N-Imidazole–boron trichloride adduct

The title compound, (I), was obtained as a colourless powder during an attempt to synthesize a product of formula B2S3 from the reaction of BCl3 with (Me3Si)2S (containing trace amounts of imidazole as a stabiliser). Recrystallization yielded crystals suitable for a diffraction study. The molecular structure of (I) is shown in Fig. 1, and selected bond lengths and angles are presented in Table 1.


Comment
The title compound, (I), was obtained as a colourless powder during an attempt to synthesize a product of formula B 2 S 3 from the reaction of BCl 3 with (Me 3 Si) 2 S (containing trace amounts of imidazole as a stabiliser). Recrystallization yielded crystals suitable for a diffraction study. The molecular structure of (I) is shown in Fig. 1, and selected bond lengths and angles are presented in Table 1.
The crystal structure of (I) may be viewed as a cross-linked hydrogen-bonded ribbon polymer (see Fig. 2). The N2-H2A donor of the imidazole makes a weak hydrogen bond with atom Cl1 in a neighbouring molecule. This interaction is supplemented by a weak interaction between C2-H2 and Cl3 of the same molecule. Although such an interaction might seem dubious, it is possible that C2 and N2 are disordered with respect to each other, leading to a disordered hydrogen bond between Cl1 or Cl3 and the two chemically feasible NH positions on the imidazole. Attempts to model this disorder were unsuccessful. A slightly stronger interaction between the N2-H2A donor and Cl2 of another neighbouring molecule cross-links the ribbons to give the overall structure.

Experimental
BCl 3 (1.0 M solution in heptane, 0.2 ml, 0.2 mmol) was added to a solution of (Me 3 Si) 2 S (0.57 ml, 0.3 mmol) in hexane (10 ml), resulting in the immediate formation of a colourless precipitate. The solution was stirred for 24 h, whereupon the solvent was removed by syringe and the resultant colourless solid was washed with hexane (3 Â 10 ml) and dried. The solid was then redissolved in CH 2 Cl 2 (10 ml), placed in a fresh Schlenk tube, layered with hexane (7 ml) and refrigerated at 243 K overnight, resulting in the formation of large colourless crystals (yield: 0.0056 g, 6%  Table 1 Selected geometric parameters (Å , ).  Hydrogen-bonding geometry (Å , ). H atoms were constrained to ideal geometries (C-H = 0.95 Å ) and refined with displacement parameters equal to 1.2 times U eq of their parent atom.
We thank the EPSRC for financial support.  The crystal structure of the title compound, viewed as a series of crosslinked hydrogen-bonded ribbon polymers. [Symmetry codes: (i) x, y À 1, z; (ii) 1 + x, y À 1, z.]

Figure 1
The molecular structure of the title compound, showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level. 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )