Dihydrogen Generation from Amine/Boranes: Synthesis, FT‐ICR, and Computational Studies

Abstract

A Fourier transform ion cyclotron resonance spectrometry (FT-ICR) study of the gas-phase protonation of ammonia-borane and sixteen amine/boranes R(1)R(2)R(3)N-BH(3) (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H(2)) are R(1)R(2)R(3)N-BH(4)(+) and R(1)R(2)R(3)N-BH(2)(+), respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6-311+G(d,p)] level) of the thermodynamic stabilities of the R(1)R(2)R(3)N-BH(3) adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R(1)R(2)R(3)N-BH(4)(+) ions is characterized by a three-center, two-electron bond between B and a loosely bound H(2) molecule. Unexpected features of some R(1)R(2)R(3)N-BH(4)(+) ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.

Two faces of triel bonds in boron trihalide complexes

“…The role of boron center in different chemical and biochemical processes is a subject of numerous studies, both experimental and theoretical ones . For example, in boron trihalides, the B‐center is characterized by the p‐orbital vacancy and the Lewis octet rule is not obeyed there .…”

Proton transfer reactions of hydrazine-boranes

“…Amine‐boranes are analogues to hydrazine‐boranes as they also contain the N―B, bond and they have gained significantly more attention. The hydrogen release reaction as well as intrinsic properties related to hydrogen storage, like gas‐phase basicity and acidity, has been studied . It has been found that complexation greatly alters the properties of amines, for example the acidity is enhanced by 30–50 kcal/mol .…”

“…It has been found that complexation greatly alters the properties of amines, for example the acidity is enhanced by 30–50 kcal/mol . The protonation centre of such complexes is located on the boron moiety, and the resulting structure can be described as R 1 R 2 R 3 N‐BH 2 + ••H 2 . The dihydrogen is connected to the remaining complex with S‐shaped 3‐centre‐2‐electron bonds, and its removal is clearly an exergonic process .…”

“…The protonation centre of such complexes is located on the boron moiety, and the resulting structure can be described as R 1 R 2 R 3 N‐BH 2 + ••H 2 . The dihydrogen is connected to the remaining complex with S‐shaped 3‐centre‐2‐electron bonds, and its removal is clearly an exergonic process . The Gibbs free energy change for the dihydrogen dissociation reaction for amine‐boranes varies from −11 to −5 kcal/mol.…”

Die Reaktionen eines P/Al‐basierten frustrierten Lewis‐Paars mit Ammoniak, Boran und Aminboranen: Adduktbildung und katalytische Wasserstoffeliminierung

“…[23] 6 verfügt über kurze intramolekulare P-H d+ ··· dÀ H-B-Kontakte (H1-H2 197.5 pm) [24] und ist stabiler als die entsprechende B-H-aktivierte Spezies (DDG = 16.2 kcal mol À1 ). Danach erfolgt Protonentransfer (H1) auf das hydridische Wasserstoffatom H2, [25] wobei der Komplex 7 mit schwach gebundenem h 2 -H 2 entsteht [24a, 26] (DDG°= 20.6; DDG = 3.6 kcal mol À1 ). 7 verliert leicht Diwasserstoff und geht in das Aminoboran-Addukt 8 über (DDG°= 0.2; DDG = À7.0 kcal mol À1 ).…”