Polyoxometalate-cyclodextrin aggregates in isolation: probing the conformer space and binding affinities

Abstract

Non-covalent interactions between polyoxometalates (POMs) and cyclodextrins (CDs) are often utilized to build novel supramolecular architectures for diverse applications. However, structure prediction becomes challenging with the increase in size and complexity of the assemblies. Herein, we investigate the conformer space of isolated non-covalent complexes of a Wells-Dawson type POM [P₂W₁₈O₆₂]^6- with cyclodextrins (γ-CD) using trapped ion mobility spectrometry (TIMS). We found that the 1 : 1 (POM : CD) complex exhibits one conformer whereas the 1 : 2 and 1 : 3 (POM : CD) complexes exhibit multiple conformers in the gas-phase, despite showing one stable form in their crystalline phase. The observation of distinct conformers in precise supramolecular aggregates of their complexes reflects the possible isomeric pathways in the growth of the assemblies. The structures of the conformers were resolved through a combination of TIMS and theoretical studies (DFT and GFN1-xTB). Moreover, we performed anion photoelectron spectroscopy (PES) studies, which revealed significant electronic stabilization of the POM anions upon complexation in the γ-CD cavity. This was evident from the increase in adiabatic detachment energy (ADE) of the [KPOM(γ-CD)₁]^5- complex compared to the [KPOM]^5- anion by ∼2.4 eV. We also estimated gas-phase binding energies between the POM and γ-CD from the PES studies. Our work provides significant insights into the geometrical and electronic gas-phase structures of POM-CD non-covalent complexes. In the future, this would allow us to precisely design their solid-state assemblies from preformed gas-phase units by mass and isomer-selected ion deposition techniques.