β-Zn₃(AsO₃)₂

In the title compound, trizinc(II) diarsenite or trizinc(II) bis­[trioxidoarsenate(III)], the constituent polyhedra that make up the structure are very distorted ZnO₄ tetra­hedra and AsO₃ trigonal pyramids. These species fuse together to generate a three-dimensional network containing unusual edge-sharing tetra­hedra, with a Zn...Zn separation of 2.903 (3) Å. The structure also features eight-ring pseudo-channels occupied by the As^III lone pairs of electrons. The title compound is a polymorph of the mineral reinerite, which also features edge-shared ZnO₄ tetra­hedra.

S1. Comment

XO₄ tetrahedra rarely share edges in crystal structures. The reason given historically is the unfavourable Coulombic repulsion that occurs due to the close contact of the central cations through the shared edge (Wells, 1962). However, a few structures containing such edge-shared tetrahedra are known, including the (non-isostructural) phases K₆Mn₂O₆ and K₆Fe₂O₆, which contain isolated [M₂O₆]^6- (M = Mn^III or Fe^III) tetrahedral pairs (Brachtel & Hoppe, 1978). For the first of these, the complete edge-shared tetrahedral pair is generated by 2/m crystallographic site symmetry and Mn···Mn = 2.780 Å (s.u.'s not supplied by authors). For K₆Fe₂O₆, the tetrahedral pair has inversion symmetry and Fe···Fe = 2.713 Å.

The phases Cs₂Co^IISiO₄ and Cs₅Co^IIISiO₆ (Hansing & Möller, 2001) contain the novel combination of edge-sharing CoO₄ and SiO₄ tetrahedra [Co···Si = 2.630 (8) and 2.7099 (19) Å for Cs₂CoSiO₄ and Cs₅CoSi₆, respectively]. The rare earth borates α-Ln₂B₄O₉ (Ln = Eu, Gd, Tb or Dy) (Emme & Huppertz, 2003) made under very high pressure (>8 GPa) contain novel B₂O₆ groups (i.e. edge-sharing BO₄ tetrahedra) with unprecedentedly short inversion-generated B···B contacts [e.g. 2.04 (2) Å in α-Gd₂B₄O₉] as part of an extended boron/oxygen network also containing corner-sharing BO₄ groups.

The mineral epididymite, of ideal composition Na₂Be₂Si₆O₁₅.H₂O (Gatta et al., 2008), contains edge-sharing BeO₄ tetrahedra [1; Be···Be = 2.3148 (5) Å] within an infinite BeO₄/SiO₄ array. Another mineral, reinerite, Zn₃(AsO₃)₂ (Ghose et al., 1977), hereinafter referred to as the α modification (space group Pbam) of this stoichiometry, contains edge-shared ZnO₄ tetrahedra [Zn···Zn = 2.8974 (13) Å; a crystallographic mirror plane generates the complete edge-shared unit] as well as vertex-sharing ZnO₄ groups.

In this paper, we report the solution-mediated synthesis and single-crystal structure of the title compound, β-Zn₃(AsO₃)₂, (I), a polymorph of reinerite which also contains edge-shared ZnO₄ tetrahedra.

Selected bond distances and angles for (I) are presented in Table 1 and its structure is illustrated in Fig. 1. There are three Zn, two As and six O atoms in the asymmetric unit of (I). Both As atoms form the centres of isolated (from each other) trigonal–pyramidal [AsO₃]^2- arsenite groups, with the unobserved As^III [Ar]3d¹⁰4s² lone pair of electrons presumed to occupy the fourth tetrahedral vertex. The mean As—O bond length and its estimated error (Taylor & Kennard, 1983) of 1.763±0.016 Å in (I) is similar to that in reinerite (1.769 Å; Ghose et al., 1977) and slightly less than that in leitite, ZnAs₂O₄ (1.784 Å; Ghose et al., 1987), which contains infinite catena-arsenite chains. The O—As—O bond angles in (I) are clustered in the narrow range 94.3 (5)–98.6 (4)°, all far smaller than the nominal tetrahedral bond angle of 109.5°, perhaps indicating that the sp³ hybridization model that justifies the pyramidal shape of the arsenite ion is simplistic. The As atoms are displaced from the planes of their three attached O atoms by 0.876 (6) (As1) and 0.888 (6) Å (As2).

