Isolation of Intermediate-Valent Ce(III)/Ce(IV) Hydrolysis Products in the Preparation of Cerium Iodates: Electronic and Structural Aspects of Ce2(IO3)6(OHx) (x (almost equal)0 and 0.44)

The layered cerium iodates Ce2(IO3)6(OHx) [x (almost equal)0 (1) and 0.44 (2)] have been prepared from the reaction of (NH4)2Ce(NO3)6 with I2O5 at 180 °C in aqueous media. The structure of these compounds consists of square antiprismatic Ce centers that are bound by bridging iodate anions and one bridging oxo anion. The structure of 2 differs from that of 1 in that the bridging oxo atom is partially protonated, resulting in significant lengthening of the Ce-O bond from 2.050(1) to 2.212 (2) (angstrom). This protonation also results in partial reduction of 1 from a formally all Ce4+ compound to an intermediate-valent ion with both Ce3+ and Ce4+ that is visually observed with a color change in the crystals from bright yellow to dark brown for 1 and 2, respectively. Magnetic susceptibility measurements on 1 show there to be a small paramagnetic impurity phase that is consistent with about 1% Ce3+ in the sample. Similar measurements on 2 show that it contains at least 22% Ce3+, determined from a measured effective moment of 1.19(5) µB/mol Ce. This result corresponds to a formulation of 2 as Ce2(IO3)6 (OH0.44). Analysis of Ce L3-edge XANES spectra supports this formulation. UV-vis diffuse reflectance spectra indicate charge-transfer or delocalization of the 4f1 electrons in 2. These results are discussed in terms of an intermediate-valent state in 2 that is composed of f states on Ce and p states on the bridging O. Crystallographic data (193 K): 1, orthorhombic, space group Pnma, a = 6.8297(4) (angstrom), b = 16.6725(9) (angstrom), c = 14.2487(8) (angstrom), Z = 4; 2, orthorhombic, space group Pnma, a = 6.8534(4) (angstrom), b = 16.989(1) (angstrom), c = 14.2404(8), Z = 4.