A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocyclic Ligand

The sluggish oxidants [Fe-IV(O)(TMC)(CH3CN)](2+) (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [Fe-IV(O)(TMCN-d(12))(OTf)](+) (TMCN-d(12)= 1,4,7,11-tetra(methyl-d(3))-1,4,7,11-tetraazacyclotetradecane) are transformed into the highly reactive oxidant [Fe-IV(O)-(TMCO)(OTf)](+) (1; TMCO=4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1. This rationalizes nature's preference for using O-rich ligand environments for the hydroxylation of strong C-H bonds in enzymatic reactions.