The current quest for increased capacity of information storage devices has fuelled intense research of molecule-based materials that can be bistable, i.e. that have two different states accessible within the same domain of environmental conditions. Numerous mononuclear coordination complexes of Fe(II) can adopt two d-electron configurations, and consequently, two spin states. Some of these complexes manifest bistability underscored by strong intermolecular interactions exerted in solid state.  Our research is focused on the investigation of molecules that can undergo spin transitions and contain multiple Fe(II) ions, and in which the transition can be affected by intramolecular interactions between the metal ions. 

We have studied pentanuclear cyanide-bridged clusters [M(tmphen)₂]₃[M'(CN)₆]₂ (M/M' = Zn/Cr (1), Zn/Fe (2), Fe/Fe (3), Fe/Co (4), and Fe/Cr (5); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline), whose design was inspired by Prussian Blue.  These complexes are prepared in the laboratory of our collaborator, Professor Kim Dunbar at Texas A&M.  The characterization of the complexes is based on a combination of Mössbauer, IR, and EPR spectroscopies, X-ray crystallography, magnetic susceptibility measurements and DFT calculations.

The clusters consist of a trigonal bipyramidal (TBP) core with three M^II ions in the equatorial positions and two M'^III ions in the axial positions.  Compounds 1–4 are isostructural and crystallize in the monoclinic space group P2₁/c.  Complex 5 crystallizes in the enantiomorphic space group P3₂21. 

The magnetic properties of compounds 1 and 2 are analogous to those observed for the individual [Cr^III(CN)₆]^3– and [Fe^III(CN)₆]^3– ions.  The Fe^II ions in compounds 3 and 4, contain Fe^II ions with a nitrogen-rich coordination, i.e. [Fe^II(tmphen)₂(NC)₂], and consequently exhibit a gradual, temperature-induced spin transition between high spin Fe^II and low spin Fe^II as determined by Mössbauer spectroscopy, magnetic measurements, and single crystal X-ray studies.  The investigation of compound 5 by these methods and by IR spectroscopy indicated that cyanide linkage isomerism occurs during cluster formation and thus contains Cr^III-NC-Fe^II-CN-Cr^III units.  The magnetic behavior of 5 at lower temperatures is characterized by weak ferromagnetic coupling between the axial Cr^III centers mediated by the equatorial diamagnetic Fe^II ions.  Mössbauer spectra collected in the presence of a high applied magnetic field have allowed, for the first time, the direct experimental observation of uncompensated spin density at diamagnetic metal ions that bridge paramagnetic metal ions. 

The framework of complexes 1-5 is similar to that of the cyanide-bridged, trigonal bipyramid heterometallic {[M(tmphen)₂]₃[M'(CN)₆]₂} (M/M' = Co/Fe (6) cluster in which we have already discovered spin transitions correlated with intramolecular electron transfer between metal ions (J. Am. Chem. Soc. 2005, 127, 6766-6779), and thus supports a remarkably broad range of physical properties.  This work has been published in the Journal of the American Chemical Society.

In the summer of 2007, Professor Doros Petasis, from Allegheny College was supported by a ACS PRF Summer Research Fellowship to conduct research in our laboratory and has studied by Mössbauer and EPR spectroscopies complex 6, as well as related complexes that have the potential to manifest a charge transfer coupled spin transition.  These results are promising and we continue our collaboration with the anticipation that a paper will be submitted in the next few months.

            We have also studied in collaboration with the group of Professor Michael Hannon from University of Birmingham, UK, a series of dinuclear tetracationic triple-helical complexes of Fe^II, [Fe₂L₃]X₄.nH₂O where X = BF₄, Cl, or PF₆ and n = 0-4, which have cylindrical shape with ~2nm length and ~1nm in diameter, and are of comparable size to protein DNA-recognition motifs such as zinc finger proteins. Each ligand L contains two bidentate units, for example pyridylimine or imidazolimine, through which it coordinates to the iron ions to form the cylinder (Figure 1).   Each iron centre has C3 symmetry and the two irons ions of each helicate are either D or L optical isomers, thereby creating either a P or an M helicate, respectively.  A detailed and careful study of these complexes showed that they manifest a multiple-step spin transition, which depends on the nature of the anion and the degree of solvation of the compounds.  The results of this study will be submitted for publication within a couple of months.