Abstract
The catalytic dehydrogenation of ammonia- and amine-boranes by a dimethylxanthene-derived frustrated Lewis pair is described. Turnover is facilitated on a thermodynamic basis by the ready release of H2 from the weakly basic PPh2-containing system. In situ NMR studies and the isolation of intermediates from stoichiometric reactions support a mechanism initiated by B-H activation, followed by end-growth BN coupling involving the terminal NH bond of the bound BN fragment and a BH bond of the incoming borane monomer.
Highlights
In recent work we have developed single component Frustrated Lewis pairs (FLPs) based on a dimethylxanthene backbone that possess a P···B separation (4.2−4.5 Å) preorganized for the facile uptake of H2
Metal complexes feature prominently among the most active catalyst systems reported to date, but other approaches, including the use of ionic liquids and Brønsted and Lewis acids/bases, have been developed.[5−9]
Targeting applications in dehydrogenation chemistry, the related PPh2 variants 2 and 3 were targeted, reasoning that the incorporation of weaker Lewis acid/base components should lead to more favorable thermodynamics for H2 loss.[17]
Summary
In recent work we have developed single component FLPs based on a dimethylxanthene backbone that possess a P···B separation (4.2−4.5 Å) preorganized for the facile uptake of H2. The phosphonium borohydride 1-(H)[2] derived from PMes2/ B(C6F5)[2] functionalized 1 has been shown to exist in equilibrium with the “free” FLP and dihydrogen at 295 K (∼20:1 1-(H)2:1 mixture in bromobenzene; Scheme 1).[16] Targeting applications in dehydrogenation chemistry, the related PPh2 variants 2 and 3 were targeted, reasoning that the incorporation of weaker Lewis acid/base components should lead to more favorable thermodynamics for H2 loss.[17] These systems can be synthesized from 4,5dibromo-9,9-dimethylxanthene using a similar approach to that employed for 1,16 and both 2 and 3 can be shown crystallographically to constitute a FLP (see SI).
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