catena-Poly[[(N,N-di­methyl­cyanamide-κN)lithium]-μ₃-bromido]

The title complex, [LiBr(C₃H₆N₂)]_n, is the unexpected product of a reaction beteween (Dipp)N(Li)SiMe₃ (Dipp = 2,6-diiso­propyl­phen­yl), Me₂NCN and CuBr. The compound is a one-dimensional polymer with a step structure derived from the association of inversion dimers, formed by bromido ligands bridging two Li⁺ cations, each of which carries a di­methyl­cyanamide ligand. The planar (LiBr)₂ unit of the polymer core has a regular rhombic shape [Li—Br—Li 77.55 (16)° and Br—Li—Br 102.45 (16)°]. These (LiBr·NCNMe₂)₂ dimers represent the repeat unit of a polymer system propagated by additional Br—Li and Li—Br bonds generating an infinite step structure along the a-axis direction.

Related literature

For examples of lithium halides solvated by Lewis bases, see: Snaith & Wright (1995); Mulvey (1991); Raston, Skelton et al. (1988), Raston, Whitaker & White (1988, 1989a,b); Edwards et al. (1993); Neumann et al. (1995); Gregory et al. (1991). For related crystal structures, see: Edwards et al. (1993); Raston, Skelton et al. (1988). A 1,3,5,7-tetra­aza­hepta­trien­yl–lithium salt was reported by Boesveld et al. (2009)

Experimental

Crystal data

• [LiBr(C₃H₆N₂)]

• M_r = 156.85

• Monoclinic, P 2₁ /c

• a = 4.2680 (8) Å

• b = 17.214 (3) Å

• c = 8.9685 (17) Å

• β = 100.089 (3)°

• V = 648.7 (2) ų

• Z = 4

• Mo Kα radiation

• μ = 6.22 mm⁻¹

• T = 200 K

• 0.35 × 0.33 × 0.32 mm

Data collection

• Bruker SMART APEX CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2000) T_min = 0.220, T_max = 0.241

• 3498 measured reflections

• 1140 independent reflections

• 965 reflections with I > 2σ(I)

• R_int = 0.032

Refinement

• R[F² > 2σ(F²)] = 0.025

• wR(F²) = 0.061

• S = 1.03

• 1140 reflections

• 67 parameters

• H-atom parameters constrained

• Δρ_max = 0.57 e Å⁻³

• Δρ_min = −0.38 e Å⁻³

Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL/PC (Sheldrick, 2008); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Experimental

S1.1. Synthesis and crystallization

Me₂NCN (0.76 mL, 9.38 mmol) was added to a solution of (Dipp)N(Li)SiMe₃ (0.60 g, 2.35 mmol) in Et₂O (30 mL) at -78°C. The resulting mixture was warmed to ca. 25°C and stirred for overnight. The resulting mixture was added dropwise into a suspension of CuBr (0.34 g, 2.35 mmol) in Et₂O (10 mL) at -78°C. The resulting mixture was warmed to ca. 25°C and stirred for 24 h, then filtered. The filtrate was concentrated in vacuo and stored at 20°C for ten days, yielding colorless crystals of the title compound (0.503 g, 68%) .

Anal. calcd. for C₆H₁₂Br₂Li₂N₄(%): C, 22.96; H, 3.85; N, 17.85. Found: C, 22.93; H, 3.89; N, 17.87. All manipulations were performed under argon using standard Schlenk and vacuum line techniques. Et₂O was dried and distilled over Na under argon prior to use. Elemental analysis is completely in agreement with the structure of the compound.

S1.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The methyl H atoms were constrained to an ideal geometry, with C—H distances of 0.98 Å and U_iso(H) = 1.5U_eq(C).

