Designing a Polyoxometalate-Incorporated Metal-Organic Framework for Reduction of Nitroarenes to Anilines by Sequential Proton-Coupled Electron Transfers.

Developing new photocatalysts for reduction of nitroarenes to anilines under mild conditions is very significant. Herein, a new polyoxometalate-based metal-organic framework (POMOF), {[Co(H2O)]2[Co2(H2O)6(TPT)][Co(TPT)PW11O39]}·3H2O·TPT (namely, CoW-TPT, TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine), was prepared by incorporating Co(II)-substituted Keggin-type anions [PCoW11O39]5- and a photosensitizer (TPT) into a framework. In this structure, the direct coordination bond between [PCoW11O39]5- and TPT ligand and π···π interactions between TPT molecules are beneficial for the separation and migration of photogenerated carriers, thus improving the photocatalytic activity of CoW-TPT. The combination of both photosensitizer TPT and the electron-storable component [PCoW11O39]5- in a cooperative photocatalysis fashion is conducive to the photocatalytic multielectron reduction of nitroarenes. CoW-TPT provided a high conversion of 94.71% in the photocatalytic reduction of nitroarenes to anilines utilizing triethanolamine as the proton source and electron donor by sequential proton-coupled electron transfers.

Photoactive Metal-Organic Framework for the Reduction of Aryl Halides by the Synergistic Effect of Consecutive Photoinduced Electron-Transfer and Hydrogen-Atom-Transfer Processes.

Consecutive photoinduced electron transfer (ConPET) has advantages on overcoming the current energetic limitation of visible-light photoredox catalysis for utilizing the energies of two photons in one catalytic cycle. A heterogeneous approach for radical chain reduction of various aryl bromides and chlorides without adding any cocatalyst is introduced by incorporating polyoxometalates (POMs) and amine catalysts into a naphthalenediimide (NDI)-based polymer. CoW-DPNDI-PYI exhibits high activity in the photocatalytic reduction of aryl halides by the synergistic effects of ConPET and hydrogen-atom-transfer (HAT) processes and an enzyme-mimicking CO2 cycloaddition reaction. The ConPET process with N,N'-bis(4-pyridylmethyl)naphthalenediimide (DPNDI) facilitates effective solar energy conversion. POMs and amine catalysts, as efficient HAT catalysts and electron donors, improve the generation of the ConPET process. The success of this work demonstrates the great application of the synergistic effects of ConPET and HAT processes in heterogeneous photocatalysis C-H arylation.

Polyoxometalate-Supported Aminocatalyst for the Photocatalytic Direct Synthesis of Imines from Alkenes and Amines.

Developing efficient photocatalysts for direct oxidative coupling of alkenes and amines with O2 under mild conditions is very significant. Herein, ZnW-PYI is well-designed by assembling a [PZnW11O39(H2O)]5- photooxidation catalyst and chiral aminocatalyst pyrrolidine-2-ylimidazole (PYI) via a coordination model. ZnW-PYI efficiently catalyzed the synthesis of imines from alkenes and amines using O2 as the oxidant through nucleophilic catalysis by employing pyrrolidine as an organocatalyst. Combining a polyoxometalate and PYI within one single framework is an effective approach not only for stabilization and heterogenization of the redox-active catalyst and aminocatalyst but also for realization of compatibility between the reaction intermediates and synergy of multiple catalytic cycles.