g -1 , Vt = 0.02 cm 3 .g -1 ) for a-1 up to highly porous for c-1 (ABET = 1120 m 2 .g -1 , Vt = 0.61 cm 3 .g -1 ) after 168 h of treatment. The collected isotherms were found to be, according to IUPAC classification, type IV isotherm presenting H2 type hysteresis loop. This behavior, commonly attributed to the presence of some mesoporosity, is in the present case more likely due to some structural swelling -as sometimes observed in this type of materials [24] or a combination of both.

In light of this porosity, we also investigated the CO2 sorption (Figure S13) and water vapor sorption (Figure 1l) properties of a-1 and c-1. As many amorphous polymers and despite its nonaccessibility to N2, a-1 was found to be slightly porous to CO2, with a total uptake (203 K, 760 Torr) of 3.9 mmol.g -1 . As expected, c-1 material exhibited a much higher total uptake in similar conditions (14.3 mmol.g -1 ). The heats of adsorption (Qst) derived from the isotherms collected at various temperatures between 258 K and 298 K were found to be comprised between 35 and 20 kJ.mol -1 for a-1 and between 25 and 19 kJ.mol -1 for c-1 (Figures S14,S15). These values are in the expected range of energies for a material without specific group expected to interact with CO2. The higher Qst observed for the amorphous material can be attributed to a smaller pores size (a-1 is nonporous to N2).

Water vapor sorption isotherms showed that the amorphous  crystalline transformation induces a gradual transition from hydrophobic to hydrophilic behavior (Figure 1l). Water uptake of a-1 increased monotonically, achieving a total uptake of only 0.04 gwater.gCOF -1 . Interestingly, the development of porosity leads to an increase of the uptake up to 0.21 gwater.gCOF -1 and 0.37 gwater.gCOF -1 after 24 h and 168 h of treatment, respectively. This behavior can be correlated with the gradual structural rearrangement from a-1 into c-1 upon treatment. The water isotherm of c-1 can be ascribed as type V, which is a typical behavior of porous materials with hydrophobic walls. [25] Once we confirmed that the chemical treatment leading to the rearrangement of the amorphous, non-porous a-1 spheres into crystalline, highly porous c-1 spheres can take place, we envisioned the use of the two-step process to confine NPs into COFs. In this process, the fast formation of a-1 spheres should allow the encapsulation of NPs at room temperature. Then, because the NPs are encapsulated and protected, the amorphous  crystalline transformation at high temperatures should occur without compromising the properties of the NPs.

To prove the efficiency of this strategy, we initially started with the encapsulation of Fe3O4 NPs (9.8 ± 3.9 nm). These NPs were synthesized using well-established methods, functionalized with polyvinylpyrrolidone (PVP) and dispersed in acetone. [26] These Fe3O4 NPs were incorporated into the acetone/acetic acid mixture of the two trigonal building blocks, and the two-step process was reproduced using the above-mentioned conditions. HAADF-STEM micrographs and XRPD studies of the solids collected after the first and second step showed the successful formation of Fe3O4@a-1 and Fe3O4@c-1 submicron spheres containing Fe3O4 NPs dispersed mostly in their center (Figure 2a,b and Figure S16). The chemical composition of Fe3O4@c-1 spheres determined by energy-dispersive X-ray (EDX) microanalysis revealed that every sphere contained iron, oxygen, nitrogen and carbon (Figure S18). The content of Fe in the composites was estimated by ICP-OES, from which a Fe3O4 NP content of 7.6 % w/w in the composite was determined. In addition, colloidal Fe3O4@c-1 spheres showed magnetic attraction when they were exposed to a magnet (Figure 2c). So, it can be concluded that NPs encapsulated within the COF spheres are the initial Fe3O4 NPs, and therefore, that the two-step process does not modify the properties of the embedded NPs. Also, the permanent porosity of Fe3O4@c-1 spheres was evaluated by N2 sorption at 77 K. As expected, the sample was found to be porous, with an apparent BET area lower than the c-1 spheres (Figure 2d, ABET = 880 m 2 .g -1 , Vt = 0.38 cm 3 .g -1 ).

We then extended the use of this process to the encapsulation of other types of NPs. Using the same strategy, c-1 spheres containing Au (9.0 ± 2.4 nm) and Pd (3.3 ± 1.1 nm) NPs were prepared (Figure 3). As shown by XRPD measurements, both composites exhibited the characteristic peaks of c-1 as well as the diffraction peaks associated to the NPs (Figure 2b). Moreover, EDX microanalysis confirmed the presence of Au and Pd in the Au@c-1 and Pd@c-1 composites, respectively (Figures S20 andS22).

