Pyridine Bases Research Articles

Synthesis and Molecular and Electronic Structures of Reduced Bis(imino)pyridine Cobalt Dinitrogen Complexes: Ligand versus Metal Reduction

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes ((Ar)PDI)CoCl(2) and ((iPr)BPDI)CoCl(2) ((Ar)PDI = 2,6-(2,6-R(2)-C(6)H(3)N=CMe)(2)C(5)H(3)N (R = (i)Pr, Et, Me); (iPr)BPDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N=CPh)(2)C(5)H(3)N) in the presence of an N(2) atmosphere furnished the corresponding neutral cobalt dinitrogen complexes ((Ar)PDI)CoN(2) and ((iPr)BPDI)CoN(2). Magnetic measurements on these compounds establish doublet ground states. Two examples, ((iPr)PDI)CoN(2) and ((iPr)BPDI)CoN(2), were characterized by X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I) oxidation state. Accordingly, the toluene solution EPR spectrum of ((iPr)PDI)CoN(2) at 23 degrees C exhibits an isotropic signal with a g value of 2.003 and hyperfine coupling constant of 8 x 10(-4) cm(-1) to the I = 7/2 (59)Co center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of ((iPr)PDI)CoN(2) was accomplished by treatment with Na[C(10)H(8)] in THF and yielded the cobalt dinitrogen anion [((iPr)PDI)CoN(2)](-). DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined N[triple bond]N infrared stretching frequencies and validate the computational methods. The electronic structures of the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell versus triplet diradical dianions.

A new approach to internal Lewis pairs featuring a phosphenium acid and a pyridine base

A bis(imino)acenaphthene (BIAN) ligand containing a pendant Lewis base has been used as a new framework to support a N-heterocyclic phosphenium cation (NHP). Reactivity studies demonstrate the ability of the ligand to act as a Lewis base, while the phosphorus centre provides a Lewis acidic site, giving new opportunities in NHP chemistry.

Calix[6]arene-picolinamide extractants for radioactive waste: effect of modification of the basicity of the pyridine N atom on the extraction efficiency and An/Ln separation

The effect of a systematic variation of the basicity of the pyridine nitrogen atom in calix[6]arene-based picolinamide ligands on the actinide(III)/lanthanide(III) separation by solvent extraction has been investigated. The distribution coefficients for trivalent metal ions (D(M)) decrease by increasing the nitric acid concentration, but for ligands (2 and 4) possessing a much less basic aromatic nitrogen atom, D(M) values are considerably higher than those of ligands (1 and 5) having more basic pyridine nuclei. Also in terms of selectivity ligands 2 and 4 behave better than ligand 1 especially at nitric acid concentrations very close to those of the nuclear waste. At [H(+)] = 1 mol L(-1), SF(Am/Eu) are still 3.23 and 1.92 for 2 and 4, respectively. A simple quantitative relationship between the efficiency of these extractants and the gas-phase basicity of suitably chosen model compounds is proposed, in order to explain the experimental extraction data, on one hand, and to orient future syntheses of ligands for An/Ln separation, on the other hand.

Corrosion Inhibition of Mild Steel in HCl by Isolated Compounds of Riccinus Communis (L.)

We have isolated three pyridine base alkaloids namely ricinine (C1), N‐demethylricinine (C2) and 4‐methoxy pyridine‐3‐carboxylic acid (C3) from methanolic extract of Riccinus Communis leaves and investigated corrosion inhibition effect on mild steel in 0.5 HCl solution using the weight loss and electrochemical techniques (Galvanostatic polarization and EIS). Polarization resistances calculated from the EIS measurements are in good agreement with those obtained from alternating current (AC) polarization measurements. The mild steel samples were also analyzed by Scanning Electron Microscopy (SEM). The results show that C1 is an excellent inhibitor for mild steel in acid medium. The inhibition was assumed to occur via adsorption of the inhibitor molecule on the metal surface. In the 298‐ 308 K temperature range, the C1, C2 and C3 adsorption follows Langmuir isotherm model. The protection efficiency increases with increasing the inhibitor concentration in the range of 250‐1000 ppm but slightly decreases with increasing temperature.

