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

The use of salicylaldehyde oxime (H 2 salox) in iron(III) carboxylate chemistry has yielded two new hexanuclear compounds [Fe 6 (µ 3-O) 2 (O 2 CPh) 10 (salox) 2 (L) 2 ]‚xMeCN‚yH 2 O [L) MeCONH 2 , x) 6, y) 0 (1); L) H 2 O, x) 2, y) 3 (2)]. Compound 1 crystallizes in the triclinic space group P1 h with (at 25°C) a) 13.210(8) Å, b) 13.87(1) Å, c) 17.04(1) Å, R) 105.79(2)°,) 96.72(2)°, γ) 116.69(2)°, V) 2578.17(2) Å 3 , 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,) 111.70(2)°, V) 10070(10) Å 3 , and Z) 4. Complexes 1 and 2 contain the [Fe 6 (µ 3-O) 2 (µ 2-OR) 2 ] 12+ core and can be considered as two [Fe 3 (µ 3-O)] triangular subunits linked by two µ 2-oximato O atoms of the salox 2ligands, which show the less common µ 3 :η 1 :η 2 :η 1 coordination mode. The benzoato ligands are coordinated through the usual syn,syn-µ 2 :η 1 :η 1 mode. The terminal MeCONH 2 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 wellresolved doublets with an average isomer shift consistent with that of high-spin Fe III ions. The two doublets, at an approximate 1:2 ratio, are characterized by different quadrupole splittings and are assigned to the nonequivalent Fe III ions of the cluster. Magnetic measurements of 2 in the 2−300 K temperature range reveal antiferromagnetic interactions between the Fe III 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.