Functional formamidines: Pyridine substituents make an exception in the usual doubly hydrogen-bonded formamidine dimer

N,N′-Dipyridylformamidine isomers crystallize differently depending on the position of the pyridyl nitrogen. The 2-pyridyl isomer self-assembles into a quadruply hydrogen-bonded dimeric macrocycle, whereas the 4-pyridyl isomer forms a hydrogen-bonded polymer. It is therefore possible to control the formation of the traditional formamidine dimer based on the positioning of basic residues.