*3.1. Crystal Structure of DPA:TA Salt*

TA and DPA in a 1:1 molar ratio were crystallized from acetonitrile and ethanol mixtures at ambient condition by slow solvent evaporation to obtain a colorless long plate crystal suitable for X-ray analysis. The X-ray single-crystal structure confirmed the formation of DPA:TA salt with approximately similar C–O bond lengths of C28-O2, 1.2538 (19), C28-O3, 1.268 (2), Å of the (COO¯) carboxylate group of TA (Figure 2a). These similarities in the bond length of C–O confirmed the transfer of an acidic proton from one of the carboxylic acidic group of TA to the N3-nitrogen atom of the tertiary amino group (chain moiety) of DPA.

DPA:TA salt crystalized in the centrosymmetric triclinic *P*-1 space group containing one protonated DPA and one TA anion in the asymmetric unit revealed that the molecular salt is in the 1:1 molar ratio. The salt pair, i.e., the protonated DPA and TA anion in the asymmetric unit linked by a strong charge, assisted the N3+–H3A···O2¯ hydrogen bond and the C9–H9···O3¯ hydrogen bond. In the crystal structure of DPA:TA salt, protonated DPA displays an intramolecular N2-H2A···N1 hydrogen bond by donating second amide hydrogen N–H to the N-atom of 2-pyridine moiety in the S<sup>1</sup> <sup>1</sup>(6) ring motif, along with other C-H···O intramolecular interactions, namely C15-H15B···O1 and C17-H17B···O1, which stabilize the conformation protonated DPA molecules in the salt, as shown in Figure 2a. The crystallographic information and geometrical parameters for the hydrogen bonding interaction are summarized in Tables 1 and 2.

**Figure 2.** (**a**)ORTEP diagram DPA:TA Salt showing the atom numbering scheme. Thermal ellipsoid drawn at 50% probability level, and H-atoms are shown as small spheres with arbitrary radii. Association between the salt pair in asymmetric unit of DPA:TA salt shown by black dotted line and in this association only carboxylate group of TA involved. Protonated DPA in salt displaying intramolecular N2-H2A∙∙∙N1, C15-H15B∙∙∙O1, C17-H17B∙∙∙O1 hydrogen bond and shown by purple color dotted line. (**b**) Conformation of protonated DPA in salt and torsional angle τ1 - C7-C14-C15-N3, τ2 – C8-C7-C14-C15, τ3 –C9-C8-C7-C14, τ4 – C13-C7-C14-C15. **Commented [M1]:** Figure 2 (a) was cut off in the first proof vesion, so, we enlarged the figure in our proof verison, but this figure have been cut off again in webpage. **Figure 2.** (**a**) ORTEP diagram of DPA:TA salt showing the atom numbering scheme. The thermal ellipsoid is drawn at 50% probability level, and H-atoms are shown as small spheres with arbitrary radii. The association between the salt pair in the asymmetric unit of DPA:TA salt is shown by the black dotted line and, in this association, only the carboxylate group of TA is involved. Protonated DPA in salt displaying the intramolecular N2-H2A···N1, C15-H15B···O1, C17-H17B···O1 hydrogen bond is shown by the purple dotted line. (**b**) Conformation of protonated DPA in salt and torsional angle τ1—C7-C14-C15-N3, τ2—C8-C7-C14-C15, τ<sup>3</sup> –C9-C8-C7-C14, τ4—C13-C7-C14-C15.




**Table 2.** Geometrical parameters of the hydrogen bond interaction in DPA:TA salt.

In the DPA:TA salt, DPA adopts conformation, where the 2-pyridine ring moiety is roughly coplanar with the chain moiety (excluding the iso-propyl moiety), and the phenyl moiety is oriented roughly perpendicular to the planar part, as shown in Figure 2b. In the DPA:TA salt, the torsional angle τ<sup>1</sup> 179.38, τ<sup>2</sup> 170.70 suggests planarity in the backbone chain, and torsional angle τ3 (12.75) suggests slight twist in the coplanarity between the chain moiety and the 2-pyridine moiety.

The dihedral angle between the phenyl and pyridine rings is 78.76(10) in DPA:TA salt, suggesting a nearly perpendicular orientation. However, the torsional value τ<sup>4</sup> (−71.52) is for the orientation of the amide group, with the planar part also being roughly perpendicular.

