21 ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2017, Vol. 43, No. 1, pp. 21–36. © Pleiades Publishing, Ltd., 2017. Original Russian Text © I. Bulhac, O. Danilescu, A. Rija, S. Shova, V.Ch. Kravtsov, P.N. Bourosh, 2017, published in Koordinatsionnaya Khimiya, 2017, Vol. 43, No. 1, pp. 23–38. Cobalt(II) Complexes with Pentadentate Schiff Bases 2,6-Diacetylpyridine Hydrazones: Syntheses and Structures I. Bulhac a , O. Danilescu a , A. Rija a , S. Shova a, b , V. Ch. Kravtsov c , and P. N. Bourosh c, * a Institute of Chemistry, Academy of Sciences of Moldova, Chisinau, Republic of Moldova b Institute of Macromolecular Chemistry “Petru Poni,” Iasi, Romania c Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, Republic of Moldova *e-mail: bourosh.xray@phys.asm.md Received April 14, 2016 Abstract—Seven new cobalt(II) complexes based on the Schiff bases, 2,6-diacetylpyridine bis(isonicotinoyl- hydrazone) (H 2 L 1 ) and 2,6-diacetylpyridine bis(nicotinoylhydrazone) (H 2 L 2 ), are synthesized and studied by X-ray diffraction analysis: [Co(H 2 L 1 )(NCS) 2 ] · 2.25H 2 O (I), [Co(H 2 L 2 )(NCS) 2 ] · CH 3 OH (II), [Co(H 2 L 2 )(NCS)(H 2 O)]NCS (III), [Co(H 4 L 1 )(NCS) 2 ](NO 3 ) 2 · 2H 2 O (IV), [Co(H 4 L 1 )(NCS) 2 ][Co(NCS) 4 ] · 0.75H 2 O (V), [Co(H 4 L 2 )(NCS) 2 ][Co(NCS) 4 ] · 1.75H 2 O (VI), and [Co(H 2 L 2 )(NCS)(CH 3 OH)] 2 [Co(NCS) 4 ] · 2CH 3 OH (VII) (CIF files CCDC 941186 (I), 1457906 (Ia), 1457905 (II), 941187 (III), 1457907 (IV), 1457908 (V), 1457909 (VI), and 941188 (VII)). The organic ligands in the complexes act as pentadentate neutral H 2 L or doubly protonated (H 4 L) 2+ coordinated through the same set of donor atoms N 3 O 2 . In all compounds I–VII, the coordination polyhedron of the Co 2+ ion in a complex with the Schiff bases has a shape of a pentagonal bipyramid. The hydrazones are arranged in the equatorial plane of the bipyramid. Its axial vertices are occupied by the nitrogen atoms of the NCS‾ anions in compounds I, II, and IV–VI and by the nitrogen atoms of NCS‾ and oxygen of the water molecule in com- pound III or methanol in compound VII. The anions or [Co(NCS) 4 ] 2 ‾ complex anions obtained by the reactions are involved along with the NCS‾ anions in the formation of compounds IV–VII. Keywords: cobalt(II) complexes, pentadentate Schiff bases with 2,6-diacetylpyridine hydrazone fragment, X-ray diffraction analysis DOI: 10.1134/S1070328417010018 INTRODUCTION The compounds of transition metal ions with vari- ous polydentate Schiff bases as chelates are most stud- ied in coordination chemistry [1, 2]. The Schiff bases containing hydrazone functional groups are attractive as ligands due to both diverse coordination modes to the metal and biological and pharmacological proper- ties of their compounds [3–7]. The results of syntheses and structural studies of the 2,6-diacetylpyridine, nic- otinoyl, and isonicotinoyl hydrazone compounds with the antimicrobial and genotoxic properties are described [8]. The analysis of the data in the Cam- bridge Crystallographic Data Centre [9] revealed that 2,6-diacetylpyridine bis(hydrazones) of benzoic acid or bis(semicarbazones) were mainly used as ligands in the transition metal compounds of this class (~70 structures). In the most cases, cobalt was used as a metal and almost all compounds were mononuclear. In all compounds, these ligands were coordinated via the pentadentate mode through the donor atoms N 3 O 2 , and the atoms of the anions or solvent mole- cules supplement the formation of coordination poly- hedra of transition metals to pentagonal bipyramids [10–17]. Attempts to modify these Schiff bases by the introduction of various terminal functional groups to enhance their denticity gave polynuclear [18–20] or polymer structures [21]. The multifunctional character of 2,6-diacetylpyri- dine hydrazones appears as different coordination modes and a variety of the obtained molecular archi- tectures: binuclear complexes with the double spiral structure in which the ligand is coordinated via the chelate bridging mode (3 + 2) [15, 16, 21], square mononuclear complexes [22], square pyramidal mononuclear complexes with the tridentate-coordi- nated ligand [15], binuclear (4 + 1) [17] and mononu- clear complexes with the octahedral environment of the metal with a metal to ligand ratio of 1 : 2 and the tridentate N,N,N-coordination mode of the ligand [23], and other. The cobalt(II) complexes in which the metal atom is seven-coordinated are interesting due to a high local anisotropy, depending mainly on the nature of the axial ligands [24]. The introduction of the terminal pyridine fragments into 2,6-diacetylpyridine hydra- zones can result in the formation of structural building 3 NO −