Bromo­di­methyl(N-methyl­pyrrolidin-2-one-O)­tin(IV)-di-[mu]-bromo-bromo­di­methyl­tin(IV)

The preparation and X-ray analysis of the title compound, [Sn₂Br₄(CH₃)₄(C₅H₉NO)], are described. The compound contains two Sn atoms in the asymmetric unit, that complexed by N-methyl­pyrrolidin-2-one being hexacoordinated (a), the other exhibiting pentacoordination (b). The most important features are three different Sn-Br bond lengths at both Sn atoms with the following values: (a) 2.5060 (9), 2.7152 (10) and 3.7118 (10) Å; (b) 2.5084 (10), 2.5279 (9) and 3.5841 (10) Å.

S1. Comment

Molecular complexes derived from 1:1 and 1:2 complexation of dimethyltin dihalides by N-methylpyrrolidinone (NMP) are well known (König et al., 2000a,b). The 2:1 complexation of dimethyltin dichloride by NMP shows pentacoordination at both Sn atoms, which are bridged by a chloro ligand, the angle Sn—Cl—Sn being 135.56 (5)°. The values of the Sn—Cl bond lengths are 2.5704 (13) and 3.1159 (13) Å (König et al., 2000c). Two bridging bromine ligands are found in the analogous adduct of dimethyltin dibromide and NMP, (I). The central atom which is complexed by NMP exhibits the octahedral geometry typically found in hexacoordinated tin complexes, whereas the other tin atom is surrounded by five ligands, generating a trigonal–bipyramidal geometry. \scheme

It is sometimes difficult to pinpoint the geometry around the central atom in certain molecular complexes. The most important parameters measured are the bond angles and bond lengths. For this reason a definition and quantitative limitation of the term bond can be made satisfactorily with the method of van der Waals radii. This term is used correctly when the measured distance between two atoms is smaller than the sum of the corresponding radii (Bondi, 1964) as for instance between Sn and Br atoms, the sum of whose van der Waals radii is calculated as 4.0–4.2 Å.

The Sn—C bond lengths of the pentacoordinated tin centre have normal values of 2.095 (5) and 2.126 (5) Å (Ho & Zuckerman, 1973). The deviation from ideal geometry in the equatorial plane is smaller than in other pentacoordinated complexes of dimethyltin dihalides and NMP (König et al. 2000b,c), as is demonstrated most clearly by the values of the angles C1—Sn1—C2, C1—Sn1—Br1 and C2—-Sn1–Br1: 132.4 (3), 106.98 (15) and 107.75 (14)°. The bromine ligands Br2 and Br4 take apical positions, the values of the Sn—Br bond lengths and the angle Br2—Sn1—Br4 being as follows: 2.5279 (9), 3.5841 (10) Å and 177.99 (3)°. The variation from the trigonal-bipyramidal geometry becomes most obvious from an extraordinarily small difference between the apical and equatorial bromine positions. The Sn1—Br1 bond has a length of 2.5084 (10) Å and is therefore only little shorter than the Sn1—Br2 bond.

The ligands in the equatorial plane are somewhat displaced towards the bridging bromine ligand Br4. This becomes evident from the values of the angles between the axial and the equatorial ligands: (a) Br2—Sn1—C1 102.38 (14)°, Br2—Sn1—C2 103.82 (15)°, Br2—Sn1—Br1 98.17 (3)°; (b) Br4—Sn1—C1 79.14 (14)°, Br4—Sn1—C2 74.17 (15)°, Br4—Sn1—Br1 82.57 (3)°.

Another difference between the chlorine and bromine derivatives lies in the angles Sn1—X4—Sn2 (X = Cl, Br). In the latter complex this angle has a value of 99.29 (3)° and thus is approximately 36° smaller than in the chlorine complex. The value of the second bridging angle Sn1—Br1—Sn2 is 100.17 (3)°.

At the hexacoordinated tin the values of the Sn—C bond lengths are 2.107 (6) Å and 2.116 (7) Å and thus lie in the normal range. The angle C3—Sn2—C4 (138.3 (3)°) is obviously smaller than the analogous angle 169.7 (4)° at the hexacoordinated central atom in cis-dibromo-trans-dimethyl-cis-bis(N-methylpyrrolidinone)tin(IV) (König et al., 2000a).

