Redetermination of the crystal structure of ThI₄

Single crystals of ThI₄, thorium(IV) tetra­iodide, were grown from thorium dioxide and aluminium triiodide. In comparison with the structure model reported previously for this compound [Zalkin et al. (1964). Inorg. Chem. 3, 639–644], we have determined the lattice parameters and fractional coordinates to a much higher precision, also leading to a better reliability factor (R = 0.029 versus 0.09). The coordination number of the Th^IV atom is eight. Its coordination polyhedron has the shape of an irregular square anti­prism. The I atoms each bridge two Th^IV atoms, resulting in the formation of infinite layers parallel to (-101) that can be described with the Niggli formula ²_∞[ThI_6/2I_2/2].

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Structure description

ThI₄ was first synthesized in 1882 (Nilson, 1882) by heating thorium metal in I₂ vapour. Its crystal structure has been known since 1964 (Zalkin et al., 1964) and IR and Raman spectra were measured in 1976 (Brown et al., 1976). The other tetra­halides of thorium, viz. ThF₄ (von Wartenberg et al., 1923), ThCl₄ (Matignon & Delepine, 1907), ThBr₄ (Nilson, 1882), and the bi- and trivalent compounds ThI₂ (Anderson & D'Eye, 1949) and ThI₃ (Hayek & Rehner, 1949) are also known. In our efforts to synthesize pure actinide halides, we have developed a chemical vapour-transport method (Deubner et al., 2017), which allowed us to obtain single crystals suitable for X-ray diffraction experiments.

The lattice parameters obtained by our single-crystal structure determination (Table 1) agree with those obtained previously (a = 13.22, b = 8.07, c = 7.77 Å, β = 98.68°, Z = 4, T = n.a.; Zalkin et al., 1964). The Th^IV atom is located on a general position and has eight iodine atoms in its irregular square-anti­prismatic coordination sphere (Fig. 1). The Th—I distances range between 3.1324 (7) and 3.2896 (7) Å and are in good agreement with the previously reported data (3.128–3.291 Å; Zalkin et al., 1964). As might be expected, the Th—I distances are comparable with those reported for the crystal structure of ThTe₂I₂ which features a similar anti-prismatic coordination sphere for the Th^IV atom [3.1445 (9)–3.2157 (7) Å; Rocker & Tremel, 2001]. The same applies for the Th⋯Th distances [4.4770 (6) Å] and the shortest I⋯I distances between the layers ranging from 4.1526 (8) to 4.2423 (8) Å [Th⋯Th distance: 4.478 (5), I⋯I distances: 4.079–4.252 (6) Å; Zalkin et al., 1964). The I—I—I angles (Table 1) of the irregular polyhedron faces are also in good agreement with the previous structure determination (Zalkin et al., 1964).

Table 1
The I—I—I angles (°) within the irregular ThI₈ coordination polyhedron

Face	Present refinement	Previous refinement (Zalkin et al., 1964)
I2—I1^iv—I3^i	76.510 (15)	76.5
I1^iv—I3^i—I4^iii	101.411 (17)	101.5
I3^i—I4^iii—I2	79.585 (15)	79.5
I4^iii—I2—I1^iv	97.949 (16)	98.0
I4—I1^ii—I2^iii	87.142 (16)	87.0
I2^iii—I3^iii—I4	91.545 (16)	91.5
I1^ii—I2^iii—I3^iii	91.485 (16)	91.4
I1^ii—I4—I3^iii	89.821 (15)	90.0
Symmetry codes: (i) x, y − 1, z; (ii) x, y − 1, z − 1; (iii) −x + , y − , −z + ; (iv) −x, −y + 1, −z + 1.

Figure 1 The irregular square-anti­prismatic coordination sphere of the Th^IV atom in the crystal structure of ThI4. Displacement ellipsoids are shown at the 95% probability level. [Symmetry codes: (i) x, y − 1, z; (ii) x, y − 1, z − 1; (iii)  − x, y − ,  − z; (iv) −x, −y + 1, −z + 1.]

With respect to the shortest Th⋯Th distance of 4.4770 (6) Å, the Th^IV atoms are bridged by three iodide atoms (a triangular face of the square anti­prism formed by the I2, I3 and I4 atoms), formally leading to the formation of one-dimensional infinite chains. These chains are in turn inter­connected by shared edges of the anti­prism (the I1 atoms), which corresponds to a Th⋯Th distance of 5.1998 (8) Å. This connection leads to infinite layers of ²_∞[ThI_6/2I_2/2], running parallel to (01) (Fig. 2). The arrangement of the Th^IV atoms within a layer corresponds to a 6³ network. Fig. 3 shows the crystal structure of the compound.

