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Synthesis, structure and Hirshfeld analysis of the potent antimicrobial [Ag(4-bromopyrazole)2ClO4] complex
Polyhedron 171 (2019) 323–329
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Synthesis, structure and Hirshfeld analysis of the potent antimicrobial [Ag(4-bromopyrazole)2ClO4] complex Saied M. Soliman a,b,⇑, Ahmed M.A. Badr a a b
Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia, 21321 Alexandria, Egypt Department of Chemistry, Rabigh College of Science and Art, King Abdulaziz University, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 10 April 2019 Accepted 19 July 2019 Available online 27 July 2019 Keywords: Silver 4-Bromopyrazole Hirshfeld DFT Antimicrobial activity
a b s t r a c t The synthesis of a new silver(I) complex, [Ag(4Brpyz)2ClO4], (where 4Brpyz = 4-bromopyrazole) has been described. Its structural aspects were identified using elemental analysis, IR and 1H NMR spectroscopy, as well as single crystal X-ray diffraction. The crystallographic analysis revealed that the complex crystallizes in the centrosymmetric monoclinic space group P2(1)/c with one [Ag(4Brpyz)2ClO4] complex per asymmetric unit and Z = 4. The asymmetric unit is comprised of one Ag(I) ion, two units of 4Brpyz as an organic ligand and one disordered perchlorate anion (part A: 28.0% and part B: 72.0%). In part A, the structure of the complex is better described as one dimensional coordination polymer, [Ag (4Brpyz)2ClO4]n, where the perchlorate ions act as a connector between the [Ag(4Brpyz)2]+ units and are also included in NAH. . .O hydrogen bonding interactions. On other hand, the major part (B) could be described as an ionic complex of the formula [Ag(4Brpyz)2]ClO4, where the free perchlorate anions are included in NAH. . .O and CAH. . .O hydrogen bonding interactions. The intermolecular interactions and the AgAN/AgAO coordinate bonds were analyzed using Hirshfeld and DFT analyses, respectively. The antimicrobial activity of the complex was compared with that of the free ligand and four commercially available antibiotics. The studied Ag(I) complex showed a lower MIC and hence better reactivity against bacteria and fungus compared to either the free ligand or any of the tested antibiotics. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Supramolecular Ag(I) complexes [1–4] have attracted the interest of researchers due to their medicinal [5] and antimicrobial applications [6–11]. Silver compounds were used in medicine before the discovery of antibiotics [12,13]. Compounds containing silver not only have promising antibacterial, antifungal, anticancer and antiviral applications [14–18], but also were used as antioxidants [19]. Also, Ag-compounds were applied in water disinfection [20] and have been used in wound dressings to avoid infections [13,21–24]. On other hand, the pyrazole nucleus is a unique structural scaffold with many applications in medicinal field [24]. Pyrazole derivatives are multipurpose biologically active compounds with various applications in a wide range of pharmacological applications. These interesting molecules have analgesic, antipyretic, antiviral, anti-inflammatory, antioxidant, anti-diabetic, anticonvul⇑ Corresponding author at: Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia, 21321 Alexandria, Egypt. Fax: +20 3 5932488. E-mail address: [email protected] (S.M. Soliman). https://doi.org/10.1016/j.poly.2019.07.026 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
sant and arrhythmic activities [25]. Some pyrazoles have more anti-inflammatory activity than indomethacin [26] and diclofenac [27]. Also, many pyrazole derivatives have been found to have good anticancer [28–32] and antimicrobial activities [33–35]. In the light of the interesting biological activity of silver(I) complexes and the pyrazole scaffold, this work presents a new selfassembled Ag(I) complex with 4-bromopyrazole (4Brpyz) as a ligand. The new complex was synthesized by the direct reaction of silver perchlorate and the 4-bromopyrazole (4Brpyz) ligand in water-alcohol mixture. The structure of this complex was characterized using FTIR, 1H NMR and single crystal X-ray diffraction. Its antimicrobial properties are discussed in comparison with the free ligand and some commercially available antibiotics. 2. Experimental 2.1. Physicochemical characterizations An Alpha Bruker instrument was used to measure the FTIR spectra (4000–400 cm 1) in KBr pellets. The 1H NMR spectrum was recorded on a JEOL JNM-ECA 500 MHz NMR spectrometer in DMSO as solvent. CHN analyses were performed using a Perkin Elmer
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2400 Elemental Analyzer. All chemicals were purchased from the Sigma-Aldrich Company. Thermogravimetric analysis was performed using a Shimadzu TGA-50 thermogravimetric analyzer in a platinum cell under a nitrogen flow of 40 mL/min and a heating rate of 10 °C/min.
Gram-negative bacteria (E. coli ATCC8739, P. aeruginosa ATCC9027) strains and one yeast (Candida albicans ATCC2091) in the Faculty of Pharmacy, Central Lab., Alexandria University, Alexandria, Egypt using the agar well diffusion method. Further details regarding the biological experiments are available in the Supplementary data.
2.2. Synthesis
3. Methods and calculations
An aqueous solution of silver(I) perchlorate (1 mmol) in 5 mL distilled water was added to 10 mL of 4-bromopyrazole solution (4Brpyz, 2 mmol) in methanol. The clear solution was left for slow evaporation at room temperature, whereupon colorless needle crystals of [Ag(4Brpyz)2ClO4] were obtained after one week. [Ag(4Brpyz)2ClO4]; (1): Yield: 95%; Anal. Calc. C6H6AgBr2ClN4O4: C, 14.38; H, 1.21; N, 11.18%. Found: C, 14.39; H, 1.19; N, 11.13%. FTIR (mmax, cm 1): 3147, 3071, 2985, 2937, 1650, 1145, 1089, 1033, 944. Ligand (4Brpyz): 3156, 3070, 2985, 2937, 1647, 1372, 1331, 1281, 1248, 1185, 1138, 1032, 945 (Fig. S1, Supplementary data); 1H NMR (500 MHz, DMSO-d6) dH (ppm): 13.22 (s, 2H, NH), 7.96 (s, 2H, Pyrazole-H), 7.56 (s, 2H, Pyrazole-H) (Fig. S2, Supplementary data).
Gaussian 09 [37], built in NBO 3.1 [38] program, was used to perform the natural bond orbital (NBO) calculations using the WB97XD method [39] and either 6-31G(d,p) or cc-PVTZ basis sets for non-metal atoms combined with cc-PVTZ-PP [40] for silver atom.
