Isothermal Calorimetric Titrations on Charge-Assisted Halogen Bonds: Role of Entropy, Counterions, Solvent, and Temperature

We have conducted isothermal calorimetric titrations
to investigate
the halogen-bond strength of cationic bidentate halogen-bond donors
toward halides, using bis­(iodoimidazolium) compounds as probes. These
data are intended to aid the rational design of halogen-bond donors
as well as the development of halogen-bond-based applications in solution.
In all cases examined, the entropic contribution to the overall free
energy of binding was found to be very important. The binding affinities
showed little dependency on the weakly coordinating counteranions
of the halogen-bond donors but became slightly stronger with higher
temperatures. We also found a marked influence of different solvents
on the interaction strength. The highest binding constant detected
in this study was 3.3 × 106 M–1.