This type of metallic NP/COF composites should be very useful for heterogeneous catalysis. However, for this to be a reality, the porous COF matrix should allow the accessibility of molecules to the embedded NPs. As a proof-of-concept, we studied this accessibility using both Au@c-1 and Pd@c-1 composites for the catalytic reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of NaBH4 as a reducing agent. The catalytic reactions were conducted by mixing two aqueous solutions of NaBH4 and 4-NP with an aqueous dispersion of Pd@c-1 (Pd: 1.1 % w/w) or Au@c-1 (Au: 3.5 % w/w), and followed by UV-vis spectroscopy (Figure 3). As control experiments, we also reproduced the same reaction using c-1 (without NPs) and amorphous Au@a-1 and Pd@a-1 hybrids as catalyst. In these latter cases, none of them showed catalytic activity (Figure 3 and Figure S24). For the Au@c-1 and Pd@c-1 hybrids, however, their catalytic activity was confirmed by the reduction of the peak at 400 nm, as well as the appearance of a new peak at 305 nm, corresponding to 4-AP (Figure 3). 4-NP was fully reduced into 4-AP after 10 min for Au@c-1 and 14 min for Pd@c-1. These results are comparable to previously reported reductions of 4-NP by NP/MOF (mainly, ZIF-8 and UiO-66) hybrids but slightly less active than a Au@carbon yolk shell nanocomposite, [27] and confirm the accessibility of molecules to the embedded NPs.

In summary, we have demonstrated an efficient and simple two-step procedure to encapsulate several types of NPs into porous and crystalline imine-based COF spheres. This strategy could be extended to a broad range of NPs and mixtures of them, and also to encapsulate molecular species. As a proof-ofconcept we have also shown the ability of Au@c-1 and Pd@c-1 hybrids to produce the catalytic reduction of 4-nitrophenol into 4aminophenol, which confirms the diffusion of reactants and products through the COF spheres to the embedded metallic NPs. This last result suggests potential applications in selective catalysis, remediation and molecular delivery.

Entry for the Table of Contents

Figure 1 .

 1 Figure 1. a,b) Representative FESEM images of a-1 (a) and c-1 (b) spheres. Insets show the corresponding photography of an aqueous colloidal solution. c,d) TEM images of a-1 (c) and c-1(d) spheres. e-h) Evolution of the roughness of the COF spheres at t = 0 h (e), 2 h (f), 24 h (g) and 168 h (h). i-l) Evolution of the crystallinity (i), porosity (j, k), and water sorption uptake (l) during the amorphous  crystalline transformation. In k, ABET (red), micropore volume (blue) and total pore volume (black). In l, water sorption isotherms of a-1 (pink), a-1 treated for 24 h (magenta), and c-1 (blue).

Figure 2 .

 2 Figure 2. a) HAADF-STEM image of single Fe3O4@c-1 sphere. b) XRPD diagrams of Fe3O4@c-1 (pink), Au@c-1 (black) and Pd@c-1 (green) composites in comparison to the simulated for the calculated from the expected eclipsed c-1 structure (red). Stars highlight the presence of the characteristic peaks of the NPs. c) Visual observation of the magnetic nature of Fe3O4@c-1 spheres. d) N2 sorption isotherms of c-1 (blue) and Fe3O4@c-1 spheres (red).

Figure 3 .

 3 Figure 3. HAADF-STEM images (first row: single sphere; second row: general view) of Pd@a-1 (a), Pd@c-1 (b), Au@a-1 (c), and Au@c-1 (d) composites. Third row shows the UV-vis evolution of 4-NP reduction using the corresponding composites as catalysts. Each spectrum was collected in intervals of 2 min.

Acknowledgements

This work was supported by the Spanish MINECO (projects PN MAT2016-77608-C3-1-P and MAT2015-65354-C2-1-R), the Catalan AGAUR (project 2014 SGR 80), and the ERC under the EU FP7 (ERC-Co 615954). V.G. is grateful to the Generalitat de Catalunya for a Beatriu de Pinós fellowship ( 2014BP-B 00155). A.Y. and ICN2 acknowledge the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2013-0295. Keywords: Covalent Organic Framework • Nanoparticles • Hybrids • Crystallization • Encapsulation


Claudia Vignatti

Javier Luis-Barrera

Vincent Guillerm

Inhar Imaz

Rubén Mas-Ballesté

José Alemán

Daniel Maspoch

Squaramide-IRMOF-16 Analogue for Catalysis: Epoxide Ring-Opening Tandem and Multicomponent Reactions

Keywords: Metal-Organic Frameworks, H-bond catalysis, Tandem and multicomponent reactions, Squaramide

Tandem and multicomponent one-pot reactions are highly attractive because they enable synthesis of target molecules in a single reaction vessel. However, they are difficult to control, as they can lead to the formation of many undesired side-products. Herein we report the use of metal-organic framework (MOF) pores decorated with organocatalytic squaramide moieties to confine ring-opening epoxide reactions of diverse substrates. Controlled mono-addition or tandem reactions inside the pores yield 1,2-aminoalcohols or 1,2,2'-aminodialcohols, respectively. In addition, this squaramide-functionalized MOF enables catalysis of highercomplexity multicomponent reactions such as the catalytic ring-opening of two different epoxides by a single amine to afford 1,2,2'aminodialcohols.

