Hexa­kis­[di­methyl­tin(IV) difluoride] potassium iodide, 6Me₂SnF₂·KI: linear rods of potassium iodide penetrating the pores in planar layers of di­methyl­tin(IV) difluoride

The hexa­gonal host–guest title compound, poly[hexa­kis­[[di­methyl­tin(IV)]-di-μ-fluorido] potassium iodide], {[Sn(CH₃)₂F₂]₆·KI}_n or (Me₂SnF₂)₆·KI, represents a layer structure of distorted {Me₂SnF_4/2} octa­hedra corner-linked via μ₂-bonding fluorine atoms. Distortion of the octa­hedra concerns not only bond lengths [d(Sn—C) = 2.089 (2) Å, d(Sn—F) = 2.077 (1)/2.080 (1), 2.252 (1)/2.266 (1) Å] but also bond angles [〈(C—Sn—C) = 162.2 (1)°, 〈(F—Sn—F) = 77.11 (7)–119.57 (5)°] giving rise to a irregular quadrilateral, pseudo-equatorial plane of fluorine atoms around the central tin atom. In the planar (001) layers, the octa­hedra are arranged according to a snub hexa­gonal tiling (sr{3,6}) resulting in small trigonal and larger hexa­gonal pores. The latter are occupied by potassium ions [d(F⋯K) = 2.702 (2) Å, 6×], which in turn form linear rods with iodine ions [d(K⋯I) = 3.6702 (2) Å, 2×) perpendicular to adjacent layers.

1. Chemical context

Di­methyl­tin(IV) difluoride, Me₂SnF₂, takes up a special position among the diorganotin(IV) dihalides as it is the only one that forms a layered structure with octa­hedrally coordinated tin atoms connected to each other via μ₂-coordinating halogen atoms (Schlemper & Hamilton, 1966). In addition, the planar layers have some unique features, such as linear fluorine bridges and tin atoms with site symmetry of 4/mmm, so that the methyl groups are also arranged exactly linearly but disordered. Despite these unusual structural properties, nothing is yet known about its potential supra­molecular properties, which is certainly also due to the fact that the compound is largely insoluble.

Like many other diorganotin(IV) difluorides, di­methyl­tin(IV) difluoride is most easily and cheaply prepared via a halide-exchange reaction of di­methyl­tin(IV) dichloride, Me₂SnCl₂, and potassium fluoride in ethanol or acetone as a solvent (Krause, 1918). By modifying these reaction conditions and using di­methyl­tin diiodide, Me₂SnI₂, instead of di­methyl­tin dichloride, it was possible for the first time to obtain not only the originally desired di­methyl­tin(IV) difluoride but also the host–guest compound of the difluoride with potassium iodide and the composition 6Me₂SnF₂·KI in a reproducible manner.

2. Structural commentary

The title compound crystallizes in the hexa­gonal space group P6/mcc with two formula units in the unit cell. The asymmetric unit (Fig. 1) consists of one tin atom and two fluorine atoms all three lying on a crystallographic mirror plane (Wyckoff letter l) and the atoms of one methyl group in general position (Wyckoff letter m). In addition, the potassium cation occupies the special position of site symmetry 6/m (Wyckoff letter b) and the iodine atom the special position of site symmetry 622 (Wyckoff letter a). Overall, the combination of these building blocks results in a supra­molecular arrangement in which linear rods of potassium iodide penetrate the pores within planar layers of di­methyl­tin(IV) difluoride.

Figure 1 Ball-and-stick model showing the connectivity scheme between the atoms in (Me2SnF2)6·KI, the atom labeling of the asymmetric unit, and some symmetry elements (m = mirror plan, gray, C2 = twofold rotation axis, red arrow, C6 = sixfold rotation axis, orange hexa­gon). With exception of the hydrogen atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn as displacement ellipsoids at the 70% probability level. Covalent bonds are drawn in orange–yellow, predominantly ionic fluorine–potassium inter­actions are visualized as dashed sticks in gray.

