Supramolecular Cd(II) complexes with (E)-N,N′-bis(2-pyridyl)iminoisoindoline (2-pyimiso): synthesis,X-ray structures,Hirshfeld surface analyses,and DFT study of [CdX2(2-pyimiso)2] (X = Cl or NCS)*

The paucity of coordination entities bearing (E)-N,N′-bis(heteroaryl)iminoisoindolines has prompted us to investigate coordination modes and supramolecular features of (E)-N,N′-bis(2-pyridyl)iminoisoindoline (2-pyimiso), a versatile iminobis(pyridyl) ligand. In this article we report the synthesis, spectroscopic characterization and crystal structure analysis of two Cd(II) : 2-pyimiso (1?:?2) bis-adducts, [CdX₂(2-pyimiso)₂] [X = Cl (1) or NCS (2)]. Our X-ray structural results reveal that 1 exhibits distorted tetrahedral coordination (four-coordinate geometry index τ₄ = 0.92), whereas 2 displays six-coordinate Cd(II) and two four-membered chelate rings (bite angles = 52.5°), each comprising one Cd–N_py [2.247(2) Å] bond and one Cd?N_imine [2.809(21) Å] secondary interaction. Remarkably, in 2 each 2-pyimiso unit binds to Cd(II) according to an unusual bidentate coordination. The contributions of the Cd–N and Cd–Cl bond valences to the total metal valence for both 1 and 2 have been evaluated to confirm the coordination modes of 2-pyimiso, which can be interpreted in terms of Jørgensen’s principle of symbiosis. X-ray structure and Hirshfeld surface analyses have shown that the crystal structure of 1 is determined by two perpendicular 1-D chains formed by weak hydrogen bonds along the a- and c-axes, whereas the supramolecular architecture of 2 exhibits 2-D sheets parallel to the ab-plane interconnected by C–H?π interactions along the c-axis. A vibrational analysis of both products has been conducted at the DFT B3LYP-D3/LACV3P** level of calculation.     

Amide-based ligands for anion coordination

Anion recognition is an active area of research in supramolecular chemistry. The rapidly increasing amount of structural data now allows anion coordination chemistry to be formalized in terms of coordination numbers and geometries based on hydrogen-bonding interactions between the host (ligand) and the guest (anion). This Minireview targets just one class of anion receptors, namely, amide-based ligands. The structural data for a series of five anion shapes are compiled according to coordination number, and distinct commonalities are observed within a given anion topology. The results also indicate a number of similarities between the coordination of anions and transition-metal ions.     

Facile and Reliable Emission-Based Nanomolar Anion Sensing by Luminescent Iridium Receptors Featuring Chelating Halogen-Bonding Sites

An anion sensor is presented that combines a bidentate hydrogen- (HB) or halogen-bonding (XB) site with a luminescent monocationic Ir fragment for strong binding of common anions (K_a up to 6×10⁴ m ⁻¹) with diagnostic emission changes. A new emission-based protocol for fast and reliable detection was derived on the basis of correction for systematic but unspecific background effects. Such a simple correction routine circumvents the hitherto practical limitations of systematic emission-based analysis of anion binding with validated open-source software (BindFit). The anticipated order of K_a values was obeyed according to size and basicity of the anions (Cl>Br=OAc) as well as the donor atom of the receptor (XB: 6×10⁴ m ⁻¹ > HB: 5×10³ m ⁻¹), and led to submicromolar limits of detection within minutes. The results were further validated by advanced NMR techniques, and corroborated by X-ray crystallographic data and DFT analysis, which reproduced the structural and electronic features in excellent agreement. The results suggest that corrected emission-based sensing may become a complementary, reliable, and fast tool to promote the use of XB in various application fields, due to the simple and fast optical determination at high dilution.     