Scheme 2. Schematic representation of the epoxide ring-opening multicomponent reaction to form the amino diols 5. All the reactions were performed on 0.1 mmol of aniline 1a and 0.1 mmol of 3f for the first step, followed by 1.0 mmol of epoxide 2a for the second step.

Tandem reactions are among the best strategies to achieve molecular complexity in a single process. [START_REF] Behr | Towards resource efficient chemistry: tandem reactions with renewables Green[END_REF] They comprise two or more consecutive independent reactions, which are catalysed by one or more catalysts. Each catalyst produces an intermediate that is further transformed by a second catalytic cycle to give the final product. This translates to lower requirements for solvent, time and energy and to less waste relative to traditional processes. Consequently, tandem reactions have peaked the interest of numerous industries, 2 especially in their solvent-free form. [START_REF] Shearouse | Alternative Solvent-Free Methodologies in the Synthesis of Pharmaceutical Drugs[END_REF] 1,2-amino alcohols (3) and 1,2,2'-aminodialcohols (4) are structural subunits that are widespread in natural products of industrial relevance (see Figure 1a). [START_REF] Nikolau | Expeditious Routes to Evernitrose and Vancosamine Derivatives and Synthesis of a Model Vancomycin Aryl Glycoside Angew[END_REF] Some of these natural products include (S, R, R, R)-Nebivolol, which is a β1-adrenergic receptor blocker; 4e Bestatin, which is an aminopeptidase inhibitor that exhibits immunomodulatory activity; 4d Sphingosine, which is a a class of cell membrane lipids; 4d and Cytoxazone, which is an immunomodulatory. 4d They are also important synthetic intermediates for biologically active compounds, [START_REF] Nikolau | Expeditious Routes to Evernitrose and Vancosamine Derivatives and Synthesis of a Model Vancomycin Aryl Glycoside Angew[END_REF] stationary phases in HPLC, [START_REF] Yuki | Novel Chiral Stationary Phases for the Resolution of the Enantiomers of Amino Acids by Ligand Exchange Chromatography[END_REF] and chiral ligands (e.g. Oxazaborolidine derivatives; Figure 1a) 6a or auxiliaries in asymmetric reactions. [START_REF] Ager | R. 1,2-Amino Alcohols and Their Heterocyclic Derivatives as Chiral Auxiliaries in Asymmetric Synthesis[END_REF] 1,2-aminoalcohols and 1,2,2'-aminodialcohols can each be readily prepared via ring-opening of epoxides by amines. However, controlling the reaction of the amine (i.e. monovs. di-addition) to the epoxide is difficult, leading to mixtures of the two types of compounds.

Herein we show that confining the aforementioned reaction to metal-organic framework (MOF) [START_REF] Huang | Multifunctional metalorganic framework catalysts: synergistic catalysis and tandem reactions[END_REF] pores decorated with or-ganocatalytic squaramide moieties enables control over the formation of the mono-or di-addition products (see below). Furthermore, it also allows for the selective synthesis of heterogeneous double-addition products via multicomponent reactions in which two different epoxides are opened by a single amine (see below). Recently, Hupp, Farha, Mirkin et al. [START_REF] Mcguirk | Turning On Catalysis: Incorporation of a Hydrogen-Bond-Donating Squaramide Moiety into a Zr Metal-Organic Framework[END_REF] and Cohen et al. [START_REF] Zhang | Design and synthesis of squaramide-based MOFs as efficient MOF-supported hydrogen-bonding organocatalysts[END_REF] demonstrated that squaramide moieties can be incorporated into MOFs by post-synthetic modification of UiO-67 and by using a tetracarboxylate squaramide-based linker to produce a new Cu(II)-based MOF showing a pore diameter of ~ 8 Å x 8 Å. Both squaramide-functionalized MOFs 10 were successfully tested as catalysts for Friedel-Crafts reactions between indoles and β-nitroalkenes. For our targeted catalytic reactions, we constructed a squaramide-functionalized IRMOF-16 analogue (hereafter called Sq_IRMOF-16) because it shows a three-dimensional mesopore system in which the squaramide moieties are totally accessible in all three dimensions and are well separated to avoid any self-quenching phenomena. In addition, the pore diameter is ~ 17 Å x 17 Å, which is sufficiently large to host the intermediates produced during the tandem reactions. The linker (3,4-dioxocyclobut-1-ene-1,2-diyl)bis(azanedyil)-pdibenzoic acid, hereafter called L1) was designed to resemble, both in topology and in length, to the p,p'-terphenyldicarboxylic acid (tpdc), which is the linker used to synthesize IRMOF-16 (Figure 1b). [START_REF] Eddaoudi | Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage[END_REF] Moreover, and in contrast to a previously reported linker [START_REF] Zhang | Design and synthesis of squaramide-based MOFs as efficient MOF-supported hydrogen-bonding organocatalysts[END_REF] in which the squaramide moiety is in the meta position to the acid, in L1 the squaramide moiety is para to the carboxylic group. Therefore, our design leads to a more acidic NH proton than in the meta case.

