Synthesis , Crystal Structure , and Magnetic Properties of Amidate and Carboxylate Dimers of Ruthenium

Solvothermal and microwave-assisted methods have been used to prepare several amidate and carboxylate complexes of the type [Ru2X(μ-NHOCC6H3-3,5-(OMe)2)4]n [X = Cl (1), Br (2), I (3)] and [Ru2X(μ-O2CC6H3-3,5-(OMe)2)4]n [X = Cl (4), Br (5), I (6)]. Complexes 4–6 have also been obtained by conventional synthesis which is ineffective to prepare the amidate compounds. However, single crystals of complexes 1–5 were obtained using the solvothermal method. The single crystal X-ray structure determination of compounds 1–5 have been carried out. All complexes display a paddlewheel-type structure with the metal atoms connected by four bridging amidate or carboxylate ligands. Chloride, bromide, or iodide anions connect the dimetallic units, producing one-dimensional zigzag chains. The magnetic properties of all compounds were studied. The magnetic moment at room temperature are in accordance with an electronic configuration with three unpaired electrons σ2π4δ2(π*δ*)3 per dimer unit. The fit of the magnetic data suggests the existence, in these complexes, of a weak antiferromagnetic intermolecular interaction between the diruthenium units mediated by the halide ligand and an appreciable zero-field splitting in the diruthenium moieties.


Introduction
The synthesis and reactivity of paddlewheel diruthenium complexes of the type Ru 2 Cl(µ-L-L) 4 (L-L = mononegative, three-atom donor ligand) have been intensively studied due to their interesting magnetic and electronic properties [1].In these complexes, the L-L ligand bridge two multiply-bonded ruthenium atoms.The most studied compound of this type of paddlewheel complexes are the chloridotetracarboxylatodiruthenium(II,III) [Ru 2 Cl(µ-O 2 CR) 4 ] (R = alkyl or aryl) [2,3].The usual method for obtaining these compounds is through a metathesis reaction of [Ru 2 Cl(µ-O 2 CMe) 4 ] with an excess of the corresponding carboxylic acid in a mixture of methanol/water (1:1) at reflux for several hours [4,5].However, the interchange reaction of the acetate ligands by amidate groups to give the analogous [Ru 2 Cl(µ-NHOCR) 4 ] cannot be carried out under mild reaction conditions [6][7][8][9][10][11][12][13].Johnson and Powell [14] demonstrated some years ago that the microwave activation is useful to prepare dimolybdenum compounds in short periods of time and with good yields.More recently, the microwave activation and the solvothermal methods have proven to be as a very effective synthetic methods for the substitution process of acetate groups in [Ru 2 Cl(µ-O 2 CMe) 4 ] by N,N-or N,O-donor ligands (N,N = triazenide, formamidinate or guanidinate; N,O = arylamidate) [15][16][17][18].On the other hand, the solvothermal reactions are carried out in a closed system at relatively high temperature and pressure and in these conditions the compounds increase their solubility.As a consequence, the slow cooling of the reaction mixture frequently leads to single crystals.This is an important advantage in the preparation of the very insoluble tetraamidatodiruthenium complexes.Due to this low solubility the number of bromo-and iodotetraamidatodiruthenium(II,III) compounds are very scarce and only recently some examples have been prepared using solvothermal methods [18].
We have used the 3,5-dimethoxybenzamidate ligand and their analogue carboxylate ligand in order to compare the properties of the amidato and carboxylato diruthenium derivatives.

Synthesis
The synthesis of the amidato complexes [Ru 2 X(µ-NHOCC 6 H 3 -3,5-(OMe) 2 ) 4 ] n [X = Cl (1), Br (2), I (3)], have been carried out by solvothermal and microwave assisted solvothermal methods.However, the thermal activation in solution does not produce the complexes as has been previously observed for analogous compounds [6][7][8][9][10][11][12][13].To achieve the substitution of the carboxylate ligands in [Ru 2 X(µ-O 2 CMe) 4 ] n (X = Cl, Br, I) by the corresponding amidate groups the presence of triethylamine and the corresponding halide salt (LiCl for 1, KBr for 2 and KI for 3) have been used in both methods [19].In all cases the substitution of the acetate ligands leads to the tetraamidate complexes 1-3 in moderate yields (35-62%) except in the microwave synthesis for complex 2 (2%).The microwave activation leads to single-phase microcrystalline powders whereas the solvothermal synthesis allows the formation of high quality single crystals.As expected, the reaction times using microwave activation are shorter than used in the solvothermal synthesis.

