Highly modulated supported triazolium-based ionic liquids: direct control of the electronic environment on Cu nanoparticles
A series of new triazolium-based supported ionic liquids (SILPs), decorated with Cu NPs, were successfully prepared and applied to the N-arylation of aryl halides with anilines. The triazoles moieties were functionalised using the copper-catalysed azide-alkyne cycloaddition. SILP surface characterisation showed a strong correlation between the triazolium cation volume and textural properties. STEM images showed well-dispersed Cu NPs onto SILP with a mean diameter varying from 3.6 to 4.6 nm depending on the triazolium cation used. In addition, XPS results suggest that the Cu(0)/Cu(I) ratio can be modulated by the electronic density of triazolium substituents. XPS and computational analysis gave mechanistic insights into the Cu NPs stabilisation pathways, where the presence of electron-rich groups attached to a triazolium ring are critical players leading to a cation pathway adsorption (Eads=72 kcal·mol-1). In contrast, less electron-rich groups favour the anion adsorption pathway (Eads= 63 kcal·mol-1). The Cu@SILP composite with electron-rich groups showed the highest activity for C-N Ullmann coupling reaction, which suggests that electron-rich groups might act as an electron-like reservoir to facilitate the oxidative addition for N-arylation. This strategy firmly suggests the strong dependence between the nature of triazolium-based SILP with the Cu NPs surface active sites, which may provide a new environment to confine and stabilise MNPs for catalytic applications.