Layered crystalline calcium phenylphosphonate—synthesis, characterization and n-alkylmonoamine intercalation

doi:10.1016/S1293-2558(02)00009-2

Solid State Sciences

Volume 4, Issue 10, October 2002, Pages 1321-1329

https://doi.org/10.1016/S1293-2558(02)00009-2 Get rights and content

Abstract

Layered calcium phosphonate was synthesized by means of the combination of a metallic salt with an aqueous phenylphosphonic acid solution to yield Ca(HO₃PC₆H₅)₂·2H₂O (CaPP1), which gives the corresponding anhydrous compound on heating. A series of n-alkylmonoamines were intercalated into the crystalline lamellar precursor, resulting in compounds with the general formula Ca(HO₃PC₆H₅)₂·xH₂N(CH₂)_nCH₃·(2−x)H₂O (n=0 to 4). Sharp and intense peaks obtained from X-ray diffraction patterns for both hydrated and anhydrous compounds are in agreement with the crystallinity presented. Thermogravimetric data showed mass losses corresponding to the release of two water molecules, in distinct stages between 300 to 520 K, followed by the organic moiety in the 570 to 880 K interval, to form the compound Ca(O₃P)₂. Two distinct DSC peaks corroborated the presence of the water molecules, in contrast to the absence of similar peaks in the anhydrous compound. The carbon:hydrogen molar ratio found for CaPP1 is 1:1 and the infrared spectrum shows the characteristic bands of the phosphonate group. The ³¹P NMR spectrum presented two distinct peaks for bonded phenylphosphonate groups at 20.0 and 12.4 ppm, due to the non-equivalence of the phosphorus atoms in the crystal. Elemental analyses for the intercalated compounds presented an increase of carbon and hydrogen percentages as the size of the amine chain increased, accompanied by a corresponding decrease of the percentage of nitrogen. The interlayer distance correlates with the number of carbon atoms of the n-alkylamine chains, a correlation which enabled inferring the interlayer distance for an unknown amine.

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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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Layered crystalline calcium phenylphosphonate—synthesis, characterization and n-alkylmonoamine intercalation

Abstract

Introduction

Section snippets

Chemicals

Synthetic procedure

Results and discussion

Conclusions

Acknowledgements

Solid State Ionics

Solid State Sci.

Inter. J. Inorg. Chem.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Polyhedron

J. Mater. Chem. Phys.

Inorg. Chim. Acta

Thermochim. Acta

Mater. Res. Bull.

J. Solid State Chem.

Progr. Inorg. Chem.

Inorg. Chem.

Acc. Chem. Res.

Chem. Rev.