Dehydrocoupling Reactions of Dimethylamine-Borane by Pt(II) Complexes: A New Mechanism Involving Deprotonation of Boronium Cations

Abstract

Coordinatively unsaturated Pt(II) complex [Pt(I(t)Bu')(I(t)Bu)](+) stabilized by N-heterocyclic carbene (NHC) ligands dehydrogenates N,N-dimethylamineborane through a mechanism that involves hydride abstraction, assisted by an amine, to yield a platinum-hydride complex [PtH(I(t)Bu')(I(t)Bu)] with concomitant formation of the boronium cation [(NHMe2)2BH2](+). This latter species is very likely in equilibrium with the THF stabilized borenium cation [(NHMe2)(THF)BH2](+), bearing an acidic NH group that is able to protonate the platinum hydride [PtH(I(t)Bu')(I(t)Bu)] releasing H2, the amino borane H2B-NMe2 and regenerating the catalytic [Pt](+) species.

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

“…The single equivalent of boronium [THF⋅BH 2 ⋅NH 3 ][BAr F 4 ] ( 5 ) formed indicates that a hydride abstraction route may be operating, as recently outlined by Conejero and co‐workers for the dehydrocoupling of H 3 B⋅NMe 2 H by cationic {Pt‐NHC} + catalysts,17 as well as that occurring in cationic Ru/Ir‐systems35 or with B(C 6 F 5 ) 3 19. We reasoned that a similar process would yield 5 by B‐H activation16 and subsequent attack by THF (Scheme 5), alongside {Rh(L R )H} that would dimerize to give neutral [Rh(L R )H] 2 (for example, complex  H ).…”

“…One equivalent of the boronium9, 17, 18, 19, 20 cation [THF⋅BH 2 ⋅NH 3 ][BAr F 4 ] was also formed ( δ ( 11 B) 0.5 (t), J BH =108 Hz; lit 19. [Et 2 O⋅BH 2 ⋅NH 3 ][BAr F 4 ] δ ( 11 B) 0.2, J BH =125 Hz).…”

“…1000 hr −1 with 1 equiv of H 2 released) 40. Sub‐catalytic in situ experiments in this solvent21 show that dimer 8 , [(BH 2 ) 2 NMeH(μ‐H)] and boronium [(NH 2 Me) 2 BH 2 ][BAr F 4 ] are present;41 the latter is suggested to arise from NMeH 2 formed from B−N bond cleavage in H 3 B⋅NMeH 2 17. Thus, it is likely that similar active species are present in THF or 1,2‐F 2 C 6 H 4 .…”

“…Deconvoluting these systems under conditions of high amine‐borane concentration is thus a significant challenge to address if precise control over the resulting polyamino‐borane is to be achieved by metal/ligand design. Nevertheless, the observation of novel and unexpected bridging amino‐borane complexes as the first‐formed species, offers tantalizing clues as to the nature of the actual catalysts; and also suggests that boronium cations may play a more general role in amine‐borane dehydrocoupling than generally appreciated 17, 19…”

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

“…[L•BHNHMe(CH 2 ) 4 ] + ) which might indicate a hydride abstraction route to form mono–cationic systems with odd numbers of hydrides. [69] …”