Generation of Powerful Tungsten Reductants by Visible Light Excitation

The homoleptic arylisocyanide tungsten complexes, W(CNXy)(6) and W(CNIph)(6) (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400-550 nm). MLCT emission (lambda(max) approximate to 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)(6) and W(CNIph)(6) with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)(6) (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [(Bu4N)-N-n][PF6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)(6)](+) and [anth](center dot-). ET from *W to benzophenone and cobalticenium also is observed in [(Bu4N)-N-n][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)(6)](+)/*W couple is -2.8 V vs Cp2Fe+/0, establishing W(CNIph)(6) as one of the most powerful photoreductants that has been generated with visible light.