Ammonia Borane Dehydrogenation Promoted by a Pincer-Square-Planar Rhodium(I) Monohydride: A Stepwise Hydrogen Transfer from the Substrate to the Catalyst

Abstract

ABSTRACT:The , and the tandem ammonia-borane dehydrogenation -cyclohexene hydrogenation. DFT calculations on the ammonia-borane dehydrogenation suggest that the process takes place by means of cis-κ 2 -PP-species, through four stages including: i) Shimoi type coordination of ammonia-borane, ii) homolytic addition of the coordinated H-B bond to afford a five-coordinate-dihydride-boryl-rhodium(III) intermediate, iii) reductive-intramolecular proton transfer from the NH 3 group to one of the hydride ligands, and iv) release of H 2 from the resulting square-planar hydridedihydrogen-rhodium(I) intermediate.

Amine–Borane Dehydropolymerization: Challenges and Opportunities

“…The quaternary B centers in H 3 B⋅NRH 2 precursors display broad (due to quadrupolar coupling) quartet resonances [for example, δ ( 11 B) −18.8 ( 1 J BH 94 Hz); R=Me, CDCl 3 )] which shift downfield in the (H 2 BNRH) n polymer [for example, δ ( 11 B) −6.7; R=Me, CDCl 3 ] where they are typically observed as broad resonances without resolvable 1 H coupling, except in the case of low molecular weight polymer or oligomeric species . Solid state and solution 11 B NMR spectroscopy has been used to propose chain branching/cross‐linking at the boron center, signaled by a resonance to lower field of the main polymer signal . 1 H and 13 C NMR spectroscopy provide information about the R group environments, and have been used to make inferences regarding polymer tacticity, although no clear correlations have yet been developed .…”

“…[16,36,62] Turnover numbers( To N) andt urnover frequencies (ToF) can be used to benchmark catalystp erformance for dehydrogenation. However,c omparisons between catalysts so far reported are not straightforward as To Fs have been calculated at one equivalent of H 2 release( ToF), [36] at 50 %c onversion (ToF 50 % ), [38] or at the maximum rate (ToF max ), [35] and hence cannotb ed irectly compared. Moreover, as To Fv alues are generally concentration dependent they should be, ideally,m easured under the same conditions using multiple data points.…”

“…[70] Solid state and solution 11 BNMR spectroscopy hasb een used to propose chain branching/ cross-linking at the boron center, signaled by ar esonance to lower field of the main polymers ignal. [35,36,38,81] 1 Ha nd 13 CNMR spectroscopy provide information aboutt he Rg roup environments,and have been used to make inferences regarding polymer tacticity, although no clear correlations have yet been developed. [1,27] 15 NNMR spectroscopy of polyaminoboranes has not been studied in detail,b ut Beweries has reported the 1 H- 15 NH MQC NMR spectrumo f( H 2 BNMeH) n ,w hich displays a cross-peak at d( 15 N) À365 with J NH % 65 Hz.…”

“…[29] Later transition-metal catalysts are most common, especially those of Rh and Ru, and earth abundant systemsh ave been developed, primarily based on iron, such as Baker's Fe(PhNCH 2 CH 2 NPh)(Cy 2 PCH 2 CH 2 PCy 2 ), [30] Manners' photochemically activated [FeCp(CO) 2 ] 2 , [31] Schneider's FeH(CO){N(CH 2 CH 2 PiPr 2 ) 2 } [32] and the related FeH(BH 4 )(CO){NH(CH 2 CH 2 PiPr 2 ) 2 }s ystem employed by Beweries. [33] Bidentate and pincerl igands feature heavily in the known catalysts ystems, in particular "POCOP", [1,25] "PNP", [32][33][34][35] and "POP" [36][37][38] constructs.…”

Lithium Hydrazinidoborane Ammoniate LiN2H3BH3·0.25NH3, a Derivative of Hydrazine Borane

“…The second one is centered at δ −6.7 ppm. It may be ascribed to N 2 B H 2 and N 3 B H environments [15]. The presence of such N 2 B H 2 and N 3 B H environments might indicate some decomposition of the starting borane, but the concomitant apparition of the B H 4 environment does not support such an assumption.…”