S2. Comment

Lithium halidies solvated by Lewis bases have been studied extensively in the past and the various crystal structures exhibit remarkable structural diversity (Snaith et al., 1995; Mulvey, 1991, Gregory et al., 1991). Monomers, dimers, tetra­mers, larger oligomers and polymers are known (Raston, Whitaker & White 1988, 1989a,b; Raston, Skelton et al. 1988; Edwards et al.,1993). Pyridines, chelating amines and Lewis bases containing oxygen usually serve as ligands (Neumann et al., 1995). A 1,3,5,7-tetra­aza­heptatrienyl-lithium salt was reported by W. Marco Boesveld (Boesveld et al., 2009) and we were attempting to synthesize a 1,3,5,7-tetra­aza­heptatrienylcopper complex by the reaction of (Dipp)N(Li)SiMe₃ (Dipp = 2,6-diiso­propyl­phenyl), Me₂NCN and CuBr. No copper complex was obtained but instead the title polymeric lithium complex (I), (C₆H₁₂Br₂Li₂N₄)_∞, was isolated from the reaction mixture. Here we present the synthesis and crystal structure of the complex (I).

A low-temperature X-ray crystallographic study shows the basic unit (Fig. 1) of the step structure of complex (I) is centrosymmetric, and to have a polymeric structure (Fig. 2) in the solid state. In the unit, atoms Li1, Br1, Li1A and Br1A are exactly co-planar and constitute a regular rhombic shape [Li—Br—Li 77.55 (16)° and Br—Li—Br 102.45 (16)° ]. The Li1—Br1 and Li1—N1 bond lengths are 2.543 (5) (av.) and 1.999 (5) Å. The bond angles N1—Li1—Br1, N1—Li1—Br1A are 113.1 (2) and 119.6 (2)°, respectively.

Related literature

For examples of lithium halides solvated by Lewis bases, see: Snaith & Wright (1995); Mulvey (1991); Raston, Skelton et al. (1988), Raston, Whitaker & White (1988, 1989a,b); Edwards et al. (1993); Neumann et al. (1995); Gregory et al. (1991). For related crystal structures, see: Edwards et al. (1993); Raston, Skelton et al. (1988). A 1,3,5,7-tetraazaheptatrienyl–lithium salt was reported by Boesveld et al. (2009).

Structure description

Lithium halidies solvated by Lewis bases have been studied extensively in the past and the various crystal structures exhibit remarkable structural diversity (Snaith et al., 1995; Mulvey, 1991, Gregory et al., 1991). Monomers, dimers, tetra­mers, larger oligomers and polymers are known (Raston, Whitaker & White 1988, 1989a,b; Raston, Skelton et al. 1988; Edwards et al.,1993). Pyridines, chelating amines and Lewis bases containing oxygen usually serve as ligands (Neumann et al., 1995). A 1,3,5,7-tetra­aza­heptatrienyl-lithium salt was reported by W. Marco Boesveld (Boesveld et al., 2009) and we were attempting to synthesize a 1,3,5,7-tetra­aza­heptatrienylcopper complex by the reaction of (Dipp)N(Li)SiMe₃ (Dipp = 2,6-diiso­propyl­phenyl), Me₂NCN and CuBr. No copper complex was obtained but instead the title polymeric lithium complex (I), (C₆H₁₂Br₂Li₂N₄)_∞, was isolated from the reaction mixture. Here we present the synthesis and crystal structure of the complex (I).

A low-temperature X-ray crystallographic study shows the basic unit (Fig. 1) of the step structure of complex (I) is centrosymmetric, and to have a polymeric structure (Fig. 2) in the solid state. In the unit, atoms Li1, Br1, Li1A and Br1A are exactly co-planar and constitute a regular rhombic shape [Li—Br—Li 77.55 (16)° and Br—Li—Br 102.45 (16)° ]. The Li1—Br1 and Li1—N1 bond lengths are 2.543 (5) (av.) and 1.999 (5) Å. The bond angles N1—Li1—Br1, N1—Li1—Br1A are 113.1 (2) and 119.6 (2)°, respectively.

