Dra. Durán-Guajardo, Rocío

Context: This study investigates the energetic and polarizability characteristics of three 1,3-dipolar cycloaddition reactions between diazene oxide and substituted ethylenes, focusing on the transition from synchronous to asynchronous mechanisms. Synchronicity analysis, using the reaction force constant, indicates that the bond evolution process becomes increasingly decoupled as the number of cyano groups increases. Polarizability analysis reveals that isotropic polarizability reaches its maximum near the transition state in all cases, while anisotropy of polarizability shifts from the transition state toward the product direction as asynchronicity increases. The larger the shift, the more asynchronous the mechanism, as refected by the weight of the transition region. A detailed examination of the parallel and perpendicular polarizability components to the newly formed sigma bonds shows that the evolution of the parallel component is closely aligned with the energetic changes along the reaction coordinate, particularly in the synchronous reaction. We have also identifed a relationship between the displacement in the maximum state of the parallel component from the transition state and the synchronicity of the mechanism. The larger the displacement, the more asynchronous the mechanism. These fndings suggest that asynchronous 1,3-dipolar cycloaddition mechanisms are characterized by a decoupling of isotropic and anisotropic polarizabilities and a shift in the maximum polarizability state of the parallel component toward the product direction. Methods: Density functional theory calculations were performed at the B3LYP/6–311+ +G(d,p)//B3LYP/6-31G(d,p) level of theory. The polarizability was calculated at each point of the reaction path, obtained using the intrinsic reaction coordinate method, as implemented in Gaussian 16.

A large set of intramolecular aminoborane-based FLPs was studied employing density functional theory in the H2 activation process to analyze how the acidity and basicity of boron and nitrogen atoms, respectively, affect the reversibility of the process. Three different linkers were employed, keeping the C–C nature in the connection between both Lewis centers: –CH2–CH2–, –CH[double bond, length as m-dash]CH–, and –C6H4–. The results show that significant differences in the Gibbs free energy of the process are found by considering all the combinations of substituents. Of the 75 systems studied, only 9 showed the ability to carry out the process reversibly (ΔGH2 in the range of −3.5 to 2.0 kcal mol−1), where combinations of alkyl/aryl or aryl/alkyl in boron/nitrogen generate systems capable of reaching reversibility. If the alkyl/alkyl or aryl/aryl combination is employed, highly exergonic (non-reversible H2 activation) and endergonic (unfeasible H2 activation) reactions are found, respectively. No appreciable differences in the linker were found, allowing us to continue the analysis with the most entropically favorable linker, the –C6H4– linker. From this, 25 different FLP systems of type 2-[bis(X)boryl]-(Y)aniline (X: H, CF3, C6F5, PFtB, FMes and Y: H, CH3, t-but, Ph, Mes) can be formed. By analyzing the electronic properties of each system, we have found that the condensed-to-boron electrophilicity index ωB+ is inversely related to the ΔGH2. Interestingly, two relationships were found; the first is for alkyl groups (Y: CH3 and t-but) and the second for aryl groups (Y: H, Ph, and Mes), which is intimately related to the proton affinity of each aniline. In addition, it is quite interesting when the frustration degree, given by B⋯N distance dB–N, is brought together with ωB+, since the Image ID:d3sc03992g-t1.gif quotient has unit energy/length corresponding to unit force; concomitantly, a measure of the FLP strength in H–H bond activation can be defined. With this finding, a rational design of this kind of FLP can be performed by analyzing the acidity of boron through condensed-to-boron electrophilicity and knowing the nature of the substituent of nitrogen according to whether the Y is alkyl or aryl, optimizing the H2 reversible activation in a rational way, which is crucial to improve the catalytic performance.

A deeper computational mechanistic study of an environmentally friendly metal-free CO2 reduction process towards obtaining methanol is presented, employing a previously tested kind of intramolecular frustrated Lewis pair (2-[bis(R)boryl]-N,N-dimethylaniline) as the catalyst and H2 as the reducing agent. The Lewis acid strength of the electrophilic boron atom was adjusted to facilitate hydride release by changing the R group, using electron-donating groups (EDGs) based on methylated aryls (Mes and Mes′) and electron-withdrawing groups (EWGs) based on fluorinated alkyls (CF3 and PFtB) and aryls (FMes and C6F5), to analyse its effect on both the H2 splitting and CO2 hydrogenation processes. The acidity of boron was measured from the local electrophilicity index obtained using conceptual density functional theory, where an excellent correlation with the Gibbs free energy of the H2 splitting process was found (R2 = 0.95), indicating that the higher the acid power of boron is, the more exergonic the H2 activation process is. The reversibility of H2 activation directly impacts the CO2 and formic acid hydrogenations, where the less exergonic the H2 splitting process is, the lower the activation energies for these hydrogenation processes are. To obtain methanol at the end, methanediol dehydration forming formaldehyde is crucial, because methanediol has a high energetic barrier, hindering the catalytic cycle from being more efficient. Conversely, formaldehyde can be easily hydrogenated to methanol in the same way as CO2 and formic acid. Finally, the catalytic activity in each case was analysed in terms of the energetic span model, where the local electrophilicity index condensed on boron shows a good linear correlation with the logarithm of the relative turnover frequency (R2 = 0.91), indicating that this reactivity index can be employed to guide the design of optimal catalytic systems to increase its catalytic activity, opening new routes directing future experiments in the field.