Directional Ionic Bonds

Covalent and ionic bonds represent two fundamental forms of bonding between atoms. In contrast to bonds with significant covalent character, ionic bonds are of limited use for the spatial structuring of matter because of the lack of directionality of the electric field around simple ions. We describe a predictable directional orientation of ionic bonds that contain concave nonpolar shields around the charged sites. Such directional ionic bonds offer an alternative to hydrogen bonds and other directional noncovalent interactions for the structuring of organic molecules and materials.

T he spatial orientation of bonding orbitals imparts directionality to bonds with significant covalent character, 1−3 thereby enabling the three-dimensional structuring of complex molecules. 4−16 Next to the directionality of covalent bonds, the presence of directional noncovalent interactions is often essential for the structure of biological and synthetic organic matter. Most notably, the directionality of hydrogen bonds is relevant for the base pairing in DNA and for other molecular recognition phenomena in biology and chemistry 17 and it is a ubiquitous design element for synthetic supramolecular chemistry. 18 −25 In comparison with hydrogen bonds, the utility of other directional noncovalent interactions, of which halogen bonds are the main representative, is more limited. 26−30 Here, we focus on imparting directionality to ionic bonds in an attempt to provide a distinct directional noncovalent interaction for the three-dimensional structuring of matter. Ionic bonds are present in ionic solids, such as sodium chloride, and are relevant for the structure of organic materials, 31−34 synthetic supramolecular assemblies, 35,36 and biological molecules, including DNA 37 and proteins. 38 In polar solvents, because of the solvation of ions, ionic bonds behave as labile noncovalent interactions and are energetically comparable to hydrogen bonds. 39−41 Furthermore, the binding of two separated ions in vacuum or the presence of multiple ionic interactions, even in water, can be energetically comparable to the strength of covalent bonds. 41,42 However, the lack of the dependence of the electrostatic attraction on the relative orientation of the cation and the anion results in the structural flexibility of ionic bonds (Figure 1a; see Figure 1b for the comparison with covalent bonding). As a result, ionic bonds are of limited use for directional connectivity, 36,43−46 unless oriented by multiple interactions, 47,48 hydrogen bonding, 19,33,36,49−59 planar π-systems, 60−62 or metal coordination. 63 Our strategy for the design of directional ionic bonds involves the placement of ions in a sterically demanding, shielding, nonpolar hydrocarbon environment that leaves the charged group exposed in one direction only (Figure 1c). A directional approach of two oppositely charged ions with such structure from the sterically accessible sides minimizes the charge separation and maximizes the Coulomb attraction. Other relative orientations result in larger separations because of the steric repulsion by the shielding backbone and are, therefore, less favorable ( Figure 1c). Directional ionic bonds might display directionality in solution and in the solid state, thus offering a new strategy for building molecules, supramolecular assemblies, and materials.
For the demonstration of the concept of directional ionic bonds, we selected N-methylpyridinium cations and arylsulfonate anions because of their potential for structural modifications at the carbon sites. The structural requirements for the construction of an effective nonpolar shield around the ionic moieties were studied by the introduction of sterically demanding 2,6-bisisopropylphenyl and 2,4,6-trisisopropylphenyl substituents at the positions 4-, 2,6-, or 2,4,6-of the central (hetero)arene groups of the two ions ( Figure 2a). The examination of the electrostatic potential maps of the ions that contain the shielding group placed in the 4-position only (C2 + and A2 − ) reveals little impact on the directionality of the approach to the charged site. However, a significant shielding of the ionic moieties is achieved when the nonpolar groups are placed in positions 2-and 6-(C3 + and A3 − ). Because of the localization of the negative charge, the −SO 3 − group appears similarly shielded by the nonpolar environments in A3 − and A4 − . However, for the shielding of the N-methylpyridinium cation, in which the positive charge is delocalized across the aromatic ring, the presence of nonpolar groups in all three positions (2,4,6-) is necessary to ensure that the charged site is exposed only from the side with the N-CH 3 group (C3 + vs C4 + ).