The Sn—Br bonds in the hexacoordinated part of the complex differ greatly and can be regarded as the adducts most interesting feature. The Sn2—Br4 bond has a value of 2.7152 (10) Å and is thus because of the bridging character of Br4 a little longer [2.6738 (10) and 2.6761 (12) Å] than in a related hexacoordinated complex (König et al., 2000a). The angle Br4—Sn2—O1 is almost linear [175.82 (14)°]. Nevertheless, the bond between the bromine ligand Br3 and Sn2 has a length of 2.5060 (9) Å which is typical of a Sn—Br bond in the equatorial plane of a trigonal bipyramid (Boer et al., 1970; König et al., 2000b). This is confirmed by the angles C3—Sn2—C4, C3—Sn2—Br3 and C4—Sn2—Br3 having the following values: 138.3 (3), 111.2 (2) and 109.48 (19)°.

As in cis-dibromo-trans-dimethyl-cis-bis(N-methylpyrrolidinone)tin(IV) (König et al., 2000b) the plane defined by the two methyl groups and the bromine ligand Br3 is displaced towards the NMP ligand. This becomes evident from the values of the following angles: (a) C3—Sn2—O1 90.1 (2)°, C4—Sn2—O1 83.2 (3)°, Br3—Sn2—O1 87.09 (12)°; (b) C3—Sn2—Br4 94.1 (2)°, C4—Sn2—Br4 93.2 (2)°, Br3—Sn2—Br4 92.11 (3)°.

The deviation from ideal geometry is closely related to the angles involving the bridging bromine ligand Br1: Br1—Sn2—C3 70.2 (2)°, Br1—Sn2—C4 71.44 (19)°, Br1—Sn2—O1 103.18 (12)°, Br1—Sn2—Br3 169.69 (3)°. The fragment C5—O1—Sn2 is not as often expected linear. The angle C5—O1—Sn2 has a value of 139.9 (5)° and is therefore within the range of known values (König, 2000). The value of the Sn2—O1 bond length is 2.271 (5) Å and thus shorter than those in related adducts (König et al., 2000a,b).

The values of the bond lengths and bond angles in the NMP ligand are comparable with those observed in other NMP coordinated organometallic compounds (Churchill & Rotella, 1979) and in free NMP (Müller et al., 1996).

S2. Experimental

The title compound is prepared by the reaction of N-methylpyrrolidinone (0.71 g, 0.69 ml, 7.3 mmol) with freshly sublimed dibromodimethylstannane (4.51 g, 14.6 mmol) derived from the reaction of dimethyltin oxide with HBr (Pfeiffer, 1902) in 10 ml of dry diethyl ether. The reaction mixture is stirred for 30 min and afterwards stored in a refrigerator at 278 K. Colourless crystals are obtained in quantitative yield after filtration and drying in vacuo, mp. 320 K. A solution of the complex (40 mg) in C₆D₆ (430 mg) gives the following values for the structure-relevant NMR parameters: ²J(¹¹⁹Sn, ¹H) = 71 Hz, ¹J(¹¹⁹Sn–¹³C) = 506 Hz and δ(¹¹⁹Sn)= 11.6 p.p.m.

S3. Refinement

The data collection covered almost the whole spere of reciprocal space. The crystal-to-detector distance was 35 mm. Crystal decay was monitored by repeating the initial frames at the end of data collection. Analysing the duplicate reflections there was no indication for any decay. Hydrogen atoms were placed in calculated positions with U_iso constrained to be 1.2U_eq of the carrier atom.

Computing details

Data collection: COLLECT (Nonius, 1999); cell refinement: DENZO and SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); software used to prepare material for publication: SHELXL97 and PARST95 (Nardelli, 1995).

(I)

Crystal data

[Sn2Br4(CH3)4(C5H9NO)]	Dx = 2.433 Mg m^−3
Mr = 716.29	Mo Kα radiation, λ = 0.71069 Å
Orthorhombic, P212121	Cell parameters from 12688 reflections
a = 7.5279 (3) Å	θ = 1.6–25.7°
b = 10.4817 (4) Å	µ = 10.72 mm^−1
c = 24.7865 (11) Å	T = 173 K
V = 1955.78 (14) Å^3	Parallelepiped, colourless
Z = 4	0.11 × 0.1 × 0.1 mm
F(000) = 1320