Crystal Structures
The structures of complexes 1-5 have been determined by single-crystal X-ray diffraction.Table 1 collects selected bond lengths and angles of these complexes.Compounds 1, 2, 4 and 5 crystallize in the monoclinic C2/c space group and the cell unit includes, in all cases, four diruthenium units with no solvate molecules.However, complex 3 crystallizes in the orthorhombic Pbca space group with eight molecules in the cell unit and without solvent molecules.
The dimetallic units of complexes 1-5 consist of two Ru atoms linked by four amidate or carboxylate bridging ligands.Figures 1 and 2 show the dimetallic units of complexes 2 and 5, respectively.The dimetallic unit of complexes 1, 3 and 4 are collected in the Supplementary Material (Figures S1-S3).In all cases the axial positions are occupied by the halide ligand giving infinite zigzag Crystals 2017, 7, 192 3 of 16 chains (Figures 3 and 4 for compounds 1 and 4, respectively).Thus, each Ru atom shows a distorted octahedral environment having a RuN 2 O 2 (amidates) and RuO 4 (carboxylates) environments in the equatorial positions and the axial sites occupied by one halide ligand and by the other Ru atom of the dimetallic unit.Ellipsoids are drawn at 50% probability level.Hydrogen atoms are omitted for clarity.
In the 1-3 amidate complexes, the Ru-X-Ru angle (Table 1) slightly decreases from the chloro [124.51(5)°] to the bromo derivative [119.24(5)°],although this decrease is more intense in the iodo derivative [101.84(3)°].This variation has been attributed to the increase of the atomic volume of the halide ligand which determines a longer distance between the dimer units allowing the decrease of the Ru-X-Ru angle [18].Similarly, in the carboxylate compounds 4 and 5, this angle is slightly acute in the bromo derivative.In all cases, the zigzag chains are parallel to each other in the crystal and each chain is surrounded by another six chains.The zigzag chains are packed only by normal van der Waals forces.The centroid-centroid distances between phenyl rings are always larger than 3.8 Å and therefore the π-π stacking has not any effect on the packing of the chains [24].Figure 5 shows the packing of the chains in complex 3.In the 1-3 amidate complexes, due to the asymmetry of the amidate ligands, four linkage isomers are possible, depending on the relative coordination of the ligands [17].However, the three complexes show a cis-RuN 2 O 2 environment similar to that observed in the other tetraamidatodiruthenium Crystals 2017, 7, 192 5 of 16 complexes described in the literature [8,9,[11][12][13]17,18].Thus, in these complexes, each diruthenium unit resides on a center of inversion, and the asymmetric unit consists of two half dimers linked by the halide ligand, resulting in zigzag chains.Therefore, each diruthenium unit adopts a cis-RuN 2 O 2 arrangement required by the inversion symmetry.Moreover, only two amidate complexes described in the literature [17,18] have a trans-RuN 2 O 2 environment, but these complexes show a linear chain arrangement.
In the 1-3 amidate complexes, the Ru-X-Ru angle (Table 1) slightly decreases from the chloro [124.51( 5) • ] to the bromo derivative [119.24(5)• ], although this decrease is more intense in the iodo derivative [101.84(3)• ].This variation has been attributed to the increase of the atomic volume of the halide ligand which determines a longer distance between the dimer units allowing the decrease of the Ru-X-Ru angle [18].Similarly, in the carboxylate compounds 4 and 5, this angle is slightly acute in the bromo derivative.In all cases, the zigzag chains are parallel to each other in the crystal and each chain is surrounded by another six chains.The zigzag chains are packed only by normal van der Waals forces.The centroid-centroid distances between phenyl rings are always larger than 3.8 Å and therefore the π-π stacking has not any effect on the packing of the chains [24].Figure 5 shows the packing of the chains in complex 3.In these structures, the dihedral angle, θ, can be considered as the angle defined by the plane occupied by the carboxylate group and the ruthenium atoms and the plane defined by the phenyl ring (Table 1, Scheme 1).
In these structures, the dihedral angle, θ, can be considered as the angle defined by the plane occupied by the carboxylate group and the ruthenium atoms and the plane defined by the phenyl ring (Table 1, Scheme 1).Scheme 1. Representation of the θ angle formed by the Ru2(µ-O2C) plane and the plane defined by the phenyl ring.
In complexes 1-5, two θ angles are found, displaying a different orientation of the phenyl rings located in cis positions.A very similar rotation θ angle was found in the complexes 1, 2, 4 and 5 (Table 1), which indicates a low influence when the amidate ligand is changed by carboxylate ligands.However, in the iodo complex 3 one θ angle (43.56) is larger than in the rest of compounds (from 18.71 to 24.18).This fact, together with the presence of the iodide ligand, could determine the packing in the solid state since this complex crystallizes in the orthorhombic Pbca space group instead of in the C2/c space observed in the other complexes described in this work.Unfortunately, it has not been possible to obtain single crystals of the iodo complex 6 to compare the results.Figure 6 shows the θ angles observed in complex 3.

