Complexes of expensive metals such as Ru(II), Os(II), and Ir(III) are used, e.g., in lighting, solar energy conversion, and photocatalysis. Replacing these metals with cheaper, more Earth-abundant metals would be useful. However, complexes of abundant first-row transition metals with a d6 valence electron configuration that show comparable photophysical and photochemical properties to the more expensive metals have been challenging to find so far. Often, the search for abundant metals that can be used in this context has focused primarily on iron and copper. Chromium has also been incorporated into luminescent materials.
Oliver S. Wenger, University of Basel, Switzerland, and colleagues have developed chromium compounds that could replace the metals osmium and ruthenium in luminescent materials and catalysts. The team found that chromium requires a more complex ligand environment: They designed an organic ligand framework for Cr(0) that is particularly bulky and rigid (example complex pictured). This environment helps to minimize energy losses and optimize the luminescent and catalytic properties.
The team obtained the complexes [Cr(LMes)3] and [Cr(LPyr)3] by reacting the isocyanide chelate ligands LMes and LPyr, which have m-terphenyl ligand backbones with mesityl or pyrenyl substituents, with CrCl3(THF)3 in the presence of Na/Hg in tetrahydrofuran (THF) at room temperature. The complexes were synthesized in 78 % and 47 % yield, respectively.
The complexes were found to be efficient catalysts for photochemical reactions, which could allow for reactions that are otherwise difficult to initiate. According to the team, the luminescent properties of the synthesized chromium complexes are nearly as good as some of the osmium compounds used so far.
- Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes,
Narayan Sinha, Christina Wegeberg, Daniel Häussinger, Alessandro Prescimone, Oliver S. Wenger,
Nat. Chem. 2023.
https://doi.org/10.1038/s41557-023-01297-9
