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

iron(II)-imidazole complexeGeometry of the [Fe(Im)6]2+ cation in the Fe(Im)6(NO3)2 salt as determined by X-ray diffraction.

iron(II)-imidazole complexeSimplified diagram of the electronic splitting of the 5T state in the [Fe(Im)]2+ cation.

Constant-Q slice along Q = 1.4 Å-1 at four temperatures; insert: close-up of the hot-band at 9.3 meV clearly indicating the temperature dependence.Insert: close-up of the hot-band at 9.3 meV clearly indicating the temperature dependence.

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.

However IN4C was ideally suited to obtain a direct spectroscopic measurement of the electronic structure of high-symmetry homoleptic complexes. 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 observed Q-dependence 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.

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