Chirality of Oxidized and Reduced Octadecamolybdodiphosphate Anions. Observation of a Pfeiffer Effect

Recently determined structures of P₂Mo₁₈0₆₂^6- and α-P₂W180₆₂^6- show that the molybdate has only Z)3 symmetry and is therefore chiral. The infrared spectra of both complexes in the solid state and in acetonitrile solution are reported and they confirm that the chirality of the molybdate is not caused by crystal packing forces. Solutions of the tetrabutylammonium salt of the heteropolymolybdate in acetonitrile in the presence of l-brucine sulfate show a significant positive Cotton effect centered at 350 nm ([M]₃₈₀ = +1.4 × 10₃ deg dm^-1 M^-1). This behavior is attributed to a Pfeiffer effect since analogous measurements with the nonchiral polyanions α- and O-P₂W18O626-, α-SiMo₁₂O₄₀^4-, and Mo₆O₁₉^2- show only minor amounts of induced optical activity. The heteropoly blue H₂P₂Mov₂Mo₁₆0₆₂^6- also exhibits a Pfeiffer effect but of diminished magnitude and no optical activity is associated with the intervalence charge-transfer transitions. An explanation of this behavior and of the apparent nonexistence of odd reduction states of P₂Mo₁₈0₆₂^6- is presented.