Photoelectron Spectroscopy and Computational Study on Microsolvated [B₁₀H₁₀]^2- Clusters and Comparisons to Their [B₁₂H₁₂]^2- Analogues

Microhydrated closo-boranes have attracted great interest due to their superchaotropic activity related to the well-known Hofmeister effect and important applications in biomedical and battery fields. In this work, we report a combined negative ion photoelectron spectroscopy and quantum chemical investigation on hydrated closo-decaborate clusters [B₁₀H₁₀]^2-·nH₂O (n = 1-7) with a direct comparison to their analogues [B₁₂H₁₂]^2-·nH₂O and free water clusters. A single H₂O molecule is found to be sufficient to stabilize the intrinsically unstable [B₁₀H₁₀]^2- dianion. The first two water molecules strongly interact with the solute forming B-H···H-O dihydrogen bonds while additional water molecules show substantially reduced binding energies. Unlike [B₁₂H₁₂]^2-·nH₂O possessing a highly structured water network with the attached H₂O molecules arranged in a unified pattern by maximizing B-H···H-O dihydrogen bonding, distinct structural arrangements of the water clusters within [B₁₀H₁₀]^2-·nH₂O are achieved with the water cluster networks from trimer to heptamer resembling free water clusters. Such a distinct difference arises from the variations in size, symmetry, and charge distributions between these two dianions. The present finding again confirms the structural diversity of hydrogen-bonding networks in microhydrated closo-boranes and enriches our understanding of aqueous borate chemistry.