The equatorial belt never has time to cool off totally before it turns back to face the sun again, but due to Mercury's spin axis being very precisely perpendicular to its orbital plane, the poles never face the Sun. Any craters there are in permanent shadow, staring out into interstellar space at the temperature of the cosmic microwave background. plus a slight contribution from starlight and gas clouds, so let's say about 5K. That's too warm for liquid helium (especially in a vacuum). It's possible that liquid hydrogen would be stable. Oxygen, Nitrogen and CO2 would be solids and water would be absolutely frigid...