Adaptive self-assembly: Environment-induced formation and reversible switching of polynuclear metallocyclophanes

Ligand 3 has been shown to self-assemble under coordination of copper(II) c ations in a 1:1 ratio in acetonitrile to give equilibrating mixtures of a [ 2 x 2] grid-type tetranuclear structure 1 and a hexanuclear achitecture of hexagonal shape 2. The latter was confirmed by determination of the crystal structure which further indicated that 2 contained acetonitrile molecules and hydroxo groups bound to the copper(II) centers, which are therefore fiv e-coordinate. The structures assigned to 1 and 2 were further supported by the spectral (mass, UV/Vis) data. The self-assembly process is strongly dep endent on the conditions of the medium. An increase in concentration in ace tronitrile increases the relative amount of hexamer 2, which appears to be the favored entity at the highest concentrations that can be reached before precipitation occurs. On the other hand, in nitromethane only the tetranuc lear complex 1 was detected by mass spectrometry. Replacement of nitrometha ne by acetonitrile and vice versa indicated the reversible switching betwee n a solution containing either 1 alone or an equilibrium mixture of 1 and 2 , respectively. In conclusion, the system described presents several remark able features: 1) self-assembly with substrate binding, 2) dynamic combinat orial structure generation, and 3) environment-induced structural switching amounting in effect to a process of adaptive self-assembly.