Mesomorphic phenanthroline derivatives: novel architectures based on hydrogen bonding

Strong hydrogen bonding has been observed in mesomorphic phenanthroline compounds engineered from diacylaminobenzene platforms: a columnar organization is found in the free ligands (n = 12 or 16) whereas a smectic arrangement is evidenced in a palladium complex (n = 16).     

Supramolecular ferroelectric liquid crystals. Hydrogen-bonded complexes between benzoic acids and chiral stilbazoles

Supramolecular ferroelectric liquid crystalline complexes have been obtained from 4-alkoxybenzoic acids and optically active trans-4-substituted-4'-stilbazoles. Chiral smectic C phases are induced by the formation of supramolecular mesogenic structure through the selective intermolecular hydrogen bond between the achiral benzoic acids and the chiral non-mesogenic stilbazoles.     

Borane-ammonia complexes stabilized by hydrogen bonding

Novel boron-ammonia complexes, wherein an NH(3) molecule is tightly bound through all four of its atoms, have been prepared and studied. The solid-state structure of ortho MOM-phenyllithium is reported. [reaction: see text]     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

On differences between hydrogen bonding and improper blue-shifting hydrogen bonding.

Twenty two hydrogen-bonded and improper blue-shifting hydrogen-bonded complexes were studied by means of the HF, MP2 and B3LYP methods using the 6-31G(d,p) and 6--311 ++G(d,p) basis sets. In contrast to the standard H bonding, the origin of the improper blue-shifting H bonding is still not fully understood. Contrary to a frequently presented idea, the electric field of the proton acceptor cannot solely explain the different behavior of the H-bonded and improper blue-shifting H-bonded complexes. Compression of the hydrogen bond due to different attractive forces-dispersion or electrostatics--makes an important contribution as well. The symmetry-adapted perturbation theory (SAPT) has been utilized to decompose the total interaction energy into physically meaningful contributions. In the red-shifting complexes, the induction energy is mostly larger than the dispersion energy while, in the case of blue-shifting complexes, the situation is opposite. Dispersion as an attractive force increases the blue shift in the blue-shifting complexes as it compresses the H bond and, therefore, it increases the Pauli repulsion. On the other hand, dispersion in the red-shifting complexes increases their red shift.     

Molecular structures of triethylamine complexes with benzoic and pivalic acids

Dissolution of benzoic or pivalic acid in excess triethylamine followed by crystallization was found to yield single crystals of 1: 1 and 1: 2 complexes, respectively. X-ray crystallography have shown that the complex with benzoic acid is an (Et₃NH)⁺(OOCPh)^? ion pair, in which the proton is transfered from the acid to the base (N?O 2.629(2) Å), and the complex with pivalic acid contains, in addition to the ion pair (N?O 2.649 Å), the homoconjugate anion (Bu^tCOO)(H)(OOCBu^t)^? (O?O 2.521 Å).     

Viscosity B -coefficients of sodium salts of some orthosubstituted benzoic acids in relation to hydrogen bonding and electronegativity

ViscosityB-coefficients of sodium salts of salicylic, thiosalicylic, anthranilic, 2,6-dihydroxy benzoic and ito-anisic acids have been measured at 298, 303 and 308 K respectively. Temperature dependence ofB-values suggest that the salts as well as their anions excepto-anisate behave as structure breakers. This is further supported by energy of activation data. The observedB-coefficients suggest probable involvement of simultaneous intra- and intermolecular hydrogen bonding between an ortho-substituent and the carboxylate group on the one hand and between the ortho-substituent and solvent water on the other. A good correlation betweenB-coefficients and the electronegativities of oxygen, nitrogen and sulphur atoms has been noted. The probable involvement of ringπ-electrons in depolymerising the water structure has been conjectured.     

