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Crystals from commercial samples of sodium cacodylate trihydrate, NaO
2As(CH
3)
2·3H
2O, were analyzed by single-crystal X-ray diffraction and two phases were identified,
viz. penta-
-aqua-disodium(I) bis(dimethylarsenate), {[Na
2(H
2O)
5](C
2H
6AsO
2)
2}
n, (I), and di-
-aqua-bis[triaquasodium(I)] bis(dimethylarsenate), [Na
2(H
2O)
8](C
2H
6AsO
2)
2, (II). Both (I) and (II) form layered structures in which hydrated Na
+ ions form layers in the
ab plane, the cacodylate ions being located in between the layers. In (I), the two non-equivalent Na
+ ions (located at twofold axes) and the three non-equivalent aqua ligands (one of which also lies on a twofold axis) form infinite polymeric layers, but in (II), layers of discrete centrosymmetric [Na
2(H
2O)
8]
2+ ions are present. One of the commercial samples analyzed contained almost exclusively crystals of the tetrahydrate (II), while another sample consisted of a mixture of the two phases.
Supporting information
CCDC references: 677077; 677078
Crystals were carefully selected from a commercial sample of `sodium cacodylate
trihydrate'.
All H-atom parameters were refined without constraints, except for the
Uiso(H) values in (I), which were set to
1.2Ueq(C) and 1.5Ueq(O). [Please
check.]
For both compounds, data collection: CrystalClear (Rigaku, 2000); cell refinement: CrystalClear (Rigaku, 2000); data reduction: CrystalClear (Rigaku, 2000); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).
(I) penta-µ-aqua-disodium(I) bis(dimethylarsenate)
top
Crystal data top
[Na2(H2O)5](C2H6AsO2)2 | F(000) = 824 |
Mr = 410.04 | Dx = 1.729 Mg m−3 |
Orthorhombic, Pbcm | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2c 2b | Cell parameters from 1585 reflections |
a = 6.1299 (11) Å | θ = 1.7–26.0° |
b = 10.7515 (18) Å | µ = 4.32 mm−1 |
c = 23.901 (4) Å | T = 100 K |
V = 1575.2 (5) Å3 | Irregular, colourless |
Z = 4 | 0.3 × 0.2 × 0.2 mm |
Data collection top
Rigaku R-AXIS IIC image-plate system diffractometer | 1585 independent reflections |
Radiation source: rotating-anode X-ray tube, Rigaku H3R | 1493 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.052 |
Detector resolution: 105 pixels mm-1 | θmax = 26.0°, θmin = 1.7° |
ϕ scans | h = −7→7 |
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000) | k = −12→13 |
Tmin = 0.255, Tmax = 0.420 | l = −29→29 |
10364 measured reflections | |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.033 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.067 | Only H-atom coordinates refined |
S = 1.18 | w = 1/[σ2(Fo2) + (0.0289P)2 + 1.7086P] where P = (Fo2 + 2Fc2)/3 |
1585 reflections | (Δ/σ)max < 0.001 |
115 parameters | Δρmax = 0.40 e Å−3 |
0 restraints | Δρmin = −0.45 e Å−3 |
Crystal data top
[Na2(H2O)5](C2H6AsO2)2 | V = 1575.2 (5) Å3 |
Mr = 410.04 | Z = 4 |
Orthorhombic, Pbcm | Mo Kα radiation |
a = 6.1299 (11) Å | µ = 4.32 mm−1 |
b = 10.7515 (18) Å | T = 100 K |
c = 23.901 (4) Å | 0.3 × 0.2 × 0.2 mm |
Data collection top