Spectroscopic Properties
The IR spectra of compounds 1-6 are very similar.The main differences between the IR spectra of the amidato (1-3) and carboxylato (4-6) complexes are (i) the presence in the amidato derivatives of a band at ca. 3350 cm −1 due to the NH stretching vibration and (ii) the strong bands observed in the 1500-1300 cm −1 region.In this region, the amidate complexes show a set of bands due to a combination of the ν(C=O) vibration (amide-I) and a mixture of the NH bending and ν(C-N) and ν(C-C) vibrations (amide-II) (Table 2).These bands are shifted to low frequencies with respect to the free amidate ligands.Complexes 4 and 5 show in this region strong bands due to the ν(COO)s and ν(COO)a stretching vibrations (Table 2).In complexes 1-5, two θ angles are found, displaying a different orientation of the phenyl rings located in cis positions.A very similar rotation θ angle was found in the complexes 1, 2, 4 and 5 (Table 1), which indicates a low influence when the amidate ligand is changed by carboxylate ligands.However, in the iodo complex 3 one θ angle (43.56) is larger than in the rest of compounds (from 18.71 to 24.18).This fact, together with the presence of the iodide ligand, could determine the packing in the solid state since this complex crystallizes in the orthorhombic Pbca space group instead of in the C2/c space observed in the other complexes described in this work.Unfortunately, it has not been possible to obtain single crystals of the iodo complex 6 to compare the results.Figure 6 shows the θ angles observed in complex 3.
In these structures, the dihedral angle, θ, can be considered as the angle defined by the plane occupied by the carboxylate group and the ruthenium atoms and the plane defined by the phenyl ring (Table 1, Scheme 1).
Scheme 1. Representation of the θ angle formed by the Ru2(µ-O2C) plane and the plane defined by the phenyl ring.
In complexes 1-5, two θ angles are found, displaying a different orientation of the phenyl rings located in cis positions.A very similar rotation θ angle was found in the complexes 1, 2, 4 and 5 (Table 1), which indicates a low influence when the amidate ligand is changed by carboxylate ligands.However, in the iodo complex 3 one θ angle (43.56) is larger than in the rest of compounds (from 18.71 to 24.18).This fact, together with the presence of the iodide ligand, could determine the packing in the solid state since this complex crystallizes in the orthorhombic Pbca space group instead of in the C2/c space observed in the other complexes described in this work.Unfortunately, it has not been possible to obtain single crystals of the iodo complex 6 to compare the results.Figure 6 shows the θ angles observed in complex 3.

