research communications
of sodium dihydrogen arsenate
aTU Wien, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria, bInstitute for Chemical Technologies and Analytics, Division of Structural Chemistry, TU Wien, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria, and cX-Ray Centre, TU Wien, Getreidemarkt 9, A-1060 Vienna, Austria
*Correspondence e-mail: matthias.weil@tuwien.ac.at
Single crystals of the title compound, Na(H2AsO4), were obtained by partial neutralization of arsenic acid with sodium hydroxide in aqueous solution. The of Na(H2AsO4) is isotypic with the phosphate analogue and the consists of two sodium cations and two tetrahedral H2AsO4− anions. Each of the sodium cations is surrounded by six O atoms of five H2AsO4− groups, defining distorted octahedral coordination spheres. In the extended structure, the sodium cations and dihydrogen arsenate anions are arranged in the form of layers lying parallel to (010). Strong hydrogen bonds [range of O⋯O distances 2.500 (3)–2.643 (3) Å] between adjacent H2AsO4− anions are observed within and perpendicular to the layers. The isotypic structure of Na(H2PO4) is comparatively discussed.
Keywords: crystal structure; isotypism; sodium arsenate; hydrogen bonding.
CCDC reference: 1575494
1. Chemical context
Arsenic acid is triprotic and thus can form various salts, depending on the degree of deprotonation (H2AsO4−, HAsO42−, AsO43−), the condensation grade of the anion (mono-, di-, tri-, polyarsenate, etc) and the amount of water incorporated in the crystal. With respect to sodium arsenates, numerous crystal structures have been determined so far, including arsenic in tetrahedral and/or in octahedral coordination by oxygen atoms. Arsenate structures with arsenic exclusively in tetrahedral coordination resemble those of the related phosphates and in some cases show isotypism with them (marked by an asterisk): Na3.25(AsO4)(OH)0.25(H2O)12* (Tillmanns & Baur, 1971), Na4(AsO4)OH (zur Loye et al., 2015), Na2(HAsO4)(H2O)7* (Baur & Khan, 1970; Ferraris et al., 1971), Na(H2AsO4)(H2O) (Ferraris et al., 1974), Na3(H2As3O10) (Driss & Jouini, 1990), Na4As2O7 (Leung & Calvo, 1973), Na(AsO3) (Liebau, 1956) and Na5(AsO5) (Haas & Jansen, 2001). Arsenate structures with arsenic in (complete or partial) octahedral coordination include Na(H2As3O9) (Driss, Jouini, Durif et al., 1988), Na3(H5As4O14) (Driss & Jouini, 1989), Na(HAs2O6) (Dung & Tahar, 1978), Na2As4O11 (Driss, Jouini & Omezzine, 1988) and Na7As11O31 (Guesmi et al., 2006). A detailed discussion of the structural principles and crystal chemical characteristics of arsenates with arsenic in octahedral coordination was given some time ago by Schwendtner & Kolitsch (2007).
Besides the Na:As 1:1 phase Na(H2AsO4)(H2O) another 1:1 phase, Na(H2AsO4), has been reported but without an additional water molecule (Fehér & Morgenstern, 1937). To our surprise, a detailed structural investigation of this salt has not yet been reported. Therefore, we started crystal growth experiments and determined its structure and report here on the results.
2. Structural commentary
The 2AsO4) is isotypic with that of Na(H2PO4) (Catti & Ferraris, 1974). The of Na(H2AsO4) comprises two Na+ cations and two tetrahedral AsO2(OH)2− groups. The Na1+ cation shows a narrow Na—O bond-length distribution in the range 2.337 (2) to 2.498 (2) Å with a distorted octahedron as the corresponding The bond-valence sum (Brown, 2002) for the Na1+ cation amounts to 1.15 valence units. The surrounding of the Na2+ cation is much more distorted, with a bond-length range from 2.338 (2) to 2.769 (3) Å under consideration of a sixfold coordination (bond-valence sum 0.92 valence units). There is an additional remote oxygen atom at a distance of 3.000 (3) Å from Na2+. Its contribution of 0.04 valence units to the bond-valence sum might be considered as too low for a significant interaction, and therefore the first coordination sphere of Na2+ is discussed as that of a considerably distorted octahedron. The two dihydrogen arsenate groups show the usual differences (Weil, 2000, 2016) between As—O and As—(OH) bonds, with two significantly shorter As—O bonds [mean 1.659 (8) Å] and two longer As—(OH) bonds [1.723 (12) Å].
