*3.2. Single-Crystal Structures*

The crystal structures of compounds **1** and **3** were solved by single crystal X-ray diffraction. All trials performed to obtain suitable single crystals for the other multicomponent solids observed in this work were unfruitful. ORTEP images of both compounds are included in Figure S7.

*9-Ethyladenine—oxalic acid (1:1) salt* (**1**). Crystal structure analysis of compound **1** reveals that this compound crystallizes in the triclinic *P*-1 space group and the asymmetric unit consists of a protonated molecule of 9ETADE and a hydrogen oxalate anion. No solvent molecules are observed in agreement with its FTIR spectrum or the TGA trace where no loss on drying was observed before melting. Moreover, the agreement between the experimental and the simulated patterns for the 9ETADE-oxalic acid (1:1) salt is excellent indicating the high purity of the bulk solid obtained by slurry, Figure 1.

The structure consists of the self-assembly of two adenine moieties through its Hoogsteen edge by N(6)–H···N(7) interactions (distance: 2.9469(18) Å; angle 161.6◦) forming a *R*<sup>2</sup> 2(10) motif (see Figure 3). Oxalate anions are connected head-to-tail through a strong hydrogen bond (distance: O(1B)–H(1B)···O(3B), 2.4728(15) Å). Furthermore, two molecules of 9ETADE interact with two hydrogen oxalate molecules through the NH2 group and N(1) to the carboxylic acids of different oxalate anions, as follows: N(6A)– H···O(2B) (distance 2.9438(17) Å; angle 139.9◦), N(6A)–H···O(3B) (distance 3.0271(18) Å; angle 128.7◦), N(1A)–H···O(1B) (distance 3.0412(17) Å; angle 124.0(16)◦) and N(1A)–H···O(4B) (distance 2.7449(16) Å; angle 158.7(18)◦) following ring motifs *R*<sup>1</sup> 2(5), and a further C(2A)– H(2A)···O(1B) interaction.

The whole structure is formed by the connection of the layers formed by 9ETADE dimers and hydrogen oxalate ions (Figure 3c) through the hydrogen bonds, as follows: C(10A)–H(10A)···O(4B) and C(8A)–H(8A)···O(2A). The complete list of H-bonding interactions is shown in Table S1.

*9-Ethyladenine—oxalic acid (2:1) salt* (**3**). Single crystals of this salt were obtained by slow evaporation of a mixture in acetonitrile—water (1:1 vol/vol). In this case, 9ETADE and oxalic acid crystallize in the monoclinic *P*21/c space group containing two protonated molecules of 9ETADE and an oxalate anion in the asymmetric unit forming a salt in a 2:1

ratio. No solvent molecules are observed, again in agreement with the TGA trace or the FTIR spectrum.

**Figure 3.** (**a**) Hydrogen bonding interactions, (**b**) layers (view along *a* axis) and (**c**) view along the vector (−2 1 0) in compound **1** (9ETADE-OXA (1:1) salt).

Two molecules of 9ETADE are bridged by an oxalate molecule through the following interactions (Figure 4): N(6A)–H···O(2B) (distance 2.743(3) Å; angle 152.7◦), N(1A)– H···O(1B) (distance 2.746(3) Å; angle 148(3)◦) and N(1A)–H···O(2B) (distance 2.968(3) Å; angle 133(3)◦). An additional hydrogen bond to a second oxalate anion is established through N(6A)–H···O(1B) (distance 2.873(3) Å; angle 156.4◦), thus constituting belts perpendicular to the **c** axis. These belts are pilled by hydrogen bonds through the methyl group from the ethyl chain to the carbonyl from the oxalate anion (distance C(11A)–H(11A)···O(2B), 3.576(5) Å). Among 9-ethyladenine molecules, only C–H···N contacts are observed (C(2A)– H(2A)···N(7A), 3.224(4) Å). Surprisingly, the N(7A)–Cg distance (3.398 Å) is shorter than the distance between mean planes for two pilled 9ETADE molecules, suggesting an attractive lone pair–π interaction (N(7A)···6-membered ring). The complete list of hydrogen bonds is included in the Supplementary Information, Table S2.

**Figure 4.** (**a**) Hydrogen bonds and (**b**) packing of the belts (view along the *b* axis, slightly tilted to improve the perspective) and (**c**) π–π stacking (view along the *c* axis) in 9ETADE-oxalic acid (2:1) salt (**3**).

The agreement between the experimental and the simulated powder diffraction patterns for the 9ETADE-oxalic acid (2:1) salt is excellent, Figure 1.

Although the crystal structures for the other multicomponent solids shown in this work were unavailable, the data collected from IR spectroscopy, thermogravimetric analyses, and the knowledge from previous crystal structures regarding to other salts and cocrystals described for this compound allow to determine their composition. So, we hypothesize that compound **4** contains a half double charge oxalate anion and a half unprotonated oxalic acid molecule per 9ETADE molecule. Additionally, for compounds **2**, **5** and **6**, a half mono-oxalate anion and a half unprotonated oxalic acid should be present, although with different contents of water molecules in each case.
