4,4′-Bipyridine | CAS 553-26-4 | ≥98%

4,4'-Bipyridine (4,4'-dipyridyl, 4,4'-bipy, γ,γ'-dipyridyl, 4-(pyridin-4-yl)pyridine) is a rigid, rod-like ditopic N-donor in which two pyridine rings are joined by a single C–C bond at their 4-positions. The two pyridyl nitrogens point outward in opposite directions, so — unlike the 2,2'-isomer, whose adjacent nitrogens chelate a single metal — 4,4'-bipyridine acts as two independent monodentate donors that bridge separate metal centres. In many solid-state structures the two rings adopt a twisted conformation rather than a fully coplanar geometry; coordination, protonation, and hydrogen bonding can change this torsion angle and tune conjugation across the biaryl axis. It is also a weak ditopic base and a reliable double hydrogen-bond acceptor, properties that underpin its dual role in coordination and supramolecular chemistry.

Preparation

4,4'-Bipyridine is accessed by reductive C–C homocoupling of pyridine through pyridyl radical-anion chemistry, with subsequent air oxidation, or more selectively by nickel- or palladium-catalysed coupling of 4-halopyridines. The same dimerised scaffold is the parent framework of the bipyridinium, or viologen, family.

Building block for metal–organic frameworks and coordination polymers

4,4'-Bipyridine is a classic rigid ditopic ligand that can act as a bridging and piillar linker for one-, two-, and three-dimensional coordination polymers and metal–organic frameworks, connecting metal nodes such as Cu(II), Zn(II), Co(II), Ni(II), and Cd(II) with defined, near-linear spacing. Its fixed geometry templates predictable topologies — zigzag and linear chains, ladders, square grids, and interpenetrated three-dimensional nets — and, combined with carboxylate or other ditopic co-ligands, builds pillared-layer frameworks studied for gas adsorption, separation, sensing, catalysis, and framework flexibility such as gate-opening and breathing. In discrete bimetallic and mixed-valence complexes, coplanar 4,4'-bipyridine bridges can mediate through-ligand electronic coupling, making the ligand useful in electron-transfer and molecular-wire model studies.

Ditopic linker in crystal engineering and cocrystals

Beyond metal coordination, the two pyridyl nitrogens make 4,4'-bipyridine a reliable hydrogen-bond acceptor for organic crystal engineering. With carboxylic acids and phenols it forms O–H···N heterosynthons, assembling one-dimensional chains and pillared hydrogen-bonded networks, and it is widely used as a rigid coformer in cocrystal design, where its predictable two-point connectivity controls packing and stoichiometry.

Precursor to viologen redox materials

Double quaternisation of the two pyridyl nitrogens with alkyl halides converts 4,4'-bipyridine into 4,4'-bipyridinium salts — the viologens — of which methyl viologen is the archetype. Viologens undergo reversible one-electron reduction to intensely coloured radical cations and further reduction to neutral forms, a fast, reversible redox couple that makes viologens widely used electron-transfer mediators and redox-active components of functional materials.

Beyond synthesis

• Electrochromic and energy-storage materials: the reversible, strongly coloured viologen redox couple underpins electrochromic displays and windows and serves as a redox-active anolyte/negolyte in aqueous organic redox-flow batteries; viologens are also studied in molecular electronics and solar-energy conversion.
• Electron-transfer mediator in solar-fuel models: methyl viologen and related viologens serve as electron relays in photocatalytic assemblies, including Ru(bpy)₃²⁺-sensitised hydrogen-evolution and CO₂-reduction model systems, where the MV²⁺/MV•⁺ couple transfers reducing equivalents to catalytic sites.
• Photochromic and stimuli-responsive MOF frameworks: bipyridinium-derived and related 4,4'-bipyridyl ligands are incorporated into coordination polymers and MOFs where electron-transfer-generated radical species give photochromic, vapochromic, or analyte-responsive behaviour.
• Host–guest and molecular-machine chemistry: viologen units derived from 4,4'-bipyridine act as electron-poor recognition sites for electron-rich macrocycles, supporting rotaxane, catenane, and mechanically interlocked molecular architectures.