Equivalence of Ethylene and Azo-Bridges in the Modular Design of Molecular Complexes: Role of Weak Interactions

Structural equivalence is a general tool applied in crystal engineering for the predictable construction of molecular assemblies. In the present contribution we analyzed the equivalence of azo (-N=N-) and ethylene (-C=C-) bridges in the modular design of organic assemblies by studying 22 molecular complexes of 4,4'-azopyridine and 1,2-bis(4-pyridyl)ethene, of which 12 are novel. Unit cell similarity index (II), as a numerical descriptor, was used to rationalize the observed equivalence/ variance in the crystal packing of related complexes. A combined structural chemistry, database analysis and computational methods unveil the fact that the identity of the primary synthons alone does not ensure isostructurality; instead a concurrent effect of the contributions from both strong and weak/dispersive forces determines the structural equivalence. A statistical analysis based on a Cambridge Structural Database survey features an apparent inverse correlation that exist between N center dot center dot center dot I and I I bond distances; a group of data points, however, deviate from this linear relation and was accounted on the basis of electrostatic potential distribution and interaction types.