Hexanuclear Iron(III) Salicylaldoximato Complexes Presenting the [Fe₆(μ₃-O)₂(μ₂-OR)₂]¹²⁺ Core: Syntheses, Crystal Structures, and Spectroscopic and Magnetic Characterization

The use of salicylaldehyde oxime (H2salox) in iron(III) carboxylate chemistry has yielded two new hexanuclear compounds [Fe6(μ3-O)2(O2CPh)10(salox)2(L)2]·xMeCN·yH2O [L = MeCONH2, x = 6, y = 0 (1); L = H2O, x = 2, y = 3 (2)]. Compound 1 crystallizes in the triclinic space group P1̄ with (at 25 °C) a = 13.210(8) Å, b = 13.87(1) Å, c = 17.04(1) Å, α = 105.79(2)°, β = 96.72(2)°, γ = 116.69(2)°, V = 2578.17(2) Å3, and Z = 1. Compound 2 crystallizes in the monoclinic space group C2/c with (at 25 °C) a = 21.81(1) Å, b = 17.93(1) Å, c = 27.72(1) Å, β = 111.70(2)°, V = 10070(10) Å3, and Z = 4. Complexes 1 and 2 contain the [Fe6(μ3-O)2(μ2-OR)2]12+ core and can be considered as two [Fe3(μ3-O)] triangular subunits linked by two μ2-oximato O atoms of the salox2- ligands, which show the less common μ3:η1:η2:η1 coordination mode. The benzoato ligands are coordinated through the usual syn,syn-μ2:η1:η1 mode. The terminal MeCONH2 ligand in 1 is the hydrolysis product of the acetonitrile solvent in the presence of the metal ions. Mössbauer spectra from powdered samples of 2 give rise to two well-resolved doublets with an average isomer shift consistent with that of high-spin FeIII ions. The two doublets, at an approximate 1:2 ratio, are characterized by different quadrupole splittings and are assigned to the nonequivalent FeIII ions of the cluster. Magnetic measurements of 2 in the 2−300 K temperature range reveal antiferromagnetic interactions between the FeIII ions, stabilizing an S = 0 ground state. NMR relaxation data have been used to investigate the energy separation between the low-lying states, and the results are in agreement with the susceptibility data.