Mechanochemically Driven C–C Bond Formation via Cu-Complex-Functionalized Polyoxoniobate under Solvent-Free Conditions

Developing a mechanochemical, solvent-free approach for forming C–C bonds in place of traditional solvent-based synthesis is significant for green chemistry. Herein, a novel Cu-complex-functionalized polyoxoniobate-based hybrid material [Cu(tpy)2]{[Cu(tpy)(H2O)]2[Nb10O28]}·5.5H2O (1, tpy = 2,2′:6′,2″-terpyridine) is successfully synthesized and thoroughly characterized. 1 displays excellent thermal and solvent stability and is highly mechanical force tolerant. Catalytic studies reveal that 1 can catalyze the Knoevenagel condensations of a series of aromatic aldehydes with malononitrile to build C–C bond compounds under mechanochemical ball milling conditions. The catalytic system is conducted with no additional heating, with a lower oscillating frequency, and without a solvent. The remarkable performance of catalyst 1 can be attributed to two factors: one is the sufficient contact between the catalyst and reagents facilitated by mechanical force, as well as the preactivated catalyst by it; the other is the synergistic effect of the Lewis acid–base sites (Cu2+ and [Nb10O28]6–) within the catalyst. The catalyst exhibits outstanding recyclability and can be applied to gram-scale reactions. To the best of our knowledge, this work represents the first example of a mechanochemically driven solvent-free organic transformation using a polyoxometalate-based catalyst.