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

Abstract 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)Bt R ], is described. The six-coordinate complexes [Ph(pz)Bt] 2 M, 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, μ eff  = 4.1  μ B , consistent with spin-crossover behavior. Quantitative analysis of the electronic spectrum of 2Ni leads to a value of D q  = 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)Bt tBu ]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, [κ 2 -Ph(pzH)Bt tBu ] CoCl 2 ·THF ( 4 ) and [κ 2 -Ph(pz)Bt tBu ] 2 Co ( 5 ), have been fully characterized.