Thermodynamic Stability Versus Kinetic Lability of ZnS4 Core

Density Functional Theory and post‐Hartree Fock calculations reveal an unusual energy profile for Zn? S and Zn? N bond dissociation reactions in several [Zn(SR)₄]^2? and [Zn(Im)(SR)₃]^? complexes. The Zn? S bond dissociation in tetrathiolate dianions, which is highly exothermic in the gas phase, proceeds through a late transition state which can be rationalized on the basis of an avoided crossing resulting from Coulomb repulsion between the anionic fragments and ligand‐to‐metal charge‐transfer in the [Zn(SR)₄]^2? complexes. When solvation models for water, DMSO, or acetonitrile are included, some complexes become stable while others are metastable, so this constitutes the first theoretical model which is in full agreement with the experimental data for various [Zn(SR)₄]^2?, [Zn(SR)₃]^?, and [Zn(Im)(SR)₃]^? complexes. The analysis given here indicates that the Zn(Cys)₄ and Zn(His)(Cys)₃ cores of numerous proteins are metastable with respect to Zn? S and Zn? N bond dissociation, respectively. This is consistent with the kinetic lability at the zinc‐centers and illustrates that in nature, thermodynamic stability is imparted upon the zinc cores by the protein environment.     

Targeting a Dark Excited State of HIV‐1 Nucleocapsid by Antiretroviral Thioesters Revealed by NMR Spectroscopy

HIV‐1 nucleocapsid (NCp7) is a two Cys₂HisCys zinc knuckle (N‐Zn and C‐Zn) protein that plays a key role in viral replication. NCp7 conformational dynamics is characterized by NMR relaxation dispersion and chemical exchange saturation transfer measurements. While the N‐Zn knuckle is conformationally stable, the C‐Zn knuckle interconverts on the millisecond timescale between the major state, in which the zinc is coordinated by three cysteines and a histidine, and two folded minor species (with populations around 1 %) in which one of the coordination bonds (Cys413‐Sγ‐Zn or His421‐N?2‐Zn) is hydrolyzed. These findings explain why antiretroviral thioesters specifically disrupt the C‐Zn knuckle by initial acylation of Cys413, and show that transient, sparsely‐populated (“dark”), excited states of proteins can present effective targets for rational drug design.     

One‐Dimensional,Cofacial Porphyrin Polymers Formed by Self‐Assembly of meso‐Tetrakis(ERE Donor) Zinc(II) Porphyrins

A series of meso‐tetrakis‐(ERE donor) zinc(II) porphyrins n Zn (ERE donor=4‐R‐3,5‐bis[(E)‐methyl]phenyl; 1 Zn: E=NMe₂, R=Br; 2 Zn: E=NMe₂, R=H; 3 Zn: E=OMe, R=Br; 4 Zn: E=OMe, R=H) have been synthesized in excellent yields. As a result of the combination of a Lewis acidic site and eight Lewis basic sites within one molecule, monomeric molecules of n Zn self‐assemble to form one‐dimensional porphyrin polymers [ n Zn]_∞ in the solid state, as confirmed for 1 Zn and 3 Zn by X‐ray crystallography. The coordination environment around the zinc(II) ions in these polymers is octahedral. They are ligated by four equatorial nitrogen atoms of the porphyrin and two apical E atoms (E=N, O) provided by the EBrE donor groups of adjacent n Zn molecules. Complexes 2 Zn and 4 Zn did not form single crystals, but solid‐state UV/Vis analysis points to the formation of similar structures. Solution UV/Vis and ¹H NMR spectroscopy indicated that interactions between 1 Zn and 2 Zn monomers in the polymers are stronger than between 3 Zn and 4 Zn monomers. Interestingly, they also revealed that the presence of a neighboring bromine atom in the EBrE donor groups has a considerable influence on the coordination properties of the benzylic N or O atoms. The zinc(II) ions of the porphyrins most likely adopt only hexacoordination in the solid state, owing to the unique predisposition of Lewis acidic and basic sites in the n Zn molecules. Several parameters of the aggregates, for example, the interplanar separation between porphyrins and the zinc–zinc distances, change as a function of the coordinating E groups. The high degree of modularity in their synthesis makes these zinc(II) porphyrins an interesting new entry in noncovalent multiporphyrin assemblies.     

