Structures and anion-binding properties of M4L6 tetrahedral cage complexes with large central cavities

Reaction of the bis-bidentate bridging ligand L(3), in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3'-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co(4)(L(3))(6)]X(8)(X =[BF(4)](-), [ClO(4)](-), [PF(6)](-) or I(-)) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co(4) tetrahedron. In every case, X-ray crystallography reveals that the 'apical' Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic)C(3) symmetry for the cages; that this structure is retained in solution is confirmed by (1)H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature (19)F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X =[BF(4)](-) and [PF(6)](-) in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol(-1) in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal-ligand bonds.     

Self-assembly and host-guest chemistry of big metallosupramolecular M4L4 tetrahedra

Metallosupramolecular tetrahedra M8[L4Ti4] are easily obtained by self-assembly from the triangular ligands L-H6 and titanoyl bis(acetylacetonate) in the presence of alkali metal carbonates as base. All the complexes can be well characterized by 1H NMR in combination with ESI FT-ICR MS. Force field calculations reveal that the tetrahedra show Ti-Ti separations of 17 angstroms ([L1(4)Ti4]8-) and 23.5 angstroms ([L2(4)Ti4]8-), respectively, leading to huge internal cavities. The cavity is readily shielded in the case of L1 but possesses big pores with the bigger ligand L2. [L1(4)Ti4]8- was used to investigate the host-guest chemistry of these container molecules and it was found that cationic organic guest species like anilinium can be introduced in the interior of the complex. Inclusion is nicely followed by NMR spectroscopy. Upon addition of one equivalent of guest the symmetry of the tetrahedron is lost but is regained after addition of significantly more than four equivalents.     

The self-assembly of a [Ga4L6](12-) tetrahedral cluster thermodynamically driven by host-guest interactions

The guest-induced synthesis of a [Ga4L6](12-) tetrahedral metal-ligand cluster resulting from a predictive design strategy is described. Each of the six dicatecholamide ligands spans an edge of the molecular tetrahedron with four Ga(III) ions at the vertices. Small cationic species not only were found to occupy the large void volume (ca. 300-400 A(3)) inside this cluster but also are necessary thermodynamically to drive cluster assembly via formation of a host-guest complex. NMe4(+), NEt4(+), and NPr4(+) all suit this purpose, and in addition the cluster exhibits a preference in the binding of these three guests: NEt4(+) is bound 300 times more strongly than NPr4(+), which is in turn bound 4 times more strongly than NMe4(+), as determined by 1H NMR spectroscopy. The K6(NEt4)6[Ga4L6] cluster was characterized by NMR spectroscopy, high- (Fourier transform ion cyclotron resonance, FT-ICR) and low-resolution electrospray ionization (ESI) mass spectrometry, elemental analysis, and single-crystal X-ray diffraction. The binding of the NEt4(+) guest molecule was confirmed in the solid state structure, which reveals that the molecule contains large channels in the solid state. As this result exemplifies, it is suggested that guest molecules will play an increasing role in the formation of larger, predesigned metal-ligand clusters.     

Resolution of chiral, tetrahedral M4L6 metal-ligand hosts

The supramolecular metal-ligand assemblies of M416 stoichiometry are chiral (M = GaIII, AlIII, InIII, FeIII, TiIV, or GeIV, H41 = N,N'-bis(2,3-dihydroxybenzoyl)-1,5-diaminonaphthalene). The resolution process of delta delta delta delta- and lambda lambda lambda lambda-[M(4)1(6)]12- by the chiral cation S-nicotinium (S-nic+) is described for the Ga(III), Al(III), and Fe(III) assemblies, and the resolution is shown to be proton dependent. From a methanol solution of M(acac)3, H(4)1, S-nicI, and KOH, the delta delta delta delta-KH3(S-nic)7[(S-nic) subset M(4)1(6)] complexes precipitate, and the lambda lambda lambda lambda-K6(S-nic)5[(S-nic) subset M(4)1(6)] complexes subsequently can be isolated from the supernatant. Ion exchange enables the isolation of the (NEt4(+))(12), (NMe4(+))(12), and K+(12) salts of the resolved structures, which have been characterized by CD and NMR spectroscopies. Resolution can also be accomplished with 1 equiv of NEt4+ blocking the cavity interior, demonstrating that external binding sites are responsible for the difference in S-nic+ enantiomer interactions. Circular dichroism data demonstrate that the (NMe4(+))(12) and (NEt4(+))(12) salts of the resolved [Ga(4)1(6)]12- and [Al(4)1(6)]12- structures retain their chirality over extended periods of time (>20 d) at room temperature; heating the (NEt4(+))(12)[Ga(4)1(6)] assembly to 75 degrees C also had no effect on its CD spectrum. Finally, experiments with the resolved K(12)[Ga(4)1(6)] assemblies point to the role of a guest in stabilizing the resolved framework.     

