AUTHOR=Sittel Steven , Naumann Robert , Heinze Katja TITLE=Molecular Rubies in Photoredox Catalysis JOURNAL=Frontiers in Chemistry VOLUME=10 YEAR=2022 URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2022.887439 DOI=10.3389/fchem.2022.887439 ISSN=2296-2646 ABSTRACT=

The molecular ruby [Cr(tpe)2]3+ and the tris(bipyridine) chromium(III) complex [Cr(dmcbpy)3]3+ as well as the tris(bipyrazine)ruthenium(II) complex [Ru(bpz)3]2+ were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4′-dimethoxycarbonyl-2,2′-bipyridine, bpz = 2,2′-bipyrazine), while [Cr(ddpd)2]3+ serves as a control system (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the CrIII driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst [Cr(tpe)2]3+ outcompetes [Cr(dmcbpy)3]3+ and even [Ru(bpz)3]2+ in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of [Cr(tpe)2]3+ in this photoredox catalytic system.