Figure 2 A section of the ^2∞[ThI6/2I2/2] layer within the crystal structure of ThI4, showing the connection of the coordination polyhedra via faces and edges. Coordination polyhedra are shown transparent in green, atomic radii are arbitrary.

Figure 3 The crystal structure of ThI4 in a projection along [010]. Displacement ellipsoids are shown at the 95% probability level.

Synthesis and crystallization

Thorium(IV) tetra­iodide was synthesized from dry thorium dioxide (3.00 g, 11.36 mmol, Merck) and sublimed aluminium iodide (9.28 g, 22.76 mmol) in an evacuated and flame-sealed borosilicate tube at 623 K with an additional in situ chemical vapour transport (Deubner et al., 2017). The temperature at the source region was 723 K and at the sink region 623 K; the length of the tube was 13 cm. Canary yellow crystals could be obtained by an additional vacuum sublimation at 723 K in an evacuated, flame-sealed borosilicate tube.

Refinement

As a starting model for the structure refinement, the atomic coordinates of the previously reported ThI₄ structure were used (Zalkin et al., 1964). Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	ThI4
Mr	739.64
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	243
a, b, c (Å)	13.1903 (16), 8.0686 (12), 7.755 (1)
β (°)	98.619 (10)
V (Å^3)	816.02 (19)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	33.29
Crystal size (mm)	0.39 × 0.26 × 0.14
Data collection
Diffractometer	Stoe IPDS 2T
Absorption correction	Numerical (X-RED and X-SHAPE; Stoe & Cie, 2009)
Tmin, Tmax	0.240, 0.622
No. of measured, independent and observed [I > 2σ(I)] reflections	5880, 2174, 1988
Rint	0.063
(sin θ/λ)max (Å^−1)	0.688
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.029, 0.071, 1.18
No. of reflections	2174
No. of parameters	47
Δρmax, Δρmin (e Å^−3)	2.82, −2.06
Computer programs: X-AREA (Stoe & Cie, 2011) X-RED (Stoe & Cie, 2009), SHELXL2018 (Sheldrick, 2015), DIAMOND (Brandenburg, 2015) and publCIF (Westrip, 2010). Coordinates taken from a previous model.

Structural data

Computing details

Data collection: X-AREA (Stoe & Cie, 2011); cell refinement: X-AREA (Stoe & Cie, 2011); data reduction: X-RED (Stoe & Cie, 2009); program(s) used to solve structure: coordinates taken from a previous model; program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2015); software used to prepare material for publication: publCIF (Westrip, 2010).

Thorium tetraiodide

Crystal data

ThI4	cell choice according to the previous literature
Mr = 739.64	Dx = 6.020 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 13.1903 (16) Å	Cell parameters from 6166 reflections
b = 8.0686 (12) Å	θ = 4.9–58.3°
c = 7.755 (1) Å	µ = 33.29 mm^−1
β = 98.619 (10)°	T = 243 K
V = 816.02 (19) Å^3	Hexagonal-block, canary yellow
Z = 4	0.39 × 0.26 × 0.14 mm
F(000) = 1208

Data collection

Stoe IPDS 2T diffractometer	2174 independent reflections
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus	1988 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.063
rotation method scans	θmax = 29.3°, θmin = 2.9°
Absorption correction: numerical (X-RED and X-SHAPE; Stoe & Cie, 2009)	h = −18→18
Tmin = 0.240, Tmax = 0.622	k = −11→9
5880 measured reflections	l = −10→10

Refinement

Refinement on F^2	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.029	w = 1/[σ^2(Fo^2) + (0.0391P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.071	(Δ/σ)max < 0.001
S = 1.18	Δρmax = 2.82 e Å^−3
2174 reflections	Δρmin = −2.06 e Å^−3
47 parameters	Extinction correction: SHELXL2018 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00252 (18)
0 constraints

