research communications
Synthesis,
and Hirshfeld surface analysis of bis(caffeinium) hexachloridoplatinum(IV) in comparison with some related compoundsaFrumkin Institute of Physical Chemistry and Electrochemistry Russian, Academy, of Sciences, 31 Leninsky Prospekt bldg 4, 119071 Moscow, Russian Federation, bPeoples' Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya, St, 117198, Moscow, Russian Federation, and cKyrgyz-Russian Slavic University, 6 Chuy Avenue, Bishkek, Kyrgyzstan
*Correspondence e-mail: den-taranee.92@mail.ru
The molecular and 8H11N4O2)2[PtCl6], synthesized from hexachloroplatinic acid and caffeine in methanol, was studied by single-crystal X-ray diffraction. The caffeinium cations form a double layer via hydrogen bonds and π-stacking interactions. The Hirshfeld surface analysis showed that the largest contribution to the crystal packing is made by H⋯H (31.2%), H⋯Cl/Cl⋯H (22.6%), O⋯H/H⋯O (21.9%) contacts for the cation and H⋯Cl/Cl⋯H (79.3%) contacts for the anion.
of the title compound, (CKeywords: crystal structure; platinum; Pt; caffeine; Hirshfeld surface analysis; hexahalide; π-stacking.
CCDC reference: 2268577
1. 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 PtIV 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. (2021, 2022).
In this work, the title compound I containing [PtCl6]2− anions and caffeinium cations was synthesized by the reaction of caffeine with H2[PtCl6] 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).
2. Structural commentary
Compound I (Fig.1a) crystallizes in the triclinic P. The (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 [PtCl6]2− anion has a slightly distorted octahedral geometry with similar Pt—Cl bond distances (Table 1).
3. Supramolecular features
Hydrogen bonds and π-stacking play a significant role in the formation of intermolecular interactions in the of I. π-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).
Similarly, π–halogen interactions (Lucas et al., 2016; Savastano et al., 2018; Frontera et al., 2011; Novikov et al., 2022) are found between the aromatic ring C6/N4/C8/N3/C2/C1 (centroid Cz) and chlorine atoms Cl1 and Cl2, with Cz ⋯ Cl distances of 3.8643 (11) and 3.7170 (11) Å, respectively, and α angles between the ring plane and the Cz⋯Cl vector of 61.82 (7) and 62.28 (7)°, respectively. It is uncertain whether such an interaction exists with Cl3 [Cz⋯Cl = 4.1102 (12) Å, α = 58.68 (8)°].
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 π-stacking interactions and weak C—H⋯O hydrogen bonds into double layers, which are connected to the anionic layers by hydrogen bonds of the N—H⋯Cl and C—H⋯C types (Table 2), the N1—H1⋯Cl3ii [symmetry code: (ii) −x + 1, −y + 1, −z] interaction being the strongest.
4. 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 dnorm surfaces are mapped over a fixed colour scale of −0.401 (red) to 1.063 (blue) for cation and −0.402 to 0.934 a.u. for anion, as illustrated in Fig. 3. Additionally, characteristic red and blue triangles indicative of π-stacking interactions are observed on the shape-index surface (Fig. 3b).
Analysis of intermolecular contacts shows that for the caffeinum cation, the largest contributions are made by H⋯H, Cl⋯H/H⋯Cl and O⋯H/H⋯O contacts (Fig. 5), and for the anion, by Cl⋯H/H⋯Cl and Cl⋯C/C⋯Cl contacts (Fig. 6). Whereas H⋯H contacts correspond to van der Waals interactions, O⋯H and Cl⋯H contacts can be described as weak hydrogen bonds. Typically, hydrogen bonds are revealed by characteristic discrete `spikes' in the fingerprint plots – indeed, such features can be observed in Fig. 4c,d,i. The structures of II and III show distributions of contacts (Figs. 5 and 6) broadly similar to that of I, if corrected for the different cation–anion ratios (1:1 for I and III, 2:1 for II).
5. 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 MHal6 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 [PtCl6]2− as the anion, the latter containing a caffeinium cation with a methylated (rather than protonated) N1 atom and a PF6− anion.