2.3. X-ray diffraction The crystallographic measurements of the studied complex were made using a Bruker D8 Quest diffractometer with graphite monochromated Mo Ka radiation. All calculations were performed using the Bruker APEX III program system and the SHELXTL program package [36a,b]. The crystal and structure solution details are listed in Table 1. Corrections for absorption were performed by SADABS
[36c].
2.4. Antimicrobial studies The antimicrobial activities were tested against Gram-positive bacteria (S. aureus ATCC6538P, B. subtilis ATCC19659) and
Table 1 Crystal data and structure refinement for the studied complex. Empirical formula Formula weight (g/mol) T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å)3 Z Density (calc.) (g/cm3) l (mm 1) F(0 0 0) Crystal size (mm3) h (°) Index ranges Reflections collected Independent reflections Completeness to theta = 25.42° Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Largest diff. peak and hole (e Å CCDC
C6H6AgBr2ClN4O4 501.29 293(2) 0.71073 monoclinic P2(1)/c
3
)
5.266(8) 17.061(3) 14.661(19) 90 91.479(7) 90 1316.8(3) 4 2.529 7.813 944 0.22 0.08 0.05 2.76–25.37 6 h 6, 20 k 18, 14 l 17 13 572 2415 [R(int) = 0.0848] 99.60% full-matrix least-squares on F2 2415/0/161 1.016 R1 = 0.0479, wR2 = 0.0976 R1 = 0.0867, wR2 = 0.1124 0.785 and 0.753 1909254
4. Results and discussion 4.1. X-ray structure description The studied Ag(I) complex crystallized in the centrosymmetric monoclinic space group P2(1)/c with one [Ag(4Brpyz)2ClO4] unit per asymmetric unit and Z = 4. The structure and atom numbering for [Ag(4Brpyz)2ClO4]n; (1a) and [Ag(4Brpyz)2]ClO4; (1b) are shown in Fig. 1. The most important bond lengths (Å) and angles (°) are given in Table 2. The asymmetric unit comprises one Ag(I) ion, two organic ligand (4Brpyz) units and one disordered perchlorate anion. Due to the disorder occurring in the perchlorate anion (part A: 28.0% and part B: 72.0%), there are two different bonding modes for this anion. In part A, the structure of the complex is better described as one dimensional coordination polymer, [Ag (4Brpyz)2ClO4]n. In this part of the studied complex, the perchlorate anion acts as a bridging ligand between two [Ag(4Brpyz)2]+ units with Ag1AO1A and Ag1AO3A distances of 2.540(3) and 2.560(3) Å, respectively. The NAAgAN, O1AAAgAO3A and OAAgAN angles are 176.1(2), 167.6(12) and 83.2(8)–94.7(10)°, respectively, indicating a distorted square planar coordination geometry for 1a (Fig. 2). On other hand, the major part (B) can be described as an ionic complex of the formula [Ag(4Brpyz)2]ClO4. The shortest Ag1. . .O1B (2.913(12) Å) and Ag1. . .O3B (2.837(11) Å) distances are too long and should be described as interactions rather than bonds. Hence, the geometry around the Ag(I) ion is considered as a linear or slightly bent configuration, augmented
Fig. 1. The asymmetric unit and atom numbering scheme of 1.
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Table 2 The most important bond distances (Å) and angles (°) of 1. Ag1AN3 Ag1AN1 Ag1. . .O1B N3AAg1AN1 N3AAg1AO1A N1AAg1AO1A
2.125(7) 2.128(6) 2.913(12) 176.1(2) 89.2(10) 94.7(10)
Ag1AO1A Ag1AO3A Ag1. . .O3B N3AAg1AO3A N1AAg1AO3A O1AAAg1AO3A
2.540(3) 2.560(3) 2.837(11) 93.0(8) 83.2(8) 167.6(12)
by two weak Ag. . .O interactions from the neighboring perchlorate anions. The Ag1AN3 and Ag1AN1 distances are found to be 2.125 (7) and 2.128(6) Å, respectively, which are close to the corresponding values (2.124 and 2.127 Å) for the structurally related [Ag (pyz)2]NO3 complex reported by Schmidbaur and coworkers [41]. Also, the two coordinated 4Brpyz ligand units show an anti-configuration with respect to one another and are slightly twisted from one another by 6.2°, in agreement with the corresponding bis(pyrazole)silver(I) nitrate complex. It seems that the presence of the bulky bromine atom at the 4-position shows almost no effect on the configuration of the ligand groups around the Ag(I) ion and the anti-configuration of the two organic ligand groups is probably adopted to maximize the intermolecular interactions which stabilize the crystal structure via hydrogen bonding interactions, as an example. It is clear from Fig. 2 that each two successive [Ag(4Brpyz)2] units are nearly perpendicular to one another. The angle between the mean planes passing through each two neighboring [Ag (4Brpyz)2] units is found to be 77.1°. Such a situation allows the silver atoms to be located below and above the stacked pyrazole moieties, which allows the formation of some Ag. . .C interactions, as shown in Fig. 3. The Ag. . .C intermolecular distances are 3.271 and 3.348 Å for the Ag1. . .C6 and Ag1. . .C3 interactions, respectively. In addition to these interactions, the complex units are packed together via H. . .O hydrogen bonds. Details regarding the hydrogen bond parameters are given in Table 3 and are shown in Fig. 3 for both parts of the complex. It is clear that the free perchlorate anions in the ionic complex (1b) are included in both of the strong NAH. . .O and weak CAH. . .O hydrogen bonding interactions (Fig. 3, lower part). On other hand, the coordinated perchlorate ions in the coordination polymer 1a are included only in strong NAH. . .O hydrogen bonds (Fig. 3, upper part). 4.2. Analysis of molecular packing Hirshfeld surfaces were performed using the Crystal Explorer 17.5 program [42,43] in order to quantify and describe the intermolecular interactions in the studied Ag(I) complex. All Hirshfeld surfaces were drawn using the same software and are shown in Fig. S3 (Supplementary data). The frequencies of the common intermolecular interactions in the crystal structure of complexes 1a and 1b are shown in Fig. 4. Table 4 summarizes the most important contacts in both parts as well as the corresponding shortest contact distances, while presentation of these contacts in part A, as an example, is shown in Fig. 5. The O. . .H hydrogen bonds
Fig. 3. Comparison between the molecular packing in complexes 1a (upper) and 1b (lower).
