Article
Thermal Dehydrogenation and Hydrolysis of BH3NH3 Catalyzed by
Cyclic (Alkyl)(amino)carbene Iridium Complexes under Mild
Conditions
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ABSTRACT: Though ammonia−borane (AB) is recognized as an excellent
hydrogen storage material, efficient dehydrogenation of AB still remains a
challenge. Herein, we report that cyclic (alkyl)(amino)carbene iridium
complexes are highly efficient for both the thermal dehydrogenation and
hydrolysis of AB under mild conditions. At 30 °C, the two processes are
completed within 15 and 5 min, releasing 2.1 and 2.8 equiv of H2 per AB,
respectively. Moreover, 2.8 equiv of H2 can be released within 10 min by
thermal dehydrogenation at 60 °C. Kinetic studies revealed that the activation
energies for thermal dehydrogenation and hydrolysis of AB are 10.7 and 8.5
kcal/mol, respectively. The catalyst can be recycled without significant loss of
activity at least six times for both processes. The reaction mechanisms were
further explored by theoretical calculations, stoichiometric reactions, and
kinetic isotope effect experiments.
group, are more nucleophilic (σ-donating) but also more
electrophilic (π-accepting) than NHCs.8 These electronic
properties give them the ability to activate a variety of bonds
and small molecules9 and to stabilize highly reactive main-
group and transition-metal diamagnetic and paramagnetic
species.10 CAACs can be used as very robust catalysts for a
variety of applications.11 Manners and co-workers used
different CAACs as hydrogen acceptors to study the
dehydropolymerization of phosphine−borane.12
Bertrand et al. and we have already reported that CAAC
copper complexes are active in AB hydrolysis,3d but we found
that they have no activity for the thermal dehydrogenation
process. Herein, we show that CAAC iridium complexes
efficiently promote both the hydrolysis and thermal dehydro-
genation of AB under mild conditions. More than 2.7 equiv of
H2 can be released in both processes. Moreover, mechanisms
for the hydrolysis and thermal dehydrogenation reactions are
proposed.
INTRODUCTION
■
Due to its high hydrogen content (19.6 wt %), nontoxicity, and
stability at normal pressure and temperature,1 ammonia−
borane, BH3·NH3 (AB), is regarded as an excellent chemical
hydrogen-storage material.2 Up to now, the dehydrogenation
of AB has been studied extensively, through two different
processes, namely hydrolysis3 and thermal dehydrogenation.4
Rh@ZSM-5-H,3g Ni/Pt@ZIF-8,3f Rh/VO2 and Ru/PC3k
3e
represent the most efficient catalysts for AB hydrolysis, since
the process can be completed in a few minutes even at room
temperature. In the case of thermal dehydrogenation, some
catalysts afford linear polyaminoborane and 1 equiv of H2,
while others can generate >2 equiv of H2 with concomitant
formation of a cyclic iminoborane trimer (borazine) and its
BN-cross-linked oligomers (polyborazylene).5 Thermal dehy-
drogenation of AB is more difficult, and only a fraction of the
hydrogen can be released in hours even at high temper-
atures.4a−d,f−j For example, AB thermal dehydrogenation
catalyzed by sterically encumbered pyridones releases only
1.9 equiv of H2 at 80 °C in 2 h.4j Using Fe pincer complexes,
2.5 equiv of H2 is released at 60 °C, but after 15 h.4g When
graphene oxide was used as the catalyst, the reaction required
100 °C to release 2.3 equiv of H2 in 8 h.6 It is quite clear that,
due to the difference in mechanisms for AB thermal
dehydrogenation and hydrolysis, the catalysts reported for
these two processes are quite different.
Received: May 20, 2021
Published: July 19, 2021
Cyclic (alkyl)(amino)carbenes (CAACs),7 in which one of
the electron π-donating and σ-withdrawing electronegative
amino substituents of NHCs is replaced by a σ-donor alkyl
© 2021 American Chemical Society
Organometallics 2021, 40, 2643−2650
2643