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

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.