Ammonia–Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNN^H Complexes

Abstract

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H 2 molecule. In this contribution, a series of Ir III complexes based on lutidine-derived CNN H pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph ( 4a ), t Bu ( 4b ), benzyl ( 4c )] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia–borane (NH 3 BH 3 , AB) in the presence of substoichiometric amounts (2.5 equiv) of t BuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a , 1.0 equiv of H 2 per mole of AB was released in 8.5 min (turnover frequency (TOF 50% ) = 1875 h –1 ), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF 50% = 55–60 h –1 ). 4a is currently the best performing Ir III homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (−d[AB]/d t = k [ 4a ]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (−d[AB]/d t = k [ 4b ][AB] 2 ). Moreover, the reactions of the derivatives 4a and 4b with an excess of t BuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b , a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with t BuOK. Complex 6 is capable of activating H 2 reversibly to yield the trihydride derivative 7 . Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal–ligand cooperation mode.

Dehydrogenation of Ammonia Borane Impacts Valence and Core Electrons: A Photoemission Spectroscopic Study

“…Hydrogen (H 2 ) storage and controlled release from high content hydrogen compounds such as ammonia borane (AB, amminetrihydridoboron or borazane) is paramount to the hydrogen energy life cycle. AB (H 3 BNH 3 ) has been considered as a promising solid hydrogen carrier as it is an excellent source of hydrogen (19.4 wt %) for fuel cell applications . AB is a nontoxic, environmentally benign, and a stable material that can be safely transported without hydrogen loss, which dictates the success of any chemical for hydrogen storage .…”

“…Although dehydrogenation of AB can be carried out under simple thermal conditions, insufficient kinetic control of the process leads to the formation of ill-defined B–N containing solids. This can be a serious drawback for the H 2 -depleted material recycling …”

“…AB (H 3 BNH 3 ) has been considered as a promising solid hydrogen carrier as it is an excellent source of hydrogen (19.4 wt %) for fuel cell applications. 1 AB is a nontoxic, environmentally benign, and a stable material that can be safely transported without hydrogen loss, which dictates the success of any chemical for hydrogen storage. 2 AB dehydrogenation renders B–N oligomers and polymers as byproducts that are suitable starting materials for the synthesis of boron nitride (BN) ceramics.…”

Recent developments on the transformation of CO2utilising ligand cooperation and related strategies

“…Other transition metal complexes with pincer ligands containing symmetrical N-heterocyclic carbene (NHC) donors were reported. These include NHCs based on a lutidine central unit, including CNN 147,148 and CNC 149 systems. These also showed interesting reactivity in the activation of small molecules via the dearomatisation–aromatisation process.…”

Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3-iPr-PNRP Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

“…14 In contrast, the understanding of the initial catalytic dehydrogenation of amine-boranes to give aminoboranes is better developed, as reaction progress can be conveniently followed by H 2 evolution as a proxy for aminoborane generation, catalyst speciation, and isotope labeling studies. There are three accepted pathways for dehydrogenation, Scheme 2: (A) stepwise or concerted inner-sphere BH/NH activation; 42−45 (B) metal−ligand cooperative (MLC) processes; 16,19,46,47 and (C) aminepromoted hydride transfer at a cationic metal center, followed by reprotonation to release H 2 . 48−50 Group 8 catalysts with the i Pr-PN H P ligand [ i Pr-PN H P = {κ 3 -(CH 2 CH 2 P i Pr 2 ) 2 NH] have been reported by Schneider and Beweries to be particularity effective for the dehydrogenation of amine-boranes, operating via MLC mechanisms (i.e., B, Scheme 2).…”