The Solvent Cage Effect:  Is There a Spin Barrier to Recombination of Transition Metal Radicals?

This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination efficiencies of photochemically generated radical cage pairs (denoted as F_cP) were measured in the presence and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect. The first method was femtosecond pump−probe transient absorption spectroscopy, which directly measured F_cP from reaction kinetics, and the second method (referred to herein as the “steady-state” method) obtained F_cP from quantum yields for the radical trapping reaction with CCl₄ as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (λ = 515 nm for the pump probe method and λ = 546 nm for the steady-state method) of Cp‘₂Mo₂(CO)₆ (Cp‘ = η⁵-C₅H₄CH₃). In addition, radical cage pairs generated from Cp‘₂Fe₂(CO)₄ and Cp*₂TiCl₂ (Cp* = η⁵-C₅(CH₃)₅) were studied by the steady-state method. The pump−probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.