Mechanically interlocked nanotubes as recyclable catalysts for Knoevenagel condensation

Single-walled carbon nanotubes (SWNTs) are a very attractive platform to build heterogeneous catalysts, benefiting from their intrinsic high surface area and their insolubility. Here, we show that SWNTs encapsulated within organic macrocycles to form mechanically interlocked rotaxane-type species (MINTs), are a good building block to graft basic nitrogenous moieties for the catalysis of the Knoevenagel condensation. The installation of the catalytically active groups is carried out after formation of the MINTs, following a modular approach. Through this chemical modification strategy, we obtain very active MINT catalysts (TOF in the range of 900-9000 h^-1). The interlocked catalysts can be recycled for at least five times by simple filtration and washing, without any appreciable loss of activity. In comparison, supramolecular controls lacking the mechanical link between the active moiety and the SWNT cannot be recycled. From a general point of view, these results prove that formation of MINTs is an interesting strategy to link catalytic molecular moieties to SWNTs, enabling their use as heterogeneous catalysts and therefore facilitating the purification of the products and the recycling of the catalyst.