Three-center Hydrogen Bonds Research Articles

Synthesis and Verification of Fluorescent pH Probes Based on 2-Quinolone Platform

Intracellular pH plays an important role in biological events, including cell metabolism, signal transduction, cell growth, apoptosis and endocytosis. The development of simple and robust proton-recognizing fluorescent probes has been a research field of high interest. In this work, we describe the design and synthesis of a 2-quinolone Schiff base as a novel acidic fluorescent probe. The design strategy of the probe is based on non-classical hydrogen bonding in the form of an intramolecular three-centered hydrogen bond (THB). pH titrations at pHs 2 and 6 indicate that there is enhancement of fluorescence with increasing proton concentration. Additionally, no interference of metal ions such as Cu2+, Fe3+, Hg2+, Zn2+, Co2+, Cd2+ and Pb2+ ions were detected during the course of these titrations.

Controlling Helix Sense at N- and C-Termini in Quinoline Oligoamide Foldamers by β-Pinene-Derived Pyridyl Moieties.

A series of quinoline oligoamide foldamers bearing a β-pinene-derived pyridyl group at the N-terminus or the C-terminus were synthesized, and the efficiencies of chiral inductions have been evaluated by 1H NMR and CD spectra. The chiral inductions were quantitative when chiral pyridyl acid was appended at the N-terminus, but were inferior when chiral pyridyl amine was appended at the C-terminus. Unexpectedly, N-oxidation on the pyridine ring at the C-terminus does not notably enhance the chiral induction efficiency in spite of the presence of three-center hydrogen bonds.

Unveiling the three-center hydrogen bond dynamic behavior in ground and excited states

The behavior of three-centered hydrogen bond (THB) in excited state is unveiled firstly. The notion proposed in experiment that the THB configuration can be stable in solution has been verified. And one more stable configuration has been found which has two intramolecular hydrogen bonds compared with THB configuration. The analysis of optimization geometrical configurations shows that the THB weakens upon photoexcitation and is apparent weaker than the two-centered hydrogen bond. The reduced electron density gradient (RDG) maps indicate that the weakening of THB compared with two-centered hydrogen bond is due to the steric repulsion of hydrogen bond donors and acceptors. In addition, fluoresce spectra indicate that the 3-position substituent of coumarin has a large blue-shift on maximum fluorescence wavelength compared with the 7-aminocoumarin C500.

Two- and Three-Centered Hydrogen Bonds Involving Organic Fluorine Stabilize Conformations of Hydrazide Halo Derivatives: NMR, IR, QTAIM, NCI, and Theoretical Evidence

The presence of two- and three-centered hydrogen bonds (HB) of the type H(N)···X-C and C═O···H(N)···X-C, respectively, involving organic fluorine in the synthesized hydrazide halo derivatives have been convincingly established by extensive multidimensional NMR studies. The stabilized conformation of the molecules involving two- and three-centered HBs derived by NMR studies have been further confirmed by density functional theory (DFT)-based calculations, such as quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI), and relaxed potential energy scan.

Hydrogen bonding and structural complexity in the Cu5(PO4)2(OH)4 polymorphs (pseudomalachite, ludjibaite, reichenbachite): combined experimental and theoretical study

