31P MAS-NMR study of R′xR″yP6O18.nH2O cyclohexaphosphates

Journal of Alloys and Compounds 325 (2001) 102–108

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P MAS-NMR study of R 9x R 99 y P6 O 18 .nH 2 O cyclohexaphosphates a, a b b C. Ben Nasr *, M. Rzaigui , I. Sobrados , J. Sanz a

´ Laboratoire de Chimie des Materiaux , Faculte´ des Sciences, 7021 Zarzouna-Bizerte, Tunisia b Instituto of Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 19 February 2001; accepted 3 April 2001

Abstract 21 1 21 Several alkali / alkaline-earth R 1 5Ca, Zn, Cd, x R y P6 O 18 .nH 2 O cyclohexaphosphates, with R 5Li, Na, K, Cs, NH 4 , Tl, DBA; R 31 have been analyzed by P MAS-NMR spectroscopy. From their NMR spectra, the number and the multiplicity of crystallographic sites have been analyzed and compared with those deduced by symmetry considerations from X-ray diffraction data. A linear dependence of the isotropic chemical shift values of NMR components on the POP angles of cycles has been obtained. However, some influence of the charge and polarizing strength of alkali and alkaline-earth cations has also been detected. Finally, principal values of the chemical shift tensor of P atoms have been analysed in terms of local symmetry and tetrahedral ring distortions.  2001 Elsevier Science B.V. All rights reserved.

Keywords: Inorganic materials; Synthesis; Crystal structure; X-Ray diffraction; Nuclear resonance

1. Introduction Cyclophosphates having a steady polymeric structure are appreciated as ion exchangers, complexing agents, detergents, catalysts, adhesives, etc. [1,2]. In the case of alkaline / alkaline-earth cyclohexaphosphates, the structure is composed of hexagonal rings formed by PO 4 tetrahedra, sharing two oxygens with tetrahedra of the ring and two oxygens with alkaline or alkaline-earth cations. In these compounds, P–O bonds joining neighbouring tetrahedra ˚ are larger than those joining tetrahedra to alkali (|1.60 A) ˚ Depending on distoror alkaline-earth cations (|1.48 A). tions of hexagonal rings, symmetry of cyclophosphates can ] ¯ to triclinic P1. vary from R3c These compounds have been extensively studied by X-ray diffraction, but few studies have been carried out with spectroscopic techniques [3–5]. High resolution NMR spectroscopy is a powerful technique for characterization of phosphates. From chemical shift values of NMR components, structural information related to tetrahedral condensation, network distortions or cations sharing oxygens with tetrahedra has been obtained [6–9]. The analysis of chemical shift anisotropies has also

*Corresponding author. E-mail address: [email protected] (C. Ben Nasr).

been used to analyze distortions of individual tetrahedra in crystalline compounds [10,11]. The aim of this work is the study of alkali / alkaline-earth cyclohexaphosphates by 31 P NMR spectroscopy. In particular, the influence of different structural factors on the chemical shift values of NMR components will be analyzed. For that, structural information previously reported, concerning crystallographic sites and arrangement of tetrahedra, will be used [12–21]. Finally, an analysis of local distortions produced in phosphorus tetrahedra, when alkali cations are changed, will be undertaken.

2. Experimental

2.1. Sample preparation The investigated compounds were obtained by neutralizing an aqueous solution of cyclohexaphosphoric acid with stoichiometric amounts of the appropriate carbonate M 2 CO 3 or MCO 3 or the amine RNH 2 . Details about the preparation of the different phosphates have been reported elsewhere [12–16]. Studied samples will be labelled in this work with the corresponding alkali or alkaline-earth symbols. The products obtained were examined by X-ray powder diffraction and infrared absorption techniques to confirm the single phase character. X-Ray diffractograms were

0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01393-7

C. Ben Nasr et al. / Journal of Alloys and Compounds 325 (2001) 102 – 108

recorded on a Philips PW 1050 / 70 diffractometer, with Cu Ka radiation at a rate of (1 / 8)8 min 21 . Infrared spectra, in the range 4000–200 cm 21 , were obtained on a PerkinElmer IR 783 spectrophotometer. Based on the structural data published, five types of cyclophosphates have been identified [12–25]. In Table 1, the structural formula, space group, unit cell parameters and number of crystallographic sites are given.