We began by synthesizing L1, using slight modifications of previously reported procedures. [START_REF] Malerich | Chiral Squaramide Derivatives are Excellent Hydrogen Bond Donor Catalysts[END_REF] Then, Sq_IRMOF-16 was synthesized by heating a mixture of L1 and Zn(NO3)2 in N,Ndimethylformamide (DMF) at 85 °C for 7 days. After this period, yellow cubic crystals of Sq_IRMOF-16 were harvested (yield = 53%). As expected, the experimental powder X-ray diffraction (PXRD) pattern of Sq_IRMOF-16 was in excellent agreement with the one calculated from the envisioned squaramide-based IRMOF-16 (Figure 1c, see also Supporting Information, Figure S1). The squaramide-based IRMOF-16 model was constructed from the experimental IRMOF-16 structure 11 by ligand replacement, respecting the symmetry of IRMOF-16 (Pm-3m space group). This step was followed by a molecular mechanics energy minimization to improve the geometry of the bonds within the framework using the Forcite tool of the Materials Studio software (Biovia). 13 Therefore, analogously to IRMOF-16, Sq_IRMOF-16 comprises a zinc-metal cluster (Zn4O) bridged by six dicarboxylate linkers that form a network with pcu topology. The network is a three-dimensional mesopore system (pore size: ~ 17 Å × 17 Å) in which the squaramide moieties point towards the pores and therefore, are totally accessible in all three dimensions (Figure 1b).

For the catalytic experiments, we carefully dried Sq_IRMOF-16 dried under inert atmosphere and then, immediately mixed it with the other reagents (Supporting Information, Figure S2). It is worth to mention that this drying step was critical, as Sq_IRMOF-16 tends to become amorphous upon exposure to vacuum, and to transform into an unknown crystalline phase upon contact with water (Supporting Information, Figure S3). In order to verify that Sq_IRMOF-16 remained stable during the catalytic processes, it was recovered from the reaction media after the catalytic runs and its crystalline phase was confirmed by XRPD (Supporting Information, Figure S4). Additional experiments proved that the catalytic activity of Sq_IRMOF-16 was not related to the degradation or leaching of molecular species under the reaction conditions. [START_REF]leaching of molecular catalytic species from Sq_IRMOF-16[END_REF] As a first approach to studying the catalytic behavior of Sq_IRMOF-16, we monitored the kinetics of the reactions of each amine (1a, R=Me, or 1b, R= t-Bu) with each epoxide (2a, R= Et or 2b, R= C10H21) at 60 ºC (Figure 2b). We introduced to the reaction medium a 5 mol% content of catalytic centers, which are included in the structure of Sq_IRMOF-16; that is, 2.9 mg of Sq_IRMOF-16 that corresponds to 0.005 mmol of catalytic units were used to catalyze the reaction of 0.1 mmol of the corresponding aniline with an excess of epoxide. Figure 2c is a plot of the kinetics for each mono-addition product, which was the major species at 8 hours of reaction. Here, the performance of Sq_IRMOF-16 was also compared with the molecular squaramide C as catalyst (Figure 2a). Using both catalysts, we studied the reaction of 1a with 2a (compare the blue dashed line with the blue solid one) and 2b (compare the red dashed line with the red solid one). The reactions barely progressed when using C, probably due to the auto-self-aggregation and poor solubility of the catalyst. In quite contrast, the use of Sq_IRMOF-16 enhanced both kinetics and yields of these reactions. Moreover, when using Sq_IRMOF-16, we observed that epoxide 2a (R 2 = Et) appeared to react better than epoxide 2b (R 2 = C10H21), as observed in Figure 2c (compare the solid blue line with the solid red one, or the solid grey line with the solid orange one). Likewise, amine 1a (R 1 = Me) typically reacted faster than amine 1b (R 1 = t-Bu), also evidenced in Figure 2c (compare the solid blue line to the solid grey one, or the solid red line to the solid orange one. Interestingly, in the case of the use of the smaller epoxide 2a in their reaction with 1a and 1b (blue and greys lines), we also found a significant amount of the dialkylated products 4a and 4b (see Supporting Information). Altogether, these observations suggest that there is a size discrimination effect when Sq_IRMOF-16 is used, which is probably due to the lower diffusion rates of the bulkier substrates. These differences confirm that the catalytic processes occur inside the pores of Sq_IRMOF-16 rather than on its external crystal surfaces. Interestingly, we observed that once the mono-addition products 3 were obtained, the bis-addition products, homo-disubstituted amino diols 4, began to form. Scheme 1 shows a series of tandem reactions of the amines 1a-b and epoxides 2a-f to form the diols 4a-g catalyzed by Sq_IRMOF-16 (5 mol%) to test its catalytic utility. Remarkably, this reaction tolerated many combinations of reagents. The times required to obtain optimized yields of a series of diols 4 correlated to the size (4a-4d; Scheme 1, top row) and/or polarity (4e-4g; Scheme 1, bottom row) of the substrates. For example, comparing the synthesis of 4a with that of 4b reveals that ethyl-epoxide (2a) reacted faster with para-methoxy aniline (1a) than with para-tert-butyl aniline (1b). Similarly to 4b, the diols 4c (from 1a and 2b) and 4d (from 1b and 2c) required 3 days and 4 days, respectively, to reach moderate yields. We attributed these low reaction rates and moderate yields to the steric bulk and hydrophobicity of the alkyl chains in epoxides 2b (R 2 = C11H23) and 2c (R 2 = C10H21), which could hamper the diffusion of each epoxide through the pores of Sq_IRMOF-16.

