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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.

In this Perspective, we highlight the emergence of target-oriented syntheses of complex molecules composed of Si–Si (oligosilanes) rather than C–C bonds. Saturated oligosilanes structurally resemble alkanes with respect to a tetrahedral geometry, a preference for a staggered conformation in linear chains, the ability to form stable small rings, and tetrahedral stereochemistry at asymmetrically functionalized Si centers. There are also critical differences, for example, differences in multiple bonding and the ability to form penta- and hexacoordinated structures, that mean that chemical reactivity and, in particular, rules for stereoselective synthesis do not cleanly translate from carbon to silicon. This Perspective will discuss recent achievements in the precise, controlled synthesis of complex molecules comprised mainly of Si–Si bonds and highlight the mechanistic insights enabling increased molecular complexity. New tools, such as electrochemical and catalytic reactions, will be discussed as well as the problem of controlling relative configuration in molecules containing multiple stereogenic-at-silicon centers. Furthermore, these synthetic achievements facilitate the discovery of new properties, including insight into light absorption, conformation, and mechanical properties.

Utilizing sunlight for photoelectrochemical carbon dioxide reduction reaction (PEC CO₂ RR) is a carbon-neutral path to valuable liquid fuels. Higher quality photoabsorbers are needed to improve the efficiency of the PEC CO₂ RR process. We show how the optoelectronic properties of sputtered ZnTe absorbers can be improved for this purpose via chloride treatments. MnCl₂ and MgCl₂ heat treatments recrystallize ZnTe absorbers to enlarge grains and improve photoluminescence. These material improvements result in the highest PEC CO₂ RR photocurrent density reported for planar ZnTe and >50% Faradaic efficiency to CO formation with diaryliodonium additive in the solution. These results pave the way to integration of polycrystalline thin-film photoabsorbers in PEC CO₂ RR systems.

Electrides are ionic crystals, with electrons acting as anions occupying well-defined lattice sites. These exotic materials have attracted considerable attention in recent years for potential applications in catalysis, rechargeable batteries, and display technology. Among this class of materials, electride semiconductors can further expand the horizon of potential applications due to the presence of a band gap. However, there are only limited reports on semiconducting electrides, hindering the understanding of their physical and chemical properties. In recent work, we initiated an approach to derive potential electrides via selective removal of symmetric Wyckoff sites of anions from existing complex minerals. Herein, we present a follow-up effort to design semiconducting electrides from parental complex sodalites. Among four candidate compounds, we found that a cubic Ca₄Al₆O₁₂ structure with the I-43m space group symmetry exhibits perfect electron localization at the sodalite cages, with a narrow electronic band gap of 1.8 eV, making it suitable for use in photocatalysis. Analysis of the electronic structures reveals that a lower electronegativity of the surrounding cations drives greater electron localization and promotes the formation of an electride band near the Fermi level. Our work proposes an alternative approach for designing new semiconducting electrides under ambient conditions and offers guidelines for further experimental exploration.

Here, a synergetic study that utilized anion photoelectron spectroscopy and high-level abinitio calculations has explored the activation of H₂O molecules by ThO₂^– molecular anions. Both experiment and theory found conclusive evidence for said activation. In the experiments, this appeared as a tell-tale directional shift in the spectral profile of the anionic complex that ruled out physisorption,i.e., ThO₂^–(H₂O), and implied chemisorption. In the computations, good agreement was found between the calculated and measured vertical detachment energies, and the atomic connectivity (the structure) of the resulting anionic complex was found to be [OTh(OH)₂]^–.

Borohydride-based electrolytes have recently emerged as promising media for the electrodeposition of electropositive metals, including rare earth (RE) elements. While the presence of supporting alkali metal cations and RE counteranions provides essential electrochemical conductivity for achieving fast metal electrodeposition, interactions between the host ligand and solvated neodymium (Nd) complexes remain unclear. This study provides insights into the coordination structure of a concentrated and directly solvated Nd salt in a lithium borohydride-supported electrolyte. Our spectroscopic results indicate that the RE coordination environment is significantly influenced by the solvation mechanism, which can vary between metathesis and complexation pathways, primarily dictated by stoichiometric factors. Under dilute conditions, nearly complete metathesis of anions leads to a high coordination number for the host ligand (borohydride), consistent with the previously reported solvated Nd speciation in chlorine-free electrolytes. In contrast, concentrated dissolution of the Nd salt in the supported electrolyte is dominated by a complexation pathway featuring a Li-ion-paired complex with a low coordination number of the host ligand. Density functional theory (DFT) calculations indicated that the observed blue shift in the borohydride vibration was the result of an increase in electron density drawn into the terminal B–H interbond region from the hydride as the coordination changed from Li to Nd. In conjunction with DFT results, vibrational analyses allowed correlation of the experimental shifts associated with changes in Nd ligation and coordination spheres, further consolidating the prevalence of highly chloride-coordinated species under concentrated conditions. In conclusion, the outcomes of this work illuminate the distinctive and heterogeneous coordination structures that the electroactive RE species can adopt at high concentrations in lithium borohydride-supported electrolytes, as a key step to comprehend the reported metal electrodeposition performance in these media.

