Click dendrimers and triazole-related aspects: catalysts, mechanism, synthesis, and functions. A bridge between dendritic architectures and nanomaterials

Abstract

One of the primary recent improvements in molecular chemistry is the now decade-old concept of click chemistry. Typically performed as copper-catalyzed azide-alkyne (CuAAC) Huisgen-type 1,3-cycloadditions, this reaction has many applications in biomedicine and materials science. The application of this chemistry in dendrimer synthesis beyond the zeroth generation and in nanoparticle functionalization requires stoichiometric use of the most common click catalyst, CuSO(4)·5H(2)O with sodium ascorbate. Efforts to develop milder reaction conditions for these substrates have led to the design of polydentate nitrogen ligands. Along these lines, we have described a new, efficient, practical, and easy-to-synthesize catalytic complex, [Cu(I)(hexabenzyltren)]Br, 1 [tren = tris(2-aminoethyl)amine], for the synthesis of relatively large dendrimers and functional gold nanoparticles (AuNPs). This efficient catalyst can be used alone in 0.1% mol amounts for nondendritic click reactions or with the sodium-ascorbate additive, which inhibits aerobic catalyst oxidation. Alternatively, catalytic quantities of the air-stable compounds hexabenzyltren and CuBr added to the click reaction medium can provide analogously satisfactory results. Based on this catalyst as a core, we have also designed and synthesized analogous Cu(I)-centered dendritic catalysts that are much less air-sensitive than 1 and are soluble in organic solvents or in water (depending on the nature of the terminal groups). These multivalent catalysts facilitate efficient click chemistry and exert positive dendritic effects that mimic enzyme activity. We propose a monometallic CuAAC click mechanism for this process. Although the primary use of click chemistry with dendrimers has been to decorate dendrimers with a large number of molecules for medicinal or materials purposes, we are specifically interested in the formation of intradendritic [1,2,3]-triazole heterocycles that coordinate to transition-metal ions via their nitrogen atoms. We describe applications including molecular recognition of anions and cations and the stabilization of transition metal nanoparticles according to a principle pioneered by Crooks with poly(amido amine) (PAMAM) dendrimers, and in particular, the control of structural and reactivity parameters in which the intradendritic [1,2,3]-triazoles and peripheral tripodal tri(ethylene glycol) termini play key roles in the click-dendrimer mediated synthesis and stabilization of gold nanoparticles (AuNPs). By varying these parameters, we have stabilized water-soluble, weakly liganded AuNPs between 1.8 and 50 nm in size and have shown large differences in behavior between AuNPs and PdNPs. Overall, the new catalyst design and the possibilities of click dendrimer chemistry introduce a bridge between dendritic architectures and the world of nanomaterials for multiple applications.