Dehydrogenation of amine–boranes with a frustrated Lewis pair

Abstract

Bulky tertiary phosphine/borane Lewis pairs P t Bu 3 /B(C 6 F 5 ) 3 react with amine-boranes to afford dehydrocoupling products and phosphonium borohydride salts.Amine-boranes are the subject of intense interest due to their potential application in hydrogen storage schemes. 1,2 A number of fast and efficient transition metal dehydrocoupling catalysts have been discovered, 3-7 and various coordination compounds relevant to the catalytic pathways have been isolated. [8][9][10] Progress has also been made in the challenging area of regeneration of spent ammonia-borane fuel. [11][12][13] Theoretical studies 14,15 of one amine-borane dehydrocoupling system employing N-heterocyclic carbenes (NHCs) as ligands 3 suggested that the free NHC could heterolytically dehydrogenate NH 3 ÁBH 3 , yielding (NH 2 BH 2 ) n and NHC-H 2 , similar to the H 2 cleavage reactivity of Bertrand's carbenes. 16 Because the reactivity of these NHC species has been compared to that of bulky phosphine/borane ''frustrated Lewis pairs'' (FLPs), it occurred to us that FLPs might also be able to dehydrogenate amine-boranes.FLPs-combinations of particularly bulky Lewis acids and bases that are unable to form traditional Lewis acid/base pairs 17 -exhibit a number of unusual reactions, [18][19][20] in particular the heterolytic cleavage of H 2 , 21-23 which has been utilized for metal-free catalytic hydrogenation of bulky imines. 24,25 Amine activation reactions have also been reported. 26,27 To our knowledge, however, no dehydrogenation reactions have been reported using these systems, 28 although calculations suggest such reactions should be possible. 29 Herein we report that the simple frustrated Lewis pair P t Bu 3 /B(C 6 F 5 ) 3 rapidly and cleanly dehydrogenates amine-boranes at room temperature, with concomitant formation of the phosphonium borohydride salt [ t Bu 3 PH][HB(C 6 F 5 ) 3 ] (Scheme 1).Initial experiments were carried out with Me 2 NHÁBH 3 , a common model for NH 3 ÁBH 3 which has desirable solubility properties and generally releases only 1 equivalent of H 2 . The frustrated Lewis pair P t Bu 3 /B(C 6 F 5 ) 3 was formed according to literature procedures in C 6 D 5 Cl, 22 and no discernable reaction was observed by NMR. The pre-formed FLP was added to a C 6 D 5 Cl solution of Me 2 NHÁBH 3 at 25 1C, giving a clear colorless solution. 30 1 H, 31 P (Fig. 1), 19 F, and 11 B (Fig. 2) The order of reagent addition is important in the dehydrocoupling reaction. If B(C 6 F 5 ) 3 and Me 2 NHÁBH 3 are dissolved Scheme 1 Fig. 2 11 B NMR spectrum shortly after FLP-mediated dehydrocoupling of Me 2 NHÁBH 3 . The underlying broad feature arises from borosilicate glass in the probe construction.

Dehydrocoupling routes to element–element bonds catalysed by main group compounds

“…30 Aluminium-phosphorus FLPs have also been used in dehydrogenic bond forming reactions. A similar mechanism was proposed in the stoichiometric dehydrocoupling of Me 2 NHBH 3 with the intermolecular FLP B(C 6 F 5 ) 3 / t Bu 3 P. 32 In this reaction the Lewis acid/base pair are not geometrically constrained and the dimeric borazane (Me 2 NBH 2 ) 2 was the major product in the reaction (rather than Me 2 NQBH 2 ), alongside smaller quantities of di-amino-borane HB(NMe 2 ) 2 . The 9.3 mol% reaction of the FLP Mes 2 PC(QC(H)Ph)Al t Bu 2 with Me 2 NHBH 3 at 45 1C in the melt resulted in rapid evolution of H 2 gas and formation of monomeric Me 2 NQBH 2 .…”

[Ir(PCy3)2(H)2(H2BNMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2

“…For example Bercaw and co-workers have noted that traces of C are formed in the dehydrocoupling of A promoted by a frustrated Lewis-pair. [50] However, our observation of C in such significant amounts when the metal can be involved at the very least shows that the metal promotes its formation.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…The authors suggested the mechanism of dehydrogenation was likely to proceed via hydride abstraction by B(C 6 F 5 ) 3 , followed by rapid deprotonation by P t Bu 3 to form the aminoborane. This then undergoes rapid oligomerisation to form the products [147]. The group of Manners used a variety of less expensive frustrated Lewis pairs to dehydrocouple H 3 B · NMe 2 H, with a combination of [Me 3 SiO 3 SCF 3 ] and 2,2,6,6-tetramethylpiperidine performing best in forming [H 2 BNMe 2 ] 2 , with only traces of side products observed.…”