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Photochemical Substitution Reactions of Iron Tricarbonyl 1,4-Dimethyltetraazadiene and Related Complexes. Behavior Consistent with the Strong Coupling Limit
Johnson, Curtis E.,Trogler, William C.
, p. 6352 - 6358 (2007/10/02)
Photosubstitution of CO in Fe(CO)3 and Fe(CO)2 proceeds via a dissociative mechanism, in contrast to corresponding thermal reactions, which are of associative character.Syntheses of Fe(CO)3, Fe(CO)2L , Fe(CO)L2 2=dmpe> and Fe3 are reported.Quantum yields for CO substitution in the tricarbonyl complex increase exponentially as a function of excitation energy from 0.08 at 578 nm to 0.53 at 313 nm.The visible and near-ultraviolet absorptions in the electronic spectrum have been attributed to ?-->?* transitions of the FeN4 metallocycle.There is a little correlation between the detailed wavelength dependence of the quantum yields and either the absorption spectrum or the nature of the lowest excited electronic states.Since the quantum yield-wavelength dependence suggests that photodissociation competes with vibrational relaxation, a weak coupling description is inappropriate.Ramifications of the strong coupling limit are explored, and we propose that the amount by which the irradiation energy exceeds the treshold for Fe-CO bond cleavage accounts for the observed wavelength dependence.Carbon monoxide photosubstitution in Fe(CO)2 and Fe(CO)3 may also be explained in the strong coupling limit.Results for the latter complex further suggest a connection between the coordination environment about iron and the quantum efficiency of the photosubstitution process.