含CH基的阴离子受体

阴离子在生命、临床及环境科学中有着重要作用,设计用于识别阴离子的人工合成受体是超分子化学的一个研究热点。识别阴离子的方法主要有氢键作用、质子转移机制、路易斯酸碱作用、以及基于不可逆的特定化学反应等。相对于传统的含NH或OH识别位点的阴离子受体,近年来含CH基阴离子受体的研究取得了长足的进展。本文详细评述了含烷基CH,苯基CH,三唑CH,咪唑鎓CH,三唑鎓CH以及CHCN基阴离子受体的设计、结构及其阴离子识别作用的研究进展。     

Differential Recognition of Anions with Selectivity towards F− by a Calix[6]arene–Thiourea Conjugate Investigated by Spectroscopy,Microscopy, and Computational Modeling by DFT

Anion recognition studies were performed with triazole‐appended thiourea conjugates of calix[6]arene (i.e., compound ⁶ L ) by absorption and ¹H NMR spectroscopy by using nineteen different anions. The composition of the species of recognition was derived from ESI mass spectrometry. The absorption spectra of compound ⁶ L showed a new band at λ=455 nm in the presence of F^? due to a charge transfer from the anion to the thiourea moiety and the absorbance increases almost linearly in the concentration range 5 to 200 μm . This is associated with a strong visual color change of the solution. Other anions, such as H₂PO₄^? and HSO₄^?, exhibit a redshift of the λ=345 nm band and the spectral changes are associated with the formation of an isosbestic point at λ=343 nm. ¹H NMR studies further confirm the binding of F^? efficiently to the thiourea group among the halides by shifting the thiourea proton signals downfield followed by their disappearance after the addition of more than one equivalent of F^?. The other anions also showed interactions with compound ⁶ L , however, their binding strength follows the order F^?>CO₃^2?>H₂PO₄^?≈CH₃COO^?>HSO₄^?. The NMR spectral changes clearly revealed the anion‐binding region of the arms in case of all these anions. The anion binding to compound ⁶ L indeed stabilizes a flattened‐cone conformation as deduced based on the calix‐aromatic proton signals and was further confirmed by VT ¹H NMR experiments. The stabilization of the flattened‐cone conformation was further augmented by the interaction of the butyl moiety of the nBu₄N⁺ counterion. The structural features of the anion‐bound species were demonstrated by DFT computations and the resultant structures carried the features that were predicted based on the ¹H NMR spectroscopic measurements. In addition, SEM images showed a marigold flower‐type morphology for compound ⁶ L and this has been transformed into a chain‐like structure of connected spherical particles in the presence of F^?. The anion‐induced microstructural features are reflective of the binding strength, size, and shape of the anions. The binding strengths of the anions by compound ⁶ L were further compared with that of compound ⁴ L , a calix[4]arene analogue of compound ⁶ L , in order to address the role of the number of arms built on the calixarene platform based on absorption spectroscopy, ¹H NMR spectroscopy, and DFT computations and it was found that compound ⁶ L is a better receptor for F^?, which extends its interactions from all the three arms.     

Ditopic Receptors based on Lower‐ and Upper‐Rim Substituted Hexahomotrioxacalix[3]arenes: Cation‐Controlled Hydrogen Bonding of Anion

Heteroditopic hexahomotrioxacalix[3]arene receptors that are capable of binding an anion and a cation simultaneously in a cooperative fashion were synthesized. The structure of one of the triamide derivatives was confirmed by single‐crystal X‐ray diffraction. The binding of alkali metals at the lower rim, and the binding of anions (chloride, bromide) at the upper rim, has been investigated by using ¹H NMR titration experiments. Alkali metal binding at the lower rim controls the calix cavity. Li⁺‐ion binding to the lower rim can improve the binding ability of anions at the upper rim amide moiety by a factor of 15, thus suggesting a strong positive allosteric effect for anion recognition. However, when a Na⁺ cation is bound to the ionophoric site on the lower rim, the calix cavity is changed from a “flattened cone” to a more‐upright form, which is favored for intramolecular hydrogen bonding between the neighboring NH and C?O groups; this change can block the inclusion of anions onto the amide moiety at the upper rim, which strongly suggests a negative allosteric effect of Na⁺‐ion binding, which controls the cooperative recognition system.     