Syntheses and Properties of meso‐Substituted Porphyrin Mesogens with Triazole Linkages and Peripheral Alkyl Chains

Discotic mesogens P/n‐M (n=12, 16, 18, M=2 H, Zn and Cu) bearing a porphyrin core, triazole linkages and peripheral 3,4,5‐trialkoxybenzyl units have been synthesized by a click‐chemistry approach. The thermal behavior, photophysical properties and morphologies of these compounds were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), XRD, UV and PL, SEM and TEM. These compounds can self‐assemble into hexagonal columnar phases in their pure states and form organogels in 1,4‐dioxane with unusually flower‐like sphere morphology. The supramolecular complexes of P/18‐Zn with C₇₀ or 4,7‐di‐4‐pydriyl‐2,1,3‐benzothadiazole can display hexagonal columnar phases too. Additionally, zinc porphyrin compounds P/n‐Zn show binding selectivity to Cu²⁺ among a series of cations in THF/H₂O.     

Mechanistic insight into initiation and chain transfer reaction of CO2/cyclohexene oxide copolymerization catalyzed by zinc?cobalt double metal cyanide complex catalysts

Although zinc? cobalt (III) double metal cyanide complex (Zn? Co (III) DMCC) catalyst is a highly active and selective catalyst for carbon dioxide (CO₂)/cyclohexene oxide (CHO) copolymerization, the structure of the resultant copolymer is poorly understood and the catalytic mechanism is still unclear. Combining the results of kinetic study and electrospray ionization‐mass spectrometry (ESI‐MS) spectra for CO₂/CHO copolymerization catalyzed by Zn? Co (III) DMCC catalyst, we disclosed that (1) the short ether units were mainly generated at the early stage of the copolymerization, and were hence in the “head” of the copolymer and (2) all resultant PCHCs presented two end hydroxyl (? OH) groups. One end ? OH group came from the initiation of zinc? hydroxide (Zn? OH) bond and the other end ? OH group was produced by the chain transfer reaction of propagating chain to H₂O (or free copolymer). Adding t‐BuOH (CHO: t‐BuOH = 2:1, v/v) to the reaction system led to the production of fully alternating PCHCs and new active site of Zn? Ot‐Bu, which was proved by the observation of PCHCs with one end ? Ot‐Bu (and ? OCOOt‐Bu) group from ESI‐MS and ¹³C NMR spectra. Moreover, Zn?OH bond in Zn? Co (III) DMCC catalyst was also characterized by the combined results from FT‐IR, TGA and elemental analysis. This work provided new evidences that CO₂/CHO copolymerization was initiated by metal? OH bond. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Structural basis for bifunctional zinc(II) macrocyclic complex recognition of thymine bulges in DNA

The zinc(II) complex of 1-(4-quinoylyl)methyl-1,4,7,10-tetraazacyclododecane (cy4q) binds selectively to thymine bulges in DNA and to a uracil bulge in RNA. Binding constants are in the low-micromolar range for thymine bulges in the stems of hairpins, for a thymine bulge in a DNA duplex, and for a uracil bulge in an RNA hairpin. Binding studies of Zn(cy4q) to a series of hairpins containing thymine bulges with different flanking bases showed that the complex had a moderate selectivity for thymine bulges with neighboring purines. The dissociation constants of the most strongly bound Zn(cy4q)-DNA thymine bulge adducts were 100-fold tighter than similar sequences with fully complementary stems or than bulges containing cytosine, guanine, or adenine. In order to probe the role of the pendent group, three additional zinc(II) complexes containing 1,4,7,10-tetraazacyclododecane (cyclen) with aromatic pendent groups were studied for binding to DNA including 1-(2-quinolyl)methyl-1,4,7,10-tetraazacyclododecane (cy2q), 1-(4-biphenyl)methyl-1,4,7,10-tetraazacyclododecane (cybp), and 5-(1,4,7,10-tetraazacyclododecan-1-ylsulfonyl)-N,N-dimethylnaphthalen-1-amine (dsc). The Zn(cybp) complex binds with moderate affinity but little selectivity to DNA hairpins with thymine bulges and to DNA lacking bulges. Similarly, Zn(dsc) binds weakly both to thymine bulges and hairpins with fully complementary stems. The zinc(II) complex of cy2q has the 2-quinolyl moiety bound to the Zn(II) center, as shown by (1)H NMR spectroscopy and pH-potentiometric titrations. As a consequence, only weak (500 μM) binding is observed to DNA with no appreciable selectivity. An NMR structure of a thymine-bulge-containing hairpin shows that the thymine is extrahelical but rotated toward the major groove. NMR data for Zn(cy4q) bound to DNA containing a thymine bulge is consistent with binding of the zinc(II) complex to the thymine N3(-) and stacking of the quinoline on top of the thymine. The thymine-bulge bound zinc(II) complex is pointed into the major groove, and there are interactions with the guanine positioned 5' to the thymine bulge.     