Thermodynamics and kinetics of the formation of the supramolecular complexes bisacetato(5,10,15,20-tetraphenylporphinate)zirconium(IV) with pyridine and imidazole

The equilibria and rates of step reactions for the formation of the supramolecular complexes of bisacetato(5,10,15,20-tetraphenylporphinate)zirconium(IV) (AcO)2ZrTPP and bioactive bases pyridine (Py) and imidazole (Im) in toluene were studied using UV-Vis and IR spectroscopy. The step stoichiometric mechanism, including the reversible coordination of two Py molecules (K1 = 1.8 × 108 l2/mol2), the equilibrium of the displacement of two AcO− into the second coordination sphere by increasing the concentration of the solvent polar component (K2 = 2.4), and the coordination of the third and fourth Py molecules in a one step with the formation of [(Py)4ZrTPP]2+ · 2(AcO)− (K3 = 2.8 × 104 l2/mol2), was verified. It was established that the spectrophotometric titration is sensible for the two-stage π-π-complexation of [(Py)4ZrTPP]2+ · 2(AcO)− with Py molecules (K4 = 29 l/mol and K5 = 1.8 l/mol). It was shown that the stronger base Im reacts irreversibly with (AcO)2ZrTPP. The thermodynamic and optical characteristics of (AcO)2ZrTPP required for using the complex in the detection of bioactive bases were studied.

Self-assembly of coordination polymers constructed from CuCN and unidentate pyridine bases

The self-assembly of K3[Cu(CN)4] and unidentate pyridine bases (L): pyridine (py), 3-methyl pyridine (3-mpy), and 2,4,6-trimethyl pyridine (tmpy) in the presence of Me3SnCl affords new coordination polymers (CPs) CuCN·0.5(py)] (1), [CuCN·0.5(3-mpy)] (2), and [CuCN·0.5(tmpy)] (3). The syntheses are achieved in H2O/acetonitrile media at room temperature. The structure of the CP 3 was characterized by X-ray single crystal analysis. It is crystallized as orthorhombic in the space group Pnma, a = 9.1065 (3) A, b = 8.6669 (3) A, and c = 12.1998 (5) A and Z = 8. The CPs 1, 2 were investigated by IR, mass, Uv–visible, and 1H-NMR spectra, as well as TGA. The CPs 1–3 are 2D-polymers consisting of 1D-(CuCN)n chain structure while the ligands alternate on both sides of the chain with associated copper atom coordination number of three. Hydrogen bonds play an essential role for developing 2D-network structure. These CPs exhibit strong fluorescent emissions in the solid state.

Experimental and Theoretical Charge Density Study of Polymorphic Isonicotinamide−Oxalic Acid Molecular Complexes with Strong O···H···N Hydrogen Bonds

Two polymorphs of the 2:1 molecular complex of isonicotinamide and oxalic acid have been characterized by combined X-ray charge density and single-crystal neutron diffraction studies at 100 K. Both polymorphs show strong O-H...N intermolecular hydrogen bonding between the acid and the pyridine base. As is typical of short, strong hydrogen bonds (SSHBs), the covalent O-H bonds are considerably elongated to 1.161(3) and 1.235(5) A, and the H...N interactions are correspondingly short at 1.398(3) and 1.313(6) A in Forms I and II, respectively. The neutron diffraction data indicate no pronounced H dynamics in the SSHBs, and in the case of Form II the SSHB can be described as quasicentered. In addition to the experimental charge densities, theoretical charge densities have been determined from ab initio calculations within the full periodic environment of the crystalline state. The SSHBs are found to be covalent in nature according to the topological analysis of the experimental and theoretical charge densities and application of the source function. Aside from the SSHBs, moderate N-H...O and weak C-H...O interactions are also present in the molecular complexes, for which hydrogen bond energies are estimated from energy densities and independent ab initio calculations. Finally, an attempt is made to evaluate the intermolecular interactions governing the manifestation of polymorphism in this compound.

