Andrew D. Burrows

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.

Mixed-component metal-organic frameworks (MC-MOFs): enhancing functionality through solid solution formation and surface modifications

Mixed-component metal–organic frameworks (MC-MOFs) are metal–organic frameworks that have different linkers or metals with the same structural role. Many of these mixed-ligand or mixed-metal MOFs are solid solutions, in which the proportions of the ligands or metals can be adjusted or even controlled. These MC-MOFs can be prepared directly, using more than one metal or ligand in the synthesis, or formed by post-synthetic modification. A second class of MC-MOFs have core–shell structures, and these can be prepared through epitaxial growth of one MOF on the surface of another or post-synthetic modification of the crystal surfaces. This review describes the syntheses, structures and properties of mixed-ligand, mixed-metal and core–shell MOFs, and highlights some of the potential benefits in functionality that these materials have.

Synthesis and post-synthetic modification of MIL-101(Cr)-NH2via a tandem diazotisation process

The functionalised metal-organic framework MIL-101(Cr)-NH(2), containing 2-aminobenzene-1,4-dicarboxylate as the linker, has been synthesised. A new tandem post-synthetic modification strategy involving diazotisation as the first step has been developed and used to introduce halo- and azo dye-functional groups into the pores.

Size-controlled synthesis of MIL-101(Cr) nanoparticles with enhanced selectivity for CO2 over N2

Nanoparticles of MIL-101(Cr) have been fabricated using a hydrothermal method for the first time. The particle size can be controlled from 19 (4) nm to 84 (12) nm, by using a monocarboxylic acid as a mediator. These nano MIL-101(Cr) materials exhibit higher selectivities for CO2 over N2 than bulk MIL-101(Cr).

THE CHEMISTRY OF GROUP 10 METAL TRIANGULO CLUSTERS

Abstract For the group 10 metals, nickel, palladium and platinum, M3 triangulo clusters display a wide chemistry which increases in scope on descending the triad. The substitution and addition chemistry of these compounds is reviewed, focusing mainly on platinum but drawing on palladium and nickel when relevant comparisons and contrasts can be made. The synthesis of triangulo clusters from monomeric compounds and bonding theories are also reviewed. Substitution reactions of [Pt3(μ-X)3Y3] clusters with CO, SO2, isocyanides, phosphines, halides and NO+ are discussed as are the addition and substitution reactions of [Pt3(μ3-CO)(μ-dppm)3]2+. Addition reactions of both types of triangulo clusters with metal fragments such as Au(PR3)+, M+, Hg and metal halides are also discussed.

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.

Zinc thiosemicarbazide dicarboxylates: the influence of the anion shape on supramolecular structure

Abstract The syntheses and crystal structures of the zinc thiosemicarbazide dicarboxylate compounds [Zn(tsc)2(OH2)2][fumarate] 2, [Zn(tsc)2(citraconate)]·H2O 3, [Zn(tsc)(μ-1,4-phenylenediacetate)] 4, [Zn(Ettsc)2(citraconate)]·3H2O 5, [Zn(Ettsc)2(Hphthalate)][Hphthalate]·H2O 6, [Zn(Metsc)2(Hphthalate)][Hphthalate]·H2O 7, [Zn(Me2tsc)2(OH2)][terephthalate]·2H2O 8 and [Zn(EtMe2tsc)2(OH2)][terephthalate] 9 are reported. The supramolecular structures of the terephthalate and fumarate compounds 2, 8 and 9 consist of chains of cations and anions, in which the ions are linked by hydrogen bonding. In contrast, compounds 3, 5, 6 and 7 contain carboxylate groups co-ordinated to the metal centre to give either neutral or monocationic species. These differences can be rationalised on the basis of the dicarboxylate structure, in particular the angle between the carboxylate vectors. Compound 4 forms co-ordination polymers in an analogous manner to thiourea derivatives.