Proton-induced coupling of a tertbutylisocyanide with a diethylaminocarbyne to yield an alkyne ligand at a single tungsten centre

Abstract

The tert-butylisocyanide substituted diethylaminocarbyne complex I(CO) 2(t-BuNC) 2 WCNEt 2 ( 1) reacts with one equivalent of HI in CH 2Cl 2 at room temperature to yield a mixture of complexes (I) 2(CO) 2(t-BuNC)W[η 2-(t-Bu)HN-CC-NEt 2] ( 2), major product, and (I) 2(CO)(t-BuNC) 2W[η 2-(t-Bu)HN-CC-NEt 2]( 3), minor product, separable by column chromatography on silica. The exclusive preparation of compound 3 can be achieved by treating I(CO)(t-BuNC) 3 WCNEt 2 ( 4) with one equivalent of HI in CH 2Cl 2 at room temperature. Complexes 2 and 3 contain a 4e-donor alkyne ligand resulting from the proton-induced coupling of one tert-butylisocyanide with the diethylaminocarbyne ligand at a single tungsten centre. Both complexes react with t-BuNC in refluxing CH 2Cl 2 to give quantitatively the cationic alkyne complex [I(CO)(t-BuNC) 3W[η 2-(t-Bu)HN-CC-NEt 2]] +I − ( 5). Thermal decarbonylation of this complex in toluene in the presence of t-BuNC at 90°C leads to the carbonyl-free alkyne complex [I(t-BuNC) 4W[η 2-(t-Bu)HN-CC-NEt 2]] +I − ( 6). The ionic nature of 5 and 6 has been confirmed by comparison of their conductivity data in 1,2-dichloroethane with NR 4 +X − salts (R = alkyl ; X = PF 6, Br, I). The composition and structure of the alkyne complexes 2, 3, 5 and 6 have been determined by total elemental analyses, IR, 1H NMR and 13C NMR spectroscopies and mass spectrometry. The spectroscopic results indicate a substantial bond delocalization and a hindered rotation of the diethylamino-group in the alkyne ligand. The barrier to this rotation is calculated to be ca 13.9-15.3 kcal mol −1 for compounds 2, 3, 5 and 6.