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

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 CoBr2 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 [SiP3]Co(N-2) (1-N-2) ([SiP3] = 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: Delta H degrees = -12.5(3) kcal mol(-1) and Delta S degrees = -26(3) cal K-1 mol(-1); N-2: Delta H degrees = -13.9(7) kcal mol(-1) and Delta S degrees = -32(5) cal K-1 mol(-1)).