pKa values in organic chemistry – Making maximum use of the available data

General and Mild Method for the Synthesis of Polythioesters from Lactone Feedstocks

Polythioesters are attracting increasing interest in applications requiring degradability or recyclability. However, few general methods exist for the synthesis of these polymers. This report presents a fast and versatile method for synthesizing polythioesters from readily available lactone feedstocks. The two-step process begins with the thionation of lactones to thionolactones, followed by the ring-opening polymerization of the thionolactones to polythioesters. Unlike previous methods that rely on harsh reagents to accomplish this transformation, we demonstrate that the mild tetrabutylammonium thioacetate is a competent initiator for polymerization. This method exhibits broad applicability, as demonstrated by the successful polymerizations of an unstrained 17-membered macrocycle and an N-substituted cyclic thionocarbamate. Furthermore, the generality of this scheme enables the synthesis of polythioesters with highly tunable properties, as demonstrated here by the synthesis of a set of polymers with glass transition temperatures spanning 180 °C. Finally, the polythioesters are efficiently depolymerized into the corresponding thiolactones.

Exchange coupling states of cobalt complexes to control proton-coupled electron transfer

The electrochemical proton reactivity of transition metal complexes receives significant attentions. A thorough understanding of proton-coupled electron transfer (PCET) pathways is essential for elucidating the mechanism behind a proton reduction reaction, and controlling the pathway is a key focus in the field of the catalyst development. Spin interactions within complexes, which arise during electron transfer, can affect significantly the PCET pathway. Herein, we explore the phenomenon of spin rearrangement during the electrochemical reorganization of high-spin cobalt complexes. Our findings reveal that opposing spin interactions, induced by different coordination environments, can alter the PCET pathway. Finally, detailed analysis of the PCET pathway allows us to propose mechanisms for proton reduction in high-spin cobalt complexes.

The Hydronaphthalide Monoanion: Isolation of the "red transient" Birch Intermediate from liquid Ammonia

Birch reactions employing alkali metals in ammonia have been a well-established method for the reduction and functionalisation of aromatic compounds for nearly 100 years. Speculations regarding intermediates in the reaction pathway have been discussed since the beginning. We hereby report the isolation of NMe4(HNaph) (1), a kinetically trapped intermediate of the Birch reaction of naphthalene and sodium in liquid ammonia. 1 has been fully characterised and has been shown to continue to react to 1,2/1,4-dihydronaphthalene - the Birch product of the reduction of naphthalene. The reactivity of 1 was investigated towards activity as an electron and hydride transfer agent with both, elements and small organic molecules. 1 demonstrates a tamed reduction potential which allows for controlled reactions such as the selective formation of hexasulphide and hexaselenide anions.

Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

A μ-oxo diiron complex, featuring the pyridine-2,6-dicarboxamide-based thiazoline-derived redox-active ligand, H2L (H2L = N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide), was synthesized and thoroughly characterized. [FeIII-(μ-O)-FeIII] showed electrocatalytic hydrogen evolution reaction activity in the presence of different organic acids of varying pKa values in dimethylformamide. Through electrochemical analysis, we found that [FeIII-(μ-O)-FeIII] is a precatalyst that undergoes concerted two-electron reduction to generate an active catalyst. Fourier transform infrared spectrum of reduced species and density functional theory (DFT) investigation indicate that the active catalyst contains a bridged hydroxo unit which serves as a local proton source for the Fe(III) hydride intermediate to release H2. We propose that in this active catalyst, the thiazolinium moiety acts as a proton-transferring group. Additionally, under sufficiently strong acidic conditions, bridged oxygen gets protonated before two-electron reduction. In the presence of exogenous acids of varying strengths, it displays electro-assisted catalytic response at a distinct applied potential. Stepwise electron-transfer and protonation reactions on the metal center and the ligand were studied through DFT to understand the thermodynamically favorable pathways. An ECEC or EECC mechanism is proposed depending on the acid strength and applied potential.

pKaH values and θH angles of phosphanes to predict their electronic and steric parameters

Phosphanes play an important role in various applications, serving as a class of organic bases with basicities spanning more than 30 orders of magnitude. Accessing comprehensive basicity data for phosphanes has been challenging due to scattered information across multiple sources and notable gaps in the existing data. In this report, we present basicities (pKaH values) of a diverse set of phosphanes, both newly measured or calculated and collected from the literature. We demonstrate that pKaH values can serve as an alternative to Tolman electronic parameters (TEP values) in evaluating the electronic properties of phosphanes. Additionally, we suggest parameters for assessing the steric properties of phosphanes without the need for preparation or calculation of metal-ligand complexes.

Highly Selective O-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket

Organocatalyzed ring-opening polymerization (ROP) is a versatile technique for synthesizing biodegradable polymers, including polyesters and polycarbonates. We introduce o-phenylene bisurea (OPBU) (di)anions as a novel class of organocatalysts that are fast, easily tunable, mildly basic, and exceptionally selective. These catalysts surpass previous generations, such as thiourea, urea, and TBD, in selectivity (kp/ktr) by 8 to 120 times. OPBU catalysts facilitate the ROP of various monomers, achieving high conversions (>95%) in seconds to minutes, producing polymers with precise molecular weights and very low dispersities (Đ ≈ 1.01). This performance nearly matches the ideal distribution expected from living polymerization (Poisson distribution). Density functional theory (DFT) calculations reveal that the catalysts stabilize the oxyanion transition state via a hydrogen bond pocket similar to the "oxyanion hole" in enzymatic catalysis. Both experimental and theoretical analyses highlight the critical role of the semirigid o-phenylene linker in creating a hydrogen bond pocket that is tight yet flexible enough to accommodate the oxyanion transition state effectively. These new insights have provided a new class of organic catalysts whose accessibility, moderate basicity, excellent solubility, and unparalleled selectivity and tunability open up new opportunities for controlled polymer synthesis.