Guest exchange dynamics in an M4L6 tetrahedral host

Guest exchange in an M(4)L(6) supramolecular assembly was previously demonstrated to proceed through a nonrupture mechanism in which guests squeeze through apertures in the host structure and not through larger portals created by partial assembly dissociation. Focusing on the [Ga(4)L(6)](12-) assembly [L = 1,5-bis(2',3'-dihydroxybenzamido)naphthalene], the host-guest kinetic behavior of this supramolecular capsule is defined. Guest self-exchange rates at varied temperatures and pressures were measured to determine activation parameters, revealing negative DeltaS and positive DeltaV values [PEt(4)(+): DeltaH = 74(3) kJ mol(-1), DeltaS = -46(6) J mol(-1) K(-1), k(298) = 0.003 s(-)); NEt(4)(+): DeltaH = 69(2) kJ mol(-1), DeltaS = -52(5) J mol(-1) K(-1), k(298) = 0.009 s(-1); NMe(2)Pr(2)(+): DeltaH = 52(2) kJ mol(-1), DeltaS = -56(7) J mol(-1) K(-1), DeltaV = +13(1) cm(3) mol(-1), k(298) = 4.4 s(-1); NPr(4)(+): DeltaH = 42(1) kJ mol(-1), DeltaS = -102(4) J mol(-1) K(-1), DeltaV = +31(2) cm(3) mol(-1), k(298) = 1.4 s(-1)]. In PEt(4)(+) for NEt(4)(+) exchange reactions, egress of the initial guest (G1) is found to be rate determining, with increasing G1 and G2 (the displacing guest) concentrations inhibiting guest exchange. This inhibition is explained by the decreased flexibility of the host imparted by exterior, or exohedral, guest interactions by both the G1 and G2 guests. Blocking the exohedral host sites with high concentrations of the smaller NMe(4)(+) cation (a weak endohedral guest) enhances PEt(4)(+) for NEt(4)(+) guest exchange rates. Finally, guest displacement reactions also demonstrate the sensitivity of guest exchange to thermodynamic endohedral guest binding affinities. When the initial guest (G1) has a weaker affinity for the host, G2 concentration dependence is observed in addition to dependence on the G2 binding strength.     

Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

Reaction of Ni(OTf)₂ with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni^II₄L₆]⁸⁺. By selectively exchanging the bound triflate from [OTf⊂Ni^II₄L₆](OTf)₇ (1), we have been able to prepare a series of host–guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [M^IIX₄⊂Ni^II₄L₆](OTf)₆, where M^IIX₄²⁻ = MnCl₄²⁻ (2), CoCl₄²⁻ (5), CoBr₄²⁻ (6), NiCl₄²⁻ (7), and CuBr₄²⁻ (8) or [M^IIIX₄⊂Ni^II₄L₆](OTf)₇, where M^IIIX₄⁻ = FeCl₄⁻ (3) and FeBr₄⁻ (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host–guest complexes 1–8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN₆} vertexes within a single Ni₄L₆ unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [Ni^II₄] complex, and between the host and the MX₄ⁿ⁻ guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX₄ⁿ⁻ guest molecules lead to stronger host–guest magnetic exchange interactions.

The tetrahedral [Ni^II₄L₆]⁸⁺ cage can reversibly bind paramagnetic MX₄^1/2− guests, inducing magnetic exchange interactions between host and guest.     