Boronic acid hydrogen bonding in encapsulation complexes

Hydrogen bonding is a key determinant of much macromolecular structure in nature, but individual donor and acceptor pairs are rarely observed in solution. Their weak interactions result in nanosecond lifetimes and rapid exchange of partners. Reversible encapsulation isolates molecules in very small spaces for milliseconds to hours and allows their characterization by NMR methods. Here we report a competitive study of hydrogen-bonding functions--carboxylic acids, primary amides, and boronic acids--within a multicomponent capsular assembly. The pairwise co-encapsulation of these molecules allows the direct observation of homodimeric boronic acids and their heterodimeric complexes with carboxylic acids and primary amides. The efficiency of boronic acids as hydrogen-bonding partners derives from their adaptable structures rather than from their intrinsic acid/base properties.     

Matrix polycondensation through hydrogen bonding interaction

Polycondensation of diesters having hydroxyl or pyridine groups was carried out with hexamethylenediamine (HMD) in the presence of (vinyl alcohol/vinyl acetate) copolymers as a matrix polymer. Apparent rates of the polycondensation of dimethyl tartrate (DMT) with HMD increased with increasing contents of PVA units in the copolymers and a strong entanglement between growing polyamide chains and PVA copolymers took place through the adsorption of HMD and DMT on the matrix copolymers. 2,6-Dimethyl pyridine dicarboxylate (2,6-DMP) reacted with HMD in the presence of the PVA copolymers or polysaccharide, while the rate enhancement effect of the matrix polymers was not significantly observed, as in the case of DMT. The effect of the matrix polymers on the polycondensation was discussed in terms of hydrogen bonding interactions.     

Interaction between biimidazole complexes of ruthenium and acetate: hydrogen bonding and proton transfer

The hydrogen bonding and deprotonation processes between four ruthenium biimidazole complexes, namely [Ru(bpy)(2)(BiimH(2))](PF(6))(2) (1, bpy is bipyridine, BiimH(2) is 2,2'-biimidazole), [Ru(bpy)(2)-(BbimH(2))](PF(6))(2) (2, BbimH(2) is 2,2'-bibenzimidazole), and [Ru(bpy)(2)(DMBbimH(2))](PF(6))(2) (3, DMBbimH(2) is 7,7'-dimethyl-2,2'-bibenzimidazole) and [Ru(bpy)(2)(TMBbimH(2))](2+) (4, TMBbimH(2) is 5,6,5',6'-tetramethyl-2,2'-bibenzimidazole), and acetate are investigated. Their hydrogen bonded adducts are indeed trapped and observed by absorption spectra and electrochemical experiments in acetonitrile solution in the presence of an excess of acetic acid for the first time. The binding constants log K(B) for these adducts are 6.74 for 1·OAc, 7.11 for 2·OAc, 7.26 for 3·OAc, and 6.99 for 4·OAc. A new approach to calculate the deprotonation constant is also developed by establishing a set of circular equilibria. The equilibrium constants for the first deprotonation step of the complexes log K(A) are 2.74 for 1, 5.19 for 2, 4.54 for 3, and 3.78 for 4. The pK(a1) values of the complexes in acetonitrile solution are calculated by subtracting log K(A) from pK(a) (HOAc in acetonitrile), giving 19.6 for 1, 17.1 for 2, 17.8 for 3, and 18.5 for 4. The degree of proton transfer (D(PT)) can be quantified by the calculation of absorption spectral and redox data, which is 0.41 for 1·OAc, 0.53 for 2·OAc, 0.57 for 3·OAc, and 0.47 for 4·OAc. Interestingly, the binding constant log K(B) (7.26) and D(PT) value (0.57) both reach their maxima at a critical point, where pK(a1) for the complex is 17.8 and ΔpK(a) for the adduct is 4.5 (ΔpK(a) = pK(a)(HOAc) - pK(a1), in acetonitrile solution). Moreover, the binding constant log K(B) shows linear correlation with the degree of proton transfer D(PT).     