The di­methyl­tin difluoride units of the title compound form exactly planar layers, as in Me₂SnF₂ itself (Schlemper & Hamilton, 1966). In contrast to the latter, the octa­hedral coordination of the tin(IV) atoms of the title compound, however, is much more distorted and the fluorine bridges are bent. Distortion of the {Me₂SnF_4/2} octa­hedron (Fig. 2) not only results from four different Sn—F distances but also from bond angles strongly deviating from 90° (Table 1). The Sn—F distances differ considerably and fall into two categories: two are very short (≃ 2.078 Å) and two are much longer (≃ 2.259 Å). In the case of Me₂SnF₂, all four Sn—F distances are the same [2.120 (5) Å]. Angular distortions in the {Me₂SnF_4/2} octa­hedron of the title compound are considerable, in particular within the tin-fluorine plane. On the one hand, there are angles that are significantly smaller [77.11 (7), 79.31 (6), 84.01 (8)°] than 90°, while one angle is significantly larger [119.57 (5)°] so that the exactly planar pseudo-equatorial plane takes the shape of an irregular quadrilateral [d(F⋯F) = 2.6524 (1), 2.7017 (1), 2.9112 (1), 3.9043 (1) Å]. The small angles result in very short fluorine–fluorine distances, which leads to a significant inter-penetration of the van der Waals spheres [r_vdW(F) = 1.47 Å; Mantina et al., 2009] of the corresponding fluorine atoms. Most remarkable, however, are the bond angles between trans-positioned atoms that increase to around 156° (Table 1).

Table 1
Selected geometric parameters (Å, °)

Sn1—F1	2.266 (1)	Sn1—F1^ii	2.080 (1)
Sn1—F2^i	2.077 (1)	Sn1—C1^iii	2.089 (2)
Sn1—F2	2.252 (1)	Sn1—C1	2.089 (2)
C1^iii—Sn1—C1	162.2 (1)	Sn1^iv—F1—Sn1	155.99 (8)
F1^ii—Sn1—F2	156.42 (5)	Sn1^v—F2—Sn1	137.11 (7)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

Figure 2 Ball-and-stick model with bond lengths and angles showing the octa­hedral coordination of the tin(IV) atom in side view (left) and top view (right). Inter­atomic fluorine⋯fluorine distances in the tin–fluorine plane (gray) are visualized on the right. [Symmetry codes used to generate equivalent atoms: (1) y − 1, −x + y, −z + 2; (2) −y + 1, x − y + 1, z; (3) x, y, −z + 2.]

So far, octa­hedral {Me₂SnF₄} building units have been found not only in Me₂SnF₂ (Schlemper & Hamilton, 1966) but also in the fluorido­stannates(IV) [Et₄N][Me₄Sn₃F₅] (Lambertsen et al., 1992) and K₂[Me₂SnF₄]·2H₂O (Ahmed et al., 2002). In the first, the two crystallographically independent building units are involved in the formation of bands whereby two fluorine atoms occupy terminal positions [d(Sn—F) = 2.026 (3) Å] and two bridging functions [d(Sn—F) = 2.115 (3)–2.272 (4) Å, 〈(Sn—F—Sn) = 150.1 (2)°/151.6 (2)°]; Sn—C distances are 2.105/2.117 Å and thus are somewhat longer than in the title compound (Table 1). The bond angles between trans-positioned ligands are all 180° in case of one tin atom and 167.0 (3)° between the carbon atoms and 175.7 (1)° between the fluorine atoms in the second tin atom. The pseudo-equatorial tin–fluorine planes are planar in both fluorido­stannates(IV) but more symmetrical than in the title compound. In [Et₄N][Me₄Sn₃F₅] (Lambertsen et al., 1992), composed of two crystallographically independent tin atoms, one plane is rectangular [d(F⋯F) = 3.027 (6), 3.002 (5) Å], and the other is trapezoid [3.091 (6)/2.969 (5), 3.044 (4)/3.044 (4) Å]. In K₂[Me₂SnF₄]·2H₂O (Ahmed et al. 2002) the plane is rectangular [d(F⋯F) = 2.958 (14), 3.012 (13) Å], too.

Both fluorine atoms in 6Me₂SnF₂·KI connect two tin atoms in a bent μ₂ coordination mode. In addition, the fluorine atom F2 is in contact with the potassium ion, but this contact [d(F⋯K) = 2.702 (1) Å] has no influence on the tin–fluorine distances, one of which is short and the other long. Only the bridging angle between the two tin atoms is reduced from 155.99 (8)° at F1 to 137.11 (7)° at F2 due to this contact.