For examples of lithium halides solvated by Lewis bases, see: Snaith & Wright (1995); Mulvey (1991); Raston, Skelton et al. (1988), Raston, Whitaker & White (1988, 1989a,b); Edwards et al. (1993); Neumann et al. (1995); Gregory et al. (1991). For related crystal structures, see: Edwards et al. (1993); Raston, Skelton et al. (1988). A 1,3,5,7-tetraazaheptatrienyl–lithium salt was reported by Boesveld et al. (2009).

Synthesis and crystallization

Me₂NCN (0.76 mL, 9.38 mmol) was added to a solution of (Dipp)N(Li)SiMe₃ (0.60 g, 2.35 mmol) in Et₂O (30 mL) at -78°C. The resulting mixture was warmed to ca. 25°C and stirred for overnight. The resulting mixture was added dropwise into a suspension of CuBr (0.34 g, 2.35 mmol) in Et₂O (10 mL) at -78°C. The resulting mixture was warmed to ca. 25°C and stirred for 24 h, then filtered. The filtrate was concentrated in vacuo and stored at 20°C for ten days, yielding colorless crystals of the title compound (0.503 g, 68%) .

Anal. calcd. for C₆H₁₂Br₂Li₂N₄(%): C, 22.96; H, 3.85; N, 17.85. Found: C, 22.93; H, 3.89; N, 17.87. All manipulations were performed under argon using standard Schlenk and vacuum line techniques. Et₂O was dried and distilled over Na under argon prior to use. Elemental analysis is completely in agreement with the structure of the compound.

Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 1. The methyl H atoms were constrained to an ideal geometry, with C—H distances of 0.98 Å and U_iso(H) = 1.5U_eq(C).

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL/PC (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The basic repeat unit of the polymer (I) showing the atom numbering scheme with displacement ellipsoids drawn at the 50% probability level. Atoms labelled with a trailing A are related to the other atoms by the symmetry operation 1-x, 1-y, -z
	Fig. 2. The expanded step polymeric structure of (I) viewed along the crystallographic c axis.

catena-Poly[[(N,N-dimethylcyanamide-κN)lithium]-µ₃-bromido]

Crystal data

[LiBr(C3H6N2)]	F(000) = 304
Mr = 156.85	Dx = 1.607 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 4.2680 (8) Å	Cell parameters from 1538 reflections
b = 17.214 (3) Å	θ = 2.4–26.5°
c = 8.9685 (17) Å	µ = 6.22 mm^−1
β = 100.089 (3)°	T = 200 K
V = 648.7 (2) Å^3	Block, colourless
Z = 4	0.35 × 0.33 × 0.32 mm

Data collection

Bruker SMART APEX CCD diffractometer	1140 independent reflections
Radiation source: fine-focus sealed tube	965 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.032
ω scans	θmax = 25.1°, θmin = 2.4°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −5→3
Tmin = 0.220, Tmax = 0.241	k = −20→18
3498 measured reflections	l = −10→10

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)] = 0.025	H-atom parameters constrained
wR(F^2) = 0.061	w = 1/[σ^2(Fo^2) + (0.0285P)^2 + 0.3312P] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max = 0.001
1140 reflections	Δρmax = 0.57 e Å^−3
67 parameters	Δρmin = −0.38 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0053 (16)

