Neutral and ionic hydrogen bonding in Schiff bases

Low-temperature, high-resolution X-ray studies of charge distributions in the three Schiff bases, the dianil of 2-hydroxy-5-methylisophthaldehyde, 3,5-dinitro-N-salicylidenoethylamine and 3-nitro-N-salicylidenocyclohexylamine, have been carried out. These structures exhibit interesting weak interactions, including two extreme cases of intramolecular hydrogen bonds that are ionic N(+)-H...O- and neutral O-H...N in nature. These two types of hydrogen bond reflect differences in geometrical parameters and electron density distribution. At the level of geometry, the neutral O-H...N hydrogen bond is accompanied by an increase in the length of the C(1)-O(1) bond, opening of the ipso-C(1) angle, elongation of the aromatic C-C bonds, shortening of the C(7)-N(2) bond and increased length of the C(1)-C(7) bond, relative to the ionic hydrogen bond type. According to the geometrical and critical point parameters, the neutral O-H...N hydrogen bond seems to be stronger than the ionic ones. There are also differences between charge density parameters of the aromatic rings consistent with the neutral hydrogen bond being stronger than the ionic ones, with a concomitant reduction in the aromaticity of the ring. Compounds with the ionic hydrogen bonds show a larger double-bond character in the C-O bond than appears in the compound containing a neutral hydrogen bond; this suggests that the electronic structure of the former pair of compounds includes a contribution from a zwitterionic canonical form. Furthermore, in the case of ionic hydrogen bonds, the corresponding interaction lines appear to be curved in the vicinity of the hydrogen atoms. In the 3-nitro-N-salicylidenocyclohexylamine crystal there exists, in addition to the intramolecular hydrogen bond, a pair of intermolecular O...H interactions in a centrosymmetric dimer unit.     

AIM analysis of intramolecular hydrogen bonding in O-hydroxy aryl Schiff bases

AIM analysis was applied to study the changes in such topological parameters as the electron density at critical points of all the bonds of the molecule during the so-called nonadiabatic proton transfer in intramolecular hydrogen bonding in o-hydroxy aryl Schiff bases. Proton transfer is presented by a stepwise elongation and fixing of the hydroxyl bond with complete optimization of the rest of the parameters of the molecule by the B3LYP/6-311++G(d,p) method. A more detailed study of electron density changes at the critical points of the chelate and phenol rings in the stepwise proton-transfer process is presented. It was shown that the dependency of the electron density at the critical point of the chelate ring on tautomeric equilibrium is of a complicated character, whereas it is linear for the phenol ring. A complex study of the changes in the total electron density at the hydrogen bond, the quasi-aromatic ring, and in the whole molecule has been accomlished. The calculations of the intramolecular hydrogen bond by means of conformational and topological methods are discussed.     

Intermolecular hydrogen bonding in the complexes of Schiff bases with acetic acid

Quantum chemical calculations are applied to study the structure and energy of the complexes of a series of Schiff bases (SBs) with acetic acid. The results of the calculations give evidence that a weak hydrogen bond occurs between the oxygen atoms of the acid and SB aromatic protons in these complexes along with the CH₃COOH…N=CH H-bond. The structure of the complexes is confirmed experimentally by measuring the nuclear Overhauser effect. The effect of the properties of SB terminal substituents on the stability of SB-acid complexes is analyzed.     

Investigations of intramolecular hydrogen bonding in three types of Schiff bases by 2H and 3H NMR isotope effects

Hydrogen bonding within the structures of three Schiff bases (1-3), obtained by condensation of 4-methoxy-, 5-methoxy- and 4,6-dimethoxysalicylaldehyde with methylamine, was investigated by measuring deuterium and tritium NMR isotope effects. The primary deuterium and tritium isotope effects (delta(XH)-delta(XD/T)) and secondary one-bond nitrogen deuterium effect appear to be very useful parameters for defining the character of intramolecular hydrogen bonds. The tritium isotope effects were also determined for nitrogen-hydrogen one-bond coupling constants for both 4-methoxy and 4,6-dimethoxy derivatives. These parameters are seen to be highly sensitive to hydrogen bond characteristics and can be used to distinguish localized and tautomeric hydrogen bonds.     

Syntheses and structural studies of Schiff bases involving hydrogen bonds

New Schiff bases have been prepared by reacting 3-hydroxy-4-pyridine- carboxaldehyde with various amines. NMR spectroscopic methods provided clear evidence that the Schiff bases exist in the solid state and in solution as hydroxyimino tautomers with the E-configuration. A study of the stabilities of the tautomeric forms and the different conformers has been carried out using density functional calculations at the B3LYP/6-31G** level.     

