Structure and reactivity of four-membered titanacycles - theoretical studies

The equilibrium geometries of a series of four-membered titanacycles with e xocyclic methylene group Cl2Ti-X-Y-C=CH2 (X:CH2, CH, O, S, N, P; Y: CH2, CH , C=O, C=CH2, N-CH3,) and two derivatives without exocyclic methylene group Cl2Ti-X-CH2-CH2 (X: CH2 O) have been calculated at the HF level of theory by using an effective core potential basis set. The geometries were compare d with data from X-ray structures for the corresponding compound Cp-2*Ti-X- Y-C=CH2. The structures of the planar four-membered titanacycles are well r epresented by the model complexes. The chloride ligands are good theoretica l substitutes for the bent metallocene system with cyclic ligand systems in the intersecting plane between the Cp* ligands. The calculated metal-carbo n bond distances are found to be shorter than in the X-ray structures of th e real molecules. Total energies are calculated at the MP2 level and are us ed to predict the reactivity of the compounds. Titanacyclobutanes with an e xocyclic methylene group are more stable than titanacyclobutanes without th is group. Oxatitanacyclobutanes are subject to cycloreversion under formati on of Cl2Ti=O and allene. Azatitanacyclobutenes undergo spontaneous ring op ening reactions. These statements are further supported by the analysis of the molecular orbitals of some selected derivatives. (C) 1999 Elsevier Scie nce S.A. All rights reserved.