trans-Di­chlorotetrakis(4-methyl­pyridine)­silicon bis­(triiodide) chloro­form solvate

At 173 K, the dication of the title compound, C₂₄H₂₈Cl₂N₄Si²⁺·2I₃^-·CHCl₃, is located on a crystallographic fourfold rotation axis. The chloro ligands occupy axial positions and the four 4-methyl­pyridine ligands lie in the equatorial plane. The almost linear I₃^- ion is located on a crystallographic mirror plane and displays two significantly different I-I bond lengths. Furthermore, chloro­form mol­ecules, which are disordered about a centre of inversion, fill the remaining gaps in the crystal structure.

S1. Comment

The extension of silicon's coordination sphere in compounds formed from silicon halides and organic nitrogen bases has been of scientific interest over a long period of time (Hensen et al., 1983; Bechstein et al., 1990; Chuit et al., 1993; Kane et al., 1998; Hensen et al., 1998; Hensen, Mayr-Stein, Stumpf et al., 2000). Neutral adducts as well as cationic complexes have been characterized. Silylium ions have played an important part in recent research efforts in the area of silicon chemistry (Belzner, 1997; Reed, 1998). Most known ionic complexes contain charge stabilizing alkyl ligands bound to the silicon centre. Only recently, the first ligand stabilized SiH₂²⁺-dications (Fleischer et al., 1996; Hensen et al., 1998) and SiCl₂²⁺-dications (Hensen, Mayr-Stein, Stumpf et al., 2000) were found. These were prepared using the strong nitrogen base N-methylimidazole as a stabilizing ligand. In the work presented in this paper we have prepared a ligand stabilized SiCl₂²⁺-dication by introducing iodine as a suitable leaving group which enabled the use of a common pyridine derivative as ligand. The title compound, (I), was synthesized from SiCl₂I₂ and the unidentate ligand 4-methylpyridine. \sch

(I) shows a sixfold coordinated silicon centre, with only insignificant deviations from ideal octahedral symmetry (Fig. 1). The four 4-methylpyridine ligands are located in the equatorial plane and two chloro ligands occupy axial positions. The SiCl₂ moiety is located on a crystallographic fourfold rotation axis, as a result of that, there is only a quarter of the molecule in the asymmetric unit. The two Si—Cl bonds are of equal length. The I₃^- ion is located on a crystallographic mirror plane, it is nearly linear, but the two I—I bonds are significantly different. A search in the Cambridge Crystallographic Database (CSD, Version 5.18, October 1999; Allen & Kennard, 1993) revealed that the two I—I bond lengths in I₃^- anions are usually equal, but different bond lengths are not uncommon. The resulting gaps in the crystal structure, located around the sites with crystallographic 2/m symmetry, are filled with chloroform molecules (see experimental section).

The only comparable compound published so far in the CSD, is trans-dichloro-tetra-(N-methylimidazolyl)silicon, of which three different crystal structures were obtained (Hensen, Mayr-Stein, Stumpf et al., 2000): one with chloride as counterion and chloroform as solvent, and two with bromide as counterion, but with chloroform and acetonitrile, respectively, as solvent. Whereas the mean Si—N bond in these structures [1.90 (1) Å] is a little bit shorter than in the title compound the mean Si—Cl bond [2.19 (1) Å] is slightly longer.

A comparison of the title compound with the related hydride structures dihydrido-tetrakis(3-picoline)-silicon dichloride (Fleischer et al., 1996), (dichloro-dihydro-dipyridyl)silane, (dichloro-dihdyro-bis(3-picoline))silane and (dihydro-tetrapyridyl)silane (Hensen et al., 1998) revealed that the Si—N bond length is nearly independent of the kind of the nitrogen base ligand, but the Si—Cl bond is considerable longer [2.2881 (4) and 2.2922 (4) Å], when the central Si atom is bonded to two H atoms instead of two nitrogen bases.

S2. Experimental

Due to the silicon halides' extreme susceptibility to hydrolysis all operations were carried out under an inert gas atmosphere. Also, exposition of SiCl₂I₂ to light was minimized. 4-Methylpyridine (5.0 mmol) was added dropwise to SiCl₂I₂ (1 mmol) in n-pentane (20 ml). The yellow precipitate was separated. Elemental analysis of the substance as well as thermochemical observations revealed that less than the expected four equivalents of the nitrogen base reacted to form a complex. Dismutation of SiCl₂I₂ (Hass et al., 1989) occurred during the preparation of the complex leading to a mixture of product complexes with varying stoichiometry with respect to halide distribution and silicon halide/methylpyridine ratio. We were able to crystallize two component complexes from the moderately soluble product powder: SiCl₄(4-methylpyridine)₂ (Hensen, Mayr-Stein, Spangenberg & Bolte, 2000) and the title compound from a hot chloroform solution that was allowed to evaporate at room temperature under vacuum over two weeks. The unexpected presence of I₃^- indicates side reactions with air.

