Synthesis and Characterization of Dithiooxamidate-Bridged Polynuclear Ni Complexes

Abstract

Mixed-valence complexes contain two metals with different formal oxidation numbers and, therefore, show mixed properties that are influenced by the electronic coupling between the two metals, which is, in turn, regulated by a bridging ligand. This is an attractive point for many researchers. Oxalate is widely used as a bridging ligand for preparing polynuclear complexes. More than 1000 complexes have been reported until now. However, dithiooxamidate, which is an oxalate analog, is less popular as a bridging ligand. Here, a new dithiooxamidate-bridged Ni-diphosphine dinuclear complex with the formula [(μ2-toxa){Ni(dppe)}2](BF4)2 (toxa = dithiooxamidate; dppe = 1,2-bis(diphenylphosphino)ethane) was prepared and characterized via single-crystal X-ray diffraction. When using 1,3-bis(diphenylphosphino)propane (dppp) instead of dppe, dinuclear, trinuclear, and tetranuclear complexes were obtained, i.e., [(μ2-toxa){Ni(dppp)}2](BF4)2, [{μ2-Ni(toxa)2}{Ni(dppp)}2](BF4)2, and [{μ3-Ni(toxa)3}{Ni(dppp)}3](BF4)2, respectively. Bidentate toxa ligands in dinuclear complexes coordinate each Ni atom as κ(S,N). However, the trinuclear and the tetranuclear complexes have the toxa ligands with κ(N,N) and κ(S,S) coordination. The [(μ2-toxa){Ni(dppe)}2](BF4)2 complex undergoes four reversible redox processes, whose analysis via a controlled-potential absorption spectrum reveals the presence of a Ni(II)-Ni(I) mixed-valence state at ∆E1/2 = 0.22 V with a comproportionation constant of 6.1 × 103.