Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2

Abstract

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-NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-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.

Keywords: N-heterocyclic carbene; copolymerization of epoxide with CO2; density functional theory; hafnium; natural bond orbitals; titanium.

Scheme 1

Previously obtained complexes containing (i) saturated NHC with group 4, and (ii) unsaturated and benzannulated NHCs ligands (a and b, respectively) with zirconium.

Scheme 2

Synthesis of NHC-Ti complexes 1a, 1b, 2a and 2b.

Figure 1

Molecular structures of (a) 1a, (b) 1b-THF (isomer A), (c) 2a and (d) 2b. Hydrogen atoms and solvent molecules are omitted for clarity. Anisotropic displacement parameters (ADP’s) are given at the 50%pobability level.

Figure 2

(a) Natural orbitals calculated for the M1a and Ma fragments and the 1a complex. (b) Natural orbitals calculated for the M1b and Mb fragments and the 1b-THF complex. The interactions between fragment orbitals leading to hybrid, bonding, or antibonding orbitals of the complexes are indicated by dashed lines.

Figure 3

Natural orbitals calculated for the M1c and Mc fragments and the 1c complex. The interactions between fragment orbitals leading to hybrid, bonding, or antibonding orbitals of the complex are indicated by dashed lines.

Scheme 3

Preparation of NHC-Hf complexes 3a, 3a’, 3a’’, 3b and 4b.

Figure 4

Molecular structure of 3b. Hydrogen atoms and pentane solvent molecule are omitted for clarity. ADP’s are given at the 50% probability level.