Photoreductive Self-Assembly from Mo_7O_(24)~(6-) to Carboxylates-Coordinated {Mo_(142)} Mo-Blue Nanoring in the Presence of Carboxylic Acids

摘要

The primary steps of the photoredox reaction between Mo_7O_(24)~(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 Mo_7O_(24)~(6-) in the presence of CH_3CO_2H induces the emissive electron spin polarization (ESP) of ~(centre dot )CH_2CO_2H and ~(centre dot)CH_3 radicals with an accompanying formation of the one-electron reduced species Mo_7O_(23)(OH)~(6-),which is demonstrated by the triplet mechanism involving the O - Mo LMCT triplet states.The prolonged photolysis of the solution containing Mo_7O_(24)~(6-) and CH_3CO_2H at pH = 3.4 leads to the formation of the acetate/propionate-coordinated {Mo_(142)} Mo-blue nanoring,Mo~v_(28)Mo~(VI)_(114)O_(429)H_(10)-(H_2O)_(49)(CH_3CO_2 ident to Ac)_5(C_2H_5CO_2 ident Pr)~(30-) (la) through the formation of the cis -configured dimeric dehydrative condensation to two-electron reduced Mo-blue (Mo_7O_(23))_2~(10) (ident to {Mo_(14)}).la is isolated as a NH_4~+/Me_3- NH~+-mixed salt which is formulated as NH_4_(27)Me_3NH_3 Mo~V_(28)Mo~(VI)_(114)O_(429)H_(10))(H_2O)_(49)(CH_3CO_2)_5(C_2H_5-CO_2) centre dot 50 +- l0 H_2O (1) by results of elementary analysis,single-crystal X-ray analysis,~1H 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)} forla,the bottom-up processes from Mo_7O_(24)~(6-) to the {Mo_(142)} ring in the coexistence of beta-Mo_8O_(26)~(4-) are discussed by (i) 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 beta-Mo_8O_(26)~(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.