A Bridging bis-Allyl Titanium Complex: Mechanistic Insights into the Electronic Structure and Reactivity

Abstract

Treatment of the dinuclear compound [{Ti(η5-C5Me5)Cl2}2(μ-O)] with allylmagnesium chloride provides the formation of the allyltitanium(III) derivative [{Ti(η5-C5Me5)(μ-C3H5)}2(μ-O)] (1), structurally identified by single-crystal X-ray analysis. Density functional theory (DFT) calculations confirm that the electronic structure of 1 is a singlet state, and the molecular orbital analysis, along with the short Ti −Ti distance, reveal the presence of a metal −metal single bond between the two Ti(III) centers. Complex 1 reacts rapidly with organic azides, RN3 (R = Ph, SiMe3), to yield the allyl μ-imido derivatives [{Ti(η5-C5Me5)(CH2CH=CH2)}2(μ-NR)(μ-O)] [R = Ph(2), SiMe3(3)] along with molecular nitrogen release. Reaction of 2 and 3 with H2 leads to the μ-imido propyl species [{Ti(η5-C5Me5)(CH2CH2CH3)}2(μ-NR)(μ-O)] [R = Ph(4), SiMe3(5)]. Theoretical calculations were used to gain insight into the hydrogenation mechanism of complex 3 and rationalize the lower reactivity of 2. Initially, the μ-imido bridging group in these complexes activates the H2 molecule via addition to the Ti −N bonds. Subsequently, the titanium hydride intermediates induce a change in hapticity of the allyl ligands, and the nucleophilic attack of the hydride to the allyl groups leads to metallacyclopropane intermediates. Finally, the proton transfer from the amido group to the metallacyclopropane moieties affords the propyl complexes 4 and 5.