Sm. Ng et al., Experimental and theoretical studies of highly fluxional TpRu(PPh3)"H2SiR3" complexes (Tp = hydridotris(pyrazolyl)borate), ORGANOMETAL, 18(13), 1999, pp. 2484-2490
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)(et
a(2)-HSiR3), the formulation of which is inferred from NMR spectroscopic da
ta. 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 betwe
en 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 ha
ve been performed to investigate the stereochemistry and the interconversio
n processes of various structural isomers for TpRu(PPh3)(H)(eta(2)-HSiR3).
The results further support the existence of a eta(2)-silane formulation fo
r these complexes. Two stable structures, with a nonclassical eta(2)-silane
ligand, have been found. These two structural isomers are found to be in e
quilibrium with a low reaction barrier (7.5 kcal/mol). The interconversion
between the enantiomeric pair of the most stable structure has almost no re
action barrier (0.5 kcal/mol). Together with the experimental findings, a m
echanistic cycle is proposed. The complexes TpRu(PPh3)(H)(eta(2)-HSiR3) rea
ct reversibly with pressurized H-2, CH3CN, and PPh3 to give TpRuH(Hz)(PPh3)
, TpRuH(CH3CN)(PPh3), and TpRuH(PPh3)(2), respectively.