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Synthesis

Graphite-bound molecules combine best of homo- and heterogeneous catalysis

Conjugated pyrazine linkage endows tunable molecular system with metal-like electron-transfer properties

by Mitch Jacoby
January 29, 2018 | APPEARED IN VOLUME 96, ISSUE 5

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Credit: J. Am. Chem. Soc.
Attaching molecular catalysts to graphite electrodes via a conjugated pyrazine linkage (black) yields a tunable moiety with catalytically favorable, metal-like electron-transfer properties.
Credit: J. Am. Chem. Soc.
Attaching molecular catalysts to graphite electrodes via a conjugated pyrazine linkage (black) yields a tunable moiety with catalytically favorable, metal-like electron-transfer properties.

Most synthetic catalysts fall squarely into one of two categories—homogeneous or heterogeneous, each with its characteristic strong points. Few catalytic systems capitalize on the best of both worlds. Now, a study led by chemists at Massachusetts Institute of Technology takes a key step in that direction (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b10723). Heterogeneous catalysts, especially ones used in electrochemistry, tend to be robust metals that efficiently bind reactant molecules and mediate catalytic reactions via electron transfer. But molecular details of the binding and active sites are tough to pin down and their structures often change during reaction. In contrast, the structures of metallo-organic molecules that serve as homogeneous solution-phase catalysts are well studied and can be modified easily via synthesis to tune catalyst performance. When used as electrocatalysts, however, electron transfer from the electrode through the catalyst to the reagent molecule is a stepwise, thermodynamically unfavorable process. To bypass that problem, a team led by MIT’s Yogesh Surendranath condenses o-phenylenediamine derivatives with o-quinone moieties commonly found on graphitic materials. The procedure attaches catalytic molecules to graphite electrodes via strong conjugated pyrazine linkages. The group’s graphite-bound catalysts, which have been used to mediate fuel-cell reactions and convert CO2 to CO, exhibit electron-transfer properties identical to those of heterogeneous metal catalysts but retain the tunability of homogeneous catalysts.

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