Crystal structure, Hirshfeld surface analysis and electrostatic potential study of naturally occurring cassane-type diterpenoid Pulcherrimin C monohydrate at 100 K

Single crystal X-ray diffraction analysis and Hirshfeld surface analysis of the title compound were carried out to analyse quantitatively the intermolecular interactions involved in the crystal packing. The electrostatic potential surface was generated over the Hirshfeld surface to visualize the potential active sites.


Chemical context
Caesalpinia pulcherrima (L) Swartz is an enduring shrub or small tree of the cassane family found in tropical regions of south and south-east Asia. It has been used ornamentally for a long time and is commonly known as Paradise flowers, Pride of Barbados and Peacock flower (Quisumbing, 1951). In addition, its parts have also been utilized as a traditional medicine in Thailand. The flowers and leaves are believed to be a cure for fever (Lotschert et al., 1983), and people in the northern regions of Thailand use its roots to treat tuberculous symptoms (Wutthithammaweach et al., 1997). Furthermore, it has also been proved that its crude DCM extract exhibits relatively strong anti-tubercular activity (Promsawan et al., 2003). A methanol extract of C. pulcherrima has been reported to have strong antibacterial activity (Parekh et al., 2006). The plant is also used to treat cardiovascular disorders, inflammation, muscular and sore pain, earache, and is known for its antipyretic, vermifugal and antimalarial activities (Patel et al., 2010;Roach et al., 2003). The present investigation deals with the isolation, single-crystal X-ray diffraction study, Hirshfeld surface analysis and electrostatic potential studies of the naturally occurring title compound, which was isolated as a monohydrate. ISSN 2056-9890

Superamolecular features and Hirshfeld surface analysis
Inter-and intramolecular interactions exert a significant influence on the geometry and properties of crystalline materials (Ferenczy et al., 2001;Putz et al., 2016) Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level.

Figure 2
Partial packing diagram of the title compound showing the formation of a chain parallel to the b axis by O-HÁ Á ÁO hydrogen bonds (dotted lines). Intramolecular C-HÁ Á ÁO hydrogen bonds (dotted lines) are also shown. Hydrogen atoms not involved in hydrogen bonding are omitted.
tional and non-conventional types of hydrogen-bonded contacts in the crystal structure of the title compound (Fig. 2, Table 1). The oxygen atom of the water molecule acts as acceptor for the hydroxyl hydrogen atom of neighboring molecule via O3-H3Á Á ÁO1W interactions, while the two hydrogens atoms interact with the hydroxyl group at atom C5 and the carbonyl functionality of neighbouring molecules via O1W-HWAÁ Á ÁO1 and O1W-HWBÁ Á ÁO8 hydrogen bonds, forming an R 2 2 (10) ring. These interactions, along with the O1-H1Á Á ÁO4 hydrogen bond, link the molecules into chains parallel to the b axis. Relatively weak C-HÁ Á Á interactions (Table 1) are also observed.
The three-dimensional Hirshfeld surface calculated for the title compound is depicted in Fig. 3. The red regions indicate areas of close contacts shorter than the sum of van der Waals radii, while the blue and white regions represents contacts having distances greater and equal to the sum of van der Waals radii, respectively. The O3-H3Á Á ÁO1W and O1-H1Á Á ÁO4 hydrogen bonds are the two interactions responsible for linking neighboring molecules (Fig. 4). The curvedness surface (Fig. 5) shows the green (flat) and blue (curved) areas, representing low and high probabilities, respectively, of forming interactions with neighbouring molecules. The highlighted regions shown correspond to those in Fig. 3. No obvious adjacent blue or red triangles are present, indicating the absence ofinteractions. The fingerprint plots are presented in Fig. 6. HÁ Á ÁH contacts are the major contributor to the Hirshfeld surface (58.1%). As a result of the presence of a water molecule in the asymmetric unit, HÁ Á ÁO interactions are observed to contribute 21.5%, with sharp spikes pointing toward the origin of the plot indicating the strength of the contacts. The contribution of CÁ Á ÁH interactions is 17.5%, whereas CÁ Á ÁO interactions are negligible (0.2%). The Hirshfeld surface mapped over electrostatic potential is shown in Fig. 7. The red regions indicate atoms with the potential to be hydrogen-bond acceptors (negative electrostatic potential), while blue regions indicate regions having atoms with positive electrostatic potential, i.e. hydrogen-bond donors.

Figure 4
Hirshfeld surface mapped over d norm for the title compound with neighbouring molecules linked via O-HÁ Á ÁO hydrogen bonds (dashed lines).

Figure 5
Hirshfeld surface mapped over shape-index for the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The water H atoms were located in a difference-Fourier map and refined with the O-H and HÁ Á ÁH distances constrained to 0.85 (1) and 1.39 (1) Å , respectively, and with U iso (H) = 1.5U eq (O). All other H atoms were positioned with idealized geometry and refined isotropically with O-H = 0.83 Å , C-H = 0.95-1.00 Å , and with U iso (H) = 1.2U eq (C) or 1.5 U eq (C-methyl, O). A rotating model was used for the methyl and hydroxy groups. Two-dimensional fingerprint plots for the title compound together with areas of Hirshfeld surfaces involved in hydrogen bonding.