Ammonia Borane Dehydrogenation Catalyzed by (κ4-EP3)Co(H) [EP3 = E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction with NH Acids

Article abstract of DOI:10.1021/acs.inorgchem.6b02673

Two Co(I) hydrides containing the tripodal polyphosphine ligand EP₃, (κ⁴-EP₃)Co(H) [E(CH₂CH₂PPh₂)₃; E = N (1), P (2)], have been exploited as ammonia borane (NH₃BH₃, AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear (¹¹B, ³¹P{¹H}, ¹H) NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect (KIE) determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, albeit with different rates and efficiency. While 1 releases 2 equiv of H₂ per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final spent fuel , 2 produces 1 equiv of H₂ only per equivalent of AB in the same reaction time, along with long-chain poly(aminoboranes) as insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++G? 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 a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the process is better defined as a sequential BH₃/NH₃ insertion process on the initially formed [Co]-NH₂BH₃ amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me₂NHBH₃ (DMAB)] has been followed via VT-FTIR spectroscopy (in the -80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the dihydrogen bonding interactions that are at the origin of the observed H₂ evolution.