Encumbering the intramolecular pi donation by using a bridge: A strategy for designing metal-free compounds to hydrogen activation

On the basis of the FLP (frustrated Lewis pair) principle, a new strategy has been proposed to construct the frustration in designing metal-free hydrogen activation compounds, by using FMO (frontier molecular orbital) analyses and quantum mechanics calculations. Unlike the known FLPs which use bulky substituents to prevent them from forming stable Lewis acid/base complexes, the new approach encumbers the intramolecular pi donation from the electron donor to the acceptor (e.g. in BH2NH2) by using a CH2 bridge (giving BH2CH2NH2). The strategy is simple and effective. Its effectiveness is demonstrated by the small hydrogen activation energy (12.0 kcal/mol) of the model molecule (BH2CH2NH2), which is significantly less than the 42.7 kcal/mol of BH2NH2 and also less than the 18.5 kcal/mol of BH2PH2 whose derivative, R2PB(C6F5)(2), has been experimentally shown to be able to activate hydrogen. We also exemplified how to use the strategy to design experimentally more realizable molecules. The example shows promises as a hydrogen activation agent. The strategy can be used to design metal-free catalysts for direct hydrogenation.