Syntheses and characterisation of tris(3-(pyridin-2-yl)-1 H- pyrazol-1-yl)methane and its bis(µ-hydroxo) dicobalt(II) complex

The new ligand tris(3-(pyridin-2-yl)-1 H -pyrazol-1-yl)methane (HC(3-Pypz) 3 , 1 ) has been synthesised by the reaction of 2-(1H-pyrazol-3-yl)pyridine with CHCl 3 and Na 2 CO 3 under phase-transfer conditions. The reaction of HC(3-Pypz) 3 with Co(BF 4 ) 2 ·6 H 2 O leads to the formation of dinuclear bis(µ-hydroxo) cobalt(II) complex [Co 2 (µ-OH) 2 (HC(3-Pypz) 3 ) 2 ][BF 4 ] 2 ·2MeOH ( 2 ). This complex was crystallographically characterised. It comprises a Co 2 (µ-OH) 2 core with bridging hydroxide ligands. The tripodal tris-(3-(pyridine-2-yl)-1 H -pyrazol-1-yl)methane ligands coordinate each cobalt atom with one bidentate arm, respectively. By this chelating and at the same time bridging coordination mode an octahedral coordination environment is formed.


Introduction
N donor stabilised cobalt complexes have been reported for a multitude of catalytic applications, e.g. as catalysts for industrially useful oxidation processes like the oxidation of alkenes 1 and for various polymerisation processes.The application in polymerisation processes ranges from ethene oligomerisation with iminopyridine cobalt complexes 2 and propyleneoxide polymerisation with phenoxide Schiff base cobalt complexes 3 to the living radical polymerisation of acrylates with tetramesitylporphyrin cobalt systems, which generate stable radicals. 4Furthermore, stoichiometric applications like the transformation of nitriles to aldehydes or cyanides have been subject of considerable interest. 5he specific N donor ligand dictates the coordination environment of cobalt and controls its reactivity by its electronic and steric properties.In this context, poly(pyrazolyl)methanes have been studied for decades.The first tris(pyrazolyl)methane was synthesised in 1937 by Hückel and Bretschneider with the reaction of pyrazole potassium salt and chloroform, 6 but the complicated synthesis hindered its wide application in coordination chemistry.In 1987 Elguero et al. have reported a revised synthesis for tris(pyrazolyl)methanes: in this protocol the pyrazole reacts with chloroform and potassium carbonate under liquid-liquid phase transfer conditions. 7y substitution of the base with excess sodium carbonate the yield could be considerably enhanced.A multitude of ligands have been synthesised according to this method, e.g.HC(pz)3, HC(3,5-Me 2 pz) 3 , HC(3-Phpz) 3 , HC(3-i Prpz) 3 and HC(3-t Bupz) 3 . 8eside these tris(pyrazolyl)methanes, a flourishing branch in synthesis and coordination chemistry of NNO scorpionate ligands was developed, based on bis(pyrazolyl)methane ligands. 9ris(pyrazolyl)methanes stabilise a great variety of coordination motifs; among these, a facial chelate coordination with local C 3 symmetry represents the most common coordination motif.In many cases, bisfacial wrapping of the metal is observed which can be avoided by using tris(pyrazolyl)methanes of the second generation.These ligands possess a sterically demanding substituent in the 3-position of the pyrazole.In a larger synthetic effort, this 3-position can also be substituted by an additional donor function.For example, Ziessel et al. synthesised in a sophisticated process tris[3-(6-carboxypyridin-2-yl)pyrazol-1-yl]methane with a substituted pyridinyl function in the 3-position. 10,11Some years earlier, Ward et al. developed the ligand phenyltris[(3-(2-pyridyl)pyrazol-1-yl])methane.In this ligand the CH head-group is replaced by a CPh group. 12In the field of bis(pyrazolyl)methanes, the analogous bis(pyrazolyl)methane (H2C(3-Pypz)2) with a pyridinyl function in the 3-position was synthesised by Ward et al. as well. 13uring our synthetic studies on poly(pyrazolyl)methanes, 14 we obtained tris(pyrazolyl)methane tris(3-(pyridin-2-yl)-1H-pyrazol-1-yl)methane (HC(3-Pypz)3, 1).Herein we report on the synthesis and characterisation of the new ligand and a structural characterisation of its cobalt(II) bis(µ-hydroxo)

