Synthesis and structural investigation of sandwich polyoxotungstates containing cerium (III/IV) and mono-lacunary Keggin tungstophosphate units

The two isostructural anions, [CeIII(α-PW11O39)2]11− and [CeIV(α-PW11O39)2]10−, both composed of CeIII/IV sandwiched by two mono-lacunary Keggin units [α-PW11O39]7−, were isolated as dimethylammonium salts and structurally characterized by single-crystal X-ray diffraction analysis. The structures of the [CeIII/IV(α-PW11O39)2]11−/10− anions exhibit two enantiomers d- and l-forms due to C2 (2) symmetry from the square-antiprismatic coordination of the central CeIII/IVO8 polyhedron. The CeIII compound crystallizes in achiral space group P21/n, while the CeIV analog in chiral space group P21 shows spontaneous resolution without any chiral auxiliaries. The correlation between the ionic radius of the central metal (M) and twist angle of the two mono-lacunary units was studied for a series of [M(α-PW11O39)2]n− (M = CeIII/IV, EuIII, ZrIV, HfIV) anions and explained in terms of steric repulsion between the O atoms in the mono-lacunary units.