New insights into H 2 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.

A large set of intramolecular aminoborane-based FLPs was studied employing density functional theory in the H 2 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: -CH 2 -CH 2 -, -CH[double bond, length as m-dash]CH-, and -C 6 H 4 -. 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 (Δ G H 2 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 H 2 activation) and endergonic (unfeasible H 2 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 -C 6 H 4 - linker. From this, 25 different FLP systems of type 2-[bis(X)boryl]-(Y)aniline (X: H, CF 3 , C 6 F 5 , PF t B, FMes and Y: H, CH 3 , 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 Δ G H 2 . Interestingly, two relationships were found; the first is for alkyl groups (Y: CH 3 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 H 2 reversible activation in a rational way, which is crucial to improve the catalytic performance.