Equivalence of Ethylene and Azo-Bridges in the Modular
Design of Molecular Complexes: Role of Weak Interactions
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Abstract
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 (−NN−)
and ethylene (−CC−) 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 (Π), 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···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