Single-molecule imaging and kinetic analysis of intermolecular polyoxometalate reactions

Jack W Jordan; Kayleigh L Y Fung; Stephen T Skowron; Christopher S Allen; Johannes Biskupek; Graham N Newton; Ute Kaiser; Andrei N Khlobystov

doi:10.1039/d1sc01874d

Single-molecule imaging and kinetic analysis of intermolecular polyoxometalate reactions

Chem Sci. 2021 Apr 26;12(21):7377-7387. doi: 10.1039/d1sc01874d.

Authors

Jack W Jordan¹, Kayleigh L Y Fung¹, Stephen T Skowron¹, Christopher S Allen^{2

3}, Johannes Biskupek⁴, Graham N Newton⁵, Ute Kaiser⁴, Andrei N Khlobystov¹

Affiliations

¹ School of Chemistry, University of Nottingham University Park Nottingham NG7 2RD UK [email protected].
² Electron Physical Science Imaging Center, Diamond Light Source Ltd. Didcot OX11 0DE UK.
³ Department of Materials, University of Oxford Oxford OX1 3HP UK.
⁴ Electron Microscopy Group of Materials Science, Ulm University 89081 Ulm Germany.
⁵ GSK Carbon Neutral Laboratories for Sustainable Chemistry, University of Nottingham Nottingham NG7 2TU UK.

Abstract

We induce and study reactions of polyoxometalate (POM) molecules, [PW₁₂O₄₀]^3- (Keggin) and [P₂W₁₈O₆₂]^6- (Wells-Dawson), at the single-molecule level. Several identical carbon nanotubes aligned side by side within a bundle provided a platform for spatiotemporally resolved imaging of ca. 100 molecules encapsulated within the nanotubes by transmission electron microscopy (TEM). Due to the entrapment of POM molecules their proximity to one another is effectively controlled, limiting molecular motion in two dimensions but leaving the third dimension available for intermolecular reactions between pairs of neighbouring molecules. By coupling the information gained from high resolution structural and kinetics experiments via the variation of key imaging parameters in the TEM, we shed light on the reaction mechanism. The dissociation of W-O bonds, a key initial step of POM reactions, is revealed to be reversible by the kinetic analysis, followed by an irreversible bonding of POM molecules to their nearest neighbours, leading to a continuous tungsten oxide nanowire, which subsequently transforms into amorphous tungsten-rich clusters due to progressive loss of oxygen atoms. The overall intermolecular reaction can therefore be described as a step-wise reductive polycondensation of POM molecules, via an intermediate state of an oxide nanowire. Kinetic analysis enabled by controlled variation of the electron flux in TEM revealed the reaction to be highly flux-dependent, which leads to reaction rates too fast to follow under the standard TEM imaging conditions. Although this presents a challenge for traditional structural characterisation of POM molecules, we harness this effect by controlling the conditions around the molecules and tuning the imaging parameters in TEM, which combined with theoretical modelling and image simulation, can shed light on the atomistic mechanisms of the reactions of POMs. This approach, based on the direct space and real time chemical reaction analysis by TEM, adds a new method to the arsenal of single-molecule kinetics techniques.