The Transition State for Surface-Catalyzed Dehalogenation: C–I Cleavage on Pd(111)

Abstract

Substituent effects have been used as a means of probing the nature of the transition state for C-I bond cleavage on the Pd(111) surface. The barriers to C-I cleavage (E ‡ C-I) have been measured in a set of 10 different alkyl and fluoroalkyl iodides (CH 3 I, CF 3 I, CH 3 CH 2 I, CF 3 CH 2 I, CF 2 HCF 2 I, CH 3 CH 2 CH 2 I, CF 3 CH 2 CH 2 I, CF 3 CF 2 CH 2 I, (CH 3) 2 CHI, and (CH 3) 3 CI) on Pd(111). These measurements were performed by adsorbing the iodides on the Pd(111) surface at low temperature (90 K) and then heating to 250 K to induce dissociation (R-I (ad) → R (ad) + I (ad)). X-ray photoemission of the I 3d 5/2 level was used to monitor the extent of reaction during heating. To influence E ‡ C-I the different alkyl and fluoroalkyl groups were chosen to give a wide range of field effect (σ F) substituent constants. By correlating E ‡ C-I with the field effect through a linear free energy relationship (E ‡ C-I = ρ F • σ F) it has been possible to compare the activation of C-I bonds on the Pd(111) surface with other dehalogenation reactions (C-Cl cleavage on Pd(111) and C-I cleavage on Ag(111)). In all cases the reaction constants (ρ F) are very small. For C-I cleavage on the Pd(111) surface ρ F = 0. These results indicate that the transition state to C-I cleavage is homolytic in the sense that it occurs early in the reaction coordinate and the reaction center in the transition state [C ••• I] ‡ is not much different from the initial state reactant. This result appears to be generally true of metal catalyzed dehalogenation reactions.

The Transition State for Carboxylic Acid Deprotonation on Cu(100)

“…This is the case for dehalogenation reactions on the Ag(111) and Pd-(111) surfaces. [4][5][6] Finally, a negative value of F F indicates that fluorination of the substituents stabilizes the transition state with respect to the adsorbed reactant. This implies that the transition state is anionic, as one would expect for the deprotonation of acids in the gas phase.…”

“…2,3 Another reaction for which substituent effects have been used as probes of the transition state is the dehalogenation of alkyl iodides on Ag(111) and Pd(111) surfaces. [4][5][6] Deiodination kinetics were measured using a series of alkyl iodides with a wide range of substituent groups. In that case the transition state was found to be homolytic or reactantlike, [RC • ‚‚‚I • ] ‡ .…”

An infrared spectroscopic and temperature-programmed desorption study of methyl iodide hydrogenation on Pd()

“…A temperature of 120 K corresponds to the multilayer desorption temperature in TPD (Fig. 1) and also that at which C-I bond cleavage takes place (between 110 and 150 K [12,14]). Thus, these features may either be assigned to compressed methyl iodide species on the Pd(1 1 1) surface [11] or to adsorbed methyl species and, most likely, to a combination of both.…”

“…The spectrum formed by heating to 170 K (when all of the multilayer has desorbed (Fig. 1), and C-I bond cleavage is complete [12,14]) shows the appearance of an additional sharper feature at 1225 cm À1 . As the surface is heated, the broad 1215-cm À1 mode loses some of its intensity and the 1225-cm À1 peak becomes sharper.…”

“…The X-ray photoelectron spectra (XPS) of methyl iodide on Pd(1 1 1) have been collected previously [12,14] and reveal that substantial C-I bond cleavage is underway by 110 K and is essentially complete by 150 K. Similar conclusions were arrived at on Pd(1 0 0) [9], where XPS similarly indicated that C-I bond cleavage occurs between 150 and 170 K, so at slightly higher temperatures than on Pd(1 1 1).…”