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The title compound, C2H7AsO2 or [As(CH3)2O(OH)], is an organic derivative of arsinic acid, and is also known by its trivial name cacodylic acid. In contrast to the first polymorph (triclinic, space group P\overline{1}, Z = 2), the current study revealed monoclinic symmetry (space group C2/c, Z = 8) for the second polymorph. The configuration of the tetra­hedral mol­ecule shows approximate Cs symmetry. Strong O—H...O hydrogen bonds connect the mol­ecules to infinite zigzag chains along [010], which are further connected by weak inter­molecular C—H...O contacts into a three-dimensional network.

Supporting information


Crystallographic Information File (CIF)
Contains datablocks I, global


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Supplementary material


Structure factor file (CIF format)
Contains datablock I


Chemical Markup Language (CML) file
Supplementary material

CCDC reference: 840987

Key indicators

  • Single-crystal X-ray study
  • T = 200 K
  • Mean [sigma](As-C) = 0.003 Å
  • R factor = 0.020
  • wR factor = 0.055
  • Data-to-parameter ratio = 23.8

checkCIF/PLATON results

No syntax errors found

Alert level C PLAT910_ALERT_3_C Missing # of FCF Reflections Below Th(Min) ..... 1 PLAT911_ALERT_3_C Missing # FCF Refl Between THmin & STh/L= 0.600 3 PLAT912_ALERT_4_C Missing # of FCF Reflections Above STh/L= 0.600 7 PLAT913_ALERT_3_C Missing # of Very Strong Reflections in FCF .... 1
Alert level G PLAT005_ALERT_5_G No _iucr_refine_instructions_details in CIF .... ?
0 ALERT level A = Most likely a serious problem - resolve or explain 0 ALERT level B = A potentially serious problem, consider carefully 4 ALERT level C = Check. Ensure it is not caused by an omission or oversight 1 ALERT level G = General information/check it is not something unexpected 0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 0 ALERT type 2 Indicator that the structure model may be wrong or deficient 3 ALERT type 3 Indicator that the structure quality may be low 1 ALERT type 4 Improvement, methodology, query or suggestion 1 ALERT type 5 Informative message, check

Comment top

The precipitation of amines from their respective organic synthesis mixtures as the ammonium salts of inorganic acids is a common practice for obtaining and purifying the desired products. However, for a couple of higher-alkylated amines, the viability of this class of compounds as phase-transfer catalysts becomes troublesome with respect to their persistent solubility which is detrimental for achieving quantitative yields following this simple synthetic protocol. Decreasing the solubility of a protonated amine can be done upon variation of the counterion which may allow for a better packing and more pronounced intermolecular interactions in the solid state. Since we intended to perform a comprehensive study involving a variety of higher-alkylated amines, we set out to optimize the yield of several established synthesis procedures by variation of the acid used for precipitation. To allow for a rationalization and tailoring of the counterions to be preferred, we determined the crystal structure of the title compound to enable comparative studies in isolated, crystalline precipitates. The crystal structure of the title compound has been determined previously (Trotter & Zobel (1965)). However, a different crystal system (triclinic, space group P1, Z = 2) was reported, suggesting that the title compound is polymorphic. Moreover, hydrogen atoms were not included in the previous refinement procedure.

The length of the As–O-bonds show a marked difference with the formal As–O-double bond being shorter by about 0.06 Å than the corresponding single bond. A projection of both methyl groups along the C—C-axis shows their hydrogen atoms to adopt an ecliptic conformation (Fig. 1).

