Michaele J. Hardie

Metal sulfonatocalix[4,5]arene complexes: bi-layers, capsules, spheres, tubular arrays and beyond

Abstract The main focus of this review is the self-assembly in aqueous solutions of bowl-shaped sodium p-sulfonatocalix[4,5]arenes with main group, transition metal and lanthanoid species, and with various organic molecules as additional supramolecular building components, for example 18-crown-6, and other macrocycles, pyridine N-oxide, amino-acids, and more. The versatility of building up new materials based on these components is demonstrated by the formation of a diverse range of complex inclusion structures assembled through π-stacking, hydrogen bonding and coordination interactions. There are up–down arrangements of calix[4]arenes in hydrophobic–hydrophobic bi-layer structures with the positively charged species and included molecules between the layers. A variant of this is prevalent in structures incorporating 18-crown-6 which, in essence, are built up of globular superanions or ionic capsules, for example {Na+⊂(18-crown-6)(H2O)n}⊂{(p-sulfonatocalix[4]arene4−)2}7−, n=0 or 2. These can crystallize, often selectively, polynuclear hydrolytic M(III) cations [M2(OH)2(H2O)8]4+, [M3(OH)4(H2O)10]5+, [M4(OH)6(H2O)12]6+, M=Cr or Rh, or [A113O4(OH)24(H2O)12]7+, depending on the pH and other synthetic parameters. 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}⊂{(M(H2O)73+)1.33(p-sulfonatocalix[4]arene4−)}2], for the smaller lanthanides, or the Ferris-wheel type structure [{La3+⊂(18-crown-6)(OH2)3}⋂{(p-sulfonatocalix[4]arene4−+2H+)}]+, for the larger lanthanide. In the presence of pyridine N-oxide, at pH 4 where the calixarenes take on 5− charge, an up–up arrangement of sulfonated calixarenes results, either assembled in icosahedral spheres, or infinite chiral, helical nano-tubes.

Self-Assembly of a 3-D Triply Interlocked Chiral [2]Catenane

A tripodal cyclotriveratrylene-based ligand with chelating 2,2‘-bipyridine arms self-assembles with either Zn(NO3)2 or Co(NO3)2 to give 3-D trigonal bipyramidal [M3L2] cage assemblies. These cages triply interlock forming [2]catenane species that have been characterized by mass spectrometry and, in the case of the Zn(II) complex, by X-ray crystallography.

Stellated polyhedral assembly of a topologically complicated Pd4L4 'Solomon cube'

Metallosupramolecular chemistry relies on self-assembly processes in which complicated species form through labile dative-covalent interactions. Two remarkable areas of this chemistry are the synthesis of topologically complicated threaded assemblies and of three-dimensional (3D) polyhedral assemblies. Very few polyhedral 3D metallosupramolecular assemblies show threaded motifs within them. Here we report an example of a new type of threaded 3D metallosupramolecular assembly built from four organic ligands and four palladium ions, a Pd4L4 so-called ‘Solomons cube’ in which interweaving and twisting of the ligands form both Solomons links and figure-of-eight ring motifs. In the solid state, six of these Pd4L4 tetramers assemble into a hollow spheroid that closely resembles a stellated truncated hexahedron. A topologically non-trivial metallosupramolecular structure is formed by a Pd4L4 complex in which interweaving and twisting of the ligands results in both Solomons Link and figure-of-eight ring motifs. In the solid state, six of these complexes assemble into a hollow spheroid that closely resembles a stellated truncated hexahedron.

SELF-ASSEMBLED SUPERANIONS : IONIC CAPSULES STABILIZED BY POLYNUCLEAR CHROMIUM(III) AQUA CATIONS

An inclusion complex of an inclusion complex: so can the superanion [{Na+⊂(18-crown-6)(OH2)2}⊂{(p-sulfonatocalix[4]arene4−)2}]7− (illustrated here) be described. These self-assembled ionnic capsules can facilitate the crystallization of polynuclear transition metal aqua ions that have different charge, size and shape, such as [Cr2(OH)2(H2O)8]4+, [Cr3(OH)4(H2O)10]5+ and [Cr4(OH)6(H2O)12]6+.

Scandium(III) Coordination Polymers Containing Capsules Based on Two p-Sulfonatocalix[4]arenes

Reactions of sodium p-sulfonatocalix[4]arene and scandium(III) tristriflate in the presence, and absence, of [18]crown-6 give the crystalline complexes [Sc2(mu-OH)2(H2O)10][Na4(H2O)8-[calix[4]arene(SO3)4]2).13 H2O and [[Sc2(mu-OH)2(H2O)8][Sc(H2O)4]2[calix[4]-arene(SO3)4-H+]2([18]crown-6).16H2O. Both complexes involve novel coordination polymers with calixarene units linked through sodium or scandium centers and also feature capsule assemblies through to the head-to-head association of calixarenes. A linear array of capsules associated with an infinite chain of aquo-bridged sodium ions, and an aquated hydroxy-bridged scandium(III) dimer, [Sc2(mu-OH)2(H2O)10]4+, are found in the absence of the crown ether. In the presence of [18]crown-6 both hydrated scandium monomers and dimers bridge between calixarenes in a two-dimensional coordination network. The crown ethers reside in cavities created by two calixarenes from adjacent polymeric sheets via a variety of supramolecular interactions(hydrogen-bonding, shape complementarity), and effectively add a third dimension to the network. The extended structure of both of these polymers is highly porous, and resembles a bilayer.

