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

A systematic investigation of coordination-driven vanadium assemblies reveals a rich structural landscape in which tetrameric metallacycles [VO(VanNH)(OR)]₄ and 1D polymeric species [VO(VanNH)(OR)]_n form selectively, depending on reaction conditions and the nature of auxiliary alkoxide ligands (VanNH = o-vanillin nicotinoylhydrazone; R = C_nH_2n+1, n = 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 'syn'/'anti' arrangements, (ii) type II with a uniform 'anti' motif, and (iii) type III composed exclusively of 'syn' units. Quantum-chemical calculations reveal that polymers adopting the alternating 'syn'/'anti' motif are thermodynamically most stable, primarily due to more exothermic enthalpies of formation. In contrast, polymeric structures dominated by 'anti' 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 NO₂-substituted benzyl alcohols using tert-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.