Abhineet Verma

Nattamai Bhuvanesh

Joseph Reibenspies

Sakharam B Tayade

Avinash S Kumbhar

Kateryna Bretosh

Jean-Pascal P Sutter

Sailaja S Sunkari
email: sailaja.sunkari1@bhu.ac.in

Structurally characterised new twisted conformer for cyclen, controlled by metal ion complexation as seen in Ni II and Cu II complexes with halides and pseudohalides

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Serendipitous self-assembly of metal ions' with diverse types of chosen ligands is a potential approach in supramolecular coordination chemistry to derive structurally versatile solids. [1,2] Such an approach, under the influence of structure directing factors, has the potential to generate interesting structures of material relevance. [3] The approach gains more prominence when the ligands are flexible and/or auxiliary anions have diverse binding modes.

Cyclen (1,4,7,10-tetraazacyclododecane) is an interesting flexible molecule, extensively explored for biological and material applications of its metal complexes, [4,5] since long. Being flexible, cyclen can adopt several conformations (Chart 1) ranging from planar to non-planar, depending on the complexation environments. Literature reported Cu and Ni metal complexes of cyclen (Chart 2) have been found to display, two conformations for cyclen, viz., a planar conformation for d 9 configurations of the Cu(II) ion and the twisted conformer as the major one for d [START_REF] Kahn | Molecular magnetism[END_REF] configurations of the Ni(II) ions. Early literature studies account for the favourable conformers of cyclen when all the ligand nitrogens' are in the basal plane, for differing coordination geometries of the metal complexes, without regard to the electronic configuration of the metal ions '.[5c-d,6,7] Interestingly, we have not come across such studies when ligand nitrogen's are not necessarily in the basal plane.

Early studies by Busch et al, [5c] focus on the planar conformation (a of chart 1) of cyclen and report the optimum M-N distances for metal ions' sitting in the planar macrocyclic cavity. Boeyens and Hancock et al, [5d] summarize the optimal M-N distances for metal complexes under different coordination geometries, with ligand nitrogens forming the basal plane that minimize repulsions, resulting in preferred conformations of the tetraaza macrocyles. According to Hancock et al, the ++++ conformer is the favoured conformer for square planar and square pyramidal geometries, while ++--(b of chart 1) conformer is the favourable conformer in octahedral geometry for cyclen in which the cyclic nitrogens form the basal plane. from metal complexation. Interestingly, the dihedral angle (~52-55°) between the ligand nitrogens' of cyclen in the series of Ni II complexes leading to the observed twisted conformation is the highest reported till date. The paper gives details regarding synthesis, structure and spectral studies for all the complexes. Further, SQUID magnetic studies for the dimeric [Ni1] complex reveal antiferromagnetic interactions between the two Ni centres.

Chart 2. Graphical presentation of the variation of dihedral angle between the cyclen nitrogens in reported Cu II & Ni II complexes, based on CSD search (version 5.38, 01-03-2019) using the search question "complexes of cyclen with transition metals" without any filters applied. Green circles and stars correspond to the systems reported in this work while red circles and stars are literature reported systems. for the diamagnetic contribution of all atoms by using Pascal's tables. [8a] The isothermal field dependence of the magnetization was recorded up to 50 kOe. Modelling was performed using PHI.

EXPERIMENTAL SECTION

[8b]

Preparation of complexes [Ni1] -[Ni4] & [Cu1] -[Cu4]:

To a methanolic solution (~ 15 ml) of cyclen (0.10 g, 0.58 mmol), Ni(CH 3 COO)

Crystallographic data collection and refinement:

Data collection was done for the title complexes as detailed in sec. 2.1. Hydrogen atoms were set as riding on the parent atoms after placing them in idealized positions around respective parent atoms. Atoms were refined with anisotropic thermal parameters except for the hydrogen atoms. The absence of additional symmetry and voids were confirmed using PLATON (ADDSYM). [9] The structures were solved by direct methods and refined by full-matrix least-squares on F 2 using the latest version of SHELX-2018. [10] The

a R 1 = Σ||F o |-|F c ||/Σ|F o |. b wR 2 = [Σ[w(F 2 o -F 2 c ) 2 ]/Σ[w (F 2 o ) 2 ]
1/2 1 and bond length and angles in ESI Table S1. Drawings are made using OLEX-II, [11] and MERCURY. [12] 3. RESULTS AND DISCUSSION

Synthesis and vibrational spectroscopic studies.

A series of Ni II and Cu II complexes have been synthesized in-situ in a step-wise manner with reasonable yields. Complexes are blue blocks with sharp melting points. The PXRD patterns (ESI, Figure S1) confirm the bulk purity of the products, as can be seen from the agreement of the experimental PXRD patterns with the simulated patterns, indicating that the crystal structures are true representatives of the bulk products.

All the complexes exhibit strong and sharp vibrational bands at 3000 -3300 cm -1 in FT-IR (ESI, Figure S2), which corresponds to the N-H symmetric and asymmetric stretching of the core moiety (cyclen) containing four N-H bonds. [13] There are other sharp bands appearing at 1455 cm -1 (m), 968 cm -1 , (s) 883 cm -1 (s) and one broad band at about 1630 cm -1 (s) corresponding to the characteristic (C-H) bands for the core moiety. Due to the presence of N 3 -group, sharp bands at 2127 cm -1 (s) and 2182 cm -1 (s) corresponding to asymmetric μ 1,3 and end-on stretching modes of azide are observed for the complexes [Ni1] & [Cu1] respectively. [14] Symmetric and asymmetric end on stretching vibrations of N(CN) 2 -have been observed at 2140 cm -1 (m) & 2225 cm -1 (m) for [Ni2] and at 2127 cm -1 (m) and 2182 cm -1 (m) for [Cu2]. [Ni3] shows sharp band at 2035 (s) corresponding to symmetric end-on stretching of NCS -group. Sharp bands at 2144 cm -1 (s) correspond to end-on symmetric stretching of coordinated SCN -, and bands at 2251 cm -1 (m) and 2287 cm -1 (m) correspond to the asymmetric stretching vibrations of uncoordinated SCN -of [Cu3]. The fingerprint region in all the complexes shows sharp bands at 526 cm -1 and 421 cm -1 corresponding to the Ni-N and Cu-N stretching frequencies respectively. The rest of the spectral patterns and band positions of all the complexes are very similar and are shown in ESI (Figure S2).

3.2

Single crystal X-ray diffraction structural studies.

[Ni(cyclen

)N 3 ] 2 [Ni1] [diazido(1,4,7,10-tetraazacyclododecan-1-yl)nickel] : Complex [Ni1]
crystallizes in the monoclinic space group C2/m. Ni II adopts distorted octahedral geometry, with three cyclen nitrogens (N2, N3, N2 I ) occupying the basal plane and the fourth position occupied by azide nitrogen (N6 I ). The axial positions of the octahedron are occupied by one of the cyclen nitrogen (N1) and nitrogen of the second azide (N4) (Figure 1a). The M-N distances are in the range 2.03 -2.12 Å, respectively (Table S1). Ni II is displaced by (d) 0.189 Å from the mean plane of the coordinating nitrogen atoms. The azide group acts as a symmetric μ-1,3 bridge and connects two Ni II centres forming a dimer, where one of the N 3 -group orients equatorially while the other tends to be axial.

The Ni-N az distances in the [Ni1] dimer are slightly lesser than double μ-1,3 azido bridged Ni II dimers in which azide is in axial -equatorial sites of the coordination polyhedron. [START_REF] Ribas | Font-Bardía, M. Two New Antiferromagnetic Nickel(II) Complexes Bridged By Azido Ligands in The cis position. Effect of The Counteranion On the Crystal Structure and Magnetic Properties[END_REF] The dihedral angle between the ligand N's in the two-cyclen moieties is 51.8° and 53.3° (Table S2). Intradimer separation between the two Ni II centres (5.313 Å) is much less than the separation observed (ranging between 7-11 Å) for reported dimeric Ni II complexes of cyclen (ESI Table S3). Neighbouring dimers are non-interacting and are arranged parallel in 3-dimensions, as shown in Figure 1b. 2a). The coordination polyhedron around Ni II is comparable to previously reported dicyanamide bridged Ni II dimer with cyclen. [START_REF][END_REF] The M-N distances are 2.06 -2.13 Å and 2.107 Å, respectively (Table S1). Ni II deviates by d = 0.167 Å from the mean plane of the coordinating nitrogen atoms. The dihedral angle between the four nitrogen of the ligand is 52.1° (Table S2). Complex [Ni3] crystallizes in a trigonal crystal system with space group R3c. The asymmetric unit contains one metal centre, one ligand and two thiocyanate anions. Three nitrogen atoms fulfil octahedral geometry of Ni II from the ligand (N1, N2 & N3) and fourth nitrogen (N5) from one of the thiocyanate in the basal plane; the fourth ligand nitrogen (N4) and nitrogen (N6) of the second thiocyanate moiety occupy the axial positions (Figure 3a). The M-N distances are in the range 2.07 -2.16 Å (Table S1). The dihedral angle between the four nitrogen atoms of the ligand is 54.6° (Table S2). Though the metal ion adopts octahedral geometry, the equatorial sites are occupied by two nitrogen atoms from the ligand (N1 & N3) and two oxygen (O1 & O2) atoms from acetate group, which defines the mean plane. In contrast, the axial positions are occupied by the other two nitrogen atoms of the ligand (N1 & N4) (Figure 4a). M-N distances are in the range 2.03-2.06 Å (Table S1). Ni II is found to be displaced by 0.142 Å from the basal plane of the coordinating nitrogens. The dihedral angle between the four nitrogen atoms of ligand is found to be 52.6 ° (Table S2). The uncoordinated bromide ions connect the monomers into a zig-zag chain through hydrogen bonding interactions (N1-H1••••Br1 : 3.346(4) Å), (Figure 4b).

