EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

Abstract

In the context of homogeneous catalysis, openshell systems are often quite challenging to characterize. Nuclear magnetic resonance (NMR) spectroscopy is the most frequently applied tool to characterize organometallic compounds, but NMR spectra are usually broad, difficult to interpret and often futile for the study of paramagnetic compounds. As such, electron paramagnetic resonance (EPR) has proven itself as a useful spectroscopic technique to characterize paramagnetic complexes and reactive intermediates. EPR spectroscopy is a particularly useful tool to investigate their electronic structures, which is fundamental to understand their reactivity. This paper describes some selected examples of studies where EPR spectroscopy has been useful for the characterization of open-shell organometallic complexes. The paper concentrates in particular on systems where EPR spectroscopy has proven useful to understand catalytic reaction mechanisms involving paramagnetic organometallic catalysts. The expediency of EPR spectroscopy in the study of organometallic chemistry and homogenous catalysis is contextualized in the introductory Sect. 1. Section 2 of the review focusses on examples of C-C and C-N bond formation reactions, with an emphasis on catalytic reactions where ligand/substrate non-innocence plays an important role. Both carbon and nitrogen centered radicals have been shown to play an important role in these reactions. A few selected examples of catalytic alcohol oxidation proceeding via related N-centered ligand radicals are included in this section as well. Section 3 covers examples of the use of EPR spectroscopy to study important commercial ethylene oligomerization and polymerization processes. In Sect. 4 the use of EPR spectroscopy to understand the mechanisms of Atom Transfer Radical Polymerization is discussed. While this review focusses predominantly on the application of EPR spectroscopy in mechanistic studies of C-C and C-N bond formation reactions mediated by organometallic catalysts, a few selected examples describing the application of EPR spectroscopy in other catalytic reactions such as water splitting, photo-catalysis, photo-redox-catalysis and related reactions in which metal initiated (free) radical formation plays a role are included as well. EPR spectroscopic investigation in this area of research are dominated by EPR spectroscopic studies in isotropic solution, including spin trapping experiments. These reactions are highlighted in Sect. 5. EPR spectroscopic studies have proven useful to discern the correct oxidation states of the active catalysts and also to determine the effective concentrations of the active species. EPR is definitely a spectroscopic technique that is indispensable in understanding the reactivity of paramagnetic complexes and in conjunction with other advanced techniques such as X-ray absorption spectroscopy and pulsed laser polymerization it will continue to be a very practical tool.

Steric Protection of Rhodium‐Nitridyl Radical Species

“…Additionally, photolysis inside the EPR machine was studied at 20 K. To our surprise, this signal resembles a typical Rh II complex, with a signal pattern and g ‐values close to the ones found for earlier reported (square planar) Rh II species that can presumably be attributed to the formation of a transient or unstable Rh II species, not detectable under the above described photochemical reaction conditions (Figure S11) . Despite the more instructive spectrum obtained in this experiment, the signal intensity is again very low, and the signal is unlikely to be representative for the bulk material.…”

Solution X-Ray Absorption Spectroscopy (XAS) for Analysis of Catalytically Active Species in Reactions with Ethylene by Homogeneous (Imido)vanadium(V) Complexes—Al Cocatalyst Systems

“…On the basis of these analysis results and results in some reaction chemistry with the proposed model complexes (including direct use of the isolated species in the catalytic reaction) as well as computational analysis, we usually propose a catalysis mechanism. Moreover, the approach by electron spin resonance (ESR) spectroscopy not by NMR spectroscopy has been employed for the paramagnetic compounds [5,6], but the method lacks the quantitative analysis in addition to a possibility of formation of "ESR silent" species. Moreover, it is difficult to obtain (clear) structural information in solution by these (NMR and ESR) methods.…”

Steric Protection of Rhodium-Nitridyl Radical Species

“…Additionally, photolysis inside the EPR machine was studied at 20 K. To our surprise, this signal resembles a typical Rh II complex, with a signal pattern and g-values close to the ones found for earlier reported (square planar) Rh II species [14] that can presumably be attributed to the formation of a transient or unstable Rh II species, not detectable under the above described photochemical reaction conditions ( Figure S11). [15] Despite the more instructive spectrum obtained in this experiment, the signal intensity is again very low, and the signal is unlikely to be representative for the bulk material.…”