Young Eun Cheon

A Comparison of the H2 Sorption Capacities of Isostructural Metal–Organic Frameworks With and Without Accessible Metal Sites: [{Zn2(abtc)(dmf)2}3] and [{Cu2(abtc)(dmf)2}3] versus [{Cu2(abtc)}3]

Various metal–organic frameworks (MOFs) have been prepared to obtain materials that show specific or multifunctional properties. Porous MOFs that contain free space where guest molecules can be accommodated are of particular interest because they can be applied in gas storage and separation, selective adsorption and separation of organic molecules, ion exchange, catalysis, sensor technology, and for the fabrication of metal nanoparticles. Secondary building units (SBUs) with a specific geometry have often been employed for the modular construction of porous MOFs as they make the design and prediction of molecular architectures simple and easy. In particular, {M2(CO2)4}-type paddlewheel clusters that can be formed from the solvothermal reaction of M ions and the appropriate carboxylic acid are widely used for the construction of porous frameworks. Three-dimensional porous frameworks with various topologies (Pt3O4, boracites, NbO, and PtS nets) can be built from paddlewheel-type metal cluster SBUs and trior tetracarboxylates, whereas pillared square-grid networks can be constructed from paddlewheel cluster SBUs and dicarboxylates in the presence of diamine ligands. Porous MOFs with accessible metal sites (AMSs) should have a higher hydrogen storage capacity than those without AMSs, although there are not yet enough experimental data to support this assumption. To determine the effect of AMSs in a MOF on H2 adsorption, the H2 uptakes should be compared for the same framework in the absence and presence of AMSs, or for two independent isostructural MOFs with and without AMSs. H2 uptake has previously been measured under several different outgassing conditions. Unfortunately, these experiments could not clearly demonstrate the effect of AMSs as the exact formula and structure at each stage were not known. Furthermore, even when coordinating solvent molecules are successfully removed with retention of the porous framework structure, the metal ion sometimes transforms its coordination geometry to the thermodynamically most stable form instead of keeping the AMSs. Herein we report two porous MOFs with the same NbOtype net topology, namely [{Zn2(abtc)(dmf)2}3]·4H2O·10dmf (1) and [{Cu2(abtc)(H2O)2}3]·10dmf·6 (1,4-dioxane) (2 ; H4abtc= 1,1’-azobenzene-3,3’,5,5’-tetracarboxylic acid ), and compare the gas adsorption data for the MOFs with and without AMSs. Heating crystals of 1 and 2 under precisely controlled conditions allowed us to prepare [{Zn2(abtc)(dmf)2}3] (1a ; SNU-4) and [{Cu2(abtc)(dmf)2}3] (2a ; SNU-5’), which have no AMSs, as well as [{Cu2(abtc)}3] (2b ; SNU-5), which has AMSs. The framework structure of 1a is the same as that of 1 and those of 2a and 2b are the same as that of 2, as evidenced by the PXRD patterns. Solid 1a, 2a, and 2b exhibit higher adsorption capabilities for N2, CO2, CH4, and H2 than other previously reported MOFs. In particular, 2b adsorbs 2.87 wt% of H2 gas at 77 K and 1 atm, which is the highest value for H2 sorption under these conditions amongst a variety of other MOFs. The N2, CO2, and CH4 adsorption capacities per unit sample volume for 2b, which has AMSs, are 140–160% higher than those for 1a and 2a, which have no AMSs. The H2 adsorption capacity of 2b is also higher than those of 1a and 2a [at 77 K and 1 atm, 2.87 wt% for 2b vs. 2.07 wt% for 1a and 1.83 wt% for 2a ; excess adsorbed H2 at 77 K and 50 bar: 5.22 wt% (total 6.76 wt%) for 2b vs. 3.70 wt% (total 4.49 wt%) for 1a], although this is mainly due to the lower molecular weight effect of 2b. The H2 sorption capacity ratios 2b/1a and 2b/2a per unit sample volume at 77 K and 1 atm are 105% and 120%, respectively, and the ratio 2b/1a at 77 K and 50 bar is 106%. Our measurements of the isosteric heat of H2 adsorption (zero-coverage isosteric heats are 7.24, 6.53, and 11.60 kJmol for 1a, 2a, and 2b, respectively) suggest that the enhanced H2 adsorption in 2b can be attributed to the stronger interaction of H2 molecules with the AMSs of the MOF. Yellowish block-shaped crystals of [{Zn2(abtc)(dmf)2}3]·4H2O·10dmf (1) were prepared by heating a dmf solution of Zn(NO3)2·6H2O and H4abtc at 100 8C for 12 h. Greenish block-shaped crystals of [{Cu2(abtc)(H2O)2}3]·10dmf·6 (1,4-dioxane) (2) were prepared by heating Cu(NO3)2·xH2O and H4abtc in a dmf/1,4-dioxane/H2O (4:3:1 v/v) mixture at 80 8C for 24 h. Solid 1 is insoluble in common organic solvents but is slightly soluble in water, where it dissociates into its building blocks. Solid 2 is insoluble in all common organic solvents and water. The temperaturedependent PXRD patterns show that the framework struc[*] Y.-G. Lee, H. R. Moon, Y. E. Cheon, Prof. M. P. Suh Department of Chemistry, Seoul National University Seoul 151-747 (Republic of Korea) Fax: (+82)2-886-8516 E-mail: mpsuh@snu.ac.kr

Multifunctional Fourfold Interpenetrating Diamondoid Network: Gas Separation and Fabrication of Palladium Nanoparticles

A fourfold interpenetrating diamondoid network, [{[Ni(cyclam)]2-(mtb)}(n)].8n H2O.4n DMF (1) (MTB=methanetetrabenzoate, DMF=dimethylformamide), has been assembled from [Ni(cyclam)][ClO4]2 (cyclam=1,4,8,11-tetraazacyclotetradecane) and methanetetrabenzoic acid (H4MTB) in DMF/H2O (7:3, v/v) in the presence of triethylamine (TEA). Despite the high-fold interpenetration, 1 generates 1D channels that are occupied by water and DMF guest molecules. Solid 1, after removal of guest molecules, exhibits selective gas adsorption behavior for H2, CO2, and O2 rather than N2 and CH4, suggesting possible applications in gas separation technologies. In addition, solid 1 can be applied in the fabrication of small Pd (2.0+/-0.6 nm) nanoparticles without any extra reducing or capping agent because a Ni II macrocyclic species incorporated in 1 reduces Pd II ions to Pd 0 on immersion of 1 in the solution of Pd(NO3)2.2H2O in MeCN at room temperature.

Recent Advances in the Dynamics of Single Crystal to Single Crystal Transformations in Metal-Organic Open Frameworks

Single crystal to single crystal transformations have been the subject of growing interest in recent years. In this article, several examples of single crystal to single crystal transformations that occur in metal–organic open frameworks (MOFs) upon guest removal and rebinding, guest exchange, and chemical oxidation are introduced. Depending on the structures of the MOFs, host framework structures can be retained or altered upon guest removal. When host framework structures are altered, significant rearrangements of the molecular components, which involve sponge-like shrinkage/swelling, sliding, swing, bending, and/or rotational motions, occurs throughout the entire crystal to retain the single crystallinity.

Proteolytic activity of Co(III) complex of 1-oxa-4,7,10-triazacyclododecane: a new catalytic center for peptide-cleavage agents

Catalytic drugs based on target-selective artificial proteases have been proposed as a new paradigm in drug design. Peptide-cleavage agents selective for pathogenic proteins of Alzheimer’s disease, type 2 diabetes mellitus or Parkinson’s disease have been prepared using the Co(III) aqua complex (Co(III)cyclen) of 1,4,7,10-tetraazacyclododecane as the catalytic center. In the present study, the Co(III) aqua complex (Co(III)oxacyclen) of 1-oxa-4,7,10-triazacyclododecane was examined in search of an improved catalytic center for peptide-cleavage agents. An X-ray crystallographic study of [Co(oxacyclen)(CO3)](ClO4), titration of Co(III)oxacyclen, and kinetic studies on the cleavage of albumin, γ-globulin, lysozyme, and myoglobin by Co(III)oxacyclen were carried out. Considerably higher proteolytic activity was observed for Co(III)oxacyclen in comparison with Co(III)cyclen, indicating that better target-selective artificial metalloproteases would be obtained using Co(III)oxacyclen as the catalytic center. The improved proteolytic activity was attributed to either steric effects or the increased Lewis acidity of the Co(III) center. The kinetic data also predicted that side effects due to the cleavage of nontarget proteins by a catalytic drug based on Co(III)oxacyclen would be insignificant.