A Zeolite‐Like Zinc Phosphonocarboxylate Framework and Its Transformation into Two‐ and Three‐Dimensional Structures

Three zinc phosphonocarboxylates, Zn₂(pbc)₂?Hdma?H₃O?2H₂O ( 1 ), Zn(pbc)?Hdma ( 2 ), and Zn_4.5(pbc)₃(OH)(H₂O)_0.5?Hdma ( 3 ) (H₃pbc=4‐phosphonobenzoic acid, dma=dimethylamine) were synthesized by the mixed solvothermal reaction of Zn(Ac)₂?2H₂O and 4‐phosphonobenzoic acid in N,N‐dimethylformamide (DMF) and water. The zigzag and ladderlike chains completely constructed by triply fused 4‐membered rings (denoted SBU‐1) are linked by the organic moieties to form the 3D zeolite‐like structure 1 and the layered structure 2 , respectively. As for structure 3 , a new second building unit (SBU‐2) formed by the inset of the [Zn₃O₁₂] trimer into the 4‐membered ring as well as SBU‐1 is observed. The connections between the two types of SBUs lead to a “zinc phosphate” layer, which is linked by the organic groups to generate a 3D pillar‐layered structure. Both solution‐mediated and solid‐state transformations of 1 to 2 and 3 were observed. A possible mechanism for the transformation is proposed. Gas sorption studies show that 1 has accessible pores for methanol and water and exhibits size selectivity for alcohols.     

Investigations of the Nature of ZnII−SiII Bonds

A series of zinc(II) silylenes was prepared by using the silylene {PhC(NtBu)₂}(C₅Me₅)Si. Whereas reaction of the silylene with ZnX₂ (X=Cl, I) gave the halide‐bridged dimers [{PhC(NtBu)₂}(C₅Me₅)SiZnX(μ‐X)]₂, with ZnR₂ (R=Ph, Et, C₆F₅) as reagent the monomers [{PhC(NtBu)₂}(C₅Me₅)SiZnR₂] were obtained. The stability of the complexes and the Zn?Si bond lengths clearly depend on the substitution pattern of the zinc atom. Electron‐withdrawing groups stabilize these adducts, whereas electron‐donating groups destabilize them. This could be rationalized by quantum chemical calculations. Two different bonding modes in these molecules were identified, which are responsible for the differences in reactivity: 1) strong polar Zn?Si single bonds with short Zn?Si distances, Zn?Si force constants close to that of a classical single bond, and strong binding energy (ca. 2.39 Å, 1.33 mdyn Å^?1, and 200 kJ mol^?1), which suggest an ion pair consisting of a silyl cation with a Zn?Si single bond; 2) relatively weak donor–acceptor Zn?Si bonds with long Zn?Si distances, low Zn?Si force constants, and weak binding energy (ca. 2.49 Å, 0.89 mdyn Å^?1, and 115 kJ mol^?1), which can be interpreted as a silylene–zinc adduct.     

Efficient Oxidation and Destabilization of Zn(Cys)4 Zinc Fingers by Singlet Oxygen

Singlet oxygen (¹O₂) plays an important role in oxidative stress in all types of organisms, most of them being able to mount a defense against this oxidant. Recently, zinc finger proteins have been proposed to be involved in its cellular detection but the molecular basis of this process still remains unknown. We have studied the reactivity of a Zn(Cys)₄ zinc finger with ¹O₂ by combinations of spectroscopic and analytical techniques, focusing on the products formed and the kinetics of the reaction. We report that the cysteines of this zinc finger are oxidized to sulfinates by ¹O₂. The reaction of the ZnS₄ core with ¹O₂ is very fast and efficient with almost no physical quenching of ¹O₂. A drastic (ca. five orders of magnitude) decrease of the Zn²⁺ binding constant was observed upon oxidation. This suggests that the Zn(Cys)₄ zinc finger proteins would release their Zn²⁺ ion and unfold upon reaction with ¹O₂ under cellular conditions and that zinc finger sites are likely targets for ¹O₂.     

