Tetra­phenyl­arsonium tetra­iodo­thallate(III)

The As and Tl atoms in title compound, (C₂₄H₂₀As)[TlI₄], sit on fourfold inversion centres, resulting in a [TlI₄]^- anion with D_2d symmetry and a Tl-I bond length of 2.7691 (3) Å. The tetrahedral anion is tetragonally distorted, being flattened in the direction of the c axis of the tetragonal unit cell, so that the two unique I-Tl-I angles differ by about 9°. In contrast, the cation has S₄ symmetry and the tetrahedral coordination geometry of the As atom is very slightly stretched in the direction of the c axis.

S1. Comment

11 different structures containing the tetraiodothallate(III) anion have been reported and of these only two have the anion on a site of high crystallographic symmetry. Often the nature of the cation keeps the space-group symmetry low (Drew et al., 1970; Glaser et al., 1982, 1983; Beno et al., 1987; Geiser et al., 1988, 1996; Riera et al., 1989; Tebbe et al., 1995; Ilyukhin et al., 2000; Slavin et al., 2000), but even CsTlI₄ crystallizes in space group P2₁/c with the anion in a general position (Thiele et al., 1986). In the salt with the Et₃S⁺ cation (Svensson et al., 2000), the [TlI₄]^- anion sits on a fourfold inversion centre, but the cation is reported to be so highly disordered that the C atoms were not included in the model, thus detracting from the precision of the geometry of the anion. The only reported ordered structure with high symmetry is that of ammonium ammonia tetraiodothallate(III) bis(18-crown-6) clathrate (Domasevitch et al., 1999), in which the [TlI₄]^- anion has crystallographic T_d symmetry and the 18-crown-6 molecule has C₃ symmetry. We were interested in finding a simpler system in which the [TlI₄]^- anion displays high crystallographic symmetry and chose (Ph₄As)⁺ as the counter-ion because the size and symmetry of the cation should be compatible with the [TlI₄]^- anion and favour crystallization of the salt in a high-symmetry space group. This cation is used frequently in organometallic syntheses, appears in over 530 of the structures in the Cambridge Structural Database (CSD, April 2002 release; Allen & Kennard, 1993) and is often found to adopt high crystallographic site symmetry in structures with amenable anions.

The structure of the title salt, (I), does indeed present the ions with high crystallographic site symmetry. The As and Tl atoms sit on fourfold inversion centres so that the cation and anion have S₄ and D_{2 d} symmetry, respectively. The Tl—I bond length (Table 1) is in agreement with those found in other structures containing [TlI₄]^- anions. For the eight such structures for which coordinates are present in the CSD, plus that of CsTlI₄, there were 31 hits for a unique Tl—I bond, with a range of bond lengths of 2.69–2.83 Å and a mean value of 2.758 (6) Å. Considering only structures with highly symmetrical [TlI₄]^- anions, the unique Tl—I bond length in the Et₃S⁺ salt is 2.757 (1) Å (Svensson et al., 2000), while it is 2.766 (2) and 2.748 (2) %A (two independent anions) in the 18-crown-6 clathrate (Domasevitch et al., 1999). The unique As—C bond length in the cation compares well with the mean value of 1.905 (1) Å for 1990 hits for this bond among 427 error-free structures in the CSD involving (Ph₄As)⁺ cations for which coordinates are present. For these hits, the range of As—C bond lengths is 1.70–2.15 Å, although over 98% of the entries are in the narrower range of 1.82–1.98 Å.

Except for the perfectly tetrahedral [TlI₄]^- anion in the 18-crown-6 clathrate (Domasevitch et al., 1999), most other examples show a very small distortion from this geometry. The range of I—Tl—I angles across the eight structures in the CSD, plus CsTlI₄, is 100–117°. Normally, this variation is because each Tl—I bond in the anion is crystallographically independent, which allows a certain degree of flexibility that can be influenced by the surrounding environment. However, even for the title compound with its four symmetry-equivalent Tl—I bonds, the anion shows significant tetragonal distortion. The [TlI₄]^- tetrahedron is flattened in the direction of the c axis of the tetragonal unit cell so that the two unique I—Tl—I angles in the anion differ by about 9° (Table 1). In contrast, for the Et₃S⁺ salt, in which the anion also has 4 symmetry, the tetragonal distortion is negligible.

