Christopher Richardson

Post‐Synthetic Modification of Tagged Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are currently attracting considerable attention, largely because of their potential for porosity, and their consequent use in applications as diverse as gas storage, catalysis, separations, and drug delivery. The first generation of MOFs were formed by linking together metal centers with simple, commercially available bridging ligands, such as 1,4-benzenedicarboxylate (bdc), but there has since been an increasing shift towards more complex structures and increased functionality. For example, MOFs in which the pores contain accessible hydrogenbonding groups, unsaturated metal centers, or chirality have been reported and studied, and the preparation of dynamic porous materials, capable of undergoing guestinduced transformations or reformations, has been explored. Another approach to forming functionalized networks is to undertake reactions on preformed MOFs, converting one solid state material into another. The incorporation of an additional functional group, a “tag”, into a linking ligand offers the opportunity to form structures in which this group is preserved during the MOF synthesis, allowing it to project into the pores or channels of the network structure. We define a “tag” as a group or functionality that is stable and innocent (that is, non-structure-defining) during MOF formation, but that can be transformed by a post-synthetic modification. This approach is shown schematically in Figure 1. A similar concept of tagging has also recently been applied in medicinal chemistry. Post-synthetic modification allows the pores in a preformed MOF to be tailored for a specific purpose, which offers the possibility of fine-tuning for selective adsorption and catalysis. The strategy also facilitates the incorporation into a MOF of functional groups that would not survive the conditions of the MOF synthesis (e.g., temperature and pH) and of functional groups that might compete with the donor groups on the bridging ligands. Given these advantages, it is surprising that there has been very little focus on postsynthetic modification of MOFs. Kim and co-workers showed that the pendant pyridyl groups in a chiral zinc network could be methylated and, very recently, Wang and Cohen, and Gamez and co-workers have both demonstrated that the amino groups in 2-amino-1,4-benzenedicarboxylate MOFs can be converted into amides or urethanes. Rosseinsky and co-workers have converted these amines into salicylidenes, and then used these to coordinate vanadium. Fujita and co-workers have shown that guest molecules can undergo similar transformations within the pores of a MOF. Herein, we report our endeavors to prepare tagged MOFs suitable for post-synthetic modification, starting from an aldehyde-modified dicarboxylate. Following seminal work from Yaghi and co-workers, it is now well-established that the octahedral zinc secondary building unit (SBU) Zn4O(O2CR)6 forms an isoreticular series of MOFs containing the same framework topology with linear dicarboxylates, such as bdc and 4,4’-biphenyldicarboxylate (bpdc). We have prepared the aldehyde-tagged dicarboxylic acid H2L 1 (2-formylbiphenyl-4,4’-dicarboxylic acid, Scheme 1), and used it in MOF synthesis. The coordinated L ligand is suitable for Figure 1. Schematic representation of the post-synthetic modification strategy for MOFs.

4,5-Di(2-pyridyl)-1,2,3-triazolate: the elusive member of a family of bridging ligands that facilitate strong metal–metal interactions

The new click-adduct 4,5-di(2-pyridyl)-1,2,3-triazole acts as a doubly-chelating anionic bridging ligand that forms dinuclear ruthenium(II) complexes which exhibit strong metal-metal interactions.

Dipyridyl β-diketonate complexes: versatile polydentate metalloligands for metal–organic frameworks and hydrogen-bonded networks

The Group 13 metal complexes [M(L(2))(3)], where M is Al or Ga and L(2) is 1,3-di(4-pyridyl)-1,3-propanedionato, are hexatopic metalloligands that have been used to prepare mixed-metal-organic frameworks containing interpenetrated primitive cubic networks. In contrast, the europium complex [Eu(HL(2))(3)(H(2)L(2))]Cl(4) x EtOH forms a hydrogen-bonded network following partial protonation of the pyridyl groups.

Dipyridyl β-diketonate complexes and their use as metalloligands in the formation of mixed-metal coordination networks

