**3. Results**

The crystalline product obtained by liquid-assisted grinding of a mixture of A, B, and C in the presence of THF shows reflections in the PXRD that cannot be attributed to the educts. The crystalline product was dissolved in nitromethane, and by cooling crystallization a single crystal was obtained (Table 1). The X-ray structure analysis confirms the successful crystallization of a ternary 2:2:1 A:B:C multicomponent system **1** (Figure 2).

**Figure 2.** Packing view of ternary 2A:2B:C multicomponent system **1.**

The central 1,2-bis(4-pyridyl)ethane building block C forms hydrogen bonds via its two aromatic nitrogen centers to hydroxy groups from each of the two neighbouring 2-methylresorcinol molecules A. The remaining hydroxy groups of A coordinate via hydrogen bonds to one nitrogen center each of the two terminal tetramethylprazine molecules. Surprisingly, the second nitrogen centers of B are not involved in any further intermolecular interactions. Consequently, the 2:2:1 A:B:C multicomponent system **1** does not form infinite chains. This behavior is observed for B in only few selected multicomponent systems. Examples are cocrystalline systems with Br-C6F4-OH12 or Br-C6F4-COOH12 [16]. The analysis of the coordination behavior of A shows that the hydrogen bonds from A to B, in comparison to A to C, have no significant differences in the distances O ... N 2750 Å and O ... N 2755 Å as well as in the angles of the ring planes A:B 75◦ and A:C 69◦.

As mentioned, the geometric environment of the involved building blocks and their intermolecular interactions are crucial parameters for the formation of higher multicomponent crystals. The one-pot reaction of all components (A, B, and C) leads to the ternary 2A:2B:C system **1**. The 2A:2B:C motif is discretely isolated by the unusual coordination behavior of the tailored B molecules. The hypothesis is that the geometric molecular environment is responsible in each aggregation step in the design of **1**. Based on differing structural environments in the crystalline products obtained by first aggregation steps, A with B and A with C, the crystal packing of 2A:2B:C—starting from A—can be built up by successive substitution of B by C or C by B. To investigate this hypothesis and to gain a deeper understanding of the formation of the multicomponent system, the cocrystals involving A and B as well A and C were crystallized and the crystal packing analyzed.

Cocrystals of **2** (A:2B) and **3** (A:C) could be obtained by grinding a 1:2 ratio (2) and 1:1 ratio (3). After that, suitable single crystals for XRD were formed after recrystallization of the crystalline powder from THF (**2**) and nitromethane (**3**). In contrast to the known crystal structure of the A:B cocrystal [8], the crystal structure of **2** represents a 2:1 cocrystal with one A and two B molecules (Figure 3). Due to the fact that the two molecules of B in **2** show explicit different structural environments, the description A:B:B' seems more appropriate for this system. The central A unit forms chains via hydrogen bonds (O1 ... N1 2.84 Å, O2 ... N2 2.85 Å) to two neighbouring B molecules. While the D distances of the hydrogen bonds are practically identical, the dihedral angles between the planes of the arene ring of A and the neighboring pyrazine rings of B show slight variations (62◦/68◦). Additionally, present B' molecules form columns with B by π-π stacking interactions. A comparison of the intermolecular potentials in the molecular arrangemen<sup>t</sup> of **2** pointed out the dominating role of the π-π stacking motif (−42 and −43 kJ/mol) in contrast to the described hydrogen bonds (−32 and −33 kJ/mol).

**Figure 3.** Packing view of (a) **2** (A:2B) O1-H1: 0.900 Å, O1 ... N3 2.84 Å, O1-H1 ... N3 161◦, O2-H2: 0.883 Å, O2 ... N2 2.85 Å, and O2-H2 ... N4 167◦.

The crystal structure **3** is built up by chains of linked 1:1 A:C cocrystals (Figure 4). Both hydroxyl groups of the central A molecule form hydrogen bonds to nitrogen centers of the two neighbouring C molecules. A comparison of both hydrogen bonds that indicates slight differences is be remarkable. In contrast to the hydrogen bond O1-H1 ... N1, O2-H2 ... N2 shows enlarged covalent O1-H2 and O2 ... N2 hydrogen bonds. Besides the slightly different hydrogen bonds to **2**, differences in dihedral angles between the planes of the arene ring of A and the neighboring pyridyl rings of B are observed (68◦/75◦).

**Figure 4.** Packing view of (b) **3** (A:C). O1-H1: 0.908 Å, O1 ... N1 2.716 Å, O1-H1 ... N1 175◦, O2-H2: 1.041 Å, O2 ... N2 2.743 Å, and O2-H2 ... N2 171◦.