The three Zn atoms in (I) form the centres of tetrahedral ZnO₄ groups. Both Zn1 and Zn2 share each of their four O-atom vertices with an As atom and a Zn atom, and unexceptional mean Zn—O distances of 1.969±0.011 and 1.965±0.027 Å arise for Zn1 and Zn2, respectively. The Zn3-centred tetrahedra form an edge-shared pair in (I) (Fig. 1) and a similar mean Zn3—O distance of 1.964±0.019 Å results. In terms of the variation ζ (Robinson et al., 1971) of the O—Zn—O bond angles from their nominal ideal values of 109.5°, the Zn2 tetrahedron displays a moderate degree of distortion with ζ = 36.6°², whereas the Zn1 and Zn3 polyhedra are grossly distorted (ζ = 161.6 and 204.5°², respectively). The displacements of the Zn atoms from the geometric centres (mean location of their four attached O atoms) are 0.19, 0.13 and 0.14 Å for Zn1, Zn2 and Zn3, respectively.

It is notable that all six asymmetric O atoms in (I) are tricoordinated to two Zn atoms plus one As atom, rather than their more common bicoordinate bridging mode. The mean Zn—O—Zn and Zn—O—As angles are 109.7±7.2 and 123.6±7.5°, respectively. For each O atom, its Zn—O—Zn angle is smaller than its two Zn—O—As angles. Atom O6 plays a key role by bridging the two Zn3 atoms in the edge-shared tetrahedra, as well as linking to As2. For nominally undistorted tetrahedra, the calculated Zn—O—Zn bond angle for an edge-shared pair is 70.5° and the Zn···Zn separation is 2.266 Å, assuming a Zn—O distance of 1.964 Å; the actual values of 94.9 (5)° and 2.903 (3) Å in (I) indicate the high degree of distortion that must occur to allow the tetrahedral edge-sharing interaction.

It seems remarkable that (I), as a polymorph of reinerite, (II), also features the very unusual situation of edge-shared ZnO₄ tetrahedra. Comparing (I) and (II) indicates that the two structures share some common features but also some significant differences. When (I) is viewed in projection down [010] (Fig. 2), the edge-shared tetrahedral pairs bridge the layers, forming a three-dimensional network. The structure features distinctive pseudo [010] channels [shortest As···As separation = 4.811 (2) Å], delineated by four ZnO₄ and four AsO₃ polyhedra; it seems that these channels could provide the required space for the As^III lone pairs. In (I), the Zn···Zn axis of the tetrahedral pair lies roughly perpendicular to the channels. The spatial requirement of lone pairs of electrons in oxo-anions has also been noted in tellurites (Brown, 1974) and selenites (Johnston & Harrison, 2001).

The structure of (II), when viewed down [001] (Fig. 3) shows similar eight-ring lone-pair channels, but the role of the edge-sharing tetrahedral pair is different: in this structure the Zn···Zn axes lie parallel to the [001] direction and hence serve to line the lone-pair channel. A comparison of the densities of (I) [4.383 Mg m^-3] and (II) [4.283 Mg m^-3] suggests that (I) may be more stable than (II) at low temperatures, in terms of the `density rule' of Burger & Ramberger (1979).

Related literature

For related literature, see: Brachtel & Hoppe (1978); Brown (1974); Burger & Ramberger (1979); Emme & Huppertz (2003); Gatta et al. (2008); Ghose et al. (1977, 1987); Hansing & Möller (2001); Johnston & Harrison (2001); Robinson et al. (1971); Taylor & Kennard (1983); Wells (1962).

S2. Experimental

ZnO (0.156 g) and NaAsO₂ (0.500 g) were placed in a 23 ml Teflon-lined autoclave with water (10 ml). The sealed vessel was heated to 423 K for 3 d and then slowly cooled to room temperature. Upon opening the autoclave, tiny rhombs and blocks of (I) were recovered, accompanied by significant amounts of unreacted ZnO.

S3. Refinement

The crystal used for data collection was very small and a rather weak diffraction pattern arose; this probbaly correlates with the rather high R_int value and a degree of noise in the final difference map; the deepest hole and highest peak are 0.93Å from As2 and 0.74Å from As2, respectively.

