Cationic and neutral ruthenium(ii) complexes bearing silyl-phosphines: synthesis, structures, and catalytic studies in selective hydroboration of carbonyl compounds

Abstract

The reactivity of ortho-silylated phosphines featuring SiMe₃ and SiR₂H (R = Me, Ph) units has been studied towards ruthenium(II) metal precursors. The reaction of phosphino-trimethylsilane (1a) with [Ru(p-cymene)Cl₂]₂ in the presence of KPF₆ afforded the cationic complex [Ru(η⁶-p-cymene)(CH₃CN)(1a)]PF₆ (3). Phosphine ligands bearing hydrosilane groups afforded P-silanol ligated ruthenium complexes [Ru(η⁶-cymene)(Ph₂PSi(CH₃)₂OH)Cl]Cl (5) and [Ru(η⁶-cymene)(Ph₂PSi(Ph)₂OH)Cl]Cl (6) via intramolecular hydroxylation of 1b and 1c. This process involves the formation of a reactive intermediate metal complex [(1b)Ru(η⁶-cymene)Cl₂] (A) which was structurally characterized. Complex 5 was subsequently converted to its neutral analog [Ru(η⁶-p-cymene)(Ph₂PSi(CH₃)₂O)Cl] (5′). Unlike alkoxy-phosphines, which are known to undergo tautomerism, silyl phosphine derivatives afford bench stable P,O-ligated ruthenium(II) complexes, indicating a unique reactivity profile of phosphine–silanols. Ruthenium complexes 3 and 5 were employed as molecular catalysts in the hydroboration of a series of aliphatic and aromatic aldehydes and ketones which yielded boronate esters. Mechanistic investigations revealed the formation of a silylated phosphine–ruthenium hydride species, which acts as the active catalyst. Furthermore, complex 3 displayed excellent chemoselective hydroboration of aldehydes over ketones at very low catalyst loadings (0.05 mol%) under solvent-free conditions.