Mithun Dhrubajyoti Mondal

Chandra Majee

Kisholoy Bhattacharya

Jérôme Long

Joulia Larionova

Marat M Khusniyarov

Muktimoy Chaudhury

Mithun Chandra Majee

Crossover from Antiferromagnetic to Ferromagnetic Exchange

published or not. The documents may come

INTRODUCTION

Dinuclear copper(II) complexes with a Cu 2 O 2 core generated by the bridging hydroxido, alkoxido, and phenoxido ligands have received wide interest in contemporary coordination chemistry because of their relevance to bioinorganic chemistry, [START_REF] Majee | Spin Coupling Concepts in Bioinorganic Chemistry[END_REF] as well as in molecular magnetism. [START_REF] Kahn | Molecular Magnetism[END_REF] Being a d 9 system with S = 1/2 ground state, the magnetic behaviour of copper (II) complexes is relatively easier to handle, both from experimental and theoretical points of view. [START_REF] Ruiz | Electronic Structure and Magnetic Behavior in Polynuclear Transition-Metal Compounds[END_REF] Bleaney and Bowers derived a theoretical expression for the magnetic exchange coupling in dinuclear copper(II) compounds for the first time [START_REF] Bleaney | The Cupric Ion in a Trigonal Crystalline Electric Field Prod[END_REF] and used that expression to explain magnetic properties and EPR signatures of a dimeric copper(II) acetate. [START_REF] Bleaney | Anomalous Paramagnetism of Copper Acetate[END_REF] Numerous exchange-coupled bi-and polynuclear transition metal complexes have since been extensively studied in order to determine magneto-structural correlations, 6 which could ultimately lead to the rational design of novel materials with desired magnetic properties. Thus, a linear correlation between the exchange coupling constant (J) and Cu II -O-Cu II bridge angle (θ) within a family of planar dihydroxido-bridged dicopper(II) complexes was demonstrated. 7 A transition from antiferromagnetic to ferromagnetic coupling (J = 0) was predicted to occur at θ ~ 97º. T u " " w -based magnetic orbitals resulting in a spin triplet ground state with minimized interelectronic repulsion.

A similar correlation has been reported also for the alkoxo-bridged dicopper(II) complexes. 8,9 Indeed, several theoretical approaches have been employed to investigate the magnetic behaviour of dicopper complexes and provide the structure-properties correlation starting by the purely qualitative approach involving the estimation of the energy splitting and finishing by more complex ab initio methods. [START_REF] Hay | Orbital Interactions in Metal Dimer Complexes[END_REF] Although many structurally characterized bis(µ-phenoxido) dicopper(II) complexes are known, except of few examples, [11][12][13] the copper(II) ions in dimers are almost exclusively antiferromagnetically coupled. 14 There are some scattered investigations on a series of bis(µphenoxido) dicopper(II) complexes 14c,15 that reveal exclusively antiferromagnetic coupling.

Thus, Thompson et al. 15 reported a linear relationship between the exchange coupling constant and phenoxide bridge Cu-OPh-Cu angle (θ). The extrapolation of the data to the ferromagnetic regime yielded the crossover point (J = 0) at θ ~ 77º, which is well below the angle reported for dihydroxido and dialkoxido complexes. [7][8][9] Sillanpää et al. 14c proposed a more realistic crossover angle θ of 89º extrapolating the antiferromagnetic coupling in a series of diphenoxido complexes towards the ferromagnetic regime. 13b Thus, the lack of compounds exhibiting ferromagnetic coupling constitutes a serious obstacle for the proper investigation of magneto-structural correlations, as reported for hydroxido-and alkoxo-bridged complexes. [7][8][9] A series of bis(µphenoxido) dicopper(II) complexes revealing both antiferromagnetic and ferromagnetic coupling would be desired for precise magneto-structural correlations.

We have an ongoing project on the chemistry and reactivity of transition metal complexes involving phenol-based ligands. 16,17 Herein we report a new family of bis-(µphenoxido)dicopper(II) complexes 1 -5 including five structurally similar tetradentate diphenol ligands (Scheme 1). Different sets of ligand substituents exert different steric pressure on the participating copper centers, which allows a large modulation of both structural parameters and magnetic properties in the series. Ultimately, we have been able to obtain antiferromagnetically and ferromagnetically spin-coupled complexes and consequently derive a precise magnetostructural correlation in bis-(µ-phenoxido)dicopper species. The structural requirements for a crossover point (J = 0) have been determined precisely by interpolating magnetic data for the first time. Density Functional Theory (DFT) calculations were also made to understand the magneto-structural correlations.

EXPRIMENTAL SECTION

Materials.

All reactions were carried out in aerobic environment with chemicals available from commercial sources and used as received. The solvents were reagent grade, dried by standard procedure, 18 and distilled under nitrogen prior to their use.

Synthesis of Ligands.

A modified procedure for the synthesis of N,N-bis(2-hydroxy-

3,5-dimethylbenzyl)-N',N'-dimethylethylene-

N,N-bis(2-hydroxy-3,5-diisopropylbenzyl)-N',N'-diisopropylethylene-1,2-diamine,

(H 2 L i-Pr,i-Pr,i-Pr [Cu 2 (L i-Pr,i-Pr,i-Pr ) 2 ]•2H 2 O (3). Complex 3 was synthesized in a manner similar to that for 1 using the ligand H 2 L i-Pr,i-Pr,i-Pr . The crystals were obtained by slow evaporation of the resulting methanol solution. We also used a desolvated sample for microanalysis as we did for compound [Cu 2 (L t-Bu,t-Bu,i-Pr ) 2 ]•H 2 O (5). Complex 5 was prepared in a manner similar to that for 1 using the ligand H 2 L t-Bu,t-Bu,i-Pr . Crystalline product was separated from the solution during the course of refluxing. Some of these crystals were of diffraction quality and used directly for X-ray crystal structure analysis. We used a desolvated sample for microanalysis as we did for compound Theoretical Calculations. Program ORCA 4.0.0.2 was used for all spin-unrestricted DFT calculations. [START_REF] Neese | ORCA 4.0.0.2 -Ab Initio, DFT and semiempirical electronic structure package[END_REF] The calculations were performed on molecular structures as obtained from crystallography without further optimizations. Single point calculations were done with B3LYP functional [START_REF] Lee | Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density[END_REF][START_REF] Becke | Density-Functional Thermochemistry. 3. The Role of Exact Exchange[END_REF] using def2-SVP and def2-TZVP basis sets [START_REF] Weigend | Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy[END_REF] for C/H and non-C/H atoms, respectively. RIJCOSX approximation with appropriate auxiliary basis sets (def2/J) [START_REF] Weigend | Accurate Coulomb-Fitting Basis Sets for H to Rn[END_REF] were employed for all calculations. Magnetic coupling was analyzed using broken-symmetry formalism. [START_REF] Ginsberg | Magnetic Exchange in Transition-Metal Complexes .12. Calculation of Cluster Exchange Coupling-Constants with the X-Alpha-Scattered Wave Method[END_REF][START_REF] Noodleman | Valence Bond Description of Anti-Ferromagnetic Coupling in Transition-Metal Dimers[END_REF][START_REF] Noodleman | Ligand Spin Polarization and Antiferromagnetic Coupling in Transition-Metal Dimers[END_REF] Corresponding orbitals were used to visualize molecular magnetic orbitals. [START_REF] Neese | Definition of Corresponding Orbitals and the Diradical Character in Broken Symmetry DFT Calculations on Spin Coupled Systems[END_REF] Molecular orbitals and spin density maps were visualized with Molekel. 31 X-ray Crystallography. which was measured at 293(2) K. No crystal decay was observed during the data collections. In all cases, absorption corrections based on multiscans using the SADABS software [START_REF]Program for Empirical Absorption Correction of Area Detector Data[END_REF] were applied. The structures were solved by direct methods [START_REF] Sheldrick | Phase Annealing in SHELX-90: Direct Methods for Larger Structures[END_REF] and refined on F 2 by a full-matrix least-

N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-N',N'-diisopropylethylene-

qu u z wR = [∑[w(F 0 2 -F c 2 ) 2 ] / ∑( 0 2 ) 2 ] ½ , R = ∑ | | 0 | - |F c | | / ∑| 0 |, S = [∑[w( 0 2 -F c 2
) 2 ] / (np)] ½ . SHELXL-2013 was used for both structure solutions and refinements. [START_REF] Sheldrick | Program for Crystal Structure Refinements[END_REF] A summary of the relevant crystallographic data and the final refinement details are given in Table 1. All non-hydrogen atoms were refined anisotropically.

The hydrogen atoms were calculated and isotropically fixed in the final refinement [d(C-H) = 0.95 Å, with the isotropic thermal parameter of U iso (H) = 1.2 U iso (C)]. The SMART and SAINT software packages [START_REF]SAINT-plus[END_REF] were used for data collection and reduction, respectively. Crystallographic diagrams were drawn using the DIAMOND software package. The Cu(II) centers in compounds 1 and 2 present a square pyramidal geometry. We have deliberately chosen to introduce three different substituents R 1 , R 2 , and R 3 in the ligand framework in order to influence the coordination geometry around the individual copper centers.

Such strategy appears to be successful since the copper centers in the later three compounds 3, 4, and 5 appear to be square planar (Scheme 2) as one of the donor amino nitrogen atoms is forced to stay away from coordination due to larger steric constrains exerted by a bulky R 3 substituent like i-Pr. Similarly, the remaining substituents R 1 and R 2 attached to the phenolate rings of the coordinated ligands also have their cumulative influence on the crystal structures of compounds 1 and 3 -5. Juxtaposition of the steric influence of all these substituents has an overall effect in tuning the average Cu-O-Cu bridge angles in the reported compounds.

Description of Crystal Structures

A slightly different crystal structure of compound [Cu 2 (L Me,Me,Me ) 2 ] 1 was reported earlier. [START_REF] Mijanuddin | Concomitant Polymorphism of an Antiferromagnetically Coupled Dicopper(II,II) Complex with Single Strand Helical Assembly: Synthesis, Structure, DSC, Magnetic and Heterogeneous Catalytic Studies[END_REF] The methodology adopted for the synthesis of that compound was also different. In our intermolecular distance is found to be equal to 10.598(3) Å.

Compound [Cu 2 (L t-Bu,Me,i-Pr ) 2 ] 4 crystallizes in the triclinic space group P-1 with two complexes accommodated per unit cell. A perspective view of the molecular structure of 4 is displayed in Figure 3. Important metrical parameters are presented in Table 2. Here also the coordination geometry around the two copper centers is distorted square planar and the trans angles around the copper centers in the basal planes vary from 165.32 3.

Electronic Spectra. The electronic spectra of these compounds were recorded in dichloromethane solution. All complexes exhibit strong absorption bands in 412-434 nm region due to phenolate to metal charge transfer transitions. 39 Much weaker bands in 636-661 nm region were assigned to d-d transitions 39 (Figure 4). Upon increasing the steric crowding of the substituents at the phenol ring, the ligand to metal charge transfer band exhibits a bathochromic shift (Table 4) due to an enhanced charge transfer from phenolate to metal ion concomitant with the increase of the inductive effect.

