An interconverting family of coordination cages and a meso-helicate; effects of temperature, concentration, and solvent on the product distribution of a self-assembly process.

William Cullen,Michael D Ward,Christopher A Hunter

doi:10.1021/ic502780b

William Cullen, Michael D Ward + Show 1 more

Open Access

https://doi.org/10.1021/ic502780b

Copy DOI

Abstract

The self-assembly between a water-soluble bis-bidentate ligand L(18w) and Co(II) salts in water affords three high-spin Co(II) products: a dinuclear meso-helicate [Co2(L(18w))3]X4; a tetrahedral cage [Co4(L(18w))6]X8; and a dodecanuclear truncated-tetrahedral cage [Co12(L(18w))18]X24 (X = BF4 or ClO4). All three products were crystallized under different conditions and structurally characterized. In [Co2(L(18w))3]X4 all three bridging ligands span a pair of metal ions; in the two larger products, there is a metal ion at each vertex of the Co4 or Co12 polyhedral cage array with a bridging ligand spanning a pair of metal ions along every edge. All three structural types are known: what is unusual here is the presence of all three from the same reaction. The assemblies Co2, Co4, and Co12 are in slow equilibrium (hours/days) in aqueous solution, and this can be conveniently monitored by (1)H NMR spectroscopy because (i) the paramagnetism of Co(II) disperses the signals over a range of ca. 200 ppm and (ii) the different symmetries of the three species give characteristically different numbers of independent (1)H NMR signals, which makes identification easy. From temperature- and concentration-dependent (1)H NMR studies it is clear that increasing temperature and increasing dilution favors fragmentation to give a larger proportion of the smaller assemblies for entropic reasons. High concentrations and low temperature favor the larger assembly despite the unfavorable entropic and electrostatic factors associated with its formation. We suggest that this arises from the hydrophobic effect: reorganization of several smaller complexes into one larger one results in a smaller proportion of the hydrophobic ligand surface being exposed to water, with a larger proportion of the ligand surface protected in the interior of the assembly. In agreement with this, (1)H NMR spectra in a nonaqueous solvent (MeNO2) show formation of only [Co2(L(18w))3]X4 because the driving force for reorganization into larger assemblies is now absent. Thus, we can identify the contributions of temperature, concentration, and solvent on the result of the metal/ligand self-assembly process and have determined the speciation behavior of the Co2/Co4/Co12 system in aqueous solution.

Highlights

The assembly of architecturally complex polyhedral coordination cages, from a combination of labile metal ions and relatively simple bridging ligands, has fascinated coordination chemists for more than 25 years.1 From Saalfrank’s early examples of M4L6 tetrahedral cages2 to Fujita’s recent Pd24L48 nanospheres,3 the synthesis, structural characterization, and guest-binding properties of these hollow metal−organic capsules have provided deep insights into control of selfassembly as well as some useful examples of functional behavior arising from the host−guest chemistry.1,4If a particular metal−ligand combination forms an assembly that is significantly more thermodynamically stable than the other possibilities, a single product is formed and isolated, and will generally have the same structure in the solid state as it does in solution behaving, in effect, like a conventional kinetically stable compound
B[MF4−12o(rLC18lnOap4−h))1.18]1XIn24t(hMese=caCgoe,s,Caus,inCdal,l all in the 2+ members of this family,1e a bridging ligand with two bidentate termini is combined with a six-coordinate metal ion, resulting in an assembly with a 2M/3L ratio that results in a polyhedral cage that has a 2:3 ratio of vertices to edges, with a metal ion occupying every vertex and a bridging ligand spanning a pair of metal ions along every edge
All three could be crystallized under different conditions and have been structurally characterized: in Co2 all three bridging ligands span both metal ions, whereas Co4 and Co12 are cages with a metal ion at each vertex and a bridging ligand spanning every edge

Summary

■ INTRODUCTION

The assembly of architecturally complex polyhedral coordination cages, from a combination of labile metal ions and relatively simple bridging ligands, has fascinated coordination chemists for more than 25 years. From Saalfrank’s early examples of M4L6 tetrahedral cages to Fujita’s recent Pd24L48 nanospheres, the synthesis, structural characterization, and guest-binding properties of these hollow metal−organic capsules have provided deep insights into control of selfassembly as well as some useful examples of functional behavior arising from the host−guest chemistry.. If a particular metal−ligand combination forms an assembly that is significantly more thermodynamically stable than the other possibilities, a single product is formed and isolated, and will generally have the same structure in the solid state as it does in solution behaving, in effect, like a conventional kinetically stable compound This is very often the case, when metal ions with strong stereoelectronic preferences (Pd2+, Pt2+) are combined with rigid bridging ligands that have a fixed, predictable arrangement of metal binding sites, as illustrated by work from the groups of Fujta,1b,3 Stang, and Shionoya.. We show how control of these parameters allows the equilibrium to be biased in favor of one component or another, using electrospray (ES) mass spectrometry (MS) and 1H NMR spectroscopy to characterize the product distribution in solution

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES