Zn-Zn Distance Research Articles

Establishing Microporosity in Open Metal−Organic Frameworks: Gas Sorption Isotherms for Zn(BDC) (BDC = 1,4-Benzenedicarboxylate)

Construction of microporous metal-organic frameworks by copolymerization of organic molecules with metal ions has received widespread attention in recent years, with significant strides made toward the development of their synthetic and structural design chemistry.1 Cognizant of the fact that access to the pores and understanding the inclusion chemistry of these materials are essential to their ultimate utility, we prepared rigid frameworks that maintain their structural integrity and porosity during anion-exchange and guest sorption from solution and in the absence of guests.2-4 Although gas sorption isotherm measurements are often used to confirm and study microporosity in crystalline zeolites and related molecular sieves,5 such studies have not been established in the chemistry of open metal-organic frameworks6 thus leaving unanswered vital questions regarding the existence of permanent porosity in this class of materials. Herein, we present the synthesis, structural characterization, and gas sorption isotherm measurements for the Zn(BDC) microporous framework of crystalline Zn(BDC)‚(DMF)(H2O) (BDC ) 1,4benzenedicarboxylate and DMF ) N,N′-dimethylformamide). Slow vapor diffusion at room temperature of triethylamine (0.05 mL) and toluene (5 mL) into a DMF solution (2 mL) containing a mixture of Zn(NO3)2‚6H2O (0.073 g, 0.246 mmol) and the acid form of BDC (0.040 g, 0.241 mmol) diluted with toluene (8 mL) yields colorless prism-shaped crystals that were formulated as Zn(BDC)‚(DMF)(H2O). X-ray single-crystal analysis8 on a sample obtained from the reaction product revealed an extended open-framework structure composed of the building unit shown in Figure 1. A total of four carboxylate units of different, but symmetrically equivalent, BDC building blocks are bonded to two zinc atoms in a di-monodentate fashion. Each zinc is also linked to a terminal water ligand to form an overall arrangement that is reminiscent of the carboxylate bridged M-M bonded molecular complexes. Although the Zn-Zn distance (Zn1-Zn1A ) 2.940 (3) A) is indicative of some M-M interaction, it does not represent an actual bond.9 The structure extends into the (011) crystallographic plane by having identical Zn-Zn units linked to remaining carboxylate functionalities of BDC to yield 2-D microporous layers. These layers are held together along the a axis by hydrogen-bonding interactions between water ligands of one layer and carboxylate oxygens of an adjacent layer as illustrated in a. Stacking of the layers in the

Structure of Zinc Oxide Particles Inserted in Nanopores of Microporous Material

The insertion of nanoparticles in the cages of microporous materials raised up a large interest, due to the applications of such systems in the field of catalysis and microelectronics, but the atomic structure of such particles, their location inside the cages and their interaction with the microporous framework is subject to controversies. EXAFS can bring accurate information so far the study of the coordination shells around the central atom can be extended beyond the first one. In zinc exchanged aluminosilicate sodalites, whose cubic structure contains one type of nanopores (6 A in diameter), it was suggested previously that some zinc oxide particles can be located in the centre of the sodalite cages. The carefull EXAFS analysis at Zn K-edge, including ab-initio calculations using the FEFF code, reveals the presence of At neighbours belonging to the framework at less than 3 A from zinc cations as well as an elongation of the Zn-Zn distance with respect to crystalline ZnO. The coordination radii and numbers are consistent with the model of three zinc cations, each of them being located almost in the plane of the 6-oxygen ring of the wall shared by two joined cages.

Multinuclearity of aqueous copper and zinc bisulfide complexes: An EXAFS investigation

Copper and zinc bisulfide complexes in NaHS solutions were investigated by Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). A goal was to investigate the possibility that bisulfide complexes are multinuclear. Copper and zinc K-edge absorption spectra were recorded in fluorescence mode at room temperature and at 80 K. Lowering temperature improved signal to noise ratios, but did not significantly change measured metal-sulfur distances or coordination numbers. In equilibrium with S-absent assemblages in the Cu-S system, dissolved copper consisted of colorless Cu(I) species having Cu-S interatomic distances of 2.30-2.33 Å (coord. no. 3 ± 1 ). Second shell Cu atoms were present at 2.72-2.75 Å ( coord. no. 1.1). The presence of second shell Cu atoms, which was confirmed by duplicate experiments using different starting materials, demonstrates that the predominant complexes in these solutions are multinuclear. The Cu-Cu distance is shorter than in most known Cu-S cluster complexes and Cu-S minerals, but is in good agreement with known Cu-Cu distances in Cu 4(RS) 2− 6 clusters. In equilibrium with a CuS + S assemblage, shorter Cu-S distances and no evidence of an ordered second shell were observed. In these polysulfide-rich solutions, different Cu complexes apparently predominate. Solutions saturated with sphalerite or amorphous ZnS precipitate yielded Zn-S and Zn-Zn distances similar to those in solid sphalerite, although the Zn-Zn coordination number was lower and second shell S was also present. In the Zn case, but not the Cu case, it is possible that the species being characterized by EXAFS were <200 Å particles not in reversible equilibrium with the solution phase. EXAFS is a promising tool for determining molecular structures of complexes of geochemical interest, but at present its application is limited to solutions containing somewhat more than 1 mM of the target element.

The ZnSb structure; A further enquiry

A redetermination of the crystal structure of ZnSb was undertaken to resolve the question raised by a short Zn-Zn distance found by a previous worker. (1) Our result, Zn-Zn distance = 2.81 Å, is in agreement with that found by Toman, (2) who employed a diffractometer and a powder sample. From the bond distances a calculation of the bond orders and valences by the method of Pauling for intermetallic compounds leads to the interpretation of ZnSb as an electron transfer compound with Zn −1 having a valence of three and Sb +1 having a valence of four. The transference of approximately one electron from Sb to Zn permits better binding through the increased valence of both atoms. In the case of the Zn-Zn interaction it appears likely that a repulsive force exists between the atoms, however, it is not certain whether the force is that due to the close approach of nonbonded atoms or that of a bond in compression. With this uncertainty in mind the effects of normal twinning and polytype formation are discussed as a possible explanation of reported (3) phase transformations. Using a single crystal and a modified powder camera the cell edges were determined at 28.4 ± 2°C to be a 0 = 6.202 Å, b 0 = 7.742 Å, c o = 8.100 A.