Phosphonate Layer Structures

Much interest has focussed on phosphonate layer structures of various kinds. This has been occasioned by the search for new catalysts, molecular sieves, NLO-type materials, and so on. 

Various zirconium phosphonate layer complexes of type Zr(RP03)2, where R = Me, Pr, Ph, CgHn, etc. have been prepared as well as ester layer complexes of type Zr(RO P03)2, R = alkyl, aryl. These 

Zirconinm snlphophosphonates snch as Zr(03P CgH4 S03H)2 have been prepared and some of these have a high proton condnctivity. 

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Zinc biphenylenebis(phosphonate) was obtained with a linear chain rather than a layer structure when the same preparative method was used as for the zinc phenylenebis(phosphonate) layer structure. The presence of base allows a solid phase transformation from the linear chain to the layer structure. 

Fig. 42.10 Ligands anchored to zirconium phosphonate-layered structures or titanium dioxide particles.

The crystal structure of zinc phenylphosphonate was solved and demonstrated the anticipated layer structure with the phenyl rings occupying the interlamellar space.409 The structure of zinc ethylphosphonate and (2-aminoethyl)phosphonate both show four-coordinate zinc centers in contrast to the coordination number of six in the phenylphosphonate compound.410... 

Organic phosphonates represent another class of anchoring agents, which react with zirconium hydroxide to form pillared structures. These are also referred to as molecularly engineered layered structures (MELS). Layered compounds of organic phosphonates of zirconium with the formula of Zr(RP03)2 have been rec-... 

Metal phosphonates are usually prepared by reaction of phosphonic acids with metal salts under hydrothermd conditions [5-7]. Layered structures are predominant for most metals, the organic group being oriented more or... 

Layered structures are also common in trivalent metal phosphonates [16-18], and practically the sole ones in tetravalent metal phosphonates. Most metal IV phosphonates, such as Zr(03PPh)2 [19, 20], present a structure in which the metal atom is coordinated to six different phosphonate groups through the oxygen atoms. 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer structures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer structure with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

Amino-carboxylic and phosphonic acids are known to form open frameworks (i.e. porous materials), particularly with first-row transition metal ions. Lanthanide diphosphonates with 3D pillared-layer structure are also known and several classes of ligands such as sim-... 

Fig. 3 A side view of the layered structure of zirconium bisfphenyl-phosphonate), with edge-to-edge interactions between phenyl rings on neighboring sheets.8 Green spheres denote phosphorus, red oxygen, and gray carbon, with ZrO,. octahedra in blue.

The metal phosphonate compounds usually adopt layered or pillared layered structures with the organic moieties filling in the inter-layer spaces [1-4]. The layered nature makes them interesting candidates to host intercalation reactions. Furthermore, the potential for the organic moieties to be modified by functional groups allows for the preparation of a number of new materials that have possible applications in areas such as catalysis, ion exchange, sensing and ion conduction [1,4-7]. 

Isostmctural M (HP04)2-H20 materials have been obtained with M = Ti, Hf, Ce, and Sn and SbH(P04)2 H2O also adopts this layered structure. In addition, a large number of layered phosphonates M (RP03)2 have been prepared in which the alkyl groups R lie between the inorganic layers in place of the OH groups in the hydrogenphosphates. 

The sample studied was a cobalt bis(phosphonate) of composition C02(O3PC6H4OC6H4PO3)-2H2O. It is one of the series of first row transition element compounds with the same general formula, but different water contents. The Cu(ll) compound structure was solved from its powder pattern, unit cell dimensions a = 8 1012(5), b = 5.3109(3), c = 29.2595(5) A. It has a layered structure in which the layers are spaced at half the c-axis dimension. The reason for the doubling of the c-axis is that in one layer the rings are tilted around the ether oxygen to the left and in the next layer to the right. The Cu atoms are 5-coordinate with square pyramidal structure. 

In contrast to the conventional approach whereby various organic groups are subsequently bound to a previously prepared surface, we have been synthesizing a broad series of anchored, layered-structure solids by precipitating the pre-derived phosphonate salts with tetravalent metal ions. The two-dimensional backbone has the zirconium phosphate structure however, substituted for hydroxylic groups are the desired organics, oriented away from the basal surfaces in a bilayered fashion in the interlayer region. 

The only anomaly we encountered, apparent in the Table, was the finding that the three-carbon bis-phosphonic acid product, which had a good elemental analysis, did not exhibit any XRD lines. We conjecture that this apparent inability to form a crystalline product results from the fact that there is no obvious way to force a conformation on a three carbon chain so that the terminal phoshonates will be in a parallel configuration, as required by a layered structure. This, then, may be an example of an intrinsically amorphous product. 

A microporous structure of one-dimensional hydrophobic channels lined by methylene groups is formed by CozCOj-GHj-POa) H O [120], but the vanadyl compound [V0]2[03p-CH2-P03l 4H2O has a layer structure. Phosphonates form various types of cavity structures [121,122],... 

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