Host–guest assembly of ligand systems for metal ion complexation. Synergistic solvent extraction of copperii and silveri by 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclodecane in combination with carboxylic acidsElectronic supplementary information (ESI) available: Further experimental details for the (non-tracer) solvent extraction studies. See http://www.rsc.org/suppdata/dt/b4/b412255k/

Host–guest formation between 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclodecane 1 and lipophilic organic carboxylic acids in chloroform has been investigated and the effect of such ligand assembly on the solvent extraction of copperii and silveri has been probed. NMR titration experiments in the absence of a metal ion confirm the formation of weak 1 1 and 1 2 macrocycle carboxylic acid assemblies in CDCl3between 1and palmitic hexadecanoic acid or 4-tert-butylbenzoic acid while difunctional salicylic acid showed a 1 2 interaction that is somewhat stronger. The interaction between the former two acids and the tetra-N-benzylated macrocycle is significantly less than that reported previously for its non-substituted parent, cyclam; a result that likely reflects the presence of the less-basic, more sterically hindered tertiary nitrogens in 1relative to the secondary nitrogens present in cyclam. Carboxylic acid-containing assemblies of this type have been used as extractants in a series of solvent extraction waterchloroform experiments. From both previous observations as well as from entropy considerations, it was anticipated that the use of a host–guest assembly of the above type for metal-ion complexation might contribute to enhanced metal ion binding and concomitant enhanced metal ion extraction. Such behaviour is postulated to arise from the components of the coordination sphere being, at least in part, assembled for complex formation. In accord with this, the use of the ligand assembly involving palmitic acidmacrocycle 1was found to lead to enhanced synergistic extraction of copperii at a metal ion concentration of 10−3mol dm−3while, for silveri, synergism was somewhat marginal at this concentration but was clearly apparent under related conditions when the silver concentration was reduced to 10−4mol dm−3. Similar behaviour towards silver was also observed when 4-tert-butylbenzoic acid was substituted for palmitic acid, while the use of salicylic acid resulted in enhanced synergistic extraction at both metal ion concentrations.