S3. Refinement

All H atoms were initially located by difference Fourier synthesis. Subsequently their positions were idealized and constrained to ride on their parent atoms with C—H(aromatic) = 0.95 and C—H(methyl) = 0.98 Å, and fixed individual displacement parameters [U(H) = 1.2 U_eq(C_aromatic) or U(H) = 1.5 U_eq(C_methyl)]. The methyl group was allowed to rotate about its local threefold axis

The C atom, the H atom and one Cl atom of the chloroform are located on a crystallographic mirror plane, one Cl atom occupies a general position. Due to the chloroform's position so close to a crystallographic inversion centre, another chloroform molecule is generated by the symmetry operation -x,1 - y,1 - z. Since the existance of both chloroform molecules is mutually exclusive, the chloroform site was refined as half occupied.

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SMART; data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 1991).

Figures

Fig. 1. Perspective view of the title compound with the atom numbering; displacement ellipsoids are at the 50% probability level.

(I)

Crystal data

C24H28Cl2N4Si^2+·2I3^−·CHCl3	Dx = 2.239 Mg m^−3
Mr = 1352.26	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/m	Cell parameters from 8014 reflections
Hall symbol: -I 4	θ = 0–25°
a = 16.241 (2) Å	µ = 5.03 mm^−1
c = 15.210 (3) Å	T = 173 K
V = 4011.8 (11) Å^3	Block, dark red
Z = 4	0.30 × 0.25 × 0.25 mm
F(000) = 2496

Data collection

Siemens CCD three-circle diffractometer	2392 independent reflections
Radiation source: fine-focus sealed tube	1957 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.048
ω scans	θmax = 27.5°, θmin = 1.8°
Absorption correction: empirical (SADABS; Sheldrick, 1996)	h = −21→21
Tmin = 0.244, Tmax = 0.284	k = −21→21
47624 measured reflections	l = −19→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)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.096	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0306P)^2 + 56.0581P] where P = (Fo^2 + 2Fc^2)/3
2392 reflections	(Δ/σ)max < 0.001
113 parameters	Δρmax = 3.21 (0.87 Å from I1) e Å^−3
0 restraints	Δρmin = −3.36 e Å^−3

Special details

Experimental. The data collection nominally covered a sphere of reciprocal space, by a combination of seven sets of exposures; each set had a different ϕ angle for the crystal and each exposure covered 0.3° in ω. The crystal-to-detector distance was 4.5 cm. Coverage of the unique set is 100% complete to at least 26.4° in θ. Crystal decay was monitored by repeating the initial frames at the end of data collection and analyzing the duplicate reflections.

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)
I1	0.14429 (4)	0.18595 (6)	0.0000	0.0680 (2)
I2	0.32278 (4)	0.21330 (3)	0.0000	0.03722 (15)
I3	0.49661 (4)	0.25259 (4)	0.0000	0.0574 (2)
Si1	0.5000	0.5000	0.25243 (16)	0.0234 (5)
Cl1	0.5000	0.5000	0.39316 (14)	0.0269 (5)
Cl2	0.5000	0.5000	0.11163 (15)	0.0313 (5)
N1	0.3882 (2)	0.4535 (2)	0.2526 (3)	0.0252 (8)
C2	0.3300 (3)	0.4824 (3)	0.3092 (3)	0.0281 (10)
H2	0.3448	0.5243	0.3498	0.034*
C3	0.2504 (3)	0.4531 (3)	0.3096 (3)	0.0293 (10)
H3	0.2114	0.4755	0.3495	0.035*
C4	0.2265 (3)	0.3909 (3)	0.2517 (3)	0.0307 (10)
C41	0.1396 (3)	0.3593 (4)	0.2482 (4)	0.0408 (13)
H41A	0.1211	0.3454	0.3077	0.061*
H41B	0.1374	0.3100	0.2111	0.061*
H41C	0.1036	0.4018	0.2235	0.061*
C5	0.2869 (3)	0.3610 (3)	0.1958 (4)	0.0327 (11)
H5	0.2739	0.3178	0.1560	0.039*
C6	0.3655 (3)	0.3930 (3)	0.1969 (3)	0.0301 (11)
H6	0.4051	0.3718	0.1570	0.036*
C1L	0.0293 (14)	0.4854 (14)	0.5000	0.066 (6)	0.50
H1L	0.0425	0.4278	0.5000	0.079*	0.50
Cl11	0.1226 (3)	0.5443 (3)	0.5000	0.0478 (10)	0.50
Cl12	−0.0250 (3)	0.5080 (4)	0.4047 (3)	0.0854 (15)	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
I1	0.0444 (4)	0.1221 (7)	0.0376 (3)	−0.0101 (4)	0.000	0.000
I2	0.0508 (3)	0.0391 (3)	0.0217 (2)	0.0024 (2)	0.000	0.000
I3	0.0513 (4)	0.0572 (4)	0.0635 (4)	−0.0111 (3)	0.000	0.000
Si1	0.0260 (8)	0.0260 (8)	0.0184 (12)	0.000	0.000	0.000
Cl1	0.0316 (7)	0.0316 (7)	0.0175 (10)	0.000	0.000	0.000
Cl2	0.0380 (8)	0.0380 (8)	0.0180 (10)	0.000	0.000	0.000
N1	0.027 (2)	0.026 (2)	0.0229 (19)	0.0008 (16)	−0.0015 (16)	−0.0035 (16)
C2	0.031 (2)	0.028 (2)	0.025 (2)	−0.0011 (19)	0.0006 (19)	−0.0052 (19)
C3	0.029 (2)	0.030 (3)	0.029 (2)	0.005 (2)	0.004 (2)	−0.003 (2)
C4	0.031 (3)	0.030 (3)	0.032 (3)	0.001 (2)	−0.006 (2)	0.002 (2)
C41	0.031 (3)	0.044 (3)	0.048 (3)	−0.004 (2)	−0.005 (2)	0.003 (3)
C5	0.038 (3)	0.028 (2)	0.032 (3)	−0.003 (2)	−0.005 (2)	−0.006 (2)
C6	0.032 (3)	0.032 (3)	0.026 (2)	0.002 (2)	0.000 (2)	−0.009 (2)
C1L	0.077 (16)	0.052 (13)	0.067 (14)	0.016 (11)	0.000	0.000
Cl11	0.042 (2)	0.061 (3)	0.041 (2)	0.0071 (19)	0.000	0.000
Cl12	0.076 (3)	0.102 (4)	0.078 (3)	−0.004 (3)	−0.035 (2)	−0.006 (3)

