Intermolecular interactions in cyanodimethylarsine and cyanodimethyl­stibine

Cyano­di­methyl­arsine, [As(CH₃)₂(CN)], and cyano­di­methyl­stibine, [Sb(CH₃)₂(CN)], have closely related, but not isomorphous, crystal structures containing XCN...XCN... chains. The N...As distance of 3.185 (3) Å is slightly shorter than the expected van der Waals distance of 3.5 Å, while the N...Sb distance of 2.862 (9) Å, compared with the expected value of 3.7 Å, is much shorter. This is consistent with Sb being a stronger Lewis acid than As.

S1. Comment

After the recognition that there were significant CN···X intermolecular interactions in (CH₃)₂AsCN (Camerman & Trotter, 1963), As(CN)₃ (Emerson & Britton, 1963), P(CN)₃ (Emerson & Britton. 1964), and CH₃As(CN)₂ (Schlemper & Britton, 1966), we attempted to extend this series to the corresponding Sb compounds.

Schlemper (1964) prepared (CH₃)₂SbCN, but found it invariably to be twinned and could only partially solve the structure. [He also prepared Sb(CN)₃, but all attempts to prepare single crystals led to amorphous solids. In addition, when he attempted to prepare CH₃Sb(CN)₂ from CH₃SbBr₂ and AgCN the product was again (CH₃)₂SbCN.] With more powerful techniques now available we decided to return to this problem and report here the structure of (CH₃)₂SbCN, (II). We have also redetermined the structure of (CH₃)₂AsCN, (I), for comparison. \sch

The atom labelling and displacement ellipsoids for both compounds are shown in Fig. 1, and the bond lengths and angles are compared in Table 1. The differences in the lengths and angles are what would be expected when Sb replaces As. The one unusual feature is that the Sb—C1 distance appears to be larger than the Sb—C2 and Sb—C3 distances; the difference is about eight s.u.s, which is marginally significant. This is opposite to the expected shortening when Csp³ is replaced by Csp.

The most significant intermolecular interactions are the N···As and N···Sb contacts, which are arranged in linear chains in both cases (see Fig. 1). The N···As distance of 3.2 Å is slightly shorter than the 3.5 Å that would be expected from the usual van der Waals radii (Pauling, 1960), but the N···Sb distance of 2.9 Å is much shorter than the van der Waals distance of 3.7 Å. The trend is what would be expected, since Sb should be a stronger Lewis acid than As. The strong N···Sb interaction is also consistent with the greater length of the Sb—C1 bond.

The linear chains formed by the N···X interactions are combined into double layers by X···X interactions (Figs. 2 and 3; Table 2). The As···As distances of 3.7 and 4.0 Å should be compared with the 4.0 Å van der Waals distance and the 3.1 Å distance found in elemental As (Donohue, 1982; the As—As single-bond distance is 2.517 Å). The Sb···Sb distances of 3.8 and 4.1 Å should be compared with the 4.4 Å van der Waals distance and the 3.4 Å distance found in elemental Sb (the Sb—Sb single-bond distance is 2.908 Å). These X···X interactions are weaker than in the pure elements but still appear to be an important part of the packing.

The crystals of the two compounds are not isomorphous but, as can be seen from Figs. 2 and 3, they are closely related. The chains parallel to the b axes and the double layers parallel to (001) are virtually the same in both structures. In both cases, the two halves of the double layers are related by centers of symmetry. The difference between the two lies in the stacking of adjacent double layers, which are related by a center of symmetry in the As compound, (I), but by a 2₁ axis in the Sb compound, (II).

S2. Experimental

Please provide brief details of syntheses of (I) and (II).

S3. Refinement

The As compound, (I), has a conventional cell, with a = 6.181 (2), b = 6.245 (2) and c = 7.875 (2) Å, and α = 78.52 (1), β = 69.69 (1) and γ = 60.43 (1)°. The matrix (110/110/001) converts this to the cell given in the data table. The change was made to facilitate comparison with the Sb structure, (II). [The matrix (010/001/100) converts the conventional cell to the orientation used by Camerman & Trotter (1963).] Schlemper (1964) examined 20 crystals of the Sb compound and found 18 obviously multiple from the diffraction patterns from precession photographs (Mo Kα radiation). The remaining two appeared to be orthorhombic, with a = 32.04 (5), b = 11.58 (2) and c = 6.21 (1) Å. With Z = 16, the calculated density (2.05 Mg m^-3) agreed reasonably with the experimental density from flotation in a CCl₄—CBr₄ mixture [2.15 (1) Mg m^-3]. However, the extinctions, for hkl reflections h+k, k+l, h+l all odd, and for hk0 reflections h+k ≠ 4n, did not fit any space group and twinning was suspected even for these two crystals. At this point, Weissenberg photographs were taken with Cu Kα radiation where, with the greater resolution, some split spots could be detected. These data fit a twinned monoclinic cell with a = 11.58 (2), b = 6.21 (1) and c = 17.03 (3) Å, and β = 109.8 (2)°; the extinctions now were appropriate for Cc or C2/c. The Sb atoms could be located but not all of the light atoms could be, and the problem was abandoned.

