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Synthesis and crystal structure of calcium hydrogen phosphite, CaHPO3

aPleasanton Ridge Research LLC, Sandia Airpark, Edgewood, New Mexico 87015, USA, and bDepartment of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, Scotland
*Correspondence e-mail: w.harrison@abdn.ac.uk

Edited by M. Weil, Vienna University of Technology, Austria (Received 31 May 2019; accepted 7 June 2019; online 14 June 2019)

The hydro­thermal synthesis and crystal structure of the simple inorganic compound CaHPO3, which crystallizes in the chiral space group P43212, are reported. The structure is built up from distorted CaO7 capped trigonal prisms and HPO3 pseudo pyramids, which share corners and edges to generate a three-dimensional network.

1. Chemical context

Calcium–phospho­rus–oxygen phases are ubiquitous in inorganic and materials chemistry. They have long been known as key components of fertilizers produced on a multi-million tonne scale (Rajan et al., 1996[Rajan, S. S. S., Watkinson, J. H. & Sinclair, A. G. (1996). Adv. Agron. 57, 77-159.]) and more recently their clinical applications as cements and biomaterials have been intens­ively studied. A recent review (Eliaz & Metoki, 2017[Eliaz, N. & Metoki, N. (2017). Materials, 10, article 334 (104 pages).]) refers to over 860 articles. Some of their other applications include use as additives in cheese making (Lucey & Fox, 1993[Lucey, J. A. & Fox, P. A. (1993). J. Dairy Sci. 76, 1714-1724.]), as environmental remediation agents (Nzihou & Sharrock, 2010[Nzihou, A. & Sharrock, P. (2010). Waste Biomass Valori. 1, 163-174.]) and as corrosion inhibitors (del Amo et al., 1999[Amo, B. del, Romagnoli, R. & Vetere, V. F. (1999). Ind. Eng. Chem. Res. 38, 2310-2314.]). Apatite, Ca5(PO4)3X (X = OH, F, Cl, ⋯), is the most abundant calcium phosphate mineral and is of great importance in mineralogy and geochemistry (Hughes & Rakovan, 2002[Hughes, J. M. & Rakovan, J. (2002). Rev. Mineral. Geochem. 48, 1-12.]).

As part of our ongoing exploratory synthetic studies, we now describe the hydro­thermal syntheses and crystal structure of CaHPO3, (I).

2. Structural commentary

The asymmetric unit of (I) has the simple composition of one Ca2+ cation and one hydrogen phosphite anion (Fig. 1[link]) with the symmetry elements of the chiral tetra­gonal space group P43212 building up the complete crystal structure. This results in the calcium ion being coordinated by seven O atoms belonging to six different HPO32− groups (the one in the arbitrarily chosen asymmetric unit is chelating via the O1,O2 edge). The Ca1—O2 distance of 2.6938 (14) Å (Table 1[link]) is notably longer than the others (mean for the six shorter distances = 2.391 Å) but it clearly qualifies as a bond based on the Brown criterion (Brown, 2002[Brown, I. D. (2002). The Chemical Bond in Inorganic Chemistry: The Bond Valence Model. Oxford University Press.]) of contributing at least 0.04 v.u. (valence units) to the bond-valence sum (BVS; Brown & Altermatt, 1985[Brown, I. D. & Altermatt, D. (1985). Acta Cryst. B41, 244-247.]) for the metal ion: the Ca1 BVS is 2.07 v.u. (expected value = 2.00 v.u.), with the long O2 bond contrib­uting 0.14 v.u. The next-nearest oxygen atom is some 3.68 Å distant from the calcium atom.

