Reductive Insertion of a Trivalent Metal Leading to the Isolation of Prussian Blue-Type EuIIIFe3[Fe(CN)6]3
Mollick, S. S.; Sur, S.; Fritsky, J. W.; Koley, N.; Storteboom, R. S.; Khoury, J. F.; Snyder, B. E. R.; Aubrey, M. L.
Reductive insertion reactions selectively modify three-dimensionally bonded solid-state structures through the addition of an elemental metal by electronation of the host structure and insertion of the metal cation into the host cavities for charge balance. However, with rare exceptions, this class of reactions has been largely restricted to insertions of monovalent metals. The reductive insertion of divalent and trivalent metal ions is intrinsically limited by their very slow diffusion through a host lattice at low temperatures and instability of a host lattice in the presence of these metals at high temperatures. These limitations originate from strong coordination of multivalent guest species to anions within the host lattice. By using coordinatively saturated anions such as cyanide in Prussian blue analogues, only relatively weak coordination modes to guest cations are available within the host structures, suggesting these host structures may participate in multivalent metalations. Herein, we report reductive insertion of europium into Berlin green Fe[Fe(CN)6] under relatively mild conditions by reaction with EuI2 at 150 °C in N-methyl-2-pyrrolidone. The formation of the half-ferrous product, EuIIIFe3[Fe(CN)6]3, is an exceptionally rare reductive insertion of a trivalent metal, and the first example of a phase pure Prussian blue (An+1/nFe[Fe(CN)6]; A = metal cation) isolated with either a lanthanide or multivalent A-site metal. The resulting compound exhibits a distribution of iron oxidation and spin states not observed in previously reported Prussian blue phases of the form MIFe[Fe(CN)6]. The valence of Eu3+ was confirmed by a combination of X-ray absorption, 57Fe Mössbauer, and magnetic susceptibility measurements, which informed a possible reaction mechanism invoking redox exchange between the ferrous host, Eu3+ guest ions, and excess Eu2+ in solution.