Morphology Effects on Free Energies of Proton-Coupled Electron Transfer in Polyoxotungstates

Polyoxotungstates have previously been established to
facilitate
the hydrogenation of small molecule substrates via hydrogen atom transfer
from reactive hydroxyl groups formed at the assembly surface. Understanding
structure–function relationships that dictate the thermochemistry
and kinetics of proton-coupled electron transfer is key to controlling
this chemistry. In this work, we combine comprehensive electrochemical
experiments and density functional theory calculations to address
how different polyoxotungstate morphologies, specifically W6O19–2, W10O32–4, SiW12O40–4, and P2W18O62–6, affect the bond dissociation free energies of surface hydroxides
(BDFE(O–H)) formed upon reduction of the assembly in acidic
media. Our results reveal increasing hydroxide bond strengths with
increasing cluster size, and that anisotropic cluster geometries result
in substantial thermodynamic differentiation of H-binding sites. We
demonstrate an excellent agreement between theory and experiments
on the reported BDFE(O–H) values and, importantly, we elucidate
how cluster size and shape affect electronic properties (local charges
and frontier molecular orbitals), giving rise to sites with increased
preference for hydrogen binding, demonstrated in higher BDFE(O–H).
Overall, this work aids the understanding and design of polyoxometalates
exhibiting surface sites with tailored interaction strengths.