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+)

Abstract

We have analyzed Mössbauer spectra of a model complex of known structure with an Fe 2+ ( S 1 =2)−μOH−Cr 3+ ( S 2 = 3 / 2 ) center ( A ) and of its Fe 3+ ( S 1 = 5 / 2 )−μO−Cr 3+ ( 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 ℋ ex = 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 ℋ eff = β S eff · g̃ eff · H + + I 1 · P̃ 1 · I 1 − β n g n H · I 1 . For complex B, the zero-field splitting S eff · D̃ eff · S eff is also included in ℋ eff . In applied fields, the 4.2 K spectra of Fe 2+ 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, Δ E Q = +2.87 mm/s, η = 0.93, and δ Fe = 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 Fe 2+ by diagonalizing the (2 S 1 + 1 = 5) × (2 S 2 + 1 = 4) matrix of the Hamiltonian ℋ = J S 1 · S 2 + + β S i · g̃ i · H } + S 1 · ã 1 · I 1 + I 1 · P̃ 1 · I 1 − β n g n H · I 1 . We determined the following parameters for Fe 2+: D 1 = +4.0 cm -1, E 1 = +0.4 cm -1, 1 / 3 Tr g̃ 1 ≳ 2.07, ã 1 / g n β n = −(10.2,3.5,15.6) T. For complex B, we found that Fe 3+ has a large quadrupole splitting (Δ E Q = −2.00 mm/s, η = 0.22) presumably as a result of anisotropic covalency due to the close proximity of the bridging O 2- . This large Δ E 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, δ Fe = 0.52 mm/s.