About this Journal Submit a Manuscript Table of Contents
International Journal of Inorganic Chemistry
Volume 2013 (2013), Article ID 987574, 6 pages
http://dx.doi.org/10.1155/2013/987574
Research Article

Nickel (II) and Iron (II) Complexes with Azole Derivatives: Synthesis, Crystal Structures and Antifungal Activities

1Department of Chemistry, University of Buea, P.O. Box 63, Buea, SWR, Cameroon
2Department of Chemistry, Higher Teachers Training College, University of Bamenda, Bambili, NWR, Cameroon
3Department of Inorganic Chemistry, Coordination Chemistry Laboratory, University of Yaounde I, P.O. Box 812, Yaoundé, Cameroon
4Department of Veterinary Medicine, National Chung Hsing University, Taichung, Taiwan
5Department of Pure and Applied Chemistry, University of Calabar, PMB 1115, Calabar, CRS, Nigeria

Received 15 May 2013; Accepted 8 July 2013

Academic Editor: Alfonso Castiñeiras

Copyright © 2013 Emmanuel N. Nfor et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Two new complexes of nickel (II) with 4-amino-3, 5-bis(pyridyl)-1, 2, 4-triazole (abpt) and iron (II) with 2-(3-phenyl-1H-pyrazole-5-yl) pyridine (phpzpy) have been synthesized and characterized by elemental analysis and IR spectroscopy. The crystal structures of the complexes have been determined by single crystal X-ray diffraction techniques. In the nickel and iron complexes, the ligands are coordinated through nitrogen atoms in bidentate manner. The ligands and their respective complexes have been tested for their antifungal activity against Aspergillus niger, Aspergillus flavus, and Candida albicans. From the study, the complexes showed enhanced activities against the tested organisms compared to the ligands.

1. Introduction

Aromatic nitrogen heterocycles represent an important class of compounds which can act as ligands towards metal ions [1]. Azoles belong to this class and are five-membered heterocyclic ligands containing two or more heteroatoms, one of which must be nitrogen. These compounds have been paid considerable attention due to their wide applicability in medicine [24].

Pyrazole and triazole derivatives are subject of many research studies due to their widespread potential biological activities such as antitumour [5, 6], anti-inflammatory [7], antipyretic [8], antivirial [9], antimicrobial [10, 11], anticonvulsant [12], antihistaminic [13], antidepressant [14, 15], insecticides, and fungicides. In coordination chemistry pyrazole-and triazole-derived ligands exhibit various coordination modes and have received considerable attention for the synthesis of transition metal complexes with various nuclearities [16]. Herein we report on the synthesis, structural determination, and antifungal activities of Ni(II) and Fe(II) complexes of abpt and phpzpy, respectively. The crystal structures of the complexes are described and compared with those of closely related structures.

2. Experimental

2.1. Materials and Physical Measurements

Elemental analysis for carbon, hydrogen, and nitrogen was performed on a Thermo Flash EA-1112 Series CHNS-O Elemental Analyzer. The IR spectra were obtained from KBr pellets in the range 4000–400 cm−1 using a Perkin-Elmer Spectrum 100 FT-IR spectrometer.

2.2. Single Crystal X-Ray Diffraction Analysis and Structure Determination

Suitable-single crystals of 1 and 2 were selected and mounted in air onto a loop. The data collection for 1 and 2 was carried out with a Bruker DUO APEX II CCD diffractometer at 173 K using an Oxford cryostream 700. Data reduction and cell refinement were performed using SAINT-Plus, [1721] and the space group was determined from systematic absences by XPREP [22] and further justified by the refinement results. Graphite-monochromated Mo K ( = 0.71073 ) radiation was used in both cases. The X-ray diffraction data have been corrected for Lorentz-polarization factor and scaled for absorption effects by multiscan using SADABS [23]. The structures were solved by direct methods, implemented in SHELXS-97 [24]. Refinement procedures by full-matrix least-squares methods based on values against all reflections have been performed by SHELXL-97, [24] including anisotropic displacement parameters for all non-H atoms. The positions of hydrogen atoms belonging to the carbon atoms Csp2 were geometrically optimized by applying a riding model. Calculations concerning the molecular geometry, the affirmation of chosen space groups, and the analysis of hydrogen bonds were performed with PLATON [25]. The molecular graphics were done with ORTEP-3[26] and Mercury (version 3.0) [27]. The crystal parameters, data collection, and refinement results for 1 and 2 are summarized in Table 1.

tab1
Table 1: Crystal data and structure refinement for compounds 1 and 2.