Magnetic Properties. Magnetic properties of the complexes 1 -5 were investigated using a SQUID magnetometer working between 1.8 and 350 K up to 7 T. The temperature dependence of T for all compounds is reported in Figure 5. resulting from a ferromagnetic coupling between the spin carriers. Such ferromagnetic interaction is ultimately confirmed by monitoring the field dependence of magnetization at 1.8 K (Figure 6), which can be fitted with a Brillouin function for a S = 1 species (g = 2.30 ± 0.02 and 2.10 ± 0.02 for 4 and 5, respectively).

To get further details, the exchange interaction between the two Cu(II) ions can be extracted using the isotropic Hamitonian, Ĥ = -JŜ 1 Ŝ 2, leading to the well-known Bleaney-Blowers equation. [START_REF] Bleaney | Anomalous Paramagnetism of Copper Acetate[END_REF] Using the PHI software, [START_REF] Chilton | A powerful New Program for the Analysis of Anisotropic Monomeric and Exchange-Coupled Polynuclear d-and f-Block Complexes[END_REF] the thermal dependence of T as well as the magnetization curves for 4 and 5 were fitted. Note that for 4 and 5, an intermolecular interaction parameter was taken into account, while for 2, both a temperature independent paramagnetism (TIP) contribution and intermolecular parameter were taken into account. The parameters from the best fits are reported in Table 5. It could be noticed than for 2, the weak value of the gyromagnetic factor may reflect the presence of a small impurity. Yet, the slope of T vs. T being mainly affected by the exchange interaction, the extracted constant could be considered as meaningful.

The nature of the magnetic exchange interactions in phenoxido dinuclear complexes has been largely studied. 14a Thus, there are several parameters affecting the magnitude and sign of the J parameter such as: the value of Cu-O-u , u… u u-O distances, 41 as well as the distortion of the copper ion geometry, 42 the effect of the asymmetry in the Cu-hydroxo bond and the out-of-plane displacement of the hydrogen atoms of the hydroxo-bridge. 9

Noticeably, there are only limited examples of phenoxido-bridged dinuclear copper complexes that exhibit ferromagnetic interactions. [11][12][13] It appears that one of the main factors controlling the sign of the exchange interactions is the Cu-O-Cu bridging angle, which is correlated with the geometry of the Cu(II) ion. 12,14c,43 In the case of hydroxo-bridged dicopper(II) complexes, it was experimentally evidenced that J varies linearly with this bridging angle through the Hatfield and Hodgson relationship. 7a A transition from antiferromagnetic to ferromagnetic coupling is therefore expected for a Cu-O-Cu angle smaller than 97°.

In the wide majority of the reported phenoxido systems, the magnetic interaction is found to be antiferromagnetic due to the important bridging angle. Thus, the complexes 1 -3 exhibit common antiferromagnetic interactions with the magnitude of J, which increases as the Cu-O-Cu angle increases. Reducing this angle induces the switching to moderate ferromagnetic interactions. In our present systems, the crossover is found for angle (θ) around 87° (Figure 7).

This value is in good agreement with the value of 90° previously reported. 11 However, we would like to point out that the strength of this ferromagnetic coupling is way larger than in other reported complexes due to smaller Cu-O-Cu angle in our systems. This can be ascribed to short Of particular interest in compounds 1-5 are the different alkyl substituents (R1, R2 and R3) with similar Hammet parameters in their ligand frameworks that support a high quality correlation, spanning a transformation from antiferro-to ferromagnetic range.

Theoretical Calculations. Density functional theory (DFT) calculations were performed on complexes 1 -5. The exchange coupling constants were calculated using a broken-symmetry approach. [START_REF] Ginsberg | Magnetic Exchange in Transition-Metal Complexes .12. Calculation of Cluster Exchange Coupling-Constants with the X-Alpha-Scattered Wave Method[END_REF][START_REF] Noodleman | Valence Bond Description of Anti-Ferromagnetic Coupling in Transition-Metal Dimers[END_REF][START_REF] Noodleman | Ligand Spin Polarization and Antiferromagnetic Coupling in Transition-Metal Dimers[END_REF] To preserve all subtle geometrical features that can heavily influence spin coupling, calculations were performed using molecular structures as determined by X-ray crystallography (vide supra) without optimization.

The calculated antiferromagnetic coupling constants in 1 -3 are in excellent agreement with experimentally obtained values (Table 6). Although we were able to reproduce ferromagnetic coupling in 4 and 5, its strength is underestimated in our calculations. In spite of considerably differing molecular structures in the series 1 -5, the Cu-Cu coupling seems to correlate with a Cu-O-Cu angle. Similarly, to related species, 14c a roughly linear correlation can be obtained (Figure 8). Thus, a crossing point of 86° corresponding to the change of the sign of the coupling constant has been obtained in our calculations. This value is in very good agreement with the experimentally obtained correlation (Figure 7) and previously reported data on related species. 14c Interestingly, the coupling in 1 -5 correlates with a hinge distortion of the Cu 2 O 2 core expressed as a Cu-O-Cu-O torsion angle. 14a Surprisingly, a linear correlation of similar quality was obtained in this case (Figure 9). These findings point to the fact that Cu-O-Cu angle and Cu-O-Cu-O torsion angle might also correlate over the whole range in our series. Indeed, an excellent linear correlation (R 2 = 0.995) between these two structural parameters could be obtained (Figure 10).

R = ∑||F o | -|F c ||/∑|F o |. b wR = [∑[w((F o 2 -F c 2 ) 2 ]/∑w(F o 2 ) 2 ]

  Elemental analyses (for C, H and N) were performed at IACS on a PerkinElmer model 2400 Series II CHNS An z . T z ( SI-MS) mode were measured on a Micromass QTOF model YA 263 mass spectrometer. The 1 H NMR spectra were recorded on a Bruker model Avance DPX-400 spectrometer using SiMe 4 as internal reference. Magnetic susceptibility data were collected with a Quantum Design MPMS-XL SQUID magnetometer working between 1.8 -350 K with the magnetic field up to 7 Tesla. The data were corrected for the sample holder and the diamagnetic contributions calculated from thePascal's constants.[START_REF] Bain | Diamagnetic corrections and Pascal's constants[END_REF]

  Figure 2a. Unlike the previous two compounds, the ligands coordinate in a tridentate/bridging

A

  gradual increase in the steric bulk of the substituents on the ligand's N atom enforces the Cu centers in these dinuclear copper complexes to adopt a square planar geometry in 3 -5 instead of a square pyramidal geometry observed in 1 and 2. The combinations of substituent thus have a modest to large influence in tuning the Cu•••Cu separations, bridging Cu-O-Cu bond angles, and hinge distortion of the Cu 2 O 2 framework (Cu-O-Cu-O torsion angle) as summarized in Table

4 and 5

 45 The room temperature T values are equal to 0.39, 0.49, 0.70, 0.84, and 0.81 cm 3 mol -1 K for compounds 1, 2, 3, 4 and 5, respectively. For compounds 4 and 5, these values are in good agreement with the theoretical value of 0.82 cm 3 mol -1 K expected for two non-interacting copper(II) ions (S = 1/2; g = 2.1). In the case of the three former compounds 1 -3, the observed lower T values originate from the occurrence of antiferromagnetic interactions, which are still operative at room temperature. Such fact is further confirmed by the decrease of T upon cooling, reflecting strong dominant antiferromagnetic interactions. At low temperature, the zero T value for complexes 1 -3 points out a diamagnetic ground state. In contrast, the temperature dependence of T for complexes shows a positive deviation upon cooling due to the presence of ferromagnetic interactions to reach a maximum close to 10 K before decreasing at low temperature, most likely due to intermolecular interactions. The maximum T values of 1.13 and 1.02 cm 3 mol -1 K for 4 and 5, respectively, are close to the value of 1.10 cm 3 mol -1 K expected for a S = 1 species (g =2.1)

  leading to large Cu-O-Cu-O torsion angle (the average dihedral angle between the Cu 2 O 2 plane and the bridging phenoxy aromatic ring plane). A switch from antiferromagnetic to ferromagnetic regime in our series can u u… u 2.71 Å ( u S3) u-O-Cu-O torsion angle of 42° (Figure S4). It could however be noticed, that below the crossing point, the magnitude of w ' .

  Thus, although both the decrease of Cu-O-Cu angle and the increase of Cu-O-Cu-O torsion angle are known to promote ferromagnetic coupling in bis(phenoxido)bridged dicopper(II) complexes, 14a here we have disclosed a correlation between those structural parameters for the first time. Further, we have analyzed the geometry of all similar known bis(phenoxido)dicopper complexes that are non-planar: they feature phenylene-CH 2 -N(R)-CH 2 -phenoxides and Cu-O-Cu-O torsion angles of non-zero. A similar linear correlation, albeit of lower quality (R 2 = 0.88) has been obtained for those data (Figure. S5). The fit is exceptionally good for structures with Cu-O-Cu-O angles higher than 10° and Cu-O-Cu angles lower than 100°. For more flat structures with Cu-O-Cu-O angles below 10°, the data become spread significantly. The antiferromagnetic coupling between two copper d(x 2 -y 2 ) type orbitals in 1 -3 is promoted by superexchange interactions through in-plane p-orbitals of bridging oxygen atoms (Figures 11, S6 and S7). Such antiferromagnetic coupling is common for this class of compounds. 14a However, relatively strong ferromagnetic coupling in 4 and 5 is rather unusual. Here, antiferromagnetic pathway by superexchange via bridging oxygen becomes significantly diminished (Figures 12, S8). Besides decreased Cu-O-Cu angle that generally leads to ferromagnetic coupling, a large hinge distortion of Cu 2 O 2 core renders two NOCuOO coordination planes strongly tilted to each other. In the extreme case, with the dihedral angle between the two NOCuOO planes attaining 90°, the antiferromagnetic pathway via superexchange vanishes completely, which should lead to very strong ferromagnetic coupling (Figure 13). In our case, significantly tilted NOCuOO coordination planes at 76.6 and 81.0° lead to relatively strong ferromagnetic coupling in 4 and 5, respectively. Thus, both structural parameters: small Cu-O-Cu angle and large Cu-O-Cu-O torsion angle in bis(phenoxido)dicopper(II) complexes, are indicative for ferromagnetic coupling, which is in agreement with previous correlations. 14aCONCLUSIONSWe have reported the synthesis and systematic variation of exchange coupling constant (J) in a series of bis-(µ-phenoxido)dicopper(II) complexes 1 -5 featuring tetradentate aminodiphenol H 2 L R1,R2,R3 ligands in which the ligands contain the same [O,N,N,O]-donor atoms but differ in substituents at phenol rings (R1, R2) and at an amine nitrogen atom (R3). The substituents are changed from methyl to tert-butyl on the phenol rings and from methyl to iso-propyl at the amine nitrogen atom, thus exerting different steric pressures on the participating copper centers. Among the substituents, R1 and R2 play a crucial role in controlling the Cu-O-u , u… u separation, and Cu-O-Cu-O torsion angle, whereas R3 controls the geometry around copper ions, changing from square pyramidal to square planar. Due to the fine tuning of structural parameters via ligand substituents, complexes 1 -3 exhibit antiferromagnetic coupling, whereas 4 and 5 reveals ferromagnetic exchange. Thus, the average Cu-O-Cu bond angle, Cu-O-Cu-46.5°, and 2.982 -2.633 Å, respectively. As a result, spin coupling changes gradually from strong antiferromagnetic (J = -395 cm -1 ) to moderate ferromagnetic (J = +53.2 cm -1 ) values. The crossover point at which the magnetic coupling is changed (J = 0) is determined at ca. 87°. Interestingly, 5 w u… u (2.633 Å) and smallest Cu-O-Cu bond angle (83.3°) and consequently a large ferromagnetic coupling constant (J = +53.2 cm -1 ) reported thus far for bis(µ-phenoxido)dicopper(II) complexes. The results of DFT calculations are in agreement with the experimentally determined crossover angle and disclose excellent magneto-structural correlations in the series 1 -5.

