NaIn(CrO₄)₂·2H₂O, the first indium(III) member of the kröhnkite family

Sodium indium(III) chromate(VI) dihydrate, NaIn(CrO₄)₂·2H₂O, synthesized from an aqueous solution at room temperature, is the first indium(III) member of the large family of compounds with kröhnkite [Na₂Cu^II(S^VIO₄)₂·2H₂O]-type chains. The crystal structure is based on infinite octa­hedral-tetra­hedral [In(CrO₄)₂(H₂O)₂]^- chains along [010], linked via charge-balancing Na⁺ cations. The slightly distorted InO₄(H₂O)₂ octa­hedra are characterized by a mean In-O distance of 2.125 Å. The CrO₄ tetra­hedra are strongly distorted (mean Cr-O = 1.641 Å). The Na atom shows an octa­hedral coordination, unprecedented among compounds with kröhnkite-type chains. The NaO₆ octa­hedra share opposite edges with the InO₄(H₂O)₂ octa­hedra to form infinite [001] chains. The hydrogen bonds are of medium strength. NaIn(CrO₄)₂·2H₂O belongs to the structural type F2 in the classification of Fleck, Kolitsch & Hertweck [Z. Kristallogr. (2002), 217, 435-443], and is isotypic with KAl(CrO₄)₂·2H₂O and MFe(CrO₄)₂·2H₂O (M = K, Tl or NH₄). All atoms are in special positions except one O atom.

S1. Comment

NaIn(CrO₄)₂·2H₂O was synthesized from an aqueous solution at room temperature as part of a comprehensive study of the crystal chemistry of the large kröhnkite [Na₂Cu^II(S^VIO₄)₂·2H₂O] family of oxysalts. The title compound is the first In^III member of this family, which comprises both natural and synthetic oxysalt compounds based on infinite octahedral–tetrahedral [M(XO₄)₂(H₂O)₂] chains, where M is either divalent (Mg, Mn, Fe, Co, Ni, Cu, Zn or Cd) or trivalent (Al, Fe, Sc, In or Tl), and where X is either pentavalent (P or As) or hexavalent (S, Se, Cr, Mo or W), as discussed in detail in our previous classification (Fleck et al., 2002) and subsequent contributions (Fleck & Kolitsch, 2003; Kolitsch & Fleck, 2005, 2006). In the kröhnkite-type chains, MO₆ octahedra are corner-linked to bridging XO₄ tetrahedra. Very small to very large mono- or divalent A atoms occupy the space between adjacent chains and provide charge balance. The resulting general formula is A_nM(XO₄)₂·2H₂O, where A = Na, K, Rb, Cs, Ag, Tl, NH₄, H or Ca (n = 1, 2).

NaIn(CrO₄)₂·2H₂O belongs to the structural type F2 in the classification of Fleck et al. (2002), and is isotypic with KAl(CrO₄)₂·2H₂O (Cudennec & Riou, 1977) and MFe(CrO₄)₂·2H₂O (M = K, Tl or NH₄) (Gravereau & Hardy, 1972). Note that the crystal structures of these previously reported chromates have been described in a non-standard setting (same space group, but with β > 120°); the title compound is described here using a standard setting.

Interestingly, NaIn(CrO₄)₂·2H₂O is not isotypic with the other known sodium metal(III) chromates containing kröhnkite-type chains, viz. NaAl(CrO₄)₂·2H₂O (Cudennec & Riou, 1977) and NaFe(CrO₄)₂·2H₂O (Hardy & Gravereau, 1970), although these two crystallize in a closely related structure type (space group C2/c; type F1 in the classification of Fleck et al., 2002). Efforts to synthesize the K and Rb analogues of the title compound from aqeuous solutions at room temperature have so far been unsuccessful.

The crystal structure of NaIn(CrO₄)₂·2H₂O is based on infinite octahedral–tetrahedral [In(CrO₄)₂(H₂O)₂]⁻ chains extending along [010], linked via charge-balancing Na⁺ cations (Figs. 1–3). The slightly distorted InO₄(H₂O)₂ octahedra are characterized by a mean In—O distance of 2.125 Å. The CrO₄ tetrahedra show a very strong bond-length distortion (Table 1), with a mean Cr—O distance of 1.641 Å. The Na atom shows a distinct octahedral coordination, with a mean Na—O bond length of 2.587 Å (Table 1; further O neighbours are at distances > 3.16 Å). Such an octahedral coordination of A atoms is unprecedented among compounds based on kröhnkite-type chains. The distorted NaO₆ octahedra share opposite edges with the InO₄(H₂O)₂ octahedra to form infinite [001] chains (Fig. 2), i.e. these octahedral–octahedral chains extend perpendicular to the octahedral–tetrahedral chains. In other kröhnkite-type sodium oxysalts, the Na atoms have one of three coordination types. Firstly, Na may have a distinct [7]-coordination [Kröhnkite, Na₂Cu(SO₄)₂·2H₂O (monoclinic, type D) (Hawthorne & Ferguson, 1975) or Na₂Mn(XO₄)₂·2H₂O (X = S or Se) (monoclinic, type D) (Wildner & Stoilova, 2003)]. Secondly, Na may have a poorly defined [7]- to [8]-coordination [Na₂M(SeO₄)₂·2H₂O (M = Zn, Co or Ni) (triclinic, type A) (Wildner & Stoilova, 2003) or Na₂Cd(SO₄)₂·2H₂O (monoclinic, type D) (Wildner & Stoilova, 2003)]. Thirdly, Na may have a [6 + 1]-coordination [Na₂Cu(SeO₄)₂·2H₂O (triclinic, type A) (Peytavin et al., 1974) or Na₂Cd(SeO₄)₂·2H₂O (monoclinic, type D) (Wildner & Stoilova, 2003)]. In this last case, even if the seventh O ligand (at about 2.7 Å) were to be neglected, the resulting distorted NaO₆ octahedra would be connected to adjacent octahedra in a different way from that in the title compound, i.e. no infinite octahedral–octahedral chains are formed.

Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O'Keeffe (1991). The bond-valence sums are 0.72 (Na), 3.29 (In), 6.09 (Cr), 0.48 (O1 = H₂O), 1.78 (O2), 1.72 (O3), and 2.06 (O4) v.u. (valence units), and thus are all reasonably close to ideal values. Although the relatively low bond-valence sum for the Na site might indicate that the Na⁺ cation is slightly too small for the void in which it is located, the equivalent displacement parameter of the Na atom does not indicate that it `rattles' within its void. The somewhat undersaturated O3 and O2 ligands are, as expected, acceptors of the two hydrogen bonds (Table 2). These bonds, which both reinforce the atomic arrangement along [001] (Figs. 1 and 2), are of medium strength (Table 2).

S2. Experimental

Tiny orange–yellow pointed prisms of the title compound crystallized at room temperature from an acidic aqueous solution (pH about 3) containing dissolved Na₂CO₃, In(NO₃)₃·H₂O and CrO₃ (Quantities or molar ratio?) in distilled water (Volume?). The crystals were accompanied by minor quantities of small yellow plates of Na₂Cr₂O₇·2H₂O (Kharitonov et al., 1969, 1970; Bulka et al., 1973) and large colourless rounded block-like crystals of NaNO₃.

S3. Refinement

All O—H distances were restrained to a length of 0.90 (5) Å. Isotropic displacement parameters of the H atoms were refined freely; the results show that atom H2 has an anomalously high U_iso value and thus appears to be disordered to some extent (it is also involved in the weaker of the two hydrogen bonds). The highest electron-density peak in NaIn(CrO₄)₂·2H₂O, 1.1 e Å⁻³, is at a distance of 0.42 Å from the O4 site. The deepest hole in the difference map, −0.9 e Å⁻³, is at a distance of 0.87 Å from the O4 site.

Computing details

Data collection: COLLECT (Nonius, 2004); cell refinement: SCALEPACK (Otwinowski et al., 2003); data reduction: SCALEPACK and DENZO (Otwinowski et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1999) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The crystal structure of NaIn(CrO4)2·2H2O in views (a) along [010], in the direction of the infinite kröhnkite-type octahedral–tetrahedral [In(CrO4)2(H2O)2]^− chains, and (b) along [110], allowing a suitable top view of the kröhnkite-type chains. InO4(H2O)2 octahedra are bridged by CrO4 tetrahedra. The intercalated Na^+ cations (shown as spheres) are octahedrally coordinated (compare Fig. 2). The hydrogen bonding is indicated as dashed lines and the unit cell is outlined.
	Fig. 2. Two views of the octahedral–octahedral chains in NaIn(CrO4)2·2H2O, (a) along [001], parallel to the chains, and (b) along [201], perpendicular to the chains. Distorted NaO6 octahedra share opposite edges with InO4(H2O)2 octahedra to form infinite [001] chains. Such octahedral–octahedral chains are unprecedented among compounds containing kröhnkite-type chains. The hydrogen bonding is indicated as dashed lines and the unit cell is outlined.
	Fig. 3. The connectivity in NaIn(CrO4)2·2H2O, shown with displacement ellipsoids at the 70% probability level. [Symmetry codes: (ii) −x,y,-z; (iii) −x,-y,-z; (iv) x,-y,z.]

sodium indium(III) chromate(VI) dihydrate

Crystal data

NaIn(CrO4)2·2H2O	F(000) = 384
Mr = 405.84	Dx = 3.286 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 806 reflections
a = 10.741 (2) Å	θ = 2.0–32.5°
b = 5.567 (1) Å	µ = 5.48 mm^−1
c = 7.497 (1) Å	T = 293 K
β = 113.78 (3)°	Prism, orange–yellow
V = 410.23 (15) Å^3	0.06 × 0.02 × 0.02 mm
Z = 2

