We study the effect of counter-ion complexation on the example of Cl(-) ions interacting with the [Co(en)(3)](3+) complex. The H-bonding of the N-H groups of the ethylenediamine (en) ligands with the Cl(-) ions may lead to giant enhancement of the VCD intensity for the N-H stretches, but may also lead to VCD sign changes in the finger print region of N-H wagging, twisting and scissoring motions. Such sign changes should not be mistaken for signatures of the presence of the other enantiomer. We elucidate the mechanism for the sign changes and give a recommendation on how to deal with this problem. We also show that the experimental spectrum is only in good accord with the calculations if complexation of 5 Cl(-) ions (two axial, three equatorial) is assumed, but not with two (axial) or three (equatorial) Cl(-) ions, thus showing the potential of VCD to be used as an experimental probe for complexation.
A ketone's carbonyl carbon is electrophilic and harbors a part of the lowest unoccupied molecular orbital of the carbonyl group, resembling a Lewis acidic center; under the right circumstances it exhibits very useful chemical reactivity, although the natural electrophilicity of the ketone's carbonyl carbon is often not strong enough on its own to produce such reactivity. Quantum chemical calculations predict that a proton shared between a ketone and the Lewis basic solvent molecule (dioxane or THF) activates carbonyl carbon to the point of enabling a facile heterolytic splitting of H . Proton-catalyzed hydrogenation of a ketone in Lewis basic solvent is the result. The mechanism involves the interaction of H with the enhanced Lewis acidity of a carbonyl carbon and the free Lewis basic solvent molecule polarizes H and enables the hydride-type attack on carbonyl carbon, which is very strongly influenced by the proton shared between a ketone and solvent. The hydride-type attack on carbon is reminiscent of the splitting of H by singlet carbenes except that, in this case, a Lewis base from the surrounding environment (solvent) is necessary for polarization of H and acceptance of the proton resulting from the heterolytic splitting of H .
Using the 6,6'-dibromo-[1,1'-binaphthalene]-2,2'-diol molecule and its vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra measured in deuterated dimethyl sulfoxide as example, we present a first detailed study of the effects induced in VCD spectra by the large-amplitude motions of solvent molecules loosely bound to a solute molecule. We show that this type of perturbation can induce significant effects in the VA and VCD spectra. We also outline a computational procedure that can effectively model the effects induced in the spectra and at the same time provide detailed structural information regarding the relative orientations of moieties involved in a solute-solvent molecular complex.
The
reaction between H2 and CO2 catalyzed
by an intramolecular frustrated Lewis pair, which is covalently bonded
to a UiO-66 metal–organic framework (MOF), is considered in
this work. Free energy surfaces (FESs) for this reaction are generated
throughout finite-temperature density functional theory (DFT) metadynamics
(MD) simulations. The simulated FESs indicate an alternative stepwise
pathway for the hydrogenation of CO2. Furthermore, indications
of weaker binding of CO2 than H2 to the Lewis
pair centers have been observed via metadynamics simulations. These
findings were unknown from the results of static-DFT calculations,
which proposed a concerted reduction of CO2. The results
of the present work may influence the design of new efficient heterogeneous Lewis
pair (LP)-functionalized MOFs to catalyze capture and conversion of
CO2 to high-value chemicals.
Enantiomeric excess (ee) in asymmetric catalysis may be strongly dependent on the solvent. The reaction product may range from an almost racemic mixture to an ee of over 90% for different solvents. We study this phenomenon for the C-C coupling reaction between nitromethane and benzaldehyde (the Henry reaction) with cinchona thiourea as the catalyst, where solvents that are strong Lewis bases induce a high ee. We show that the effect of the solvent does not consist of a change in the reaction mechanism. Instead, the solvation "prepares" the molecule, which is very flexible, in a specific conformation. The reaction barriers in this conformer are not lower than for other conformers, but are sufficiently differentiated between the enantiomers to give rise to a large ee. It is the strong Lewis basicity of the solvent that leads to the clear preference in solution for the "asymmetric" conformer. Although general rules or predictions for how solvent effects could be harnessed to produce a desired ee in general would be hard to formulate, this study does show that it is in this case (and presumably in many other cases as well) specific solute-solvent interactions rather than effects of the dielectric continuum of the solvent that are the root cause of the solvent effect. This is in agreement with experiment for the Henry reaction.
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