The metastable type-II clathrate Na_24-δGe₁₃₆ was obtained from Na₁₂Ge₁₇ by applying a two-step procedure. At first, Na₁₂Ge₁₇ was reacted at 70 °C with a solution of benzophenone in the ionic liquid (IL) 1,3-dibutyl-2-methylimidazolium-bis(trifluoromethylsulfonyl) azanide. The IL was inert towards Na₁₂Ge₁₇, but capable of dissolving the sodium salts formed in the redox reaction. By annealing at 340 °C under an argon atmosphere, the X-ray amorphous intermediate product was transformed to crystalline Na_24-δGe₁₃₆ (δ≈2) and α-Ge in an about 1 : 1 mass ratio. The product was characterized by X-ray powder diffraction, chemical analysis, and ²³Na solid-state NMR spectroscopy. Metallic properties of Na_24-δGe₁₃₆ were revealed by a significant Knight shift of the ²³Na NMR signals and by a Pauli-paramagnetic contribution to the magnetic susceptibility. At room temperature, Na_24-δGe₁₃₆ slowly ages, with a tendency to volume decrease and sodium loss.