Reactions of the titanaallene intermediate [Cp*Ti-2=C=CH2] with isonitriles: An approach to the chemistry of radialene type molecules

The titanaallene intermediate [Cp*Ti-2=C=CH2] (2), obtained via methane eli mination from Cp*Ti-2(CH=CH2)Me (1), reacts with an excess of cyclohexyliso nitrile (CyNC), to give the five-membered metallacycle Cp*2TiC(=NCy)C(=NCy) C(=CH2)C=NCy (4), which exhibits a radialene substructure. This reaction oc curs 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 az abutatriene complex (Cp*Ti-2(CNAr)(eta (2)-H2CC=C=NAr) (5). The X-ray struc ture analysis of 5 reveals a pentacoordinated geometry consisting of a eta (2)-C,C-azabutatriene moiety stabilized by a further ArNC ligand. Whereas 5 is stable in the solid state (dec 102-105 degreesC), in solution (20 degre esC) 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 inserti on of two isonitrile molecules into the Ti-CH2 bond in complex 5, to the fi ve-membered ring complex 8, Cp*2TiC(=NAr)C(=NAr)CH2C=C=NAr. The molecular s tructure of 8; shows a planar titanacyclopentane. Density functional theory calculations confirm the terminal C,C-coordination mode as found in 5 as t he preferred arrangement against the C,N- or internal C,C-coordination mode s.