of Na(HIn the 2AsO4) the AsO2(OH)2 tetrahedra are arranged in layers lying parallel to (010) with the Na+ cations approximately on the same level (Fig. 1). Strong, asymmetric hydrogen bonds [O⋯O distances between 2.500 (3) and 2.643 (3) Å, Table 1] between each of the OH groups of the two dihydrogen arsenate tetrahedra and O atoms of adjacent tetrahedra significantly contribute to the crystal packing. These hydrogen bonds are both within a layer and towards adjacent layers (Fig. 1).
of Na(HThe differences between the isotypic arsenate and phosphate structures can mainly be seen in the X—O bond lengths of the anions (X = As, mean of 1.69 Å; X = P, mean of 1.55 Å), with Δmax(X—O) of 0.15 Å between arsenate and phosphate tetrahedra. The difference with respect to the Na—O distances in the two structures is less pronounced, with Δmax(Na—O) = 0.10 Å. Relevant bond lengths of the isotypic crystal structures of Na(H2AsO4) and Na(H2PO4) (Catti & Ferraris, 1974) are compiled in Table 2. A more quantitative comparison of the two crystal structures with the help of the COMPSTRU routine (de la Flor et al., 2016) revealed the following values: The degree of (S), i.e. the spontaneous strain (sum of the squared eigenvalues of the strain tensor divided by 3), is 0.0159; the maximum distance (dmax.), i.e. the maximal displacement between the atomic positions of paired atoms, is 0.1920 Å for atom pair O1; the arithmetic mean (dav) of the distances of all atom pairs is 0.1108 Å; the measure of similarity (Δ) (Bergerhoff et al., 1999) is a function of the differences in atomic positions (weighted by the multiplicities of the sites) and the ratios of the corresponding lattice parameters of the structures and amounts to 0.049.
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3. Synthesis and crystallization
The title compound was prepared following a procedure by Fehér & Morgenstern (1937). An arsenic acid solution (ca 65%wt) was partly neutralized with diluted NaOH solution using methyl red as indicator. The resulting solution was concentrated by heating. Standing of the solution overnight on a warm plate (ca 313 K) afforded colourless crystals with a lath-like form and maximal edge lengths of 0.5 mm.
4. Refinement
Crystal data, data collection and structure . Starting coordinates and labelling of atoms were taken from the isotypic Na(H2PO4) structure (Catti & Ferraris, 1974). Hydrogen atoms were clearly discernible from difference maps and were refined with distance restraints d(O—H) = 0.85 (1) Å.
