**1. Introduction**

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]. In their work, each of these researchers developed molecular systems that exhibited selectivity in binding to alkali metal ions [2–8]. In the following decades, 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. Gautum Desiraju's pioneering work encouraged researchers to consider the crystal as a supramolecular entity in which a variety of complementary interactions control the arrangemen<sup>t</sup> of molecules [11,12]. In this context, Ward exploited complementary hydrogen bonding interactions between anionic sulphonates and guanidinium cations to generate an impressive series of network materials that represent an outstanding example of true crystal engineering [13,14]. All of the above examples rely upon di fferent types of complementary interactions between components to yield supramolecular assemblies. In this current work, the serendipitous formation of a novel series of crystalline, supramolecular compounds is described, in which the tris (catechol) molecule, cyclotricatechylene (H6ctc, **I**, Scheme 1), participates

in all of the types of associations indicated in the examples listed above i.e., alkali metal binding, cage formation and hydrogen bonding within a highly symmetric extended network. The resulting compounds represent remarkable examples of high symmetry multi-component supramolecular assemblies in which a synergy exists between the different types of complementary interactions that govern their formation.

**Scheme 1.** Cyclotricatechylene (**I**) and cyclotriveratrylene (**II**).

Cyclotricatechylene possesses 3-fold symmetry and commonly adopts a bowl-shaped conformation with hydroxylic groups located on the rim of the bowl. The compound is closely related to its hexamethyl counterpart, cyclotriveratrylene (**II**, Scheme 1), which has received considerable attention with respect to its ability to host gues<sup>t</sup> molecules within the bowl-shaped cavity [15,16]. In the last 12 years, interest in the supramolecular chemistry of cyclotricatechylene has grown and a variety of metal-based derivatives have been synthesised and structurally characterised. The structures of these assemblies depend upon the manner in which the metal ion interacts with the tris (catechol) ligand. For example, transition metal ions (M) such as VIV (in the form of vanadyl) [17] and CuII [18] combine with ctc6− to produce M6ctc4 anionic tetrahedral cages in which ctc hexaanions are located at the vertices of a tetrahedron. The catecholate arms extend along the tetrahedron edges and are linked by metal centres, each of which is linked to a pair of catecholate groups as indicated in Figure 1a. Recent work has demonstrated that 5-coordinate Si centres can fulfil the role of the metal centre to generate robust covalent cages [19,20].

**Figure 1.** Metallosupramolecular assemblies involving cyclotricatechylene, (**a**) the anionic cage [Cu6(ctc)4]<sup>12</sup><sup>−</sup>; C black, O red, Cu blue and (**b**) the anionic clam [Cs(H5ctc)(H4ctc)]<sup>2</sup><sup>−</sup>; C black, O red, Cs green, black and white striped connections represent hydrogen bonds, hydrogen atoms omitted for clarity.

In addition to the cages of the type described above, H6ctc, in various stages of deprotonation, is able to form fascinating clam-like structures when it is combined with large alkali metal ions, in particular, Rb+ and Cs+ [21]. In these structures, pairs of partially deprotonated Hnctc(6−n)− units assemble through complementary hydrogen bonding interactions between hydroxyl groups which results in a rim-to-rim arrangemen<sup>t</sup> as shown in Figure 1b. A Rb+ or Cs+ ion occupies the

internal cavity of the assembly and interacts only with the aromatic surfaces of the Hnctc(6−n)− units. A computational investigation of the stability of these clam-like structures, under di fferent levels of protonation, was recently reported [22]. The a ffinity of ctc units for alkali metal ions is also 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 original intention of the study was to investigate the interaction of large alkali metal cations (Rb+ or Cs+) with the internal aromatic surfaces of cyclotricatechylene, in various states of deprotonation, and determine how simple anions such as phosphate and sulphate impact upon the nature of the assembly. 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.