Stabilization of copper(II) thiosulfonate coordination complexes through cooperative hydrogen bonding interactions

A series of copper(II) thiosulfonate complexes have been prepared via the reaction of [Cu(Me 3tren)(OH 2)](ClO 4) 2 (Me 3tren = tris(2-methylaminoethyl)amine) with three thiosulfonate ligands (RSO 2S (-), where R = Me, Ph, and MePh) and characterized by microanalysis, FTIR spectroscopy, and X-ray crystallography. In these complexes, the distorted trigonal bipyramidal copper(II) coordination sphere is occupied by four amine nitrogen atoms from the tripodal tetramine ligand and an apically bound sulfur atom from the thiosulfonate ligand. By using the tripodal tetramine ligand the oxidation of the thiosulfonate has been restricted, allowing the isolation of the complexes. The Cu-S distances were found to be similar to those in related thiosulfate complexes, indicating coordinative interactions of similar strength. Two types of intramolecular hydrogen bonding interactions were evident which enhance the binding of the thiosulfonate to the copper(II) center. These interactions, which involve two amine N-H groups and either one or two thiosulfonate oxygens, were found to be weaker than in the corresponding thiosulfate complexes. The complex formation constants for the thiosulfonate complexes (log K f = 0.3-0.7) were found to be two orders of magnitude lower than compared to the thiosulfate analogues. This correlates well with a lower strength of intramolecular hydrogen bonding.     

Supramolecular liquid crystals induced by intermolecular hydrogen bonding between benzoic acid and 4-(alkoxyphenylazo) pyridines

Supramolecular liquid crystalline complexes have been obtained from binary mixtures of 4-(alkoxyphenylazo) pyridines and 4-octyloxylbenzoic acid. None of the pyridine-based derivatives is mesomorphic, but the hydrogen-bonded complexes are. Their liquid crystalline properties were investigated by DSC, polarizing optical microscopy, FTIR spectroscopy and powder X-ray diffraction. The complexes exhibit enantiotropic smectic C phases.     

Supramolecular liquid crystals induced by intermolecular hydrogen bonding between benzoic acid and 4-(alkoxyphenylazo) pyridines

Supramolecular liquid crystalline complexes have been obtained from binary mixtures of 4-(alkoxyphenylazo) pyridines and 4-octyloxylbenzoic acid. None of the pyridine-based derivatives is mesomorphic, but the hydrogen-bonded complexes are. Their liquid crystalline properties were investigated by DSC, polarizing optical microscopy, FTIR spectroscopy and powder X-ray diffraction. The complexes exhibit enantiotropic smectic C phases.     

Structure and hydrogen bonding in polyhydrated complexes of guanine

Abstract Molecular structure of complexes of guanine with 12, 13, 16, and 17 water molecules were calculated using B3LYP/6-311G(d,p) level of theory. Interaction with water results in some deformation of geometrical parameters of guanine, which can be described as contribution of zwitter-ionic resonant form into the structure of DNA base. Saturation of water binding sites within guanine creates possibilities for the formation of the N···H–O hydrogen bond where the nitrogen atom of amino group acts as proton acceptor. The NBO analysis of guanine–water interactions reveals that hydrogen bonds involving the N(3) and N(7) atoms of guanine represent a case of mixed N···H–O/π···H–O hydrogen bonds where contribution of π-system into total energy of interaction varies from 3% to 41%. This contribution significantly depends on orientation of the hydrogen atom of water molecule with respect to plane of purine bicycle and influence of neighboring water molecules. Graphical Abstract      

Supramolecular liquid crystals formed by hydrogen bonding between a benzocrown-bearing stilbazole and carboxylic acids

The mesomorphism of hydrogen bonded complexes formed between 4'-carboxybenzo-15-crown-5 stilbazolyl ester (CBCSE) as proton acceptor and carboxylic acids as proton donors is discussed. CBCSE is a monotropic mesogen, forming a nematic phase upon quench cooling. A total of 32 hydrogen bonded complexes has been studied. Hydrogen bonding with carboxylic acids stabilizes the nematic phase, and/or induces a smectic A (SmA) phase. CBCSE forms 1:1 complexes (molar ratio) with alkanoic acids (fatty acids) and 2:1 complexes with alkanedioic acids. None of these proton donors is a mesogen itself, but the hydrogen bonded complexes are. The influence of the chain or spacer length on the transition temperatures is discussed. Besides the homologous series of the alkanoic and alkanedioic acids, the following carboxylic acids were used in this study: diglycolic acid, pyridine-2,6-dicarboxylic acid, 4-dodecyloxybenzoic acid, 3,4-bis(dodecyloxy)benzoic acid, 2,3,4-tris(dodecyloxy)benzoic acid and 3,4,5-tris(dodecyloxy)benzoic acid, phthalic acid, isophthalic acid and terephthalic acid.     

