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 (
N,N
-bis(2-pyridylmethyl)-
N
-bis(2-pyridyl)methylamine) framework has been synthesized by replacing the two pyridylmethyl arms with corresponding (
N
-methyl)pyrazolylmethyl units to form [
N-
bis(1-methyl-2-pyrazolyl)methyl-
N
-(
bis
-2-pyridylmethyl)amine] (L1). The oxidation of the iron(II) precursor (N2Py2Pz)Fe
II
(OTf)
2
(
1
) with (
t
BuSO
2
)C
6
H
4
IO at 298 K leads to the formation of the [Fe
IV
(O)(N2Py2Pz)]
2+
intermediate (
2
) with a near-IR band at 750 nm (ε
M
= 250 M
−1
cm
−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
2
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.