Permethyltitanocene derivatives with naked chalcogen ligands: Synthesis of [(Cp₂*Ti)₂(μ-E)] and [Cp ₂*Ti(η²-E₂)] and the role of the terminal chalcogenides [Cp₂*Ti(E)] in their interconversion (E = Se, Te)

Permethyltitanocene hydride, [Cp₂*TiH], reacts with elemental selenium or tellurium to give the products [(Cp₂*Ti) ₂(μ-E)] (E = Se, 1a; Te, 1b), [Cp₂*Ti(η ²-E₂)] (E = Se, 2a; Te, 2b) and [Cp ₂*Ti(η²-Se₃)] (3), depending on the equivalency of the chalcogen employed. Dinuclear compounds 1 are paramagnetic and have D_2d (idealized) structures, as shown by X-ray structural analysis of μ-telluride 1b; they may be converted to diamagnetic dichalcogenides 2 through further reaction with the appropriate chalcogen. Derivatives 2 are monomeric in the solid state, as shown by X-ray structural analysis of ditelluride 2b, and in solution, as demonstrated by multinuclear NMR spectroscopy. Combination of diselenide 2a and ditelluride 2b results in partial redistribution to the mixed species [Cp₂*Ti(η ²-SeTe)], suggesting dimeric structures of formula [Cp ₂*Ti(μ-E-E)₂TiCp₂*] may be accessible in solution. The dichalcogenides and the triselenide may be converted back to complexes 1 by treatment with a chalcogen-abstracting agent. The possible involvement of monomeric terminal chalcogenides [Cp₂* Ti(E)] in the interconversion of 1 and 2 was probed experimentally and computationally by means of Density Functional Theory calculations on [Cp ₂M(E)] (M = Ti, E = O, S, Se, Te; M = Zr, E = O, Te). Several unsuccessful attempts to generate and trap [Cp₂*Ti(Te)] are described. The results of these studies suggest that [Cp₂* Ti(Te)] has a very weak Ti-Te bond and a readily accessible triplet excited state. These factors, along with the small size of titanium, render this member of the [Cp₂*M(E)] family of complexes difficult to trap with Lewis bases, in contrast to many other congeners in the series of Group 4 terminal chalcogenides.