Mössbauer Spectroscopy of Spin-Coupled Iron−Chromium Complexes:  μ-Hydroxo−Bis(μ-acetato)-Bridged Iron(2+)−Chromium(3+) and μ-Oxo−Bis(μ-acetato)-Bridged Iron(3+)−Chromium(3+)

We have analyzed Mössbauer spectra of a model complex of known structure with an Fe2+(S 1=2)−μOH−Cr3+(S 2=3/2) center (A) and of its Fe3+(S 1 = 5/2)−μO−Cr3+(S 2=3/2) analog (B). These μ-hydroxo and μ-oxo bridged binuclear metal centers display unusual magnetic properties as found in several diiron−oxo proteins. Our results confirm antiferromagnetic spin coupling between Fe and Cr ions which results in S eff = 1/2 and S eff = 1 ground states for A and B, respectively. The isotropic exchange ℋex = J S 1·S 2 is weaker for the μ-hydroxo (J ≈ 21 cm-1) than for the μ-oxo (J ≈ 275 cm-1) complex. Spectra recorded at 4.2 K, in fields of 0.22−4.7 T, have been analyzed with the effective spin Hamiltonian for the ground state ℋeff = βS eff·g̃ eff·H + + I 1·P̃ 1·I 1 − β ngn H·I 1. For complex B, the zero-field splitting S eff·D̃ eff·S eff is also included in ℋeff. In applied fields, the 4.2 K spectra of Fe2+ in A showed hyperfine splittings which allowed the determination of the following S eff = 1/2 Hamiltonian parameters: 1/3 Tr g̃ eff ≈ 2.00, = −(18.3,5.6,25.0) T, ΔEQ = +2.87 mm/s, η = 0.93, and δFe = 1.21 mm/s. The weak coupling of A allows the zero-field splitting to mix higher spin manifolds with the ground state doublet, and, to obtain intrinsic parameters, we also calculated the spectra of Fe2+ by diagonalizing the (2S 1 + 1 = 5) × (2S 2 + 1 = 4) matrix of the Hamiltonian ℋ = J S 1·S 2 + + βS i ·g̃i ·H} + S 1·ã 1·I 1 + I 1·P̃ 1·I 1 − β ngn H·I 1. We determined the following parameters for Fe2+: D 1 = +4.0 cm-1, E 1 = +0.4 cm-1, 1/3 Tr g̃ 1 ≳ 2.07, ã 1/gn β n = −(10.2,3.5,15.6) T. For complex B, we found that Fe3+ has a large quadrupole splitting (ΔEQ = −2.00 mm/s, η = 0.22) presumably as a result of anisotropic covalency due to the close proximity of the bridging O2-. This large ΔE Q is comparable to values found in diiron−oxo proteins. Spectra of B in applied fields also displayed hyperfine splittings, and the following S eff = 1 Hamiltonian parameters could be deduced: D eff = +3.9 cm-1, E eff = +1.7 cm-1, 1/3 Tr g̃ eff = 2.01, = −(33.8,30.9,35.8) T, δFe = 0.52 mm/s.