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The Ever‐Surprising Chemistry of Boron: Enhanced Acidity of Phosphine⋅Boranes
Author(s) -
Hurtado Marcela,
Yánez Manuel,
Herrero Rebeca,
Guerrero Andrés,
Dávalos Juan Z.,
Abboud JoséLuis M.,
Khater Brahim,
Guillemin JeanClaude
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802307
Subject(s) - deprotonation , chemistry , boranes , chloride , medicinal chemistry , inorganic chemistry , protonation , phosphine , gas phase , ionization , lewis acids and bases , ion , boron , organic chemistry , catalysis
The acidity‐enhancing effect of BH 3 in gas‐phase phosphine ⋅ boranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas‐phase acidities (GA) of MePH 2 and MePH 2 ⋅ BH 3 differ by about 118 kJ mol −1 (see picture).The gas‐phase acidity of a series of phosphines and their corresponding phosphine ⋅ borane derivatives was measured by FT‐ICR techniques. BH 3 attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol −1 ). This acidity‐enhancing effect of BH 3 is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High‐level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH 3 association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH 3 group is present. Our study also shows that deprotonation of ClCH 2 PH 2 and ClCH 2 PH 2 ⋅ BH 3 is followed by chloride departure. For the latter compound deprotonation at the BH 3 group is found to be more favorable than PH 2 deprotonation, and the subsequent loss of Cl − is kinetically favored with respect to loss of Cl − in a typical S N 2 process. Hence, ClCH 2 PH 2 ⋅ BH 3 is the only phosphine ⋅ borane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.