A combined kinetic and DFT study of the uncatalyzed isomerization of cationic solvent complexes of the type cis-[Pt(R')(S)(PR3)2]+ (R' ) linear and branched alkyls or aryls and S ) solvents) to their trans isomers has shown that the reaction goes through the rate-determining dissociative loss of the weakly bonded molecule of the solvent and the interconversion of two geometrically distinct T-shaped 14-electron three-coordinate intermediates. The Pt-S dissociation energy is strongly dependent on the coordinating properties of S and independent of the nature of R'. The energy barrier for the fluxional motion of [Pt(R')- (PR3)2]+ is comparatively much lower ( 8-21 kJ mol-1). The presence of beta-hydrogens on the alkyl chain (R') Et, Prn, and Bun) produces a great acceleration of the reaction rate. This accelerating effect has been defined as the beta-hydrogen kinetic effect, and it is a consequence of the stabilization of the transition state and of the cis-like three-coordinate [Pt(R')(PR3)2]+ intermediate through an incipient agostic interaction. The DFT optimization of [Pt(R')(PMe3)2]+ (R' ) Et, Prn, and Bun) reproduces a classical dihapto Pt....eta2-HC agostic mode between the unsaturated metal and a dangling C-H bond. The value of the agostic stabilization energy (in the range of 21-33 kJ mol-1) was estimated by both kinetic and computational data and resulted in being independent of the length of the hydrocarbon chain of the organic moiety. A better understanding of such interactions in elusive reaction intermediates is of primary importance in the control of reaction pathways, especially for alkane activation by metal complexes.
The beta-Hydrogen Kinetic Effect / R. Romeo, G. D’Amico, E. Sicilia, N. Russo, S. Rizzato. - In: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. - ISSN 0002-7863. - 129:17(2007), pp. 5744-5755.
|Titolo:||The beta-Hydrogen Kinetic Effect|
RIZZATO, SILVIA (Ultimo)
|Settore Scientifico Disciplinare:||Settore CHIM/03 - Chimica Generale e Inorganica|
|Data di pubblicazione:||2007|
|Digital Object Identifier (DOI):||http://dx.doi.org/10.1021/ja0702162|
|Appare nelle tipologie:||01 - Articolo su periodico|