Scheme 2 .

 2 Scheme 2. Synthetic scheme for bis(µ-phenoxido)dicopper(II) complexes 1 -5.

Figure 1 .

 1 Figure 1. Partially labeled POV-Ray (in ball and stick form) diagram showing the atom labeling

Figure 2 .

 2 Figure 2. (a) Partially labeled POV-Ray (in ball and stick form) diagram showing the atom

Figure 3 .

 3 Figure 3. Partially labelled POV-Ray (in ball and stick form) diagram showing the atom

Figure 4 .

 4 Figure 4. Uv-Vis spectra of complexes 1 -5 recorded in dichloromethane.

Figure 5 .

 5 Figure 5. Temperature dependence of T for compounds 1 -5 measured under a 1000 Oe dc

Figure 6 .

 6 Figure 6. Field dependence of the magnetization at 1.8 K for compounds 4 and 5. The solid lines

Figure 7 .Figure 8 .

 78 Figure 7. Correlation between experimentally determined exchange coupling constant J and Cu-O-Cu angle in 1 -5.

Figure 9 .

 9 Figure 9. Correlation between calculated exchange coupling constant J and Cu-O-Cu-O torsion

Figure 10 .

 10 Figure 10. Correlation between Cu-O-Cu angle and Cu-O-Cu-O torsion angle in 1 -5.

Figure 11 .

 11 Figure 11. Spin density map for 1; a broken symmetry state obtained from B3LYP calculations

Figure 12 .

 12 Figure 12. Spin density map for 4; a triplet state obtained from B3LYP calculations.

Figure 13 .

 13 Figure 13. Spin density maps for 1 (left) and 5 (right) revealing more coplanar and more

1,2-diamine, (H 2 L Me,Me,Me

	with 20 mL of cold methanol and re-crystallized from dichloromethane/methanol (1:1 v/v) mixture. Yield: 80%; mp 174 °C. Anal. Calc. for C 22 H 32 N 2 O 2 : C, 74.12; H, 9.05; N, 7.86. Found: C, 74.28; H, 9.19; N, 7.85%. 1 H NMR (400 MHz, CDCl 3 , TMS, δ/ ): 2.20 ( , 12H, -) H 3 CN: m/z = 357.55 (100%, M+H + ). Other N 2 O 2 ligands have been synthesized following the same procedure. N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N',N'-diethylethylene-1,2-diamine, (H 2 L Me,Me,Et ). Yield: 82%; mp 170 °C. Anal. Calcd for C 24 H 36 N 2 O 2 : C, 74.96; H, 9.44; N, 7.28. Found: C, 74.90; H, 9.64; N, 7.34%. 1 H NMR (400 MHz, CDCl 3 , TMS, δ/ ): 1.11 [ , 6H, H 3 of N(CH 2 CH 3 ) 2 ], 2.19 [s, CH (d, 2H, aryl), 6.85 (d, 2H, aryl), 9.46 (broad, 2H, OH). SI-MS ( 12 H, CH 3 of Ph(CH 3 ) 2 ], 2.53 (q, 4H, CH 2 of N(CH 2 CH 3 ) 2 ), 2.62 (t, 4 H, CH 2 of (Et) 2 N-(CH 2 ) 2 -

) is described here as a prototype. To a solution

of 2,4-dimethyl phenol (2.44 g, 20 mmol

) in methanol

(30 mL)

were added N,N-dimethylethylenediamine (0.88 g, 10 mmol) and paraformaldehyde (60 mg, 20 mmol). The resulting solution was refluxed for 15 hr. It was then cooled to room temperature and rotary evaporated to ca 10 mL volume. The resulting white precipitate was filtered off, washed 3 ),

2.30 (s, 6H, of N(CH 3 )

2 ), 2.55 (s, 4H, of (CH 3 ) 2 N-(CH 2 ) 2 -),

3.57

(s, 4H, Ar-CH 2 ),

6.67

),

3.56 (s, 4H

, Ar-CH 2 ),

6.68 (s, 2H, aryl), 6.85 (s, 2H, aryl), 9.32 (broad, 2H, OH)

. SI-MS (positive) in CH 3 CN: m/z = 385.4 (100%, M+H + ).

1,2- diamine, (H 2 L t-Bu,Me,i-Pr )

	CH 2 -N(R) 2 ], 3.24 [m, 2H, CH of N{CH(CH 3 ) 2 } 2 ], 3.52 (s, 4H, Ar-CH 2 ), 6.68 (d, 2H, aryl), 6.96
	( , 2H,	). SI-MS (	)	H 3 CN: m/z = 497.8 (100%, M+H + ).
	N,N
			. Yield: 62%; mp 162 °C. Anal. Calcd for C 32 H 52 N 2 O 2 : C, 77.37; H,
	10.55; N, 5.64. Found: C, 77.90; H, 10.38; N, 5.59%. 1 H NMR (400 MHz, CDCl 3 , TMS, δ/	):
	1.05 [d, 12H, CH 3 of N(CH(CH 3 ) 2 ) 2 ], 1.36 (s, 18H, CH 3 of t-butyl), 2.22 [s, 6H, CH 3 of
	Ph(CH 3 )], 2.24 [t, 2H, CH 2 of (i-Pr) 2 N-CH 2 -CH 2 -N(R) 2 ], 2.77 [t, 2H, CH 2 of (i-Pr) 2 N-CH 2 -

-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N',N'-diisopropylethylene-1,2-diamine, (H 2 L t-Bu,t-Bu,i-Pr )

	. Yield: 55%; mp 165 °C. Anal. Calcd for C 38 H 64 N 2 O 2 : C, 78.57; H, 11.10; N,
	4.82. Found: C, 78.50; H, 11.01; N, 4.80%. 1 H NMR (400 MHz, CDCl 3 , TMS, δ/	): 1.07 { ,
	12H, CH 3 of N(CH(CH 3 ) 2 ) 2 }, 1.27 (s, 18H, CH 3 of t-butyl), 1.38 (s, 18H, CH 3 of t-butyl), 2.59
	[t, 2H, CH 2 of (i-Pr) 2 N-CH 2 -CH 2 -N(R) 2 ], 2.79 [t, 2H, CH 2 of (i-Pr) 2 N-CH 2 -CH 2 -N(R) 2 ], 3.27
	[m, 2H, CH of N{CH(CH 3 ) 2 } 2 ], 3.58 (s, 4H, Ar-CH 2 ), 6.69 (d, 2H, aryl), 7.18 (d, 2H, aryl) 8.82
	(broad, OH). SI-MS (	)	H 3 CN: m/z = 581.8 (100%, M+H + ).

Preparation of Complexes. Safety Note! Perchlorate

  salts of metal complexes are potentially explosive and should be handled in small quantities with sufficient care.

20 [Cu 2 (L Me,Me,Me ) 2 ] (

	1). The ligand H 2 L Me,Me,Me (89 mg, 0.25 mmol) was dissolved in
	methanol (30 mL). To this were added Et 3 (68 μL, 0.50	)	u( O 4 ) 2 •6H 2 O (90 mg,
	0.25 mmol. The resulting dark brown solution was refluxed for 2 h and filtered over a celite bed.
	The filtrate solution was left in the air for slow evaporation. The product was obtained as a
	brown crystalline solid within 2 -3 days. The crystals were filtered off, washed with cold
	methanol and air dried. Some of these crystals were of diffraction quality and used directly for
	X-ray crystal structure analysis. Yield: 70 mg (65%). Anal. Calcd for C 44 H 60 N 4 O 4 Cu 2 : C, 63.21;
	H, 7.23; N, 6.70. Found: C, 63.25; H, 7.26; N, 6.67%. FT-IR bands (KBr pellet, cm -1 ): 2956(m),
	1610(m), 1477(s), 1321(m), 1249(m), 1161(m), 858(m), 800(m). V-( H 2 Cl 2 ) [λ max / nm (ε,
	L mol -1 cm -1 )]: 412(5100), 473(3900), 640(880).

[Cu 2 (L Me,Me,Et ) 2 ]•CH 2 Cl 2 (

  2). Complex 2 was synthesized following a similar procedure as that described above for 1 using the ligand H 2 L Me,Me,Et and CuCl 2 •2H 2 O as metal ion precursor

	instead of Cu(ClO 4 ) 2 •6H 2 O. The compound was recrystallized from 1:1 (v/v)
	dichloromethane/methanol mixture. The compound is prone to solvent loss. Drying under
	vacuum for a long time afforded a fully desolvated analytical-grade sample that was used
	subsequently for microanalysis. Yield: 60%. Anal. Calcd for C 48 H 68 N 4 O 4 Cu 2 : C, 64.62; H, 7.68;
	N, 6.28. Found: C, 64.66; H, 7.83; N, 6.27%. FT-IR bands (KBr pellet, cm -1 ): 2918(m),
	1610(m), 1473(s), 1321(m), 1245(m), 1159(m), 858(m), 798(m). V-( H 2 Cl 2 ) [λ max / nm (ε,
	L mol -1 cm -1 )]: 410(4900), 474(3600), 636(980).

  2. Yield: 54%. Anal. Calcd for C 76 H 124 N 4 O 4 Cu 2 , C,71.04; H, 9.73; N, 4.36. Found:

	C, 70.92; H, 9.44; N, 4.34%. FT-IR bands (KBr pellet, cm -1 ): 2958(s), 1604(m), 1471(s),
	1301( ), 1238( ), 877(w) 820(w). V-( H 2 Cl 2 ) [λ max /	(ε, L	-1 cm -1 )]: 434(5300),
	533(2500), 661(1400), 840(870).

Physical Measurements. IR spectroscopic measurements were made on samples pressed into KBr pellets using a Shimadzu 8400S FT-IR spectrometer while for UV-visible spectral measurements; a PerkinElmer Lambda 950 UV/vis/NIR spectrophotometer was employed.

  Suitable crystals of 1 (brown block, 0.14×0.12×0.08 mm 3 ), 2 (brown block, 0.16×0.12×0.09 mm 3 ), 3 (brown block, 0.10×0.09×0.08 mm 3 ), 4 (brown block,

0.16×0.15×0.10 mm 3 ) and 5 (brown block, 0.12×0.08×0.06 mm 3 ) were mounted on glass fibres coated with perfluoropolyether oil before mounting. Intensity data for the aligned crystals were measured employing a Bruker SMART APEX II CCD diffractometer equipped with a monochromatized Mo K α radiation (λ = 0.71073 Å) source at 150(2) K except for compound 2

  [START_REF] Diamond | Visual Crystal Structure Information System, version 3.1[END_REF]

Table 1 .

