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-(iPr2PO)2C6H3]­Fe­(PMe2R)2H (R = Me, 1; R = Ph, 2) or [2,6-(iPr2PO)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 (kNH3BH3/kND3BD3 = 3.7) is the product of individual KIEs (kNH3BH3/kND3BH3 = 2.0 and kNH3BH3/kNH3BD3 =
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.