Molecular trimeric ruthenium carboxylate clusters (Ru-3 clusters) have been introduced into the pore channels of mesoporous metal-organic framework chromium terephthalate [MIL-101(Cr)] by employing a facile two-step post-synthetic strategy in which diamine hooks anchored on the framework metal nodes of the MOF are used to covalently immobilize the Ru-3 clusters. The catalytic activity of the isolated Ru-3 clusters in the pore channels of the MOF was significantly improved compared to the bulk counterpart.