Reactions of the Titanaallene Intermediate [Cp*2TiCCH2] with Isonitriles: An Approach to the Chemistry of Radialene Type Molecules

The titanaallene intermediate [Cp* 2 Ti C CH 2 ] ( 2 ), obtained via methane elimination from Cp* 2 Ti(CH CH 2 )Me ( 1 ), reacts with an excess of cyclohexylisonitrile (CyNC), to give the five-membered metallacycle ( 4 ), which exhibits a radialene substructure. This reaction occurs by a [2+1] addition followed by subsequent insertion of two molecules of isonitrile. A [2+1] addition product is isolated from the reaction of 1 with 2,6-dimethylphenylisonitrile (ArNC) in 1:2 ratio in the form of the azabutatriene complex [Cp* 2 Ti(CNAr)( η 2 -H 2 CC C NAr) ( 5 ). The X-ray structure analysis of 5 reveals a pentacoordinated geometry consisting of a η 2 -C,C-azabutatriene moiety stabilized by a further ArNC ligand. Whereas 5 is stable in the solid state (dec 102−105 °C), in solution (20 °C) one molecule of isonitrile is released. Subsequent intramolecular C−H bond activation and C−C bond formation afford product 7 . The latter can be directly isolated from reaction of 2 with 1 equiv of ArNC. The treatment of the titanaallene species 2 with 3 equiv of ArNC leads, through the insertion of two isonitrile molecules into the Ti−CH 2 bond in complex 5, to the five-membered ring complex 8, The molecular structure of 8 shows a planar titanacyclopentane. Density functional theory calculations confirm the terminal C,C-coordination mode as found in 5 as the preferred arrangement against the C,N- or internal C,C-coordination modes.