“Click” Assemblies and Redox Properties of Arene- and Gold- Nanoparticle-Cored Triazolylbiferrocene-Terminated Dendrimers Amalia Rapakousiou, Rodrigue Djeda, Maxime Grillaud, Na Li, Jaime Ruiz, and Didier Astruc* ISM, UMR CNRS No. 5255, Univ. Bordeaux, 33405 Talence Cedex, France * S Supporting Information ABSTRACT: Large dendritic assemblies terminated by organo- metallic groups that possess a rich redox chemistry and stability in two or more oxidation states are highly desired as electron-reservoir systems, sensors, and redox catalysts. Here the synthesis and click (CuAAC) chemistry of ethynyl biferrocene including branching onto dendrons, arene-cored dendrimers, and gold nanoparticles are developed, and the role of the 1,2,3-triazole linkers and redox chemistry of these assemblies are discussed including the properties and stabilities of the redox states. ■ INTRODUCTION Macromolecules containing redox-active units have recently been largely developed due in particular to their electrochromic and polyelectrolyte properties. 1 Among the various transition- metal sandwich complexes that display remarkable redox properties, 2 ferrocenyl derivatives are the most frequently studied, because of the richness of ferrocene chemistry and its easy oxidation to ferricenium. 3 In the ferrocene family, biferrocene 4 derivatives present a great interest, as they include three easily accessible oxidation states, a characteristic that makes their redox properties much richer than those of the parent ferrocene compounds. In addition, the biferrocenium cation is extremely robust because the electron-releasing ferrocenyl group stabilizes the neighboring ferricenium cation. Indeed, it is well known that biferrocenium belongs to the class 2 of the mixed- valent compounds according to Robin and Day’s classification following the localization of the valence at the infrared time scale. 5 Therefore, biferrocenyl units have often been incorpo- rated in various nanostructures such as dendrimers and nanoparticles. 6 Ferrocenyl-terminated dendrimers 7 and nano- particle-cored ferrocenyl-terminated branched assemblies 8 rep- resent a class of metallodendrimers 9 that have proved useful especially for redox sensing, 10 although ferricenium or related Fe(III) analogues were rarely isolated and studied. 11 Biferrocenyl dendrimers have been constructed with up to eight electronically communicated silicon-bridged biferrocenyl groups in the periphery where stable modified electrodes were prepared with interacting ferrocenes. 5a Water-soluble poly- (propyleneimine) biferrocenyl dendrimers complexed with β-CD were studied as well as their adsorption at self-assembled monolayers on gold for molecular printboards. 5b Redox-active biferrocenes entrapped in polyphenylazomethine dendrimers were studied with an impressive control of reversible encapsulation/release due to the stability of the biferrocenium cation, 5c whereas molecular junctions were also prepared from biferrocenyl- terminated dendrimers. 5d Biferrocenyl-terminated diaminobutane poly(propyleneimine) dendrimers were also synthesized by Casado’s group, who demonstrated electrochemical anion sensing and successful immobilization onto electrode surfaces. 5e Finally, dendritic tris- and tetra-biferrocenes linked by rigid ethynylaryl spacers were synthesized through Sonogashira coupling and homocoupling reactions displaying an electron-rich chemistry. 5f Gold nanoparticles (AuNPs) are a proli fic field with applications in optics, sensing, nanomedicine, and catalysis, 12 and Nishihara’s group has extensively studied AuNPs with biferrocene-terminated thiolate ligands forming thin redox-active films by electrodeposition. 7 Additionally, biferrocene-functionalized terpyridine octanethiols and the studies of their self-organization chemisorbed on AuNPs were reported. 13 Click methodology, 14 in particular the copper-catalyzed azide-alkyne cycloaddition (CuAAC) forming 1,2,3-triazoles (trz), 14b-g has recently appeared as one of the most powerful and practical means to form metallocene-terminated nanoassemblies. The triazolyl linkage is significant for redox recognition and subsequent stabilization of gold and palladium nanoparticles for catalysis, 15 and in many cases it shows remarkable biocompat- ibility. 16 Triazolyl-metallocenyl dendrimers have been success- fully synthesized by a CuAAC-type click reaction using the Sharpless catalyst and showed various applications in redox recognition and catalysis. 17 Herein, we report the detailed synthesis of ethynylbiferrocene and its CuAAC-type click reactions providing access to triazolylbiferrocenyl nanosystems. 18 Therefore, three generations Received: October 9, 2014 Article pubs.acs.org/Organometallics © XXXX American Chemical Society A dx.doi.org/10.1021/om501031u | Organometallics XXXX, XXX, XXX-XXX