Structural, EPR, and Mössbauer
Characterization
of (μ-Alkoxo)(μ-Carboxylato)Diiron(II,III) Model Complexes
for the Active Sites of Mixed-Valent Diiron Enzymes
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Abstract
To obtain structural and spectroscopic models for the
diiron(II,III)
centers in the active sites of diiron enzymes, the (μ-alkoxo)(μ-carboxylato)diiron(II,III)
complexes [Fe<sup>II</sup>Fe<sup>III</sup>(<i>N</i>-Et-HPTB)(O<sub>2</sub>CPh)(NCCH<sub>3</sub>)<sub>2</sub>](ClO<sub>4</sub>)<sub>3</sub> (<b>1</b>) and [Fe<sup>II</sup>Fe<sup>III</sup>(<i>N</i>-Et-HPTB)(O<sub>2</sub>CPh)(Cl)(HOCH<sub>3</sub>)](ClO<sub>4</sub>)<sub>2</sub> (<b>2</b>) (<i>N</i>-Et-HPTB = <i>N,N,N</i>′<i>,N</i>′-tetrakis(2-(1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane)
have been prepared and characterized by X-ray crystallography, UV–visible
absorption, EPR, and Mössbauer spectroscopies. Fe1–Fe2
separations are 3.60 and 3.63 Å, and Fe1–O1–Fe2
bond angles are 128.0° and 129.4° for <b>1</b> and <b>2</b>, respectively. Mössbauer and EPR studies of <b>1</b> show that the Fe<sup>III</sup> (<i>S</i><sub>A</sub> = 5/2) and Fe<sup>II</sup> (<i>S</i><sub>B</sub> = 2)
sites are antiferromagnetically coupled to yield a ground state with <i>S</i> = 1/2 (<i>g</i> <b>=</b> 1.75, 1.88, 1.96);
Mössbauer analysis of solid <b>1</b> yields <i>J</i> = 22.5 ± 2 cm<sup>–1</sup> for the exchange coupling
constant (H = <i>J</i><b>S</b><sub>A</sub>·<b>S</b><sub>B</sub> convention). In addition
to the <i>S</i> = 1/2 ground-state spectrum of <b>1</b>, the EPR signal for the <i>S</i> = 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 <sup>57</sup>Fe magnetic hyperfine
interactions at the Fe<sup>III</sup> site is larger than normally
observed in mononuclear complexes and arises from admixing <i>S</i> > 1/2 excited states into the <i>S</i> =
1/2
ground state by zero-field splittings at the two Fe sites. Analysis
of the “<i>D</i>/<i>J</i>” mixing
has allowed us to extract the zero-field splitting parameters, local <i>g</i> 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 <b>1</b> 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 <i>g</i> values from the free electron value (<i>g</i> = 2) for the antiferromagnetically coupled diiron(II,III) core in
complex <b>1</b> are predominantly determined by the anisotropy
of the effective <i>g</i> values of the ferrous ion and
only to a lesser extent by the admixture of excited states into ground-state
ZFS terms (<i>D</i>/<i>J</i> mixing). The results
for <b>1</b> are discussed in the context of the data available
for diiron(II,III) clusters in proteins and synthetic diiron(II,III)
complexes