Understanding the Supramolecular Self‐Assembly of the Fullerene Derivative PCBM on Gold Surfaces

We report a DFT study on the self‐assembly of the fullerene derivative PCBM on the Au(111) surface. Recent STM experiments (Angew. Chem. 2007 , 119, 8020–8023^[1]) show a coverage‐dependent transition of the adsorption and self‐assembly of PCBM on this surface. To understand the origin of this observation, we compute the geometries and relative energies of ten PCBM dimers and four tetramers. The calculations show that the self‐assembly of PCBM at high coverage is mainly controlled by hydrogen bonding between the PCBM tails. Due to the large size of the fullerene cage, the hydrogen bonds are formed far away from the surface; hence they are very similar to those found in the gas phase. This picture successfully explains the observed site‐insensitive adsorption at high coverage and the 2D arrangement of PCBM on the surface.     

Building a Better Halide Receptor: Optimum Choice of Spacer,Binding Unit,and Halosubstitution

Quantum calculations are used to measure the binding of halides to a number of bipodal dicationic receptors, constructed as a pair of binding units separated by a spacer group. A number of variations are studied. A H atom on each binding unit (imidazolium or triazolium) is replaced by Br or I. Benzene, thiophene, carbazole, and dimethylnaphthalene are considered as spacer groups. Each receptor is paired with halides F^?, Cl^?, Br^?, and I^?. Substitution with I on the binding unit yields a large enhancement of binding, as much as 13 orders of magnitude; a much smaller increase occurs for substitution with Br. Imidazolium is a more effective binding agent than is triazolium. Benzene and dimethylnaphthalene represent the best spacers, followed by thiophene and carbazole. F^? binds much more strongly than do the other halides, which obey the order Cl^?>Br^?>I^?.     

Anion selectivity in zwitterionic amide-functionalised metal salt extractants

Amide-functionalised salen ligands capable of extracting metal salts have been synthesised and characterised. Single-crystal X-ray structure determinations of complexes of NiSO4, [Ni(L)(SO4)], confirm that the ionophores are in a zwitterionic form with Ni(II) bound in the deprotonated salen moiety and the SO4(2-) ion associated with protonated pendant N'-amidopiperazine groups. Treatment of [Ni(L)(SO4)] with base removes the protons from the pendant amido-amine group resulting in loss of the SO4(2-) ion and formation of metal-only complexes of type [Ni(L-2H)], which have been characterized by single-crystal X-ray diffraction. Three of the ligands with solubilities suitable for solvent extraction studies show loading and stripping pH-profiles that are suitable for the recovery of CuSO4 or CuCl2 from industrial leach solutions. The copper-only complexes, [Cu(L-2H)], are selective for Cl- over SO4(2-) in both solvent extraction and bulk liquid membrane transport experiments and were found to bind Cl- in two steps via the formation of a 1:1:1 [Cu(L-H)Cl] assembly, followed by a 1:1:2 [Cu(L)Cl2] assembly as the pH of the aqueous phase is lowered. The anion transport selectivity was evaluated for a number of other mono-charged anions and interestingly the ligands were found to display a preference for the Br- ion. To probe the influence of the Hofmeister bias on the selectivity of anion complexation, single-phase potentiometric titration experiments were employed to investigate the binding of SO4(2-) and Cl- by one of the copper only complexes, [Cu(L-2H)] in 95 %/5 % MeOH/water. Under these conditions selectivity was reversed (SO4(2-)>Cl-) confirming that the Hofmeister bias, which reflects the relative hydration energies of the anions, dominates the selectivity of anion extraction from aqueous media into CHCl3.     

Anion Receptors Containing ‐NH Binding Sites: Hydrogen‐Bond Formation or Neat Proton Transfer?

