Amine functionalized quinolinium polyoxometalates as highly active heterogeneous catalysts for solvent-free CO2 cycloaddition and Knoevenagel condensation reactions

Abstract

Introducing task-specific catalytic sites through rational design is paramount while developing a multifunctional catalyst for organic transformation reactions. In this work, we utilized rarely explored quinolinium counterions to build a new class of quinolinium-polyoxometalate (POM) hybrids and test their catalytic activities in various organic transformation reactions. We introduced a task-specific ‘–NH₂’ functional group onto the quinolinium moiety to enhance basic and hydrogen bonding sites towards the catalytic reactions and developed a series of POM-hybrids (ACMQ)₄[H₂V₁₀O₂₈] (hybrid 1), (ACMQ)₄[SiMo₁₂O₄₀] (hybrid 2) and (ACMQ)₄[SiW₁₂O₄₀] (hybrid 3) (where ACMQ = 4-amino-7-chloro-1-methylquinolin-1-ium) starting from common POM precursors. These hybrids were tested as catalysts for two organic transformation reactions: the cycloaddition of CO₂ to epichlorohydrin (ECH) to form epichlorohydrin carbonate, and the Knoevenagel condensation of benzaldehyde with malononitrile to yield 2-benzylidene malononitrile. Hybrid 1, containing the decavanadate cluster, showed the best catalytic activity among the hybrids tested toward the CO₂ cycloaddition reaction, yielding cyclic carbonates in 96% yield with a turnover number (TON) of 834 at ambient temperature and pressure in solvent-free, neat conditions. This hybrid also showed the best catalytic performance with high conversion (96%) and a high turnover of 5647 in the Knoevenagel condensation reaction of benzaldehyde with malononitrile at room temperature using an eco-friendly solvent, ethanol. A control compound, hybrid 4 ((DCMQ)₄[H₂V₁₀O₂₈]), prepared using the decavanadate cluster and a quinolinium counterion, 4,7-dichloro-1-methylquinolin-1-ium (DCMQ) bearing a –Cl moiety in place of the –NH₂ on ACMQ, showed negligible catalytic activity in both these reactions, emphasizing the role of ‘–NH₂’ functionality of the counterions in determining the better catalytic performance of hybrid 1. Additionally, hybrid 1 showed structural stability in up to five catalytic cycles in both reactions. The presence of multiple catalytic sites on hybrid 1, i.e., the basic oxygen surface of the clusters and the ‘–NH₂’ functional group of the quinolinium counterions that can act as both basic sites as well as hydrogen bond donors to activate different substrates, is expected to play a significant role in its catalytic performance. Most importantly, in this study, the hybrid catalysts were synthesized in water at room temperature, and the catalytic reactions were conducted either under neat conditions or in ethanol, marking a significant step toward sustainability in catalyst synthesis and reactions.