details are summarized in Table 3
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Supporting information
CCDC reference: 1575494
https://doi.org/10.1107/S2056989017013470/hb7706sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989017013470/hb7706Isup2.hkl
Data collection: APEX3 (Bruker, 2016); cell
SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: coordinates taken from isotypic structure; program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).Na(H2AsO4) | F(000) = 624 |
Mr = 163.93 | Dx = 2.996 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
a = 7.0528 (14) Å | Cell parameters from 2433 reflections |
b = 13.798 (3) Å | θ = 3.1–31.8° |
c = 7.4792 (15) Å | µ = 9.32 mm−1 |
β = 93.02 (3)° | T = 100 K |
V = 726.8 (3) Å3 | Lath, colourless |
Z = 8 | 0.12 × 0.08 × 0.01 mm |
Bruker APEXII CCD diffractometer | 1890 reflections with I > 2σ(I) |
ω and φ scans | Rint = 0.052 |
Absorption correction: multi-scan (SADABS; Krause et al., 2015) | θmax = 32.6°, θmin = 2.9° |
Tmin = 0.534, Tmax = 0.746 | h = −10→10 |
11092 measured reflections | k = −20→20 |
2651 independent reflections | l = −11→11 |
Refinement on F2 | Primary atom site location: isomorphous structure methods |
Least-squares matrix: full | Hydrogen site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.030 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.059 | w = 1/[σ2(Fo2) + (0.020P)2 + 0.0156P] where P = (Fo2 + 2Fc2)/3 |
S = 1.04 | (Δ/σ)max = 0.001 |
2651 reflections | Δρmax = 0.87 e Å−3 |
125 parameters | Δρmin = −0.86 e Å−3 |
4 restraints |
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. |
x | y | z | Uiso*/Ueq | ||
As1 | 0.32467 (4) | 0.36827 (2) | 0.84669 (4) | 0.00611 (7) | |
As2 | 0.82170 (4) | 0.37010 (2) | 0.50756 (4) | 0.00595 (7) | |
Na1 | 0.85834 (16) | 0.40338 (8) | −0.00598 (15) | 0.0091 (2) | |
Na2 | 0.34819 (17) | 0.39825 (9) | 0.26282 (16) | 0.0134 (3) | |
O1 | 0.2417 (3) | 0.26833 (14) | 0.7478 (3) | 0.0108 (4) | |
O2 | 0.5315 (3) | 0.34069 (15) | 0.9726 (3) | 0.0099 (4) | |
O3 | 0.1910 (3) | 0.42983 (14) | 0.9809 (3) | 0.0078 (4) | |
O4 | 0.4015 (3) | 0.44833 (15) | 0.6878 (3) | 0.0124 (4) | |
O5 | 0.9202 (3) | 0.26110 (14) | 0.5709 (3) | 0.0113 (4) | |
O6 | 0.8540 (3) | 0.45079 (14) | 0.6715 (3) | 0.0098 (4) | |
O7 | 0.9303 (3) | 0.40531 (15) | 0.3188 (3) | 0.0093 (4) | |
O8 | 0.5931 (3) | 0.34459 (14) | 0.4613 (3) | 0.0082 (4) | |
H1 | 0.545 (7) | 0.2797 (14) | 0.970 (6) | 0.055 (15)* | |
H2 | 0.462 (5) | 0.415 (3) | 0.619 (5) | 0.038 (13)* | |
H3 | 1.026 (4) | 0.270 (3) | 0.627 (6) | 0.071 (18)* | |
H4 | 0.995 (5) | 0.457 (2) | 0.332 (6) | 0.045 (13)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
