Tricaesium tris­(oxalato-[kappa]²O¹,O²)chromate(III) dihydrate

The title compound, Cs₃[Cr(C₂O₄)₃]·2H₂O, has been synthesized for the first time and the spatial arrangement of the cations and anions is compared with those of the other members of the alkali metal series. The structure is built up of alternating layers of either the D or L enantio­mers of [Cr(oxalate)₃]^3-. Of note is that the distribution of the [Cr(oxalate)₃]^3- enantio­mers in the Li⁺, K⁺ and Rb⁺ tris(oxalato)chromates differs from those of the Na⁺ and Cs⁺ tris­(oxalato)chromates, and also differs within the corresponding BEDT-TTF [bis­(ethyl­enedithio)tetra­thia­fulvalene] conducting salts. The use of tris­(oxalato)chromate anions in the crystal engineering of BEDT-TTF salts is discussed, wherein the salts can be paramagnetic superconductors, semiconductors or metallic proton conductors, depending on whether the counter-cation is NH₄⁺, H₃O⁺, Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺. These materials can also be superconducting or semiconducting, depending on the spatial distribution of the D and L enantio­mers of [Cr(oxalate)₃]^3-.

S1. Comment

The title compound, Cs₃[Cr(C₂O₄)₃].2H₂O, (I), represents a new salt of the tris(oxalato)chromate(III) anion, as this is the first study of the caesium analogue and the first report of any tris(oxalato)metallate with Cs⁺ cations. The first examples of [Cr(oxalate)₃]^3- were studied by Werner (1912). The [Cr(oxalate)₃]^3- anion has more recently been used as a component in molecular radical cation salts of BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] (Martin et al., 1997), which have been widely studied because of the opportunities they afford for making new combinations of physical properties through crystal engineering. The series of BEDT-TTF salts containing tris(oxalato)chromate anions can be paramagnetic superconductors, semiconductors or metallic proton conductors, depending on whether the counter-cation is NH₄⁺, H₃O⁺, Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺ (Martin et al., 2007). These materials can also be superconducting or semiconducting, depending on the spatial distribution of the D and L enantiomers of [Cr(oxalate)₃]^3- (Martin et al., 1999). It is the opportunities for crystal engineering afforded by [Cr(oxalate)₃]^3- that have led us to synthesise the caesium analogue, (I), which completes the series of alkali metal tris(oxalato)chromates.

In (I), the Cr^III cation is coordinated by three bidentate oxalate groups in an approximate octahedral geometry (Fig. 1). The Cr—O distances fall in the range 1.967 (3)–1.984 (3) Å and the internal O—Cr—O angles of the chelate rings are 82.67 (13), 82.33 (13) and 82.29 (13)°, similar to those observed in the alkali metal tris(oxalato)chromate salts of lithium (Sekine et al., 1994), sodium (Bulc et al., 1982), rubidium (van Niekerk & Schoening, 1952) and potassium (Taylor, 1978). The Cs⁺ compound crystallises in the triclinic space group P1 and has two formula units in the asymmetric unit. The lithium analogue also crystallises in space group P1 with Z = 2, the sodium analogue in the monoclinic space group C2/c with Z = 8, and the potassium and rubidium analogues in the monoclinic space group P2₁/c, with Z = 4.

