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

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}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}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.