Table 1
Selected bond lengths (Å)

Ca1—O1i 2.2870 (13) Ca1—O2 2.6938 (14)
Ca1—O2ii 2.3600 (14) P1—O1 1.5173 (13)
Ca1—O2iii 2.3907 (13) P1—O3 1.5192 (13)
Ca1—O3iv 2.4014 (12) P1—O2 1.5423 (13)
Ca1—O3v 2.4192 (13) P1—H1 1.30 (2)
Ca1—O1 2.4869 (14)    
Symmetry codes: (i) [x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+{\script{1\over 4}}]; (ii) [x+{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+{\script{1\over 4}}]; (iii) y, x, -z; (iv) [y-{\script{1\over 2}}, -x+{\script{1\over 2}}, z+{\script{1\over 4}}]; (v) x, y-1, z.
[Figure 1]
Figure 1
The asymmetric unit of (I) expanded to show the complete calcium coordination polyhedron (50% displacement ellipsoids). See Table 1[link] for symmetry codes.

The calcium coordination environment in (I) can be described as a distorted mono-capped trigonal prism with O1i/O2/O3iv and O2ii/O2iii/O3v forming the ends of the prism and the Ca1—O1 bond protruding through the twisted rectangular face formed by atoms O2/O2iii/O2ii/O3iv (see Table 1[link] for symmetry codes). The dihedral angle between the end-faces noted in the previous sentence is 6.90 (9)°, and the calcium ion is displaced from them by 1.5607 (8) and 1.4454 (9) Å, respectively. The dihedral angles subtended by O1i/O2/O3iv and O1/O2/O3iv (the latter being the triangle formed by the protruding atom O2 and the common edge with the prism-end) is 36.14 (7)°; the equivalent value for O2ii/O2iii/O3v and O1/O2ii/O2iii is 41.37 (4)°. These data, especially the second value, are in very good agreement with the ideal value of 41.5° for a capped trigonal prism with C2v symmetry built up from hard spheres (Lewis & Lippard, 1975[Lewis, D. L. & Lippard, S. J. (1975). J. Am. Chem. Soc. 97, 2697-2702.]). Finally, it may be noted that the calcium ion is displaced from the centroid of its seven associated O atoms by 0.34 Å approximately away from the O1/O2 edge of the chelating phosphite group.

The HPO32– hydrogen phosphite group in (I) displays its normal (Loub, 1991[Loub, J. (1991). Acta Cryst. B47, 468-473.]) tetra­hedral (including the H atom) shape with a mean P—O separation of 1.526 Å. The bond to O2 is clearly longer than the others (Table 1[link]), which might correlate with the fact that O2 bonds to three calcium cations, whereas O1 and O3 bond to two. The O—P—O angles are notably distorted with O1—P1—O2 (the chelating atoms to the adjacent calcium ion) some 8° smaller than the other two angles. The P atom is displaced by 0.4200 (9) Å from the plane of its attached O atoms, which is typical (Holmes et al., 2018[Holmes, W., Cordes, D. B., Slawin, A. M. Z. & Harrison, W. T. A. (2018). Acta Cryst. E74, 1411-1416.]). As usual, the hydrogen atom of the phosphite group shows no propensity to form hydrogen bonds and in (I) atom H1 `points into space' with its nearest neighbour being another H1 atom at 2.08 (4) Å. The mean P—O bond length in the `type e' (isolated) HPO32– groups surveyed by Loub (1991[Loub, J. (1991). Acta Cryst. B47, 468-473.]) of 1.517 Å is slightly shorter than the value for (I) but the mean P—H separation of 1.30 Å established by Loub is identical to the refined value for (I).

Each of the three unique O atoms in (I) has a different coordination environment: O1 bonds to two Ca cations and one P atom in an approximate T-shape with Ca—O—Ca = 108.30 (5) and Ca—O—P = 99.88 (7) and 150.21 (8)°. The environment of O2 can be described as a distorted OPCa3 tetra­hedron [range of angles = 91.01 (6)–131.49 (8); mean = 107.0°] whereas O3 bonds to two Ca and one P atom in an approximate trigonal arrangement [Ca—O—Ca = 103.77 (5) and Ca—O—P = 127.66 (7) and 128.07 (7)°; bond-angle sum = 359.5°].

The extended structure of (I) in polyhedral representation is shown in Fig. 2[link]. The linkage of the CaO7 and HPO3 polyhedra generates a dense three-dimensional network in which each calcium cation is surrounded by five others linked via edges with Ca⋯Ca separations clustered in the narrow range of 3.6938 (5)–3.8712 (5) Å. There appear to be small voids in the structure but these correspond to the P—H vertices and a PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]) analysis did not reveal any free space in the structure.