Synthesis
In general, tris(pyrazolyl)methanes are synthesised under liquid-liquid phase transfer conditions by the reaction of the corresponding pyrazole, chloroform and excess base. 7,8For the synthesis of the ligand tris(3-(pyridin-2-yl)-1H-pyrazol-1-yl)methane (HC(3-Pypz)3, 1) the substituted pyrazole 2-(1H-pyrazol-3-yl)pyridine 15 is used (Scheme 1a).With tetrabutylammoniumbromide as phase-transfer catalyst and sodium carbonate, we obtained the highest yields in this reaction.At first, the pyrazole reacted with the base sodium carbonate in water in an exothermic reaction under presence of the phase-transfer catalyst.After cooling and addition of CHCl3 to the solution, the reaction mixture was refluxed for three days.After conventional work up, we obtained a mixture of regioisomers of the products in which the pyridinyl function is either in the 3-or in the 5-position.This fact is manifested in the NMR spectra of the crude product which show only a small yield of the desired product.This undesired isomerisation is known and so a further isomerisation step is performed. 8,10,11Therefore the mixture of isomers was dissolved in boiling toluene.Addition of a catalytic amount of p-toluenesulfonic acid leads to the complete isomerisation of all regioisomers to the desired 3-isomer.Hence, a purification by column chromatography is not necessary (Scheme 1b).The final purification step proceeds as follows: after filtration of the solution at room temperature a red brown precipitate was collected.This precipitate was extensively washed with water, ethanol and hexane to afford the pure ligand tris The complex [Co2(µ-OH)2(HC(3-Pypz)3)2][BF4]2•2MeOH (2, Figure 1) was obtained by mixing equimolar amounts of Co(BF4)2•6H2O in acetonitrile and methanol and the ligand tris(3-(pyridin-2-yl)-1H-pyrazol-1-yl)methane in dichloromethane (Scheme 2).This complex crystallised by gas phase diffusion of diethyl ether.1) is the first structurally characterised metal complex with tris(3-(pyridin-2-yl)-1H-pyrazol-1yl)methane, a ligand of the aforementioned second generation.Compound 2 crystallises orthorhombic in the space group Pbca.The geometric centre of the cation lies on a crystallographic inversion centre.Ward et al. 13 reported the dinuclear complex [Co2(H2C(3-Pypz)2)2(µ-OH)2][ClO4]2•MeCN (C1, Scheme 3) with the corresponding bis(pyrazolyl)methane as ligand.The ligand H2C(3-Pypz)2 contains two pyridinyl/pyrazolyl moieties bridged by a methylene backbone.In this complex each cobalt(II) ion is octahedrally coordinated.Analogously to 2, both cobalt ions are cooordinated by two bidentate pyridinyl/pyrazolyl chelate ligand arms (one from each ligand) and two bridging hydroxide oxygen atoms.The complexes 2 and C1 show great similarity in their key geometric parameters (Table 2 The ligand in C2 (Scheme 3) is similar to the ligand presented herein, but in this ligand the CH head-group is replaced by a CPh group. 