客体分子所处晶穴结构不同对甲烷水合物稳定性的影响

采用分子动力学方法模拟了SⅠ型甲烷水合物受热分解微观过程,并对水合物分解过程中不同晶穴结构内客体分子对甲烷水合物稳定性的作用进行了研究.通过最终构象、均方位移和势能等性质的变化规律对分别缺失大晶穴和小晶穴中客体分子的2种水合物体系随模拟温度升高稳定性的变化进行了分析.模拟结果显示,随温度的上升,水合物稳定性逐渐下降直至彻底分解;而水合物分解速度与2种晶穴各自部分晶穴占有率相关,不能简单的通过整体晶穴占有率表示.对比相同注热过程中2种水合物体系分解状况,发现位于大晶穴内的客体分子对水合物稳定性影响更大,缺失大晶穴内客体分子的水合物更容易随温度升高而分解.     

pH-Controlled selection between one of three guests from a mixture using a coordination cage host

We demonstrate the use of a simple pH swing to control the selection of one of three different guests from aqueous solution by a coordination cage host. Switching between different guests is based on the fact that neutral organic guests bind strongly in the cage due to the hydrophobic effect, but for acidic or basic guests, the charged (protonated or deprotonated) forms are hydrophilic and do not bind. The guests used are adamantane-1,3-dicarboxylic acid (H₂A) which binds at low pH when it is neutral but not when it is deprotonated; 1-amino-adamantane (B) which binds at high pH when it is neutral but not when it is protonated; and cyclononanone (C) whose binding is not pH dependent and is therefore the default guest at neutral pH. Thus an increase in pH can reversibly switch the host between the three different bound states cage·H₂A (at low pH), cage·C (at medium pH), and cage·B (at high pH) in succession.     

An octanuclear coordination cage with a 'cuneane' core--a topological isomer of a cubic cage

The octanuclear coordination cage [Ni(8)(L(14Naph))(12)](BF(4))(16) has the core structure of a 'cuneane'--a toplogical isomer of a cube--with a metal ion at each of the eight vertices and bridging ligand spanning each of the twelve edges; this is the only possible 8-vertex polyhedron other than a cube that will form a cage in which each metal is connected to three others.     

Understanding behaviour of vitamin-C guest binding with the cucurbit[6]uril host

The present study describes non-covalent interaction and complexation behaviour of sodium ascorbate (SA) with cucurbit[6]uril (CB[6]) at neutral pH in aqueous Na₂SO₄ solution. The interaction behaviour is investigated using various analytical techniques like NMR, UV–Vis, fluorescence, TGA and DRS. The substantial increase in the intensity of emission and absorption spectra of sodium ascorbate is observed. The Benesi–Hildebrand evaluation method is used to determine the stoichiometry and equilibrium constant of the cucurbit[6]uril–sodium ascorbate complex, which suggested the 1:1 complex. Time-dependent ¹H NMR, ¹³C CP MAS and CD studies also echoed non-covalent interaction between SA with CB[6].     

Sequence-selective recognition of peptides within the single binding pocket of a self-assembled coordination cage

The single binding pocket of a self-assembled Pd6L4 coordination cage recognizes oligopeptides in a highly sequence-selective fashion. In particular, the Trp-Trp-Ala sequence is strongly bound by the cavity (Ka >/=106 M-1). Tripeptides possessing the same residues but in different sequences (i.e., Trp-Ala-Trp and Ala-Trp-Trp) show much poorer affinity. Even singly mutated tripeptides with aromatic-aromatic-aliphatic sequences of the residues (e.g., Trp-Trp-Gly and Trp-Tyr-Ala) are not recognized efficiently. X-ray analysis and NMR reveal that all residues of the Trp-Trp-Ala sequence cooperatively interact with the cage via CH-pi and pi-pi interactions.     

