Bis(norbornadiene)rhodium(I) tetrafluoroborate
[CAS# 36620-11-8]

Identification

• Classification: Organic raw materials >> Organometallic compound >> Organic rhodium
• Name: Bis(norbornadiene)rhodium(I) tetrafluoroborate
• Synonyms: Bis[eta-(2,5-norbornadiene)]rhodium(I) tetrafluoroborate
• Molecular Formula: C₁₄H₁₆BF₄Rh
• Molecular Weight: 373.99
• CAS Registry Number: 36620-11-8
• EC Number: 625-230-4
• SMILES: [B-](F)(F)(F)F.C1C2C=CC1C=C2.C1C2C=CC1C=C2.[Rh]

Properties

• Melting point: 136.9-139.9 ºC

Safety Data

• Hazard Symbols: GHS02;GHS05;GHS07 DangerGHS02;GHS05
• Hazard Statements: H228-H242-H290-H302+H312+H332-H302-H312-H314-H315-H319-H332-H413
• Precautionary Statements: P210-P234-P235-P240-P241-P260-P261-P264-P264+P265-P270-P271-P273-P280-P301+P317-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P351+P338-P305+P354+P338-P316-P317-P321-P330-P332+P317-P337+P317-P362+P364-P363-P370+P378-P390-P403-P405-P406-P410-P411-P420-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1B	H314
Acute toxicity	Acute Tox.	4	H302
Serious eye damage	Eye Dam.	1	H318
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Acute toxicity	Acute Tox.	4	H332
Flammable solids	Flam. Sol.	1	H228
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413
Skin irritation	Skin Irrit.	2	H315
Skin corrosion	Skin Corr.	1C	H314
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Acute toxicity	Acute Tox.	4	H312

• Transport Information: UN 1759

Discovory and Applicatios

Bis(norbornadiene)rhodium(I) tetrafluoroborate is a well-characterized organometallic complex with the formula \[Rh(nbd)₂]⁺BF₄⁻, where "nbd" stands for norbornadiene (bicyclo\[2.2.1]hepta-2,5-diene). This compound consists of a rhodium(I) metal center coordinated to two molecules of norbornadiene, with tetrafluoroborate (BF₄⁻) as a non-coordinating counterion. It appears as a stable yellow-orange crystalline solid under standard conditions and is soluble in polar organic solvents such as acetone and dichloromethane.

The compound was first prepared and structurally characterized in the mid-20th century as part of broader efforts to investigate the coordination chemistry of transition metals with dienes. Norbornadiene, with its strained bicyclic structure and conjugated diene system, forms particularly stable η⁴-complexes with low-valent transition metals, including rhodium(I). Bis(norbornadiene)rhodium(I) tetrafluoroborate is obtained by reacting rhodium(I) precursors such as \[RhCl(nbd)]₂ with silver tetrafluoroborate (AgBF₄) in the presence of an excess of norbornadiene, which serves as both a ligand and a stabilizing agent for the monomeric complex.

In terms of structure, the rhodium(I) center adopts a square planar geometry typical of d⁸ metal complexes. Each norbornadiene ligand coordinates to the metal via its two double bonds, acting as a chelating diene in an η⁴-fashion. The tetrafluoroborate anion remains non-coordinated and serves to balance the positive charge on the metal-ligand complex.

Bis(norbornadiene)rhodium(I) tetrafluoroborate is widely used as a precursor to other rhodium complexes in homogeneous catalysis. Its significance lies in the fact that the norbornadiene ligands are labile and can be readily displaced by a variety of neutral or anionic ligands, allowing the formation of catalytically active species. For example, phosphine ligands such as triphenylphosphine (PPh₃) or bidentate diphosphines can displace one or both norbornadiene ligands to generate rhodium-phosphine complexes, which are active in hydrogenation, hydroformylation, and C–C coupling reactions.

In catalytic applications, bis(norbornadiene)rhodium(I) tetrafluoroborate serves as a source of low-valent rhodium in reactions requiring precise control of the coordination environment. It has been utilized in asymmetric hydrogenation, especially when combined with chiral phosphine ligands, and has demonstrated high activity and selectivity in the hydrogenation of functionalized olefins and imines. The resulting rhodium-phosphine complexes have been instrumental in fine chemical synthesis and pharmaceutical research.

Beyond hydrogenation, this rhodium complex has also been studied in the context of hydrosilylation, carbonylation, and borylation reactions. Its reactivity with small molecules such as CO, H₂, and olefins allows it to act as a versatile platform for mechanistic studies of transition metal catalysis.

Because of its cationic nature and relatively open coordination sphere, \[Rh(nbd)₂]⁺ serves as a model complex in the study of ligand substitution kinetics, oxidative addition, and reductive elimination processes. Its role in the development of catalytic cycles and mechanistic pathways has been well documented in organometallic chemistry literature.

Bis(norbornadiene)rhodium(I) tetrafluoroborate is handled under inert atmosphere conditions, typically in a glovebox or using Schlenk techniques, due to its sensitivity to air and moisture. Prolonged exposure to light or oxygen can lead to decomposition or ligand displacement. Storage in sealed, light-protected containers under nitrogen or argon is recommended for maintaining its stability.

In summary, bis(norbornadiene)rhodium(I) tetrafluoroborate is a fundamental rhodium(I) complex with established importance in synthetic and catalytic chemistry. Its ease of ligand substitution, stability in organic solvents, and well-defined reactivity make it an essential intermediate for the preparation of a wide range of rhodium-based catalysts.

References

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DOI: 10.1038/s42004-024-01313-0

2022. P-Chirogenic Alkylphosphonium Salts. Science of Synthesis, 142, 1.
URL: https://science-of-synthesis.thieme.com/app/text/?id=SD-142-00229

2019. Metal�Organic Frameworks in Asymmetric Catalysis: Recent Advances. Russian Journal of Organic Chemistry, 55, 6.
DOI: 10.1134/s1070428019060101