Rigaku R-AXIS IIC image-plate system diffractometer | 1585 independent reflections |
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000) | 1493 reflections with I > 2σ(I) |
Tmin = 0.255, Tmax = 0.420 | Rint = 0.052 |
10364 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.033 | 0 restraints |
wR(F2) = 0.067 | Only H-atom coordinates refined |
S = 1.18 | Δρmax = 0.40 e Å−3 |
1585 reflections | Δρmin = −0.45 e Å−3 |
115 parameters | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
C1 | 0.3585 (6) | 0.1594 (3) | 0.41682 (14) | 0.0186 (6) | |
C2 | 0.2396 (5) | −0.1048 (3) | 0.46167 (13) | 0.0192 (6) | |
As1 | 0.18837 (4) | 0.01275 (2) | 0.402786 (10) | 0.00707 (11) | |
Na1 | 0.1353 (2) | 0.28419 (13) | 0.2500 | 0.0098 (3) | |
Na2 | 0.6967 (2) | 0.09170 (14) | 0.2500 | 0.0107 (3) | |
O1 | −0.0764 (3) | 0.04993 (18) | 0.40040 (8) | 0.0121 (4) | |
O2 | 0.2782 (3) | −0.05114 (17) | 0.34272 (8) | 0.0101 (4) | |
O3 | 0.7111 (3) | −0.07794 (18) | 0.31658 (9) | 0.0118 (4) | |
O4 | 0.8610 (3) | 0.2188 (2) | 0.31589 (8) | 0.0121 (4) | |
O5 | 0.3110 (5) | 0.0934 (3) | 0.2500 | 0.0119 (6) | |
H1A | 0.316 (5) | 0.191 (3) | 0.4522 (15) | 0.014* | |
H1B | 0.497 (6) | 0.137 (3) | 0.4179 (13) | 0.014* | |
H1C | 0.328 (5) | 0.220 (3) | 0.3867 (15) | 0.014* | |
H2A | 0.189 (5) | −0.073 (3) | 0.4958 (15) | 0.014* | |
H2B | 0.387 (6) | −0.121 (3) | 0.4636 (14) | 0.014* | |
H2C | 0.163 (5) | −0.182 (3) | 0.4541 (14) | 0.014* | |
H3 | 0.579 (6) | −0.077 (3) | 0.3245 (14) | 0.018* | |
H4 | 0.778 (6) | −0.049 (4) | 0.3397 (16) | 0.018* | |
H5 | 0.285 (5) | 0.048 (3) | 0.2786 (14) | 0.018* | |
H6 | 0.884 (6) | 0.168 (3) | 0.3473 (15) | 0.018* | |
H7 | 0.819 (6) | 0.279 (4) | 0.3260 (16) | 0.018* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1 | 0.0211 (15) | 0.0168 (15) | 0.0179 (15) | −0.0023 (12) | −0.0043 (13) | −0.0054 (12) |
C2 | 0.0246 (17) | 0.0204 (15) | 0.0126 (14) | 0.0107 (13) | 0.0033 (13) | 0.0053 (12) |
As1 | 0.00877 (16) | 0.00727 (16) | 0.00517 (16) | 0.00090 (9) | −0.00079 (10) | −0.00025 (10) |
Na1 | 0.0097 (7) | 0.0086 (7) | 0.0109 (8) | −0.0008 (5) | 0.000 | 0.000 |
Na2 | 0.0139 (8) | 0.0083 (7) | 0.0098 (7) | −0.0011 (6) | 0.000 | 0.000 |
O1 | 0.0106 (9) | 0.0162 (10) | 0.0095 (9) | 0.0040 (8) | −0.0006 (8) | −0.0009 (7) |
O2 | 0.0121 (9) | 0.0097 (9) | 0.0086 (9) | 0.0002 (8) | 0.0015 (8) | −0.0015 (7) |
O3 | 0.0087 (9) | 0.0153 (10) | 0.0113 (10) | 0.0010 (8) | −0.0007 (8) | −0.0022 (8) |
O4 | 0.0177 (10) | 0.0078 (10) | 0.0109 (10) | 0.0010 (8) | 0.0005 (8) | −0.0025 (8) |
O5 | 0.0170 (15) | 0.0110 (14) | 0.0076 (13) | 0.0004 (11) | 0.000 | 0.000 |
Geometric parameters (Å, º) top
C1—As1 | 1.920 (3) | Na1—Na2 | 4.016 (2) |
C1—H1A | 0.95 (4) | Na2—O4 | 2.316 (2) |
C1—H1B | 0.88 (4) | Na2—O5 | 2.364 (3) |
C1—H1C | 0.99 (3) | Na2—O3 | 2.422 (2) |
C2—As1 | 1.918 (3) | Na2—Na1iv | 3.393 (2) |
C2—H2A | 0.94 (4) | Na2—Na1v | 3.463 (2) |
C2—H2B | 0.92 (3) | Na2—H3 | 2.64 (3) |
C2—H2C | 0.97 (3) | Na2—H4 | 2.67 (4) |
As1—O1 | 1.6725 (19) | Na2—H5 | 2.66 (3) |
As1—O2 | 1.6840 (18) | O3—H3 | 0.83 (4) |
