Amine Boranes Dehydrogenation Mediated by an Unsymmetrical Iridium Pincer Hydride: (PCN) vs (PCP) Improved Catalytic Performance

The
IrIII hydride (tBuPCN)­IrHCl
(1) containing the tridendate unsymmetrical pincer ligand tBuPCN– {tBuPCN­(H) = 1-[3-[(di-tert-butyl­phosphino)­methyl]­phenyl]-1H-pyrazole} has been exploited as ammonia borane (NH3BH3, AB) and amine boranes dehydrogenation catalyst
in THF solution at ambient temperature. 1 releases one
H2 equivalent per AB equivalent, with concomitant cyclic
poly­(amino­boranes) formation [B-(cyclo­triborazanyl)-amine-borane
(BCTB) and cyclo­triborazane (CTB)] as the final “spent
fuel”. 1 has been found to have superior catalytic
activity than its symmetrical analogue (tBuPCP)­IrHCl, with recorded TOF values of 580 h–1 (AB in THF) and 401 h–1 (DMAB in toluene) at ambient
temperature. The reaction has been analyzed experimentally through
multinuclear [11B, 31P­{1H}, 1H] NMR and IR spectroscopy, kinetic rate measurements, and
kinetic isotope effect determination with deuterated AB isotopologues.
The hydride/​borohydride intermediate (tBuPCN)­IrH­(η2-BH4) (2) is the catalyst resting state formed during the dehydrogenation
process; it is detected by a variable-temperature multinuclear NMR
of the reaction course (in the 190–323 K range). A DFT modeling
of the reaction mechanism using DMAB as substrate has been performed
with the geometry optimization in toluene at the M06 level of theory.
The combination of the kinetic and computational data reveals that
a simultaneous B–H/N–H activation occurs in the presence
of 1, after the preliminary amine borane coordination
to the metal center.