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 s- and p-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC p-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.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

“…However, this weak bond may be enhanced by harnessing anionic linkers. This strategy has allowed the isolation of a number of interesting metal complexes − of the early transition metals, − the lanthanides, − and the actinides. − …”

A Bifunctional NHC-Aryloxido Titanium Catalyst for the Ring-Opening Polymerization of ε-Caprolactone and an Unusual Fragmentation of Its Ligand Backbone

“…4 Its benzimidazol-2-ylidene analogue C does the same. 5 The [Ti]−OPr i complex of B catalyzes the ringopening polymerization (ROP) of rac-lactide. 6 Homoleptic D fits two ( Ar OCO Ar ) 2− units onto a single Ti center.…”

Metallocene catalysts for the ring-opening co-polymerisation of epoxides and cyclic anhydrides

“…[39][40][41] However, there are only limited reports of the Group 4 metals (Ti, Zr, Hf ) being used in this transformation; perhaps surprising given there are multiple studies detailing their efficacy in the ROP of cyclic esters [42][43][44][45][46] and the ROCOP of epoxides and CO 2 . [47][48][49][50][51][52][53][54][55] In 1999, Endo and co-workers reported the use of titanium alkoxide catalysts in the ROCOP of oxetane and anhydrides, whilst heterogeneous zirconium phosphate materials with intercalated amines have also been demonstrated to catalyse the ROCOP of glycidyl phenyl ether and hexahydro-4-methylphthalic anhydride. [56][57][58][59] Furthermore, Chakraborty, Chand and colleagues have detailed the use of 8-hydroxyquinolinate based ligands complexed to titanium and zirconium in the copolymerisation of maleic anhydride and tert-butyl glycidyl ether.…”