Water-Halogen Interactions in Chlorine and Bromine Clathrate Hydrates: An Example of Multidirectional Halogen Bonding

There are unresolved questions in our understanding of the clathrate hydrates with halogen molecule guests. In this work, we describe the synthesis and single crystal X-ray structural determination of cubic structure I (sI) Cl-2, tetragonal structure III (sIII) Br-2, and mixed cubic sI Cl-2-Br-2 clathrate hydrate phases. Unlike the previous X-ray structural determination of the pure chlorine clathrate hydrate by Pauling and Marsh (Proc. Nat. Acad. Sci. USA 1952, 38, 112), we report that 32.5% of the clathrate small cages are occupied by Cl-2 molecules. Pure Br-2 produces the unique tetragonal sIII clathrate hydrate whose structure is refined from single-crystal measurements, identifying disordered Br-2 guest positions in cages and giving structural details not previously identified. In mixed sI clathrate hydrate of Cl-2 and Br-2, we observed that Br-2 guests fit into the large sI cages. The introduction of Br-2 in the Cl-2 hydrate lattice causes severe distortions because of strong Br center dot center dot center dot OH2 interactions. In these three hydrate structures, based on accepted van der Waals radii, the effective diameters of the clathrate hydrate cages are smaller than the size of the Cl-2 and Br-2 guest molecules. The accommodation of these guests in the cages and the short O center dot center dot center dot Cl and O center dot center dot center dot Br contact distances reveal halogen bonding between the water oxygen atoms and the dihalogen guests. The X-ray structural determinations of the length of the Cl-Cl and Br-Br bonds of the Cl-2 and Br-2 guests show significant librational shortening compared to free Cl-2 and Br-2 molecules.