PHOTOCHEMISTRY OF (FULVALENE)TETRACARBONYLDIRUTHENIUM AND ITS DERIVATIVES - EFFICIENT LIGHT ENERGY-STORAGE DEVICES

Broad-band irradiation (lambda(max)=350 nm) of FvRu(2)(CO)(4) (1, Fv=e ta(5):eta(5)-bicyclopentadienyl) resulted in rapid isomerization to co lorless u(2)-eta(1):eta(5)-cyclopentadienyl)(2)Ru-2(CO)(4) (2) in a no vel process involving a formal dinuclear oxidative addition to a C-C b ond. The product reverted to 1 upon heating in solution or in the soli d 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-d(8) revealed the absence of label scrambling, pointing to intramolecular pathways. The quantum yield (0.15) was unaffected by the presence of C Cl4, and no chlorination products were observed under these conditions . Irradiation of solutions of 1 or 2 with 300 nm light provided u(2)-e ta(1):eta(5)-cyclopentadienyl)(2)Ru-4(CO)(6) (6) or, in the presence o f alkynes, the adducts FvRu(2)(CO)(3)(RCCR) (8-10, R = H, C6H5, CO2CH3 ). Heating 1 and PR3 (R = CH2CH3, CH3, or OCH3) yielded FvRu(2)(CO)(3) (PR3) (12-14), in which a fluxional process occurs characterized by in tramolecular terminal to bridging carbonyl exchange. While 12 and 13 w ere inert, compound 14 rapidly and reversibly afforded the P(OCH3)(3)- substituted analog of 2 (15) upon irradiation with UV light. The two d iastereomeric 3,3'-di-tert-butyl-substituted fulvalene analogs of 1 (1 9) underwent the same reaction sequence with complete retention of ste reochemistry, via the diastereomeric photoproducts 20. A double regioc hemical labeling experiment proceeded with retention of connectivity a nd stereochemistry. A concerted mechanism for the photoisomerization i s consistent with the experimental observations, but a biradical pathw ay 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 di sordered transition state. A sequence involving reversible CO loss was ruled out through a crossover experiment with 2-(CO)-C-13. Kinetic an d labeling experiments point to a change in mechanism when the thermal reversion of 2 to 1 was run under CO (similar to 1 atm). The occurren ce 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 FvRu(2)(CO)(4)Cl-2) by yet another mechanism. As in the potoi somerization of 1, the thermal reversion of 2 may follow a concerted p athway, although biradical intermediates cannot be excluded.