Cyanoacetylenes and cyanoacetylides: versatile ligands in organometallic chemistry

Abstract

The nitrile like lone pair of the cyanoacetylene PhCCCN (1) has been found to coordinate readily to the Ru(PPh₃)₂Cp fragment, to give [Ru(NCCCPh)(PPh₃)₂Cp]PF₆ (2) which may be considered as an “extended” derivative of the more common benzonitrile complex [Ru(NCPh)(PPh₃)₂Cp]PF₆ (3). Reaction of 1 with [Co₂(CO)₆(dppm)] readily forms the μ,η² complex [Co₂(μ,η²-PhC₂CN)(CO)₄(dppm)] (4), which reacts with [RuCl(PPh₃)₂Cp] to give the mixed metal species [{Co₂(μ,η²-PhC₂CN){Ru(PPh₃)₂Cp}(CO)₄(μ-dppm)}]PF₆ (5). The η¹(N) bonded PhCCCN ligand is labile, being displaced by NCMe at ambient temperature to afford [Ru(NCMe)(PPh₃)₂Cp]PF₆, or by tcne to give trans-[{Ru(PPh₃)₂Cp}₂(μ-tcne)}][PF₆]₂ (9). The metallocyanoacetylide [Ru(CCCN)(PPh₃)₂Cp] (6) was prepared by lithiation (BuLi) of [Ru(CCH)(PPh₃)₂Cp] followed by treatment with PhOCN. Coordination of the metal fragments Ru(PPh₃)₂Cp or Fe(dppe)Cp to the N terminus in 6 occurs readily to give the homo- or hetero-bimetallic cations [{Cp(PPh₃)₂Ru}(μ-CCCN){ML₂Cp}]⁺, which were isolated as the PF₆ salts [ML₂Cp = Ru(PPh₃)₂Cp (7); Fe(dppe)Cp (8)]. The crystal structures of 2–5, 7 and 9 are reported. The electrochemical response of these complexes suggests there are considerable electronic interactions between the heterometallic end-caps in 8 through the polarised C₃N bridge.