A High-Valent Nonheme Iron Intermediate. Structure and Properties of [Fe₂(μ-O)₂(5-Me-TPA)₂] (ClO₄)₃

In our efforts to model the oxygen activation chemistry of methane monooxygenase (MMO) and the R2 protein of ribonucleotide reductase (RNR), we have discovered a transient green species (3) in the reaction of H₂O₂with a (μ-oxo)diiron(III) TPA complex (TPA = tris(2-pyridylmethyl)amine). Our studies show that the precursor to 3 is [Fe₂O(TPA)₂(OH)(H₂O)](ClO₄)₃(2a), which can be obtained by the treatment of [Fe₂O(TPA)₂(H₂O)(ClO₄)]-(ClO₄)₃ (1) with an equivalent of base. Crystallographic studies show that 1 has a nearly linear (μ-oxo)diiron(III) core with terminal aqua and perchlorato ligands (∠Fe-(μ-O)-Fe = 174.1(4)°), while 2c, the 5-Et-TPA analogue of 2a, has a bent (μ-oxo)diiron(III) core that is supported by an H₃O₂^- bridge. The presence of an H₃O₂^- bridge in the latter is indicated by the short O–O separation (2.464(9) Å), the Fe—Fe distance of 3.346(9) Å, and the Fe—(μ-O)-Fe angle of 136.3(3)°. Thus treatment of 1 with an equivalent of base results in the replacement of the bound perchlorate with hydroxide and the bending of the Fe-O-Fe unit to form 2. That the bent Fe-O-Fe core persists in solution is indicated by its UV—vis features and NMR spectra that reflect distinct TPA coordination modes about the individual iron sites. The green intermediate 3 is generated by the reaction of 2, [Fe₂O(L)₂(OH)(H₂O)](ClO₄)₃ (L = TPA, 5-Me-TPA, and 5-Et-TPA), with H₂O₂in CH₃CN at -40 °C; when 5-Me-TPA is used as the tripodal ligand, 3b can be isolated as a solid upon standing overnight at —40 °C. Complex 3b exhibits electronic absorption features at 366 (∊ = 7900 M^-1 cm^-1) and 616 nm (∊ = 5200 M^-1 cm^-1) and an S = 3/2 EPR spectrum with g values at 4.45, 3.90, and 2.01. It exhibits one sharp Mössbauer doublet with ΔEQ = 0.49 mm/s and δ = 0.12 mm/s at 100 K, which accounts for 90% of the iron in the solid. Elemental analysis and electrospray ionization mass spectrometry show that 3b is a dinuclear complex best formulated as [Fe₂(O)₂(5-Me-TPA)₂](ClO₄)₃. This dinuclear formulation is corroborated by magnetic susceptibility measurements showing that 3b has a high-temperature moment of 3.9 μ _B/2Fe, corresponding to the S = 3/2 center observed by EPR. The formula for 3b suggests two unique properties: (a) that it has an Fe₂(μ-O)₂ core, and (b) that it is formally Fe^IIIFe^IV. The presence of an Fe₂(μ-O)₂core in 3b is indicated by its EXAFS spectrum, which requires the inclusion of an Fe scatterer at 2.89 Å for a satisfactory fit. It is further supported by the observation of resonance-enhanced Raman features at 676 and 656 cm^-1 (both of which shift to 634 cm^-1 with added H₂¹⁸O), which are associated with an Fe₂O₂breathing mode by analogy to those observed for Mn₂O₂complexes. The high-valent nature of 3b is corroborated by the ca. 3 eV upshift of its higher X-ray absorption K-edge relative to that of 2b and the reduction of 3b to the diiron(III) state at —40 °C by chemical (ferrocene titration) and cyclic voltammetric (E_1/2= 0.96 V vs NHE) methods. Thus, 3b represents a bis(μ-oxo)-diiron complex with a formally Fe^IIIFe^IV valence state. Complex 3b has an unusual electronic structure. EPR, magnetization, and Mössbauer studies show that 3b has an S = 3/2 ground state with a large and nearly axial zero-field splitting, D = 35 ± 15 cm^-1 and E/D = 0.04. The Mössbauer data show that 3 contains two equivalent iron sites which have unusually small magnetic hyperfine interactions, A = (—7.8, —7.9, —6.5) MHz. A variety of exchange coupling models are considered to describe the electronic properties of 3b; these include Fe^IIIFe^III sites coupled to a ligand radical and valence-delocalized Fe^IIIFe^IV centers. Among the models considered, the only one that could possibly explain the observed site equivalence, isomer shift, and other properties consists of a valence-delocalized low-spin (S = 1/2) Fe^III-low-spin (S=1) Fe^IV pair coupled by Heisenberg as well as double exchange; however, detailed theoretical studies of double exchange interactions involving low-spin iron sites are required before such an assignment can be made. Whatever its electronic structure, 3b is the only well-characterized high-valent nonheme iron species that is derived from the reaction of H₂O₂and a (μ-oxo)diiron(III) complex. As such, it is relevant to the transient species observed in the oxidation chemistry of MMO and RNR R2 and provides a synthetic example of how a high-valent state can be attained in a nonheme environment.