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
Li1	0.2655 (11)	0.4372 (3)	0.0594 (6)	0.0289 (11)
Br1	0.81425 (7)	0.485193 (18)	0.18525 (3)	0.03230 (16)
N1	0.2914 (7)	0.32132 (16)	0.0517 (3)	0.0475 (8)
N2	0.5102 (8)	0.18972 (16)	0.0853 (3)	0.0509 (8)
C1	0.3896 (8)	0.25987 (19)	0.0667 (4)	0.0352 (8)
C2	0.3986 (11)	0.1281 (2)	−0.0201 (5)	0.0672 (12)
H2A	0.2384	0.1487	−0.1023	0.101*
H2B	0.3037	0.0868	0.0327	0.101*
H2C	0.5779	0.1070	−0.0621	0.101*
C3	0.7254 (9)	0.1720 (2)	0.2260 (5)	0.0581 (11)
H3A	0.7997	0.2206	0.2775	0.087*
H3B	0.9081	0.1426	0.2035	0.087*
H3C	0.6130	0.1410	0.2916	0.087*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Li1	0.025 (3)	0.022 (3)	0.038 (3)	0.0034 (19)	0.001 (2)	0.004 (2)
Br1	0.0248 (2)	0.0371 (2)	0.0339 (2)	0.00073 (13)	0.00223 (13)	0.00497 (14)
N1	0.052 (2)	0.0287 (17)	0.057 (2)	0.0056 (14)	−0.0034 (15)	0.0025 (14)
N2	0.068 (2)	0.0252 (16)	0.053 (2)	0.0129 (14)	−0.0071 (16)	−0.0022 (14)
C1	0.035 (2)	0.032 (2)	0.0363 (19)	−0.0027 (15)	−0.0003 (14)	−0.0015 (14)
C2	0.108 (4)	0.036 (2)	0.062 (3)	−0.003 (2)	0.027 (2)	−0.0161 (19)
C3	0.050 (2)	0.058 (3)	0.065 (3)	0.0178 (19)	0.003 (2)	0.019 (2)

Geometric parameters (Å, º)

Li1—N1	1.999 (5)	N2—C1	1.312 (4)
Li1—Br1^i	2.535 (5)	N2—C2	1.445 (5)
Li1—Br1^ii	2.541 (5)	N2—C3	1.457 (5)
Li1—Br1	2.553 (5)	C2—H2A	0.9800
Li1—Li1^iii	3.179 (9)	C2—H2B	0.9800
Li1—Li1^ii	3.251 (10)	C2—H2C	0.9800
Br1—Li1^iv	2.535 (5)	C3—H3A	0.9800
Br1—Li1^ii	2.541 (5)	C3—H3B	0.9800
N1—C1	1.137 (4)	C3—H3C	0.9800
N1—Li1—Br1^i	113.1 (2)	C1—N1—Li1	161.0 (3)
N1—Li1—Br1^ii	119.6 (2)	C1—N2—C2	121.0 (3)
Br1^i—Li1—Br1^ii	102.45 (16)	C1—N2—C3	118.4 (3)
N1—Li1—Br1	106.6 (2)	C2—N2—C3	119.9 (3)
Br1^i—Li1—Br1	114.03 (19)	N1—C1—N2	178.5 (4)
Br1^ii—Li1—Br1	100.67 (17)	N2—C2—H2A	109.5
N1—Li1—Li1^iii	135.1 (3)	N2—C2—H2B	109.5
Br1^i—Li1—Li1^iii	51.30 (14)	H2A—C2—H2B	109.5
Br1^ii—Li1—Li1^iii	51.15 (14)	N2—C2—H2C	109.5
Br1—Li1—Li1^iii	118.2 (2)	H2A—C2—H2C	109.5
N1—Li1—Li1^ii	127.7 (3)	H2B—C2—H2C	109.5
Br1^i—Li1—Li1^ii	119.2 (2)	N2—C3—H3A	109.5
Br1^ii—Li1—Li1^ii	50.50 (13)	N2—C3—H3B	109.5
Br1—Li1—Li1^ii	50.17 (13)	H3A—C3—H3B	109.5
Li1^iii—Li1—Li1^ii	83.2 (2)	N2—C3—H3C	109.5
Li1^iv—Br1—Li1^ii	77.55 (16)	H3A—C3—H3C	109.5
Li1^iv—Br1—Li1	114.03 (19)	H3B—C3—H3C	109.5
Li1^ii—Br1—Li1	79.33 (17)

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