Polar Self-Assembly: Steric Effects Leading to Polar Mixed-Ligand Coordination Cages
✍ Scribed by Jianyong Zhang; Philip W. Miller; Mark Nieuwenhuyzen; Stuart L. James
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 254 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
We present a highly unusual example of self‐assembly, specifically a polar, mixed‐ligand cage which forms in preference to symmetrical homo‐ligand products, and which suggests that steric effects can be exploited to obtain novel non‐uniform polyhedral cages. In particular, reaction between the bulky tripodal triphosphine 2,4,6‐tris(diphenylphosphino)triazine, L1, the non‐bulky tripodal trinitrile 2,4,6‐tris(cyanomethyl)trimethylbenzene, L2 and silver hexafluoroantimonate, AgSbF~6~, in a 3:1:4 ratio gives the mixed‐ligand aggregate [Ag~4~(L1)~3~(L2)(SbF~6~)]^3+^, 1‐SbF~6~, instantly as the only product in quantitative yield. The X‐ray crystal structure of complex 1‐SbF~6~ is consistent with the suspected solution‐state structure. The cage derives from trigonal‐pyramidal geometry, the basal vertices of which are defined by three bulky triphosphines, L1, and the apical vertex by the non‐bulky trinitrile, L2. There is apical elongation amounting to 19 % in comparison to the ideal uniform tetrahedron. The cage also encapsulates an SbF~6~ anion. ^19^F NMR spectra in solution for the analogous PF~6~ complex [Ag~4~(L1)~3~(L2)(PF~6~)]^3+^, 1‐PF~6~, confirm that one anion is also encapsulated in solution. The synthesis of the analogous CF~3~SO~3~^−^ complex, [Ag~4~(L1)~3~(L2)(OTf)]^3+^, 1‐OTf, in solution is also described, although 1‐PF~6~ and 1‐OTf could not be isolated due to slow decomposition in solution. The selective formation of these mixed‐ligand cages is discussed in terms of ligand–ligand and ligand‐included anion steric repulsions, which we propose prevent the formation of the competing hypothetical homo‐ligand tetrahedral structure [Ag~4~(L1)~4~(SbF~6~)]^3+^, and thus favour the mixed ligand cage. “Cage cone angles” for L1 and L2 are estimated at 115 ° and 101 °, respectively. Variable‐temperature ^31^P NMR spectroscopy shows that complex 1‐SbF~6~ and the related previously reported partial tetrahedral complex [Ag~4~(L1)~3~(anion)]^3+^ undergo dynamic twisting processes in solution between enantiomeric C~3~‐symmetry conformations.