In the crystal structure, O—H···O hydrogen bonds as well as intermolecular C—H···O contacts, whose range falls by about 0.2 Å below the sum of the van-der-Waals radii of the atoms participating, are present. The hydrogen bonds are formed between the H atom of the hydroxyl group as donor and the formally double-bonded oxygen atom and connect the molecules to zigzag chains along [010]. The C—H···O contacts are supported by one hydrogen atom per methyl group each as the donor atom. While for one methyl group the double bonded O atom acts as acceptor and gives rise to the formation of centrosymmetric cacodylic acid dimers, the oxygen atom of the hydroxyl group acts as acceptor for the other methyl group. In this case, too, the formation of centrosymmetric cacodylic acid dimers can be observed. In total, the molecules are connected to a three-dimensional network in the crystal structure. In terms of graph-set analysis (Etter et al. (1990); Bernstein et al. (1995)), the descriptor for the classical hydrogen bonds is C11(4) on the unitary level while both C—H···O contacts necessitate a R22(8) descriptor on the same level (Fig. 2).

The packing of the title compound in the crystal structure is shown in Figure 3.

Related literature top

For the crystal structure of the triclinic polymorph of the title compound, see: Trotter & Zobel (1965). For graph-set analysis of hydrogen bonds, see: Etter et al. (1990); Bernstein et al. (1995).

Experimental top

The compound was obtained commercially (KEK). Crystals suitable for the X-ray diffraction study were taken directly from the provided product.

Refinement top

The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C—As bonds to best fit the experimental electron density (HFIX 137 in the SHELX program suite (Sheldrick, 2008)), with U(H) set to 1.5Ueq(C). The H atom of the hydroxyl group was found from a difference Fourier map and allowed to rotate with a fixed angle around the O—As bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite (Sheldrick, 2008)), its U(H) set to 1.5Ueq(O).

Computing details top

Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2010); data reduction: SAINT (Bruker, 2010); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound, with atom labels and anisotropic displacement ellipsoids drawn at the 50% probability level.
[Figure 2] Fig. 2. Intermolecular contacts, viewed along [001]. Blue dashed lines indicate classical O–H···O hydrogen bonds, green dashed lines C—H···O contacts. Symmetry operators: i) -x + 1/2, -y + 1/2, -z + 1; ii) -x + 1/2, y - 1/2, -z + 1/2; iii) -x + 1/2, y + 1/2, -z + 1/2; iv) -x + 1, -y + 1, -z + 1.
[Figure 3] Fig. 3. Molecular packing of the title compound, viewed along [010]; anisotropic displacement ellipsoids are drawn at the 50% probability level.
dimethylarsinic acid top
Crystal data top
[As(CH3)2O(OH)]F(000) = 544
Mr = 138.00Dx = 1.955 Mg m3
Monoclinic, C2/cMelting point = 468–469 K
Hall symbol: -C 2ycMo Kα radiation, λ = 0.71069 Å
a = 15.764 (9) ÅCell parameters from 6920 reflections
b = 6.494 (5) Åθ = 2.2–28.4°
c = 11.302 (4) ŵ = 7.09 mm1
β = 125.86 (3)°T = 200 K
V = 937.7 (10) Å3Block, colourless
Z = 80.49 × 0.42 × 0.39 mm
Data collection top
1166 independent reflections
Radiation source: fine-focus sealed tube1117 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.040
ϕ and ω scansθmax = 28.4°, θmin = 3.5°
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
h = 2020
Tmin = 0.608, Tmax = 1.000k = 87
7792 measured reflectionsl = 1515
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.020Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.055H-atom parameters constrained
S = 1.20 w = 1/[σ2(Fo2) + (0.0242P)2 + 1.2973P]
where P = (Fo2 + 2Fc2)/3
1166 reflections(Δ/σ)max = 0.001
49 parametersΔρmax = 0.31 e Å3
0 restraintsΔρmin = 0.80 e Å3
Crystal data top
[As(CH3)2O(OH)]V = 937.7 (10) Å3
Mr = 138.00Z = 8
Monoclinic, C2/cMo Kα radiation
a = 15.764 (9) ŵ = 7.09 mm1
b = 6.494 (5) ÅT = 200 K
c = 11.302 (4) Å0.49 × 0.42 × 0.39 mm
β = 125.86 (3)°
Data collection top
1166 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
1117 reflections with I > 2σ(I)
Tmin = 0.608, Tmax = 1.000Rint = 0.040
7792 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0200 restraints
wR(F2) = 0.055H-atom parameters constrained
S = 1.20Δρmax = 0.31 e Å3
1166 reflectionsΔρmin = 0.80 e Å3
49 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
As10.338221 (14)0.33757 (3)0.373791 (19)0.01723 (9)
O10.37470 (13)0.5820 (2)0.44526 (17)0.0268 (3)
O20.20889 (12)0.3139 (2)0.27582 (18)0.0260 (3)
C10.4078 (2)0.1666 (3)0.5414 (3)0.0293 (5)
C20.38735 (18)0.2918 (4)0.2580 (2)0.0270 (4)
Atomic displacement parameters (Å2) top
As10.01664 (13)0.01855 (13)0.01733 (13)0.00089 (6)0.01041 (10)0.00034 (6)
O10.0276 (8)0.0202 (7)0.0229 (7)0.0003 (6)0.0092 (6)0.0043 (6)
O20.0164 (7)0.0298 (8)0.0314 (8)0.0006 (6)0.0137 (7)0.0059 (6)
C10.0327 (12)0.0311 (12)0.0269 (11)0.0066 (9)0.0189 (10)0.0092 (8)
C20.0269 (11)0.0353 (11)0.0279 (10)0.0045 (9)0.0211 (9)0.0053 (9)
Geometric parameters (Å, º) top
As1—O21.6617 (19)C1—H1B0.9800
As1—O11.7201 (19)C1—H1C0.9800
As1—C21.895 (2)C2—H2A0.9800
As1—C11.895 (2)C2—H2B0.9800
O2—As1—O1109.90 (8)As1—C1—H1C109.5
O2—As1—C2111.32 (10)H1A—C1—H1C109.5
O1—As1—C2108.04 (9)H1B—C1—H1C109.5
O2—As1—C1112.19 (10)As1—C2—H2A109.5
O1—As1—C1103.46 (10)As1—C2—H2B109.5
C2—As1—C1111.56 (11)H2A—C2—H2B109.5
Hydrogen-bond geometry (Å, º) top
O1—H1···O2i0.841.692.528 (2)172
C1—H1A···O2ii0.982.523.481 (3)167
C2—H2B···O1iii0.982.483.354 (3)148
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x+1/2, y+1/2, z+1; (iii) x+1, y+1, z+1.

