Use of the tetrahydroborate ligand as "gate-keeper" and protected hydride ligand: preparation and study of alkyl hydride and acyl hydride complexes of ruthenium(II)

Complex 3, [Ru(eta2-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(eta1-BH4)(CO)(Et)L2L'] (4a), [Ru(eta2-BH4)(COEt)L2L'] (5a-d), and [Ru(eta1-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(eta2-BH4)(CO)HL2], , with ethene and then CO yields propanal, but turning this into a catalytic cycle is hindered by the greater readiness of to yield propanal non-catalytically (reacting with CO) than catalytically (reacting with H2).