The new data for (II) could be indexed as a C-centered monoclinic cell (the one reported in the data table) or as an F-centered orthorhombic cell, which can be obtained from the monoclinic cell using the matrix (100/010/102). As found in the earlier work, the As positions could be found readily in the monoclinic cell but only some of the light atoms could be found. Rotating the monoclinic cell by 180° around the a axis produces a pseudo-merohedral twin related by the twin law (100 /010/101). Continuing the solution and refinement treating the crystal as a merohedral twin led to a chemically reasonable solution with a twin fraction of 0.475 (3).

Crystals of both (I) and (II) were too irregular in shape to allow face-indexed absorption corrections. While the SADABS (Sheldrick, 1996) absorption corrections were significant, they were not completely adequate. This problem led to the large peaks in the final difference maps. In both cases, all of the large peaks were close to the heavy atom. Problems with the absorption corrections were also the most likely reason for the elongated displacement ellipsoid for atom C1 in (II). H atoms were placed geometrically, with C—H = 0.98 Å, and refined as riding atoms, with U_iso(H) = 1.5U_eq(C). Are these the correct constraints?

Computing details

For both compounds, data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 1994); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. Views of the molecules of (I) (the As compound) and (II) (the Sb compound). Displacement ellipsoids are shown at the 50% probability level. In both cases, a second molecule shows the N···X intermolecular interaction.
	Fig. 2. The packing of (I) viewed along the b axis. The intermolecular As···As contacts are shown as dotted lines.
	Fig. 3. The packing of (II) viewed along the b axis. The intermolecular Sb···Sb contacts are shown as dotted lines.

(I) cyanodimethylarsine

Crystal data

[As(CH3)2CN]	Z = 4
Mr = 131.01	F(000) = 256
Triclinic, C1	Dx = 1.756 Mg m^−3
a = 10.738 (2) Å	Mo Kα radiation, λ = 0.71073 Å
b = 6.253 (2) Å	Cell parameters from 2568 reflections
c = 7.875 (2) Å	θ = 2.8–27.5°
α = 98.30 (3)°	µ = 6.68 mm^−1
β = 108.39 (3)°	T = 173 K
γ = 90.68 (3)°	Needle, colorless
V = 495.6 (2) Å^3	0.50 × 0.15 × 0.10 mm

Data collection

Siemens SMART area-detector diffractometer	1125 independent reflections
Radiation source: fine-focus sealed tube	1058 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
ω scans	θmax = 27.5°, θmin = 2.8°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996; Blessing, 1995)	h = −13→13
Tmin = 0.28, Tmax = 0.51	k = −8→8
2927 measured reflections	l = −10→10

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)] = 0.033	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.090	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.07P)^2 + 0.11P] where P = (Fo^2 + 2Fc^2)/3
1125 reflections	(Δ/σ)max = 0.001
46 parameters	Δρmax = 1.66 e Å^−3
0 restraints	Δρmin = −1.33 e Å^−3

Special details

Refinement. The structure is refined in space group C-1 to facilitate comparison with the analogous antimony compound.

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

	x	y	z	Uiso*/Ueq
As1	0.83363 (3)	0.04319 (4)	0.35626 (4)	0.02295 (16)
C3	0.9127 (4)	0.0553 (6)	0.1661 (5)	0.0335 (7)
H3A	1.0084	0.0515	0.2171	0.050*
H3B	0.8912	0.1896	0.1156	0.050*
H3C	0.8784	−0.0691	0.0701	0.050*
C1	0.8271 (3)	−0.2706 (5)	0.3337 (5)	0.0273 (6)
C2	0.6502 (3)	0.0542 (6)	0.2111 (5)	0.0326 (7)
H2A	0.5944	0.0497	0.2880	0.049*
H2B	0.6255	−0.0701	0.1135	0.049*
H2C	0.6385	0.1886	0.1590	0.049*
N1	0.8243 (3)	−0.4534 (5)	0.3301 (5)	0.0397 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As1	0.0222 (2)	0.0195 (2)	0.0264 (2)	0.00164 (13)	0.00633 (15)	0.00468 (14)
C3	0.0375 (19)	0.0303 (17)	0.0395 (18)	0.0052 (14)	0.0195 (15)	0.0110 (14)
C1	0.0235 (16)	0.0265 (16)	0.0339 (16)	0.0041 (12)	0.0097 (13)	0.0094 (12)
C2	0.0227 (16)	0.0280 (16)	0.0426 (18)	0.0042 (12)	0.0027 (14)	0.0083 (14)
N1	0.042 (2)	0.0286 (19)	0.051 (2)	0.0041 (15)	0.0170 (16)	0.0115 (15)

