{Mo₉₆La₈} Eggshell Ring and Self-Assembly to {Mo₁₃₂} Keplerate through Mo-blue Intermediate, Involved in UV-Photolysis of [Mo₇O₂₄]⁶⁻/Carboxylic Acid System at pH 4

The prolonged UV-photolysis of aqueous solutions containing [Mo7O24]6− and C2H5CO2H (as electron donor) at pH 3.9−4.1 generates the carboxylate-coordinated {Mo132} Keplerate (1a) isolated as a formamidinium/ammonium-mixed salt, [HC(NH2)2]26(NH4)28[MoV60MoVI72O372(H2O)48(C2H5CO2)36(iC3H7CO2)6]·16H2O (1), through the Mo-blue intermediate (2). The coordination of 2 to La3+ gives rise to the formation of the chain structure of the C2-symmetric {Mo96La8} eggshell rings, formulated by H22[MoV20MoVI76O301(H2O)29{La(H2O)6}2]{La(H2O)5}6]·54.5H2O (3). The eggshell-ring geometry results from the insertion of [MoVI2O7(H2O)]2− (spacer) into the equator outer ring of the wheel-shaped Mo-blue, and 10 {(MoVI)(MoVI5)} pentagonal subunits alternately above and below the equator outer ring are connected by eight La3+ and two {MoVI2} linkers within two inner rings. The neighboring eggshell rings are linked through two Mo−O−Mo bonds formed by dehydrative condensation between the {MoVI2} linkers to result in the chain structure. Together with the results of the elemental analysis and IR, electronic absorption, 13C NMR, and ESI-MS spectra for 2, the ring profile analysis of 3 let us identify 2 with a carbolylate-coordinated Mo-blue ring of high nuclearity. The MoVI→MoV photoreductive change of 2 to the 60-electron reduced Keplerate in the presence of C2H5CO2H involves both degradation of the outer ring and splitting of the binuclear linkers, which leads to the formation of [(MoVI)MoVI5O21(H2O)4(carboxylate)]7− pentagonal subunits and [MoV2O4(carboxylate)]+/[MoVO2(carboxylate)1/2]0.5+-mixed linkers for 1.