Synthesis and Characterization of the New Fac-and Mer-[M (Caf)₃(SCN)₃],OH₂

Transition metal Complexes have received great attention for many years, because of their catalytic, biological activity, essentially due to interaction with their heavy metal ions, bonding through nitrogen and oxygen. The interest in metal complexes of Purina and development of the field of bioinorganic chemistry have increased substantially, since it has been reorganized that many of the complexes may serve as models for biologically important species, i.e. the caffeine is a strong antioxidant that prevent DNA damage. The interaction between nucleic acid and metal ions constitutes a field with multidisciplinary characters such as the bioinorganic chemistry or inorganic biochemistry. N-heterocyclic carbines (NHCs) have been isolated in 1991 [1], and have become ubiquitous in coordination chemistry particularly in the field of catalysis [2-7]. In more recent years, metal NHCs, have shown promise as antimicrobial (AgI-NHCs) and as antitumor (Pd (II), Cu (II), Au (I) and Rh (II) NHCs) agents [8-12]. Caffeine (Figure 1) is the more thoroughly studied methylxanthines to date. Caffeine was first isolated from sea and coffee in the early 1820 [13-15]. Caffeine is widely perceived as a central nervous system (CNS) stimulant. It acts like a brain cortex stimulant and is usually sought for by those looking for a general sense of mental energy with increased awareness and wakefulness improved clear thinking and attenuated fatigue. It was reported as enhancing a wide range of exercise activities from those relying on explosive strength to short term [16]. Caffeine has a centrosymmetric Cs point group. Its number of normal modes of vibration can be distributed as r , = vib27 A' + 12 A''. It's known from the literature that the coordination of the caffeine with nitrogen and oxygen atoms which is accompanied by elimination of the mirror plane a_h and by a whole series of changes in the infrared spectrum [17]. We report here the synthesis and characterization of some new fac- and mer- [M(caf)₃X₃]; M= Cr(III), Fe(III) and Ru(III) containing caffeine and thiocyanato as legends. The synthesized complexes are characterized by molar conductance, infrared, EPR and electronic spectroscopic analysis.

Figure 1: Structure of caffeine (1,3,7-trimethyl-3,7-dihydro-purine-2,6-dione).

All chemicals were obtained from commercial sources and were used without purifications: (FeCl₃, 6H₂O BDH; CrCl₃^ 6H₂O BDH; RuCl₃(H₂O)₃ Sigma Aldrich), caffeine Sigma Aldrich, potassium thiocyanate Sigma Aldrich, Ethanol and DMSO Sigma Aldrich. Infrared spectra were recorded as KBr pellets on a JASCO FT-IR 660 plus spectrophotometer in the range of 4000-400cm^-1 at 298K while the UV-Visible spectra were obtained on a Shimadzu UV-1800 Spectrophotometer. The EPR spectrum was recorded on a conventional X band Burke ER 200D spectrometer operating at 9.5GHz. Conductivity measurements were performed at 25 °C in DMSO using Hatch HQ430d flexi.

Synthesis of the fac- and mer- caffeine complexes [M(caf)₃(SCN)₃OH₂, where caf = caffeine and M= Cr(III), Fe(III) and Ru(III)

To a solution of caffeine (3mmol) and potassium thiocyanate KSCN (3mmol) in ethanol, was added a solution of metal salts (Fe(III) and Cr(III)) (lmmol) in ethanol. The obtained solution was refluxed for 4h, after which the solution is concentrated, filtered and dried.

Characterization of fac- and mer- caffeine complexes [M(caf)₃(SCN)₃]OH<₂ Where caf = caffeine and M= Cr(III), Fe(III) and Ru(HI)

The analytical data and some physical properties of the fac- and mer- caffeine complexes [M(caf)₃(SCN)₃] OH₂; M= Cr³⁺, Fe³⁺ and Ru³⁺ are collected in Table 1. The complexes are stable in air, soluble in DMSO and DMF. Molar conductance values of the fac- and mer- caffeine complexes in DMSO (10-3M solution at 25 °C) were (7.3-18.6)Ω^-1cm² mol^-1 indicating, their non-electrolytic nature [18] (Table 1).

Table 1: Physico-chemical data of the fac-and mer-caffeine complexes.

Infrared spectra

In order to study binding mode of the caffeine and thiocyanato ion to Chromium(III), Ruthenium(III) and Iron (III) in the new complexes Figure 2 &Table 2

Figure 2: Infrared spectrum of [Fe(caf)₃(SCN)₃],OH₂ in KBr.

