Synthesis, Structural Characterisation and Electrochemical Studies of Neutral Alkenylcarbyne Tungsten Complexes Bearing Chelating Bidentate and Tridentate Phosphanes

Abstract

Neutral alkenylcarbyne complexes [X = SCN; n = 1 (2a), 4 (2b). X = OCN; n = 1 (2c), 4 (2d)] were prepared by reaction of the acetonitrile complexes [n = 1 (1a), 4 (1b)] with M[OCN] or M[SCN] (M = Na, K, Bu4N), and were fully characterised. The synthesis of complex 3, containing the tridentate phosphane triphos [triphos = PPh(CH2CH2PPh2)2], is also described. The structures of 2b and 3 were determined by X-ray diffraction methods. In both complexes, the coordination of the W atom was octahedral, with the N-bonded thiocyanate (2b) or the chlorine (3) anions trans to the alkenylcarbyne group. The W−N and W−Cl bond lengths were significant longer than the average found in the literature; this indicates the remarkable trans influence of the alkenylcarbyne moiety. The electrochemical behaviour of complexes 2a−d and 3, as well as that of the related compounds D (2e: X = F, n = 4), (2f: X = Cl, n = 1), (2g: X = S2P(OEt)2, n = 1) and A (4: n = 1) was investigated by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprotic media and at a Pt electrode. They underwent irreversible anodic and cathodic processes, the former usually being multi-electronic in the time scale of CPE and involving anodically induced proton dissociation from the alkenylcarbynes. These ligands behave as remarkably strong π-electron acceptors and from the linear relationship between the oxidation potential of the first anodic wave and the electrochemical PL ligand parameter, it was possible to estimate, for the first time for a carbyne-containing metal centre, that is, the site {(dppe)(CO)2W≡C−R}, the values of the electrochemical electron richness (Es) and the polarisability (β) parameters. These values indicate that this site has low electron richness and low polarisability; this is accounted for by the extensive π-electron acceptance of the alkenylcarbyne ligand. PL was also estimated for the S2P(OEt)2− ligand, which was shown to act as a rather strong net electron releaser.