Spectroscopic Interrogation of the Reduction of Model Chromium Precatalysts for Olefin Oligomerization

Chromium precatalysts typically undergo in situ or ex situ activation to enable olefin oligomerization. The activation processes significantly impact catalytic reactivity, but the conditions used often involve low concentrations of chromium, increasing the difficulty of obtaining mechanistic insights. Here, we describe investigations into the redox chemistry accessible to a family of CrIII precatalysts for ethylene oligomerization. Cyclic voltammetry (CV) studies of the CrIII complexes 1, 2, and 3 (Cr­(P,N)­Cl3(THF), where (P,N) represents one of a series of bidentate phosphorus- and nitrogen-containing ligands and THF is inner-sphere tetrahydrofuran) reveal single quasi-reversible reductions for each of the complexes, all near −1.5 V versus ferrocenium/ferrocene (Fc+/0). CV also indicates minor formation of heterogeneous material upon electrochemical reduction; these processes were quantified with electrochemical quartz crystal microbalance (EQCM) studies and X-ray photoelectron (XP) spectroscopy. Electron paramagnetic resonance (EPR; at X-band) studies of the paramagnetic CrIII complexes are reported as well as studies of the in situ reduction of the CrIII complexes. Treatment of the CrIII complexes with decamethyl­cobaltocene (Cp*2Co), triethylaluminum (AlEt3), or modified methylaluminoxane (MAO) primarily results in production of S = 2 CrII species. Minor quantities (ca. 1% conversion) of CrI species are also produced. The structure of the CrII form of 1 was obtained from single-crystal X-ray diffraction analysis and is consistent with the proposed reactivity pattern. Titration of 1 with Cp*2Co reveals isosbestic behavior in UV–vis spectra consistent with one-electron reduction of 1, confirming these findings. Notably, however, reduction of 1 with 2 equiv of Cp*2Co in the presence of AlEt3 reveals that the system can cleanly undergo further reduction by at least two electrons. Taken together, these results provide insights into the redox chemistry accessible to CrIII precatalysts for ethylene oligomerization.