Organometallic Photocatalysis Promoted by Visible Light (Sunlight) : Photo-redox Catalysis and Difunctional Dinuclear Catalyst System

Organometallic photocatalysis developed in the authors' laboratory, which is mediated by [Ru(bipy)(3)](2+) (TB)-type sensitizers, is reviewed. After characteristics of photophysical properties of TB ( (1) effective MLCT transition caused by visible light, (2) long lifetime of the excited triplet states, and (3) redox activities of the excited states) are briefly summarized, discussion is devoted to two types of catalytic systems. One is photoredox catalysis, where the photoexcited TB species serves as a 1e-oxidant as well as a 1e-reductant (SET (single electron transfer) processes) in a single catalytic cycle. The reactions are further divided into two types: reductive and oxidative quenching cycles. The former is effective for generation of radical species from electron-rich substrates such as amine and organoborate, whereas the latter providing carbon-centered radicals is effective for C-C bond formation, in particular, C-CF3 bond formation by the action of electrophilic trifluoromethylating reagents. The present photoredox-catalytic system turns out to be very green in terms of (1) generation of organic radicals under mild conditions without use of harmful reagents, (2) no need of special equipment, (3) no need of sacrificial redox reagent (redox-neutral and atom-economic), and (4) sunlight-promoted reactions (no need of energy from oil and coal). The other catalytic system is difunctional dinuclear catalyst system consisting of a TB-type photo-harvesting unit and a reactive Pd center connected by a bipyrimidine bridge. The dinuclear catalyst turns out to be effective for dimerization/oligomerization/polymerization of olefins, and the photoexcitation promotes insertion of olefin into the Pd-C bond. Studies on the substituent effects and DFT analysis reveal that photoexcitation at the TB-like moiety induces MLCT transition toward the bridging bmp part, which causes perturbation of the electronic structure of the reactive Pd center to promote the insertion.