Mossbauer evidence for antisymmetric exchange in a diferric synthetic complex and diferric methane monooxygenase

The dinuclear iron cluster of the oxidized hydroxylase component of methane monooxygenase (MMOH) contains two antiferromagnetically coupled high- spin ferric ions (H = JS(A)·S(B), S(A) = S(B) = 5/2, J = 15 cm^-1). Previous Mossbauer studies revealed that the electronic ground state of the cluster contains a paramagnetic admixture; this is manifested in magnetic hyperfine splittings that are larger by about 10% than those attributable to the applied magnetic field. This observation cannot be explained by anisotropic Zeeman interactions, zero-field splittings, or anisotropic exchange. Here we report Mossbauer and magnetization studies of the (μ- phenoxo)bis(μ-carboxylato)diiron(III) complex, [Fe₂(HXTA)(O₂CCH₃)2]-, 1; HXTA = N,N'-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-carboxymethylglycine). Like MMOH, complex 1 contains a pair of antiferromagnetically coupled high- spin ferric ions (our magnetization data yield J = 20 ± 2 cm^-1), and its 4.2 K Mossbauer spectra also exhibit increased magnetic splittings. Studies of the Ga(III)Fe(III) analogue of 1 revealed no unusual properties of the high-spin Fe(III) site, suggesting that the increased magnetic splittings are attributes of the pair rather than properties of the local sites. The Mossbauer spectra of 1 recorded in parallel applied field exhibit nuclear Δm = 0 transitions, indicating the presence of an interaction that produces at each iron site spin expectation values in directions perpendicular to the applied field. Analysis of the data of 1 shows that the unusual spectral features of the complex, and by extension those of MMOH, reflect the presence of antisymmetric exchange, d·(SA x S(B)); we obtained [d] = 2.2 ± 0.7 cm^{- 1} for complex 1 and [Id] 1.5 cm^-1 for MMOH. This study shows that Mossbauer spectroscopy can be a sensitive tool for determining antisymmetric exchange interactions.