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997), and SORTAV (Blessing, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The asymmetric unit of (I), expanded to show the coordination environments of the Zn atoms and the edge-sharing Zn3O4 groups. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 - x, 1 - y, 1 - z; (ii) 1 - x, -y, 1 - z; (iii) x, 1/2 - y, z - 1/2; (iv) -x, y + 1/2, 1/2 - z; (v) x, 3/2 - y, z - 1/2; (vi) x, y + 1, z.]
	Fig. 2. Projection down [010] of the structure of (I), with the ZnO4 groups represented as polyhedra. In the electronic version of the paper, the Zn1- and Zn2-centred vertex-sharing tetrahedra are lilac, and the Zn3-centred edge-sharing groups are green.
	Fig. 3. Projection down [001] of the structure of reinerite (redrawn from the data of Ghose et al., 1977), with the ZnO4 groups represented as polyhedra. For colour key, see Fig. 2.

trizinc(II) bis[trioxidoarsenate(III)]

Crystal data

Zn3(AsO3)2	F(000) = 816
Mr = 441.95	Dx = 4.383 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 4282 reflections
a = 8.2438 (8) Å	θ = 2.9–27.5°
b = 5.1781 (4) Å	µ = 20.44 mm^−1
c = 15.8222 (16) Å	T = 120 K
β = 97.445 (5)°	Block, colourless
V = 669.71 (11) Å^3	0.02 × 0.01 × 0.01 mm
Z = 4

Data collection

Nonius KappaCCD area-detector diffractometer	1547 independent reflections
Radiation source: fine-focus sealed tube	921 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.154
ω and ϕ scans	θmax = 27.6°, θmin = 3.4°
Absorption correction: multi-scan (SADABS; Bruker, 2003)	h = −9→10
Tmin = 0.685, Tmax = 0.905	k = −6→6
7371 measured reflections	l = −20→20

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.064	w = 1/[σ^2(Fo^2) + (0.0596P)^2 + 1.1085P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.152	(Δ/σ)max < 0.001
S = 1.04	Δρmax = 1.59 e Å^−3
1547 reflections	Δρmin = −1.92 e Å^−3
101 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0027 (5)

Special details

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Zn1	0.5107 (2)	0.0498 (3)	0.31146 (11)	0.0157 (5)
Zn2	0.1301 (2)	0.6450 (3)	0.25995 (10)	0.0141 (5)
Zn3	0.3905 (2)	0.6274 (3)	0.43363 (11)	0.0153 (5)
As1	0.16665 (18)	0.1336 (3)	0.39382 (10)	0.0140 (4)
As2	0.25838 (18)	0.4589 (3)	0.63057 (9)	0.0144 (4)
O1	0.0765 (13)	0.0023 (19)	0.2960 (7)	0.019 (2)
O2	0.2229 (11)	0.440 (2)	0.3564 (6)	0.017 (2)
O3	0.3610 (12)	−0.0178 (19)	0.3961 (6)	0.017 (2)
O4	0.4115 (12)	0.3056 (19)	0.7011 (6)	0.017 (2)
O5	0.2789 (11)	0.7673 (18)	0.6766 (6)	0.012 (2)
O6	0.3835 (12)	0.504 (2)	0.5510 (6)	0.018 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0156 (9)	0.0142 (9)	0.0167 (9)	−0.0004 (7)	0.0005 (7)	0.0017 (7)
Zn2	0.0131 (9)	0.0139 (8)	0.0147 (9)	−0.0011 (7)	−0.0011 (7)	0.0017 (7)
Zn3	0.0147 (9)	0.0147 (9)	0.0157 (9)	−0.0001 (7)	−0.0009 (7)	0.0017 (7)
As1	0.0140 (8)	0.0131 (8)	0.0144 (8)	0.0010 (6)	−0.0003 (6)	−0.0006 (6)
As2	0.0123 (8)	0.0140 (8)	0.0168 (8)	0.0007 (6)	0.0018 (6)	−0.0010 (6)
O1	0.023 (6)	0.010 (5)	0.023 (6)	0.007 (4)	0.001 (4)	0.001 (4)
O2	0.007 (5)	0.031 (6)	0.013 (5)	−0.001 (4)	−0.002 (4)	0.002 (4)
O3	0.010 (5)	0.018 (5)	0.025 (6)	−0.006 (4)	0.007 (4)	−0.005 (5)
O4	0.017 (6)	0.016 (5)	0.017 (5)	−0.002 (4)	−0.006 (4)	0.001 (4)
O5	0.010 (5)	0.008 (5)	0.019 (5)	−0.006 (4)	0.010 (4)	−0.007 (4)
O6	0.015 (6)	0.032 (6)	0.007 (5)	0.003 (5)	0.000 (4)	0.005 (5)