All three of the Cs⁺ cations form close contacts with both the oxalate and the water O atoms. The numerous Cs—O close contacts to oxalate O atoms are in the range 3.02 (1)–3.51 (1) Å, with each of the three Cs⁺ cations forming seven such Cs—O contacts, including a single short contact to a water O atom [3.11 (1), 3.11 (1) and 3.27 (1) Å]. There is hydrogen bonding between the oxalate O atoms and three of the H atoms on the two water molecules, while the fourth H atom forms a hydrogen bond with the O atom of a neighbouring water molecule (Table 1). By comparison, the lithium [Cr(oxalate)₃]^3- analogue has six water molecules, four of which are coordinated to Li⁺ cations and two tetrahedrally hydrogen-bonded to other water molecules or oxalate ligands. The three Li⁺ cations have octahedral, square-pyramidal and tetrahedral coordination geometries. The sodium salt contains five water molecules. Four of these form a separate layer containing polymeric chains of H₂O molecules and Na⁺ cations between the [Cr(oxalate)₃]^3- layers, whilst a fifth resides in cavities between these layers. The four water molecules in the layers with Na⁺ cations form two interlayer hydrogen bonds, while the fifth H₂O molecule forms hydrogen bonds with both the H₂O/Na⁺ layer and the [Cr(oxalate)₃]^3- layer. The Na⁺ cations in the H₂O/Na⁺ layers form close contacts with water O atoms within the layer and also with oxalate O atoms in the [Cr(oxalate)₃]^3- layer, whilst the other Na⁺ cations form only close contacts with oxalate O atoms in the anion layer. The potassium salt contains 2.7 water molecules, all of which form hydrogen bonds to two K⁺ cations and two oxalate O atoms. Two K⁺ cations are coordinated to six oxalate O atoms and two H₂O molecules, whilst the third K⁺ cation is coordinated to four oxalate O atoms and two H₂O molecules. The rubidium salt contains three water molecules which are hydrogen bonded to a single oxalate O atom each. Two H₂O are also hydrogen-bonded to each other, whilst the third is coordinated to two Rb⁺ cations. Two Rb⁺ cations are coordinated to six oxalate O atoms and two H₂O molecules, whilst the third is coordinated to five oxalate O atoms and two H₂O molecules.

The structure of Cs₃Cr(C₂O₄)₃.2H₂O is built up of layers of [Cr(oxalate)₃]^3- anions in the bc plane that stack in the a direction, with each layer consisting exclusively of only a single enantiomer (Fig. 2) of either D- or L-[Cr(oxalate)₃]^3- anions and adjacent layers consisting exclusively of the opposing enantiomer. Na₃Cr(oxalate)₃.5H₂O is also built up of layers of opposing D- or L-[Cr(oxalate)₃]^3- anions in the c direction, with adjacent layers having opposing enantiomers (Fig. 3). These [Cr(oxalate)₃]^3- layers are segregated by layers built up of polymeric chains of H₂O and Na⁺ cations. The anion packing in Li₃Cr(oxalate)₃.6H₂O does not consist of layers of a single enantiomer. Instead, pairs of D- and L-[Cr(oxalate)₃]^3- anions form columns in the a direction segregated by the H₂O/Li⁺ layers. The packing of [Cr(oxalate)₃]^3- anions in the potassium and rubidium salts is similar, with pairs of opposing enantiomers in the bc plane rather than enantiomers occupying discrete layers.

In summary, the [Cr(oxalate)₃]^3- anion has now been studied with a series of counter-cations including five alkali metal cations. It has been postulated that the structures of the NH₄⁺ and K⁺ salts differ due to the ability of the ammonium cation to form tetrahedral hydrogen bonds, and no conclusions could be drawn on the effects of changing the size of the cation (van Niekerk & Schoening, 1952). Whilst the alkali metal atomic radii increase in size in the order Li–Na–K–Rb–Cs, we have observed no direct correlation between cation size and packing of the [Cr(oxalate)₃]^3- layer. As expected, the smaller members of the family contain more coordinated water than the larger members. There is also no relationship between cation size and the chirality of the layers, with the Na⁺ and Cs⁺ salts having segregated layers of each enantiomer whilst the Li⁺, K⁺ and Rb⁺ salt layers contain mixtures of both enantiomers. The structures of the segregated layers of enantiomers in the Na⁺ and Cs⁺ salts differ greatly, as can be seen by the packing of [Cr(oxalate)₃]^3- anions in the Na⁺ salt in Fig. 3 compared with the Cs⁺ salt in Fig. 2.

The [Cr(oxalate)₃]^3- anion has been used as a component in molecular radical cation salts of BEDT-TTF to study the effects of small structural changes upon the bulk physical properties. The Li⁺, K⁺ and Rb⁺ salts all give a hexagonal packing arrangement of [Cr(oxalate)₃]^3- anions and cations, with a guest solvent molecule able to fit into the hexagonal cavity which differs in size depending on which cation is present. This can lead to metallic, semiconducting or superconducting behaviour. Interestingly, it is the Na⁺ and Cs⁺ salts which have given non-hexagonal packing arrangements in semiconducting BEDT-TTF salts (Martin et al., 2007 and 2008), both salts having no layer segration by enantiomer. The Na⁺ salt contains separate layers of BEDT-TTF, [Cr(oxalate)₃]^3- anions or Na⁺/H₂O, whilst the salt grown from Cs₃[Cr(oxalate)₃].2H₂O contains only layers of BEDT-TTF or H₂O/[Cr(oxalate)₃]^3-, with no Cs⁺ being included in the structure.

