Roles of a tetrahydroborate ligand in a facile route to ruthenium(ii) ethyl hydride complexes, and a kinetic study of ethane reductive elimination

Abstract

The tetrahydroborate ligand in [Ru(η²-BH₄)(CO)H(PMe₂Ph)₂], 1, allows conversion under very mild conditions to [Ru(CO)(Et)H(PMe₂Ph)₃], 7, by way of [Ru(η²-BH₄)(CO)Et(PMe₂Ph)₂], 4. Deprotection of the hydride ligand in 7 (by BH₃ abstraction) occurs only in the final step, thus preventing premature ethane elimination. A deviation from the route from 4 to 7 yields [Ru(η²-BH₄)(COEt)(PMe₂Ph)₃], 6, but does not prevent ultimate conversion to 7. Modification of the treatment of 4 yields an isomer of 7, 10. Both isomers eliminate ethane at temperatures above 250 K: the immediate product of elimination, thought to be [Ru(CO)(PMe₂Ph)₃], 11, can be trapped as [Ru(CO)(PMe₂Ph)₄], 12, [Ru(CO)H₂(PMe₂Ph)₃], 3a, or [Ru(CO)(CCCMe₃)H(PMe₂Ph)₃], 13. The elimination is a simple first-order process with negative ΔS^‡ and (for 7) a normal kinetic isotope effect (k_H/k_D = 2.5 at 287.9 K). These results, coupled with labelling studies, rule out a rapid equilibrium with a σ-ethane intermediate prior to ethane loss.