When the amide‐containing receptor 1 ⁺ is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH₃COO^?, F^?, H₂PO₄^?), it undergoes deprotonation of the ‐NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl^?, Br^?, NO₃^?), 1 ⁺ establishes true hydrogen‐bond interactions of decreasing intensity. The less acidic receptor 2 ⁺ undergoes neat proton transfer with only the more basic anions CH₃COO^? and F^?, and establishes hydrogen‐bond interactions with H₂PO₄^?. An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and ¹H NMR spectra, is proposed.     

Understanding sterol-membrane interactions part I: Hartree-Fock versus DFT calculations of 13C and 1H NMR isotropic chemical shifts of sterols in solution and analysis of hydrogen-bonding effects

1H and 13C NMR chemical shifts are exquisitely sensitive probes of the local environment of the corresponding nuclei. Ultimately, direct determination of the chemical shifts of sterols in their membrane environment has the potential to reveal their molecular interactions and dynamics, in particular concerning the hydrogen-bonding partners of their OH groups. However, this strategy requires an accurate and efficient means to quantify the influence of the various interactions on chemical shielding. Herein the validity of Hartree-Fock and DFT calculations of the 13C and 1H NMR chemical shifts of cholesterol and ergosterol are compared with one another and with experimental chemical shifts measured in solution at 500 MHz. A computational strategy (definition of basis set, simpler molecular models for the sterols themselves and their molecular complexes) is proposed and compared with experimental data in solution. It is shown in particular that the effects of hydrogen bonding with various functional groups (water as a hydrogen-bond donor and acceptor, acetone) on NMR chemical shifts in CDCl3 solution can be accurately reproduced with this computational approach.     

Spin–Spin Coupling Constants Transmitted through Ir?H⋅⋅⋅H?N Dihydrogen Bonds

Relativity matters: Calculations of NMR shielding tensors and spin–spin coupling constants transmitted through Ir? H???H? N dihydrogen bonds are presented. The picture shows one of the simplified models employed. It is shown that the spin–orbit relativistic effects influence the NMR shielding constants far more than the spin–spin coupling constants.

     

Preorganized Anion Traps for Exploiting Anion–π Interactions: An Experimental and Computational Study

1,3‐Bis(pentafluorophenyl‐imino)isoindoline (A^F) and 3,6‐di‐tert‐butyl‐1,8‐bis(pentafluorophenyl)‐9H‐carbazole (B^F) have been designed as preorganized anion receptors that exploit anion–π interactions, and their ability to bind chloride and bromide in various solvents has been evaluated. Both receptors A^F and B^F are neutral but provide a central NH hydrogen bond that directs the halide anion into a preorganized clamp of the two electron‐deficient appended arenes. Crystal structures of host–guest complexes of A^F with DMSO, Cl^?, or Br^? (A^F:DMSO, A^F:Cl^?, and ${{\rm A}{{{\rm F}\hfill \atop 2\hfill}}}$ :Br^?) reveal that in all cases the guest is located in the cleft between the perfluorinated flaps, but NMR spectroscopy shows a more complex situation in solution because of E,Z/Z,Z isomerism of the host. In the case of the more rigid receptor B^F, Job plots evidence 1:1 complex formation with Cl^? and Br^?, and association constants up to 960 M ^?1 have been determined depending on the solvent. Crystal structures of B^F and B^F:DMSO visualize the distinct preorganization of the host for anion–π interactions. The reference compounds 1,3‐bis(2‐pyrimidylimino)isoindoline (A^N) and 3,6‐di‐tert‐butyl‐1,8‐diphenyl‐9H‐carbazole (B^H), which lack the perfluorinated flaps, do not show any indication of anion binding under the same conditions. A detailed computational analysis of the receptors A^F and B^F and their host–guest complexes with Cl^? or Br^? was carried out to quantify the interactions in play. Local correlation methods were applied, allowing for a decomposition of the ring–anion interactions. The latter were found to contribute significantly to the stabilization of these complexes (about half of the total energy). Compounds A^F and B^F represent rare examples of neutral receptors that are well preorganized for exploiting anion–π interactions, and rare examples of receptors for which the individual contributions to the binding energy have been quantified.     

