High Capacity Hydrogen Adsorption in Cu(II) Tetracarboxylate Framework Materials: The Role of Pore Size, Ligand Functionalization, and Exposed Metal Sites

A series of isostructural metal-organic framework polymers of composition [Cu-2(L)(H2O)(2)] (L=tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 6(4).8(2) topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H-2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H-2 adsorption of 77.8 mg g(-1) equivalent to 7.78 wt% [wt% = (weight of adsorbed H-2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H-2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sites for this material: at the exposed Cu(II) coordination site, at the pocket formed by three {Cu-2} paddle wheels, and at the cusp of three phenyl rings. Systematic virial analysis of the H2 isotherms suggests that the H2 binding energies at these sites are very similar and the differences are smaller than 1.0 kJ mol(-1), although the adsorption enthalpies for H-2 at the exposed Cu(II) site are significantly affected by pore metrics. Introducing methyl groups or using kinked ligands to create smaller pores can enhance the isosteric heat of adsorption and improve H-2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H-2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.