Characterisation of the ligand-field splitting of Fe(II) for a biologically relevant ligand

The imidazole ring functions as a ligand towards transition metal ions in a number of biologically important molecules. In particular, it is found coordinated to iron in haemoglobin, myoglobin, and cytochrome c. The majority of previous studies on iron(II)-imidazole complexes have focused on model systems, resembling the naturally occurring low-symmetry environments, which are necessarily so complicated that the role of the individual ligands is difficult to determine. The approach of this work was to study high-symmetry homoleptic complexes, in order to obtain an understanding of the iron-imidazole interaction by the determination of the ground-state electronic structure.

A direct spectroscopic measurement of the electronic structure can be obtained using inelastic neutron scattering (INS), and IN4 is ideally suited. The ligand-field in this case has both a spin-orbit coupling and a symmetry-imposed trigonal field contribution; by directly probing the electronic states using INS one can characterise the effect of imidazole ligation to the metal. The ground-state to first excited-state transition (I) at ca. 2 meV had been previously observed on FOCUS at PSI, alluding to the presence of a hot-band at ca. 9 meV on the basis of existing models of ligand-field splitting of the 5T2g (Oh) ground term. IN4 data collected on a 5 g sample of the fully-deuterated nitrate salt clearly shows the increase in intensity of both the induced de-population of the excited state and the hot band as a function of temperature, with a concurrent decrease in I due to thermal de-population of the ground-state.

The Q-dependence of these features is consistent with the free-ion form factor of Fe(II) and is easily discriminated from the low-lying phonons growing up around 5 meV. This experimental result provides a complete characterisation of the ground term splitting of imidazole ligation to Fe(II) for the first time. In addition, by using atomic overlap model bonding parameters, it will be applicable to lower symmetry transition metal sites in biologically active systems.

References: G. Carver et al, Inorg. Chem. 42, 5771 (2003);