Synthesis of a Base-Free Titanium Imido and a Transient Alkylidene from a Titanocene Dinitrogen Complex. Studies on TiNR Hydrogenation, Nitrene Group Transfer, and Comparison of 1,2-Addition Rates

The synthesis and reactivity of the end-on bound dinitrogen complex [(η5-C5H3-1,3-(SiMe3)2)2Ti]2(μ2,η1,η1-N2) is described. The solid state structure of the dinitrogen compound reveals a weakly activated end-on bound N2 ligand with an N−N bond length of 1.164(5) Å. Displacement of the N2 ligand by organic azides has been used to prepare monomeric, base-free titanocene imido complexes, (η5-C5H3-1,3-(SiMe3)2)2TiNR (R = SiMe3, 2,4,6-Me3-C6H2). While unreactive toward C−H bonds, the Ti−N linkage is readily hydrogenated and participates in group transfer reactions with unsaturated organic molecules such as carbon monoxide and benzophenone. Reaction of the N2 complex with Ph2CN2 allowed isolation of (η5-C5H3-1,3-(SiMe3)2)2Ti(N2CPh2), which exists as a mixture of interconverting η2 and η1 isomers in solution. The diazoalkane complex also participates in "imido-like" reactivity, producing (η5-C5H3-1,3-(SiMe3)2)2Ti(NHNCPh2)H upon addition of H2. Changing the diazoalkane to Me3SiCHN2 resulted in isolation of the double cyclometalated titanocene (η5-C5H3-η1-1-SiMe2CH2-3-SiMe3)2Ti, arising from facile intramolecular C−H activation of the cyclopentadienyl substituent by a transient titanocene alkylidene.