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

Preparation and Reactivity Study of Chromium(III), Iron(II), and Cobalt(II) Complexes of 1,3-Bis(imino)benzimidazol-2-ylidene and 1,3-Bis(imino)pyrimidin-2-ylidene

Chromium­(III), iron­(II), and cobalt­(II) complexes of bis­(imino)­benzimidazol-2-ylidene and bis­(imino)­pyrimidin-2-ylidene were successfully prepared by reaction of either the benzimidazolium or pyrimidinium salts or the corresponding copper complexes with the respective metal halide. X-ray diffraction analysis of the Cr­(III) complex of the pyrimidin-2-ylidene ligand demonstrated, for the first time, the ability of bis­(imino)­carbene-type ligands to coordinate to metal centers in a tridentate fashion. The coordination mode of these ligands was surprisingly highly dependent on the nature of both the metal and the ligand itself. The activity of these complexes in ethylene polymerization was assessed under ambient conditions (room temperature and 1 atm of C₂H₄) using methylaluminoxane as cocatalyst. In contrast to the iron and cobalt complexes, both chromium complexes were active in ethylene polymerization

Preparation and Reactivity Study of Chromium(III), Iron(II), and Cobalt(II) Complexes of 1,3-Bis(imino)benzimidazol-2-ylidene and 1,3-Bis(imino)pyrimidin-2-ylidene

Chromium­(III), iron­(II), and cobalt­(II) complexes of bis­(imino)­benzimidazol-2-ylidene and bis­(imino)­pyrimidin-2-ylidene were successfully prepared by reaction of either the benzimidazolium or pyrimidinium salts or the corresponding copper complexes with the respective metal halide. X-ray diffraction analysis of the Cr­(III) complex of the pyrimidin-2-ylidene ligand demonstrated, for the first time, the ability of bis­(imino)­carbene-type ligands to coordinate to metal centers in a tridentate fashion. The coordination mode of these ligands was surprisingly highly dependent on the nature of both the metal and the ligand itself. The activity of these complexes in ethylene polymerization was assessed under ambient conditions (room temperature and 1 atm of C₂H₄) using methylaluminoxane as cocatalyst. In contrast to the iron and cobalt complexes, both chromium complexes were active in ethylene polymerization

Synthesis, Characterization, and Ethylene Polymerization Studies of Chromium, Iron, and Cobalt Complexes Containing 1,3-Bis(imino)-N-Heterocyclic Carbene Ligands

New chromium­(III), iron­(II), and cobalt­(II) complexes of acyclic 1,3-bis­[(2,6-dimethylphenylimino)­ethyl and benzyl]­imidazol-2-ylidene were prepared from the corresponding silver or copper adduct as transmetalating agent or from in situ deprotonation of the parent imidazolium salt. The catalytic activities of all three complexes were evaluated for ethylene polymerization at atmospheric pressure and room temperature with activation by methylaluminoxane. The Cr­(III) complexes were found to be the most active, with a rate of 35 kg PE mol^–1 Cr h^–1

Synthesis, Characterization, and Ethylene Polymerization Studies of Chromium, Iron, and Cobalt Complexes Containing 1,3-Bis(imino)-N-Heterocyclic Carbene Ligands

New chromium­(III), iron­(II), and cobalt­(II) complexes of acyclic 1,3-bis­[(2,6-dimethylphenylimino)­ethyl and benzyl]­imidazol-2-ylidene were prepared from the corresponding silver or copper adduct as transmetalating agent or from in situ deprotonation of the parent imidazolium salt. The catalytic activities of all three complexes were evaluated for ethylene polymerization at atmospheric pressure and room temperature with activation by methylaluminoxane. The Cr­(III) complexes were found to be the most active, with a rate of 35 kg PE mol^–1 Cr h^–1

Ammonia Activation by a Nickel NCN-Pincer Complex featuring a Non-Innocent N-Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

A Ni0-NCN pincer complex featuring a six-membered N-heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its NiII precursor. It retained chloride in the square-planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni0 complex rapidly activated ammonia at room temperature, in a ligand-assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent-dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non-innocent behavior of the NHC ligand.peerReviewe