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

Two isostructural, nonclassical Co­(H2) complexes are prepared from their Co­(N2) precursors using tris­(phosphino)­silyl and tris­(phosphino)­borane ancillary ligands. Comproportionation of CoBr2 and Co metal in the presence of TPB (tris-(o-diisopropylphophinophenyl)­borane) gives (TPB)­CoBr (4). One-electron reduction of 4 triggers N2 binding to give (TPB)­Co­(N2) (2-N2) which is isostructural to previously reported [SiP3]­Co­(N2) (1-N2) ([SiP3] = tris-(o-diisopropylphosphinophenyl)­silyl). Both 1-N2 and 2-N2 react with 1 atm H2 to generate thermally stable H2 complexes 1-H2 and 2-H2, respectively. Both complexes are characterized by a suite of spectroscopic techniques in solution and by X-ray crystallography. The H2 and N2 ligands in 2-H2 and 2-N2 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 (H2: ΔHo = −12.5(3) kcal mol–1 and ΔSo = −26(3) cal K–1 mol–1; N2: ΔHo = −13.9(7) kcal mol–1 and ΔSo = −32(5) cal K–1 mol–1).