The bridging of the tin atoms by the fluorine atoms leads to a layered arrangement of the di­methyl­tin(IV) difluoride building blocks, whereby the symmetrically related methyl groups [d(Sn—C) = 2.089 (2) Å] are almost perpendicular to the exactly planar tin–fluorine plane (Fig. 3). In the layers, the tin atoms are arranged in such a way that slightly distorted triangles [d(Sn⋯Sn) = 4.0298 (1)/4.2507 (2)/4.7220 (1) Å] and regular hexa­gons form a semi-regular 3-3-3-3-6 tessellation (Fig. 4). On each vertex of this snub hexa­gonal tiling (Schläfli-symbol sr{3,6}), there are four triangles and one hexa­gon. While the bridging fluorine atoms fill the space in the triangles practically seamlessly, this is not the case in the hexa­gons. The resulting pores [d(F⋯F) = 5.403 (1) Å] are large enough to incorporate potassium cations. As a result of the special position of the potassium cation, it is hexa­gonal–bipyramidally coordinated by six equatorially bound fluorine atoms [d(K⋯F) = 2.702 (2) Å] and two axially bound iodine anions [d(K⋯I) = 3.6702 (2) Å] (Fig. 5), resulting in linear rods of potassium iodine extending along [001]. In potassium fluoride, KF, and potassium iodide, KI, the potassium atoms are octa­hedrally coordinated (both adopt the NaCl structure type), and the corresponding potassium–halide distances are d(K⋯F) = 2.672 (3) Å (a = 5.334 (3) Å, T = 295 (2) K; Broch et al., 1929), and d(K⋯I) = 3.529 Å (a = 7.059 Å, T = 295 (2) K; Teatum & Smith, 1957) and 3.5328 (2) Å (a = 7.0655 (2), T = 295 (2) K; Hambling, 1953), respectively, indicating predominantly ionic bonding within the rods and between the potassium cations and the fluorine atoms of the tin–fluorine layers. Calculations of the bond lengths based on the ionic radii lead to similar results, i.e. K⋯I distances are even slightly shorter (3.58 Å with r_K[6] = 1.52 Å, r_I[6] = 2.06 Å; Shannon, 1976). In the fluorido­stannate(IV) K₂[Me₂SnF₄]·2H₂O the K⋯F contacts are about 0.1 Å shorter [2.595 (12), 2.617 (2) Å].

Figure 3 Space filling model showing the construction principle of a Me2SnF2 layer in relation to the unit cell (red) in top view (above) and side view (below). Atoms are visualized as single-colored or truncated, two-colored spheres according to their van der Waals radii and cut-offs based on the inter­section of the two spheres with cut-off faces showing the color of the inter­penetrating atom. Color code/van der Waals radii used: Sn = orange–yellow/2.17 Å, F = green/1.47 Å, C = dark gray/1.70 Å, H = white/1.1 Å.

Figure 4 Details of the tessellation pattern in the Me2SnF2 layers of the title compound resulting from the positions of the tin atoms positioned in the corners of the polygons.

Figure 5 Ball-and-stick model (left) and space-filling model (right) of hexa­gonal–bipyramidal coordination of the potassium ion with K—F and K—I atom distances. In the space-filling model, atoms are visualized as single-colored or truncated, two-colored spheres according to their van der Waals radii and cut-offs based on the inter­section of the two spheres with cut-off faces showing the color of the inter­penetrating atom. Color code/van der Waals radii used: K = blue/2.17 Å, F = green/1.47 Å, I = violet/2.06 Å.

The distances between the Sn—F planes are c/2 = 7.3404 (6) Å (Fig. 6), while they are somewhat smaller [7.08 (2) Å] in Me₂SnF₂. The slightly longer distance in the title compound is due to the fact that the methyl groups of two neighboring layers are directly opposite each other, while they are laterally offset in the guest-free difluoride. There are no inter­actions of the iodine anions except with the potassium cations. They are, however, regularly surrounded in a hexa­gonal prismatic shape from six symmetry-equivalent hydrogen atoms [H13] in a distance of 3.446 Å that is significant longer than the sum (3.08 Å) of the van der Waals radii (Mantina et al. 2009) of iodine (1.98 Å) and hydrogen (1.10 Å).

Figure 6 Space-filling model (3 × 2 × 1 unit cells) based on the van der Waals radii of tin (magenta, 2.17 Å), fluorine (green, 1.47 Å), carbon (dark gray, 1.70 Å), hydrogen (white, 1.10 Å) and ionic radii of potassium (blue, 1.52 Å), iodine (violet, 2.06 Å) that describes the packing of the Me2SnF2 layers; layer spacing in Å.

3. Synthesis and crystallization

0.80 g (1.99 mmol) of Me₂SnI₂ were dissolved at room temperature in 20 ml of ethanol to which a solution of 0.10 g (2.08 mmol) KF in 10 ml of water was added while stirring. The colorless, voluminous precipitate of Me₂SnF₂ that formed immediately was filtered off after 20 min and washed twice with 5 ml toluene, yield: 0.24 g (1.29 mmol, 65%). After a few days during which a large part of the solvents had evaporated, the host–guest compound 6Me₂SnF₂·KI crystallized out of the remaining reaction solution, yield: 55 mg.

