Mechanical Robustness in Fused Polyhedral Borane Crystals through Persistent Weak Intermolecular Interactions

Abstract

Borane clusters represent a unique class of molecular materials featuring electron-deficient bonding and diverse polyhedral architectures. While their electronic, photophysical, and chemical properties have been extensively explored, the influence of polyhedral architecture on their mechanical behavior remains largely unknown. Here, we investigate the nanomechanical properties of two prototypical-docosahydrooctadecaborane isomers (syn-B18H22 and anti-B18H22) through nanoindentation experiments, density functional theory (DFT) calculations, crystallographic and energy-framework analyses. The crystals exhibit exceptional mechanical robustness, displaying elastic modulus (E) of ∼20 GPa for syn-B18H22 (syn -B18) and ∼17 GPa for anti-B18H22 (anti -B18), despite being composed exclusively of boron and hydrogen atoms and lacking conventional strong hydrogen-bonding networks. Structural analyses reveal that both isomers adopt three-dimensional interlocked packing arrangements sustained by multiple B···μHBB, B-H···B, and B···B contacts. In contrast, the parent single-polyhedral borane crystal of B10H14 exhibits a substantially lower elastic modulus (∼9 GPa) and has lower-dimensional interaction networks in the solid state. The structure-mechanical property correlation presented here establishes a clear relationship between polyhedral architecture, intermolecular interaction topology, and mechanical performance in borane molecular crystals. More broadly, this work identifies macropolyhedral fusion as a structural design strategy for enhancing the mechanical robustness of cluster-based molecular solids and expands the emerging materials chemistry of borane-derived crystalline materials.