Copper(I) Cuboctahedral Coordination Cages: Host–Guest Dependent Redox Activity

Abstract

The self-assembly of coordination cages containing internal cavities with well-defined shape and size has achieved increasing prominence not only because of their aesthetic discrete structures, but also owing to their promising functionalities as metalated containers for storage, recognition, delivery, catalysis, or as molecular reactors. Recent advances have revealed that enantioselective guest binding or stabilization of coordinatively unsaturated metal complexes can be accomplished by coordination cages, and an unusual regioselective Diels–Alder reaction could be facilitated by coordination hosts. These results may further inspire chemists to design and synthesize effective self-assembled container molecules capable of activating reactivity relying on the host–guest chemistry. So far, the construction of coordination cages has mainly involved partially blocked metal ions such as Pd and Pt or “naked” metal ions of high coordination number (4 or 6). Cage structures are less common for the low-coordinationnumber Ag, Au, and Cu ions, which can afford the unique trigonal coordination mode not accessible to other metal ions. Particularly, the Cu ion is rarely used in the cage structure assembly, probably because of its redox instability at ambient atmosphere. Nevertheless, incorporation of the redox-robust Cu ion may be able to install reactive sites into the host molecules, or to activate reactivity of the substrate molecules, which is essential to an effective molecular reactor. It has been known that many Cu-containing enzymes perform a variety of critical biological functions, and their synthetic models for C H bond activation has long been an important research objective. Herein, we report the assembly of a series of Cu cuboctahedral coordination cages by using a bulky triangular ligand and different Cu slats, which show redox stability relying on counteranions and reactivity towards arene C H bond activation depending on the host– guest adaptability. The triangular tris-monodentate ligand possessing three rotatable benzimidazole (Bim) arms, 1,3,5-tris(1-benzylbenzimidazol-2-yl)benzene (L), was prepared by substitution of 1,3,5-tris(2-benzimidazolyl)benzene. As depicted in Scheme 1, reaction of L with Cu ion at room temperature readily resulted in formation of cage structure {guest [Cu4L4]·X·solvent} (guest=ClO4 , X= 3ClO4 , 1a ; guest= I , X= 3I , 2a ; guest=MeOH, X= 4CF3SO3 , 3a, and X= 4MeC6H4SO3 , 4a). For 1–2a, Cu salts were used, whereas for 3–4a the Cu ion originated from rapid in situ reduction of Cu (see below). Interestingly, the Cu complexes 3–4a can be slowly oxidized to Cu complexes within several days with concomitant hydroxylation of L to 2,4,6-tris(1-benzylbenzimidazol-2-yl)phenol (LOH) at ambient temperature, giving the dinuclear complex [Cu2(LO)2(CF3SO3)2](CF3SO3)2·solvent (3b) and the tetranuclear complex [Cu4(LO)2(H2O)2(MeC6H4SO3)4] (4b). In contrast, Cu +