The iron(III) and aluminium(III) complexes of 1,3-di(4-pyridyl)propane-1,3-dionato (dppd) and 1,3-di(3-pyridyl)propane-1,3-dionato (dmppd), [Fe(dppd)(3)] 1, [Fe(dmppd)(3)] 2, [Al(dppd)(3)] 3 and [Al(dmppd)(3)] 4 have been prepared. These complexes adopt molecular structures in which the metal centres contain distorted octahedral geometries. In contrast, the copper(II) and zinc(II) complexes [Cu(dppd)(2)] 5 and [Zn(dmppd)(2)] 6 both form polymeric structures in which coordination of the pyridyl groups into the axial positions of neighbouring metal centres links discrete square-planar complexes into two-dimensional networks. The europium complex [Eu(dmppd)(2)(H(2)O)(4)]Cl·2EtOH·0.5H(2)O 7 forms a structure containing discrete cations that are linked into sheets through hydrogen bonds, whereas the lanthanum complex [La(dmppd)(3)(H(2)O)]·2H(2)O 8 adopts a one-dimensional network structure, connected into sheets by hydrogen bonds. The iron complexes 1 and 2 act as metalloligands in reactions with silver(I) salts, with the nature of the product depending on the counter-ions present. Thus, the reaction between 1 and AgBF(4) gave [AgFe(dppd)(3)]BF(4)·DMSO 9, in which the silver centres link the metalloligands into discrete nanotubes, whereas reactions with AgPF(6) and AgSbF(6) gave [AgFe(dppd)(3)]PF(6)·3.28DMSO 10 and [AgFe(dppd)(3)]SbF(6)·1.25DMSO 11, in which the metalloligands are linked into sheets. In all three cases, only four of the six pyridyl groups present on the metalloligands are coordinated. The reaction between 2 and AgNO(3) gave [Ag(2)Fe(dmppd)(3)(ONO(2))]NO(3)·MeCN·CH(2)Cl(2)12. Compound 12 adopts a layer structure in which all pyridyl groups are coordinated to silver centres and, in addition, a nitrate ion bridges between two silver centres. A similar structure is adopted by [Ag(2)Fe(dmppd)(3)(O(2)CCF(3))]CF(3)CO(2)·2MeCN·0.25CH(2)Cl(2)13, with a bridging trifluoroacetate ion playing the same role as the nitrate ion in 12.

Selective incorporation of functional dicarboxylates into zinc metal–organic frameworks

Zinc(II) nitrate reacts with different ratios of 1,4-benzenedicarboxylic acid (H(2)bdc) and 2-halo-1,4-benzenedicarboxylic acid (H(2)bdc-X, X = Br or I) to give [Zn(4)O(bdc)(3-x)(bdc-X)(x)], in which preferential incorporation of bdc is observed. The selective incorporation is related to crystal growth rates, and the proportion of incorporated bdc-X rises with increasing reaction time.

Subtle structural variation in copper metal-organic frameworks: syntheses, structures, magnetic properties and catalytic behaviour

Two new copper metal-organic frameworks containing 5-nitro-1,3-benzenedicarboxylate (5-nbdc) have been prepared from the reaction between Cu(NO(3))(2).3H(2)O and H(2)(5-nbdc) in DMF at different temperatures. Single crystal X-ray structures of {[Cu(2)(5-nbdc)(2)(DMF)(2)].2DMF}(infinity) () and {[Cu(2)(5-nbdc)(2)(DMF)(2)].3(1/3)DMF}(infinity) () revealed similar sheet structures, containing triangular and hexagonal pores, but differences in the stacking of the sheets. Magnetic measurements on and are consistent with antiferromagnetic dimers containing a small quantity of paramagnetic impurity. The desolvated forms of and were applied as Lewis acid catalysts in the acetylation of methyl 4-hydroxybenzoate. When the reaction between Cu(NO(3))(2).3H(2)O and H(2)(5-nbdc) was carried out in a mixture of DMF and water, the reaction gave metallomacrocycles of formula [Cu(6)(5-nbdc)(6)(H(2)O)(12)(DMF)(6)] (). These assemble through hydrogen-bonding interactions to form a gross structure in which the macrocycle pores align into channels. The reaction between Cu(NO(3))(2).3H(2)O and 5-methylsulfanylmethyl-1,3-benzenedicarboxylic acid, H(2)(5-msbdc), in DMF-water gave {[Cu(2)(5-msbdc)(2)(OH(2))(2)].3DMF}(infinity) (), which contains similar sheets to those in and , whereas the reaction with 5-amino-1,3-benzenedicarboxylic acid, H(2)(5-abdc), gave {[Cu(2)(5-abdc)(2)(DMF)(2)]}(infinity) (), which has a previously reported network based on sheets containing rhombohedral pores. The reaction between Cu(NO(3))(2).3H(2)O and 2-methoxy-1,3-benzenedicarboxylic acid, H(2)(2-mbdc), in DMF gave [Cu(2)(2-mbdc)(2)(DMF)(2)] (). The presence of the substituent in the 2-position removes the co-planarity of the carboxylate groups, and the sheet structure adopted by contains rhomboidal pores.

Continuous adsorption and biotransformation of micropollutants by granular activated carbon-bound laccase in a packed-bed enzyme reactor

Laccase was immobilized on granular activated carbon (GAC) and the resulting GAC-bound laccase was used to degrade four micropollutants in a packed-bed column. Compared to the free enzyme, the immobilized laccase showed high residual activities over a broad range of pH and temperature. The GAC-bound laccase efficiently removed four micropollutants, namely, sulfamethoxazole, carbamazepine, diclofenac and bisphenol A, commonly detected in raw wastewater and wastewater-impacted water sources. Mass balance analysis showed that these micropollutants were enzymatically degraded following adsorption onto GAC. Higher degradation efficiency of micropollutants by the immobilized compared to free laccase was possibly due to better electron transfer between laccase and substrate molecules once they have adsorbed onto the GAC surface. Results here highlight the complementary effects of adsorption and enzymatic degradation on micropollutant removal by GAC-bound laccase. Indeed laccase-immobilized GAC outperformed regular GAC during continuous operation of packed-bed columns over two months (a throughput of 12,000 bed volumes).

