Layered crystalline calcium phenylphosphonate—synthesis, characterization and n-alkylmonoamine intercalation
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Introduction
Metal phosphonates constitute a class of layered crystalline compounds in which the organic groups are covalently attached to the inorganic backbone in dense packing with an inorganic arrangement. This kind of compound forms a relatively new class of layered structures, consisting of a sequence of alternating inorganic and organic layers, and has received significant interest in recent years, as well-established in excellent reviews [1], [2]. The beginnings of this research field involved zirconium phosphonate [1], [3] and the chemical properties of these new compounds stimulated extensive exploitation [1], [4].
The great advantage of these compounds comes, firstly, from their preparative procedures, at low temperatures, enabling the incorporation of functional groups into the layers without disturbing the inorganic backbone. These compounds are also potentially useful for supramolecular assembly [1]. Nowadays, such sets of properties cause relevant interest for the materials chemist. Thus, a number of compounds exhibit properties such as ion exchangers, sorbents, sensors, proton conductors, nonlinear optical materials, photochemically active materials, catalysts, and hosts for intercalation of a variety of guest molecules [1], [2], [4], [5], [6].
The majority of organophosphonate compound preparation methods employ the direct reaction of a metallic salt, with the desired alkyl- or arylphosphonic acids [1], [2], [7], [8], [9]. The first organic derivatives were focused on tetravalent metals, as previously observed with phosphates, whose structure determination as α-Zr(C6H5PO3)2 was first studied through the powder diffraction method [10].
Research groups devoted considerable attention to preparations of new materials prior to exploring intercalation reactions with zirconium phosphonates. As occurred with the great majority of lamellar compounds, the intercalation process does not affect the original inorganic structure, or the van der Waals interactive forces, which normally maintain the sequence of lamella. However, polar organic molecules can be inserted inside the interlamellar position through an acid-base interactive process. Such an operation makes orientation easy by accommodating them into the interlayer space. An important property associated with the layered compounds is the maintenance of the final layered structure with the entrance of the guest molecule, which causes an increase in the interlayer distance, during intercalation into the cavity [1], [2], [6].
A decade after discovery of zirconium phosphonate a series of similar divalent compounds, with the general formula M(O3PR)·H2O have been synthesized. The majority of them have six coordinated metal atoms [1] and some layered alkaline earth organophosphonates, such as magnesium [11], calcium [12] and barium [13], were also studied. However, a particular interest is devoted to the chemistry of calcium phosphonates as part of their use as drugs for diagnosis and therapy for bone diseases and also for calcium metabolism. Recently, anhydrous calcium phenylphosphonate has had its structure determined, showing the inorganic framework of CaO8 polyhedra with the metal in coordination eight and the phenyl groups pointing out of the layer [14].
The aim of this publication is to explore the syntheses and characterization of the compound Ca(HO3PC6H5)2·2H2O, its anhydrous derivative, and the structural effect caused by n-alkylmonoamines interaction into the free interlamellar space.
Section snippets
Chemicals
All chemicals used were of reagent grade. Doubly distilled water was used throughout the experiments. Phenylphosphonic acid (Aldrich), calcium chloride (Fisher), and sodium hydroxide (Vetec) were used for all preparations. Amines (Fluka) of the general formula CH3(CH2)nNH2 (n=0 to 4), that is, methyl-, ethyl-, n-propyl-, n-butyl- and n-pentylamines, were used without further purification.
Synthetic procedure
Hydrated calcium phenylphosphonate (CaPP1) was prepared by reacting 60.0 cm3 of a 1.0 mol dm−3 aqueous
Results and discussion
Calcium, phosphorus, carbon and hydrogen elemental analyses for the synthesized compound CaPP1 were determined as 10.2, 15.8, 37.0 and 3.1%, respectively. These values are very close to the calculated amounts of 10.25, 15.90, 36.96 and 3.12% for the proposed empirical formula Ca(HO3PC6H5)2·2H2O, as listed in Table 1. It can be dehydrated on heating, enabling the preparation of the anhydrous compound [14]. However, other chemical layered compounds such as Ca(O3PR)2 [12], [14], Ca(HO3PR)2·3H2O
Conclusions
Anhydrous and hydrated lamellar crystalline calcium phenylphosphonates can be used as host supports for organic polar molecules. From 31P NMR the hydrated form of this host compound presents two chemically non-equivalent groups of phosphorus and, from elemental analysis, the minimum formula of Ca(HO3PC6H5)2·2H2O was established.
An increase in amine concentration causes a decrease in the interlayer distance, to reach a constant distance in a well-defined plateau due to the substitution of water
Acknowledgements
The authors are indebted to CNPq for fellowships and to FAPESP for financial support.
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