Structural, EPR, and M枚ssbauer Characterization of (渭-Alkoxo)(渭-Carboxylato)Diiron(II,III) Model Complexes for the Active Sites of Mixed-Valent Diiron Enzymes

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• 作者：Feifei Li ; Mrinmoy Chakrabarti ; Yanhong Dong ; Karl Kauffmann ; Emile L. Bominaar ; Eckard M眉nck ; Lawrence Que ; Jr.
• 刊名：Inorganic Chemistry
• 出版年：2012
• 出版时间：March 5, 2012
• 年：2012
• 卷：51
• 期：5
• 页码：2917-2929
• 全文大小：622K
• 年卷期：v.51,no.5(March 5, 2012)
• ISSN：1520-510X

文摘

To obtain structural and spectroscopic models for the diiron(II,III) centers in the active sites of diiron enzymes, the (渭-alkoxo)(渭-carboxylato)diiron(II,III) complexes [Fe^IIFe^III(N-Et-HPTB)(O₂CPh)(NCCH₃)₂](ClO₄)₃ (1) and [Fe^IIFe^III(N-Et-HPTB)(O₂CPh)(Cl)(HOCH₃)](ClO₄)₂ (2) (N-Et-HPTB = N,N,N鈥?i>,N鈥?tetrakis(2-(1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane) have been prepared and characterized by X-ray crystallography, UV鈥搗isible absorption, EPR, and M枚ssbauer spectroscopies. Fe1鈥揊e2 separations are 3.60 and 3.63 脜, and Fe1鈥揙1鈥揊e2 bond angles are 128.0掳 and 129.4掳 for 1 and 2, respectively. M枚ssbauer 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); M枚ssbauer analysis of solid 1 yields J = 22.5 卤 2 cm^鈥? for the exchange coupling constant ( = JS_A路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 ⁵⁷Fe 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 鈥?i>D/J鈥?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 DFT 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.