7 research outputs found
RECENT CASES
Hydride abstraction from monocationic
hydride bridged salts [HĀ(H<sub>2</sub>BāL)<sub>2</sub>]<sup>+</sup> [BĀ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]ĀÆ (L = Lewis
base) generates an observable
primary borenium cation for L = <i>i</i>Pr<sub><i>2</i></sub>NEt, but with L = Me<sub>3</sub>N, Me<sub>2</sub>NPr, or several <i>N</i>-heterocyclic carbenes, highly reactive dicationic dimers
are formed
Phosphine Modulation for Enhanced CO<sub>2</sub> 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 ā18.6 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 CO<sub>2</sub> 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 ā18.6 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 CāH Borylation and Related Reactions of BāH Boron Cations
Catalytic
procedures are described for the amine-directed borylation
of aliphatic and aromatic tertiary amineāboranes. Sequential
double borylation is observed in cases where two or more CāH
bonds are available that allow 5-center or 6-center intramolecular
borylation. The HNTf<sub>2</sub>-catalyzed borylation of benzylamineāboranes
provides a practical means for the synthesis of ortho-substituted
arylboronic acid derivatives, suitable for SuzukiāMiyaura 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 HNTf<sub>2</sub> (Tf = trifluoromethanesulfonyl (CF<sub>3</sub>SO<sub>2</sub>)) 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 Ļ-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 moleculeās 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 operationsīønamely,
through-space electron delocalization in triangular macrocyclic molecules
that exhibit a single well-defined voltage profileīøcompared
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 1.95ā1.60
V for reduction and (ii) 1.60ā1.95 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 UVāVis 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 C<sub>60</sub>
Although
pristine C<sub>60</sub> 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 C<sub>60</sub> molecules, albeit
usually with little or no control over their long-range order. Herein,
an extended viologen-based cyclophaneīøExBox<sub>2</sub><sup>4+</sup>īøhas been employed as a molecular receptor which,
not only binds C<sub>60</sub> 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<sup>ā7</sup> S cm<sup>ā1</sup>, even without any evacuation
of oxygen, and matches the conductivity of high-quality, unfunctionalized
C<sub>60</sub>-based materials that typically require stringent high-temperature
vaporization techniques, along with the careful removal of oxygen
and moisture, prior to measuring their conductance