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

Abstract

Two Co(I) hydrides containing the tripodal polyphosphine ligand EP, (κ-EP)Co(H) [E(CHCHPPh); E = N (1), P (2)], have been exploited as ammonia borane (NHBH, AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear (B, P{H}, H) 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 H per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final "spent fuel", 2 produces 1 equiv of H 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 BH/NH insertion process on the initially formed [Co]-NHBH amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and MeNHBH (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 H evolution.

Amine‐Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α‐Diimine Cobaltates

“…Waterman and co‐workers reported that cyclopentadienylcobalt complexes Cp R Co(CO) 2 I (R=H, Me; Figure ) catalyze ammonia borane (AB) dehydrogenation at elevated temperatures (65 °C) . The authors performed catalytic transfer hydrogenations with styrenes, alkynes, and olefins with an excess (8 equiv) of AB at 65 °C within 6 h. Tripodal polyphosphine cobalt(I) hydrides (Figure ), recently reported by Shubina and co‐workers, exhibited similar activity in the AB dehydrogenation . A mechanism was proposed based on DFT calculations.…”

“…Various dehydrogenation and transfer hydrogenation protocols have been developed with precious metal catalysts, and the underlying mechanisms have been thoroughly studied . By contrast, dehydrogenations are far less advanced with the abundant and cheaper late 3d metals, despite the recent progress with Ti, Mn, Fe, Co, and Ni catalysts …”

“…Although a number of iron catalysts for amine‐borane dehydrogenations have been studied recently, effective cobalt catalysts are scarce . To the best of our knowledge, only three well‐defined molecular cobalt catalysts have been reported to date.…”

Amine–Borane Dehydropolymerization: Challenges and Opportunities

“…Schneider has compared the pre‐catalysts RuH(PMe 3 ){N((CH 2 ) 2 P i Pr 2 ) 2 } and RuH 2 (PMe 3 ){NR((CH 2 ) 2 P i Pr 2 ) 2 } (R=H, Me) in the dehydropolymerization of H 3 B⋅NH 3 , wherein subtle changes in catalyst structure resulted in mechanistic changes based on experimental and computational findings . Dehydrogenation of ammonia‐borane by the cobalt catalysts CoH{E(CH 2 CH 2 PPh 2 ) 3 } produces (H 2 BNH 2 ) n when E=P, but when E=N two equivalents of H 2 are liberated and B‐(cyclotriborazanyl)amine–borane (BCTB), borazine, and polyborazylenes are obtained, which was attributed to the coordinative flexibility of the NP 3 ligand . Further understanding of how catalyst structure influences mechanism will be essential for understanding and optimization of reactivity for the tailored production of polymeric materials.…”

“…As such, the mechanism does not involve an anchoring M−N (or M−B) bond for chain propagation, and it is also proposed that H 2 does not act to modify the polymer chain length. An unusual mechanistic pathway calculated by Rossin and Shubina involves B−N cleavage chain‐growth, in which BH 3 and NH 3 moieties are added individually at a cobalt tetraphosphine center …”