A computational study of substituent effects on the stability and geometry of carbazole‐pyridine complexes

Hydrogen bonding between carbazole and pyridine is known to quench fluorescence emission of carbazole. Three carbazolopyridinophanes—compounds composed of carbazole and pyridine subunits such that an intramolecular hydrogen bond may exist between them—have been pursued as reversible fluorescent sensors that detect given analytes through fluorescence restoration. However, these sensors exhibit background fluorescence believed to be related to the proportion of non‐hydrogen‐bonded conformers present. In this computational investigation, the potential energy surfaces of various hydrogen‐bonded carbazole:pyridine complexes are investigated using density functional theory with the intent of explaining the observed background fluorescence for the carbazolopyridinophanes. The results indicate carbazolopyridinophane conformers most resembling the geometry of their corresponding free carbazole:pyridine complexes exhibit the least background fluorescence.     

Acid‐Induced Shift of Intramolecular Hydrogen Bonding Responsible for Excited‐State Intramolecular Proton Transfer

The significant progress recently achieved in designing smart acid‐responsive materials based on intramolecular charge transfer inspired us to utilize excited‐state intramolecular proton transfer (ESIPT) for developing a turn‐on acid‐responsive fluorescent system with an exceedingly large Stokes shift. Two ESIPT‐active fluorophores, 2‐(2‐hydroxyphenyl)pyridine (HPP) and 2‐(2‐hydroxyphenyl)benzothiazole (HBT), were fused into a novel dye (HBT‐HPP) fluorescent only in the protonated state. Moreover, we also synthesized three structurally relevant control compounds to compare their steady‐state fluorescence spectra and optimized geometric structures in neutral and acidic media. The results suggest that the fluorescence turn‐on was caused by the acid‐induced shift of the ESIPT‐responsible intramolecular hydrogen bond from the HPP to HBT moiety. This work presents a systematic comparison of the emission efficiencies and basicity of HBT and HPP for the first time, thereby utilizing their differences to construct an acid‐responsive smart organic fluorescent material. As a practical application, red fluorescent letters can be written using the acid as an ink on polymer film.     

Colourimetric Carboxylate Anion Sensors Derived from Viologen‐Based Receptors

A series of tri‐ and tetrapodal viologen‐based anion receptors showing a colourimetric response to carboxylates, such as acetate, have been synthesised. Alteration of the anion binding sites allows for binding site competition within a receptor. This results in a delayed colourimetric response for urea derivatives compared with pyridinium systems because the anions are initially bound to the periphery of the receptor, away from the viologen unit. DFT calculations and experimental measurements allow the colour change to be assigned to an anion–receptor charge‐transfer process, facilitated by the exceptionally low reduction potential of the cationic host compounds. Evidence for electron transfer to give the viologen radical cation is also seen in some cases.     

Rational Design of a Nonbasic Molecular Receptor for Selective NH4+/K+ Complexation in the Gas Phase

A new molecular receptor ( 1 ) for ammonium recognition has been designed and constructed by using only carbon atoms. This molecular receptor can co‐exist in two different isoenergetic conformations but, upon complexation, the conformers are no longer isoenergetic, and a basket‐shaped conformation becomes clearly more stable. The pre‐organised tetrahedral structure of this basket‐shaped molecule favours the complexation of ammonium ions by N? H???π interactions with the four phenyl groups of the host. A similar behaviour is not observed in a similar, but less pre‐organised, reference molecule. ESI‐MS competition experiments show that 1 is able to bind NH₄⁺ over K⁺ selectively. This is the first example of a neutral molecular receptor that shows a remarkable NH₄⁺/K⁺ selectivity. DFT‐calculations provide insight into the nature of host–guest interactions of both 1? NH₄⁺ and 1? K⁺ complexes as well as in the mechanism involved in multiple cation–π interactions and the influence of these interactions on the conformational stability and the selective binding of the host.