Turning over on sticky balls: preparation and catalytic studies of surface-functionalized TiO₂ nanoparticles

We have investigated the reactivity of rhodium(iii) complex-functionalized TiO₂ nanoparticles and demonstrate a proof-of-principle study of their catalytic activity in an alcohol oxidation carried out under aqueous conditions water in air. TiO₂ nanoparticles (NPs) have been treated with (4-([2,2':6',2''-terpyridin]-4'-yl)phenyl)phosphonic acid, 1, to give the functionalized NPs (1)@TiO₂. Reaction between (1)@TiO₂ NPs and either RhCl₃·3H₂O or [Rh₂(μ-OAc)₄(H₂O)₂] produced the rhodium(iii) complex-functionalized NPs Rh(1)₂@TiO₂. The functionalized NPs were characterized using thermogravimetric analysis (TGA), matrix-assisted laser desorption ionization (MALDI) mass spectrometry, ¹H NMR and FT-IR spectroscopies; the single crystal structures of [Rh(1)₂][NO₃]₃·1.25[H₃O][NO₃]·2.75H₂O and of a phosphonate ester derivative were determined. ¹H NMR spectroscopy was used to follow the reaction kinetics and to assess the recyclability of the NP-supported catalyst. The catalytic activity of the Rh(1)₂@TiO₂ NPs was compared to that of a homogeneous system containing [Rh(1)₂]³⁺, confirming that no catalytic activity was lost upon surface-binding. Rh(1)₂@TiO₂ NPs were able to withstand reaction temperatures of up to 100 °C for 24 days without degradation.