A novel blue luminescent high-spin iron(III) complex with interlayer O–HCl bridging: Synthesis, structure and spectroscopic studies

A blue luminescent high-spin (<i>s</i> = 5/2) iron(III) complex, [FeIII(HL¹)₂]Cl (<b>2</b>), was synthesized with the acyclic tridentate salicylaldehyde 2-pyridyl hydrazone ligand, H₂L¹ (<b>1</b>), and it can subsequently be deprotonated and methylated into blue luminescent iron(III) complexes, [FeIII(L²)₂]Cl (<b>3</b>) and [Fe^III (MeL¹)₂]Cl (<b>4</b>) respectively. On excitation at 390 nm, the ligand and the complexes display fluorescence in the blue region (λ_em = 440–450 nm) of the spectrum. The association constant, (<i>K</i>_ass = 4.6421 × 10⁴ at 298 K) and the thermodynamic parameters for complex <b>2</b> have been determined by UV–Vis spectroscopy. The iron(III)–iron(II) reduction potentials lie near – 0.2 V versus SCE. The X-ray crystal structure of the complex, [Fe^III (HL¹)₂]Cl • H₂O (<b>2</b>), was determined, revealing meridional binding of the two ligands affording a <i>cis</i>-FeN₄O₂ geometry with alternate chlorine and water molecules at the interface of the main molecule, resulting in interesting interlayer O–HCl bridging.