Crystal structure of sodium di-hydrogen arsenate.

Single crystals of the title compound, Na(H2AsO4), were obtained by partial neutralization of arsenic acid with sodium hydroxide in aqueous solution. The crystal structure of Na(H2AsO4) is isotypic with the phosphate analogue and the asymmetric unit consists of two sodium cations and two tetra-hedral H2AsO4- anions. Each of the sodium cations is surrounded by six O atoms of five H2AsO4- groups, defining distorted octa-hedral coordination spheres. In the extended structure, the sodium cations and di-hydrogen 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.

Chemical context

Arsenic acid is triprotic and thus can form various salts, depending on the degree of deprotonation (H2AsO4 −, HAsO4 2−, AsO4 3−), 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.

Structural commentary

The crystal structure of Na(H2AsO4) is isotypic with that of Na(H2PO4) (Catti & Ferraris, 1974 ▸). The asymmetric unit 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 coordination polyhedron. 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].

In the crystal structure of Na(H2AsO4) 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 ▸).

Figure 1

The crystal structure of Na(H2AsO4) in a projection along [100]. All atoms are depicted with displacement ellipsoids at the 97% probability level. Di­hydrogen arsenate tetra­hedra are given in polyhedral representation, Na+ cations as single ellipsoids without bonds to surrounding O atoms. H⋯O hydrogen bonds are illustrated with green lines.

Table 1

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O2—H1⋯O8i	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⋯O1ii	0.85 (2)	1.73 (2)	2.566 (3)	171 (5)
O7—H4⋯O6iii	0.85 (2)	1.66 (2)	2.500 (3)	169 (4)

Symmetry codes: (i) ; (ii) ; (iii) .

The 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 lattice distortion (S), i.e. the spontaneous strain (sum of the squared eigenvalues of the strain tensor divided by 3), is 0.0159; the maximum distance (d max.), i.e. the maximal displacement between the atomic positions of paired atoms, is 0.1920 Å for atom pair O1; the arithmetic mean (d av) 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.

Table 2

Comparison of bond lengths (Å) in the title compound and the isotypic phosphate analogue (Catti & Ferraris, 1974 ▸)

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 + , z − ; (iii) x + 1, y, z − 1; (iv) x, y, z − 1; (v) x − 1, y, z.

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.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. 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.

Table 3

Experimental details

Crystal data
Chemical formula	Na(H2AsO4)
M r	163.93
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	100
a, b, c (Å)	7.0528 (14), 13.798 (3), 7.4792 (15)
β (°)	93.02 (3)
V (Å3)	726.8 (3)
Z	8
Radiation type	Mo Kα
μ (mm−1)	9.32
Crystal size (mm)	0.12 × 0.08 × 0.01
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.534, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	11092, 2651, 1890
R int	0.052
(sin θ/λ)max (Å−1)	0.758
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.030, 0.059, 1.04
No. of reflections	2651
No. of parameters	125
No. of restraints	4
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.87, −0.86

Computer programs: APEX3 and SAINT (Bruker, 2016 ▸), SHELXL2016 (Sheldrick, 2015 ▸), ATOMS (Dowty, 2006 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2056989017013470/hb7706sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989017013470/hb7706Isup2.hkl

CCDC reference: 1575494

Additional supporting information: crystallographic information; 3D view; checkCIF report

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)

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)