Controlling the formation of metallosupramolecular assemblies by metal ionic radiiw

Controlling the structure of multi-component assemblies is one of the leading challenges for the supramolecular chemist. One of the simplest assemblies is the dinuclear doublestranded helicate, and the rules that govern the formation of this species are largely established. The formation of the helicates’ higher nuclearity cousin, the cyclic helicate, is conversely less well understood. One of the major problems in the formation of these higher nuclearity assemblies is that the design principles that apply to helicate formation, i.e. using a ligand that contains two binding domains that coordinate different metal ions, equally apply to the formation of cyclic helicates. For the larger cyclic species to preside in solution, the formation of the entropically favoured dimer has to be prevented and this can be achieved by intermolecular interactions (e.g. templation by anions) or by intramolecular interactions which stabilise the formation of the cyclic species relative to its double-stranded alternative. As an example of the first of these approaches, in the work carried out by Ward et al., a ligand with two bidentate domains separated by a 1,8-naphthalenediyl spacer was reported to form a simple mononuclear species with Cu(CF3SO3), but in the presence of tetrafluoroborate, a tetranuclear cyclic helicate [Cu4L4] 4+ was observed. Hannon et al., on the other hand, demonstrated that a metal ion’s preference for different coordination geometries could affect the self-assembly outcome. In this case a bis-bidentate ligand containing a 1,3-bis(aminomethyl)phenyl spacer formed linear dimers with tetrahedral metal ions and trinuclear circular helicates with octahedral metal ions. Other reports have cited inter-strand CH p interactions as the principal driving force for the preferential formation of high complexity cyclic assemblies over their dimeric counterparts. There are also examples of intermediate systems where the self-assembly of ligands and metal ions results in a dynamic combinatorial library where a number of oligomers are formed in solution i.e. [MxLx] n+ where x= 2, 3, 4, 5, etc. In this communication we describe how the formation of either dinuclear double-stranded or pentanuclear circular helicates can be controlled by inter-ligand steric interactions which, in turn, are governed by the size of the metal ion. This approach allows for the specific formation of either of the two structures and gives valuable insight into some of the factors which control the formation of cyclic helicates. The ligand L, which was prepared by the reaction of 2,20-bipyridine-6-thioamide with 1,3-di(a-bromoacetyl)benzene, contains two tridentate binding domains separated by a phenylene ring (Fig. 1). Reaction of this ligand with Cd(ClO4)2 6H2O in MeNO2 results in a colourless solution, from which crystals are formed upon slow diffusion of dichloromethane. In the ESI mass spectrum, peaks at m/z 1076 ({[Cd2(L )](ClO4)3} ) and 1629 ({[Cd2(L )2](ClO4)3} ), with the appropriate isotope pattern for their charged states, were present indicating the formation of the dinuclear doublestranded helicate. Formation of the complex [Cd2(L )2](ClO4)4 was confirmed by a single crystal X-ray diffraction study (Fig. 2).z In the solid-state the ligand partitions into two tridentate domains, each comprising a thiazole–pyridyl–pyridyl