Convergent approach for direct cross-coupling enabled by flash irreversible generation of cationic and anionic species

In biosynthesis multiple kinds of reactive intermediates are generated, transported, and reacted across different parts of organisms, enabling highly sophisticated synthetic reactions. Herein we report a convergent synthetic approach, which utilizes dual intermediates of cationic and carbanionic species in a single step, hinted at by the ideal reaction conditions. By reactions of unsaturated precursors, such as enamines, with a superacid in a flow microreactor, cationic species, such as iminium ions, are generated rapidly and irreversibly, and before decomposition, they are transported to react with rapidly and independently generated carbanions, enabling direct C-C bond formation. Taking advantage of the reactivity of these double reactive intermediates, the reaction take place within a few seconds, enabling synthetic reactions which are not applicable in conventional reactions.

Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.

Thermoswitchable catalysis to inhibit and promote plastic flow in vitrimers

Acid-base catalysis is a common strategy to induce covalent bond exchanges in dynamic polymer networks. Strong acids or strong bases can promote rapid network rearrangements, and are simultaneously preferred catalysts for chemical reactions where maximum efficiency at the lowest possible temperature is aimed for. However, within the context of dynamic polymer networks, the incorporation of highly active catalysts can negatively affect the longer term application potential. Network dynamicity can diminish through catalyst ageing or quenching and highly active catalysts may prematurely activate bond exchanges, leading to dimensional instability and thus low creep resistance of the polymer networks. Herein, we present several examples where we explicitly explored weak acids (carboxylic acids) as catalysts for dynamic bond exchanges, using vinylogous urethanes (VU) as a well-understood protic acid catalysed vitrimer chemistry. Surprisingly, we have found that the sought-after long-term stability offered by a weak acid does not necessarily bring lower activity at high temperature. In fact, the weak acids show a remarkable thermoswitchable catalytic behaviour, going from an inactive hydrogen bonded state to an active state where the polymer matrix is protonated, with a profound impact on the network reactivity and rheology. Carboxylic acids with different electronic or steric environments show clear reactivity trends and their fine-tuning resulted in the most thermally responsive VU vitrimers studied to date. Our findings point out that catalyst choice and design for vitrimers is only poorly informed by catalyst performance in more traditional chemical reactions (in solvent), and that a more tailored catalyst design holds great promise for the field of vitrimers.

Phenol as proton shuttle and buffer for lithium-mediated ammonia electrosynthesis

Ammonia is a crucial component in the production of fertilizers and various nitrogen-based compounds. Now, the lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising approach for ammonia synthesis at ambient conditions. The proton shuttle plays a critical role in the proton transfer process during Li-NRR. However, the structure-activity relationship and design principles for effective proton shuttles have not yet been established in practical Li-NRR systems. Here, we propose a general procedure for verifying a true proton shuttle and established design principles for effective proton shuttles. We systematically evaluate several classes of proton shuttles in a continuous-flow reactor with hydrogen oxidation at the anode. Among the tested proton shuttles, phenol exhibits the highest Faradaic efficiency of 72 ± 3% towards ammonia, surpassing that of ethanol, which has been commonly used so far. Experimental investigations including operando isotope-labelled mass spectrometry proved the proton-shuttling capability of phenol. Further mass transport modeling sheds light on the mechanism.

Thermodynamic and Kinetic Activity Descriptors for the Catalytic Hydrogenation of Ketones

Activity descriptors are a powerful tool for the design of catalysts that can efficiently utilize H2 with minimal energy losses. In this study, we develop the use of hydricity and H- self-exchange rates as thermodynamic and kinetic descriptors for the hydrogenation of ketones by molecular catalysts. Two complexes with known hydricity, HRh(dmpe)2 and HCo(dmpe)2, were investigated for the catalytic hydrogenation of ketones under mild conditions (1.5 atm and 25 °C). The rhodium catalyst proved to be an efficient catalyst for a wide range of ketones, whereas the cobalt catalyst could only hydrogenate electron-deficient ketones. Using a combination of experiment and electronic structure theory, thermodynamic hydricity values were established for 46 alkoxide/ketone pairs in both acetonitrile and tetrahydrofuran solvents. Through comparison of the hydricities of the catalysts and substrates, it was determined that catalysis was observed only for catalyst/ketone pairs with an exergonic H- transfer step. Mechanistic studies revealed that H- transfer was the rate-limiting step for catalysis, allowing for the experimental and computation construction of linear free-energy relationships (LFERs) for H- transfer. Further analysis revealed that the LFERs could be reproduced using Marcus theory, in which the H- self-exchange rates for the HRh/Rh+ and ketone/alkoxide pairs were used to predict the experimentally measured catalytic barriers within 2 kcal mol-1. These studies significantly expand the scope of catalytic reactions that can be analyzed with a thermodynamic hydricity descriptor and firmly establish Marcus theory as a valid approach to develop kinetic descriptors for designing catalysts for H- transfer reactions.

