Ruthenium and osmium are among the rarest and most expensive stable elements on the planet, yet they are widely used in metal complexes that emit light and mediate photochemical reactions¹. It would be a lot better if more abundant metals could be used instead, but this is difficult. The coordination environments developed for precious metals are usually not directly adaptable to non-noble metals and often result in much inferior photophysical and photochemical properties². For example, when ruthenium(II) is replaced by iron(II) in the prototypical tris(2,2’-bipyridine) coordination environment, the lifetime of the photoactive excited state decreases by 7 orders of magnitude from roughly 500 nanoseconds to 50 femtoseconds^1,3. Photoluminescence can then no longer occur, and photocatalysis is not possible any more from the respective excited state.

From iron via manganese to chromium

The photophysically and photochemically most useful forms of ruthenium and osmium in octahedral coordination complexes are their +II oxidation states, in which they adopt the low-spin d⁶ valence electron configuration. Isoelectronic iron(II) has been investigated extensively as an alternative, but until now only one luminescent iron(II) complex with a very short excited-state lifetime has been reported¹. Moving leftward in the periodic table, the same low-spin d⁶ electron configuration can be obtained with manganese in the +I oxidation state and chromium in the zero-valent state. Earlier we reported two luminescent manganese(I) complexes, but their photoluminescence quantum yields and excited-state lifetimes were roughly a factor of 100 below what is typically reached with photoactive ruthenium(II) compounds⁴. Now, we developed two chromium(0) complexes featuring photoluminescence quantum yields and excited-state lifetimes that are very similar as for a well-known osmium(II) reference compound.

A tailor-made packaging for chromium

The new chromium(0) complexes owe their favorable properties to chelate ligands that make the coordination environment very stiff and enable good electron delocalization in the photoactive excited state. These two design principles minimize energy losses due to unwanted molecular vibrations, and the luminescent and catalytic properties can be optimized. The chromium atoms are shielded particularly well in these complexes, making them remarkably inert towards oxidation and leading to good photo-stability. Encased in these rigid organic frameworks, chromium(0) is therefore easier to handle than its low-valent oxidation state might suggest⁵.

 Photocatalysis under red light

 Owing to their low oxidation state, the new chromium(0) complexes become very strong electron donors when excited. Red light readily triggers electron transfer to reaction partners, which previously often required higher energy input in the form of blue or UV light. This can be exploited for photocatalytic reactions involving the light-driven cleavage of carbon-halogen bonds.

References

1. Sinha, N. & Wenger, O. S. Photoactive Metal-to-Ligand Charge Transfer Excited-States in 3d⁶ Complexes with Cr⁰, Mn^I, Fe^II, and Co^III. J. Am. Chem. Soc. 145, 4903-4920 (2023).
2. Wegeberg, C. & Wenger, O. S. Luminescent First-Row Transition Metal Complexes. JACS Au 1, 1860-1876 (2021).
3. Wenger, O. S. Photoactive Complexes with Earth-Abundant Metals. J. Am. Chem. Soc. 140, 13522-13533 (2018).
4. Herr, P.; Kerzig, C.; Larssen, C. B.; Häussinger, D. & Wenger, O. S. Manganese(I) complexes with metal-to-ligand charge transfer luminescence and photoreactivity. Nat. Chem. 13, 956-962 (2021).
5. Wegeberg, C.; Häussinger, D.; Wenger, O. S. Pyrene-Decoration of a Chromium(0) Tris(diisocyanide) Enhances Excited State Delocalization: A Strategy to Improve the Photoluminescence of 3d⁶ Metal Complexes. J. Am. Chem. Soc. 143, 15800-15811 (2021).