Bis(2-methyl-2-propan­ammonium) chromate

A new organic chromate, [(CH₃)₃CNH₃]₂[CrO₄], associated with the monoprotonated 2-methyl-2-propan­amine mol­ecule, has been synthesized. The structure of the compound consists of discrete chromate ions stacked in layers perpendicular to the (010) plane, separated by organic layers containing [(CH₃)₃CNH₃]⁺ groups. The cohesion and stability of the structure are ensured by a two-dimensional network of hydrogen bonds in the (001) plane, where the O atoms of the anion are acceptors from the 2-methyl-2-propan­ammonium N-H groups.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.003 Å
• Disorder in solvent or counterion
• R factor = 0.036
• wR factor = 0.090
• Data-to-parameter ratio = 15.0

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



PLAT_302  Alert C Anion/Solvent Disorder .......................      17.00 Perc.


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 1 Alert Level C = Please check

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S1. Comment

The present description of the bis(2-methyl-2-propanammonium)chromate structure is part of an investigation of materials resulting from interaction between chromic acid and organic molecules, as amines and aminoalcohols. Among the investigated materials, we have previously described 2,2-dimethyl-1,3-propanediammonium chromate (Chebbi et al., 2000), 4-ammonio-2,2,6,6-tetramethylpiperidinium chromate dihydrate (Chebbi & Driss, 2001) and 1,4-butanediammonium chromate (Chebbi & Driss, 2002). Furthermore, the literature gives some examples of CrO₄²⁻ associated to organic cation: 2(CN₃H₆)⁺·CrO₄²⁻ (Cygler et al. 1976) and 2[(CH₃)₄N]⁺·CrO₄²⁻.xH₂O (x = 0.5 or 2; Sorehkin et al., 1978). Two components, inorganic CrO₄²⁻ and organic [(CH₃)₃CNH₃]⁺, build up the atomic arrangement of [(CH₃)₃CNH₃]₂[CrO₄], (I) (Fig. 1). Six chromate layers per cell coexist parallel to (001) planes at z_i = 2i + 1/12 (0 ≤ i ≤ 5), while the organic groups provide the cohesion in layers through N—H···O hydrogen bonds (Fig. 2). The network of CrO₄²⁻ lies between two networks of cations without interconnection between successive layers. The CrO₄²⁻ anion has local 3 m symmetry instead of regular −43m symmetry and the charge is compensated by the 2-methyl-2-propanammonium cations. The Cr atom is delocalized in two positions, separated by 0.9 Å and having an occupancy of 0.5 each. The Cr—O distances within the distorted tetrahedron CrO₄ vary from 1.620 (5) to 1.632 (2) Å. In addition, the O—Cr—O angle values differ significantly from the ideal value 109.5° and vary from 106.77 (4) to 112.03 (3)°. These values are generally observed for this type of tetrahedron (Gerault et al., 1987; Bars et al., 1977; Stephens et al., 1969; Brauer et al., 1990). With regard to the geometry of the anion, there is a slight dissymmetry in the Cr—-O bond lengths; the Cr–O1 bond is significantly shorter than the other three. This probably reflects the fact that atom O1 is not involved in any hydrogen bond. The 2-methyl-2-propanammonium groups establish hydrogen bond involving the H atoms of the NH₃ groups: one N—H···O bond has an N···O distance of 2.806 (2) Å and participates in the cohesion of the two-dimensional network. The N—C and C—C distances, and C—C—N and C—C—C angles in this organic group are comparable whith those observed for other compounds (Cygler et al. 1976; Chebbi et al., 2000; Chebbi & Driss, 2001, 2002).

S2. Experimental

The title compound was prepared from a 1:2:100 mixture of CrO₃ (2 g), C₄H₁₁N (4.84 g) and H₂O (36 g). Yellow single crystals suitable of X-ray analysis were obtained from the solution by slow evaporation of the solvent at room temperature.

