Abstract
A serendipitous discovery has led to the generation of a family of four compounds in which six components combine to form symmetric metal-cyclotricatechylene (H6ctc) cages. The four compounds, which have the compositions, [Cs((CH3)2CO)6][K4(H6ctc)4(H2O)8][Cs4(H2O)6](PO4)3, [Rb((CH3)2CO)6][Rb2K2(H6ctc)4(H2O)6][Rb4(H2O)6](PO4)3, [Cs((CH3)2CO)6][K4(H6ctc)4(H2O)8]-[Cs(H2O)9](SO4)3 and [Rb((CH3)2CO)6][Rb2K2(H6ctc)4(H2O)6][Rb(H2O)9](SO4)3 possess cubic symmetry that arises from the complementary interactions that govern the assembly of the components. The cage cavities contain water molecules and either one or four large alkali metal ions (either Rb+ or Cs+) which interact with the internal aromatic surfaces of the cage. Each cage is linked to six tetrahedral anions (PO43− or SO42−) through 24 equivalent hydrogen bonds and each anion bridges a pair of cages through eight such hydrogen bonds. An unusual octahedral complex M((CH3)2CO)6+ (M = Rb or Cs), in which the M-C=O link is linear, appears to be a key structural component. A feature of this family of crystalline compounds is the presence of a range of complementary interactions which combine to generate materials that exhibit high crystallographic symmetry.
Highlights
The emergence of the field of supramolecular chemistry was recognised with the award of the 1987 Nobel Prize in Chemistry to Cram, Lehn and Pedersen “for their development and use of molecules with structure-specific interactions of high selectivity” [1]
The field of metallosupramolecular chemistry proved to be a rich and fertile area with researchers such as Fujita producing a stunning array of cage-type compounds in which metal-centres are linked by bridging ligands [9,10]
In addition to metal-ligand assemblies, expansion of the field of supramolecular chemistry included the area of crystal engineering
Summary
The emergence of the field of supramolecular chemistry was recognised with the award of the 1987 Nobel Prize in Chemistry to Cram, Lehn and Pedersen “for their development and use of molecules with structure-specific interactions of high selectivity” [1]. Gautum Desiraju’s pioneering work encouraged researchers to consider the crystal as a supramolecular entity in which a variety of complementary interactions control the arrangement of molecules [11,12]. The affinity of ctc units for alkali metal ions is apparent in the metal-like assemblies of the type depicted in Figure 1a with the large alkali metal cations associating with the internal aromatic surfaces of the cage. In this current work, we report further investigations into supramolecular assemblies formed from the combination of alkali metal cations with H6ctc, this time in the presence of the tetrahedral oxyanions, sulphate and phosphate. The high symmetry products obtained from this synthetic investigation were completely unexpected and display a remarkable level of complementarity involving the six components from which each of the compounds is assembled
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