A synthetic approach combining hydrothermal and ionothermal (eutectic mixture of choline chloride and malonic acid) procedures is proposed that allowed the isolation of the first lanthanide-organic frameworks with residues of 1,4-phenylenebis(methylene)-diphosphonic acid (H 4 pmd), [Ln(Hpmd)(H 2 O)] (where Ln 3+ ) Ce 3+ and Pr 3+ ), exhibiting an unprecedented trinodal topology with 3-and 8-connected nodes. The structural details were unveiled from single-crystal X-ray diffraction and the materials were characterized using standard techniques.Metal-organic frameworks (MOFs) are an attractive class of materials that has received a considerable amount of worldwide attention over the last two decades. 1 This surge has been primarily motivated by their intriguing structural architectures 2 and, more recently, from their potential applications. 3 Remarkably, the most widely employed synthetic strategy is the solvo(hydro)thermal method inherited from zeolite chemistry. More recently, ionothermal synthesis has been used with great success to isolate novel MOFs or coordination complexes, 4-6 zeolites 7 and nanomaterials. 8 Depending on the choice of ionic liquids (ILs) different materials can be isolated and, to date, 1-ethyl-3-methyl imidazolium bromide (EMIm-Br) is the most commonly employed IL solvent and structure-directing agent (SDA). 4 However, the corresponding cation is usually included in the final product to balance the crystal charge and its removal usually implies a destruction of the framework. Following our interest in the synthesis and structural characterization of MOFs, 9 in particular those with lanthanides, here we wish to describe a simple and effective method that combines hydrothermal and ionothermal synthetic concepts (hydro-ionothermal synthesis, HI): eutectic mixtures of choline chloride and malonic acid (CM, melting point 10°C) 10 having a small amount of intentionally added distilled water were used as the solvent media in the reaction of lanthanide ions with a bisphosphonic acid source. CM mixtures have been profitably used in the preparation of a number of d-block oxalophosphates and oxalo-phosphonates, 6 but their use with lanthanides and flexible chelating ligands is unknown to date.The reaction between tetraethyl-p-xylylenebisphos-phonate (texbp) and LnCl 3 · 6H 2 O (Ln 3+ ) Ce 3+ and Pr 3+ ) in preprepared homogeneous eutectic mixtures of CM, 11 led to the isolation of phase-pure crystalline materials which were formulated as [Ln(Hpmd)(H 2 O)] (where Ln 3+ ) Ce 3+ for 1 and Pr 3+ for 2; H 4 pmd ) 1,4-phenylenebis(methylene)diphosphonic acid, see Scheme 1) on the basis of single-crystal X-ray diffraction studies 12 and CHN elemental composition studies in combination with EDS data. Even though a handful of frameworks with d-and p-block elements and H 4-x pbp -x residues have been reported, 13-16 the compounds described here constitute the first examples of framework materials with this ligand coordinated to lanthanide centers. The bulk materials are highly crystalline even tho...