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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-.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270110014861/sq3237sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270110014861/sq3237Isup2.hkl
Contains datablock I

CCDC reference: 782523

Comment top

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 CrIII 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 top

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).

Experimental top

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).

Refinement top

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 Uiso(H) (in the range 1.2–1.5 times Ueq of the parent atom), after which the positions were refined with riding constraints.

Computing details top

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 top
[Figure 1] Fig. 1. The asymmetric unit of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
[Figure 2] 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.
[Figure 3] 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 top
Crystal data top
Cs3[Cr(C2O4)3]·2H2OZ = 2
Mr = 750.80F(000) = 682
Triclinic, P1Dx = 3.081 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 8.2299 (5) ÅCell parameters from 4324 reflections
b = 10.2956 (6) Åθ = 3.0–29.0°
c = 10.9902 (5) ŵ = 7.43 mm1
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) Å3
Data collection top
Xcalibur, Sapphire3
diffractometer
3326 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.018
ω/2θ scansθmax = 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 = 810
Tmin = 0.743, Tmax = 1.000k = 1313
5562 measured reflectionsl = 1114
3631 independent reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.025H-atom parameters constrained
wR(F2) = 0.059 w = 1/[σ2(F2) + ( 0.03P)2 + 4.43P],
where P = (max(Fo2,0) + 2Fc2)/3
S = 0.98(Δ/σ)max = 0.002
3621 reflectionsΔρmax = 0.91 e Å3
217 parametersΔρmin = 1.62 e Å3
0 restraints
Crystal data top
Cs3[Cr(C2O4)3]·2H2Oγ = 77.221 (5)°
Mr = 750.80V = 809.14 (9) Å3
Triclinic, P1Z = 2
a = 8.2299 (5) ÅMo Kα radiation
b = 10.2956 (6) ŵ = 7.43 mm1
c = 10.9902 (5) ÅT = 120 K
α = 63.662 (5)°0.22 × 0.15 × 0.13 mm
β = 89.775 (4)°
Data collection top
Xcalibur, Sapphire3
diffractometer
3631 independent reflections
Absorption correction: multi-scan
[CrysAlis PRO (Oxford Diffraction, 2009). Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm]
3326 reflections with I > 2σ(I)
Tmin = 0.743, Tmax = 1.000Rint = 0.018
5562 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0250 restraints
wR(F2) = 0.059H-atom parameters constrained
S = 0.98Δρmax = 0.91 e Å3
3621 reflectionsΔρmin = 1.62 e Å3
217 parameters
Special details top

