2024-03-29T05:44:09Zhttp://digital.csic.es/dspace-oai/requestoai:digital.csic.es:10261/131972021-06-14T12:06:28Zcom_10261_34com_10261_5col_10261_287
The ever-surprising chemistry of boron: enhanced acidity of phosphine·boranes
Hurtado, Marcela
Yáñez, Manuel
Herrero, Rebeca
Guerrero, Andrés
Dávalos, J.Z.
Abboud, José Luis M.
Khater, Brahim
Guillemin, Jean-Claude
Ab initio calculations
Acidity
Gas-phase reactions
Ion cyclotron resonance
Phosphanes
8 pages, 4 figures, 1 scheme.-- PMID: 19274692 [PubMed].-- Supporting information (14 pages) available at: http://www3.interscience.wiley.com/cgi-bin/fulltext/122246930/sm001.pdf?PLACEBO=IE.pdf
The acidity-enhancing effect of BH(3) in gas-phase phosphineboranes 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 phosphineborane 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 phosphineborane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.
This work has been supported by Grants CTQ 2006-10178/BQU and BQU2003-05827 from Ministerio de Educación y Ciencia (CSIC) , the CNES and PCMI (INSU-CNRS) (ENSCR), by the DGI Project No. BQU2003-00894, Consolider on Molecular Nanoscience, CSD2007-00010, and by the Project MADRISOLAR, Ref.: S-0505/PPQ/0225 of the Comunidad Autónoma de Madrid. A generous allocation of computing time at the CCC of the UAM is also acknowledged.
Peer reviewed
2009-05-26T07:13:07Z
2009-05-26T07:13:07Z
2009-03-09
artículo
http://purl.org/coar/resource_type/c_6501
Chemistry - A European Journal 15(18): 4622-4629 (2009)
0947-6539
http://hdl.handle.net/10261/13197
10.1002/chem.200802307
en
http://dx.doi.org/10.1002/chem.200802307
none
918459 bytes
application/pdf
John Wiley & Sons