A Functionalized Spiro[chlorin‐porphyrin]‐Type ‘Dimer’ Dizinc Complex from Rapid [4+2] Self‐cycloaddition of a Conjugated [β,β′‐Bis(methylene)porphyrinato]zinc

Thermal extrusion of SO₂ from β,β′‐sulfolenoporphyrins is an effective method for in situ generation of β,β′‐bis(methylene)porphyrin which remained unobserved in typical synthetic applications but underwent quickly efficient [4+2]‐cycloaddition reactions with dienophiles.We now report the thermal extrusion of SO₂ from the symmetrical (tetra‐β,β′‐sulfolenoporphyrinato)zinc 1?Zn in the absence of a dienophile (Scheme). In the event, the thermally in situ generated conjugated diene underwent a [4+2] self‐cycloaddition, to give the {spirobi[tri‐β,β′‐sulfolenoporphyrinato]}dizinc 4?2Zn . This chiral (racemic) spirobiporphyrinoid dizinc complex represents the combination of the closely positioned and interacting chromophores of a (porphyrinato)zinc and of a (β‐methylene‐β,β′‐dihydroporphyrinato)zinc. It carries six sulfoleno moieties that are still available for further SO₂ extrusion and cycloaddition reactions.     

Synthesis and Self‐Organization of Zinc β‐(Dialkoxyphosphoryl)porphyrins in the Solid State and in Solution

The first synthesis and self‐organization of zinc β‐phosphorylporphyrins in the solid state and in solution are reported. β‐Dialkoxyphosphoryl‐5,10,15,20‐tetraphenylporphyrins and their Zn^II complexes have been synthesized in good yields by using Pd‐ and Cu‐mediated carbon–phosphorous bond‐forming reactions. The Cu‐mediated reaction allowed to prepare the mono‐β‐(dialkoxyphosphoryl)porphyrins 1 Zn – 3 Zn starting from the β‐bromo‐substituted zinc porphyrinate ZnTPPBr (TPP=tetraphenylporphyrin) and dialkyl phosphites HP(O)(OR)₂ (R=Et, iPr, nBu). The derivatives 1 Zn – 3 Zn were obtained in good yields by using one to three equivalents of CuI. When the reaction was carried out in the presence of catalytic amounts of palladium complexes in toluene, the desired zinc derivative 1 Zn was obtained in up to 72 % yield. The use of a Pd‐catalyzed C? P bond‐forming reaction was further extended to the synthesis of β‐poly(dialkoxyphosphoryl)porphyrins. An unprecedented one‐pot sequence involving consecutive reduction and phosphorylation of H₂TPPBr₄ led to the formation of a mixture of the 2,12‐ and 2,13‐bis(dialkoxy)phosphorylporphyrins 5 H₂ and 6 H₂ in 81 % total yield. According to the X‐ray diffraction studies, 1 Zn and 3 Zn are partially overlapped cofacial dimers formed through the coordination of two Zn centers by two phosphoryl groups belonging to the adjacent molecules. The equilibrium between the monomeric and the dimeric species exists in solutions of 1 Zn and 3 Zn in weakly polar solvents according to spectroscopic data (UV/Vis absorption and NMR spectroscopy). The ratio of each form is dependent on the concentration, temperature, and traces of water or methanol. These features demonstrated that zinc β‐phosphorylporphyrins can be regarded as new model compounds for the weakly coupled chlorophyll pair in the photosynthesis process.     

