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.