Rhodium(II) acetate dimer
[CAS# 15956-28-2]

Identification

• Classification: Organic raw materials >> Organometallic compound >> Organic rhodium
• Name: Rhodium(II) acetate dimer
• Synonyms: Tetrakis[micron-(acetato-O:O')]dirhodium
• Molecular Formula: C₈H₁₂O₈Rh₂
• Molecular Weight: 441.99
• CAS Registry Number: 15956-28-2
• EC Number: 240-084-8
• SMILES: CC(=O)[O-].CC(=O)[O-].CC(=O)[O-].CC(=O)[O-].[Rh+2].[Rh+2]

Properties

• Melting point: 205 °C

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H315-H319-H412
• Safety Statements: P264-P264+P265-P273-P280-P302+P352-P305+P351+P338-P321-P332+P317-P337+P317-P362+P364-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Eye irritation	Eye Irrit.	2A	H319
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412

Discovery and Applications

Rhodium(II) acetate dimer, commonly represented as Rh₂(OAc)₄, where OAc denotes the acetate ligand (CH₃COO⁻), is a well-known coordination compound consisting of two rhodium centers bridged by four acetate groups. The compound appears as a dark red crystalline solid and is soluble in various organic solvents such as chloroform, dichloromethane, and benzene.

The synthesis of rhodium(II) acetate dimer typically involves the reduction of rhodium(III) chloride in the presence of sodium acetate or acetic acid under reflux conditions. The process results in the formation of the Rh₂(OAc)₄ dimer with rhodium in the +2 oxidation state. This dimeric complex is characterized by a direct metal-metal bond between the two rhodium centers, which is a notable feature of this compound and contributes to its unique chemical properties.

Structurally, rhodium(II) acetate dimer adopts a paddlewheel geometry, where the two rhodium atoms are connected by a Rh–Rh bond and each rhodium is coordinated octahedrally by four bridging acetate ligands in the equatorial positions. The axial positions are typically occupied by solvent molecules or other ligands in solution. The metal-metal bond length is relatively short, reflecting strong bonding interaction.

This compound has significant importance in homogeneous catalysis and organic synthesis. It is widely used as a catalyst or catalyst precursor in carbene and nitrene transfer reactions, including cyclopropanation, C–H insertion, and aziridination. The rhodium(II) acetate dimer facilitates the formation of reactive metal-carbenoid or metal-nitrenoid intermediates, enabling selective transformations with high efficiency and stereoselectivity.

Rhodium(II) acetate dimer also finds application in the synthesis of complex natural products and pharmaceuticals due to its ability to mediate challenging bond formations under mild conditions. It serves as a valuable tool in organometallic chemistry and mechanistic studies related to transition metal-mediated processes.

The compound should be handled with care, as rhodium complexes can be toxic and irritant. It is generally stable under normal laboratory conditions but sensitive to strong oxidants and reducing agents. Proper protective equipment, including gloves and eye protection, should be used when working with this material.

In summary, rhodium(II) acetate dimer is a dark red paddlewheel dimeric complex with bridging acetate ligands and a direct Rh–Rh bond. It is a versatile and widely used catalyst in organic synthesis for carbene and nitrene transfer reactions, valued for its unique structural and electronic properties that enable selective and efficient transformations.

References

2011. Vinyl Diazophosphonates as Precursors to Quaternary Substituted Indolines and Cyclopentenes. Organic Letters, 13(2).
DOI: 10.1021/ol102931n

2014. Synthesis of 3-Amino-3-hydroxymethyloxindoles and 3-Hydroxy-3-hydroxymethyloxindoles by Rh2(OAc)4-Catalyzed Three-Component Reactions of 3-Diazooxindoles with Formaldehyde and Anilines or Water. The Journal of organic chemistry, 79(8).
DOI: 10.1021/jo500307n

2014. One-Pot Protocol to Functionalized Benzopyrrolizidine Catalyzed Successively by Rh2(OAc)4 and Cu(OTf)2: A Transition Metal-Lewis Acid Catalysis Relay. Organic letters, 16(24).
DOI: 10.1021/ol503247t