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.