Exploring N activation using novel Lewis acid/base pairs: computational insight into frustrated Lewis pair reactivity.

The activation of dinitrogen (N) is a crucial step in synthesizing nitrogen-based compounds and remains a significant challenge due to its strong triple bond. Currently, industrial N conversion relies on the Haber-Bosch process, a highly energy-intensive method that utilizes transition metal-based catalysts. Frustrated Lewis pairs (FLPs) have emerged as a promising alternative for N activation without the need for transition metals. In this work, we employ density functional theory (DFT) to investigate the activation of N by transition metal-free Lewis acids (LAs) and bases (LBs). Our study demonstrates that LAs play a crucial role in capturing N and determining the thermodynamics of activation, while LBs play a complementary role by reducing the bond order of the N molecule, thereby promoting activation. The efficiency of N capture is directly linked to the electroaccepting characteristics of the LAs. A principal component analysis (PCA) reveals that the key factors influencing the electroaccepting power of LAs are the degree of pyramidalization and orbital occupation at the acidic site, as well as the local electrophilicity index. The LA-N interaction is found to be electrostatic with partially covalent character. Among the 21 LAs analyzed, triptycene-based systems exhibit the highest stability in forming LA-N complexes, highlighting their potential as effective N-capturing agents. However, the N triple bond remains largely intact, necessitating the involvement of LBs in LA-N-LB complexes for full activation, in a "push-pull" mechanism. Six LBs are analyzed in complexes with the most promising LAs. Bonding analysis indicates that the LB-N interaction can be regarded as a covalent bond, which may explain the main role of the LB in the reduction of the N bond order. Furthermore, the bond activation is significantly enhanced by increasing the nucleophilicity of the LB. Among all the LA-LB pair combinations, only three exhibit the defining characteristics of frustrated Lewis pairs (FLPs), with moderate interaction energies and substantial LA-LB distances. Our findings suggest that FLPs composed of triptycene-based LAs and tris--butylphosphine represent the most promising candidates for N activation.