**4. Concluding Remarks**

The synthesis and X-ray characterization of two new cocrystals of mandelic acid and pyridylcarboxamides are reported herein. There is competition and interplay of the hydrogen bonding functional groups during binary cocrystallization. The results observed suggest that the hydroxy carboxylic acid forms reliable synthons to afford cocrystals with the pyridinecarboxamides. For the formation of tetramers in **1** and **3**, a two-stage-based crystallization mechanism is proposed. The first of acid–amide or amide–amide molecular recognition, respectively, and the second per association, by hydrogen bond, heterodimers or homodimers, are symmetrically related. The energetic features of the H-bonds were studied using the QTAIM and MEP surface analyses evidencing that the COOH···N, O H-bonds are the strongest. Some recurrent motifs, such as *R*<sup>2</sup> <sup>2</sup>(7) between carboxy group and the pyridine ring and the self-assembled *R*<sup>2</sup> <sup>2</sup>(8) motif between the carboxamide groups, are described and analyzed energetically. We believe that the estimation of individual contributions by means of QTAIM analysis reported herein is useful in terms of rationalizing the interactions and for future design and synthesis of cocrystals.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/cryst12020142/s1, Figure S1: 1H NMR spectra in DMSO-d6 of cocrystals **1** (upper) and **3** (lower) comparative with their coformers; Figure S2: 1H NMR spectrum of **DL-H2ma**; Figure S3: 1H NMR spectrum of **pic**; Figure S4. 1H NMR spectrum of (**pic**)-(**D-H2ma**) (**1**); Figure S5: 1H NMR spectrum of **inam**; Figure S6: 1H NMR spectrum of (**inam**)-(**L-H2ma**) (**3**); Figure S7: 13C NMR spectrum of DL-H2ma; Figure S8: 13C NMR spectrum of **pic**; Figure S9: 13C NMR spectrum of (**pic**)-(**D-H2ma**) (**1**); Figure S10: 13C NMR spectrum of **inam**; Figure S11: 13C NMR spectrum of (**inam**)-(**L-H2ma**) (**3**); Figure S12: XRPD patterns of the solid forms of **1** and **3**, obtained at room temperature. The XRPD patterns of the cocrystals match well with the simulated XRPD. Table S1: Hydrogen bond parameters [Å, ◦] for cocrystals. Letters included as superscripts refer to symmetry codes shown in text and figures; Figure S13: IR spectrum of (**pic**)-(**D-H2ma**) (**1**); Figure S14: IR Spectrum of (**inam**)-(**L-H2ma**) (**3**).

**Author Contributions:** Conceptualization, I.G.-S., R.T.-I. and J.M.G.-P.; methodology, all authors; software, A.F.; validation, A.C., A.F. and J.N.-G.; formal analysis, I.G.-S., R.T.-I. and J.M.G.-P.; investigation, R.T.-I. and J.M.G.-P.; writing—original draft preparation, A.C. and A.F.; writing—review and editing, all authors; visualization, I.G.-S., R.T.-I. and J.M.G.-P.; supervision, A.C., J.N.-G. and A.F.; project administration, A.C., J.N.-G. and A.F.; funding acquisition, A.C. and A.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We thank the research groups GI-1580 (USC, Xunta de Galicia) and FQM-283 (Junta de Andalucía) and the "Centre de Tecnologies de la Informació" (Universitat de les Illes Balears, UIB) for free allocation of computer time.

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