Nicotinoyl hydrazone-directed vanadium assemblies: metallosupramolecular isomerism, polymorphism, and catalytic oxidation performance.

A systematic investigation of coordination-driven vanadium assemblies reveals a rich structural landscape in which tetrameric metallacycles [VO(VanNH)(OR)] and 1D polymeric species [VO(VanNH)(OR)] form selectively, depending on reaction conditions and the nature of auxiliary alkoxide ligands VanNH = -vanillin nicotinoylhydrazone; R = CH, = 1-5. Additionally, several polymorphs and structural isomers are identified, along with a rare reversible, temperature-dependent phase transformation. The assemblies can be classified into three distinct structural types based on the intramolecular orientation of the pyridyl moieties: (i) type I featuring alternating "/" arrangements, (ii) type II with a uniform " motif, and (iii) type III composed exclusively of " units. Quantum-chemical calculations reveal that polymers adopting the alternating "/" motif are thermodynamically most stable, primarily due to more exothermic enthalpies of formation. In contrast, polymeric structures dominated by " arrangements become favored for longer alkoxo ligands, whereas the tetranuclear metallocycles exhibit only modest stabilization across the alkoxide series. The pronounced structural diversity arises from a subtle interplay among electronic, steric, and crystal-packing effects that governs phase selection and crystallization behavior. In the catalytic oxidation of Cl- and NO-substituted benzyl alcohols using -butyl hydroperoxide (TBHP), the tetranuclear complexes initially exhibit high selectivity toward aldehyde formation, while the polymeric species achieve superior overall conversion. The results highlight a fundamental trade-off between catalytic activity and selectivity, strongly modulated by catalyst nuclearity and alkoxo ligand length. The insights gained provide valuable design principles for developing vanadium-based assemblies with tunable properties.