β-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 (≈8−21 kJ mol-1). The presence of β-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 β-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····η2-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.