2.3. Antifungal Activity

The synthesized abpt, phpzpy, and their metal complexes 1 and 2 were screened in vitro for their antifungal activity against three fungi: Aspergillus niger, Aspergillus flavus, and Candida albicans and evaluated by poisoned food technique [28, 29]. The molds were grown on Sabouraud dextrose agar (SDA) at 25°C for 7 days and used as inocula. DMSO was used as the negative control while fluconazole was used as the positive control. The experiments were performed in triplicates. Diameters of fungal colonies were measured and expressed as percent mycelial inhibition: where is average diameter of fungal colony in negative control sets and is the average diameter fungal colony in experimental sets.

2.4. Synthesis of Ligands

The ligand 4-amino-3, 5-bis(2-pyridyl)-1,2, 4-triazole (abpt) was prepared in two steps from commercially available precursors, as previously described [3032], while its counterpart 2-(3-phenyl-1H-pyrazole-5-yl)pyridine(phpypz) was equally synthesized as previously reported [33].

2.5. Synthesis of [Ni(abpt)2(OH)2]ClO42− (1)

To a solution of 4-amino-3, 5-bis(pyridyl)-1,2,4-triazole (abpt) (238 mg, 1 mmol) in methanol (20 cm3) Ni(ClO4)·6H2O (0.375 mg, 1 mmol) was added in water/methanol (1 : 1) solution (10 cm3) with continuous stirring for 2 h. The product was filtered and allowed to evaporate slowly at room temperature resulting the formation of dark red crystals suitable for single crystal X-ray diffraction studies after one week. Yield 80%. Anal. Calcd.: C, 64.17; H, 2.67; N, 3.74. Found; C, 64.00; H, 2.69; N, 3.44%. IR absorption bands (KBr, cm−1): 3387s, 3260s, 2108m, 2070s, 2041vs, 1608s, 1589s, 1433 and 1088w, 777s, 731m, 633s.

2.6. Synthesis of [Fe (phpypz)2(NCS)2] (2)

To a methanolic solution (15 mL) of [Fe(H2O)6](ClO4)2 (153 mg; 0.42 mmol) (and a pinch of ascorbic acid) a solution of KNCS (82 mg; 0.84 mmol) was added in the same solvent (15 mL) under nitrogen. The mixture was stirred for 15 min then filtered (to remove KClO4) via a cannula into a schlenk containing a solution of phpzpy (18 mg; 0.84 mmol) in methanol. Water (5 mL) was added, and the solution was stirred for 1 h at room temperature, then left to evaporate under nitrogen. Single crystal suitable for X-ray diffraction was obtained after two days as yellow block. Yield 75%. Anal. Calcd.: C, 67.94; H, 4.15; N, 5.28. Found; C, 67.93; H, 3.95; N, 5.63%. IR absorption bands (KBr, cm−1): 3205sh, 3053s, 2084s, 1615s, 1571m, 1510w, 1459w, 1432m, 1291w, 1250w, 1061w, 968w, 927w, 772s, 707w, 636w, 471w.

3. Results and Discussion

3.1. Antifungal Activity

The ligands abpt and phpzpy, their complexes, standard drug (fluconazole), and DMSO solvent were screened separately for their antifungal activity against Aspergillus flavus, Aspergillus niger, and Candida albicans. The ligands were found to show antifungal activity, and their metal complexes show enhanced activity compared to the free ligands. Of the four chemical compounds screened for their antifungal activity, three of them showed more than 50% inhibition of mycelial growth against the three fungal strains while one showed less than 50% mycelial growth against the fungi compared to the standard drug with greater than 70% inhibition (Table 3). The overtone’s concept [34] and Tweedy’s chelation theory [35] can be used to explain the enhancement in antifungal activity of the metal complexes. Chelation considerably reduces the polarity of the metal ion because of partial sharing of its positive charge with the donor group and possible delocalization within the entire chelate ring system that is formed during coordination. Such chelation could enhance the lipophilic character of the central metal atom and hence increase the hydrophobic character and liposolubility of the complex favouring its permeation through the lipid layers of the cell membrane.

