Regulating the catalytic activity of multi-Ru-bridged polyoxometalates based on differential active site environments with six-coordinate geometry and five-coordinate geometry transitions?

Nanoscale Pub Date: 2021-04-12 DOI: 10.1039/D1NR01447A

Abstract

Five-coordinate geometry around ruthenium with highly exposed active sites has attracted intensive scientific interest due to its superior properties and extensive applications. Herein, we report a series of structurally controllable multi-Ru-bridged polyoxometalates, K5NaH10[{Ru4(H2O)n}(WO2)4(AsW9O33)4]·mH2O {1, 1-dehyd-373K, 1-dehyd-473K, 1-dehyd-573K; n = 4, m = 36; n = 4, m = 6; n = 4, m = 0; n = 0, m = 0} fabricated through a feasible assembly strategy using arsenotungstate {2, KNa12H17Cl2(As4W40O140)·29H2O} as a structure-directing unit. Systematic characterization methods identified that the six-coordinate geometry can successfully transform into five-coordinate geometry about active sites (Ru) by removing aqua ligands under high reaction temperatures. All the multi-Ru-bridged polyoxometalates demonstrated strong stability and catalytic effectiveness in the transformation of 1-(4-chlorophenyl)ethanol to 4′-chloroacetophenone under very mild conditions. 1-dehyd-573K, specifically, achieves the best catalytic effectiveness with a turnover frequency (TOF) = 25?100·h?1 owing to its unique five-coordinate geometry on the Ru sites. To our knowledge, 1-dehyd-573K outperforms other POM-based catalysts in the oxidative catalysis of 1-(4-chlorophenyl)ethanol. The heterogeneous polyoxometalates were also proven to be strongly reusable, with their structural integrities well maintained after multiple-cycle catalytic reactions.

Graphical abstract: Regulating the catalytic activity of multi-Ru-bridged polyoxometalates based on differential active site environments with six-coordinate geometry and five-coordinate geometry transitions
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