SPECTROSCOPIC STUDIES OF THE ELECTRONIC-STRUCTURE OF IRON(III) TRIS(CATECHOLATES)

Single-crystal polarized absorption and magnetic circular dichroism (MCD) have been employed to elucidate the electronic structure of the iron(III) tris(catecholate) complex [Fe(cat)3]3- to investigate the bonding in ferric enterobactin and similar catechol siderophores. In addition to [Fe(cat)3]3-, two related complexes, [Fe(TRENCAM)]3-, a catecholamide, and [Fe(eta)3]3-, a terephthalamide, have been studied by MCD to provide a perturbation on the electronic structure of [Fe(cat)3]3- and to also determine differences in bonding between the three complexes. The ligand-to-metal charge-transfer (CT) band characteristic of the catecholate siderophores is found for [Fe(cat)3]3- to be composed of two overlapping x,y polarized transitions at 18414 and 22018 cm-1. These transitions are ligand pi-to metal d in nature; assignments of these and four other CT transitions and an experimental energy order for the molecular orbitals of the complexes are made in D3 symmetry. It is evident from analysis of the spectra that a significant contribution to the Fe-O interaction in these complexes is made by pi-bonding. An estimate of 10Dq as approximately 13 000 cm-1 is made for the iron(III) tris(catecholates) from the transition energy of the spin forbidden ligand field (LF)6A1 --> 4T1 transition observed in the MCD. A value of 10Dq is also obtained from the CT transition energies and is determined to be approximately 63% of the value obtained from the LF transitions. This reduction indicates that the CT excited state is best described as an iron(II) species. Analysis of the CT spectra indicates that the electronic structure of the two complexes [Fe(cat)3]3- and [Fe(TRENCAM)]3- are similar, whereas [Fe(eta)3]3- shows a higher degree of sigma- and pi-interactions in Fe-O bonding. The contribution of differences in bonding to the known thermodynamic stability constants is discussed.