Unraveling Ligand-Driven Isomerism in Redox-Active Mixed-Ligand Polyoxometalates Using Ion Mobility Spectrometry.

Abstract

Redox-active metal-oxide clusters that combine atomic precision with tunable solubility and electronic structure are emerging as functional components for energy storage, catalysis, and quantum materials. Polyoxovanadate (POV)-alkoxides, in particular, offer a modular platform where ligand shell composition can be tailored without disrupting the core structure. However, structural characterization of mixed-ligand POV-alkoxides is hindered by the presence of compositional and isomeric heterogeneity. Here, we report the structural investigation of mixed-ligand Lindqvist-type POV-alkoxides using ion mobility-mass spectrometry (IM-MS) coupled with density functional theory (DFT) and collision cross section (CCS) simulations. We examine two series of clusters, [V₆O₇(OR¹)_12-<i>x</i>(OR²)_<i>x</i>] (<i>x</i> = 0-8), with ligands of varying steric bulk: methoxy/ethoxy (MeO/EtO) and ethoxy/ethoxyethyl (EtO/EOE). While the MeO/EtO series shows a linear increase in CCS with ligand substitution, the EtO/EOE series exhibits a curved trend and broadened CCS distributions, indicating the formation of multiple low-energy isomers. DFT results reveal that this behavior arises from the preferential extension and clustering of bulky EOE ligands on one side of the core. Comparison of experimental and simulated CCS distributions further indicates that high-CCS isomers are not formed under solvothermal conditions. Instead, the as-synthesized solution contains isomers in which the bulky EOE ligands cluster on one side of the hexavanadate core. These findings demonstrate the power of gas-phase structural analysis to resolve subtle isomeric preferences in complex molecular assemblies and offer a framework for understanding ligand-directed assembly in redox-active metal-oxide clusters.