Na1—O5 | 2.317 (3) | O3—H4 | 0.75 (4) |
Na1—O3i | 2.370 (2) | O4—H6 | 0.94 (4) |
Na1—O4ii | 2.409 (2) | O4—H7 | 0.74 (4) |
Na1—Na2ii | 3.393 (2) | O5—H5 | 0.86 (3) |
Na1—Na2iii | 3.463 (2) | | |
| | | |
As1—C1—H1A | 108 (2) | Na1iv—Na2—Na1v | 110.28 (4) |
As1—C1—H1B | 108 (2) | O4—Na2—Na1 | 93.93 (7) |
H1A—C1—H1B | 109 (3) | O5—Na2—Na1 | 30.59 (7) |
As1—C1—H1C | 108.0 (19) | O3—Na2—Na1 | 114.80 (6) |
H1A—C1—H1C | 111 (3) | Na1iv—Na2—Na1 | 111.39 (5) |
H1B—C1—H1C | 113 (3) | Na1v—Na2—Na1 | 138.33 (5) |
As1—C2—H2A | 110 (2) | O4—Na2—H3 | 93.7 (7) |
As1—C2—H2B | 109 (2) | O4vii—Na2—H3 | 169.2 (8) |
H2A—C2—H2B | 110 (3) | O5—Na2—H3 | 74.5 (8) |
As1—C2—H2C | 110 (2) | O3vii—Na2—H3 | 86.4 (7) |
H2A—C2—H2C | 108 (3) | O3—Na2—H3 | 18.2 (8) |
H2B—C2—H2C | 109 (3) | Na1iv—Na2—H3 | 129.4 (7) |
O1—As1—O2 | 112.69 (9) | Na1v—Na2—H3 | 55.0 (8) |
O1—As1—C2 | 109.97 (12) | Na1—Na2—H3 | 96.9 (8) |
O2—As1—C2 | 107.66 (11) | O4—Na2—H4 | 73.0 (9) |
O1—As1—C1 | 109.67 (12) | O4vii—Na2—H4 | 143.1 (8) |
O2—As1—C1 | 107.84 (12) | O5—Na2—H4 | 101.0 (8) |
C2—As1—C1 | 108.91 (15) | O3vii—Na2—H4 | 95.4 (9) |
O5—Na1—O3i | 111.66 (9) | O3—Na2—H4 | 16.1 (8) |
O3iii—Na1—O3i | 84.37 (11) | Na1iv—Na2—H4 | 101.4 (8) |
O5—Na1—O4ii | 93.80 (9) | Na1v—Na2—H4 | 53.4 (8) |
O3iii—Na1—O4ii | 91.19 (7) | Na1—Na2—H4 | 116.8 (7) |
O3i—Na1—O4ii | 154.01 (10) | H3—Na2—H4 | 28.4 (10) |
O4vi—Na1—O4ii | 81.67 (11) | O4—Na2—H5 | 110.0 (7) |
O5—Na1—Na2ii | 80.12 (8) | O4vii—Na2—H5 | 133.9 (8) |
O3iii—Na1—Na2ii | 134.13 (6) | O5—Na2—H5 | 18.5 (7) |
O4vi—Na1—Na2ii | 43.02 (5) | O3vii—Na2—H5 | 94.0 (8) |
O5—Na1—Na2iii | 134.99 (9) | O3—Na2—H5 | 74.4 (8) |
O3iii—Na1—Na2iii | 44.33 (5) | Na1iv—Na2—H5 | 149.4 (8) |
O4ii—Na1—Na2iii | 119.09 (7) | Na1v—Na2—H5 | 96.5 (8) |
Na2ii—Na1—Na2iii | 144.89 (5) | Na1—Na2—H5 | 43.8 (8) |
O5—Na1—Na2 | 31.27 (8) | H3—Na2—H5 | 56.3 (11) |
O3i—Na1—Na2 | 88.97 (7) | H4—Na2—H5 | 82.5 (11) |
O4ii—Na1—Na2 | 116.61 (7) | Na1v—O3—Na2 | 92.54 (8) |
Na2ii—Na1—Na2 | 111.39 (5) | Na1v—O3—H3 | 123 (2) |
Na2iii—Na1—Na2 | 103.72 (4) | Na2—O3—H3 | 96 (2) |
O4—Na2—O4vii | 85.70 (11) | Na1v—O3—H4 | 122 (3) |
O4—Na2—O5 | 115.49 (8) | Na2—O3—H4 | 101 (3) |
O4—Na2—O3vii | 151.08 (10) | H3—O3—H4 | 111 (3) |
O4—Na2—O3 | 88.95 (7) | Na2—O4—Na1iv | 91.78 (8) |
O4vii—Na2—O3 | 151.08 (10) | Na2—O4—H6 | 105 (2) |
O5—Na2—O3 | 92.43 (8) | Na1iv—O4—H6 | 126 (2) |
O3vii—Na2—O3 | 82.15 (11) | Na2—O4—H7 | 126 (3) |
O4—Na2—Na1iv | 45.20 (6) | Na1iv—O4—H7 | 102 (3) |
O5—Na2—Na1iv | 141.98 (9) | H6—O4—H7 | 108 (4) |
O3—Na2—Na1iv | 115.48 (7) | Na1—O5—Na2 | 118.14 (13) |
O4—Na2—Na1v | 115.73 (7) | Na1—O5—H5 | 115 (2) |
O5—Na2—Na1v | 107.74 (9) | Na2—O5—H5 | 100 (2) |
O3—Na2—Na1v | 43.13 (5) | | |
Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) x−1, y, z; (iii) −x+1, y+1/2, z; (iv) x+1, y, z; (v) −x+1, y−1/2, z; (vi) x−1, y, −z+1/2; (vii) x, y, −z+1/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O3—H3···O2 | 0.83 (4) | 1.917 | 2.742 (3) | 172.69 |
O3—H4···O1iv | 0.75 (4) | 2.010 | 2.757 (3) | 171.36 |