A monocationic dicopper(I,I) nitrite complex [Cu₂(μ-κ¹:κ¹-O₂N)DPFN][NTf₂] (2) (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine, NTf₂^– = N(SO₂CF₃)₂^–), was synthesized by treatment of a dicopper acetonitrile complex, [Cu₂(μ-MeCN)DPFN][NTf₂]₂ (1), with tetrabutylammonium nitrite ([nBu₄N][NO₂]). DFT calculations indicate that 2 is one of three linkage isomers that are close in energy and presumably accessible in solution. Reaction of the μ-κ¹:κ¹-O₂N complex with p-TolSH produces nitrous acid (HONO) and the corresponding dicopper thiolate species via an acid–base exchange reaction. Notably, treatment of 2 with HNTf₂ results in N–O bond cleavage in the putative, HONO-ligated complex to form the more thermodynamically favorable nitrosyl-bridged dicopper complex [Cu₂(μ-NO)(μ-OH)DPFN][NTf₂]₂ (4). This scission can be reversed via deprotonation of the hydroxy ligand with KO^tBu. X-ray diffraction studies confirmed the solid-state molecular structures of 2 and 4. DFT calculations were used to construct a reaction coordinate diagram detailing formation of the μ-NO complex and to describe its electronic structure. The nitrosyl ligand in 4 is chemically labile, as demonstrated by its ready displacement in reactions with CO or NO₂^–.

Solid polymer electrolytes have yet to achieve the desired ionic conductivity (>1 mS/cm) near room temperature required for many applications. This target implies the need to reduce the effective energy barriers for ion transport in polymer electrolytes to around 20 kJ/mol. In this work, we combine information extracted from existing experimental results with theoretical calculations to provide insights into ion transport in single-ion conductors (SICs) with a focus on lithium ion SICs. Through the analysis of temperature-dependent ionic conductivity data obtained from the literature, we evaluate different methods of extracting energy barriers for lithium transport. The traditional Arrhenius fit to the temperature-dependent ionic conductivity data indicates that the Meyer–Neldel rule holds for SICs. However, the values of the fitting parameters remain unphysical. Our modified approach based on recent work (Macromolecules 2023, 56, 15, 6051), which incorporates a fixed pre-exponential factor, reveals that the energy barriers exhibit temperature dependence over a wide range of temperatures. Using this approach, we identify anions leading to the energy barriers <30 kJ/mol, which include trifluoromethane sulfonimide (TFSI), fluoromethane sulfonimide (FSI), and boron-based organic anions. In our efforts to design the next generation of anions, which can exhibit the energy barriers <20 kJ/mol, we have performed density functional theory (DFT) based calculations to connect the chemical structures of boron-based anions via the binding energy of cation (lithium)-anion pairs with the experimentally derived effective energy barriers for ion hopping. Not only have we identified a correlation between the binding energy and the energy barriers, but we also propose a strategy to design new boron-based anions by using the correlation. This combined approach involving experiments and theoretical calculations is capable of facilitating the identification of promising new anions, which can exhibit ionic conductivity >1 mS/cm near room temperature, thereby expediting the development of novel superionic single-ion conducting polymer electrolytes.

The reaction of AnCl₄(DME)_x (An = Th, x = 2; An = U, x = 0) with 4 equiv of NaNR₂ (R = SiMe₃) in THF at 65 °C results in the formation of [An{N(R)(SiMe₂CH₂)}(NR₂)₂] (An = U, 1; An = Th, 2), and not the reported monomeric actinide hydrides, [AnH(NR₂)₃], as expected. Furthermore, both complexes 1 and 2 were characterized by X-ray crystallography. Surprisingly, their unit cell parameters are remarkably close to those reported for [AnH(NR₂)₃], suggesting that the original crystals of [AnH(NR₂)₃] were, in fact, [An{N(R)(SiMe₂CH₂)}(NR₂)₂], but were misidentified. Reduction of 1 with 1.1 equiv of KC₈ in THF, in the presence of 1 equiv of 2.2.2-cryptand, results in the formation of [K(2.2.2-cryptand)][U{N(R)(SiMe₂CH₂)}(NR₂)₂] (3) in good yield. Likewise, the reaction of 1 with 1 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) results in the formation of the BAC adduct, [(BAC)U{N(R)(SiMe₂CH₂)}(NR₂)₂] (4), in moderate yield. Finally, the addition of H₂ (10 bar) to 2 in C₆D₆ at room temperature results in the formation of the targeted monomeric hydride, [ThH(NR₂)₃], in 32% yield, according to integrations against an internal standard. However, removal of the H₂ atmosphere results in rapid reformation of 2. In contrast, the addition of H₂ (10 bar) to 1 in C₆D₆ at room temperature results in no apparent reaction.

We report the synthesis and characterization of a series of high- and low-spin dicobalt complexes of the ^tBuPNNP expanded pincer ligand. Reacting this dinucleating ligand in its neutral form with two equiv of CoCl₂(tetrahydrofuran)_1.5 yields a high-spin dicobalt complex featuring one Co inside and one Co outside of the dinucleating pocket. Performing the same reaction in the presence of two equivalents of KOtBu provides access to a high-spin dicobalt complex wherein both Co centers are bound within the PNNP pocket, and this complex also features a bridging OtBu ligand. Reacting either of the high-spin complexes with excess diethyl silane affords a low-spin dicobalt complex containing two unusual bridging Si-based ligands. These complexes were investigated using NMR spectroscopy, XAS, single crystal X-ray structure determination, and computational methods, showing that the Si-based ligands are best described as base-stabilized silylenes.