Related literature

For related literature, see: Bulc et al. (1982); Martin et al. (1997, 1999, 2007, 2008); Niekerk & Schoening (1952); Sekine et al. (1994); Taylor (1978); Werner (1912).

S2. Experimental

Dark-blue plate crystals of the title compound with well developed {010} faces were grown by slow evaporation from an aqueous solution of oxalic acid dihydrate, caesium oxalate monohydrate and caesium dichromate (7:2:1).

S3. Refinement

The H atoms were all located in a difference map. They were initially refined with soft restraints on the bond lengths and angles to regularise their geometry (O—H = 0.82 Å) and U_iso(H) (in the range 1.2–1.5 times U_eq of the parent atom), after which the positions were refined with riding constraints.

Computing details

Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO (Oxford Diffraction, 2009); data reduction: CrysAlis PRO (Oxford Diffraction, 2009); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS (Betteridge et al., 2003).

Figures

	Fig. 1. The asymmetric unit of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. The structure of (I), viewed down the a axis, showing the packing of the [Cr(oxalate)3]^3- anions, water molecules and Cs^+ cations. In the electronic version of the journal, the colour scheme is O red, C grey, Cr pink, Cs blue and H white.
	Fig. 3. The packing of a layer containing a single enantiomer of [Cr(oxalate)3]^3- in Na3[Cr(oxalate)3].5H2O (Bulc et al., 1982). In the electronic version of the journal, the colour scheme is O red, C grey and Cr pink [Blue?].

Tricaesium tris(oxalato-κ²O¹,O²)chromate(III) dihydrate

Crystal data

Cs3[Cr(C2O4)3]·2H2O	Z = 2
Mr = 750.80	F(000) = 682
Triclinic, P1	Dx = 3.081 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 8.2299 (5) Å	Cell parameters from 4324 reflections
b = 10.2956 (6) Å	θ = 3.0–29.0°
c = 10.9902 (5) Å	µ = 7.43 mm^−1
α = 63.662 (5)°	T = 120 K
β = 89.775 (4)°	Plate, blue
γ = 77.221 (5)°	0.22 × 0.15 × 0.13 mm
V = 809.14 (9) Å^3

Data collection

Xcalibur, Sapphire3 diffractometer	3326 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.018
ω/2θ scans	θmax = 29.1°, θmin = 3.0°
Absorption correction: multi-scan [CrysAlis PRO (Oxford Diffraction, 2009). Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm]	h = −8→10
Tmin = 0.743, Tmax = 1.000	k = −13→13
5562 measured reflections	l = −11→14
3631 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.025	H-atom parameters constrained
wR(F^2) = 0.059	w = 1/[σ^2(F^2) + ( 0.03P)^2 + 4.43P], where P = (max(Fo^2,0) + 2Fc^2)/3
S = 0.98	(Δ/σ)max = 0.002
3621 reflections	Δρmax = 0.91 e Å^−3
217 parameters	Δρmin = −1.62 e Å^−3
0 restraints

Special details

Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems open-flow nitrogen cryostat (Cosier & Glazer, 1986) with a nominal stability of 0.1 K.

Cosier, J. & Glazer, A. M. (1986). J. Appl. Cryst. 19, 105–107.

