Spin Frustration in the Triradical Trianion of a Naphthalenediimide Molecular Triangle

Crystalline supramolecular frameworks consisting of charged molecules, held together by hydrogen bonds and Coulomb interactions, have attracted great interest because of their unusual structural, chemical, electronic, and magnetic properties. Herein, we report the preparation, structure, and magnetic properties of the triradical trianion of a shape-persistent chiral equilateral molecular triangle having three naphthalene-1,4:5,8-bis­(dicarboximide)­s ((+)-NDI-Δ^3(−•)). Single-crystal X-ray diffraction of its tris­(cobalto­cenium) salt ([(+)-NDI-Δ^3(−•)­(CoCp₂⁺)₃]) reveals accessible one-dimensional tubular cavities, and variable-temperature electron paramagnetic resonance spectroscopy shows that a dilute solution of [(+)-NDI-Δ^3(−•)­(CoCp₂⁺)₃] in an organic glass has a spin-frustrated doublet ground state and a thermally accessible quartet state. Furthermore, SQUID magnetometry from 5 to 300 K of solid [(+)-NDI-Δ^3(−•)­(CoCp₂⁺)₃] shows ferromagnetic ordering with a Curie temperature <i>T</i>_C = 20 K. The successful preparation of hybrid ionic materials comprising macrocyclic triradical trianions with spin-frustrated ground states and accessible 1D pores offers routes to new organic spintronic materials

Synthesis of Ex^<i>n</i>Box Cyclophanes

A rapid and efficient synthesis of the extended bipyridinium-based class of cyclophanesthat is, <b>Ex</b>^{<i><b>n</b></i>}<b>Box</b>^<b>4+</b> (<i>n</i> = 0–3), where <i>n</i> is the number of <i>p</i>-phenylene rings inserted between the pyridinium ringsis demonstrated, resulting in much higher yields of products along with a reduced output of oligomeric byproducts. Although each cyclophane can be synthesized readily without the use of a precise stoichiometric amount of template, <b>ExBox</b>^<b>4+</b> can be prepared in 66% yield (following crystallization) using six equivalents of pyrene in a template-directed protocol. This new methodology has been employed to synthesize, in modest yield, a nearly 2.5 nm long cyclophane consisting of 12 aromatic rings

Synthesis of Ex^<i>n</i>Box Cyclophanes

A rapid and efficient synthesis of the extended bipyridinium-based class of cyclophanesthat is, <b>Ex</b>^{<i><b>n</b></i>}<b>Box</b>^<b>4+</b> (<i>n</i> = 0–3), where <i>n</i> is the number of <i>p</i>-phenylene rings inserted between the pyridinium ringsis demonstrated, resulting in much higher yields of products along with a reduced output of oligomeric byproducts. Although each cyclophane can be synthesized readily without the use of a precise stoichiometric amount of template, <b>ExBox</b>^<b>4+</b> can be prepared in 66% yield (following crystallization) using six equivalents of pyrene in a template-directed protocol. This new methodology has been employed to synthesize, in modest yield, a nearly 2.5 nm long cyclophane consisting of 12 aromatic rings

Employment trends survey 1998

SIGLEAvailable from British Library Document Supply Centre-DSC:8737.834(1998) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Cation-Dependent Gold Recovery with α‑Cyclodextrin Facilitated by Second-Sphere Coordination

Herein, we report an alkali metal cation-dependent approach to gold recovery, facilitated by second-sphere coordination with eco-friendly α-cyclodextrin (α-CD). Upon mixing eight salts composed of Na⁺, K⁺, Rb⁺, or Cs⁺ cations and [AuX₄]⁻ (X = Cl/Br) anions with α-, β-, or γ-CD in water, co-precipitates form selectively from the three (out of 24) aqueous solutions containing α-CD with KAuBr₄, RbAuBr₄, and CsAuBr₄, from which the combination of α-CD and KAuBr₄ affords the highest yield. Single-crystal X-ray analyses reveal that in 20 of the 24 adducts CD and [AuX₄]⁻ anions form 2:1 sandwich-type second-sphere adducts driven partially by [C–H···X–Au] interactions between [AuX₄]⁻ anions and the primary faces of two neighboring CDs. In the adduct formed between α-CD and KAuBr₄, a [K­(OH₂)₆]⁺ cation is encapsulated inside the cavity between the secondary faces of two α-CDs, leading to highly efficient precipitation owing to the formation of a cation/anion alternating ion wire residing inside a continuous α-CD nanotube. By contrast, in the other 19 adducts, the cations are coordinated by OH groups and glucopyranosyl ring O atoms in CDs. The strong coordination of Rb⁺ and Cs⁺ cations by these ligands, in conjunction with the stereo­electronically favorable binding of [AuBr₄]⁻ anions with two α-CDs, facilitates the co-precipitation of the two adducts formed between α-CD with RbAuBr₄ and CsAuBr₄. In order to develop an efficient process for green gold recovery, the co-precipitation yield of α-CD and KAuBr₄ has been optimized regarding both the temperature and the molar ratio of α-CD to KAuBr₄

