In the face of rising atmospheric carbon dioxide (CO₂) emissions from fossil fuel combustion, the hydrogen evolution reaction (HER) continues to attract attention as a method for generating a carbon-neutral energy source for use in fuel cells. Since some of the best-known catalysts use precious metals like platinum, which have low natural abundance and high cost, developing efficient Earth abundant transition metal catalysts for HER is an important objective. Building off previous work with transition metal catalysts bearing 2,2'-bipyridine-based ligand frameworks, this work reports the electrochemical analysis of a molecular nickel(II) complex, which can act as an electrocatalyst for the HER with a Faradaic efficiency for H₂ of 94 ± 8% and turnover frequencies of 103±6 s⁻¹ when pentafluorophenol is used as a proton donor. Computational studies of the Ni catalyst suggest that non-covalent interactions between the proton donor and ligand heteroatoms are relevant to the mechanism for electrocatalytic HER.

The addition of non-benzenoid quinones, acenapthenequinone or aceanthrenequinone, to the 9-carbene-9-borafluorene monoanion (1) affords the first examples of dianionic 10-membered bora-crown ethers (2-5), which are characterized by multi-nuclear NMR spectroscopy (1H, 13C, 11B), X-ray crystallography, elemental analysis, and UV-Vis spectroscopy. These tetraoxadiborecines have distinct absorption profiles based on the positioning of the alkali metal cations. When compound 4, which has a vacant C4B2O4 cavity, is reacted with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, a color change from purple to orange serves as a visual indicator of metal binding to the central ring, whereby the Na+ ion coordinates to four oxygen atoms. A detailed theoretical analysis of the calculated reaction energetics is provided to gain insight into the reaction mechanism for the formation of 2-5. These data, and the electronic structures of proposed intermediates, indicate that the reaction proceeds via a boron enolate intermediate.

A family of eight compounds of the general formula [(C₈H₉NO)₂MX₂] or [(C₈H₉NO)₂(H₂O)₂MX2], (M = Ni, Co, Cu, Zn; X = Cl, Br) has been prepared and the compounds characterized by combustion analysis, IR, single-crystal X-ray diffraction and variable temperature magnetization measurements. [[(C₈H₉NO)₂(H₂O)_nMX2], (1, n = 0, M = Cu, X = Cl; 2, n = 0, M = Cu, X = Br; 3, n = 2, M = Ni, X = Cl; 4, n = 2, M = Ni, X = Br; 5, n = 2, M = Co, X = Cl; 6, n = 2, M = Co, X = Br; 7, n = 0, M = Zn, X = Cl; 8, n = 0, M = Zn, X = Br.) The eight compounds crystallize in three distinct space groups and have coordination number of either four (compounds 1, 2, 7, and 8) or six (compounds 3–6). Compounds 1 and 2 are slightly distorted square planar, compounds 3–6 are slightly distorted octahedral, and compounds 7 and 8 are slightly distorted tetrahedral. All eight compounds form chains either through bihalide interactions (1 and 2) or systems of hydrogen bonds (3–8). Chains are linked into layers through short halide…halide (1, 2, 7) and both traditional and non-traditional hydrogen bonds. The complexes have also been studied via variable temperature magnetic susceptibility measurements. Data for Cu(II) complexes 1 and 2 the 1D-Heisenberg uniform chain model with J/k_B of −13.4(6) K and −14.3(4) K, respectively, with antiferromagnetic interchain interactions (θ = −4.1(5) K, −2.5(5) K, respectively) following the Hamiltonian. The Ni(II) and Co(II) compounds showed temperature dependent moments which were well-modeled as arising due to single-ion anisotropy.

Bosonic Dirac materials are testbeds for dissipationless spin-based electronics. In the quasi two-dimensional honeycomb lattice of CrX₃ (X = Cl, Br, I), Dirac magnons have been predicted at the crossing of acoustical and optical spin waves, analogous to Dirac fermions in graphene. Here we show that, distinct from CrBr₃ and CrI₃, gapless Dirac magnons are present in bulk CrCl₃, with inelastic neutron scattering intensity at low temperatures approaching zero at the Dirac K point. Upon warming, magnon-magnon interactions induce strong renormalization and decreased lifetimes, with a ~25% softening of the upper magnon branch intensity from 5 to 50 K, though magnon features persist well above T_N. Moreover, on cooling below ~50 K, an anomalous increase in the a-axis lattice constant and a hardening of a ~26 meV phonon feature are observed, indicating magnetoelastic and spin-phonon coupling arising from an increase in the in-plane spin correlations that begins tens of Kelvin above T_N.

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.