Synthesis, characterization and antimicrobial activities of Co(III) and Ni(II) complexes with 5-methyl-3-formylpyrazole-N(4)-dihexylthiosemicarbazone (HMP_zNHex₂): X-ray crystallography and DFT calculations of [Co(MP_zNHex₂)₂]ClO₄·1.5H₂O (I) and [Ni(HMP_zNHex₂)₂]Cl₂·2H₂O (II)

A new pyrazole containing ligand, 5-methyl-3-formylpyrazole-N(4)-dihexylthiosemicarbazone (HMP_zNHex₂), and a host of its cobalt(III) and nickel(II) complexes, [Co(MP_zNHex₂)₂]X·mH₂O and [Ni(HMP_zNHex₂)₂]X₂·nH₂O (X= Cl, Br, ClO₄, BF₄ and NO₃; m = 0 for Cl, Br, BF₄ & NO₃ and 1.5 for ClO₄; n = 0 for Br, ClO₄, BF₄ & NO₃ and 2 for Cl), respectively, have been synthesized and characterized by elemental analyses, magnetic measurements (polycrystalline state), ¹H NMR (for the ligand and its Co(III) complexes), electronic and IR spectral parameters. All the reported Co(III) and Ni(II) complexes are cationic in nature and behaving as 1:1 and 1:2 electrolytes, respectively, in MeOH. Electronic spectral data of the complexes categorize them as havingdistorted octahedral coordination geometry. IR spectral features (4000-/450 cm^-1) specify a monodeprotonated / neutral tridentate (NNS) function of the ligand, HMP_zNHex₂ coordinating to the Co(III) / Ni(II) via the pyrazolyl (tertiary) ring nitrogen, azomethine nitrogen and thiolato / thioketo sulfur atom. ¹H NMR spectral data (in CDCl₃ at 400 MHz) for the primary ligand and those of its Co(III) complexes are in agreement with the proposition of bonding sites evidenced from IR data. The single crystal x-ray data of I (C2/c (#15); monoclinic) and II (P-1 (#2); triclinic) have confirmed a CoN₄S₂ and a NiN₄S₂ octahedral coordination, respectively. The pair of monoprotic / neutral coordinating ligands is more or less orthogonal to each other in the complex species. The data obtained from DFT calculations are reasonably in agreement with the UV-Vis spectral assignment and the structures of the complex species. Although the ligand and the tested metal ion complexes are capable of inhibiting microbial growth, the cobalt complexes can be promoted as better antimicrobial agents.