Modular\nand Stepwise Synthesis of a Hybrid Metal–Organic\nFramework for Efficient Electrocatalytic Oxygen Evolution

Abstract

The paddle-wheel\ntype cluster Co₂­(RCOO)₄­(L^T)₂ (R = substituent group, L^T = terminal ligand),\npossessing unusual metal coordination geometry\ncompared with other cobalt compounds, may display high catalytic activity\nbut is highly unstable especially in water. Here, we show that with\njudicious considerations of the host/guest geometries and modular\nsynthetic strategies, the labile dicobalt clusters can be immobilized\nand stabilized in a metal–organic framework (MOF) as coordinative\nguests. The Fe­(na)₄(L^T) fragment in the MOF\n[{Fe₃(μ₃-O)­(bdc)₃}₄{Fe­(na)₄(L^T)}₃] (H₂bdc\n= 1,4-benzenedicaboxylic acid, Hna = nicotinic acid) can be removed\nto give [{Fe₃(μ₃-O)­(bdc)₃}₄] with a unique framework connectivity possessing suitable\ndistribution of open metal sites for binding the dicobalt cluster\nin the form of Co₂(na)₄(L^T)₂. After two-step, single-crystal to single-crystal, postsynthetic\nmodifications, a thermal-, water-, and alkaline-stable MOF [{Fe₃(μ₃-O)­(bdc)₃}₄­{Co₂­(na)₄­(L^T)₂}₃] containing the desired dicobalt cluster was obtained, giving\nextraordinarily high electrocatalytic oxygen evolution activity in\nwater at pH = 13 with overpotential as low as 225 mV at 10.0 mA cm^–2.