[Figure 2]
Figure 2
Polyhedral view of the packing in (I) viewed down [110].

3. Database survey

A survey of the Inorganic Crystal Structure Database (ICSD; Belsky et al., 2002[Belsky, A., Hellenbrandt, M., Karen, V. L. & Luksch, P. (2002). Acta Cryst. B58, 364-369.]), updated to March 2019, for compounds containing Ca, P, O and H and no other elements revealed 142 matches. The vast majority of these are phosphates (containing tetra­hedral PVO4 groups) and many of them are apatite derivatives. When the presence of any other element alongside Ca/P/O/H was allowed in the search, no fewer than 554 hits arose.

The closest analogues to (I) are the PIII-containing phases calcium bis­(di­hydrogen phosphite) monohydrate, Ca(H2PO3)2·H2O [reported first by Larbot et al. (1984[Larbot, A., Durand, J. & Cot, L. (1984). Z. Anorg. Allg. Chem. 508, 154-158.]) (ICSD reference 36285) and then by Mahmoudkhani & Langer (2001a[Mahmoudkhani, A. H. & Langer, V. (2001a). Acta Cryst. E57, i19-i21.]; ICSD 280575) and calcium hydrogen phosphite monohydrate, CaHPO3·H2O (Mahmoudkhani & Langer, 2001b[Mahmoudkhani, A. H. & Langer, V. (2001b). Phosphorus Sulfur Silicon, 176, 83-94.]; ICSD 411737). The water mol­ecule coordinates to the calcium cation in both compounds and O—H⋯O hydrogen bonds (from the OH moiety of the H2PO3 group and the water mol­ecule in 280575 and from the water mol­ecule in 411737) are prominent features of the crystal structures. It is notable that both phases feature a CaO7 coordination polyhedron with one chelating phosphite group: in 411737 its distorted capped trigonal–prismatic shape is similar to that seen in (I) whereas in 280575 it is closer to a penta­gonal bipyramid. The overall topology of the Ca/P/O bonding network in 411737 is layered but in 280575 it is three-dimensional.

4. Synthesis and crystallization

A mixture of 2.36 g (10.0 mmol) Ca(NO3)2·4H2O, 0.52 g (6.0 mmol) H3PO3 and 0.47 g (4.0 mmol) NH4ClO4 were dissolved in 10 ml H2O then mixed with 4.0 g 15 N NH4OH and loaded into a 23 ml Teflon cup. This was heated in a stainless steel pressure vessel for seven days at 473 K and cooled to room temperature over a few hours. Product recovery by vacuum filtration and rinsing with deionized water yielded 0.67 g (5.6 mmol; 93% yield based on Ca) of sparkling colourless prisms of (I).

A calculated X-ray powder pattern for (I) based on the single-crystal structure model was found to be in excellent agreement with its measured powder pattern (see supporting information): no `hits' were found in a search against the JCPDS database of powder patterns. ATR–FTIR (diamond window, cm−1) for (I): 2467w, 2436m (P—H stretch); 1151s, 1056vs, 979vs, 588vs, 511s, 448s (phosphite P—O stretches and bends) [for the spectrum, see supporting information; for peak assignments, see Fridrichová et al. (2012[Fridrichová, M., Němec, I., Matulková, I., Gyepes, R., Borodavka, F., Kroupa, J., Hlinka, J. & Gregora, I. (2012). Vib. Spectrosc. 63, 485-491.])].

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2[link]. Atom H1 was located in a difference map and its position and Uiso value were freely refined. The absolute structure of the crystal chosen for data collection is well-defined in space group P43212 (No. 96) although the bulk sample presumably also consists of equal amounts of the other enanti­omer (space group P41212, No. 92).

