Metal-Organic Cages Assembled From {Ni₆} Nodes for Selective CO₂ Reduction

Constructing cluster-based nodes and using them as catalytic sites can bring new structure and function for metal-organic cages (MOCs). However, successful examples are very limited. Herein, we found that a 2-mercapto-5-methyl-1,3,4-thiadiazole (Hmmt) based {Ni₆} subunit is robust in building such MOCs. The {Ni₆} node is based on the central Ni₄ cubane and two wing Ni(II) ions with virtually three or four connected features. When various carboxylate ligands are incorporated the triangular prismatic [Ni₁₈(Hmmt)₃(mmt)₁₂(BPDC)₆(CH₃O)₁₂(H₂O)₆(DMF)₆] (4/MOC-18N, H₂BPDC = biphenyl-4,4′-dicarboxylic acid), tetrahedral-like [Ni₂₄(Hmmt)₄(mmt)₁₆(BTC)₄(CH₃O)₁₆Cl₄(H₂O)₄(DMF)₅(CH₃OH)₃] (5/MOC-24N, H₃BTC = 1,3,5-benzenetricarboxylic acid) and the octahedral [Ni₃₆(Hmmt)₆(mmt)₂₄(BTC)₈(CH₃O)₂₄(H₂O)₁₂(DMF)₁₂] (6/MOC-36N) can be isolated. Interestingly, MOCs 4–6 can uptake CO₂ in solid states. Moreover, they can effectively and selectively convert CO₂ into CO under visible light owing to the active wing Ni(II) ions, which are ancillarily coordinated with solvent molecules. All of them show turnover frequencies larger than 3500 µmol·g⁻¹·h⁻¹ and selectivity higher than 90%. This is a much higher performance compared to the usage of cage space for CO₂ reduction in previously reported MOCs and competitive to most Ni(II)-based simple coordination complexes. As such, this work may open a new paradigm for designing node based catalytic function for MOCs.