In general, cyclen adopts a twisted conformation in all the structures (Table S2), similar to that observed in other Ni-cyclen complexes with diverse anions (Chart 2). The trends in Ni-N cy bond lengths are similar for all complexes and are in accordance with other reported Ni-cyclen complexes (ESI Table S3). However, the dihedral angle between the cyclen nitrogens (~52-55°) observed here is much higher than the angle (35-40°) in the Ni II complexes of cyclen reported so far (chart 2), thus revealing a new conformational angle and geometry for the twisted conformation of cyclen in Ni II complexes. (1,4,7,10-tetraazacyclododecan-1-yl)copper]: Monomeric [Cu1] crystallizes in the monoclinic space group P2 1 /n with a distorted square pyramidal geometry around the metal centre. The asymmetric unit contains one metal centre, one ligand and two azide moieties. The ligand nitrogens (N1, N2, N3 & N4) occupy the basal plane, and azide nitrogen (N5) of one moiety occupies the pyramidal position (Figure 5a) of the metal ion. Cu-N cy and Cu-N az bond lengths are 2.01-2.04 Å and 2.10 Å, respectively (Table S1). Cu II is displaced by 0.5330( 6) Å from the basal plane. The dihedral angle between the four nitrogen atoms of the ligand is found to be 2.7° (Table S2). Both coordinated and uncoordinated azides are involved in hydrogen bonding interactions with the amino hydrogens of the ligand. The coordinated azides connect the symmetry-related monomers into a dimer through amino N cy - H---N az (H---A

[Cu(cyclen)N

3 ]N 3 [Cu1] [azido

[Cu 2 (cyclen) 2 (SCN) 2 ](SCN) 2 [Cu3]

[ (1,4,7,10-tetraazacyclododecan-1-yl)(isothiocyanato)copper]: Complex [Cu3] crystallizes in orthorhombic space group P2 1 2 1 2 1 . The asymmetric unit contains two independent [Cu(cyclen)(NCS)] + moieties and two uncoordinated NCS -units. While the metal ion exhibits square pyramidal geometry, the two units are distinct by the coordination mode of the thiocyanate moiety in the pyramidal position. In the first unit, the distorted square pyramidal geometry is fulfilled by the four nitrogen atoms of the ligand forming the basal plane (N1A, N2A, N3A & N4A), while the fifth axial site is occupied by the S (S1A) of the thiocyanate moiety (Figure 6a). However, in the second unit, the distorted square pyramidal geometry of the metal center is fulfilled by the ligand nitrogens (N1B, N2B, N3B & N4B) and the pyramidal position is occupied by the nitrogen of second thiocyanate moiety, (N5B). Displacement of Cu II from the basal plane is 0.556 Å & 0.589 Å in both units, respectively. The dihedral angle between four nitrogen atoms of the ligand is 0.2 ° and 0.0 ° for the two independent units, respectively (Table S2). The two cyclen units are arranged in a flip-flop manner as pairs connected through hydrogen bond interactions (N cy -H---N SCN = 2.960 Å) between ring amino hydrogens of the first unit and the N end of the coordinated thiocyanate of the second unit (red dashed lines in figure 6c). 7a). The Cu II ion is displaced by 0.593(2) Å from the basal plane. The dihedral angle for the four ligand nitrogen atoms is found to be 0.0 ° (Table S2). In the asymmetric unit, there is another bromide ion (Br2) that is uncoordinated and connects the neighbouring [Cu(cyclen)Br] + units into a 2-D network through hydrogen bonding interactions (N1-H1••••Br2 = 3.319 Å) (Figure 7b).

Structural Comparisons between Ni & Cu Series.

The diverse shapes exhibited by all the metal complexes may be attributed to the conformational flexibility of the ligand to support the metal ions' geometrical preferences arising out of CFSE, supported by the shape of the anion. Cyclen being flexible, when put in solution containing metal ions' and anions, adjusts its conformation to suit the metal ions' geometrical preferences, thereby adopting a twisted conformation with Ni II ion and a planar conformation with Cu II ion, that maximises the CFSE of the respective metal ions. By doing so, it supports the metal complex attain diverse shapes in association with the anion, which may be depicted as a cartoon representation as an azide With a larger twist in the ligand conformation (dihedral angle of ligand nitrogens = ~ 52-55°), this anionic effect is more pronounced in the Ni II series. However, with a planar ligand around the Cu(II) ion (dihedral angle of ligand nitrogens = ~ 0-3°), the gross shape of the molecule remains the same (an inverted umbrella -cartoon representation) similar to other reported complexes (ESI, Table S4)., with a minimal effect of the anion that shows up in the packing patterns only.

The planar conformation (++++) of cyclen in all the copper complexes is in accordance with the predictions of Hancock et al. for square pyramidal geometry. [6] However, in Ni II complexes, cyclen displays several conformations as ++++ in Ni1 and Ni3; +++-in Ni2; and -+++ in Ni4, very much different from the predictions (++--) of Hancock et al for an octahedral geometry of a metal ion, where all the ligand amino nitrogens are in the basal plane. The variations in ligand conformation in our case appears to arise from the difference in planar atoms for the octahedral geometry, where only three amino nitrogens of the ligand are in basal plane and the fourth position is occupied by different anionic atoms. The modelling performed with a dimer model for two S = 1 centres (reference Hamiltonian is = -( )) lead to an interaction parameter of J = -82.1 ± 0.2 cm -1 for g = 2.0. The obtained ) exchange parameter is in accordance with values reported for related compounds [START_REF] Ribas | Font-Bardía, M. Two New Antiferromagnetic Nickel(II) Complexes Bridged By Azido Ligands in The cis position. Effect of The Counteranion On the Crystal Structure and Magnetic Properties[END_REF][START_REF][END_REF][START_REF] Bai | Chelating Schiff Base Assisted Azide-Bridged Mn(II), Ni(II) and Cu(II) Magnetic Coordination Polymers[END_REF] and confirms the strong Ni-Ni antiferromagnetic interaction mediated by the azido-bridges.

CONCLUSION

Self-assembly of Ni II or Cu II with conformationally flexible cyclen in the presence of pseudohalides has resulted in title complexes which are characterised structurally. All the Ni II complexes display a twisted conformer for the ligand with a large dihedral angle (52-55°, Table S2 ) with no anion effect on the molecular shape. In the absence of J-T distortion, in octahedral Ni II complexes, the ligand is highly distorted (dihedral angle = ~52-55°) to support the octahedral geometry of the metal ion, thus attaining a twisted conformation, in which the ligand nitrogens' occupy three positions of the square plane, and anionic nitrogen occupy the remaining one position, as seen through X-ray structures. Due to the positioning of anionic nitrogens' in the square plane, depending on the anion, different structures were obtained for Ni-cyclen-X complexes, viz., "Dumb bell-shaped structure" with azide [Ni1], "Tom cat structure" with dicyanamide [Ni2], "Jerry mouse structure" with thiocyanate [Ni3] and "Wine-Glass structure" with bromide [Ni4] ions. The [Ni1] dumbbell shows antiferromagnetic interactions (J = -82.1 ± 0.2 cm -1 ) between the end-to-end azide bridged Ni centers.

The present study involving the serendipitous self-assembly approach is successful in providing a new conformation for cyclen with a large dihedral angle (~52-55°), the highest reported so far between its ring nitrogens. The increased dihedral angle of the ligand in its non-planar mode of coordination to support Ni II octahedral geometry, in association with diversely shaped anions, resulted in interesting structures under self-assembling conditions. The examples are important as there are no literature reports predicting the conformations of cyclen for octahedral geometry in which not all its ring nitrogens' form the basal plane. The work provides scope for in-depth theoretical studies in finding out all the possible conformations for cyclen in octahedral coordination when all ligand nitrogens are not necessarily in basal plane of the metal ion. More examples may be helpful in developing functional molecules and study their functionality based on variations in structure.

Chart 1 .

 1 Possible conformations of coordinated tetraaza macrocycle (cyclen) when (a) planar and (b) twisted. The + indicates that the hydrogen of the NH group is above the plane of the flattened macrocycle, while the -indicates that it is below. Continuing with our interest in synthesizing new molecules with a focus on their magnetic behaviour, our attempts with Ni II , Cu II , cyclen and various linkers based on halides and pseudo halides have resulted in complexes ([Ni1]-[Ni4] & [Cu1]-[Cu4]) having distinctly different conformations for cyclen. The observed conformations, i.e. planar (in [Cu1]-[Cu4]) and twisted (in [Ni1]-[Ni4]), have been understood to be due to the change in the dihedral angle of the ring nitrogens (N1 to N4) resulting

2. 1

 1 Material and methods: Cyclen, Ni(OAc) 2 •4H 2 O, Cu(OAc) 2 •2H 2 O were purchased from TCI chemicals, whereas NaN 3 , KSCN, NaN(CN) 2 & KBr were procured from Sigma Aldrich and used without further purification. Perkin Elmer Spectrum-2 FTIR spectrometer was employed for FT-IR data collection as KBr pellets in the region 4000 to 400 cm -1 . CHN analysis were performed on an Exeter analytical Inc. model: CE 440 elemental analyzer. A BRUKER Quest X-ray (fixed-Chi geometry) diffractometer Mo-I radiation (Kα = 0.71073 Å) was employed for single crystal X-ray data collection for complexes [Ni1]-[Ni4] at 110 K; PANalytical XRD equipped with Rigaku two circle diffractometer using Mo-Kα radiation at 300 K for [Cu1] and a Bruker Kappa APEX-II CCD using D8 X-ray generator, Cryoflex (model Tr-60) Mo-Kα radiation at 300 K for [Cu3] and [Cu4]. Powder X-ray diffraction (PXRD) data were recorded and collected on a Rigaku Smartlab powder type diffractometer with Cu Kα1 (λ = 1.5406 Å; power 1600 W, 40 kV, 40 mA) at a scan speed of five degrees per minute. The magnetic measurement for [Ni1] was carried out with an MPMS-5 QUANTUM DESIGN SQUID magnetometer between 300 and 2 K with an applied field of 1 kOe. Data have been corrected for sample holder and

Scheme 1 .

 1 Scheme 1. Schematic representation for the preparation of complexes. M = Ni II for [Ni1]-[Ni4] and M = Cu II for [Cu1]-[Cu4] whereas X = N 3 -, N(CN) 2 -, SCN -and Br -respectively.

  refinement converged to final values of R1 & wR2 = 0.0587 & 0.1399 for [Ni1]; 0.0485 & 0.1524 for

Figure 1 .

 1 Figure 1. (a) (left) ORTEP of [Ni1] showing atoms as 40 % probability ellipsoids. Ring hydrogens have been omitted for clarity. (right) Corresponding Polyhedral drawing showing the coordination environment around the metal center. (b) Packing of dimers in the unit cell. Symmetry codes: I = x, -y, z; II = 1-x, y, 1-z; III = 1-x, -y, 1-z.3.2.2. [Ni(cyclen)(N(CN) 2 ) 2 ] [Ni2]: bis(N-cyanocyanamido)(1,4,7,10-tetraazacyclododecan-1-yl)nickel]: Complex [Ni2] crystallizes in orthorhombic space group Pbca. Monomeric [Ni2] is in a distorted octahedral geometry with three cyclen nitrogens (N1, N2 & N3) occupying the basal plane and the fourth position occupied by nitrogen of one of the anionic dicyanamide (N5). Axial positions of the octahedron are occupied by one of the cyclen nitrogen (N4) and nitrogen (N8) of the second dicyanamide group (Figure2a). The coordination polyhedron around Ni II is comparable to previously reported dicyanamide bridged Ni II dimer with cyclen.[START_REF][END_REF] The M-N distances are 2.06 -2.13 Å and 2.107 Å, respectively (TableS1). Ni II deviates by d = 0.167 Å from the mean plane of the coordinating nitrogen atoms. The dihedral angle between the four nitrogen of the ligand is 52.1° (TableS2). Individual monomers act as hydrogen bond donors (through N cy -H) and acceptors (through dicyanamide N's). They are interconnected with weak hydrogen bonding interactions with neighbouring monomers (N1-H1••••N10 : 3..023(5) Å; N4-H4••••N10 Å : 3.167(5) Å; N3-H3••••N7 : 3.046(5) Å; C8-H8B••••N7 : 3.590(5) Å) (Figure 2b), leading to a 2-D layer arrangement in the unit cell.

Figure 2 .

 2 Figure 2. (a) (left) Ortep of [Ni2] showing atoms as 40 % probability ellipsoids. Ring hydrogens have been omitted for clarity; (right) Coordination polyhedron around the metal center; (b) Packing of hydrogen bonded monomers in the unit cell.3.2.3. [Ni(cyclen)(SCN) 2 ] [Ni3][(1,4,7,10-tetraazacyclododecan-1-yl)diisothiocyanatonickel] : Complex[Ni3] crystallizes in a trigonal crystal system with space group R3c. The asymmetric unit contains one metal centre, one ligand and two thiocyanate anions. Three nitrogen atoms fulfil octahedral geometry of Ni II from the ligand (N1, N2 & N3) and fourth nitrogen (N5) from one of the thiocyanate in the basal plane; the fourth ligand nitrogen (N4) and nitrogen (N6) of the second thiocyanate moiety occupy the axial positions (Figure3a). The M-N distances are in the range 2.07 -2.16 Å (TableS1). The dihedral angle between the four nitrogen atoms of the ligand is 54.6° (TableS2). The metal ion deviates by 0.121 Å from the mean plane of the coordinating nitrogens. Terminal sulphur atoms of the coordinated thiocyanate moieties are involved in bifurcated hydrogen bonding interactions with the amino hydrogens of the neighbouring cyclen units (N2-H1••••S2 : 3.481(4) Å; N4-H4••••S2 : 3.553(4) Å; N1-H1••••S2 : 3.620(4) Å; N3-H3••••S1 : 3.578(6) Å), giving a trigonal packing of the monomeric units when viewed down the b axis of the unit cell (Figure 3b).

  The metal ion deviates by 0.121 Å from the mean plane of the coordinating nitrogens. Terminal sulphur atoms of the coordinated thiocyanate moieties are involved in bifurcated hydrogen bonding interactions with the amino hydrogens of the neighbouring cyclen units (N2-H1••••S2 : 3.481(4) Å; N4-H4••••S2 : 3.553(4) Å; N1-H1••••S2 : 3.620(4) Å; N3-H3••••S1 : 3.578(6) Å), giving a trigonal packing of the monomeric units when viewed down the b axis of the unit cell (Figure 3b).

Figure 3 .

 3 Figure 3. (a) (left) Ortep of [Ni3] showing atoms as 40 % probability ellipsoids. Ring hydrogens are omitted for clarity; (right) Coordination polyhedron around the metal centre; (b) Packing of monomers in the unit cell.

Figure 4 .

 4 Figure 4. (a) (left) Ortep of [Ni4] showing atoms as 40 % probability ellipsoids. Ring hydrogens have been omitted for clarity; (right) Coordination polyhedron around the metal center. (b) Hydrogen bonded zig-zag chain of monomers in the unit cell. Symmetry code: I = x, -y, z.

  1.994 Å) hydrogen bonding interactions (red dashed lines, fig. 5b). The uncoordinated azides not only connect the monomers of the same symmetry into a hydrogen-bonded chain involving amino N-H••••N az (N1-H1••••N10 : 2.973(18); N3-H3••••N8 : 3.052(18) Å) hydrogen bonds but also connect the symmetry-related dimers to neighbouring dimers (N4-H4••••N10 : 2.972(17) Å), leading to a 2-D layer (black dashed lines, figure 5b).

Figure 5 .

 5 Figure 5. (a) (left) Ortep of [Cu1] showing atoms as 40 % probability ellipsoids. Ring hydrogens have been omitted for clarity; (right) Coordination polyhedron around the metal center; (b) Packing of H-bonded dimers into a 2-D layer in the unit cell.

  The uncoordinated thiocyanate moieties connect the neighbouring flipped pairs into a 2-D layer arrangement in the unit cell through hydrogen-bonding interactions with ring amino hydrogens. (N4A-H4A••••S1C : 3.318(5) Å; N1B-H1B••••N1C : 2.957(8) Å; N3A-H3A••••S1D : 3.439(6); N3B-H3B••••S1D : 3.338(5) Å, black lines in fig. 6b).

Figure 6 .

 6 Figure 6. (a) (left) Ortep of [Cu3] showing atoms as 40 % probability ellipsoids. Ring hydrogens have been omitted for clarity; (right) Coordination polyhedron around the metal center; (b) 2-D layer arrangement of flipped pairs.3.2.7. [Cu(cyclen)Br]Br [Cu4][(1,4,7,10-tetraazacyclododecan-1-yl)copper(II) bromide]: Complex [Cu4] crystallizes in the tetragonal space group P4/nmm with square pyramidal geometry around the metal center. Nitrogens of ligand occupy the four equatorial positions (N1, N2, N3 & N4), forming the basal plane whereas bromide ion (Br1) occupies the pyramidal position (Figure7a). The Cu II ion is displaced by 0.593(2) Å from the basal plane. The dihedral angle for the four ligand nitrogen atoms is found to be 0.0 ° (TableS2). In the asymmetric unit, there is another bromide ion (Br2) that is uncoordinated and connects the neighbouring [Cu(cyclen)Br] + units into a 2-D network through hydrogen bonding interactions (N1-H1••••Br2 = 3.319 Å) (Figure7b).

Figure 7 .

 7 Figure 7. (left) Ortep of [Cu4] showing atoms as 40 % probability ellipsoids. Ring hydrogens have been omitted for clarity; (right) Coordination polyhedron around the metal center. (b) Packing of monomers through hydrogen bonding interactions. Symmetry code: I = x, 3/2-y, z; II = 3/2-x, 3/2-y, z; III = 3/2-x, y, z

Scheme 2 : 3 . 4 .Figure 8 :

 2348 Scheme 2: Schematic presentation of the diverse shapes arising out of metal ion's coordination supported by anions.3.4. Magnetic Studies of Ni1.The magnetic behaviour for the azide-bridged Ni dimer, [Ni1] has been investigated in the temperature domain 2 -300 K. The temperature dependence of χ M T (χ M stands for the molar magnetic susceptibility) for [Ni1] is characteristic for strong antiferromagnetic interaction between the two Ni centres (Figure8a). The value for χ M T at 300 K, 1.45 cm 3 mol -1 K, is below the Curie contribution of 2 cm 3 mol -1 K expected for two Ni (S = 1, g = 2.0) and steadily decrease as T is lowered to reach almost zero below 20 K. The related χ M = f(T) shows a broad maximum at 110 K (Figure8(b)).(a)(b)

  Symmetry codes used to generate equivalent atoms: I = x, -y, z; II = -x+1, y, -z+1; III = -x+1, -y, -z+1 for [Ni1]; I 1/2-X, 1-Y, 1/2+Z; II X, 3/2-Y, 1/2+Z; III -1/2+X, 3/2-Y, 1-Z ; IV 1-X, -1/2+Y, 3/2-Z for [Ni2]; I 1/3+Y-X, 2/3-X, -1/3+Z; II 1-Y, 1-X, -1/2+Z; III -1/3+X, 1/3+X-Y, -1/6+Z;IV 2/3-Y, 1/3+X+Y, 1/3+Z for [Ni3]; I = x, -y, z for [Ni4]; I =-x+1, -y+1, -z+1; II = x-1, y, z for [Cu1]; I -1/2+x, ½-y, 1-z; II 2-x, 1/2+y, ½-z; III 2-x, -1/2+y, ½-z for [Cu3]; I = x, 3/2-y, z for [Cu4].TableS2. Dihedral angle between four nitrogen atoms of cyclen and deviation of metal from the basal plane in the title complexes. The ligands nitrogen's are shown in ink blue and anionic nitrogen in teal blue color.` Selected distances and angles for reported complexes of Ni(II) with cyclen.

  2

  The reaction mixture was stirred overnight at room temperature (25 °C). The clear mother liquor was filtered into a small test tube, which is placed inside a bigger stoppered test tube partially filled with ether. Slow diffusion of ether into the inner tube maintained at 18 °C, afforded single crystals suitable for x-ray structural studies. Suitable single crystals were obtained within 2 days in case of NiII complexes and one week for Cu II complexes. Yield: [Ni1] = 0.23 g (76.6 %, 0.47 mmol), M.P. 214 (±1)°C. Anal. Calcd. for C 16 H 40 N 14 Ni 2 (MW = 546.04): C, 35.20; H, 7.38; N, 35.92.

	•4H 2 O (0.15 g, 0.58 mmol) was added for the preparation of
	complexes [Ni1]-[Ni4] and Cu(CH 3 COO) 2 •2H 2 O (0.16 g, 0.58 mmol) for complexes [Cu1]-[Cu4]. After
	stirring for 10 minutes, NaN 3 (0.06 g, 1.16 mmol), or KSCN (0.08 g, 1.16 mmol), or NaN(CN) 2 (0.08 g,
	1.16 mmol) or KBr (0.12 g, 1.16 mmol) was respectively added for the preparation of corresponding
	complexes. Found: C, 34.92; H, 7.41; N, 35.42. [Ni2] = 0.28 g (87.5 %, 0.77 mmol), M.P. 209(±1)°C. Anal. Calcd. for
	C 12 H 20 N 10 Ni (MW = 363.09): C, 39.70; H, 5.55; N, 38.58. Found: C, 39.43; H, 5.34; N, 38.83. [Ni3] =
	0.26 g (81.2 %, 0.74 mmol), M.P. 212(±1)°C. Anal. Calcd. for C 10 H 20 N 6 S 2 Ni (MW = 347.15): C, 34.60; H,
	5.81; N, 24.21. Found: C, 35.02; H, 5.25; N, 23.94. [Ni4] = 0.32 g (91.6 %, 0.84 mmol), M.P. 182(±1)°C.
	Anal. Calcd. for C 10 H 23 N 4 O 2 BrNi (MW = 369.94): C, 32.47; H, 6.27; N, 15.15. Found: C, 32.24 ; H, 5.92;
	90 . and
	[Cu4] = 0.32 g (84.2 %, 0.81 mmol) M.P. 201(±1)°C. Anal. Calcd. for C 8 H 20 N 4 Br 2 Cu (MW = 395.64): C,
	24.29; H, 5.10; N, 14.16. Found: C, 24.01; H, 4.94; N, 14.31. Procedure for the preparation of
	complexes is presented schematically in Scheme 1. All the complexes were completely characterized
	by FT-IR, CHN, PXRD, in addition to SCXRD. Due to poor quality of crystals, SCXRD data for [Cu2] could
	not be collected.

N, 15.24. [Cu1] = 0.30 g (93.7 %, 0.79 mmol), M.P. 235(±1)°C. Anal. Calcd. for C 8 H 20 N 10 Cu (MW = 319.88): C, 41.82; H, 7.02; N, 31.04. Found: C, 41.43; H, 7.23; N, 31.35. [Cu2] = 0.31 g (91.2 %, 0.86 mmol), M.P. 215(±1)°C. [Cu3] = 0.28 g (82.3 %, 0.79 mmol), M.P. 230(±1)°C. Anal. Calcd. for C 10 H 20 N 6 S 2 Cu (MW = 351.98): C

, 34.12; H, 5.73; N, 23.88. Found: C, 33.93; H, 5.34; N, 23.

Table 1 .