Improved pH measurement of mobile phases in reversed-phase liquid chromatography

Mobile phase pH is a critically important parameter in reversed-phase liquid chromatographic (RPLC) separations involving analytes that display acidic or basic properties in the pH range used for the mobile phase. The main problem in measuring mobile phase pH lies in the fact that RPLC mobile phases are typically aqueous-organic mixtures. In addition to experimental difficulties, the pH values refer to different aqueous-organic compositions that cannot be correctly compared. Given this situation, the unified pH (wabspH, also termed as ) based on the absolute chemical potential of the solvated proton has been proposed as a rigorous way of characterising mobile phase acidity that is fully inter-comparable between mobile phases of any composition. Here we report the wabspH values of 78 reversed-phase liquid chromatography-mass spectrometry mobile phases that were carefully measured by potential differences in a symmetric cell with two glass electrode half-cells and almost ideal ionic liquid triethylamylammonium bis((trifluoromethyl)sulfonyl)imide [N2225][NTf2] salt bridge with multiple overlapping measurements. The system of altogether 300 ΔwabspH values was anchored to the pH value of standard pH 7.00 aqueous buffer solution. The consistency standard deviation of the whole set of measurements was 0.09 pH units. In addition to the differential potentiometric reference method, simpler measurement methods that use double junction reference or double junction combined electrodes were tested and were found suitable for routine laboratories if high accuracy is not required.

Co-Catalyzed Hydrofluorination of Alkenes: Photocatalytic Method Development and Electroanalytical Mechanistic Investigation

The hydrofluorination of alkenes represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)-F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and the poor nucleophilicity of fluoride. In this study, we present a new Co(salen)-catalyzed hydrofluorination of simple alkenes utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a photoredox-mediated polar-radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radiopharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calculations, unveiled the involvement of both carbocation and CoIV-alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene.

Radical-Smiles Rearrangement by a Vitamin B2-Derived Photocatalyst in Water

Herein, we report a catalytic radical-Smiles rearrangement system of arene migration from ether to carboxylic acid with riboflavin tetraacetate (RFT), a readily available ester of natural vitamin B2, as the photocatalyst and water as a green solvent, being free of external oxidant, base, metal, inert gas protection, and lengthy reaction time. Not only the known substituted 2-phenyloxybenzoic acids substrates but also a group of naphthalene- and heterocycle-based analogues was converted to the corresponding aryl salicylates for the first time. Mechanistic studies, especially a couple of kinetic isotope effect (KIE) experiments, suggested a sequential electron transfer-proton transfer processes enabled by the bifunctional flavin photocatalyst.

Synthesis and evaluation of aromatic stationary phases based on linear solvation energy relationship model for expanded application in supercritical fluid chromatography

In the last decade, the separation application based on aromatic stationary phases has been demonstrated in supercritical fluid chromatography (SFC). In this paper, four aromatic stationary phases involving aniline (S-aniline), 1-aminonaphthalene (S-1-ami-naph), 1-aminoanthracene (S-1-ami-anth) and 1-aminopyrene (S-1-ami-py) were synthesized based on full porous particles (FPP) silica, which were not end-capped for providing extra electrostatic interaction. Retention mechanism of these phases in SFC was investigated using a linear solvation energy relationship (LSER) model. The aromatic stationary phases with five positive parameters (a, b, s, e and d+) can provide hydrogen bonding, π-π, dipole-dipole and cation exchange interactions, which belong to the moderate polar phases. The LSER results obtained using routine test solutes demonstrated that the aforementioned interactions of four aromatic stationary phases were influenced by the type and bonding density of the ligand, but to a certain extent. Furthermore, the LSER data verified that the S-1-ami-anth column based on full porous particles silica had higher cation exchange capacity (d+ value), compared to the commercialized 1-AA column (based on the ethylene-bridged hybrid particles). The relationship between the d+ value and SFC additive was quantitatively proved so as to regulate electrostatic interaction reasonably. This value was greatly increased by phosphoric acid, slightly increased by trifluoroacetic acid and formic acid, but significantly reduced by ammonium formate and diethylamine. Taking the S-1-ami-naph column as an example, better peek shape of the flavonoids was obtained after the addition of 0.1 % phosphoric acid in MeOH while isoquinoline alkaloids were eluted successfully within 11 min after adding 0.1 % diethylamine in MeOH. Combined with the unique π-π interaction and controllable electrostatic interaction, the aromatic stationary phases in this study have been proven to have expandable application potential in SFC separation.

Acid-base equilibrium in non-aqueous medium: colorimetric visualization, estimation of acidity constants and construction of molecular logic gates

A reversible acid-base probe, (N1E, N4E)-N1, N4-bis((Z)-3-(4-(dimethylamino)phenyl)allylidene)benzene-1,4-diamine (MM1), is introduced for the colorimetric visualization of acid-base equilibria in non-aqueous media. MM1 displays reversible acidochromic behavior, showing exciting colorimetric change varying from weak to strong acid. Also, we have fabricated a colorimetric paper strip-based test kit to visualize acid-base equilibria. A dipstick experiment has been demonstrated to visualize the acid-base equilibria in the gaseous state. This acid-base probe has also been employed to estimate the pKa values of several acidic compounds in a non-aqueous medium using overlapping indicator methods. Based on reversible acidochromic UV-visible absorption spectral and colorimetric behavior, we have constructed a reconfigurable dual input and dual output combinational logic circuit and set-reset memorized device employing acid and base as chemically encoded inputs and corresponding optical outputs. The current report evokes a new protocol for developing various reversible acidochromic probes and its implication for constructing opto-chemical molecular logic gates and estimating the acid dissociation constants of various acidic compounds in non-aqueous media.

