dirhodiumII

Dirhodium(II) refers to a class of dinuclear rhodium compounds in which two rhodium centers are in the +2 oxidation state and share a direct metal–metal interaction. The archetypal member is dirhodium(II) tetraacetate, Rh2(OAc)4, which is typically described as a paddlewheel complex. In this arrangement four carboxylate ligands bridge the two Rh atoms, and each Rh center also bears axial ligands, giving a characteristic Rh2 core with a Rh–Rh bond. The general family is represented by Rh2(O2CR)4, with the R group varying among carboxylate substituents.

Structure and properties: Dirhodium(II) carboxylates feature a dinuclear Rh2 unit whose electronic and steric environment is governed by the bridging carboxylates. The Rh–Rh bond and the carboxylate ligands lead to distinctive reactivity patterns, including tolerance to a range of solvents and functional groups. Many derivatives employ bulky or chiral carboxylates to tune reactivity and selectivity for catalytic applications.

Synthesis and availability: These compounds are prepared from rhodium precursors under conditions that promote formation of the dinuclear core and exchange of carboxylate ligands. Dirhodium(II) carboxylates are widely used in research and are available commercially, with numerous variants prepared to optimize performance for specific transformations.

Applications: The most prominent use of dirhodium(II) complexes is as catalysts for carbene-transfer reactions, derived from diazo compounds. They enable cyclopropanation of alkenes, as well as X–H, N–H, and C–H insertion reactions, often with high stereochemical control when chiral carboxylates are employed. The variety of Rh2 catalysts allows tuning of activity, selectivity, and substrate scope for organic synthesis.

See also: Dirhodium(II) carboxylate catalysts; Cyclopropanation; Carbene transfer; Diazo compounds.