Studies on metal-organic frameworks of Cu(II) with isophthalate linkers for hydrogen storage.

Hydrogen (H2) is a promising alternative energy carrier because of its environmental benefits, high energy density, and abundance. However, development of a practical storage system to enable the "Hydrogen Economy" remains a huge challenge. Metal-organic frameworks (MOFs) are an important class of crystalline coordination polymers constructed by bridging metal centers with organic linkers. MOFs show promise for H2 storage owing to their high surface area and tuneable properties. In this Account, we summarize our research on novel porous materials with enhanced H2 storage properties and describe frameworks derived from 3,5-substituted dicarboxylates (isophthalates) that serve as versatile molecular building blocks for the construction of a range of interesting coordination polymers with Cu(II) ions. We synthesized a series of materials by connecting linear tetracarboxylate linkers to {Cu(II)2} paddlewheel moieties. These materials exhibit high structural stability and permanent porosity. Varying the organic linker modulates the pore size, geometry, and functionality to control the overall H2 adsorption. Our top-performing material in this series has a H2 storage capacity of 77.8 mg g(-1) at 77 K, 60 bar. H2 adsorption at low, medium, and high pressures correlates with the isosteric heat of adsorption, surface area, and pore volume, respectively. Another series, using tribranched C3-symmetric hexacarboxylate ligands with Cu(II), gives highly porous (3,24)-connected frameworks incorporating {Cu(II)2} paddlewheels. Increasing the length of the hexacarboxylate struts directly tunes the porosity of the resultant material from micro- to mesoporosity. These materials show exceptionally high H2 uptakes owing to their high surface area and pore volume. The first member of this family reported adsorbs 111 mg g(-1) of H2, or 55.9 g L(-1), at 77 K, 77 bar, while at 77 K, 1 bar, the material adsorbs 2.3 wt % H2. We and others have since achieved enhanced H2 adsorption in these frameworks using combinations of polyphenyl groups linked by alkynes. The maximum storage achieved for one of the enhanced materials is 164 mg g(-1) at 77 K, 70 bar, but because of its low density, its volumetric capacity is only 45.7 g L(-1). We attribute the significant adsorption of H2 at low pressures to the arrangement of the {Cu24(isophthalate)24} cuboctahedral cages within the polyhedral structure. Free metal coordination positions are the first binding sites for D2, and these frameworks have two types of Cu(II) centers, one with its vacant site pointing into the cuboctahedral cage and another pointing externally. D2 molecules bind first at the former position and then at the external open metal sites. Design of ligands and complexes is key for enhancing and maximizing H2 storage, and although current materials operate at 77 K, research continues to explore routes to high capacity H2 storage materials that can function at higher temperatures.


Conspectus
Hydrogen (H 2 ) is a promising alternative energy carrier due to its environmental benefits, high energy density and its abundance. However, development of a practical storage system to enable the "Hydrogen Economy" remains a huge challenge. Metal-organic frameworks (MOFs) are an important class of crystalline coordination polymers constructed by bridging metal centers with organic linkers, and show promise for H 2 storage due to their high surface area and tuneable properties. We summarize our research on novel porous materials with enhanced H 2 storage properties, and describe frameworks derived from 3,5-substituted dicarboxylates (isophthalates) that serve as versatile molecular building blocks for the construction of a range of interesting coordination polymers with Cu(II) ions.
A series of materials has been synthesised by connecting linear tetracarboxylate linkers to {Cu(II) 2 } paddlewheel moieties. These (4,4)-connected frameworks adopt the fof-topology in which the Kagomé lattice layers formed by {Cu(II) 2 } paddlewheels and isophthalates are pillared by the bridging ligands. These materials exhibit high structural stability and permanent porosity, and the pore size, geometry and functionality can be modulated by variation of the organic linker to control the overall H 2 adsorption properties. NOTT-103 shows the highest H 2 storage capacity of 77.8 mg g −1 at 77 K, 60 bar among the fof-type frameworks. H 2 adsorption at low, medium and high pressures correlates with the isosteric heat of adsorption, surface area and pore volume, respectively.
Tri-branched C 3 -symmetric hexacarboxylate ligands with Cu(II) give highly porous (3,24)-connected frameworks incorporating {Cu(II) 2 } paddlewheels. These ubt-type frameworks comprise three types of polyhedral cage: a cuboctahedron, truncated tetrahedron and a truncated octahedron which are fused in the solid state in the ratio 1:2:1, respectively. Increasing the length of the hexacarboxylate struts directly tunes the porosity of the resultant material from micro-to mesoporosity. These materials show exceptionally high H 2 uptakes owing to their high surface area and pore volume. NOTT-112, the first reported member of this family reported, adsorbs 111 mg g −1 of H 2 at 77 K , 77 bar. More recently, enhanced H 2 adsorption in these ubt-type frameworks has been achieved using combinations of polyphenyl groups linked by alkynes to give an overall gravimetric gas capacity for NU-100 of 164 mg g −1 at 77 K, 70 bar. However, due to its very low density NU-100 shows a lower volumetric capacity of 45.7 g L -1 compared with 55.9 g L -1 for NOTT-112, which adsorbs 2.3 wt% H 2 at 1 bar, 77K. This significant adsorption of H 2 at low pressures is attributed to the arrangement of the {Cu 24 (isophthalate) 24 } cuboctahedral cages within the polyhedral structure. Free metal coordination positions are the first binding sites for D 2 , and in these ubt-type frameworks there are two types of Cu(II) centres, one with its vacant site pointing into the cuboctahedral cage and another pointing externally. D 2 molecules bind first at the former position, and then at the external open metal sites.
However, other adsorption sites between the cusp of three phenyl groups and a Type I pore window in the framework are also occupied.
Ligand and complex design feature strongly in enhancing and maximising H 2 storage, and, although current materials operate at 77 K, research continues to explore routes to high capacity H 2 storage materials that can function at higher temperatures.