 1 Important Crystal Data for title Complexes. Empirical formula C 16 H 40 N 14 Ni 2 C 12 H 20 N 10 Ni C 10 H 20 N 6 S 2 Ni C 10 H 23 BrN 4 O 2 Ni C 8 H 20 N 10 Cu C 10 H 20 N 6 S 2 Cu C 8 H 20 Br 2 N 4 Cu

	Complex		[Ni1]	[Ni2]	[Ni3]	[Ni4]	[Cu1]	[Cu3]	[Cu4]
	CCDC No.		1889902	1889904	1889951	1889905	1890811	1890876	1890877
	F.W.					546.04	363.09	347.15	369.94	319.88	351.98	395.64
	Space group	C2/m	Pbca	R 3 c	Pnma	P2 1 /n	P2 1 2 1 2 1	P4/nmm
	Crystal system	Monoclinic	Orthorhombic Trigonal	Orthorhombic Monoclinic	Orthorhombic Tetragonal
	a (Å)					13.2618(9)	12.8420(6)	22.9171(16) 15.1276(11)	8.3568(4)	12.8292(9)	9.5813(6)
	b (Å)					8.7178(6)	14.8998(6)	22.9171(16) 8.9116(7)	14.4655(7)	13.3387(10) 9.5813(6)
	c (Å)					13.2822(9)	16.5728(7)	15.2853(11) 11.1889(8)	11.2207(5)	18.2848(14) 7.5171(5)
	α, β, γ (°)		90,109.205(2), 90 90, 90, 90	90, 90,120 90, 90, 90	90,92.84(12), 90 90, 90, 90	90, 90, 90
	V (Å 3 )					1450.14(17)	3171.1(2)	6952.2(11) 1508.39(19)	1354.74(11)	3129.0(4)	690.08(10)
	T (K)					110.0	110.0	110.0	110.0	273	110	300
	Z					2	8	18	4	4	8	2
	D calc (mg m	-3 )	1.251	1.521	1.492	1.629	1.568	1.494	1.904
	μ (mm	-1 )		1.327	1.240	1.522	3.936	1.618	1.659	7.351
	GOF on F 2		1.115	1.079	1.068	1.182	1.076	1.007	1.064
	R (F o 2 )	a			0.0587	0.0485	0.0486	0.0434	0.0220	0.0616	0.0288
	Rw (F o 2 )	b		0.1399	0.1524	0.1178	0.0814	0.0525	0.0683	0.0327

[

  Ni2]; 0.0486 & 0.1178 for [Ni3]; 0.0434 & 0.0814 for [Ni4]; 0.0220 & 0.0525 for [Cu1]; 0.0616 & 0.0683 for [Cu3] and 0.0288 & 0.0327 for [Cu4] respectively. Important crystal data are presented in Table

  ) between the cyclen nitrogens, not reported till now, while the CuII complexes display a planar conformation, as predicted by Hancock et al. The variation in the cyclen conformers as seen in Ni II complexes (++++ in Ni1 and Ni3; +++-in Ni2; and -+++ in Ni4) and Cu II (++++) complexes has been explained by the changes in the dihedral angle of the ligand nitrogens. The change in ligand conformation from twisted in Ni II to planar in Cu II complexes has been understood in terms of the metal ions' geometrical preferences imposed by CFSE. The Jahn-Teller effect in Cu II allows the sustenance of planar conformation of cyclen by adopting a square pyramidal geometry where all four N atoms of cyclen complex to Cu II in the square plane and different anions (N 3 -, SCN -& Br -) occupy the pyramidal position resulting in a gross inverted umbrella structure for all the complexes ([Cu1]-[Cu4]

ACKNOWLEDGEMENTS

This work is supported by funding from DST, New Delhi, India (Project code: DST/YSS/2015/000993). The infrastructural facilities at Department of Chemistry, Mahila Maha Vidyalaya, Banaras Hindu University and instrumental facilities at Department of Chemistry, Institute of Science, Banaras Hindu University, are gratefully acknowledged.

Electronic Supplementary Information

Structurally Characterised

Table S1. Selected bond lengths (A> ), angles (°) and important hydrogen bonding interactions (A> ) for title complexes. 13) O2-Ni-N3 99.17( 9)

Hydrogen Bonding Interactions

Hydrogen Bonding Interactions

Hydrogen Bonding Interactions

Hydrogen Bonding Interactions


Jiantao Zhao

Christopher Sauvage

Frédérique Bitton

Mathilde Causse

Meta-analysis of GWAS provides new insights into genetic control of tomato flavor

Tomato flavor has changed over the course of long-term domestication and intensive breeding. To understand the genetic control of flavor related traits, we performed a metaanalysis of genome-wide association studies (GWAS) for 18 traits, using 775 tomato accessions and 2,316,117 SNPs from three GWAS panels (Sauvage et al., 2014;Bauchet et al., 2017 andTieman et al., 2017). In a first step, we imputed SNP data to increase genome coverage for two out of three panels and ran EMMAX software for each panel separately. From each run, the genomic inflation factor and standard errors of the beta coefficients were implemented in two meta-GWAS models: the inverse variance-weighted fixed effect and the Han Eskin random-effect model that takes heterogeneity across studies into account. Then, from the meta-GWAS results, we conducted a statistical fine-mapping of the candidate loci following a heuristic linkage disequilibrium (LD) approach. We discovered 305 significant associations for the contents of sugars, acids, amino acids and flavor-related volatiles. We showed that fruit citrate and malate contents have been impacted by selection during domestication and improvement, while sugar content has undergone less stringent selection. Results suggest that it may be possible to significantly increase volatiles that positively contribute to consumer preferences while reducing unpleasant volatiles, by selection of the relevant allele combinations. Our results provide genetic insights into the influence of human selection on tomato flavor and demonstrate the benefits obtained from meta-analysis.


Céline Pichon

Carine Duhayon

Emilie Delahaye

Jean-Pascal Sutter

Discrete versus 1D compounds based on pentagonal bipyramid coordination geometries: a matter of solubility?

Keywords: Chain compounds, Cyanidometallates, Heterometallic complexes, Heptacoordination, Magnetic properties

A series of heterometallic compounds based on the association of heptacoordinated trans-dicyanido Cr(III) complex [Cr(L)(CN) 2 ]K and divalent [Mn II (H 2 L n )Cl 2 ] complexes incorporating pentadentate ligands with a planar N 3 O 2 coordination pocket and various R substituents illustrate the subtle effect of R on the selective crystallization of discrete or 1-D heterometallic cyanidobridged compounds.

INTRODUCTION

The building-block approach is an efficient strategy for the synthesis of molecular magnetic materials with specific properties. 1 The versatility of coordination complexes combined to a careful choice of building blocks allows to tune the dimensionality (from discrete to 3-D), the exchange interactions, and provides the possibility to implement additional properties such as optic, photosensitivity, porosity to the magnetic material. 2 This approach was applied to heptacoordinated complexes with pentagonal bipyramid coordination geometry selected for their substantial magnetic anisotropy with axial Zero-Field Splitting (ZFS) D parameter ranging between -20 and 40 cm -1 depending of the electronic configuration of the 3d metal ion. 3 Lanthanide complexes in this surrounding, especially the Dy(III) derivatives, exhibit stronger axial magnetic anisotropies leading to Single-Molecule Magnet (SMM) properties with very high energy barriers up to 1815 K. 4 These complexes exhibit structural robustness of the coordination sphere due to pentadentate ligands stabilizing the equatorial plan making them good building blocks. 5 Recent examples of low-dimensional heterometallic systems (discrete or coordination polymers) made up with [Cr(L)(CN) 2 ] -and heptacoordinated Mn(II), 6 Fe(II) [START_REF]O3-Mn1-O4 84[END_REF] and Ni(II) 8 complexes illustrated this approach. The Isingtype magnetic anisotropy (D < 0) for Fe(II) and Ni(II) derivatives and their ferromagnetic interactions with Cr(III) units resulted in SMMs and SCMs behaviors. [START_REF]O3-Mn1-O4 84[END_REF]8 Associations of [Cr(L)(CN) 2 ] -with [Fe(H 2 L 1 )Cl 2 ] (Scheme 1) evidenced the importance of stoichiometry to isolate either the discrete trinuclear [Cr 2 Fe] or the [CrFe] chain. 7a A series of trinuclear compounds of general formula [{Cr(L)(CN) 2 } 2 M(H 2 L N3O2R )] (M = Mn and Fe, R = NH 2 , cyclohexyl, mandelic) was also obtained in water or H 2 O/MeOH mixtures with 2 equivalents of chromium complex vs. the divalent entity. 7b Based on this observation, we were interested to understand the conditions promoting formation (i.e. propagation of oligomeric species) and crystallization of chains. Our approach was to study the influence of the peripheral groups R of the pentadentate ligands H 2 L N3O2R on the species solubilities. Indeed, it is crucial to control the crystallization of heterometallic species (discrete or 1D) in order to obtain pure samples. Mn(II) complexes with NH 2 , cyclohexyl and mandelic substituents (Scheme 1) have been selected to check the effect of steric hindrance and intermolecular interactions on the heterometallic associations. Their syntheses, crystal structures, and magnetic behaviors are described herein.

Syntheses.

Associations between [Cr(L)(CN) 2 ] -and [Mn(L n )Cl 2 ] complexes shown on Scheme 1 were performed with various Cr/Mn ratio by interdiffusion of Mn(II) and Cr(III) solutions in different solvents with the goal to crystallize heterometallic chains. In the case of L 1 , crystallization in concentrated methanol medium was fast (one hour) affording the dinuclear complex [Cr(L)(CN) 2 Mn(H 2 L 1 )(MeOH)](PF 6 )⋅H 2 O⋅2MeOH (1). It was not possible to identify the chain [Cr(L)(CN) 2 Mn(H 2 L 1 )] + even working with a default of [Cr(L)(CN) 2 ] -.

With the ligand H 2 L 2 , the chain [Cr(L)(CN) 2 Mn(H 2 L 2 )](PF 6 )•2H 2 O•MeOH (2,
ratio Cr/Mn 1:1) was achieved in a 50:50 water/methanol medium whereas a similar chain

[Cr(L)(CN) 2 Mn(H 2 L 2 )][Cr(L)(CN) 2 ]•5.5H 2 O•3MeOH
(3) but with one Cr(III) complex as anion was preferentially crystallized in the absence of PF 6 -anions.

In the case of H 2 L 3 , the nature of the solvent modified the species crystallized. In H Solid state infrared spectra for the heterometallic complexes showed a noticeable shift of the cyanide bands towards larger wavenumbers compared to 2130 cm -1 for [Cr(L)(CN) 2 ]K. 5 Characteristic bands were found at 2127 and 2150 cm -1 for 1, 2148 and 2153 cm -1 for 2, 2151 and 2160 cm -1 for 3 and 2133 and 2161 cm -1 for 4. We can notice that the shift between the two cyanide bands is more pronounced in 1 which can be explained by one terminal and one bridging CN group in this complex whereas the others systems have both bridging Cr(III) units linked by both cyanides. 5,6 Finally, PF 6 -anions were detected in 1 and 2 with strong bands around 845 cm -1 .

Crystal structures.

