1,1,1,1,4,4,4,4-Octa­carbonyl-2,2,3,3,5,5,6,6-octa­methyl-cyclo-2,3,5,6-tetra­anti­mony-1,4-dichromium

The structure of the title compound, alternatively called bis­(μ-tetra­methyl­distibinediyl)bis­(tetra­carbonyl­chromium), [Cr₂Sb₄(CH₃)₈(CO)₈], consists of two Me₄Sb₂ bridging units between Cr(CO)₄ complex fragments. The centre of the mol­ecule is located on a special position of 2/m symmetry. This is the first characterized Sb₄Cr₂ heterocycle.

S1. Comment

Distibines act as donor ligands through the lone pairs of the antimony atoms to one or two transition metal complex fragments (Breunig & Fichtner, 1979, 1981; Breunig & Knobloch, 1980; von Seyerl & Huttner, 1980; Breunig et al., 1981, 2003; Bernal et al., 1984; Weber, Huttner et al., 1985; Weber, Zsolnai et al., 1985; Dickson et al., 1993; Breunig & Pawlik, 1995; Sharma et al., 1995; Balázs et al., 2003; Lorenz et al., 2005). From the reaction of [Cr(CO)₄(nbd)] [nbd = norbornadiene (C₇H₈)] and tetramethyldistibine, a polymeric complex [Cr(CO)₄(µ-Sb₂Me₄)]_n with a trans arrangement of the distibine ligands was obtained (Breunig & Knobloch, 1980). We report here the structure of {[Cr(CO)₄]₂(µ-Sb₂Me₄)₂} (I), the first complex featuring two bridging distibine ligands between transition metal complex fragments. (I) is a cyclic isomer of the polymer [Cr(CO)₄(µ-Sb₂Me₄)]_n (Breunig & Knobloch, 1980).

A thermal ellipsoid representation of the crystal structure of (I) is shown in Fig. 1. The centre of the molecule is located on a special position of 2/m symmetry. The Sb—Sb bond lengths in (I) are in the range of values found in the analogous complexes {[Cr(CO)₅]₂(µ-Sb₂Me₄)} [2.8097 (9) Å] (Breunig et al., 2003) and {[Cr(CO)₅]₂(µ-Sb₂Ph₄)} [2.865 (4) Å] (von Seyerl & Huttner, 1980). In addition, the Sb—Cr bond lengths are of the same magnitude as other reported values: 2.621 (8) and 2.629 (9) Å in {[Cr(CO)₅]₂(µ-Sb₂Me₄)} (Breunig et al., 2003) and 2.626 (1) Å in {[Cr(CO)₅]₂(µ-Sb₂Ph₄)} (von Seyerl & Huttner, 1980).

The geometries of the Cr(CO)₄ fragments are normal, the bond angles being close to the ideal value of 90°. The C—Sb—Sb, C—Sb—C bond angles are larger than in the uncoordinated distibine Me₄Sb₂ [mean C—Sb—Sb 93.6 (6), mean C—Sb—C 93.5 (15)°] (Ashe et al., 1984), and are in the range of values found for the corresponding angles in {[Cr(CO)₅]₂(µ-Sb₂Me₄)} [C—Sb—Sb 96.41 (11)—102.44 (11); C—Sb—C 99.20 (18)–100.34 (16)°] (Breunig et al., 2003). The geometrical parameters of antimony are consistent with the known transition from being close to a p³ configuration to an sp³ hybridization of the coordinating atoms in pnicogen ligands by complexation.

The distibine fragments in (I) adopt a synclinal conformation, with a torsion angle Cr—Sb—Sb—Cr of 66.88 (4)° [cf. ideal antiperiplanar conformation with Cr—Sb—Sb—Cr 180° in {[Cr(CO)₅]₂(µ-Sb₂Me₄)} (Breunig et al., 2003) and {[Cr(CO)₅]₂(µ-Sb₂^tBu₂Cl₂)} (Weber, Zsolnai et al., 1985); anticlinal conformation with Cr—Sb—Sb—Cr 130.98 (36)° in {[Cr(CO)₅]₃(Sb₂^tBu₂)} (Weber, Huttner et al., 1985)]. The antimony and chromium atoms form a six-membered heterocycle in a chair conformation. There are transannular contacts [3.7133 (9) Å] between the antimony atoms of the parallel distibine units shorter than the sum of the van der Waals radii [Σ_rvdW(Sb,Sb) = 4.0 Å] (Bondi, 1964).

The crystals consist of stacks of molecules of (I), which are directed along the a axis, each stack being surrounded by six neighbouring stacks. There are contacts of 2.73 and 2.74 Å between the carbonyl oxygen atoms and the hydrogen atoms of the methyl groups of neighbouring molecules, both inside and between stacks.

