Synthesis, crystal structure and Hirshfeld surface analysis of bis(caffeinium) hexachloridoplatinum(IV) in comparison with some related compounds

In the crystal, the cations and anions form a layered structure via strong N—H⋯Cl, weak C—H⋯Cl and C—H⋯O hydrogen bonds and π-stacking interactions.


Chemical context
Caffeine is a biologically active compound involved in a number of biochemical processes (Costa et al., 2010;Santos et al., 2010;Herman & Herman, 2012). Some sources consider it the most common medicine in the world, constantly used by the population (Knapik et al., 2022). It is known that caffeine compounds are able to exert a strong influence on the action of various pharmaceutical drugs (Traganos et al., 1991). Currently, an active search is underway for platinum-based pharmaceutical drugs, primarily those with antitumor activity (Dilruba & Kalayda, 2016). In this regard, it seemed important to us to study the interaction of caffeine with the chemical forms of platinum used in the pharmaceutical industry. In addition, platinum is actively used as a catalyst in chemical reactions, including various fields of fine organic synthesis (Blaser & Studer, 2007;Zhang et al., 2006;Seselj et al., 2015). Study of interaction of Pt IV with various heterocyclic organic molecules is of great importance in the context of search for new catalytic reactions and synthetic routes. Studies on the interaction of hexachloroplatinates with various biological organic compounds have been performed before, for example by Novikov et al. (2021Novikov et al. ( , 2022. In this work, the title compound I containing [PtCl 6 ] 2À anions and caffeinium cations was synthesized by the reaction of caffeine with H 2 [PtCl 6 ] in methanol and structurally characterized, using Hirshfeld surface analysis to estimate relative contribution of non-covalent intermolecular interactions in comparison with similar compounds, bis(3-carboxypyridinium) hexachloroplatinum RECJAO (II; Novikov et al., 2022) and methylcaffeinium hexafluorophospate AXUQIT (III; Kascatan-Nebioglu et al., 2004).

Structural commentary
Compound I (Fig.1a) crystallizes in the triclinic space group P1. The unit cell (Fig. 1b) contains two caffeinium cations and one centrosymmetric hexachloroplatinate anion with a platinum atom in a special position 1a. In the imidazole ring of the caffeine molecule, the nitrogen N1 atom is protonated. The cation, including the methyl groups, has a flat geometry (maximum deviation for non-hydrogen atoms 0.030 Å ). The [PtCl 6 ] 2À anion has a slightly distorted octahedral geometry with similar Pt-Cl bond distances (Table 1).

Supramolecular features
Hydrogen bonds and -stacking play a significant role in the formation of intermolecular interactions in the crystal structure of I. -stacking is observed between the six-membered pyrimidine rings. Pairs of parallel cations related by an inversion centre, are stacked with interplanar separation of 3.404 (3) Å (Fig. 1b).
The crystal packing in I can be represented as cationic and anionic layers parallel to the (001) plane (Fig. 2). The caffeinium cations are linked by -stacking interactions and weak C-HÁ Á ÁO hydrogen bonds into double layers, which are connected to the anionic layers by hydrogen bonds of the N-HÁ Á ÁCl and C-HÁ Á ÁC types (Table 2), the N1-H1Á Á ÁCl3 ii [symmetry code: (ii) Àx + 1, Ày + 1, Àz] interaction being the strongest.

Hirshfeld surface analysis
Crystal Explorer 21 was used to calculate the Hirshfeld surfaces (HS) and two-dimensional fingerprint plots (Figs. 3 and 4). The donor and acceptor groups are visualized using a standard (high) surface resolution and d norm surfaces are mapped over a fixed colour scale of À0.401 (red) to 1.063 (blue) for cation and À0.402 to 0.934 a.u. for anion, as illus- Symmetry code: (i) Àx; Ày; Àz.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.43, update of November 2022;Groom et al., 2016) revealed 13 unique structures with caffeinium cations, but none of them contained anions of MHal 6 type. The closest analogues of I found were II and III (see above), the former containing N-protonated 3-carboxypyridine (nicotinic acid) as the cation and [PtCl 6 ] 2À as the anion, the latter containing a caffeinium cation with a methylated (rather than protonated) N1 atom and a PF 6 À anion.

Synthesis and crystallization
A saturated solution of dried caffeine in 5 mL of methanol was prepared, to which a few drops of a concentrated solution of hexachloroplatinic acid in hydrochloric acid were added. After one week, the yellow crystals of I that formed were extracted from the solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Reflections with resolution > 5 Å , obscured by the beamstop (beam diameter 0.6 mm), were excluded from the refinement. The methyl groups C5H 3 and C7H 3 were refined as rigid bodies rotating around N-C Two-dimensional fingerprint plots for the cations and anions in I.

Figure 5
Percentage contributions of intermolecular interactions for the cations in I and similar compounds.

Figure 6
Percentage contributions of intermolecular interactions for the anions in I and similar compounds.

Figure 3
Hirshfeld surface of the caffeinium cation mapped over (a) d norm and (b) shape-index.
The H atoms at N1 and C3 were refined isotropically.

Bis(1,3,7-trimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-9-ium) hexachloridoplatinum(IV)
Crystal data (C 8  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq Occ. (