Deciphering the mechanism of O₂ reduction with electronically tunable non-heme iron enzyme model complexes

A homologous series of electronically tuned 2,2′,2′′-nitrilotris(N-arylacetamide) pre-ligands (H₃L^R) were prepared (R = NO₂, CN, CF₃, F, Cl, Br, Et, Me, H, OMe, NMe₂) and some of their corresponding Fe and Zn species synthesized. The iron complexes react rapidly with O₂, the final products of which are diferric mu-oxo bridged species. The crystal structure of the oxidized product obtained from DMA solutions contain a structural motif found in some diiron proteins. The mechanism of iron mediated O₂ reduction was explored to the extent that allowed us to construct an empirically consistent rate law. A Hammett plot was constructed that enabled insightful information into the rate-determining step and hence allows for a differentiation between two kinetically equivalent O₂ reduction mechanisms.