Hydrogen bonding in the Cu5(PO4)2(OH)4 polymorphs pseudomalachite, ludjibaite and reichenbachite has been studied by low-temperature single-crystal X-ray diffraction (XRD; pseudomalachite) and solid-state density functional theory (DFT; pseudomalachite, ludjibaite, reichenbachite) calculations. Pseudomalachite at 100 K is monoclinic, P21/c, a = 4.4436(4), b = 5.7320(5), c = 16.9300(15) A, β = 91.008(8)°, V = 431.15(7) A3 and Z = 2. The structure has been refined to R 1 = 0.025 for 1383 unique observed reflections with |F o| ≥ 4σF. DFT calculations were done with the CRYSTAL14 software package. For pseudomalachite, the difference between the calculated and experimental H sites does not exceed 0.152 A. Structural configurations around hydroxyl groups in all three polymorphs show many similarities. Each OH5 group is involved in a three-center (bifurcated) hydrogen bond with the H···A distances in the range of 2.141–2.460 A and the D–H···A angles in the range of 122.41°–139.30°, whereas each OH6 group forms a four-center (trifurcated) bond (H···A = 2.093–2.593 A; D–H···A = 122.79°–137.71°). The crystal structures of the Cu5(PO4)2(OH)4 polymorphs are based on three-dimensional frameworks of Cu and P polyhedra. The copper-centered octahedra share edges to form two-dimensional layers parallel to (100) in all three structures. The layers have square voids above and beneath PO4 tetrahedra that link adjacent layers by sharing O atoms with two CuO6 octahedra each. From the topological point of view, none of the polymorphs can be obtained from another by a displacive transformation, and therefore pseudomalachite, ludjibaite and reichenbachite can be viewed as combinatorial polymorphs. According to information-based structural complexity considerations, the three phases are very similar in their configurational entropies and preferential crystallization of one phase over another cannot be entropy driven and is probably governed by other mechanisms that may involve such factors as structures of prenucleation clusters, chemical admixtures, etc.

Hydrogen bonding system in euchroite, Cu2(AsO4)(OH)(H2O)3: low-temperature crystal-structure refinement and solid-state density functional theory modeling

Hydrogen bonding in euchroite has been studied by means of low-temperature single-crystal X-ray diffraction (XRD) and solid-state density functional theory (DFT) calculations. The mineral is orthorhombic, P212121, a = 10.0350(8), b = 10.4794(8), c = 6.1075(5) A, V = 642.27(9) A3, and Z = 4. The structure has been refined to R 1 = 0.036 for 2436 unique observed reflections with |F o| ≥ 4σ F . DFT calculations were performed with the CRYSTAL14 software package. The basic features of the crystal structure of euchroite are the same as described by previous authors. There are two symmetrically-independent Cu sites octahedrally coordinated by O atoms. The CuO6 octahedra are strongly distorted containing four short (1.927–2.012 A) and two long (2.360–2.797 A) bonds each, in agreement with the expected Jahn-Teller distortion of an octahedrally-coordinated Cu2+ cation. There is one symmetrically-independent As site that is tetrahedrally coordinated by four O atoms to form an arsenate oxyanion, AsO4 3−. The structure is based upon chains of edge-sharing CuO6 octahedra running parallel to [001]. The chains are linked by AsO4 tetrahedra into a three-dimensional framework, which is stabilized by hydrogen bonds formed from OH and H2O groups. The coordinates of H atoms determined by single-crystal X-ray diffraction and those calculated using DFT are very similar. The distance Δ between experimental and theoretical H positions does not exceed 0.250 A, except for the H72 site, for which Δ = 0.609 A. The hydrogen bonding scheme in euchroite is rather complex and involves a combination of relatively strong two-center hydrogen bonds as well as few three-center (bifurcated) hydrogen bonds. The largest difference between the XRD and DFT results involves the H72 atom of the H2O7 molecule and can be assigned to the effect of temperature, which favors a strong linear hydrogen bond at 0 K (calculated) and a bifurcated three-center bond at 100 K (measured). The Cu-H2O configurations are bent for the H2O7 and H2O6 groups and are almost planar for the H2O8 groups.

Unveiling the peculiar hydrogen bonding behavior of solvated N-heterocyclic carbenes.

