Spectroscopic and Computational Study of the Protonation Equilibria of Amino-Substituted benzo[b]thieno[2,3-b]pyrido[1,2-a]benzimidazoles as Novel pH-Sensing Materials

We present the synthesis and analytical, spectroscopic and computational characterization of three amino-substituted benzo[b]thieno[2,3-b]pyrido[1,2-a]benzimidazoles as novel pH probes with a potential application in pH-sensing materials. The designed systems differ in the number and position of the introduced isobutylamine groups on the pentacyclic aromatic core, which affects their photophysical and acid-base properties. The latter were investigated by UV-Vis absorption and fluorescence spectroscopies and interpreted by DFT calculations. An excellent agreement in experimentally measured and computationally determined pKa values and electronic excitations suggests that all systems are unionized at neutral pH, while their transition to monocationic forms occurs at pH values between 3 and 5, accompanied by substantial changes in spectroscopic responses that make them suitable for detecting acidic conditions in solutions. Computations identified imidazole imino nitrogen as the most favorable protonation site, further confirmed by analysis of perturbations in the chemical shifts of 1H and 13C NMR, and showed that the resulting basicity emerges as a compromise between the basicity-reducing effect of a nearby nitrile and a favorable contribution from the attached secondary amines. With this in mind, we designed a system with three amino substituents for which calculations predict pKa = 7.0 that we suggest as an excellent starting point for a potential pH sensor able to capture solution changes during the transition from neutral towards acidic media.

Preferred protonation site of a series of sulfa drugs in the gas phase revealed by IR spectroscopy

Sulfa drugs are an important class of pharmaceuticals in the treatment of bacterial infections. The amido/imido tautomerism of these molecules in their neutral form has been widely discussed in the literature. Here, we study the protonation preferences of sulfa drugs upon electrospray ionization (ESI) using IR action spectroscopy of the ionized gas-phase molecules in a mass spectrometer. Our set of molecules includes sulfanilamide (SA), the progenitor of the family of sulfa drugs, and the actual, sulfonamide nitrogen substituted, sulfa drugs sulfamethoxazole (SMX), sulfisoxazole (SIX), sulfamethizole (SMZ), sulfathiazole (STZ), sulfapyridine (SP) and sulfaguanidine (SG). IR multiple photon dissociation (IRMPD) spectra were recorded for the protonated sulfa drugs using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) and an optical parametric oscillator/amplifier (OPO/OPA) as well as the FELIX free electron laser (FEL) as IR sources. The OPO provides tunable IR radiation in the NH stretch region (3100–3700 cm$$^{-1}$$ - 1 ), while the FEL covers the fingerprint region (520–1750 cm$$^{-1}$$ - 1 ). Comparison of experimental IR spectra with spectra predicted using density functional theory allowed us to determine the gas-phase protonation site. For SA, the sulfonamide NH$$_2$$ 2 group was identified as the protonation site, which contrasts the situation in solution, where the anilinic NH$$_2$$ 2 group is protonated. For the derivative sulfa drugs, the favored protonation site is the nitrogen atom included in the heterocycle, except for SG, where protonation occurs at the sulfonamide nitrogen atom. The theoretical investigations show that the identified protonation isomers correspond to the lowest-energy gas-phase structures.

Atomic-scale evidence for highly selective electrocatalytic N−N coupling on metallic MoS2

Molybdenum sulfide (MoS2) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS2 significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS2 has a protonation site with a pKa of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS2 and induces sequential proton−electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS2, expanding the application of TMDs to selective electrocatalysis.

Towards a theoretical understanding of alkaloid-extract Cytisine derivatives of Retama monosperma (L.) Boiss. Seeds, as eco-friendly inhibitor for carbon steel corrosion in acidic 1M HCl solution