As1 | 0.00551 (13) | 0.00631 (14) | 0.00645 (13) | 0.00067 (11) | −0.00029 (10) | −0.00017 (11) |
As2 | 0.00609 (13) | 0.00599 (14) | 0.00564 (13) | −0.00119 (11) | −0.00081 (10) | 0.00025 (11) |
Na1 | 0.0087 (6) | 0.0086 (5) | 0.0099 (6) | 0.0004 (4) | −0.0002 (4) | 0.0003 (4) |
Na2 | 0.0161 (6) | 0.0132 (6) | 0.0106 (6) | 0.0027 (5) | −0.0038 (5) | −0.0001 (5) |
O1 | 0.0101 (10) | 0.0078 (10) | 0.0141 (11) | −0.0005 (8) | −0.0031 (8) | −0.0037 (8) |
O2 | 0.0077 (10) | 0.0068 (9) | 0.0147 (11) | 0.0005 (8) | −0.0044 (8) | 0.0006 (8) |
O3 | 0.0062 (10) | 0.0094 (9) | 0.0078 (10) | 0.0016 (7) | 0.0007 (7) | −0.0011 (7) |
O4 | 0.0150 (11) | 0.0112 (10) | 0.0114 (11) | 0.0040 (9) | 0.0059 (9) | 0.0033 (8) |
O5 | 0.0105 (10) | 0.0064 (10) | 0.0163 (11) | −0.0006 (8) | −0.0054 (9) | 0.0012 (8) |
O6 | 0.0101 (10) | 0.0110 (10) | 0.0083 (10) | −0.0033 (8) | 0.0012 (8) | −0.0034 (8) |
O7 | 0.0122 (10) | 0.0091 (9) | 0.0070 (9) | −0.0034 (9) | 0.0038 (7) | −0.0008 (8) |
O8 | 0.0058 (9) | 0.0078 (9) | 0.0108 (10) | −0.0011 (7) | −0.0013 (7) | 0.0011 (8) |
As1—O3 | 1.6484 (19) | Na1—O6iv | 2.498 (2) |
As1—O1 | 1.657 (2) | Na2—O8 | 2.338 (2) |
As1—O4 | 1.730 (2) | Na2—O3iv | 2.371 (2) |
As1—O2 | 1.736 (2) | Na2—O1ii | 2.419 (2) |
As2—O6 | 1.663 (2) | Na2—O6i | 2.586 (2) |
As2—O8 | 1.668 (2) | Na2—O2iv | 2.703 (3) |
As2—O7 | 1.711 (2) | Na2—O4i | 2.769 (3) |
As2—O5 | 1.713 (2) | Na2—O7v | 3.000 (3) |
Na1—O3i | 2.337 (2) | O2—H1 | 0.847 (19) |
Na1—O5ii | 2.376 (2) | O4—H2 | 0.828 (18) |
Na1—O3iii | 2.382 (2) | O5—H3 | 0.847 (19) |
Na1—O7 | 2.456 (2) | O7—H4 | 0.849 (19) |
Na1—O2iv | 2.459 (2) | ||
O3—As1—O1 | 120.05 (10) | O1ii—Na2—O4i | 157.97 (8) |
O3—As1—O4 | 107.40 (10) | O6i—Na2—O4i | 73.32 (7) |
O1—As1—O4 | 109.92 (11) | O2iv—Na2—O4i | 90.18 (7) |
O3—As1—O2 | 105.89 (10) | O8—Na2—O7v | 128.32 (8) |
O1—As1—O2 | 109.06 (10) | O3iv—Na2—O7v | 72.63 (7) |
O4—As1—O2 | 103.17 (10) | O1ii—Na2—O7v | 74.48 (7) |
O6—As2—O8 | 112.80 (10) | O6i—Na2—O7v | 52.53 (6) |
O6—As2—O7 | 111.60 (10) | O2iv—Na2—O7v | 129.53 (8) |
O8—As2—O7 | 111.00 (10) | O4i—Na2—O7v | 125.50 (7) |
O6—As2—O5 | 110.24 (10) | As1—O1—Na2vi | 131.96 (12) |
O8—As2—O5 | 104.21 (10) | As1—O2—Na1vii | 135.72 (11) |
O7—As2—O5 | 106.56 (10) | As1—O2—Na2vii | 86.99 (8) |
O3i—Na1—O5ii | 161.36 (9) | Na1vii—O2—Na2vii | 109.31 (9) |
O3i—Na1—O3iii | 90.20 (7) | As1—O2—H1 | 107 (3) |
O5ii—Na1—O3iii | 89.30 (8) | Na1vii—O2—H1 | 104 (3) |
O3i—Na1—O7 | 86.19 (8) | Na2vii—O2—H1 | 111 (3) |
O5ii—Na1—O7 | 75.23 (8) | As1—O3—Na1i | 130.46 (11) |
O3iii—Na1—O7 | 83.49 (8) | As1—O3—Na2vii | 101.02 (9) |
O3i—Na1—O2iv | 102.04 (8) | Na1i—O3—Na2vii | 100.03 (8) |