New anticancer zinc(II) complexes comprising thiosemicarbazones of saturated ring: structure,DNA/protein binding,DNA cleavage,topoisomerase‐1 inhibition and anti‐proliferation studies

The reaction of the thiosemicarbazones (CH₂)₄C?NN(H)C(?S)NHR (R = H, Me) with zinc(II) acetate in methanolic solution proceeds readily under mild conditions to form stable mononuclear complexes Zn[(CH₂)₄C?NN?C(S)NHR]₂. DNA interaction studies show that the zinc(II) complexes bind to DNA via groove mode and exhibit efficient DNA cleavage activity in the presence of hydrogen peroxide. Also, the complexes display a binding affinity to bovine serum albumin protein with K_BSA values of ca 10⁵ M^?1. Topoisomerase catalytic inhibition studies suggest that both complexes are efficient topoisomerase‐I impeders. Furthermore, the anti‐proliferative effects of the two complexes on five human tumor cell lines (Caki‐2, MCF‐7, CaSki, NCI‐H322M and Co‐115) indicate that both complexes have the potential to act as effective anticancer drugs. Copyright © 2016 John Wiley & Sons, Ltd.     

Luminescence Enhancement based Sensing of L‐Cysteine by Doped Quantum Dots

The interaction of a presynthesized orange emitting Mn²⁺‐doped ZnS quantum dots (QDs) with L‐Cysteine (L?Cys) led to enhance emission intensity (at 596 nm) and quantum yield (QY). Importantly, the Mn²⁺‐doped ZnS QDs exhibited high sensitivity towards L?Cys, with a limit of detection of 0.4±0.02 μM (in the linear range of 3.3–13.3 μM) and high selectivity in presence of interfering amino acids and metal ions. The association constant of L?Cys was determined to be 0.36×10⁵ M^?1. The amplified passivation of the surface of Mn²⁺‐doped ZnS QDs following the incorporation and binding of L?Cys is accounted for the enhancement in their luminescence features. Moreover, the luminescence enhancement‐based detection will bring newer dimension towards sensing application.     

Peptide‐Based Carbohydrate Receptors

A broad spectrum of physiological processes is mediated by highly specific noncovalent interactions of carbohydrates and proteins. In a recent communication we identified several cyclic hexapeptides in a dynamic combinatorial library that interact selectively with carbohydrates with high binding constants in water. Herein, we report a detailed investigation of the noncovalent interaction of two cyclic hexapeptides (Cys‐His‐Cys (which we call HisHis) and Cys‐Tyr‐Cys (which we call TyrTyr)) with a selection of monosaccharides and disaccharides in aqueous solution. The parallel and antiparallel isomers of HisHis or TyrTyr were synthesized separately, and their interaction with monosaccharides and disaccharides in aqueous solution was studied by isothermal titration calorimetry, NMR spectroscopic titrations, and circular dichroism spectroscopy. From these measurements, we identified particularly stable complexes (K_a>1000 M ^?1) of the parallel isomer of HisHis with N‐acetylneuraminic acid and with methyl‐α‐D ‐galactopyranoside as well as of both isomers of TyrTyr with trehalose. To gain further insight into the structure of the peptide–carbohydrate complexes, structure prediction was performed using quantum chemical methods. The calculations confirm the selectivity observed in the experiments and indicate the formation of multiple intermolecular hydrogen bonds in the most stable complexes.     

Protein‐Based Capacitive Biosensors: a New Tool for Structure‐Activity Relationship Studies

The present work reports a new application of a protein‐based capacitive biosensor as an in vitro assay for the selectivity study of the bacterial periplasmic protein MerP and four MerP variants. The modified MerP proteins were produced by site‐directed mutagenesis of the heavy metal associated motif (HMA). The MerP and modified MerPs selectivity for copper, zinc, cadmium and mercury bivalent ions were investigated and compared. The variations in the proteins affinity were related to the primary structure of the HMA motifs. Key amino acids for copper coordination of metalloproteins that contain the metal binding sequence Gly‐Met‐Thr‐Cys‐xxx‐xxx‐Cys were identified. The results brought insights valid for Menkes and Wilson ATPases. The protein‐based capacitive biosensors were a simple and useful tool for studying structure‐activity relationships of proteins.     

