Structure and reactivity of four-membered titanacycles—theoretical studies

Abstract

The equilibrium geometries of a series of four-membered titanacycles with exocyclic methylene group Cl 2 Ti–X–Y–C CH 2 (X: CH 2, CH, O, S, N, P; Y: CH 2, CH, CO, CCH 2, N–CH 3) and two derivatives without exocyclic methylene group Cl 2 Ti–X–CH 2–C H 2 (X: CH 2, O) have been calculated at the HF level of theory by using an effective core potential basis set. The geometries were compared with data from X-ray structures for the corresponding compound Cp* 2 Ti–X–Y–C CH 2. The structures of the planar four-membered titanacycles are well represented by the model complexes. The chloride ligands are good theoretical substitutes for the bent metallocene system with cyclic ligand systems in the intersecting plane between the Cp* ligands. The calculated metal–carbon bond distances are found to be shorter than in the X-ray structures of the real molecules. Total energies are calculated at the MP2 level and are used to predict the reactivity of the compounds. Titanacyclobutanes with an exocyclic methylene group are more stable than titanacyclobutanes without this group. Oxatitanacyclobutanes are subject to cycloreversion under formation of Cl 2TiO and allene. Azatitanacyclobutenes undergo spontaneous ring opening reactions. These statements are further supported by the analysis of the molecular orbitals of some selected derivatives.