Structure and reactivity of diamidoiron(III) hydroperoxide. The mechanism of oxygen-atom transfer to ammonia

Abstract

Diamidoiron( 111) hydroperoxide has been examined as a highly simplified model for the hydroperoxide intermediate in hemoproteins such as peroxidases and cytochrome P-450. Equilibrium structures and transition states were optimized at the UHF and UMP2 levels using the 3-21G basis and the Wachters-Hay basis set (augmented with an f function on iron). The MP2 level was found to be essential for a proper description of the geometry and bonding. Diamidoiron(II1) hydroperoxide is a peroxo-bridged structure with Fe-0 bond lengths of 1.98 and 2.15 A. A CASSCF/ 3-21G geometry optimization also afforded a cyclic FeOOH structure with an Fe-0-0 bond angle of 70.4O. Similar calculations on (HO)3TiOOH yielded a bridged structure in excellent agreement with X-ray data. The barrier for oxygen transfer from diamidoiron(II1) hydroperoxide to ammonia is only 10.2 kcal/mol (MP4 energies using the Wachters-Hay basis), a barrier lower than that for LiOOH (20.0 kcallmol) and peroxyformic acid (17.8 kcal/mol). Protonated diamidoiron(II1) hydroperoxide is no longer bridged (R(Fe-O) = 2.01 and 2.85 A) and has a negative barrier for oxygen donation to NH3. Although these highly abbreviated models have many shortcomings, the present calculations suggest that in certain cases oxygen atom transfer from iron hemoproteins could precede the formation of the ferryl complex.