Table 2 Spectral data of the fac and mer-caffeine complexes [M(caf)₃(SCN)₃],OH₂; M=Cr³⁺, Fe³⁺ and Ru³⁺

The infrared spectrum of the free caffeine [19] (Table 2) is characterized by absorption peaks at 3110cm'¹ and 2954cm'¹ assigned to the v(C-H) stretching vibrations in imidazole ring and v(CH₃) stretching vibration in imidazole and pyrimidine rings respectively. A strong bands are observed at 1700cm^-1 and 1660cm^{- 1} which are characteristic of the asymmetric and symmetric v(C=O) stretching vibration in pyrimidine ring respectively. The v(C=C) and v(C=N) vibrations of the free caffeine that occur at 1600cm" ¹ and 1548cm’¹ for imidazole ring. The different modes of the methyl bending and rocking are observed in (1470-1025)cm^-1 range. In the infrared spectra of all fac-and mer-caffeine complexes, the characteristic bands of the free caffeine are shifted to lower frequencies. The v(C-H) stretching vibrations in imidazole ring and methyl groups in pyrimidine ring are shifted to higher frequencies with the higher intensity in the fac-and mer- caffeine complexes. The stretching frequencies of the symmetric and asymmetric vs(C=O), vas(C=O), v(C=C), v(C=N), 5HCN which appeared in (1694- 1699)cm^-1, (1641-1655)cm^-1, (1553-1598)cm^-1 and (1540-1550) cm^-1 ranges respectively [22,23] are shifted to lower frequencies in the fac-and mer-caffeine complexes. The methyl vibrations attached with nitrogen bending and rocking atom in the side chain N-CH₃ vibration which are found to be observed around (1340-1496) cm^-1 and (978-1077)cm^-1 respectively are shifted in the fac-and mer-caffeine complexes. Bands of variable intensity are observed in the region (1100-1000)cm^-1 due to in plane deformation vibration in the imidazole and pyrimidine rings. The medium and strong vibrational bands in the regions (974-978)cm^-1 and (800- 803)cm^-1 are assigned to the 5 (imid) +v (N-C) and p (pyrimid) +5 (C=O) respectively [24,25]. However, during the complexes formation of the caffeine, the imidazole ring gets more frequency changes with higher intensity (Table 2), suggesting that the caffeine coordinate through the imidazole vibration N9 atom, and acting as monodentate ligand [26]. The thiocyanato ligand in the fac- and mer- caffeine complexes absorbed at five frequencies [27]. Strong vibrations in the (2034-2122)cm^-1 range for [Fe(caf)₃(SCN)₃] and [Ru(caf)₃(SCN)₃] are due to v(SCN) stretching vibration and may attributed to the presence of the thiocyanato in the coordination sphere of these complexes. The multiplicity of the band indicated two isomers which are distinguishable by the infrared spectroscopy [27]. The five absorptions are characteristic for mer and fac isomers in the new complexes. The thiocyanato ligand can coordinate the metal through either the nitrogen or the sulphur. The free thiocyanate ion absorbs at about 2052cm^-1 and 750cm^-1 which are due to v (SCN) and v (C-S) stretching vibrations [28].These peaks are shifted to higher frequencies in the complexes suggesting the coordination through the sulphur atom (M-SCN). Therefore, the spectra show five bands in (2034-2090) cm^-1 range attributed to two isomers mer [M(caf)₃(SCN)₃] (2A1+B1) with three actives bands in IR (2A1+B1) and the fac isomer has two actives bands in IR (A1+E) [27]. New bands in (555-586)cm^-1 range are assigned to u (M-N) vibration [22].

Table 3: UV-Visible data of the fac-and mer-caffeine complexes [M(caf)₃(SGN)₃]GH₂; M= Gr^3+;, Fe^3+; and Ru³⁺.

Figure 3: Electronic spectrum of [Cr(caf)₃(SCN)₃],OH₂, in DMSO.

All absorptions were fully assigned in Table 3 and the electronic spectrum of [Cr(caf)₃(SCN)₃] is shown in Figure 3. The UV-Visible data ofthe thiocyanato and caffeine ligands [29] were compared with those of the fac- and mer-caffeine complexes [M(caf)₃(SCN)₃],OH₂ ; M= Cr³⁺, Fe³⁺ and Ru³⁺. The electronic spectra of the thiocyanato and caffeine ligands [29] show the absorption bands in the UV region can be attributed to and n^n* transitions which were shifted to higher wavelength upon coordination. Therefore, new bands at longer wavelength observed in (366-756)nm range may be assigned to d-d transitions.

EPR spectra

Figure 4: EPR spectrum of [Fe(caf)₃(SCN)₃]OH₂ at room temperature on X band in solid state

The X-band EPR spectrum of the complex [Fe(caf)₃(SCN)₃] OH₂, in the solid state recorded at room temperature are shown in Figure 4. ESR spectrum of the complex shows only an intense and broad signal without hyperfine splitting (giso=2.151). The shape of the spectrum is consistent with octahedral environment around Fe (II) ion and the higher g value for the investigated complex, when compared to that of free electron (g = 2.0023) revealing an appreciable covalence of metal-ligands (M-N9 and M-SCN) bonding characteristic of octahedral stereochemistry [30-32] (Table 3).

New fac- and mer- caffeine complexes [M(caf)₃(SCN)₃],OH₂; where caf=caffeine, M= Cr³⁺, Fe³⁺, Ru³⁺ were synthesized and characterized on the basis of molar conductance, infrared, UV- Visible EPR. The spectroscopic data suggest that the M (III) caption lies on a fac-and mer-arranged octahedral coordination geometry provided by sulfur atoms of three thiocyanato ligands and nitrogen atoms of three coordinated caffeine molecules and hydration water molecule.

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