103

NMR components were determined from the position of the side band that does not change in spectra taken with different spinning rates. The mean error of the chemical shift values was 0.2 ppm. The analysis of MAS-NMR spectra was carried out using the Bruker program WINFIT [26]. For a given spinning rate, the intensities of the side bands are computed by Herzfeld and Berger’s method [27]. The optimization procedure is based on a non linear least squares method that fits the experimental envelope. In this process, the spinning rate and the position, linewidth and intensity of components are automatically determined. However, chemical shift anisotropies (Ds ) and asymmetry parameters (h ) of NMR components must be determined by a trial and error procedure. siso , Ds and h of each component are given in Table 2. For quantitative purposes, the sum of the integrated intensities of all side bands corresponding to each component have been calculated. Relative intensities of different components are given in the second column of the table.

2.2. NMR technique 31

P MAS-NMR spectra were obtained on a solid-state high-resolution Bruker MSL 400 spectrometer operating at 161.96 MHz. The spinning rates used were 4, 7 and 10 kHz. The p / 2 pulse was 6 ms and the time interval between successive scans, 50 s. The number of scans was in the range 40–400. All measurements were carried out at room temperature, with H 3 PO 4 (85%) as an external standard reference. Isotropic chemical shift values (siso ) of

Table 1 Structural formula, unit cell parameters, space group and number of crystallographic sites in cyclophosphates analyzed [12–25] Structural formula

Li 6 P6 O 18 .5H 2 O Na 6 P6 O 18 .6H 2 O K 6 P6 O 18 .3H 2 O K 6 P6 O 18 (C 5 H 6 NH 3 ) 6 P6 O 18 .6H 2 O (DBA) 6 P6 O 18 .6H 2 O Li 3 Na 3 P6 O 18 .12H 2 O Li 3 K 3 P6 O 18 .H 2 O Li 3 Rb 3 P6 O 18 .H 2 O Li 3 Cs 3 P6 O 18 .2H 2 O Li 3 (NH 4 ) 3 P6 O 18 .H 2 O Li 3 Tl 3 P6 O 18 .H 2 O Li 2 (NH 4 )P6 O 18 .4H 2 O

Unit cell parameters a a

b b

9.490 107.16 11.582 6.803

8.069 113.84 18.544 17.45 107.18 15.753 11.669 110.78 23.957 106.44 10.474 15.047 15.100 7.687 93.86 15.013 15.060 15.824 106.26 15.669 106.82 16.042 107.03 7.528 115.68 9.172 105.28 11.862 103.22 8.764 106.63 11.501 90.80

15.753 12.369 114.91 10.725 10.474 15.047 15.100 7.948 113.4 15.013 15.060 9.429

Li 2 K 4 P6 O 18 .2H 2 O

7.401

Li 2 Cs 4 P6 O 18 .4 H 2 O

9.579

Li 2 Tl 4 P6 O 18 .2H 2 O

18.348

Na 4 ZnP6 O 18 .6H 2 O

9.276 71.63 7.309

K 2 Ca 2 P6 O 18 .6H 2 O K 2 Zn 2 P6 O 18 . 8H 2 O K 2 Cd 2 P6 O 18 .8H 2 O

7.234 114.14 7.446 97.56

Space group

Number of crystallographic sites

Ref.