Geometric parameters (Å, º)

I1—I2	2.9327 (10)	C41—H41B	0.9800
I2—I3	2.8944 (10)	C41—H41C	0.9800
Si1—N1	1.967 (4)	C5—C6	1.379 (7)
Si1—N1^i	1.967 (4)	C5—H5	0.9500
Si1—N1^ii	1.967 (4)	C6—H6	0.9500
Si1—N1^iii	1.967 (4)	C1L—C1L^iv	1.06 (4)
Si1—Cl1	2.140 (3)	C1L—Cl12^v	1.455 (5)
Si1—Cl2	2.142 (3)	C1L—Cl12^iv	1.455 (5)
N1—C6	1.348 (6)	C1L—Cl12	1.735 (12)
N1—C2	1.362 (6)	C1L—Cl12^vi	1.735 (12)
C2—C3	1.378 (7)	C1L—Cl11	1.79 (3)
C2—H2	0.9500	C1L—H1L	0.9600
C3—C4	1.395 (7)	Cl11—Cl12^v	2.309 (7)
C3—H3	0.9500	Cl11—Cl12^iv	2.309 (7)
C4—C5	1.386 (7)	Cl12—Cl12^v	0.852 (8)
C4—C41	1.502 (7)	Cl12—C1L^iv	1.455 (5)
C41—H41A	0.9800	Cl12—Cl11^iv	2.309 (7)
I3—I2—I1	175.98 (3)	C6—C5—H5	119.5
N1—Si1—N1^i	90.000 (1)	C4—C5—H5	119.5
N1—Si1—N1^ii	179.8 (3)	N1—C6—C5	122.4 (5)
N1^i—Si1—N1^ii	90.001 (1)	N1—C6—H6	118.8
N1—Si1—N1^iii	90.001 (1)	C5—C6—H6	118.8
N1^i—Si1—N1^iii	179.8 (3)	C1L^iv—C1L—Cl12^v	85.7 (8)
N1^ii—Si1—N1^iii	89.998 (1)	C1L^iv—C1L—Cl12^iv	85.7 (8)
N1—Si1—Cl1	89.92 (13)	Cl12^v—C1L—Cl12^iv	169.9 (17)
N1^i—Si1—Cl1	89.92 (13)	C1L^iv—C1L—Cl12	56.7 (6)
N1^ii—Si1—Cl1	89.92 (13)	Cl12^v—C1L—Cl12	29.3 (4)
N1^iii—Si1—Cl1	89.92 (13)	Cl12^iv—C1L—Cl12	142.4 (14)
N1—Si1—Cl2	90.08 (13)	C1L^iv—C1L—Cl12^vi	56.7 (6)
N1^i—Si1—Cl2	90.08 (13)	Cl12^v—C1L—Cl12^vi	142.4 (14)
N1^ii—Si1—Cl2	90.08 (13)	Cl12^iv—C1L—Cl12^vi	29.3 (4)
N1^iii—Si1—Cl2	90.08 (13)	Cl12—C1L—Cl12^vi	113.3 (12)
Cl1—Si1—Cl2	180.0	C1L^iv—C1L—Cl11	121 (3)
C6—N1—C2	117.3 (4)	Cl12^v—C1L—Cl11	90.0 (11)
C6—N1—Si1	122.1 (3)	Cl12^iv—C1L—Cl11	90.0 (11)
C2—N1—Si1	120.6 (3)	Cl12—C1L—Cl11	108.4 (10)
N1—C2—C3	122.4 (4)	Cl12^vi—C1L—Cl11	108.4 (10)
N1—C2—H2	118.8	C1L^iv—C1L—H1L	129.5
C3—C2—H2	118.8	Cl12^v—C1L—H1L	94.8
C2—C3—C4	120.6 (5)	Cl12^iv—C1L—H1L	94.8
C2—C3—H3	119.7	Cl12—C1L—H1L	108.7