Data collection

Nonius KappaCCD diffractometer	2662 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.073
Graphite monochromator	θmax = 25.7°, θmin = 1.6°
Detector resolution: 10 vertical, 18 horizontal pixels mm^-1	h = −9→8
443 frames via ω–rotation (Δω=1°) at different κ–angles and two times 75 s per frame scans	k = −12→12
12688 measured reflections	l = −29→30
3713 independent 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)] = 0.036	H-atom parameters constrained
wR(F^2) = 0.065	Calculated w = 1/[σ^2(Fo^2) + (0.0131P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.90	(Δ/σ)max = 0.001
3713 reflections	Δρmax = 0.78 e Å^−3
154 parameters	Δρmin = −0.98 e Å^−3
0 restraints	Absolute structure: Flack (1983); 1570 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.117 (10)

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
Sn1	0.10583 (7)	0.94668 (5)	0.94349 (2)	0.04193 (16)
Sn2	0.51534 (7)	0.78031 (5)	0.81081 (2)	0.03900 (15)
Br1	0.17872 (12)	1.02190 (7)	0.85000 (4)	0.0538 (2)
Br2	−0.14618 (11)	1.09923 (8)	0.96301 (3)	0.0548 (3)
Br3	0.73656 (12)	0.60304 (8)	0.80141 (4)	0.0603 (3)
Br4	0.45214 (11)	0.72139 (8)	0.91574 (3)	0.0529 (2)
O1	0.5551 (7)	0.8176 (5)	0.7213 (2)	0.0490 (15)
N1	0.6502 (8)	0.8472 (5)	0.6358 (3)	0.0409 (16)
C1	0.3072 (7)	1.0141 (5)	0.9944 (2)	0.049 (2)
H1A	0.4110	0.9619	0.9901	0.074*
H1B	0.3353	1.1008	0.9852	0.074*
H1C	0.2677	1.0102	1.0312	0.074*
C2	−0.0216 (7)	0.7664 (5)	0.9367 (2)	0.053 (2)
H2A	0.0653	0.7018	0.9291	0.079*
H2B	−0.0821	0.7459	0.9697	0.079*
H2C	−0.1059	0.7703	0.9077	0.079*
C3	0.6417 (10)	0.9544 (6)	0.8290 (3)	0.049 (2)
H3A	0.5820	1.0227	0.8107	0.073*
H3B	0.7633	0.9509	0.8174	0.073*
H3C	0.6376	0.9688	0.8673	0.073*
C4	0.2625 (9)	0.7160 (7)	0.7853 (3)	0.052 (2)
H4A	0.1789	0.7254	0.8142	0.077*
H4B	0.2699	0.6277	0.7751	0.077*
H4C	0.2241	0.7655	0.7549	0.077*
C5	0.6769 (11)	0.8386 (6)	0.6885 (3)	0.0377 (19)
C6	0.8702 (9)	0.8582 (7)	0.6999 (3)	0.051 (2)
H6A	0.9184	0.7868	0.7201	0.076*
H6B	0.8890	0.9358	0.7204	0.076*
C7	0.4791 (11)	0.8291 (8)	0.6097 (3)	0.063 (3)
H7A	0.3929	0.8031	0.6360	0.095*
H7B	0.4895	0.7644	0.5825	0.095*
H7C	0.4421	0.9078	0.5934	0.095*
C8	0.8068 (10)	0.8728 (7)	0.6040 (3)	0.048 (2)
H8A	0.7995	0.9562	0.5872	0.072*
H8B	0.8218	0.8087	0.5761	0.072*
C9	0.9577 (11)	0.8678 (9)	0.6439 (4)	0.068 (3)
H9A	1.0299	0.9442	0.6413	0.101*
H9B	1.0324	0.7941	0.6371	0.101*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.0445 (3)	0.0342 (3)	0.0471 (4)	−0.0002 (3)	0.0000 (3)	−0.0009 (3)
Sn2	0.0382 (3)	0.0346 (3)	0.0441 (3)	0.0020 (3)	−0.0012 (3)	−0.0009 (3)
Br1	0.0619 (6)	0.0411 (5)	0.0582 (6)	0.0075 (4)	0.0140 (5)	0.0095 (4)
Br2	0.0531 (6)	0.0583 (6)	0.0530 (6)	0.0159 (5)	−0.0019 (5)	−0.0074 (4)
Br3	0.0662 (6)	0.0511 (5)	0.0635 (6)	0.0247 (5)	0.0027 (5)	0.0057 (5)
Br4	0.0591 (6)	0.0526 (5)	0.0470 (5)	0.0011 (4)	0.0048 (5)	0.0049 (4)
O1	0.051 (4)	0.060 (4)	0.036 (3)	0.008 (3)	0.005 (3)	0.010 (3)
N1	0.041 (4)	0.035 (4)	0.046 (5)	−0.001 (3)	0.000 (4)	0.002 (3)
C1	0.056 (5)	0.038 (5)	0.054 (6)	0.006 (4)	−0.008 (5)	−0.011 (4)
C2	0.063 (5)	0.040 (4)	0.055 (6)	−0.021 (4)	0.008 (5)	0.006 (4)
C3	0.055 (5)	0.038 (5)	0.053 (6)	−0.017 (4)	−0.002 (4)	−0.003 (4)
C4	0.043 (5)	0.055 (5)	0.057 (6)	−0.003 (4)	0.000 (4)	−0.010 (4)
C5	0.051 (5)	0.024 (4)	0.038 (5)	0.006 (4)	−0.009 (5)	0.001 (4)
C6	0.037 (5)	0.050 (5)	0.065 (6)	−0.006 (4)	−0.020 (5)	−0.001 (4)
C7	0.054 (6)	0.069 (6)	0.067 (6)	−0.001 (5)	−0.020 (5)	0.007 (5)
C8	0.063 (6)	0.041 (5)	0.039 (5)	−0.003 (4)	0.012 (5)	0.005 (4)
C9	0.049 (6)	0.075 (6)	0.079 (7)	−0.020 (5)	0.002 (6)	−0.019 (6)

