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Nonstoichiometric pseudoprotic ionic liquids (NPPILs) are an emerging class of ionic liquids with interesting physical properties and intriguing prospects for technological applications. However, fundamental questions remain about the proton transfer equilibria that underlie their ionic character. We use a combination of nuclear magnetic resonance spectroscopy, infrared spectroscopy, and small-angle X-ray scattering to characterize the equilibria of trihexylamine/butyric acid and water/butyric acid mixtures. This combination of techniques offers considerable insight into how proton transfer changes with the composition of the mixture. Further, we construct a model based on information from ¹H and ¹³C NMR, which yields numerical values for the concentrations of all ions present in the trihexylamine/butyric acid mixture, and demonstrate that the results of the model are supported by data from other physical measurements. This is the first quantitative calculation of ionic concentrations in an NPPIL.

The ability to selectively reduce CO₂ to a particular product or mixture of products is expected to play a key role in mitigation strategies aiming to alleviate the devastating impact of this greenhouse gas in our climate and oceans. Among those, the production of liquid solar fuels from CO₂ and H₂O will likely need cascade strategies involving multiple catalysts carrying out different functions. This will require that the catalysts doing the initial CO₂ reduction steps deliver the right product or products to downstream catalysts. CO, H₂ and formate are the most common products in CO₂ reduction by molecular catalysts. Here, in this work, we demonstrate control over the selectivity of C₁ products in photochemical CO₂ reduction with the same catalyst, simply by changing the redox potential of the photosensitizer and/or the water concentration. Turnover numbers for CO generation with one of the photosensitizers under anhydrous conditions reached 85,000, one of the largest values reported to date. A combination of experimental results and DFT calculations show that control of the selectivity is achieved, in part, due to the interplay between regimes under kinetic or thermodynamic control. These regimes are largely dictated by the proton sources and the CO₂ reduction byproducts generated.

The hydrolysis of thermoplastic poly(ester urethane) (PEU) is convoluted by its block copolymer phase structure and competing hydrolytic sensitivities of multiple functional groups. The exact pathways for water ingress, water interaction with the material and ultimately the kinetics and order of functional group hydrolysis remain to be refined. Additional diagnostics are needed to enable deeper insight and deconvolution of material changes. In combination with GPC results, a promising analytical technique – two-dimensional correlation spectroscopy (2D-COS) – has been reviewed and applied to analyze FTIR spectra of hydrolyzed PEUs aged under various conditions, such as exposure time, temperature, and relative humidity. 2D-COS allows the complex role of water with distinct intermediate steps to be established, plus it emphasizes the initial stages of PEU hydrolysis at more susceptible functional groups. As a complication for the raw material, ATR IR detected some talc on the surface of commercial PEU beads and pressed sheets thereof, which can interfere with water ingress and thereby retards PEU hydrolysis, particularly in its natural form or moderate aging at lower temperatures (e.g., below the melting point of PEU). As aging temperature increases above the melting temperature, even traces of water trapped inside the PEU are sufficient to initiate the hydrolysis, which then progresses strongly with increasing temperatures. Feedback from 2D-COS analysis confirms that PEU hydrolysis starts at esters in the soft-segments before those in the urethane linkage become susceptible. Only when the molecular weight of PEU is below a critical molar mass (M_c) will the hydrolysis occur in parallel in the hard-segments since protective morphological phase structures are then absent. The current observations demonstrate unexpected behavior that may result from 'unknown' additives in polymer degradation, the temporal and group-specific hydrolysis of PEU as a function of locally available water molecules, the order of reactivity of susceptible functional groups, and the importance of changes in molecular weight coupled with the phase structure of the polymer.

Reagents capable of concerted proton–electron transfer (CPET) reactions can access reaction pathways with lower reaction barriers compared to stepwise pathways involving electron transfer (ET) and proton transfer (PT). To realize reductive multielectron/proton transformations involving CPET, one approach that has shown recent promise involves coupling a cobaltocene ET site with a protonated arylamine Brønsted acid PT site. This strategy colocalizes the electron/proton in a matter compatible with a CPET step and net reductive electrocatalysis. To probe the generality of such an approach a class of C,C'-diaryl-ocarboranes is herein explored as a conceptual substitute for the cobaltocene subunit, with an arylamine linkage still serving as a colocalized Brønsted base suitable for protonation. The featured ocarborane (PhCbPh^N) can be reduced and protonated to generate an N–H bond with a weak effective bond dissociation free energy (BDFE_eff) of 31 kcal/mol, estimated with measured thermodynamic data. This N–H bond is among the lowest measured element–H bonds for analyzed nonmetal compounds. Distinct solid-state crystal structures of the one- and two-electron reduced forms of diaryl-o-carboranes are disclosed to gain insight into their well-behaved redox characteristics. The singly reduced, protonated form of the diaryl-o-carborane can mediate multi-ET/PT reductions of azoarenes, diphenylfumarate, and nitrotoluene. In contrast to the aforementioned cobaltocene system, available mechanistic data disclosed herein support these reactions occurring by a rate-limiting ET step and not a CPET step. A relevant hydrogen evolution reaction (HER) reaction was also studied, with data pointing to a PT/ ET/PT mechanism, where the reduced carborane core is itself highly stable to protonation.

