Synthesis, Characterization and Reactivity Study of Bis (Imino)-N-Heterocyclic Carbene Transition Metal Complexes

Three generations of the first 1,3-bis(imino) N-heterocyclic carbene (NHC) ligand precursors were synthesized, isolated and characterized. The synthetic methodologies of the ligand precursors were controlled by the iminic carbon substituents. The corresponding complexes of Cr(III), Fe(II), Co(II), Pd(II), and Zn(II) were prepared from the in situ deprotonation of the NHC ligand precursors or from the related Cu(I) or Ag(I) adducts. The NHC ring fragment and iminic carbon substituents had a significant impact on the solid-state structure of these complexes in which mono-, bi- and tridentate coordination modes were observed.
The catalytic activities of chromium, iron and cobalt complexes of 1,3-bis(imino) NHC ligands were evaluated in ethylene polymerization. The activities of chromium(III) complexes of imidazol-2-ylidene showed slightly enhanced activities with a relatively electron-poor phenyl group (compared to methyl) installed on the iminic carbons. These results suggest that a decrease in the electron-donating or an increase in the π-accepting capability of the ligand may produce more active olefin polymerization catalysts. The ligand scaffold was then modified by introducing a benzimidazole moiety to reduce σ-electron donating and increase the π-accepting ability of the ligand and this may lead to a more electropositive metal center. Although these ligands were designed as a tridentate ligand, such coordination mode could not be achieved in the transition metal complexes of imidazole-2-ylidene and benzimidazol-2-ylidene. Steric and electronic parameters perhaps prevent them from adopting this coordination fashion. The five-membered ring of the carbene was then replaced by a six-membered ring of pyrimidin-2-ylidene to achieve a tridentate coordination mode. DFT calculations were performed to assess the electronic properties of the bis(imino)-NHC ligands. The pyrimidin-2-ylidene and the benzimidazol-2-ylidene are predicted to be the best σ–donor and the best π-acceptor of these NHC ligands based on their energy of the highest occupied and the lowest unoccupied molecular orbitals, respectively.