Salts of Hydrates of Imiquimod, an Immune Response Modifier

Imiquimod free base 1, an immune response modifier, its hydrochloride salt 2, hydrochloride salt monohydrate 3, salts with maleic acid 4, fumaric acid 5, succinic acid 6, adipic acid 7, and 4-hydroxybenzoic acid 8 have been prepared and characterized by single crystal X-ray diffraction. The carboxylic acid salts 4−8 were obtained by solution crystallization experiments of imiquimod hydrochloride with carboxylic acid group containing molecules. Imiquimod contains a 2-aminopyridine moiety which is known to form salts when crystallized with acidic carboxylic acid moiety containing molecules. During the crystallization, hydrochloride was liberated from the imiquimod and the basic pyridine moiety was protonated by the carboxylic acids. Dicarboxylic acid salts 5−7 crystallized in 2:1 stoichiometry; however, 2, 4, and 8 crystallized in a 1:1 ratio. Imiquimod maleate salt 4 crystallized as sesquihydrate, whereas imiquimod fumarate (5) crystallized as hexahydrate. Two aliphatic-α,ω-dicarboxylic acid salts of imiqu...

Pyrrolidine–pyridine base catalysts for the enantioselective Michael addition of ketones to chalcones

A series of pyrrolidine–pyridine base organocatalysts have been developed and 3a found to be a highly effective catalyst for the asymmetric Michael addition reactions of ketones to chalcones. The reaction generated the corresponding products 1, 5-diketones in excellent diastereoselectivities (up to >99:1 dr) and enantioselectivities (up to 100% ee).

Thermodynamics of mixtures containing amines. IX. Application of the concentration–concentration structure factor to the study of binary mixtures containing pyridines

Binary mixtures formed by a pyridine base and an alkane, or an aromatic hydrocarbon, or a 1-alkanol have been studied in the framework of the concentration–concentration structure factor, S CC (0), formalism. Deviations between experimental data and those provided by the DISQUAC model are discussed. Systems containing alkanes are characterized by homocoordination. In pyridine + alkane mixtures, S CC (0) decreases with the chain length of the longer alkanes, due to size effects. For a given alkane, S CC (0) also decreases with the number of CH 3– groups in the pyridine base. This has been interpreted assuming that the number of amine–amine interactions available to be broken upon mixing also decreases similarly, probably as steric hindrances exerted by the methyl groups of the aromatic amine increase with the number of these groups. Homocoordination is higher in mixtures with 3,5-dimethylpyridine than in those with 2,6-dimethylpyridine. That is, steric effects exerted by methyl groups in positions 3 and 5 are stronger than when they are in positions 2 and 6. Similarly, from the application of the DISQUAC (dispersive–quasichemical) model, it is possible to conclude that homocoordination is higher in systems with 3- or 4-methylpyridine than in those involving 2-methylpyridine. Systems including aromatic hydrocarbons are nearly ideal, which seems to indicate that there is no specific interaction in such solutions. Mixtures with 1-alkanols show heterocoordination. This reveals the existence of interactions between unlike molecules, characteristic of alkanol + amine mixtures. Methanol systems show the lowest S CC (0) values due, partially, to size effects. This explains the observed decrease of homocoordination in such solutions in the order: pyridine > 2-methylpyridine > 2,6-dimethylpyridine. Moreover, as the energies of the OH–N hydrogen bonds are practically independent of the pyridine base considered when mixed with methanol, it suggests that size effects are predominant over steric hindrances to the creation of the OH–N hydrogen bonds, which are expected to increase with the number of methyl groups in the aromatic amine. For a given 1-alkanol (≠methanol), S CC (0) varies in the sequence: pyridine > methyl pyridine ≈ 2,6-dimethylpyridine. For alkyl pyridines, stability seems to be independent of position and number of alkyl groups attached to the aromatic ring of the amine. Mixtures with isomeric 2-alkanols show lower heterocoordination, as the hydroxyl group is more sterically hindered than in 1-alkanols.

