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.


Synthesis and crystallization
Me 2 NCN (0.76 mL, 9.38 mmol) was added to a solution of (Dipp)N(Li)SiMe 3 (0.60 g, 2.35 mmol) in Et 2 O (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 Et 2 O (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 C 6 H 12 Br 2 Li 2 N 4 (%): 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. Et 2 O was dried and distilled over Na under argon prior to use. Elemental analysis is completely in agreement with the structure of the compound.

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).
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)   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

Figure 2
The expanded step polymeric structure of (I) viewed along the crystallographic c axis.

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (2) Geometric parameters (Å, º)