Properties of prussian blue materials manifested in molecular complexes: Observation of cyanide linkage isomerism and spin-crossover behavior in pentanuclear cyanide clusters

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) were prepared by combining [M'(III)(CN)(6)](3-) anions with mononuclear complexes of M-II ions with two capping tmphen ligands. 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 P2(1)/c. Complex 5 crystallizes in the enantiomorphic space group P3(2)21. The magnetic properties of compounds 1 and 2 reflect the contributions of the individual [Cr-III(CN)(6)](3-) and [Fe-III(CN)(6)](3-) ions. The Fe-II ions in compounds 3 and 4 exhibit a gradual, temperature-induced spin transition between high spin (HS) and low spin (LS), as determined by the combination of Mossbauer spectroscopy, magnetic measurements, and single-crystal X-ray studies. The investigation of compound 5 by these methods and by IR spectroscopy indicates that cyanide linkage isomerism occurs during cluster formation. The magnetic behavior of 5 is determined by weak ferromagnetic coupling between the axial Cr-III centers mediated by the equatorial diamagnetic Fe-II ions. Mossbauer spectra collected in the presence of a high applied field have allowed, for the first time, the direct experimental observation of uncompensated spin density at diamagnetic metal ions that bridge paramagnetic metal ions.