Stable tetranuclear copper cage as efficient heterogeneous catalyst for C−C and C−N bond forming reactions
Two tetranuclear copper cages with tubular type, [Cu 4 (BMBA) 4 ((CH 3 ) 2 N) 2 (H 2 O) 2 ] ( 1 ) and [Cu 4 (BMBA) 4 (Bipy)(H 2 O) 2 ] ( 2 ), were successfully constructed by the reaction of Cu(NO 3 ) 2 with dicarboxylic acid 4,4′-[( ethylazanediyl ) bis ( methylene )]-bis[(1,1′-biphenyl)-2-carboxylic acid] (H 2 BMBA) 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 (C C) and Clauson-Kaas (C N) reactions.
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