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 [RuCl₂(PNP)] (<b>1</b>) was synthesized from the binuclear [RuCl₂(<i>p</i>-cym)]₂ with a PNP-type ligand (PNP = 3,6-di-<i>tert</i>-butyl-1,8-bis(di<i>iso</i>propylphosphino)methyl)-9<i>H</i>-carbazole - (Cbzdiphos <i>ⁱ</i> ^Pr)H) in a toluene solution, within 20 h at 110 °C, producing a green solid, which was precipitated with a 1/1 mixture of <i>n</i>- pentane/HMDSO. The complex was characterized by NMR-¹H, ¹³C, and ³¹P{¹H}, 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 <b>1</b> was investigated in the presence of potassium triethylborohydride (KBEt₃H, in THF solution of 1.0 mol L^-1) and ammonium formate (NH₄HCO₂), producing an <i>in situ</i> hydride complex and a formate intermediate species coordinated to the ruthenium center. The complex <b>1</b>, loaded with 0.08%, catalyzed the decomposition of ammonium formate (AF) into H₂, CO₂, and NH₃ in THF solutions at 80 °C, with 94% of H₂ and TOF = 206 h^-1 (molar ratio [Ru]/AF = 1/1204). The catalytic activity increased remarkably for the decomposition of formic acid (FA) as a substrate to produce H₂ and CO₂. In the HMDSO solution at 80 °C, a conversion of 100% was obtained in relation to H₂ and TOF = 3010 h^-1 (molar ration [Ru]/FA/NEt₃ = 1/1204/843). In an equimolar mixture of AF/FA in HMDSO solution at 80 °C, without additives, the complex <b>1</b> catalyzed the decomposition of both with 100% of H₂ and TOF = 987 h^-1 (molar ratio [Ru]/AF/FA= 1/602/602). Under the later conditions, as well as upon AF decomposition, carbamic acid [HO(C=O)NH₂] was obtained as a coproduct of a secondary reaction between NH₃ and CO₂ (yield = 50% in relation to the amount of AF). A kinetic study for decomposing FA, in the range of 60-100 °C, provided Δ<i>S</i>‡ = -9.7 e.u, Δ<i>G</i>‡ = 13.35 kJ mol^-1, and <i>E</i> _a = 64 kJ mol^-1, suggesting that the mechanism is more associative than for the known complexes.