6. Synthesis and crystallization
A I that formed were extracted from the 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 of7. Refinement
Crystal data, data collection and structure . Reflections with resolution > 5 Å, obscured by the beamstop (beam diameter 0.6 mm), were excluded from the The methyl groups C5H3 and C7H3 were refined as rigid bodies rotating around N—C bonds [Uiso(H) refined], C4H3 as rotationally disordered between two orientations with occupancies of 0.62 (4) and 0.38 (4) [Uiso(H) = 1.2Ueq(C)], with C—H 0.96 Å in each case. The H atoms at N1 and C3 were refined isotropically.
details are summarized in Table 3Supporting information
CCDC reference: 2268577
https://doi.org/10.1107/S2056989023005157/zv2027sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989023005157/zv2027Isup2.hkl
Data collection: APEX3 (Bruker, 2018); cell
SAINT v7.68A (Bruker, 2013); data reduction: SAINT v7.68A (Bruker, 2013); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Olex2 1.5 (Dolomanov et al., 2009); software used to prepare material for publication: Olex2 1.5 (Dolomanov et al., 2009).(C8H11N4O2)2[PtCl6] | Z = 1 |
Mr = 798.20 | F(000) = 386 |
Triclinic, P1 | Dx = 2.135 Mg m−3 |
a = 7.8800 (2) Å | Mo Kα radiation, λ = 0.71073 Å |
b = 8.1542 (2) Å | Cell parameters from 4212 reflections |
c = 10.5374 (3) Å | θ = 3.1–29.8° |
α = 95.784 (2)° | µ = 6.34 mm−1 |
β = 91.525 (2)° | T = 296 K |
γ = 112.472 (1)° | Plate, orange |
V = 620.92 (3) Å3 | 0.18 × 0.08 × 0.02 mm |
Bruker Kappa APEXII area-detector diffractometer | 3573 reflections with I > 2σ(I) |
φ and ω scans | Rint = 0.035 |
Absorption correction: multi-scan (SADABS; Krause et al., 2015) | θmax = 30.0°, θmin = 3.5° |
Tmin = 0.734, Tmax = 1.000 | h = −11→11 |
9764 measured reflections | k = −11→11 |
3616 independent reflections | l = −14→14 |
Refinement on F2 | Primary atom site location: dual |
Least-squares matrix: full | Hydrogen site location: mixed |
R[F2 > 2σ(F2)] = 0.022 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.041 | w = 1/[σ2(Fo2) + (0.0171P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.04 | (Δ/σ)max < 0.001 |
3616 reflections | Δρmax = 0.45 e Å−3 |
173 parameters | Δρmin = −0.89 e Å−3 |
1 restraint |
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. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Pt1 | 0.000000 | 0.000000 | 0.000000 | 0.02050 (4) | |