 1 Summary of the Crystallographic Data for the Dicopper(II) Complexes 1 -5.

	Parameters	1	2	3	4	5
	Composition	C 44 H 60 N 4 O 4	C 49 H 70 Cl 2 N 4	C 68 H 108	C 64 H 100	C 76 H 124
		Cu 2	O 4 Cu 2	N 4 O 6 Cu 2	N 4 O 4 Cu 2	N 4 O 5 Cu 2
	Formula wt.	836.04	976.06	1204.66	1116.55	1300.86
	Crystal System	orthorhombic Triclinic	monoclinic triclinic	Monoclinic
	Space group	Pnna	P -1	P 2 1 /c	P -1	C 2/c
	a, Å	14.278(4)	12.6394(7)	15.660(2)	13.3104(13) 41.334(3)
	b, Å	23.385(6)	13.0315(8)	25.044(3)	14.6501(13) 13.8244(9)
	c, Å	12.942(4)	15.7942(9)	17.802(3)	17.9756(16) 29.215(2)
	α, deg	90	93.946(10)	90	72.847(2)	90
	β, deg	90	102.586(10) 103.149(5) 71.4641(18) 114.807(2)
	γ, deg	90	103.539(10) 90	71.9747(18) 90
	V, Å 3	4321(2)	2448.6(2)	6798.6(16) 3084.3(5)	15153.5(18)
	ρ calc , Mg m -3	1.285	1.324	1.177	1.202	1.140
	Temp, K	150(2)	293(2)	150(2)	150(2)	150(2)
	λ (Mo K α ), Å	0.71073	0.71073	0.71073	0.71073	0.71073
	Z	4	2	4	2	8
	2θ max [ ο ]	46.22	61.858	50.00	59.27	50.396
	Reflections	30428/	27395/	51815/	34826/	71312/
	Collected/Unique	3020	11729	11963	14517	13580
	F(000)/ μ mm -1	1768/ 1.029	1030/ 1.024 2600/	1204/ 0.737 5648/ 0.610
				0.676
	R int / GOF on F 2	0.1263/	0.0230/	0.1610/	0.0259/	0.0568/
		1.243	1.273	0.963	1.033	1.316
	No. of parameters 250	562	745	691	865
	R1 a (F 0 ), wR2 b (F 0 )	0.0565,	0.0504,	0.0651,	0.0386,	0.0605,
	(all data)	0.1542	0.1699	0.1487	0.1280	0.1789
	Largest diff. peak,	0.366,	0.983,	0.945,	0.565,	1.187,
	Deepest hole, eÅ -3	-0.675	-0.787	-0.719	-0.469	-0.676
	a

Table 4 .

 4 Electronic Spectral Data from the Complexes 1 -5. Me,Me,Et ) 2 ]•CH 2 Cl 2 (2) 410 (4880), 474 (3600), 636 (990). [Cu 2 (L i-Pr,i-Pr,i-Pr ) 2 ]•2H 2 O (3) 426 (6800), 634 (1500), 775 (1060) [Cu 2 (L t-Bu,Me,i-Pr ) 2 ] (4) 435 (5430), 658 (1250), 830 (630) [Cu 2 (L t-Bu,t-Bu,i-Pr ) 2 ]•H 2 O (5) 434(5280), 533(2480), 661(1400), 840(872)

	1/2

Table 5 .

 5 Structural and Magnetic Fit Parameters for Complexes 1 -5.

	Complexes Average Cu-	Cu•••Cu	J (cm 1 )	g	zJ(cm 1 )	TIP
		O-Cu Angle (º)	Distance				(cm 3 .mol 1 )
			(Å)
	1	98.63	2.9822(16) 395.1 ±	2.237 ±	-	-
				0.6	0.005
	2	97.92	2.9927(4) 259.4 ±	1.784 ±	2.0 ± 0.8	(4.2 ±
				0.8	0.006		0.08)10 3
	3	94.39	2.8730(2) 185.4 ±	2.242 ±	-	-
				0.4	0.002
	4	86.95	2.7205(4)	+46 ± 2	2.122 ±	0.009±	-
					0.004	0.007
	5	83.27	2.6327(6)	+53.2±	2.0390 ±	0.030±	-
				0.4	0.0009	0.002

Table 6 .

 6 Calculated and Experimentally Determined Exchange Coupling Constants [J, cm -1 ]. Scheme 1. Synthesis of iminodiphenol ligands.

	Complexes	calc.	exp.
	1	-323	395.1 ± 0.6
	2	-249	259.4 ± 0.8

i-Pr,i-Pr,i-Pr i-Pr i-Pr i-Pr H 2 L t-Bu,Me,i-Pr t-Bu Me i-Pr H 2 L t-Bu,t-Bu,i-Pr t-Bu t-Bu i-Pr

ACKNOWLEDGMENTS

This work was supported by the Council of Scientific and Industrial Research (CSIR), New Delhi (Grant No. 01/2557/12-EMR-II). Three of us (DM, MCM and KB) also thank the CSIR for the award of Research Fellowships. The single crystal X-ray diffraction data were collected on an instrument supported by DST, New Delhi, as a National Facility at IACS under the IRHPA program. J.Lo and J.La thank the University of Montpellier and CNRS for financial support and PAC of ICGM for magnetic measurements. M.M.K. acknowledges the Deutsche Forschungsgemeinschaft (Research Grant KH 279/3) for financial support and Prof. Karsten Meyer (University Erlangen-Nürnberg) for his continuous support throughout the years.

ASSOCIATED CONTENT

Supporting Information

Partially labelled POV-Ray diagram showing the atom labelling scheme in complexes 1 and 5 (Figures S1, S2), Cu•••Cu Separation (Å) vs Coupling constant (J) plot (Figure S3), Cu-O-Cu-O Torsion Angle (º) vs Coupling constant (J) plot (Figure S4), Linear correlation between Cu-O-Cu angle and Cu-O-Cu-O torsion angle for non-planar bis(phenoxido)dicopper complexes (Figure S5), Spin density map for 2; a broken symmetry state obtained from B3LYP calculations (Figure S6), Spin density map for 3; a broken symmetry state obtained from B3LYP calculations (Figure S7, Spin density map for 4; a triplet state obtained from B3LYP calculations (Figure S8).

Accession Codes

CCDC 1859903-1859907 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www. ccdc.cam.ac.uk


Jérôme Long

Grigorii G Skvortsov

Anton V Cherkasov

Konstantin A Lyssenko

Andrei I Poddel'sky

Yannick Guari

Joulia Larionova

Alexander A Trifonov

Synthesis, structure and magnetic properties of a series of Ln(III) complexes with radical-anionic iminopyridine ligands: effect of the lanthanide ion over the slow relaxation of the magnetization

We report the synthesis, structure and magnetic properties investigation of a series of homoleptic Ln(III) complexes coordinated by radical-anionic iminopyridine ligands of general formula [Ln(IPy) 3 ]•solv (IPy = iminopyridine; Ln = Tb, Dy, Er, Y, Gd). The dysprosium analogue exhibits a zero-field Single-Molecule Magnet (SMM) behavior.

Owing to their exceptional magnetic properties, lanthanide coordination and organometallic chemistries have recently taken a decisive turn in materials science. In this line of thought, Single-Molecule Magnets (SMMs) based on lanthanide ions have totally changed the molecular magnetism landscape and are often considered as future candidates for high-density information storage or quantum computing. [1][2][3][4] In such complexes, the high magnetic anisotropy of the lanthanide ion in a particular environment gives rise the appearance of an energy barrier, , separating two opposites magnetic states and resulting in a superparamagnetic behaviour. Yet, designing high performance SMMs requires to accurately control the coordination environment of the lanthanide ions in order to maximize the anisotropy and crystal-field splitting, while minimizing the Quantum Tunnelling of the Magnetization (QTM). This latter provides underbarrier relaxation pathways that limit the SMM performances, such as the energy barrier and magnetic coercivity. Although the physical origin with respect to QTM is different, Raman and direct processes provide also underbarrier relaxations that reduce the capacity of the SMMs. The coordination environment around the lanthanide ion depends on both electronic and symmetry considerations generated by the contiguous ligands. Taking advantage of this strategy, several lanthanide complexes exhibiting very high anisotropic barriers of few thousands of cm 1 have been reported within the last few years, [5][6][7][8][9] while a dysprosium metallocene complexes shows a high temperature hysteresis 8, 10-11 or that could even overcome liquid nitrogen's boiling temperature.

12 Although QTM is known to depend on symmetry, dipolar and hyperfine interactions, 13 the presence of intermolecular exchange interactions could greatly influence the relaxation dynamics in a complex fashion. On one hand, sometimes the presence of a neighbouring paramagnetic centre to the lanthanide site can produce an internal field that could suppress the tunnelling. 14 On the other hand, enhancement of the relaxation dynamics has also been observed in dinuclear systems exhibiting crystallographically independent lanthanide sites due to the creation of a local transverse magnetic moment. [15][16] In addition, the strength of the exchange interaction also influences the energy splitting between the ground and excited states in polynuclear systems which should be as high as large as possible to obtain large energy barrier systems. With this in mind, several exchange biased systems showing genuine SMM behaviour have been reported.

17-19

Due to the intern character of the f orbitals, radical ligands could be viewed as particularly appropriate to promote significant interactions and study the influence over the slow relaxation of the magnetization. semiquinoates, 24 verdazyls, 25 triazinyl, 26 bipyrimidyl, 27 pyridylpyrazine, 28 indigo, 29 or dinitrogen have been reported. 30 For instance, the latest example has hold the record for the highest coercitivity of 14 K for many years. 30

Among radical-lanthanide systems with great potentialities, ytterbium-radical metallocenes with various heterocyclic amines have been studied. 31 Remarkably, an exchange coupling larger than hundreds of cm 1 has been reported in several ytterbium metallocenes complexes. [32][33] Despite such advantages, the investigation has been solely dedicated to ytterbium-based complexes and the use of highly anisotropic Kramers ions, such as Dy 3+ or Er 3+ , particularly suitable to design SMMs has not been investigated. For instance, iminopyridines (IPy) belong to redox active ligands, but their coordination chemistry with such anisotropic lanthanide ions remains unexplored. Their great tunability in terms of both, steric and electronic properties, allows tailoring the coordination environment of the lanthanide site. In recent years, the reactions of various ytterbocenes with diimino ligands bearing various substituents were reported. [34][35][36][37][38] Among these complexes, a paramagnetic homoleptic Yb 3+ complex coordinated by three iminopyridyl radical-anions has been obtained through the oxidative cleavage of the  5 YbCp bond (Cp = cyclopentadienyl). This complex shows a reduced magnetic moment at room temperature that suggests a strong radicals/ytterbium antiferromagnetic interaction.

39 Following this, we report in this communication the synthesis, structure and magnetic studies of four homoleptic lanthanide complexes based on iminopyridine (Ipy) ligands. Remarkably, the dysprosium analogue reveals a zero-field slow relaxation reflecting a genuine SMM behaviour.

Complexes Ln(IPy .-) 3 (Ln = Tb (1), Dy (2), Er (3), Y (4), Gd( 5))

were prepared by treating the potassium derivative of the related iminopyridine with LnCl 3 in 3:1 molar ratio (Scheme 1).

Scheme 1. Synthesis of complexes 15.

