Synthesis and characterization of Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) complexes with ligands of N,O and N,S donor atoms containing long chain pendant arms and study of their antimicrobial activities

Abstract

Transition metal chelates of dithiocarbazic acid, its S-alkyl/aryl esters and their Schiff bases have been studied, mainly due to their potential anticancer, antibacterial, antifungal, antimoebic, nonlinear optical and insecticidal activities. The number of ligands synthesized continues to increase because of the intriguing observation that different ligands show different biological properties, although they may differ only slightly in their molecular structures. It is well known that coordination of metals to Schiff base ligands may enhance their biological activities. The electrochemical behavior and photoluminescence properties of many Schiff bases and of corresponding metal complexes are reported in the literature. In view of this, the present work focuses on the synthesis of three new bidentate Schiff base ligands, formed by the condensation of S-hexyldithiocarbazate with 4-methylbenzaldehyde, thiophene-2-carboxaldeyde and 4-methoxybenzaldehyde, and on their Ni(II), Cu(II), Zn(II), Cd(II), Pd(II) and Pb(II) complexes. The synthesized ligands and the complexes have been characterized by various physico-chemical and spectroscopic methods such as 1H and 13C NMR, mass, IR and UV-Vis spectra, elemental analysis, magnetic moment, conductance and melting point measurements. A total of six X-ray single crystals structures have been determined in this work. In order to provide more insight into the behavior of these compounds in solution, the photoluminescence properties and the electrochemical analyses have also been performed. Ligands and metal complexes structurally characterized by X-ray single crystal diffraction study are here reported. Both ligands 1 and 14 crystallize in triclinic system, space group 𝑃1����, and unit cell parameters and cell volume are as follows: Crystal data of ligand 1: a = 4.79244(9), b = 11.3790(2), c = 14.5382(3) Å,  = 100.1666(7), β = 91.2117(7), γ = 94.6754(7)°, V = 777.26(3) Å3, Z = 2. Crystal data of ligand 14: a = 4.55596(8), b = 12.4224(3), c = 14.9619(3) Å,  = 75.7300(9), β = 84.7599(10), γ = 84.6141(9)°, V = 814.99 (3) Å3, Z = 2. In both the ligands 1 and 14, the dithiocarbazate group adopts an E configuration with respect to the C=N bond of the benzylidene moiety. The β-nitrogen and the thioketo sulfur are trans located with respect to the C(9)-N(2) bond. Each molecule is in its thione tautomeric (also confirmed by IR and NMR) and the co-planarity of atoms (with the exception of the S-hexyl chain) indicates an electron delocalization within it. Crystal data of Ni(II) complex 2: triclinic system, space group 𝑃1����, a = 4.6738(3), b = 10.5132(5), c = 16.4789(8), Å,  = 86.522(3), β = 84.850(3), γ = 79.057(3)°, V = 791.00(7) Å3, Z = 1. Crystal data of Pd(II) complex 6: monoclinic system, space group C2, a = 18.3559(11), b = 9.6747(5), c = 10.3368(6) Å, β = 116.810(2)°, V = 1638.37(16) Å3, Z = 2. The X-ray crystallographic structural analysis of 2 and 6 confirmed that the two Schiff bases, in their deprotonated imino thiolate form, act as chelating ligands to the metal center via the azomethine nitrogen N(1) and thiolate sulfur S(1) atoms. In complex 2, the Ni-S(1) and Ni-N(1) bond distances are of 2.1777(11) and 1.933(4) Å, respectively, with a S(1)-Ni-N(1’) chelating angle of 86.06(10)°, whereas in complex 6 the Pd-S(1) and Pd-N(1) coordination bond distances are of 2.264(4) and 2.154(12) Å, respectively, with an N(1)-Pd-S(1) chelating angle of 83.2(3)°. The complex 2 stacks at a distance of 4.6738(3) Å (along axis a), which exclude any significant interactions between the aromatic rings. The most interesting structural feature between these complexes is the different configuration of ligands, that is trans and cis in the square planar geometry of NiL2, 2 and PdL2, 6 complex, respectively. In addition the configuration of dithiocarbazate group about the imine bond N1=C8 is E in the Pd(II) complex 6 and Z in Ni(II) complex 2. In fact the torsion angle N2−N1−C8−C5 in 6 is 172.1(14)°, different from that observed in the nickel derivative 2 of 1.2(7)°. This allows an approach between the rings of the methylbenzylidene moieties, with a centroid-to-centroid distance of 4.114(8) Å, indicating a very weak π-π interaction…………………………………………..