Structure Characterization and Properties of K‑Containing Copper Hexacyanoferrate

Copper hexacyanoferrate,
CuII[FeIII(CN)6]2/3·nH2O, was synthesized, and varied amounts
of K+ ions were inserted via reduction by K2S2O3 (aq). Ideally, the reaction can be written
as CuII[FeIII(CN)6]2/3·nH2O + 2x/3K+ + 2x/3e– ↔ K2x/3CuII[FeIIxFeIII1–x(CN)6]2/3·nH2O. Infrared, Raman, and Mössbauer spectroscopy studies show
that FeIII is continuously reduced to FeII with
increasing x, accompanied by a decrease of the a-axis of the cubic Fm3̅m unit cell. Elemental analysis of K by inductively coupled plasma
shows that the insertion only begins when a significant fraction,
∼20% of the FeIII, has already been reduced. Thermogravimetric
analysis shows a fast exchange of water with ambient atmosphere and
a total weight loss of ∼26 wt % upon heating to 180 °C,
above which the structure starts to decompose. The crystal structures
of CuII[FeIII(CN)6]2/3·nH2O and K2/3Cu­[Fe­(CN)6]2/3·nH2O were
refined using synchrotron X-ray powder diffraction data. In both,
one-third of the Fe­(CN)6 groups are vacant, and the octahedron
around CuII is completed by water molecules. In the two
structures, difference Fourier maps reveal three additional zeolitic
water sites (8c, 32f, and 48g) in the center of the cavities formed
by the −Cu–N–C–Fe– framework. The
K-containing compound shows an increased electron density at two of
these sites (32f and 48g), indicating them to be the preferred positions
for the K+ ions.