The tetragonal distortion of the tetrahedral coordination geometry about the As atom in the (Ph₄As)⁺ cation is less severe, with only a 2.5° difference between the unique C—As—C angles. The distortion is also in the opposite direction to that in the anion, so that the cation is slightly stretched in the direction of the c axis of the unit cell. For the 427 error-free structures in the CSD, the C—As—C angle for 3140 hits ranges from 97.5 to 119.3°, although over 96% of the entries are in the narrower range of 105.0–114.0°.

Compound (I) is not isostructural with any other known related structure. There are no entries in the CSD for any (Ph₄As)⁺ salts which crystallize in space group I4₁/a, nor are there entries for any halothallate(III) salts having this space group. No related (Ph₄P)⁺ salts are recorded either. Cotton et al. (1965) reported that powder diffraction measurements on (Ph₄As)[TlCl₄] showed that it is isostructural with (Ph₄As)[FeCl₄] (Zaslow & Rundle, 1957; Cotton & Murillo, 1975), but these compounds crystallize in the space group I4.

S2. Experimental

The title compound was prepared exactly as described by Cotton et al. (1965), and its melting point (413 K) and microanalytical data were identical with the reference values. Suitable crystals were grown by slow evaporation of a solution of (I) in acetone.

S3. Refinement

The largest peak of residual electron density was 1.73 Å from atom H3. All H atoms were placed in geometrically idealized positions (C—H = 0.95 Å) and constrained to ride on their parent atoms, with U_iso(H) = 1.2U_eq(C). One reflection was omitted from the final refinement because its observed intensity was much lower than the calculated value as a result of being partially obscured by the beam stop.

Computing details

Data collection: COLLECT (Nonius, 2000); cell refinement: DENZO–SMN (Otwinowski & Minor, 1997); data reduction: DENZO–SMN and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97; molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2002).

Figures

Fig. 1. View of the structure of the title compound drawn with the c axis of the unit cell running horizontally from left to right and showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 5/4 - y, 1/4 + x, 5/4 - z; (ii) 1 - x, 3/2 - y, z; (iii) 5/4 - y, 1/4 + x, 1/4 - z.]

tetraphenylarsonium tetraiodothallate(III)

Crystal data

(C24H20As)[TlI4]	Dx = 2.540 Mg m^−3
Mr = 1095.26	Melting point: 413 K
Tetragonal, I41/a	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -I 4ad	Cell parameters from 21361 reflections
a = 14.7686 (3) Å	θ = 2.0–30.0°
c = 13.1319 (3) Å	µ = 11.10 mm^−1
V = 2864.22 (11) Å^3	T = 160 K
Z = 4	Prism, red
F(000) = 1960	0.20 × 0.12 × 0.08 mm

Data collection

Nonius KappaCCD diffractometer	2100 independent reflections
Radiation source: Nonius FR591 sealed tube generator	1748 reflections with I > 2σ(I)
Horizontally mounted graphite crystal monochromator	Rint = 0.076
Detector resolution: 9 pixels mm^-1	θmax = 30.0°, θmin = 2.8°
ϕ and ω scans with κ offsets	h = −20→20
Absorption correction: numerical (Coppens et al., 1965)	k = −20→19
Tmin = 0.243, Tmax = 0.458	l = −18→16
22651 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.031	H-atom parameters constrained
wR(F^2) = 0.078	w = 1/[σ^2(Fo^2) + (0.0288P)^2 + 7.2222P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.001
2099 reflections	Δρmax = 1.13 e Å^−3
69 parameters	Δρmin = −1.77 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00031 (7)

Special details

Experimental. Solvent used: Cooling Device: Oxford Cryosystems Cryostream 700 Crystal mount: glued on a glass fibre Mosaicity (°.): 0.729 (2) Frames collected: 216 Seconds exposure per frame: 50 Degrees rotation per frame: 2.0 Crystal-Detector distance (mm): 30.0