O5ii—Na1—O2iv | 80.75 (8) | As1—O3—Na1viii | 122.85 (11) |
O3iii—Na1—O2iv | 166.71 (9) | Na1i—O3—Na1viii | 89.80 (7) |
O7—Na1—O2iv | 102.26 (9) | Na2vii—O3—Na1viii | 110.53 (9) |
O3i—Na1—O6iv | 79.94 (8) | As1—O4—Na2i | 128.10 (12) |
O5ii—Na1—O6iv | 118.47 (8) | As1—O4—H2 | 105 (3) |
O3iii—Na1—O6iv | 83.21 (8) | Na2i—O4—H2 | 99 (3) |
O7—Na1—O6iv | 160.71 (8) | As2—O5—Na1vi | 134.83 (12) |
O2iv—Na1—O6iv | 93.76 (8) | As2—O5—H3 | 111 (3) |
O8—Na2—O3iv | 156.69 (9) | Na1vi—O5—H3 | 113 (3) |
O8—Na2—O1ii | 86.89 (8) | As2—O6—Na1vii | 122.07 (11) |
O3iv—Na2—O1ii | 90.22 (8) | As2—O6—Na2i | 128.58 (11) |
O8—Na2—O6i | 122.07 (8) | Na1vii—O6—Na2i | 90.36 (7) |
O3iv—Na2—O6i | 77.53 (7) | As2—O7—Na1 | 137.29 (12) |
O1ii—Na2—O6i | 126.96 (8) | As2—O7—Na2ix | 126.26 (11) |
O8—Na2—O2iv | 92.76 (8) | Na1—O7—Na2ix | 90.85 (7) |
O3iv—Na2—O2iv | 63.95 (7) | As2—O7—H4 | 114 (3) |
O1ii—Na2—O2iv | 81.03 (8) | Na1—O7—H4 | 102 (3) |
O6i—Na2—O2iv | 132.91 (8) | Na2ix—O7—H4 | 61 (3) |
O8—Na2—O4i | 73.31 (7) | As2—O8—Na2 | 137.60 (11) |
O3iv—Na2—O4i | 104.07 (8) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, −y+1/2, z−1/2; (iii) x+1, y, z−1; (iv) x, y, z−1; (v) x−1, y, z; (vi) x, −y+1/2, z+1/2; (vii) x, y, z+1; (viii) x−1, y, z+1; (ix) x+1, y, z. |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H1···O8vi | 0.85 (2) | 1.75 (2) | 2.595 (3) | 175 (5) |
O4—H2···O8 | 0.83 (2) | 1.82 (2) | 2.643 (3) | 178 (4) |
O5—H3···O1ix | 0.85 (2) | 1.73 (2) | 2.566 (3) | 171 (5) |
O7—H4···O6x | 0.85 (2) | 1.66 (2) | 2.500 (3) | 169 (4) |
Symmetry codes: (vi) x, −y+1/2, z+1/2; (ix) x+1, y, z; (x) −x+2, −y+1, −z+1. |
Bond | Na(H2AsO4) | Na(H2PO4) |
Na1—O3i | 2.337 (2) | 2.355 (1) |
Na1—O5ii | 2.376 (2) | 2.406 (1) |
Na1—O3iii | 2.382 (2) | 2.371 (1) |
Na1—O7 | 2.456 (2) | 2.501 (1) |
Na1—O2iv | 2.459 (2) | 2.436 (1) |
Na1—O6iv | 2.498 (2) | 2.564 (1) |
Na2—O8 | 2.338 (2) | 2.334 (1) |
Na2—O3iv | 2.371 (2) | 2.369 (1) |
Na2—O1ii | 2.419 (2) | 2.433 (1) |
Na2—O6i | 2.586 (2) | 2.601 (1) |
Na2—O2iv | 2.703 (3) | 2.600 (1) |
Na2—O4i | 2.769 (3) | 2.730 (1) |
Na2—O7v | 3.000 (3) | 2.930 (1) |
As/P1—O3 | 1.6484 (19) | 1.499 (1) |
As/P1—O1 | 1.657 (2) | 1.508 (1) |
As/P1—O4 | 1.730 (2) | 1.592 (1) |
As/P1—O2 | 1.736 (2) | 1.597 (1) |
As/P2—O6 | 1.663 (2) | 1.523 (1) |
As/P2—O8 | 1.668 (2) | 1.519 (1) |
As/P2—O7 | 1.711 (2) | 1.562 (1) |
As/P2—O5 | 1.713 (2) | 1.572 (1) |
Symmetry codes (i) -x+1, -y+1, -z+1; (ii) x, -y+1/2, z-1/2; (iii) x+1, y, z-1; (iv) x, y, z-1; (v) x-1, y, z. |
Acknowledgements
The X-ray centre of TU Wien is acknowledged for financial support of this study.
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