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

Reduction of the 1,3-disubstituted titanocene complexes, (eta(5)-C5H3-1,3-Pr-i(2))(2)TiI or rac, meso-(eta(5)-C5H3-1-Pr-i-3-Me)(2)TiI, with excess 0.5% sodium amalgam under an N-2 atmosphere furnished the corresponding titanocene dinitrogen compounds, [(eta(5)-C5H3-1,3-Pr-i(2))(2)Ti](2)(mu(2),eta(2)eta(2)-N-2) and [(eta(5)-C5H3-1-Pr-i-3-Me)Ti](2)(mu(2),eta(2)eta(2)-N-2). Crystallographic studies on both molecules revealed side-on bound, [N-2](2-) ligands with N-N distances of 1.226(5) and 1.216(5) angstrom, 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, [(eta(5)-C5H3-1,3-Me-2)(2)Ti], resulted in crystallization of a trimetallic titanium dinitrogen complex with an activated mu(3),eta(2)eta(1),eta(1)-N-2 ligand with in N-N distance of 1.320(3) angstrom. Hydrogenation of the isomeric titanocene dimethyl complex, (eta(5)-C5H3-1,2-Me-2)(2)TiMe2, in the presence of dinitrogen did not result in N-2 coordination but rather furnished the bimetallic titanium compound, (eta(5)-C5H3-1,2-Me-2)(2)Ti(mu(2)-H)Ti(eta(5)-C5H3-1,2-Me-2)(eta(5),eta(1)-C5H2-1,2-Me-2), resulting from C-H activation of a cyclopentadienyl ring position. Addition of PhC CPh furnished (eta(5)-C5H3-1,2-Me-2)(2)Ti(eta(2)-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.