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

Abstract

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-d 3 ), 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-d 3 led to an S = 3/2, Cr­(III) product (2-d 3 ) that maintained spectroscopic features in HYSCORE for the CD3 group, indicative of retention of the Cr-alkyl bond. Following protonolysis of 1-h 3 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.

Union carbide polymerization catalysts: from uncovering active site structures to designing molecularly-defined analogs

“…The CW EPR spectrum 41 of 4-SiO 2 ( Fig. 7A , black) shows the presence of HS (120–240 mT region) and a LS (300–400 mT region) Cr( iii ) species.…”

Controlling Polyethylene Molecular Weights and Distributions Using Chromium Complexes Supported by SNN-Tridentate Ligands

“…Therefore, the development of tunable catalytic systems that could produce different polyethylenes is highly desirable but challenging. , One of the industrially most relevant catalyst is the chromium-based Phillips catalyst (CrO x /SiO 2 ). − Discovered in 1951 by Hogan and Banks at Phillips Petroleum, this heterogeneous catalyst allows the manufacture of one-third of the high-density polyethylene (HDPE) commercialized, which is the most widely used polymer . Although the Phillips catalyst has been studied for over 70 years by both academic and industrial scientists, the nature of the initiation process, the coordination environment and the oxidation state of the metal in the active species, and the catalytic mechanism remain a hot and sometimes controversial topic. − Furthermore, much effort has been dedicated to the development of well-defined homogeneous Cr-based catalysts, ,− that are more amenable to detailed spectroscopic and structural studies than heterogeneous catalysts. In particular, multidentate ligands containing N, O, S, and/or P donor atoms have allowed a better control of the metal coordination sphere and a fine-tuning of its stereochemical features toward improved catalytic properties .…”

Alkene oligomerization via metallacycles: Recent advances and mechanistic insights

“…Similarly, Agapie and co-workers made a Pulse EPR analysis (HYSCORE & ENDOR) on a cationic Cr III system (Scheme 29). [71] After addition of ethylene (100eq.) on that complex, a Cr I species was detected (not fully characterized) among others.…”