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 1, the presence of free hydroxyl groups stabilized the molecular species through intramolecular O-H•••N type hydrogen bond interactions. In compound 2, the free amino groups are involved in both intramolecular N-H•••N type and intermolecular N-H•••O type hydrogen bond interactions. Compound 1 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 2 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.