Di­methyl­tin(IV) diiodide, Me₂SnI₂, was prepared from di­methyl­tin(IV) oxide, Me₂SnO, and ammonium iodide, NH₄I, in a molar ratio of 1:2 via the release of water and ammonia (Fig. 7). For this purpose, 2.97 g (18 mmol) Me₂SnO and 6.26 g (43.2 mmol) NH₄I were suspended in 200 ml toluene and the mixture heated to boiling under reflux in a soxhlet apparatus. The water formed during the reaction was removed using silica gel in an extraction sleeve. After 8 h, the mixture was filtered off hot and most of the solvent was distilled off. During the evaporation of the remaining solvent, di­methyl­tin(IV) diiodide crystallized out as very large, dark-yellow crystals over the course of 2 d. Yield: 4.13 g (10.26 mmol; 57%). ¹H NMR (250 MHz, CDCl₃): δ, ⁿJ(^119/117Sn–¹H) (ppm, Hz) 1.57,62 (s, CH₃); ¹³C NMR (250 MHz, CDCl₃): δ,ⁿJ(^119/117Sn–¹³C) (ppm, Hz) 6.02, 387.3/370.2 (CH₃)/¹J); analysis: calculated for C₂H₆I₂Sn (402.59): C 5.97, H 1.50, found C 5.99, H 1.48%.

Figure 7 Reaction equation for the formation of Me2SnI2.

4. Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 2. Methyl H atoms were placed geometrically and allowed to ride on the C atom (AFIX 137; Sheldrick, 2015) with d(C—H) = 0.98 Å) and a common U_iso(H) parameter.

Table 2
Experimental details

Crystal data
Chemical formula	[Sn(CH3)2F2]6·KI
Mr	1286.55
Crystal system, space group	Hexagonal, P6/mcc
Temperature (K)	100
a, c (Å)	11.0616 (4), 14.6807 (6)
V (Å^3)	1555.65 (13)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	5.94
Crystal size (mm)	0.22 × 0.14 × 0.05
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.451, 0.712
No. of measured, independent and observed [I > 2σ(I)] reflections	51729, 662, 635
Rint	0.080
(sin θ/λ)max (Å^−1)	0.660
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.013, 0.028, 1.17
No. of reflections	662
No. of parameters	35
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å^−3)	0.65, −0.40
Computer programs: APEX2 and SAINT (Bruker, 2009), SHELXS (Sheldrick 2008), SHELXL (Sheldrick, 2015), DIAMOND (Brandenburg, 2006), Mercury (Macrae et al. (2020) and publCIF (Westrip, 2010).

Supporting information

Computing details

Poly[hexakis[[dimethyltin(IV)]-di-µ-fluorido] potassium iodide]

Crystal data

[Sn(CH3)2F2]6·KI	Dx = 2.747 Mg m^−3
Mr = 1286.55	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6/mcc	Cell parameters from 9284 reflections
a = 11.0616 (4) Å	θ = 2.8–29.1°
c = 14.6807 (6) Å	µ = 5.94 mm^−1
V = 1555.65 (13) Å^3	T = 100 K
Z = 2	Plate, colourless
F(000) = 1176	0.22 × 0.14 × 0.05 mm

Data collection

Bruker APEXII CCD diffractometer	635 reflections with I > 2σ(I)
φ and ω scans	Rint = 0.080
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θmax = 28.0°, θmin = 2.8°
Tmin = 0.451, Tmax = 0.712	h = −14→14
51729 measured reflections	k = −14→14
662 independent reflections	l = −19→19

Refinement

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.013	w = 1/[σ^2(Fo^2) + (0.0086P)^2 + 1.4878P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.028	(Δ/σ)max = 0.001
S = 1.17	Δρmax = 0.65 e Å^−3
662 reflections	Δρmin = −0.39 e Å^−3
35 parameters	Extinction correction: SHELXL (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00069 (7)
Primary atom site location: structure-invariant direct methods