Table 2
Experimental details

Crystal data
Chemical formula CaHPO3
Mr 120.06
Crystal system, space group Tetragonal, P43212
Temperature (K) 100
a, c (Å) 6.67496 (6), 12.9542 (2)
V3) 577.18 (1)
Z 8
Radiation type Mo Kα
μ (mm−1) 2.49
Crystal size (mm) 0.20 × 0.11 × 0.10
 
Data collection
Diffractometer Rigaku AFC12 CCD
Absorption correction Multi-scan (CrysAlis PRO; Rigaku, 2017[Rigaku (2017). CrysAlis PRO. Rigaku Corporation, Tokyo, Japan.])
Tmin, Tmax 0.811, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections 7615, 668, 667
Rint 0.030
(sin θ/λ)max−1) 0.649
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.014, 0.037, 1.27
No. of reflections 668
No. of parameters 51
H-atom treatment All H-atom parameters refined
Δρmax, Δρmin (e Å−3) 0.28, −0.38
Absolute structure Flack x determined using 230 quotients [(I+)−(I)]/[(I+)+(I)] (Parsons et al., 2013[Parsons, S., Flack, H. D. & Wagner, T. (2013). Acta Cryst. B69, 249-259.])
Absolute structure parameter −0.006 (16)
Computer programs: CrysAlis PRO (Rigaku, 2017[Rigaku (2017). CrysAlis PRO. Rigaku Corporation, Tokyo, Japan.]), SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]), ORTEP-3 for Windows (Farrugia, 2012[Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849-854.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).

Supporting information


Computing details top

Data collection: CrysAlis PRO (Rigaku, 2017); cell refinement: CrysAlis PRO (Rigaku, 2017); data reduction: CrysAlis PRO (Rigaku, 2017); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).