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
Tl	0.5000	0.7500	0.6250	0.03529 (11)
I	0.63475 (2)	0.83385 (2)	0.51273 (2)	0.04527 (12)
As	0.5000	0.7500	0.1250	0.02783 (17)
C1	0.5776 (3)	0.6805 (3)	0.2104 (3)	0.0311 (7)
C2	0.5391 (3)	0.6346 (3)	0.2921 (3)	0.0409 (9)
H2	0.4762	0.6401	0.3057	0.049*
C3	0.5935 (4)	0.5807 (4)	0.3537 (4)	0.0488 (11)
H3	0.5678	0.5484	0.4092	0.059*
C4	0.6849 (3)	0.5742 (3)	0.3339 (4)	0.0466 (10)
H4	0.7219	0.5370	0.3757	0.056*
C5	0.7233 (3)	0.6213 (3)	0.2539 (3)	0.0424 (9)
H5	0.7867	0.6173	0.2420	0.051*
C6	0.6698 (3)	0.6746 (3)	0.1908 (3)	0.0360 (8)
H6	0.6959	0.7065	0.1352	0.043*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl	0.03596 (13)	0.03596 (13)	0.03397 (17)	0.000	0.000	0.000
I	0.04061 (18)	0.0507 (2)	0.04452 (18)	−0.00312 (12)	0.00632 (11)	0.00371 (12)
As	0.0283 (2)	0.0283 (2)	0.0268 (3)	0.000	0.000	0.000
C1	0.0345 (19)	0.0310 (18)	0.0279 (16)	−0.0005 (14)	−0.0037 (14)	−0.0003 (13)
C2	0.038 (2)	0.048 (2)	0.037 (2)	0.0001 (19)	0.0027 (16)	0.0070 (17)
C3	0.055 (3)	0.051 (3)	0.041 (2)	0.001 (2)	−0.008 (2)	0.012 (2)
C4	0.051 (3)	0.042 (2)	0.047 (2)	0.005 (2)	−0.019 (2)	0.0012 (19)
C5	0.035 (2)	0.042 (2)	0.051 (2)	0.0029 (17)	−0.0111 (17)	−0.0081 (18)
C6	0.037 (2)	0.035 (2)	0.0366 (19)	−0.0021 (16)	−0.0044 (16)	−0.0047 (15)

Geometric parameters (Å, º)

Tl—I	2.7691 (3)	C3—H3	0.9500
As—C1	1.904 (4)	C4—C5	1.383 (7)
C1—C6	1.389 (6)	C4—H4	0.9500
C1—C2	1.390 (6)	C5—C6	1.389 (6)
C2—C3	1.391 (6)	C5—H5	0.9500
C2—H2	0.9500	C6—H6	0.9500
C3—C4	1.377 (7)
I—Tl—I^i	106.469 (6)	C4—C3—H3	120.2
I—Tl—I^ii	115.660 (13)	C2—C3—H3	120.2
C1—As—C1^ii	107.8 (2)	C3—C4—C5	120.8 (4)
C1—As—C1^iii	110.31 (11)	C3—C4—H4	119.6
C6—C1—C2	120.9 (4)	C5—C4—H4	119.6
C6—C1—As	121.0 (3)	C4—C5—C6	120.3 (4)
C2—C1—As	118.1 (3)	C4—C5—H5	119.9
C1—C2—C3	119.5 (4)	C6—C5—H5	119.9
C1—C2—H2	120.3	C1—C6—C5	118.9 (4)
C3—C2—H2	120.3	C1—C6—H6	120.6
C4—C3—C2	119.7 (4)	C5—C6—H6	120.6
C1^iii—As—C1—C6	−14.4 (3)	As—C1—C2—C3	177.7 (4)
C1^ii—As—C1—C6	−134.9 (4)	C1—C2—C3—C4	0.9 (7)
C1^iv—As—C1—C6	104.6 (4)	C2—C3—C4—C5	0.4 (8)
C1^iii—As—C1—C2	166.6 (3)	C3—C4—C5—C6	−1.3 (7)
C1^ii—As—C1—C2	46.1 (3)	C2—C1—C6—C5	0.5 (6)
C1^iv—As—C1—C2	−74.4 (3)	As—C1—C6—C5	−178.5 (3)
C6—C1—C2—C3	−1.3 (7)	C4—C5—C6—C1	0.8 (6)

Symmetry codes: (i) −y+5/4, x+1/4, −z+5/4; (ii) −x+1, −y+3/2, z; (iii) −y+5/4, x+1/4, −z+1/4; (iv) y−1/4, −x+5/4, −z+1/4.