beta-Hydrogen Kinetic Effect.
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 ( approximately 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 approximately 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.
Romeo
Raffaello
R
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Università di Messina, Salita Sperone, 31-Vill. S. Agata-98166 Messina, Italy. rromeo@unime.it
D'Amico
Giuseppina
G
Sicilia
Emilia
E
Russo
Nino
N
Rizzato
Silvia
S
D003460:Crystallization; D018360:Crystallography, X-Ray; D004583:Electrons; D006859:Hydrogen / Q000737:chemistry*; D007202:Indicators and Reagents; D007536:Isomerism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D012997:Solvents; D013816:Thermodynamics
D007202:Indicators and Reagents; D012997:Solvents; D006859:Hydrogen