Cl1 | −0.04784 (10) | 0.01026 (9) | 0.21599 (5) | 0.03560 (15) | |
Cl2 | 0.30304 (9) | 0.03978 (10) | 0.04685 (6) | 0.03557 (14) | |
Cl3 | 0.08989 (10) | 0.30803 (8) | 0.01376 (6) | 0.03532 (14) | |
O1 | 0.5394 (3) | 0.1187 (3) | 0.3683 (2) | 0.0474 (5) | |
O2 | 0.1510 (3) | 0.4010 (3) | 0.4685 (2) | 0.0517 (6) | |
N1 | 0.5642 (3) | 0.6187 (3) | 0.1713 (2) | 0.0340 (5) | |
H1 | 0.656 (3) | 0.646 (4) | 0.126 (3) | 0.053 (10)* | |
N2 | 0.3378 (3) | 0.6487 (3) | 0.2696 (2) | 0.0317 (5) | |
N3 | 0.5758 (3) | 0.3628 (3) | 0.2700 (2) | 0.0296 (5) | |
N4 | 0.3502 (3) | 0.2641 (3) | 0.42037 (19) | 0.0289 (5) | |
C1 | 0.3648 (4) | 0.5055 (3) | 0.3126 (2) | 0.0271 (5) | |
C2 | 0.5073 (4) | 0.4880 (3) | 0.2510 (2) | 0.0269 (5) | |
C3 | 0.4589 (4) | 0.7135 (4) | 0.1859 (3) | 0.0378 (7) | |
H3 | 0.475 (4) | 0.812 (4) | 0.150 (3) | 0.046 (9)* | |
C4 | 0.1984 (5) | 0.7171 (4) | 0.3115 (3) | 0.0485 (8) | |
H4A | 0.127241 | 0.645910 | 0.373253 | 0.058* | 0.38 (4) |
H4B | 0.258237 | 0.838989 | 0.349513 | 0.058* | 0.38 (4) |
H4C | 0.118735 | 0.711220 | 0.239191 | 0.058* | 0.38 (4) |
H4D | 0.208901 | 0.818169 | 0.268052 | 0.058* | 0.62 (4) |
H4E | 0.077905 | 0.625090 | 0.291792 | 0.058* | 0.62 (4) |
H4F | 0.217407 | 0.752859 | 0.402113 | 0.058* | 0.62 (4) |
C5 | 0.7299 (4) | 0.3460 (4) | 0.2023 (3) | 0.0461 (8) | |
H5A | 0.817041 | 0.334293 | 0.262454 | 0.097 (9)* | |
H5B | 0.788825 | 0.450488 | 0.160388 | 0.097 (9)* | |
H5C | 0.684455 | 0.242150 | 0.139850 | 0.097 (9)* | |
C6 | 0.2746 (4) | 0.3915 (3) | 0.4059 (2) | 0.0306 (5) | |
C7 | 0.2650 (4) | 0.1360 (4) | 0.5105 (3) | 0.0387 (7) | |
H7A | 0.226895 | 0.194389 | 0.581227 | 0.051 (5)* | |
H7B | 0.352395 | 0.090814 | 0.541266 | 0.051 (5)* | |
H7C | 0.159753 | 0.038818 | 0.468151 | 0.051 (5)* | |
C8 | 0.4901 (4) | 0.2398 (4) | 0.3536 (2) | 0.0312 (6) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Pt1 | 0.02068 (7) | 0.02260 (7) | 0.01919 (6) | 0.00823 (5) | 0.00384 (4) | 0.00678 (4) |
Cl1 | 0.0415 (4) | 0.0448 (4) | 0.0214 (3) | 0.0164 (3) | 0.0078 (2) | 0.0081 (2) |
Cl2 | 0.0232 (3) | 0.0458 (4) | 0.0396 (3) | 0.0136 (3) | 0.0015 (2) | 0.0125 (3) |
Cl3 | 0.0420 (4) | 0.0236 (3) | 0.0410 (3) | 0.0114 (3) | 0.0123 (3) | 0.0088 (2) |
O1 | 0.0488 (13) | 0.0419 (12) | 0.0622 (14) | 0.0242 (11) | 0.0122 (10) | 0.0258 (10) |
O2 | 0.0584 (15) | 0.0519 (14) | 0.0569 (13) | 0.0294 (12) | 0.0344 (11) | 0.0201 (11) |
