Peter Hubberstey

Inorganic crystal engineering using self-assembly of tailored building-blocks

Abstract The use of transition metal complexes of bridging bidentate ligands to construct predictable, multi-dimensional infinite networks is an area of chemistry which has received ever-increasing attention over recent years. This article will review the advances that have been made in this field of research and will illustrate how ligand design and the properties of the transition metal and counter-anion can be used to control network geometry and thus crystal structure. The range of network topologies and structural motifs that have been constructed thus far will be outlined with particular emphasis upon how specific arrays can be prepared via rational design of molecular building-blocks. The unusual phenomenon of interpenetration, or polycatenation, will be discussed and methods to achieve control over this effect will be highlighted.

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 [Cu2(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 64.82 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 H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g(-)- equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. 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 {Cu2} 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 H2 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 H2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.

Cation-induced kinetic trapping and enhanced hydrogen adsorption in a modulated anionic metal–organic framework

Metal–organic frameworks (MOFs)—microporous materials constructed by bridging metal centres with organic ligands—show promise for applications in hydrogen storage, which is a key challenge in the development of the ‘hydrogen economy’. Their adsorption capacities, however, have remained insufficient for practical applications, and thus strategies to enhance hydrogen–MOF interactions are required. Here we describe an anionic MOF material built from In(iii) centres and tetracarboxylic acid ligands (H4L) in which kinetic trapping behaviour—where hydrogen is adsorbed at high pressures but not released immediately on lowering the pressure—is modulated by guest cations. With piperazinium dications in its pores, the framework exhibits hysteretic hydrogen adsorption. On exchange of these dications with lithium cations, no hysteresis is seen, but instead there is an enhanced adsorption capacity coupled to an increase in the isosteric heat of adsorption. This is rationalized by the different locations of the cations within the pores, determined with precision by X-ray crystallography. Porous metal–organic frameworks are promising for hydrogen storage applications, but adsorption capacities have remained too low for practical use. Now, the adsorption behaviour of such a framework has been modulated by exchanging cations within its pores resulting in either kinetic trapping or enhanced hydrogen affinity.

A partially interpenetrated metal–organic framework for selective hysteretic sorption of carbon dioxide

The selective capture of carbon dioxide in porous materials has potential for the storage and purification of fuel and flue gases. However, adsorption capacities under dynamic conditions are often insufficient for practical applications, and strategies to enhance CO(2)-host selectivity are required. The unique partially interpenetrated metal-organic framework NOTT-202 represents a new class of dynamic material that undergoes pronounced framework phase transition on desolvation. We report temperature-dependent adsorption/desorption hysteresis in desolvated NOTT-202a that responds selectively to CO(2). The CO(2) isotherm shows three steps in the adsorption profile at 195 K, and stepwise filling of pores generated within the observed partially interpenetrated structure has been modelled by grand canonical Monte Carlo simulations. Adsorption of N(2), CH(4), O(2), Ar and H(2) exhibits reversible isotherms without hysteresis under the same conditions, and this allows capture of gases at high pressure, but selectively leaves CO(2) trapped in the nanopores at low pressure.

Exceptionally high H2 storage by a metal-organic polyhedral framework

The desolvated polyhedral framework material NOTT-112 shows an excess H(2) uptake of 7.07 wt% between 35 and 40 bar at 77 K, and a total H(2) uptake of 10 wt% at 77 bar and 77 K.

Hydrogen storage in metal–organic frameworks

The role of porous metal–organic frameworks (MOFs) as storage materials for hydrogen (H2) and the current state-of-the-art in this rapidly developing field are reviewed. The significant potential of MOFs to effectively and reversibly store H2 illustrates the great promise that crystal engineering of solid-state materials gives to the next generation of applied multi-functional materials.

Exceptional Thermal Stability in a Supramolecular Organic Framework: Porosity and Gas Storage

