Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

High-Field EPR for the study of transition metal ion complexes
Contributors: A.L. Barra, C. Duboc, P. Neugebauer
In the last decades, EPR has undergone a profound renewal, with the advent of High-Field and High-
Frequency EPR (HF-EPR) as well as the development of pulsed techniques. HF-EPR has proven to be very
powerful for the study of transition metal complexes from mononuclear systems to Single-Molecule Magnets
(SMM). The domain has now grown up to establishing relations between electronic and molecular structures
not only for mononuclear complexes but also for spin clusters and it allows determining the structural
parameters governing the magnetic anisotropy. This is also an essential step to understand the reaction
mechanisms for paramagnetic moieties from enzymes. Here we report our main results obtained during these
last four years with the application of HF-EPR to topics of interest to chemistry and biology.
Mononuclear complexes
With its multifrequency operation, our HF-EPR spectrometer is particularly suited for the study of
spin systems with S>1/2. Among these, the non Kramers ions, EPR-silent at standard EPR frequencies due to
large Zero-Field Splittings (ZFS), are the main focus of study. An illustrative example involves two Ni(II)
complexes (S=1), with very similar ligands derived from galactose which possess three O-CH2-pyridine
pendant arms that chelate the Ni ion. [Cha07]. The analysis of the EPR spectra recorded at several
temperatures and frequencies shows a striking change of sign for the axial term of ZFS (D parameter)
whereas the rhombicity (E/D) is the same. Ligand field calculation for both complexes, within the Angular
Overlap Model, allowed understanding the change of sign for D. The structural difference responsible for it
results from the nature of the sugar scaffolds. In one case, the sugar scaffold imposes an intramolecular
hydrogen bond with one of the atoms linked to the Ni II which leads to a distorted coordination sphere and a
positive D, while the absence of such a hydrogen bond in the other complex results in a less distorted
environment around the Ni center and leads to a negative D value.
HF-EPR is also useful for the study of half-integer spin systems with S>1/2, e.g. allowing
approaching the high field limit where spectra are easy to analyze. Such studies have been performed on
Mn(II) or Fe(III) complexes. In the case of Mn(II) numerous mononuclear complexes have been described.
However, few studies existed on the precise determination of their electronic parameters, most of them being
obtained from EPR spectroscopy. Several mononuclear Mn(II) complexes have been studied by HF-EPR,
allowing the determination of the ZFS parameters, and.the results were then analyzed with DFT model
calculations [Duboc8]. They reveal that D is closely connected to the coordination number (5 or 6) of the
Mn(II) ion, D being dominated by spin-orbit contributions for 5-coordiantion whereas spin-spin interaction
gives the leading part for 6-coordination. In the case of halide complexes, D is correlated with the nature of
the halide anion
These studies illustrate the interest of combining HF-EPR spectroscopy with theoretical approaches
relying on molecular calculation such as AOM or DFT. Beyond obtaining magneto-structural correlations for
the magnetic anisotropy, this approach is fundamental for defining the best suited starting blocks for building
up SMM with large anisotropies (bottom-up approach) or for the understanding of reactivity (for catalysis or
for metalloenzymes when the metal center is the active site).
The previous examples deal with synthetic complexes, where quantities can be large. Taking
advantage of the Quasi-Optical configuration of the HF-EPR spectrometer, it has been possible to extend this
approach to a mononuclear high-spin ferrous iron centre in a protein. Rubredoxins are small iron-sulfur
proteins (~6 kDa) that contain a mononuclear tetrahedral [Fe-4S] site. A frozen solution of the mutant Rm
A44S of Pyrococcus abyssi in its reduced state has been measured at several frequencies and temperatures,
leading to the first EPR determination of the magnetic anisotropy of a ferrous iron centre from a protein
[Barra6]. It illustrates the interest of the Quasi-Optical multifrequency approach for the study of biologically
relevant metal centres with large ZFS.
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