Modular and Stepwise Synthesis of a Hybrid Metal–Organic Framework for Efficient Electrocatalytic Oxygen Evolution

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