Experimental and theoretical studies of highly fluxional TpRu(PPh3)'H2SiR3' complexes (Tp = hydridotris(pyrazolyl)borate)

Reactions of TpRuH(CH3CN)(PPh3) with free silanes HSiR3 (R-3 = Et-3, (EtO)(3), Ph-3, HEt2, HPh2, and H2Ph) in THF yield the complexes TpRu(PPh3)(H)(eta(2)-HSiR3), the formulation of which is inferred from NMR spectroscopic data. The chemical equivalence of the two hydrogen atoms of TpRu(PPh3)(H)(eta(2)-HSiR3) down to -100 degrees C is attributed to rapid fluxionality between the two, i.e., TpRu(PPh3)(H-a)(eta(2)-HbSiR3) reversible arrow TpRu(PPh3)(H-b)(eta(2)-HaSiR3). Molecular orbital calculations at the B3LYP level have been performed to investigate the stereochemistry and the interconversion processes of various structural isomers for TpRu(PPh3)(H)(eta(2)-HSiR3). The results further support the existence of a eta(2)-silane formulation for these complexes. Two stable structures, with a nonclassical eta(2)-silane ligand, have been found. These two structural isomers are found to be in equilibrium with a low reaction barrier (7.5 kcal/mol). The interconversion between the enantiomeric pair of the most stable structure has almost no reaction barrier (0.5 kcal/mol). Together with the experimental findings, a mechanistic cycle is proposed. The complexes TpRu(PPh3)(H)(eta(2)-HSiR3) react reversibly with pressurized H-2, CH3CN, and PPh3 to give TpRuH(Hz)(PPh3), TpRuH(CH3CN)(PPh3), and TpRuH(PPh3)(2), respectively.