The SALSAC approach: comparing the reactivity of solvent-dispersed nanoparticles with nanoparticulate surfaces

We demonstrate that the 'surface-as-ligand, surface-as-complex' (SALSAC) approach that we have established for annealed nanoparticulate TiO2 surfaces can be successfully applied to nanoparticles (NPs) dispersed in solution. Commercial TiO2 NPs have been activated by initial treatment with aqueous HNO3 followed by dispersion in water and heating under microwave conditions. We have functionalized the activated NPs with anchoring ligands 1-4; 1-3 contain one or two phosphonic acid anchoring groups and 4 has two carboxylic acid anchors; ligands 1, 2 and 4 contain 6,6'-dimethyl-2,2'-bipyridine (Me2bpy) metal binding domains and 3 contains a 2,2':6',2''-terpyridine (tpy) unit. Ligand functionalization of the activated NPs has been validated using infrared and 1H NMR spectroscopies, and thermogravimetric analysis. NPs functionalized with 1, 2 and 4 react with [Cu(MeCN)4][PF6] and those with 3 react with FeCl2·4H2O; metal binding has been investigated using solid-state absorption spectroscopy and scanning electron microscopy (SEM). Competitive binding of ligands 1-4 to TiO2 NPs has been investigated and shows preferential binding of phosphonic acid over carboxylic acid anchors. For the phosphonic acids, the binding orders are 3 > 1 > 2 which is rationalized in terms of relative pK a values (phosphonic acid and [HMe2bpy]+ or [Htpy]+) and the number of anchoring groups in the ligands. Ligand exchange between ligand-functionalized NPs and homoleptic metal complexes gives NPs functionalized with heteroleptic copper(i) or iron(ii) complexes.