Emergent Ion-Gated Binding of Cationic Host–Guest Complexes within Cationic M₁₂L₂₄ Molecular Flasks

“Molecular flasks” are well-defined supra­molecular cages that can encapsulate one or more molecular guests within their cavities and, in so doing, change the physical properties and reactivities of the guests. Although molecular flasks are powerful tools for manipulating matter on the nano­scale, most of them are limited in their scope because of size restrictions. Recently, however, increasingly large and diverse supra­molecular cages have become available with enough space in their cavities for larger chemical systems such as polymers, nano­particles, and bio­molecules. Here we report how a class of metallo­supra­molecular cages known as M₁₂L₂₄ polyhedra have been adapted to serve as nano­meter-scale containers for solutions of a pseudo­rotaxane host–​guest complex based on a tetra­cationic cyclo­phane host, cyclo­bis­(paraquat-<i>p</i>-phenylene) (CBPQT⁴⁺), and a 1,5-dioxy­naphthalene (DNP) guest. Remarkably, the hierarchical integration of pseudo­rotaxanes and M₁₂L₂₄ super­hosts causes the system to express stimulus-responsive behavior, a property which can be described as emergent because neither the DNP⊂​CBPQT⁴⁺ nor the M₁₂L₂₄ assemblies exhibit this behavior independently. The DNP-containing M₁₂L₂₄ molecular flasks are effectively “sealed off” to CBPQT⁴⁺ until ions are added as a stimulus to “open” them. The electrolyte stimulus reduces the electro­static screening distance in solution, allowing favorable DNP⊂​CBPQT⁴⁺ host–​guest inter­actions to overcome repulsive Coulombic inter­actions between the cationic M₁₂L₂₄ cages and CBPQT⁴⁺ rings. This unusual example of ion-gated transport into chemical nano­containers is reminiscent of trans­membrane ion channels which act as gates to the cell, with the important difference that this system is reversible and operates at equilibrium

A Water-Soluble pH-Triggered Molecular Switch

A bistable donor–acceptor [2]­catenane, which is composed of a crown ether containing a hydroquinone unit and a 1,5-diaminonaphthalene unit, interlocked mechanically by cyclobis­(paraquat-<i>p</i>-phenylene) as its tetrachloride, exists as a mixture of translational isomers, both in the solid state and in aqueous solution. UV/vis and ¹H NMR spectroscopies demonstrate that this isomeric mixture can be switched in water in the presence of hydrochloric acid to afford a single diprotonated derivative in which only the hydroquinone unit resides inside the cavity of the tetracationic cyclophane. Treatment with 1,4-diazabicyclo[2.2.2]­octane resets the molecular switch

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

Shuttling Rates, Electronic States, and Hysteresis in a Ring-in-Ring Rotaxane

The trisradical recognition motif between a 4,4′-bipyridinium radical cation and a cyclo-bis-4,4′-bipyridinium diradical dication has been employed previously in rotaxanes to control their nanomechanical and electronic properties. Herein, we describe the synthesis and characterization of a redox-active ring-in-ring [2]­rotaxane <b>BBR·</b>8PF₆ that employs a tetraradical variant of this recognition motif. A square-shaped bis-4,4′-bipyridinium cyclophane is mechanically interlocked around the dumbbell component of this rotaxane, and the dumbbell itself incorporates a smaller bis-4,4′-bipyridinium cyclophane into its covalently bonded structure. This small cyclophane serves as a significant impediment to the shuttling of the larger ring across the dumbbell component of <b>BBR</b>^<b>8+</b>, whereas reduction to the tetraradical tetracationic state <b>BBR</b>^{<b>4(+•)</b>} results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, <b>BBR</b>·8PF₆ exhibits qualitatively similar behavior to its predecessors that interconvert between hexacationic and trisradical tricationic states. The rigid preorganization of two bipyridinium groups within the dumbbell of <b>BBR</b>·8PF₆ confers, however, two distinct properties upon this rotaxane: (1) the rate of shuttling is reduced significantly relative to those of its predecessors, resulting in marked electrochemical hysteresis observed by cyclic voltammetry for switching between the <b>BBR</b>^<b>8+</b><b>/BBR</b>^{<b>4(+•)</b>} states, and (2) the formally tetraradical form of the rotaxane, <b>BBR</b>^{<b>4(+•)</b>}, exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within <b>BBR</b>^{<b>4(+•)</b>}, has a long enough lifetime to be characterized by ¹H NMR spectroscopy

Efficient Long-Range Stereochemical Communication and Cooperative Effects in Self-Assembled Fe₄L₆ Cages

A series of large, optically active Fe₄L₆ cages was prepared from linear 5,5′-bis­(2-formylpyridines) incorporating varying numbers (<i>n</i> = 0–3) of oligo-<i>p</i>-xylene spacers, chiral amines, and Fe^II. When a cage was constructed from the ligand bridged by one <i>p</i>-xylene spacer (<i>n</i> = 1) and a bulky chiral amine, both a homochiral Fe₂L₃ helicate and Fe₄L₆ cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe₄L₆ cage was observed. In the case of larger cages (<i>n</i> = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands’ geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands