2,2'-Bipyridyl: Properties, Applications & Lab Guide

2,2'-Bipyridyl is a versatile bidentate nitrogen-donor ligand widely used in coordination chemistry, catalysis, electrochemistry, and analytical science for its exceptional ability to form stable chelate complexes with a broad range of transition metals.

CAS Number

366-18-7

Molecular Formula

C₁₀H₈N₂

Molecular Weight

156.18 g/mol

Appearance

White to off-white crystalline powder

Melting Point

69–73 °C

Coordination Mode

Bidentate, N,N-donor

Aladdin SKU

D108977

Chemical Structure and Physical Properties

2,2'-Bipyridyl (also written 2,2'-bipyridine, commonly abbreviated as bipy or bpy) consists of two pyridine rings connected by a single C–C bond at their 2-positions. The two nitrogen atoms are oriented on the same side of the molecule in the chelating (s-trans to s-cis rotational) conformation, enabling simultaneous coordination to a single metal center through both nitrogen lone pairs.

Key physical data include a molecular weight of 156.18 g/mol (molecular formula C₁₀H₈N₂, CAS 366-18-7), a melting point of approximately 69–73 °C, and moderate solubility in common organic solvents such as ethanol, acetone, chloroform, and dichloromethane. Solubility in water is limited at room temperature but increases upon complex formation or in acidic aqueous media. The compound appears as a white to off-white crystalline solid and exhibits a characteristic UV absorption around 280–285 nm in solution.

Coordination Chemistry and Complex Formation

2,2'-Bipyridyl is one of the most extensively studied bidentate chelating ligands in inorganic and organometallic chemistry. It readily coordinates to a wide variety of transition metals—including Fe, Ru, Co, Ni, Cu, Os, Rh, and Ir—forming thermodynamically stable five-membered chelate rings. The resulting tris(bipyridyl) complexes, such as [Ru(bpy)₃]²⁺, are landmark compounds in photochemistry and electron-transfer research.

The chelate effect confers significantly greater thermodynamic stability on bipy complexes compared to analogous mono-dentate pyridine complexes. The ligand can also act as a π-acceptor through back-donation into its low-lying π* orbitals, making it valuable in stabilizing low-oxidation-state metal centers. This dual σ-donor/π-acceptor character underpins its utility across diverse chemical contexts.

Key Laboratory and Industrial Applications

Analytical Chemistry: 2,2'-Bipyridyl is a classical colorimetric reagent for the determination of iron(II). The intensely colored red-orange tris complex [Fe(bipy)₃]²⁺ (ε ≈ 8,650 L mol⁻¹ cm⁻¹ at ~522 nm) enables sensitive and selective spectrophotometric quantification of Fe²⁺ in water, biological fluids, and industrial samples.

Photochemistry and Solar Energy: The archetypal complex [Ru(bpy)₃]²⁺ is a benchmark photosensitizer in studies of light-driven electron transfer, artificial photosynthesis, and dye-sensitized solar cells (DSSCs). Its long-lived metal-to-ligand charge-transfer (MLCT) excited state (~600 ns in deaerated water) makes it ideal for solar energy conversion research.

Catalysis: Bipy-metal complexes serve as catalysts and pre-catalysts in a broad range of organic transformations, including atom-transfer radical polymerization (ATRP), cross-coupling reactions, and asymmetric synthesis. Copper–bipy systems are especially prevalent in controlled radical polymerization.

Electrochemistry: The reversible redox behavior of bipy complexes makes them useful as electrochemical mediators, reference compounds, and components in molecular electronics and sensors.

Handling, Storage, and Safety Guidelines

Handling: 2,2'-Bipyridyl should be handled in a well-ventilated area or fume hood. Avoid inhalation of dust or vapors, skin contact, and ingestion. Use appropriate personal protective equipment (PPE) including nitrile gloves, safety glasses, and a lab coat.

Storage: Store in a tightly sealed container away from light, moisture, and incompatible materials (strong oxidizers, strong acids). Recommended storage temperature is room temperature (15–25 °C) in a dry environment. Under these conditions, the reagent is stable for extended periods.

Disposal: Dispose of waste material in accordance with local, regional, and national regulations governing chemical waste. Do not pour down drains. Consult the Safety Data Sheet (SDS) for full hazard information, GHS classification, and emergency procedures before use.

Frequently asked questions

What is 2,2'-Bipyridyl used for in analytical chemistry?

2,2'-Bipyridyl is widely used as a colorimetric reagent for the selective spectrophotometric determination of iron(II). It forms a highly colored red-orange tris-chelate complex with Fe²⁺ that absorbs strongly near 522 nm, allowing sensitive quantification of iron in water, environmental, biological, and industrial samples.

Why is [Ru(bpy)₃]²⁺ so important in photochemistry?

[Ru(bpy)₃]²⁺ is a benchmark photosensitizer because it absorbs visible light strongly, undergoes efficient intersystem crossing to a long-lived triplet MLCT excited state (lifetime ~600 ns in deaerated aqueous solution), and can participate in both oxidative and reductive electron-transfer quenching pathways. These properties make it ideal for studying artificial photosynthesis, solar energy conversion, and photoredox catalysis.

How should 2,2'-Bipyridyl solutions be prepared for metal-complex synthesis?

2,2'-Bipyridyl dissolves readily in common organic solvents such as ethanol, methanol, acetone, and acetonitrile. For aqueous reactions, acidification or the use of co-solvents aids dissolution. Weigh out the reagent under ambient conditions, dissolve in the chosen solvent, and use freshly prepared solutions where possible. Confirm concentration by UV spectroscopy if precise stoichiometry is required.

Is 2,2'-Bipyridyl the same as 2,2'-bipyridine?

Yes. 2,2'-Bipyridyl and 2,2'-bipyridine are two names for the same compound (CAS 366-18-7), both referring to the molecule composed of two pyridine rings joined at the 2-positions. The abbreviations bipy and bpy are used interchangeably in the scientific literature.

What metals form stable complexes with 2,2'-Bipyridyl?

2,2'-Bipyridyl forms stable complexes with a broad range of transition metals, including iron (Fe²⁺/Fe³⁺), ruthenium (Ru²⁺), copper (Cu²⁺/Cu⁺), cobalt (Co²⁺/Co³⁺), nickel (Ni²⁺), osmium (Os²⁺), rhodium (Rh³⁺), and iridium (Ir³⁺), among others. The bidentate chelation creates thermodynamically favorable five-membered metallacycles.

References

1. Constable, E. C. (1994). 2,2'-Bipyridine: A molecule for all seasons. Chemical Society Reviews, 23, 205–212.
2. Juris, A. et al. (1988). Ru(II) polypyridine complexes: photophysics, photochemistry, electrochemistry, and chemiluminescence. Coordination Chemistry Reviews, 84, 85–277.
3. Matyjaszewski, K. & Xia, J. (2001). Atom Transfer Radical Polymerization. Chemical Reviews, 101(9), 2921–2990. doi:10.1021/cr940534g
4. Schilt, A. A. (1969). Analytical Applications of 1,10-Phenanthroline and Related Compounds. Pergamon Press, Oxford. [Background reference for bipyridyl iron colorimetry context]

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