] P1

3

[21]

Ccmb P2 1 /m

2 3

[22] [20]

Pa3

2 3

[23] [14]

3

[17]

1 1 1 3

[24] [24] [17] [13]

P1 R3 R3 P2 1 /c

1 1 3

[17] [17] [12]

8.539

P2 1 /a

3

[17]

7.890

P2 1 /c

3

[17]

15.170

C2 /c

3

[18]

7.484 115.55 12.335

3

[15]

P1 P2 1 /n

3

[19]

3

[25]

6

[16]

˚ c (A) g (8) 7.810 65.19 10.482 9.195 15.753 10.180 79.03 13.314 41.685 12.779 12.779 9.567 83.01 12.826 12.812 7.930

9.913 84.85 14.299 82.01

P1 P2 1 /m R3c R3 R3

P1 P1

C. Ben Nasr et al. / Journal of Alloys and Compounds 325 (2001) 102 – 108

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Table 2 Intensity (I), isotopic chemical shift (siso ), anisotropy (Ds ) and asymmetry (h ) parameters of 31 P MAS-NMR components in cyclophosphates Structural formula

I (%)

siso (ppm)

Ds (ppm)

h

Li 6 P6 O 18 .5H 2 O

31 69 37 63 33 33 33 11 58 15 15 33 33 34 32 68 100 50 50 100 32 34 34 100 50 50 21 21 58 28 36 36 36 33 30 30 31 38 30 36 33 31 39 37 28 36 36 14 14 22 30 9 8

220.2 222.5 222.8 218.0 219.2 221.5 222.3 218.3 218.9 225.5 226.5 220.7 219.8 223.5 221.5 227.1 220.5 222.6 223.8 225.1 216.7 219.8 221.3 225.1 225.4 226.8 217.9 223.3 224.8 219.3 221.3 222.1 218.7 221.5 224.6 222.0 224.1 225.5 218.7 219.8 224.7 217.5 219.4 220.4 223.0 224.9 226.2 218.8 228.0 222.4 224.8 226.5 228.6

2135 2135 2150 2134 2142 2135 2136 2150 2152 2141 140 2143 2145 2139 2134 2145 2122 2139 2140 2139 2137 2144 2145 2130 2139 2139 2130 2136 2136 2148 2138 2138 2142 2135 2136 2139 2146 2145 2128 2134 2130 2130 2139 2150 2139 2131 2124 2130 2142 2140 2133 2130 2137

0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.3 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.6 0.5 0.5 0.5 0.4 0.5 0.5 0.5 0.4 0.4 0.5 0.6 0.5 0.4 0.5 0.5 0.4 0.5 0.5 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.5 0.6 0.5 0.5 0.5 0.5 0.5 0.5

Na 6 P6 O 18 .6H 2 O K 6 P6 O 18 .3H 2 O

K 6 P6 O 18

(C 6 H 5 NH 3 ) 6 P6 O 18 .6H 2 O

(DBA) 6 P6 O 18 .6H 2 O Li 3 Na 3 P6 O 18 .12H 2 O Li 3 K 3 P6 O 18 .H 2 O Li 3 Rb 3 P6 O 18 .H 2 O Li 3 Cs 3 P6 O 18 .2H 2 O

Li 3 (NH 4 ) 3 P6 O 18 .H 2 O Li 3 Tl 3 P6 O 18 .H 2 O Li 2 (NH 4 )P6 O 18 .4H 2 O

Li 2 K 4 P6 O 18 .2H 2 O

Li 2 Cs 4 P6 O 18 .4 H 2 O

Li 2 Tl 4 P6 O 18 .2H 2 O

Na 4 ZnP6 O 18 .6H 2 O

K 2 Ca 2 P6 O 18 .6H 2 O

K 2 Zn 2 P6 O 18 .8H 2 O

K 2 Cd 2 P6 O 18 .8H 2 O

3. Results 31

P MAS-NMR spectra of the analyzed phosphates are given in Figs. 1 and 2. In all cases, the spectra are formed

