Coordination frameworks

To this point, coordination complexes that are individual entities that pack together in the solid state and exist as separated units in solution have been our focus. A burgeoning area of inorganic chemistry is the synthesis and application of substances in which ligands act as bridges to create extended structures in the solid state. 

FIGURE 9.36 Examples of Linking Groups in Metal-Organic Frameworks, (a) 

FIGURE 9.38 (a) Crystal Structure of Metal-Organic Polyhedron Composed of Cu(ll) and 5-Bromo-1,3-benzenedicarboxylate. 

FIGURE 939 (a) IZn O] cluster building block that comprises MOF-5. (b) Crystal structure of MOF-5. (Structures generated from CIF data). 

A detailed classification system for networks has been devised using three-letter codes to describe topologies, for example dia for networks based on the diamond structure, pcu for primitive cubic, bcu for body-centered cubic, and so forth.  

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Microporous inorganic materials dominated historically by the 2eohtes and alumosilicates, and the great variety of more recent nonoxide and coordination framework materials should also be mentioned here (171—174) but not discussed in detail. This type of molecular recognition is usually known as molecular sieving. 

In Sn(II) complexes, the tin atom was located above the di-Schiff base coordination plane, while in Sn(IV) complexes, it was coplanar with the imine coordination framework. The position of the metal was supported by X-ray data. For the compounds studied, the 119Sn chemical shift values varied from 501.4 to —1015.9 ppm. Increase in the coordination number from Sn(II) to Sn(IV) led to an increase in the tin shielding. The differences of up to 3.0 ppm between 5119Sn values for the complexes, being derivatives of R,R and S,S 1,2-diaminocyclohexane, were observed. 

Coordination frameworks of pyridazino[4,5-ri]pyridazine have revealed a pronounced ability for anion Jt interactions <06CC4808>. 

The above-mentioned interest in the synthesis of extended 1-D, 2-D and 3-D networks containing SCO centres, which is driven largely by a desire to maximise steric and electronic cooperativity between metal centres, has arisen in parallel with a much broader range of research interests in 1-D, 2-D and 3-D coordination framework materials. Within this broader per-... 

The transformation T we adopt is induced by the wave function normalization condition which, in terms of the weights, reads w + W3 = 1. From (3.5), it is apparent that if T sends the vvm set into a new set wm with ivi = vvi + iv3 = 1 as one of its elements, then both the first row and the first column of the transformed polarization component of the solvent force constant matrix K, "/ = T. Kp°r. T (T = T) are zero, since the derivatives of wi are zero. Given the normalization condition and the orthogonality requirement — with the latter conserving the original gauge of the solvent coordinates framework — one can calculate T for any number of diabatic states [42], The transformation for the two state case is... 

Fig. 2.12 Force balance on a small planar surface of area A that is oriented with an outward-normal unit vector n in an orthogonal cylindrical coordinate framework.

The influence of calcination and different hydrothermal treatments on the coordination of aluminum in MCM-41 molecular sieves is investigated by 27A1 MAS NMR and FTIR spectroscopy using ammonia as probe molecule. Samples are further characterized by XRD and 29Si MAS NMR spectroscopy. It is found that non-tetrahedral Al species formed during calcination can be reversible transformed into tetrahedral coordinated framework Al by hydrothermal treatment. Re-inserted Al gives rise to Bronsted acidity. 

These NMR results are confirmed by the IR spectra of ammonium-exchanged Al-MCM-41. In the spectral range of NH deformation modes, a vibration band at ca. 1450 cm 1 is observed which is assigned to the deformation mode of ammonium ions (BS band). Its presence evidences the incorporation of A1 into tetrahedrally coordinated framework positions, because ammonium ion exchange requires a negatively charged framework. The intensity of the NH4+ deformation mode increases with increasing aluminum content until a Si/Al (gel) ratio of 7.3. 

