The storage of H2 in a safe and compact form represents a significant current challenge,[1] and there is wide-ranging interest in materials that can store and release H2 with fast kinetics and high reversibility over multiple cycles.[2] Porous coordination frameworks have become competitors to other porous materials, such as zeolites [3] and carbon materials (for example, activated carbon or nanotubes),[4] with recent studies confirming that these frameworks can store considerable quantities of H2 at 78 K.[5–11] Most studies of H2 adsorption in coordination frameworks focus on the lowpressure region (0–1 bar) and, therefore, do not fully address the relationship between porosity and storage capacity. Although recent high-pressure volumetric measurements on some coordination frameworks revealed a correlation between maximum uptake and surface area,[9] the study involved several coordination frameworks with different structure types, and the influence of pore size and shape on guest adsorption was not investigated systematically. Herein, we report the structures of three close structural analogues, along with studies of high-pressure H2 adsorption by these materials, to establish a route to higher H2 storage capacity. In coordination frameworks, the metal cations and carboxylate ligands can form a range of multinuclear nodes with predefined geometries (for example, the binuclear paddle-wheel units {Zn2 (O2CR) 4}[12] and {Cu2 (O2CR) 4}[5, 10, 13](4-connected), the trinuclear units {Ni3O (O2CR) 6},{Fe3O-(O2CR) 6},[6, 14] and {Cr3O (O2CR) 6}[11](6-connected), and the tetranuclear unit {Zn4O (O2CR) 6}[7, 15](6-connected)), which are largely dependent upon the metal cation and the reaction stoichiometry. The three coordination-framework materials in this study are based on biphenyl, terphenyl, and quaterphenyl tetracarboxylic acids. By varying the length of the organic backbone of these ligands, we obtain the desired structural analogues, in terms of framework composition and topology, and thereby investigate the correlation of pore size with gas-adsorption behavior.

Biphenyl-3, 3’, 5, 5’-tetracarboxylic acid (H4L1; Figure 1) was synthesized by the oxidation of 3, 3’, 5, 5’-tetramethylbiphenyl with KMnO4. Terphenyl-3, 3’’, 5, 5’’-tetracarboxylic acid (H4L2; Figure 1) and quaterphenyl-3, 3’’’, 5, 5’’’-tetracarboxylic acid (H4L3; Figure 1) were synthesized by the Suzuki coupling of diethylisophthalate-5-boronic acid and dibromobenzene (for H4L2) or dibromobiphenyl (for H4L3). Solvothermal reaction of H4L1, H4L2, or H4L3 with Cu (NO3) 2· 2.5 H2O in a slightly acidified mixture of DMF/1, 4-dioxane/H2O afforded the solvated framework compounds [Cu2 (L1)(H2O) 2](1),[Cu2 (L2)(H2O) 2](2), and [Cu2 (L3)(H2O) 2](3), respectively (Figure 1). Acidic reaction solutions are necessary to obtain crystalline products; aqueous HCl is the most effective acid for this purpose. We were unable to reproduce the preparation of 1 in pure crystalline form by following the reported procedure,[5] but with our experimental procedures, the complex can be synthesized in good yield, in a highly crystalline form.