Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization

Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.

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