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).

本研究通过X射线晶体学、热分析及计算方法，对碘元素（I2）与中性1,4-二氮杂双环[2.2.2]辛烷（DABCO）及其单烷基化PF6-盐之间的卤素键合（XB）进行了探讨。研究发现，DABCO能够与I2形成1:1和1:2的复合物，展现出异常强烈的卤素键合（ΔEcp = −73.0 kJ/mol），在1:1复合物（1a）中表现出极短的N···I距离（2.37 Å）。在更受青睐的1:2复合物（1b）中，卤素键合作用相较于1a有所减弱[ΔEcp = −64.4 kJ/mol和d(N···I) = 2.42 Å]。单烷基化DABCO盐（2PF6–7PF6）与I2形成相应的1:1 XB复合物，类似于母体自由碱DABCO，但X射线衍射和计算（M05-2X/def2-TZVPP）几何参数表明，卤素键合作用相对较弱，但依然可被视为中等强度的卤素键合。单烷基DABCO复合物的固态堆积受到亲脂性烃链长度的显著影响，长尾阳离子表现出增强的亲疏水性。然而，部分原因在于母体单烷基DABCO PF6-盐的亲疏水性，其I2复合物表现出对I2的可逆结合，这种结合原本是非多孔的晶体晶格。这一现象通过2PF6–7PF6及其相应I2复合物的热分析和X射线粉末衍射研究得到了验证。通过调节烷基链的长度，I2的释放温度可以从75°C（[4···I2]­PF6）调节至100°C（[7···I2]­PF6）。此外，这些高度稳定的（在正常实验室条件下可保存数月）I2复合物可以通过三种不同的途径制备：溶液混合法、机械化学研磨法以及气固反应法（即I2蒸汽与PF6-盐的固态反应）。