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Can an Amine Be a Stronger Acid than a Carboxylic Acid? The Surprisingly High Acidity of Amine-Borane Complexes.
MedLine Citation:
PMID:  23060273     Owner:  NLM     Status:  Publisher    
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 BH(3) 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 BH(3) 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 BH(3) group also contributes to the increased stability of the deprotonated species.
Ana Martín-Sómer; Al Mokhtar Lamsabhi; Manuel Yáñez; Juan Z Dávalos; Javier González; Rocío Ramos; Jean-Claude Guillemin
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-10-11
Journal Detail:
Title:  Chemistry (Weinheim an der Bergstrasse, Germany)     Volume:  -     ISSN:  1521-3765     ISO Abbreviation:  Chemistry     Publication Date:  2012 Oct 
Date Detail:
Created Date:  2012-10-12     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9513783     Medline TA:  Chemistry     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Copyright Information:
Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Departamento de Química, Facultad de Ciencias, Módulo13. Universidad Autónoma de Madrid. Cantoblanco, 28049-Madrid (Spain), Fax: (+34) 91-497-5238.
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