Related literature

(type here to add)

For related literature, see: Ashe, Ludwig, Oleksyszyn & Huffman (1984); Balázs et al. (2003); Bernal et al. (1984); Bondi (1964); Breunig & Fichtner (1979, 1981); Breunig & Knobloch (1980); Breunig & Pawlik (1995); Breunig et al. (2003); Breunig, Fichtner & Knobloch (1981); Bruker (1998); Dickson et al. (1993); Lorenz et al. (2005); Seyerl & Huttner (1980); Sharma et al. (1995); Weber, Huttner, Scheidsteger & Zsolnai (1985); Weber, Zsolnai & Huttner (1985).

S2. Experimental

Crystals of (I) were obtained from a solution of 2,6-Mes₂C₆H₃Sb(SbMe₂)₂ (Mes = 2,4,6-Me₃C₆H₂) and [Cr(CO)₄(nbd)] in a 1:1 molar ratio in benzene, in a sealed NMR tube at ambient temperature. The synthesis has not been reproduced in preparative scale.

S3. Refinement

During the space-group determination using XPREP (Bruker, 1998), the reflections -2,0,3; 0,0,3 and -2,0,1 were removed. The hydrogen atoms were placed in calculated positions (C—H = 0.98 Å) using a riding model with thermal parameters set at 1.5 times those of the carbon atoms directly attached, U_iso(H)=1.5U_eq(C). The methyl groups were allowed to rotate. The largest residual electron density is located 1.22 Å from the hydrogen atom H4C and the deepest hole is located at 0.88 Å from the antimony atom Sb1.

Computing details

Data collection: Stoe IPDS1 (Stoe, 1999); cell refinement: Stoe IPDS1 (Stoe, 1999); data reduction: Stoe IPDS1 (Stoe, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg & Putz, 2006); software used to prepare material for publication: WinGX (Farrugia, 1999) and publCIF (Westrip, 2007).

Figures

Fig. 1. Graphical representation of the molecular structure of (I). Hydrogen atoms are omitted for clarity. Displacement ellipsoids are drawn at 50% probability. Symmetry codes: (i) -x + 1, y, -z + 1; (ii) x, -y + 1, z; (iii) -x + 1, -y + 1, -z + 1.

1,1,1,1,4,4,4,4-Octacarbonyl-2,2,3,3,5,5,6,6-octamethyl-cyclo-2,3,5,6- tetraantimony-1,4-dichromium

Crystal data

[Cr2Sb4(CH3)8(CO)8]	F(000) = 872
Mr = 935.35	Dx = 2.32 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 4362 reflections
a = 13.125 (3) Å	θ = 1.7–26.1°
b = 11.332 (2) Å	µ = 4.80 mm^−1
c = 10.185 (2) Å	T = 173 K
β = 117.91 (3)°	Prism, orange
V = 1338.6 (6) Å^3	0.2 × 0.2 × 0.1 mm
Z = 2

Data collection

Stoe IPDS-1 diffractometer	1367 independent reflections
Radiation source: fine-focus sealed tube	1173 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.036
ϕ scans	θmax = 26.0°, θmin = 2.3°
Absorption correction: part of the refinement model (ΔF) (Walker & Stuart, 1983)	h = −16→16
Tmin = 0.447, Tmax = 0.646	k = −13→13
4676 measured reflections	l = −12→12

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)] = 0.024	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.056	H-atom parameters constrained
S = 0.99	w = 1/[σ^2(Fo^2) + (0.0352P)^2] where P = (Fo^2 + 2Fc^2)/3
1367 reflections	(Δ/σ)max = 0.001
78 parameters	Δρmax = 1.07 e Å^−3
0 restraints	Δρmin = −0.55 e Å^−3