3.2. Description of the Crystal Structures

The structure of complex 1 with the atomic numbering scheme is shown in Figure 1. The bond lengths and bond angles for the complex 1 are listed in Table 2. Complex 1 crystallizes in the monoclinic P21/n space group, and the coordination geometry around the Ni(II) center can be described as a six-coordinated distorted octahedron where four nitrogen atoms from two bidentate abpt ligands and two oxygen atoms from water molecule occupy the six coordination with the chlorate anion as the counter ion in the outer sphere. The average Cl–O bond distance is 1.37.

tab2
Table 2: Selected bond lengths (Å) and bond angles (°) of 1 and 2.
tab3
Table 3: Antifungal activity of synthesized compounds.
987574.fig.001
Figure 1: Molecular diagram of complex salt 1 with the atom numbering scheme (hydrogen atoms are omitted for clarity)

The Ni–N1py and Ni–N2trz bond length values are 2.105 and 2.039 respectively, while the Ni–O1 bond length is 2.109 which are similar to those of other nickel complexes reported elsewhere [30, 36, 37]. The Ni–N distances of 2.039 to 2.105 are considerably shorter than the Ni–O1 of 2.109 indicating the stronger ability of the nitrogen to bond to the Ni than the oxygen atom. Furthermore the longer Ni–Npy distance compared to its Ni–Ntrz counterpart is consistent with other systems of this ligand type [38]. The two water molecules occupy the apical position whereas; the two triazole ligands occupy the equatorial plane. The bond angles between apical axis and equatorial plane in complex 1 vary from 90°, indicating distortion of the octahedral structure. The N1–Ni1–N2 bond angles varies from 51.2° to 79.2° while the O1–Ni1–N1 bond angle is 89.8° and O1–Ni–N2 is 90.4°. Furthermore the O1–Ni–O1 bond angle is 180° which are all comparable to other nickel (II) complexes with similar ligands [36, 37]. There exists the intra molecular and intermolecular hydrogen bond in the complex 1. The intra molecular hydrogen bond exists between N6–N5–H and the intermolecular bond between O4O1–H and O3N5–H as shown in Figure 2(a). The chlorate anions are held in the vacant space between the abpt ligand wings of two complex cations by O3N5–H and O3O3 and O4O1–H with bond distances of 2.87, 3.036 and 2.309 Å respectively as shown in Figure 2(b). An electrostatic interaction O3O3 with bond distance 3.04 is observed.

fig2
Figure 2: (a) Intra- and intermolecular hydrogen bond of complex 1. (b) Packing diagram of the complex salt 1 where chlorate anions are shown in space-filling model.

The structure of the complex 2 with the atom numbering scheme is depicted in Figure 3. The bond lengths and bond angles are listed in Table 2. This complex crystallizes in the monoclinic C2/c space group. The coordination geometry around the Fe(II) is a six coordinate distorted octahedron where four nitrogen atoms from the two bidentate neutral phpzpy ligand and two nitrogen atoms from the thiocyanate ligands occupy the six coordination sites. The bond distance of Fe–Npy= 2.250 is longer than that of the Fe–Npz = 2.13 as would be expected for ligands of this type [38], whereas the Fe–Nthio = 2.107 is considerably shorter than the Fe–Npz and Fe–Npy counterpart due to the stronger ability of the thiocyanate ion to bind the Fe in comparison to the neutral bidentate ligand. The N4–C008 bond distance is 1.159 and S2–C008 distance is 1.618. These observed C–N and C–S bond length values which fall in the 1.133 Å to 1.159 and 1.612–1.168 range, for complex 2, have been reported in other thiocyanato complexes [39].

987574.fig.003
Figure 3: Molecular diagram of complex 2 with the atom-numbering scheme.

