Isocyanide Insertion Reactions with Organometallic Group 4 Tropocoronand Complexes:  Formation of η²-Iminoacyl, Enediamido, η²-Imine, and μ-Imido Products

The reactivity of the tropocoronand complexes [M(TC-3,n)R₂] (M = Zr(IV), Hf(IV); n = 3, 5; R = CH₂Ph, Ph) with aryl and alkyl isocyanides is described. Alkyl isocyanides induced migration of both benzyl groups to afford the bis(iminoacyl) species [Hf(TC-3,n){η²-(R)NC(CH₂Ph)}₂] (n = 5, R = Cy (1), n-Bu (2); n = 3, R = Cy (3)). Different products were isolated with aryl isocyanides, depending on the rate of substrate addition. Treatment of [M(TC-3,n)(CH₂Ph)₂] with excess 2,6-dimethylphenyl isocyanide (ArNC) generated the enediamido = Zr (4), Hf (5)) via coupling of two bis(iminoacyl) groups, whereas slow step-wise addition produced = Zr, n = 3 (6); M = Hf, n = 3 (7), n = 5 (8)). The η²-imine an intermediate in the formation of 7, was isolated and characterized, but the corresponding zirconium species was unstable in solution, decomposing to form the imido-bridged compound {[Zr(TC-3,3)]₂(μ-NAr)₂} (10). Altering the nature of the carbon donor ligands also influenced the reactivity. Addition of both aryl and alkyl isocyanides to solutions of [Zr(TC-3,3)Ph₂] (11) afforded the η²-imine complexes R = Cy (12), Ar (13)), the stability of which may be due in part to interactions between the metal center and a phenyl ring. Electron-withdrawing R substituents enhance this effect. All new compounds were characterized by X-ray crystallography, and plausible mechanisms for the migration reactions are outlined.