Design, formation and properties of tetrahedral M(4)L(4) and M(4)L(6) supramolecular clusters

The rigid tris- and bis(catecholamide) ligands H(6)A, H(4)B and H(4)C form tetrahedral clusters of the type M(4)L(4) and M(4)L(6) through self-assembly reactions with tri- and tetravalent metal ions such as Ga(III), Fe(III), Ti(IV) and Sn(IV). General design principles for the synthesis of such clusters are presented with an emphasis on geometric requirements and kinetic and thermodynamic considerations. The solution and solid-state characterization of these complexes is presented, and their dynamic solution behavior is described. The tris-catecholamide H(6)A forms M(4)L(4) tetrahedra with Ga(III), Ti(IV), and Sn(IV); (Et(3)N)(8)[Ti(4)A(4)] crystallizes in R3(-)c (No. 167), with a = 22.6143(5) A, c = 106.038(2) A. The cluster is a racemic mixture of homoconfigurational tetrahedra (all Delta or all Lambda at the metal centers within a given cluster). Though the synthetic procedure for synthesis of the cluster is markedly metal-dependent, extensive electrospray mass spectrometry investigations show that the M(4)A(4) (M = Ga(III), Ti(IV), and Sn(IV)) clusters are remarkably stable once formed. Two approaches are presented for the formation of M(4)L(6) tetrahedral clusters. Of the bis(catecholamide) ligands, H(4)B forms an M(4)L(6) tetrahedron (M = Ga(III)) based on an "edge-on" design, while H(4)C forms an M(4)L(6) tetrahedron (M = Ga(III), Fe(III)) based on a "face-on" strategy. K(5)[Et(4)N](7)[Fe(4)C(6)] crystallizes in I43(-)d (No. 220) with a = 43.706(8) A. This M(4)L(6) tetrahedral cluster is also a racemic mixture of homoconfigurational tetrahedra and has a cavity large enough to encapsulate a molecule of Et(4)N(+). This host-guest interaction is maintained in solution as revealed by NMR investigations of the Ga(III) complex.     

Diastereoselective formation and optical activity of an M4L6 cage complex

A chiral bridging ligand affords a single diastereoisomer of tetrahedral M4L6 cage complex in which the optical rotation of each ligand is increased by a factor of 5 on coordination.     

Reversible photoswitching self-assembly of azobenzene-functionalized hyperbranched polyglycerol induced by host-guest chemistry

Reversible assembly and disassembly of rod-like large complex micelles have been achieved by applying photoswitching of supramolecular inclusion and exclusion of azobenzene-functionalized hyperbranched polyglycerol and α-cyclodextrin as driving force, promising a versatile system for self-assembly switched by light. Hydrogen-nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectroscopy were applied to characterize the azobenzene-functionalized hyperbranched polyglycerol. Atomic force microscopy (AFM), transmission electron microscopy (TEM) and dynamic laser light scattering (DLS) were employed to investigate and track the morphology of the rod-like large complex micelles before and after irradiation of UV light.     

An expanded neutral M4L6 cage that encapsulates four tetrahydrofuran molecules

A large 4,4'-biphenylene-spaced bis-β-diketone ligand is demonstrated to form a neutral tetrahedral M(4)L(6) metal-organic cage that encloses a volume of 844 ?(3) and encapsulates four tetrahydrofuran guest molecules.     

Synthesis and crystal structure of an M4L6 tetrahedral cage with outward‐facing pockets from a substituted pyrazolyl–pyridine ligand

The self‐assembly of metal–polydentate ligands to give supramolecular tetrahedral complexes is of considerable current interest. A new ligand, 4‐benzyl‐2‐[1‐(2‐{[3‐(4‐benzylpyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]methyl}benzyl)‐1H‐pyrazol‐3‐yl]pyridine (L), with chelating pyrazolyl–pyridine units substituted on the 4‐position of the pyridyl ring with benzyl units, has been synthesized and fully characterized. The self‐assembly of L with cobalt(II) gave rise to a tetrahedral cage (hexakis{μ‐4‐benzyl‐2‐[1‐(2‐{[3‐(4‐benzylpyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]methyl}benzyl)‐1H‐pyrazol‐3‐yl]pyridine}perchloratotetracobalt(II) octakis(perchlorate) acetonitrile undecasolvate, [Co₄(ClO₄)(C₃₈H₃₂N₆)₆](ClO₄)₇·11CH₃CN) with approximate T symmetry. The X‐ray crystal structure of the cage, i.e. [Co₄L₆ClO₄](ClO₄)₇, shows that the substituted benzyl groups are oriented away from the centres of their respective ligands towards the Co^II vertices, making small outward‐facing pockets from three benzyl rings at the corners of the tetrahedron.     