Introduction
Hydrogen (H 2 ) is a promising energy carrier due to the absence of any carbon emissions at the point of use. H 2 has a high energy density (33.3 kWh/kg) compared to hydrocarbons (12.4−13.9 kWh/kg), but the development of new H 2 storage materials has become one of the major technological barriers and challenges to realising the ''Hydrogen Economy''. 1 Solid-state H 2 storage systems based on chemisorption and physisorption have been extensively studied over recent years, but none has satisfied the DOE's 2017 targets for H 2 storage systems: 5.5 wt% in gravimetric terms and 40 g L −1 in volumetric capacity of H 2 at an operating temperature of -40−60 o C and at pressure below 100 atm. 2 Physisorption of H 2 in porous solids is an attractive option since it can show fast kinetics and favourable thermodynamics in adsorption and release cycles. 3 Porous metal-organic frameworks (MOFs) are an important class of crystalline coordination polymer solids constructed from metal centers bridged by organic linkers, and are being intensively studied for H 2 storage due to their high internal surface areas and pore volumes. 4 The modular nature MOFs allows tuning of framework topologies, pore size and geometry to enhance H 2 adsorption properties. 5 MOFs with very high surface area (>3000 m 2 g −1 ) show significant H 2 uptake but only at low temperatures (usually 77 K) owing to low isosteric heats of adsorption involved (typically 5−8 kJ mol −1 ). Strategies to enhance the H 2 binding in these porous hosts include generating frameworks with narrow pores such that the greater overlapping potentials of the pore walls increase the H 2 −framework interactions, 6 incorporation of exposed metal sites to afford strong binding sites for H 2 , 7 doping with metal ions 8 such as Li + and Mg 2+ , and cation exchange to introduce strong electrostatic fields within the cavities, 9,10 and doping of frameworks with metal nanoparticles to increase H 2 uptake via spillover. 4 In this Account, we describe our research in the synthesis of framework materials derived from isophthalate linkers to paddlewheel {Cu 2 (OOCR) 4 } moieties. We describe synthetic strategies to enhance the H 2 adsorption capacity and binding energies in these porous hosts via the assembly of poly-aromatic linear, tri-and tetra-branched isophthalate-containing linkers with varied geometries, and we discuss neutron diffraction studies that have probed preferred D 2 binding sites within these systems.

Porous MOFs Containing Isophthalate Linkers with Cu(II) Paddlewheels
A large number of Cu(II) paddlewheel-based MOFs with various topologies showing permanent porosity and stability have been constructed via the self-assembly of aromatic carboxylates and Cu(II) salts. 11,12 Variation of the linkers can efficiently introduce different pore metrics and functionalities. The coordinated solvent molecules on the Cu(II) paddlewheel can be removed via heat treatment in vacuo after solvent exchange 5 or by treatment with supercritical CO 2 13 affording materials incorporating exposed Cu (

Tetracarboxylate frameworks
A range of rigid aromatic tetracarboxylate struts (  (Table 1). Due to the nature of physisorbed H 2 in these porous materials, frameworks with high surface area often show high H 2 adsorption capacities. Thus, in the series NOTT-100 to NOTT-102, 16 the pore volume and surface area increase with the increasing length of the ligand backbone, leading to an increase in overall H 2 adsorption capacities at saturation. MOFs with large pores require high pressure to achieve saturation. However, there may be an optimum pore size and pores dimensions that optimise H 2 uptake capacities.

