The First Organometallic Dendrimers: Design and Redox Functions
Résumé
This review summarizes our original organometallic route to stars, dendrimers, metallostars and metallodendrimers and the redox functions of these macromolecules in catalysis and anionic recognition. The synthesis of metal-sandwich stars and dendritic cores was achieved using the CpM+ induced polyallylation and polybenzylation of polymethylbenzenes (M = Fe or Ru) and pentamethylcyclopentadienyl ligands (M = Co or Rh). Subsequent functionalization of the polyallyl dendritic cores yielded polyols which are precursors of polyiodo, polymesylates, polynitriles, polyamines and polybenzaldehaldehyde cores. The synthesis of dendrimers up to 144-nitrile and 243-allyl was subsequently achieved starting from mesitylene. Functionalization of the polybenzyl dendritic cores was achieved by regiospecific Friedel-Crafts reactions (acetylation, chlorocarbonylation) in the para position. Various metallodendrimers were synthesized with amidoferrocene, amidocobaltocenium and FeCp*(η 6-N-alkylaniline)+ termini in which the redox centers show a reversible behavior and are all independent as observed by cyclic voltammetry. The 9-, 18- and 24-amidometallocene dendrimers were used for the recognition of the oxo anions H2PO 4 − and HSO 4 − by cyclic voltammetry, whereas a 24-iron-alkylaniline dendrimer was efficient to recognize Cl− and Br− anions by 1H NMR with sharp dendritic effects. Differences between the responses to the different anions were large and the largest effects were found for the 18-Fc dendrimer (dendritic effect). A water-soluble star-shaped hexa-iron redox catalyst was as efficient as the mononuclear species for the cathodic reduction of NO 3 − and NO 2 − in water. In conclusion, metallostars are suitable for catalysis, and metallodendrimers present optimal topologies for molecular recognition. These specific functions related to the topologies cannot be interchanged between the metallostars and the metallodendrimers with optimized efficiency in the present examples.