Step-by-Step Replacement of Cyano Groups by Tricyanovinyls-The Influence on the Acidity

Acid-base properties are the simplest expression of compounds' coordinating ability. In the present work, we studied in silico how the gas-phase Brønsted acidity (GA) of several polycyano-substituted compounds change when cyano (CN) groups are replaced by 1,2,2-tricyanovinyl (TCNV) groups in (iso)cyanic acid, dicyanoamine, cyanoform, and hydrogen tetracyanoborate. Different tautomers and conformers/isomers are included in this study. Gas-phase acidity values are compared with the acidities of various acids, including percyanated protonated monocarba-closo-dodecaborate (carborane acid) and dodecaborate, as well as hydrogen cyanide and 1,2,2-tricyanoethene. An estimation of acetonitrile (MeCN), dimethylsufoxide (DMSO), and 1,2-dichloroethane (DCE) acidities is presented using the COSMO-RS method and correlation analysis. The strongest acid with four TCNV groups shows remarkable acidic properties.

Base-catalyzed diastereodivergent thia-Michael addition to chiral β-trifluoromethyl-α,β-unsaturated N-acylated oxazolidin-2-ones

Base-catalyzed diastereodivergent thia-Michael addition of thiols to chiral β-trifluoromethyl-α,β-unsaturated N-acylated oxazolidin-2-ones is reported. By tuning the base-catalyst (i-Pr2NEt, DABCO, or P2-t-Bu), a range of chiral thia-Michael adducts was synthesized in good yields with high diastereoselectivities. A plausible mechanism was proposed on the basis of the experimental results. This work is complementary to the existing methods offering advantages, e.g., switchable diastereoselectivity using a readily synthesized chiral starting material, a cheap and readily available base catalyst, and a simple and practical operation, enabling synthetic application in organic synthesis.

Metal-Ligand Proton Tautomerism, Electron Transfer, and C(sp3)-H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex

The para-N-pyridyl-based PCP pincer proligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN-tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners, e.g., (tBuPCP)IrHCl and derivatives, 2-H is highly air-sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp3)-H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 2-H is calculated to undergo metal-ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (with a negative charge on Ir) and a p-N-pyridylidene (a remote N-heterocyclic carbene) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (ΔΔG° = -31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2• which is observed by NMR spectroscopy. 2• can be further oxidized to give cationic Ir(III) complex, 2+, which can oxidatively add a phosphino-t-butyl C-H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of 2+ would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergo facile β-H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation driven by one-electron oxidation and deprotonation, enabled by MLPT.

Kinetics and Mechanism of Azole n-π*-Catalyzed Amine Acylation

Azole anions are highly competent in the activation of weak acyl donors, but, unlike neutral (aprotic) Lewis bases, are not yet widely applied as acylation catalysts. Using a combination of in situ and stopped-flow 1H/19F NMR spectroscopy, kinetics, isotopic labeling, 1H DOSY, and electronic structure calculations, we have investigated azole-catalyzed aminolysis of p-fluorophenyl acetate. The global kinetics have been elucidated under four sets of conditions, and the key elementary steps underpinning catalysis deconvoluted using a range of intermediates and transition state probes. While all evidence points to an overarching mechanism involving n-π* catalysis via N-acylated azole intermediates, a diverse array of kinetic regimes emerges from this framework. Even seemingly minor changes to the solvent, auxiliary base, or azole catalyst can elicit profound changes in the temporal evolution, thermal sensitivity, and progressive inhibition of catalysis. These observations can only be rationalized by taking a holistic view of the mechanism and a set of limiting regimes for the kinetics. Overall, the analysis of 18 azole catalysts spanning nearly 10 orders of magnitude in acidity highlights the pitfall of pursuing ever more nucleophilic catalysts without regard for catalyst speciation.

Solution Studies of a Water-Stable, Trivalent Antimony Pnictogen Bonding Anion Receptor with High Binding Affinities for CN-, OCN-, and OAc. Inorg Chem 2023. [PMID: 37499143 DOI: 10.1021/acs.inorgchem.3c01887] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis.

Synthesis, Structural Characterization, and CO2 Reactivity of a Constitutionally Analogous Series of Tricopper Mono-, Di-, and Trihydrides

The formation of hydrides at heterogeneous copper surfaces results in dramatic structural and reactivity changes, yet the morphologies of these materials and their respective roles in catalysis are not well understood. Of particular interest is the reactivity of heterogeneous copper hydrides with carbon dioxide (CO₂), an early mechanistic branching point in the CO₂ reduction reaction. Herein, we report the synthesis, characterization, and reactivity of tricopper compounds supported by a facially biased, chelating tris(carbene) ligand scaffold. This sterically bulky environment affords access to an isolable series of tricopper hydrides: [LCu₃H]²⁺ (4), [LCu₃H₂]⁺ (3), and LCu₃H₃ (6). Single-crystal X-ray diffraction and solution NMR spectroscopy studies reveal both geometric flexibility within the Cu₃ core and fluxionality of hydride ligands across the Cu₃ cluster, providing both atomically precise experimental analogues of static surface species and emulating dynamic ligand behavior proposed for surfaces. Electronic structure calculations serve as a predictor of hydricity, which was likewise benchmarked experimentally via both protonolysis and hydride abstraction reactions. Increased hydride number (and commensurately lower cluster charge) results in more hydridic complexes, with a thermodynamic hydricity range spanning >30 kcal/mol. These thermochemical studies serve as an accurate predictor of CO₂ reactivity. Together, this Cu₃H_x series exhibits the structure/reactivity relationships proposed for catalytically active copper surfaces, validating the application of carefully designed molecular clusters toward elucidating mechanisms in surface science.