In oxazole (167) (95MI5) and isoxazole (168) [88ZN(B)328] N-M coordination prevails. The possibility of O coordination (169) is less likely. Examples of the C-coordinated derivatives of oxazole (170) and isoxazole (171) are known [89JOM(372)287]. The complexes of composition CrL2 based on 3-methyl-5-phenyl- and 3,5-diphenylisoxazole were assigned a polymeric structure with the -coordinated framework (78ZOB418). The other example is the interaction of the cyano complexes [M(CN) (cp)(dppe)] (M = Fe, Ru) or [Fe(cp)(dppe)(CNH)]Br with gem-dicyano-epoxide to afford the oxazol-2-yl complexes with the C-coordination mode... 

The scheme has been repeatedly questioned in the light of X-ray and IR spectroscopic results which do not support it. Based on their IR measurements, Jacobs and Beyer (204) proposed an extra-framework (A1,0) species acting as a Lewis acid, in place of the hypothetical three-coordinated framework Al. 

For our calculations we choose a spherical coordinate framework with the polar axis directed along the vector //eff- Then the basic vectors of the problem are expressed as... 

Resuming the main line of our consideration, let us show how to consistently take into account the effect of the particle magnetic anisotropy by solving the Brown equation (4.90). Taking n as the polar axis of the coordinate framework, we recover the situation considered as an illustration in Section II.B. Namely, the dimensionless particle magnetization is expressed as Eq. (4.54) and the particle energy as Eq. (4.55). Then the nonstationary solution of the kinetic equation (4.90), which is equivalent to Eq. (4.27), is presented in the form of expansion (4.56) whose amplitudes satisfy Eqs. (4.60) and (4.61). 

Remark 2. For a mechanically unmovable particle, the Landau-Lifshitz equation (4.3) is written in the coordinate framework Ojdy z , which is quiescent with respect to the crystal axes of the sample. As soon as one considers a ferroparticle suspended in a liquid matrix, then the rotational degrees of freedom... 

Formula (4.370) leads to a simple expression for the macroscopic optical anisotropy. If the Oz axis of the coordinate framework coincides with the applied held direction h, then the uniaxial symmetry condition yields (n2) = (n2) and vx = vy. Hence for the refractive index anisotropy (birefringence) we have... 

NMR-characterization. 27 Corbin et al. (35) were able to show by a systematic study that Z/A1 MAS NMR gives the true Si/Al ratio with a mean error of 10 %, if two conditions are met a) The amount of paramagnetic species is less than 0.05 % and b) the sample does not contain "NMR-invisible" aluminium. Chemical analyses of the samples under study showed that condition one is fulfilled. If samples contain "NMR-invisible" aluminium a difference between the concentration determined by chemical analysis and the framework aluminium concentration determined by NMR should be observed. From the absence of such a difference we conclude that "NMR-invisible" aluminium species do not exist in our samples. Also a line at the position of about 0 ppm due to octahedrally coordinated non-framework aluminium and a broad line at about 30 ppm due to tetrahedrally coordinated nonframework aluminium (36) could not be observed. The values for the concentration of framework aluminium atoms derived from the intensities of the line at about 60 ppm (see below) are in good agreement with those corresponding to the amount of alumina used in the synthesis mixtures. In conclusion, through the Al MAS NMR measurements it was possible to show that all aluminium atoms are incorporated in tetrahedrally oxygen coordinated framework positions. 

In order to check this conclusion further, a number of (Si,B)-ZSM-5 zeolites were prepared via direct synthesis. The crystals of B-ZSM-5 are all very well developed with the length of about 200 pm (Figure 2). In the as-synthesized (Si,B)-ZSM-5 the chemical shift of boron is -3.95 ppm with FWMH=94 Hz [Figure 3 (b)]. The signal is narrow and symmetric because of small quadrupole interactions of tetrahedral B. The signals at -3.9 ppm in the spectra of boronated samples are also narrow (91 to 115 Hz, see Table I and Figure 1) and are clearly due to tetrahedrally coordinated framework boron. 

Properties of zeolites are intimately related to the type of occupancy of the tetrahedral sites. Modification of the composition of the framework by increasing the silicon content increases the thermal stability of the samples. The catalytically active centres in zeolites are the acidic (Bronsted) hydroxyl groups associated with tetrahedrally coordinated framework aluminium atoms. Catalytic activity is thus strongly dependent on the concentration and location of aluminium in the framework. 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

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