Data collection

Nonius KappaCCD area-detector diffractometer	804 independent reflections
Radiation source: fine-focus sealed tube	708 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.015
ϕ and ω scans	θmax = 32.5°, θmin = 4.0°
Absorption correction: multi-scan (SCALEPACK; Otwinowski et al., 2003)	h = −15→16
Tmin = 0.735, Tmax = 0.898	k = −8→8
1474 measured reflections	l = −11→11

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.060	w = 1/[σ^2(Fo^2) + (0.03P)^2 + 1.25P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
804 reflections	Δρmax = 1.12 e Å^−3
49 parameters	Δρmin = −0.90 e Å^−3
2 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0040 (8)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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
Na	0.0000	0.0000	0.5000	0.0410 (7)
In	0.0000	0.0000	0.0000	0.01388 (12)
Cr	0.11112 (5)	0.5000	0.31447 (7)	0.01379 (13)
O1	0.2171 (3)	0.0000	0.0714 (4)	0.0231 (6)
O2	0.2492 (3)	0.5000	0.2785 (5)	0.0281 (6)
O3	0.1479 (3)	0.5000	0.5428 (4)	0.0373 (8)
O4	0.0193 (2)	0.2525 (5)	0.2181 (4)	0.0542 (9)
H1	0.271 (5)	0.0000	0.193 (6)	0.039 (15)*
H2	0.240 (14)	0.0000	−0.028 (14)	0.17 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na	0.0288 (13)	0.072 (2)	0.0226 (13)	0.000	0.0111 (10)	0.000
In	0.01485 (17)	0.01127 (17)	0.01448 (18)	0.000	0.00484 (12)	0.000
Cr	0.0144 (3)	0.0150 (3)	0.0110 (3)	0.000	0.0041 (2)	0.000
O1	0.0166 (12)	0.0347 (16)	0.0160 (13)	0.000	0.0044 (10)	0.000
O2	0.0212 (13)	0.0278 (15)	0.0380 (18)	0.000	0.0147 (13)	0.000
O3	0.0347 (17)	0.060 (2)	0.0137 (13)	0.000	0.0058 (12)	0.000
O4	0.0385 (13)	0.0631 (18)	0.0761 (19)	−0.0321 (12)	0.0388 (14)	−0.0595 (16)

Geometric parameters (Å, º)

In—O4	2.102 (2)	Cr—O4^iv	1.679 (2)
In—O4^i	2.102 (2)	Na—O2^v	2.531 (3)
In—O4^ii	2.102 (2)	Na—O2^vi	2.531 (3)
In—O4^iii	2.102 (2)	Na—O4^vii	2.615 (3)
In—O1^i	2.172 (3)	Na—O4^ii	2.615 (3)
In—O1	2.172 (3)	Na—O4^viii	2.615 (3)
Cr—O3	1.594 (3)	Na—O4	2.615 (3)
Cr—O2	1.611 (3)	O1—H1	0.86 (4)
Cr—O4	1.679 (2)	O1—H2	0.88 (5)
O2^v—Na—O2^vi	180.00 (9)	O4^ii—In—O4^iii	180.00 (12)
O2^v—Na—O4^vii	83.10 (9)	O4—In—O1^i	87.19 (9)
O2^vi—Na—O4^vii	96.90 (9)	O4^i—In—O1^i	92.81 (9)
O2^v—Na—O4^ii	96.90 (9)	O4^ii—In—O1^i	87.19 (9)
O2^vi—Na—O4^ii	83.10 (9)	O4^iii—In—O1^i	92.81 (9)
O4^vii—Na—O4^ii	180.0	O4—In—O1	92.81 (9)
O2^v—Na—O4^viii	83.10 (9)	O4^i—In—O1	87.19 (9)
O2^vi—Na—O4^viii	96.90 (9)	O4^ii—In—O1	92.81 (9)
O4^vii—Na—O4^viii	65.03 (10)	O4^iii—In—O1	87.19 (9)
O4^ii—Na—O4^viii	114.97 (10)	O1^i—In—O1	180.00 (15)
O2^v—Na—O4	96.90 (9)	O3—Cr—O2	109.49 (17)
O2^vi—Na—O4	83.10 (9)	O3—Cr—O4	108.09 (13)
O4^vii—Na—O4	114.97 (10)	O2—Cr—O4	110.41 (10)
O4^ii—Na—O4	65.03 (10)	O3—Cr—O4^iv	108.09 (13)
O4^viii—Na—O4	180.0	O2—Cr—O4^iv	110.41 (10)
O4—In—O4^i	180.00 (12)	O4—Cr—O4^iv	110.3 (2)
O4—In—O4^ii	83.9 (2)	In—O1—H1	117 (4)
O4^i—In—O4^ii	96.1 (2)	In—O1—H2	116 (9)
O4—In—O4^iii	96.1 (2)	H1—O1—H2	127 (10)
O4^i—In—O4^iii	83.9 (2)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O3^v	0.86 (4)	1.81 (4)	2.662 (4)	168 (6)
O1—H2···O2^ix	0.88 (5)	1.92 (6)	2.789 (5)	168 (13)

Symmetry codes: (v) −x+1/2, −y+1/2, −z+1; (ix) −x+1/2, −y+1/2, −z.