by several lines in the 216 / 232 ppm range, with their corresponding satellite spinning side bands spaced at equal intervals (spinning rate of the sample expressed in ppm). As the chemical environment of P atoms is similar in these phosphates, the existence of several lines in the region analyzed indicates the presence of several crystallographic sites in the unit cell of these cyclophosphates. In particular, 31 P MAS-NMR spectra can be formed by a unique component (Li 3 K 3 and Li 3 Rb 3 of Fig. 1), two components with relative intensities 2.1 (Na 6 and K 6 of Fig. 1; Li 2 K 4 and DBA 6 of Fig. 2), three components with intensities 1:1:1 (Na 4 Zn and K 2 Zn 2 of Fig. 2) and six components (K 2 Cd 2 of Fig. 2). Spinning side band patterns are very large (100 / 2200 ppm), indicating that chemical shift anisotropies are much more important than residual P–P, P–M or P–H dipolar interactions. The 31 P NMR signals analyzed exhibit chemical shift patterns (experimental envelope) formed by two maxima, suggesting axial distortions in tetrahedra; however, in most cases patterns are more complex (0.4,h , 0.6) indicating that P environments display lower symmetries. Isotropic siso values, anisotropies Ds and asymmetry parameters h of NMR components are given in Table 2.

4. Discussion Cyclophosphates are formed by tetrahedra sharing two corners with neighbouring tetrahedra (Q 2 units in Lippmaa’s notation) [28], and two other ones interacting with alkali or alkaline-earth cations. According to this, the isotropic chemical shift values siso are higher than those corresponding to monophosphates (210 / 5 ppm) or diphosphates (5 / 20 ppm) of alkali or alkaline-earth cations [6– 11]. Chemical shift values of cyclophosphates are similar to those obtained previously in polyphosphates, indicating that siso values are mainly defined by tetrahedral condensation of phosphates. 31 P NMR spectra of the samples analyzed are formed by several components. As the chemical environment of all P atoms is similar in these phosphates, resolved components must correspond to different crystallographic sites occupied by phosphorus. In the case of cyclophosphates, it is interesting to relate the number of NMR components to the symmetry of hexagonal rings. In general, it is observed that the number of components increases when the symmetry of cyclophosphates decreases: 1. In phosphates with rhombohedral symmetry R3¯ (Li 3 Na 3 , Li 3 K 3 , Li 3 Rb 3 , Li 3 (NH 4 ) 3 and Li 3 Tl 3 ), the 31 P NMR spectra display only one line. According to the structure, the axis 3¯ could be disposed perpendicularly at the center of the rings. Sometimes, two poorly resolved components are detected, indicating the possible existence of a lower symmetry; however, the

C. Ben Nasr et al. / Journal of Alloys and Compounds 325 (2001) 102 – 108

105

Fig. 1. 31 P MAS-NMR spectra of Li 3 K 3 P6 O 18 .H 2 O (a), Li 3 Rb 3 P6 O 18 .H 2 O (b), Na 6 P6 O 18 .6H 2 O (c) and K 6 P6 O 18 .3H 2 O (d) cyclohexaphosphates, recorded at 4 and 7 kHz.

separation between components is always weak (Li 3 K 3 cyclohexaphosphate of Fig. 1a). 2. In the orthorhombic Na 6 , the symmetry of rings is 2 /m (space group Ccmb; Fig. 1c) and in the hydrated monoclinic K 6 , the symmetry is m (space group P2 1 /m; Fig. 1d). In both cases, spectra should be formed by two components with relative intensities 2:1; however the different broadening of two components suggests an incipient splitting of the larger component. 3. In the case of the remaining monoclinic cyclohex-

aphosphates: (Li 2 K 4 ; space group P2 1 /a), (Li 2 Cs 4 and Li 2 (NH 4 ) 4 ; space group P2 1 /c), (K 2 Ca 2 ; space group P2 1 /n), (Li 2 Tl 4 ; space2 group C2 /c) the internal symmetry of the cycle is 1 and the number of the lines in the NMR spectra should be 3. Once again, two of three lines are very close and their resolution is sometimes difficult (Li 2 K 4 of 2 Fig. 2a). 4. In the triclinic P1 cyclohexaphosphates, the phosphate rings are again centro-symmetric. From this fact, 31 P MAS-NMR spectra are formed by three components

Fig. 2. 31 P MAS-NMR spectra of Li 2 K 4 P6 O 18 .2H 2 O (a), DBA 6 P6 O 18 .6H 2 O (b), Na 4 ZnP6 O 18 .6H 2 O (c), K 2 Zn 2 P6 O 18 .8H 2 O (d) and K 2 Cd 2 P6 O 18 .8H 2 O cyclophosphates, recorded at 4, 7 and 10 kHz.

