RECENT CASES

Hydride abstraction from monocationic hydride bridged salts [H­(H₂B–L)₂]⁺ [B­(C₆F₅)₄]¯ (L = Lewis base) generates an observable primary borenium cation for L = <i>i</i>Pr_<i>2</i>NEt, but with L = Me₃N, Me₂NPr, or several <i>N</i>-heterocyclic carbenes, highly reactive dicationic dimers are formed

Phosphine Modulation for Enhanced CO₂ Capture: Quantum Mechanics Predictions of New Materials

It is imperative to develop efficient CO2 capture and activation technologies to combat the rising levels of deleterious greenhouse gases in the atmosphere. Using Quantum Mechanics methods (Density Functional Theory), we propose and evaluate several metal-free and metal-containing phosphines that provide strong CO2 binding under ambient conditions. Depending on the electron donating capacity of the phosphine and the ability of the P-bound ligands to hydrogen bond to the CO2, we find that the CO2 binding can be as strong as −18.6 kcal/mol downhill, which should be quite adequate for ambient conditions. We explore some modifications of the phosphine to improve CO2 binding, and we elucidate which chemical descriptors correlate directly with CO2 binding energy. Specifically, we find that charge accumulation on the CO2 unit of the CO2-bound adduct has the greatest correlation with CO2 binding affinity. Finally, we probe the mechanism for CO2 reduction to CO and methanol in aqueous media

Phosphine Modulation for Enhanced CO₂ Capture: Quantum Mechanics Predictions of New Materials

It is imperative to develop efficient CO2 capture and activation technologies to combat the rising levels of deleterious greenhouse gases in the atmosphere. Using Quantum Mechanics methods (Density Functional Theory), we propose and evaluate several metal-free and metal-containing phosphines that provide strong CO2 binding under ambient conditions. Depending on the electron donating capacity of the phosphine and the ability of the P-bound ligands to hydrogen bond to the CO2, we find that the CO2 binding can be as strong as −18.6 kcal/mol downhill, which should be quite adequate for ambient conditions. We explore some modifications of the phosphine to improve CO2 binding, and we elucidate which chemical descriptors correlate directly with CO2 binding energy. Specifically, we find that charge accumulation on the CO2 unit of the CO2-bound adduct has the greatest correlation with CO2 binding affinity. Finally, we probe the mechanism for CO2 reduction to CO and methanol in aqueous media

Electrophilic C–H Borylation and Related Reactions of B–H Boron Cations

Catalytic procedures are described for the amine-directed borylation of aliphatic and aromatic tertiary amine–boranes. Sequential double borylation is observed in cases where two or more C–H bonds are available that allow 5-center or 6-center intramolecular borylation. The HNTf₂-catalyzed borylation of benzylamine–boranes provides a practical means for the synthesis of ortho-substituted arylboronic acid derivatives, suitable for Suzuki–Miyaura cross-coupling applications

Borenium Ion Catalyzed Hydroboration of Alkenes with N-Heterocyclic Carbene-Boranes

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-1-butene with the N-heterocyclic carbene (NHC)-derived borane <b>2</b> and catalytic HNTf₂ (Tf = trifluoromethanesulfonyl (CF₃SO₂)) effects hydroboration at room temperature. With 3-hexene, surprisingly facile migration of the boron atom from C(3) of the hexyl group to C(2) was observed over a time scale of minutes to hours. Oxidative workup gave a mixture of alcohols containing 2-hexanol as the major product. A similar preference for the C(2) alcohol was observed after oxidative workup of the 3-octene and 1-cyclohexyl-1-butene hydroborations. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations, and the C(2) selective migrations are attributed to the four-center interconversion of borenium cations with cationic NHC-borane-olefin π-complexes

Redox-Active Macrocycles for Organic Rechargeable Batteries

Organic rechargeable batteries, composed of redox-active molecules, are emerging as candidates for the next generation of energy storage materials because of their large specific capacities, cost effectiveness, and the abundance of organic precursors, when compared with conventional lithium-ion batteries. Although redox-active molecules often display multiple redox states, precise control of a molecule’s redox potential, leading to a single output voltage in a battery, remains a fundamental challenge in this popular field of research. By combining macrocyclic chemistry with density functional theory calculations (DFT), we have identified a structural motif that more effectively delocalizes electrons during lithiation events in battery operationsnamely, through-space electron delocalization in triangular macrocyclic molecules that exhibit a single well-defined voltage profilecompared to the discrete multiple voltage plateaus observed for a homologous macrocyclic dimer and an acyclic derivative of pyromellitic diimide (PMDI). The triangular macrocycle, incorporating three PMDI units in close proximity to one another, exhibits a single output voltage at 2.33 V, compared with two peaks at (i) 2.2 and 1.95–1.60 V for reduction and (ii) 1.60–1.95 and 2.37 V for oxidation of the acyclic PMDI derivative. By investigating the two cyclic derivatives with different conformational dispositions of their PMDI units and the acyclic PMDI derivative, we identified noticeable changes in interactions between the PMDI units in the two cyclic derivatives under reducing conditions, as determined by differential pulse voltammetry, solution-state spectroelectrochemistry, and variable-temperature UV–Vis spectra. The numbers and relative geometries of the PMDI units are found to alter the voltage profile of the active materials significantly during galvanostatic measurements, resulting in a desirable single plateau for the triangular macrocycle. The present investigation reveals that understanding and controlling the relative conformational dispositions of redox-active units in macrocycles are key to achieving high energy density and long cycle-life electrodes for organic rechargeable batteries

Semiconducting Single Crystals Comprising Segregated Arrays of Complexes of C₆₀

Although pristine C₆₀ prefers to adopt a face-centered cubic packing arrangement in the solid state, it has been demonstrated that noncovalent-bonding interactions with a variety of molecular receptors lead to the complexation of C₆₀ molecules, albeit usually with little or no control over their long-range order. Herein, an extended viologen-based cyclophaneExBox₂⁴⁺has been employed as a molecular receptor which, not only binds C₆₀ one-on-one, but also results in the columnar self-assembly of the 1:1 inclusion complexes under ambient conditions. These one-dimensional arrays of fullerenes stack along the long axis of needle-like single crystals as a consequence of multiple noncovalent-bonding interactions between each of the inclusion complexes. The electrical conductivity of these crystals is on the order of 10^–7 S cm^–1, even without any evacuation of oxygen, and matches the conductivity of high-quality, unfunctionalized C₆₀-based materials that typically require stringent high-temperature vaporization techniques, along with the careful removal of oxygen and moisture, prior to measuring their conductance