To investigate the hydrogen bonding and its dynamics of N-heterocyclic carbenes (NHCs) in solution, a molecular mechanical force field was fitted for the homologous series of 1,3-dialkylimidazol-2-ylidenes. During the exploration of the potential energy surface of the water/1,3-dimethylimidazol-2-ylidene system, it was observed for the first time that the carbene is prone to interaction with hydrogen bond donor molecules also from the rather unusual "on top" orientation, where the direction of the interplay is perpendicular to the plane of the NHC's ring. The fitting of the force field parameters for imidazol-2-ylidenes was found to be the best in the case of a two-site model, which reproduces not only the strength, but also the direction dependency of hydrogen bonding. With the aid of this tool, curious, hitherto unknown types of hydrogen bonding could be unveiled for NHCs. In the case of non-hydrogen bonding solvents, carbenes tend to form short lived, but structurally influental hydrogen bonds between each other via ring hydrogen atoms and the divalent carbon atoms. The chemically highly important hydrogen bond dynamics of NHCs was found to be facilitated by three center hydrogen bonding, where two alcohol molecules bind to a carbene, which is allowed only by the aforementioned relatively strong interaction between the NHC and the hydrogen bond donor in the "on top" orientation. The latter finding has significant effects on processes that involve this kind of replacement, such as the selective transesterification reactions, and the mechanism of proton exchange on azolium rings.

Structural Basis for the Structure–Activity Behaviour of Oxaliplatin and its Enantiomeric Analogues: A Molecular Dynamics Study of Platinum-DNA Intrastrand Crosslink Adducts

The discrimination of Pt-GG adducts by mismatch repair proteins, DNA damage-recognition proteins, and translation DNA polymerases was thought to be vital in determining the toxicity, efficacy, and mutagenicity of platinum anti-tumour drugs. Studies on cis-diammine-Pt-GG (from cisplatin and carboplatin) and trans-R,R-diaminocyclohexane (DACH)-Pt-GG indicated that these proteins recognized the differences in conformation and conformational dynamics of Pt-DNA complexes. However, the structural basis of enantiomeric DACH-Pt-GG forms is unclear. Molecular dynamics simulations results presented here reveal that the conformational dynamics between trans-R,R-DACH-Pt-GG, trans-S,S-DACH-Pt-GG, cis-DACH-Pt-GG and undamaged DNA are distinct and depend on the chirality of DACH though their major conformations are similar. Trans-DACH-Pt was found to be energetically favoured over cis-DACH-Pt to form DNA adducts. Moreover, oxaliplatin and its cis-DACH analogues were found to preferentially form hydrogen bonds on the 3′ side of the Pt-GG adduct, whereas the S,S-DACH-Pt preferred the 5′ side. A three-centre hydrogen bond formed between cis1-DACH-Pt and DNA was observed, and the differences in hydrogen bond formation are highly correlated with differences in DNA conformational dynamics. Based on these results, it is suggested that the different bioactivities of oxaliplatin and its enantiomeric analogues were controlled by the difference in hydrogen bonds formation dynamics between DNA and the Pt moiety. Our molecular dynamics approach was demonstrated to be applicable to the study of stereoisomer conformations of platinum-DNA model, thereby suggesting its potential application as a tool for the study and design of new effective platinum-based drugs.

High-Resolution Crystal Structures of Protein Helices Reconciled with Three-Centered Hydrogen Bonds and Multipole Electrostatics