The ligand phpzpy is not completely planar. The pyridyl and phenyl groups are slightly twisted with respect to the pyrazole. The angles between the rings py-pz = 27.84(7)°, py-ph = 63.51(6)°, and pz-ph = 19.97(5)° in the complex. The N2–Fe1–N1 and N2–Fe1–N4 bond angles are 73.66(8)° and 87.64°. Furthermore the N4–Fe1–N4, N4–Fe1–N1, and N4–Fe1–N2 are 99.9°, 90.9°, and 87.6°, respectively. In the crystal packing (Figure 4(a)) intermolecular hydrogen bonds C008-S1H5 exist in the complex.

fig4
Figure 4: (a) Intermolecular hydrogen bond of complex 2. (b) Packing diagram of complex 2 along the c-axis.

4. Conclusion

We have synthesized and characterized by elemental analysis, IR, and XRD spectroscopic techniques Ni(II) and Fe(II) complexes with azole derivatives abpt and phpypz, respectively. The complexes and their ligands have been tested for their antifungal activity against three fungal strains A. niger, A. flavus, and C. albicans, and the complexes have shown enhanced activity compared to their parent ligands.

5. Further Material

Crystallographic data for the structures reported in this paper have been deposited with Cambridge Crystallographic Data Center as supplementary publication no. CCDC-875789 and 875790. Copies of the data can be obtained free of charge from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax (44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Acknowledgments

One of the authors, Emmanuel N. Nfor is thankful to the Common wealth Scholarship Commission (CSC) for Research Fellowship and Professor Andrew D. Burrows of the Department of Chemistry, University of Bath, UK, for hosting the fellowship.

