Competition between Hydrogen and Halogen Bonds: The Effect of Solvent Volume
Jean Lombard, T. Le Roex, Delia A. Haynes
Abstract
The effect of crystallization method and solvent volume on the competition between hydrogen and halogen bond formation was investigated through co-crystallization experiments carried out by adding the common ditopic acceptor molecule, 1,2-bis(4-pyridyl)ethane, to both a halogen-bond donor molecule and a hydrogen-bond donor molecule simultaneously. The formation of a halogen-bonded co-crystal of 1,2-bis(4-pyridyl)ethane and 1,4-diiodotetrafluorobenzene (1·3) is compared to the formation of three structurally similar hydrogen-bonded co-crystals of 1,2-bis(4-pyridyl)ethane and hydroquinone (2a·3), 2-fluorohydroquinone (2b·3), and 2,3,5,6-tetrafluorohydroquinone (2c·3). Different crystallization methods, including sonic slurry crystallization, neat grinding, liquid-assisted grinding, and co-sublimation, were chosen as they differ, among other things, in the amount of solvent that is used. We observe the known preference toward the halogen-bonded co-crystal when polar solvents are used, but the selectivity is much less pronounced as the amount of solvent is reduced. Competition experiments carried out by vacuum sublimation exclude all solvent, and all selectivity for one form over another is eliminated. The amount of solvent used, and not just its polarity, is therefore critical when co-crystallization is attempted.