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 (Å2) top
xyzUiso*/Ueq
Cs10.24540 (3)1.40826 (3)0.37553 (3)0.0105
Cs20.69122 (3)1.03653 (3)0.33926 (3)0.0124
Cs30.17374 (3)1.72892 (3)0.10003 (3)0.0115
Cr40.22095 (8)1.09160 (7)0.23561 (7)0.0072
O50.0406 (4)1.2776 (3)0.1568 (3)0.0109
C60.0508 (5)1.3697 (5)0.2030 (4)0.0095
O70.0465 (4)1.4936 (3)0.1676 (3)0.0127
C80.2044 (5)1.3157 (5)0.3112 (4)0.0101
O90.2811 (4)1.1766 (3)0.3532 (3)0.0113
O100.2451 (4)1.4022 (4)0.3465 (3)0.0159
O110.1677 (4)1.0415 (3)0.0895 (3)0.0099
C120.2503 (5)1.0898 (5)0.0156 (4)0.0108
O130.2270 (4)1.0800 (4)0.1212 (3)0.0167
C140.3837 (5)1.1697 (5)0.0008 (4)0.0112
O150.3835 (4)1.1824 (3)0.1113 (3)0.0102
O160.4730 (4)1.2176 (4)0.0911 (3)0.0184
O170.0684 (4)0.9793 (3)0.3519 (3)0.0115
C180.1220 (5)0.8372 (5)0.3934 (4)0.0114
O190.0362 (4)0.7456 (4)0.4400 (4)0.0179
C200.3115 (5)0.7886 (5)0.3772 (4)0.0098
O210.3855 (4)0.8979 (3)0.3349 (3)0.0112
O220.3753 (4)0.6587 (4)0.4026 (3)0.0164
O230.3219 (4)1.5124 (4)0.0218 (4)0.0214
O240.6059 (5)1.3623 (4)0.3025 (4)0.0243
H2320.38101.42630.01960.0321*
H2420.50661.37600.31940.0365*
H2310.24061.51250.06410.0321*
H2410.60281.41550.21900.0365*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cs10.01255 (13)0.01076 (13)0.00794 (13)0.00429 (10)0.00199 (9)0.00333 (10)
Cs20.00942 (13)0.01633 (15)0.01053 (13)0.00297 (10)0.00038 (10)0.00530 (11)
Cs30.01112 (13)0.00979 (13)0.01416 (14)0.00417 (10)0.00049 (10)0.00511 (11)
Cr40.0078 (3)0.0070 (3)0.0063 (3)0.0028 (2)0.0008 (2)0.0022 (3)
O50.0107 (15)0.0105 (15)0.0112 (15)0.0011 (12)0.0022 (12)0.0053 (13)
C60.009 (2)0.013 (2)0.0077 (19)0.0058 (16)0.0031 (15)0.0041 (17)
O70.0126 (15)0.0096 (15)0.0147 (16)0.0012 (12)0.0016 (12)0.0052 (13)
C80.010 (2)0.014 (2)0.0053 (19)0.0037 (17)0.0005 (15)0.0032 (17)
O90.0153 (15)0.0088 (15)0.0094 (15)0.0024 (12)0.0008 (12)0.0039 (13)
O100.0225 (17)0.0161 (17)0.0140 (16)0.0095 (14)0.0023 (13)0.0090 (14)
O110.0117 (15)0.0094 (15)0.0096 (14)0.0051 (12)0.0020 (12)0.0040 (12)
C120.011 (2)0.008 (2)0.012 (2)0.0005 (16)0.0008 (16)0.0041 (17)
O130.0221 (17)0.0193 (18)0.0133 (16)0.0094 (14)0.0022 (13)0.0095 (14)
C140.010 (2)0.010 (2)0.010 (2)0.0014 (16)0.0005 (16)0.0021 (17)
O150.0137 (15)0.0117 (15)0.0073 (14)0.0070 (12)0.0026 (12)0.0043 (12)
O160.0151 (16)0.028 (2)0.0123 (16)0.0115 (14)0.0049 (13)0.0060 (15)
O170.0101 (15)0.0109 (15)0.0115 (15)0.0033 (12)0.0039 (12)0.0030 (13)
C180.012 (2)0.012 (2)0.010 (2)0.0039 (16)0.0007 (16)0.0039 (18)
O190.0100 (15)0.0143 (17)0.0267 (19)0.0061 (13)0.0036 (13)0.0054 (15)
C200.010 (2)0.012 (2)0.0057 (19)0.0025 (16)0.0012 (15)0.0022 (17)
O210.0092 (14)0.0094 (15)0.0114 (15)0.0022 (12)0.0003 (12)0.0017 (13)
O220.0173 (17)0.0094 (16)0.0198 (17)0.0038 (13)0.0049 (13)0.0041 (14)
O230.0171 (17)0.0216 (19)0.0283 (19)0.0024 (14)0.0039 (14)0.0147 (16)
O240.0249 (19)0.026 (2)0.0247 (19)0.0111 (16)0.0064 (15)0.0115 (17)
Geometric parameters (Å, º) top
Cr4—O51.975 (3)C12—C141.558 (6)
Cr4—O91.970 (3)C14—O151.296 (5)
Cr4—O111.969 (3)C14—O161.219 (5)
Cr4—O151.984 (3)O17—C181.291 (5)
Cr4—O171.966 (3)C18—O191.230 (5)
Cr4—O211.972 (3)C18—C201.564 (6)
O5—C61.275 (5)C20—O211.300 (5)
C6—O71.237 (5)C20—O221.225 (5)
C6—C81.560 (6)O23—H2320.826
C8—O91.295 (5)O23—H2310.816
C8—O101.223 (5)O24—H2420.833
O11—C121.294 (5)O24—H2410.831
C12—O131.229 (5)
O5—Cr4—O982.68 (13)O9—C8—O10125.9 (4)
O5—Cr4—O1191.74 (13)Cr4—O9—C8113.4 (3)
O9—Cr4—O11169.10 (13)Cr4—O11—C12115.1 (3)
O5—Cr4—O1593.94 (13)O11—C12—O13125.2 (4)
O9—Cr4—O1588.75 (13)O11—C12—C14114.1 (4)
O11—Cr4—O1582.29 (12)O13—C12—C14120.7 (4)
O5—Cr4—O1791.23 (13)C12—C14—O15113.4 (4)
O9—Cr4—O1797.77 (13)C12—C14—O16120.9 (4)
O11—Cr4—O1791.68 (13)O15—C14—O16125.7 (4)
O15—Cr4—O17172.15 (13)Cr4—O15—C14115.1 (3)
O5—Cr4—O21172.73 (13)Cr4—O17—C18112.0 (3)
O9—Cr4—O2194.89 (13)O17—C18—O19125.2 (4)
O11—Cr4—O2191.73 (13)O17—C18—C20113.4 (4)
O15—Cr4—O2192.85 (13)O19—C18—C20121.4 (4)
O17—Cr4—O2182.29 (13)C18—C20—O21113.2 (4)
Cr4—O5—C6114.4 (3)C18—C20—O22120.2 (4)
O5—C6—O7125.8 (4)O21—C20—O22126.6 (4)
O5—C6—C8114.2 (4)Cr4—O21—C20110.6 (3)
O7—C6—C8120.0 (4)H232—O23—H231109.5
C6—C8—O9113.4 (4)H242—O24—H241105.2
C6—C8—O10120.7 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O23—H232···O150.832.252.963 (7)145
O24—H242···O100.832.152.974 (7)173
O23—H231···C6i0.822.583.333 (7)154
O23—H231···O7i0.821.992.809 (7)178
O24—H241···O23ii0.832.102.886 (7)158
Symmetry codes: (i) x, y+3, z; (ii) x+1, y+3, z.

