Hydrogen Production by the Ruthenium(II) Complex Bearing a Bulky PNP Ligand: A Catalyst for the Decomposition of Formic Acid and/or Ammonium Formate.

The five-coordinate complex [RuCl(PNP)] () was synthesized from the binuclear [RuCl(-cym)] with a PNP-type ligand (PNP = 3,6-di--butyl-1,8-bis(dipropylphosphino)methyl)-9-carbazole - (Cbzdiphos )H) in a toluene solution, within 20 h at 110 °C, producing a green solid, which was precipitated with a 1/1 mixture of - pentane/HMDSO. The complex was characterized by NMR-H, C, and P{H}, mass spectroscopy-LIFDI, FTIR, UV/vis spectroscopy, and cyclic voltammetry, as well as a description of the optimized structure by DFT calculation. The reactivity of was investigated in the presence of potassium triethylborohydride (KBEtH, in THF solution of 1.0 mol L) and ammonium formate (NHHCO), producing an hydride complex and a formate intermediate species coordinated to the ruthenium center. The complex , loaded with 0.08%, catalyzed the decomposition of ammonium formate (AF) into H, CO, and NH in THF solutions at 80 °C, with 94% of H and TOF = 206 h (molar ratio [Ru]/AF = 1/1204). The catalytic activity increased remarkably for the decomposition of formic acid (FA) as a substrate to produce H and CO. In the HMDSO solution at 80 °C, a conversion of 100% was obtained in relation to H and TOF = 3010 h (molar ration [Ru]/FA/NEt = 1/1204/843). In an equimolar mixture of AF/FA in HMDSO solution at 80 °C, without additives, the complex catalyzed the decomposition of both with 100% of H and TOF = 987 h (molar ratio [Ru]/AF/FA= 1/602/602). Under the later conditions, as well as upon AF decomposition, carbamic acid [HO(C=O)NH] was obtained as a coproduct of a secondary reaction between NH and CO (yield = 50% in relation to the amount of AF). A kinetic study for decomposing FA, in the range of 60-100 °C, provided Δ‡ = -9.7 e.u, Δ‡ = 13.35 kJ mol, and = 64 kJ mol, suggesting that the mechanism is more associative than for the known complexes.