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]­Cl₂·<i>n</i>H₂O, were synthesized and structurally and spectrally characterized, and their magnetic properties were investigated (1_–2H corresponds to doubly deprotoned Schiff-base pentadentate ligands <b>1a</b>, <i>N</i>,<i>N</i>′-bis­(2-hydroxybenzylidene)-1,5-diamino-3-azapentane, <b>1b</b>, <i>N</i>,<i>N</i>′-bis­(3-ethoxy-2-hydroxybenzylidene)-1,7-diamino-4-azaheptane, or <b>1c</b>, <i>N</i>,<i>N</i>′-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 <b>3b</b> and <b>3c</b>. Ligands 1a_–2H in the complex cation of <b>3a</b> 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 {Fe^III₃(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 (<i>s</i> = 5/2), while the very central Fe­(II) site is in the low-spin state (<i>s</i> = 0). At low temperature, the ferromagnetic exchange interactions stay evident for all three complexes. Mössbauer spectra of compounds <b>3a</b> and <b>3b</b> 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)

Bistability of Fc-PTM-Based Dyads: The Role of the Donor Strength

Bistability of valence tautomeric donor–acceptor dyads formed by covalently linking a ferrocene-based electron-donor and the perchlorotriphenylmethyl radical, as the electron-acceptor, is tuned via a chemical modification of the ferrocene group. Specifically, the methylation of the ferrocene unit increases the strength of the donor group stabilizing the zwitterionic state in polar solvents and leading to an intriguing coexistence of neutral and zwitterionic species in solvents of intermediate polarity. Bistability in the crystalline phase is demonstrated by temperature dependent Mössbauer spectra. This complex and interesting behavior is quantitatively rationalized in the framework of a bottom-up modeling strategy. Optical spectra in solution are first analyzed to extract and parametrize an effective two-state molecular model, which is then adopted to rationalize the observed bistability in the solid state as due to cooperative electrostatic interchromophore interactions

Bistability of Fc-PTM-Based Dyads: The Role of the Donor Strength

Bistability of valence tautomeric donor–acceptor dyads formed by covalently linking a ferrocene-based electron-donor and the perchlorotriphenylmethyl radical, as the electron-acceptor, is tuned via a chemical modification of the ferrocene group. Specifically, the methylation of the ferrocene unit increases the strength of the donor group stabilizing the zwitterionic state in polar solvents and leading to an intriguing coexistence of neutral and zwitterionic species in solvents of intermediate polarity. Bistability in the crystalline phase is demonstrated by temperature dependent Mössbauer spectra. This complex and interesting behavior is quantitatively rationalized in the framework of a bottom-up modeling strategy. Optical spectra in solution are first analyzed to extract and parametrize an effective two-state molecular model, which is then adopted to rationalize the observed bistability in the solid state as due to cooperative electrostatic interchromophore interactions