Russian doll assembled superanion capsule–metal ion complexes: combinatorial supramolecular chemistry in aqueous media

Abstract

The focus of this perspective is the self assembly in aqueous solutions (pH < 3.5) of the synthons sodium p-sulfonatocalix[4]arene and 18-crown-6, or related macrocycles, into globular superanions or ionic capsules, for example {Na ؉ ʚ(18-crown-6)(OH 2 ) n }ʚ{(p-sulfonatocalix[4]arene(4Ϫ)) 2 } 7؊ , n ‫؍‬ 0 or 2. These can crystallise, often selectively, polynuclear hydrolytic metal(III) cations [M 2 (OH) 2 (H 2 O) 8 ] 4؉ , [M 3 (OH) 4 (H 2 O) 10 ] 5؉ , [M 4 (OH) 6 -(H 2 O) 12 ] 6؉ , M ‫؍‬ Cr or Rh, or [Al 13 O 4 (OH) 24 (H 2 O) 12 ] 7؉ , depending on the pH and other synthetic parameters, which are established using a combinatorial approach. These superanions are ambivalent; two calixarenes shroud a crown ether with a central sodium ion bound also by two trans-water molecules or two oxygen centres of sulfonate groups from each of the calixarenes. Electrostatic repulsion between the negatively charged calixarenes is compensated by the large polynuclear cations interacting with the negatively charged hydrophilic equatorial region of the capsule, and some protonation of the sulfonate groups. Similarly, protonated cyclam forms a capsule with two calixarenes and crystallises [Cr 2 (OH) 2 (H 2 O) 8 ] 4؉ . Lanthanide(III) ions form a range of complexes at specific pH in the presence of the calixarene and crown ether, including complexes containing the capsule [{18-crown-6}ʚ {(M(H 2 O) 7 3؉ ) 1.33 (p-sulfonatocalix[4]arene(4؊))} 2 ], for the smaller lanthanides, or the ferris wheel type structure [{La 3؉ ʚ(18-crown-6)(OH 2 ) 3 }ʝ{(p-sulfonatocalix[4]arene-(4؊) ؉ 2H ؉ )}] ؉ , for the larger lanthanide. Only some of the continuous structures have an up-down bilayer arrangement of calixarenes.

Synthesis of experimental models for molecular inorganic geochemistry—A review with examples

“…A simple anion, such as nitrate or sulfate, can be used in combination with the macrocyclic molecule, in this case a crown ether molecule, to break up the solvated structure of the cation. Thus, Cr(III) oligomers have also been isolated as an 18-crown-6 adduct, albeit with a sulfonate counterion (see Drljaca et al, 1997Drljaca et al, , 1999aDrljaca et al, ,b, 2000Hardie and Raston, 2000;Schmitt et al, 2001Schmitt et al, , 2002.…”

“…5. The Cr(III) tetramer was trapped and crystallized for structural characterization by Drljaca et al (2000) using a combination of macrocycle supramolecules to disrupt the hydration sheath of water molecules (see Drljaca et al, 1997Drljaca et al, , 1999aDrljaca et al, ,b, 2000Hardie and Raston, 2000;Schmitt et al, 2001Schmitt et al, , 2002 for examples; this figure was adapted from Hardie and Raston, 2000). gibbsite (a-Al(OH) 3 ), while the hydrolysis of Fe(III) commonly yields oxo-bridged structures, like hematite (aFe 2 O 3 ) or goethite (a-FeOOH).…”

Metallo-supramolecular capsules

“…The p-sulfonatocalix[n]arenes have been shown to assemble with other molecular components into numerous different supramolecular arrangements including 2D or 3D coordination polymers, amino acid complexes and 'Ferris wheels' for example [66,67]. Although this is the case, it is the assembly of p-sulfonatocalix [4]arene (SO 3 [4]) into dimeric 'Russian doll', or nanometre scale capsules (of icosahedral or cuboctahedral geometry) built from 12 calixarenes that are of special interest here [68][69][70].…”

Self-Assembly of Trigonal-Prismatic Metallocages Encapsulating BF4 or CuI32 as Anionic Guests: Structures and Mechanism of Formation

“…An additional Cu + ion appears to be accommodated in the cage, however, this Cu + ion is triangularly surrounded by three iodide anions, therefore, the guest molecule really is the CuI 3 2− ion (Figure 2). To the best of our knowledge, such an inclusion of an anionic metal complex into a discrete cationic metallocage has not been reported before; it may be compared with Stang's host–guest molecular squares incorporating coordination of Ag + ions by the “π‐tweezer effect”15a–c or other known ion–cryptate and ion–crown‐ether encapsulating systems 15d–e. Outside the cage there are neutral molecules, and most importantly, an interesting [Cu 2 I 4 ] 2− counterion16 in which two Cu + ions are symmetrically bridged by two out of the four I − ligands leading to the dimer shown in Figure 3.…”