Theoretical and experimental evidence for non-linear hydrogen bonds in protein helices is ubiquitous. In particular, amide three-centered hydrogen bonds are common features of helices in high-resolution crystal structures of proteins. These high-resolution structures (1.0 to 1.5 Å nominal crystallographic resolution) position backbone atoms without significant bias from modeling constraints and identify Φ = -62°, ψ = -43 as the consensus backbone torsional angles of protein helices. These torsional angles preserve the atomic positions of α-β carbons of the classic Pauling α-helix while allowing the amide carbonyls to form bifurcated hydrogen bonds as first suggested by Némethy et al. in 1967. Molecular dynamics simulations of a capped 12-residue oligoalanine in water with AMOEBA (Atomic Multipole Optimized Energetics for Biomolecular Applications), a second-generation force field that includes multipole electrostatics and polarizability, reproduces the experimentally observed high-resolution helical conformation and correctly reorients the amide-bond carbonyls into bifurcated hydrogen bonds. This simple modification of backbone torsional angles reconciles experimental and theoretical views to provide a unified view of amide three-centered hydrogen bonds as crucial components of protein helices. The reason why they have been overlooked by structural biologists depends on the small crankshaft-like changes in orientation of the amide bond that allows maintenance of the overall helical parameters (helix pitch (p) and residues per turn (n)). The Pauling 3.613 α-helix fits the high-resolution experimental data with the minor exception of the amide-carbonyl electron density, but the previously associated backbone torsional angles (Φ, Ψ) needed slight modification to be reconciled with three-atom centered H-bonds and multipole electrostatics. Thus, a new standard helix, the 3.613/10-, Némethy- or N-helix, is proposed. Due to the use of constraints from monopole force fields and assumed secondary structures used in low-resolution refinement of electron density of proteins, such structures in the PDB often show linear hydrogen bonding.

High- and low-temperature phases in isostructural 4-chloro-3-nitroaniline and 4-iodo-3-nitroaniline.

The structures of 4-chloro-3-nitroaniline, C6H5ClN2O2, (I), and 4-iodo-3-nitroaniline, C6H5IN2O2, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single-crystal X-ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high-temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene-ring planes at two different orientations. In the low-temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room-temperature cell. Each of the low-temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low-temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low-temperature phases. It seems that competition between the primary amine-nitro N-H···O hydrogen bonds which form three-centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N-H···N hydrogen bonding of moderate strength which results in the graph-set motif C(3). This graph-set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high- and the low-temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high-temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C60, o328-o330].

Solid State Structure and Solution Thermodynamics of Three-Centered Hydrogen Bonds (O∙∙∙H∙∙∙O) Using N-(2-Benzoyl-phenyl) Oxalyl Derivatives as Model Compounds

Intramolecular hydrogen bond (HB) formation was analyzed in the model compounds N-(2-benzoylphenyl)acetamide, N-(2-benzoylphenyl)oxalamate and N1,N2-bis(2-benzoylphenyl)oxalamide. The formation of three-center hydrogen bonds in oxalyl derivatives was demonstrated in the solid state by the X-ray diffraction analysis of the geometric parameters associated with the molecular structures. The solvent effect on the chemical shift of H6 [δH6(DMSO-d6)–δH6(CDCl3)] and Δδ(ΝΗ)/ΔT measurements, in DMSO-d6 as solvent, have been used to establish the energetics associated with intramolecular hydrogen bonding. Two center intramolecular HB is not allowed in N-(2-benzoylphenyl)acetamide either in the solid state or in DMSO-d6 solution because of the unfavorable steric effects of the o-benzoyl group. The estimated ΔHº and ΔSº values for the hydrogen bonding disruption by DMSO-d6 of 28.3(0.1) kJ·mol−1 and 69.1(0.4) J·mol−1·K−1 for oxalamide, are in agreement with intramolecular three-center hydrogen bonding in solution. In the solid, the benzoyl group contributes to develop 1-D and 2-D crystal networks, through C–H∙∙∙A (A = O, π) and dipolar C=O∙∙∙A (A = CO, π) interactions, in oxalyl derivatives. To the best of our knowledge, this is the first example where three-center hydrogen bond is claimed to overcome steric constraints.

Crescent aromatic oligothioamides as highly selective receptors for copper(II) ion