Modulation with acetonitrile of the dynamics of guest binding to the two distinct binding sites of cholate aggregates

Bile salt aggregates are supramolecular systems containing two different binding sites. The effect of the addition of acetonitrile on the specificity and dynamics of guest binding to the two binding sites of cholate aggregates was studied. The protection of guests included in the aggregate from interaction with ions in the aqueous phase was evaluated from quenching of the singlet and triplet excited states of guest molecules bound to the cholate aggregates. The dynamics of guest binding to the primary and secondary binding sites of the cholate aggregates were determined at increasing acetonitrile mole fractions. The structure of the aggregates was not significantly altered provided the cholate concentrations were higher than 20 mM and the acetonitrile mole fraction did not exceed 0.033 (9.1% v/v). These results show that acetonitrile can be used to modulate the solubility of guests in the aggregates and to manipulate the residence time of guests in the primary and secondary binding sites.     

DABCO-Induced self-assembly of a trisporphyrin double-decker cage: thermodynamic characterization and guest recognition

This paper describes the thermodynamic characterization of the self-assembly of a Zn trisporphyrin induced by coordination with 1,4-diazabicyclo[2.2.2]octane (DABCO) to form a stable 2:3 double-decker molecular coordination cage that recognizes benzene-1,3,5-tricarboxamides. The self-assembly process has been studied using UV-vis and (1)H NMR spectroscopy and quantitatively characterized in terms of a single stability constant that describes the strength of the individual coordination interactions and two effective molarities (EM) that describe the additional stability imparted by intramolecular cyclization. The EM values of the two consecutive cyclic intramolecular interactions are very similar. At micromolar concentrations, the formation of the fully assembled coordination cage is highly favored over the formation of intermediate species stabilized by fewer interactions, and so self-assembly is an all-or-nothing process. In contrast, at millimolar concentrations, the relative stability of intermediate species increases, leading to a stepwise self-assembly process, and a 2:2 intermediate can be clearly identified using (1)H NMR spectroscopy. The molecular recognition of benzene-1,3,5-tricarboxamides by the cage was investigated using (1)H NMR spectroscopy. The tricarboxamides bind inside the central cavity of the cage complex, and isothermal titration calorimetry (ITC) allowed the quantification of the stoichiometry and binding affinities.     

A kinetic study of guest displacement reactions on a host-guest complex with a photoswitchable calixarene

The displacement processes of several guests, incorporated in a calixarene host system, were investigated in the gas phase by electrospray ionization-Fourier transform-ion cyclotron resonance (ESI-FT-ICR) mass spectrometry. The complexes resulting from a resorcin[4]arene host with ammonia and sec-butylamine guests were isolated in an ICR-cell, separately using both states of the photoswitch as well as two reference systems for the open and closed forms of the photoswitchable host. The isolated complexes were forced to exchange the guest by using methylamine, ethylamine and sec-butylamine, resulting in different reaction rates for all the measured systems. Especially, the reaction rates of both states of the photoswitch are dependent on the provided guest. Potential side effects like proton exchanges were examined by an H/D-exchange experiment. The results were investigated and supported by quantum chemical calculations (DFT).     

Cyclic dimer of a fused porphyrin zinc complex as a novel host with two pi-electronically coupled binding sites

Upon complexation with 4,4[prime or minute]-bipyridine, a cyclic dimer of a fused porphyrin zinc complex, having two pi-electronically coupled binding sites, shows a strong negative cooperativity in the second guest binding, to allow stepwise formation of 1 : 1 and 1 : 2 inclusion complexes.     

Supramolecular structure of N,N-Bis(2-hydroxybenzyl)alkylamine: flexible molecular assembly framework for host without guest and host with guest

N,N-Bis(2-hydroxybenzyl)alkylamine derivatives form a cage-like assembly consisting of two molecules via inter- and intramolecular hydrogen bonds. The derivatives exhibit themselves as host to accept copper-ion guests under the double-oxygen-bridged dimeric system. Quantum chemical calculation suggested that the host-guest interaction is based on a charge-transfer coordination. Comparison of the crystal structures before and after complexation clarifies a rare example of a host-guest compound where the hosts maintain their cage framework through the change of hydrogen bonds to coordination bonds.     

