Photochemistry of (fulvalene)tetracarbonyldiruthenium and its derivatives: Efficient light energy storage devices

Broad-band irradiation (λ(max) = 350 nm) of FvRu₂(CO)₄ (1, Fv = η⁵:η⁵-bicyclopentadienyl) resulted in rapid isomerization to colorless (μ₂-η¹:η⁵-cyclopentadienyl)₂Ru₂(CO)₄ (2) in a novel process involving a formal dinuclear oxidative addition to a C-C bond. The product reverted to 1 upon heating in solution or in the solid state, under the latter conditions with an enthalpy change of -29.8 (1.5) kcal mol^-1. Mechanistic studies with a mixture of 1 and 1-d₈ revealed the absence of label scrambling, pointing to intramolecular pathways. The quantum yield (0.15) was unaffected by the presence of CC1₄, and no chlorination products were observed under these conditions. Irradiation of solutions of 1 or 2 with 300 nm light provided Fv(μ₂-η¹:η⁵-cyclopentadienyl)₂Ru₄(CO)₆ (6) or, in the presence of alkynes, the adducts FvRu₂(CO)₃(RCCR) (8-10, R = H, C₆H₅, CO₂CH₃). Heating 1 and PR₃ (R = CH₂CH₃, CH₃, or OCH₃) yielded FvRu₂(CO)₃(PR₃) (12-14), in which a fluxional process occurs characterized by intramolecular terminal to bridging carbonyl exchange. While 12 and 13 were inert, compound 14 rapidly and reversibly afforded the P(OCH₃)₃- substituted analog of 2 (15) upon irradiation with UV-light. The-two diastereomeric 3,3'-di-tert-butyl-substituted fulvalene analogs of 1 (19) underwent the same reaction sequence with complete retention of stereochemistry, via the diastereomeric photoproducts 20. A double regiochemical labeling experiment proceeded with retention of connectivity and stereochemistry. A concerted mechanism for the photoisomerization is consistent with the experimental observations, but a biradical pathway cannot be ruled out. Kinetic data for the isomerizations of 2, 15, 20a, and 20b to their respective metal-metal-bonded Fv precursors were determined. The entropies of activation (+7 to +21 eu) suggested a disordered transition state. A sequence involving reversible CO loss was ruled out through a crossover experiment with 2-¹³CO. Kinetic and labeling experiments point to a change in mechanism when the thermal reversion of 2 to 1 was run under CO(~l atm). The occurrence of ligand-induced C-C coupling was indicated through studies of the reactivity of 2 with P(CH₃)₃. Photoisomer 2 reacts with excess CCl₄ to give FvRu₂(CO)₄C1₂) by yet another mechanism. As in the potoisomerization of 1, the thermal reversion of 2 may follow a concerted pathway, although biradical intermediates cannot be excluded.