All the compounds have been crystallized and their crystal structures have been solved. General crystallographic data are gathered in Table 1. The metal-ligand bond distances and angles are similar to that found in the starting materials, 9 further details can be found in the Experimental section with selected bond distances and angles. In all the heterometallic compounds, the metal centers are heptacoordinated with a coordination polyhedron close to bipyramid pentagonal geometry (PBP, Table S1). The actual deformations of the PBP coordination geometries have been analyzed by Continuous Shape

Measures 10 performed with SHAPE 11 (Table S1). The metal sites are connected through cyanide bridges and the compounds are cationic chains differing by peripheric groups and/or anions except the complex [Cr(L)(CN) 2 Mn(H 2 L 1 )(MeOH)](PF 6 )⋅H 2 O⋅2MeOH (1) which is a discrete complex. 1 is a dinuclear species with one cyanide of the chromium complex connecting the two metallic centers while the second one is terminal (Figure 1). The molecule is slightly bent due to a Mn-N≡C angle of 160.4°. For the Mn(II) atom, a methanol molecule is coordinated in axial position in trans vs. the nitrogen atom of the CN bridge (Mn-N = 2.210 Å). However, the organization of the dinuclear entities in the crystal packing is close to chains of dinuclears propagating along the c axis through short hydrogen bonds occurring between terminal nitrogen of the free cyanide and the H atom of the methanol coordinated to a neighbouring dinuclear (N⋅⋅⋅C = 2.75 Ǻ, Figure S1b). The crystal structure for the chain [Cr(L)(CN) 2 Mn(H 2 L 2 )](PF 6 ) (2) confirmed the formation of a coordination polymer (Figure 2) constituted of alternating Cr and Mn centers linked by the cyanide ligands associated to an PF 6 -anion and crystallized with solvent molecules (2 H 2 O and 1 MeOH molecules). The Cr and Mn centers are heptacoordinated with a pentagonal bipyramid polyhedral shape (Figure 2 and Table S1). The axial positions of the The compounds 3 and 4 are also cyanide-bridged Cr-Mn cationic chains (Figures 3 andS5). Their charge is this time compensated by a chromium complex and they crystallized with solvent molecules (5.5 H 2 O and 3 MeOH for 3 and 6 water molecules for 4). The chains display a similar molecular arrangement with closely related bond distances and angles (Tables S6, S7, S8 andS9). They are quite linear with average Mn-N-C and Cr1-C-N angles of 160.5/172.7° (3); 161/172.4° (4). The steric hindrance of the peripheral group on the Mn site is the main difference resulting in an almost perpendicular orientation of the mandelic groups with both substituents on the same side vs. the pentadentate plane (Figure 3) while the core of the H 2 L 3 ligand is perfectly planar in 3 (Figure S5). The chains are propagating along the a axis and are packed in layers in the (a,b) plane with the anions intercalated between the layers (Figures S7b and Figure S7c). The shortest metal-metal distance is 8.75 Ǻ for 3

between Cr atoms of adjacent chains (Figure S5c) while it equals 9.00 Å for 4 between Cr atoms of the anion and Mn sites of the chain (Figure S7c). Finally, these two chains crystallized in chiral space groups which is not surprising for 4 as we started from a chiral building block. In 3, we can exclude any spontaneous resolution effect as we did not observe the presence of CD signals from solid state circular dichroism measurement (Figure S8). This compound crystallized then as a racemic mixture. Magnetic properties. The dc magnetic susceptibility (χ M stands for the molar magnetic susceptibility) versus temperature and the magnetization (M) versus field were investigated for all compounds to probe the impact of substituents on intra-and inter-molecular exchange interactions.

For the dinuclear system 1, the χ M T product was 5.98 cm 3 Kmol -1 at 300 K. A theoretical Curie contribution of 6.25 cm 3 Kmol -1 is expected for one Cr(III) and one Mn(II) in the absence of exchange interactions. This value decreased to 1.65 cm 3 Kmol -1 at 3.6 K followed by a slight increase to 1.77 cm 3 Kmol -1 at 2 K (Figure 4). Such a behavior is characteristic for an antiferromagnetic Cr-Mn interaction, in agreement with similar cyanidobridged Cr-Mn systems. 5 The resulting S = 1 ground spin state implies a χ M T signal tending to 1 cm 3 Kmol -1 , which is not reached for 1. The larger experimental values combined with the slight increase below 5 K are reproducible as two batches showed these features.

Contamination of the samples by traces of a chain derivative is discarded by PXRD (see Figure S2). The observed behavior can be attributed to intermolecular interactions by means of the hydrogen bonds linking Cr and Mn sites of adjacent molecules (Figure S1b). Modeling of the thermal dependence of χ M T was performed with an expression of the susceptibility derived from the Hamiltonian given equation 2. The contribution of intermolecular interactions was considered in the mean-field approximation. [START_REF] O'connor | [END_REF] The best fit to the experimental behavior yielded J = -6.36 ± 0.06 cm -1 , g = 1.97 ± 0.01, zJ' = +0.27 ± 0.02 cm -1 .

The strength of the Cr-Mn exchange interaction is similar to reported values. [5] The magnetization (M) curve at 2 K of 2 (Figure S11) shows a very fast increase of the magnetization for weak fields and a smoother variation for higher field tending to 2.13 µ B , close to 2 µ B expected for a S = 1 ground state.

= -.

(Equation 1) with J the exchange interaction between Mn and Cr sites 2) where zj' is the intermolecular interaction, N is the Avogadro number, g is the Landé factor and µ B is the Bohr magneton.

= / 1 - ² (Equation
For the 1D compounds 2, 3 and 4, the magnetic features are characteristic of ferrimagnetic spin chains (Figures 4 andS9). The χ M T values found at 300 K are respectively, 5.79, 4.45 and 7.72 cm 3 mol -1 K. Upon cooling, this value decreases to a minimum around respectively, 13, 10 and 16 K before increasing steeply for lower temperatures (Figure S10).

The classical model developed for an Heisenberg linear chain with alternating spins 13 was used to reproduce these magnetic behaviors. A good agreement with the experimental data was reached with the following sets of parameters: J = -5.97 ± 0.05 cm -1 and g = 1.98 ± 0.01 for 2, J = -5.06 ± 0.05 cm -1 and g = 1.97 ± 0.01 for 3 and J = -7.46 ± 0.09 cm -1 and g = 1.98 ± 0.01 for 4 (Figures 4 andS11). These values of Cr(III)-Mn(II) exchange interactions are similar with the ones found in homologous heptacoordinated 6,7b systems and are in the same range of order compared to cyanide-bridged hexacoordinated Cr-Mn species. 14 The magnetization for 2 is characterized by an S-shaped trace with a slop bending at 830 Oe before a rapid increase of M to reach saturation value of 2.16 µ B (Figure S11a), suggesting a metamagnetic behavior. To probe this effect, χ M as a function of the temperature was measured under different fields between 100 and 4000 Oe (Figure S12a) and magnetization curves were recorded between 3 and 8 K (Figure S12b). The phase diagram (H,T) was constructed from the maxima on χ vs. T and dM/dH vs. H curves (Figure 5). This metamagnetic behavior is likely the result of weak interchain interaction attributed to the short separation between the chains. Here the peripheral groups of the ligand play an important role, for 2 the steric hindrance of the NH 2 groups is not sufficient to screen the intermolecular exchange interactions between the chains. For these chains, no metamagnetic behavior was observed (Figure S11b).

These magnetic systems validate the strength of Cr-CN-Mn exchange interactions previously determined in trinuclear complexes 7b but also underline the importance of bulky substituents to shortcut interchain exchange pathways with diamagnetic scaffolds screening the exchange interactions.

CONCLUSION

The pentadentate ligands considered herein are interesting platforms to synthesize heptacoordinated complexes with various functional groups (R) at the periphery. These substituents allow introducing in the complexes an additional functionality like chirality 7b,15 but they also appear to play an important role during the crystallization process. This work and our previous results [START_REF]O3-Mn1-O4 84[END_REF] illustrate the subtle effect of R on the selective crystallization of discrete or 1D heterometallic compounds possibly formed during the assemblage progression.

Reagents stoichiometry and solvent conditions are the experimental parameters chemists typically adjust to target a product but the species actually crystallizing is also driven by solubility considerations; and this can be greatly dependent on the R substituents as illustrated in this study.

The R groups also contribute to the crystal packings and hence the intra-and inter-molecular contacts. The first may mostly contribute to the bending of the polynuclear compounds 7b while the seconds are promoting additional pathways for magnetic interactions, but both contribute to the overall magnetic behaviors of the compounds.

Within this family of ligands, the R-substituents should be considered as an additional parameter with which chemists can fine-tune the selectivity of the reaction and the magnetic behavior.

EXPERIMENTAL SECTION.

Materials K with an applied field of 1 kOe and corrected for the diamagnetic contribution sample by using Pascal's tables [START_REF] Kahn | Molecular Magnetism[END_REF] and for the sample holder. The field dependences of the magnetization were measured at 2 K with dc magnetic field up to 5 T. The absence of ferromagnetic impurities was checked by measurement of M vs. H at 100 K. The Circular Dichroism (CD) spectrum was recorded with a Jacso J-815 spectropolarimeter on pellets of compound diluted in KBr (1.25 % of compound). The experiment was performed at room temperature in a 900-300 nm wavelength range, 2 nm data pitch, 1 nm bandwidth, 100 nm/min scanning speed, continuous scanning mode, 4 s DIT and no accumulation.

Syntheses: [Cr(L)(CN) 2 Mn(H 2 L 1 )(MeOH)](PF 6 )⋅H 2 O⋅2MeOH, 1:

A solution of K[Cr(L)(CN) 2 ]•2H 2 O
(0.015 g, 0.03 mmol) in 0.5 mL MeOH was placed in a 1 cm diameter tube.

[Mn(H 2 L [Mn(H 2 L 3 )Cl 2 ]•2H 2 O (0.007 g, 0.01 mmol) dissolved in MeOH (2 mL) was slowly layered on the top. Orange single crystals suitable for X-ray data collection were isolated after 2 days.

Yield: 6 mg (28 %). IR (KBr, cm -1 ): 2161 (w), Single-crystal X-ray diffraction: Single crystals suitable for X-ray diffraction were coated with paratone oil and mounted onto the goniometer. The X-ray crystallographic data were obtained at low temperature from an Apex2 Bruker (MoKα radiation source) or a Gemini Oxford-Diffraction (CuKα radiation source) diffractometer, equipped with an Oxford Cryosystem. The structures have been solved by direct methods using Superflip and refined by means of least-square procedures on F using the PC version of the program CRYSTALS.

The scattering factors for all the atoms were used as listed in the International Tables for Xray Crystallography. Absorption correction was performed using a multi-scan procedure.

When it was possible, all non-hydrogen atoms were refined anisotropically. The H atoms were usually located in a difference map except some on solvate molecules, but those attached to carbon atoms were systematically repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry and Uiso (H) (in the range 1.2-1.5 times Ueq of the parent atom), after which the positions were refined with riding constraints. For 3, it was not possible to resolve some diffuse electrondensity residuals (enclosed solvent molecules). Some solvent molecules were squeezed with the SQUEEZE facility from PLATON: 2.5H 2 O•1MeOH (per asymmetric unit). The structure for 1 presented a statistic disorder that concerns part of one ligand and a PF 6 -anion. The crystal of 4 was poorly diffracting, leading to a lack of data at high angle and a low theta full.

However Crystallographic informations for all the complexes are gathered in Table 1; the CIF files are also available from The Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk/data_request/cif) under the references CCDC-2095429 to 2095432.

ORTEP plots and selected bond distances and angles are given in the supporting information.