Complexes 15 were obtained as moisture and air-sensitive dark green crystalline solids in 75, 72, 70, 73 and 71 % yields, respectively. X-Ray single crystal diffraction analysis (Fig. 1, Table S1) shows that compounds 15 crystallize in the hexagonal space group as the solvates 1•0.5(THF), 2•0.5(Toluene), 3•0.5(Toluene) and 4•0.5(Toluene) and 5•0.5(toluene). For the sake of clarity, only the crystal structure of 2 will be described. The molecular structure of 2 is shown in Figure 1. The coordination sphere is composed of six nitrogen atoms giving a hexacoordinated geometry close to a distorted octahedron as indicated by the SHAPE 40 analysis (Table S2).

The Dy-N Py distance of 2.424(2) Å is slightly longer than Dy-N imine one of 2.392(2) Å. The bonding situation within the planar diimino fragment N Py -C-C-N imine in 2 is consistent with the radical anionic form of the ligand (N imine -C 1.339(3) Å; C-C 1.394(3) Å; N Py -C 1.383(2) Å). 34 The crystal packing shows that the shortest intermolecular distance Dy-Dy is equal to 8.306(2) Å, while no H-bonds have been detected (Fig. S1).

The radical-anionic nature of the iminopyridine ligands was further confirmed with Electron Paramagnetic Resonance (EPR) spectroscopy. While complexes 13 do not exhibit an observable EPR spectrum, 4 incorporating the diamagnetic Y 3+ exhibits a broad line (ΔE = 62 G) at room temperature with g = 2.002 without any hyperfine splitting and visible anisotropy (Fig. S2). The g-factor value is typical for ligand-centered radicals.

The radical-anionic nature of the ligands was also corroborated by the direct current (dc) magnetic properties' investigations of 4 incorporating the diamagnetic Y 3+ ion, which exhibits intra-and intermolecular antiferromagnetic interactions between the radicals (S = ½) (Fig. S3, see ESI for details). The T vs T curves for 15 show a gradual then abrupt decrease from around 30 K, most likely associated with the thermal depopulation of the ± m J levels from the Ln 3+ ions combined with intra and intermolecular antiferromagnetic interactions (Fig. S3). The dc study of 5 presenting isotropic Gd 3+ ion reveals that the intramolecular radical-radical interactions seem to be much greater than the Gd 3+ -radical ones. The slow relaxation of the magnetization was investigated by alternate currents (ac) measurements. Under a zero dc field, only the dysprosium analogue 2 shows a significant signal of out-of-phase susceptibility (") component. The frequency dependence of " at different temperatures shows the presence of a broad single peak (Fig. 2). Such out-of-phase signals could be observed up to 30 K on the temperature dependence of the ac susceptibilities (Fig. S4). Fitting of the Cole-Cole plots (Fig. S5) with a generalized Debye model show large values of the  parameter (i.e. 0.42) at low temperature, indicating an important distribution of relaxation times (Table S3). Further insights into the relaxation dynamics could be obtained by analysing the temperature dependence of the relaxation time, . A clear deviation from the linearity could be observed at low temperature, pointing out a divergence from a thermally activated process. Hence, the all data range could be modelled using the following equation:

 1 =  0 1 exp(/kT) + CT m +  1
QTM (Eq. 1). 41 The first term accounts for a thermally activated process, while the second and third ones stand for two-phonon Raman and QTM, respectively. Surprisingly, the Raman process is found inoperative 42 and as a consequence, the magnetization relaxes through a combination of thermally activated and QTM processes (Table 1). Considering solely a Raman and QTM processes leads to poor fitting confirming that the relaxation involves readily a thermally activated relaxation. In order to shortcut this latter, the field dependence of the relaxation time was monitored at 15 K (Fig. S6). The complex behaviour could not be modelled with the usual equation

 1 = DH 4 T + B 1 /(1+B 2 H²) + K (Eq. 2),
for which the first term accounts for the direct process (for Kramers-ion), the second one stands for the QTM, while the K constant accounts for the field-independent Raman and thermally activated process. However, the data clearly shows that the field showing the highest relaxation time is 1000 Oe. The frequency dependence of the ac susceptibilities under this dc field shows the presence of a broad peak for " (Fig. S7-S8, Table S4), while the temperature dependence of  confirms the decrease of the QTM (Fig. 3). The temperature dependence of  could be fitted by taking into account an additional direct process: 4). The value of  (Table 1) is found comparable with respect to the zero-field data. The large distribution of the relaxation times may be explained by the presence of the crystallographic disorder of solvent molecules and/or ligands, as well as by impact of inter and intramolecular interactions. In contrast, the analogues 1 and 3 do not show a significant outof-phase component even in the presence of a dc field (Fig. S9-S10). The observed behaviour follows the usual trend expected by electrostatic model and considering the angular dependence of the 4f electronic density for the Ln 3+ ions. While Dy 3+ is a Kramers' oblate ion for which the electronic density could be stabilized by an axial crystal-field, the Er 3+ ion Kramers ion exhibits a prolate density which could hardly be stabilized the IPy  ligands. On the other hand, Tb 3+ is an oblate ion, however its non-Kramers nature (considering no exchange interaction) requires a particular geometry in order to observe a slow relaxation of the magnetization. 44 Since the presence of strong exchange interaction would modify the ground state, this suggests that the relaxation is mainly dominated by the single-ion anisotropy of the dysprosium rather than a coupled state with the radicals. This is in line with the observation of strong antiferromagnetic coupling between the radicals in 4 and 5. In contrast to the strongly coupled N 2 3 radical bridged Tb complex, such lanthanide ion's dependence has been also observed in weakly exchange systems based on r bipyrimidyl or pyridyl-pyrazine ligands. [27][28] However, the presented systems exhibit a crystalloprahically imposed trigonal symmetry which should provide mixed m J states. Although, zero-field slow relaxation in six-coordinate lanthanide complexes is rather scarce, [45][46][47] complex 2 is the first example of a zero-field undiluted SMM with such imposed symmetry. This suggests that despite being weak, the exchange interactions may affect the relaxation. In summary, we have shown in this communication that the use of radical anion of IPy as ligand enables the formation of homoleptic magnetic complexes with a series of lanthanide ions. The study of the magnetic properties indicates that the dysprosium analogue shows a zero-field SMM behaviour for which out-of-phase signals could be still observed up to 30 K. The great tunability of such ligands in terms of both, steric and electronic factors should offer a careful tweaking of the

 1 =  0 1 exp(/kT) + CT m + AT n (Eq.

Fig. 1 .

 1 Fig. 1. Molecular structure of 2. Colour code: orange, Dy; blue, N; grey, C; Hydrogen atoms have been omitted for clarity.

Fig. 2 .

 2 Fig. 2. Frequency dependence of ' and " for 2 under a zero dc-field.

Fig. 3 .

 3 Fig. 3. Temperature dependence of the relaxation time for 2 using the ac susceptibility data at 0 Oe and 1000 Oe. The solid line represents the fit with Eq. 1 (0 Oe) or Eq. 3 (1000 Oe).

43 Table 1 :

 431 Fit parameters of the temperature dependence of the relaxation time for 2.

	Compound	 (cm 1 )	0 (s)	m	C (s 1 .K m )	QTM (ms)	A (s -1 .K -1 )
	2 (0 Oe)	28 ± 4	(3 ± 1)  10 5	9*	-	1.0 ± 0.1	-
	2 (1000 Oe)	34 ± 3	(2.1 ± 0.5)  10 6	9*	(2.5 ± 10 10 0.8) 	-	43 ± 5

coordination environment in order to improve the slow relaxation features. The authors thank the University of Montpellier, CNRS for financial support, PAC of ICGM for magnetic measurements and the Analytical Center of The G.A. Razuvaev IOMC RAS for the X-ray study. The Russian co-authors thank the Russian Science Foundation (grant 17-73-30036). The X-ray study of 24 has been carried out in the framework of the Russian state assignment (Тheme № 44.2, Reg. № АААА-А16-116122110053-1) using the equipment of The Analytical Center of IOMC RAS.


Dr Stanislav V Konnov

Dr Florent Dubray

Dr Edwin B Clatworthy

Prof Cassandre Kouvatas

Prof Jean-Pierre Valentin Gilson

Prof Jean-Pierre Dath

Dr Delphine Minoux

Dr Cindy Aquino

Valentin Valtchev

Dr Simona Moldovan

Dr Siddardha Koneti

Dr Nikolai Nesterenko

Svetlana Mintova
email: svetlana.mintova@ensicaen.fr

Dr Svetlana Valtchev

Mintova

Novel Strategy for the Synthesis of Ultra-Stable Single-Site Mo-ZSM-5 Zeolite Nanocrystals

Keywords: Mo single-sites, zeolite, synthesis, methane conversion, hydrogen production, thermal stability, regenerability

Introduction

As the world undergoes a decades-long transformation of the energy industry to low or zero carbon-based resources and fuels, natural gas (mainly CH4) and biomass will play increasingly important roles in transitioning away from traditional fossil fuel feedstocks such as coal, light and heavy oils. [1] The shift in focus to other energy sources will require the modification of existing and the development of emerging industrial processes. Zeolites, already a key player in catalysis and separations in oil refining and petrochemistry [1a] are poised to play an important role in such technologies if the stability of metal sites in zeolite-based catalysts can be insured. [2] Moreover, dedicated crude-to-chemicals technologies will start to boost the production of chemicals from oil at refineries from ≈ 10-15% at present to ≈ 30-80% in the coming decades and many traditional facilities are already considering changing their product slate from transportation fuels to chemical feedstocks. [START_REF] Chadwick | Crude-to-Chemicals: Opportunity or Threat?[END_REF] Such scenarios would benefit from the development of novel and stable metal-containing zeolite catalysts.