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

	x	y	z	Uiso*/Ueq
Cs1	0.24540 (3)	1.40826 (3)	−0.37553 (3)	0.0105
Cs2	0.69122 (3)	1.03653 (3)	0.33926 (3)	0.0124
Cs3	0.17374 (3)	1.72892 (3)	0.10003 (3)	0.0115
Cr4	0.22095 (8)	1.09160 (7)	0.23561 (7)	0.0072
O5	0.0406 (4)	1.2776 (3)	0.1568 (3)	0.0109
C6	0.0508 (5)	1.3697 (5)	0.2030 (4)	0.0095
O7	−0.0465 (4)	1.4936 (3)	0.1676 (3)	0.0127
C8	0.2044 (5)	1.3157 (5)	0.3112 (4)	0.0101
O9	0.2811 (4)	1.1766 (3)	0.3532 (3)	0.0113
O10	0.2451 (4)	1.4022 (4)	0.3465 (3)	0.0159
O11	0.1677 (4)	1.0415 (3)	0.0895 (3)	0.0099
C12	0.2503 (5)	1.0898 (5)	−0.0156 (4)	0.0108
O13	0.2270 (4)	1.0800 (4)	−0.1212 (3)	0.0167
C14	0.3837 (5)	1.1697 (5)	−0.0008 (4)	0.0112
O15	0.3835 (4)	1.1824 (3)	0.1113 (3)	0.0102
O16	0.4730 (4)	1.2176 (4)	−0.0911 (3)	0.0184
O17	0.0684 (4)	0.9793 (3)	0.3519 (3)	0.0115
C18	0.1220 (5)	0.8372 (5)	0.3934 (4)	0.0114
O19	0.0362 (4)	0.7456 (4)	0.4400 (4)	0.0179
C20	0.3115 (5)	0.7886 (5)	0.3772 (4)	0.0098
O21	0.3855 (4)	0.8979 (3)	0.3349 (3)	0.0112
O22	0.3753 (4)	0.6587 (4)	0.4026 (3)	0.0164
O23	0.3219 (4)	1.5124 (4)	−0.0218 (4)	0.0214
O24	0.6059 (5)	1.3623 (4)	0.3025 (4)	0.0243
H232	0.3810	1.4263	0.0196	0.0321*
H242	0.5066	1.3760	0.3194	0.0365*
H231	0.2406	1.5125	−0.0641	0.0321*
H241	0.6028	1.4155	0.2190	0.0365*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs1	0.01255 (13)	0.01076 (13)	0.00794 (13)	−0.00429 (10)	0.00199 (9)	−0.00333 (10)
Cs2	0.00942 (13)	0.01633 (15)	0.01053 (13)	−0.00297 (10)	0.00038 (10)	−0.00530 (11)
Cs3	0.01112 (13)	0.00979 (13)	0.01416 (14)	−0.00417 (10)	0.00049 (10)	−0.00511 (11)
Cr4	0.0078 (3)	0.0070 (3)	0.0063 (3)	−0.0028 (2)	0.0008 (2)	−0.0022 (3)
O5	0.0107 (15)	0.0105 (15)	0.0112 (15)	−0.0011 (12)	−0.0022 (12)	−0.0053 (13)
C6	0.009 (2)	0.013 (2)	0.0077 (19)	−0.0058 (16)	0.0031 (15)	−0.0041 (17)
O7	0.0126 (15)	0.0096 (15)	0.0147 (16)	−0.0012 (12)	0.0016 (12)	−0.0052 (13)
C8	0.010 (2)	0.014 (2)	0.0053 (19)	−0.0037 (17)	0.0005 (15)	−0.0032 (17)
O9	0.0153 (15)	0.0088 (15)	0.0094 (15)	−0.0024 (12)	−0.0008 (12)	−0.0039 (13)
O10	0.0225 (17)	0.0161 (17)	0.0140 (16)	−0.0095 (14)	0.0023 (13)	−0.0090 (14)
O11	0.0117 (15)	0.0094 (15)	0.0096 (14)	−0.0051 (12)	0.0020 (12)	−0.0040 (12)
C12	0.011 (2)	0.008 (2)	0.012 (2)	−0.0005 (16)	0.0008 (16)	−0.0041 (17)
O13	0.0221 (17)	0.0193 (18)	0.0133 (16)	−0.0094 (14)	0.0022 (13)	−0.0095 (14)
C14	0.010 (2)	0.010 (2)	0.010 (2)	−0.0014 (16)	−0.0005 (16)	−0.0021 (17)
O15	0.0137 (15)	0.0117 (15)	0.0073 (14)	−0.0070 (12)	0.0026 (12)	−0.0043 (12)
O16	0.0151 (16)	0.028 (2)	0.0123 (16)	−0.0115 (14)	0.0049 (13)	−0.0060 (15)
O17	0.0101 (15)	0.0109 (15)	0.0115 (15)	−0.0033 (12)	0.0039 (12)	−0.0030 (13)
C18	0.012 (2)	0.012 (2)	0.010 (2)	−0.0039 (16)	0.0007 (16)	−0.0039 (18)
O19	0.0100 (15)	0.0143 (17)	0.0267 (19)	−0.0061 (13)	0.0036 (13)	−0.0054 (15)
C20	0.010 (2)	0.012 (2)	0.0057 (19)	−0.0025 (16)	−0.0012 (15)	−0.0022 (17)
O21	0.0092 (14)	0.0094 (15)	0.0114 (15)	−0.0022 (12)	0.0003 (12)	−0.0017 (13)
O22	0.0173 (17)	0.0094 (16)	0.0198 (17)	−0.0038 (13)	0.0049 (13)	−0.0041 (14)
O23	0.0171 (17)	0.0216 (19)	0.0283 (19)	−0.0024 (14)	−0.0039 (14)	−0.0147 (16)
O24	0.0249 (19)	0.026 (2)	0.0247 (19)	−0.0111 (16)	0.0064 (15)	−0.0115 (17)

