Mixed-valence heptanuclear iron complexes with ferromagnetic interaction.

Three new Prussian blue analogues, heptanuclear mixed-valence iron complexes of the type [Fe(II)(CN)(6){Fe(III)(1(-2H))}(6)]Cl(2)·nH(2)O, were synthesized and structurally and spectrally characterized, and their magnetic properties were investigated (1(-2H) corresponds to doubly deprotoned Schiff-base pentadentate ligands 1a, N,N'-bis(2-hydroxybenzylidene)-1,5-diamino-3-azapentane, 1b, N,N'-bis(3-ethoxy-2-hydroxybenzylidene)-1,7-diamino-4-azaheptane, or 1c, N,N'-bis(3-methoxy-2-hydroxybenzylidene)-1,6-diamino-3-azahexane). These compounds were formed by assembling the [Fe(CN)(6)](4-) building block with mononuclear complexes of the [Fe(1(-2H))Cl] type. X-ray structure analysis revealed that the complexes adopt a star-like architecture: the Fe(II) ion lies at the very center, and on its octahedral nodes the Fe(III) sites are coordinated in the Fe(II)-C≡N-Fe(III) manner. The Schiff-base pentadentate ligand moiety 1(-2H) coordinates a single Fe(III) center in two complexes 3b and 3c. Ligands 1a(-2H) in the complex cation of 3a adopt an unusual coordination mode: three donor atoms of the same ligand (one O and two N) coordinate one Fe(III), whereas the remaining N' and O' donor atoms coordinate the neighboring Fe(III) center creating the {Fe(ON(2))(N'O')N″} chromophore involving two 1a(-2H) ligand moieties. Moreover, three Fe(III) centers are interconnected with three 1a(-2H) ligands in such a manner that two {Fe(III)(3)(1a(-2H))(3)} units form two intramolecular rings. Magnetic investigation of the heptanuclear complexes revealed the high-spin state of all six Fe(III) coordination sites (s = 5/2), while the very central Fe(II) site is in the low-spin state (s = 0). At low temperature, the ferromagnetic exchange interactions stay evident for all three complexes. Mössbauer spectra of compounds 3a and 3b revealed a presence of two different doublets for both compounds: the major doublet is related to six Fe(III) high-spin coordination sites and the minor doublet refers to the low-spin very central Fe(II).