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

Abstract

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 (ΔEcp = −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 [ΔEcp = −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).