The condensation of two pyridine-2-carboxaldehyde molecules and one ammonium cation produces 3-(pyridyne-2-yl)imidazo[1,5-a]-pyridine ligands which have the indolizine ring:

Complexes with 3-(pyridyne-2-yl)imidazo[1,5-a]-pyridine ligands by spontaneous dimerization of pyridine-2-carboxaldehyde within the coordination sphere of manganese(II) in a one-pot reaction.
Celedonio M. Álvarez, Lucía Álvarez-Miguel, Raúl García-Rodríguez and Daniel Miguel.
Dalton Trans., 2012,41,7041-7046. DOI: 10.1039/C2DT30453H