A novel magnesium metal-organic framework formulated as [Amim] 2 [Mg 3 (1,4-NDC) 4 (MeIm) 2 (H 2 O) 2 ]·2H 2 O (1) (Amim=1-propylene-3-methylimidazolium, 1,4-NDCH 2 =1,4-naphthalene dicarboxylic acid, MeIm=1-methyl imidazole) was synthesized in an ionothermal reaction by using an ionic liquid 1-propylene-3-methylimidazolium chloride (AmimCl) as solvent and reactant. The coordinated MeIm was in situ produced by the decomposition of AmimCl during the ionothermal synthesis. Single-crystal X-ray analysis reveals that 1 crystallizes in the monoclinic system, space group P2 1 /n. Its structure is composed of linear trinuclear magnesium clusters as secondary building units and features an anionic three-dimensional (3D) framework with 4 24 ·6 4 topology built upon 8-connected net. The thermal stability of 1 has also been studied.