contributed by 26.3 and 28.9% to the whole intermolecular interactions in 1a and 1b, respectively. The shortest hydrogen bond occurring in both parts is O4. . .H2, with a contact distance of 2.181 Å. Also, the results revealed the presence of some Br1. . .Br2 (3.656 Å) and Ag1. . ..C6 (3.271 Å) interactions, with contact distances slightly shorter than the van der Waals radii sum of the two elements (3.70 and 3.42 Å, respectively). The Hirshfeld surface analysis indicated the presence of significantly short O2A. . .Br1 (3.302 Å) and O2B. . .Br1 (3.349 Å) contacts in 1a and 1b, respectively. In addition, the Ag1. . .O3 contacts appeared as red spots in the dnorm maps, indicating that these interactions are significantly strong (Fig. 5). On other hand, the shape index and curvedness maps shown in Fig. S3 (Supplementary data), as well as the almost negligible C. . .C, C. . .N and N. . .N contacts, reveal the insignificance of p–p stacking interactions amongst the complex units.
4.3. FTIR spectra and TGA analysis The FTIR spectra of the free 4Brpyz ligand and its Ag(I) complex are shown in Fig. S1 (Supplementary data). The FTIR spectrum of the complex showed little variations compared to that for the free ligand. This is usually common for Ag(I) complexes with N-donor ligands which have relatively weak AgAN interactions. In 4Brpyz, the mNAH vibrational band was detected at 3156 cm 1, which
Fig. 2. The one dimensional coordination polymer of 1a.
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Table 3 Hydrogen bonds [Å and °] for complex 1. DAH A
DAH (Å)
H A (Å)
D A (Å)
DAH A (°)
N2AH2. . .O3A(i) N2AH2. . .O4(i) N4AH4A. . .O4(ii) N4AH4A. . .O2B N4AH4A. . .O4(ii) N2AH2. . .O4(i) C3AH3. . .O3B(iii) C6AH6. . .O2B(ii)
0.86 0.86 0.86 0.86 0.86 0.86 0.93 0.93
2.397 2.328 2.595 2.393 2.595 2.328 2.675 2.719
2.97(3) 3.18(1) 3.24(1) 3.01(1) 3.24(1) 3.18(1) 2.96(1) 3.32(1)
124.8 172.2 132.4 129.2 132.4 172.2 98.9 123.2
(i) x,1.5 y, 1/2 + z; (ii) 1 + x,y,z; (iii) 1 + x,1.5 y,1/2 + z.
Fig. 4. The common contacts and their percentages in the crystal structure of the studied Ag(I) complex.
Table 4 The important intermolecular contacts and the shortest interaction distances (Å) in both parts of the studied silver(I) complex. Contact
Shortest contact (Å)
%
Shortest contact (Å)
5.2 26.3 3.0 7.9 3.5
3.271 2.181 2.838 3.349 3.656
Part A; 1a Ag1. . .C6 O4. . .H2 Ag1. . .O3 Br1. . .O2 Br1. . .Br2
3.271 2.181 2.562 3.302 3.656
%
Part B; 1b 5.7 28.9 2.4 8.1 3.7
Fig. 5. Presentation of the most important Br. . .Br (A), Br. . .O (B), Ag. . .O (C), Ag. . .C (D) and O. . .H (E) intermolecular contacts in part A (1a); for more details see Table 4.
shifted slightly to a lower wavenumber in the Ag(I) complex (3147 cm 1). Also, the mC@N band appeared at 1647 and 1650 cm 1 for 4Brpyz and its Ag-complex, respectively. A new broad triple split band in the range 1145–1033 cm 1 was assigned to the perchlorate anion vibrations in 1. Thermogravimetric (TGA) analyses of the free 4Brpyz ligand and its silver(I) perchlorate complex are shown in Fig. 6. It is clear
that the Ag(I) complex is thermally more stable than the free ligand. 4Brpyz decomposed completely in the temperature range 80–170 °C in one decomposition step, leaving no residue. In contrast, the silver(I) complex decomposed at a higher temperature of 213 °C, leaving a residue of 26.8% at the end of the thermal decomposition (302 °C), probably corresponding to silver(I) chloride, with a theoretical mass percent of 28.6%.
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Table 6 The net interaction energies (kcal/mol) of the AgAN and AgAO/Ag. . .O interactions. NBOi
NBOj
1a
LP(N1) LP(N3) LP(O1) LP(O3)
LP*(Ag1) LP*(Ag1) LP*(Ag1) LP*(Ag1)
90.40 89.93 31.18 31.08
1b (85.25) (83.99) (32.65) (33.22)
83.85 86.62 16.31 20.00
(80.57) (81.43) (17.22) (21.17)
Values inside and outside parentheses for the cc-PVTZ and 6-31G(d,p) basis sets, respectively.
Fig. 6. TGA analyses of 4Brpyz and its AgClO4 complex.
4.4. NBO analyses In order to simulate the coordination environment around the Ag(I) ion in the structure of the studied complex, we selected the trimeric unit shown in Fig. S4 (Supplementary data) to perform the DFT studies. The results presented here are for the central Ag (I)-complex unit in this structure. The natural charge at this Ag(I) ion is reduced to 0.624 and 0.636 e for 1a and 1b, respectively. It is slightly less in 1a because in this part of the complex, the perchlorate anions are closer to the Ag(I) ion, which compensates its positive charge more than that in 1b (Table 5). In both parts, the two organic ligand molecules transfer almost the same net negative electron density to the silver(I) ion. The two perchlorate anions have a higher net negative charge in 1b compared to 1a as a consequence of the stronger interaction between the Ag(I) and ClO4 ions in the latter compared to the former. The natural orbitals included in the AgAN and AgAO coordination interactions are listed in detail in Table S1 (Supplementary data). Table 6 summarized the net interaction energies of these bonds. These results show that the two AgAN coordination bonding interactions are slightly different. There is one slightly weaker AgAN bond than the other in both parts of the complex. The same is true for the AgAO bonds in part A of this complex (1a). On the other hand, the two Ag. . .O interactions are more different compared to each other in part B (1b) and are weaker than those in 1a. It is obvious that the perchlorate ion in 1b has an ionic nature (16.31–21.17 kcal/mol) while it is coordinated for 1a (31.08– 33.22 kcal/mol). As a result of these interactions between the ligand donor atoms NBOs to the Ag(I) NBOs (Fig. 7), the outer most shell electronic distribution of the Ag(I) ion is reduced to [core] 5S0.364d9.865p0.15 and 5S0.384d9.855p0.12, for 1a and 1b, respectively using the 6-31G(d,p) and cc-PVTZ-PP basis set combinations. Natural bond order calculations of the Ag. . .O interactions in both parts revealed these facts very well. The bond order values for the Ag1AO1 and Ag1AO3 bonds in part A are 0.276 and 0.249,
Table 5 The net natural charges for the different complex fragments of 1 using the WB97XD method.
