Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNN Complexes.

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H molecule. In this contribution, a series of Ir complexes based on lutidine-derived CNN pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (), Bu (), benzyl ()] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia-borane (NHBH, AB) in the presence of substoichiometric amounts (2.5 equiv) of 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 , 1.0 equiv of H per mole of AB was released in 8.5 min (turnover frequency (TOF) = 1875 h), while complexes and (0.8 mol %) exhibited lower catalytic activities (TOF = 55-60 h). is currently the best performing Ir 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 (-d[AB]/d = []). Conversely, the reaction with is second order in AB and first order in the catalyst (-d[AB]/d = [][AB]). Moreover, the reactions of the derivatives and with an excess of BuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of , a monodeprotonated (at the pincer NHC-arm) species is observed upon reaction with BuOK. Complex is capable of activating H reversibly to yield the trihydride derivative . Finally, DFT calculations of the first AB dehydrogenation step catalyzed by has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal-ligand cooperation mode.