Supramolecular liquid crystals formed by hydrogen bonding between a benzocrown-bearing stilbazole and carboxylic acids

The mesomorphism of hydrogen bonded complexes formed between 4'-carboxybenzo-15-crown-5 stilbazolyl ester (CBCSE) as proton acceptor and carboxylic acids as proton donors is discussed. CBCSE is a monotropic mesogen, forming a nematic phase upon quench cooling. A total of 32 hydrogen bonded complexes has been studied. Hydrogen bonding with carboxylic acids stabilizes the nematic phase, and/or induces a smectic A (SmA) phase. CBCSE forms 1:1 complexes (molar ratio) with alkanoic acids (fatty acids) and 2:1 complexes with alkanedioic acids. None of these proton donors is a mesogen itself, but the hydrogen bonded complexes are. The influence of the chain or spacer length on the transition temperatures is discussed. Besides the homologous series of the alkanoic and alkanedioic acids, the following carboxylic acids were used in this study: diglycolic acid, pyridine-2,6-dicarboxylic acid, 4-dodecyloxybenzoic acid, 3,4-bis(dodecyloxy)benzoic acid, 2,3,4-tris(dodecyloxy)benzoic acid and 3,4,5-tris(dodecyloxy)benzoic acid, phthalic acid, isophthalic acid and terephthalic acid.     

Hydrated alizarin complexes: hydrogen bonding and proton transfer

We investigated the hydrogen bonding structures and proton transfer for the hydration complexes of alizarin (Az) produced in a supersonic jet using fluorescence excitation (FE), dispersed laser induced fluorescence (LIF), visible-visible hole burning (HB), and fluorescence detected infrared (FDIR) spectroscopy. The FDIR spectrum of bare Az with two O-H groups exhibits two vibrational bands at 3092 and 3579 cm(-1), which, respectively, correspond to the stretching vibration of O1-H1 that forms a strong intramolecular hydrogen bond with the C9=O9 carbonyl group and the stretching vibration of O2-H2 that is weakly hydrogen-bonded to O1-H1. For the 1:1 hydration complex Az(H(2)O)(1), we identified three conformers. In the most stable conformer, the water molecule forms hydrogen bonds with the O1-H1 and O2-H2 groups of Az as a proton donor and proton acceptor, respectively. In the other conformers, the water binds to the C10=O10 group in two nearly isoenergetic configurations. In contrast to the sharp vibronic peaks in the FE spectra of Az and Az(H(2)O)(1), only broad, structureless absorption was observed for Az(H(2)O)(n) (n≥ 2), indicating a facile decay process, possibly due to proton transfer in the electronic excited state. The FDIR spectrum with the wavelength of the probe laser fixed at the broad band exhibited a broad vibrational band near the O2-H2 stretching vibration frequency of the most stable conformer of Az(H(2)O)(1). With the help of theoretical calculations, we suggest that the broad vibrational band may represent the occurrence of proton transfer by tunnelling in the electronic ground state of Az(H(2)O)(n) (n≥ 2) upon excitation of the O2-H2 vibration.     

Self-assembly of supramolecular complexes based on hydrogen bonding

Self-assembly of hydrogen-bonding recognition complexes—2.5-bis(alkylamino)-1,4-benzoquinones were studied, and thermotropic liquid crystalline behavior of 2.5-bis(dodecylamino)-1,4-benzoquinone was further investigated. The obtained results showed that a ribbon-like backbone in this system plays an important role in keeping the layered supramolecular structure.