The examination of a set of previously reported crystal structures of substituted N-methylpyridinium arylsulfonate salts confirms that a wide range of relative orientations of the two ions is possible (C1 + A1 − , Figure 2b). This structural flexibility is expected for a bond with high ionic character ( Figure 1a) and is consistent with the nondirectional distribution of the electrostatic potential of ions C1 + and A1 − (Figure 2a). Furthermore, the molecular structures of ion pairs C2 + A4 − and C4 + A2 − , which each contain one insufficiently shielded ion, display a strongly bent arrangement of the anion and the cation, while in the case of C3 + A3 − , the anion approaches the cation from the second exposed site rather than from the side with the N-CH 3 group (Figure 2b). However, as indicated by the analysis of the electrostatic potential maps, the restricted access to the charged areas between positions 2-and 6-in cation C4 + and anion A3 − results in the predicted geometry of the ionic bond ( Figure  2c). The geometric consequence of the closest possible approach of the two ionic moieties, which maximizes the Coulomb interaction, is a close-to-linear arrangement of the backbones of the two ions in ion pair 1. Because of the predictable and spatially well-defined molecular assembly, which contrasts the usual lack of directionality of ionic bonds that are not supported by hydrogen bonds 49−51 (Figures 1a  and 2b), we propose that the ionic bond in 1 (Figure 2c) can be considered as a realization of a directional ionic bond (Figure 1c). The ionic bond in 1 is distinct from other types of directional bonding because it does not rely on the covalent contribution for the directionality of bonding, although oriented electrostatic potential also plays a role in halogen and hydrogen bonds. 64− 68 The generality of directional ionic bonds is supported by the observation of the expected bonding geometry for ions with varied shielding substituents at the positions 2-and 6-(2−6, Figure 3a). To compare the directionality of different ionic bonds, we define angles α and β as shown in Figure 3a, with higher directionality of the ionic bond corresponding to the values of the angles that are closer to 180°. A comparison of the α and β angles for all ionic bonds discussed here is given in the two-dimensional plot in Figure 3b. While the ion pairs shown in Figure 2b, which do not contain sufficient shielding around the ions, give more varied and lower values for both of the angles (red circles), a narrower distribution closer to 180°i s observed for directional ionic bonds (blue circles, the additional data points represent the examples discussed below). The deviation of the angles in the directional ionic bonds from 180°is a combined result of the lack of perfect shielding of the charged sites, crystal packing effects, and the presence of H 2 O molecules in some of the structures. Additionally, dispersion interactions are likely responsible for the tendency toward orientations that maximize the contact surface between the shielding substituents on the anion and those on the cation (2 and 3). With larger shielding groups, the central (hetero)arene planes of the ions are twisted toward a dihedral angle of 90°to give interlocked structures 4−6 in which the ionic bonds appear further shielded, but the α and β angles remain similar to those in 1−3.
The study of the ionic bond in 3 in nonpolar solvents benzene-d 6 and toluene-d 8  − to the solution of 3 in C 6 D 6 results in the loss of the cation−anion ROE correlations, thereby demonstrating the cleavage of the directional ionic bond by the exogenous ion pair ( Figure S1). Furthermore, the cation−anion correlations are not observed in the polar solvent DMSO-d 6 , which indicates ion separation or potentially a less directional ion pairing (Figure 3c, bottom).
The SAPT analysis 70 of the truncated structures indicates that the presence of the shielding groups contributes around 13% to the total interaction energy in 3 ( Figure S2a, 3 vs 3 trunc A) with the dispersion and induction contributions being largely offset by the steric repulsion (3 trunc B and 3 trunc C). The electrostatic contribution dominates the interaction in the directional ionic bond and it is comparable to the energy between two point charges centered on the N and S atoms ( Figure S2c; for a comparison with a simpler ion pair and hydrogen-and halogen-bonded systems, see Figure S2d). 71 However, larger shielding groups provide higher stabilization of the interaction (25% estimated in the case of 6, Figure S2b).
