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

Abstract

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]·4H₂O 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]_n4 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.