12The copper complex C2 shows a square-pyramidal coordination environment and the ligand coordinates in a tetradentate manner.The fifth ligand is a solvent molecule.The Cu-Npy bond lengths (2.059(3) and 2.051(3) Å) and the Cu-Npz bond lengths (1.944(3) and 1.950(3) Å) are significantly shorter.With the anionic tris(pyrazolyl)borate HB(3-Pypz)3 -the cobalt complex C3 (Scheme 3) has been reported with a trigonal prism as coordination environment for the cobalt atom. 16The three Co-Npy bond lengths (2.259(5), 2.281(5) and 2.284(5) Å) are considerably longer than the Co-Npz bond lengths (2.060(5), 2.061(5) and 2.066(6) Å).In comparison to 2, the Co-Npz bond lengths are significantly longer whereas the Co-Npy bond lengths are shorter.When the larger europium(III) ion is incorporated into a complex as in the complex cation C4, the coordination geometry of europium is monocapped square antiprismatic (Scheme 3). 17The three Npy donors and both oxygen donor atoms of the solvent describe a pentagonal plane.Also the three Npz donors and one Npy donor form one square plane and the next square plane is formed by the two oxygen donors, the fluoride ion and one Npz donor.A Npy donor is the cap of the mono-capped square antiprism.The Eu-Npz bond lengths (2.533 (11), 2.529(10), 2.551(10) Å) and the Eu-Npy bond lengths (2.648 (11), 2.659(11), 2.690(10) Å) are larger in comparison to those of 2 as expected for a lanthanide ion with higher coordination number.In the cobalt complexes with the unsubstituted ligands HC(py)3 (C5) and HC(pz)3 (C6) (Scheme 3), 18,19 the octahedral coordination environment of the cobalt ion is spanned by Co-N-bond lengths of Co-Npy 2.109(2) Å and Co-Npz 2.114(3), 2.108(3), 2.122(2) Å.
In principle, the ligand tris(3-(pyridin-2-yl)-1H-pyrazol-1-yl)methane allows a trigonal prismatic coordination environment, but DFT studies show that the cobalt(II) ion is too small for a trigonal prismatic coordination which is revealed by the Co-N bond lengths of the hypothetic modell (BP86/def2-TZVP, Figure 2) with Co-Npy 2.141, 2.069 and 3.434 Å and Co-Npz 1.881, 1.884 and 2.376 Å.The system favours a 4+1+1 coordination or after rearrangement an octahedral coordination with bridging ligands.For a more detailed analysis of the electronic structure of the ligand the Mulliken charges and the NBO charges have been determined.The resulting charges are summarised in Table 2.These charges do not represent absolute charges but the trends give an impression of electronic effects.The advantage of NBO charges over Mulliken charges lies in their greater independence of the basis sets. 20In comparison of pyridinyl functions versus pyrazolyl functions, the pyridinyl donors represent the stronger donor because the charge of the pyridinyl N donor (Mulliken and NBO) is more negative.This is a further argument for the observed coordination in which the pyridinyl donors coordinate closely to the metal.