Self-assembly and anion-exchange properties of a discrete cage and 3D coordination networks based on cage structures

By using tridentate ligand 4-(3-pyridinyl)-1,2,4-triazole (pytrz), cage-like complexes of {[Cu(mu2-pytrz)2](ClO4)(SO4)0.5C2H5OH.0.25 H2O}6 (1), {[Cu3(mu3-pytrz)4(mu2-Cl)2(H2O)2](ClO4)2Cl(2).2 H2O}n (2), and {[Cu3(mu3-pytrz)3(mu3-O)(H2O)3](ClO4)2.5(BF4)(1.5)5.25 H2O}n (3) have been synthesized with different copper(II) salts. Complex 1 represents the second example of a M6L12 metal-organic octahedron with an overall Th symmetry. Complex 2 is constructed from a 3(8) cage-building unit (CBU) and each CBU connects six neighboring cages to give the first 3D metal-organic framework (MOF) based on octahedral M6L12. Complex 3 is built from Cu24(pytrz)12 CBUs with the trinuclear copper clusters serving as second building units (SBUs) and decorating each corner of the M24L12 polyhedron. The Cu24(pytrz)12 building unit is linked by extra ligands to give an extended 3D framework that has the formula Cu24(pytrz)24 and possesses a CaB6 topology. The mixed anions ClO4- and BF4- in 3 are both included in the inner cavity of the cage and can be completely exchanged by ClO4- through the open windows of the cage, as evidenced by the crystal structure of the 3D MOF {[Cu3(mu3-pytrz)3(mu3-O)(H2O)3](ClO4)(4)4.5 H2O}n (4). Complex 4 can also be synthesized when employing 1 as a precursor in an extensive study of the anion-exchange reaction. This represents the first successful conversion of a discrete cage into a 3D coordination network based on a cage structure. Complex 2 remains invariable during anion-exchange reactions because uncoordinated Cl- ions are located in the comparatively small inner cavity.     

On the interaction of two different types of ligands binding to the same molecule part I: basics and the transfer of the decoupled sites representation to systems with n and one binding sites

The decoupled sites representation (DSR) for one type of ligand allows to regard complex overall titration curves as sum of classical Henderson-Hasselbalch (HH) titration curves. In this work we transfer this theoretical approach to molecules with different types of interacting ligands (e.g. protons and electrons), prove the existence of decoupled systems for n ₁ and one binding sites for two different ligands, and point out some difficulties and limits of this transfer. A major difference to the DSR for one type of ligand is the loss of uniqueness of the decoupled system. However, all decoupled systems share a unique set of microstate probabilities and each decoupled system corresponds to a certain permutation of these microstate probabilities. Moreover, we show that the titration curve of a certain binding site in the original system can be regarded as linear combination of the titration curves of the individual sites of the decoupled system if the weights of the linear combination are substituted by functions in the activity of the second ligand. In the underlying model with only pairwise interaction, an important observation of our theoretical investigation is the following: Even though the binding sites of ligand L ₁ may not interact directly, they can show secondary interaction due to the interaction with the second type of ligand. This means, if the activity of the second ligand is fixed and we regard the 1-dimensional titration curve of an individual binding site for ligand L ₁ depending on its activity, we may observe a strong deviation from the classical HH shape in spite of non-interacting sites for ligand L ₁.     

On the interaction of different types of ligands binding to the same molecule Part II: systems with n to 2 and n to 3 binding sites

In the first part of this work we formulated the decoupled sites representation for two different types of ligands and highlighted special properties of the case of n binding sites for ligand L ₁ and one binding site for ligand L ₂. Moreover, for this case, we identified the microstate constants as unique components all decoupled molecules share. In the second part on hand, we investigate the cases with (n, 2) and (n, 3) binding sites. As it is difficult to solve the system of equations occurring when a molecule with more than one binding site for both ligands shall be decoupled, we present applicable calculation methods which exploit the special structure of the system of equations. Moreover, we investigate which unique properties all decoupled molecules share and show that for two different decoupled molecules with the same binding polynomial, not all microstate constants of a certain macrostate are permutations of the microstate constants of the other molecule.     