Chirality manipulation of supramolecular self-assembly based on the host-guest chemistry of cyclodextrin

Chiral materials have been of the interests of scientists for nearly a century. People have endeavored a great effort to manipulate the chirality of various self-assembled materials. Among these efforts, cyclodextrins are used only in recent years, although it has long been recognized that the chirality of cyclodextrin can be transferred to the guest. In this review, we for the first time summarize the recent advancement of the supramolecular chirality manipulation on the basis of the host-guest chemistry of cyclodextrins. By using the simple Harata-Kodaka's rule, natural cyclodextrins can be exploited in a dynamic manner to create chirality inversion materials through crystalline self-assembly, which is facile and environment-friendly. What is more, we also discussed the remarks on future outlooks at the end of this article and expect it to stimulate a rapid development on both the theory and application level.     

Characterization of self-assembled supramolecular [Ga4L6] host-guest complexes by electrospray ionization mass spectrometry

Self-assembled supramolecular host-guest complexes have been characterized by electrospray ionization mass spectrometry. The spectra obtained by use of a Q-TOF instrument equipped with a Z-spray ion source show primarily the 3- and 4- charge states of the assemblies. The assemblies have the general formula [guest subset Ga4L6]11- where L represents the chelating bidentate catechol ligand 1,5-bis(2',3'-dihydroxy-benzamido)naphthalene and guests are tetramethyl ammonium (Me4N+), tetraethyl ammonium (Et4N+), tetra-n-propyl ammonium (Pr4N+) and decamethylcobaltocenium (Cp*2Co+) cations. For the first time, the mass spectrum of the empty assembly [Ga4L6]12- is reported. This article also reports that provided the electrospray ion source is capable of preserving noncovalent interactions, it is possible to observe host-guest complexes containing both weak binding guests as well as sterically demanding guests in the mass spectra. The present data suggest that electrospray mass spectrometry is a powerful tool for characterization of supramolecular host-guest complexes.     

Hexagonal terpyridine--ruthenium and -iron macrocyclic complexes by stepwise and self-assembly procedures

Methods for the self-assembly, as well as directed construction, of hexaruthenium metallomacrocycles employing bisterpyridine building blocks are described. Self-assembly is effected by a combination of equimolar mixtures of bismetalated and nonmetalated bis(terpyridinyl) monomers each possessing the requisite planar, 60 degrees, terpyridine-metal-terpyridine connectivity. Stepwise synthesis of the identical hexamer is also discussed and used to aid in verification of the self-assembled product. Preparation and analysis of the related FeII metallomacrocycle are detailed and its TEM image confirms the hexameric structure. Characterization of the metalated products includes cyclic voltammetry along with the routine analytical techniques.     

Origins of cooperative noncovalent host-guest chemistry in mixed valence complexes

The electronic effects resulting from noncovalent host-guest interactions between calix[6]arene and a ruthenium dimer, [Ru3O(OAc)6(CO)(ppy)]2-mu-pz (ppy=4-phenyl pyridine, pz=pyrazine), are presented. The noncovalent interaction is between the calix[6]arene and the ppy ligands of the dimer. The dimer can bind 2 equiv of calix[6]arene. The complex [Ru3O(OAc)6(CO)(ppy)]2-mu-pz forms a highly stable mixed valence ion with strong electronic coupling between the two Ru3 clusters. The strength of the electronic interaction is found to be moderated by calix[6]arene binding. Addition of calix[6]arene to the mixed valence ion causes the electronic coupling to decrease. The binding of calix[6]arene is found to be cooperative. The origins of cooperative binding are developed in terms of the potential energy surfaces associated with the symmetric and asymmetric mixed valence ion. In particular, it is found that symmetry breaking (through the binding of a single calix[6]arene) destabilizes the mixed valence state. Restoration of symmetry (through the binding of a second calix[6]arene) increases the stability of the mixed valence ion and provides an additional driving force for the binding of the second calix[6]arene.     