Experimental Section
General.The substituted pyrazole 2-(1H-pyrazol-3-yl)pyridine was synthesised according to the published procedure. 15Solvents and other chemicals were commercially available and were used without further purification.IR spectra were recorded on a Nicolet FT-IR spectrometer P510. 1 H and 13 C NMR spectra were recorded at 500 and 125 MHz, respectively, on a BRUKER Avance 500 instrument.The NMR signals were calibrated to the residual signals of the deuterated solvents (CDCl3 δH = 7.26 ppm).Mass spectra were recorded on a Finnigan MAT 95 (EI-MS, 70 eV) or on a Finnigan TSQ (ESI-MS).Elemental analyses for C, H and N were obtained using an Elementar analysator vario MICRO Cube (ligand) or a LECO CHNS-932 analysator (complex).

Synthesis of Tris(3-(pyridin-2-yl)-1H-pyrazol-1-yl)methane (1)
2-(1H-Pyrazol-3-yl)pyridine 15 (13.0 g, 90 mmol) and tetra-n-butylammonium bromide (1.4 g, 4.3 mmol) were suspended in distilled water (219 ml).With vigorous stirring, sodium carbonate (60 g, 0.56 mol) was added gradually to the reaction mixture.At room temperature chloroform (70 ml) was added and the flask was equipped with a reflux condenser.This mixture was heated at reflux for three days during which it became a dark emulsion.The mixture was allowed to cool to room temperature and filtered to remove the solid sodium carbonate.The organic layer was separated from the aqueous layer and the latter was extracted with diethylether (3×100 ml).The combined organic layers were washed with distilled water (2×200 ml) and dried over sodium sulphate.The solution was filtered and the solvent was removed under reduced pressure.The resulting red-brown residue was dissolved in the minimum amount of hot toluene and poured into a flask containing a small amount of p-toluenesulfonic acid (0.18 g).The dark solution was heated at reflux temperature for one day.After it had cooled to room temperature, a dark precipitate appeared that was filtered.The residue was suspended in water (100 ml) and stirred overnight.Then the red brown residue was filtered, washed with water, extensively with ethanol and hexane, to afford pure tris(3-(pyridin-2-yl)-1H-pyrazol-1-yl)methane (HC(3-Pypz) 3 , 1).Sand brown powder, yield 45%, 6.0 g.

Crystal structure analysis
Crystal data for compound 2 are presented in Table 3. X-ray diffraction data was collected with a Bruker-AXS SMART APEX CCD 21 using MoKα radiation (λ = 0.71073 Å) and a graphite monochromator.Data reduction and absorption correction was performed with SAINT and SADABS. 21,22The structure was solved by direct and conventional Fourier methods and all non-hydrogen atoms refined anisotropically with fullmatrix least-squares based on F² (SHELXTL 21,22 ).Hydrogen atoms were derived from difference Fourier maps and placed at idealised positions, riding on their parent C atoms, with isotropic displacement parameters Uiso(H) = 1.2Ueq(C) and 1.5Ueq(C methyl).All methyl groups were allowed to rotate but not to tip.Besides the BF 4 -anions two molecules methanol exist in the unit cell per formula unit in disordered manner.It was possible to model the disordered solvent molecules in an adequate manner, the data set was treated with the SQEEZE facility of PLATON. 23,24Full crystallographic data (excluding structure factors) for 2 have been deposited with the Cambridge Crystallographic Data Centre as supplementary no.CCDC -742818.Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fay: (+44)1223-336-033;e-mail: deposit@ccdc.cam.ac.uk).

Computational details
Density functional theory (DFT) calculations were performed with the program suite Gaussian 03 25 or with the program Turbomole. 26,27The geometry of 1 was optimised using the B3LYP 28,29 hybrid DFT functional and the 6-31g(d) basis set on all atoms as implemented in Gaussian.The Mulliken charge of each atom was calculated by a Mulliken population analysis.The NBO charge of each atom was calculated by a natural bond orbital analysis. 30,31he geometry of the hypothetical model complex was optimised using the BP86 pure functional 28,32 and the def2-TZVP 33 basis set as implemented in Turbomole on all atoms.

Figure 1
shows the molecular structure of [Co2(µ-OH)2(HC(3-Pypz)3)2] 2+ in crystals of [Co2(µ-OH)2(HC(3-Pypz)3)2][BF4]2•2 MeOH.This structure illustrates that each ligand acts as a chelate for each cobalt atom and as a bridge between the cobalt centres, with the two ligands in a non-helical arrangement.Additionally, two hydroxide ions bridge both cobalt centres yielding a Co 2 (µ-OH) 2 rhomb.Each cobalt(II) atom is therefore octahedrally coordinated by two bidentate N pyridinyl/pyrazolyl donor functionalities (one from each ligand) and two bridging hydroxide oxygen atoms.The third potentially bidentate ligand arm of each of the ligands points into the periphery.The Co-N bond lengths (2.172 -2.140 Å) are indicative of high-spin cobalt(II) ions.The bond lengths of the cobalt ions to the harder hydroxide ligands are significantly shorter (2.034(1) and 2.044(1) Å).The Co•••Co separation amounts to 3.095(3) Å.

Figure 2 .
Figure 2. Hypothetical model of a trigonal prismatic coordination.