Powder X-Ray diffraction (PXRD) patterns were recorded in transmission mode using capillary tubes on a XPert Pro (Theta-Theta mode) Panalytical diffractometer with λ(Cu Kα1,Kα2 ) = 1.54059 and 1.54439 Å.

SUPPORTING MATERIAL

Crystallographic and geometric information; polyhedral shape analyses of the coordination spheres; powder X-ray diffraction diagrams and additional magnetic data. The crystallographic information for the structures has been deposited at CCDC with references CCDC-2095431 (1), CCDC-2095430 (2), CCDC-2095429 (3), CCDC-2095432 (4). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. a Including co-crystallized solvent molecules (except for 3 for which some solvent molecules were squeezed), b Table S3. Selected bond angles of 1 (°).

R 1 = Σ||F o | -|F c ||/Σ|F o | ; c wR 2 = [Σ(w(F o 2 -F c 2 ) 2 )/Σ ([w(F o 2 ) 2 ]

Scheme 1 :

 1 Scheme 1: Sketch of the heptacoordinated metal complexes and heterometallic systems along with their numbering.

  Figure 1. Molecular structure of the dinuclear complex [Cr(L)(CN) 2 Mn(H 2 L 1 )(MeOH)](PF 6 )⋅H 2 O⋅2MeOH, 1 omitting the crystallographic disorder on a cyclohexyl group and the solvent molecules.

  manganese ion are occupied by the nitrogen atoms of the bridging cyanides (Mn1-N = 2.197 to 2.230 Å). The chains are slightly bent with average Mn-N-C angle close to 158.5° and Cr-C-N angles of 175°. The chains propagate along the a axis (Figures S3b and S3c); the shortest inter-chain metal-metal separation is found between two Mn atoms separated by 8.81 Å.

Figure 2 .

 2 Figure 2. Molecular structure of the chain [Cr(L)(CN) 2 Mn(H 2 L2)](PF 6 )•2H 2 O•MeOH, 2. The solvates and one counter-ion are not shown for clarity.

Figure 3 .

 3 Figure 3. Molecular structure of the chains a) [Cr(L)(CN) 2 Mn(H 2 L2)][Cr(L)(CN) 2 ], 3 and b) [Cr(L)(CN) 2 Mn(H 2 L3)][Cr(L)(CN) 2 ], 4. The solvates are not shown for clarity. The phase purity of the compounds was checked by Powder X-Ray Diffraction (PXRD) as shown on Figures S2, S4, S6 and S9. Experimental and calculated diffractograms are globally well superposed except small diffraction peaks corresponding to trinuclears for 3 and 4. It was not possible to get samples without these additional traces of trinuclear complexes,

Figure 4 .

 4 Figure 4. Experimental (O/) and calculated (─) temperature dependence of χ M T for 1 and 2, the best fit parameters are discussed in the text.

Figure 5 .

 5 Figure 5. Phase diagram (H,T) for 2. The black line is a guide for the eyes. The magnetization value for 3 and 4 at 5 Tesla is respectively 4.85 and 5 µ B corresponding to a ground state S = 1 of the CrMn chain subunit plus the contribution of the Cr anion (S = 3/2).

Figure S1 .

 S1 Figure S1. (a) Asymmetric unit of [Cr(L)(CN) 2 Mn(H 2 L 1 )(MeOH)](PF 6 ), 1 with thermal ellipsoids fixed at 30 %. Crystal packing of 1 highlighting the organization by pairs with views (b) in the (b,c) plane and (c) in the (a,c) plane. The dashed red lines materialize the shortest intermolecular metal-metal distance (indicated in Å). Color scheme: C = grey, N = blue, O = red, Cr = green, Mn = yellow.

Figure S2 .

 S2 Figure S2. Comparison of the experimental powder X-ray diffractogram measured on polycrystalline powder introduced with their mother solution in capillary tube with the calculated ones of 1.

Figure S3 .Figure S4 .Figure S5 .Figure S6 .Figure S8 .S15Figure S9 .

 S3S4S5S6S8S9 Figure S3. (a) Asymmetric unit of [Cr(L)(CN) 2 Mn(H 2 L 2 )](PF 6 )•2H 2 O•MeOH, 2 with thermal ellipsoids fixed at 30 % and without solvent molecules. Crystal packing of 2 (b) along the (b,c) plane perpendicularly to the chain propagation and (c) along the a axis. The dashed red lines materialize the shortest intermolecular metal-metal distance (indicated in Å). Color scheme: C = grey, H = black, N = blue, O = red, Cr = green, Mn = yellow.

Figure S10 .

 S10 Figure S10. Experimental (O) and calculated (─) temperature dependence of χ M T = f(T) curves of 3 and 4. The best fit parameters are discussed in the text.

  CN) 2 } 2 Mn(H 2 L 3 )]•13H 2 O 7b described in a previous work has been crystallized with two equivalents of Cr(III) complex in water.

	2 O/MeOH	50:50,	a	cationic	chain	with	one	chromium	as	anion,

[Cr(L)(CN) 2 Mn(H 2 L 3 )][Cr(L)(CN) 2 ]•6H 2 O (4), was the main product with traces of the trinuclear species even when working with a stoechiometry 1:1. The trinuclear [{Cr(L)(

  An orange solid slowly appeared during the addition. The suspension was stirred overnight at room temperature. The solid was filtered and washed with a minimum of a mixture H 2 O/MeOH 50:50 followed by Et 2 O (5 mL), yielding 58 mg (91 % based on Cr) of 4.Based on PXRD, this orange polycrystalline solid incorporated a small amount of the trinuclear complex {Cr 2 Mn(L 3 )} 7b , see FigureS6. Conditions to grow single crystals suitable for X-ray data collection: a solution of K[Cr(L)(CN) 2 ]•2H 2 O (0.015 g, 0.03 mmol) in 3 mL H 2 O was placed in a 1 cm diameter tube and layered with 1 mL of H 2 O/MeOH 50:50.

	1266 (m), 1199 (sh), 1177 (m), 1136 (sh), 1104 (w), 1084 (w), 1071 (w), 1048 (m), 1025 (w), Elemental analysis (%) calcd. for C 61 H 61 Cr 2 MnN 21 O 10
	1014 (w), 994 (w), 953 (w), 846 (s), 810 (sh), 740 (w), 714 (m), 683 (m). Elemental analysis ([Cr(L)(CN) 2 Mn(H 2 L 2 )][Cr(L)(CN) 2 ]•4H 2 O): C 52.06; H 4.37; N 20.90; found: C 52.12; H
	(%) calcd. for C 50 H 62 CrF 6 MnN 12 O 7 P ([Cr(L)(CN) 2 Mn(H 2 L 1 )(MeOH)](PF 6 )•H 2 O): C 50.25; H 3.78; N 20.13.
	5.23; N 14.07; found: C 50.11; H 5.09; N 13.75.
	[Cr(L)(CN) 2 Mn(H 2 L 3 )][Cr(L)(CN) 2 ]•6H 2 O, 4: [Mn(H 2 L 3 )Cl 2 ]•2H 2 O (0.022 g, 0.03 mmol) in
	[Cr(L)(CN) 2 Mn(H 2 L 2 )](PF 6 )•2H 2 O•MeOH, 2: K[Cr(L)(CN) 2 ] (0.044 g, 0.07 mmol) in 10 mL 6 mL MeOH was added dropwise to a solution of K[Cr(L)(CN) 2 ]•2H 2 O (0.045 g, 0.08 mmol)
	MeOH was added dropwise to [Mn(H 2 L 2 )Cl 2 ].2H 2 O (0.064 g, 0.15 mmol) and NH 4 PF 6 (0.8 in 9 mL H 2 O.
	g, 0.49 mmol) dissolved in 6 mL H 2 O and 6 mL MeOH. A red/brown solid slowly appeared
	during the addition. The suspension was stirred overnight at room temperature. The
	polycrystalline solid was filtered and washed with MeOH (4 mL) followed by Et 2 O (5 mL),
	yielding 58 mg (79 % based on Cr) of 2. Phase purity of this polycrystalline solid was
	confirmed by PXRD, see Figure S4. Conditions to get single crystals suitable for X-ray
	structure analysis: A solution of [Mn(H 2 L 2 )Cl 2 ] (0.014 g, 0.03 mmol) and NH 4 PF 6 (0.02 g,
	2153 (w), 2148 (w), 1682 (m), 1634 (w), 1614 (w), 1601 (w), 1586 (w), 1558 (sh), 1519 (s),
	1495 (sh), 1455 (w), 1435 (w), 1419 (w), 1379 (s), 1306 (w), 1270 (w), 1214 (sh), 1196 (m),
	1179 (w), 1163 (w), 1149 (w), 1138 (w), 1122 (w), 1096 (m), 1073 (w), 1051 (m), 1025 (w),
	1013 (w), 938 (sh), 907 (w), 844 (s), 817 (sh), 812 (w), 802 (w), 758 (w), 739 (w), 716 (m),
	685 (w), 645 (w). Elemental analysis (%) calcd. for C 36 H 36 CrF 6 MnN 14 O 5 P
	([Cr(L)(CN) 2 Mn(H 2 L 2 )](PF 6 )•H 2 O): C 43.38; H 3.64; N 19.68; found: C 43.59; H 3.60; N
	19.55.
	[Cr(L)(CN) 2 Mn(H 2 L 2 )][Cr(L)(CN) 2 ]•5.5H 2 O•3MeOH,	3:	A	solution	of
	[Mn(H 2 L 2 )Cl 2 ].2H 2 O (0.036 g, 0.10 mmol) in 4 mL H 2 O was placed in a 1 cm diameter tube
	and layered with 2 mL of H 2 O/MeOH 50:50. K[Cr(L)(CN) 2 ] (0.040 g, 0.06 mmol) dissolved
	1 )Cl 2 ].0.5H 2 O (0.013 g, 0.02 mmol) dissolved in MeOH (0.5 mL) was slowly in 4 mL MeOH was layered on top. Orange crystals were isolated after 3 days which were
	layered on the top. Orange crystals were isolated after 1 night. Yield: 11 mg (30 %). IR (KBr, filtered and washed with MeOH (4 mL) followed by Et 2 O (5 mL), yielding 12 mg (10 %

cm -1 ): 3351 (w), 3210 (w), 2932 (m), 2857 (m), 2150 (w), 2127 (sh), 1651 (m), 1637 (m), 1586 (m), 1561 (m), 1519 (s), 1494 (sh), 1451 (m), 1433 (m), 1382 (s), 1341 (w), 1304 (w), 0.12 mmol) in 1 mL H 2 O and 1 mL MeOH was placed in a 1 cm diameter hemolysis tube and layered with 2 mL of H 2 O/MeOH (1:1). A solution of [Cr(L)(CN) 2 ]K (0.007 g, 0.012 mmol) in 2 mL MeOH was carefully added on the top. After 2 days, dark orange crystals appeared. IR (KBr, cm -1 ): 3643 (w), 3521 (m), 3401 (m), 3350 (m), 3274 (sh), 3205 (sh), 2919 (m), based on Cr) of 3. IR (KBr, cm -1 ): 2963 (w), 2927 (w), 2908 (w), 2160 (w), 2151 (sh), 1674 (m), 1634 (m), 1599 (w), 1587 (w), 1516 (m), 1494 (w), 1454 (w), 1417 (w), 1373 (m), 1259 (s), 1196 (w), 1162 (w), 1091 (m), 1015 (m), 838 (sh), 785 (s), 738 (m), 712 (m), 683 (m).

