Multi-frequency, high-field EPR as a powerful tool to accurately determine zero-field splitting in high-spin transition metal coordination complexes

Electron paramagnetic resonance at multiple, high frequencies (95-700 GHz) and at correspondingly high magnetic fields (up to 25 T), known as HFEPR, is a relatively new technique. There have been an increasing number of applications of HFEPR, such as in organic radical chemistry and in materials science. The focus of this review, however, is on the application of HFEPR to transition metal coordination chemistry, in particular to mononuclear complexes, as opposed to clusters that are relevant to single-molecule magnets. There are many complexes of paramagnetic transition metal ions for which conventional EPR (fields below 2 T, frequencies not exceeding 35 GHz) is less than ideal. Primarily, such systems are high-spin (i.e., S > 1/2), wherein the effects of zero-field splitting can make the complex either "EPR-silent" using conventional EPR, or make the EPR spectrum not particularly informative. Examples of the former are many integer-spin (non-Kramers) ions such as Mn(III) and Fe(II), while the latter case is exemplified by high-spin Fe(111). We will review here the use of HFEPR to study high-spin transition metal complexes, generally of the first row. For half-integer high-spin systems, we will review only those where the large magnitude of zero-field splitting necessitates the use of HFEPR. We will generally not discuss systems in which the zero-field splitting is in most cases very small and conventional EPR is extensively employed. The experimental and analytical methods for the accurate determination of zero-field splitting and other spin Hamiltonian parameters from HFEPR studies of these systems will be described. Comparison with other physical methods such as magnetometry, magnetic circular dichroism (MCD), and Mossbauer effect spectroscopy will also be made. We will further give selected examples how ligand-field theory can be used to provide information on chemical bonding and geometry, based on analysis of the spin Hamiltonian parameters well established by HFEPR. (c) 2006 Elsevier B.V All rights reserved.