Octahedral HSiCl3 and HSiCl2Me Adducts with Pyridines

Stable solid adducts of substituted pyridines (Rpy) with HSiCl(3) and HSiCl(2)Me were prepared in high yields under aprotic and anaerobic conditions at room temperature. The octahedral complexes of HSiCl(3) underwent dismutation reactions in polar solvents. In contrast, the HSiCl(2)Me(Rpy)(2) adducts were not susceptible to dismutation under comparable conditions, but they tended to dissociate more easily because of the reduced Lewis acidity of HSiCl(2)Me relative to HSiCl(3). The bonding between silicon and its surrounding ligands is highly ionic, as can be seen from QTAIM and charge distribution analyses. (29)Si CP/MAS spectra in combination with quantum-chemical calculations show that the lowest shielding is along the Cl-Si-Cl axis. The other two components of the shielding tensor are oriented along the N-Si-N and H-Si-Cl/Me axes. It is known that many reactions of (hydrido)chlorosilanes are catalyzed by pyridine bases. Therefore, the results presented here provide a basis for better control of these reactions, especially chlorine substitution and hydrosilylation.

The Effect of H2 on Chichibabin Condensation Catalyzed by Pure ZSM-5 and Pt/ZSM-5 for Pyridine and 3-Picoline Synthesis

The effects of H2 in the feed gasses on the Chichibabin condensation of ammonia with formaldehyde and acetaldehyde for pyridine and 3-picolines synthesis were studied in this work. It was observed that over Pt/ZSM-5 and H-ZSM-5 catalyst hydrogen can moderate the deactivation, depressed the coke formation, slightly lower the initial yield of pyridine bases and increase the selectivity of pyridine over 3-picoline. These effects cannot be simply attributed to the reactant dilution by H2, while are similar as those of spilt-over hydrogen on a Pt/ZSM-5 catalyst on this reaction. These phenomena are taken as the evidence to support the hypothesis that molecular H2 is activated by the pure zeolite during the Chichibabin condensation.

Binding Catalysis and Inhibition by Metal Ions and Protons in the Enolization of Phenylacetylpyrazine

A study of the enolization of phenylacetylpyrazine (PzCOCH(2)Ph) catalyzed by acid, base and metal ions in aqueous solution shows, unusually, that metal ions are more effective catalysts than protons, e.g., for zinc k(Zn)/k(H) = 600. Such behavior contrasts with that of the structurally related phenacylpyridine (PyCH(2)COPh) for which k(Zn)/k(H) = 0.0065. To interpret this difference, equilibrium constants for the tautomerization of phenylacetylpyrazine and for binding of protons and metal ions to its keto tautomer and enolate anion have been measured or estimated and are compared with existing measurements for phenacylpyridine. A tautomeric constant, K(E) = 1.2 x 10(-3) (pK(E) = 2.9), is derived by combining forward and reverse rate constants for enolization measured, respectively, by iodination or bromination of the keto tautomer and relaxation of the less stable enol. For the keto tautomer, NMR measurements yield a pK(a) = -0.90 for N-protonation, and spectrophotometric measurements give pK(a) = 11.90 for ionization to an enolate anion. For the enol, pK(a) values of 0.44 and -4.80 for mono- and diprotonation are obtained from the pH profile for ketonization and absorbance measurements for the transient enol reactant. Binding constants for metal ions (Cu(2+), Ni(2+), Zn(2+), Co(2+), and Cd(2+)) are derived from the saturation of their catalysis of the ketonization reaction. It is found that ketonization is efficiently catalyzed by metal ions but inhibited by acid. These findings, and the striking difference from phenacylpyridine, are ascribed to differences in thermodynamic driving force arising from stronger binding of the proton to the more basic pyridine than pyrazine nitrogen atom in both the reactant keto tautomer and in the enaminone or zwitterion product of the rate-determining (proton transfer) step of the enolization.

