Structural, EPR, and Mössbauer 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 (μ-alkoxo)­(μ-carboxylato)­diiron­(II,III)
complexes [FeIIFeIII(N-Et-HPTB)­(O2CPh)­(NCCH3)2]­(ClO4)3 (1) and [FeIIFeIII(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 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 1 and 2, respectively. Mössbauer and EPR studies of 1 show that the FeIII (SA = 5/2) and FeII (SB = 2)
sites are antiferromagnetically coupled to yield a ground state with S = 1/2 (g = 1.75, 1.88, 1.96);
Mössbauer analysis of solid 1 yields J = 22.5 ± 2 cm–1 for the exchange coupling
constant (H = JSA·SB 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 FeIII 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 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.