Synthesis and characterization of novel Schiff bases containing pyrimidine unit

The work involves synthesis of novel Schiff base derivatives containing a pyrimidine unit starting with chalcones. 4-Aminoacetophenone was reacted with 4-nitrobenzaldehyde or 4-chlorobenzaldehyde in basic medium giving chalcones, [I]_a and [I]_b, respectively, by Claisen-Schemidt reaction. The chalcones [I]_a and [I]_b were reacted with urea in HCl medium giving oxopyrimidines, [II]_a and [II]_b. They were also reacted with thiourea in basic medium to give thioxopyrimidines, [III]_a and [III]_b. The novel mono and bis Schiff bases, [VIII]_na, [VIII]_nb, [IX]_na, [IX]_nb, [X]_na, [X]_nb, [XI]_na, and [XI]_nb were synthesized by the reaction of pyrimidine derivatives; oxopyrimdines, [II]_a and [II]_b and thioxopyrimidines, [III]_a and [III]_b with 4-(4′-n-alkoxybenzoloxy)benzaldehyde [VI] and polymethylene-α,ω-bis-4-oxybenzaldehydes [VII]_m, respectively, in dry benzene using drops of glacial acetic acid as a catalyst. The synthesized compounds were characterized by melting points, elemental analysis, FTIR, and ¹H NMR spectroscopy.     

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.     

Orientational properties of liquid-crystalline 4-n-alkyloxybenzilidene-4'-fluoroanilines

The orientational order of 4-n-hexyloxybenzilidene-4'-fluoroaniline, (FAB-OC6) and its 4-n-heptyloxy analogue, (FAB-OC7), studied by means of ²H NMR and ¹⁹F NMR is reported. The entire temperature range of the liquid-crystalline phases (S_B, S_A and N for FAB-OC6, S_B and S_A for FAB-OC7) have been investigated. The results are discussed in relation to previous X-ray diffraction measurements. The chemical shift tensor components of the fluorine bond, evaluated from the ¹⁹F NMR spectra are compared with previous results.     

Supramolecular liquid-crystalline materials: molecular self-assembly and self-organization through intermolecular hydrogen bonding

Supramolecular liquid-crystals are molecular complexes formed from different and independent molecular species through specific molecular interactions such as hydrogen bonding. We have recently developed new types of H-bonded liquid-crystalline materials obtained by molecular self-assembly processes: (1) doubly H-bonded liquid-crystalline complexes through a molecular recognition process between 2,6-bis(acylamino)pyridines and benzoic acids, (2) liquid-crystalline polymer blends involving an H-bonding interaction between poly(4-vinylphenol) and a thermotropic main-chain polyester containing a lateral pyridyl substituent, (3) liquid-crystalline networks built through hydrogen bonds between multifunctional H-bonding components. These new materials may bridge a gap between liquid crystals and supramolecular systems.     

Ruthenium(II) carbonyl complexes containing tridentate Schiff bases

New ruthenium(II) complexes, [Ru(CO)(B)(LL)(PPh₃)] (where, LL = tridentate Schiff bases; B = PPh₃, pyridine, piperidine or morpholine) have been prepared by reacting [RuHCl(CO)(PPh₃)₃] or [RuHCl(CO)(PPh₃)₂(B)] with Schiff bases containing donor groups (O, N, X) viz., salicylaldehyde thiosemicarbazone (X = S), salicylaldehyde semicarbazone (X = O), o-hydroxyacetophenone thiosemicarbazone (X = S) and o-hydroxyacetophenone semicarbazone (X = O). The new complexes were characterised by elemental analysis, spectral (i.r., ¹H- and ³¹P-n.m.r.), data.     

Nuclear quantum effects and hydrogen bonding in liquids

We have employed ab initio path integral molecular dynamics simulations to investigate the role of nuclear quantum effects on the strength of hydrogen bonds in liquid hydrogen fluoride. Nuclear quantum effects are shown to be responsible for a stronger hydrogen bond and an enhanced dipole-dipole interaction, which lead, in turn, to a shortening of the H...F intrachain distance. The simulation results are analyzed in terms of the electronic density shifts with respect to a purely classical treatment of the nuclei. The observed enhanced hydrogen-bond interaction, which arises from a coupling of intra- and intermolecular effects, should be a general phenomenon occurring in all hydrogen-bonded systems.     

Condensation of schiff bases with compounds containing labile hydrogen atoms

5,6-Benzoquinoline derivatives are formed by the reaction of cycloheptanone with arylidene--naphthylamines. The order of activity of cyclic ketones in this reaction (C₅ > C₆ > C₇) is discussed, and the UV spectra are presented.See [11] for communication IV.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 519–523, April, 1971.     

