Conformational Preferences of trans-1,2- and cis-1,3-Cyclohexanedicarboxylic Acids in Water and Dimethyl Sulfoxide as a Function of the Ionization State As Determined from NMR Spectroscopy and Density Functional Theory Quantum Mechanical Calculations

Recent advances in structures and applications of coordination polymers based on cyclohexanepolycarboxylate ligands

Coordination polymers (CPs) are emerging crystalline materials constructed by metal entities and organic ligands through coordination bonds, containing infinite coordination units in one, two, or three dimensions. Here an overview...

Dynamics and Entropy of Cyclohexane Rings Control pH-Responsive Reactivity

Activation entropy (ΔS ^‡) is not normally considered the main factor in determining the reactivity of unimolecular reactions. Here, we report that the intramolecular degradation of six-membered ring compounds is mainly determined by the ΔS ^‡, which is strongly influenced by the ring-flipping motion and substituent geometry. Starting from the unique difference between the pH-dependent degradation kinetics of geometric isomers of 1,2-cyclohexanecarboxylic acid amide (1,2-CHCAA), where only the cis isomer can readily degrade under weakly acidic conditions (pH < 5.5), we found that the difference originated from the large difference in ΔS ^‡ of 16.02 cal·mol^-1·K^-1. While cis-1,2-CHCAA maintains a preference for the classical chair cyclohexane conformation, trans-1,2-CHCAA shows dynamic interconversion between the chair and twisted boat conformations, which was supported by both MD simulations and VT-NMR analysis. Steric repulsion between the bulky 1,2-substituents of the trans isomer is one of the main reasons for the reduced energy barrier between ring conformations that facilitates dynamic ring inversion motions. Consequently, the more dynamic trans isomer exhibits much a larger loss in entropy during the activation process due to the prepositioning of the reactant than the cis isomer, and the pH-dependent degradation of the trans isomer is effectively suppressed. When the ring inversion motion is inhibited by an additional methyl substituent on the cyclohexane ring, the pH degradability can be dramatically enhanced for even the trans isomer. This study shows a unique example in which spatial arrangement and dynamic properties can strongly influence molecular reactivity in unimolecular reactions, and it will be helpful for the future design of a reactive structure depending on dynamic conformational changes.

Reaction mechanism of the selective reduction of CO2 to CO by a tetraaza [CoIIN4H]2+ complex in the presence of protons

The tetraaza [CoIIN4H]2+ complex (1) is remarkable for its ability to selectively reduce CO2 to CO with 45% Faradaic efficiency and a CO to H2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO2 reduction. The selectivity for CO2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO2, and bicarbonate. After reduction of the catalyst by two electrons, formation of [CoIN4H]+-CO2- is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [CoIN4H]+-CO2H, followed by reaction with CO2 to form [CoIIN4H]2+-CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO2 as a Lewis acid to assist in C-O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1. Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO2.