O5—H5···O2 | 0.86 (3) | 1.865 | 2.714 (3) | 170.58 |
O4—H6···O1iv | 0.94 (4) | 1.809 | 2.743 (3) | 171.46 |
O4—H7···O2iii | 0.74 (4) | 1.964 | 2.693 (3) | 171.73 |
Symmetry codes: (iii) −x+1, y+1/2, z; (iv) x+1, y, z. |
(II) di-µ-aqua-bis[triaquasodium(I)] bis(dimethylarsenate)
top
Crystal data top
[Na2(H2O)8](C2H6AsO2)2 | F(000) = 944 |
Mr = 464.08 | Dx = 1.640 Mg m−3 |
Monoclinic, C2/c | Melting point: not measured K |
Hall symbol: -C 2yc | Mo Kα radiation, λ = 0.71073 Å |
a = 11.516 (3) Å | Cell parameters from 5714 reflections |
b = 8.838 (2) Å | θ = 2.2–25.5° |
c = 19.635 (5) Å | µ = 3.65 mm−1 |
β = 109.907 (8)° | T = 303 K |
V = 1879.1 (8) Å3 | Block, colourless |
Z = 4 | 0.25 × 0.15 × 0.10 mm |
Data collection top
Rigaku R-AXIS IIc image-plate system diffractometer | 1652 independent reflections |
Radiation source: rotating-anode X-ray tube, Rigaku RU-H3R | 1517 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.032 |
Detector resolution: 105 pixels mm-1 | θmax = 25.5°, θmin = 2.2° |
ϕ scans | h = −13→13 |
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000) | k = −10→10 |
Tmin = 0.373, Tmax = 0.696 | l = −23→23 |
5714 measured reflections | |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.018 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.044 | All H-atom parameters refined |
S = 1.00 | w = 1/[σ2(Fo2) + (0.0278P)2] where P = (Fo2 + 2Fc2)/3 |
1652 reflections | (Δ/σ)max = 0.001 |
125 parameters | Δρmax = 0.28 e Å−3 |
0 restraints | Δρmin = −0.20 e Å−3 |
Crystal data top
[Na2(H2O)8](C2H6AsO2)2 | V = 1879.1 (8) Å3 |
Mr = 464.08 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 11.516 (3) Å | µ = 3.65 mm−1 |
b = 8.838 (2) Å | T = 303 K |
c = 19.635 (5) Å | 0.25 × 0.15 × 0.10 mm |
β = 109.907 (8)° | |
Data collection top
Rigaku R-AXIS IIc image-plate system diffractometer | 1652 independent reflections |
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000) | 1517 reflections with I > 2σ(I) |
Tmin = 0.373, Tmax = 0.696 | Rint = 0.032 |
5714 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.018 | 0 restraints |
wR(F2) = 0.044 | All H-atom parameters refined |
S = 1.00 | Δρmax = 0.28 e Å−3 |
1652 reflections | Δρmin = −0.20 e Å−3 |
125 parameters | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
C1 | 0.1060 (2) | 1.04565 (19) | 0.17722 (11) | 0.0425 (4) | |
H1A | 0.1371 | 1.1313 | 0.1588 | 0.064* | |
H1B | 0.1294 | 1.0538 | 0.2289 | 0.064* | |
H1C | 0.0176 | 1.0430 | 0.1561 | 0.064* | |
C2 | 0.34923 (17) | 0.8666 (2) | 0.19694 (11) | 0.0383 (4) | |
H2A | 0.3817 | 0.9539 | 0.1805 | 0.057* | |
H2B | 0.3832 | 0.7767 | 0.1837 | 0.057* | |
H2C | 0.3711 | 0.8706 | 0.2487 | 0.057* | |
O1 | 0.11507 (11) | 0.71695 (12) | 0.18493 (6) | 0.0275 (2) | |
O2 | 0.13842 (10) | 0.85626 (10) | 0.06215 (6) | 0.0243 (2) | |
O3 | 0.13255 (14) | 0.76248 (17) | 0.33138 (8) | 0.0424 (3) | |
O4 | 0.33169 (13) | 1.08614 (14) | 0.50682 (8) | 0.0355 (3) | |
O5 | 0.37936 (13) | 0.64835 (14) | 0.50201 (7) | 0.0277 (3) | |
O6 | 0.41984 (15) | 0.94080 (17) | 0.38455 (9) | 0.0394 (3) | |