C. Ben Nasr et al. / Journal of Alloys and Compounds 325 (2001) 102 – 108

106

Table 3 ˚ OPO angles (degrees), tetrahedral distortion indexes ID(PO), ID(OPO) and mean kPOPl angles, deduced from published data Interatomic PO distances (A), [12–21] Structural formula

P–O m

ID(P–O)

O–P–O m

ID(OPO)

kP–O–Pl

Li 6 P6 O 18 .5H 2 O

1.534 1.538 1.538 1.543 1.542 1.548 1.536 1.538 1.536 1.532 1.537 1.536 1.539 1.542 1.538 1.540 1.539 1.540 1.548 1.548 1.546 1.545 1.535 1.558 1.543 1.548 1.545 1.542 1.543 1.546 1.530 1.532 1.527 1.531 1.528 1.531

0.039 0.038 0.039 0.033 0.041 0.046 0.037 0.040 0.042 0.042 0.042 0.042 0.040 0.042 0.042 0.037 0.040 0.042 0.040 0.039 0.037 0.039 0.039 0.036 0.035 0.038 0.038 0.039 0.041 0.044 0.038 0.040 0.037 0.038 0.035 0.040

109.2 109.2 109.1 109.2 109.3 109.6 109.3 109.9 109.2 109.3 109.2 109.1 109.2 109.2 109.3 109.1 109.1 109.2 109.1 109.3 109.2 109.2 109.2 109.3 109.3 109.1 109.1 109.2 109.1 109.2 109.1 109.3 109.2 109.1 109.3 109.1

0.036 0.040 0.039 0.041 0.034 0.033 0.038 0.042 0.039 0.044 0.032 0.037 0.037 0.039 0.044 0.042 0.043 0.043 0.044 0.038 0.037 0.041 0.036 0.039 0.035 0.035 0.043 0.033 0.037 0.039 0.037 0.044 0.038 0.042 0.038 0.039

133.1 132.4 132.4 139.0 130.9 128.3 133.6 132.6 133.4 139.3 137.8 136.8 130.0 131.5 129.4 129.1 131.0 132.3 129.6 134.8 131.7 132.3 134.0 129.2 128.5 132.8 130.4 124.7 126.0 126.7 130.0 132.2 129.0 131.2 134.0 131.3

K 6 P6 O 18 .3H 2 O

(C 6 H 5 NH 3 ) 6 P6 O 18 .6H 2 O

(DBA) 6 P6 O 18 .6H 2 O

Li 3 Cs 3 P6 O 18 .2H 2 O

Li 2 (NH 4 )P6 O 18 .4H 2 O

Li 2 K 4 P6 O 18 .2H 2 O

Li 2 Tl 4 P6 O 18 .2H 2 O

Na 4 ZnP6 O 18 .6H 2 O

K 2 Ca 2 P6 O 18 .6H 2 O

K 2 Cd 2 P6 O 18 .8H 2 O

(Li 3 Cs 3 ; Na 4 Zn in Fig. 2c; K 2 Zn 2 in Fig. 2d). Similar results were obtained in Li 6 and (C 6 H 5 NH 3 ) 6 (Fig. 2b); however the resolution of the three components is poor in these phosphates. In the case of cyclohexaphosphate K 2 Cd 2 (Fig. 2e), the existence of two independent rings doubles the number of components. On the other hand, distortions of the polyhedra are responsible for observed chemical shift anisotropies and for detection of spinning side band patterns covering important regions of 31 P NMR spectra. Spectral regions occupied by these bands are proportional to tetrahedra distortions; from this fact, NMR patterns could be used to monitor distortions. In order to analyze this point, the experimental envelopes were deconvoluted, determining for each component siso , Ds and h parameters (Table 2). In cyclophosphate groups P–O bonds are considerably ˚ than external ones (|1.48 A), ˚ involved longer (|1.60 A) in alkali or alkaline-earth cation interactions. From this

