Immobilisation of rhodium acetonitrile complexes in ordered mesoporous silica

Rhodium acetonitrile complexes have been immobilised in the mesoporous silica MCM-41 by pore volume impregnation of a solution of the compound [Rh₂(CH₃CN)₁₀](BF₄)₄ in acetonitrile. Powder XRD and N₂ adsorption-desorption analysis of the derivatised material confirmed that the textural properties of the hexagonally ordered host were preserved during the grafting experiment and that the channels remained accessible. The BET specific surface area decreased from 1047 to 820 m² g^-1, while the specific total pore volume decreased from 0.85 to 0.64 cm³ g^-1. The functionalised material was also characterised by MAS NMR (¹¹B, ¹³C and ²⁹Si), FTIR and Raman spectroscopy, Rh K-edge EXAFS spectroscopy, UV-Vis and luminescence spectroscopy. These studies revealed the existence of surface-attached dimeric species, with a rhodium-rhodium interatomic distance (2.58 Å) similar to that in the compound [Rh₂(CH₃CN)₁₀](BF₄)₄ (2.61 Å). Exposure of this material to light promotes irreversible photodissociation of the dimeric species to give immobilised monomeric species.