Bis(cyclopentadienyl) titanium dinitrogen chemistry: Synthesis and characterization of a side-on bound haptomer

The titanocene dinitrogen complex [(eta(5)-C5H2-1,2,4-Me-3)(2)Ti](2)(mu(2),eta(2),eta(2)-N-2) (1-N-2) has been prepared both by dihydrogen reductive elimination from the in situ generated monohydride derivative and by alkali metal reduction of the corresponding monoiodide. In the solid state, 1-N-2 contains a side-on bound N-2 ligand with an N-N bond length of 1.216(3) A, consistent with a significant contribution from a two-electron-reduced diazenido ([N-2](2-)) fragment. Attempts to maintain side-on coordination of the N-2 ligand but increase the wedge dihedral by introducing isopropyl or tert-butyl substituents induced a change in hapticity to end-on and a less activated bridging dinitrogen ligand. The ansa-titanocene dinitrogen complexes rac-[Me2Si(eta(5)-C5H2-2-SiMe3-4-Bu-t)(2)Ti](2)(mu(2),eta(1),eta(1)-N-2) (4-N-2) and [Me2Si(eta(5)-C5Me4)(eta(5)-C5H3-3-Bu-t)Ti](2)(mu(2),eta(1),eta(1)-N-2) (5-N-2) have also been prepared and crystallographically characterized and shown to have weakly activated, end-on dinitrogen ligands. The latter example coordinates two additional equivalents of N-2 upon cooling under a dinitrogen atmosphere in pentane solution.