Synthesis, crystal structure and photoluminescence of the salts Cation⁺ [M(caffeine)Cl]⁻ with Cation⁺=NⁿBu₄⁺, AsPh₄⁺ and M==Zn(II), Pt(II)

2.1 Synthesis of (NⁿBu₄)[Zn(caffeine)Cl₃] (1)

ZnCl₂ and caffeine were dissolved in water at relatively high concentrations (ca. 2 g each in 20 mL) and heated for 5 min. After cooling, an aqueous solution of (NⁿBu₄)triflate was added. This mixture was kept in a refrigerator overnight until colorless crystals precipitated, which were collected by filtration and washed with a small amount of ice-cold water and dried in a vacuum; this product was recrystallized from water: (NⁿBu₄)[Zn(caffeine)Cl₃]×3H₂O. – Analysis (C₂₄H₅₂Cl₃N₅O₅Zn): Calcd. C 43.5, H 7.9, N 10.6; found C 42.5, H 6.8, N 11.4%.

2.2 Synthesis of (AsPh₄)[Pt(caffeine)Cl₃] (2)

To an aqueous solution of K[Pt(caffeine)Cl₃] [3] was added an aqueous solution of (AsPh₄)Cl. A yellow product precipitated. It was collected by filtration, washed with water and dried in a vacuum: (AsPh₄)[Pt(caffeine)Cl₃]. – Analysis (C₃₂H₃₀AsCl₃N₄O₂Pt): Calcd. C 43.7, H 3.5, N 6.4; found C 43.9, H 3.5, N 6.0%.

2.3 X-ray crystallography

Suitable crystals were fixed on MITIGEN holders using a perfluorinated oil and mounted on a GV50 Rigaku diffractometer with a TitanS2 detector for the data collection of (1), respectively on a Supernova Agilent diffractometer equipped with an Atlas detector for the data collection of (2). The crystals were kept at a steady temperature of 123 K during data collection. The structures were solved with the Shelxt [5] structure solution program using Olex2 [6] as the graphical interface. The model was refined with version 2018/3 of Shelxl [7] using least-squares minimizations. All non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were calculated geometrically and refined using the riding model.

Data reduction, scaling and absorption corrections were performed using CrysAlis Pro (Rigaku, V1.171.39.37b, 2017). Numerical absorption corrections based on Gaussian integrations over a multifaceted crystal model were applied.

Crystal data and further details of the data collections and structure refinements are summarized in Table 1.

CCDC 1963741 and 1963740 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Synthesis and crystal and molecular structures

The synthesis of (NⁿBu₄)[Zn(caffeine)Cl₃] led in the first step to the formation of [Zn(H₂O)₆][Zn(caffeine)Cl₃]₂×2H₂O, which precipitated as colorless crystals from the aqueous solution. Its crystal structure was in agreement with results of a previous study [1] with the exception of small deviations of bond lengths and angles. In order to obtain the structure of the isolated anion [Zn(caffeine)Cl₃]⁻ without any interference by the cation [Zn(H₂O)₆]²⁺, it was tried to isolate the desired anion with a big organic cation. However, upon addition of an aqueous solution of (AsPh₄)Cl it was not the desired salt (AsPh₄)[Zn(caffeine)Cl₃] but (AsPh₄)₂[ZnCl₄] which precipitated. It was characterized by a preliminary crystal structure determination. Owing to the kinetic lability of simple zinc complexes, the caffeine ligand was obviously replaced by chloride, which was present in big excess after the addition of (AsPh₄)Cl. Another reagent was more suitable. Upon addition of (NⁿBu₄)triflate, the salt (NⁿBu₄)[Zn(caffeine)Cl₃]×3H₂O could be isolated. Upon recrystallization from CH₂Cl₂/hexane the salt (NⁿBu₄)[Zn(caffeine)Cl₃] without solvent was obtained. Suitable crystals were selected and structurally characterized (Fig. 1). A further advantage of the cation (NⁿBu₄)⁺ is its electronic innocence. It does not participate in any low-energy electronic transition in distinction to (AsPh₄)⁺ (see below).

Fig. 1:

Solid-state molecular structure of the anion [Zn(caffeine)Cl₃]⁻ in (NⁿBu₄)[Zn(caffeine)Cl₃] (1). Displacement ellipsoids are plotted at the 50% probability level. Selected bond lengths and angles (Å, °): Zn–Cl2 2.2220(4), Zn–Cl3 2.2551(4), Zn–N1 1.343(2), N1–C2 1.343(2), N1–C5 1.3689(19); Cl2–Zn–Cl3 116.219(16), N1–Zn–Cl2 106.05(4), C2–N1–Zn 117.46(10), C5–N1–Zn 138.06(10), C2–N1–C5 104.15(12).

The synthesis of K[Pt(caffeine)Cl₃] [3] and its crystal structure have been reported before [4]. Since platinum(II) complexes are kinetically stable, the isolation of the anion [Pt(caffeine)Cl₃]⁻ can be easily achieved with the cation (AsPh₄)⁺. Upon addition of an aqueous solution of (AsPh₄)Cl, the salt (AsPh₄)[Pt(caffeine)Cl₃] precipitated. Upon recrystallization from CH₂Cl₂-hexane, yellow crystals suitable for a crystal structure determination were obtained (Fig. 2). The crystal contained some solvent (CH₂Cl₂ and H₂O), which is taken into account according to its statistical distribution (see Table 1).

