A fluorosurfactant and photoreducible CuII-tren click catalyst: surfactant and catalytic properties at liquid/liquid interfaces

Abstract

The fluorous copper(II) complex [Cu^II(trenRf₆)3-benzoylbenzoate]3-benzoylbenzoate 2, composed of a highly fluorophilic tris(2-aminoethyl)amine ligand and two 3-benzoylbenzoates as counterions and photosensitizers, was synthesized from the dinuclear complex [Cu^II₂(3-benzoylbenzoate)₄(H₂O)₂] 1 which was characterized by X-ray analysis. Complex 2, which is highly soluble in perfluorocarbons, moderately soluble in organic solvents while insoluble in water, was found to be a very effective fluorosurfactant. At the air/water interface it formed a Langmuir film, which upon compression slowly collapsed at about 28 mN m⁻¹, which corresponds to a surface area of about 220 Ų per molecule. Tensiometric measurements revealed that 2 is more rapidly adsorbed at the diisopropyl ether (DIPE)/water interface than the perfluorodecalin (PFD)/water one, leading to a decrease of the interfacial tensions of about 14 mN m⁻¹ and 40 mN m⁻¹, respectively. Photoreduction of 2 occurs effectively in H-donating solvents such as THF and DIPE, or even in PFD ensuring that an electron donor, such as propargyl alcohol, is present in a separate aqueous phase. Complex 2, when combined with light (365 nm), catalyzes the click reaction between the azide 3 and alkyne 4 under homogeneous conditions (methanol), to afford the disaccharide 5. Under emulsified biphasic DIPE/water or PFD/water conditions, the reactions proceeded well. However, it was shown that a fast and significant amount of copper and 3-benzoylbenzoate counterion was transferred into the aqueous phase, and that most of the catalysis could be ascribed to a copper species solubilised in the aqueous phase, and not to the fluorous copper complex accumulated at the interface.