Electron paramagnetic resonance (EPR) spectroscopy is an extremely versatile technique for the characterisation of homogeneous catalysts involving paramagnetic centres. The paramagnetic parent pre-catalyst, the activated catalyst itself or any resulting reactive intermediates, can all be monitored in situ in order to delineate the role of the formal metal oxidation state and the influence of ligand structure on the resulting catalytic activity. In this review of our recent results, we describe how highly reactive, low valent transition metal complexes of formal oxidation states Ni(I), Fe(I) and Cr(I) coordinated by N-heterocyclic carbene (NHC) or phosphine ligands and which are active for a range of cross-coupling reactions or ethylene oligomerisation, have been investigated by EPR. Analysis of the EPR spectra revealed (i) how the influence of the NHC ring size affects the electronic properties of the Ni(I) centre, (ii) how low spin Fe(I) intermediates are catalytically competent on-cycle species in cross-coupling reactions and (iii) how intramolecular structural rearrangements involving Cr(I) centres occur following the addition of a co-catalyst to the reaction medium. These results demonstrate the utility of EPR to probe the structure–reactivity relationships in paramagnetic homogeneous catalysts, providing information not readily accessible by other techniques.

The Jahn–Teller distorted Cu(II) complex [Cu(en)2](OTf)2 1 (en = 1,2-diaminoethane) has been reported and characterised using X-ray crystallography, EPR and ENDOR spectroscopy, and DFT calculations. The solid state structure shows an intra- and inter-molecular hydrogen-bonded network via the N–H groups and the coordinated triflate anions. CW and pulsed EPR/ENDOR were used to determine the spin Hamiltonian parameters of the Cu(II) complex, which were in excellent agreement with the DFT. The structure of the complex, as determined by angular selective ENDOR, is also in good agreement with the crystal structure, confirming the axial coordination of the counter-ion(s) in the frozen solution. The small 14N superhyperfine couplings are also consistent with the sp3 hybridised nature of the coordinating nitrogens. These results show that the correlation between the 14N hyperfine coupling and hybridisation of donor nitrogens can be useful to determine not only the coordination around the Cu(II) metal centre but also the nature of the donor in unknown Cu(II) systems.

ENDOR spectroscopy and DFT calculations have been used to thoroughly investigate the ligand hyperfine couplings for the bis(acetylacetonato)–copper(II) complex [Cu(acac)2] in frozen solution. Solutions of [Cu(acac)2] were prepared under anhydrous conditions, and EPR revealed that the g and CuA values were affected by traces of water present in the solvent. The ligand HAi hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons (HAi = 1.35, −1.62, −2.12 MHz; aiso = −0.80 MHz) and smaller couplings to the fully averaged methyl group protons (HAi = −0.65, 1.658, −0.9 MHz; aiso = 0.036 MHz) were identified by simulation of the angular selective ENDOR spectra and confirmed by DFT. Since the barrier to methyl group rotation was estimated to be ca. 5 kJ mol−1 by DFT, rapid rotation of these –CH3 groups, even at 10 K, leads to an averaged value of HAi. However, variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. Using CW Q-band ENDOR, these additional couplings were characterised by the hyperfine parameters HAi = 3.45, 2.9, 2.62 MHz, aiso = 2.99 MHz and assigned to a hindered methyl group rotation. This hindered rotation of a sub-set of methyl groups occurs in 120° jumps, such that a large Adip and aiso component is always observed. Whilst the majority of the methyl groups undergo free rotation, a sub-set of methyl groups experience hindered rotation in frozen solution, through proton tunnelling. This hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.

X- and Q-band EPR and ENDOR spectroscopy was used to study the structure of a series of heteroleptic and homoleptic copper bis(oxazoline) complexes, based on the (−)-2,2′-isopropylidenebis[(4S)-4-phenyl-2-oxazoline] ligand and bearing different counterions (chloride versus triflate); labelled [CuII(1a–c)]. The geometry of the two heteroleptic complexes, [CuII(1a)] and [CuII(1c)], depended on the choice of counterion. Formation of the homoleptic complex was only evident when the CuII(OTf)2 salt was used (CuII(Cl)2 inhibited the transformation from heteroleptic to homoleptic complexes). The hyperfine and quadrupole parameters for the surrounding ligand nuclei were determined by ENDOR. Well resolved 19F and 1H couplings confirmed the presence of both coordinated water and TfO− counterions in [Cu(1a)].

X- and Q-band EPR and ENDOR spectroscopy was used to study the structure of a series of heteroleptic and homoleptic copper bis(oxazoline) complexes, based on the (−)-2,2′-isopropylidenebis[(4S)-4-phenyl-2-oxazoline] ligand and bearing different counterions (chloride versus triflate); labelled [CuII(1a–c)]. The geometry of the two heteroleptic complexes, [CuII(1a)] and [CuII(1c)], depended on the choice of counterion. Formation of the homoleptic complex was only evident when the CuII(OTf)2 salt was used (CuII(Cl)2 inhibited the transformation from heteroleptic to homoleptic complexes). The hyperfine and quadrupole parameters for the surrounding ligand nuclei were determined by ENDOR. Well resolved 19F and 1H couplings confirmed the presence of both coordinated water and TfO− counterions in [Cu(1a)].

ENDOR spectroscopy and DFT calculations have been used to thoroughly investigate the ligand hyperfine couplings for the bis(acetylacetonato)–copper(II) complex [Cu(acac)2] in frozen solution. Solutions of [Cu(acac)2] were prepared under anhydrous conditions, and EPR revealed that the g and CuA values were affected by traces of water present in the solvent. The ligand HAi hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons (HAi = 1.35, −1.62, −2.12 MHz; aiso = −0.80 MHz) and smaller couplings to the fully averaged methyl group protons (HAi = −0.65, 1.658, −0.9 MHz; aiso = 0.036 MHz) were identified by simulation of the angular selective ENDOR spectra and confirmed by DFT. Since the barrier to methyl group rotation was estimated to be ca. 5 kJ mol−1 by DFT, rapid rotation of these –CH3 groups, even at 10 K, leads to an averaged value of HAi. However, variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. Using CW Q-band ENDOR, these additional couplings were characterised by the hyperfine parameters HAi = 3.45, 2.9, 2.62 MHz, aiso = 2.99 MHz and assigned to a hindered methyl group rotation. This hindered rotation of a sub-set of methyl groups occurs in 120° jumps, such that a large Adip and aiso component is always observed. Whilst the majority of the methyl groups undergo free rotation, a sub-set of methyl groups experience hindered rotation in frozen solution, through proton tunnelling. This hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.

The Jahn–Teller distorted Cu(II) complex [Cu(en)2](OTf)2 1 (en = 1,2-diaminoethane) has been reported and characterised using X-ray crystallography, EPR and ENDOR spectroscopy, and DFT calculations. The solid state structure shows an intra- and inter-molecular hydrogen-bonded network via the N–H groups and the coordinated triflate anions. CW and pulsed EPR/ENDOR were used to determine the spin Hamiltonian parameters of the Cu(II) complex, which were in excellent agreement with the DFT. The structure of the complex, as determined by angular selective ENDOR, is also in good agreement with the crystal structure, confirming the axial coordination of the counter-ion(s) in the frozen solution. The small 14N superhyperfine couplings are also consistent with the sp3 hybridised nature of the coordinating nitrogens. These results show that the correlation between the 14N hyperfine coupling and hybridisation of donor nitrogens can be useful to determine not only the coordination around the Cu(II) metal centre but also the nature of the donor in unknown Cu(II) systems.