Electrophilic Addition to Cyanide Ligands in Tungsten(II) Four-Electron-Donor Alkyne Complexes

Abstract

Extended reflux of Tp‘(CO)(I)W(RC⋮CR‘) (Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) complexes in ethyl acetate in the presence of silver cyanide produces Tp‘(CO)(CN)W(RC⋮CR‘) complexes in moderate to high yields. A single-crystal X-ray structure of Tp‘(CO)(CN)W(PhC⋮CMe) shows that the alkyne is aligned parallel to the M−CO axis. Electrophilic attack at the nitrogen of the cyanide ligand with methyl triflate or triflic acid yields cationic isocyanide complexes, whereas attack with HBF4 results in neutral BF3 adduct complexes. Coordination of the Tp‘(CO)(PhC⋮CMe)W+ fragment to the cyanide nitrogen of Tp‘(CO)(CN)W(HC⋮CBun) to form a CN-bridged dinuclear complex has been achieved. Variable-temperature 1H NMR measurements reflect acetylene rotation barriers in the Tp‘(CO)(L)W(HC⋮CH)n+ series (L = CO, CNH, CNMe {n = 1}; CN-, CNBF3- {n = 0}) and, thus, quantify π-effects for the cyanide and isocyanide ligands with respect to steric considerations. Extended Hückel molecular orbital calculations were carried out on a series of model complexes, [H3(CO)(L)W(HC⋮CH)]n- (L = CO, CNH {n = 1}; CN-, H- {n = 2}) to augment the experimental data.