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 H2O2 with a (mu-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 (mu-oxo)diiron(III) core with terminal aqua and perchlorato ligands (angle Fe-(mu-O)-Fe = 174.1(4)degrees), while 2c, the 5-Et-TPA analogue of 2a, has a bent (mu-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) Angstrom), the Fe-Fe distance of 3.346(9) Angstrom, and the Fe-(mu-O)-Fe angle of 136.3(3)degrees. 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 H2O2 in CH3CN at -40 degrees C; when 5-Me-TPA is used as the tripodal ligand, 3b can be isolated as a solid upon standing overnight at -40 degrees C. Complex 3b exhibits electronic absorption features at 366 (epsilon = 7900 M(-1) cm(-1)) and 616 nm (epsilon = 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 Mossbauer doublet with Delta E(Q) = 0.49 mm/s and delta = 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 [Fe-2(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 mu(B)/2Fe, corresponding to the 5 = 3/2 center observed by EPR. The formula for 3b suggests two unique properties: (a) that it has an Fe-2(mu-O)(2) core, and (b) that it is formally (FeFeIV)-Fe-III. The presence of an Fe-2(mu-O)(2) core in 3b is indicated by its EXAFS spectrum, which requires the inclusion of an Fe scatterer at 2.89 Angstrom 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 (H2O)-O-18), which are associated with an Fe2O2 breathing mode by analogy to those observed for Mn2O2 complexes. 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 degrees C by chemical (ferrocene titration) and cyclic voltammetric (E(1/2) = 0.96 V vs NHE) methods. Thus, 3b represents a bis(mu-oxo)-diiron complex with a formally (FeFeIV)-Fe-III valence state. Complex 3b has an unusual electronic structure. EPR, magnetization, and Mossbauer 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 Mossbauer 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 (FeFeIII)-Fe-III sites coupled to a ligand radical and valence-delocalized (FeFeIV)-Fe-III 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 H2O2 and a (mu-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.