Asymmetric Hydrogenation of Enamides

An asymmetric Hantzsch ester mediated transfer hydrogenation of aromatic enamides catalyzed by a combination of chiral phosphoric acid 1 and acetic acid is reported. Mechanistic assumptions involve the acid-dependent generation of iminium ions 2 and 3, wherein the phosphoric acid activated intermediate 2 exclusively undergoes further reaction. The role of the achiral acid apparently lies in the perpetuation of a sufficient quantity of the iminium species, as well as in the neutralization of the basic pyridine byproduct.

CD38 in Bovine Lung: A Multicatalytic NADase

We report the kinetics and molecular properties of CD38 purified from bovine lung microsomal membranes after its solubilization with Triton X-100. The enzyme was found to be a novel member of a multicatalytic NAD(+)-glycohydrolase (NADase, EC 3.2.2.6). It was able to utilize NAD( + ) in different ways, producing nicotinamide (Nam) and either adenosine diphosphoribose (ADPR, NADase activity) or cyclic ADPR (cADPR, cyclase activity); it also catalyzed the hydrolysis of cADPR to ADPR (cADPR, hydrolase activity). In addition, the enzyme catalyzed the pyridine base exchange reaction with conversion of NAD( + ) into NAD analogues. These data are evidence that CD38 is involved in the regulation of both NAD(+) and calcium-mobilizing agents, the concentration resulting in an essential enzyme that plays a key role in cellular energy and signal-transduction systems.

Exploring the Chemistry of Electron-Accepting Molecules in the Cavities of the Basic Microporous P4VP Polymer by in situ FTIR Spectroscopy

A systematic in situ FTIR investigation of the interaction of several molecules characterized by a different electron accepting ability (or acidity) with the basic pyridine units contained in a commercial poly(4-vinylpyridine) system characterized by a high surface area is here reported, with the final goal to fully understand the role of basicity in the catalytic processes. This approach, for the first time applied to polymeric materials, has only some similarities with the method usually adopted in the investigation of catalytic acid sites hosted inside oxidic porous materials (such as zeolites), where the probes are molecules characterized by an increased basicity. The experimental results are confirmed by ab initio theoretical calculations conducted on the 4-methylpyridine, which is the simplest molecule resembling the pyridine moiety in the P4VP system.

An Enolisable Barbiturate with Adjustable Hydrogen-Bonding Structure for UV/Vis Detection of Nucleic Acid Bases and Related Compounds

The use of hydrogen-bonding patterns in the same way as is known from DNA building blocks is a challenge for the construction of novel types of suitable chromophoric probes. This feature has been utilised for the construction of a novel type of UV/Vis probe for detection of supramolecular AAD or DAD sequences (A=hydrogen bond acceptor, D=hydrogen bond donor). Here we report on the structure of the enolisable chromophore 1-n-butyl-5-(4-nitrophenyl)barbituric acid (1), which has an adjustable hydrogen-bonding pattern. The position of the keto-enol equilibrium of this dye is strongly influenced both by the solvent polarity and by the chemical environment. Furthermore, the recognition properties of the barbiturate were examined by the use of seven artificial receptors: the pyridine bases 2,6-diaminopyridine (DAP), 2,6-diacetamidopyridine (DAC) and 2,6-bis(trifluoroacetamido)pyridine (TFA), as well as the nucleic acid bases 9-ethyladenine (EtAd), 9-ethylguanine (EtGu), 1-n-butylcytosine (BuCy) and 1-n-butylthymine (BuTy). It was found that 1 can interact with these bases either through acid-base interaction or by hydrogen-bonding complexation. The balance between the interactions is dependent both on the basicity strength and on the presence of a suitable recognition sequence in the base. The induced formation of the enol form of 1 thus causes a significant UV/Vis shift as function of the nature of the base.