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

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

	x	y	z	Uiso*/Ueq
Th1	0.18352 (2)	0.01507 (3)	0.17681 (3)	0.01185 (10)
I1	0.05855 (4)	0.90966 (7)	0.80945 (6)	0.01692 (12)
I2	0.18029 (4)	0.25357 (6)	0.49875 (6)	0.01548 (12)
I3	0.09726 (4)	0.69187 (6)	0.32569 (6)	0.01769 (13)
I4	0.15132 (4)	0.36459 (6)	0.00132 (6)	0.01833 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Th1	0.01200 (14)	0.00591 (15)	0.01672 (14)	−0.00018 (8)	−0.00090 (8)	−0.00035 (8)
I1	0.0137 (2)	0.0170 (3)	0.0190 (2)	0.00123 (17)	−0.00083 (15)	−0.00413 (17)
I2	0.0177 (2)	0.0117 (2)	0.0173 (2)	−0.00222 (16)	0.00343 (15)	−0.00095 (16)
I3	0.0134 (2)	0.0108 (3)	0.0290 (3)	0.00061 (16)	0.00340 (17)	0.00347 (18)
I4	0.0218 (2)	0.0100 (2)	0.0202 (2)	−0.00236 (17)	−0.00671 (17)	0.00211 (16)

Geometric parameters (Å, º)

Th1—I4	3.1324 (7)	Th1—I3^iii	3.2269 (6)
Th1—I3^i	3.1340 (6)	Th1—I1^iv	3.2675 (6)
Th1—I2	3.1576 (6)	Th1—I4^iii	3.2896 (7)
Th1—I1^ii	3.1857 (7)	Th1—Th1^v	4.4770 (6)
Th1—I2^iii	3.2041 (6)	Th1—Th1^vi	5.1998 (8)
I4—Th1—I3^i	151.181 (15)	I3^iii—Th1—I4^iii	71.069 (16)
I4—Th1—I2	77.164 (17)	I1^iv—Th1—I4^iii	125.668 (16)
I3^i—Th1—I2	99.624 (17)	I4—Th1—Th1^v	47.262 (12)
I4—Th1—I1^ii	80.418 (16)	I3^i—Th1—Th1^v	143.752 (15)
I3^i—Th1—I1^ii	86.534 (16)	I2—Th1—Th1^v	45.695 (11)
I2—Th1—I1^ii	143.446 (15)	I1^ii—Th1—Th1^v	126.717 (13)
I4—Th1—I2^iii	117.155 (16)	I2^iii—Th1—Th1^v	118.545 (15)
I3^i—Th1—I2^iii	82.309 (17)	I3^iii—Th1—Th1^v	44.426 (10)
I2—Th1—I2^iii	144.308 (10)	I1^iv—Th1—Th1^v	99.896 (13)
I1^ii—Th1—I2^iii	72.070 (15)	I4^iii—Th1—Th1^v	87.161 (14)
I4—Th1—I3^iii	70.287 (15)	I4—Th1—Th1^vi	76.002 (12)
I3^i—Th1—I3^iii	138.169 (14)	I3^i—Th1—Th1^vi	78.151 (12)
I2—Th1—I3^iii	81.582 (15)	I2—Th1—Th1^vi	108.886 (14)
I1^ii—Th1—I3^iii	117.173 (16)	I1^ii—Th1—Th1^vi	36.853 (10)
I2^iii—Th1—I3^iii	74.250 (16)	I2^iii—Th1—Th1^vi	106.387 (14)
I4—Th1—I1^iv	77.136 (15)	I3^iii—Th1—Th1^vi	141.428 (14)
I3^i—Th1—I1^iv	74.466 (15)	I1^iv—Th1—Th1^vi	35.786 (10)
I2—Th1—I1^iv	74.449 (15)	I4^iii—Th1—Th1^vi	147.277 (14)
I1^ii—Th1—I1^iv	72.639 (17)	Th1^v—Th1—Th1^vi	118.313 (9)
I2^iii—Th1—I1^iv	138.513 (15)	Th1^vii—I1—Th1^iv	107.361 (17)
I3^iii—Th1—I1^iv	143.026 (16)	Th1—I2—Th1^v	89.455 (15)
I4—Th1—I4^iii	133.886 (15)	Th1^viii—I3—Th1^v	89.458 (15)
I3^i—Th1—I4^iii	69.451 (15)	Th1—I4—Th1^v	88.361 (15)
I2—Th1—I4^iii	73.192 (16)	Th1—I4—I1^ix	161.270 (12)
I1^ii—Th1—I4^iii	140.814 (17)	Th1^v—I4—I1^ix	76.457 (11)
I2^iii—Th1—I4^iii	74.312 (15)

Symmetry codes: (i) x, y−1, z; (ii) x, y−1, z−1; (iii) −x+1/2, y−1/2, −z+1/2; (iv) −x, −y+1, −z+1; (v) −x+1/2, y+1/2, −z+1/2; (vi) −x, −y, −z; (vii) x, y+1, z+1; (viii) x, y+1, z; (ix) x, y+1, z−1.