Paramagnetic organometallic Cr(II)/Cr(III) redox-active catalysts

The synthesis and reactivity of well-defined paramagnetic organometallic complexes of the first-row transition metals is an area of increasing research, as new applications of these species are discovered. While most inorganic chemists studying paramagnetic organochromium compounds have been investigating their application for olefin polymerization, redox-active chromium catalysts have been developed for CC bond forming reactions for organic synthetic applications. In particular, a variety of chiral ligands have been introduced for the coupling of organic halides and aldehydes by chromium reagents, the Nozaki–Hiyama–Kishi (NHK) reaction. This article describes some of the coordination chemistry of Cr(II) and Cr(III) that led to these asymmetric NHK catalysts and that continues to underpin the unique reactivity profile of these reagents, as well as reviewing recent progress in the development of these and other paramagnetic organometallic catalysts based on more than one oxidation state of chromium.