Table 1 .

 1 Selected crystallographic data and refinement parameters of 1-5.

	Formula a Mw (g.mol -1 )			1 C 51 H 65 CrF 6 MnN 12 O 8 P 1226.04	2 C 37 H 42 CrF 6 MnN 14 O 7 P 1046.72	3 C 63 H 67 Cr 2 MnN 21 O 1 1 1453.28	4 C 75 H 75 Cr 2 MnN 19 O 1 4 1625.46
	Crystal system			Monoclinic	Monoclinic	Monoclinic	Triclinic
	Space group			P2 1 /c	P2 1 /c	Pc	P1
	T (K)			180	100	100	123
	a (Å)			14.772(3)	10.688(1)	21.005(1)	10.287(1)
	b (Å)			17.671(2)	25.152(1)	10.750(1)	10.548(1)
	c (Å) α (°) β (°) γ (°) V (Å 3 )			23.482(4) 90 91.024(5) 90 6128.6(20)	16.853(1) 90 103.299(2) 90 4409.3(3)	16.763(1) 90 96.030(2) 90 3764.3(3)	19.439(1) 75.152(1) 76.839(1) 83.95(1) 1982.8(1)
	Z			4	4	2	1
	ρ calcd. cm -3 ) µ (mm -1 ) Collected reflns	(g	1.33 0.486 (Mo Kα) 63446	1.58 0.660 (Mo Kα) 94579	1.28 0.516 (Mo Kα) 133493	1.36 4.112 (Cu Kα) 32202
	Unique reflns			13382	9047	20914	10169
	R int Nb of parameters	0.107 703	0.064 604	0.047 884	0.026 1010
	Nb of			7049, n=2.1	6801, n=3	18046, n=3	9750, n=3
	reflns(I≥nσ )
	Final	R 1 ,	0.0885, 0.0714	0.0361, 0.0367	0.0340, 0.0363	0.0394, 0.0474
	wR 2 (I≥nσ) b,c
	Flack parameter		/	/	0.002(7)	0.001(3)
	Δρ min /			-0.79/1.40	-0.53/0.73	-0.44/0.57	-0.24/1.42
	Δρ max GOF			0.83	1.062	1.082	0.954
	CCDC number			2095431	2095430	2095429	2095432

Table S1 .

 S1 Results of Continuous Shape Measures calculation using SHAPE 1 with the closest geometries for each metal center highlighted in blue.

	Heptacoordinated centers 2
	HP-7	D 7h Heptagon
	HPY-7 C 6v Hexagonal pyramid
	PBPY-7 D 5h Pentagonal bipyramid
	COC-7 C 3v Capped octahedron
	CTPR-7 C 2v Capped trigonal prism
	JPBPY-7 D 5h Johnson pentagonal bipyramid J13
	JETPY-7 C 3v Johnson elongated triangular pyramid J7
	Metal center	HP-7	HPY-7 PBPY-7 COC-7 CTPR-7	JPBPY-7 JETBPY-7
	Cr1@1	34.224 24.372	0.247	7.814	6.191	3.451	23.874
	Mn1@1	32.231 22.221	0.718	7.096	5.355	3.515	21.209
	Cr1@2	33.345 22.815	0.439	6.724	5.138	3.634	22.838
	Mn1@2	32.962 22.724	0.468	7.451	5.597	3.625	22.067
	Cr1@3	33.597 22.584	0.476	7.236	5.741	3.674	22.245
	Cr2@3	34.275 24.121	0.281	7.862	6.117	3.467	23.735
	Mn1@3	32.375 21.723	0.675	7.352	5.469	3.899	20.878
	Cr1@4	32.399 23.590	0.522	6.568	4.662	3.645	22.559
	Cr2@4	32.443 24.662	0.342	7.345	5.424	3.569	22.265
	Mn1@4	32.910 19.315	1.177	6.822	5.229	3.748	19.904

1 M. Llunell, D. Casanova, J. Cirera, P. Alemany, S. Alvarez, SHAPE: Program for the stereochemical analysis of molecular fragments by means of continuous shape measures and associated tools, 2.1; University of Barcelona: Barcelona, 2013. 2 D. Casanova, P. Alemany, J. M. Bofill, S.

Alvarez, Chem. Eur. J., 2003, 9, 1281

ACKNOWLEDGEMENTS

This work was supported by the French National Research Agency, ANR, (grant ANR-17-CE07-0007). Authors are grateful to J.-F. Meunier, Laure Vendier (LCC) and Charles-Louis Serpentini (IMRCP) for technical assistance in magnetic, powder X-Ray data and CD collections, respectively.

Supporting Information

Discrete versus 1D compounds based on pentagonal bipyramid coordination geometries: a matter of solubility? S6. Selected bond lengths for 3. Table S7. Selected bond angles for 3. Figure S6. Comparison of the experimental and calculated powder X-ray diffractogram of 3. Figure S7. Molecular structure views for [Cr(L)(CN) 2 Mn(H 2 L 3 )][Cr(L)(CN) 2 ]•6H 2 O, 4. Table S8. Selected bond lengths for 4. Table S9. Selected bond angles for 4. Figure S8. Solid state circular dichroism spectra of 3.


Laurie Barthe

Audrey Denicourt-Nowicki

Alain Roucoux
email: alain.roucoux@ensc-rennes.fr

Karine Philippot
email: karine.philip-pot@lcc-toulouse.fr

Bruno Chaudret

Mehrdji Hemati

Model arenes hydrogenation with silica-supported rhodium nanoparticles: The role of the silica grains and of the solvent on catalytic activities

Silica-supported rhodium-based nanoheterogeneous catalysts were easily prepared by impregnation with a pre-stabilized colloidal suspension. The resulting catalysts contain rhodium nanoparticles welldispersed in the silica pores with a mean size of 5 nm. Influence of the silica grains size and of the solvent was investigated in arenes hydrogenation. It appeared that the size of the silica grains has a minimal influence on the reaction rate but the supported nanocatalysts displayed higher TOFs in hexane than in water.

Introduction

Transition-metal nanoparticle science is a strategic research area in material development due to their physical and chemical properties. In particular, the recent period has known a growing interest for the controlled synthesis of nanometric size metal species and their application in various academic or industrial areas. Compared with homogeneous catalytic systems, these nanospecies have for example proved to be efficient in the hydrogenation of arene derivatives [1]. From now on, noble metal nanoparticles constitute an unavoidable family of nanocatalysts for hydrogenation under mild conditions [2] at the border between homogeneous and heterogeneous chemistry, thus leading to the formation of key intermediates for fine chemistry [3].

For economical and environmental purposes, the catalyst recovery is of paramount importance in sustainable fine chemicals production. According to the approaches developed in homogeneous catalysis to easily recover catalysts; several techniques have essen-tially been developed through the heterogenization of the active species. In all cases, the first investigation is to immobilize the efficient species onto various support materials. This strategy offers the advantages to combine both high and specific activity with easy and environmentally friendly product separation.

In this context, the combination of metallic nanoparticles with a solid support provides a huge scope for the development of highly active and stable catalysts, which could potentially be recyclable [START_REF] Astruc | [END_REF]. Successful supported metal nanoparticles on inorganic oxides carriers are a mainstay of commercial heterogeneous catalysts and are employed in numerous industrial reactions [START_REF] Ponec | Catalysis by Metals and Alloys[END_REF] and fuel cell technologies [6]. Several authors have reported the development of supported metal nanoclusters on solid supports by methodologies such as adsorption [7], diffusion of pre-formed nanoparticles into pores via sonication [8], sol-gel [9], vapor-phase deposition of organometallic complexes [10]. Our methodology relies on the deposit onto an inorganic matrix of a pre-stabilized aqueous colloidal suspension, which has been well characterized in terms of organization and size [11]. Several authors have also reported modified wetness impregnation methods for the preparation of supported nanocatalysts [12]. In this paper, we describe the behaviour of pore-controlled silica-supported nanocatalysts in arenes hydrogenation. Particularly, we investigated the influence of solvents and grain sizes on the rate and kinetic effect in the hydrogenation of functionalized arene derivatives.

Experimental

Starting materials

Rhodium chloride hydrate was obtained from Strem Chemicals. Sodium borohydride and all aromatic substrates were purchased from Aldrich or Fluka and were used without further purification. Water was distilled twice before use by conventional method. The surfactant HEA-16Cl was prepared as previously described [13]. Two commercial silicas from Merck were used as received. They display similar physical properties (specific area: 500 m 2 /g; porous volume: 0.8 cm 3 /g; average pore diameter: 5.5 nm) but different grains sizes [80 lm (Si80) and 120 lm (Si120)].

Analytical procedures

TEM-analysis

After embedding of the sample in a resin (Spurr R1032) and treatment at 70 °C during one night for polymerization, the solid has been cut into thin lamellas (50 nm) with a microtom. A TEManalysis was then realized after deposition of a lamella onto a carbon covered cupper grid using a Hitachi HU-12A microscope operating at 120 kV with a resolution of 4.5 Å, at the service ''Centre de microscopie électronique appliqué à la biologie de la faculté de médecine" at Toulouse.

Gas chromatography

All analyses of catalytic reactions were performed using a Carlo Erba GC 6000 with a FID detector equipped with a Factor Four column (30 m, 0.25 mm i.d.).

Synthesis of the aqueous rhodium(0) suspension

To an aqueous solution of surfactant HEA-16Cl (95 mL, 7.6 Â 10 À3 mol L À1 ) sodium borohydride (36 mg, 9.5 Â 10 À4 mol) was added. Then, this solution was added under vigorous stirring to an aqueous solution (5 mL) of RhCl 3 , 3H 2 O (100 mg, 3.8 Â 10 À4 mol) to obtain an aqueous Rh(0) colloidal suspension (100 mL). The reduction occurs instantaneously as revealed by a color change from red to black. The obtained suspensions are stable for several months as confirmed by TEM (the sizes of the particles remain unmodified over this time frame [13]).

Synthesis of the silica-supported rhodium nanoparticles

A suspension of silica (18.5 g) in 40 ml of deionized water was stirred vigorously during 2 h. Furthermore, 50 ml of Rh(0)-HEA-16Cl (1.9 Â 10 À4 mol of Rh) were added under vigorous stirring. The system was kept under stirring for 2 h. After filtration and three water washings, the grey powder was dried overnight at 60 °C. The rhodium loading of the materials were determined by ICP-AES in the CNRS center at Vernaison. Similar results were obtained before and after catalysis with 0.08 wt% of Rh showing the absence of metal leaching .These results were homogeneous what kind of silica used, namely Si120 and Si80. In all cases, a reproducible value about 0.08 wt% was obtained by ICP-analysis near the theoretical value of 0.1 wt%.