Ag 2(4Brpyz) 2ClO4
1a
1b
0.664 (0.624) 0.225 (0.234) 1.779 (-1.719)
0.671 (0.636) 0.234 (0.239) 1.807 (-1.754)
Values inside and outside parentheses for the cc-PVTZ and 6-31G(d,p) basis sets, respectively.
Fig. 7. The overlaps among the occupied natural orbitals of the donor atoms and the empty anti-bonding natural orbitals of the Ag(I) ion using the WB97XD method. The green/yellow set are for the donor atoms NBOs and red/violet colors combination are for the Ag(I) NBOs. For simplicity, the AgAN interactions of one part of the complex are presented (upper) while the Ag. . .O interactions of both parts (lower) are given, showing the weaker orbital overlaps in 1b compared to 1a. Hydrogen atoms are omitted for more clarity. (Color online.)
respectively while the corresponding values in part B are lower (0.181 and 0.195, respectively). 4.5. Antimicrobial activity In the present study, in vitro antimicrobial activities of the [Ag (4Brpyz)2ClO4] complex and the 4Brpyz ligand in comparison to Ciprofloxacin, Clotrimazol, Streptomycin and Cephradine antibiotics were tested by the use of the agar well-diffusion method in DMSO as solvent. The antimicrobial activity was estimated as the minimum inhibitory concentration (MIC; mg/ml). The antimicrobial activity results (MIC) of the [Ag(4Brpyz)2ClO4] complex and the free 4Brpyz ligand compared to the selected antibiotics are collected in Table 7. The complex and the ligand showed good antibacterial activity against the studied microorganisms in comparison with standard antibiotics. As can be seen clearly from Table 7, the [Ag(4Brpyz)2ClO4] complex is more efficient as an antibacterial agent against all bacteria and the fungus C. albicans than the free ligand. The MIC values for the [Ag(4Brpyz)2ClO4] complex against the Gram-positive bacteria and the yeast C. albicans are 9 mg/ml and in the range of 9–13 for the Gram-negative bacteria. These values are generally lower than the MIC values of the free ligand (12–15 mg/ml) indicating higher antibacterial and antifungal action of the [Ag(4Brpyz)2ClO4] complex compared to the free 4Brpyz ligand. Interestingly, the biological profile of the studied Ag(I) complex is significantly higher than the commercial antibiotics used in the biological experiments. In addition, the [Ag(4Brpyz)2ClO4] complex could be used as antifungal agent
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Table 7 The MIC (mg/ml) of [Ag(4Brpyz)2ClO4] and 4Brpyz against different microbes compared with the selected standard antibiotics. Compound
S. aureus
4Brpyz AgClO4 [Ag(4Brpyz)2ClO4] Ciprofloxacin Clotrimazole Streptomycin Cephradine
14.1 ± 0.12 4.0 ± 0.05 9.0 ± 0.11 30.3 ± 0.23 – 128.2 ± 0.21 64.2 + 0.20
B. subtilis
Gram-positive bacteria
P. aeruginosa
E. coli
C. albicans
12.0 ± 0.11 6.1 ± 0.08 8.9 ± 0.09 30.2 ± 0.23 – 31.8 ± 0.20 128.2 ± 0.21
11.9 ± 0.09 7.0 ± 0.07 8.9 ± 0.09 – 17.2 ± 0.18 – –
Gram-negative bacteria 15.0 ± 0.09 5.0 ± 0.06 9.1 ± 0.10 30.1 ± 0.20 – – –
against the yeast C. albicans. Therefore, this complex has the advantage of being able to be used as both an antibacterial and an antifungal agent.
4.6. 1H NMR spectra The 1H NMR spectrum of the free 4Brpyz ligand looks completely different from that of the studied Ag(I) complex (Fig. S2, Supplementary data). While the complex showed three well separated proton signals: one for the NH proton (d 13.22 ppm) and two for the CH protons (d 7.96 and 7.56 ppm). The free ligand showed no clear signal for the NH proton and the two CH protons appeared as one signal at d 7.73 ppm. Such variations leave no doubt about the absence of free 4Brpyz in the solution medium of 1 or at least the complex retains its identity completely or partially but to high extent in solution. Interestingly, the absence of a clear split signal for coordinated DMSO at about d 2.5 ppm could be a sufficient evidence on the absence or little existence of solvated Ag(I) or Ag4Brpyz complex cations. In this regard, the high antimicrobial action of the studied complex could be attributed mainly to the presence of [Ag(4Brpyz)2]+ species in solution and excludes – at least partially – the presence of a cooperative effect between the free silver(I) ion and free 4-bromopyrazole in solution.
5. Conclusion A new [Ag(4Brpyz)2ClO4] self-assembled coordination complex was synthesized and characterized. The X-ray structure analysis showed that the [Ag(4Brpyz)2ClO4] complex is comprised of one Ag(I) ion coordinating two 4-bromopyrazole ligands via one of their nitrogen atoms and one disordered perchlorate ion, leading to two parts of this complex. In one part, the AgAO(perchlorate) bonds are significantly short and the perchlorate anion acts as a connector between the [Ag(4Brpyz)2]+ cations, leading to the formation of a one dimensional coordination polymer of the formula [Ag(4Brpyz)2ClO4]n. In the other part, the Ag. . .O distances are significantly longer and could not be considered as a bond, leading to the ionic [Ag(4Brpyz)2]ClO4 complex. Hirshfeld analysis showed that the O. . .H (26.3–28.9%), Br1. . .Br2 (3.656 Å) and Ag1. . ..C6 (3.271 Å) interactions are the main contacts contributing in the packing of the studied complex. The complex was screened using the agar well diffusion method for its in vitro antimicrobial activity against a range of Gram-positive and Gram-negative bacteria, as well as the fungus C. albicans. Its antimicrobial actions are higher than the free ligand and a selected set of known antibiotics.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.07.026.