Na1 | 0.27390 (6) | 0.86194 (6) | 0.43443 (4) | 0.02757 (16) | |
As1 | 0.173618 (13) | 0.863667 (15) | 0.153124 (7) | 0.01910 (8) | |
H3A | 0.137 (2) | 0.746 (3) | 0.2950 (14) | 0.055 (8)* | |
H3B | 0.068 (3) | 0.753 (3) | 0.3327 (13) | 0.055 (8)* | |
H4A | 0.276 (3) | 1.109 (3) | 0.5193 (16) | 0.058 (8)* | |
H4B | 0.342 (2) | 1.162 (3) | 0.4851 (15) | 0.060 (8)* | |
H5A | 0.371 (2) | 0.570 (3) | 0.4824 (12) | 0.038 (6)* | |
H5B | 0.449 (2) | 0.658 (2) | 0.5194 (13) | 0.037 (6)* | |
H6A | 0.411 (2) | 1.013 (3) | 0.3684 (13) | 0.043 (7)* | |
H6B | 0.492 (3) | 0.940 (3) | 0.4146 (16) | 0.068 (8)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1 | 0.0568 (12) | 0.0292 (8) | 0.0532 (11) | 0.0011 (8) | 0.0339 (10) | −0.0085 (8) |
C2 | 0.0247 (9) | 0.0536 (12) | 0.0328 (9) | −0.0058 (7) | 0.0049 (7) | 0.0064 (7) |
O1 | 0.0321 (6) | 0.0273 (5) | 0.0286 (6) | −0.0053 (5) | 0.0173 (5) | −0.0002 (4) |
O2 | 0.0264 (6) | 0.0286 (6) | 0.0187 (5) | 0.0019 (4) | 0.0087 (5) | 0.0001 (4) |
O3 | 0.0286 (8) | 0.0722 (10) | 0.0284 (7) | −0.0055 (6) | 0.0124 (6) | −0.0105 (6) |
O4 | 0.0425 (8) | 0.0281 (6) | 0.0454 (7) | 0.0008 (6) | 0.0273 (6) | 0.0016 (6) |
O5 | 0.0216 (7) | 0.0292 (7) | 0.0309 (6) | 0.0001 (5) | 0.0072 (5) | −0.0046 (5) |
O6 | 0.0408 (9) | 0.0373 (8) | 0.0465 (8) | 0.0009 (6) | 0.0234 (7) | 0.0092 (7) |
Na1 | 0.0288 (4) | 0.0281 (3) | 0.0274 (3) | 0.0001 (2) | 0.0116 (3) | 0.0005 (2) |
As1 | 0.01939 (11) | 0.02204 (11) | 0.01779 (11) | −0.00102 (5) | 0.00883 (7) | −0.00146 (5) |
Geometric parameters (Å, º) top
C1—As1 | 1.9152 (17) | O3—H3A | 0.75 (3) |
C1—H1A | 0.9600 | O3—H3B | 0.75 (3) |
C1—H1B | 0.9600 | O4—Na1 | 2.3969 (15) |
C1—H1C | 0.9600 | O4—H4A | 0.78 (3) |
C2—As1 | 1.9095 (19) | O4—H4B | 0.82 (3) |
C2—H2A | 0.9600 | O5—Na1 | 2.3878 (14) |
C2—H2B | 0.9600 | O5—H5A | 0.78 (2) |
C2—H2C | 0.9600 | O5—H5B | 0.76 (2) |
O1—As1 | 1.6771 (10) | O6—Na1 | 2.3218 (15) |
O2—As1 | 1.6931 (12) | O6—H6A | 0.71 (2) |
O3—Na1 | 2.2965 (16) | O6—H6B | 0.84 (3) |
| | | |
As1—C1—H1A | 109.5 | Na1—O6—H6A | 116.2 (19) |
As1—C1—H1B | 109.5 | Na1—O6—H6B | 112.8 (19) |
H1A—C1—H1B | 109.5 | H6A—O6—H6B | 107 (3) |
As1—C1—H1C | 109.5 | O3—Na1—O6 | 98.10 (7) |
H1A—C1—H1C | 109.5 | O3—Na1—O5 | 105.12 (6) |
H1B—C1—H1C | 109.5 | O6—Na1—O5 | 99.64 (6) |
As1—C2—H2A | 109.5 | O3—Na1—O4 | 144.03 (6) |
As1—C2—H2B | 109.5 | O6—Na1—O4 | 84.68 (6) |
H2A—C2—H2B | 109.5 | O5—Na1—O4 | 109.75 (6) |
As1—C2—H2C | 109.5 | O3—Na1—O5i | 90.97 (6) |
H2A—C2—H2C | 109.5 | O6—Na1—O5i | 164.22 (5) |
H2B—C2—H2C | 109.5 | O5—Na1—O5i | 90.34 (5) |
Na1—O3—H3A | 131 (2) | O4—Na1—O5i | 80.43 (5) |
Na1—O3—H3B | 115.0 (19) | O3—Na1—Na1i | 101.16 (5) |
H3A—O3—H3B | 114 (3) | O6—Na1—Na1i | 144.33 (6) |
Na1—O4—H4A | 108.0 (18) | O5—Na1—Na1i | 46.33 (4) |
Na1—O4—H4B | 115 (2) | O4—Na1—Na1i | 96.58 (5) |
H4A—O4—H4B | 103 (2) | O5i—Na1—Na1i | 44.01 (3) |
Na1—O5—Na1i | 89.66 (5) | O1—As1—O2 | 112.39 (5) |
Na1—O5—H5A | 118.1 (16) | O1—As1—C2 | 109.94 (7) |
Na1i—O5—H5A | 103.1 (16) | O2—As1—C2 | 108.19 (8) |
Na1—O5—H5B | 114.9 (15) | O1—As1—C1 | 107.96 (8) |
