Mono and dinuclear tungsten alkenyl-carbyne complexes bridged by cyanide and diisocyanide ligands: Synthesis, electrochemical- and W-183-NMR studies

Neutral trans-cyanide alkenylcarbyne complexes 2a and 2b have been prepared by reaction of the complex 1a and 1b with NaCN or [Bu4N]CN. The reaction o f complexes 2a and 2b with an equimolar amount of the acetonitrile complexe s 1a and 1b in CH2Cl2 leads to the cationic cyanide-bridged bis(alkenylcarb yne) di-tungsten complexes 3a-d. Diisocyanide-bridged bis(alkenylcarbyne) d i-tungsten complexes 4a and 4b have been synthesized by the reaction of com plexes 1a and 1b with 0.5 equivalents of the diisocyanide 1,4-(CN)(2)C6H4. IR as well as H-1-, P-31{H-1}-, C-13{H-1}-, and W-183-NMR data are reported . The spectroscopic data show that in the dinuclear complexes 3a-d, the bri dging CN group and the alkenylcarbyne units are located in trans positions, while in the dinuclear complexes 4a and b, the isocyanide groups of the br idging ligand 1,4-(CN)(2)C6H4 and the two alkenylcarbyne moieties are cis. The W-183 chemical shifts of complexes 2a, 2b, 3a-d, 4a, and 4b were obtain ed through two-dimensional indirect P-31, W-183 NMR recording techniques. A downfield shifting of W-183 resonances of the cyanide-bridged dinuclear co mplexes 3a-d with respect to the mononuclear ones, 2a and 2b, was observed. The delta(183)W of isocyanide bridging dinuclear complexes 4a and 4b appea r at higher field than those of the corresponding mononuclear cyanide 2a an d 2b in accordance with the higher pi-acceptor electron properties of the i socyanide ligand. The electrochemical behaviour of all the complexes has be en investigated by cyclic voltammetry and controlled potential electrolysis in aprotic media and at a Pt (or vitreous C) electrode. Complexes 1, 2, or 3 undergo multi-electron irreversible oxidation processes involving anodic ally induced proton dissociation from the alkenylcarbyne ligands, and irrev ersible cathodic processes are also observed for all the complexes. The ord er of the redox potentials reflects that of the net electron pi-acceptor/si gma-donor character of the ligands and the ligating alkenylcarbynes are sho wn to behave as remarkably strong pi-electron accepters (even stronger than CO).