Salicylaldoxime (H 2salox) in iron(III) carboxylate chemistry: Synthesis, X-ray crystal structure, spectroscopic characterization and magnetic behavior of trinuclear oxo-centered complexes

Abstract

Two new neutral trinuclear oxo-centered complexes, [Fe 3(μ 3-O)(O 2CPh) 5(salox)L 1L 2] [L 1 = L 2 = MeOH ( 1), L 1 = EtOH, L 2 = H 2O ( 2)] are reported. In both complexes, the central oxygen atom is bridging the three iron(III) ions, which form a triangle with an average edge of ∼3.3 Å. The central [Fe 3(μ 3-O)] 7+ core is planar. The benzoato ligands are coordinated through the usual syn, syn μ 2:η 1:η 1 mode and the salicylaldoximato ligand shows the common μ 2:η 1:η 1:η 1 coordination mode. Two of the iron(III) ions have an O 6 coordination environment while the third has an O 5N one. The two crystallographically independent trimers in the structure of 1 ( a and b, respectively) through intermolecular interactions form polymeric chains of the …a–a–b–b… type, along the ac diagonal of the unit cell. In 2, again through intermolecular interactions, the trimers form layers lying parallel to the [1 0 1] plane. Mössbauer spectra at room temperature from polycrystalline samples of 1 and 2 give rise to two well-resolved doublets with an average isomer shift consistent with high spin iron(III) ions. The two doublets, at 2:1 ratio, are characterized by different quadrupole splitting and are assigned to the nonequivalent iron(III) ions of the clusters. Magnetic measurements of 1 and 2 in the 2–300 K temperature range show antiferromagnetic interactions between the iron(III) ions in the trimers stabilizing a S = 1/2 ground state. The derived values for the exchange interaction constants fall in the range usually found in [Fe 3(μ 3-O)] 7+ clusters. Solid state X-band EPR spectra of 1 and 2 at liquid helium temperatures give rise to broad lines extending several hundred Gauss, indicating weak intermolecular interactions. EPR spectra from 1 and 2 in an acetone glass at 4.2 K exhibit a strong symmetric signal at g ∼ 2.0 which is attributed to the S = 1/2 ground state of the clusters.