Experimental details

Crystal data
Chemical formula[As(CH3)2O(OH)]
Crystal system, space groupMonoclinic, C2/c
Temperature (K)200
a, b, c (Å)15.764 (9), 6.494 (5), 11.302 (4)
β (°) 125.86 (3)
V3)937.7 (10)
Radiation typeMo Kα
µ (mm1)7.09
Crystal size (mm)0.49 × 0.42 × 0.39
Data collection
DiffractometerBruker APEXII CCD
Absorption correctionMulti-scan
(SADABS; Bruker, 2008)
Tmin, Tmax0.608, 1.000
No. of measured, independent and
observed [I > 2σ(I)] reflections
7792, 1166, 1117
(sin θ/λ)max1)0.668
R[F2 > 2σ(F2)], wR(F2), S 0.020, 0.055, 1.20
No. of reflections1166
No. of parameters49
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.31, 0.80

Computer programs: APEX2 (Bruker, 2010), SAINT (Bruker, 2010), SHELXS97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2008), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Selected bond lengths (Å) top
As1—O21.6617 (19)As1—C21.895 (2)
As1—O11.7201 (19)As1—C11.895 (2)
Hydrogen-bond geometry (Å, º) top
O1—H1···O2i0.841.692.528 (2)172
C1—H1A···O2ii0.982.523.481 (3)167
C2—H2B···O1iii0.982.483.354 (3)148
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x+1/2, y+1/2, z+1; (iii) x+1, y+1, z+1.

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