A 5,000-fold increase in the HAT reactivity of a nonheme Fe ^IV =O complex simply by replacing two pyridines of the pentadentate N4Py ligand with pyrazoles

A pentadentate [N5] ligand (N2Py2Pz) based on the classic N4Py (<i>N,N</i>-bis(2-pyridylmethyl)-<i>N</i>-bis(2-pyridyl)methylamine) framework has been synthesized by replacing the two pyridylmethyl arms with corresponding (<i>N</i>-methyl)pyrazolylmethyl units to form [<i>N-</i>bis(1-methyl-2-pyrazolyl)methyl-<i>N</i>-(<i>bis</i>-2-pyridylmethyl)amine] (L1). The oxidation of the iron(II) precursor (N2Py2Pz)Fe^II(OTf)₂ (<b>1</b>) with (^tBuSO₂)C₆H₄IO at 298 K leads to the formation of the [Fe^IV(O)(N2Py2Pz)]²⁺ intermediate (<b>2</b>) with a near-IR band at 750 nm (ε_M = 250 M^-1cm^-1) and a t_1/2 ~ 2 min at 298 K. The introduction of the less basic pyrazolylmethyl ligands in place of two pyridylmethyl units generates Fe^IV=O intermediate <b>2</b> that exhibits a cyclohexane oxidation rate of 0.29 s^-1 at 298 K, which is 5,000-fold faster than that observed for the classic Fe^IV(O)N4Py parent complex and 40,000-fold more reactive than the least reactive Fe^IV(O)N2Py2Q' complex in this series (Py = pyridine, Q' = isoquinoline) recently reported by Nordlander.