13.0 ± 0.10 3.9 ± 0.05 13.0 ± 0.14 30.5 ± 0.25 – 63.9 ± 0.16 96.0 ± 0.22
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Synthesis, structure and Hirshfeld analysis of the potent antimicrobial [Ag(4-bromopyrazole)2ClO4] complex Saied M. Soliman a,b,⇑, Ahmed M.A. Badr a a b
Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia, 21321 Alexandria, Egypt Department of Chemistry, Rabigh College of Science and Art, King Abdulaziz University, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 10 April 2019 Accepted 19 July 2019 Available online 27 July 2019 Keywords: Silver 4-Bromopyrazole Hirshfeld DFT Antimicrobial activity
a b s t r a c t The synthesis of a new silver(I) complex, [Ag(4Brpyz)2ClO4], (where 4Brpyz = 4-bromopyrazole) has been described. Its structural aspects were identified using elemental analysis, IR and 1H NMR spectroscopy, as well as single crystal X-ray diffraction. The crystallographic analysis revealed that the complex crystallizes in the centrosymmetric monoclinic space group P2(1)/c with one [Ag(4Brpyz)2ClO4] complex per asymmetric unit and Z = 4. The asymmetric unit is comprised of one Ag(I) ion, two units of 4Brpyz as an organic ligand and one disordered perchlorate anion (part A: 28.0% and part B: 72.0%). In part A, the structure of the complex is better described as one dimensional coordination polymer, [Ag (4Brpyz)2ClO4]n, where the perchlorate ions act as a connector between the [Ag(4Brpyz)2]+ units and are also included in NAH. . .O hydrogen bonding interactions. On other hand, the major part (B) could be described as an ionic complex of the formula [Ag(4Brpyz)2]ClO4, where the free perchlorate anions are included in NAH. . .O and CAH. . .O hydrogen bonding interactions. The intermolecular interactions and the AgAN/AgAO coordinate bonds were analyzed using Hirshfeld and DFT analyses, respectively. The antimicrobial activity of the complex was compared with that of the free ligand and four commercially available antibiotics. The studied Ag(I) complex showed a lower MIC and hence better reactivity against bacteria and fungus compared to either the free ligand or any of the tested antibiotics. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Supramolecular Ag(I) complexes [1–4] have attracted the interest of researchers due to their medicinal [5] and antimicrobial applications [6–11]. Silver compounds were used in medicine before the discovery of antibiotics [12,13]. Compounds containing silver not only have promising antibacterial, antifungal, anticancer and antiviral applications [14–18], but also were used as antioxidants [19]. Also, Ag-compounds were applied in water disinfection [20] and have been used in wound dressings to avoid infections [13,21–24]. On other hand, the pyrazole nucleus is a unique structural scaffold with many applications in medicinal field [24]. Pyrazole derivatives are multipurpose biologically active compounds with various applications in a wide range of pharmacological applications. These interesting molecules have analgesic, antipyretic, antiviral, anti-inflammatory, antioxidant, anti-diabetic, anticonvul⇑ Corresponding author at: Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia, 21321 Alexandria, Egypt. Fax: +20 3 5932488. E-mail address: [email protected] (S.M. Soliman). https://doi.org/10.1016/j.poly.2019.07.026 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
sant and arrhythmic activities [25]. Some pyrazoles have more anti-inflammatory activity than indomethacin [26] and diclofenac [27]. Also, many pyrazole derivatives have been found to have good anticancer [28–32] and antimicrobial activities [33–35]. In the light of the interesting biological activity of silver(I) complexes and the pyrazole scaffold, this work presents a new selfassembled Ag(I) complex with 4-bromopyrazole (4Brpyz) as a ligand. The new complex was synthesized by the direct reaction of silver perchlorate and the 4-bromopyrazole (4Brpyz) ligand in water-alcohol mixture. The structure of this complex was characterized using FTIR, 1H NMR and single crystal X-ray diffraction. Its antimicrobial properties are discussed in comparison with the free ligand and some commercially available antibiotics. 2. Experimental 2.1. Physicochemical characterizations An Alpha Bruker instrument was used to measure the FTIR spectra (4000–400 cm 1) in KBr pellets. The 1H NMR spectrum was recorded on a JEOL JNM-ECA 500 MHz NMR spectrometer in DMSO as solvent. CHN analyses were performed using a Perkin Elmer
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2400 Elemental Analyzer. All chemicals were purchased from the Sigma-Aldrich Company. Thermogravimetric analysis was performed using a Shimadzu TGA-50 thermogravimetric analyzer in a platinum cell under a nitrogen flow of 40 mL/min and a heating rate of 10 °C/min.
Gram-negative bacteria (E. coli ATCC8739, P. aeruginosa ATCC9027) strains and one yeast (Candida albicans ATCC2091) in the Faculty of Pharmacy, Central Lab., Alexandria University, Alexandria, Egypt using the agar well diffusion method. Further details regarding the biological experiments are available in the Supplementary data.
2.2. Synthesis
3. Methods and calculations
An aqueous solution of silver(I) perchlorate (1 mmol) in 5 mL distilled water was added to 10 mL of 4-bromopyrazole solution (4Brpyz, 2 mmol) in methanol. The clear solution was left for slow evaporation at room temperature, whereupon colorless needle crystals of [Ag(4Brpyz)2ClO4] were obtained after one week. [Ag(4Brpyz)2ClO4]; (1): Yield: 95%; Anal. Calc. C6H6AgBr2ClN4O4: C, 14.38; H, 1.21; N, 11.18%. Found: C, 14.39; H, 1.19; N, 11.13%. FTIR (mmax, cm 1): 3147, 3071, 2985, 2937, 1650, 1145, 1089, 1033, 944. Ligand (4Brpyz): 3156, 3070, 2985, 2937, 1647, 1372, 1331, 1281, 1248, 1185, 1138, 1032, 945 (Fig. S1, Supplementary data); 1H NMR (500 MHz, DMSO-d6) dH (ppm): 13.22 (s, 2H, NH), 7.96 (s, 2H, Pyrazole-H), 7.56 (s, 2H, Pyrazole-H) (Fig. S2, Supplementary data).
Gaussian 09 [37], built in NBO 3.1 [38] program, was used to perform the natural bond orbital (NBO) calculations using the WB97XD method [39] and either 6-31G(d,p) or cc-PVTZ basis sets for non-metal atoms combined with cc-PVTZ-PP [40] for silver atom.