Experimental details

Crystal data
Chemical formulaCs3[Cr(C2O4)3]·2H2O
Mr750.80
Crystal system, space groupTriclinic, P1
Temperature (K)120
a, b, c (Å)8.2299 (5), 10.2956 (6), 10.9902 (5)
α, β, γ (°)63.662 (5), 89.775 (4), 77.221 (5)
V3)809.14 (9)
Z2
Radiation typeMo Kα
µ (mm1)7.43
Crystal size (mm)0.22 × 0.15 × 0.13
Data collection
DiffractometerXcalibur, Sapphire3
diffractometer
Absorption correctionMulti-scan
[CrysAlis PRO (Oxford Diffraction, 2009). Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm]
Tmin, Tmax0.743, 1.000
No. of measured, independent and
observed [I > 2σ(I)] reflections
5562, 3631, 3326
Rint0.018
(sin θ/λ)max1)0.684
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.025, 0.059, 0.98
No. of reflections3621
No. of parameters217
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.91, 1.62

Computer programs: CrysAlis PRO (Oxford Diffraction, 2009), SIR92 (Altomare et al., 1994), CRYSTALS (Betteridge et al., 2003), CAMERON (Watkin et al., 1996).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O23—H232···O150.832.252.963 (7)145
O24—H242···O100.832.152.974 (7)173
O23—H231···C6i0.822.583.333 (7)154
O23—H231···O7i0.821.992.809 (7)178
O24—H241···O23ii0.832.102.886 (7)158
Symmetry codes: (i) x, y+3, z; (ii) x+1, y+3, z.
 

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