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

Lakshmi Suresh,Jonas B Ekeli,Vidar R Jensen,Ralte Lalrempuia,Karl W Törnroos,Erwan Le Roux,Francis Gillis-D'Hamers

doi:10.3390/molecules25194364

Lakshmi Suresh, Jonas B Ekeli + Show 5 more

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https://doi.org/10.3390/molecules25194364

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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.

Highlights

In the past few decades, N-heterocyclic carbenes (NHCs) have emerged as privileged ancillary ligands that, in particular, have been explored in combination with low-to-medium valent late-transition-metalsMolecules 2020, 25, 4364; doi:10.3390/molecules25194364 www.mdpi.com/journal/moleculesMolecules 2020, 25, x FOR PEER REVIEWMolecules 2020, 25, 4364 transition-metals due to their strong σ-donor capacity, structural diversity, and their successful use in organometallic catalysis [1,2,3,4,5,6,7,8]
To further investigate the structural differences of the complexes and their relation to the electronic properties of the NHC ligands, we studied the ligands and the complexes using density functional theory (DFT) and natural bond orbital (NBO) analyses
The Ti-Ccarbene distances with which these ligands bind to the metal vary considerably, and these differences manifest themselves, via trans influence and other “ripple effects”, in significant variations in the other metal-ligand bond distances. These structural differences and their relation to the metal-NHC bonds and the electronic properties of the ligands have been studied for titanium complexes 1a, 1b-THF, and 1c using DFT and NBO analyses

Summary

Introduction

In the past few decades, N-heterocyclic carbenes (NHCs) have emerged as privileged ancillary ligands that, in particular, have been explored in combination with low-to-medium valent late-transition-metalsMolecules 2020, 25, 4364; doi:10.3390/molecules25194364 www.mdpi.com/journal/moleculesMolecules 2020, 25, x FOR PEER REVIEWMolecules 2020, 25, 4364 transition-metals due to their strong σ-donor capacity, structural diversity, and their successful use in organometallic catalysis [1,2,3,4,5,6,7,8]. High-valent early-transition-metal NHC complexes have due to their strong structural diversity, and successful in organometallic received much less σ-donor attentioncapacity,. NHC complexes have received much these metal centers [2,5,9,11,12,14] Their dissociation from oxophilic metals was partially less attention which to a large anionic extent iscarbon, due to nitrogen, their easeand of dissociation from these metal prevented by [9,10,11,12,13,14,15], designing multidentate oxygen-functionalized centers [2,5,9,11,12,14]. Anchoring ligands metals to oxophilic metals has proved by to designing multidentate anionic carbon, nitrogen, and oxygen-functionalized ligands [5,13,15,16,17]

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