2.3. X-ray diffraction The crystallographic measurements of the studied complex were made using a Bruker D8 Quest diffractometer with graphite monochromated Mo Ka radiation. All calculations were performed using the Bruker APEX III program system and the SHELXTL program package [36a,b]. The crystal and structure solution details are listed in Table 1. Corrections for absorption were performed by SADABS
[36c].
2.4. Antimicrobial studies The antimicrobial activities were tested against Gram-positive bacteria (S. aureus ATCC6538P, B. subtilis ATCC19659) and
Table 1 Crystal data and structure refinement for the studied complex. Empirical formula Formula weight (g/mol) T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å)3 Z Density (calc.) (g/cm3) l (mm 1) F(0 0 0) Crystal size (mm3) h (°) Index ranges Reflections collected Independent reflections Completeness to theta = 25.42° Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Largest diff. peak and hole (e Å CCDC
C6H6AgBr2ClN4O4 501.29 293(2) 0.71073 monoclinic P2(1)/c
3
)
5.266(8) 17.061(3) 14.661(19) 90 91.479(7) 90 1316.8(3) 4 2.529 7.813 944 0.22 0.08 0.05 2.76–25.37 6 h 6, 20 k 18, 14 l 17 13 572 2415 [R(int) = 0.0848] 99.60% full-matrix least-squares on F2 2415/0/161 1.016 R1 = 0.0479, wR2 = 0.0976 R1 = 0.0867, wR2 = 0.1124 0.785 and 0.753 1909254
4. Results and discussion 4.1. X-ray structure description The studied Ag(I) complex crystallized in the centrosymmetric monoclinic space group P2(1)/c with one [Ag(4Brpyz)2ClO4] unit per asymmetric unit and Z = 4. The structure and atom numbering for [Ag(4Brpyz)2ClO4]n; (1a) and [Ag(4Brpyz)2]ClO4; (1b) are shown in Fig. 1. The most important bond lengths (Å) and angles (°) are given in Table 2. The asymmetric unit comprises one Ag(I) ion, two organic ligand (4Brpyz) units and one disordered perchlorate anion. Due to the disorder occurring in the perchlorate anion (part A: 28.0% and part B: 72.0%), there are two different bonding modes for this anion. In part A, the structure of the complex is better described as one dimensional coordination polymer, [Ag (4Brpyz)2ClO4]n. In this part of the studied complex, the perchlorate anion acts as a bridging ligand between two [Ag(4Brpyz)2]+ units with Ag1AO1A and Ag1AO3A distances of 2.540(3) and 2.560(3) Å, respectively. The NAAgAN, O1AAAgAO3A and OAAgAN angles are 176.1(2), 167.6(12) and 83.2(8)–94.7(10)°, respectively, indicating a distorted square planar coordination geometry for 1a (Fig. 2). On other hand, the major part (B) can be described as an ionic complex of the formula [Ag(4Brpyz)2]ClO4. The shortest Ag1. . .O1B (2.913(12) Å) and Ag1. . .O3B (2.837(11) Å) distances are too long and should be described as interactions rather than bonds. Hence, the geometry around the Ag(I) ion is considered as a linear or slightly bent configuration, augmented
Fig. 1. The asymmetric unit and atom numbering scheme of 1.
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Table 2 The most important bond distances (Å) and angles (°) of 1. Ag1AN3 Ag1AN1 Ag1. . .O1B N3AAg1AN1 N3AAg1AO1A N1AAg1AO1A
2.125(7) 2.128(6) 2.913(12) 176.1(2) 89.2(10) 94.7(10)
Ag1AO1A Ag1AO3A Ag1. . .O3B N3AAg1AO3A N1AAg1AO3A O1AAAg1AO3A
2.540(3) 2.560(3) 2.837(11) 93.0(8) 83.2(8) 167.6(12)
by two weak Ag. . .O interactions from the neighboring perchlorate anions. The Ag1AN3 and Ag1AN1 distances are found to be 2.125 (7) and 2.128(6) Å, respectively, which are close to the corresponding values (2.124 and 2.127 Å) for the structurally related [Ag (pyz)2]NO3 complex reported by Schmidbaur and coworkers [41]. Also, the two coordinated 4Brpyz ligand units show an anti-configuration with respect to one another and are slightly twisted from one another by 6.2°, in agreement with the corresponding bis(pyrazole)silver(I) nitrate complex. It seems that the presence of the bulky bromine atom at the 4-position shows almost no effect on the configuration of the ligand groups around the Ag(I) ion and the anti-configuration of the two organic ligand groups is probably adopted to maximize the intermolecular interactions which stabilize the crystal structure via hydrogen bonding interactions, as an example. It is clear from Fig. 2 that each two successive [Ag(4Brpyz)2] units are nearly perpendicular to one another. The angle between the mean planes passing through each two neighboring [Ag (4Brpyz)2] units is found to be 77.1°. Such a situation allows the silver atoms to be located below and above the stacked pyrazole moieties, which allows the formation of some Ag. . .C interactions, as shown in Fig. 3. The Ag. . .C intermolecular distances are 3.271 and 3.348 Å for the Ag1. . .C6 and Ag1. . .C3 interactions, respectively. In addition to these interactions, the complex units are packed together via H. . .O hydrogen bonds. Details regarding the hydrogen bond parameters are given in Table 3 and are shown in Fig. 3 for both parts of the complex. It is clear that the free perchlorate anions in the ionic complex (1b) are included in both of the strong NAH. . .O and weak CAH. . .O hydrogen bonding interactions (Fig. 3, lower part). On other hand, the coordinated perchlorate ions in the coordination polymer 1a are included only in strong NAH. . .O hydrogen bonds (Fig. 3, upper part). 4.2. Analysis of molecular packing Hirshfeld surfaces were performed using the Crystal Explorer 17.5 program [42,43] in order to quantify and describe the intermolecular interactions in the studied Ag(I) complex. All Hirshfeld surfaces were drawn using the same software and are shown in Fig. S3 (Supplementary data). The frequencies of the common intermolecular interactions in the crystal structure of complexes 1a and 1b are shown in Fig. 4. Table 4 summarizes the most important contacts in both parts as well as the corresponding shortest contact distances, while presentation of these contacts in part A, as an example, is shown in Fig. 5. The O. . .H hydrogen bonds
Fig. 3. Comparison between the molecular packing in complexes 1a (upper) and 1b (lower).
contributed by 26.3 and 28.9% to the whole intermolecular interactions in 1a and 1b, respectively. The shortest hydrogen bond occurring in both parts is O4. . .H2, with a contact distance of 2.181 Å. Also, the results revealed the presence of some Br1. . .Br2 (3.656 Å) and Ag1. . ..C6 (3.271 Å) interactions, with contact distances slightly shorter than the van der Waals radii sum of the two elements (3.70 and 3.42 Å, respectively). The Hirshfeld surface analysis indicated the presence of significantly short O2A. . .Br1 (3.302 Å) and O2B. . .Br1 (3.349 Å) contacts in 1a and 1b, respectively. In addition, the Ag1. . .O3 contacts appeared as red spots in the dnorm maps, indicating that these interactions are significantly strong (Fig. 5). On other hand, the shape index and curvedness maps shown in Fig. S3 (Supplementary data), as well as the almost negligible C. . .C, C. . .N and N. . .N contacts, reveal the insignificance of p–p stacking interactions amongst the complex units.
