Due to the rise in atmospheric carbon dioxide (CO₂) concentrations, there is a need for the development of new strategies to enhance the selectivity and activity of the electrocatalytic conversion of CO₂ to value-added products. The incorporation of redox mediators (RMs) as cocatalysts to enhance the transfer of redox equivalents during catalysis has been gaining more attention in recent years across a variety of small molecule transformations. We have shown that using Cr-centered complexes with sulfone-based RMs leads to an enhancement of CO₂ reduction electrocatalysis under protic conditions via an inner-sphere mechanism. In these cocatalytic systems, an oxygen atom of the reduced RM binds to the Cr center to form a key intermediate stabilized by pancake bonding between the reduced aromatic components of the catalyst ligand backbone and the RM. This interaction facilitates the transfer of an electron and accesses a more kinetically favorable reaction pathway. Here, we show that expanding the aromatic character of the ligand backbone of the catalyst as well as the RM can cause a greater enhancement of coelectrocatalytic activity. These results suggest that further activity improvements can be achieved by focusing on the kinetic and thermodynamic parameters which control association between the catalyst and RM.

Three 4-(2’-pyridyl)imidazole (4-pyim) complexes of copper(II) have been synthesized and studied structurally and magnetically. The structures of [CuCl₂(4-pyim)] (1), [CuCl(4-pyim)₂]₂Cl₂(H₂O)₁₀ (2), and [Cu(CuCl₄)(4-pyim)₂][Cu(H₂O)(4-pyim)₂](CuCl₄)(H₂O)₄ (3) are reported. Single-crystal X-ray diffraction measurements show that 1 crystallizes in the monoclinic space group P2₁/n with a four-coordinate Cu(II) ion forming dimers via semi-coordinate bonds to bridging chloride ions. The structure of 1 shows the copper and chloride ions disordered over two sites. Compound 2 crystallizes in the triclinic space group P – 1 with five-coordinate Cu(II) ions in a highly distorted geometry between square pyramidal and trigonal bipyramidal. It has an extensive hydrogen bonding network created by 10 lattice water molecules, chloride ions, and nitrogen atoms in the ligands. Compound 3 crystallizes in the monoclinic space group Cc with both four- and five-coordinate Cu(II) ions present in the lattice; the five-coordinate Cu(II) ions display highly distorted geometries. All three compounds have hydrogen bonding and π-stacking interactions among the 4-pyim rings. Magnetic susceptibility data were collected on 1. Magnetic susceptibility data of 1 shows that it exhibits modest antiferromagnetic interactions which are best fit using a honeycomb model [(2J = −2.6(2) K), 2J’ = −1.6(2) K, H = -2JΣs_i·s_j]. Disorder in the crystal structure decreases the rate of growth of the correlation length at low temperatures, lowering the temperature of the expected maximum in χ below the range of the data.

The syntheses, molecular structures, reactivities, and computational assessment of dipotassium diboratapentacene isomers are described (1 and 2). These compounds represent the first examples of aromatized diboraacenes where the boron atoms are spatially separated in different rings of the acene framework. Both 1 and 2 react with carbon dioxide (CO₂) via diastereoselective carboxylation of the pentacene backbone that likely proceeds by a frustrated Lewis pair-like mechanism. The placement of the boron atoms and the reactivity studies provide a platform for later stage functionalization of diboraacenes beyond the central ring of the polycyclic aromatic hydrocarbon core.

We report the dimerization and oligomerization of ethylene using bis(phosphino)boryl supported Ni(II) complexes as catalyst precursors. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type (^RPBP)NiBr (R = tBu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 mol_ethylene·mol_Ni^–1·s^–1 for the formation of butenes with 41(1)% selectivity for 1-butene using (^PhPBP)NiBr, and 68 mol_ethylene·mol_Ni^–1·s^–1 for butenes production with 87.2(3)% selectivity for 1-butene using (^tBuPBP)NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and (^tBuPBP)NiBr as the precatalyst, ethylene oligomerization to form C₄ through C₂₀ products was achieved, while the use of (^PhPBP)NiBr as the pre-catalyst retained selectivity for C₄ products. Our studies suggest that the ethylene dimerization is not initiated by Ni hydride or alkyl intermediates. Rather, our studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni–B unit of the (^RPBP)Ni catalysts is proposed as a key element of the Ni catalysis.

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.