Voltammetric Detection of Vanillylmandelic Acid and Homovanillic Acid Using Urea-Derivative-Modified Graphite Electrode

Vanillylmandelic acid (VMA) and homovanillic acid (HVA) are diagnostic markers of neuroblastoma. The purpose of this study was to understand the reason for the discrimination of structural analogues (VMA and HVA) onto a graphite electrode coated with an electrochemically oxidized urea derivative. Density functional theory calculations (DFT), FTIR spectroscopic measurements, and electrochemical impedance spectroscopic measurements were used in this work. Density functional theory calculations (DFT) were used to identify the most suitable binding sites of the urea derivative and to describe possible differences in its interaction with the studied analytes. The FTIR measurement indicated the enhancement and disappearance of NH vibrations on graphite and platinum surfaces, respectively, that could be connected to a different orientation and thus provide accessibility of the urea moiety for the discrimination of carboxylates. Additionally, the higher the basicity of the anion, the stronger the hydrogen-bonding interaction with -NH-groups of the urea moiety: VMA (pK_b = 10.6, K_Ads = (5.18 ± 1.95) × 10⁵) and HVA (pK_b = 9.6, K_Ads = (4.78 ± 1.58) × 10⁴). The differential pulse voltammetric method was applied to detect VMA and HVA as individual species and interferents. As individual analytes, both HVA and VMA can be detected at a concentration of 1.99 × 10^-5 M (RSD ≤ 0.28, recovery 110-115%).

Acid-Base and Photocatalytic Properties of the CeO2-Ag Nanocomposites

In this work, CeO₂ nanoparticles, as well as CeO₂ nanocomposites with plasmonic silver nanoparticles, were synthesized using a simple sol-gel process. The concentration of silver in the composites varied from 0.031-0.25 wt%. Cerium hydroxide dried gel was calcined at temperatures from 125 to 800 °C to obtain CeO₂. It was shown that, at an annealing temperature of 650 °C, single-phase CeO₂ nanopowders with an average particle size in the range of 10-20 nm can be obtained. The study of acid-base properties showed that with an increase in the calcination temperature from 500 to 650 °C, the concentration of active centers with pKa 9.4 and 6.4 sharply increases. An analysis of the acid-base properties of CeO₂-Ag nanocomposites showed that with an increase in the silver concentration, the concentration of centers with pKa 4.1 decreases, and the number of active centers with pKa 7.4 increases. In a model experiment on dye photodegradation, it was shown that the resulting CeO₂ and CeO₂-Ag nanopowders have photocatalytic activity. CeO₂-Ag nanocomposites, regardless of the silver concentration, demonstrated better photocatalytic activity than pure nanosized CeO₂.

Understanding the activity of glucose oxidase after exposure to organic solvents

Long-term stability of enzymes in organic solvents is one of the most challenging problems in modern biotechnology and chemical industries. However, the resistance of enzymes to organic solvents is not very well understood so far. Herein, the effects of apolar, chlorinated, and polar organic solvents on the activity and structure of glucose oxidase from Aspergillus niger were systemically investigated using spectrophotometric activity assay of this enzyme and absorption and chiroptical spectroscopy. Molecular dynamics simulations and correlation of the activity with properties of the organic solvents were employed to understand the effects of organic solvents on the enzyme. The experimental and theoretical results showed that apolar solvents reduce the enzyme activity because they facilitate its aggregation through inter-enzymatic salt bridges. Moreover, polar solvents strongly coordinate with amino acid residues in the glucose binding pocket and prevent binding of the substrates. We found that this enzyme is stable in pure apolar and chlorinated solvents and these solvents can be used for the functionalization of its residues. This work provides an in depth understanding at the molecular level of the impact of various pure organic solvents on the structure and dynamics of glucose oxidase and the regulation of its catalytic activity.

Probing the reactivity of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with metal cations and acids in acetonitrile by electrochemistry and UV-Vis spectroscopy

2,2-Diphenyl-1-picrylhydrazyl (DPPH) is certainly one of the most widely used free radicals in several applications, because of its high stability. Unfortunately, there are few works dealing with its stability in the presence of many chemical species that coexist during chemical processes. In this work, the stability of DPPH was investigated by electrochemistry and UV-Vis spectroscopy in the presence of some metal cations (Cu²⁺ and Zn²⁺) and acids (HClO₄ and HNO₃) in acetonitrile. In the presence of Cu²⁺, DPPH was oxidized to DPPH⁺ with the formation of an equivalent amount of Cu⁺. With Zn²⁺, DPPH undergoes a slow disproportionation with the formation of Zn(DPPH)⁺ and DPPH⁺, certainly favored by the acidity of the metal cation. This hypothesis was subsequently confirmed by studying the stability of DPPH in the presence of HClO₄. This acid of appreciable strength in acetonitrile (pK_a = 1.83) causes a fast disproportionation of DPPH with the formation of DPPH-H and DPPH⁺. This mechanism was confirmed both by UV-Vis spectroscopy and by electrochemistry, with a stoichiometry corresponding to 2 equivalents of DPPH for about 1 equivalent of HClO₄. In the presence of nitric acid, which is about 10⁷ weaker than HClO₄ in acetonitrile, the disproportionation was much slower. These preliminary results are proof that many chemical species are likely to react with DPPH and indirectly induce sources of bias during its application, especially when evaluating antioxidant properties.

