Terpyridine

In chemistry, terpyridine (2,2';6'2"-terpyridine) is a polypyridine compound in which three pyridine molecules are bound with a single bond. Its molecular formula is C15H11N3.

Terpyridine was first synthesized by G. Morgan and F. H. Burstall in 1932. It was called tripyridyl at that time.

Synthesis
Terpyridine was initially synthesized from the oxidative coupling of pyridines. This method, however, has low-yield reactions and other oligomers and isomers are created, so it was necessary to separate the mixtures. In 1991, L. E. Guise et al. reported the more efficient synthesis of terpyridine from the condensation of 2-acetylpyridine and enaminone, which is prepared by the reaction of 2-acetylpyridine with N,N-dimethylformamide dimethyl acetal.

An alternative is to react 2-acetylpyridine with a base, carbon disulfide and methyl iodide to form PyCOCH=C(SMe)2, this is then reacted with 2-acetylpyridine and a base to form a 1,5-diketone, this diketone is then reacted with ammonium acetate to form a terpyridine which is then treated with Raney nickel to remove the thioether group.

Another alternative is to form a BTP and then to react this with norbornadiene to convert the triazine rings to pyridine rings.

Substituted terpyridines are also synthesized from palladium-catalyzed cross-coupling reactions.

Properties
Terpyridine is a tridentate ligand and forms a complex with a transition metal ion in the same way as other polypyridine compounds, such as 2,2'-bipyridine and 1,10-phenanthroline. In particular, to six-coordinated metal ions, such as cobalt or iron, two terpyridines can be coordinated. These kinds of complexes, called bisterpyridine complexes, do not have any enantiomers, so they differ from bipyridine complexes. The steric structures of bisterpyridine complexes are therefore easy to control.

The terpyridine complexes, like other polypyridine complexes, exhibit characteristic optical and electrochemical properties: metal-to-ligand charge transfer (MLCT) in the visible light region, reversible reduction and oxidation, and fairly intense luminescence. In these properties, however, slight differences exist between terpyridine complexes and bipyridine complexes.

The fact that terpy can act as a pi-acceptor enables terpy and bipyridine to stabilise 'abnormal' oxidation states. For instance with two terpy ligands it is possible to form in acetonitrile cobalt(III) and cobalt(I) complexes which are stable on the time scale of seconds. Also it is possible to form nickel(I) and nickel(III) complexes.

In addition to the metal atoms in these complexes undergoing redox reactions, the ligands can accept an electron in one electron reductions.

Related compounds
The 2,6-bis-(1,2,4-triazinyl) pyridines are able to bind to metals in a similar way to 2,2';6'2"-terpyridine. These 2,6-bis-(1,2,4-triazinyl) pyridines (BTPs) contain both pyridine and triazine rings. They are made by the reactions of hydrazine and a 1,2-diketone with pyridine-2,6-dicarbonitrile. These compounds were first made by F.H. Case, but in recent years they have been of interest as possible extraction agents for the trivalent minor actinides in a SANEX process. For more details of the SANEX process please see nuclear reprocessing.