A series of crescent aromatic oligothioamides (4, 6, 8, 15, and 18) bearing different number of sulfur atoms were designed and synthesized via thionation of their corresponding aromatic oligoamides (3, 5, and 7) using Lawesson’s reagent. The X-ray structure of a trimeric analogue (13) revealed the presence of intramolecular three-center hydrogen bonds that are responsible for the rigidification of the molecular backbone. The extraction by these novel receptors toward some representative heavy metal cations (Zn2+, Cd2+, Co2+, Ni2+, Pb2+, and Cu2+) and alkali and alkaline earth metal cations (Li+, Na+, K+, Rb+, Cs+, Ca2+, and Sr2+) demonstrated high efficiency (83.5%–96.4%) and superior selectivity for Cu2+ over other selected metal cations. Particularly, the extractability was correlated to both the number of sulfur atoms and orientation of thiocarbonyl groups as revealed in the order: 6 > 4 > 18 > 15. This is in stark contrast to the oligoamides that only gave much lower extractability (5.9%–16.4%), suggestive of the importance of replacement of carbonyl oxygen atoms with sulfur atoms in the extraction of Cu2+. The complexation behavior of 4, 6, and 8 with Cu2+ was also examined by UV-Vis and NMR techniques.

Molecular Complexes of Diethyl N,N′-1,3-Phenyldioxalamate and Resorcinols: Conformational Switching through Intramolecular Three-Centered Hydrogen-Bonding

The mechanochemically induced complexation between diethyl N,N′-1,3-phenyldioxalamate tweezers and resorcinol, orcinol, 4,6-di-tert-butyl-1,3-benzenediol, and 4-hexyl-1,3-benzenediol is described. IR-spectroscopy, X-ray powder diffraction, 13C CPMAS, and single X-ray diffraction allowed establishing the structures of the complexes as hydrogen-bonded heterodimers and heterotetramers. Complexation occurs through O–H···O═C hydrogen-bonding interactions with the participation of phenolic OH and amide carbonyl groups. The initial conformation and steric factors coming from the 1,3-benzenediols exert a strong influence on the final structure of the complex formed. Complexation twists both oxalyl arms by 180°, strengthens the intramolecular (amide)CO···H(Csp2)···OC(amide) three-centered hydrogen bond, and moves apart the oxalyl arms to allow the accommodation of the 1,3-benzenediol inside the cavity. The supramolecular architectures of the complexes in 1-D are directed by R12(6), R22(10), and R12(6) adjacent hyd...

Three-dimensional hydrogen-bonded assembly in 2,2′-disulfanylidene-5,5′-biimidazolidinylidene-4,4′-dione–dimethylformamide–water (3/2/4)

The title compound, 3C6H4N4O2S2·2C3H7NO·4H2O, comprises three 2,2'-disulfanylidene-5,5'-biimidazolidinylidene-4,4'-dione molecules, two dimethylformamide molecules and four water molecules arranged around a crystallographic inversion centre. The non-H atoms within the 5,5'-biimidazolidinylidene molecule are coplanar and these molecules aggregate through N-H···S hydrogen-bonding interactions with cyclic motifs [graph set R2(2)(8)], giving two-dimensional ribbon structures which are close to being parallel. The two independent water molecules associate to form centrosymmetric cyclic hydrogen-bonded (H2O)4 tetrameric units [graph set R4(4)(8)]. The ribbon structures extend along the a axis and are linked through the water tetramers and the dimethylformamide molecules by a combination of two- and three-centre hydrogen bonds, giving an overall three-dimensional structure.

Features of three-centered hydrogen bonding in di(vinylpyrrolyl)pyridine and di(vinylpyridyl)-pyrrole from the ab initio calculation data and QTAIM analysis

Within the calculation at the MP2/6-311++G(d,p) level and QTAIM analysis the structural investigation of 2,6-bis-[2-(1H-pyrrole-2-yl)-vinyl]-pyridine and 2,5-bis-[2-(pyridine-2-yl)-vinyl]-1H-pyrrole is performed. It is found that a symmetric three-centered hydrogen bond forms in the first molecule. The components of this bond are substantially weakened in comparison with the two-centered hydrogen bond in the Z-isomer of 2-[2-(1H-pyrrole-2-yl)-vinyl]-pyridine. This weakening is due to the steric interaction of two hydrogen bond donors. At the same time, more rigid steric conditions for the formation of a three-centered interaction in 2,5-bis-[2-(pyridine-2-yl)-vinyl]-1H-pyrrole results in actual dissociation of one of the components of the three-center hydrogen bond accompanied by the strengthening of the retained component. This provides the possibility of rocking vibrations of the hydrogen bond in the latter case due to the alternate entrance of hydrogen bond acceptors into the coordination sphere of the hydrogen atom.