Reductive Elimination Reactions in Gold(III) Complexes Leading to C(sp3)-X (X = C, N, P, O, Halogen) Bond Formation: Inner-Sphere vs SN2 Pathways

The reactions leading to the formation of C-heteroatom bonds in the coordination sphere of Au(III) complexes are uncommon, and their mechanisms are not well known. This work reports on the synthesis and reductive elimination reactions of a series of Au(III) methyl complexes containing different Au-heteroatom bonds. Complexes [Au(CF₃)(Me)(X)(PR₃)] (R = Ph, X = OTf, OClO₃, ONO₂, OC(O)CF₃, F, Cl, Br; R = Cy, X = Me, OTf, Br) were obtained by the reaction of trans-[Au(CF₃)(Me)₂(PR₃)] (R = Ph, Cy) with HX. The cationic complex cis-[Au(CF₃)(Me)(PPh₃)₂]OTf was obtained by the reaction of [Au(CF₃)(Me)(OTf)(PPh₃)] with PPh₃. Heating these complexes led to the reductive elimination of MeX (X = Me, Ph₃P⁺, OTf, OClO₃, ONO₂, OC(O)CF₃, F, Cl, Br). Mechanistic studies indicate that these reductive elimination reactions occur either through (a) the formation of tricoordinate intermediates by phosphine dissociation, followed by reductive elimination of MeX, or (b) the attack of weakly coordinating anionic (TfO^- or ClO₄^-) or neutral nucleophiles (PPh₃ or NEt₃) to the Au-bound methyl carbon. The obtained results show for the first time that the nucleophilic substitution should be considered as a likely reductive elimination pathway in Au(III) alkyl complexes.

HAT Catalysts to Facilitate Acid Mediated Cleavage of Biaryl C-C Bonds in Binaphthols

A method to cleave the C-C biaryl bond of binaphthyl derivatives under reductive conditions is described. Triflic acid employed together with a catalytic HAT reagent, 2-ethyl-9,10-dihydroxyanthracene, that is regenerated using H2 with catalytic Pd/C yielded monomeric products in improved yields. A range of substrates is disclosed, and kinetic analyses provide insight into the mechanism of aryl-aryl activation.

Universal Trends between Acid Dissociation Constants in Protic and Aprotic Solvents

pKa values in non-aqueous solvents are of critical importance in many areas of chemistry. Our knowledge is, despite their relevance, still limited to the most fundamental properties and few pKa values in the most common solvents. Taking advantage of a recently introduced computationally efficient procedure we computed the pKa values of 182 compounds in 21 solvents. This data set is used to establish for the first time universal trends between all solvents. Our computations indicate, that the total charge of the molecule and the charge of the acidic group combined with the Kamlet-Taft solvatochromic parameters are sufficient to predict pKa values with at least semi- quantitative accuracy. We find, that neutral acids such as alcohols are strongly affected by the solvent properties. This is contrasted by cationic acids like ammonium ions whose pKa is often almost completely independent from the choice of solvent.

Synthesis and Characterization of Newly Designed and Highly Solvatochromic Double Squaraine Dye for Sensitive and Selective Recognition towards Cu2

Synthesis and characterization of a novel and zwitterionic double squaraine dye (DSQ) with a unique D-A-A-D structure is being reported. Contrary to the conventional mono and bis-squaraine dyes with D-A-D and D-A-D-A molecular frameworks reported so far, DSQ dye demonstrated strong solvatochromism allowing for the multiple ion sensing using a single probe by judicious selection of the suitable solvent system. The DSQ dye exhibited a large solvatochromic shift of about 200 nm with color changes from the visible to NIR region with metal ion sensitivity. Utilization of a binary solvent consisted of dimethylformamide and acetonitrile (1:99, v/v), highly selective detection of Cu²⁺ ions with the linearity range from 50 μM to 1 nM and a detection limit of 6.5 × 10^-10 M has been successfully demonstrated. Results of the Benesi-Hildebrand and Jobs plot analysis revealed that DSQ and Cu²⁺ ions interact in the 2:1 molecular stoichiometry with appreciably good association constant of 2.32 × 10⁴ M^-1. Considering the allowed limit of Cu²⁺ ions intake by human body as recommended by WHO to be 30 μM, the proposed dye can be conveniently used for the simple and naked eye colorimetric monitoring of the drinking water quality.

