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

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.