Synthesis And Catalytic Application Of Subnano Metal Clusters Templated By G-/C-Rich Oligonucleotides

Subnano metal clusters, which consist of several to tens of metal atoms andpossess dimensions smaller than1nm, usually exhibit unique catalytic properties dueto their high surface to volume ratio and large fraction of under-coordinated surfaceatoms. Aiming to elevate catalytic performance, reduce cost and fulfill therequirement of green chemistry and eco-friendliness, this research focuses on thepreparation of highly catalytic subnano Pd and Ag-Pd bimetallic clusters withpolymorphic DNA as templates in aqueous solution at ambient temperature.Meanwhile, the influences of DNA conformation on the structure and properties ofmetal clusters were investigated systematically.Firstly, highly active subnano Pd clusters were synthesized using i-motif DNAIm1as the template. It is indicated that Pd1~Pd5clusters are generated at [Pd]/[base]ratio of0.2, Pd8~Pd9clusters at [Pd]/[base] ratio of0.5and large nanoparticles withthe size about2.6nm are formed at [Pd]/[base] ratio of2.0. The i-motif-stabilizedPd8~Pd9clusters show high catalytic activity towards the reduction of4-nitrophenolwith relative rate constant value of2034min-1(mM Pd)-1, which is much larger thanthat of Pd nanocatalysts templated by G-quadruplex or duplex DNA and peptides orother polymers reproted in literatures. Experimetal characterizations and DFTcalculations disclose that the unique structure of i-motif with consecutivehemiprotonated CH+·C pairs can efficiently ligate Pd ions on the N3sites of cytosinesand the synthesized Pd clusters consist of metallic Pd atoms as well as positivelycharged Pd that is ligated by nucleobases, which is capable of facilitating theactivation of nitryl group of4-nitrophenol.Secondly, the size, oxidation state and acitivity of Pd clusters were finely tunedthrough changing the loop sequces of Im1to Im-AAA, Im1-TTA and Im1-TTT. At[Pd]/[base] ratio of0.2, the nuclearities of dominating Pd clusters templated by thesefour DNA are smaller than6and the percentages of positive Pd are higher than70%,which result in extremely low activity towards the reduction of4-nitrophenol. At[Pd]/[base] ratio of0.5, Both of Im1and Im1-TTA generate Pd8as the main cluster,which consists of about50%positive Pd and shows high activity towards the samereaction. In contrast, Im1-AAA produces Pd10as the major species with63%of positive Pd, while for Im1-TTT template, in addition to the dominating Pd6cluster，large Pd nanoparticles are also generated, which leads to decrease of percentage ofpositive Pd to46%. Pd nanocatalysts templated by the latter two templates bothexhibit very low activity towards the reduction of4-nitrophenol.Finally, Ag-Pd bimetal clusters were produced with G-rich DNA as templates,which exhibit synergistic catalysis towards the reduction of4-nitrophenol. TheAg5Pd1and Ag4Pd2clusters synthesized at the Ag/Pd ratio of4.0possess the highestapparent rate constant of75min-1, which is the highest value reported so far,120times higher than that of Ag cluster and75times higher than that of Pd clustertemplated by G-rich DNA. The Ag-Pd bimtallic clusters remain their synergisticcatalysis when replacing4-nitrphenol with nitrobenzene or4-nitrobenzoic acid as thesubstrate. Further researches indicate that this synergistic catalysis can be mainlyascribed to bifunctional effect, i.e. Pd accelerates the hydrogenation of nitro group tohydroxyamino group and Ag promote the hydrogenation of hydroxyamino group toamino group. What’s more, Pd has excellent hydrogen adsorption capacity so that itcan facilitate the intermediate storage of hydrogen radicals. The intimate contact ofAg and Pd in the clusters benefits quick transfer of the reactants in each step, whichleads to the extremely high acitivity of Ag-Pd clusters toward the reduction of nitrogroup.