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 (λmax ≈ 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, [Bun4N]­[PF6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W­(CNIph)6]+ and [anth]•–. ET from *W to benzophenone and cobalticenium also is observed in [Bun4N]­[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.