N1 | 0.0342 (13) | 0.0317 (12) | 0.0351 (12) | 0.0088 (10) | 0.0109 (10) | 0.0131 (9) |
N2 | 0.0337 (12) | 0.0253 (11) | 0.0357 (11) | 0.0106 (10) | 0.0019 (9) | 0.0054 (9) |
N3 | 0.0285 (12) | 0.0289 (11) | 0.0327 (11) | 0.0110 (9) | 0.0066 (9) | 0.0085 (8) |
N4 | 0.0331 (12) | 0.0278 (11) | 0.0258 (10) | 0.0103 (9) | 0.0052 (8) | 0.0090 (8) |
C1 | 0.0280 (13) | 0.0218 (12) | 0.0303 (12) | 0.0079 (10) | 0.0015 (10) | 0.0040 (9) |
C2 | 0.0288 (13) | 0.0236 (12) | 0.0251 (11) | 0.0062 (10) | 0.0009 (9) | 0.0049 (9) |
C3 | 0.0453 (18) | 0.0281 (14) | 0.0393 (15) | 0.0110 (13) | 0.0048 (12) | 0.0129 (11) |
C4 | 0.0430 (18) | 0.0396 (18) | 0.071 (2) | 0.0243 (15) | 0.0078 (16) | 0.0092 (15) |
C5 | 0.0368 (17) | 0.0457 (18) | 0.062 (2) | 0.0201 (14) | 0.0206 (14) | 0.0134 (15) |
C6 | 0.0328 (14) | 0.0288 (13) | 0.0283 (12) | 0.0097 (11) | 0.0043 (10) | 0.0033 (10) |
C7 | 0.0433 (17) | 0.0378 (16) | 0.0320 (13) | 0.0087 (13) | 0.0098 (12) | 0.0172 (11) |
C8 | 0.0290 (14) | 0.0296 (14) | 0.0329 (13) | 0.0081 (11) | 0.0001 (10) | 0.0082 (10) |
Pt1—Cl1i | 2.3153 (6) | N4—C7 | 1.464 (3) |
Pt1—Cl1 | 2.3153 (6) | N4—C8 | 1.389 (3) |
Pt1—Cl2i | 2.3161 (6) | C1—C2 | 1.358 (4) |
Pt1—Cl2 | 2.3161 (6) | C1—C6 | 1.433 (3) |
Pt1—Cl3 | 2.3222 (6) | C3—H3 | 0.89 (3) |
Pt1—Cl3i | 2.3222 (6) | C4—H4A | 0.9600 |
O1—C8 | 1.213 (3) | C4—H4B | 0.9600 |
O2—C6 | 1.213 (3) | C4—H4C | 0.9600 |
N1—H1 | 0.846 (18) | C4—H4D | 0.9600 |
N1—C2 | 1.370 (3) | C4—H4E | 0.9600 |
N1—C3 | 1.335 (4) | C4—H4F | 0.9600 |
N2—C1 | 1.380 (3) | C5—H5A | 0.9600 |
N2—C3 | 1.312 (4) | C5—H5B | 0.9600 |
N2—C4 | 1.469 (4) | C5—H5C | 0.9600 |
N3—C2 | 1.354 (3) | C7—H7A | 0.9600 |
N3—C5 | 1.468 (3) | C7—H7B | 0.9600 |
N3—C8 | 1.389 (3) | C7—H7C | 0.9600 |
N4—C6 | 1.399 (3) | ||
Cl1i—Pt1—Cl1 | 180.0 | C1—C2—N1 | 107.3 (2) |
Cl1—Pt1—Cl2 | 89.97 (2) | N1—C3—H3 | 125 (2) |
Cl1—Pt1—Cl2i | 90.03 (2) | N2—C3—N1 | 109.9 (2) |
Cl1i—Pt1—Cl2 | 90.03 (2) | N2—C3—H3 | 125 (2) |
Cl1i—Pt1—Cl2i | 89.97 (2) | N2—C4—H4A | 109.5 |
Cl1—Pt1—Cl3i | 89.93 (2) | N2—C4—H4B | 109.5 |
Cl1—Pt1—Cl3 | 90.07 (2) | N2—C4—H4C | 109.5 |
Cl1i—Pt1—Cl3 | 89.93 (2) | H4A—C4—H4B | 109.5 |
Cl1i—Pt1—Cl3i | 90.07 (2) | H4A—C4—H4C | 109.5 |
Cl2—Pt1—Cl2i | 180.0 | H4B—C4—H4C | 109.5 |
Cl2i—Pt1—Cl3 | 90.52 (3) | H4D—C4—H4E | 109.5 |
Cl2—Pt1—Cl3 | 89.48 (3) | H4D—C4—H4F | 109.5 |
Cl2i—Pt1—Cl3i | 89.48 (3) | H4E—C4—H4F | 109.5 |
Cl2—Pt1—Cl3i | 90.52 (3) | N3—C5—H5A | 109.5 |
