On the origin of reversible hydrogen activation by phosphine-boranes

Mechanistic insights into the factors responsible for the reversible hydrogen-activation ability exhibited by an aryl phosphine-borane system ((CH(3))(2)P-C(6)F(4)-B(CF(3))(2)) are presented. A detailed evaluation of the energies of various intermediates, generated by the addition of molecular hydrogen, and their interconverting barriers have been carried out using ab initio and DFT methods. Several rearrangement possibilities of the H(2)-phosphino-borane adduct have been investigated so as to unravel the lower energy pathways that convert the initial adduct to a series of other intermediates. The initial adduct formed by the heterolytic addition of a molecular hydrogen across the C-B bond is identified to undergo a series of rearrangement reactions until it terminates at the C-P end of the molecule. Among the possible 1,n-migrations (for which n=1-5), 1,2-proton migrations are found to possess lower energy transition states, whereas 1,2-hydride (in a zwitterionic intermediate) and 1,4-proton-coupled electron transfers exhibited much higher energy transition states. The minimum energy pathway for the transfer of a proton and hydride from the C-B bond to the C-P bond is found to involve a cascade of 1,2-proton transfers followed by a 1,2-hydride migration and finally a 1,4-proton-coupled electron transfer. The higher energy pathways identified for the hydride transfer suggest the possibility of a cascade of reversible proton migrations from a thermodynamically stable intermediate (M(a)). Possible uptake of two hydrogen molecules by the phosphine-borane system is additionally considered in the present study, in which relatively higher barriers than those with one molecule of hydrogen are observed. The computed thermodynamic parameters are found to be in accordance with the experimental observations, in which the uptake and storage of molecular hydrogen are carried out at lower temperatures whereas the liberation demands elevated temperatures.