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 mu-hydroxo bridging ligand, [{(salten)Fe}(2)-(OH)][B(C6H5)(4)](x)center dot(CH3CN)(x)center dot(H2O)(y) (1) [H(2)salten = 4-azaheptane-1,7-bis(salicylideneiminate)], is reported. The complex has been characterized with X-ray crystallography, FTIR, magnetic susceptibility measurements, and Mossbauer spectroscopy. The data have been compared with the results of DFT calculations on both 1 and a model with an unsupported mu-oxo bridge (2) to verify the formulation of the complex as a mu-hydroxo-bridged species. The X-ray structure [Fe-O(H) = 1.997(1) angstrom and Fe-O(H)-Fe = 159 degrees] is consistent with the DFT-optimized geometry of 1 [Fe-O(H) = 2.02 angstrom and Fe-O(H)-Fe = 151 degrees]; the Fe-O(H) distance in 1 is about 0.2 angstrom longer than the Fe-O separations in the optimized geometry of 2 (1.84 angstrom) and in the crystallographic structures of diiron(III) compounds with unsupported mu-oxo bridges (1.77-1.81 angstrom). 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 H-ex = JS(1)center dot S-2). Nearly the same J value is obtained by analyzing the temperature dependence of the Mossbauer spectra (J = 43 cm(-1); other parameters: delta = 0.49 mm s(-1), Delta E-Q = -0.97 mm s(-1), and eta = 0.45 at 4.2 K). The experimental J values and Mossbauer parameters agree very well with those obtained from DFT calculations for the mu-hydroxo-bridged compound (J = 46 cm(-1), delta = 0.48 mm s(-1), Delta E-Q = -1.09 mm s(-1), and eta = 0.35). The exchange coupling constant in 1 is distinctly different from the value J approximate to 200 cm(-1) calculated for the optimized mu-oxo-bridged species, 2. The increased exchange-coupling in 2 arises primarily from a decrease in the Fe-O bond length.