A New Zinc(II) Thiocyanate Complex of a Pyridine-4-carbohydrazide Schiff Base: Crystal Structure and Redox Properties of a Related Copper(II) Complex

Abstract

A new zinc(II) complex, ZnCl(SCN)(H₂L)₂, derived from the Schiff base ligand N′-[(E)-(2-hydroxy)methylidene]pyridine-4-carbohydrazide (H₂L), was synthesised and structurally characterised by single-crystal X-ray diffraction. The complex crystallises in the orthorhombic space group Ama2, with unit-cell parameters a = 26.0306(2) Å, b = 16.0094(1) Å, and c = 6.840(1) Å. The asymmetric unit comprises two neutral H₂L ligands coordinated to the Zn²⁺ ion through their pyridine nitrogen atoms, one thiocyanate anion (SCN⁻) coordinated through nitrogen, one chloride anion, and one lattice water molecule. The zinc centre adopts a distorted tetrahedral geometry, with Zn–N bond lengths of 1.979(4) and 2.052(3) Å, a Zn–Cl bond length of 2.1919(12) Å, and bond angles that depart markedly from the ideal tetrahedral value (103–125°). To complement the structural study, the redox behaviour of the related copper(II) complex [Cu(H₂L)Cl·H₂O]Cl (L-4) was investigated by cyclic voltammetry in acetonitrile containing 0.1 M TEATFB, using a Pt disk working electrode (r = 2 mm), a stainless-steel wire counter electrode, and an Ag/AgCl reference electrode. Over the potential range −1.0 to +1.0 V versus SCE and at scan rates of 10–50 mV s⁻¹, the voltammograms displayed a single quasi-reversible redox couple, characterised by a sharp cathodic peak at −0.75 V and a broader anodic peak at −0.45 V versus SCE. The observation of a single redox wave indicates a uniform Cu(II) coordination environment, consistent with the structural and spectroscopic data obtained for the complex.