Electrospray Ionization Mass Spectrometry Studies on the Mechanism of Hydrosilylation of Terminal Alkynes Using an N-Heterocyclic Carbene Complex of Iridium, Allow Detection/Characterization of All Reaction Intermediates^⊥

A new pyridine-4,5-dichloroimidazol-2-ylidene complex of Ir(I) has been obtained, and its catalytic activity toward hydrosilylation of phenylacetylene and 4-aminophenylacetylene has been studied. The addition of HSiMe2Ph to the latter Ir(I) species afforded the corresponding IrIII-NHC hydrosilylated species, whose molecular structure was determined. This species and its nonchlorinated analogue have been used as catalyst of a series of reactions designed for elucidating the catalytic processes. The mechanism of the catalytic hydrosilylation of phenylacetylene has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and tandem MS/MS, allowing the detection of all the reaction intermediates leading to both the hydrosilylation and the dehydrogenative-silylation products. The ratio of the silylation/hydrosilylation products depends on the excess of phenylacetylene added to the reaction mixture.