Spectroscopic Properties
The IR spectra of compounds 1-6 are very similar.The main differences between the IR spectra of the amidato (1-3) and carboxylato (4-6) complexes are (i) the presence in the amidato derivatives of a band at ca. 3350 cm −1 due to the NH stretching vibration and (ii) the strong bands observed in the 1500-1300 cm −1 region.In this region, the amidate complexes show a set of bands due to a combination of the ν(C=O) vibration (amide-I) and a mixture of the NH bending and ν(C-N) and ν(C-C) vibrations (amide-II) (Table 2).These bands are shifted to low frequencies with respect to the free amidate ligands.Complexes 4 and 5 show in this region strong bands due to the ν(COO)s and ν(COO)a stretching vibrations (Table 2).

Spectroscopic Properties
The IR spectra of compounds 1-6 are very similar.The main differences between the IR spectra of the amidato (1-3) and carboxylato (4-6) complexes are (i) the presence in the amidato derivatives of a band at ca. 3350 cm −1 due to the NH stretching vibration and (ii) the strong bands observed in the 1500-1300 cm −1 region.In this region, the amidate complexes show a set of bands due to a combination of the ν(C=O) vibration (amide-I) and a mixture of the NH bending and ν(C-N) and ν(C-C) vibrations (amide-II) (Table 2).These bands are shifted to low frequencies with respect to the free amidate ligands.Complexes 4 and 5 show in this region strong bands due to the ν(COO) s and ν(COO) a stretching vibrations (Table 2).The UV-Vis-NIR spectra of compounds 1-6 show three main bands (Table 3).The bands observed in the range 321-346 nm are assigned to a charge transfer ligand-metal of the type σ(axial ligand) → σ*(Ru 2 ).The most characteristic bands observed in diruthenium complexes in the ranges 467-500 and 995-1153 nm are assigned to π(RuO/N,Ru 2 ) → π*(Ru 2 ) and δ(Ru 2 ) → δ*(Ru 2 ) according to previous studies [17,18,25,26].

