Use of the tetrahydroborate ligand as “gate-keeper” and protected hydrideligand: preparation and study of alkyl hydride and acyl hydride complexes of ruthenium(ii)

Abstract

Complex 3, [Ru(η²-BH₄)(CO)(Et)L₂] (L = PMe₂Ph) can be converted by nucleophiles L′ {a, PMe₂Ph; b, P(OMe)₃; c, Me₃CNC; d, CO} to alkyl and acyl complexes [Ru(η¹-BH₄)(CO)(Et)L₂L′] (4a), [Ru(η²-BH₄)(COEt)L₂L′] (5a–d), and [Ru(η¹-BH₄)(COEt)L₂L′₂] (7d and isomers 7c and 10c). Deprotection can then be achieved under conditions mild enough to allow study of the resulting alkyl hydride complexes [Ru(CO)(Et)HL₂L′] (1a, 1b) and acyl hydride complexes [Ru(COEt)HL₂L′₂] (8c, 8d) prior to elimination of ethane and propanal respectively, with formation of ruthenium(0) complexes [Ru(CO)L₂L′₂] (6a, 6b, 6d). With Me₃CNC, however, the final product is (depending on the solvent used) [Ru(CNCMe₃)₂{C(H)NCMe₃}(COEt)L₂] (9c) or [Ru(CNCMe₃)₃(COEt)L₂]⁺ (11c). Successive treatment of [Ru(η²-BH₄)(CO)HL₂], 2, with ethene and then CO yields propanal, but turning this into a catalytic cycle is hindered by the greater readiness of 5d to yield propanal non-catalytically (reacting with CO) than catalytically (reacting with H₂).