Can an Amine Be a Stronger Acid than a Carboxylic Acid? The Surprisingly High Acidity of Amine-Borane Complexes

• Published in: Chemistry : a European journal Vol. 18; no. 49; pp. 15699 - 15705
• Main Authors: Martín-Sómer, Ana, Lamsabhi, Al Mokhtar, Yáñez, Manuel, Dávalos, Juan Z., González, Javier, Ramos, Rocío, Guillemin, Jean-Claude
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 03.12.2012; WILEY‐VCH Verlag; Wiley Subscription Services, Inc; Wiley-VCH Verlag
• Subjects: ab initio calculations; acidity; Acids; amine-borane complexes; Amines; Analytical chemistry; Aniline; Boranes; Carboxylic acids; Chemical Sciences; Chemistry; Complexation; electrospray mass spectrometry; extended Cooks kinetic method; Gas phases; Ionization; Mass spectrometry; Mathematical analysis; Organic chemistry; ab initio calculations; electrospray mass spectrometry; acidity; extended Cooks kinetic method; amine-borane complexes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201202192

The gas‐phase acidity of a series of amine–borane complexes has been investigated through the use of electrospray mass spectrometry (ESI‐MS), with the application of the extended Cooks kinetic method, and high‐level G4 ab initio calculations. The most significant finding is that typical nitrogen bases, such as aniline, react with BH3 to give amine–borane complexes, which, in the gas phase, have acidities as high as those of either phosphoric, oxalic, or salicylic acid; their acidity is higher than many carboxylic acids, such as formic, acetic, and propanoic acid. Indeed the complexation of different amines with BH3 leads to a substantial increase (from 167 to 195 kJ mol−1) in the intrinsic acidity of the system; in terms of ionization constants, this increase implies an increase as large as fifteen orders of magnitude. Interestingly, this increase in acidity is almost twice as large as that observed for the corresponding phosphine–borane analogues. The agreement between the experimental and the G4‐based calculated values is excellent. The analysis of the electron‐density rearrangements of the amine and the borane moieties indicates that the dative bond is significantly stronger in the N‐deprotonated anion than in the corresponding neutral amine–borane complex, because the deprotonated amine is a much better electron donor than the neutral amine. On the top of that, the newly created lone pair on the nitrogen atom in the deprotonated species, conjugates with the BN bonding pair. The dispersion of the extra electron density into the BH3 group also contributes to the increased stability of the deprotonated species. It's all relative: The complexation of boranes to amines leads to a dramatic increase in the acidity of the amine. Thus, typical nitrogen bases, such as aniline, give amine–borane complexes, which are nitrogen‐based acids that are as acidic as phosphoric acid in the gas phase.