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Structure and Magnetic Properties of a Non-Heme Diiron Complex Singly Bridged by a Hydroxo Group

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posted on 2006-08-21, 00:00 authored by Josseline Jullien, Gergely Juhász, Pierre Mialane, Eddy Dumas, Cédric R. Mayer, Jérôme Marrot, Eric Rivière, Emile L. Bominaar, Eckard Münck, Francis Sécheresse
The synthesis of the first singly bridged non-heme diiron complex with a μ-hydroxo bridging ligand, [{(salten)Fe}2(OH)][B(C6H5)4]·(CH3CN)x·(H2O)y (1) [H2salten = 4-azaheptane-1,7-bis(salicylideneiminate)], is reported. The complex has been characterized with X-ray crystallography, FTIR, magnetic susceptibility measurements, and Mössbauer spectroscopy. The data have been compared with the results of DFT calculations on both 1 and a model with an unsupported μ-oxo bridge (2) to verify the formulation of the complex as a μ-hydroxo-bridged species. The X-ray structure [Fe−O(H) = 1.997(1) Å and Fe−O(H)−Fe = 159°] is consistent with the DFT-optimized geometry of 1 [Fe−O(H) = 2.02 Å and Fe−O(H)−Fe = 151°]; the Fe−O(H) distance in 1 is about 0.2 Å longer than the Fe−O separations in the optimized geometry of 2 (1.84 Å) and in the crystallographic structures of diiron(III) compounds with unsupported μ-oxo bridges (1.77−1.81 Å). The formulation of 1 as a hydroxo-bridged compound is also supported by the presence of an O−H stretch band in the FTIR spectrum of the complex. The magnetic susceptibility measurements of 1 reveal antiferromagnetic exchange (J = 42 cm-1 and Hex = JS1·S2). Nearly the same J value is obtained by analyzing the temperature dependence of the Mössbauer spectra (J = 43 cm-1; other parameters:  δ = 0.49 mm s-1, ΔEQ = −0.97 mm s-1, and η = 0.45 at 4.2 K). The experimental J values and Mössbauer parameters agree very well with those obtained from DFT calculations for the μ-hydroxo-bridged compound (J = 46 cm-1, δ = 0.48 mm s-1, ΔEQ = −1.09 mm s-1, and η = 0.35). The exchange coupling constant in 1 is distinctly different from the value J ≈ 200 cm-1 calculated for the optimized μ-oxo-bridged species, 2. The increased exchange-coupling in 2 arises primarily from a decrease in the Fe−O bond length.

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