Sol–gel materials with trapped trinuclear class-II mixed-valence macrocyclic complexes that mimic their solution redox behaviour

Abstract

The redox chemistry of mixed-valence Co^III/Fe^II/Co^III complexes in solution and trapped in sol–gel matrices has been compared. Whereas the redox S₂O₈²⁻/OH⁻ reactivity cycling of the complexes [{L_nCo^III(µ-NC)}₂Fe^II(CN)₄]²⁺ physically immobilized into the pore network of TEOS-derived xerogels, where L_n represents a polyaza macrocycle, mimic perfectly the reactivity shown in solution without leaching of the complex to the reaction medium, all attempts to immobilize smaller and negatively charged dinuclear [{L_nCo^III(µ-NC)}Fe^II(CN)₅]⁻ related complexes have been found unsuccessful. A fine tuning of the pore size of the matrix, the electrostatic interactions between the matrix’s surface and the complexes, as well as possible chemical adduct formation between the OR groups of the matrix and the cyanide ligands, seem to be responsible for the effects observed. The chemical preparation of the mixed-valence complexes inside the sol–gel matrix has also been tried by using a silylated modified L_n ligand with four hydrolysable-Si(OEt)₃ groups. Following the anchoring of the ligand to a TEOS-derived matrix, the initial formation of the L_nCo^III building block has been achieved, but diffusion of the negatively charged [Fe(CN)₆]⁴⁻ species to the {L_nCo^III}³⁺ centre is severely hindered unless high acidity is used. Nevertheless, under such acidic conditions the mixed-valence [{L_nCo^III(µ-NC)}Fe^II(CN)₅]⁻ compounds have been successfully formed inside the matrix and have been characterized by IR and UV-Vis spectroscopy. Despite this fact, the redox S₂O₈²⁻/OH⁻ reactivity cycling carried out on the latter materials has proved to be much more slow than for xerogels containing the trinuclear [{L_nCo^III(µ-NC)}₂Fe^II(CN)₄]²⁺ species physically trapped.