Geometric parameters (Å, º)

Zn1—O5^i	1.964 (9)	As1—O2	1.776 (11)
Zn1—O3	1.967 (10)	As1—O3	1.780 (10)
Zn1—O4^ii	1.967 (10)	As2—O6	1.743 (10)
Zn1—O4^iii	1.978 (9)	As2—O5	1.755 (9)
Zn2—O2	1.933 (9)	As2—O4	1.761 (9)
Zn2—O1^iv	1.959 (10)	O1—Zn2^vii	1.959 (10)
Zn2—O5^v	1.966 (10)	O1—Zn2^viii	2.002 (10)
Zn2—O1^vi	2.002 (10)	O3—Zn3^viii	1.937 (10)
Zn3—O3^vi	1.937 (10)	O4—Zn1^ii	1.967 (10)
Zn3—O6^i	1.968 (10)	O4—Zn1^ix	1.978 (9)
Zn3—O6	1.973 (10)	O5—Zn1^i	1.964 (9)
Zn3—O2	1.977 (9)	O5—Zn2^x	1.966 (10)
Zn3—Zn3^i	2.903 (3)	O6—Zn3^i	1.968 (10)
As1—O1	1.765 (10)
O5^i—Zn1—O3	130.2 (4)	O1—As1—O3	97.1 (5)
O5^i—Zn1—O4^ii	99.5 (4)	O2—As1—O3	97.3 (5)
O3—Zn1—O4^ii	98.3 (4)	O6—As2—O5	98.3 (5)
O5^i—Zn1—O4^iii	99.3 (4)	O6—As2—O4	94.3 (5)
O3—Zn1—O4^iii	116.4 (4)	O5—As2—O4	97.2 (4)
O4^ii—Zn1—O4^iii	111.6 (3)	As1—O1—Zn2^vii	119.6 (6)
O2—Zn2—O1^iv	112.4 (4)	As1—O1—Zn2^viii	121.5 (5)
O2—Zn2—O5^v	116.0 (4)	Zn2^vii—O1—Zn2^viii	115.5 (5)
O1^iv—Zn2—O5^v	111.5 (4)	As1—O2—Zn2	131.3 (5)
O2—Zn2—O1^vi	111.2 (4)	As1—O2—Zn3	115.1 (5)
O1^iv—Zn2—O1^vi	105.4 (3)	Zn2—O2—Zn3	113.1 (5)
O5^v—Zn2—O1^vi	99.0 (4)	As1—O3—Zn3^viii	120.1 (5)
O3^vi—Zn3—O6^i	116.4 (4)	As1—O3—Zn1	123.4 (5)
O3^vi—Zn3—O6	125.3 (4)	Zn3^viii—O3—Zn1	108.2 (5)
O6^i—Zn3—O6	85.1 (5)	As2—O4—Zn1^ii	125.0 (5)
O3^vi—Zn3—O2	103.1 (4)	As2—O4—Zn1^ix	124.7 (6)
O6^i—Zn3—O2	118.8 (4)	Zn1^ii—O4—Zn1^ix	109.9 (4)
O6—Zn3—O2	108.6 (4)	As2—O5—Zn1^i	121.0 (5)
O3^vi—Zn3—Zn3^i	134.0 (3)	As2—O5—Zn2^x	117.0 (5)
O6^i—Zn3—Zn3^i	42.6 (3)	Zn1^i—O5—Zn2^x	116.4 (4)
O6—Zn3—Zn3^i	42.5 (3)	As2—O6—Zn3^i	120.3 (5)
O2—Zn3—Zn3^i	122.9 (3)	As2—O6—Zn3	144.6 (6)
O1—As1—O2	98.6 (4)	Zn3^i—O6—Zn3	94.9 (5)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+1, −y, −z+1; (iii) x, −y+1/2, z−1/2; (iv) −x, y+1/2, −z+1/2; (v) x, −y+3/2, z−1/2; (vi) x, y+1, z; (vii) −x, y−1/2, −z+1/2; (viii) x, y−1, z; (ix) x, −y+1/2, z+1/2; (x) x, −y+3/2, z+1/2.