S3. Refinement

H atoms of methyl groups were placed at calculated positions and refined as riding, with C—H = 0.96 Å. Atom H4 bonded to N was located in difference Fourier calculations and was refined isotropically. The Cr atom is disordred over two positions, separated by about 0.9 Å, and these positions were refined with half occupancies. One non-equivalent O atom was located around the Cr atom and its position was refined with a site occupancy of 0.5

Computing details

Data collection: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); cell refinement: CAD-4 EXPRESS; data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecular structure of (I) (PLATON; Spek, 1990). Atoms Cr1 and O1 have a site-occupation factor of 0.5.
	Fig. 2. Projection along the c direction of (I), with dashed lines indicating hydrogen bonds.

Bis(2-methyl-2-propanammonium) chromate

Crystal data

(C4H12N)2[CrO4]	Dx = 1.239 Mg m^−3
Mr = 264.29	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 15 reflections
Hall symbol: -R 3 2"c	θ = 11–14°
a = 6.720 (1) Å	µ = 0.81 mm^−1
c = 54.330 (9) Å	T = 293 K
V = 2124.8 (6) Å^3	Hexagonal prism, yellow
Z = 6	0.28 × 0.25 × 0.14 mm
F(000) = 852

Data collection

Enraf-Nonius CAD-4 diffractometer	359 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.057
Graphite monochromator	θmax = 27.0°, θmin = 3.6°
ω/2θ scans	h = 0→8
Absorption correction: ψ scan (North et al., 1968)	k = −8→0
Tmin = 0.802, Tmax = 0.893	l = −69→69
1936 measured reflections	2 standard reflections every 120 min
524 independent reflections	intensity decay: 0.4%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.091	H atoms treated by a mixture of independent and constrained refinement
S = 1.37	w = 1/[σ^2(Fo^2) + (0.02P)^2 + 0.149P] where P = (Fo^2 + 2Fc^2)/3
524 reflections	(Δ/σ)max < 0.001
35 parameters	Δρmax = 0.17 e Å^−3
0 restraints	Δρmin = −0.16 e Å^−3

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	Occ. (<1)
Cr1	0.6667	0.3333	0.07467 (2)	0.0349 (3)	0.50
O1	0.6667	0.3333	0.04485 (9)	0.0600 (12)	0.50
O2	0.4342 (3)	0.1009 (3)	0.0833	0.0648 (7)
N1	0.3333	0.6667	0.06612 (5)	0.0441 (6)
C1	0.3333	0.6667	0.03843 (6)	0.0548 (8)
C2	0.2382 (6)	0.4187 (4)	0.03014 (5)	0.0860 (9)
H1	0.3359	0.3629	0.0361	0.102 (11)*
H2	0.2325	0.4117	0.0125	0.096 (9)*
H3	0.0862	0.3254	0.0367	0.109 (11)*
H4	0.372 (4)	0.808 (3)	0.0728 (3)	0.060 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr1	0.0242 (3)	0.0242 (3)	0.0561 (6)	0.01211 (15)	0.000	0.000
O1	0.0616 (18)	0.0616 (18)	0.057 (2)	0.0308 (9)	0.000	0.000
O2	0.0394 (7)	0.0394 (7)	0.0868 (15)	−0.0019 (9)	0.0153 (7)	−0.0153 (7)
N1	0.0391 (8)	0.0391 (8)	0.0542 (15)	0.0195 (4)	0.000	0.000
C1	0.0554 (11)	0.0554 (11)	0.0536 (17)	0.0277 (6)	0.000	0.000
C2	0.100 (3)	0.0780 (17)	0.0782 (17)	0.044 (2)	0.0000 (17)	−0.0242 (14)

Geometric parameters (Å, º)

Cr1—O1	1.620 (5)	C1—C2	1.524 (3)
Cr1—O2	1.632 (2)	C2—H1	0.96
N1—C1	1.505 (4)	C2—H2	0.96
N1—H4	0.925 (18)	C2—H3	0.96
O1—Cr1—O2	106.77 (4)	C1—C2—H2	109.5
O2^i—Cr1—O2	112.03 (3)	H1—C2—H2	109.5
C1—N1—H4	113.2 (12)	C1—C2—H3	109.5
N1—C1—C2	107.17 (17)	H1—C2—H3	109.5
C2^ii—C1—C2	111.67 (15)	H2—C2—H3	109.5
C1—C2—H1	109.5

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H4···O2^iii	0.92 (2)	1.88 (2)	2.806 (2)	176 (2)

Symmetry code: (iii) x, y+1, z.