Stable tetranuclear copper cage as efficient heterogeneous catalyst for C−C and C−N bond forming reactions

Abstract

Two tetranuclear copper cages with tubular type, [Cu4(BMBA)4((CH3)2N)2(H2O)2] (1) and [Cu4(BMBA)4(Bipy)(H2O)2] (2), were successfully constructed by the reaction of Cu(NO3)2 with dicarboxylic acid 4,4′-[(ethylazanediyl)bis(methylene)]-bis[(1,1′-biphenyl)-2-carboxylic acid] (H2BMBA) under solvothermal conditions. The complexes 1 and 2 exhibit a similar cage structure, formed by four ligands connecting two binuclear copper paddle-wheel secondary building units (SBUs). Interestingly, the complex 2 was obtained by appropriately binding of 4, 4′-bipyridine (Bipy) molecule on the axial positions of two Cu paddle-wheels inside the cage. For this reason, the complex 2 exhibits excellent stability in some common organic solvents at room temperature and aqueous solutions with a pH of 2 to 12. Upon thermal activation of complex 2, the coordinated water molecules can be removed, and its exposed metal active-sites show Lewis acid character, which can be used an efficient heterogeneous catalysis of Knoevenagel condensation (CC) and Clauson-Kaas (CN) reactions.