General procedure for hydrogenation under atmospheric hydrogen pressure

Reactions were carried out under standard conditions (20 °C, 1 atm of H 2 ). A 25 ml round bottom flask, charged with SiO 2 -supported Rh(0) catalyst (1 g, 10 ml of water) and a magnetic stirrer, was connected with a gas burette (500 mL) with a flask to balance the pressure. The flask was closed by a septum, and the system filled with hydrogen. The appropriate aromatic substrate ([Substrate]/[Rh] = 100) was injected through the septum and the mixture stirred at 1500 min À1 . The reaction was monitored by gas chromatography analyses. Turnover frequencies (TOFs) were determined for 100% conversion.

Results and discussion

3.1. Preparation of silica-supported rhodium(0) nanoparticles In opposition to classical preparation methods of supported nanocatalysts which involve repeated oxidation and reduction steps under severe conditions, our approach consists of the simple impregnation of silica particles using a pre-stabilized rhodium aqueous colloidal suspension.

Colloidal suspensions of rhodium nanoparticles were prepared by chemical reduction of an aqueous solution of RhCl 3 Á 3H 2 O in the presence of two equivalents of N,N-dimethyl-N-cetyl-N-(2hydroxyethyl) ammonium chloride salt 1 as protecting agent (Fig. 1) [13]. Then, an adequate amount of silica was added to the colloidal suspension under vigorous stirring. The suspension coloration change from black to colorless evidences the quantitative adsorption of rhodium nanoparticles on the silica grains. After filtration, the surfactant was eliminated by water washings and the dark powder dried overnight at 60 °C, leading to silica-supported rhodium(0) nanoparticles as a fine grey powder. The metal loading of the composite materials was determined by ICP-analysis as 0.08 wt% whatever is the silica employed. This value corresponds well to the expected one showing that the metal is significantly deposited onto the silica support. During this preparation, all the steps were performed at room temperature, under air, without any gas treatment or calcination step. Two silica were used, namely Si120 and Si80. These silica are quite similar in terms of specific area, porous volume and pores diameter (see § 2.1). The difference concerns the silica grains which are smaller for Si80 than for Si120 to estimate their impact during nanoparticles deposit, clustering and catalytic activities.

3.2. Catalytic activity in the hydrogenation of arene derivatives 3.2.1. Influence of the solvent and of the silica on the catalytic activity

We have examined the influence of several solvents (hexane, water, dichloromethane) in the hydrogenation of toluene as model substrate at room temperature and under atmospheric hydrogen pressure. The nature of catalytic species has previously been checked with mercury poisoning experiments. The results provide good evidence, in light of prior literature [13,14], that the Rh(0) nanoparticles are the true catalysts without leached molecular species. Consequently, nanoheterogeneous catalysts synthesized with silicas Si80 and Si120 were used to compare the influence of their grain size (80 lm and 120 lm, respectively).

The conversion was determined by gas chromatography analysis. The catalytic performances are reported in Table 1. The TOFs are defined as moles of H 2 consumed per moles of rhodium per hour. In water, the total conversion of toluene was observed with higher TOFs (ca. 160 h À1 ; Entries 2 and 3) than with the surfactant-stabilized aqueous suspension of rhodium nanoparticles (TOF = 53 h À1 , Entries 1). These results show the positive kinetic effect in aqueous media of the catalyst's heterogenization. However, dichloromethane does not constitute a good reaction medium for toluene hydrogenation since, only 24-30% conversion of toluene was achieved in 5.5 h in this solvent (Entries 6 and 7). Finally, very improved catalytic activity was obtained in hexane with a total conversion in methylcyclohexane and a TOF of 450 h À1 (Entries 4 and 5). Kinetic investigations have shown a linear variation of the conversion with time (25% and 50% in a reaction time of 12 and 25 mm, respectively) which corresponds to zero-order. Hexane is thus a good solvent compared to water. It may enable a better solubilization of substrate and product and also a better diffusion of the substrate towards the metallic surface. In the opposite, in water, the silica rhodium nanoparticles are surrounded by water molecules, slowing down the access of the lipophilic substrate to the metallic species. Besides these results, no significative difference was observed in terms of catalytic activity between the two supports Si80 or Si120 demonstrating that the silica grains size is not an important parameter in conditions used.

Hydrogenation of various arene derivatives in hexane and water

The catalytic study was extended to typical monofunctionalized and disubstituted arene derivatives, such as anisole and o-xylene, using Rh(0)-Si80 and Rh(0)-Si120 catalysts, in water and hexane (Table 2).

In all cases, whatever the silica and the solvent are, a total conversion of the substrate was observed. No real difference in catalytic activity was noticed between the two families of catalysts prepared with silica Si80 and Si120 showing here again the minimal influence of the silica grain size. As previously observed with toluene, for anisole and o-xylene hydrogenations, a better activity was obtained in hexane as in water due to better substrate solubility and a better diffusion to the active metal centers. In fact, TOFs are three times higher in hexane compared to water (Entries 5 and 7, 9 and 11). We obtained relevant TOFs (470 h À1 ) in anisole hydrogenation without by-product such as phenol or cyclohexanone (Entries 5,6). In all cases (Entries 9-12). The hydrogenation of oxylene leads to the formation of 1,2-dimethylcyclohexane with a classical 92/8 cis/trans ratio for nanoheterogeneous catalysts A similar ratio (95/5) has already been observed with colloidal suspension of rhodium nanoparticles [13]. In comparison with monosubstituted substrates, reaction time for o-xylene hydrogenation significantly increased showing the steric hindrance impact.

Characterization of silica-supported rhodium(0) nanoparticles

Both silica-supported rhodium nanoparticle systems were characterized by TEM-analysis after microtomy preparation. The transmission electron micrographs and the particle size histograms obtained by observation of a cross section of silica spheres containing metal nanoparticles are shown on Fig. 2.

These experiments show that a well uniform diffusion of the surfactant-stabilized rhodium colloidal suspension in the silica matrix has occurred, leading to a homogeneous dispersion of rhodium particles with an average size of 5 nm. Whatever the grain size of silica is, we observed similar mean sizes and size distributions, thus justifying the low influence of the grain size during impregnation with the aqueous suspension. Compared to the size of the nanoparticles in the aqueous colloidal suspension (2.4 nm) [13], the particle size increase during the immobilization step, can be explained by their clustering inside the pores of the silica matrix, whose size is ca. 5.5 nm for the two silica studied. The pores size of the matrix controls thus the rhodium nanoparticles size and further their catalytic activities in hydrogenation reactions.

To check the efficiency of the nanoparticles immobilization and the absence of leaching, recycling experiments were performed with silica-supported nanocatalysts Rh-Si80 and Rh-Si120 in toluene hydrogenation in both hexane and water at 20 °C and under 1 atm H 2 . The catalyst was recovered by simple filtration and dried at 60 °C before another run with new addition of substrate. No significant loss of activity in both solvents was observed, showing the efficient adsorption of the particles on the silica and the reusability of our system. Similar catalytic lifetimes of silica-capped rhodium nanoparticles for model arenes hydrogenation have already been reported in a previous study [11]. Finally, TEM-analysis of Rh-Si120 sample (Fig. 2b) after recycling showed no modification in particle size, with a similar average size of 5 nm (Fig. 3). This observation justifies the narrow catalytic activity observed in the second run.

Summary

In conclusion, we developed a simple procedure to synthesize silica-supported rhodium nanoparticles by wet impregnation from a surfactant-stabilized aqueous colloidal suspension. Catalysts were prepared without calcination step. The nanoclusters are well-dispersed with a homogeneous mean size of 5 nm controlled by the pores size of the silica. The obtained activities can be explained with the very homogeneous size dispersion of rhodium particles inside silica pores probably due to the initial surfactantstabilization allowing a better control of the particle size in contrast with traditional supported catalysts which are usually concentrated on a localised area of the support. Indeed, a recent study has shown the influence of surfactant for dispersions of rhodium in mesoporous aluminosilicates [15]. Finally, they display an interesting activity in arenes hydrogenation in hexane and can be reused in successive runs. We also observed no real influence of silica grain size on the catalytic activity.

Fig. 1 .

 1 Fig.1. Practical one-pot silica-supported rhodium nanoparticles synthesis.

10 Fig. 2 .Fig. 3 .

 1023 Fig. 2. Transmission electron microscopy of silica-supported rhodium nanoparticles performed on the central zone at 120 kV. Micrograph and metal size distribution of (a) Rh-Si80 and (b) Rh-Si120.

Table 1

 1 Influence of the solvent and the grain size of the silica in the catalytic hydrogenation of toluene. a Turnover frequency defined as mol of H 2 per mol of rhodium per h.d Rhodium suspension (3.8 Â 10 À5 mol).

	CH 3
			Rh-Si80/120 or Rh-NPs		CH 3
			Solvent, 1atm H 2 , RT
	Entry	Catalyst	Solvent	Time (h)	Conv. b (%)	TOF (h À1 ) c
	1	Rh NPs d	H 2 O	5.7	100	53
	2	Rh-Si80	H 2 O	2.3	100	163
	3	Rh-Si120	H 2 O	2.5	100	150
	4	Rh-Si80	Hexane	0.9	100	417
	5	Rh-Si120	Hexane	0.83	100	452
	6	Rh-Si80	CH 2 Cl 2	5.5	31	21
	7	Rh-Si120	CH 2 Cl 2	5.5	33	23
	a Conditions. Catalyst: 1 g of Rh-SiO 2 , toluene, [S]/[M] = 100, 10 mL solvent, 1 atm
	H 2 , RT. b Determined by GC analysis.

c

Table 2

 2 Catalytic hydrogenation of arene derivatives in hexane and water with silica-supported rhodium nanoparticles. a

			R
			R'	Hexane or Water, 1atm H 2 , RT Rh-Si80/120	R' R
			R = CH 3, OCH 3
			R' =H, CH 3
	Entry	Substrate	Catalyst	Solvent	Product (%) b		Time (h)	TOF (h À1 ) c
	1	Toluene	Rh-Si80	Hexane	Methylcyclohexane (100)		0.9	417
	2	Toluene	Rh-Si120	Hexane	Methylcyclohexane (100)		0.83	452
	3	Toluene	Rh-Si80	Water	Methylcyclohexane (100)		2.3	163
	4	Toluene	Rh-Si120	Water	Methylcyclohexane (100)		2.5	150
	5	Anisole	Rh-Si80	Hexane	Methoxycyclohexane (100)		0.8	469
	6	Anisole	Rh-Si120	Hexane	Methoxycyclohexane (100)		0.8	469
	7	Anisole	Rh-Si80	Water	Methoxycyclohexane (100)		2.5	150
	8	Anisole	Rh-Si120	Water	Methoxycyclohexane (100)		2.5	150
	9	o-Xylene	Rh-Si80	Hexane	1,2-Dimethylcyclohexane (cis/trans=92/8)	2.1	179
	10	o-Xylene	Rh-Si120	Hexane	1,2-Dimethylcyclohexane (cis/trans=92/8)	2.1	179
	11	o-Xylene	Rh-Si80	Water	1,2-Dimethylcyclohexane (cis/trans=92/8)	6.8	55
	12	o-Xylene	Rh-Si120	Water	1,2-Dimethylcyclohexane (cis/trans=92/8)	7.5	50

a Conditions. Catalyst: 1 g of Rh-SiO 2 , [S]/[M] = 100, 10 mL solvent, 1 atm H 2 , RT, stirred at 1500 min À1 .

b

Determined by GC analysis.

c

Turnover frequency defined as mol of H 2 per mol of rhodium per h.