In the above reactions, we also found that the bulkier epoxide 2e reacted at a similar reaction rate than did the smaller epoxide 2a. We ascribed this fact to the greater polarity of the -CH2OTBDMS group in 2e relative to the -Et group in 2a, which may help the diffusion of 2e through the pores of Sq_IRMOF-16. Consistent with our hypothesis, the more polar epoxides 2f (R 2 = CH2HNBoc) and 2g (R 2 = CH2CO2Et) gave near-total conversion (yields > 90%) to their corresponding diols 4f and 4g, respectively, after only 22 h.

We next evaluated the capacity of Sq_IRMOF-16 to catalyze multicomponent reactions of higher complexity. To this end, we used three reagents (one amine reacted sequentially with two epoxides) to generate heterogeneous diols in one-pot multicomponent reactions. This approach typically requires less energy and generate less waste than step-reactions which needs multiple purification processes. However, a drawback of one-pot reactions for heterogeneous additions is that they demand strict control of the chemistry. In our case, to avoid the formation of undesired side-products, a single mono-addition intermediate 3 had to be generated first. Once 3 had been formed in the reaction media, via one pot process (i.e. without any purification), other epoxides can be added to obtain the desired hetero-disubstituted amino diols 5 (Scheme 2).

We began by reacting para-methoxy aniline (1a) and epoxide (S)-2f in the presence of Sq_IRMOF-16 for 8 hours to produce the non-isolated intermediate 3f. Then, the enantiopure epoxide (S)-2a was added to the reaction medium to give the desired amino-diol (S,S)-5a. Similarly, we synthesized the amino-diol (S,R)-5b using the same conditions as for 5a, except that instead of (S)-2a, we used (R)-2a. In both cases, we found a substantial amount of the homo-substituted product 4g (~ 20%) in the crude mixture. These results indicate that Sq_IRMOF-16 can indeed catalyze multicomponent reactions, including diastereo-divergent ones. In conclusion, we have synthesized a squaramide-functionalized IRMOF-16 analogue, Sq_IRMOF-16, for use as a catalyst in the ring-opening of epoxides by nucleophilic amines. Sq_IRMOF-16 does not undergo the self-aggregation phenomena usually observed for squaramides in solution; in fact, this heterogeneous catalyst is superior to its molecular squaramide analogue. The pores in Sq_IRMOF-16 are sufficiently large to catalyze the ring-opening of diverse epoxides using different amines. We have demonstrated the catalytic activity of Sq_IRMOF-16 in the synthesis of simple, tandem and multicomponent epoxide ring-openings under solvent-free conditions and in good yields. The evidences suggest that these reactions are confined to the squaramide-functionalized pores, as Sq_IRMOF-16 shows size-and polarity-discrimination effects. Given that many organocatalytic moieties can be introduced into MOF pores, we are confident that MOF-based catalysts such as Sq_IRMOF-16 should help to expand the scope of heterogeneous catalysis in one-pot reactions.