For example, the dehydrogenation of propane to propylene employs alumina-supported catalysts, [START_REF] Sattler | [END_REF] however, state of the art synthetic procedures have demonstrated several new strategies to improve the activity and stability of zeolite-supported catalysts such as the encapsulation of metal atoms and clusters during the zeolite synthesis, [5] stabilisation of the metal phase in the zeolite channels by alkali metals, [2] and the use of frameworkmetal sites to stabilise extra-framework active sites. [6] Similar to the dehydrogenation of propane, there are no commercial zeolite-based catalysts for the non-oxidative conversion of methane to hydrogen and industrially important chemicals (olefins, aromatics). This process is a direct route to decarbonize methane to petrochemicals while co-producing a clean energy vector -hydrogen. A big competitive advantage relative to the other routes to produce hydrogen from methane (e.g. steam reforming process) is in no direct CO2 emission and, as a consequence, no requirement for CO2 capture. The route is also more carbon efficient and requires lower capital investments relative to syngas routes, which are the only industrial processes currently available to transform methane to petrochemicals. This reaction faces two critical issues: (1) low conversion due to unfavorable thermodynamics and (2) catalyst deactivation due to coke deposition at high temperatures (> 700 °C) causing irreversible damage to the zeolite structure. Recent studies on metal-containing zeolites for the non-oxidative conversion of methane, such as Mo-impregnated ZSM-5, have focussed on the optimization of the metal loading, reaction process, and catalyst regeneration. [7] For this process to be industrially viable, the catalysts envisioned will have to possess, among other features, a low density of similar active sites (single sites), short diffusion path lengths of guest molecules (nanosized crystalline domains) to minimize secondary reactions (e.g. coking), shape selectivity (selective production of high-value aromatics such as p-xylene) and superior hydrothermal stability. It is clear that such catalysts will have to operate outside the current "comfort zone" of conventional industrial catalytic processes. At present, metal containing zeolites operating at high temperature in the presence of steam suffer from irreversible damages including:

(1) the loss of dispersion of the metal phase, (2) str collapse, (3) strong dealumination due to the reaction between volatile metals and framework Al, and (4) hydrolysis of Al bonds. Such challenges are seldom addressed in the current literature. Thermal and hydrothermal stability of zeolites and currently available methodologies for their stabilization remain important issues for the industrial scale application of novel materials or even known zeolite catalysts. In addition to the recent examples for propane dehydrogenation mentioned earlier reported methods for the stabilization of zeolites include the addition of phosphorous in MFI, [8] ion-exchange with rare metals in FAU, [9] dealumination/silication by (NH treatments and/or partial dealumination combined with extraction of Al extra-framework species (USY). [10] Most of these stabilization strategies of zeolites are based on defect healing, or the creation of substantially "defect free" structures. Very recently, Iyoki et al. demonstrated the stabilization of h zeolites (SiO2/Al2O3 > 240, BEA, MFI and MOR liquid-mediated treatment consisting of hydroxide anions, fluoride anions and organic structure directing cations, without requiring the addition of silylating agents. The treated zeolit were able to withstand high temperature steaming conditions of 900-1150 °C. [11] In this work we report for the first time the synthesis of novel single-site Mo-containing ZSM with superior stability (up to 1000 °C) by treatment of nanosized zeolite with sodium molybdate unde pressure. Mo is atomically dispersed in the framework of an aluminium containing MFI zeolite ([Mo] homogeneously distributed through the crystals. Mo effectively heals silanol defects and the resulting catalyst demonstrates stable activity over consecutive catalytic reactions. The exceptional stability and performance of the containing nanozeolite for H2 and higher hydrocarbons production from CH4 under methane dehydroaromatization (MDA) conditions was demonstrated. The single containing ZSM-5 nanozeolite is stable under oxidative, reductive, and steaming conditions.

Results and Discussion

Synthesis and comprehensive characterization of Mo containing zeolites

The single-site [Mo]-ZSM-5 zeolite sample was sy following the novel procedure outlined in the experimental section and presented in Scheme S1. The synthesis approach is based on the treatment of calcined Mo-free ZSM zeolite with sodium molybdate at 90 °C for 5 days under autogenous pressure, followed by purification and calcination (550 °C for 6 h). The prepared [Mo]-ZSM-5 contains 0.5 wt.% of Mo according to EDS-TEM and ICP analyses. The reference sample, R-ZSM-5, was prepared by incipient wetness impregnation of the parent Mo-free ZSM-5 nanozeolite with 1 wt.% of Mo according to standard procedures rep MDA literature. [12] In contrast to the direct synthesis approac and superior hydrothermal stability. It is clear that such catalysts t "comfort zone" of conventional industrial catalytic processes. At present, metalcontaining zeolites operating at high temperature in the presence of steam suffer from irreversible damages including:

(1) the loss of dispersion of the metal phase, (2) structural due to the reaction between volatile metals and framework Al, and ( 4) hydrolysis of Al-O-Si bonds. Such challenges are seldom addressed in the current literature. Thermal and hydrothermal stability of zeolites and the currently available methodologies for their stabilization remain important issues for the industrial scale application of novel materials or even known zeolite catalysts. In addition to the recent examples for propane dehydrogenation mentioned earlier, reported methods for the stabilization of zeolites include the exchange with rare-earth dealumination/silication by (NH4)2SiF6 treatments and/or partial dealumination combined with extraction Most of these stabilization strategies of zeolites are based on defect healing, or the creation of substantially "defect free" structures. Very demonstrated the stabilization of high-silica I and MOR type) using a mediated treatment consisting of hydroxide anions, fluoride anions and organic structure directing cations, without requiring the addition of silylating agents. The treated zeolites were able to withstand high temperature steaming conditions of In this work we report for the first time the "top-down" containing ZSM-5 nanozeolite with superior stability (up to 1000 °C) by treatment of calcined nanosized zeolite with sodium molybdate under autogenous pressure. Mo is atomically dispersed in the framework of an aluminium containing MFI zeolite ([Mo]-ZSM-5) and homogeneously distributed through the crystals. Mo effectively heals silanol defects and the resulting catalyst demonstrates ctivity over consecutive catalytic reactions. The exceptional stability and performance of the single-site Moand higher hydrocarbons under methane dehydroaromatization e single-site Mo-5 nanozeolite is stable under oxidative, Synthesis and comprehensive characterization of Mo-5 zeolite sample was synthesized following the novel procedure outlined in the experimental section and presented in Scheme S1. The synthesis approach is free ZSM-5 nanosized zeolite with sodium molybdate at 90 °C for 5 days under pressure, followed by purification and calcination 5 contains 0.5 wt.% of TEM and ICP analyses. The reference 5, was prepared by incipient wetness 5 nanozeolite with 1 wt.% of Mo according to standard procedures reported in the In contrast to the direct synthesis approach reported earlier by our group, [13] the current method allows stabilization of single metal species in already synthesized aluminium containing MFI type (ZSM-5) zeolites (Scheme S1). This new procedure can be applied to any and aluminosilicate zeolites including commercially available materials.

The structural properties of the novel single zeolite sample were characterized by in spectroscopic analyses. The XRD patterns of the single [Mo]-ZSM-5 and reference R-ZSM-5 zeolite samples are depicted in Figure 1A. The [Mo]-ZSM-5 sample is monoclinic (P21/n) while the R-ZSM-5 is orthorhombic ( untreated calcined Mo-free ZSM-5 was orthorhombic and the XRD pattern is provided in Figure S1 for comparison. In addition, orthorhombic symmetry was maintained for the pristine ZSM that was subjected to the same treatment as that used for the preparation of the [Mo]-ZSM-5 sample, how presence of sodium molybdate (Figure S1). The characteristic splitting of the diffraction peaks at 23. 3, 23.8, and 24.5° 2 in the XRD pattern of the single-site [Mo]-ZSM- the current method allows stabilization of single metal species in already synthesized 5) zeolites (Scheme S1). procedure can be applied to any calcined pure silica and aluminosilicate zeolites including commercially available The structural properties of the novel single-site [Mo]-ZSM-5 zeolite sample were characterized by in-depth diffraction and oscopic analyses. The XRD patterns of the single-site 5 zeolite samples are 5 sample is monoclinic 5 is orthorhombic (Pnma). Pristine 5 was orthorhombic and the XRD pattern is provided in Figure S1 for comparison. In addition, orthorhombic symmetry was maintained for the pristine ZSM-5 that was subjected to the same treatment as that used for the 5 sample, however, without the presence of sodium molybdate (Figure S1). The characteristic splitting of the diffraction peaks at 23. 3, 23.8, and 24.5° 2θ in the -5 zeolite sample is ZSM-5 zeolite samples in the clearly seen (Figure 1B); stars marked the splitting of the single peaks typical for orthorhombic R-ZSM-5. The monoclinic symmetry is an indication for the isomorphous substitution of a metal with an ionic radius larger than Si in the MFI framework, however, the specific T-site locations occupied by Mo in the ZSM-5 framework were not determined. [13] The Le Bail profile refinements of the XRD patterns are summarized in Table S1. clearly seen (Figure 1B); stars marked the splitting of the single 5. The monoclinic symmetry is an indication for the isomorphous substitution of a metal with an ionic radius larger than Si in the MFI framework, site locations occupied by Mo in the The Le Bail profile refinements of the XRD patterns are summarized in Table S1.

site [Mo]-ZSM-5 zeolite sample further indicates the substitution of Si IV (ionic radius: the MFI framework. [13] The morphology, size and elemental composition of both 5 are studied by STEM-HAADF (Figure 2). The statistical analysis of the particle size (measured for about 200 particles) is presented as inserts in the 2B. They reveal the average particles size is ZSM-5 samples. The most striking differences between the samples are found in the morphology of the particles. The crystals of [Mo]-ZSM-5 nd sharp edges, whereas in the case 5 crystals, the outer surface is less regular and the inner crystals morphology is marked by the presence of mesopores (darker circular areas). The sharpness of the edges e interpreted as a signature of its monocrystalline nature, whilst the irregular outer 5 crystals are associated with their polycrystallinity. The polycrystalline nature of the R-ZSM-5 crystals is also associated with the presence of an abundant number of defects (notably the mesopores), which is further confirmed by NMR ( 1 H and 29 Si MAS NMR), and IR spectroscopy (see below). The atomically dispersed Mo in [Mo] ZSM-5 (seen as light points in the micrographs) are highlighted with circles in the STEM-HAADF micrographs, and confirm the presence of homogeneously distributed Mo atoms within the zeolite crystals (Figure 2A). The size of Mo light points in STEM HAADF micrographs is similar in the two samples, however, they differ substantially in their stability under high temperature treatment (up to 1000 ºC) and steaming. The chemical composition of the single-site [Mo]-ZSMby ICP and confirmed by EDS-TEM. The elemental distributions in the [Mo]-ZSM-5 and R-ZSM-5 samples S2. The shape and morphology of the pristine ZSM examined by STEM-EDS (Figure S3). The agglomerated crystals have irregular shapes, and a mean size of 170 nm. The crystals in the reference sample (R-ZSM and size (Figure S2); the distribution of both Si and Al in the pristine and reference sample is homogeneous suggesting that no changes under impregnation occurred.

Further structural information of the single sample is provided by 29 Si MAS NMR spectroscopy (Figure 3A). The results from the 29 Si and 27 Al NMR analyses are consistent with the IR observations performed in the silanol region for the two samples (Figure 4A). Sample [Mo] contains a negligible amount of isolated silanols represented by the peak at 3745 cm -1 , and no silanol nests. In comparison, sample R-ZSM-5 contains a significant amount of isolated silanols and silanol nests represented by the highly intense peaks at 3745 cm -1 and 3510 cm -1 , respectively. The peak corresponding to bridging silanol species at 3610 cm samples remains unchanged, confirming the preservation of Brönsted acid sites (Figure 4A). Additionally, the low amount of silanol defects in the [Mo]-ZSM-5 is confirmed by spectroscopy (Figure 4B); the samples were dehydrated at 200 °C overnight prior to the measurements to remove the zeolitic water. One peak at 2.3 ppm corresponding to isol 5 particles contain additional mesopores and surface defects. The presence of Al in the framework of both samples is Al MAS NMR spectroscopy (Figure 3B). The 5 samples contain a sharp peak at 60 ppm corresponding to Al in tetrahedral coordination, and no octahedral species are detected at 0 ppm.