C4—C3—H3	119.7	Cl12^vi—C1L—H1L	108.7
C5—C4—C3	116.3 (5)	Cl11—C1L—H1L	109.3
C5—C4—C41	121.6 (5)	C1L—Cl11—Cl12^v	39.05 (15)
C3—C4—C41	122.1 (5)	C1L—Cl11—Cl12^iv	39.05 (15)
C4—C41—H41A	109.5	Cl12^v—Cl11—Cl12^iv	77.7 (3)
C4—C41—H41B	109.5	Cl12^v—Cl12—C1L^iv	93.9 (9)
H41A—C41—H41B	109.5	Cl12^v—Cl12—C1L	56.8 (6)
C4—C41—H41C	109.5	C1L^iv—Cl12—C1L	37.6 (14)
H41A—C41—H41C	109.5	Cl12^v—Cl12—Cl11^iv	122.8 (7)
H41B—C41—H41C	109.5	C1L^iv—Cl12—Cl11^iv	50.9 (11)
C6—C5—C4	121.1 (5)	C1L—Cl12—Cl11^iv	75.3 (8)
N1^i—Si1—N1—C6	−48.1 (5)	Cl12^iv—C1L—Cl11—Cl12^v	−169.9 (17)
N1^ii—Si1—N1—C6	−138 (100)	Cl12—C1L—Cl11—Cl12^v	−23.3 (4)
N1^iii—Si1—N1—C6	132.1 (3)	Cl12^vi—C1L—Cl11—Cl12^v	−146.6 (16)
Cl1—Si1—N1—C6	−138.0 (4)	C1L^iv—C1L—Cl11—Cl12^iv	84.9 (8)
Cl2—Si1—N1—C6	42.0 (4)	Cl12^v—C1L—Cl11—Cl12^iv	169.9 (17)
N1^i—Si1—N1—C2	132.8 (3)	Cl12—C1L—Cl11—Cl12^iv	146.6 (16)
N1^ii—Si1—N1—C2	43 (100)	Cl12^vi—C1L—Cl11—Cl12^iv	23.3 (4)
N1^iii—Si1—N1—C2	−47.1 (4)	C1L^iv—C1L—Cl12—Cl12^v	169 (3)
Cl1—Si1—N1—C2	42.8 (4)	Cl12^iv—C1L—Cl12—Cl12^v	169 (3)
Cl2—Si1—N1—C2	−137.2 (4)	Cl12^vi—C1L—Cl12—Cl12^v	174.1 (19)
C6—N1—C2—C3	−1.2 (7)	Cl11—C1L—Cl12—Cl12^v	53.7 (12)
Si1—N1—C2—C3	178.0 (4)	Cl12^v—C1L—Cl12—C1L^iv	−169 (3)
N1—C2—C3—C4	0.9 (8)	Cl12^iv—C1L—Cl12—C1L^iv	0.001 (9)
C2—C3—C4—C5	0.3 (7)	Cl12^vi—C1L—Cl12—C1L^iv	4.7 (16)
C2—C3—C4—C41	−178.0 (5)	Cl11—C1L—Cl12—C1L^iv	−116 (3)
C3—C4—C5—C6	−1.3 (8)	C1L^iv—C1L—Cl12—Cl11^iv	−43 (3)
C41—C4—C5—C6	177.1 (5)	Cl12^v—C1L—Cl12—Cl11^iv	147.3 (13)
C2—N1—C6—C5	0.2 (7)	Cl12^iv—C1L—Cl12—Cl11^iv	−43 (3)
Si1—N1—C6—C5	−179.0 (4)	Cl12^vi—C1L—Cl12—Cl11^iv	−38.6 (12)
C4—C5—C6—N1	1.1 (8)	Cl11—C1L—Cl12—Cl11^iv	−159.0 (10)
C1L^iv—C1L—Cl11—Cl12^v	−84.9 (8)

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