Fig. 2:

Solid-state molecular structure of the anion [Pt(caffeine)Cl₃]⁻ of (AsPh ₄)[Pt(caffeine)Cl₃] (2). Displacement ellipsoids 50%. Selected bond lengths and angles (Å, °): Pt–Cl1 2.2998(11), Pt–Cl3 2.3102(9), Pt–N1 2.030(3), N1–C6 1.367(5), N1–C7 1.335(5); Cl2–Pt–Cl3 91.78(4), Cl1–Pt–Cl3 176.41(4), N1–Pt–Cl2 177.65(10), C7–N1–C6 106.2(3).

In the anion [Zn(caffeine)Cl₃]⁻ the Zn²⁺ cation is, as expected, pseudo-tetrahedrally surrounded by caffeine and three chloride ions, while in the anion [Pt(caffeine)Cl₃]⁻, Pt(II) carries the four ligands in a square-planar arrangement owing to its low-spin d⁸ electron configuration. In comparison to previous determinations of the structures of Zn(II) and Pt(II) caffeine complexes, the present work does not reveal any conspicuous features of the molecular structures.

3.2 Electronic (absorption and emission) spectra

Some scattered observations of the electronic spectra of caffeine, including the fluorescence, have been reported [8], [9], [10], but a rather clear picture and interpretation has not emerged. Our observations and conclusions are the following.

Caffeine does not display long-wavelength absorptions, which extend to the visible spectral region. Irrespective of the solvent it shows an intense band (Fig. 3) at λ_max=275.1 nm (ε=1×10⁴ mol⁻¹ cm⁻¹) [11]. This absorption is assigned to a π–π* transition. It has been suggested that π–π* and n–π* states of caffeine are close in energy [12]. Owing to much lower intensities of n–π* absorptions, there is no distinct band of this type visible, but a very diffuse inflection appears at ~300 nm. This may be attributed to this n–π* transition. The photoluminescence spectrum of caffeine supports this assignment.

Fig. 3:

Absorption, excitation and emission spectrum of caffeine in water (1-cm quartz cell).

The corresponding emission (Fig. 3) appears at λ_max=372 nm. It does not overlap with the π–π* absorption, but the excitation band with λ_max=307 nm may coincide with the nπ* absorption at ~300 nm. Moreover, the excitation and emission bands display a distinct overlap. These observations suggest, that the emission originates from the lowest n–π* singlet. Accordingly, it is a caffeine fluorescence.

The electronic spectra of (NⁿBu₄)[Zn(caffeine)Cl₃] (Fig. 4) are very similar to those of the free caffeine ligand. The absorption (λ_max=275.1 nm, ε~10⁴), excitation (λ_max=305 nm) and emission (λ_max=370 nm) spectra of the complex closely resemble those of the free caffeine ligand (Fig. 3). In the solid state of the complex, the excitation appears at λ_max=322 nm and the emission (fluorescence) at λ_max=404 nm.

Fig. 4:

Absorption, excitation and emission spectrum of (NⁿButyl₄)[Zn(caffeine)Cl₃] in water (1-cm quartz cell).

The spectral assignments of the free caffeine ligand can be certainly applied also to the zinc complex. In this case, these spectral features are attributed to caffeine IL (intraligand) transitions. This is not at all surprising, because zinc(II) complexes do not display LF (ligand field) and/or CT absorptions in the UV/visible spectral region.

The spectral situation of the anion [Pt(caffeine)Cl₃]⁻ is more complicated, because the cation (AsPh₄)⁺ absorbs also in the UV region and obscures all spectral features of the caffeine IL type with one exception. The yellow color of the complex originates from a less intense but distinct absorption at λ_max=360 nm (ε~10²). This band is assigned to a spin-allowed LF transition, which appears in the spectrum of the anion [Pt(NH₃)Cl₃]⁻ at λ_max=346 nm (ε=109) [13]. Although the LF strength of caffeine is not known, the spectral shift of 14 nm is certainly reasonable. In order to evaluate the electronic spectra of the anion [Pt(caffeine)Cl₃]⁻, the potassium salt has to be used as a reference since K⁺ does not interfere. Again, the LF band at 360 nm reveals the presence of the [Pt(caffeine)Cl₃]⁻ anion. However, the caffeine IL bands can now be identified (Fig. 5).

Fig. 5:

Absorption, excitation and emission spectrum of K[Pt(caffeine)Cl₃] in water (1-cm quartz cell).

The general pattern of absorption, excitation and emission (fluorescence) bands of the platinum complex is the same as that of free caffeine (Fig. 3) and of the zinc complex (Fig. 4). However, in detail there are distinct differences, which are based partially on the fact that platinum certainly forms rather covalent bonds in distinction to the zinc complex, which is more ionic. First of all, the π–π* absorption appears at λ_max=275.1 nm, precisely the same position as that of the free ligand and the IL absorption of the zinc complex. Obviously, this π–π* transition of the caffeine ligand is not affected by complex formation. The excitation band of [Pt(caffeine)Cl₃]⁻ at λ_max=343 nm undergoes a clear red shift compared to the zinc complex (305 nm). This applies also to the emission (Pt: 415 nm, Zn: 370 nm). Accordingly, the lone pair of electrons at the unsubstituted N atom of the imidazole ring of caffeine forms a strong sigma bond to Pt(II) which thus destabilizes the lone pair and consequently shifts the n–π* IL transition to lower energies. Nevertheless, the covalent interaction between platinum and caffeine seems to be relatively weak since there is no heavy-atom effect of platinum in operation. Such an effect should be accompanied by a fluorescence quenching and the appearance of a phosphorescence at much longer wavelengths. A similar behavior has been observed before for a few other Pt(II) complexes [14], [15].

In conclusion, the complex anion [Pt(caffeine)Cl₃]⁻ has been characterized by its electronic spectra which are attributed to caffeine IL transitions and a low energy LF transition.