Zirconium Phosphonates of aType

The preparation of Zr phosphonates is closely related to the methods employed for the preparation of a-Zr(HPO4)2 • H2O, i.e. refluxing of amorphous precipitates with solutions containing the chosen phosphonic acids, and the direct precipitation method in the presence of Zr fluorocomplexes and the suitable H2O3PR acid.

As already mentioned, the layer structure arises from the concatenation of ZrO6 octahedra and O3PR tetrahedra similar to that present in a-zirconium phosphate. Due to the short lateral distance between adjacent O3P-R groups on each side of the a-layer

(5.3 A) interpenetration of the R-groups belonging to adjacent layers cannot occur for steric reasons and a double film of R-groups is expected for all the members of this class. Therefore these organic derivatives have a layered structure similar to that of zirconium benzenephosphonate (see Figure 13) or zirconium carboxyethanphosphonate (see Figure 14), two typical compounds of the class.

A list of selected a-layered phosphonates is given in Table 4. Note that the compounds contain a variety of functional groups. By choosing appropriate organic groups attached to the phosphorus atom, it is possible to vary the acid properties of the phosphon-ates from neutral (e.g. P-CH3) or weakly acid (e.g. P-CH2COOH) to strongly acid (e.g. P-C6H4SO3H) or even to basic (e.g. P-C2H4NH2), or to anchor the amino acid chiral group. The nature of the covalently

Figure 14 Computer-generated representation of the sequence of two layers of a-Zr(HOOCCH2CH2PO3)2. (Data from Alberti G, Costantino U, Casciola M, Vivani R and Peraio A (1991) Solid State Ionics 46: 61-68, with permission from Elsevier Science.)

present on the surfaces of the layers of the parent a-Zr(HPO4)2. However, more voluminous groups may be attached to the a-layers if their dimensions are compensated by introducing small groups R' (R' being H, OH, CH3) to obtain compounds of formula Zr(RPO3) 24J(R'PO3)x. These mixed component phases are of great interest since a very special type of complexing agent or redox couple may be fixed to the layers. A selection of the multicomponent phases prepared to date is given in Table 5.

Zirconium diphosphonates, of general formula Zr(O3P-R-PO3), in which adjacent inorganic layers of the a-type are covalently joined to each other by divalent organic groups, may also be obtained. These zirconium phosphates do not possess interlayer micro-porosity, because the distance between adjacent pillars is 5.3 A and the van der Waals diameter of the alkyl or aryl pillar is about 4 A. It is however possible to create microporosity in the interlayer region if some pillars are replaced by small O3P-H groups, and if the pillar has been suitably designed. By using a pillar with bases, such as 3,3',5,5'-tetramethylbiphenyl-diphosphonic acid, a pillared compound exhibiting a high phosphite percentage and interlayer micro-porosity of 375m2g4J, has recently been prepared.

attached groups depends on the imagination and ability of the chemist to synthesize the appropriate phos-phonic acids.

The only limitation to synthesis is the use of organic groups with a cross-section equal to or less than 24 A2. This is the free area around each P-OH group

Table 4 Interlayer distances of some zirconium £v's-monophos-phonates and organophosphates with a-layered structure

Compound

Interlayerdistance (As)

Zr(O3PCH3)2

8.9

Zr(O3PCH2OH)2H2O

10.1

Zr(O3PCH2Cl)2

10.1

Zr(O3PCH2CN)2

10.8

Zr(O3PC3H7)2

14.0

Zr(O3P(CH2)2COCl)2

13.5

Zr(O3PCH2COOH)2

11.3

Zr(O3P(CH2)2COOH)2

13.0

Zr(O3P(CH2)3COOH)2

15.0

Zr(O3PCH=CH2)

10.6

Zr(O3PCH2SO3H)2

15.4

Zr[(O3PO)(CH2CH2O)nPO3]

Zr[(O3PO)(CH2CH2NH2)2] • 2HCl

14.3

Zr[HOOCCH(NH2)CH2OPO3]2

14.5

Table 5 Compositions and interlayer distances of some derivatives of a-zirconium phosphate with two different pendant groups

Compound

Interlayer

distance (A)

ZKO3POHWO3PHK34

6.5

Zr(O3POH)1.15(O3PC6H5)0.85

12.4

Zr(O3POH)(O3PC2H4COOH)

12.9

Zr(O3PCH2OH)(O3PH)

7.0

Zr(O3PC2H4COOH)1 25(O3PCH2OH)0 75

13.6

Zr(O3PC6H5)(O3PH)

10.5

Zr(O3PC6H4SO3H)0 85(O3PC2H5)1 15 • 3.7H2O

18.5

Zr(O3PC6H4SO3H)0.97(O3PCH2OH)1.03 • 4.9H2O

19.6

A computer-generated structural model of this micro-porous pillared compound is shown in Figure 15.

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