fact, tetrahedra could display axial distortions and NMR powder patterns would be formed by two maxima (sII and sI values). However, in all cases, the asymmetry parameter h is near 0.5 (Table 2), indicating that the local symmetry of the tetrahedra is lower. These results agree with the point symmetry 1, deduced for P sites in structural analyses published on these compounds [12–25]. In order to analyze distortions on PO 4 tetrahedra, dispersions on PO distances and POP angles have been calculated from structural XRD data reported in these compounds. For that, ID(PO) and ID(OPO) parameters were calculated, according to expressions (Table 1)

O uPO 2 PO u /OuPO u ID(OPO) 5O uOPO 2 OPO u /O uOPO u ID(PO) 5

i

m

i

i

i

i

i

m

i

i

where PO i and OPOi stand for PO distances and OPO

C. Ben Nasr et al. / Journal of Alloys and Compounds 325 (2001) 102 – 108

angles of a given tetrahedron and PO m and OPO m are the mean values in the tetrahedron. Obtained ID(PO) and ID(OPO) values are always near 0.38, showing that most tetrahedra display similar distortions (Table 3). According to this fact, deduced Ds values are always near 2135 ppm and h values are close to 0.5. From this fact, anisotropies associated with tetrahedral distortions cannot be used to assign NMR components to different crystallographic sites. However, siso values of NMR components changes appreciably in cyclophosphates, indicating that factors other than tetrahedral condensation are affecting this parameter. Among structural factors invoked to explain differences in siso values, P–O–P angles (geometric factor) [6] and the cation type, sharing oxygens with tetrahedra (chemical factor) [6,10], are the most relevant. In a first stage of the analysis, POP angles defining the phosphate chains have been considered. For that, the average of the two POP angles defined by each P atom with neighbouring tetrahedra have been calculated in 11 cyclophosphates analyzed (Table 3). When siso values corresponding to NMR components are plotted against POP angles, it is observed that the correlation is reasonably good (Fig. 3), suggesting that both parameters must be related [29]. As POP angles depend on disposition of contiguous tetrahedra, the conformation of cyclophosphates is found to be a significant parameter in the analysis. However, the scattering of the points is important in Fig. 3, indicating that other factors also affect the chemical shift parameter. In the case of the phosphates with divalent

107

cations Zn, Cd and Ca, NMR components are shifted towards more negative values (Fig. 3). This indicates that the charge and the ionic radius (polarizing strength) of cations also affect the position of components. Consequently, a representation of the chemical shifts against P–O–P angles is insufficient and more elaborated analyses should be used [30].

5. Conclusions The 31 P MAS-NMR study of cyclohexaphosphates has shown that the chemical shift of NMR components depends not only on P–O–P angles but on the polarizing strength of cations that shares oxygens with PO 4 tetrahedra. P atoms located at different crystallographic sites display different chemical shifts. From the analysis of NMR spectra, the structural information deduced from X-ray diffraction technique was confirmed. In particular, the number and the multiplicity of the NMR components agree with predictions deduced from the symmetry of P6 O 18 rings. Tetrahedral distortions and local symmetry of P environments were also discussed in terms of chemical shift anisotropies (Ds and h parameters) of 31 P NMR components. In this analysis, it was shown that in all cases tetrahedral distortions are considerable (Ds | 2135 ppm) and point symmetries low (h |0.5).

Fig. 3. Correlation between chemical shift values, siso , of NMR components and POP angles obtained in previous studies [12–25]. In this plot, siso values of cyclophosphates with monovalent (h) and divalent (s) cations are differentiated.

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