Geometric parameters (Å, º)

Sn1—C1	2.095 (5)	C2—H2C	0.9601
Sn1—C2	2.126 (5)	C3—H3A	0.9600
Sn1—Br1	2.5084 (10)	C3—H3B	0.9600
Sn1—Br2	2.5279 (9)	C3—H3C	0.9600
Sn1—Br4	3.5841 (10)	C4—H4A	0.9600
Sn2—C3	2.107 (6)	C4—H4B	0.9600
Sn2—C4	2.116 (7)	C4—H4C	0.9600
Sn2—O1	2.271 (5)	C5—C6	1.497 (10)
Sn2—Br3	2.5060 (9)	C6—C9	1.541 (11)
Sn2—Br4	2.7152 (10)	C6—H6A	0.9700
Sn2—Br1	3.7118 (10)	C6—H6B	0.9700
O1—C5	1.245 (9)	C7—H7A	0.9600
N1—C5	1.324 (9)	C7—H7B	0.9600
N1—C8	1.444 (9)	C7—H7C	0.9600
N1—C7	1.454 (9)	C8—C9	1.507 (10)
C1—H1A	0.9600	C8—H8A	0.9700
C1—H1B	0.9600	C8—H8B	0.9700
C1—H1C	0.9600	C9—H9A	0.9700
C2—H2A	0.9600	C9—H9B	0.9700
C2—H2B	0.9599
C1—Sn1—C2	132.4 (3)	H2A—C2—H2C	109.5
C1—Sn1—Br1	106.98 (15)	H2B—C2—H2C	109.5
C2—Sn1—Br1	107.75 (14)	Sn2—C3—H3A	109.5
C1—Sn1—Br2	102.38 (14)	Sn2—C3—H3B	109.5
C2—Sn1—Br2	103.82 (15)	H3A—C3—H3B	109.5
Br1—Sn1—Br2	98.17 (3)	Sn2—C3—H3C	109.5
C1—Sn1—Br4	79.14 (14)	H3A—C3—H3C	109.5
C2—Sn1—Br4	74.17 (15)	H3B—C3—H3C	109.5
Br1—Sn1—Br4	82.57 (3)	Sn2—C4—H4A	109.5
Br2—Sn1—Br4	177.99 (3)	Sn2—C4—H4B	109.5
C3—Sn2—C4	138.3 (3)	H4A—C4—H4B	109.5
C3—Sn2—O1	90.1 (2)	Sn2—C4—H4C	109.5
C4—Sn2—O1	83.2 (3)	H4A—C4—H4C	109.5
C3—Sn2—Br3	111.2 (2)	H4B—C4—H4C	109.5
C4—Sn2—Br3	109.48 (19)	O1—C5—N1	123.0 (7)
O1—Sn2—Br3	87.09 (12)	O1—C5—C6	128.1 (8)
C3—Sn2—Br4	94.1 (2)	N1—C5—C6	108.9 (7)
C4—Sn2—Br4	93.2 (2)	C5—C6—C9	104.8 (7)
O1—Sn2—Br4	175.82 (14)	C5—C6—H6A	110.8
Br3—Sn2—Br4	92.11 (3)	C9—C6—H6A	110.8
C3—Sn2—Br1	70.2 (2)	C5—C6—H6B	110.8
C4—Sn2—Br1	71.44 (19)	C9—C6—H6B	110.8