Figure 1 .

 1 Figure 1. a) Schematic illustration of the introduction of squaramide moieties in MOFs for catalysing Friedel-Craft reactions with nitroalkanes (previous works) and simple, tandem and multicomponent epoxide ring-openings under solvent-free conditions (this work). b) Representation of the linkers tpdc and L1, and of the structure of Sq_IRMOF-16, in which the squaramide moieties have been highlighted in red. EWG refers to electron withdrawing groups. c) XRPD diffractogram of Sq_IRMOF-16 (red), compared with the simulated powder pattern obtained from the structural model (black).

Scheme 1 .

 1 Scheme 1. Schematic representation of the epoxide ring-opening tandem reaction (top) and representation of the molecular structures of the synthesized homo-disubstituted amino diols 4 (bottom). All the reactions were performed on a 0.1 mmol of aniline 1 and 1.0 mmol of epoxide 2. In the case of 2b, 2c and 2e were used 0.4 mmol of epoxide under solvent free conditions.[b] The corresponding mono-addition products were also detected in the crude mixture.

Figure 2 .

 2 Figure 2. a) Representation of the molecular structure of catalyst C. b) Schematic representation of the epoxide ring-opening mono-addition reactions. c) Kinetics plots for ring-openings of an epoxide (2a or 2b) by an amine (1a or 1b), using either MOF-Sq (solid lines) or C (dashed lines) as catalyst (in both cases, 5 mol% of catalytic units). Reactions were run at 60 ºC, using an excess of epoxide (200 µL) as solvent. Yield was measured by GC-MS and based on an internal standard.

ACKNOWLEDGMENT

This work was supported by the Spanish MINECO (projects PN MAT2015-65354-C2-1-R and CTQ2015-64561-R), the Catalan AGAUR (project 2014 SGR 80), the ERC under the EU FP7 (ERC-Co 615954 and ERC-CG 647550), and European Union's Horizon 2020 research and innovation programme under grant agreement No 685727. It was also funded by the CERCA Programme / Generalitat de Catalunya. ICN2 acknowledges the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2013-0295.

ASSOCIATED CONTENT

Supporting Information. Experimental procedures, material characterization and catalytic studies are included in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org."

AUTHOR INFORMATION Corresponding Author

* E-mail: inhar.imaz@icn2.cat; ruben.mas@uam.es; daniel.maspoch@icn2.cat; jose.aleman@icn2.cat

Author Contributions

# These authors contributed equally.


Vincent Guillerm

Thais Grancha

Inhar Imaz

Judith Juanhuix

Daniel Maspoch

Zigzag Ligands for Transversal Design in Reticular Chemistry: Unveiling New Structural Opportunities for Metal-Organic Frameworks

Herein we describe the topological influence of zigzag ligands in the assembly of Zr(IV) metal-organic frameworks (MOFs).

Through a transversal design strategy using reticular chemistry, we were able to synthesize a family of isoreticular Zr(IV)-based MOFs exhibiting the bcurather than the fcutopology. Our findings underscore the value of the transversal parameter in organic ligands for dictating MOF architectures.

 However, there are numerous reports demonstrating that the angle in bent ligands strongly influences the assembly of metal-organic polyhedra 13d,23 _ENREF_47 and MOFs. 24 Another point regarding use of linear ligands is that deliberate incorporation of steric hindrance adjacent to a given functional group can force it to tilt away from its preferred orientation, resulting in a twisted ligand (Scheme 1), which Scheme 1. Four Different Geometries for Dicarboxylic Ligands.

in turn provides access to topologies that are generally not easily accessible. 25 Finally, although a few isolated structures with zigzag ligands 26 or 3-connected ligand with broken collinearity 27 have been reported, to the best of our knowledge, there have not yet been any reports on the potential use of their unique shape for rational design of MOFs.