References

  1. J. Reedjik, in Comprehensive Coordination Chemistry, G. Wilkinson, R. D. Gillard, and J. A. McCleverty, Eds., vol. 2, Pergamon Press, Oxford, UK, 1987.
  2. D. Sanglard, K. Kuchler, F. Ischer, J.-L. Pagani, M. Monod, and J. Bille, “Mechanisms of resistance to azole antifungal agents in Candida albicans isolates from AIDS patients involve specific multidrug transporters,” Antimicrobial Agents and Chemotherapy, vol. 39, no. 11, pp. 2378–2386, 1995. View at Scopus
  3. J. E. Sjöström, J. Fryklund, T. Kühler, and H. Larsson, “In vitro antibacterial activity of omeprazole and its selectivity for Helicobacter spp. are dependent on incubation conditions,” Antimicrobial Agents and Chemotherapy, vol. 40, no. 3, pp. 621–626, 1996. View at Scopus
  4. F. C. Odds, G. Dams, G. Just, and P. Lewi, “Susceptibilities of Candida spp. to antifungal agents visualized by two-dimensional scatterplots of relative growth,” Antimicrobial Agents and Chemotherapy, vol. 40, no. 3, pp. 588–594, 1996. View at Scopus
  5. H. D. H. Showalter, J. L. Johnson, J. M. Hoftiezer et al., “Anthrapyrazole anticancer agents. Synthesis and structure-activity relationships against murine leukemias,” Journal of Medicinal Chemistry, vol. 30, no. 1, pp. 121–131, 1987. View at Scopus
  6. S. A. F. Rostom, M. A. Shalaby, and M. A. El-Demellawy, “Polysubstituted pyrazoles—part 5. Synthesis of new 1-(4-chlorophenyl)-4-hydroxy-1H-pyrazole-3-carboxylic acid hydrazide analogs and some derived ring systems. A novel class of potential antitumor and anti-HCV agents,” European Journal of Medicinal Chemistry, vol. 38, pp. 959–974, 2003.
  7. A. K. Tewari and A. Mishra, “Synthesis and anti-inflammatory activities of N4, N5-disubstituted-3-methyl-1H-pyrazolo[3,4-c]pyridazines,” Bioorganic & Medicinal Chemistry, vol. 9, pp. 715–718, 2001.
  8. R. H. Wiley and P. Wiley, Pyrazolones, Pyrazolidones and Derivatives, John Wiley and Sons, New York, NY, USA, 1964.
  9. S. L. Janus, A. Z. Magdif, B. P. Erik, and N. Claus, “Synthesis of triazenopyrazole derivativesas potential inhibitors of HIV-1,” Monatshefte für Chemie, vol. 130, pp. 1167–1173, 1999.
  10. V. Michon, C. Herve du Penhoat, F. Tombret, J. M. Gillardin, F. Lepage, and L. Berthon, “Preparation, structural analysis and anticonvulsant activity of 3- and 5-aminopyrazole N-benzoyl derivatives,” European Journal of Medicinal Chemistry, vol. 30, no. 2, pp. 147–155, 1995. View at Publisher · View at Google Scholar · View at Scopus
  11. R. Boehm and C. Karow, “Biologically active triazoles,” Die Pharmazie, vol. 36, no. 4, pp. 243–247, 1981. View at Scopus
  12. I. Yildirim, N. Ozdemir, Y. Akçamur, M. Dinçer, and O. Andaç, “4-Benzoyl-1,5-diphenyl-1H-pyrazole-3-carboxylic acid methanol solvate,” Acta Crystallographica, vol. 61, pp. 256–258, 2005.
  13. D. M. Bailey, P. E. Hansen, A. G. Hlavac et al., “3,4-Diphenyl-1H-pyrazole-1-propanamine antidepressants,” Journal of Medicinal Chemistry, vol. 28, no. 2, pp. 256–260, 1985. View at Scopus
  14. C. K. Chu and S. J. Cutler, “Chemistry and antiviral activities of acyclonucleosides,” Journal of Heterocyclic Chemistry, vol. 23, no. 2, pp. 289–319, 1986. View at Scopus
  15. S. C. Bahal, B. L. Dubey, N. Nath, and J. K. Srivastava, “Synthesis, characterisation and fungitoxicity of the complexes of Hg(II), Cd(II), Cu(II) and Ag(I) with 3-o-tolyoxymethyl-4-aryl-5-mercapto-1,2,4-triazole,” Inorganica Chimica Acta, vol. 91, pp. L43–L45, 1984.
  16. M. R. Grimmett, in Obshchaya Comprehensive Organic Chemistry, D. Barton and W. D. Ollis, Eds., vol. 8, Pergamon, Oxford, UK, 1979.
  17. A. L. Gavrilova and B. Bosnich, “Principles of mononucleating and binucleating ligand design,” Chemical Reviews, vol. 104, no. 2, pp. 349–383, 2004. View at Publisher · View at Google Scholar · View at Scopus
  18. P. J. Steel, “Aromatic nitrogen heterocycles as bridging ligands; a survey,” Coordination Chemistry Reviews, vol. 106, pp. 227–265, 1990. View at Publisher · View at Google Scholar
  19. R. Mukherjee, “Coordination chemistry with pyrazole-based chelating ligands: molecular structural aspects,” Coordination Chemistry Reviews, vol. 203, pp. 151–218, 2000. View at Publisher · View at Google Scholar
  20. J. Klingele, S. Dechert, and F. Meyer, “Polynuclear transition metal complexes of metal-bridging compartmental pyrazolate ligands,” Coordination Chemistry Reviews, vol. 253, no. 21-22, pp. 2698–2741, 2009. View at Publisher · View at Google Scholar · View at Scopus
  21. S. Trofimenko, Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands, University College Press, London, UK, 1999.
