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

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–, –CHCH–, 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.