Rhodium(III) 2,4-pentanedionate
[CAS# 14284-92-5]

Identification

• Classification: Organic raw materials >> Organometallic compound >> Organic rhodium
• Name: Rhodium(III) 2,4-pentanedionate
• Synonyms: Rhodium (III) acetylacetonate; Tris(2,4-pentanedionato-O,O')-rhodium
• Molecular Formula: C₁₅H₂₁O₆Rh
• Molecular Weight: 400.23
• CAS Registry Number: 14284-92-5
• EC Number: 238-192-5
• SMILES: C/C(=C/C(=O)C)/[O-].C/C(=C/C(=O)C)/[O-].C/C(=C/C(=O)C)/[O-].[Rh+3]

Properties

• Melting point: 260 °C (Expl.)
• Boiling point: 280 °C (Decomposes) (Expl.)

Safety Data

• Hazard Symbols: GHS07;GHS08 Warning
• Risk Statements: H302-H312-H315-H319-H332-H335-H361
• Safety Statements: P203-P261-P264-P264+P265-P270-P271-P280-P301+P317-P302+P352-P304+P340-P305+P351+P338-P317-P318-P319-P321-P330-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute toxicity	Acute Tox.	4	H302
Acute toxicity	Acute Tox.	4	H312
Eye irritation	Eye Irrit.	2	H319
Acute toxicity	Acute Tox.	4	H332
Skin irritation	Skin Irrit.	2	H315
Reproductive toxicity	Repr.	2	H361
Specific target organ toxicity - single exposure	STOT SE	3	H335
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413

Discovery and Applications

Rhodium(III) 2,4-pentanedionate, commonly referred to as rhodium(III) acetylacetonate or Rh(acac)₃, is a coordination complex in which a rhodium(III) ion is chelated by three bidentate acetylacetonate ligands (acac). The acac ligands are derived from 2,4-pentanedione and coordinate to the metal center through their two oxygen atoms. This complex has the molecular formula Rh(C₅H₇O₂)₃ and is typically obtained as a dark red or reddish-brown crystalline solid.

The synthesis of Rh(acac)₃ typically involves reacting a rhodium(III) salt, such as rhodium(III) chloride or rhodium(III) nitrate, with acetylacetone in the presence of a base, which deprotonates the diketone and facilitates coordination to the rhodium center. This procedure is often carried out in aqueous or alcoholic solution and may involve a reflux step to ensure complete reaction. The resulting complex is purified by recrystallization from organic solvents such as ethanol or acetone.

Structurally, rhodium(III) acetylacetonate adopts an octahedral geometry in which the three acac ligands occupy the six coordination sites around the central Rh³⁺ ion. Each ligand forms a five-membered chelate ring with the metal, providing a highly stable and symmetrical complex. The complex is typically low-spin due to the strong-field nature of the acac ligands, and it is diamagnetic. The compound is stable in air and moisture, and it has good solubility in non-polar and moderately polar organic solvents.

Rhodium(III) acetylacetonate is widely used as a precursor in organometallic and coordination chemistry. It serves as a starting material for synthesizing a variety of rhodium complexes by ligand substitution reactions, where one or more acac ligands are replaced by phosphines, nitrogen ligands, or other donor molecules. Its thermal and chemical stability makes it useful for generating rhodium-based catalysts in situ.

In catalysis, Rh(acac)₃ has been employed as a precursor in reactions such as hydroformylation, hydrogenation, and C–H activation. Under reductive conditions, it can be converted into Rh(I) species, which are often the catalytically active forms. It has also been investigated in asymmetric catalysis, where chiral ligands are introduced to replace one or more acac groups, yielding chiral rhodium complexes.

Additionally, Rh(acac)₃ is used in materials science, particularly in the preparation of rhodium-containing thin films or nanoparticles via chemical vapor deposition (CVD), sol–gel processing, or thermal decomposition. These materials have potential applications in electronics, catalysis, and sensor technology.

From a safety standpoint, rhodium(III) acetylacetonate is handled with standard laboratory precautions. It should be stored in a cool, dry place away from strong acids and oxidizing agents. Although rhodium compounds are generally of low toxicity, care should be taken to avoid inhalation of dust or prolonged skin contact.

In summary, rhodium(III) 2,4-pentanedionate is a stable, octahedral complex used extensively as a precursor in coordination chemistry, homogeneous catalysis, and materials science. Its three acac ligands provide strong chelation to the rhodium center, enabling versatile reactivity and facilitating the preparation of a wide range of rhodium-based compounds and catalytic systems.

References

none