The resistivity versus temperature measurement is commonly used for identifying temperature-induced phase change and the resulting hysteresis loop. While the resistance is influenced by both the density of states and the carrier lifetimes, the Seebeck coefficient is influenced predominantly by the density of states, and hence is a better probe of the phase of the material. Here, 1T′ - T_d temperature-induced phase transition in MoTe₂ is studied using temperature-dependent X-ray diffraction, resistivity, and Seebeck coefficient measurements. A more distinct hysteresis is observed when measuring the Seebeck coefficient which is consistent with direct measurements of the crystallographic angle using the temperature-dependent X-ray diffraction. The Seebeck and electrical resistivity measurements indicate a competing contribution of the electrons and holes. The contribution of electron pockets becomes more dominant when molybdenum atoms are replaced by tungsten. In MoTe₂, a topologically induced enhancement of the Nernst coefficient is observed at low temperatures, and a relatively large phase-transition induced Thomson coefficient of 111 μV⋅K^-1 is measured at 254 K which is larger than the Seebeck coefficient measured in the entire temperature range.

Bismuth complexes stabilized by carbon-based donor ligands are underserved by their instability, often due to facile ligand dissociation and deleterious protonolysis. Herein, we show that the ortho-bismuthination of hexaphenylcarbodiphosphorane enables a robust framework with geometrically constrained carbone–bismuth bonding interactions, which are highly tunable by cationization. The carbodiphosphorane bismuth halides (1 and 2) are remarkably air-stable and feature unprecedented trans^carboneC–Bi–X ligation, resulting in highly elongated Bi–X bonds. In contrast to known carbone–bismuth complexes, hydrolytic activation of the carbone yields well-defined organobismuth complexes, and subsequent dehydrohalogenation is feasible using potassium bis(trimethylsilyl)amide or N-heterocyclic carbenes. The redox-flexibility of this framework was evaluated in the high catalytic activity of 1 and 2 for silylation of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) under mild conditions (50 °C, 24–96 h) and low catalyst loadings (5–10 mol %), which suggests the accessibility of short-lived hydridic and radical bismuth species. The reaction of 1, PhSiH₃, and tris(pentafluorophenyl)borane (BCF) yields the first crystallographically characterized bismuth hydridoborate complex as an ionic species (9), presumably by BCF-mediated hydride abstraction from an unobserved [Bi]–H intermediate. All isolated compounds have been characterized by heteronuclear NMR spectroscopy and X-ray crystallography, and the bonding situation in representative complexes (1, 2, 5, and 9) were further evaluated using density functional theory.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann–Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr₃ and [(^pyCDC)PnBr₂][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr₃ (1) with one or two equivalents of AgNTf₂ (NTf₂ = bis(trifluoromethanesulfonyl)imide) or AgSbF₆ gives stibaalkene mono- and dications of the form [(CDC)SbBr_3–n][A]_n (2–4; n = 1,2; A = NTf₂ or SbF₆). The stibaalkene trication [(CDC)₂Sb][NTf₂]₃ (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6–9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (^pyCDC) was used as a ligand to access [(^pyCDC)PnBr₂][Br] (10, 12) and trications [(^pyCDC)Pn][NTf₂]₃ (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC–Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf₂]₂ (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF₃, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF₅ (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

A pyrazine-2-carboxylate (pzCO₂) complex of copper(II) has been synthesized, studied structurally and magnetically, and compared with structurally similar compounds. The structure of [CuCl(pzCO₂)] (1) is reported and compared with the known structures of [Cu(pzCO₂)₂] (2) and [Cu(pzCO₂)₂(H₂O)₂] (3). Single-crystal X-ray diffraction measurements show that 1 crystallizes in the monoclinic space group Pc, with two crystallographically independent five-coordinate Cu(II) ions in a geometry close to square-pyramidal. It has a bilayer structure in the packing. Magnetic susceptibility data of 1 show that it exhibits weak ferromagnetic interactions (2 J = 2.26(7) K). In contrast, magnetic susceptibility data of 2 and 3 show weak antiferromagnetic interactions.

