It is a long-thought proposal that dense light-element molecular hydrides, such as diborane (${\mathrm{B}}_2{\mathrm{H}}_6$) and methane (${\mathrm{CH}}_4$), offer an ideal platform to search for phonon-mediated superconductors. However, these hydrides are often unstable under sufficiently high pressure, e.g., ${\mathrm{B}}_2{\mathrm{H}}_6$ decomposed into BH and ${\mathrm{H}}_2$ at pressures of above 153 GPa, which are unlikely to exhibit high superconductivity. Here, we find a feasible route to stabilize these light-element molecular hydrides with high superconductivity under high pressure by high-throughput structure searches and first-principles calculations. We uncover a series of stable H-rich rare-earth ($R$) metal based boron hydrides $R{\mathrm{B}}_2{\mathrm{H}}_{10}$ with polydiborane networks. Strikingly, ${\mathrm{YB}}_2{\mathrm{H}}_{10}$ is predicted to be a high-temperature superconductor with unprecedentedly critical temperature ( $T_c$ ) of up to 93 K under 150 GPa. The present findings open a route to stabilize the unstable diborane by bringing the additional $R$ metals into the lattice under high pressure, as well as tuning the superconductivity among diborane-based hydrides and other similar dense light-element molecular hydrides.

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• Pressure effects
• Structural properties
• Superconducting phase transition
• High-temperature superconductors
• Density functional theory
• First-principles calculations