Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes:  Delocalization of Fe d-Electrons into the Porphyrin Ligand

Abstract

Hemes (iron porphyrins) are involved in a range of functions in biology including electron transfer, small molecule binding and transport, and O 2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin π → π* transitions which obscure the metal center. Recently we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e. differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (p) ring both in terms of P→Fe σ and π donation and Fe→P π back-bonding. We find that π donation to Fe(III) is much larger than π back-bonding from Fe(II), indicating that a hole super-exchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.

Simulations of iron K pre-edge X-ray absorption spectra using the restricted active space method

“…18,36,37 In the L-edge XAS spectra, both complexes have intense peaks that can be assigned to back-donation. However, it is not clear whether such features can be detected in K pre-edge spectra.…”

“…For symmetric systems it often achieves excellent agreement with experimental data through a multi-parameter fit to the experimental spectrum. [35][36][37] However, the number of model parameters increases with decreasing symmetry. Moreover, additional parameters are required to describe the 3d-4p orbital hybridization.…”

Intrinsic deviations in fluorescence yield detected x-ray absorption spectroscopy: the case of the transition metal L2,3edges

“…Charge transfer states have a different degree of localization and it can be expected that the ratio between radiative and non-radiative channels is modified. This implies that systems with significant charge transfer intensity, in particular involving both ligand-metal and metal-ligand charge transfer in molecular π-systems 29,30 , can show modified results. On the other hand, we do not expect significant modifications of the results in case of bulk oxides.…”

Electronic structure of Fe- vs. Ru-based dye molecules

“…4 and 7-11. For three-dimensional complexes, the electronic structure [12][13][14][15][16][17] and charge transfer [18][19][20][21][22][23][24] have been investigated using many spectroscopic [25][26][27][28][29][30][31] and theoretical [32][33][34][35][36][37][38][39][40] methods.…”