Cl3i—Pt1—Cl3 | 180.0 | N3—C5—H5B | 109.5 |
C2—N1—H1 | 128 (2) | N3—C5—H5C | 109.5 |
C3—N1—H1 | 124 (2) | H5A—C5—H5B | 109.5 |
C3—N1—C2 | 107.7 (2) | H5A—C5—H5C | 109.5 |
C1—N2—C4 | 125.7 (2) | H5B—C5—H5C | 109.5 |
C3—N2—C1 | 108.2 (2) | O2—C6—N4 | 122.2 (2) |
C3—N2—C4 | 126.1 (2) | O2—C6—C1 | 126.5 (3) |
C2—N3—C5 | 123.2 (2) | N4—C6—C1 | 111.2 (2) |
C2—N3—C8 | 117.9 (2) | N4—C7—H7A | 109.5 |
C8—N3—C5 | 118.8 (2) | N4—C7—H7B | 109.5 |
C6—N4—C7 | 116.2 (2) | N4—C7—H7C | 109.5 |
C8—N4—C6 | 127.2 (2) | H7A—C7—H7B | 109.5 |
C8—N4—C7 | 116.5 (2) | H7A—C7—H7C | 109.5 |
N2—C1—C6 | 131.1 (2) | H7B—C7—H7C | 109.5 |
C2—C1—N2 | 106.9 (2) | O1—C8—N3 | 120.5 (3) |
C2—C1—C6 | 122.0 (2) | O1—C8—N4 | 122.1 (2) |
N3—C2—N1 | 128.6 (2) | N4—C8—N3 | 117.3 (2) |
N3—C2—C1 | 124.1 (2) | ||
N2—C1—C2—N1 | −0.4 (3) | C5—N3—C2—N1 | −0.8 (4) |
N2—C1—C2—N3 | 179.0 (2) | C5—N3—C2—C1 | 179.9 (3) |
N2—C1—C6—O2 | −0.4 (5) | C5—N3—C8—O1 | −1.5 (4) |
N2—C1—C6—N4 | −179.0 (2) | C5—N3—C8—N4 | 177.2 (2) |
C1—N2—C3—N1 | 0.3 (3) | C6—N4—C8—O1 | −175.8 (3) |
C2—N1—C3—N2 | −0.6 (3) | C6—N4—C8—N3 | 5.4 (4) |
C2—N3—C8—O1 | 175.7 (2) | C6—C1—C2—N1 | −178.6 (2) |
C2—N3—C8—N4 | −5.5 (3) | C6—C1—C2—N3 | 0.7 (4) |
C2—C1—C6—O2 | 177.4 (3) | C7—N4—C6—O2 | 3.3 (4) |
C2—C1—C6—N4 | −1.2 (3) | C7—N4—C6—C1 | −178.1 (2) |
C3—N1—C2—N3 | −178.7 (3) | C7—N4—C8—O1 | 0.3 (4) |
C3—N1—C2—C1 | 0.6 (3) | C7—N4—C8—N3 | −178.4 (2) |
C3—N2—C1—C2 | 0.0 (3) | C8—N3—C2—N1 | −178.0 (3) |
C3—N2—C1—C6 | 178.1 (3) | C8—N3—C2—C1 | 2.8 (4) |
C4—N2—C1—C2 | −179.4 (3) | C8—N4—C6—O2 | 179.4 (3) |
C4—N2—C1—C6 | −1.4 (4) | C8—N4—C6—C1 | −2.0 (4) |
C4—N2—C3—N1 | 179.8 (3) |
Symmetry code: (i) −x, −y, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···Cl3ii | 0.85 (2) | 2.45 (2) | 3.296 (2) | 174 (3) |
C3—H3···Cl2ii | 0.89 (3) | 2.81 (3) | 3.455 (3) | 131 (3) |
C5—H5C···Cl2 | 0.96 | 2.91 | 3.563 (3) | 127 |
C7—H7B···O1iii | 0.96 | 2.44 | 3.346 (4) | 157 |
Symmetry codes: (ii) −x+1, −y+1, −z; (iii) −x+1, −y, −z+1. |
Acknowledgements
The X-ray diffraction experiment was carried out at the Centre of Shared Use of Physical Methods of Investigation of IPCE RAS.
Funding information
Funding for this research was provided by: Ministry of Science and Higher Education of the Russian Federation (award No. 122011300061-3). This work was supported by the RUDN University Strategic Academic Leadership Program.
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