Dihydrogen Binding to Isostructural S = 1/2 and S = 0 Cobalt Complexes

Abstract

Two isostructural, nonclassical Co(H(2)) complexes are prepared from their Co(N(2)) precursors using tris(phosphino)silyl and tris(phosphino)borane ancillary ligands. Comproportionation of CoBr(2) and Co metal in the presence of TPB (tris-(o-diisopropylphophinophenyl)borane) gives (TPB)CoBr (4). One-electron reduction of 4 triggers N(2) binding to give (TPB)Co(N(2)) (2-N(2)) which is isostructural to previously reported [SiP(3)]Co(N(2)) (1-N(2)) ([SiP(3)] = tris-(o-diisopropylphosphinophenyl)silyl). Both 1-N(2) and 2-N(2) react with 1 atm H(2) to generate thermally stable H(2) complexes 1-H(2) and 2-H(2), respectively. Both complexes are characterized by a suite of spectroscopic techniques in solution and by X-ray crystallography. The H(2) and N(2) ligands in 2-H(2) and 2-N(2) are labile under ambient conditions and the binding equilibria are observable by temperature-dependent UV/vis. A van't Hoff analysis allows for the ligand binding energetics to be determined (H(2): ΔH(o) = -12.5(3) kcal mol(-1) and ΔS(o) = -26(3) cal K(-1) mol(-1); N(2): ΔH(o) = -13.9(7) kcal mol(-1) and ΔS(o) = -32(5) cal K(-1) mol(-1)).