The effect of radical size and mass on the cage recombination efficiency of photochemically generated radical cage pairs

This study explored the effect of radical size, chain length, and mass on t he radical cage effect. Radical cage pairs of the type [(RCp)(CO)(3)M-., M- .(CO)(3)(CpR)] (M = Mo or W; CpR = various substituted cyclopentadienyl lig ands) were generated by photolysis (lambda = 540 nm) of the metal-metal ben ds in (RCp)(2)M-2(CO)(6) The cage recombination efficiencies (denoted as F- cP) for the radical cage pairs were obtained by extracting them from quantu m yield measurements for the photoreactions with CCl4 (a metal-radical trap ) as a function of solvent system viscosity. For the series of molecules (R 3SiOCH2CH2Cp)(2)Mo-2(CO)(6), (R = Me, i-Pr, n-Pr, n-Hx), the F-cP values we re linearly proportional to mass(1/2)/radius(2), in agreement with the pred ictions of Noyes' cage effect theory. It is also demonstrated that the diff erence in the cage recombination efficiencies between the [(MeCp)(CO)(3)Mo- ., Mo-.(CO)(3)(CpMe)] and [(MeCp)(CO)(3)W-., W-.(CO)(3)(CpMe)] cage pairs c annot be ascribed to the different masses of the radicals. Rather, the diff erence is shown to be attributable to differences in the metal-metal bond e nergies or to differences in the spin-orbit coupling. In another comparison , F-cP at any viscosity for [(MeCp)(CO)(3)Mo-., Mo-.(CO)(3)(CpMe)] was show n to be identical to that of [Cp*(CO)(3)Mo-., Mo-.(CO)(3)Cp*] (Cp* = eta(5) -C5Me5) in tetrahydrofuran (THF)/tetraglyme solution. Rotation of the MeCp ring is fast compared to the time scale of diffusive separation (k(dP)) and radical recombination (k(cP)), and hence the effective volumes of the radi cals in the solvent cage are nearly identical, which leads to similar F-cP values.