Nickel-rich layered oxides stand as ideal cathode candidates for high specific capacity and energy density next-generation lithium-ion batteries. However, increasing the Ni content significantly exacerbates structural degradation under high operating voltage, which greatly restricts large-scale commercialization. While strategies are being developed to improve cathode material stability, little is known about the effects of electrolyte–electrode interaction on the structural changes of cathode materials. Here, using LiNiO₂ in contact with electrolytes with different proton-generating levels as model systems, we present a holistic picture of proton-induced structural degradation of LiNiO₂. Through ab initio molecular dynamics calculations based on density functional theory, we investigated the mechanisms of electrolyte deprotonation, protonation-induced Ni dissolution, and cathode degradation and the impacts of dissolved Ni on the Li metal anode surfaces. We show that the proton-transfer reaction from electrolytes to cathode surfaces leads to dissolution of Ni cations in the form of NiOOH_x, which stimulates cation mixing and oxygen loss in the lattice accelerating its layered-spinel–rock-salt phase transition. Migration of dissolved Ni²⁺ ions to the anode side causes their reduction into the metallic state and surface deposition. This work reveals that interactions between the electrolyte and cathode that result in protonation can be a dominant factor for the structural stability of Ni-rich cathodes. Considering this factor in electrolyte design should be of benefit for the development of future batteries.

Surfactant monolayers at sea spray aerosol (SSA) surfaces regulate various atmospheric processes including gas transfer, cloud interactions, and radiative properties. Most experimental studies of SSA employ a simplified surfactant mixture of long-chain fatty acids (LCFAs) as a proxy for the sea surface microlayer (SSML) or SSA surface. However, MCFAs make up nearly 30% of the FA fraction in nascent SSA. Given that LCFA monolayers are easily disrupted upon the introduction of chemical heterogeneity (such as mixed chain lengths), simple FA proxies are unlikely to represent realistic SSA interfaces. Integrating experimental and computational techniques, we characterize the impact that partially-soluble medium-chain fatty acids (MCFAs) have on the properties of atmospherically-relevant LCFA mixtures. We explore the extent to which the MCFA lauric acid (LA) is surface stabilized by varying acidity, salinity, and monolayer composition. We also discuss the impacts of pH on LCFA-assisted LA retention, where the presence of LCFAs may shift the surface-adsorption equilibria of laurate—the conjugate base—towards higher surface activities. Molecular dynamic simulations suggest a mechanism for the enhanced surface retention of laurate. We conclude that increased FA heterogeneity at SSA surfaces promotes surface activity of soluble FA species, altering monolayer phase behavior and impacting climate-relevant atmospheric processes.

The immobilization of molecular electrocatalysts on conductive electrodes is an appealing strategy for enhancing their overall activity relative to those of analogous molecular compounds. Here, in this study, we report on the interfacial electrochemistry of self-assembled two-dimensional nanosheets of graphene nanoribbons (GNR-2DNS) and analogs containing a Rh-based hydrogen evolution reaction (HER) catalyst (RhGNR-2DNS) immobilized on conductive electrodes. Proton-coupled electron transfer (PCET) taking place at N-centers of the nanoribbons was utilized as an indirect reporter of the interfacial electric fields experienced by the monolayer nanosheet located within the electric double layer. The experimental Pourbaix diagrams were compared with a theoretical model, which derives the experimental Pourbaix slopes as a function of parameter f, a fraction of the interfacial potential drop experienced by the redox-active group. Interestingly, our study revealed that GNR-2DNS was strongly coupled to glassy carbon electrodes (f = 1), while RhGNR-2DNS was not (f = 0.15). We further investigated the HER mechanism by RhGNR-2DNS using electrochemical and X-ray absorption spectroelectrochemical methods and compared it to homogeneous molecular model compounds. RhGNR-2DNS was found to be an active HER electrocatalyst over a broader set of aqueous pH conditions than its molecular analogs. We find that the improved HER performance in the immobilized catalyst arises due to two factors. First, redox-active bipyrimidine-based ligands were shown to dramatically alter the activity of Rh sites by increasing the electron density at the active Rh center and providing RhGNR-2DNS with improved catalysis. Second, catalyst immobilization was found to prevent catalyst aggregation that was found to occur for the molecular analog in the basic pH. Overall, this study provides valuable insights into the mechanism by which catalyst immobilization can affect the overall electrocatalytic performance.