Assembly of a coordination cage with four aromatic channel receptors on the outside

Silver salts and triphosphine ligands with biphenyl substituents assemble to give coordination cages with four external aromatic channel receptors in a pseudo-tetrahedral arrangement.     

Reconstructing reactivity in dynamic host–guest systems at atomistic resolution: amide hydrolysis under confinement in the cavity of a coordination cage

Spatial confinement is widely employed by nature to attain unique efficiency in controlling chemical reactions. Notable examples are enzymes, which selectively bind reactants and exquisitely regulate their conversion into products. In an attempt to mimic natural catalytic systems, supramolecular metal–organic cages capable of encapsulating guests in their cavity and of controlling/accelerating chemical reactions under confinement are attracting increasing interest. However, the complex nature of these systems, where reactants/products continuously exchange in-and-out of the host, makes it often difficult to elucidate the factors controlling the reactivity in dynamic regimes. As a case study, here we focus on a coordination cage that can encapsulate amide guests and enhance their hydrolysis by favoring their mechanical twisting towards reactive molecular configurations under confinement. We designed an advanced multiscale simulation approach that allows us to reconstruct the reactivity in such host–guest systems in dynamic regimes. In this way, we can characterize amide encapsulation/expulsion in/out of the cage cavity (thermodynamics and kinetics), coupling such host–guest dynamic equilibrium with characteristic hydrolysis reaction constants. All computed kinetic/thermodynamic data are then combined, obtaining a statistical estimation of reaction acceleration in the host–guest system that is found in optimal agreement with the available experimental trends. This shows how, to understand the key factors controlling accelerations/variations in the reaction under confinement, it is necessary to take into account all dynamic processes that occur as intimately entangled in such host–guest systems. This also provides us with a flexible computational framework, useful to build structure–dynamics–property relationships for a variety of reactive host–guest systems.

Encapsulation of guests in metal–organic cages allows control over chemical reactivity. Focusing on the hydrolysis of amides, here we show an effective molecular simulation approach to reconstruct reactivity in host–guest systems in dynamic regimes.     

Self-assembled hetero-bimetallic coordination cage and cation-clusters with micro(2)-Cl bridging using a flexible two-arm ferrocene amide linker

One flexible, discrete coordination cage [Cu(2)(3-BPFA)(4)(H(2)O)(2)](ClO(4))(4).4CH(3)OH (), and two cation-clusters with micro(2)-Cl bridging [Ni(2)(micro-Cl)(3-BPFA)(4)(H(2)O)(2)](ClO(4))(3) () and [Co(2)(micro-Cl)(3-BPFA)(4)(H(2)O)(2)](ClO(4))(4).4CH(3)OH (), containing the ferrocenyl functionality were prepared via coordination-driven self-assembly and Cl-anion template from Cu(II), Ni(II) and Co(II) salts and a flexible two-arm molecule 1,1-bis[(3-pyridylamino)carbonyl]ferrocene (3-BPFA).     

Silica nanoparticles functionalised with cation coordination sites and fluorophores for the differential sensing of anions in a quencher displacement assay (QDA)

In conjunction with quenching metal ions, silica nanoparticles carrying terpyridine coordination sites and sulforhodamine B signalling units were employed for the differential fluorometric recognition of anions.     

Poly(ε-caprolactone) macroligands with β-diketonate binding sites: synthesis and coordination chemistry

Dibenzoylmethane (dbm) initiators with one and two alcohol sites were used to generate dbm end-functionalized and dbm-centered poly(ε-caprolactone) macroligands (dbmPCL and dbmPCL₂) with low polydispersities (∼1.1). Chelation of polymeric ligands to metal ions (Eu³⁺, Fe³⁺, Ni²⁺ and Cu²⁺) produced metal-centered star polymers, which were characterized by UV-vis and fluorescence spectroscopy, as well as gel permeation chromatography.