Na1i—O5—H5B | 126.6 (17) | O2—As1—C1 | 108.77 (7) |
H5A—O5—H5B | 105 (2) | C2—As1—C1 | 109.56 (9) |
Symmetry code: (i) −x+1/2, −y+3/2, −z+1. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O3—H3A···O1 | 0.75 (3) | 2.104 | 2.842 (2) | 168.65 |
O3—H3B···O1ii | 0.75 (3) | 2.045 | 2.788 (2) | 168.71 |
O4—H4A···O2iii | 0.78 (3) | 2.059 | 2.835 (2) | 171.69 |
O4—H4B···O2iv | 0.82 (3) | 2.005 | 2.823 (2) | 175.19 |
O5—H5A···O2v | 0.78 (2) | 2.070 | 2.849 (2) | 175.02 |
O5—H5B···O2vi | 0.76 (2) | 2.061 | 2.810 (2) | 169.77 |
O6—H6A···O1iv | 0.71 (2) | 2.053 | 2.758 (2) | 176.06 |
O6—H6B···O4vii | 0.84 (3) | 2.101 | 2.939 (2) | 173.91 |
Symmetry codes: (ii) −x, y, −z+1/2; (iii) x, −y+2, z+1/2; (iv) −x+1/2, y+1/2, −z+1/2; (v) −x+1/2, y−1/2, −z+1/2; (vi) x+1/2, −y+3/2, z+1/2; (vii) −x+1, −y+2, −z+1. |
Experimental details
| (I) | (II) |
Crystal data |
Chemical formula | [Na2(H2O)5](C2H6AsO2)2 | [Na2(H2O)8](C2H6AsO2)2 |
Mr | 410.04 | 464.08 |
Crystal system, space group | Orthorhombic, Pbcm | Monoclinic, C2/c |
Temperature (K) | 100 | 303 |
a, b, c (Å) | 6.1299 (11), 10.7515 (18), 23.901 (4) | 11.516 (3), 8.838 (2), 19.635 (5) |
α, β, γ (°) | 90, 90, 90 | 90, 109.907 (8), 90 |
V (Å3) | 1575.2 (5) | 1879.1 (8) |
Z | 4 | 4 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 4.32 | 3.65 |
Crystal size (mm) | 0.3 × 0.2 × 0.2 | 0.25 × 0.15 × 0.10 |
|
Data collection |
Diffractometer | Rigaku R-AXIS IIC image-plate system diffractometer | Rigaku R-AXIS IIc image-plate system diffractometer |
Absorption correction | Multi-scan (CrystalClear; Rigaku, 2000) | Multi-scan (CrystalClear; Rigaku, 2000) |
Tmin, Tmax | 0.255, 0.420 | 0.373, 0.696 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 10364, 1585, 1493 | 5714, 1652, 1517 |
Rint | 0.052 | 0.032 |
(sin θ/λ)max (Å−1) | 0.617 | 0.606 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.033, 0.067, 1.18 | 0.018, 0.044, 1.00 |
No. of reflections | 1585 | 1652 |
No. of parameters | 115 | 125 |
H-atom treatment | Only H-atom coordinates refined | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.40, −0.45 | 0.28, −0.20 |
Selected geometric parameters (Å, º) for (I) topC1—As1 | 1.920 (3) | Na1—O3i | 2.370 (2) |
C2—As1 | 1.918 (3) | Na1—O4ii | 2.409 (2) |
As1—O1 | 1.6725 (19) | Na2—O4 | 2.316 (2) |
As1—O2 | 1.6840 (18) | Na2—O5 | 2.364 (3) |
Na1—O5 | 2.317 (3) | Na2—O3 | 2.422 (2) |
| | | |
O1—As1—O2 | 112.69 (9) | O3i—Na1—O4ii | 154.01 (10) |
O1—As1—C2 | 109.97 (12) | O4iv—Na1—O4ii | 81.67 (11) |
O2—As1—C2 | 107.66 (11) | O4—Na2—O4v | 85.70 (11) |
O1—As1—C1 | 109.67 (12) | O4—Na2—O5 | 115.49 (8) |
O2—As1—C1 | 107.84 (12) | O4—Na2—O3v | 151.08 (10) |
C2—As1—C1 | 108.91 (15) | O4—Na2—O3 | 88.95 (7) |
O5—Na1—O3i | 111.66 (9) | O4v—Na2—O3 | 151.08 (10) |
O3iii—Na1—O3i | 84.37 (11) | O5—Na2—O3 | 92.43 (8) |
O5—Na1—O4ii | 93.80 (9) | O3v—Na2—O3 | 82.15 (11) |
O3iii—Na1—O4ii | 91.19 (7) | | |
Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) x−1, y, z; (iii) −x+1, y+1/2, z; (iv) x−1, y, −z+1/2; (v) x, y, −z+1/2. |
Hydrogen-bond geometry (Å, º) for (I) top