Dual Ir^III/L_nNi^II metallaphotoredox catalyzed C(sp³)–C(sp²) cross-coupling reactions are widely assumed to proceed by photoinduced single electron transfer steps due to the highly oxidizing Ir^III* excited state (Ir^III = [Ir(dF(CF₃)ppy)₂(dtbbpy)]⁺[PF₆]^–; dF(CF₃)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; L_n = dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). Using time-resolved absorption and emission spectroscopy, we reveal that energy transfer between Ir^III* and various LnNi^II precatalysts and intermediates with k_q ≥ 10⁸ M^–1 s^–1 also drives catalysis. Specifically, the excited states of L_nNi^II dihalide precatalysts/organometallic intermediates accessible by energy transfer appear to drive bond homolysis, halogen radical elimination, and reductive elimination reactions that facilitate formation of cross-coupled products. Energy transfer dynamics consequently circumvent the need for photoinduced electron transfer, thereby extending substrate scopes to coupling partners that cannot be oxidized by Ir^III*. Within a cross-electrophile coupling model reaction between 4-bromobenzotrifluoride and bromocyclohexane, energy transfer activates the L_nNi^II precatalyst at early reaction times before nucleophilic reductants are present. In the absence of Ir^III, direct excitation of L_nNi^II(Br)₂ also activates the precatalyst to form a L_nNi^II(Br)(Aryl) intermediate. To compare energy transfer and electron transfer kinetics, we determined rate constants for reductive quenching by Br^– (k_SET = 4.1 × 10⁸ M^–1 s^–1) and for the subsequent electron transfer from reduced Ir^III•– to L_nNi^II(Br)₂ (k_SET = 4.1 × 10⁷ M^–1 s^–1) using Stern-Volmer analysis and pulse radiolysis, respectively. Energy transfer rate constants are competitive with the electron transfer rate constants and energy transfer is a parallel pathway within metallaphotoredox catalysis. Exploiting the energy transfer mechanism, we demonstrate highly selective cross-electrophile coupling between 4-chlorobenzotrifluoride and bromocyclohexane to form exclusively cross-coupled product. Here, with alkyl-trifluoroborate nucleophiles that do not reductively quench IrIII* emission, transmetalation with L_nNi^II(Br/Cl)(Aryl) followed by energy transfer also drives excited state reductive elimination to form C(sp³)–C(sp²) cross-coupled product. Similarly, energy transfer rather than Ni^II oxidation drives C(sp²)–OR reductive elimination, despite the strongly oxidizing ability of Ir^III*. In total, these reactions demonstrate energy transfer processes from Ir^III* to L_nNi^II in metallaphotoredox catalysis that can unlock alternative reactive pathways.

We report the [^natMn/⁵²Mn]Mn(II) complexes of the macrocyclic chelators PYAN [3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane] and CHXPYAN [(4¹R,4²R,10¹R,10²R)-3,5,9,11-tetraaza-1,7(2,6)-dipyridina-4,10(1,2)-dicyclohexanacyclododecaphane]. The X-ray crystal structures of Mn-PYAN and Mn-CHXPYAN evidence distorted octahedral geometries through coordination of the nitrogen atoms of the macrocycles. Cyclic voltammetry studies evidence reversible processes due to the Mn(II)/Mn(III) pair, indicating that the complexes are resistant to oxidation. CHXPYAN forms a more thermodynamically stable and kinetically inert Mn(II) complex than PYAN. Radiochemical studies with the radioactive isotope manganese-52 (⁵²Mn, t_1/2 = 5.6 days) evidenced better radiochemical yields for CHXPYAN than for PYAN. Both [⁵²Mn]Mn(II) complexes remained stable in mouse and human serum, so in vivo stability studies were carried out. Positron emission tomography/computed tomography scans and biodistribution assays indicated that [⁵²Mn]Mn-PYAN has a distribution pattern similar to that of [⁵²Mn]MnCl₂, showing persistent radioactivity accumulation in the kidneys. Conversely, [⁵²Mn]Mn-CHXPYAN remained stable in vivo, clearing quickly from the liver and kidneys.

Electrical doping of organic semiconductors (OSCs) can be achieved using simple one-electron reductants and oxidants as n- and p-dopants, respectively, but for such dopants, increased doping strength is accompanied by increased sensitivity to ambient moisture and/or oxygen. “Indirect” or “complex” dopants—defined here as those that generate OSC radical cations or anions via pathways more complex than a single simple electron transfer, i.e., by multistep reactions—represent a means of circumventing this problem. Further, this review highlights the importance of understanding the reaction mechanisms by which such dopants operate for: (i) ensuring a researcher knows the composition of a doped material; (ii) predicting the thermodynamic feasibility of achieving doping with related dopant:OSC combinations; and (iii) predicting whether thermodynamically feasible doping reactions are likely to be rapid or slow, or to require subsequent activation. The mechanistic information available to date for some of the wide variety of complex n- and p-dopants that have been reported is then reviewed, emphasizing that in many cases our knowledge is far from complete.