pH-Independent Selective Formation of VO₂⁺ Motif Incorporating a Family of Hydrazone Ligands: Synthesis, Structure, and Luminescence-Based Sensing Studies toward Selective Metal Ions

Two new dioxidovanadium(V) compounds with the general formula [V^VO₂(L)] (L = hydrazone ligand) were synthesized using three different methods (acidic, neutral, and basic media) with either V^IVOSO₄. 5H₂O, [V^IVO(acac)₂], or NH₄V^VO₃ as the starting material and hydrazone ligands. In both cases, five coordinated V⁵⁺ ions adopted distorted square pyramidal geometry through the coordination of two oxido ligands and one hydrazone ligand. In compound <b>1</b>, the presence of free hydroxyl groups stabilized the molecular species through intramolecular O-H•••N type hydrogen bond interactions. In compound <b>2</b>, the free amino groups are involved in both intramolecular N-H•••N type and intermolecular N-H•••O type hydrogen bond interactions. Compound <b>1</b> showed highly selective luminescence turn-on behavior, along with a 33-nm blue shift in the presence of Al³⁺ ions in an aqueous medium. The experimental limit of detection (LOD) was found to be 66 nM. Whereas compound <b>2</b> showed luminescence quenching behavior in the presence of Fe³⁺, Al³⁺, and Cr³⁺ ions. The LODs were observed to be 312, 408, and 280 nM for Fe³⁺, Al³⁺, and Cr³⁺ ions, respectively. The luminescence response mechanisms of both compounds in the presence of metal ions have been correlated with molecular-level interactions.