Characterization of Cr-Hydrocarbyl Species via Pulse EPR in the Study of Ethylene Tetramerization Catalysis

The characterization of complexes
involved in chromium catalysis
is challenging due to the paramagnetism of Cr in its common oxidation states. Here, we demonstrate
the utility of pulse electron paramagnetic resonance (pulse EPR) techniques
in assigning structural features of Cr organometallic complexes relevant
to ethylene tetramerization. An S = 3/2, Cr­(III)
bisaryl-methyl ethylene tetramerization precatalyst (1) has been selected for characterization by CW and pulse EPR spectroscopies.
Using an isotopically labeled Cr-CD3 complex (1-d3), the methyl ligand was confirmed to remain
bound to Cr in solution by detection of 2H couplings in
X-band hyperfine sublevel correlation (HYSCORE) spectroscopy. Protonolysis
of 1-d3 led to an S = 3/2, Cr­(III) product (2-d3) that maintained spectroscopic features in HYSCORE for the CD3 group, indicative of retention of the Cr-alkyl bond. Following
protonolysis of 1-h3 and subsequent
reaction with ethylene, an S = 1/2 Cr­(I) species
with an ethylene-derived ligand was generated, supporting a mechanism
involving this Cr oxidation state. Additionally, the pulse EPR characterization
of a Cr­(I) allyl-diene complex was performed for comparison. This
is the first direct observation of hydrocarbyl ligands on Cr using
pulse EPR methods. The methods described here are broadly applicable
to Cr, first-row transition metals and other open-shell organometallic
catalytic systems.