3-Acetyl-1-phenylthiourea

In the crystal structure of title compound, C9H10N2OS, there are two symmetry-independent mol­ecules, each having an intra­molecular N—H⋯O hydrogen bond generating an S(6) ring motif. The benzene rings and the virtually planar acetyl­thoiurea fragments [r.m.s. deviations = 0.0045 and 0.0341 Å] are oriented at dihedral angles of 50.71 (6) and 62.79 (6)° in the two mol­ecules. In the crystal, N—H⋯S and N—H⋯O hydrogen bonds link mol­ecules via cyclic R 2 2(8) and R 2 2(12) motifs into a one-dimensional polymeric network extending along [101]. The intra- and inter­molecular N—H⋯O inter­actions are part of a three-center hydrogen bond. A C—H⋯S inter­action also occurs.

The double H-atom acceptability of the P=O group in newXP(O)(NHCH2C6H4-2-Cl)2phosphoramidates [X= C6H5O– and CF3C(O)NH–]: a database analysis of compounds having a P(O)(NHR) group

In the hydrogen-bond patterns of phenyl bis(2-chlorobenzylamido)phosphinate, C(20)H(19)Cl(2)N(2)O(2)P, (I), and N,N'-bis(2-chlorobenzyl)-N''-(2,2,2-trifluoroacetyl)phosphoric triamide, C(16)H(15)Cl(2)F(3)N(3)O(2)P, (II), the O atoms of the related phosphoryl groups act as double H-atom acceptors, so that the P=O···(H-N)(2) hydrogen bond in (I) and the P=O···(H-N(amide))(2) and C=O···H-N(C(O)NHP(O)) hydrogen bonds in (II) are responsible for the aggregation of the molecules in the crystal packing. The presence of a double H-atom acceptor centre is a result of the involvement of a greater number of H-atom donor sites with a smaller number of H-atom acceptor sites in the hydrogen-bonding interactions. This article also reviews structures having a P(O)NH group, with the aim of finding similar three-centre hydrogen bonds in the packing of phosphoramidate compounds. This analysis shows that the factors affecting the preference of the above-mentioned O atom to act as a double H-atom acceptor are: (i) a higher number of H-atom donor sites relative to H-atom acceptor centres in molecules with P(=O)(NH)(3), (N)P(=O)(NH)(2), C(=O)NHP(=O)(NH)(2) and (NH)(2)P(=O)OP(=O)(NH)(2) groups, and (ii) the remarkable H-atom acceptability of this atom relative to the other acceptor centre(s) in molecules containing an OP(=O)(NH)(2) group, with the explanation that the N atom bound to the P atom in almost all of the structures found does not take part in hydrogen bonding as an acceptor. Moreover, the differences in the H-atom acceptability of the phosphoryl O atom relative to the O atom of the alkoxy or phenoxy groups in amidophosphoric acid esters may be illustrated by considering the molecular packing of compounds having (O)(2)P(=O)(NH) and (O)P(=O)(NH)(N)groups, in which the unique N-H unit in the above-mentioned molecules almost always selects the phosphoryl O atom as a partner in forming hydrogen-bond interactions. The P atoms in (I) and (II) are in tetrahedral coordination environments, and the phosphoryl and carbonyl groups in (II) are anti with respect to each other (the P and C groups are separated by one N atom). In the crystal structures of (I) and (II), adjacent molecules are linked via the above-mentioned hydrogen bonds into a linear arrangement parallel to [100] in both cases, in (I) by forming R(2)(2)(8) rings and in (II) through a combination of R(2)(2)(10) and R(2)(1)(6) rings.

