A series of lanthanide tartrate metal-organic frameworks (MOFs) were synthesized and structurally characterized. These include the three-dimensional (3D) frameworks {[Ln-(C 4 H 4 O 6 )(H 2 O) 3 ]Cl·2H 2 O} n (C 4 H 4 O 6 ), tartrate; Ln = La (1), Nd (2), and Pr (3) and the 2D framework {[Ce 2 (C 4 H 4 O 6 ) 3 (H 2 O) 5 ]·2.5H 2 O} n (4). Using compounds 3 and 4 as representatives, proton con-
IntroductionMetal-organic frameworks (MOFs), a class of polymeric complexes built by metal ion/cluster nodes and organic linkers, have unique features such as long-range structural order, high crystallinity, and large surface area from the highly regular porous structures. MOF materials have attracted much interest due to their diverse and useful applications including for selective gas adsorption and separation, heterogeneous catalysis, sensory devices, electrical conductivity, magnetism, and drug delivery. [1][2][3][4][5][6] In the past decade, MOFs have also been studied for ion conduction, especially as proton conductors for potential applications in fuel cell technologies. [7][8][9][10][11] In this capacity, MOFs possess clear advantages over conventional proton conductors, such as organic polymers and inorganic solids. Specifically, organic polymer based proton conductors (e.g. Nafion) are always amorphous which suggest the low regularity in the polymeric structure. [12] As a result, determination of the pathway for proton conduction is difficult. Establishing the structure-property relationship, critical for the improvement of materials performance through informed synthesis, is thus a challenge. On the other hand, proton-conducting inorganic solids, such as metal oxides and perovskites, require high working temperature in the range of 400-1000°C despite their welldefined and long-range ordered structures. [10] Furthermore, the limited choices of metal ions and matching anions make tuning the structure and proton-conducting ability of the purely inor- [a]