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 (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).