Side-on Dinitrogen Complexes of Titanocenes with Disubstituted Cyclopentadienyl Ligands: Synthesis, Structure, and Spectroscopic Characterization

Reduction of the 1,3-disubstituted titanocene complexes,
(η5-C5H3-1,3-iPr2)2TiI or rac, meso-(η5-C5H3-1-iPr-3-Me)2TiI, with excess 0.5% sodium amalgam under an N2 atmosphere
furnished the corresponding titanocene dinitrogen compounds, [(η5-C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-N2) and [(η5-C5H3-1-iPr-3-Me)­Ti]2(μ2,η2,η2-N2). Crystallographic
studies on both molecules revealed side-on bound, [N2]2‑ ligands with N–N distances of 1.226(5) and
1.216(5) Å, respectively. Variable temperature magnetic susceptibility
studies established population of a triplet ground state at ambient
temperature that is slightly higher in energy than the singlet. Reducing
the size of the 1,3-cyclopentadienyl substituents to methyl groups,
[(η5-C5H3-1,3-Me2)2Ti], resulted in crystallization of a trimetallic titanium
dinitrogen complex with an activated μ3,η2,η1,η1-N2 ligand
with an N–N distance of 1.320(3) Å. Hydrogenation of the
isomeric titanocene dimethyl complex, (η5-C5H3-1,2-Me2)2TiMe2, in
the presence of dinitrogen did not result in N2 coordination
but rather furnished the bimetallic titanium compound, (η5-C5H3-1,2-Me2)2Ti­(μ2-H)­Ti­(η5-C5H3-1,2-Me2)­(η5,η1-C5H2-1,2-Me2), resulting from C–H
activation of a cyclopentadienyl ring position. Addition of PhCCPh
furnished (η5-C5H3-1,2-Me2)2Ti­(η2-PhCCPh), demonstrating
that the C–H bond activation event was reversible. By contrast,
a bridging formyl complex was obtained following addition of five
equivalents of CO, highlighting the availability of hydride insertion
chemistry.