Magnetic Properties
The magnetic moments at room temperature of complexes 1-6 are in the range of 4.04 to 4.68 µ B .These values are in accordance with the presence of three unpaired electrons per dimer unit in a ground-state configuration σ 2 π 4 δ 2 (π*δ*) 3 .This configuration was proposed by Norman and col.[27] as the basis for theoretical studies.In all complexes the molar magnetic susceptibility increases with decreasing temperature.However, the magnetic moment slowly decreases until about 100 K and then strongly decreases until 2 K.This behavior can be ascribed to the existence, in these type of complexes, of a strong zero-field splitting (ZFS), typical in diruthenium compounds, together with a non-negligible antiferromagnetic coupling between the dimetallic units mediated by the halide ligands or through-space.
The zero-field splitting effect on the susceptibility can be quantified by considering the Hamiltonian H D = S × D × S, as described by O'Connor [28] which leads to the equation, The temperature independent paramagnetism (TIP) has been also introduced giving, The antiferromagnetic coupling has been considered as a perturbation by using the molecular field approximation [28], Finally, the consideration of a paramagnetic impurity (P) leads to the final equation, In these equations, the parameters N, g, zJ, k and D have the usual meanings.Using this model, a good agreement between the experimental and calculated curves of the molar susceptibility and the magnetic moment was obtained.Figure 7, shows the experimental and calculated curves for complex 1.Similar curves have been obtained for complexes 2-6 (Figures S4-S8 in the Supplementary Material.The magnetic parameters (g, zJ, D, TIP, P and σ 2 ) obtained in the fits of the magnetic data of complexes 1-6 are collected in Table 4.In these equations, the parameters N, g, zJ, k and D have the usual meanings.Using this model, a good agreement between the experimental and calculated curves of the molar susceptibility and the magnetic moment was obtained.Figure 7, shows the experimental and calculated curves for complex 1.Similar curves have been obtained for complexes 2-6 (Figures S4-S8 in the Supplementary Material.The magnetic parameters (g, zJ, D, TIP, P and σ 2 ) obtained in the fits of the magnetic data of complexes 1-6 are collected in Table 4.There are no appreciable differences between the magnetic data of the amidate and carboxylates derivatives.The D values, ranging from 49.43 to 58.89 cm −1 , are similar to those found for analogous diruthenium complexes [3,17,18,29,30].The low zJ values, from −0.95 to −3.62 cm −1 , are in accordance with a weak antiferromagnetic interaction between the dimetallic units mediated by the halide ligands in a zigzag chain [3,18].The low Ru-X-Ru angles (from 101.84° to 124.89°) prevent a good orbital overlap between the diruthenium units and the halide ions, leading to the weak antiferromagnetic coupling.These values are lower than that observed in other diruthenium compounds which give linear chains, in the solid state, where the electronic communication is more favored [3,17,18,29].Although a linear correlation between the strength of the magnetic coupling and the Ru-X/Ru-X-Ru ratio has been established [18,31] compound 2 does not correlate well because the zJ is the lowest of the five compounds.However, this fact is not unusual because the magnetic coupling value and the Ru-X distance and the Ru-X-Ru angle depend on many factors and many other complexes are far from linearity.There are no appreciable differences between the magnetic data of the amidate and carboxylates derivatives.The D values, ranging from 49.43 to 58.89 cm −1 , are similar to those found for analogous diruthenium complexes [3,17,18,29,30].The low zJ values, from −0.95 to −3.62 cm −1 , are in accordance with a weak antiferromagnetic interaction between the dimetallic units mediated by the halide ligands in a zigzag chain [3,18].The low Ru-X-Ru angles (from 101.84 • to 124.89 • ) prevent a good orbital overlap between the diruthenium units and the halide ions, leading to the weak antiferromagnetic coupling.These values are lower than that observed in other diruthenium compounds which give linear chains, in the solid state, where the electronic communication is more favored [3,17,18,29].Although a linear correlation between the strength of the magnetic coupling and the Ru-X/Ru-X-Ru ratio has been established [18,31] compound 2 does not correlate well because the zJ is the lowest of the five compounds.However, this fact is not unusual because the magnetic coupling value and the Ru-X distance and the Ru-X-Ru angle depend on many factors and many other complexes are far from linearity.
Microwave reaction was carried out in an ETHOS ONE microwave oven using TFM Teflon closed vessels equipped with temperature sensor and pressure control.Solvothermal synthesis was carried out in a Memmert Universal Oven UFE 400 using Teflon-lined stainless steel autoclaves.
Elemental analyses were done by the Microanalytical Service of the Universidad Complutense de Madrid.FT-IR spectra were recorded in a Perkin Elmer Spectrum 100 with a universal ATR accessory with a spectral range of 4000-650 cm −1 .Electronic spectra of the complexes in the solid state were acquired on a Cary 5G spectrophotometer equipped with a Praying Mantis accessory for diffuse reflectance measurements.The reflectance data were converted by the instrument software to the F(R∞) values according to the Kubelka-Munk function.Magnetization measurements at variable temperature were obtained with a Quantum Design MPMSXL SQUID (Superconducting Quantum Interference Device) magnetometer over a temperature range of 2-300 K at the constant field of 1 T. Molar susceptibility values calculated from magnetization data were corrected for the diamagnetic contribution of both the sample holder and the compound to the susceptibility.The molar diamagnetic corrections for the complexes were calculated on the basis of Pascal's constants.Crushed crystals were used in the magnetic measurements to ensure the homogeneity of the sample.Microwave assisted synthesis (method a).Chloridotetrakis(acetato)diruthenium(II,III) (0.12 g, 0.25 mmol), 3,5-dimethoxybenzamide (0.27 g, 1.50 mmol), lithium chloride (0.08 g, 2 mmol), triethylamine (0.25 mL) and absolute ethanol (8 mL) were added into a 85 mL TFM Teflon vessel with magnetic stirrer bar.The vessel was sealed with a lid equipped with temperature and pressure sensors and placed in the microwave oven.Reaction mixture was then treated by a three-step program consisting of (i) 15 min heating ramp up to 130 Solvothermal synthesis (method b).MeOH was used as solvent.Same reagents and quantities used in method (a) were added to a 23 mL Teflon-lined autoclave and stirred several minutes to become homogenised.The reactor was closed and heated under a three-step program consisting of (i) 2 h heating ramp up to 130  (2) Microwave assisted synthesis (method a).Bromidotetrakis(acetato)diruthenium(II,III) (0.13 g, 0.25 mmol), 3,5-dimethoxybenzamide (0.27 g, 1.50 mmol), potassium bromide (0.24 g, 2 mmol), triethylamine (0.25 mL) and absolute ethanol (8 mL) were added into a 85 mL TFM Teflon vessel with magnetic stirrer bar.The vessel was sealed with a lid equipped with temperature and pressure sensors, and placed in the microwave oven.Reaction mixture was then treated by a three-step program consisting of (i) 15 min heating ramp up to 120 • C; (ii) 16 h isotherm at 120 • C; and (iii) 20 min cooling ramp up to room temperature.A brown suspension was obtained.Solid is obtained by filtration and washed twice with 10 mL of cold ethanol and with 10 mL of water.Yield: 2%.

