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New Insights into the Mechanism of Proton Transfer to Hydride Complexes: Kinetic and Theoretical Evidence Showing the Existence of Competitive Pathways for Protonation of the Cluster [W 3 S 4 H 3 (dmpe) 3 ] + with Acids
Author(s) -
Algarra Andrés G.,
Basallote Manuel G.,
Feliz Marta,
FernándezTrujillo M. Jesús,
Llusar Rosa,
Safont Vicent S.
Publication year - 2006
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.200500695
Subject(s) - protonation , hydride , kinetic energy , proton , cluster (spacecraft) , mechanism (biology) , chemistry , computational chemistry , chemical physics , physics , computer science , nuclear physics , metal , ion , organic chemistry , quantum mechanics , programming language
The reaction of the hydride cluster [W 3 S 4 H 3 (dmpe) 3 ] + ( 1 , dmpe=1,2‐bis(dimethylphosphanyl)ethane) with acids (HCl, CF 3 COOH, HBF 4 ) in CH 2 Cl 2 solution under pseudo‐first‐order conditions of excess acid occurs with three kinetically distinguishable steps that can be interpreted as corresponding to successive formal substitution processes of the coordinated hydrides by the anion of the acid (HCl, CF 3 COOH) or the solvent (HBF 4 ). Whereas the rate law for the third step changes with the nature of the acid, the first two kinetic steps always show a second‐order dependence on acid concentration. In contrast, a single kinetic step with a first‐order dependence with respect to the acid is observed when the experiments are carried out with a deficit of acid. The decrease in the T 1 values for the hydride NMR signal of 1 in the presence of added HCl suggests the formation of an adduct with a WH⋅⋅⋅HCl dihydrogen bond. Theoretical calculations for the reaction with HCl indicate that the kinetic results in CH 2 Cl 2 solution can be interpreted on the basis of a mechanism with two competitive pathways. One of the pathways consists of direct proton transfer within the WH⋅⋅⋅HCl adduct to form WCl and H 2 , whereas the other requires the presence of a second HCl molecule to form a WH⋅⋅⋅HCl⋅⋅⋅HCl adduct that transforms into WCl, H 2 and HCl in the rate‐determining step. The activation barriers and the structures of the transition states for both pathways were also calculated, and the results indicate that both pathways can be competitive and that the transition states can be described in both cases as a dihydrogen complex hydrogen‐bonded to Cl − or HCl 2 − .