Special details

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
C1	0.6517 (5)	0.5	0.9467 (6)	0.0303 (12)
C2	0.8118 (4)	0.6170 (5)	0.9177 (5)	0.0398 (11)
C3	0.7730 (5)	0.5	0.6748 (7)	0.0348 (13)
C4	0.6187 (5)	0.8432 (4)	0.6852 (6)	0.0438 (11)
H4A	0.6898	0.8505	0.7795	0.066*
H4B	0.5596	0.8954	0.6863	0.066*
H4C	0.634	0.8654	0.6031	0.066*
C5	0.4114 (4)	0.6845 (4)	0.6924 (6)	0.0387 (10)
H5A	0.437	0.6954	0.7987	0.058*
H5B	0.3629	0.6138	0.6574	0.058*
H5C	0.367	0.7536	0.6376	0.058*
Sb1	0.55915 (2)	0.66384 (2)	0.65639 (3)	0.02674 (10)
Cr1	0.70963 (7)	0.5	0.80719 (9)	0.0272 (2)
O1	0.6229 (4)	0.5	1.0357 (5)	0.0463 (11)
O2	0.8752 (3)	0.6908 (4)	0.9881 (4)	0.0592 (10)
O3	0.8167 (5)	0.5	0.5997 (6)	0.0500 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.023 (3)	0.030 (3)	0.032 (3)	0	0.009 (2)	0
C2	0.034 (2)	0.057 (3)	0.032 (2)	−0.008 (2)	0.0182 (19)	−0.003 (2)
C3	0.035 (3)	0.032 (3)	0.035 (3)	0	0.014 (3)	0
C4	0.051 (3)	0.029 (2)	0.051 (3)	−0.008 (2)	0.024 (2)	−0.005 (2)
C5	0.037 (2)	0.042 (3)	0.045 (3)	0.000 (2)	0.025 (2)	−0.0007 (19)
Sb1	0.02856 (15)	0.02384 (14)	0.02756 (15)	−0.00119 (12)	0.01291 (11)	−0.00108 (10)
Cr1	0.0252 (4)	0.0311 (5)	0.0246 (4)	0	0.0111 (4)	0
O1	0.047 (3)	0.059 (3)	0.040 (3)	0	0.027 (2)	0
O2	0.053 (2)	0.077 (3)	0.044 (2)	−0.034 (2)	0.0202 (18)	−0.0204 (19)
O3	0.061 (3)	0.052 (3)	0.055 (3)	0	0.042 (3)	0

Geometric parameters (Å, º)

C1—O1	1.134 (8)	C4—H4C	0.98
C1—Cr1	1.900 (6)	C5—Sb1	2.148 (4)
C2—O2	1.161 (6)	C5—H5A	0.98
C2—Cr1	1.846 (5)	C5—H5B	0.98
C3—O3	1.151 (8)	C5—H5C	0.98
C3—Cr1	1.887 (7)	Sb1—Cr1	2.6205 (9)
C4—Sb1	2.148 (4)	Sb1—Sb1^i	2.8157 (11)
C4—H4A	0.98	Cr1—C2^ii	1.846 (5)
C4—H4B	0.98	Cr1—Sb1^ii	2.6205 (9)
O1—C1—Cr1	176.4 (5)	C4—Sb1—Sb1^i	97.32 (14)
O2—C2—Cr1	179.1 (4)	C5—Sb1—Sb1^i	97.44 (14)
O3—C3—Cr1	176.8 (6)	Cr1—Sb1—Sb1^i	121.89 (3)
Sb1—C4—H4A	109.5	C2—Cr1—C2^ii	91.8 (3)
Sb1—C4—H4B	109.5	C2—Cr1—C3	89.91 (19)
H4A—C4—H4B	109.5	C2^ii—Cr1—C3	89.91 (19)
Sb1—C4—H4C	109.5	C2—Cr1—C1	88.55 (18)
H4A—C4—H4C	109.5	C2^ii—Cr1—C1	88.55 (18)
H4B—C4—H4C	109.5	C3—Cr1—C1	177.8 (3)
Sb1—C5—H5A	109.5	C2—Cr1—Sb1^ii	177.44 (14)
Sb1—C5—H5B	109.5	C2^ii—Cr1—Sb1^ii	88.94 (16)
H5A—C5—H5B	109.5	C3—Cr1—Sb1^ii	92.55 (14)
Sb1—C5—H5C	109.5	C1—Cr1—Sb1^ii	89.01 (12)
H5A—C5—H5C	109.5	C2—Cr1—Sb1	88.94 (16)
H5B—C5—H5C	109.5	C2^ii—Cr1—Sb1	177.44 (14)
C4—Sb1—C5	100.00 (19)	C3—Cr1—Sb1	92.55 (13)
C4—Sb1—Cr1	117.64 (15)	C1—Cr1—Sb1	89.01 (12)
C5—Sb1—Cr1	118.02 (13)	Sb1^ii—Cr1—Sb1	90.23 (4)
C4—Sb1—Cr1—C2	11.4 (2)	C4—Sb1—Cr1—C1	100.0 (2)
C5—Sb1—Cr1—C2	−108.7 (2)	C5—Sb1—Cr1—C1	−20.08 (19)
Sb1^i—Sb1—Cr1—C2	131.09 (13)	Sb1^i—Sb1—Cr1—C1	−140.34 (12)
C4—Sb1—Cr1—C3	−78.5 (2)	C4—Sb1—Cr1—Sb1^ii	−171.03 (16)
C5—Sb1—Cr1—C3	161.5 (2)	C5—Sb1—Cr1—Sb1^ii	68.92 (15)
Sb1^i—Sb1—Cr1—C3	41.22 (14)	Sb1^i—Sb1—Cr1—Sb1^ii	−51.34 (4)

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