Mechanistic studies of ammonia borane dehydrogenation catalyzed by iron pincer complexes

A series of iron bis(phosphinite) pincer complexes with the formula of [2,6-((i)Pr2PO)2C6H3]Fe(PMe2R)2H (R = Me, 1; R = Ph, 2) or [2,6-((i)Pr2PO)2-4-(MeO)C6H2]Fe(PMe2Ph)2H (3) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes 1-3 release 2.3-2.5 equiv of H2 per AB in 24 h. Among the three iron catalysts, 3 exhibits the highest activity in terms of both the rate and the extent of H2 release. The initial rate for the dehydrogenation of AB catalyzed by 3 is first order in 3 and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (k(NH3BH3)/k(ND3BD3) = 3.7) is the product of individual KIEs (k(NH3BH3)/k(ND3BH3) = 2.0 and k(NH3BH3)/k(NH3BD3) = 1.7), suggesting that B-H and N-H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing trans PMe3 or PMe2Ph (relative to the hydride) by AB. Loss of NH3 from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H2 release at the late stage of the dehydrogenation reaction.