  22. SAINT-Plus, Version 7.12, “Bruker AXS Inc,” Madison, Wis, USA, 2004.
  23. Bruker. XPREP2, Version 6.14, “Bruker AXS Inc,” Madison, Wis, USA, 2003.
  24. G. M. Sheldrick, SADABS, University of Gottingen, Germany, 1996.
  25. G. M. Sheldrick, “A short history of SHELX,” Acta Crystallographica A, vol. 64, pp. 112–122, 2008. View at Publisher · View at Google Scholar
  26. A. L. J. Spek, “Single-crystal structure validation with the program PLATON,” Applied Crystallography, vol. 36, pp. 7–13, 2003. View at Publisher · View at Google Scholar
  27. L. J. Farrugia, “ORTEP-3 for Windows—a version of ORTEP-III with a Graphical User Interface (GUI),” Journal of Applied Crystallography, vol. 30, p. 565, 1997. View at Publisher · View at Google Scholar
  28. K. Singh, Y. Kumar, P. Puri, C. Sharma, and K. R. Aneja, “Synthesis, spectroscopic, thermal and antimicrobial studies of Co(II), Ni(II), Cu(II) and Zn(II) complexes with Schiff base derived from 4-amino-3-mercapto-6-methyl-5-oxo-1,2,4-triazine,” Medicinal Chemistry Research, vol. 21, no. 8, pp. 1708–1716, 2012. View at Publisher · View at Google Scholar
  29. A. W. Bauer, W. M. Kirby, J. C. Sherris, and M. Turck, “Antibiotic susceptibility testing by a standardized single disk method,” American Journal of Clinical Pathology, vol. 45, no. 4, pp. 493–496, 1966. View at Scopus
  30. M. H. Klingele, P. D. W. Boyd, B. Moubaraki, K. S. Murray, and S. Brooker, “Probing the dinucleating behaviour of a Bis-bidentate ligand: synthesis and characterisation of some di- and mononuclear Cobalt(II), Nickel(II), Copper(II) and Zinc(II) complexes of 3,5-Di(2-pyridyl)-4-(1H-pyrrol-1-yl)-4H-1,2,4-triazole,” European Journal of Inorganic Chemistry, vol. 2006, pp. 573–589, 2006. View at Publisher · View at Google Scholar
  31. S. K. Mandal, H. J. Clase, J. N. Bridson, and S. Ray, “Synthesis, structure and electrochemistry of a novel dinuclear cobalt(II) complex containing pyrrole units: an unusual reductive electropolymerization of the dinuclear cobalt(II) complex,” Inorganica Chimica Acta, vol. 209, no. 1, pp. 1–4, 1993. View at Scopus
  32. J. F. Geldard and F. Lions, “The organic chemistry of a new weak field tridentate chelating agent. 3,5-Di(2-pyridyl)-1,2,4-triazole,” The Journal of Organic Chemistry, vol. 30, pp. 318–319, 1965. View at Publisher · View at Google Scholar
  33. J. Pons, A. Chadghan, J. Casabo, A. Alvarez-Larena, J. Francesc Piniella, and J. Ros, “Synthesis and structural characterisation of new binuclear Pd(II) complexes with both bridging and terminal pyrazolate ligands,” Inorganic Chemistry Communications, vol. 3, no. 6, pp. 296–299, 2000. View at Publisher · View at Google Scholar · View at Scopus
  34. Y. Anjaneyulu and R. P. Rao, “Preparation, characterization and antimicrobial activity studies on some ternary complexes of Cu(II) with acetylacetone and various salicylic acids,” Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, vol. 16, no. 3, pp. 257–272, 1986. View at Publisher · View at Google Scholar
  35. B. G. Tweedy, “Plant extracts with metal ions as potential antimicrobial agents,” Phytopathology, vol. 55, pp. 910–914, 1964.
  36. F. S. Keij, R. A. G. de Graaff, J. G. Haasnoot, and J. Reedijk, “Synthesis and co-ordination chemistry of a novel dinucleating chelating triazole ligand. The crystal structure bis-µ-[4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole-N, µ-N1,µ-N1,N,]-bis[aquachloro-nickel(II)] dichloride tetrahydrate,” Journal of the Chemical Society, Dalton Transactions, pp. 2093–2097, 1984.
  37. P. J. Van Koningsbruggen, D. Gatteschi, R. A. G. De Graaff, J. G. Haasnoot, J. Reedijk, and C. Zanchini, “Isotropic and anisotropic magnetic exchange interactions through μ-N1,N2 1,2,4-triazole and μ-sulfato bridges: X-ray crystal structure, magnetic properties, and single-crystal EPR study of (μ-4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole-N′,N1,N 2,N)(μ-sulfato-O,O′)[(sulfato-O)aquacopper(II)] triaquacopper(II) hydrate,” Inorganic Chemistry, vol. 34, no. 21, pp. 5175–5182, 1995. View at Scopus
  38. M. H. Klingele and S. Brooker, “The coordination chemistry of 4-substituted 3,5-di(2-pyridyl)-4H-1,2,4-triazoles and related ligands,” Coordination Chemistry Reviews, vol. 241, pp. 119–132, 2003. View at Publisher · View at Google Scholar
  39. J. G. Małecki, J. Mroziński, and K. Michalik, “Structural, spectroscopic and magnetic properties of Mn(II), Co(II) and Ni(II) complexes with 2-hydroxy-6-methylpyridine ligand,” Polyhedron, vol. 30, pp. 1806–1814, 2011. View at Publisher · View at Google Scholar