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) Cl2, tetragonal structure III (sIII) Br2, and mixed cubic sI Cl2–Br2 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 Cl2 molecules.
Pure Br2 produces the unique tetragonal sIII clathrate
hydrate whose structure is refined from single-crystal measurements,
identifying disordered Br2 guest positions in cages and
giving structural details not previously identified. In mixed sI clathrate
hydrate of Cl2 and Br2, we observed that Br2 guests fit into the large sI cages. The introduction of Br2 in the Cl2 hydrate lattice causes severe distortions
because of strong Br···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 Cl2 and Br2 guest
molecules. The accommodation of these guests in the cages and the
short O···Cl and O···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 Cl2 and Br2 guests
show significant librational shortening compared to free Cl2 and Br2 molecules.