Reaction of β-amino-β-(pyrid-4-yl)acrylonitrile with the aromatic dicarboxaldehydes 9,10-bis(4-formylphenyl)anthracene and terephthalaldehyde affords the dihydropyridyl products 9,10-bis(4-((3,5-dicyano-2,6-dipyridyl)dihydropyridyl)phenyl)anthracene (L(1)) and 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene (L(2)), respectively. In the solid state [L(1)]·2.5DMF·3MeOH (SOF-1) crystallizes in the monoclinic space group P2(1)/c and forms a 3D stable supramolecular organic framework via strong N-H···N(py) hydrogen bonds and π-π interactions. The material incorporates pyridyl-decorated channels and shows permanent porosity in the solid state. The pore volumes of the desolvated framework SOF-1a calculated from the N(2) isotherm at 125 K and the CO(2) isotherm at 195 K are 0.227 and 0.244 cm(3) g(-1), respectively. The N(2) absorption capacity of SOF-1a at 77 K is very low, with an uptake of 0.63 mmol g(-1) at 1 bar, although saturation N(2) adsorption at 125 K is 6.55 mmol g(-1) (or 143 cm(3) g(-1)). At ambient temperature, SOF-1a shows significant CO(2) adsorption with approximately 3 mol of CO(2) absorbed per mole of host at 16 bar and 298 K, corresponding to 69 cm(3) g(-1) at STP. SOF-1a also adsorbs significant amounts of C(2)H(2), with an uptake of 124 cm(3) (STP) g(-1) (5.52 mmol g(-1)) at 1 bar at 195 K. Methane uptake at 195 K and 1 bar is 69 cm(3) (STP) g(-1). Overall, gas adsorption measurements on desolvated framework SOF-1a reveal not only high capacity uptakes for C(2)H(2) and CO(2), compared to other crystalline molecular organic solids, but also an adsorption selectivity in the order C(2)H(2) > CO(2) > CH(4) > N(2). Overall, C(2)H(2)(270 K)/CH(4)(273 K) selectivity is 33.7 based on Henrys Law constant, while the C(2)H(2)(270 K)/CO(2)(273 K) ratio of uptake at 1 bar is 2.05. The less bulky analogue L(2) crystallizes in the triclinic space group P1 as two different solvates [L(2)]·2DMF·5C(6)H(6) (S2A) and [L(2)]·2DMF·4MeOH (S2B) as pale yellow tablets and blocks, respectively. Each L(2) molecule in S2A participates in two N-H···O hydrogen bonds between dihydropyridyl rings and solvent DMF molecules. Packing of these layers generates 1D nanochannels along the crystallographic a and b axes which host DMF and benzene molecules. In S2B, each L(2) ligand participates in hydrogen bonding via an N-H···O interaction between the N-H of the dihydropyridyl ring and the O of a MeOH and also via an N···H-O interaction between the N center of a pyridine ring and the H-O of a second MeOH molecule. The presence of the L(2)-HOMe hydrogen bonds prevents ligand-ligand hydrogen bonding. As a result, S2B crystallizes as one-dimensional chains rather than as an extended 3D network. Thermal removal of solvents from S2A results in conversion to denser phase S2C which shows no effective permanent porosity.

Twelve-connected porous metal–organic frameworks with high H2 adsorption

The twelve-connected metal-organic frameworks {[Ni(3)(OH)(L)(3)].n(solv)}(infinity) and {[Fe(3)(O)(L)(3)].n(solv)}(infinity) [LH(2) = pyridine-3,5-bis(phenyl-4-carboxylic acid)] have been prepared and characterised: these materials can be desolvated to form porous materials that show adsorption of H(2) up to 4.15 wt% at 77 K.

Enhancement of H2 adsorption in Li+-exchanged co-ordination framework materials

H(2) adsorption in (Me(2)NH(2))[In(L)] is enhanced by exchange of Me(2)NH(2)(+) for Li(+) cations; the Li(+)-exchanged material displays a lower isosteric heat for H(2) adsorption than the parent material, indicating that the increase in H(2) capacity is due to an increase in the accessible pore volume on cation exchange, while the lower adsorption enthalpy is consistent with increased pore size.

Enhancement of H2 Adsorption in Coordination Framework Materials by Use of Ligand Curvature

Solvothermal reaction of the ligands H(4)L(110) ((2,7-phenanthrenediyl)diisophthalic acid) and H(4)L(111) ([2,7-(9,10-dihydrophenanthrenediyl)]diisophthalic acid) with Cu(NO(3))(2) x 2.5 H(2)O in a slightly acidified mixture of DMF/1,4-dioxane/H(2)O afforded the solvated framework compounds [Cu(2)(L(110))(H(2)O)(2)](DMF)(7.5)(H(2)O)(5) (NOTT-110) and [Cu(2)(L(111))(H(2)O)(2)](DMF)(7.5)(H(2)O)(5) (NOTT-111), respectively. Crystal structure determinations confirmed that NOTT-110 and NOTT-111 have the same NbO framework structure, differing only at the 9 and 10 positions of the phenanthrene group. The BET surface areas for desolvated NOTT-110 and NOTT-111 were estimated to be 2960 and 2930 m(2) g(-1), respectively. Compared with their phenyl analogues, introduction of phenanthrene groups to these porous Cu(II)-carboxylate framework materials leads to an enhancement of H(2) adsorption. Thus, the H(2) isotherms for desolvated NOTT-110 and NOTT-111 confirm 2.64 and 2.56 wt % total H(2) uptake, respectively, at 1 bar and 78 K. NOTT-110 shows a high total H(2) storage capacity of 7.62 wt % at 55 bar and 77 K (8.5 wt % at saturation) with a total volumetric capacity of 46.8 g L(-1) at 55 bar and 77 K.