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.