Controlling the formation of metallosupramolecular assemblies by metal ionic radii

Abstract

The formation of either dinuclear double-stranded or pentanuclear circular helicates from a ligand containing two tridentate domains separated by a phenylene unit can be controlled by inter-ligand steric interactions which themselves are governed by the size of the metal ion.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. [1][2][3][4][5][6][7] 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) 8 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(CF 3 SO 3 ), but in the presence of tetrafluoroborate, a tetranuclear cyclic helicate [Cu 4 L 4 ] 4+ was observed. 9 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. 10 Other reports have cited inter-strand CHÁ Á Áp interactions as the principal driving force for the preferential formation of high complexity cyclic assemblies over their dimeric 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 1 , which was prepared by the reaction of 2,2 0 -bipyridine-6-thioamide with 1,3-di(a-bromoacetyl)benzene, contains two tridentate binding domains separated by a phenylene ring (Fig. 1) 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

Head‐To‐Tail and Heteroleptic Pentanuclear Circular Helicates

“…The distribution of species in the ESI-MS spectrum of [Cu 5 (L 2 ) 5 ](ClO 4 ) 10 is strikingly similar to that observed for the previously reported zinc-based circular helicate [Zn 5 (L 1 ) 5 ]-(OTf) 10 , [7] the solid-state structure of which was shown to be quantitatively retained in solution. Nonetheless, we have conducted tandem MS experiments to ascertain whether the lower nuclearity species observed in the ESI-MS spectrum of [Cu 5 (L 2 ) 5 ](ClO 4 ) 10 are present in solution or merely products of gas-phase fragmentation in the ion source.…”

“…[6] In a recent report, we showed that bis(tridentate) ligand L 1 (Scheme 1) forms pentanuclear circular helicates with small transition-metal dications. [7] Formation of the observed structure was driven by repulsion between the central phenyl groups of L 1 , which are brought too close to one another in the putative dimer. Since this appeared to be a Figure 1.…”

“…[6] In a recent report, we showed that bis(tridentate) ligand L 1 (Scheme 1) forms pentanuclear circular helicates with small transition-metal dications. [7] Formation of the observed structure was driven by repulsion between the central phenyl groups of L 1 , which are brought too close to one another in the putative dimer. Since this appeared to be a robust approach for generating circular helicates, we decided to explore whether L 1 , and its variants L 2 and L 3 (Scheme 1), could be used to further diversify structural complexity in circular helicates.…”

A Remarkable Solvent Effect on the Nuclearity of Neutral Titanium(IV)‐Based Helicate Assemblies

“…A slow diffusion of n-pentane into the resulting orange mixture led to red crystals, which were collected by filtration (yield: 76 %). [13][14][15][16][17][18][19][20][21][22][23][24][25][69][70][71]137.3,137.7,138.6,139.4,141.0,143.0,143.4,159.5,160.4,160.8,161 (4) ; a = 90, b = 105.4120 (10), g = 908; V = 27 747.1(9) (2) 3 ; T = 173(2) K; Z = 2, 1 cald = 1.166 Mg m 3 ; reflections collected = 80 349, R int = 0.0848; refinement factors: R1 = 0.1083, Rw = 0.2870, GOF = 1.100. CCDC 905356 contains the supplementary crystallographic data for this paper.…”

“…A few other routes to select the nuclearity of polymetallic d‐block or lanthanide helicates that incorporate diimine‐based ligands have also been explored 16. In these examples, the selection of a given assembly is governed by the choice of coordination geometry and ionic radii of the metal ions,15b, 17, 18 fine‐tuning of the ligand design,19 and post‐coordination of a ligand to a double‐stranded helicate assembly 20. Other reports also mention the role of interstrand CH⋅⋅⋅π interactions to govern the formation of circular helicates over dimeric assemblies 19.…”