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

Abstract

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 FcP) 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 FcP from reaction kinetics, and the second method (referred to herein as the "steady-state" method) obtained FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (lambda = 515 nm for the pump probe method and lambda = 546 nm for the steady-state method) of Cp'2Mo2(CO)6 (Cp' = eta(5)-C5H4CH3). In addition, radical cage pairs generated from Cp'2Fe2(CO)4 and Cp*2TiCl2 (Cp* = eta(5)-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.

Cage Escape Competes with Geminate Recombination during Alkane Hydroxylation by the Diiron Oxygenase AlkB

“…The intercalation of solvent molecules between organometallic radical pairs has been directly observed to occur in the picosecond time regime. [19,20] Rapid reorganization of active-site water is also thought to be involved in the complicated diffusional kinetics that occur after photodissociation of heme protein axial ligands. [21,22] For photodissociated [heme-Fe II NO] and [heme-Fe II O 2 ] in myoglobin, ligand recombination and cage escape are competitive.…”

“…The intercalation of solvent molecules between organometallic radical pairs has been directly observed to occur in the ps time regime. [19,20] Rapid reorganization of active site water is also thought to be involved in the complicated diffusional kinetics that occur after photodissociation of heme protein axial ligands. [21,22] [23] Thus, even though there is almost no enthalpic barrier to ligand recombination, there is an entropic driving force that favors ligand diffusion into adjacent protein cavities where NO, for example, is observed to persist for up to a nanosecond.…”

A two‐step reaction scheme leading to singlet carbene species that can be detected under matrix conditions for the reaction of Zr(3F) with either CH3F or CH3CN

“…For instance, two sequential radical reactions similar to those proposed here could be helpful for rationalizing the results obtained by Harris et al for the recombination efficiencies of photochemically generated radical cage pairs containing Mo, Fe, and Ti in viscous solvents. These authors have suggested that no spin barrier exists to radical–radical recombination . Although not conclusive because the matrix has not been modeled explicitly, this scheme could represent an alternative to the two‐state and multistate reactivity models for the theoretical description of reaction systems involving transition‐metal species in confined media.…”

“…These authors have suggested that no spin barrier exists to radical-radical recombination. [40] Although not conclusive because the matrix has not been modeled explicitly, this scheme could represent an alternative to the two-state and multistate reactivity models for the theoretical description of reaction systems involving transition-metal species in confined media. [41,42] Conclusions A theoretical study using a gas-type model for the reactions Zr 1 CH 3 F and Zr 1 CH 3 CN was performed to rationalize the product distributions revealed from IR matrix-isolation determinations.…”

Should External Heavy-Atom Effects be Expected in Transition Metal Complexes?

“…Tyler and coworkers investigated the presence of a spin barrier to the geminate recombination 17-electron radicals in complexes of Fe, Mo, and Ti. 29 Recombination efficiencies were found to be similar in the presence or absence of an external heavy atom perturber, from which it was concluded that no spin barrier was present; another possible interpretation exists, which is that an external heavy atom effect may not have been significantly observed. A combined experimental/computational study by our group also investigated the activation of stannanes (Sn-H bonds) by a series triplet, first-row transition metal reaction intermediates.…”