4.3. FTIR spectra and TGA analysis The FTIR spectra of the free 4Brpyz ligand and its Ag(I) complex are shown in Fig. S1 (Supplementary data). The FTIR spectrum of the complex showed little variations compared to that for the free ligand. This is usually common for Ag(I) complexes with N-donor ligands which have relatively weak AgAN interactions. In 4Brpyz, the mNAH vibrational band was detected at 3156 cm 1, which
Fig. 2. The one dimensional coordination polymer of 1a.
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Table 3 Hydrogen bonds [Å and °] for complex 1. DAH A
DAH (Å)
H A (Å)
D A (Å)
DAH A (°)
N2AH2. . .O3A(i) N2AH2. . .O4(i) N4AH4A. . .O4(ii) N4AH4A. . .O2B N4AH4A. . .O4(ii) N2AH2. . .O4(i) C3AH3. . .O3B(iii) C6AH6. . .O2B(ii)
0.86 0.86 0.86 0.86 0.86 0.86 0.93 0.93
2.397 2.328 2.595 2.393 2.595 2.328 2.675 2.719
2.97(3) 3.18(1) 3.24(1) 3.01(1) 3.24(1) 3.18(1) 2.96(1) 3.32(1)
124.8 172.2 132.4 129.2 132.4 172.2 98.9 123.2
(i) x,1.5 y, 1/2 + z; (ii) 1 + x,y,z; (iii) 1 + x,1.5 y,1/2 + z.
Fig. 4. The common contacts and their percentages in the crystal structure of the studied Ag(I) complex.
Table 4 The important intermolecular contacts and the shortest interaction distances (Å) in both parts of the studied silver(I) complex. Contact
Shortest contact (Å)
%
Shortest contact (Å)
5.2 26.3 3.0 7.9 3.5
3.271 2.181 2.838 3.349 3.656
Part A; 1a Ag1. . .C6 O4. . .H2 Ag1. . .O3 Br1. . .O2 Br1. . .Br2
3.271 2.181 2.562 3.302 3.656
%
Part B; 1b 5.7 28.9 2.4 8.1 3.7
Fig. 5. Presentation of the most important Br. . .Br (A), Br. . .O (B), Ag. . .O (C), Ag. . .C (D) and O. . .H (E) intermolecular contacts in part A (1a); for more details see Table 4.
shifted slightly to a lower wavenumber in the Ag(I) complex (3147 cm 1). Also, the mC@N band appeared at 1647 and 1650 cm 1 for 4Brpyz and its Ag-complex, respectively. A new broad triple split band in the range 1145–1033 cm 1 was assigned to the perchlorate anion vibrations in 1. Thermogravimetric (TGA) analyses of the free 4Brpyz ligand and its silver(I) perchlorate complex are shown in Fig. 6. It is clear
that the Ag(I) complex is thermally more stable than the free ligand. 4Brpyz decomposed completely in the temperature range 80–170 °C in one decomposition step, leaving no residue. In contrast, the silver(I) complex decomposed at a higher temperature of 213 °C, leaving a residue of 26.8% at the end of the thermal decomposition (302 °C), probably corresponding to silver(I) chloride, with a theoretical mass percent of 28.6%.
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Table 6 The net interaction energies (kcal/mol) of the AgAN and AgAO/Ag. . .O interactions. NBOi
NBOj
1a
LP(N1) LP(N3) LP(O1) LP(O3)
LP*(Ag1) LP*(Ag1) LP*(Ag1) LP*(Ag1)
90.40 89.93 31.18 31.08
1b (85.25) (83.99) (32.65) (33.22)
83.85 86.62 16.31 20.00
(80.57) (81.43) (17.22) (21.17)
Values inside and outside parentheses for the cc-PVTZ and 6-31G(d,p) basis sets, respectively.
Fig. 6. TGA analyses of 4Brpyz and its AgClO4 complex.
4.4. NBO analyses In order to simulate the coordination environment around the Ag(I) ion in the structure of the studied complex, we selected the trimeric unit shown in Fig. S4 (Supplementary data) to perform the DFT studies. The results presented here are for the central Ag (I)-complex unit in this structure. The natural charge at this Ag(I) ion is reduced to 0.624 and 0.636 e for 1a and 1b, respectively. It is slightly less in 1a because in this part of the complex, the perchlorate anions are closer to the Ag(I) ion, which compensates its positive charge more than that in 1b (Table 5). In both parts, the two organic ligand molecules transfer almost the same net negative electron density to the silver(I) ion. The two perchlorate anions have a higher net negative charge in 1b compared to 1a as a consequence of the stronger interaction between the Ag(I) and ClO4 ions in the latter compared to the former. The natural orbitals included in the AgAN and AgAO coordination interactions are listed in detail in Table S1 (Supplementary data). Table 6 summarized the net interaction energies of these bonds. These results show that the two AgAN coordination bonding interactions are slightly different. There is one slightly weaker AgAN bond than the other in both parts of the complex. The same is true for the AgAO bonds in part A of this complex (1a). On the other hand, the two Ag. . .O interactions are more different compared to each other in part B (1b) and are weaker than those in 1a. It is obvious that the perchlorate ion in 1b has an ionic nature (16.31–21.17 kcal/mol) while it is coordinated for 1a (31.08– 33.22 kcal/mol). As a result of these interactions between the ligand donor atoms NBOs to the Ag(I) NBOs (Fig. 7), the outer most shell electronic distribution of the Ag(I) ion is reduced to [core] 5S0.364d9.865p0.15 and 5S0.384d9.855p0.12, for 1a and 1b, respectively using the 6-31G(d,p) and cc-PVTZ-PP basis set combinations. Natural bond order calculations of the Ag. . .O interactions in both parts revealed these facts very well. The bond order values for the Ag1AO1 and Ag1AO3 bonds in part A are 0.276 and 0.249,
Table 5 The net natural charges for the different complex fragments of 1 using the WB97XD method.