NOTT-103 incorporates a naphthalene group in the linker and has a pore size intermediate between NOTT-101 and
NOTT-102. However, NOTT-103 shows a higher H 2 storage capacity: 65.1 mg g −1 at 77 K, 20 bar, 77.8 mg g −1 at 60 bar. 17 Low pressure H 2 adsorption capacity is strongly correlated to the interaction of H 2 with the framework, and the NOTT series of frameworks with accessible open Cu(II) sites shows high H 2 uptakes exceeding 2.2 wt% at 1 bar, 77 K, higher than the H 2 capacities in most other MOFs without exposed metal sites under the same conditions. 17 The role of open metal sites in NOTT-101 in binding H 2 has been confirmed by neutron powder diffraction studies, which also reveal that, in addition to open metal sites, pore functionality and geometry can affect the H 2 affinity.
Small pores generate higher H 2 affinity due to the overlap potential from opposite pore walls compared to larger pores. 21  suggesting that the bulkier and non-conjugated 9,10-hydrophenanthrene substituent provides stronger binding sites for H 2 molecules. 18

Hexacarboxylate frameworks
Increasing the length of the organic struts in fof-type networks is an effective methodology for generating structures with large pores and high surface area and giving enhanced H 2 adsorption capacities. However, when the ligand bridges are lengthened beyond a certain point, this strategy appears to fail due to the onset of interpenetration, which reduces the available pore volume and appears to also lower structural stability. 17 We argued that a more highly-connected network topology might be less likely to form interpenetrated structures. Also, when fused within a network structure, highly-connected metal-organic polyhedra may better maintain their intrinsic porosity on tessellation in 3D space. 22 Maximising the surface area, incorporating open metal sites and optimising the pore size and geometry are all essential routes to achieve high H 2 adsorption capacity in framework materials. Taking these strategies into consideration, we designed a series of elongated rigid hexacarboxylate ligands ( Figure 5) for the construction of frameworks with {Cu(II) 2 } paddlewheel units. [23][24][25][26][27] In all of these isostructural frameworks, the hexacarboxylate linker comprises of three coplanar isophthalate units All the ubt-type frameworks show high pore volumes and BET surface areas (Table 2) and significant total H 2 storage capacities at high pressures (Figure 7). NOTT-112, the first system of this type reported, 23  The volumetric H 2 capacity is a critical criterion for practical transportation applications, and of course the density of materials plays an important role in defining this capacity. Increasing the length of the hexacarboxylate spacers in the above ubt frameworks generates materials with enhanced porosity in terms of pore volume. However, the crystal density drops dramatically when large organic spacers are employed. Thus, although NU-100 has ultrahigh porosity, it shows a total volumetric H 2 uptake of only 45.7 g L −1 at 70 bar, 77 K, 32,33 lower than the shorter-linked analogues NOTT-115 (49.3 g L −1 , 60 bar, 77 K) 25 and medium-sized NOTT-112 (55.9 g L −1 at 77 bar, 77 K) 24 (Table 2).  (3,24)-connected net to form the fcc-packing of the cuboctahedra, as this type of close packing will be inhibited if there is steric hindrance and repulsion between the two closest axial ligands in the Cu(II) paddlewheel in the two closest cuboctahedra (Figure 8). 27 Thus, modifying the shape of the hexacarboxylate linker by introducing an angular component to the three co-planar isophthalate arms emanating from the C 3 -symmetric central core, results in a different type of tight packing of the cuboctahedra as observed in NOTT-122. 27 NOTT-122 shows body centered tetragonal (bct)-packing of cuboctahedra and the highest H 2 adsorption capacity of 2.61 wt% at 77 K, 1 bar among all the (3,24)-connected frameworks. The combined effects of the closed bct-packing of the cuboctahedra with exposed Cu(II) sites are both responsible for the high H 2 adsorption capacity in NOTT-122.

Octacarboxylate frameworks
Four isophthalates can be linked at the 1-position to a tetrotopic organic unit to form an octacarboxylate linker.
We have thus combined 35

Neutron powder diffraction studies
A detailed understanding of the H 2 adsorption sites within framework materials is vital for establishing structure-performance correlations and developing materials with enhanced properties. We employed neutron powder diffraction (NPD) to elucidate the site-specific interactions of H 2 within frameworks. NPD data were collected for NOTT-101 at different D 2 loadings and Rietveld refinement revealed three different hydrogen binding sites. 17 The exposed Cu(II) sites are the first strongest binding site with a distance of Cu•••D 2 (centroid) of 2.50(3) Å: this is slightly longer than that observed in HKUST-1 37 (2.40 Å), but is clearly not of the "Kubas" type σ-bond binding. Two other adsorption sites were identified at higher loadings, both coinciding with a 3-fold symmetry axis: one is located in the middle of the triangular {(Cu 2 ) 3 (isophthalate) 3 } window, while the other is in the cusp of three phenyl rings (Sites II and III, Figure 10). NPD studies on gas-loaded NOTT-112 revealed there are differences between the Cu•••H 2 interactions at the two Cu(II) sites in the same paddlewheel unit. 24 The first and most strongly-bound site (site A1, Figure 11) was found at

Conclusions and Outlook
In this Account, we have described our recent work on the rational design of MOF materials for applications in   a Derived from N 2 isotherms. b Pore diameters estimated from Dubinin-Astakhov analysis. c 1 bar and 20 bar H 2 adsorption data were obtained by gravimetric methods, and 60 bar data were obtained by volumetric methods. wt% = 100(weight of adsorbed H 2 )/(weight of host)). d The pore diameter of PCN-46 was calculated using the Horvath-Kawazoe model.