**4. Discussion**

If the formation of the described ternary multicomponent crystal **1** in a one-pot-synthesis is due to the different environments during the molecular recognition of A, B, and C in the crystallization process, **1** should also be synthesizable from the cocrystals **2** and **3**. In both cocrystals (**2** and **3**), the building block A shows significant differences in the molecular environment (Figure 5). In order to verify the assumption, appropriate crystallization experiments were carried out. Cocrystal **2** was grinded with C, and cocrystal **3** was grinded with B. Additionally, a physical mixture of B and C was grinded with A. The powder diffractograms of all three crystallization experiments show exclusively the reflections of the expected ternary multicomponent system **1**. It seems, when attempting to design a multicomponent crystal of nth degree, it is helpful to take a closer look at the (n − 1)th degree predecessors.

**Figure 5.** General proposed formation process of a ternary multicomponent crystal.

The hypothesis that differences in the molecular environment during the molecular recognition in the crystallization process are responsible for the formation of multicomponent crystal has been demonstrated in this study by using the building blocks 2-methylresorcinol (A), tetramethylpyrazine B and 1,2-bis(4- pyridyl)ethane C. The carefully designed cocrystals **2** (A:2B) and **3** (A:C) with differences in their molecular environment are suitable starting materials for successfully synthesizing the ternary multicomponent crystal **1** (2A:2B:C). Furthermore, the results indicate that Ostwald's rule of stages can be applied to solid state synthesis just as well as it can be applied to crystallization from solution.

**Author Contributions:** Project administration, supervision, and single crystal structure determination, K.M.; investigation and original draft preparation, D.K.; X-ray powder diffraction and solid state analysis, C.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the DFG Open Access Publication Funds of the Ruhr-Universität Bochum.

**Conflicts of Interest:** The authors declare no conflict of interest.
