Electronic Structure and Reactivity of High-Spin Iron−Alkyl- and −Pterinperoxo Complexes

Abstract

The spectroscopic properties and electronic structure of the four-coordinate high-spin [Fe^III(L3)(OO^tBu)]⁺ complex (1; L3 = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate; ^tBu = tert-butyl) are investigated and compared to the six-coordinate high-spin [Fe(6-Me₃TPA)(OH_x)(OO^tBu)]^x+ system (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2) studied earlier [Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc.2001, 123, 12802−12816]. Complex 1 is characterized by Raman features at 889 and 830 cm^-1 which are assigned to the O−O stretch (mixed with the symmetric C−C stretch) and a band at 625 cm^-1 that corresponds to ν(Fe−O). The UV−vis spectrum shows a charge-transfer (CT) transition at 510 nm from the alkylperoxo (v = vertical to C−O−O plane) to a d orbital of Fe(III). A second CT is identified from MCD at 370 nm that is assigned to a transition from (h = horizontal to C−O−O plane) to an Fe(III) d orbital. For the TPA complex the CT is at 560 nm while the CT is to higher energy than 250 nm. These spectroscopic differences between four- and six-coordinate Fe(III)−OOR complexes are interpreted on the basis of their different ligand fields. In addition, the electronic structure of Fe−OOPtn complexes with the biologically relevant pterinperoxo ligand are investigated. Substitution of the tert-butyl group in 1 by pterin leads to the corresponding Fe(III)−OOPtn species (2), which shows a stronger electron donation from the peroxide to Fe(III) than 1. This is related to the lower ionization potential of pterin. Reduction of 2 by one electron leads to the Fe(II)−OOPtn complex (3), which is relevant as a model for potential intermediates in pterin-dependent hydroxylases. However, in the four-coordinate ligand field of 3, the additional electron is located in a nonbonding d orbital of iron. Hence, the pterinperoxo ligand is not activated for heterolytic cleavage of the O−O bond in this system. This is also evident from the calculated reaction energies that are endothermic by at least 20 kcal/mol.