Ternary TiO 2 /ZnO/NiO Nanocatalyst Mediated Green Synthesis of Pyrazole Heterocycles: Insights From Computational Modeling and Fluorescence Studies

Abstract

ABSTRACT In this study, a novel ternary nanocatalytic system TiO 2 /ZnO/NiO was synthesized via sol gel method and characterized using UV–visible DRS, XPS, XRD, and SEM techniques. This nanocatalyst promoted the solvent‐free, eco‐friendly synthesis of pyrazole heterocycles through a one pot reaction of aldehyde, active methylene compounds and phenyl hydrazine, which involves Knoevenagel condensation followed by Michael addition reactions. This approach notably leads to excellent yields, shorter reaction times, simple work‐up procedures and eminent reusability of catalyst for up to five successive reaction cycles. These pyrazoles were characterized using FT‐IR, 1 HNMR, 13 CNMR, and MALDI‐MS spectroscopic techniques. Fluorescence studies established that all pyrazoles showed good emission properties and selective metal ion sensing behavior through effective pyrazole‐metal ion complexation. Compound 4c was characterized by fluorescence enhancement and selective detection of Co 2+ ions. Density functional theory studies including frontier‐molecular‐orbital and molecular‐electrostatic‐potential analyses, supported the reaction mechanism and revealed orbital controlled cyclization with a narrowed HOMO–LUMO gap at the rate‐limiting transition state. MEP analysis locates complementary π‐hole/π‐well pairs that pre‐organize the phenyl‐hydrazine nucleophile, reducing the entropic barrier and affording a 72% yield. Combined experimental and theoretical results emphasize the potential of this catalytic system for sustainable heterocyclic synthesis and development of metal‐ion responsive pyrazole fluorophores.