Geometric parameters (Å, º)

Cr4—O5	1.975 (3)	C12—C14	1.558 (6)
Cr4—O9	1.970 (3)	C14—O15	1.296 (5)
Cr4—O11	1.969 (3)	C14—O16	1.219 (5)
Cr4—O15	1.984 (3)	O17—C18	1.291 (5)
Cr4—O17	1.966 (3)	C18—O19	1.230 (5)
Cr4—O21	1.972 (3)	C18—C20	1.564 (6)
O5—C6	1.275 (5)	C20—O21	1.300 (5)
C6—O7	1.237 (5)	C20—O22	1.225 (5)
C6—C8	1.560 (6)	O23—H232	0.826
C8—O9	1.295 (5)	O23—H231	0.816
C8—O10	1.223 (5)	O24—H242	0.833
O11—C12	1.294 (5)	O24—H241	0.831
C12—O13	1.229 (5)
O5—Cr4—O9	82.68 (13)	O9—C8—O10	125.9 (4)
O5—Cr4—O11	91.74 (13)	Cr4—O9—C8	113.4 (3)
O9—Cr4—O11	169.10 (13)	Cr4—O11—C12	115.1 (3)
O5—Cr4—O15	93.94 (13)	O11—C12—O13	125.2 (4)
O9—Cr4—O15	88.75 (13)	O11—C12—C14	114.1 (4)
O11—Cr4—O15	82.29 (12)	O13—C12—C14	120.7 (4)
O5—Cr4—O17	91.23 (13)	C12—C14—O15	113.4 (4)
O9—Cr4—O17	97.77 (13)	C12—C14—O16	120.9 (4)
O11—Cr4—O17	91.68 (13)	O15—C14—O16	125.7 (4)
O15—Cr4—O17	172.15 (13)	Cr4—O15—C14	115.1 (3)
O5—Cr4—O21	172.73 (13)	Cr4—O17—C18	112.0 (3)
O9—Cr4—O21	94.89 (13)	O17—C18—O19	125.2 (4)
O11—Cr4—O21	91.73 (13)	O17—C18—C20	113.4 (4)
O15—Cr4—O21	92.85 (13)	O19—C18—C20	121.4 (4)
O17—Cr4—O21	82.29 (13)	C18—C20—O21	113.2 (4)
Cr4—O5—C6	114.4 (3)	C18—C20—O22	120.2 (4)
O5—C6—O7	125.8 (4)	O21—C20—O22	126.6 (4)
O5—C6—C8	114.2 (4)	Cr4—O21—C20	110.6 (3)
O7—C6—C8	120.0 (4)	H232—O23—H231	109.5
C6—C8—O9	113.4 (4)	H242—O24—H241	105.2
C6—C8—O10	120.7 (4)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O23—H232···O15	0.83	2.25	2.963 (7)	145
O24—H242···O10	0.83	2.15	2.974 (7)	173
O23—H231···C6^i	0.82	2.58	3.333 (7)	154
O23—H231···O7^i	0.82	1.99	2.809 (7)	178
O24—H241···O23^ii	0.83	2.10	2.886 (7)	158

Symmetry codes: (i) −x, −y+3, −z; (ii) −x+1, −y+3, −z.