Chapter 9
Multinuclear NMR Study of Host-Guest Interactions in Sodalites
Downloaded by STANFORD UNIV GREEN LIBR on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch009
G. Engelhardt Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany
The framework of sodalites consists of a perfectly periodic array of all-spacefilling[4 6 ]polyhedra (sodalite orß-cages)formed by a network of corner-sharingTO tetrahedra (T = Si and/or Al). The cages may accomodate a large variety of guest species such as cations, anions, water, or organic molecules. The interactions of the guests with the host framework in aluminosilicate (T = Si, Al), and silica sodalites (T = Si) are studied by multinuclear solid-state NMR spectroscopy. Si and Al NMR provide information on the framework structure, and Na, H, and C NMR is applied to characterise the cagefillings.Structural phase transitions of the hostframeworkand the dynamics of the guest species are studied by variable temperature NMR. The potential of NMR in providing detailed information on the sodalite structure which complements the results of diffraction techniques is demonstrated by selected sodalite compositions containing various cations and anions, water, organic molecules, or paramagnetic Na clusters as guest species. 6
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The host structure of sodalites consists of a three-dimensional, four-connected tetrahedralframeworkformed by comer-sharing T0 tetrahedra. While sodalite-type frameworks are known for a large variety of T-atoms (e.g., Be, Zn, B, Ga, Ge, P, As, Si, Al), in the present study only alummosilicate sodalites (T = Si, Al), and silica sodalites (T = Si) are considered. Figure la shows the basic polyhedral building unit of sodalites known as sodalite cage or β-cage. The T-atoms form [4 6 ] polyhedra (truncated octahedra) which belong to thefiveFedorov solids and can be connected to a space-filling array as shown in Figure lb. The cages may enclathrate a vast variety of cationic, anionic and/or neutral guest species as depicted in one of the cages in Figure lb by a central anion tetrahedralry surrounded by four cations. Depending on the type and distribution of guest species, sodalites may exhibit unique optical, electronic, magnetic and other interesting properties rendering this class of compounds attractive, e.g., as matrices for metal and semiconductor clusters in the 4
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©1998 American Chemical Society In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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267 quantum size regime ("nanocomposites"). Such clusters and other guest species may be formed by selective chemical reactions inside the β-cages and the related "intracage chemistry" has recently found considerable interest. It is obvious that the specific structure and properties of a certain sodalite composition are strongly affected by the interaction between the host framework and the enclathrated guests. The steric and coordination requirements of the guest species together with Coulomb and Van der Waals interactions or hydrogen bonds are important factors determining the resulting structure. A number of powerful techniques are available to investigate host-guest interactions in sodalites which include x-ray and neutron diffraction, elastic and inelastic neutron scattering, IR and Raman spectroscopy, EXAFS and XANES, and others. A valuable complement to these techniques is solid-state NMR spectroscopy which provides detailed information on the local structure around the respective NMR nucleus (/). The sodalite framework can be studied by Si and Al NMR, and even 0 NMR may be applied but 0 isotopic enriched materials have to be used rendering this technique expensive and impracticable for routine use. A number of other nuclei are suitable to investigate the guest species, e.g., Na for the Na cations, *H for Hfi, OH" and other proton containing species, and C for organic molecules. In this paper, solid-state NMR spectroscopy is applied to study selected sodalite compositions comprising aluminoslilicate sodalites (T = Si, Al) containing mainly Na* cations, mono- as well as divalent anions, and paramagnetic Na clusters, and silica sodalites (T = Si only) enclathrating various organic guest species. The work presented here has been carried out in close cooperation with a number of scientistsfromdifferent laboratories. Their names and locations are given at the beginning of each subchapter. 29
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Aluminosilicate sodalites with diamagnetic guest species (J.Felsche, P.Sieger, Konstanz, Germaixy)
The majority of aluminosilicate sodalites have a 1:1 ratio of strictly alternating silicon and aluminum atoms in theframeworkand posses a cubic structure. The negative charges introduced by the (A10 )" tetrahedra and, if present, by additional guest anions in the sodalite cages are balanced by alkali metal cations, preferably by Na . The general unit cell composition is given by [Na A] [SiA10 ] for monovalent anions A, and [Na A'][Na ][SiA10 ] for divalent anions A'. In this notation, the first brackets contain the cagefillings,while the last brackets describe the sodalite framework. Aluminosilicate sodalites can be prepared with a single type of anion or with different anions in the cages (see below). Furthermore, the β-cage can also contain hydroxy groups and/or water molecules according to the general composition [Na ^(OH) (H 0)J [SiA10 ] (2). 4/2
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Si NMR of the aluminosilicate framework. The Si MAS NMR spectra of cubic aluminosilicate sodalites with uniform cagefillingsconsist of a single narrow line (typical linewidth 0.5-2 ppm), indicating the crystallographic equivalence of all Si atoms in theframework.The position of this line, i.e. the Si chemical shift, 6(Si), depends sensitively on the type of guest species within the cages. It has been shown already in earlier work (3) that this dependency is related to changes in the SiOAl bond angles due to expansion or contraction of the β-cages induced by the different space requirements of the cage contents. Figure 2 presents a plot of 5(Si) versus the SiOAl bond angle, a, for 33 29
In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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Figure 1. The structure of sodalites. (a) Single sodalite cage, (b) space filling array of sodalite cages (oxygen atoms omitted).
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Figure 2. Plot of Si chemical shifts versus SiOAl bond angles of alummosilicate sodalites with different cage contents. The guest species in the 0-cages are indicated for selected compositions.
In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
269 sodalite compositions takenfromour own work andfromthe literature (3, 4, 5, 6). The shift range extendsfrom-76.4 ppm for lithium chloride sodalite to -97.3 ppm for potassium perchlorate sodalite (5), corresponding to a range of α from 125.6° to 156.6°, respectively. The linear correlation between 8(Si) and α observed previously for a smaller set of data (3) is clearly confirmed: linear regression yields 5(Si) =-0.62oc -1.09 with a correlation coefficient of 0.991 and a standard deviation of 0.62 ppm Since, in cubic structures, α is related to the lattice parameter, a