Here we provide the first-ever report on the use of zigzag ligands to create isoreticular Zr-bcu-MOFs. By employing zigzag ligands, we anticipated the introduction of a transversal parameter in reticular chemistry. The shape of these ligands can be defined by four main geometric parameters (Figure 1a): height (h); width (w); carboxylate-to-carboxylate distance (c-c), which is equal to (h 2 +w 2 ) 1/2 ; and angle (α), which is defined by c-c and h and is equal to arctan (w/h). Therefore, unlike the use of linear ligands, which can only be made taller or shorter (by adjusting the height; see Figure 1), our transversal reticular chemistry enables also stretching of the ligand in the transversal direction, varying the width (w, Figure 1). By breaking the collinearity of the carboxylate binding directions, our approach enables topological possibilities different from the default ones. To begin exploring transversal reticular chemistry, we chose to work with trans,trans-muconic acid (tmuc), a zigzag ligand with a height of 5.8 Å and a width of 2.1 Å (Figure 2a). Reaction of tmuc and ZrCl4 in DMF and trifluoroacetic acid at 120 ºC for 3 days afforded colorless crystals suitable for single-crystal X-ray diffraction (SCXRD). SCXRD analysis (performed on the XALOC beamline of the ALBA synchrotron 28 ) revealed the formation of a 3D network of formula Zr6O4(OH)4(tmuc)8(H2O)8 (hereafter called Zr-bcu-tmuc), which crystallizes in a tetragonal system. As commonly observed in Zr(IV)-based MOFs, the inorganic building unit in Zr-bcu-tmuc is the ubiquitous Zr6O4(OH)4(OOC)12-x(H2O)2x (x = 4) hexanuclear cluster. In this framework, each of these units is connected to eight others through eight bridging zigzag tmuc ligands adopting overall an 8-connected, bcu topology (Figure 1c,d).

If most Zr(IV)-based MOFs built up from dicarboxylate linear ligands adopt the fcu topology, the use of a zigzag ligand permitted deviation from it and favored the formation, somewhat unexpectedly, of the bcu network. Interestingly, there are previous reports that tmuc, when acting as a linear ligand, can also form the standard Zr-fcu-MOF (Figure 2). 14 Here, we would like to mention that the bcu topology actually derives from the fcu topology: it consists of an fcu network in which one-third (four out of 12) of the bridges are absent, resulting in a similar face-centered-cubic packing in which the connectivity of the 12-c nodes is reduced to 8 (Figure 2c). Thus, we attributed the topological deviation from fcu to a bcu to the zigzag conformation of tmuc. Indeed, inspection of the octahedral cage of Zr-bcu-tmuc compared to that from Zr-fcu-tmuc 14 reveals that the zigzag conformation of tmuc ligands in Zr-bcu-tmuc does not match the perfect alignment of the clusters in the structure (Figure 2b, purple arrows). Moreover, although the distances between the clusters bridged by tmuc ligands in Zr-bcu-tmuc (15.0 Å) are comparable to that in Zr-fcu-muc (14.6 Å), the cluster-cluster gaps that are not filled by tmuc in the former appear to be too wide (16.8 Å) to accommodate additional tmuc ligands. These unoccupied ligand "voids" in Zr-bcu-tmuc create 1D channels along the c axis whose pore size was estimated to be 7.0 Å (Figure 1d). In light of these apparent voids and free channels in Zr-bcu-tmuc, we evaluated its porosity by measuring its N2 sorption at 77 K, obtaining values of 640 m 2 /g for BET area and 0.27 cm 3 /g for total pore volume (Figures S19,S20).

A detailed trigonometric study of the Zr-bcu-tmuc crystal structure confirmed that formation of this MOF is indeed governed by both the height and the width of the zigzag ligands (Figures S13-S18). From this study, we estimated their contribution to the cell parameters as follows:

𝑎 = 𝑎 ≈ 2(𝑎 2 + 𝑎 2 ) cos(43 -𝑎) + 8.7

𝑎 ≈ 2𝑎 2 + 𝑎 2 sin(43 -𝑎) + 13.1

Consequently, we reasoned that in addition to the classical elongation parameter (height [h]) used in reticular chemistry, the transversal parameter (width [w]) of the zigzag ligands could be exploited to construct a family of isoreticular Zr-bcu MOFs. Indeed, the above equations suggest that, for a set of zigzag ligands having a similar width, the effects of h on the cell parameters should follow the same trend as in classical reticular chemistry; that is, as the height of the ligand increases, so should the cell parameters (Figure S18). However, for a set of zigzag ligands of similar height, the transversal parameter reveals its influence on the cell parameters: thus, as the width increases, the a and b axes (i.e. the parameters that define the channel diameter) increase, whereas the c axis (i.e. the cluster-cluster distance along z) decreases (Figure 3).

To confirm this hypothesis and to illustrate the power of transversal reticular chemistry, we carefully selected three additional ligands of similar height ( 7.9 Å) but increasing width: 2,6-naphthalene dicarboxylic acid (26ndc; w = 1.1 Å), 2,2'-bipyridine-4,4'-dicarboxylic acid (22bipy44dc; w = 3.7 Å) and azobenzene-3,3'-dicarboxylic acid (azo33; w = 5.7 Å). We reasoned that combining such features in a series of isoreticular MOFs would enable inhibition of the height effect, thereby providing the first-ever case of purely transversal reticular chemistry.

