Coupling of σ-acetylide ligands at Group 4 metallocene complexes to yield methylenecyclopropene-type frameworks

Treatment of a variety of bis(alkynyl)metallocenes Cp2M(-C=C-R)(2) (M = Ti, Zr, Hf; R = CH3, n-C3H7, n-C4H10, cyclo-C6H11) with tris(pentatluorophenyl)borane yields the metallocene(mu-RC4R)borate betaines 3. These seem to be in an endothermic equilibrium with their methylenecyclopropene derived isomers-by means of an intramolecular alkyne insertion reaction-which are very effectively trapped by the reaction with tert-butylisocyanide to yield the complexes 6 that exhibit a methylenecyclopropane derived sigma-ligand framework. The zirconocene compounds 6a (R = CH3), 6c (R = n-C3H7), 6d (R = n-C4H9), and 6e (R = cyclo-C6H11) were characterized by X-ray crystal structure analyses. The hydrolysis of 6a, 6c and 6d gave the respective metal free systems 7 that exhibit a pronounced cyclopropenylium-borare-betaine character. (C) 1999 Elsevier Science S.A. All rights reserved.