Intramolecular halogen bonds between aryl halide donors and suitable acceptors, such as carbonyl or quinolinyl groups, held in proximity by 1,2-aryldiyne linkers, provide triangular structures in the solid state. Aryldiyne linkers provide a nearly ideal template for intramolecular halogen bonding as minor deviations from alkyne linearity can accommodate a variety of halogen bonding interactions, including O···Cl, O···Br, O···I, N···Br, and N···I. Halogen bond lengths for these units, observed by single crystal X-ray crystallography, range from 2.75 to 2.97 Å. Internal bond angles of the semirigid bridge between halogen bond donor and acceptor are responsive to changes in the identity of the halogen, the identity of the acceptor, and the electronic environment around the halogen, with the triangles retaining almost perfect co-planarity in even the most strained systems. Consistency between experimental results and structures predicted by M06-2X/6-31G* calculations demonstrates the efficacy of this computational method for modeling halogen-bonded structures of this type.
A series of arylene ethynylene compounds has been generated in order to study the differences between strong and weak intramolecular halogen bonding in solution. With strong intramolecular halogen bonding, the presence of electron‐withdrawing fluorine substituents near the halogen‐bond donor has a significant impact on the halogen bond and consequently on 13C NMR chemical shifts. UV/Vis studies suggest increased conjugation of the arylene ethynylene backbone when strong halogen bonds are present. UV/Vis and NMR signatures of intramolecular halogen bonding are diminished significantly when electron‐withdrawing fluorine substituents are absent, however. This change in behavior is further illustrated by differences in crystallization tendencies with the presence or absence of electron‐withdrawing substituents. Calculations using the M06‐2X functional provide some insight into the energies associated with both strong and weak intramolecular halogen bonding.
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