The stereoelectronic effects of N-heterocyclic carbene (NHC) and cyclic(alkyl)(amino) carbene (CAAC) coordination to calcium silylamides and amidoboranes have been investigated. The straightforward complexation of a sterically unencumbered NHC (i.e., N,N′-diisopropyl-2,3-dimethylimidazol-2-ylidine) and {Ca[N(SiMe₃)₂]₂}₂ or (THF)₂Ca[N(SiMe₃)₂]₂ in equimolar amounts afforded (NHC)Ca[N(SiMe₃)₂]₂ (2) or (NHC)(THF)Ca[N(SiMe₃)₂]₂ (4), respectively. Spectroscopic analyses reveal negligible electronic differences in 2 and 4, and the latter can be desolvated under prolonged vacuum. Similarly, CAAC complexation to {Ca[N(SiMe₃)₂]₂}₂ afforded (CAAC)Ca[N(SiMe₃)₂]₂ (5) as the first crystallographically characterized CAAC–Ca coordination complex, but this compound is thermally unstable and rapidly decomposes to intractable mixtures. The reaction of {Ca[N(SiMe₃)₂]₂}₂ and HNMe₂BH₃ afforded the bis(amidoborane) [Ca(NMe₂BH₃)₂]_n (6), which does not react with bulky carbenes but readily complexes with unencumbered NHCs with subsequent non-innocent participations in calcium-mediated amine borane dehydrocoupling. Significantly, (NHC)₂Ca(NMe₂BH₃)₂ (7) decomposes under mild heating (50 °C, 16 h) to form [CaH₂]_n and the hydride-rich complex [(NHC–BH₂NMe₂)Ca(NMe₂BH₃)₂]₂ (8). Compound 8 was independently prepared from the reaction of 6 and NHC–BH₂NMe₂ and is remarkably thermally stable in refluxing benzene (85 °C, 24 h). In the absence of carbenes, the dehydrocoupling of 6 and HNMe₂BH₃ afforded (THF)₂Ca(NMe₂BH₃)(NMe₂BH₂NMe₂BH₃) (9), but subsequent reactions with NHC resulted in the immediate abstraction and migration of Me₂N═BH₂ toward the formation of 8.

Electrocatalyst design and optimization strategies continue to be an active area of research interest for the applied use of renewable energy resources. The electrocatalytic conversion of carbon dioxide (CO₂) is an attractive approach in this context because of the added potential benefit of addressing its rising atmospheric concentrations. In previous experimental and computational studies, we have described the mechanism of the first molecular Cr complex capable of electrocatalytically reducing CO₂ to carbon monoxide (CO) in the presence of an added proton donor, which contained a redox-active 2,2′-bipyridine (bpy) fragment, CrN₂O₂. The high selectivity for CO in the bpy-based system was dependent on a delocalized Cr^II(bpy^•–) active state. Subsequently, we became interested in exploring how expanding the polypyridyl ligand core would impact the selectivity and activity during electrocatalytic CO₂ reduction. Here, we report a new CrN₃O catalyst, Cr(tpy^tbupho)Cl₂ (1), where 2-(2,2′:6′,2″-terpyridin-6-yl)-4,6-di-tert-butylphenolate = [tpy^tbupho]⁻, which reduces CO₂ to CO with almost quantitative selectivity via a different mechanism than our previously reported Cr(^tbudhbpy)Cl(H₂O) catalyst. Computational analyses indicate that, although the stoichiometry of both reactions is identical, changes in the observed rate law are the combined result of a decrease in the intrinsic ligand charge (L₃X vs L₂X₂) and an increase in the ligand redox activity, which result in increased electronic coupling between the doubly reduced tpy fragment of the ligand and the Cr^II center. The strong electronic coupling enhances the rate of protonation and subsequent C–OH bond cleavage, resulting in CO₂ binding becoming the rate-determining step, which is an uncommon mechanism during protic CO₂ reduction.

The impact of the exact spatial arrangement of the alkali metal on the electronic properties of 9-carbene-9-borafluorene monoanions is assessed, and a series of [K][9-CAAC-9-borafluorene] complexes (1–4) have been isolated (CAAC = cyclic(alkyl)(amino) carbene, (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene). Compound 1, which contains [B]–K(THF)₃ interactions, is compared to charge-separated 2–4, which were prepared by capturing the potassium cations with 18-crown-6, 2.2.2-cryptand, or 1,10-phenanthroline. Notably, the ¹¹B NMR spectra of charge-separated borafluorene monoanions 2–4 show distinct low-field signatures compared to 1. Theoretical calculations indicate that charge separation may be exploited to influence the nucleophilic and electron transfer properties of 9-carbene-9-borafluorene monoanions. When [K(2.2.2-cryptand)][9-CAAC-9-borafluorene] (3) is reacted with 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione, the carbene ligand is displaced, and new air-stable R₂BO₂ spirocycles are formed (5 and 6, respectively). Remarkably, compounds 5 and 6 display fluorescence under UV light in both the solid and solution phases with quantum yields of up to 20%. In addition, a drastic red-shift in the emission color is observed in 6 because of the presence of the nitrogen atoms on the phenanthroline moiety. Mechanistic insights into the formation of these spirocycles are also described based on density functional theory calculations.