Density functional computations of alkynylation of ethanimine catalyzed by chiral zinc(II)‐complexes

The alkynylation of ethanimine catalyzed by chiral zinc(II)‐complexes was studied by means of the density functional theory (DFT). All the intermediates and transition states were optimized completely at the B3LYP/6‐31G(d,p) level. Calculation results confirm that the alkynylation of ethanimine is exothermic and the total released energy is about ?13 kJ/mol. The formation of the catalyst–alkynyl complexes M4 is the rate‐determining step for this alkynylation, and the formation of the catalyst–amine complexes M5 is the chirality‐limiting step for this alkynylation. The transition states for the chirality‐limiting step have a H? O? Zn? C? C? N six‐membered ring. The dominant products predicted theoretically for this alkynylation are, respectively, S‐amine for ethanimine anti and R‐amine for ethanimine syn . © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Nitrogen‐Doped Carbon Nanotube Arrays for High‐Efficiency Electrochemical Reduction of CO2: On the Understanding of Defects,Defect Density,and Selectivity

Nitrogen‐doped carbon nanotubes (NCNTs) have been considered as a promising electrocatalyst for carbon‐dioxide‐reduction reactions, but two fundamental chemistry questions remain obscure: 1) What are the active centers with respect to various defect species and 2) what is the role of defect density on the selectivity of NCNTs? The aim of this work is to address these questions. The catalytic activity of NCNTs depends on the structural nature of nitrogen in CNTs and defect density. Comparing with pristine CNTs, the presence of graphitic and pyridinic nitrogen significantly decreases the overpotential (ca. ?0.18 V) and increases the selectivity (ca. 80 %) towards the formation of CO. The experimental results are in congruent with DFT calculations, which show that pyridinic defects retain a lone pair of electrons that are capable of binding CO₂. However, for graphitic‐like nitrogen, electrons are located in the π* antibonding orbital, making them less accessible for CO₂ binding.     

A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions

Reaction of Zn(OAc)₂ ? 2 H₂O with 2,6‐diisopropylphenyl phosphate (dippH₂) in the presence of pyridine‐4‐carboxaldehyde (Py‐4‐CHO) in methanol resulted in the isolation of a tetrameric zinc phosphate cluster [Zn(dipp)(Py‐4‐CH(OH)(OMe))]₄ ? 4 MeOH ( 1 ) with four hemiacetal moieties stabilized on the double‐4‐ring inorganic cubane cluster. The change of solvent from methanol to acetonitrile leads to the formation of [Zn(dipp)(Py‐4‐CHO)]₄ ( 2 ), in which the coordinated Py‐4‐CHO retains its aldehydic form. Dissolution of 1 in CD₃CN readily converts it to the aldehydic form and yields 2 . Similarly 2 , which exists in the aldehyde form in CD₃CN, readily converts to the hemiacetal form in CD₃OD/CH₃OH. Compound 1 is an unprecedented example in which four hemiacetals have been stabilized on a single molecule in the solid state retaining its stability in solution as revealed by its ¹H NMR spectrum in CD₃OD. The solution stability of 1 and 2 has further been confirmed by ESI‐MS studies. To generalize the stabilization of multiple hemiacetals on a single double‐four‐ring platform, pyridine‐2‐carboxaldehyde (Py‐2‐CHO) was used as the auxiliary ligand in the reaction between zinc acetate and dippH₂, leading to isolation of [Zn(dipp)(Py‐2‐CH(OH)(OMe))]₄ ( 3 ). Understandably, recrystallization of 3 from acetonitrile yields the parent aldehydic form, [Zn(dipp)(Py‐2‐CHO)]₄ ( 4 ). Single‐crystal X‐ray diffraction studies reveal that supramolecular bonding, aided by hydrogen‐bonding interactions involving the hemiacetal functionalities (C?OH, C?OMe, and C?H), are responsible for the observed stabilization. The hemiacetal/aldehyde groups in 1 and 2 readily react with p‐toluidine, 2,6‐dimethylaniline, and 4‐bromoaniline to yield the corresponding tetra‐Schiff base ligands, [Zn(dipp)(L)]₄ (L=4‐methyl‐N‐(pyridin‐4‐ylmethylidene)aniline ( 5 ), 2,6‐dimethyl‐N‐(pyridin‐4‐ylmethylene)‐aniline ( 6 ), and 4‐bromo‐N‐(pyridin‐4‐ylmethylene)aniline ( 7 )). Isolation of 5 – 7 opens up further possibilities of using 1 and 2 as new supramolecular synthons and ligands.