p-Aryl-Substituted Iminopyridine Chromium Complexes as Precatalysts for Ethylene Polymerization: The Question of the Physical Oxidation State Examined by Experimental and Density Functional Study in the Absence of Structural Data

Abstract

Investigations into iminopyridine chromium complexes have unveiled their potential as precatalysts for the polymerization of (di)olefins (Organometallics 2018, 37, 4827–4840). A Cr-to-ligand electron transfer, with the ligand (L) in the monoanionic radical state and Cr in a physical trivalent oxidation state, proved to be fundamental for facilitating ethylene polymerization by the complex with the unsubstituted aldimine (Cr-pH, where X = H). In the absence of structural data, we embarked on a detailed study by ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy and density functional theory to elucidate the structural and electronic features driving electron transfer in Cr-pH. The Cr-to-L electron transfer is facilitated by the presence of two tetrahydrofurans in the metal’s coordination sphere in the monoligated form or by the formation of a bis-ligated species. The UV–vis–NIR spectra of Cr-pH, coupled with electrospray ionization mass spectroscopy data, indicate the coexistence of multiple species. Successively, we enlarged the library of chromium complexes by appending different substituents (i.e., CH₃, OCH₃, and CF₃) at the p-N-aryl 4-position. While Cr-pCH₃ and Cr-pOCH₃ display electronic features analogous to those of Cr-pH, Cr-pCF₃ stands out as an exception, likely containing only monoligated species. Cr-pCF₃ emerged as the most active catalyst, producing ultra-high-molecular weight poly(ethylene) with a unimodal and narrow molecular weight distribution event at 40 °C.