Effect of alkaline and atom-planting treatment on the catalytic performance of ZSM-5 catalyst in pyridine and picolines synthesis

Abstract For the gas phase synthesis of pyridine and picolines, a successive alkaline and atom-planting treatment was employed to tailor the H-ZSM-5, and a catalyst containing hierarchical pore and framework Ga cations was prepared. Desilication was the major consequence of the alkaline treatment, while a slight dealumination happened simultaneously. A large number of meso- and macropores were introduced into the H-ZSM-5. The generated silanol groups, formed octahedral aluminum and decreased zeolite framework Si/Al caused decrease of the strong acid sites and increase of the weak acid sites and Lewis/Bronsted (L/B) ratio. Subsequent atom-planting treatment consumed the silanol groups and generated the bridged hydroxyl groups Si(OH)Ga, which resulted in decrease of the L/B ratio and the weak acid sites and increase of the strong acid sites. The catalytic performance shows that the introduced hierarchical pores increased the stability of the catalysts. The pyridine bases yield were not influenced by the generation of silanol groups and weak acid sites, while they were increased by the generation of Si(OH)Ga. The change of the P/3P has the same trend as that of L/B. The coke-introduced selectivation caused the gradual increase of P/3P during the deactivation of the original H-ZSM-5.

Behavior of a Deactivating HZSM-5 with Varying Al3+ Content in Vapor Phase Aminocyclization

Aminocyclization of aldehydes to synthesize pyridine bases is an interesting reaction with HZSM-5 catalyst undergoing rapid deactivation. The catalyst characterization studies like NH3-TPD, XRD, TGA/DTA, Pore size distribution, SEM, CHNS, IR for the fresh and deactivated catalyst at various Si/Al ratio content as evidenced by the striking difference in the behavior of HZSM-5 catalyst in terms of its physicochemical characteristics, coke formation tendencies and higher selectivity to pyridine formation.

EPR Study of the Photolysis of Methyl- and Adenosylcobinamides in the Presence of Phosphine and Pyridine Bases. Evidence for the Need of a Judicious Choice of Irradiation Temperature and Solvent to Assess Ligand Binding

The products of anaerobic photolysis of methylcobinamide (MeCbi+) and adenosylcobinamide (AdoCbi+) in the presence of phosphine and pyridine bases have been studied in methanol, at two irradiation temperatures (77 and 300 K) by EPR spectroscopy. The presence of cobalt(II) species and organic radicals in solution is indicative that homolytic cleavage of the Co−C bond is induced upon irradiation with visible light. The EPR results obtained upon photolysis of cobinamide solutions in the presence of several cobinamide:phosphorus base molar ratios show that only for a large excess of base is the 1:1 adduct quantitatively formed, and is, in those experimental conditions, the single Co(II) species in solution. For molar ratios below 1:20 the EPR spectra allow identification of a mixture of the 1:1 adduct and a species with no phosphorus ligand bound to the cobalt(II) center. For pyridine derivatives, (a) in equimolar solutions of cobinamide:nitrogen base, EPR spectra exhibit one signal characteristic of the five-coordinate species, and (b) in the presence of a large excess of base a less intense EPR signal characteristic of a 1:2 adduct is observed together with that of the 1:1 adduct. Photolysis in frozen matrix allowed us to confirm that phosphines do not show a propensity to bind to the organocobalt(III)(corrin)+ compounds but do bind to the [CoII(corrin)]+ species formed upon photolysis, thus evidencing the reactivity of the latter. Another aspect concerning ligand binding is the fact that the choice of the solvent may be determinant in the assignment of coordination numbers. EPR parameters and spin densities calculated for the Co(II) species are in agreement with those described for a 2A1 ground state and vary according to basicity of pyridine bases and with Tolman cone angle for the phosphines, confirming that both electronic and steric effects are important to determine the type of adducts formed and the usefulness of EPR parameters to get insight into structural changes regarding the coordination of axial ligands. The possibility of establishing such correlations is recently becoming more important, as in many of the processes catalyzed by B12 cofactors displacement of axial ligands is observed and proved to be determinant to the enzymatic mechanism.