Investigation of Fe(III) Complexes with Aminoguanidine Schiff Base Ligands as Biomimetic Catalysts and Sulphide Sensors

Two high spin Fe(III) complexes, [Fe(L¹)₂]Cl ⋅ H₂O (1) and [Fe(L²)₂]Cl ⋅ 4H₂O ⋅ 0.5MeOH (2), of Schiff base ligands of aminoguanidine with salicylaldehyde and pyridoxal (isolated as the hydrochloride salts L¹H₂⁺Cl⁻ and L²H₃²⁺Cl⁻₂ respectively) are reported. X-ray crystal structure of both the complexes along with their spectroscopic and variable temperature magnetic properties are also investigated. It is found that complex 2 shows stronger zero field splitting (D=9.5 cm⁻¹) than complex 1 (D=5.5 cm⁻¹), probably due to greater distortion of the Fe(III) coordination polyhedron in complex 2. TD-DFT calculations are used to assign the electronic spectra of the complexes. Both the ligands show fluorescence at 450–460 nm with lifetime of nanosecond order and quantum yield of 0.04–0.07 at room temperature. Both the Fe(III) complexes are found to efficiently catalyze the aerial oxidation of DTBC to DTBQ with the turn over numbers 100–155 h⁻¹, which is among the highest for mononuclear Fe(III) complexes. The complexes also act as very good fluorometric sensor for S²⁻, and the limit of detection (LOD) is in the octa-molar region. The pH and temperature dependences of the sensing are also investigated.