Volatilization Potential of Per- and Poly-fluoroalkyl Substances from Airfield Pavements and during Recycling of Asphalt

Per- and poly-fluoroalkyl substances (PFAS) in water are typically present in their ionic (nonvolatile) forms; however, these can transition to their nonionic (volatile) forms when in contact with organic solvents and organic matrices. In particular, when PFAS are dissolved in organic solvents such as residues left from firefighting foams, fuels, and bitumen present in asphalt, the equilibrium between ionic and nonionic forms can trend toward more volatile nonionic forms of PFAS. We assessed the volatility of common PFAS based on calculated and available experimental data across ambient temperature ranges experienced by airfield pavements and at elevated temperatures associated with reworking asphalts for reuse. Volatilities are shown to be comparable to hydrocarbons in the semivolatile range, suggesting that volatilization is a viable loss mechanism for some PFAS that are nonvolatile in water. The present study points to future investigative needs for this unexplored mass loss mechanism and potential exposure pathway. Environ Toxicol Chem 2022;41:2202-2208. © 2022 Commonwealth of Australia. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Universal control of proton concentration using an electrochemically generated acid compatible with miniaturization

Controlling locally produced acidity in miniaturized spaces is of high importance to manage simultaneous chemical reactions. Here, we present a platform that hosts miniaturized micro-reactors each one enabling electrochemical control of the acidity in ∼nL volumes. We demonstrated the local control of chemical reactions with the deprotection of strong acid labile groups in a region of 150 μm of diameter of upstanding glass using high proton concentrations (∼10-1 M) and the acidity contrasts between the cell region and the outside. We demonstrated accurate control of the proton concentration in aqueous and organic solvents and the control of chemical reactions in organic electrolytes achieved with a sulfonated tetrafluoroethylene-based membrane, which isolates the acid generating electrodes from the reagents in the solution. The quantitative control of the acidity by faradaic currents was demonstrated by the calibration of carboxyfluorescein adjusted with external titrations and with a tautomer transition occurring at pH 4.2. To the best of our knowledge, this platform shows the best control of acidity in the smallest volume reported so far.

Efficient pKa Determination in a Nonaqueous Solvent Using Chemical Shift Imaging

pK_a is an important property of a molecule which impacts many fields, such as drug design, catalysis, reactivity, and environmental toxicity. It is often necessary to measure pK_a in nonaqueous media due to the poor solubility of an analyte in water, for example, many compounds of pharmaceutical interest. Although NMR methods to measure pK_a in water are well established, determining pK_a in organic solvents is laborious and problematic. We present an efficient one-shot method to determine the pK_a of an analyte in an organic solvent in a single measurement. Diffusion of an acid into a basic solution of the analyte and a set of pH indicators establishes a pH gradient in the NMR tube. The chemical shift of a pH sensitive resonance of the analyte and the pH of the solution are then determined simultaneously as a function of position along the pH gradient by recording a chemical shift image of the NMR tube. The pK_a of the analyte is then determined using the Henderson-Hasselbalch equation. The method can be implemented in any laboratory with a gradient equipped NMR high-field spectrometer and is demonstrated for a range of pharmaceutical compounds and inorganic phosphazene bases.

Dual-Silane Premodified Silica Nanoparticles-Synthesis and Interplay between Chemical, Mechanical, and Curing Properties of Silica-Rubber Nanocomposites: Application to Tire Tread Compounds

In silica-rubber based nanocomposites, a single organo-silicon is often used to compatibilize and covalently link silica to rubber. In this work, we have investigated the impact, at micro- and macroscales, of the decoupling of the hydrophobization and the coupling activity of silane by pretreating silica with two different silane chemistries. The first one, a mercaptosilane, is the coupling agent that promotes a covalent link between silica and rubber during the sulfur-mediated vulcanization reaction. The second one, an alkylsilane, aims to improve the silica dispersion. For both kind of silanes, we have varied the chain length and studied at macroscale the dynamic mechanical properties through the key indicators that are E'' as loss modulus, E' as storage modulus, and their respective ratio tan δ. The shorter silanes combination yielded an improvement in terms of wet grip indicators with tan δ at 0 °C increasing from 0.205 to 0.237 while maintaining rolling resistance indicators at the same level. We have evaluated the impact of the silane chemistry onto the cross-linking reactivity within the fabricated rubber-based nanocomposites by using moving-dye rheometer measurements (MDR). By purposely using atomic force microscopy (AFM), we have studied the silica dispersion in the matrix and the rubber/silica interface and provided the rationale explanation of the mechanical properties observed at the macroscale. AFM observation pointed out the existence of a soft interface around silica fillers when long alkylsilanes were used. We infer that this interface impacts the polymer-filler dynamic and subsequently affects the mechanical properties of the composite material.

How to Predict the pK a of Any Compound in Any Solvent

Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the pK a of rather few compounds in the most common solvents, and a simple yet truly general computational procedure to predict pK a's of any compound in any solvent is still missing. In this contribution, we describe such a procedure. Our method requires only the experimental pK a of a reference compound in water and a few standard quantum-chemical calculations. This method is tested through computing the proton solvation energy in 39 solvents and by comparing the pK a of 142 simple compounds in 12 solvents. Our computations indicate that the method to compute the proton solvation energy is robust with respect to the detailed computational setup and the construction of the solvation model. The unscaled pK a's computed using an implicit solvation model on the other hand differ significantly from the experimental data. These differences are partly associated with the poor quality of the experimental data and the well-known shortcomings of implicit solvation models. General linear scaling relationships to correct this error are suggested for protic and aprotic media. Using these relationships, the deviations between experiment and computations drop to a level comparable to that observed in water, which highlights the efficiency of our method.

9-Phosphatriptycene Derivatives: From Their Weak Basicity to Their Application in Frustrated Lewis Pair Chemistry

The accurate prediction of the basicity of tertiary phosphines in acetonitrile and water is reported by the linear correlation between computed ΔpK_a's obtained by density functional theory (DFT) and experimental values extracted from the literature. This method is applied to the prediction of pK_a values of 9-phosphatriptycene derivatives and showed that they are weaker Brønsted bases than their triphenylphosphine analogues. This lower reactivity is attributed to their high pyramidalization that increases their lone pair 3s character, stabilizing its energy level. Their potential application in frustrated Lewis pair chemistry is then considered by investigating the hydrogenation of 1,1-diphenylethylene by the tris(pentafluorophenyl)borane/1-chloro-9-phosphatriptycene frustrated Lewis pair.