In the crystal structure of DPA:TA salt, two inversion-symmetry related protonated DPA molecules form an amide homodimer synthon via a pair of strong N–H···O hydrogen bonds in the R<sup>2</sup> <sup>2</sup>(8) ring motif, and they are listed in Table 2. In this dimeric association, protonated DPA donates amide hydrogen N2–H2B to the amide C=O1 oxygen of inversion-symmetry related protonated DPA molecules in the dimeric N2–H2B···O1 hydrogen bond, whereas the second hydrogen of amide N2–H2A engaged in the intramolecular N2–H2A···N1 nitrogen bonds with the N1-atom of the 2-pyridine moiety in the S<sup>1</sup> <sup>1</sup>(6) ring motif. This homodimer of protonated DPA molecules were linked to two TA anion via a strong charge assisted N3+–H3A···O2¯ hydrogen bond and the C9–H9···O3¯ interaction to form a centrosymmetric dimeric unit comprising two protonated DPA and two TA anion, as shown in Figure 3a. In this association, N+3–H3A (protonated tertiary amino nitrogen) hydrogen of the protonated DPA donates hydrogen to carboxylate (COO¯) O2-oxygen of the TA anion in the N3+–H3A···O2¯ hydrogen bond, and the C9–H9 Hydrogen of the 2-pyridine moiety donates hydrogen to carboxylate (COO¯) O3-oxygen of the TA anion in the C9–H9···O3 interaction. Thus, in this association, both carboxylate (COO¯) O2-, O3-oxygen of TA anion are engaged in hydrogen bonding with protonated DPA molecules, as shown in Figure 3a. The closely associated TA anion forms a one-dimensional (1D) chain using a linear and strong O4–H4A···O3¯ hydrogen bond, as shown in Figure 3b.

Such dimeric units are extended through linear and strong O4–H4A···O3¯ and short and non-linear C20–H20B···O4 hydrogen bonds with the neighboring unit translated dimeric units along the *b*-axis to generate a ladder-like network where the protonated DPA dimer units join the 1D chains of the TA anion, as shown in Figure 4. In this association, the O3¯ oxygen atom of the carboxylate anion of TA accepts hydrogen from the carboxyl OH (O4–H4A) of the neighboring unit translated TA anion in the O4–H4A···O3¯ hydrogen bond along the *b*-axis. Whereas, in turn, carboxyl hydroxyl (O4–H4A) O4-oxygen accept C20–H20B hydrogen of unit translated protonated DPA molecules in short and non-linear C20–H20B···O4 hydrogen bonds.

**Figure 3.** (**a**) Dimeric unit of DPA:TA salt. In this, the inversion center related asymmetric unit of DPA:TA extends through the N−H···O hydrogen bond in the R<sup>2</sup> 2 (8) ring motif in *ac*-diagonally. (**b**) TA anion linked to neighboring unit translated TA anion through a strong and linear O4– H4A···O3¯ hydrogen bond to form a one-dimensional (1D) chain of TA anion alon the *b*-axis. Dotted lines indicate the non-covalent interaction (hydrogen atoms not involved in the hydrogen bonding were removed for clarity).

**Figure 4.** Linking of neighboring unit translated dimeric units along the *b*-axis through O4–H4···O3¯ and C20–H20B···O4 hydrogen bonding interaction to form a ladder structure along the *b*-axis. In this packing, the protonated DPA dimer is held between the 1D chain of the TA anion.

Such 1D chains of dimeric units (ladder-like structures), assembled along the *a*-axis to generate two-dimensional (2D) layer packing through C16–H16···O3¯, C21–H21C···O5 interaction, generate a 2D layer. In this association, the *iso*-propyl moiety of protonated DPA molecules are involved in C–H···O hydrogen bonding by donating C16–H16 and C21–H21 hydrogen to the carboxylate oxygen O3 and carboxyl (C=O5) oxygen of the neighboring 1D chain of dimeric units along the *a*-axis, as shown in Figure 5a. Whereas the packing of such 1D chains of dimeric units (ladder-like structures) along the *ac*-diagonal are done through relatively weak and longer C–H···O interaction by donating C19–H19 and C20–H20 hydrogen of the *iso*-propyl moiety of protonated DPA molecules to the carboxylate oxygen O2 of the TA anion from the neighboring 1D chain of dimeric units through C19–H19···O2¯ and C20–H20C···O2¯ interaction, as shown in Figure 5b.

**Figure 5.** Packing of the 1D dimeric unit chain (**a**) along the *a*-axis through C16–H16···O3¯, C21–H21C···O5 interaction to generate a two-dimensional (2D) layer (**b**) along the *ac*-diagonal through C19–H19···O2¯ and C20–H20C···O2¯ interaction to generate a 2D layer.

Packing the view down the *b*-axis, in this packing, the dimeric unit is associated with the neighboring dimeric unit along the *a*-axis (parallel to the *ac*-diagonal) through short C16–H16···O3¯ interaction; in this association, the *iso*-propyl moiety of the protonated DPA donates C16–H16 hydrogen to the carboxylate O3-oxygen of the TA anion of the neighboring dimeric unit. Whereas, along the *ac*-diagonal dimeric unit, the DPA:TA associated centrosymmetric combine with the neighboring dimeric unit through relatively weaker and longer C–H···O interactions by donating C19–H19 and C20–H20C hydrogen of the *iso*-propyl moiety of the protonated DPA molecules to the carboxylate O2-oxygen of the TA anion from the neighboring dimeric unit through C19–H19···O2¯ and C20–H20C···O2¯ interactions, as shown in Figure 6. Packing of the 1D chain of dimeric units in the *ac*-plane creates a solvent assessable void and void space ~22 Å<sup>3</sup> per molecule (asymmetric unit) in the unit cell, calculated by using the contact surface from Mercury 2020, 2.0 software.

**Figure 6.** Packing of dimeric unit view down the *b*-axis. The neighboring dimeric unit is associated through C–H···O interaction.
