Structural, EPR, and Mossbauer Characterization of (μ-Alkoxo)(μ-Carboxylato)Diiron(II,III) Model Complexes for the Active Sites of Mixed-Valent Diiron Enzymes

To obtain structural and spectroscopic models for the diiron(II,III) centers in the active sites of diiron enzymes, the (mu-alkoxo)(mu-carboxylato)diiron(II,III) complexes [(FeFeIII)-Fe-II(N-Et-HPTB)(O2CPh)(NCCH3)(2)](ClO4)(3) (1) and [(FeFeIII)-Fe-II(N-Et-HPTB)(O2CPh)(Cl)(HOCH3)](ClO4)(2) (2) (N-Et-HPTB = N,N,N',N'-tetrakis (2- (1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane) have been prepared and characterized by X-ray crystallography, UV-visible absorption, EPR, and Mossbauer spectroscopies. Fe1-Fe2 separations are 3.60 and 3.63 angstrom, and Fe1-O1-Fe2 bond angles are 128.0 degrees and 129.4 degrees for 1 and 2, respectively. Mossbauer and EPR studies of 1 show that the Fe-III (S-A = 5/2) and Fe-II (S-B = 2) sites are antiferromagnetically coupled to yield a ground state with S = 1/2 (g = 1.75, 1.88, 1.96); Mossbauer analysis of solid 1 yields J = 22.5 +/- 2 cm(-1) for the exchange coupling constant (H = J(SA)center dot S-B convention). In addition to the S = 1/2 ground-state spectrum of 1, the EPR signal for the S = 3/2 excited state of the spin ladder can also be observed, the first time such a signal has been detected for an antiferromagnetically coupled diiron(II,III) complex. The anisotropy of the Fe-57 magnetic hyperfine interactions at the Fe-III site is larger than normally observed in mononuclear complexes and arises from admixing S > 1/2 excited states into the S = 1/2 ground state by zero-field splittings at the two Fe sites. Analysis of the D/J mixing has allowed us to extract the zero-field splitting parameters, local g values, and magnetic hyperfine structural parameters for the individual Fe sites. The methodology developed and followed in this analysis is presented in detail. The spin Hamiltonian parameters of 1 are related to the molecular structure with the help of D FT calculations. Contrary to what was assumed in previous studies, our analysis demonstrates that the deviations of the g values from the free electron value (g = 2) for the antiferromagnetically coupled diiron(II,III) core in complex 1 are predominantly determined by the anisotropy of the effective g values of the ferrous ion and only to a lesser extent by the admixture of excited states into ground-state ZFS terms (D/J mixing). The results for 1 are discussed in the context of the data available for diiron(II,III) clusters in proteins and synthetic diiron(II,III) complexes.