Photoreductive self-assembly from [Mo7O24]6- to carboxylates-coordinated {Mo142} Mo-blue nanoring in the presence of carboxylic acids

The primary steps of the photoredox reaction between [Mo7O24](6-) and carboxylic acid electron (and proton) donors in aqueous solutions are investigated by the chemically induced dynamic electron spin polarization (CIDEP) spectroscopy. The excitation of the O -> Mo ligand-to-metal charge-transfer (LMCT) bands of [Mo7O24](6-) in the presence of CH3CO2H induces the emissive electron spin polarization (ESP) of center dot CH2CO2H and center dot CH3 radicals with an accompanying formation of the one-electron reduced species [Mo7O23(OH)](6-), which is demonstrated by the triplet mechanism involving the O -> Mo LMCT triplet states. The prolonged photolysis of the solution containing [Mo7O24](6-) and CH3CO2H at pH = 3.4 leads to the formation of the acetate/propionate-coordinated {Mo-142} Mo-blue nanoring, [(Mo28Mo114O429H10)-Mo-V-O-VI-(H2O)(49)(CH3CO2 equivalent to Ac)(5)(C2H5CO2 equivalent to Pr)](30-) (1a) through the formation of the cis-configured dimeric dehydrative condensation to two-electron reduced Mo-blue [(Mo7O23)(2)](10-) (equivalent to{Mo-14}). la is isolated as a [NH4](+)/[Me-3-NH](+)-mixed salt which is formulated as [NH4](27)[Me3NH](3)[(MO28Mo114O429H10)-Mo-V-O-VI(H2O)(49)(CH3CO2)(5)(C2H5CO2)]center dot 150 +/- 10H(2)O (1) by results of elementary analysis, single-crystal X-ray analysis, H-1 NMR, IR, and UV/Vis measurements, and manganometric redox titration. Based on the building-block sequence of {Mo-20(Ac)(Pr)(1/4)}{Mo-21(Ac)(1/2)(Pr)(1/4)}{Mo-20(Ac)}{Mo-20(Ac)}{Mo-21(Ac)(1/2)(Pr)(1/4)}{Mo-20(Ac)(Pr)(1/4)}{Mo-20} for 1a, the bottom-up processes from [Mo7O24](6-) to the {Mo-142} ring in the coexistence of beta-[Mo8O26](4-) are discussed by G) the stabilization of the molecular curvature Of {Mo-14} through both the intramolecular transfer of monomolybdates and the intermolecular transfer of monomolybdates as degradation fragments of P-[Mo8O26](4-), to yield {Mo-21} and {Mo-20} building blocks, (ii) the outer-ring formation resulting from seven successive two-electron-photoreductive condensations among {Mo-21} and {Mo-20}, and (iii) inner-ring formation resulting from eight successive dehydrative condensations between monomolybdate linkers attached to the neighboring head Mo sites.