We prepared single crystals of the isoreticular Zr-bcu-MOFs Zrbcu-26ndc, Zr-bcu-22bipy44dc, Zr-bcu-azo33 by using similar conditions as for Zr-bcu-tmuc but replacing tmuc with either 26ndc, 22bipy44dc or azo33, respectively. For each synthesis, the reaction time and the amount of modulator were slightly adjusted to optimize crystal quality (see Supporting Information). As expected, SCXRD analysis confirmed the formation of the targeted family of isoreticular Zr-bcu-MOFs (Figure 1c,d, Figures andTables S2-S4).

Due to the similar height of the three ligands, the general dimensions of the resulting MOFs were governed mainly by the width (i.e. the transversal parameter) of the corresponding zigzag ligands. In fact, their cell parameter values were in strong agreement with the theoretical values obtained with the equations extracted from the parent model structure of Zr-bcu-tmuc (vide supra), as illustrated in Figure 3 and in Table S5. Thus, the a and b cell dimensions, and the pore size, increased with increasing width: 17.5 and 7.3 Å (a and b value and pore size) for Zr-bcu-26ndc; 21.2 and 9.8 Å for Zr-bcu-22bipy44dc; and 22.2 and 12.0 Å for Zr-bcu-azo33 (Figure 1d). Contrariwise, the c parameter (or cluster-to-cluster distance) decreased with increasing width: 22.6 Å for Zr-bcu-26ndc, 18.9 Å for Zr-bcu-22bipy44dc and 15.8 Å for Zr-bcu-azo33 (Figure 3).

In summary, we have reported a new design approach to access novel isoreticular MOFs based on disrupting the collinearity of the carboxylate groups in 2-connected ligands. Our work has revealed that the transversal parameter in organic ligands can be modulated for reticular synthesis of MOFs, as demonstrated in our rational synthesis of four isoreticular Zr-bcu-MOFs using zigzag ligands. This parameter provides an additional degree of structural fine-tuning in MOFs by reticular chemistry, enabling deviations from default structures such as the fcu topology typically observed for Zr(IV) MOFs with 2-connected, linear ligands. By reducing the connectivity of the inorganic building blocks from the ideal 12 down to 8 to create ordered defects, our approach could become complementary to -or even substitute -the classical monotopic ligand (modulator) addition for MOF synthesis.

Figure 1 .

 1 Figure 1. a) Characteristic distances and angle in zigzag ligands. b) Schematic of transversal reticular chemistry, showing three ligands with comparable height and variable widths. c) Representation of the cages associated with the structurally distinct MOFs Zr-bcu-tmuc, Zr-bcu-26ndc, Zr-bcu-22bipy44dc and Zr-bcu-azo33. d) Structural view of the MOFs Zr-bcu-tmuc, Zr-bcu-26ndc, Zr-bcu-22bipy44dc and Zr-bcu-azo33 along the c axis.

Figure 2 .

 2 Figure 2. a) View of the two conformations adopted by tmuc in Zr-fcu-muc (linear; green) and Zr-bcu-tmuc (zigzag; purple). b) Comparison of the regular octahedral cage in Zr-fcu-muc, in which all cluster distances are equal, and the distorted octahedral cage in Zr-bcu-tmuc, in which longer distances correspond to ligand voids. c) Comparison of fcu-a and bcu-a networks.

Figure 3 .

 3 Figure 3. Comparison of the theoretical parameters a, b and c for the Zr-bcu-MOFs, with the experimental values for height: 7.9 Å (Zrbcu-26ndc, Zr-bcu-22bipy44dc and Zr-bcu-azo33) or 5.8 Å (Zr-bcu-tmuc).

ACKNOWLEDGMENT

This work was supported by the EU FP7 ERC-Co 615954, the Spanish MINECO (project PN MAT2015-65354-C2-1-R), and the Catalan AGAUR (project 2014 SGR 80). It was also funded by the CERCA Program/Generalitat de Catalunya. ICN2 acknowledges the support of MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2013-0295. V.G. is grateful to the Generalitat de Catalunya for a Beatriu de Pinós fellowship (2014 BP-B 00155) (Marie Curie Fellowship, EU-funded project ITHACA). The authors thank Jorge Albalad for the TGA measurements.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b07050. Detailed synthesis, structural data, FTIR spectra, PXRD diagrams, TGA curves and adsorption data.

AUTHOR INFORMATION

Corresponding Authors

* vincent.guillerm@icn2.cat * daniel.maspoch@icn2.cat

Notes

The authors declare no competing financial interests.