Synthesis and characterization of square planar nickel(II) - Arylthiolate complexes with the biphenyl-2,2'-dithiolate ligand

Two new Ni(II)S₄ complexes with the biphenyl-2,2'-dithiolate ligand (L) are reported. The dinuclear complex 1, [Ni₂L₃]^2-, was formed in the reaction of 2-3 equiv of Na₂L and [NiCl₄]^2- and the mononuclear complex [NIL₂]^2- (2) by using 4-10 equiv of Na₂L. Complexes 1 and 2 have been crystallographically characterized. (Et₄N)₂[1]· 0.5S₂Ph₂, CH₃CN: C₆₀H₇₁·N₃Ni₂S₇, triclinic, P1, a = 13.806(2) Å, b = 14.267(2) Å, c = 16.873(2) Å, α = 69.263(10)°, β = 69.267(8)°, γ = 83.117(10)°, Z = 2, R₁ = 0.0752 (wR2 = 0.2011). (Et₄N)(Na·CH₃CN)[2]: C₃₄H₃₉N₂NaNiS₄, triclinic, P1, a = 9.9570(10) Å, b = 13.2670(10) Å, c = 13.9560(10) Å, α = 108.489(7)°, β = 90.396(6)°, γ = 103.570(4)°, Z = 2, R1 = 0.0390 (wR2 = 0.0995). Both complexes are square planar about the nickel ion in the solid state as well as in solution. Most Ni(II)-thiolate complexes are square planar except the tetrahedral mononuclear complexes with monodentate arylthiolate ligands that cannot force a square planar geometry. The ligand (L) has some flexibility to change its bite angle via the phenyl-phenyl bond and should not force a planar geometry on its complexes either. Therefore, it is interesting that 2 has adopted a square planar structure. Complex 2 readily converts to I in solution when not in the presence of excess L in a process that is presumably similar to that known for other mononuclear, bidentate ligated Ni(II) complexes. Both complexes, at least in the solid state, appear to have an inclination to bind another metal ion on one face of the complex (Ni²⁺ in 1, Na⁺ in 2). We hope to take advantage of this in future work to synthesize relevant model complexes for the active sites of the nickel-iron hydrogenases after suitable modifications are made to L.