Cooperative Reactivity in an Extended-Viologen-Based Cyclophane.

A tetracationic pyridinium-based cyclophane with a box-like geometry, incorporating two juxtaposed alkyne functions, has been synthesized. The triple bonds are reactive through cycloadditions toward dienes and azides, promoted by the electron-withdrawing nature of the pyridinium rings, as well as by the strain inherent in the cyclophane. The cycloadditions proceeded in high yields, with the cyclophane reacting faster than its acyclic analogue. While the cyclophane contains two reactive triple bonds, there is no evidence for a stable monofunctional intermediate-only starting material and the difunctional product have been detected by (1)H NMR spectroscopy. Molecular modeling of the energy landscape reveals a lower barrier for the kinetically favored second cycloaddition compared with the first one. This situation results in tandem cascading reactions within rigid cyclophanes, where reactions at a first triple bond induce increased reactivity at a distal second alkyne.

Allosteric Modulation of Substrate Binding within a Tetracationic Molecular Receptor.

The synthesis and recognition phenomena of a tetracationic molecular receptor that possesses a nanometer-sized molecular cavity are described. The host-guest properties of the molecular receptor can be tuned and modulated allosterically, where the association of a heterotropic effector at the periphery of the molecule serves to modulate its affinity for the globular, electron-rich guest that resides within its molecular cavity. This stimuli-responsive host-guest behavior was observed in both the solution phase and the crystalline solid state, and can be reversed with high fidelity by sequestration of the effector molecule.

Catenation through a Combination of Radical Templation and Ring-Closing Metathesis.

Synthesis of an electrochemically addressable [2]catenane has been achieved following formation by templation of a [2]pseudorotaxane employing radically enhanced molecular recognition between the bisradical dication obtained on reduction of the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and the radical cation generated on reduction of a viologen disubstituted with p-xylylene units, both carrying tetraethylene glycol chains terminated by allyl groups. This inclusion complex was subjected to olefin ring-closing metathesis, which was observed to proceed under reduced conditions, to mechanically interlock the two components. Upon oxidation, Coulombic repulsion between the positively charged and mechanically interlocked components results in the adoption of a co-conformation where the newly formed alkene resides inside the cavity of the tetracationic cyclophane. (1)H NMR spectroscopic analysis of this hexacationic [2]catenane shows a dramatic upfield shift of the resonances associated with the olefinic and allylic protons as a result of them residing inside the tetracationic component. Further analysis shows high diastereoselectivity during catenation, as only a single (Z)-isomer is formed.

Energy and Electron Transfer Dynamics within a Series of Perylene Diimide/Cyclophane Systems.

Artificial photosynthetic systems for solar energy conversion exploit both covalent and supramolecular chemistry to produce favorable arrangements of light-harvesting and redox-active chromophores in space. An understanding of the interplay between key processes for photosynthesis, namely light-harvesting, energy transfer, and photoinduced charge separation and the design of novel, self-assembling components capable of these processes are imperative for the realization of multifunctional integrated systems. We report our investigations on the potential of extended tetracationic cyclophane/perylene diimide systems as components for artificial photosynthetic applications. We show how the selection of appropriate heterocycles, as extending units, allows for tuning of the electron accumulation and photophysical properties of the extended tetracationic cyclophanes. Spectroscopic techniques confirm energy transfer between the extended tetracationic cyclophanes and perylene diimide is ultrafast and quantitative, while the heterocycle specifically influences the energy transfer related parameters and the acceptor excited state.

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.

Solvent-dependent self-assembly behaviour and speciation control of Pd6L8 metallo-supramolecular cages.

The C3-symmetric chiral propylated host-type ligands (±)-tris(isonicotinoyl)-tris(propyl)-cyclotricatechylene (L1) and (±)-tris(4-pyridyl-4-benzoxy)-tris(propyl)-cyclotricatechylene (L2) self-assemble with Pd(II) into [Pd6L8](12+) metallo-cages that resemble a stella octangula. The self-assembly of the [Pd6(L1)8](12+) cage is solvent-dependent; broad NMR resonances and a disordered crystal structure indicate no chiral self-sorting of the ligand enantiomers in DMSO solution, but sharp NMR resonances occur in MeCN or MeNO2. The [Pd6(L1)8](12+) cage is observed to be less favourable in the presence of additional ligand, than is its counterpart, where L=(±)-tris(isonicotinoyl)cyclotriguaiacylene (L1 a). The stoichiometry of reactant mixtures and chemical triggers can be used to control formation of mixtures of homoleptic or heteroleptic [Pd6L8](12+) metallo-cages where L=L1 and L1 a.

Coordination polymers utilizing N-oxide functionalized host ligands.

Pyridyl functionalized host molecules are oxidized to their N-oxide analogues and form a series of coordination polymers and discrete complexes with transition metal cations. Complex {[Ag(3)(NMP)(6)(L1)(2)]·3(ClO(4))}(∞) where L1 = tris(isonicotinoyl-N-oxide)cyclotriguaiacylene, NMP = N-methylpyrrolidone, is a three-dimensional (3-D) 3,6-connected coordination polymer of pyrite-like (pyr) topology and features ligand unsupported argentophilic interactions, while two-dimensional (2-D) 3,6-connected coordination polymers with the rarely reported kagome dual (kgd) topology are found for [M(L1)(2)](2+) where M = Zn, Cd, Cu. Ligand L2 = tris(nicotinoyl-N-oxide)cyclotriguaiacylene forms a 2-D coordination polymer with 4(4) (sql) grid topology in complexes {[M(L2)(2)(DMF)(2)]·2ClO(4)·8(DMF)}(∞) M = Cd or Cu, DMF = N,N'-dimethylformamide, and a double-linked chain structure in {[Co(L2)(2)(DMF)(2)]·2NO(3)·4(DMF)·H(2)O}(∞), and both types of structure feature hand-shake self-inclusion motifs either within or between the polymers. 2-D coordination networks with 6(3) (hcb) topologies are found in complexes {[M(L3)(NO(3))(2)]·2(DMF)}(∞) (M = Cd, Zn) and {[Cu(5)(L3)(2)Cl(10)(NMP)(4)]}(∞) where L3 = tris(2-pyridylmethyl)cyclotriguaiacylene, while [Ag(2)(L3)(2)(NMP)(4)]·2(BF(4))·2(NMP) has a discrete dimeric structure which again shows hand-shake host-guest interactions supported by π-π stacking.