Because of their availability, lower cost, biodegradability, respect for health, environmental standards and their extremely high efficiency [Benmessaoud Left D, Zertoubi M, Irhzo A, Azzi M, Revue: Huiles et extraits de plantes comme inhibiteurs de corrosion pour différents métaux et alliages dans le milieu acide chlorhydrique, J Mater Environ Sci 4:855–866, 2013], the green corrosion inhibitors are currently the most sought after candidates. The ultimate aim of this work is to study the selectivity and reactivity of a series of three molecules, namely Cytisine, Dehydro-Cytisine and N-methyl Cytisine which are the major constituents of Retama monosperma seeds alkaloid extract (AERS) as eco-friendly inhibitors of corrosion of carbon steel in acidic medium HCl (1M), using the density functional theory (DFT) at B3LYP/aug-cc-pvdz levels. Indeed, this alkaloid extract AERS has proven experimentally its corrosion inhibitory power for carbon steel in 1 M HCl even at lower concentration (94.42% at 400[Formula: see text]mg/L). Besides, its adsorption on the steel surface in 1 M HCl solution followed Langmuir’s isotherm [El Hamdani N, Fdil R, Tourabi M, Jama C, Bentiss F, Alkaloids extract of Retama monosperma (L.) Boiss. seeds used as novel eco-friendly inhibitor for carbon steel corrosion in 1 M HCl solution: Electrochemical and surface studies, Appl Surf Sci 357:1294–1305, 2015, doi: 10.1016/j.apsusc.2015.09.159; El Hamdani N, Contribution a la valorisation des parties aériennes de Retama Monosperma, phytochimie, activités antimicrobiennes et propriétés anticorrosives, Universite Chouaib Doukkali Faculte des sciences El jadida, 2015]. Herein, the major aim of this work is to investigate the likelihood of the carbon steel corrosion being inhibited by the protonated species in the aqueous acidic solution. For this purpose, after proving that the most favorable protonation site for protonation was N9, we have evaluated the structural, global and local reactivity descriptors, partial atomic charge in terms of Natural bond orbital (NBO) of the three protonated inhibitors at the level of N9, which are the most stable in the aqueous acidic solution. NBO analysis and the thermodynamic properties such as entropy, enthalpy, heat capacity and Gibbs free energy were also done. In conclusion, it was observed that three protonated molecules showed a high propensity to adsorb to the mild steel surface with a priority of the molecule d-CytN9H+.

Synthesis, Characterization and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand

<div>A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal </div><div>pendadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3, </div><div>CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These </div><div>diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also </div><div>remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic </div><div>techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic </div><div>cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-</div><div>cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-</div><div>hexenyl group to the methylcyclopentyl derivative over the course of several hours. </div><div>Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a </div><div>very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can </div><div>accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent, </div><div>leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for </div><div>Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to </div><div>divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand </div><div>as opposed to protonation of the methyl group. The produce of protonation at nitrogen is </div><div>the dimeric species 4 which was prepared via separate synthesis and characterized by Xray crystallography.</div>

Synthesis, Characterization and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pendadentate Ligand

<div>A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal </div><div>pendadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3, </div><div>CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These </div><div>diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also </div><div>remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic </div><div>techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic </div><div>cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-</div><div>cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-</div><div>hexenyl group to the methylcyclopentyl derivative over the course of several hours. </div><div>Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a </div><div>very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can </div><div>accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent, </div><div>leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for </div><div>Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to </div><div>divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand </div><div>as opposed to protonation of the methyl group. The produce of protonation at nitrogen is </div><div>the dimeric species 4 which was prepared via separate synthesis and characterized by Xray crystallography.</div>

Synthesis, Characterization and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand

<div>A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal </div><div>pendadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3, </div><div>CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These </div><div>diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also </div><div>remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic </div><div>techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic </div><div>cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-</div><div>cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-</div><div>hexenyl group to the methylcyclopentyl derivative over the course of several hours. </div><div>Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a </div><div>very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can </div><div>accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent, </div><div>leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for </div><div>Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to </div><div>divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand </div><div>as opposed to protonation of the methyl group. The produce of protonation at nitrogen is </div><div>the dimeric species 4 which was prepared via separate synthesis and characterized by Xray crystallography.</div>

Microhydration of protonated biomolecular building blocks: protonated pyrimidine

The protonation site and evolution of the hydration network in microsolvated protonated pyrimidine clusters, H+Pym–(H2O)n with n = 1–4, has been explored by infrared spectroscopy and density functional theory calculations.

UV Spectrophotometric Determination of Thermodynamic Dissociation Constants of Some Aromatic Hydrazones in Acid Media

The spectral behavior of some p-nitro-p-substituted benzoylhydrazones in the perchloric acid media was followed, applying the UV spectroscopy. The position of the absorption maximum in the spectra was defined in acidic media and the electronic transitions were discussed, as well (7<pH<1). The equilibrium between neutral and protonated form was investigated in the ethanol-water (V/V, 1:1) solutions. The observed changes in the UV spectra suggested that protonation process took place in one step. The pH region of protonation ranges between 1.4 and 2.9. Using the changes in the UV spectra which appear as a result of the protonation reaction the stoichiometric dissociation constants were determined numerically (pKBH+ = n·pH + logI) and graphically (intercept of the dependence of logI on pH). Thermodynamic dissociation constants were estimated as an intercept of dependence of pKBH+ on square root of the ionic strength. In order to achieve that, measurements were performed at different ionic strengths: 0.1, 0.25 and 0.5 mol/dm3, adjusted with sodium perchlorate. The obtained thermodynamic pKBH+ values ranged between 2.07 and 2.58. In order to predict proton transfer at a given pH, semiempirical methods AM1 and PM3 were applied. The influence of the substituents present in the p-position of the benzene ring on pKBH+ values of investigated hydrazones was discussed, too. Total energy, binding energy, enthalpy of formation, Gibbs energies of formation, atomic charge and proton affinity values were used to predict protonation site in hydrazone molecule. Furthermore, the stability and the proton affinity of the isomers (E and Z) in which hydrazones exist and their protonated forms were defined.