Antibacterial activity of ruthenium(II) carbonyl complexes containing tetradentate Schiff bases

Several new hexa-coordinated ruthenium(II) complexes of the type [Ru(CO)(LL)(B)] (where, LL = anthacac, anthdibm, 2-amtpacac or 2-amtpdibm; B = PPh₃ or py or pip or morph) have been prepared by reacting [RuHCl(CO)(PPh₃)₃] or [RuHCl(CO)(PPh₃)₂(B)] with tetradentate Schiff bases such as bis(anthranilic acid)acetylacetimine (H₂-anthacac), bis(anthranilic acid) dibenzoylmethimine (H₂-anthdibm), bis(2-aminothiophenol) acetylacetimine (H₂-2-amptacac) or bis(2-aminothiophenol) dibenzoylmethimine (H₂-2-amtpdibm). The complexes have been characterised by elemental analyses and spectral (i.r., electronic spectra, ¹H- and ³¹P-n.m.r.) data. An octahedral structure has been tentatively proposed for the complexes, which were also tested for their antibacterial properties.     

Intramolecular hydrogen bond investigations in some Schiff bases using C-C and N-C coupling constants

Five Schiff bases, derived from substituted salicylaldehydes and methylamine, have been investigated by analysis of C-C and N-C coupling constants to check potential of those spectral parameters in intramolecular hydrogen bonds research. Two remaining imines, without OH substituent in position 2, were applied as model compounds for imine structure. The one-bond C-C couplings in aromatic ring provide valuable information about bond orders and correlate with bond lengths obtained by X-ray. The one-bond heteronuclear C-N couplings can be used very easily to distinguish between imine and enamine form of Schiff bases. Additionally the two-bond N-C couplings supply interesting information about geometry of the investigated molecules.     

One-pot synthesis of supramolecular polymer containing quadruple hydrogen bonding units

A facile, efficient technique was built to synthesize a supramolecular material containing quadruple hydrogen bonding sites. The current approach presented here involves a single-step reaction between the amine of precursor, e.g. methyl isocytosine (MIC) and the epoxy group of polymer, e.g. poly(ethylene glycol diglycidyl ether) (PEG DGE, M_n = 526 g/mol, as verified using ¹H NMR and FT-IR spectroscopy. Wide angle X-ray scattering (WAXS), UV/visible spectroscopy and differential scanning calorimeter (DSC) clearly show that the product is not a simple mixture of two components, but the supramolecular polymer containing quadruple hydrogen bonding sites. Complex melt viscosities reveal that mechanical properties of the supramolecular polymer are enhanced by more than 10⁴ times compared to the pristine low molecular weight polymer, giving rise to the significant change of physical state from liquid to solid. Current approach also illustrates an advantageous route because it does not need the selective use of monofunctionalized precursor and not produce a dead, difunctionalized precursor.     

Self-assembly of multilayer films containing gold nanoparticles via hydrogen bonding

Polymer/Au nanoparticle multilayer ultrathin films are fabricated via hydrogen-bonding interaction by a layer-by-layer technique. The Au nanoparticles surface-modified with pyridine groups of poly(4-vinylpyridine) (PVP) are prepared in dimethyl formamide (DMF). Transmission electron microscopy (TEM) image shows that uniform nanoparticles are dispersed in the PVP chains. Poly(3-thiophene acetic acid) (PTAA) and poly(acrylic acid) (PAA) are utilized to form hydrogen bonds with PVP, respectively. Considering the pH-sensitive dissociation behavior of PTAA and PAA, we investigate the release behavior of the Au-containing multilayers at different pH values in this work. UV-vis spectroscopy and atomic force microscopy (AFM) are employed to monitor the buildup and the release of the multilayers. The results indicate that in the films assembled with gold nanoparticles, the polymers are difficult to be removed from the substrate. The interaction between the gold particles and the neighboring PVP chains is responsible for the phenomenon. Gold particles act as physical cross-link points in the multilayers. Due to the additional interaction caused by the gold nanoparticles in the films except the hydrogen-bonding interaction between PTAA (or PAA) and PVP, the stability of the Au-containing multilayer film is ensured even though the changes in pH values may result in the break of the hydrogen bonds.     

Complexes of hydrazide Schiff bases

Summary 1,2-Diphenyl-2-(benzoylhydrazino)ethanol (DBE) and 1,2-diphenyl-2-(p-nitrobenzoylhydrazino)ethanol (DPNBE) form complexes of general formulae [M(H₂L)₂]X₂ and [M₂L₂] (where M = Co or Ni, H₂L and L are the neutral and dibasic forms of the ligands and X = Cl or NO₃). The analytical, spectral, magnetic and physical studies show the complexes to be of two types, namely monomeric cationic species prepared in neutral media and neutral binuclear oxygen-bridged species formed in alkaline solution. The ligands are tridentate, coordinating through carbonyl (or enolic) oxygen, azomethine nitrogen and alcoholic oxygen atoms.     

Solid-state synthesis of Schiff bases

A new preparative method for the synthesis of Schiff bases by the solid-state interaction of crystalline organic and organometallic aldehydes and amines is suggested. The possibility of direct synthesis of aldimines in the solid state from amine salts without isolation of the free amines is demonstrated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2056–2059, November, 1997.