Synthetic Heme Selenolate: Spectral and Reactivity Differences from Synthetic Heme Thiolate

Abstract

The relationship between the potent catalytic reactivity of cytochrome P450 and the electron-donating character of the axial thiolate ligand has long been investigated. Here, we synthesize the first synthetic iron porphyrin coordinated by selenolate (Se-SR), which is a better electron-donor than thiolate. At ordinary temperature, the spin state of heme selenolate Se-SR is lower (S = 3/2) than that of our previously synthesized SR complex (S = 5/2), which is structurally identical with complex Se-SR except that the axial ligand is thiolate. The N-O stretching mode of the NO- Se-SR complex clearly indicates that the electron-donating trans-effect of selenolate is larger than that of thiolate. A cyclic voltammogram of Se-SR revealed that the FeIII/FeIV redox couple of Se-SR is lower than that of SR, but the two potentials are unexpectedly identical in the case of the FeII/FeIII redox couple. The catalytic activity of complex Se-SR is 13-fold higher than that of SR complex in substituted phenol oxidation with cumene hydroperoxide, which may reflect the O-O bond cleavage rate. On the other hand, the results of alkane-alkene competitive oxidation indicate that the active intermediate derived from SR has a greater preference than that derived from Se-SR for oxidizing alkane, although Se-SR gave a higher total product yield. Thus, the active intermediate derived from heme thiolate is more effective than that derived from heme selenolate for alkane hydroxylation. This is consistent with the emergence in nature of heme thiolate structure, not heme selenolate, in cytochrome P450, based on its function in alkane hydroxylation.