The transition state for metal-catalyzed dehalogenation: C-I bond cleavageon Ag(111)

The kinetics of C-I bond cleavage on the Ag(111) surface have been measured in a set of 10 alkyl and fluoroalkyl iodides. All the iodides adsorb molec ularly on the Ag(111) surface at low temperatures and then dissociate durin g heating at temperatures below 200 K. X-ray photoemission spectroscopy was used to monitor the rate of C-I cleavage during heating. Estimation of the barrier to C-I cleavage (DeltaE(C-I)(double dagger)) indicates that it is sensitive to the nature of the alkyl or fluoroalkyl substituent groups bond ed to the alpha -carbon atom. The barriers can be correlated to the field a nd polarizability substituent constants (sigma (F) and sigma (alpha)) of th e alkyl groups and reveal that the reaction constants for C-I cleavage are quite low (rhoF = -17 +/- 1 kJ/mol and rho (alpha) = -11 +/- 2 kJ/mol). Thi s suggests that the transition state for C-I cleavage is slightly anionic w ith respect to the adsorbed iodide (RC-I --> [RCdelta-. . .I](double dagger )) or, in other words, that the electron density on the carbon atom of the C-I bond is slightly greater in the transition state than in the reactant i odide. The magnitudes of the reaction constants, however, are relatively lo w. The implication is that the transition state to C-I cleavage is fairly h omolytic in the sense that it occurs early in the reaction coordinate and i s like the initial state iodide.