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

The five-coordinate\ncomplex [RuCl<sub>2</sub>(PNP)] (<b>1</b>) was synthesized from\nthe binuclear [RuCl<sub>2</sub>(<i>p</i>-cym)]<sub>2</sub> 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\n– (Cbzdiphos<i><sup>i</sup></i><sup>Pr</sup>)H) in\na toluene solution, within 20 h at 110 °C,\nproducing a green solid, which was precipitated with a 1/1 mixture\nof <i>n</i>- pentane/HMDSO. The complex was characterized\nby NMR<sup>1</sup>H, <sup>13</sup>C, and <sup>31</sup>P{<sup>1</sup>H}, mass spectroscopyLIFDI, FTIR, UV/vis spectroscopy,\nand cyclic voltammetry, as well as a description of the optimized\nstructure by DFT calculation. The reactivity of <b>1</b> was\ninvestigated in the presence of potassium triethylborohydride (KBEt<sub>3</sub>H, in THF solution of 1.0 mol L<sup>–1</sup>) and ammonium\nformate (NH<sub>4</sub>HCO<sub>2</sub>), producing an <i>in situ</i> hydride complex and a formate intermediate species coordinated to\nthe ruthenium center. The complex <b>1</b>, loaded with 0.08%,\ncatalyzed the decomposition of ammonium formate (AF) into H<sub>2</sub>, CO<sub>2</sub>, and NH<sub>3</sub> in THF solutions at 80 °C,\nwith 94% of H<sub>2</sub> and TOF = 206 h<sup>–1</sup> (molar\nratio [Ru]/AF = 1/1204). The catalytic activity increased remarkably\nfor the decomposition of formic acid (FA) as a substrate to produce\nH<sub>2</sub> and CO<sub>2</sub>. In the HMDSO solution at 80 °C,\na conversion of 100% was obtained in relation to H<sub>2</sub> and\nTOF = 3010 h<sup>–1</sup> (molar ration [Ru]/FA/NEt<sub>3</sub> = 1/1204/843). In an equimolar mixture of AF/FA in HMDSO solution\nat 80 °C, without additives, the complex <b>1</b> catalyzed\nthe decomposition of both with 100% of H<sub>2</sub> and TOF = 987\nh<sup>–1</sup> (molar ratio [Ru]/AF/FA= 1/602/602). Under the\nlater conditions, as well as upon AF decomposition, carbamic acid\n[HO(CO)NH<sub>2</sub>] was obtained as a coproduct of a secondary\nreaction between NH<sub>3</sub> and CO<sub>2</sub> (yield = 50% in\nrelation to the amount of AF). A kinetic study for decomposing FA,\nin the range of 60–100 °C, provided Δ<i>S</i>‡ = −9.7 e.u, Δ<i>G</i>‡ = 13.35\nkJ mol<sup>–1</sup>, and <i>E</i><sub>a</sub> = 64\nkJ mol<sup>–1</sup>, suggesting that the mechanism is more\nassociative than for the known complexes.