My space colonies are mainly located onto large, partialy terraformed moons in our solar system. They still travel using rockets but due to their vessels' large size they need very powerful propulsion system. Their fuel is metastable Helium. For this reason I need a way to store it for long period of time. Helium gas is monoatomic so it has a tendency to leak, plus my colonist would need extremely large tanks; cryogenic liquid storage is expensive and dangerous. I know Helium forms compounds despite it being a noble gas. Which of those (or hypothetical others) are stable liquid/solid in "non extreme" conditions for long storage onto the moons?
By non extreme I mean close to 1 atm and 20 °C, or the closest possible.
EDIT: The stored Helium doesn't necesarily need to be in the metastable state.