Abstract
In the context of homogeneous catalysis, open-shell 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.
Highlights
Organometallic chemistry and associated catalytic reactions have long been dominated by the study of diamagnetic complexes, using Nuclear magnetic resonance (NMR) as the main characterization technique
This paper describes some selected examples of studies where electron paramagnetic resonance (EPR) spectroscopy has been useful for the characterization of open-shell organometallic complexes
The expediency of EPR spectroscopy in the study of organometallic chemistry and homogenous catalysis is contextualized in the introductory Sect
Summary
Organometallic chemistry and associated catalytic reactions have long been dominated by the study of diamagnetic complexes, using NMR as the main characterization technique. Examples where the g-tensor can be misleading are some ligand-radical complexes of heavy transition metals such as iridium, for which SOC constants are large In such cases even a small spin density at the metal can cause a substantial g-anisotropy, underlining the importance of combining EPR spectroscopy with DFT property calculations to determine spin density distributions [5, 6]. For most metallo-enzymes, g-tensor and HFI information is (partially) lost due to g-strain and A-strain caused by a distribution of geometries with similar spectral features that overlap, leading to additional signal broadening This amplifies the necessity of advance pulse-techniques compared to many organometallic complexes measured in frozen solution.
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