Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate–Tris(bipyridine)ruthenium Hybrid Compounds

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Summary:
The authors from Dalian University of Technology & Northeast Normal University developed four polyoxometalate–tris(bipyridine)ruthenium hybrid photocatalysts with strong visible-light harvesting, achieving >99 % conversion and 98 % selectivity for aerobic amine-to-imine oxidation within 25 min.

Background:
1. To address the low efficiency of visible-light amine oxidation, previous researchers explored TiO₂, g-C₃N₄, MOFs, etc., yet suffered from long reaction times, high O₂ pressure, or poor recyclability.
2. The authors herein proposed ionic self-assembly of [Ru(bpy)₃]²⁺ with Lindqvist/Keggin POMs, obtaining recyclable heterogeneous catalysts that operate in air at room temperature.

Research Content:
1. Synthesis: Slow-diffusion or drop-wise precipitation of [Ru(bpy)₃]Cl₂ with (TBA)₂[M₆O₁₉], (TBA)₄[Mo₈O₂₆] or (TBA)₄[W₁₀O₃₂] in MeCN/DMF.
2. Characterizations:
   1) BET—microporous hybrids (no detailed values).
   2) SEM/TEM—nanocrystals 50–200 nm; PXRD confirms phase purity.
   3) UV–vis DRS: broad 450–600 nm absorption; TRPL τ drops from 245 ns (Ru complex) to 1.73 ns (hybrid); transient photocurrent 1 > 3 > 4 > 2; EIS shows lowest Rct for 1.
3. Application: 0.6 mol % catalyst, 10 W 445 nm LED, air, 25 °C, MeCN; benzylamine conversion >99 %, TOF 392 h⁻¹, five-cycle stability retained.
4. Mechanism: Visible-light excitation → electron transfer from [Ru(bpy)₃]²⁺* to POM LUMO → O₂ activation to ¹O₂ & O₂•⁻; holes oxidize amine to cation radical, followed by H-abstraction and coupling to imine.

Outlook:
The work delivers readily recyclable, noble-metal-based hybrids that set a new benchmark for green imine synthesis under ambient conditions and offers a versatile platform for POM–photosensitizer photocatalysis.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate–Tris(bipyridine)ruthenium Hybrid Compounds
Authors: Haiyan An, Huiyun Luo, Tieqi Xu, Shenzhen Chang, Yanhong Chen, Qingshan Zhu, Yaohui Huang, Huaqiao Tan, Yang-Guang Li
DOI: 10.1021/acs.inorgchem.2c01243
Link: https://doi.org/10.1021/acs.inorgchem.2c01243

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