Further investigations into tetrahedral M4L6 cage complexes containing guest anions: new structures and NMR spectroscopic studies

A series of ligands LPh, Lnaph and Lanth, which contain two bidentate pyrazolyl–pyridine termini separated by an aromatic (1,2-phenyl, 2,3-naphthyl or 2,3-anthracenyl, respectively) spacer have been used to prepare tetrahedral cage complexes of the form [M4L6]Xn, in which a bis-bidentate bridging ligand spans each of the six edges of the M4 tetrahedron and one anion is bound in the central cavity. Several new examples have been structurally characterised, including an example with a new ligand (Lanth), the first example with a second-row transition metal ion [Cd(II)], and the first example of a cage containing a dianionic guest (hexafluorosilicate). The series of structurally similar Co(II) complexes [Co4L6(BF4)](BF4)7 (L = LPh, Lnaph and Lanth) have been examined in detail by NMR spectroscopy. The 1H NMR spectra are highly shifted between −110 and +90 ppm, but the spectra can be completely assigned by correlation of measured T1 relaxation times with distances of the protons in the complexes from the paramagnetic Co(II) centres. 1H DOSY measurements have been used to estimate diffusion constants which confirm the structural integrity of the cages in solution, and 19F DOSY measurements on the anions show that (i) the trapped [BF4]− anion diffuses at the same rate as the cage superstructure surrounding it, indicating that it is trapped inside the cage cavity; and (ii) the ‘free’ [BF4]− anions have diffusion rates consistent with substantial retardation due to ion-pairing with the 7+ complex cation.

A versatile six-component molecular capsule based on benign synthons: selective confinement of a heterogeneous molecular aggregate

Solvent interactions in the formation of novel H-bonded capsules, comprised of four substituted terpyridines and two C-methylcalix[4]resorcinarenes (both synthons synthesized using “green” methodology), are shown to determine the size and shape of the guest cavity. Two representative host-guest complexes showing centred and off-centred apexes with respect to the calixarenes are described in detail, showing the inclusion of four guest molecules in the solid state.

Coordination Networks with Carborane Anions: Ag(i) and Nitrogen Bridging Ligands

New crystalline coordination networks based on Ag(i) and N-bridging ligands — pyrazine (pyz), 4,4′-bipyridine (bpy), and 2,3-bis-(2-pyridyl)pyrazine (bppz) — incorporating carborane monoanions (CB11H12)− or [Co(C2B9H11)2]− were isolated and characterized using single crystal X-ray diffraction, microanalysis, and infrared spectroscopy. All the complexes with the bpy and bppz ligands reveal 1D coordination infinite chains involving the Ag(i) and the ligands, and layered extended structures with rows of carborane anions between the layers of chains. Interestingly, two polymorphs of the complex [Ag(bpy)(CH3CN)][Co(C2B9H11)2] were observed. The complex [Ag(pyz)(CH3CN)2][Co(C2B9H11)2] shows a 1D coordination polymer, while the pyz complex with the smaller carborane anion {[Ag(pyz)](CB11H12)} exhibits a 3D coordination network structure with four Ag···H−B interactions between the silver centre and carborane anions.

An infinite chainmail of M6L6 metallacycles featuring multiple Borromean links

Borromean rings or links are topologically complex assemblies of three entangled rings where no two rings are interlinked in a chain-like catenane, yet the three rings cannot be separated. We report here a metallacycle complex whose crystalline network forms the first example of a new class of entanglement. The complex is formed from the self-assembly of CuBr2 with the cyclotriveratrylene-scaffold ligand (±)-tris(iso-nicotinoyl)cyclotriguaiacylene. Individual metallacycles are interwoven into a two-dimensional chainmail network where each metallacycle exhibits multiple Borromean-ring-like associations with its neighbours. This only occurs in the solid state, and also represents the first example of a crystalline infinite chainmail two-dimensional network. Crystals of the complex were twinned and have an unusual hollow tubular morphology that is likely to result from a localized dissolution-recrystallization process. Metallacycles made up of six copper ions and six cyclotriguaiacylene-based ligands form a unique topological entanglement in the solid state. Individual metallacycles are interwoven into an infinite 2D chainmail network where each one forms multiple Borromean-ring-like associations with its neighbours. Crystals of the complex grow in an unusual tubular morphology.

Metallo-Cryptophanes Decorated with Bis-N-Heterocyclic Carbene Ligands: Self-Assembly and Guest Uptake into a Nonporous Crystalline Lattice

Pd3L2 metallo-cryptophane cages with cyclotriveratrylene-type L ligands can be stabilized by use of a bis-N-heterocyclic carbene as an auxiliary cis-protecting ligand, while use of more common protecting chelating ligands such as ethylenediamine saw a Pd3L2 to Pd6L8 rearrangement occur in solution. The crystalline Pd3L2 complexes act as sponges, taking up 1,2-dichorobenzene or iodine in a single-crystal-to-single-crystal fashion despite not exhibiting conventional porosity.