Trinuclear Rhodium Complexes and Their Relevance for Asymmetric Hydrogenation

Various trinuclear rhodium complexes of the type [Rh3(PP)3(μ3‐OH)x(μ3‐OMe)2−x]BF4 (where PP=Me‐DuPhos, dipamp, dppp, dppe; different ligands and μ‐bridging anions) are presented, which are formed upon addition of bases such as NEt3 to solvate complexes [Rh(PP)(solvent)2]BF4. They were extensively characterized by X‐ray diffraction and NMR spectroscopy (103Rh, 31P, 13C, 1H). Their in situ formation resulting from basic additives (NEt3) or basic prochiral olefins (without addition of another base) can cause deactivation of the asymmetric hydrogenation. This effect can be reversed by means of acidic additives. Basic effects: Stable trinuclear rhodium complexes are presented as possible diolefin‐free precatalysts for asymmetric hydrogenation (see structure). It is shown that basic additives such as NEt3 commonly used to manipulate enantioselectivities can lead to a deactivation by formation of the respective trinuclear complexes, and even prochiral olefins can initiate the formation of trinuclear complexes without other basic additives. However, acidic additives can reverse this effect.