A ribbon-like metallopolymer from 1,4-bis(2-pyridyl)buta-1,3-diyne and silver(I) nitrate

Abstract Reaction of 1,4-bis(2-pyridyl)buta-1,3-diyne with silver(I) nitrate in methanol results in the assembly of a one-dimensional metallopolymer in high yield. An X-ray crystal structure shows this to have a ribbon-like shape, with two crystallographically independent ligands bridging silver atoms separated by ~ 8.7 A.

The synthesis, structures and reactions of zinc and cobalt metal–organic frameworks incorporating an alkyne-based dicarboxylate linker

The reaction of zinc(II) nitrate and 4,4′-ethynylenedibenzoic acid (H2edb) in DMF at 80 °C gave the metal–organic framework material [Zn4O(edb)3(H2O)2]·6DMF 1 in which edb ligands connect Zn4O centres into a doubly-interpenetrated cubic network with a similar topology to observed with other linear dicarboxylates in the IRMOF series. Analysis of the nitrogen isotherm revealed the material to have a BET surface area of 1088 m2 g−1, which is approximately one-third of the value calculated from GCMC simulations, suggesting incomplete activation or pore blocking in the activated material. The reaction of cobalt(II) nitrate and H2edb in DMF gave [Co3(edb)3(DMF)4]·2.6DMF 2. The structure of 2 is based on Co3(O2CR)6 linear secondary building units that are linked by the edb ligands into a two-dimensional network. When 2 was placed under vacuum, a colour change from pale pink to deep blue was observed, which is consistent with loss of the coordinated DMF molecules. When treated with [Co2(CO)8], crystals of 1 turned dark red, and IR analysis is consistent with coordination of Co2(CO)6 fragments to the alkyne groups. However, the colour change was restricted to the external crystal surfaces. This is a likely consequence of partial framework collapse, which occurs following coordination of Co2(CO)6 to the alkyne groups. Coordination changes the preferred angle between carboxylate groups in the edb ligand, which in turn introduces strain into the network.

Silver coordination networks and cages based on a semi-rigid bis(isoxazolyl) ligand

The semi-rigid ligand 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene (bisox) reacts with a range of silver(i) salts to give products in which the anions dictate the structure. The reactions with AgNO(3) and AgO(2)CCF(3) both lead to compounds in which the anions are coordinated to the silver centres. Thus, the structure of [Ag(2)(NO(3))(2)(bisox)] 1 contains helical silver-nitrate chains that are linked into sheets by bridging bisox ligands, whereas the structures of [Ag(O(2)CCF(3))(bisox)]·0.5X (2a, X = MeOH; 2b, X = MeCN) consist of sheets in which Ag(2)(μ-O(2)CCF(3))(2) dimers act as 4-connecting nodes. In these structures bisox adopts the S-conformation, with the nitrogen donor atoms anti to each other. The reactions of bisox with AgClO(4) and AgBF(4) in methanol give the compounds [Ag(2)(bisox)(3)]X(2) (3, X = ClO(4); 5, X = BF(4)), the structures of which contain triply-interpenetrated sheets with Borromean links and ligands in the S-conformation. Recrystallisation of these compounds from acetonitrile-diethyl ether gives [Ag(2)(bisox)(3)]X(2)·xEt(2)O (4, X = ClO(4), x = 1; 6, X = BF(4), x = 1.2). The structures of 4 and 6 contain similar triply-interpenetrated sheets to those in 3 and 5, though these are sandwiched between sheets of discrete Ag(2)(bisox)(3) cages, in which the bisox ligands are in the C-conformation, with the nitrogen donor atoms syn to each other. Diethyl ether molecules project through the faces of the cages and template cage formation. Both 4 and 6 lose diethyl ether on heating in vacuum, and convert into 3 and 5, respectively. This solid state transformation requires a change in conformation of half the bisox ligands, with conversion of 6 into 5 occurring more readily than conversion of 4 into 3. The reactions of bisox with AgPF(6) and AgSbF(6) in methanol give mixtures of products from which [Ag(bisox)(2)]X·0.5bisox (7, X = PF(6); 8, X = SbF(6)) can be isolated. Both 7 and 8 have structures containing one-dimensional chains, in which the bisox ligands adopt C-conformations and interconnect distorted tetrahedral silver centres in a pairwise manner generating macrocycles. Additional uncoordinated bisox molecules lie within half of these macrocyclic rings. Recrystallisation of the crude AgSbF(6)/bisox reaction mixture from acetonitrile-diethyl ether gives [Ag(bisox)(2)]SbF(6)9, the structure of which consists of a triply-interpenetrated flattened diamondoid network. A similar structure was observed for [Ag(bisox)(2)]CF(3)SO(3)10, which is formed from the reaction of AgO(3)SCF(3) and bisox in methanol.