Kinetic studies on the first dihydrogen aquacomplex, [Ru(H2)(H2O)5]2+: Formation under H2 pressure and catalytic H/D isotope exchange in water

The ruthenium(II) hexaaqua complex [Ru(H2O)6]2+ reacts with dihydrogen under pressure to give the η2-dihydrogen ruthenium(II) pentaaqua complex [Ru(H2)(H2O)5]2+.The complex was characterized by 1H, 2H and 17O NMR: δH = −7.65 ppm, JHD = 31.2 Hz, δO = −80.4 ppm (trans to H2) and δO = −177.4 ppm (cis to H2).The H–H distance in coordinated dihydrogen was estimated to 0.889 Å from JHD, 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: kf = (1.7 ± 0.2) × 10−3 kg mol−1 s−1 and Keq = 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 (Id).In deuterated water, one can observe that [Ru(H2)(H2O)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 η2-dihydrogen ligand was determined as k1 = (0.24 ± 0.04) × 10−3 s−1.The upper and lower limits of the pKa of the coordinated dihydrogen ligand have been estimated:3 < pKa < 14.