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
Sn1	0.25750 (2)	0.84066 (2)	1.0000	0.00817 (7)
F1	0.45960 (15)	0.83530 (15)	1.0000	0.0150 (3)
F2	0.26458 (14)	1.04772 (14)	1.0000	0.0145 (3)
C1	0.2871 (2)	0.8695 (2)	0.85944 (14)	0.0177 (4)
H11	0.2895	0.7897	0.8325	0.044 (5)*
H12	0.3755	0.9552	0.8472	0.044 (5)*
H13	0.2101	0.8772	0.8326	0.044 (5)*
K1	0.0000	1.0000	1.0000	0.0162 (3)
I1	0.0000	1.0000	1.2500	0.02125 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.00640 (10)	0.00645 (9)	0.01217 (10)	0.00359 (7)	0.000	0.000
F1	0.0102 (7)	0.0154 (8)	0.0228 (8)	0.0091 (6)	0.000	0.000
F2	0.0094 (7)	0.0068 (6)	0.0266 (8)	0.0035 (6)	0.000	0.000
C1	0.0192 (9)	0.0217 (9)	0.0142 (9)	0.0116 (8)	0.0012 (7)	0.0010 (7)
K1	0.0073 (4)	0.0073 (4)	0.0338 (7)	0.00366 (18)	0.000	0.000
I1	0.02021 (14)	0.02021 (14)	0.0233 (2)	0.01011 (7)	0.000	0.000

Geometric parameters (Å, º)

Sn1—F1	2.266 (1)	K1—F2^vi	2.702 (1)
Sn1—F2^i	2.077 (1)	K1—F2^vii	2.702 (1)
Sn1—F2	2.252 (1)	K1—F2^i	2.702 (1)
Sn1—F1^ii	2.080 (1)	K1—I1	3.6702 (2)
Sn1—C1^iii	2.089 (2)	K1—I1^vi	3.6702 (2)
Sn1—C1	2.089 (2)	C1—H11	0.9800
K1—F2	2.702 (1)	C1—H12	0.9800
K1—F2^iv	2.702 (1)	C1—H13	0.9800
K1—F2^v	2.702 (1)
F2^i—Sn1—F1^ii	79.31 (6)	F2^vi—K1—F2^i	120.000 (1)
F2^i—Sn1—C1^iii	96.47 (5)	F2^vii—K1—F2^i	180.0
F1^ii—Sn1—C1^iii	97.19 (5)	F2^iv—K1—F2	120.0
F2^i—Sn1—C1	96.46 (5)	F2^v—K1—F2	60.0
F1^ii—Sn1—C1	97.19 (5)	F2^vi—K1—F2	180.0
C1^iii—Sn1—C1	162.2 (1)	F2^vii—K1—F2	120.0
F2^i—Sn1—F2	77.11 (7)	F2^i—K1—F2	60.000 (1)
F1^ii—Sn1—F2	156.42 (5)	F2^iv—K1—I1	90.0
C1^iii—Sn1—F2	85.50 (5)	F2^v—K1—I1	90.0
C1—Sn1—F2	85.50 (5)	F2^vi—K1—I1	90.0
F2^i—Sn1—F1	163.31 (5)	F2^vii—K1—I1	90.0
F1^ii—Sn1—F1	84.01 (8)	F2^i—K1—I1	90.0
C1^iii—Sn1—F1	85.56 (5)	F2—K1—I1	90.0
C1—Sn1—F1	85.56 (5)	F2^iv—K1—I1^vi	90.0
F2—Sn1—F1	119.57 (5)	F2^v—K1—I1^vi	90.0
Sn1^viii—F1—Sn1	155.99 (8)	F2^vi—K1—I1^vi	90.0
Sn1^v—F2—Sn1	137.11 (7)	F2^vii—K1—I1^vi	90.0
Sn1^v—F2—K1	114.35 (6)	F2^i—K1—I1^vi	90.0
Sn1—F2—K1	108.53 (5)	F2—K1—I1^vi	90.0
F2^iv—K1—F2^v	180.0	I1—K1—I1^vi	180.0
F2^iv—K1—F2^vi	60.0	K1^ix—I1—K1	180.0
F2^v—K1—F2^vi	120.0	Sn1—C1—H11	109.5
F2^iv—K1—F2^vii	120.000 (1)	Sn1—C1—H12	109.5
F2^v—K1—F2^vii	60.0	H11—C1—H12	109.5
F2^vi—K1—F2^vii	60.0	Sn1—C1—H13	109.5
F2^iv—K1—F2^i	60.0	H11—C1—H13	109.5
F2^v—K1—F2^i	120.000 (1)	H12—C1—H13	109.5

Symmetry codes: (i) y−1, −x+y, −z+2; (ii) −y+1, x−y+1, z; (iii) x, y, −z+2; (iv) −x+y−1, −x+1, z; (v) x−y+1, x+1, −z+2; (vi) −x, −y+2, −z+2; (vii) −y+1, x−y+2, z; (viii) −x+y, −x+1, z; (ix) y−1, x+1, −z+5/2.