Are intramolecular frustrated Lewis pairs also intramolecular catalysts? A theoretical study on H2 activation

We investigate computationally a series of intramolecular frustrated Lewis pairs (FLPs), with the general formula Mes(2)PCHRCH(2)B(C6F5)(2), that are known from the literature to either activate molecular hydrogen (FLPs with R = H (1) or Me (4)), or remain inert (FLPs with R = Ph (2) or SiMe3 (3)). The prototypical system Mes(2)PCH(2)CH(2)B(C6F5)(2) (1) has been described in the literature (Grimme et al., Angew. Chem., Int. Ed., 2010; Rokob et al., J. Am. Chem. Soc., 2013) as an intramolecular reactant that triggers the reaction with H-2 in a bimolecular concerted fashion. In the current study, we show that the concept of intramolecular H-2 activation by linked FLPs is not able to explain the inertness of the derivative compounds 2 and 3 towards H-2. To cope with this, we propose an alternative intermolecular mechanism for the investigated reaction, assuming stacking of two open-chain FLP conformers, and formation of a dimeric reactant with two Lewis acid-base domains, that can split up to two hydrogen molecules. Using quantum-chemical methods, we compute the reaction profiles describing these alternative mechanisms, and compare the derived predictions with earlier reported experimental results. We show that only the concept of intermolecular H-2 activation could explain both the activity of the FLPs having small substituents in the bridging molecular region, and the inertness of the FLPs with a bulkier substitution, in a consistent way. Importantly, the intermolecular H-2 activation driven by intramolecular FLPs indicates the key role of steric factors and noncovalent interactions for the design of metal-free systems that can efficiently split H-2, and possibly serve as metal-free hydrogenation catalysts.