Polysulfonylamine, CXCII [1]. Polynäre Verbindungen aus Di(4-fluorbenzolsulfonyl) amin (FAH), 1,1,3-Trimethylharnstoff (TrMU) und Dimethylamin (Me2NH): Bildung und Strukturen des Cokristalls FAH·TrMU (Z′ = 1), des Salzes Me2NH2 +·FA– (Z′ = 6) und des Salz-Cokristalls FAH·TrMU·Me2NH2 +·FA– (Z′ = 1). Bemerkungen zur Oxophobie von C–F-Gruppen in Kristallstrukturen / Polysulfonylamines, CXCII. Polynary Compounds Based upon Di(4-fluorobenzenesulfonyl)amine (FAH), 1,1,3-Trimethylurea (TrMU) and Dimethylamine (Me2NH): Formation

Cocrystallization of di(4-fluorobenzenesulfonyl)amine, (4-F-C6H4SO2)2NH (= FAH), with 1,3,3- trimethylurea (TrMU) from dichloromethane/petroleum ether afforded the molecular cocrystal FAH・TrMU (1, monoclinic, P21/n, Z′ = 1) and the salt cocrystal FAH・TrMU・Me2NH2 +・FA− (2, monoclinic, P21, Z′ = 1). The minor product 2 resulted from a hydrolysis reaction of TrMU and was obtained by serendipity. The salt component of 2, Me2NH2 +・FA− (3, monoclinic, C2, Z′ = 6), was prepared by metathesis of [Me2NH2]Cl with Ag[FA] and is not isomorphous to its previously reported congeners Me2NH2 +・(4-X-C6H4SO2)2N− (4 - 7 for X = Cl, Br, I or Me, all monoclinic, Cc, Z′ = 1). The three new structures display one-dimensional arrays (catemers) based upon classical two- or three-centre hydrogen bonds that use N-H or N+-H groups as donors and S=O, C=O or N− groups as acceptors. In 1, the catemers consist of alternating FAH and TrMU molecules, in 3 of alternating Me2NH2 + and FA− ions (six independent formula units), in 2 of alternating Me2NH2 + and FA− ions (one independent unit) and FAH・ ・ ・ TrMU・ ・ ・ heterodimers acting as side-groups to the ionic polymer. The packings of 1 - 3 are completely devoid of short fluorine-oxygen contacts below the van derWaals limit, as are all the known crystal structures containing FAH or FA− entities (“oxophobia” of the C-F groups). The Z′ = 6 structure of 3 may be interpreted as a stratagem to avoid short F・ ・ ・O contacts, in contrast to the Z′ = 1 structures of 4 - 6, which exhibit hydrogen-bonded ion catemers similar to the catemer of 3, and two halogen bonds C-X・ ・ ・O=S per formula unit

Self-Complementary Quadruply Hydrogen-Bonded Duplexes Based on Imide and Urea Units

The quadruply hydrogen-bonded duplexes based on an imide-urea structure preorganized by three-center hydrogen bonds were found to associate via bifurcated hydrogen bonds. (1)H NMR dilution experiments revealed the high stability of the homodimer in apolar solvent (K(dim) > 10(5) M(-1) in CDCl(3)) and enhancement of association ability due to electron-withdrawing substituent effects. The ready synthetic availability and adjustable association affinity via electronic effects may render these association units potentially applicable in constructing supramolecular architectures.

Synthesis and C2-symmetric structure of a cyclotetrapeptide composed of anthranilic acid and leucine

A chiral cyclotetrapeptide (1) was synthesized from 2-nitrobenzoic acid and leucine. A single-crystal X-ray of the compound revealed a C(2) -symmetric bowl-shaped structure. The cyclic compound had a unique hydrogen-bonding network composed of three-centered hydrogen bonds and bifurcated hydrogen bonds between NH and CO of anthranilic residue. The NMR spectra and molecular modeling of 1 also suggested the chiral bowl structure.