The title complex ( tBuPCP)­IrH­(Cl) (1; tBuPCP = κ3-2,6-(CH2PtBu2)2C6H3) appeared to be moderately active in NHMe2·BH3 (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (tBuPCP)­IrHCl­(η1-BH3·NHMe2) (3) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)­IrH2(η1-BH3·NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and tert-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of 1) the hydride complex (tBuPCP)­IrH­(μ2-H2BH2) (5) was identified, which is not a dormant catalytic species but the steady-state intermediate formed as a result of the B–N bond breaking. DMAB dehydrogenation yields the borazane monomer H2BNMe2 (detected by 11B NMR); dimerization of this species gives the final product [H2BNMe2]2 and (tBuPCP)­IrH4 as the catalyst resting state. The scenario of B–N bond cleavage in DMAB leading to byproducts of dehydrogenation such as bis­(dimethylamino)­hydroborane and (tBuPCP)­IrH­(μ2-H2BH2) (5) is proposed. The results obtained allow us to suggest the mechanism of catalytic DMAB dehydrocoupling that could be generalized to other processes.

The Ir III hydride ( tBu PCN)IrHCl (1) containing the tridendate unsymmetrical pincer ligand tBu PCN − { tBu PCN(H) = 1-[3-[(ditert-butylphosphino)methyl]phenyl]-1H-pyrazole} has been exploited as ammonia borane (NH 3 BH 3 , AB) and amine boranes dehydrogenation catalyst in THF solution at ambient temperature. 1 releases one H 2 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 ( tBu PCP)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 [ 11 B, 31 P{ 1 H}, 1 H] NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect determination with deuterated AB isotopologues. The hydride/borohydride intermediate ( tBu PCN)IrH(η 2 -BH 4 ) (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.

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