Ag 2(4Brpyz) 2ClO4
1a
1b
0.664 (0.624) 0.225 (0.234) 1.779 (-1.719)
0.671 (0.636) 0.234 (0.239) 1.807 (-1.754)
Values inside and outside parentheses for the cc-PVTZ and 6-31G(d,p) basis sets, respectively.
Fig. 7. The overlaps among the occupied natural orbitals of the donor atoms and the empty anti-bonding natural orbitals of the Ag(I) ion using the WB97XD method. The green/yellow set are for the donor atoms NBOs and red/violet colors combination are for the Ag(I) NBOs. For simplicity, the AgAN interactions of one part of the complex are presented (upper) while the Ag. . .O interactions of both parts (lower) are given, showing the weaker orbital overlaps in 1b compared to 1a. Hydrogen atoms are omitted for more clarity. (Color online.)
respectively while the corresponding values in part B are lower (0.181 and 0.195, respectively). 4.5. Antimicrobial activity In the present study, in vitro antimicrobial activities of the [Ag (4Brpyz)2ClO4] complex and the 4Brpyz ligand in comparison to Ciprofloxacin, Clotrimazol, Streptomycin and Cephradine antibiotics were tested by the use of the agar well-diffusion method in DMSO as solvent. The antimicrobial activity was estimated as the minimum inhibitory concentration (MIC; mg/ml). The antimicrobial activity results (MIC) of the [Ag(4Brpyz)2ClO4] complex and the free 4Brpyz ligand compared to the selected antibiotics are collected in Table 7. The complex and the ligand showed good antibacterial activity against the studied microorganisms in comparison with standard antibiotics. As can be seen clearly from Table 7, the [Ag(4Brpyz)2ClO4] complex is more efficient as an antibacterial agent against all bacteria and the fungus C. albicans than the free ligand. The MIC values for the [Ag(4Brpyz)2ClO4] complex against the Gram-positive bacteria and the yeast C. albicans are 9 mg/ml and in the range of 9–13 for the Gram-negative bacteria. These values are generally lower than the MIC values of the free ligand (12–15 mg/ml) indicating higher antibacterial and antifungal action of the [Ag(4Brpyz)2ClO4] complex compared to the free 4Brpyz ligand. Interestingly, the biological profile of the studied Ag(I) complex is significantly higher than the commercial antibiotics used in the biological experiments. In addition, the [Ag(4Brpyz)2ClO4] complex could be used as antifungal agent
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Table 7 The MIC (mg/ml) of [Ag(4Brpyz)2ClO4] and 4Brpyz against different microbes compared with the selected standard antibiotics. Compound
S. aureus
4Brpyz AgClO4 [Ag(4Brpyz)2ClO4] Ciprofloxacin Clotrimazole Streptomycin Cephradine
14.1 ± 0.12 4.0 ± 0.05 9.0 ± 0.11 30.3 ± 0.23 – 128.2 ± 0.21 64.2 + 0.20
B. subtilis
Gram-positive bacteria
P. aeruginosa
E. coli
C. albicans
12.0 ± 0.11 6.1 ± 0.08 8.9 ± 0.09 30.2 ± 0.23 – 31.8 ± 0.20 128.2 ± 0.21
11.9 ± 0.09 7.0 ± 0.07 8.9 ± 0.09 – 17.2 ± 0.18 – –
Gram-negative bacteria 15.0 ± 0.09 5.0 ± 0.06 9.1 ± 0.10 30.1 ± 0.20 – – –
against the yeast C. albicans. Therefore, this complex has the advantage of being able to be used as both an antibacterial and an antifungal agent.
4.6. 1H NMR spectra The 1H NMR spectrum of the free 4Brpyz ligand looks completely different from that of the studied Ag(I) complex (Fig. S2, Supplementary data). While the complex showed three well separated proton signals: one for the NH proton (d 13.22 ppm) and two for the CH protons (d 7.96 and 7.56 ppm). The free ligand showed no clear signal for the NH proton and the two CH protons appeared as one signal at d 7.73 ppm. Such variations leave no doubt about the absence of free 4Brpyz in the solution medium of 1 or at least the complex retains its identity completely or partially but to high extent in solution. Interestingly, the absence of a clear split signal for coordinated DMSO at about d 2.5 ppm could be a sufficient evidence on the absence or little existence of solvated Ag(I) or Ag4Brpyz complex cations. In this regard, the high antimicrobial action of the studied complex could be attributed mainly to the presence of [Ag(4Brpyz)2]+ species in solution and excludes – at least partially – the presence of a cooperative effect between the free silver(I) ion and free 4-bromopyrazole in solution.
5. Conclusion A new [Ag(4Brpyz)2ClO4] self-assembled coordination complex was synthesized and characterized. The X-ray structure analysis showed that the [Ag(4Brpyz)2ClO4] complex is comprised of one Ag(I) ion coordinating two 4-bromopyrazole ligands via one of their nitrogen atoms and one disordered perchlorate ion, leading to two parts of this complex. In one part, the AgAO(perchlorate) bonds are significantly short and the perchlorate anion acts as a connector between the [Ag(4Brpyz)2]+ cations, leading to the formation of a one dimensional coordination polymer of the formula [Ag(4Brpyz)2ClO4]n. In the other part, the Ag. . .O distances are significantly longer and could not be considered as a bond, leading to the ionic [Ag(4Brpyz)2]ClO4 complex. Hirshfeld analysis showed that the O. . .H (26.3–28.9%), Br1. . .Br2 (3.656 Å) and Ag1. . ..C6 (3.271 Å) interactions are the main contacts contributing in the packing of the studied complex. The complex was screened using the agar well diffusion method for its in vitro antimicrobial activity against a range of Gram-positive and Gram-negative bacteria, as well as the fungus C. albicans. Its antimicrobial actions are higher than the free ligand and a selected set of known antibiotics.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.07.026.
13.0 ± 0.10 3.9 ± 0.05 13.0 ± 0.14 30.5 ± 0.25 – 63.9 ± 0.16 96.0 ± 0.22
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