Kinetic Aspects for the Reduction of CO<sub>2</sub> and CS<sub>2</sub> with Mixed-Ligand Ruthenium(II) Hydride Complexes Containing Phosphine and Bipyridine

A new water-soluble ruthenium hydride complex [Ru­(H)­(bpy)<sub>2</sub>(PTA)]­PF<sub>6</sub> (bpy = 2,2′-bipyridine, PTA = 1,3,5-triaza-7-phosphaadamantane) (<b>1a</b>) was prepared. <b>1a</b> reacted with CO<sub>2</sub> and CS<sub>2</sub> to give the corresponding formate and dithioformate complexes, respectively. Both the insertions of CO<sub>2</sub> and CS<sub>2</sub> into the Ru–H bond of <b>1a</b> followed second-order kinetics. The second-order rate constant (<i>k</i><sub>2</sub>) of CO<sub>2</sub> insertion reaction varied from (9.40 ± 0.41) × 10<sup>–4</sup> M<sup>–1</sup> s<sup>–1</sup> in acetone to (1.13 ± 0.08) × 10<sup>–1</sup> M<sup>–1</sup> s<sup>–1</sup> in methanol; moreover, the ln­(<i>k</i><sub>2</sub>) is in good linear relationship with the acceptor number (AN) of the solvent used. Although, the <i>k</i><sub>2</sub> of CS<sub>2</sub> insertion reaction ranged from (3.43 ± 0.10) M<sup>–1</sup> s<sup>–1</sup> in methanol to (24.0 ± 0.5) M<sup>–1</sup> s<sup>–1</sup> in <i>N</i>,<i>N</i>-dimethylformamide, which is 1000 times faster than CO<sub>2</sub> insertion. Generally, the <i>k</i><sub>2</sub> of CS<sub>2</sub> insertion increased with the static dielectric constant (<i>D</i><sub>s</sub>) of the reaction medium investigated. For comparison purposes, we further investigated the reactivity of [Ru­(H)­(bpy)<sub>2</sub>(PPh<sub>3</sub>)]­PF<sub>6</sub> (PPh<sub>3</sub> = triphenylphosphine) (<b>1b</b>) with CO<sub>2</sub> and CS<sub>2</sub>. <b>1b</b> reacted with CO<sub>2</sub> slowly in the methanol with a <i>k</i><sub>2</sub> of (1.46 ± 0.09) × 10<sup>–3</sup> M<sup>–1</sup> s<sup>–1</sup>, yielding a formate complex [Ru­(η<sup>1</sup>-OC­(H)O)­(bpy)<sub>2</sub>(PPh<sub>3</sub>)]­PF<sub>6</sub> (<b>2b</b>). The reaction of <b>1b</b> with CS<sub>2</sub> is 1000 times faster than that of CO<sub>2</sub>. The structures of <b>1a</b>, <b>1b</b>, and <b>2b</b> were determined by X-ray crystallographic analysis.