Two heptacopper(II) disk complexes with a [Cu(7)(µ(3)-OH)(4)(µ-OR)(2)](8+) core.

The reaction of CuX(2) (X(-) ≠ F(-)) salts with 1 equiv of 3-pyridyl-5-tert-butylpyrazole (HL) in basic methanol yields blue solids, from which disk complexes of the type [Cu(7)(µ(3)-OH)(4)(µ-OR)(2)(µ-L)(6)](2+) and/or the cubane [Cu(4)(µ(3)-OH)(4)(HL)(4)](4+) can be isolated by recrystallization under the appropriate conditions. Two of the disk complexes have been prepared in crystalline form: [Cu(7)(µ(3)-OH)(4)(µ-OCH(2)CF(3))(2)(µ-L)(6)][BF(4)](2) (2) and [Cu(7)(µ(3)-OH)(4)(µ-OCH(3))(2)(µ-L)(6)]Cl(2)·xCH(2)Cl(2) (3·xCH(2)Cl(2)). The molecular structures of both compounds as solvated crystals can be described as [Cu⊂Cu(6)(µ-OH)(4)(µ-OR)(2)(µ-L)(6)](2+) (R = CH(2)CF(3) or CH(3)) adducts. The [Cu(6)(µ-OH)(4)(µ-OR)(2)(µ-L)(6)] ring is constructed of six square-pyramidal Cu ions, linked by 1,2-pyrazolido bridges from the L(-) ligands and by basal, apical-bridging hydroxy or alkoxy groups, while the central Cu ion is bound to the four metallamacrocyclic hydroxy donors in a near-regular square-planar geometry. The L(-) ligands project above and below the metal ion core, forming two bowl-shaped cavities that are fully (R = CH(2)CF(3)) or partially (R = CH(3)) occupied by the alkoxy R substituents. Variable-temperature magnetic susceptibility measurements on 2 demonstrated antiferromagnetic interactions between the Cu ions, yielding a spin-frustrated S = (1)/(2) magnetic ground state that is fully populated below around 15 K. Electrospray ionization mass spectrometry, UV/vis/near-IR, and electron paramagnetic resonance measurements imply that the heptacopper(II) disk motif is robust in organic solvents.

A metal-organic framework immobilised iridium pincer complex.

An iridium pincer complex has been immobilised in the metal-organic framework NU-1000 using a technique called solvent assisted ligand-incorporation (SALI). The framework proved to be stable under the conditions required to activate the iridium complex and spectroscopic investigations showed formation of the catalytically active iridium dihydride. The Ir-pincer modified NU-1000 is an active catalyst for the condensed phase hydrogenation of a liquid alkene (1-decene and styrene) and shows enhanced activity with respect to a homogeneous analogue. Additionally, the Ir-pincer immobilised inside NU-1000 operated as an efficient heterogenous catalyst under flow conditions.

Controlling the assembly of cyclotriveratrylene-derived coordination cages.

A review of the emerging field of cyclotriveratrylene-derived coordination cages is presented. Ligand-functionalised cyclotriveratrylene (CTV) derivatives self-assemble with a range of metal cations to afford coordination cages, polymers and topologically non-trivial constructs, such as [2]catenanes and a self-entangled cube. Increased control over their self-assembly allows for the controlled and predictable formation of well-defined coordination cages for application in host-guest and recognition chemistry, with surfactant binding and single-crystal-to-single-crystal (SCTSC) uptake of small-molecule guests being observed.

Encapsulation of sodium alkyl sulfates by the cyclotriveratrylene-based, [Pd(6)L(8)](12+) stella octangula cage.

We have previously described a cyclotriveratrylene (CTV)-based complex, [Pd6L8](12+), that forms a highly symmetric stella octangula cage. Here we report on the ability of this system to host sodium salts of three alkyl sulfates; octyl (SOS), dodecyl (SDS) and tetradecyl (STS). (1)H NMR chemical shift and diffusion coefficient measurements reveal that two molecules of alkyl sulfate reside in the cage, though rapid exchange between cage and bulk solvent is apparent. Host : guest association constants have been determined and support for the internalisation of the sulfates is available from 2D ROESY and NOESY data.

New insights into the aggregation of silver pyrazolides using sterically hindered bidentate pyrazole ligands.

Recrystallisation of Ag[L(1)] (HL(1) = 3{5}-[pyrid-2-yl]-5{3}-tert-butylpyrazole) in the presence of halide anions leads to two polymorphs of [Ag(3)(µ-Br)(µ-L(1))(2)], which differ in their mode of supramolecular association, and the cluster [Ag(10)(µ-L(1))(8)]Cl(2). In contrast, Ag[L(2)] (HL(2) = 3{5}-[isoquinol-1-yl]-5{3}-tert-butyl-pyrazole) crystallises as a cyclic tetrameric molecule.

M3L2 metallo-cryptophanes: [2]catenane and simple cages.

Crystalline M(3)L(2) complexes with either single cage or triply interlocking [2]catenane chiral structures are formed the self-assembly of host-like ligands with transition metals.