A mononuclear carboxylate-rich oxoiron(iv) complex: A structural and functional mimic of TauD intermediate 'J'

The pentadentate ligand ⁿBu-P2DA (2(b), ⁿBu-P2DA = N-(1′,1′-bis(2-pyridyl)pentyl)iminodiacetate) was designed to bind an iron center in a carboxylate-rich environment similar to that found in the active sites of taurine dioxygenase (TauD) and other α-ketoglutarate- dependent mononuclear non-heme iron enzymes. The iron(ii) complex ⁿBu₄N[Fe^II(Cl)(ⁿBu-P2DA)] (3(b)-Cl) was synthesized and crystallographically characterized to have a 2-pyridine-2-carboxylate donor set in the plane perpendicular to the Fe-Cl bond. Reaction of 3(b)-Cl with N-heterocyclic amines such as pyridine or imidazole yielded the N-heterocyclic amine adducts [Fe^II(N)( ⁿBu-P2DA)]. These adducts in turn reacted with oxo-transfer reagents at -95 °C to afford a short-lived oxoiron(iv) complex [Fe ^IV(O)(ⁿBu-P2DA)] (5(b)) in yields as high as 90% depending on the heterocycle used. Complex 5(b) exhibits near-IR absorption features (λ_max = 770 nm) and Mössbauer parameters (δ = 0.04 mm s^-1; ΔE_Q = 1.13 mm s^-1; D = 27 ± 2 cm^-1), characteristic of an S = 1 oxoiron(iv) species. Direct evidence for an FeO bond of 1.66 Å was found from EXAFS analysis. DFT calculations on 5(b) in its S = 1 spin state afforded a geometry-optimized structure consistent with the EXAFS data. They further demonstrated that the replacement of two pyridine donors in [Fe^IV(O)(N4Py)]²⁺ (N4Py = N,N-(bis(2-pyridyl)methyl)N-bis(2-pyridylmethyl)amine) with carboxylate donors in 5(b) decreased the energy gap between the ground S = 1 and the excited S = 2 states, reflecting the weaker equatorial ligand field of 5(b) and accounting for its larger D value. Complex 5(b) reacted readily with dihydrotoluene, methyldiphenylphosphine and ferrocene at -60 °C, and in all cases was approximately a 5-fold more reactive oxidant than [Fe ^IV(O)(N4Py)]²⁺. The reactivity differences between these two complexes may arise from a combination of electronic and steric factors. Carboxylate-rich 5(b) represents the closest structural mimic reported thus far of the oxoiron(iv) intermediate ('J') found in TauD and provides us with vital insights into the role carboxylate ligands play in modulating the spectroscopic and reactivity properties of the non-heme oxoiron(iv) moiety.