Post-synthetic cation exchange in anionic metal-organic frameworks; a novel strategy for increasing the catalytic activity in solvent-free condensation reactions

In recent years anionic MOFs (metal-organic frameworks) have only been scarcely explored for application in heterogeneous catalysis. In this work an anionic metal-organic framework, [HDMA](2)[Zn-2(BDC)(3)(DMA)(2)]center dot 6DMF, has been prepared and its cations, HDMA(+), were exchanged by Ni2+, Na+, Li+, tetraethylammonium (TEA(+)) and tetrapropylammonium (TPA(+) ) by a post-synthetic process. By the strategy developed here, we have been able to prepare cation exchanged anionic MOFs as novel heterogeneous catalytic systems for solvent-free Knoevenagel condensation reaction between benzaldehyde and malononitrile. These catalysts were easily isolated from the reaction mixture and were reused several times without a significant degradation in activity.