Mechanism of an Alkyl‐Dependent Photochemical Homolysis of the Re–Alkyl Bond in [Re(R)(CO)3(α‐diimine)] Complexes via a Reactive σπ* Excited State

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Abstract
MLCT excitation of the complexes [Re(R)(CO)3(α‐diimine)] (R = Me, Et, benzyl (Bz); α‐diimine = iPr‐PyCa, R′‐DAB) results in the homolysis of the Re‐R bond leading to the formation of radicals R. and [Re(CO)3(α‐diimine)]. as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low (10−2) and depends on the temperature and excitation wave‐length, whereas for R = Et and Bz the quantum yield is near unity and independent of T and λexc. The reaction is shown to proceed via a σ(Re‐R)π* excited state that is rapidly (< 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time‐resolved IR and UV/Vis absorption spectra studied in the ns‐μs and ps‐μs time domains, respectively, show that the σπ* excited state is rather long‐lived (τ ≈ 250 ns) in noncoordinating solvents; the dissociation of the Re‐R bond from this state is strongly accelerated by polar or coordinating solvents (τ< 20 ps). The σπ* excited state is spectroscopically characterized by a (presumably σπ* → MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground‐state values. The relative energies of the MLCT and reactive σπ* states, controlled by the nature of the alkyl lig‐and, determine the photoreactivity of the complexes.

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