Kinetic studies on the first dihydrogen aquacomplex, [Ru(H 2)(H 2O) 5] 2+: Formation under H 2 pressure and catalytic H/D isotope exchange in water

Abstract

The ruthenium(II) hexaaqua complex [Ru(H 2O) 6] 2+ reacts with dihydrogen under pressure to give the η 2-dihydrogen ruthenium(II) pentaaqua complex [Ru(H 2)(H 2O) 5] 2+.The complex was characterized by 1H, 2H and 17O NMR: δ H = −7.65 ppm, J HD = 31.2 Hz, δ O = −80.4 ppm ( trans to H 2) and δ O = −177.4 ppm ( cis to H 2).The H–H distance in coordinated dihydrogen was estimated to 0.889 Å from J HD, which is close to the value obtained from DFT calculations (0.940 Å).Kinetic studies were performed by 1H and 2H NMR as well as by UV–Vis spectroscopy, yielding the complex formation rate and equilibrium constants: k f = (1.7 ± 0.2) × 10 −3 kg mol −1 s −1 and K eq = 4.0 ± 0.5 mol kg −1.The complex formation rate with dihydrogen is close to values reported for other ligands and thus it is assumed that the reaction with dihydrogen follows the same mechanisn ( I d).In deuterated water, one can observe that [Ru(H 2)(H 2O) 5] 2+ catalyses the hydrogen exchange between the solvent and the dissolved dihydrogen.A hydride is proposed as the intermediate for this exchange.Using isotope labeling, the rate constant for the hydrogen exchange on the η 2-dihydrogen ligand was determined as k 1 = (0.24 ± 0.04) × 10 −3 s −1.The upper and lower limits of the p K a of the coordinated dihydrogen ligand have been estimated:3 < p K a < 14.