A Three-Stage Mechanistic Model for Ammonia–Borane Dehydrogenation by Shvo’s Catalyst

Abstract

We propose a mechanistic model for three-stage dehydrogenation of ammonia borane (AB) catalyzed by Shvo’s cyclopentadienone-ligated ruthenium complex. We provide evidence for a plausible mechanism for catalyst deactivation, the transition from fast catalysis to slow catalysis, and relate those findings to the invention of a second-generation catalyst that does not suffer from the same deactivation chemistry. The primary mechanism of catalyst deactivation is borazine-mediated hydroboration of the ruthenium species that is the active oxidant in the fast catalysis case. This transition is characterized by a change in the rate law for the reaction and changes in the apparent resting state of the catalyst. Also, in this slow catalysis situation, we see an additional intermediate in the sequence of boron, nitrogen species, aminodiborane. This occurs with concurrent generation of NH3, which itself does not strongly affect the rate of AB dehydrogenation.

The Synthesis, Characterization and Dehydrogenation of Sigma‐Complexes of BN‐Cyclohexanes

“…Perhaps most closely related to the system under discussion here are dimer/monomer equilibria operating for Shvo’s catalyst in both amine–borane dehydrogenation and hydrogen transfer reactions. [76, 77] In these cases a dimer is suggested to be in equilibrium with two, different, monomers (of equal relative concentration); one of which is active in catalysis.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…14). H 3 B · NH 3 is required to convert 66 back into 63, which is the rate-limiting step in this slow regime, and, thus, the reaction becomes first order in [H 3 B · NH 3 ] [117].…”

The Simplest Amino‐borane H2B=NH2 Trapped on a Rhodium Dimer: Pre‐Catalysts for Amine–Borane Dehydropolymerization

“…Turning to the pure and well‐characterized dimer 4 , initial rate measurements in a closed system (4 mol % rhodium, THF) were more informative, and a first‐order dependence for either H 3 B⋅NH 3 or H 3 B⋅NMeH 2 , as well as catalyst 4 , were measured for the early pseudo zero‐order phase of catalysis (Supporting Information, Figures S19 and 20). Such behavior is not consistent with a rapid dimer–monomer equilibrium for which an order of [ 4 ] 1/2 would be expected,22, 36, 42 a view supported by the stoichiometric reactions with acetonitrile or toluene (see above). Under these conditions complexes  2 or 4 do not promote full conversion of amine‐borane (for 4 , 70 % conversion of H 3 B⋅NH 3 after 10 hrs).…”