Geometric parameters (Å, º)

As1—C1	1.943 (3)	C3—H3C	0.9800
As1—C2	1.948 (4)	C1—N1	1.139 (4)
As1—C3	1.950 (3)	C2—H2A	0.9800
C3—H3A	0.9800	C2—H2B	0.9800
C3—H3B	0.9800	C2—H2C	0.9800
C1—As1—C2	94.51 (15)	H3B—C3—H3C	109.5
C1—As1—C3	94.99 (14)	N1—C1—As1	176.3 (3)
C2—As1—C3	98.36 (16)	As1—C2—H2A	109.5
As1—C3—H3A	109.5	As1—C2—H2B	109.5
As1—C3—H3B	109.5	H2A—C2—H2B	109.5
H3A—C3—H3B	109.5	As1—C2—H2C	109.5
As1—C3—H3C	109.5	H2A—C2—H2C	109.5
H3A—C3—H3C	109.5	H2B—C2—H2C	109.5

(II) cyanodimethylstibine

Crystal data

[Sb(CH3)2CN]	F(000) = 656
Mr = 177.84	Dx = 2.150 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 11.313 (7) Å	Cell parameters from 2092 reflections
b = 6.175 (4) Å	θ = 2.6–25.0°
c = 16.701 (11) Å	µ = 4.86 mm^−1
β = 109.673 (9)°	T = 173 K
V = 1098.6 (12) Å^3	Irregular, colorless
Z = 8	0.4 × 0.3 × 0.2 mm

Data collection

Siemens SMART area-detector diffractometer	968 independent reflections
Radiation source: fine-focus sealed tube	959 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.051
ω scans	θmax = 25.1°, θmin = 1.3°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996; Blessing, 1995)	h = −13→12
Tmin = 0.20, Tmax = 0.38	k = 0→7
1063 measured reflections	l = 0→19

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)] = 0.038	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.097	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.076P)^2 + 3.19P] where P = (Fo^2 + 2Fc^2)/3
968 reflections	(Δ/σ)max = 0.006
49 parameters	Δρmax = 2.19 e Å^−3
0 restraints	Δρmin = −1.62 e Å^−3

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

	x	y	z	Uiso*/Ueq
Sb1	0.83284 (5)	0.07875 (15)	0.42951 (3)	0.0246 (2)
C1	0.8225 (12)	−0.2804 (14)	0.4146 (5)	0.035 (2)
N1	0.8214 (12)	−0.4601 (14)	0.4109 (6)	0.047 (2)
C2	0.6378 (9)	0.1055 (17)	0.3533 (7)	0.042 (2)
H2A	0.6217	0.0218	0.3009	0.064*
H2B	0.6169	0.2579	0.3392	0.064*
H2C	0.5860	0.0489	0.3853	0.064*
C3	0.9041 (10)	0.1064 (19)	0.3243 (6)	0.041 (2)
H3A	0.8823	0.2490	0.2977	0.061*
H3B	0.8665	−0.0068	0.2823	0.061*
H3C	0.9956	0.0894	0.3456	0.061*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sb1	0.0280 (3)	0.0167 (3)	0.0271 (3)	0.0003 (3)	0.0068 (3)	−0.00020 (18)
C1	0.060 (6)	0.015 (4)	0.036 (4)	0.018 (5)	0.024 (5)	0.001 (3)
N1	0.054 (5)	0.023 (6)	0.061 (5)	−0.005 (5)	0.016 (5)	0.002 (4)
C2	0.032 (5)	0.035 (5)	0.054 (6)	−0.002 (4)	0.006 (4)	−0.014 (5)
C3	0.055 (6)	0.031 (5)	0.045 (6)	0.006 (4)	0.030 (5)	0.000 (4)

Geometric parameters (Å, º)

Sb1—C2	2.150 (10)	C2—H2B	0.9800
Sb1—C3	2.172 (9)	C2—H2C	0.9800
Sb1—C1	2.230 (9)	C3—H3A	0.9800
C1—N1	1.112 (12)	C3—H3B	0.9800
C2—H2A	0.9800	C3—H3C	0.9800
C2—Sb1—C3	95.8 (4)	H2A—C2—H2C	109.5
C2—Sb1—C1	90.3 (4)	H2B—C2—H2C	109.5
C3—Sb1—C1	90.4 (4)	Sb1—C3—H3A	109.5
N1—C1—Sb1	176.8 (10)	Sb1—C3—H3B	109.5
Sb1—C2—H2A	109.5	H3A—C3—H3B	109.5
Sb1—C2—H2B	109.5	Sb1—C3—H3C	109.5
H2A—C2—H2B	109.5	H3A—C3—H3C	109.5
Sb1—C2—H2C	109.5	H3B—C3—H3C	109.5