Coordinatively unsaturated binuclear clusters of rhodium. The reactivity of [{Pri2P(CH2)nPPri2}Rh]2(μ-H)2 (n = 2, 3, and 4) with dihydrogen, and their use in the catalytic hydrogenation of olefins

Abstract

The coordinatively unsaturated binuclear rhodium hydrides [{Pri2P(CH2)nPPri2}Rh]2(μ-H)2 (n = 2: 1b; n = 3: 1c; n = 4: 1d) react rapidly with dihydrogen to yield the fluxional binuclear tetrahydrides [{Pri2P(CH2)nPPri2}RhH]((μ-H)3[Rh{Pri2P(CH2)nPPri2}] 2b–d(n = 2–4). The dihydride 1b was structurally characterized by single crystal X-ray diffraction. While the tetrahydrides 2b and 2c were found to be stable only in solution in the presence of excess dihydrogen, 2d was stable in the solid state. Three separate exchange processes were characterized for 2b–d via 1H and 31P{1H} NMR spectroscopy: intermolecular exchange with free dihydrogen, and two intramolecular processes exchanging the four hydride ligands. The geometry about the rhodium centres in the limiting structures involved in these processes is affected by the chelate ring size of the ancillary diphosphine ligand. A "rocking" process observed at low temperatures for each of these tetrahydrides has a ΔG≠ of <7.0(5) kcal/mol, 8.6(5) kcal/mol, and 11.5(5) kcal/mol for 2b, 2c, and 2d, respectively. The dihydrides 1b and 1c were found to catalyze the hydrogenation of 1-hexene. Turnover numbers of 850–950 h−1 and 700 h−1 were observed for 1b and 1c, respectively, along with isomerization side reactions. Catalyst concentration studies on the hydrogenation of styrene using 1c as the catalyst precursor revealed a decrease in turnover frequency with increasing total metal concentration, suggesting that the active catalyst is a mononuclear species. Chemical evidence suggests that a pathway involving binuclear intermediates is also available, but that in the 1c system at least, a pathway utilizing mononuclear species as intermediates predominates. Keywords: binuclear rhodium hydrides, homogeneous catalysis.