Conformations and Binding Modes of 2,3,5,6-Tetra(2‘-pyridyl)pyrazine

The compound 2,3,5,6-tetra(2‘-pyridyl)pyrazine (tppz) is a remarkably versatile electron donor that displays a wide variety of binding modes. Quantum mechanical calculations on the 14 low energy conformations of tppz indicate a preference for four conformations in methanol and two others in either the gas phase or benzene. The prevalence of a species in solution and its predicted solubility, based on simple dipole moment considerations, provides an explanation for the isolation of different tppz polymorphs from the two solvents. Using a simple system for describing the tppz geometry, a review of previously reported crystal structures of complexes containing tppz as a ligand was conducted. The literature reveals a variety of tppz binding modes, including mono-, bi-, and tridentate coordination to a variety of Lewis acids. In addition to small molecular complexes, tppz-containing macrocycles and one-, two-, and three-dimensional extended chain systems have been characterized. Upon the basis of the analysis of tppz coordination behavior, several new binding modes are proposed that could be used for the construction of novel tppz complexes and supramolecular systems.