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.