Photochemistry of ansa-zirconocenes: ethylene-bis(1-indenyl)- and ethylene-bis(4,7-dimethyl-1-indenyl) zirconium dichlorides

This paper reports the full details of photochemical experiments on ansa ethylene-bis(indenyl) zirconium dichlorides derivatives. Photolysis of the 50 : 50 meso : racemic isomeric mixtures of ethylene-bis(1-indenyl) zirconium dichloride [EBIZrCl2] and ethylene-bis(4,7-dimethyl-1-indenyl) zirconium dichlorides [EBDMIZrCl2] produces a photostationary state made up of racemic (60%) and meso (40%) isomers. ESR spin trapping experiments showed the photogeneration of ligand- and zirconium-centered radicals. It is proposed that the meso-to-racemic photoconversion mechanism first involves the cleavage of one of the two metal-indenyl bonds. This generates a biradical species that quickly converts to the more stable products such as the racemic isomers of EBIZrCl2 and EBDMIZrCl2. The study of the electronic spectra of EBIZrCl2, EBDMIZrCl2 and comparison with those of Cp2ZrCl2, Cp-2*ZrCl2 and Ind(2)ZrCl(2) showed that the reactive excited states of ansa-zirconocenes are associated with LMCT transitions. (C) 1999 Elsevier Science S.A. All rights reserved.