Catching the First Oligomerization Event in the Catalytic Formation of Polyaminoboranes: H3B·NMeHBH2·NMeH2 Bound to Iridium

Effect of the phosphine steric and electronic profile on the Rh-promoted dehydrocoupling of phosphine-boranes

The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine–boranes H₃B·PR₂H [R = 3,5-(CF₃)₂C₆H₃; p-(CF₃)C₆H₄; p-(OMe)C₆H₄; adamantyl, Ad] and H₃B·PCyH₂ to form the metal-bound linear diboraphosphines H₃B·PR₂BH₂·PR₂H and H₃B·PRHBH₂·PRH₂, respectively, are reported. Reaction of [Rh(L)(η⁶-FC₆H₅)][BAr^F₄] [L = Ph₂P(CH₂)₃PPh₂, Ar^F = 3,5-(CF₃)₂C₆H₃] with 2 equiv of H₃B·PR₂H affords [Rh(L)(H)(σ,η-PR₂BH₃)(η¹-H₃B·PR₂H)][BAr^F₄]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η²-PR₂·BH₂PR₂·BH₃)][BAr^F₄]. With electron-withdrawing groups on the phosphine–borane there is the parallel formation of the products of B–P cleavage, [Rh(L)(PR₂H)₂][BAr^F₄], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H₃B·P(Ad)₂H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine–borane complex is formed, [Rh(L){η²-H₃B·P(Ad)₂H}][BAr^F₄], that undergoes B–P bond cleavage to give [Rh(L){η¹-H₃B·P(Ad)₂H}{P(Ad)₂H}][BAr^F₄]. The relative rates of dehydrocoupling of H₃B·PR₂H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P–B bond cleavage. H₃B·PCyH₂ undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P–H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.

The effect of changing the steric and electronic profile of the phosphine of primary and secondary phosphine−boranes H₃B·PR₂H (R = H, aryl, adamantyl) on the Rh-catalyzed dehydrocoupling to form metal-bound linear diboraphosphines is reported.

Dehydrogenation of a tertiary amine-borane by a rhenium complex

Photolysis of CpRe(CO)₃ in the presence of H₃BNEt₃ results in the dehydrogenation of the borane and formation of trans-CpRe(CO)₂(H)₂.

Multiple metal-bound oligomers from Ir-catalysed dehydropolymerisation of H3B·NH3 as probed by experiment and computation

Multiple metal-bound oligomers in the dehydropolymerisation of H₃B·NH₃ have been observed experimentally and the mechanism of oligomerisation probed computationally.

The roles of dihydrogen bonds in amine borane chemistry

A dihydrogen bond (DHB) is an electrostatic interaction between a protonic hydrogen and a hydridic hydrogen. Over the past two decades, researchers have made significant progress in the identification and characterization of DHBs and their properties. In comparison with conventional hydrogen bonds (HBs), which have been widely used in catalysis, molecular recognition, crystal engineering, and supramolecular synthesis, chemists have only applied DHBs in very limited ways. Considering that DHBs and conventional HBs have comparable strength, DHBs could be more widely applied in chemistry. Over the past several years, we have explored the impact of DHBs on amine borane chemistry and the syntheses and characterization of amine boranes and ammoniated metal borohydrides for hydrogen storage. Through systematic computational and experimental investigations, we found that DHBs play a dominant role in dictating the reaction pathways (and thus different products) of amine boranes where oppositely charged hydrogens coexist for DHB formation. Through careful experiments, we observed, for the first time, a long-postulated reaction intermediate, ammonia diborane (AaDB), whose behavior is essential to mechanistic understanding of the formation of the diammoniate of diborane (DADB) in the reaction of ammonia (NH3) with tetrahydrofuran borane (THF·BH3). The formation of DADB has puzzled the boron chemistry community for decades. Mechanistic insight enabled us to develop facile syntheses of aminodiborane (ADB), ammonia borane (AB), DADB, and an inorganic butane analog NH3BH2NH2BH3 (DDAB). Our examples, together with those in the literature, reinforce the fact that DHB formation and subsequent molecular hydrogen elimination are a viable approach for creating new covalent bonds and synthesizing new materials. We also review the strong effects of DHBs on the stability of conformers and the hydrogen desorption temperatures of boron-nitrogen compounds. We hope that this Account will encourage further applications of DHBs in molecular recognition, host-guest chemistry, crystal engineering, supramolecular chemistry, molecular self-assembly, chemical kinetics, and the syntheses of new advanced materials.

Metal-free dehydrogenation of amine-boranes by an N-heterocyclic carbene

The dehydrogenation of primary and secondary amine-boranes (RNH(2)·BH(3) and R(2)NH·BH(3); R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-(i)Pr(2)C(6)H(3)) as a stoichiometric dehydrogenation agent. In the case of primary amine-boranes, carbene-bound adducts IPr·BH(2)-NH(R)-BH(3) were obtained in place of the desired polymers [RNH-BH(2)](n). The secondary amine-borane (i)Pr(2)NH·BH(3) participated in dehydrogenation chemistry with IPr to afford the aminoborane [(i)Pr(2)N=BH(2)] and the dihydroaminal IPrH(2) as products. Attempts to induce H(2) elimination from the arylamine-borane DippNH(2)·BH(3) yielded a reaction mixture containing the known species IPr·BH(2)NHDipp, IPr·BH(2)NH(Dipp)-BH(3), free DippNH(2) and IPrH(2). The new hindered aryl-amine borane adduct Ar*NH(2)·BH(3) [Ar* = 2,6-(Ph(2)CH)(2)-4-MeC(6)H(2)] underwent a reaction with IPr to give IPr·BH(3) and free Ar*NH(2), consistent with the presence of a weaker N-B dative bond in Ar*NH(2)·BH(3) relative to its less hindered amine-borane analogues.