Mixed-Valence Heptanuclear Iron Complexes with Ferromagnetic Interaction

Three new Prussian blue analogues, heptanuclear mixed-valence iron complexes of the type [FeII(CN)6{FeIII(1–2H)}6]­Cl2·nH2O, 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 FeII–CN–FeIII 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(ON2)­(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 {FeIII3(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).