Al NMR analyses are consistent with the IR observations performed in the silanol or the two samples (Figure 4A). Sample [Mo]-ZSM-5 contains a negligible amount of isolated silanols represented by , and no silanol nests. In comparison, 5 contains a significant amount of isolated ts represented by the highly intense , respectively. The peak corresponding to bridging silanol species at 3610 cm -1 in both samples remains unchanged, confirming the preservation of ally, the low amount of 5 is confirmed by 1 H MAS NMR spectroscopy (Figure 4B); the samples were dehydrated at the measurements to remove the zeolitic water. One peak at 2.3 ppm corresponding to isolated Al MAS NMR spectra of samples [Mo]-silanol species is present in the spectrum of [Mo] no peaks in the range 4-5.5 ppm are observed which is attributed to the absence of silanol nests. For the R reference sample, broad peaks at 2.3, and 4.0 to isolated silanols, and silanol nests and/or bridging hydroxyl groups respectively are observed (Figure 4B).

The location and state of the Mo are further studied by MAS NMR spectroscopy. The spectra of adsorbed trimethylphosphine oxide (TMPO) on the single and reference R-ZSM-5 samples are presented in Figure 5. The presence of Mo in the framework positions zeolite is confirmed by the resonance at 45 ppm which is consistent with previous work, [13] while a small amount of silanols are also observed (50 ppm). [15] Two Brønsted acid sites (64 and 74 ppm) as well as crystalline (43 pp (29 ppm) TMPO are present, indicating that TMPO acidic and metallic sites. [15c, 16] In contrast, for the Mo impregnated R-ZSM-5 sample, Brønsted acid sites (64 and 74 ppm) and silanols (50 ppm) are present, however, no framework Mo is detected as the resonance at 45 ppm is not (Figure 5). silanol species is present in the spectrum of [Mo]-ZSM-5, while 5.5 ppm are observed which is nests. For the R-ZSM-5 reference sample, broad peaks at 2.3, and 4.0-5.5 ppm related to isolated silanols, and silanol nests and/or bridging hydroxyl tively are observed (Figure 4B). [14] The location and state of the Mo are further studied by 31

Catalytic test of Mo-containing ZSM-5 zeolite samples: non oxidative high temperature CH4 conversion to H2 and higher hydrocarbons

The catalytic performances of the Mo single-site [Mo] and reference R-ZSM-5 samples were assessed for the high temperature non-oxidative conversion of CH atmospheric pressure, 0.3 g of catalyst, WHSV 1.2 h (Figure 6). The reference sample, R-ZSM-5, rapidly deactivates until the conversion of CH4 plateaus after 7 h. In contrast, the single-site [Mo]-ZSM-5 zeolite sample with only 0.5 % Mo shows exceptional stability after three consecutive reaction regeneration cycles. It is worth noting that for [Mo] deal with Mo-sites which remain within the zeolite framework allowing for the presence of either non-carburized or partially carburized Mo-sites. The location and retention of Mo in the zeolite framework results in the high stability of the catalyst, allowing it to operate under severe conditions over multiple reaction-regeneration cycles. However, further in required to determine the exact structure and evolution of the actives sites. Mo remains in the framework structure of the [Mo] ZSM-5 zeolite and does not undergo any discernible changes over the three reaction-regeneration cycles, vide infra methane conversion to hydrogen and the detailed product yields of the two catalysts are compared in Figure 6B. The [Mo] sample affords a significantly higher yield of hydrogen and ethylene in comparison to the R-ZSM-5 reference sample. It is also worth noting that the [Mo]-ZSM-5 sample relatively high proportion of ethylene in the C2 fraction The production of C3-C4 hydrocarbons is negligible for both samples.

To assess the stability of the Mo in the single 5 zeolite sample, the catalysts were subjected to multiple cycles of reaction (MDA, i.e. reducing conditions) and regeneration (calcination, i.e. oxidative conditions). Under typical MDA operating conditions, the classical Mo containing zeolite (R ZSM-5) deactivates due to the sintering and migration of molybdenum carbide species as a consequence of weak interactions with the zeolite. [17] During the exothermic catalyst regeneration, the molybdenum carbide is oxidized, and steam is generated from the oxidation of coke. Under such conditions the Mo species become mobile and react with framework Al to form aluminium molybdates. These new Mo species cannot form phase is highly beneficial, in particular for catalysts working under harsh conditions (e.g. high temperatures and continuous oxidation/reduction cycles).

As shown above, the single-site metal species incorporated into the zeolite framework results in stable catalytic performance. Further investigation of the thermal and hydrothermal stability of the single-site Mo-containing [Mo]-ZSM-5 zeolite and the reference sample is provided below. Emphasis is placed on the characterization of the samples in an attempt to describe the structural evolution of the catalysts under harsh reaction and regeneration conditions.

Stability and Mo-containing zeolite samples.

The stability of the single-site [Mo]-ZSM-5 and the reference ZSM-5 zeolite samples was evaluated after the catalytic reaction and subsequent recycling treatments. The samples subjected to 3 catalytic (reaction-regeneration) cycles are denoted with "(3 cycles)", while the samples that are subjected to 3 catalytic cycles followed by steaming are denoted with "(S)". patterns of the calcined ([Mo]-ZSM-5) and used ([Mo] cycles)) samples after 3 catalytic cycles (methane conversion and regeneration at 750 °C) clearly show that all the Bragg peaks are identical, confirming the preservation of the structure (Figure S5). A marginal decrease of the unit cell volume for [Mo]-ZSM-5 (3 cycles) is observed, suggesting that the sample underwent a degree of dealumination (Table S1). In contrast, the unit cell volume of R-ZSM-5 sample decreased substantially from 5359.283 to 5344.653 Å 3 , which is indicative of significant structural change occurring in the sample R (3 cycles). Remarkably, the 29 This suggests that the Brønsted sites due to framework impregnated ZSM-5 (R-ZSM-5 sample) are more resilient to dealumination than in pure ZSM-5. This discrepancy could be attributed to the presence of a protective molybdenum oxide shell around the framework Al species in the R consistent with recent work reporting a preferential location of Mo near framework Al species in ZSM-5. [19] Additionally, the stability of the samples after 3 catalytic cycles was studied after a subsequent steaming treatment performed at 550 °C. The XRD patterns and spectra of the [Mo]-ZSM-5 (S) and R-ZSM-5 (S) samples (Figure 7) establishes the outstanding hydrothermal stability of the framework Mo species in [Mo]-ZSM-5 zeolite. All peaks are identical for the calcined [Mo]-ZSM phase is highly beneficial, in particular for catalysts working high temperatures and continuous site metal species incorporated eolite framework results in stable catalytic performance. Further investigation of the thermal and hydrothermal stability of 5 zeolite and the reference sample is provided below. Emphasis is placed on the tion of the samples in an attempt to describe the structural evolution of the catalysts under harsh reaction and 5 and the reference [R]-5 zeolite samples was evaluated after the catalytic reaction and subsequent recycling treatments. The samples subjected to regeneration) cycles are denoted with "(3 cycles)", while the samples that are subjected to 3 catalytic followed by steaming are denoted with "(S)". The XRD 5) and used ([Mo]-ZSM-5 (3 cycles)) samples after 3 catalytic cycles (methane conversion and regeneration at 750 °C) clearly show that all the Bragg peaks are identical, confirming the preservation of the crystalline ginal decrease of the unit cell 5 (3 cycles) is observed, suggesting that the sample underwent a degree of dealumination (Table S1). In 5 sample decreased , which is indicative of significant structural change occurring in the sample R-ZSM-5

Si MAS NMR spectrum of the 5 (3 cycles) sample is almost identical to 5 sample (Figure S6). The 29 Si 5 (3 cycles) still displays highly species, in stark contrast to Rspecies are observed. This further demonstrates the exceptional stability of the 5 zeolite under reductive (catalytic reaction) and oxidative (regeneration) conditions. The peaks in the spectrum of 5 (3 cycles) sample is related to the degree of nation, the conservation of Mo in the structure, and the absence of silanols. The occurrence of dealumination in both the 5 (3 cycles) samples was Al MAS NMR spectra by the presence of pecies at 0 ppm (Figure S7). 7A). In addition, the of the [Mo]-ZSM-5 (S) sample (Figure 7B) displays highly resolved Q 4 species and no Q 3 species, demonstrating that no silanol species were generated after the combined react regeneration cycles and steaming treatments despite the simultaneous occurrence of dealumination. These observations further support the conclusion that the Mo atoms are retained in the MFI framework. This is the first example of a zeolite structure that contains heteroelements, such as Mo, able to withstand such harsh treatment (3 high temperature reaction/regeneration cycles in MDA, followed by steaming at 550 °C). In contrast, the R-ZSM-5 (S) sample displays both unresolved Q species attributed to the presence of silanol species. The formation of aluminium molybdates and the 5 (S) samples, while the diffraction pattern of 5 (S) sample reveals the appearance of several . In addition, the 29 Si MAS NMR spectra 5 (S) sample (Figure 7B) displays highly species, demonstrating that no silanol species were generated after the combined reactionregeneration cycles and steaming treatments despite the simultaneous occurrence of dealumination. These observations further support the conclusion that the Mo atoms are retained in the MFI framework. This is the first example of a zeolite structure at contains heteroelements, such as Mo, able to withstand such harsh treatment (3 high temperature reaction/regeneration cycles in MDA, followed by steaming at 550 °C). In contrast, the 5 (S) sample displays both unresolved Q supra. Importantly, no silanols are observed after the reaction regeneration cycles and subsequent steaming of the [Mo] 5 sample according to the 31 P MAS NMR spectra (Figur even though dealumination occurs. This demonstrates that atomically dispersed Mo introduced into the MFI zeolite framework stabilizes the crystalline structure and prevents the formation of silanols. This is further supported by the fact that, despite the occurrence of dealumination, the resonance at 45 ppm remains unchanged, suggesting that Mo is retained in the framework.

Conclusion

Here we report the synthesis of a novel single ZSM-5 zeolite using a Mo-free ZSM subjected to treatment with sodium molybdate under autogenous pressure. The single-site [Mo] contains highly disperse Mo (0.5 wt.%) according to EDS and ICP analyses; the Mo]-ZSM-5 zeolite was compared with a reference sample prepared with 1 wt.% Mo by the classical impregnation method (R-ZSM-5). The properties of the single site Mo containing ZSM-5 zeolite were characterized at the atomistic and molecular level using diffraction, micros spectroscopy methods.

The novel single-site Mo-containing nanozeolite possesses atomically dispersed frameworkhomogenously distributed through the zeolite crystals. The introduction of atomically dispersed Mo in the zeolite framework heals silanol defects resulting in an extremely stable resistant to severe structural degradation. The novel single Mo-containing ZSM-5 nanozeolite exhibits superior performance for CH4 conversion to H2 and higher hydrocarbons operating under successive reductive and oxidative atmospheres at high temperature (850 and 750 °C), as well as demonstrating exceptional structural stability under hydrothermal steaming conditions.

In summary, zeolites have so far been used successfully and extensively in oil refining and petrochemistry, and will continue to play an important role by meeting the challenges in these fields. However, the operating conditions (e.g. temperatures in the 750-850 °C range) of some emergin will push zeolites outside this "comfort zone" and successful zeolitic catalysts will require exceptional thermal and hydrothermal resilience.