D—H···A | D—H | H···A | D···A | D—H···A |
O3—H3···O2 | 0.83 (4) | 1.917 | 2.742 (3) | 172.69 |
O3—H4···O1vi | 0.75 (4) | 2.010 | 2.757 (3) | 171.36 |
O5—H5···O2 | 0.86 (3) | 1.865 | 2.714 (3) | 170.58 |
O4—H6···O1vi | 0.94 (4) | 1.809 | 2.743 (3) | 171.46 |
O4—H7···O2iii | 0.74 (4) | 1.964 | 2.693 (3) | 171.73 |
Symmetry codes: (iii) −x+1, y+1/2, z; (vi) x+1, y, z. |
Hydrogen-bond geometry (Å, º) for (II) top
D—H···A | D—H | H···A | D···A | D—H···A |
O3—H3A···O1 | 0.75 (3) | 2.104 | 2.842 (2) | 168.65 |
O3—H3B···O1i | 0.75 (3) | 2.045 | 2.788 (2) | 168.71 |
O4—H4A···O2ii | 0.78 (3) | 2.059 | 2.835 (2) | 171.69 |
O4—H4B···O2iii | 0.82 (3) | 2.005 | 2.823 (2) | 175.19 |
O5—H5A···O2iv | 0.78 (2) | 2.070 | 2.849 (2) | 175.02 |
O5—H5B···O2v | 0.76 (2) | 2.061 | 2.810 (2) | 169.77 |
O6—H6A···O1iii | 0.71 (2) | 2.053 | 2.758 (2) | 176.06 |
O6—H6B···O4vi | 0.84 (3) | 2.101 | 2.939 (2) | 173.91 |
Symmetry codes: (i) −x, y, −z+1/2; (ii) x, −y+2, z+1/2; (iii) −x+1/2, y+1/2, −z+1/2; (iv) −x+1/2, y−1/2, −z+1/2; (v) x+1/2, −y+3/2, z+1/2; (vi) −x+1, −y+2, −z+1. |
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The first salts of cacodylic acid were prepared by Robert Bunsen in 1842 during his famous studies on the cacodyl radical in 1837–1843 (Bunsen, 1891). Bunsen informed Berzelius, who wrote an account for the results in his annual report on the progress of chemistry for 1842 (Berzelius, 1842), which is the first publication on the subject. Although more than 160 years have passed, none of the salts are found in the Cambridge Structural Database (CSD; Version 5.28 of November 2006; Allen 2002). Sodium cacodylate trihydrate is today commercially available; one important use is in buffer solutions which find use in protein crystallization (Ericsson et al., 2006; Izaac et al., 2006). Examination of a commercial sample showed that it contained crystals of two distinct morphologies, mainly round irregularly shaped crystals (some of which displayed well developed faces), but also a few crystal plates. It was possible to select crystals suitable for single-crystal X-ray diffraction, and the plates were identified as the expected trihydrate, [{Na(H2O)3}∞]{O2As(CH3)2}∞, (I), but the irregular crystals were identified as a tetrahydrate, [Na2(H2O)8]{O2As(CH3)2}2, (II). Another commercial sample was shown by powder X-ray diffraction to consist almost exclusively of the tetrahydrate, (II). The weight loss on drying this sample at 433 K overnight is also consistent with the expected weight loss on dehydratization of the tetrahydrate. A tetrahydrate of sodium cacodylate does not appear to be officially available on a commercial basis. SciFinder Scholar 2006 (American Chemical Society, 2005) lists 29 commercial sources for sodium cacodylate; of these, 11 are given simply as `sodium cacodylate' and may be the anhydrous product, 6 are given as `sodium cacodylate hydrate' and 12 as the trihydrate. Care should be taken on using sodium cacodylate, when molar mass is a matter of importance.
There are two non-equivalent Na ions in the structure of (I), Na1 and Na2, both located on twofold axes (Fig. 1). All of the three water molecules in (I) are coordinated to Na ions and give rise to infinite layers in the ab plane. Both Na1 and Na2 are five-coordinate, with coordination geometries intermediate between trigonal–bipyramidal and square-pyramidal. The coordination geometry around atom Na1 is perhaps closer to square-pyramidal, while Na2 is perhaps closer to trigonal–bipyramidal coordination geometry. The aqua ligands are bridging between atoms Na1 and Na2. Atom O5 forms a single bridge between atoms Na1 and Na2; atoms O3 and O3(x, y, 1/2 - z) form two bridges between atoms Na2 and Na1(1 - x, -1/2 + y, 1/2 - z); and atoms O4 and O4(x, y, 1/2 - z) form two bridges between atoms Na2 and Na1(1 + x, y, z). The resulting layer is depicted in Fig. 2. The cacodylate anions are located in between the Na/H2O layers and form hydrogen bonds to the layers. Atom O2 interacts with atoms H3, H5 and H7(1 - x, -1/2 + y, z); atom O1 interacts with atoms H4(-1 + x, y, z) and H6(-1 + x, y, z). All of these H atoms are in the same layer. There are no directed attractive forces between the anions; one reason is that only relatively weak C—H···O interactions could form. The geometry of the cacodylate anion is unremarkable, with only a slight degree of distortion from ideal tetrahedral coordination geometry around the central As atom.
In the crystal structure of (II), the Na+ ions and water molecules give rise to discrete dinuclear cations (Fig. 3) rather than the infinite layers present in (I). As in (I), atom Na1 is five-coordinate and the coordination geometry is best described as trigonal–bipyramidal, distorted towards square-pyramidal geometry. Atoms Na1 and Na1vii [symmetry code: (vii) 1/2 - x, 3/2 - y, 1 - z] are linked by two µ2-aqua ligands corresponding to atoms O5 and O5vii. One of the two bridging aqua ligands is in an equatorial position and the other is in an axial position in the coordination environment of atom Na1. The resulting bis(µ2-aqua)(hexaaqua)disodium ion appears to be very rare in combination with organic counter-ions; there is only one further example in the CSD (Laborda et al., 2004). A number of similar ions are, however, found in the CSD, where the Na atoms are six-coordinate.
As in the structure of (I), the crystal structure of (II) is built up by layers of Na+ ions and water molecules in the ab plane {although in the form of discrete [Na2(H2O)8]2+ ions}, with the anions appearing between the layers, the components being linked by hydrogen bonds. Within the asymmetric unit, there is one short contact between the anion and the cation (O1—H3A; Table 3). In addition, atom O1 forms two short intermolecular contacts, to H3B(-x, y, 1/2 - z) and H6A(1/2 - x, -1/2 + y, 1/2 - z), and atom O2 forms four short intermolecular contacts, to H5B(-1/2 + x, 3/2 - y, -1/2 + z), H4B(1/2 - x, -1/2 + y, 1/2 - z), H5A(1/2 - x, 1/2 + y, 1/2 - z) and H4A(x, 2 - y, -1/2 + z) (Table 3). The cation forms four equivalent short contacts (2.10 Å), beside those involving O1 and O2: O4···H6bviii [symmetry code: (viii) 1 - x, 2 - y, 1 - z], H6b–O4viii, O4vii–H6bix [symmetry code: (ix) -1/2 + x, -1/2 + y, z], and O4ix–H6Bvii. The crystal structure of (II) is depicted in Fig. 4 (along the a axis) and in Fig. 5 (along the b axis). The formation of layers in the ab plane is most easily seen from Fig. 4.