Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

Two new pentadentate {N5} donor ligands
based on the N4Py (N4Py = N,N-bis­(2-pyridylmethyl)-N-bis­(2-pyridyl)­methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)­methyl-N-(2-pyridyl)­methyl-N-(bis-2-pyridyl methyl)­amine]
(L1) and [N-bis­(1-methyl-2-benzimidazolyl)­methyl-N-(bis-2-pyridylmethyl)­amine] (L2), where one or two pyridyl arms of N4Py have been
replaced by corresponding (N-methyl)­benzimidazolyl-containing
arms. The complexes [FeII(CH3CN)­(L)]2+ (L = L1 (1); L2 (2)) were synthesized,
and reaction of these ferrous complexes with iodosylbenzene led to
the formation of the ferryl complexes [FeIV(O)­(L)]2+ (L = L1 (3); L2 (4)), which
were characterized by UV–vis spectroscopy, high resolution
mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives
at room temperature of 40 h (L = L1) and 2.5 h (L = L2).
The redox potentials of 1 and 2, as well
as the visible spectra of 3 and 4, indicate
that the ligand field weakens as ligand pyridyl substituents are progressively
substituted by (N-methyl)­benzimidazolyl moieties.
The reactivities of 3 and 4 in hydrogen-atom
transfer (HAT) and oxygen-atom transfer (OAT) reactions show that
both complexes exhibit enhanced reactivities when compared to the
analogous N4Py complex ([FeIV(O)­(N4Py)]2+),
and that the normalized HAT rates increase by approximately 1 order
of magnitude for each replacement of a pyridyl moiety; i.e., [FeIV(O)­(L2)]2+ exhibits the highest rates. The second-order HAT rate constants
can be directly related to the substrate C–H bond dissociation
energies. Computational modeling of the HAT reactions indicates that
the reaction proceeds via a high spin transition state.