Thermodynamically controlled products: novel motif of metallohelicates stabilized by inter- and intra-helix hydrogen bonds

Two temperature and/or time dependent self-assembly processes of maleic dianion (male), thiourea and zinc(II) or cadmium(II) ions have been studied. By prolonging the reaction time and heating, the kinetic product [{Zn(male)(tu)(H2O)}(2)]. 4H(2)O 1 can be converted into the thermodynamic product [{Zn(male)(tu)(2)}(n)] 2. In the case of the Cd-maleate-tu system, the thermodynamic product with helical formation [{(male)Cd(tu)(2)}(2)(male)Cd(tu)(H2O)(2)](n) 4 was obtained from the kinetically controlled product [Cd(Hmale)(2)(tu)(2)(H2O)(2)] 3 by prolonging the reaction time. The thermodynamic helicate 2 bears a new metallohelicate motif in which the helical structure is induced and stabilized by interhelix hydrogen bonding. On the contrary, 4 also bears an infinite metallohelicate motif where the helix is induced and stabilized by intrahelix hydrogen bonding and a 3-D network is formed through hydrogen bonds and short S ... S contacts among neighboring helixes. All the complexes were characterized by X-ray crystallography.