The Effect of Radical Size and Mass on the Cage Recombination Efficiency of Photochemically Generated Radical Cage Pairs

Abstract

This study explored the effect of radical size, chain length, and mass on the radical cage effect. Radical cage pairs of the type [(RCp)(CO)3M•, •M(CO)3(CpR)] (M = Mo or W; CpR = various substituted cyclopentadienyl ligands) were generated by photolysis (λ = 540 nm) of the metal−metal bonds in (RCp)2M2(CO)6. The cage recombination efficiencies (denoted as F cP) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl4 (a metal-radical trap) as a function of solvent system viscosity. For the series of molecules (R3SiOCH2CH2Cp)2Mo2(CO)6 (R = Me, i-Pr, n-Pr, n-Hx), the F cP values were linearly proportional to mass 1/ 2 /radius2, in agreement with the predictions of Noyes' cage effect theory. It is also demonstrated that the difference in the cage recombination efficiencies between the [(MeCp)(CO)3Mo•, •Mo(CO)3(CpMe)] and [(MeCp)(CO)3W•, •W(CO)3(CpMe)] cage pairs cannot be ascribed to the different masses of the radicals. Rather, the difference is shown to be attributable to differences in the metal−metal bond energies or to differences in the spin−orbit coupling. In another comparison, F cP at any viscosity for [(MeCp)(CO)3Mo•, •Mo(CO)3(CpMe)] was shown to be identical to that of [Cp*(CO)3Mo•, •Mo(CO)3Cp*] (Cp* = η5-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 radicals in the solvent cage are nearly identical, which leads to similar F cP values.

Microviscosity and wavelength effects on radical cage pair recombination

“…The concentration of Cp 0 2 Mo 2 ðCOÞ 6 was changed for each experimental wavelength (k = 546 nm, k = 436 nm, k = 404 nm, and k = 366 nm) so that the resultant solutions had an absorbance Scheme 2. Photolysis of Cp 0 2 Mo 2 ðCOÞ 6 leads to Cp 0 Mo(CO) 3 radicals by way of a [Cp 0 (CO) 3 Mo Å , Å MoCp 0 (CO) 3 ] cage pair. Scheme 1.…”

“…In solution, irradiation into either absorption band causes Mo-Mo photolysis to form a [Cp 0 (CO) 3 Mo Å , Å MoCp 0 (CO) 3 ] cage pair followed by formation of Cp 0 Mo(CO) 3 free radicals (Scheme 2) [3]. Mo-CO bond dissociation also occurs when the 390 nm band is irradiated [12], but it is noted that the steady-state method for determining F cP is independent of side reactions [3] and consequently the presence of a Mo-CO bond dissociation process does not affect the accurate determination of F cP values for the radical cage pairs formed by Mo-Mo bond homolysis [13]. In the presence of CCl 4 , the Cp 0 (CO) 3 Mo Å free radicals react to form Cp 0 (CO) 3 …”

“…In contrast, the steady-state method measures net F cP values that are a combination of both primary and secondary caging. Non-geminate cage effects are effectively eliminated from the steady-state method by trapping all free radicals with an excess of CCl 4 [3].…”

“…The solvent cage effect is vital to understanding the reactivity of radicals generated by bond homolysis reactions in solution [1][2][3]. The term ''cage effect'' refers to the phenomenon that the probability of recombination of a radical pair is greater in solution than in the gas phase.…”

“…It is not clear, however, if this same dependence of F cP on wavelength holds for larger polyatomic molecules because energy transfer to the large number of vibrational modes available in such molecules may be very fast. To investigate this question, we studied the dependence of F cP on the irradiation wavelength for the [Cp 0 (CO) 3 Mo Å , Å MoCp 0 (CO) 3 ] radical cage pair, generated by photolysis of Cp 0 2 Mo 2 (CO) 6 (Scheme 2). The results of our study are reported herein.…”