The single-site Mo-containing ZSM example of a zeolite material able to operate "outside" of the comfort zone. This novel synthesis approach can be applied for the introduction of other metals (e.g. V, W, Cu, Zn) in other calcined zeolites including pure silica and aluminosilicates. even though dealumination occurs. This demonstrates that atomically dispersed Mo introduced into the MFI zeolite framework stabilizes the crystalline structure and prevents the formation of silanols. This is further supported by the fact that, te the occurrence of dealumination, the resonance at 45 that Mo is retained in the Here we report the synthesis of a novel single-site [Mo]free ZSM-5 nanosized zeolite ted to treatment with sodium molybdate under site [Mo]-ZSM-5 zeolite contains highly disperse Mo (0.5 wt.%) according to EDS-TEM 5 zeolite was compared with a .% Mo by the classical 5). The properties of the single-5 zeolite were characterized at the atomistic and molecular level using diffraction, microscopy and taining nanozeolite possesses -molybdenum atoms homogenously distributed through the zeolite crystals. The sed Mo in the zeolite framework heals silanol defects resulting in an extremely stable material, resistant to severe structural degradation. The novel single-site 5 nanozeolite exhibits superior performance and higher hydrocarbons operating under successive reductive and oxidative atmospheres at high temperature (850 and 750 °C), as well as demonstrating exceptional structural stability under hydrothermal steaming been used successfully and extensively in oil refining and petrochemistry, and will continue to play an important role by meeting the challenges in these fields. However, the operating conditions (e.g. temperatures in 850 °C range) of some emerging industrial processes will push zeolites outside this "comfort zone" and successful zeolitic catalysts will require exceptional thermal and containing ZSM-5 represents a strong e to operate "outside" of the comfort zone. This novel synthesis approach can be applied for the introduction of other metals (e.g. V, W, Cu, Zn) in other zeolites including pure silica and aluminosilicates.

A single-site Mo-containing nanosized ZSM-5 zeolite site Mo-containing nanosized ZSM-5 zeolite displays superior thermal (1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stabi and preventing the formation of silanols.

. 5 zeolite with atomically dispersed framework-molybdenum is synthesized. The single

Figure 1 .

 1 Figure 1. XRD patterns of [Mo]-ZSM-5 and R-Z range of (A) 5-45 °2θ and (B) 22-26.5 °2θ

  The larger unit-cell volume of the single-site[Mo] sample further indicates the substitution of Si 0.26 Å) for Mo VI (ionic radius: 0.59 Å) in the MFI framework. The morphology, size and elemental composition of both samples [Mo]-ZSM-5 and R-ZSM-5 are studied by STEM HAADF (Figure2). The statistical analysis of the particle size (measured for about 200 particles) is presented as inserts in the Figure2Aand 2B. They reveal the average particles size is about 150 ( + /-20) nm for both Mo-ZSM-5 and R-ZSM The most striking differences between the samples are found in the morphology of the particles. The crystals of [Mo] exhibit smooth surfaces and sharp edges, whereas in the case of R-ZSM-5 crystals, the outer surface is less regular and the inner crystals morphology is marked by the presence of mesopores (darker circular areas). The sharpness of the edges in the case of specimen [Mo]-ZSM-5 can be interpreted as a signature of its monocrystalline nature, whilst the irregular outer edges of the R-ZSM-5 crystals are associated with their polycrystallinity. The polycrystalline nature of the R crystals is also associated with the presence of an number of defects (notably the mesopores), which is further

Figure 2 .

 2 Figure 2. STEM-HAADF micrographs of (a) [Mo]-ZSM-5: left scale bar = 100 nm, right scale bar = 10 nm, and (B) R bar = 10 nm. Inset: particle size distribution.

  The spectrum of the calcined [Mo]-ZSM resolved Q 4 ((SiO)4Si) and no Q 3 indicating an almost total absence of silanols. In comparison, the reference R-ZSM-5 sample displays unresolved Q species, indicating the presence of silanol defects. These results are consistent with the HRTEM observations revealing the formation of [Mo]-ZSM-5 crystals with smooth surface, while the 5: left scale bar = 100 nm, right scale bar = 10 nm, and (B) R-ZSM-5: left scale bar = 100 nm, right scale Si MAS NMR), and IR spectroscopy (see below). The atomically dispersed Mo in [Mo]-5 (seen as light points in the micrographs) are highlighted HAADF micrographs, and confirm the presence of homogeneously distributed Mo atoms within the zeolite crystals (Figure 2A). The size of Mo light points in STEM-HAADF micrographs is similar in the two samples, however, they their stability under high temperature treatment (up to 1000 ºC) and steaming. The chemical -5 crystals is determined TEM. The elemental distributions 5 samples are shown in Figure S2. The shape and morphology of the pristine ZSM-5 was EDS (Figure S3). The agglomerated crystals have irregular shapes, and a mean size of 170 nm. The ZSM-5) have a similar shape d size (Figure S2); the distribution of both Si and Al in the pristine and reference sample is homogeneous suggesting that no changes under impregnation occurred. Further structural information of the single-site [Mo]-ZSM-5 Si MAS NMR spectroscopy (Figure 3A). ZSM-5 sample reveals well-((SiO)3Si-OH) species, st total absence of silanols. In comparison, the 5 sample displays unresolved Q 4 and Q 3 species, indicating the presence of silanol defects. These results are consistent with the HRTEM observations revealing the tals with smooth surface, while the 5: left scale bar = 100 nm, right scale R-ZSM-5 particles contain additional mesopores and surface defects. The presence of Al in the framework of both samples is confirmed by 27 Al MAS NMR spectroscopy (Figure 3B). The spectra for both the [Mo]-ZSM-5 and R-ZSM-5 sam a sharp peak at 60 ppm corresponding to Al in tetrahedral coordination, and no octahedral species are detected at 0 ppm.

Figure 3 .

 3 Figure 3. (A) 29 Si MAS NMR and (B) 27 Al MAS NMR spectra of samples [Mo] ZSM-5 and R-ZSM-5.

Figure 4 .

 4 Figure 4. (A) FTIR spectra and (B) 1 H MAS NMR spectra of dehydrated [Mo] ZSM-5 and R-ZSM-5 samples.

Figure 5 .

 5 Figure 5. 31 P MAS NMR spectra of TMPO adsorbed on samples [Mo] and R-ZSM-5.

P

  Figure 6. (A) CH4 conversion after the first and third consecutive cycles of reaction-regeneration as a function of time on stream for the [Mo] reference R-ZSM-5 catalysts and (B) total CH4 conversion (X: milligrams per gram of catalyst or %), products yield (milligrams per gram of catalyst) corresponding to 2-12 h time gap of calculation over the [Mo] ZSM-5. Reaction conditions: T = 850 °C, atmospheric pressure, WHSV = 1.2 h -1 , time on stream window = 2-12 h.

  Si MAS NMR spectrum of the single-site [Mo]-ZSM-5 (3 cycles) sample is almost identical to that of the calcined [Mo]-ZSM-5 sample (Figure MAS NMR spectra of [Mo]-ZSM-5 (3 cycles) still displays highly resolved Q 4 species and no Q 3 species, in stark contrast to R ZSM-5 (3 cycles) where both Q 3 and Q 4 species are observed. This further demonstrates the exceptional stability of th framework Mo in the [Mo]-ZSM-5 zeolite under reductive (catalytic reaction) and oxidative (regeneration) conditions. The intensity and high resolution of the Q 4 peaks in the spectrum of the [Mo]-ZSM-5 (3 cycles) sample is related to the degree of dealumination, the conservation of Mo in the structure, and the absence of silanols. The occurrence of dealumination in both the [Mo]-ZSM-5 (3 cycles) and R-ZSM-5 (3 cycles) samples was revealed in the 27 Al MAS NMR spectra by the presence of octahedral aluminum species at 0 ppm (Figure S 27 Al MAS NMR spectroscopy, dealumination occurred to a slightly greater degree for the [Mo]-ZSM-5 (3 cycles) sample in comparison to the R-ZSM-5 (3 cycles) sample.

  Based on Al MAS NMR spectroscopy, dealumination occurred to a 5 (3 cycles) sample in 5 (3 cycles) sample. This suggests Al in the Mo 5 sample) are more resilient to 5. This discrepancy could be attributed to the presence of a protective molybdenum oxide shell around the framework Al species in the R-ZSM-5 sample stent with recent work reporting a preferential location of after 3 catalytic cycles was studied after a subsequent steaming treatment The XRD patterns and 29 Si MAS NMR 5 (S) samples (Figure tstanding hydrothermal stability of the 5 zeolite. All the Bragg ZSM-5 and the steamed [Mo]-ZSM-5 (S) samples, while the diffraction pattern of the R-ZSM-5 (S) sample reveals the ap Bragg peaks (Figure

Figure 7 .

 7 Figure 7. (A) XRD patterns, and (B) 29 Si MAS NMR spectra of samples [Mo] ZSM-5 (S) and R-ZSM-5 (S). Diffraction peaks marked by "*" at 20.8, 26.6, 36.5, 40.2, and 42.4 ° 2θ on sample R-ZSM formation of catalytically inactive aluminium molybdate species.

  Figure 8. (A) 27 Al MAS NMR spectra of dehydrated [Mo] ZSM-5 (S) samples, and (B) 31 P MAS NMR spectra of TMPO adsorbed on dehydrated [Mo]-ZSM-5 (S) and R-ZSM-5 (S) samples.

  5 (S) sample wereAl MAS NMR analysis (Figure8A) due to of a peak at 14 ppm, which was not 5 (S) sample. Indeed, in the case of 5 (S) sample, a portion of the tetrahedral Al (53 ppm) is converted to distorted framework Al and/or pentaoctahedral Al. Critically, in stark contrast with the R-5 (S) sample. Indeed, it has been reported that MoO3 reacts with framework Al above 550 °C to form aluminum Anderson entities.[7a] Consequently, the absence of aluminum molybdate in [Mo]-Mo atoms remain atomically 5 after steam treatment. P MAS NMR spectra (using TMPO) of the steamed samples are depicted in Figure 8B. The number of Brønsted acid sites in 5 (S) sample decreased substantially after quent formation of framework aluminium, evidenced from both 31 P MAS NMR Al MAS NMR (Figure 8A) spectroscopy, vide Al MAS NMR spectra of dehydrated [Mo]-ZSM-5 (S) and Rspectra of TMPO adsorbed on

zeolite displays superior thermal (1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite, molybdenum is synthesized. The single-5 zeolite displays superior thermal (1000 °C), hydrothermal (steaming), and catalytic (converting lity, maintaining the atomically disperse Mo, structural integrity of the zeolite,

Acknowledgements

Financial support from TOTAL and Industrial Chair ANR "Nanoclean Energy" is acknowledged as well as from the Normandy Region through his RIN Recherche Program. Authors declare no competing interest in this work. GENESIS is supported by the Région Haute-Normandie, the Mé Rouen Normandie, the CNRS via LABEX EMC and the French National Research Agency as a part of the program . Importantly, no silanols are observed after the reactionregeneration cycles and subsequent steaming of the [Mo]-ZSM-P MAS NMR spectra (Figure 8B),

om TOTAL and Industrial Chair ANR-TOTAL "Nanoclean Energy" is acknowledged as well as from the Normandy Region through his RIN Recherche Program. Authors declare no competing interest in this work. GENESIS is Normandie, the Métropole Rouen Normandie, the CNRS via LABEX EMC and the French National Research Agency as a part of the program "Investissements d'avenir" with the reference ANR-11-EQPX-0020. The 111 Project (Grant No. B17020) providing opportunities for future collaboration with Jilin University, China is acknowledged.