New family of ferric spin clusters incorporating redox-active ortho-dioxolene ligands.

Seven new di-, tri-, tetra-, and hexanuclear iron complexes that incorporate a polydentate Schiff base and variously substituted catecholate ligands have been synthesized from the trinuclear precursor [Fe(3)(OAc)(3)(L)(3)] (1), where LH(2) = 2-[[(2-hydroxyethyl)imino]phenylmethyl]-phenol. These were isolated as the compounds [Fe(3)(OAc)(Cat)(L)(3)] (2), [Fe(6)(OAc)(2)(Cat)(4)(L)(4)] (3), [Fe(4)(3,5-DBCat)(2)(L)(4)] (4), [Bu(4)N][Fe(4)(OAc)(3,5-DBCat)(4)(L)(2)] (5a, 5(-) is the complex monoanion [Fe(4)(OAc)(3,5-DBCat)(4)(L)(2)](-)), [Fe(4)(OAc)(3,5-DBCat)(3)(3,5-DBSQ)(L)(2)] (6), [Fe(2)(Cl(4)Cat)(2)(L)(LH(2))(H(2)O)] (7), and [Et(3)NH](2)[Fe(2)(Cl(4)Cat)(2)(L)(2)] (8a, 8(2-) is the complex dianion [Fe(2)(Cl(4)Cat)(2)(L)(2)](2-)), where CatH(2) = catechol; 3,5-DBCatH(2) = 3,5-di-tert-butyl-catechol; 3,5-DBSQH = 3,5-di-tert-butyl-semiquinone, and Cl(4)CatH(2) = tetrachlorocatechol. While compounds 2-4, 5a, 7, and 8a were obtained by directly treating 1 with the appropriate catechol, compound 6 was synthesized by chemical oxidation of 5a. These compounds have been characterized by single crystal X-ray diffraction, infrared and UV-visible spectroscopy, voltammetry, UV-visible spectroelectrochemistry, and magnetic susceptibility and magnetization measurements. An electrochemical study of the three tetranuclear complexes (4, 5(-), and 6) reveals multiple reversible redox processes due to the o-dioxolene ligands, in addition to reductive processes corresponding to the reduction of the iron(III) centers to iron(II). A voltammetric study of the progress of the chemical oxidation of compound 5a, together with a spectroelectrochemical study of the analogous electrochemical oxidation, indicates that there are two isomeric forms of the one-electron oxidized product. A relatively short-lived neutral species (5) that possesses the same ligand arrangement as complex 5(-) is the kinetic product of both chemical and electrochemical oxidation. After several hours, this species undergoes a significant structural rearrangement to convert to complex 6, which appears to be largely driven by the preference for the 3,5-DBSQ(-) ligand to bind in a non-bridging mode. Variable temperature magnetic susceptibility measurements for compounds 3, 4, 5a, 6, 7, and 8a reveal behavior dominated by pairwise antiferromagnetic exchange interactions, giving rise to a poorly isolated S = 0 ground state spin for compound 3, well-isolated S = 0 ground state spins for complexes 4, 5(-), 7 and 8(2-), and a well-isolated S = 1/2 ground state spin for complex 6. The ground state spin values were confirmed by low temperature variable field magnetization measurements. The thermal variation of the magnetic susceptibility for compounds 3, 4, 5a, 6, 7, and 8a were fitted and/or simulated using the appropriate Hamiltonians to derive J values that are consistent with magnetostructural correlations that have been reported previously for alkoxo-bridged ferric complexes.