Abstract
Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H2 molecule. In this contribution, a series of IrIII complexes based on lutidine-derived CNNH pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (4a), tBu (4b), benzyl (4c)] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia–borane (NH3BH3, AB) in the presence of substoichiometric amounts (2.5 equiv) of tBuOK. 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 H2 per mole of AB was released in 8.5 min (turnover frequency (TOF50%) = 1875 h–1), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF50% = 55–60 h–1). 4a is currently the best performing IrIII 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]/dt = k[4a]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (−d[AB]/dt = k[4b][AB]2). Moreover, the reactions of the derivatives 4a and 4b with an excess of tBuOK (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 tBuOK. Complex 6 is capable of activating H2 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.
Highlights
The controlled release of H2 from high content hydrogen compounds used as H2-storage systems is paramount to the use of this gas as an energy vector in the so-called Hydrogen Economy.[1,2] In this context, ammonia−borane (NH3BH3, AB) has received increasing attention as a hydrogen storage material because of its high available hydrogen content (19.6 wt % H), moisture kinetic stability and easy H2 thermal release.[3]
AB dehydrogenation can be performed under simple thermal conditions,[5] insufficient kinetic control of the process leads to the formation of ill-defined B−N containing solids
We report on the synthesis of a series of Ir complexes based on lutidine-derived CNNH ligands bearing secondary amino and N-heterocyclic carbenes (NHCs) side-arms and their catalytic performance in AB dehydrogenation
Summary
The controlled release of H2 from high content hydrogen compounds used as H2-storage systems is paramount to the use of this gas as an energy vector in the so-called Hydrogen Economy.[1,2] In this context, ammonia−borane (NH3BH3, AB) has received increasing attention as a hydrogen storage material because of its high available hydrogen content (19.6 wt % H), moisture kinetic stability and easy H2 thermal release.[3]. AB dehydrogenation can be performed under simple thermal conditions,[5] insufficient kinetic control of the process leads to the formation of ill-defined B−N containing solids. This is a serious drawback for the H2-depleted material recycling. The use of a catalyst allows for a fine control of the rate and extent of H2 production, as well as the nature of the H2-depleted byproducts. The H2 release temperature is lowered, with respect to pure AB, and the process can be performed with a lower energetic impact
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