Calcium hydrogen phosphite top
Crystal data top
CaHPO3Dx = 2.763 Mg m3
Mr = 120.06Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P43212Cell parameters from 6820 reflections
a = 6.67496 (6) Åθ = 3.0–27.5°
c = 12.9542 (2) ŵ = 2.49 mm1
V = 577.18 (1) Å3T = 100 K
Z = 8Prism, colourless
F(000) = 4800.20 × 0.11 × 0.10 mm
Data collection top
Rigaku AFC12 CCD
diffractometer
668 independent reflections
Confocal mirrors, HF Varimax monochromator667 reflections with I > 2σ(I)
Detector resolution: 28.5714 pixels mm-1Rint = 0.030
ω scansθmax = 27.5°, θmin = 3.4°
Absorption correction: multi-scan
(CrysAlis PRO; Rigaku, 2017)
h = 88
Tmin = 0.811, Tmax = 1.000k = 88
7615 measured reflectionsl = 1616
Refinement top
Refinement on F2All H-atom parameters refined
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0206P)2 + 0.1946P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.014(Δ/σ)max < 0.001
wR(F2) = 0.037Δρmax = 0.28 e Å3
S = 1.26Δρmin = 0.38 e Å3
668 reflectionsExtinction correction: SHELXL2014 (Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
51 parametersExtinction coefficient: 0.056 (4)
0 restraintsAbsolute structure: Flack x determined using 230 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013)
Primary atom site location: structure-invariant direct methodsAbsolute structure parameter: 0.006 (16)
Hydrogen site location: difference Fourier map
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ca10.31831 (5)0.10351 (5)0.11917 (3)0.00616 (14)
P10.29735 (7)0.57084 (7)0.10542 (3)0.00575 (15)
H10.220 (4)0.625 (4)0.1928 (19)0.013 (6)*
O10.4785 (2)0.43959 (19)0.12612 (11)0.0080 (3)
O20.1356 (2)0.4401 (2)0.05361 (10)0.0078 (3)
O30.3393 (2)0.76729 (19)0.05029 (9)0.0081 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ca10.0058 (2)0.00652 (19)0.0061 (2)0.00010 (12)0.00026 (13)0.00022 (12)
P10.0057 (2)0.0056 (2)0.0059 (2)0.00049 (15)0.00009 (17)0.00001 (15)
O10.0070 (6)0.0078 (5)0.0092 (5)0.0004 (5)0.0005 (5)0.0000 (5)
O20.0070 (6)0.0086 (6)0.0079 (5)0.0012 (5)0.0002 (5)0.0001 (5)
O30.0100 (6)0.0067 (6)0.0076 (6)0.0003 (5)0.0002 (5)0.0005 (5)
Geometric parameters (Å, º) top
Ca1—O1i2.2870 (13)P1—O31.5192 (13)
Ca1—O2ii2.3600 (14)P1—O21.5423 (13)
Ca1—O2iii2.3907 (13)P1—H11.30 (2)
Ca1—O3iv2.4014 (12)O1—Ca1ii2.2869 (13)
Ca1—O3v2.4192 (13)O2—Ca1i2.3600 (14)
Ca1—O12.4869 (14)O2—Ca1iii2.3907 (13)
Ca1—O22.6938 (14)O3—Ca1vi2.4014 (12)
P1—O11.5173 (13)O3—Ca1vii2.4192 (13)
O1i—Ca1—O2ii149.66 (5)O1—Ca1—O256.90 (4)
O1i—Ca1—O2iii111.93 (5)O1—P1—O3115.75 (8)
O2ii—Ca1—O2iii95.90 (4)O1—P1—O2107.96 (8)
O1i—Ca1—O3iv81.56 (5)O3—P1—O2114.40 (8)
O2ii—Ca1—O3iv74.49 (4)O1—P1—H1109.0 (12)
O2iii—Ca1—O3iv161.56 (5)O3—P1—H1104.1 (11)
O1i—Ca1—O3v87.17 (5)O2—P1—H1104.9 (11)
O2ii—Ca1—O3v89.43 (5)P1—O1—Ca1ii150.21 (8)
O2iii—Ca1—O3v73.62 (4)P1—O1—Ca199.88 (7)
O3iv—Ca1—O3v121.07 (5)Ca1ii—O1—Ca1108.30 (5)
O1i—Ca1—O1122.62 (4)P1—O2—Ca1i120.90 (7)
O2ii—Ca1—O173.14 (4)P1—O2—Ca1iii131.49 (8)
O2iii—Ca1—O178.87 (4)Ca1i—O2—Ca1iii105.94 (5)
O3iv—Ca1—O183.24 (5)P1—O2—Ca191.01 (6)
O3v—Ca1—O1145.62 (5)Ca1i—O2—Ca199.78 (5)
O1i—Ca1—O270.38 (4)Ca1iii—O2—Ca192.99 (5)
O2ii—Ca1—O2130.00 (4)P1—O3—Ca1vi127.66 (7)
O2iii—Ca1—O277.54 (5)P1—O3—Ca1vii128.07 (7)
O3iv—Ca1—O296.39 (4)Ca1vi—O3—Ca1vii103.77 (5)
O3v—Ca1—O2133.13 (5)
O3—P1—O1—Ca1ii11.3 (2)O3—P1—O2—Ca1iii53.71 (12)
O2—P1—O1—Ca1ii140.99 (16)O1—P1—O2—Ca118.17 (8)
O3—P1—O1—Ca1149.73 (6)O3—P1—O2—Ca1148.60 (6)
O2—P1—O1—Ca120.05 (8)O1—P1—O3—Ca1vi78.88 (11)
O1—P1—O2—Ca1i120.21 (8)O2—P1—O3—Ca1vi47.62 (13)
O3—P1—O2—Ca1i109.36 (9)O1—P1—O3—Ca1vii110.53 (10)
O1—P1—O2—Ca1iii76.72 (11)O2—P1—O3—Ca1vii122.98 (9)
Symmetry codes: (i) x1/2, y+1/2, z+1/4; (ii) x+1/2, y+1/2, z+1/4; (iii) y, x, z; (iv) y1/2, x+1/2, z+1/4; (v) x, y1, z; (vi) y+1/2, x+1/2, z1/4; (vii) x, y+1, z.
 

Acknowledgements

We thank Kirstie McCombie for collecting the powder pattern, Sarah Ferrandin for collecting the IR spectrum and the EPSRC National Crystallography Service (University of Southampton) for the X-ray data collection. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract DE-AC52–06 N A25396) and Sandia National Laboratories (Contract DE-NA-0003525).

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