Synthesis and magnetic behavior of the tetrahedral cage complex [(cyclen)4V4(CN)6]6+

Reaction of [(cyclen)V(CF(3)SO(3))(2)](CF(3)SO(3)) with 4 equiv. of Et(4)N(CN) in DMF generates the seven-coordinate complex [(cyclen)V(CN)(3)], while a reaction employing just 1.5 equiv. produces a tetrahedral cage complex, [(cyclen)(4)V(4)(CN)(6)](6+), in which antiferromagnetic coupling leads to an S= 0 ground state.     

Pillar[6]arenes: From preparation,host-guest property to self-assembly and applications

Pillar[n]arenes primarily comprise pillar[5]arenes and pillar[6]arenes, which belong to the new class of supramolecular macrocyclic hosts. Pillar[n]arenes have aroused wide attention because of their highly rigid and symmetrical architectures, controllable cavity size, and wide applications in a wide variety of areas. Although pillar[6]arene is difficult to synthesize, numerous studies have been conducted on it. In this review, the strategies to synthesize and functionalize pillar[6]arenes are investigated systematically. In addition, their host-guest properties in organic solvents and in aqueous solution are described. Moreover, pillar[6]arenes applied in different fields (e.g., molecular recognition, drug release, cancer therapy, and gas separation) are clarified. Hopefully, this study is capable of arousing more attention from increasing scientists to study large-cavity pillar[n]arenes.     

Cyclic tetranuclear and hexanuclear palladium(II) complexes and their host-guest chemistry

Cyclic tetrameric complexes have been prepared by the reaction of Pd(en)Cl2 or Pd(dapol)Cl2, or their nitrato analogues, with Na2(5'GMP) in aqueous solution, where en=1,2-diaminoethane, dapol=1,3-diamino-2-propanol, and 5'GMP=guanosine 5'-monophosphate. Addition of certain small molecules containing hydrophobic groups resulted in the expansion of the tetramer to a cyclic hexamer with strong bonding of one guest per hexameric host. At pH 5-6, the guest molecule can be a cation, anion, or neutral, and those species containing trimetylsilyl and t-butyl groups bonded the most strongly. The size of the central cavity of the [Pd(en)(5'GMP)]6 host has been estimated to be 5.2 A. Formation of the host-guest complex caused a large upfield shift (Deltadelta) of 2.5-2.9 ppm in the 1H NMR spectrum of the most highly affected guest protons, which were those in closest proximity to the guanine nucleobases. NOESY spectra were used to determine the interaction sites between the host and the guest. Apparent association constants determined at 26 degrees C and pD 5.4 for the [Pd(en)(5'GMP)]6-DSS and [Pd(en)(5'GMP)]6-t-butanol systems, where DSS is 3-(trimethylsilyl)-1-propanesulfonate anion, were 1.36+/-0.11x10(4) and 2.74+/-0.95x10(4) M(-3/2), respectively. The Pd(dapol)-5'GMP system forms hexameric host-guest complexes, similar in nature to those of the Pd(en)-5'GMP system. The molecular and crystal structures of Pd(dapol)Cl2 are also reported.     

Syntheses, structural characterization, and host-guest chemistry of ethynylcrown ether containing polynuclear gold(I) complexes

Two novel ethynylcrown ether containing di- and tetranuclear gold(I) complexes have been synthesized and structurally characterized; their binding ability toward various metal ions has also been studied.     

Assembly of supermolecular complexes from the tripodal ligand titmb: assembly of a large M6L8 cage from 14 components

The coordinatively saturated, nanometer-sized M6L8 complex [Pd6(titmb)8]Cl(12).2H2O (titmb = 1,3,5-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene) was obtained by assembly of six Pd(II) ions with eight flexible titmb tripodal ligands; structural analysis shows that these eight titmb are in a disordered cube configuration and the six Pd atoms are in a disordered octahedral configuration; the inner cavity of the cage is estimated to have a volume of 1000 A3, large enough to encapsulate eight Cl- anions.