different anions determined the resulting assemblies as polymer, planar dinuclear, or
propeller-type dinuclear structures. Whereas the Ag(I)-bppn complexes gave rise to
grid-type structures regardless of the anion, which is attributed to the electron-rich πligand favors the maximized π-π stacking at the expense of anion-π interactions.
Taking the Fe(II) metallacycles as examples, Dunbar and coworkers [35, 36] applied
19 F NMR spectroscopy to provide evidence that anion-π interactions are the main
driving force in the templating process leading to particular structures in solution.
Whereas random Fe(II)/bytz oligomers were formed in the presence of nontemplating anions such as CF 3 SO 3
À
, closed polygons were favored in the presence
of templating anions including BF 4
À , AsF 6
À , and SbF 6
À . At elevated temperature,
the considerable broadening
19 F NMR resonance indicated rapid exchange of the
encapsulated anions with free species, while lowering the temperature led to broadening of
19 F NMR resonance of polygons, and a second distinct
19 F NMR resonance
corroborating the presence of encapsulated anions occurred. From the low activation
energy determined, the authors concluded that anions acted as templates rather than
merely diffusing into preformed cages.
In another report by Choi et al. [37], they set up mechanochemical reactions of
3,6-dimethoxy-s-tetrazine (dmotz) with AgCF 3 SO 3 and AgClO 4 . Different structures such as 1D linear polymer ([Ag(dmotz)(CF 3 SO 3 )] n ) or 2D grid polymer ([Ag
(dmotz) 2 (ClO 4 )] n ) were obtained depending on the specific anions applied.
Intermolecular anion-π interactions between tetrazine ring and CF 3 SO 3
À , for example, led to the tightening of 1D linear chains, whereas ClO 4
À was surrounded with
four tetrazine rings which provided the 2D structure.
10.5 Self-Assembly with Anion as Primary Building Blocks
As the main challenge in anion-templated self-assemblies (vide supra) is that it is
difficult to distinguish the contribution of anion-π and coordination interactions,
using charge-neutral π receptor as one building component to probe anion-π-directed
assembly is therefore particularly intriguing. However, due to the flexible directionality of anion-π non-covalent bond, this task is very challenging, and the early
examples that appeared in literature were mainly observed in crystal structures.
The rational design of anion-π-controlled self-assembly based on charge-neutral
building blocks emerged until very recently.
10.5.1 Self-Assembly with Electron-Deficient Arenes
In 2008, Kochi et al. [38] examined the behavior of a series of planar π-acids toward
various types of polyatomic anions by means of crystallography. They demonstrated
from about 20 crystal structures that similar repeating anion/π-acid arrangement
consisting of a vertical one-dimensional (1D) stack of π-acid alternatively interspersed with the π-bonded anions (Fig. 11). Anions interacted with the carbon atom
of π-acid molecule forming the so-called σ-type interaction. The location of anions
262
D.-X. Wang
propeller-type dinuclear structures. Whereas the Ag(I)-bppn complexes gave rise to
grid-type structures regardless of the anion, which is attributed to the electron-rich πligand favors the maximized π-π stacking at the expense of anion-π interactions.
Taking the Fe(II) metallacycles as examples, Dunbar and coworkers [35, 36] applied
19 F NMR spectroscopy to provide evidence that anion-π interactions are the main
driving force in the templating process leading to particular structures in solution.
Whereas random Fe(II)/bytz oligomers were formed in the presence of nontemplating anions such as CF 3 SO 3
À
, closed polygons were favored in the presence
of templating anions including BF 4
À , AsF 6
À , and SbF 6
À . At elevated temperature,
the considerable broadening
19 F NMR resonance indicated rapid exchange of the
encapsulated anions with free species, while lowering the temperature led to broadening of
19 F NMR resonance of polygons, and a second distinct
19 F NMR resonance
corroborating the presence of encapsulated anions occurred. From the low activation
energy determined, the authors concluded that anions acted as templates rather than
merely diffusing into preformed cages.
In another report by Choi et al. [37], they set up mechanochemical reactions of
3,6-dimethoxy-s-tetrazine (dmotz) with AgCF 3 SO 3 and AgClO 4 . Different structures such as 1D linear polymer ([Ag(dmotz)(CF 3 SO 3 )] n ) or 2D grid polymer ([Ag
(dmotz) 2 (ClO 4 )] n ) were obtained depending on the specific anions applied.
Intermolecular anion-π interactions between tetrazine ring and CF 3 SO 3
À , for example, led to the tightening of 1D linear chains, whereas ClO 4
À was surrounded with
four tetrazine rings which provided the 2D structure.
10.5 Self-Assembly with Anion as Primary Building Blocks
As the main challenge in anion-templated self-assemblies (vide supra) is that it is
difficult to distinguish the contribution of anion-π and coordination interactions,
using charge-neutral π receptor as one building component to probe anion-π-directed
assembly is therefore particularly intriguing. However, due to the flexible directionality of anion-π non-covalent bond, this task is very challenging, and the early
examples that appeared in literature were mainly observed in crystal structures.
The rational design of anion-π-controlled self-assembly based on charge-neutral
building blocks emerged until very recently.
10.5.1 Self-Assembly with Electron-Deficient Arenes
In 2008, Kochi et al. [38] examined the behavior of a series of planar π-acids toward
various types of polyatomic anions by means of crystallography. They demonstrated
from about 20 crystal structures that similar repeating anion/π-acid arrangement
consisting of a vertical one-dimensional (1D) stack of π-acid alternatively interspersed with the π-bonded anions (Fig. 11). Anions interacted with the carbon atom
of π-acid molecule forming the so-called σ-type interaction. The location of anions
262
D.-X. Wang