O1—Sn2—Br1	103.18 (12)	H6A—C6—H6B	108.9
Br3—Sn2—Br1	169.69 (3)	N1—C7—H7A	109.5
Br4—Sn2—Br1	77.58 (2)	N1—C7—H7B	109.5
Sn1—Br1—Sn2	100.17 (3)	H7A—C7—H7B	109.5
Sn2—Br4—Sn1	99.29 (3)	N1—C7—H7C	109.5
C5—O1—Sn2	139.9 (5)	H7A—C7—H7C	109.5
C5—N1—C8	115.3 (7)	H7B—C7—H7C	109.5
C5—N1—C7	124.4 (7)	N1—C8—C9	104.5 (6)
C8—N1—C7	120.3 (7)	N1—C8—H8A	110.9
Sn1—C1—H1A	109.3	C9—C8—H8A	110.9
Sn1—C1—H1B	109.7	N1—C8—H8B	110.9
H1A—C1—H1B	109.5	C9—C8—H8B	110.9
Sn1—C1—H1C	109.5	H8A—C8—H8B	108.9
H1A—C1—H1C	109.5	C8—C9—C6	105.8 (6)
H1B—C1—H1C	109.5	C8—C9—H9A	110.6
Sn1—C2—H2A	109.5	C6—C9—H9A	110.6
Sn1—C2—H2B	110.2	C8—C9—H9B	110.6
H2A—C2—H2B	109.5	C6—C9—H9B	110.6
Sn1—C2—H2C	108.6	H9A—C9—H9B	108.7
C1—Sn1—Br1—Sn2	80.35 (15)	C3—Sn2—O1—C5	57.4 (7)
C2—Sn1—Br1—Sn2	−66.55 (15)	C4—Sn2—O1—C5	−163.8 (8)
Br2—Sn1—Br1—Sn2	−173.97 (3)	Br3—Sn2—O1—C5	−53.8 (7)
Br4—Sn1—Br1—Sn2	4.15 (2)	Br4—Sn2—O1—C5	−132.9 (16)
C3—Sn2—Br1—Sn1	−104.6 (2)	Br1—Sn2—O1—C5	127.1 (7)
C4—Sn2—Br1—Sn1	92.2 (2)	Sn2—O1—C5—N1	174.7 (5)
O1—Sn2—Br1—Sn1	170.22 (14)	Sn2—O1—C5—C6	−5.3 (13)
Br3—Sn2—Br1—Sn1	−4.71 (19)	C8—N1—C5—O1	179.6 (6)
Br4—Sn2—Br1—Sn1	−5.57 (3)	C7—N1—C5—O1	−2.2 (11)
C3—Sn2—Br4—Sn1	72.6 (2)	C8—N1—C5—C6	−0.3 (8)
C4—Sn2—Br4—Sn1	−66.31 (19)	C7—N1—C5—C6	177.9 (7)
O1—Sn2—Br4—Sn1	−97.0 (17)	O1—C5—C6—C9	175.0 (7)
Br3—Sn2—Br4—Sn1	−175.97 (3)	N1—C5—C6—C9	−5.0 (8)
Br1—Sn2—Br4—Sn1	3.88 (2)	C5—N1—C8—C9	5.7 (8)
C1—Sn1—Br4—Sn2	−114.62 (15)	C7—N1—C8—C9	−172.6 (7)
C2—Sn1—Br4—Sn2	105.23 (15)	N1—C8—C9—C6	−8.3 (9)
Br1—Sn1—Br4—Sn2	−5.66 (3)	C5—C6—C9—C8	8.2 (9)
Br2—Sn1—Br4—Sn2	106.3 (9)