Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2
| Autor: | Ralte Lalrempuia, Lakshmi Suresh, Francis Gillis-D'Hamers, Jonas B. Ekeli, Karl W. Törnroos, Erwan Le Roux, Vidar R. Jensen |
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| Jazyk: | angličtina |
| Rok vydání: | 2020 |
| Předmět: |
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Molecular Grønn kjemi hafnium Molecular Conformation Pharmaceutical Science Article Catalysis Polymerization Analytical Chemistry Bærekraftig katalyse lcsh:QD241-441 chemistry.chemical_compound lcsh:Organic chemistry Coordination Complexes Heterocyclic Compounds Cyclohexenes Drug Discovery Polymer chemistry Moiety Molecule titanium Physical and Theoretical Chemistry Homoleptic Inorganic chemistry: 442 [VDP] density functional theory Utnyttelse av CO2 Green Chemistry natural bond orbitals Chemistry Ligand Organic Chemistry Utilization of CO2 Carbon Dioxide N-heterocyclic carbene copolymerization of epoxide with CO2 Uorganisk kjemi: 442 [VDP] Sustainable catalysis Chemistry (miscellaneous) Molecular Medicine Methane Carbene Cyclohexene oxide Natural bond orbital |
| Zdroj: | Molecules, Vol 25, Iss 4364, p 4364 (2020) Molecules Volume 25 Issue 19 |
| ISSN: | 1420-3049 |
| Popis: | Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal&ndash NHC bond distances and bond strengths are governed by ligand-to-metal &sigma and &pi donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC &pi acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC. |
| Databáze: | OpenAIRE |
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