Moessbauer Evidence for Antisymmetric Exchange in a Diferric Synthetic Complex and Diferric Methane Monooxygenase

• Published in: Journal of the American Chemical Society 1998-09, Vol.120 (34), p.8739-8746
• Main Authors: Kauffmann, KE, Popescu, C V, Dong, Y, Lipscomb, J D, Que, L Jr, Muenck, E
• Format: Article
• Language: English
• Subjects: magnetization; Moessbauer spectroscopy
• ISSN: 0002-7863
• DOI: 10.1021/ja981065t

The dinuclear iron cluster of the oxidized hydroxylase component of methane monooxygenase (MMOH) contains two antiferromagnetically coupled high-spin ferric ions (H = JS sub(A), times S sub(B), S sub(A) = S sub(B) = 5/2, J = 15 cm super(-1)). Previous Moessbauer 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 Moessbauer and magnetization studies of the ( mu -phenoxo)bis( mu -carboxylato)diiron(III) complex, [Fe sub(2)(HXTA)(O sub(2)CCH sub(3)) sub(2)] super(-), 1; HXTA = N,N'-(2-hydroxy-5-methyl-l,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 plus or minus 2 cm super(-1)), and its 4.2 K Moessbauer spectra also exhibit increased magnetic splittings. Studies of the Ga super(III)Fe super(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 Moessbauer spectra of 1 recorded in parallel applied field exhibit nuclear Delta m = 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 times (S sub(A) x S sub(B)); we obtained |d| = 2.2 plus or minus 0.7 cm super(-1) for complex 1 and |d| approximately 1.5 cm super(-1) for MMOH. This study shows that Moessbauer spectroscopy can be a sensitive tool for determining antisymmetric exchange interactions.