On the Reactivity of Mononuclear Iron(V)oxo Complexes

Ferric tetraamido macrocyclic ligand (TAML)-based catalysts [Fe{C6H4-1,2-(NCOCMe2NCO)(2)CR2}(OH2)]PPh4 [1; R = Me (a), Et (b)] are oxidized by m-chloroperoxybenzoic acid at -40 degrees C in acetonitrile containing trace water in two steps to form Fe(V)oxo complexes (2a,b). These uniquely authenticated Fe-V(O) species comproportionate with the Fe-III starting materials 1a,b to give mu-oxo-(Fe-IV)(2) dimers. The comproportionation of 1a-2a is faster and that of 1b-2b is slower than the oxidation by 2a,b of sulfides (p-XC6H4SMe) to sulfoxides, highlighting a remarkable steric control of the dynamics. Sulfide oxidation follows saturation kinetics in [p-XC6H4SMe] with electron-rich substrates (X = Me, H), but changes to linear kinetics with electron-poor substrates (X = Cl, CN) as the sulfide affinity for iron decreases. As the sulfide becomes less basic, the Fe-IV/Fe-III ratio at the end of reaction for 2b suggests a decreasing contribution of concerted oxygen-atom transfer (Fe-V -> Fe-III) concomitant with increasing electron transfer oxidation (Fe-V -> Fe-IV). Fe-V is more reactive toward PhSMe than Fe-IV by 4 orders of magnitude, a gap even larger than that known for peroxidase Compounds I and II. The findings reinforce prior work typecasting TAML activators as faithful peroxidase mimics.