Following the observation of the directionality in the solid state and in nonpolar solvents, we next show the potential of directional ionic bonds for the spatial organization of matter over distances of several nanometers. The modular synthetic approach to the ions in 1−6 ( Figures S3 and S4) enables the connection of additional molecular units to the directional ionic bond at the para-positions of the shielding substituents that are present at the back sides of the cation and the anion (Figures S5 and S6). The approximate geometric collinearity of the covalent bonds at these sites enables the use of directional ionic bonds as structurally well-defined linear bonds (Figure 4a). For example, linear and bent supramolecular systems, such as 7 and 8 (Figure 4a), can be accessed by the use of two directional ionic bonds and 1,4-and 1,3disubstituted benzene groups as spacers. The ROESY analysis of 7 and 8 reveals a similar interaction pattern to that of 3, although the signals between anion-CH 3 a and cation-CH 3 c,e appear stronger, which suggests a more flexible orientation of the cation (Figure 4b; see Figure 3d for the labeling of the groups). Water molecules can bind to the oxygen atoms of arylsulfonate anions under air, and the presence of a water molecule is observed in the solid-state structure of 7. However, the approximate linear geometry remains preserved ( Figure  4c), which shows that the directionality of directional ionic bonds can be maintained in the presence of strong hydrogen bond donors. Furthermore, directional ionic bonds can be applied to the molecular architecture at the nanoscale level, as shown with the molecular structure of 9, in which the binding of two palladium-porphyrin groups through two directional ionic bonds leads to a separation of the metal centers by 6.5 nm (Figure 4a,d). The precise spatial organization of functional molecular units, such as porphyrins, over large distances is relevant in synthetic and biological systems, such as molecular light-harvesting devices. 72−74 The use of more complex multi-ionic units enables access to structurally well-defined materials with directional ionic bonds, as exemplified by the formation of a two-dimensional hexagonal structure in 10 ( Figure 4e). The hexagons in 10 are composed of six directional ionic bonds, with the long and the short diagonals of the hexagon measuring 5.2 and 4.6 nm, respectively. In the crystal structure, the 2D layers form an ABCA′B′C′ stacking pattern (where ′ indicates a glide plane relationship) in which cavities are present between the ions of non-neighboring layers (Figure 4e, Figure S7). Transmission electron microscopy (TEM) shows the formation of thin plates upon drop casting (Figure 4e, Figure S8), while the thermal analysis of the constituent ions demonstrates partial decomposition upon the heating of 10 at 250°C for 1 h (see the Supporting Information). Porous materials, such as metal− organic frameworks, 75−77 covalent organic frameworks, 78 and (charge-assisted) 54−56 hydrogen-bonded organic frameworks, 21,22,59 have a range of applications in diverse areas of chemistry and can have geometries related to that in 10. 52,79,80 However, directional ionic bonds offer a complementary strategy for the construction of organic materials by utilizing otherwise nondirectional Coulomb interactions between two ions as a geometrically well-defined linear bonding element.
Directional ionic bonds, which are demonstrated with the structures of 1−10, open new avenues for the structuring of organic matter by imparting directionality to ionic bonding, which is a fundamental 81−83 and omnipresent mode of chemical bonding. Several properties of directional ionic bonds, including the binding strength and the modulation with solvent polarity and competing ions, are comparable with the structural and dynamical properties of hydrogen bonds. Therefore, the use of directional ionic bonds might be envisioned in diverse areas of supramolecular and materials chemistry. Additionally, as demonstrated with the structures of 7−10, directional ionic bonds connect molecular fragments in The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c01030.
Experimental details, characterization spectra, single crystal data, and computational details (PDF)