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

The halogen bonding (XB) between elemental iodine (I₂) 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 I₂ showing an exceptionally strong halogen bond (ΔE^cp = −73.0 kJ/mol) with extremely short N···I distance (2.37 Å) in the 1:1 complex (1a). In the more favored 1:2 complex (1b), the XB interaction was found to be slightly weaker [ΔE^cp = −64.4 kJ/mol and d(N···I) = 2.42 Å] as compared to 1a. The monoalkylated DABCO salts (2PF₆–7PF₆) form corresponding 1:1 XB complexes with I₂ {[2···I₂]­PF₆–([7···I₂]­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 I₂ complexes exhibit a reversible binding of I₂ into their originally nonporous crystal lattices. This was verified by thermal analysis and X-ray powder diffraction studies of 2PF₆–7PF₆ and their corresponding I₂ complexes. By varying the length of the alkyl chain, the release temperature of I₂ can be tuned from 75 °C ([4···I₂]­PF₆) to 100 °C ([7···I₂]­PF₆). Furthermore, these highly stable (preservable for months in normal laboratory conditions) I₂ 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., I₂ vapor to solid PF₆^– salts).