Halogen Bonding-Based Catch and Release: Reversible Solid-State Entrapment of Elemental Iodine with Monoalkylated DABCO Salts

The halogen bonding (XB) between elemental iodine (I-2) and neutral 1,4-diazabicyclo[2.2.2]octane (DABCO) and its monoalkylated PF6- salts was studied by X-ray crystallographic, thermoanalytical, and computational methods. DABCO was found to form both 1:1 and 1:2 complexes with I-2 showing an exceptionally strong halogen bond (Delta E-cp = -73.0 kJ/mol) with extremely short N center dot center dot center dot I distance (2.37 angstrom) in the 1:1 complex (1a). In the more favored 1:2 complex (1b), the XB interaction was found to be slightly weaker [Delta E-cp = -64.4 kJ/mol and d(N center dot center dot center dot I) = 2.42 angstrom] as compared to la. The monoallcylated DABCO salts (2PF(6)-7PF(6)) form corresponding 1:1 XB complexes with I-2 ([2 center dot center dot center dot I-2]PF6 ([7 center dot center dot center dot I-2]PF6} similarly to the parent free base DABCO, but both X-ray diffraction and calculated (M05-2X/def2-TZVPP) geometrical parameters indicate that the XB interactions are somewhat weaker than with DABCO itself but can nonetheless be considered as moderately strong halogen bonds. The solid -state packing of the monoalkyl DABCO complexes is greatly affected by the length of the lipophilic hydrocarbon chain as the long-tail cations show increasing amphiphilic character. However, partly as a consequence of the amphiphilic nature of parent monoalkyl DABCO PF6- salts, their I-2 complexes exhibit a reversible binding of I-2 into their originally nonporous crystal lattices. This was verified by thermal analysis and X-ray powder diffraction studies of 2PF(6)-7PF(6) and their corresponding I-2 complexes. By varying the length of the alkyl chain, the release temperature of I-2 can be tuned from 75 degrees C ([4 center dot center dot center dot I-2]PF6) to 100 degrees C ([7 center dot center dot center dot I-2]PF6). Furthermore, these highly stable (preservable for months in normal laboratory conditions) I-2 complexes can be prepared with three different routes: by mixing in solution, by mechanochemical grinding of the components, and via gas-to-solid reaction (i.e., I-2 vapor to solid PF6- salts).