<i>p</i>‑Aryl-Substituted Iminopyridine\nChromium Complexes as Precatalysts for Ethylene Polymerization: The\nQuestion of the Physical Oxidation State Examined by Experimental\nand Density Functional Study in the Absence of Structural Dat

Investigations into iminopyridine chromium complexes\nhave unveiled\ntheir potential as precatalysts for the polymerization of (di)olefins\n(<i>Organometallics</i> <b>2018</b>, <i>37</i>, 4827–4840). A Cr-to-ligand electron transfer, with the ligand\n(L) in the monoanionic radical state and Cr in a physical trivalent\noxidation state, proved to be fundamental for facilitating ethylene\npolymerization by the complex with the unsubstituted aldimine (<b>Cr</b>-<i>p</i><b>H</b>, where X = H). In the\nabsence of structural data, we embarked on a detailed study by ultraviolet–visible–near-infrared\n(UV–vis–NIR) spectroscopy and density functional theory\nto elucidate the structural and electronic features driving electron\ntransfer in <b>Cr</b>-<i>p</i><b>H</b>. The\nCr-to-L electron transfer is facilitated by the presence of two tetrahydrofurans\nin the metal’s coordination sphere in the monoligated form\nor by the formation of a bis-ligated species. The UV–vis–NIR\nspectra of <b>Cr</b>-<i>p</i><b>H</b>, coupled\nwith electrospray ionization mass spectroscopy data, indicate the\ncoexistence of multiple species. Successively, we enlarged the library\nof chromium complexes by appending different substituents (i.e., CH<sub>3</sub>, OCH<sub>3</sub>, and CF<sub>3</sub>) at the <i>p</i>-<i>N</i>-aryl 4-position. While <b>Cr</b>-<i>p</i><b>CH</b><sub><b>3</b></sub> and <b>Cr</b>-<i>p</i><b>OCH</b><sub><b>3</b></sub> display\nelectronic features analogous to those of <b>Cr</b>-<i>p</i><b>H</b>, <b>Cr</b>-<i>p</i><b>CF</b><sub><b>3</b></sub> stands out as an exception, likely\ncontaining only monoligated species. <b>Cr</b>-<i>p</i><b>CF</b><sub><b>3</b></sub> emerged as the most active\ncatalyst, producing ultra-high-molecular weight poly(ethylene) with\na unimodal and narrow molecular weight distribution event at 40 °C.