Original Basic Activation for Enhancing Silica Particle Reactivity: Characterization by Liquid Phase Silanization and Silica-Rubber Nanocomposite Properties

Silica fillers are used in various nanocomposites in combination with silanes as a reinforcing filler. In tire technology, silica is generally functionalized before (pre-treated) or during mixing (in-situ silanization or post-treated). In both cases, a soft base catalyst (e.g., triethylamine or diphenyl guanidine, DPG) is typically used to accelerate and increase the yield of the silane/silica coupling reaction. In this study, we investigated how pre-treatments of silica particles with either strong amine or hydride bases impact the silanization of silica prior to or during SBR mixing for silica-rubber nanocomposite fabrication. Our findings are supported by molecular characterization (solid state ²⁹Si NMR, ¹H NMR and TGA), and scanning electron microscopy. In addition, the impact of these silica pre-treatments on a nanocomposite’s mechanical properties was evaluated using dynamic mechanical analysis (DMA).

Inducing a pH-dependent conformational response by competitive binding to Zn2+ of a series of chiral ligands of disparate basicity

Molecules that change shape in response to environmental conditions are central to biological molecular communication devices and their synthetic chemical analogues. Here we report a molecular system in which a series of chiral anionic ligands of differing basicity are selectively protonated according to the pH of the medium. A cationic circular dichroism (CD) reporter complex responds to anion binding by selecting one of two alternative enantiomeric conformations. Exploiting the principle that less basic anions have, in general, weaker electrostatic interactions than more basic anions, a set of three chiral acids with large (>5 unit) pK a differences and differing configurations were sequentially deprotonated in acetonitrile by addition of base, allowing the most basic anion in the mixture at any time to bind to the reporter complex. A characteristic CD output resulted, which changed in sign as the next-most basic anion was revealed by the next deprotonation in the series. Four cycles of switching between three ligand-bound states were achieved with minimal changes in signal magnitude, by alternating addition of base and acid. The pH-dependent conformational response was used to transduce a signal by appending to the binding site a 2-aminoisobutyric acid (Aib) oligomer, whose M or P helical conformation depended on the chirality of the bound ligand, and was reported by a remote 13C-labelled NMR reporter group. The multicomponent system thus converts a pH signal into a programmable conformational response which induces a remote spectroscopic effect.

Convenient and accurate insight into solution-phase equilibria from FlowNMR titrations

Solution phase titrations are made easy by multi-nuclear FlowNMR spectroscopy with automated, continuous titre addition to give accurate insights into Brønsted acid/base, hydrogen bonding, Lewis acid/base and metal/ligand binding equilibria under native conditions.

Acid catalysis through N-protonation in undistorted carboxamides: improvement of amide proton sponge acylating ability

Acid catalysis of weakly distorted or undistorted carboxamides in acyl-migration reactions proceeding through N-protonation is the process with low probability in contrast to O-protonation. This circumstance made the experimental study...

Determination of the aqueous pKa of very insoluble drugs by capillary electrophoresis: Internal standards for methanol-water extrapolation

A fast determination of acidity constants (pK_a) of very insoluble drugs has become a necessity in drug discovery process because it often produces molecules that are highly lipophilic and sparingly soluble in water. In this work the high throughput internal standard capillary electrophoresis (IS-CE) method has been adapted to the determination of pK_a of water insoluble compounds by measurement in methanol/aqueous buffer mixtures. For this purpose, the reference pK_a values for a set of 46 acid-base compounds of varied structure (internal standards) have been established in methanol-water mixtures at several solvent composition levels (with a maximum of 40% methanol). The IS-CE method has been successfully applied to seven test drugs of different chemical nature with intrinsic solubilities lower than 10^-6 M. pK_a values have been determined at different methanol/aqueous buffer compositions and afterwards Yasuda-Shedlovsky extrapolation method has been applied to obtain the aqueous pK_a. The obtained results have successfully been compared to literature ones obtained by other methods. It is concluded that the IS-CE method allows the determination of aqueous pK_a values using low proportions of methanol, becoming then more accurate in the extrapolation procedure than other reference methods.

Novel lipophilic fluorophores with highly acidity-dependent two-photon response

Lipophilic fluorophores are widely implemented in nonlinear microscopy; however, few existing membrane-specific probes combine high brightness of two-photon excited fluorescence (2PEF) with pH sensitivity. Here we describe four novel two-photon excited fluorophores, based on a coumarin 151 core structure, where lipophilicity is induced by a covalently attached phosphazene moiety. Changing the environmental acidity using trifluoromethanesulfonic (triflic) acid leads to profound changes in the linear fluorescence and 2PEF characteristics, due to chromophores' switching between neutral- and protonated forms. We characterize this dependence by measuring the two-photon absorption (2PA) spectra over the region λ 2PA = 550 - 1000 nm, observing 2PA cross sections of σ 2PA = 10 - 20 GM, with associated 2PEF brightness of 10 - 13 GM, in neutral solutions of both acetonitrile and n -octanol. Although quantum chemical modelling and NMR measurements show that, at high chromophore concentrations, protonation may be accompanied by a dimerization process, these dimers likely do not form at the lower concentrations used in optical spectroscopy.