SPECTROSCOPIC AND ELECTRONIC-STRUCTURE STUDIES OF MET-HEMERYTHRIN MODEL COMPLEXES - A DESCRIPTION OF THE FERRIC-OXO DIMER BOND

A combination of theoretical and spectral techniques have been used to probe the electronic structure of mu-oxo non-heme Fe dimer complexes which model the active site found in the oxygen carrier hemerythrin. Variable temperature electronic absorption (including single crystal polarized and orientationally averaged), variable temperature magnetic circular dichroism (MCD), and variable temperature resonance Raman spectroscopies have been used to assign all of the spectral features present in these complexes. These spectral studies are complemented by both high spin and broken symmetry SCF-X alpha-SW calculations. Together, a description of the highly covalent bonding in these complexes has been generated. The unique UV/vis absorption spectra of the Fe-oxo dimer unit is dominated by low energy intense oxo --> Fe3+ ligand to metal charge-transfer (LMCT) transitions. These transitions appear at much lower energy than expected due to the presence of very large excited-state antiferromagnetic exchange in the LMCT transitions from the bridging oxo,stoup. The different spin components of these oxo LMCT transitions can be identified from their large temperature dependence exhibited in the different spectroscopies listed above. The observed excited state splittings have been analyzed using a recently developed valence bond configuration interaction (VBCI) model to describe this bridging-ligand charge-transfer excited state antiferromagnetism and probe specific superexchange pathways in the ground and excited states. This VBCI model indicates an inverted charge-transfer ordering Scheme with the in the Fe(III)-O-Fe(III) plane pi CT transition lowest in energy. The low energy of these transitions and the high covalency of the Fe(III)-O-Fe(III) bond are also sources of the higher intensity of the ligand field transitions relative to those of monomeric Fe3+ complexes. The ligand field excited-state splitting for the (6)A(1) --> (4)A(1)-transition, which is ferromagnetic, is analyzed using the Tanabe model. of exchange and demonstrates the importance of a mixed pi/sigma (Fe d(yz) - Fe d(z2)) magnetic exchange pathway. The nature of the Fe-oxo bond in the Fe(III)-O-Fe(III) unit is analyzed and its' strength is mainly due to strong sigma bonding interactions of the oxo p(z) orbital with the 4s and 4p orbitals of the iron. The high stability of the mu-oxo iron dimer bond provides significant insight into differences in the O-2 reactivity among the different binuclear non-heme proteins hemerythrin, methane monooxygenase, and ribonucleotide reductase.