Synthesis of Complexes
Solvothermal synthesis (method b).Same reagents and quantities used in method (a) were added to a 23 mL Teflon-lined autoclave and stirred several minutes to become homogenised.The reactor was closed and heated under a three-step program consisting of (i) 2 h heating ramp up to 100  (3) Microwave assisted synthesis (method a).Iodotetrakis(acetato)diruthenium(II,III) (0.14 g, 0.25 mmol), 3,5-dimethoxybenzamide (0.27 g, 1.50 mmol), potassium iodide (0.34 g, 2 mmol), triethylamine (0.25 mL) and absolute ethanol (8 mL) were added into a 85 mL TFM Teflon vessel with magnetic stirrer bar.The vessel was sealed with a lid equipped with temperature and pressure sensors, and placed in the microwave oven.Reaction mixture was then treated by a three-step program consisting of (i) 15 min heating ramp up to 100 Solvothermal synthesis (method b).Same reagents and quantities used in method (a) were added to a 23 mL Teflon-lined autoclave and stirred several minutes to become homogenised.The reactor was closed and heated under a three-step program consisting of (i) 2 h heating ramp up to 100  (4) Microwave assisted synthesis (method a).Chloridotetrakis(acetato)diruthenium(II,III) (0.12 g, 0.25 mmol), 3,5-dimethoxybenzoic acid (0.27 g, 1.50 mmol) and methanol (8 mL) were added into a 85 mL TFM Teflon vessel with magnetic stirrer bar.The vessel was sealed with a lid equipped with temperature and pressure sensors, and placed in the microwave oven.Reaction mixture was then treated by a three-step program consisting of (i) 15 min heating ramp up to 130 Solvothermal synthesis (method b).Same reagents and quantities used in method (a) were added to a 23 mL Teflon-lined autoclave and stirred several minutes to become homogenised.The reactor was closed and heated under a three-step program consisting of (i) 2 h heating ramp up to 85

Crystal Structure Determination
Details of the data collection and crystal structure refinement for 1-5 are given in Tables 5 and 6.Data collection for 2, 4 and 5 was carried out at room temperature on a Bruker Smart CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) operating at 50 kV and 35 mA for 2 and 5, and 50 kV and 30 mA for 4. The data were collected over a hemisphere of the reciprocal space by combination of three exposure sets.Each exposure of 20 s covered 0.3 in ω.The cell parameters were determined and refined by a least-squares fit of all reflections.The first 100 frames were recollected at the end of the data collection to monitor crystal decay, and no appreciable decay was observed.
Data collection for 1 and 3 was carried out at room temperature on a Xcalibur-Atlas CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) operating at 50 kV and 40 mA.The exposure time were 19.62 s and 75 s in ω respectively.
The structures were solved by direct methods and refined by full-matrix least-square procedures on F 2 (SHELXL-97) [35].All non-hydrogen atoms were refined anisotropically.In complex 1, C18 has been splitted in two positions to solve the disorder.All hydrogen atoms were included in their calculated positions and refined riding on the respective carbon atoms.Mercury CSD 3.9 [36] was used for molecular graphics.
CCDC 1556299-1556303 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.This material is available free of charge via the Internet at http: //pubs.acs.org.

Scheme 1 .
Scheme 1. Representation of the θ angle formed by the Ru 2 (µ-O 2 C) plane and the plane defined by the phenyl ring.

Table 2 .
Selected bands in the IR spectra for compounds 1-6.
a Not clearly observed.

Table 5 .
Crystal data and structure refinement for 1-3.