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}<sub>2</sub>(OH)][B(C<sub>6</sub>H<sub>5</sub>)<sub>4</sub>]·(CH<sub>3</sub>CN)<i><sub>x</sub></i>·(H<sub>2</sub>O)<i><sub>y</sub></i> (<b>1</b>) [H<sub>2</sub>salten = 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 <b>1</b> and a model with an unsupported μ-oxo bridge (<b>2</b>) 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 <b>1</b> [Fe−O(H) = 2.02 Å and Fe−O(H)−Fe = 151°]; the Fe−O(H) distance in <b>1</b> is about 0.2 Å longer than the Fe−O separations in the optimized geometry of <b>2</b> (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 <b>1</b> 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 <b>1</b> reveal antiferromagnetic exchange (<i>J</i> = 42 cm<sup>-1</sup> and <i>H</i><sub>ex</sub> = <i>J</i><b>S</b><sub>1</sub>·<b>S</b><sub>2</sub>). Nearly the same <i>J</i> value is obtained by analyzing the temperature dependence of the Mössbauer spectra (<i>J</i> = 43 cm<sup>-1</sup>; other parameters:  δ = 0.49 mm s<sup>-1</sup>, Δ<i>E</i><sub>Q</sub> = −0.97 mm s<sup>-1</sup>, and <i>η</i> = 0.45 at 4.2 K). The experimental <i>J</i> values and Mössbauer parameters agree very well with those obtained from DFT calculations for the μ-hydroxo-bridged compound (<i>J</i> = 46 cm<sup>-1</sup>, δ = 0.48 mm s<sup>-1</sup>, Δ<i>E</i><sub>Q</sub> = −1.09 mm s<sup>-1</sup>, and <i>η</i> = 0.35). The exchange coupling constant in <b>1</b> is distinctly different from the value <i>J</i> ≈ 200 cm<sup>-1</sup> calculated for the optimized μ-oxo-bridged species, <b>2</b>. The increased exchange-coupling in <b>2</b> arises primarily from a decrease in the Fe−O bond length.