New insights into H2 activation by intramolecular frustrated Lewis pairs based on aminoboranes: the local electrophilicity index of boron as a suitable indicator to tune the reversibility of the process

César Barrales-Martínez; Rocío Durán; Pablo Jaque

doi:10.1039/d3sc03992g

New insights into H₂ activation by intramolecular frustrated Lewis pairs based on aminoboranes: the local electrophilicity index of boron as a suitable indicator to tune the reversibility of the process

Chem Sci. 2023 Oct 3;14(42):11798-11808. doi: 10.1039/d3sc03992g. eCollection 2023 Nov 1.

Authors

César Barrales-Martínez¹, Rocío Durán¹, Pablo Jaque^{2

3}

Affiliations

¹ Instituto de Investigación Interdisciplinaria (I3), Vicerrectoría Académica and Centro de Bioinformática, Simulación y Modelado (CBSM), Facultad de Ingeniería, Universidad de Talca Campus Lircay Talca 3460000 Chile cesar.barrales@utalca.cl.
² Departamento de Química Orgánica y Fisicoquímica, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile Sergio Livingstone 1007, Independencia Santiago Chile.
³ Centro de Modelamiento Molecular, Biofísica y Bioinformática, CM2B2, Universidad de Chile Sergio Livingstone 1007, Independencia Santiago Chile.

Abstract

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 (ΔG_H₂ 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 ω_B⁺ is inversely related to the ΔG_H₂. 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 d_B-N, is brought together with ω_B⁺, since the 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.