Are intramolecular frustrated Lewis pairs also intramolecular catalysts? A theoretical study on H 2 activation

• Published in: Physical chemistry chemical physics : PCCP 2015, Vol.17 (16), p.10687-10698
• Main Authors: Liu Zeonjuk, Lei, St. Petkov, Petko, Heine, Thomas, Röschenthaler, Gerd-Volker, Eicher, Johannes, Vankova, Nina
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/C5CP00368G

We investigate computationally a series of intramolecular frustrated Lewis pairs (FLPs), with the general formula Mes 2 PCHRCH 2 B(C 6 F 5 ) 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 SiMe 3 ( 3 )). The prototypical system Mes 2 PCH 2 CH 2 B(C 6 F 5 ) 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.