18.5.2
Halides
In 1997, Martin and Greenwood [104] introduced a new generation of microporous
compounds utilizing halides as replacements for the normal oxygen or chalcogenide anions. New materials were formed from metal halides such as zinc dichloride and copper chloride; Zn and Cu are both known to exhibit tetrahedral geometries with chlorine. Three compounds, named CZX-n, have been synthesized
under solvothermal conditions (160
C) in benzene. The framework of CZX-1 is
isostructural with the aluminosilicate sodalite (SOD), while CZX-2 and CZX-3
adopt new framework topologies. All three structures are based on ZnCl 4 and
CuCl 4 tetrahedra sharing corners, as in the aluminosilicate zeolites, and the organic molecules are trapped in the micropores. One interesting property of these
microporous halides is their high capacity to adsorb methanol into the framework.
They can adsorb up to three equivalents per framework unit, but the addition of
more methanol results in dissolution of the structure and the formation of a colloidal solution.
18.5.3
Nitrides
The existence of condensed framework structures based upon ceramic nitrides, e.g.
the silicon nitrides of composition Si 3 N 4 , led to the idea that the synthesis of openframework nitrides might also be feasible. The first success in this area was the
preparation of the phosphorus nitride, Zn 7 [P 12 N 24 ]Cl 2 , by Schnick in 1992 [105].
This was achieved by means of a high temperature (7–800
C) solid state reaction
between ZnCl 2 and HPN 2 , and yielded a small pore open-framework with the sodalite structure. A similar reaction, in which a source of oxygen (OP(NH 2 ) 3 ) was
also included in the reaction mixture, has more recently led to the discovery of a
phosphorus oxonitride with the same structure [106]. The preparation of openframework silicon nitrides requires more severe conditions, but Schnick successfully achieved this objective in 1997 with the synthesis of the small pore, zeolitic
Ba 2 Nd 7 Si 11 N 23 [107]. In this instance, reaction between the metals and silicon
imide, Si(NH) 2 , under a nitrogen atmosphere in an RF furnace operating at temperatures between 900 and 1650
C, was necessary. By analogy with the progress in
other areas, it seems probable that much milder reaction conditions will be necessary if the preparation of large pore nitrides is to be achieved.
18.5.4
Binary Metal Oxides
Whereas the development of crystalline, open-frameworks based upon silicates,
phosphates and related materials has progressed at an ever increasing pace, the
synthesis of simple binary oxides with periodic open structures has been less suc18.5 Chalcogenides, Halides, Nitrides and Oxides 607
Halides
In 1997, Martin and Greenwood [104] introduced a new generation of microporous
compounds utilizing halides as replacements for the normal oxygen or chalcogenide anions. New materials were formed from metal halides such as zinc dichloride and copper chloride; Zn and Cu are both known to exhibit tetrahedral geometries with chlorine. Three compounds, named CZX-n, have been synthesized
under solvothermal conditions (160
C) in benzene. The framework of CZX-1 is
isostructural with the aluminosilicate sodalite (SOD), while CZX-2 and CZX-3
adopt new framework topologies. All three structures are based on ZnCl 4 and
CuCl 4 tetrahedra sharing corners, as in the aluminosilicate zeolites, and the organic molecules are trapped in the micropores. One interesting property of these
microporous halides is their high capacity to adsorb methanol into the framework.
They can adsorb up to three equivalents per framework unit, but the addition of
more methanol results in dissolution of the structure and the formation of a colloidal solution.
18.5.3
Nitrides
The existence of condensed framework structures based upon ceramic nitrides, e.g.
the silicon nitrides of composition Si 3 N 4 , led to the idea that the synthesis of openframework nitrides might also be feasible. The first success in this area was the
preparation of the phosphorus nitride, Zn 7 [P 12 N 24 ]Cl 2 , by Schnick in 1992 [105].
This was achieved by means of a high temperature (7–800
C) solid state reaction
between ZnCl 2 and HPN 2 , and yielded a small pore open-framework with the sodalite structure. A similar reaction, in which a source of oxygen (OP(NH 2 ) 3 ) was
also included in the reaction mixture, has more recently led to the discovery of a
phosphorus oxonitride with the same structure [106]. The preparation of openframework silicon nitrides requires more severe conditions, but Schnick successfully achieved this objective in 1997 with the synthesis of the small pore, zeolitic
Ba 2 Nd 7 Si 11 N 23 [107]. In this instance, reaction between the metals and silicon
imide, Si(NH) 2 , under a nitrogen atmosphere in an RF furnace operating at temperatures between 900 and 1650
C, was necessary. By analogy with the progress in
other areas, it seems probable that much milder reaction conditions will be necessary if the preparation of large pore nitrides is to be achieved.
18.5.4
Binary Metal Oxides
Whereas the development of crystalline, open-frameworks based upon silicates,
phosphates and related materials has progressed at an ever increasing pace, the
synthesis of simple binary oxides with periodic open structures has been less suc18.5 Chalcogenides, Halides, Nitrides and Oxides 607