Coordination chemistry of pyrazolylbis((alkylthio)methyl)borates: Hybrid nitrogen and sulfur donor ligands

The synthesis of iron(II), cobalt(II) and nickel(II) complexes supported by chelating borate ligands containing one pyrazole and two thioethers, phenyl(pyrazolyl)bis((alkylthio)methyl)borates, [Ph(pz)BtR], is described. The six-coordinate complexes [Ph(pz)Bt]2M, M = Fe (1Fe), Co (1Co) and Ni (1Ni), form exclusively the cis isomers as confirmed by X-ray diffraction analyses. Whereas 1Co and 1Ni are high spin, 1Fe exhibits a room temperature magnetic moment, μeff = 4.1 μB, consistent with spin-crossover behavior. Quantitative analysis of the electronic spectrum of 2Ni leads to a value of Dq = 1086 cm−1, reflective of a ligand field strength somewhat weaker than those imposed by the related tridentate borate ligands Tp or PhTt. Replacement of the methylthioether substituent with the sterically more demanding tert-butylthioether leads to the isolation of [Ph(pz)BttBu]MX, M = Co, X = Cl (2Co); M = Ni, X = Cl (2Ni) or acac (3). The solid state structures of 2Co and 2Ni are chloride-bridged dimers. Additional high-spin cobalt(II) complexes, accessible under distinct preparative conditions, [κ2-Ph(pzH)BttBu] CoCl2·THF (4) and [κ2-Ph(pz)BttBu]2Co (5), have been fully characterized.