(Z)-2,2,2-Trichloro-N 2-cyanoacetamidine

The title compound, C3H2Cl3N3, crystallizes as the Z isomer with respect to the C=N bond. The –C(NH2)=NCN functional group is effectively planar (r.m.s. deviation = 0.016 Å), with only the three Cl atoms out of the molecular plane. A strong network of N—H⋯N hydrogen bonds forms dimers which are associated into ribbons in the crystal structure. Hydrogen bonding is suspected to be the cause of the near-equivalence of the formal C—N and C=N bonds (ΔCN = 0.008 Å)

The title compound, C 3 H 2 Cl 3 N 3 , crystallizes as the Z isomer with respect to the C N bond. The -C(NH 2 ) NCN functional group is effectively planar (r.m.s. deviation = 0.016 Å ), with only the three Cl atoms out of the molecular plane. A strong network of N-HÁ Á ÁN hydrogen bonds forms dimers which are associated into ribbons in the crystal structure. Hydrogen bonding is suspected to be the cause of the near-equivalence of the formal C-N and C N bonds (Á CN = 0.008 Å )

Comment
The stucture of the title compound, (I), is shown in Fig. 1. Molecular dimensions are available in the archived CIF. Structure (I) crystallizes as the Z isomer with respect to the imino bond (Fig. 1). The structure is essentially planar except for the CCl 3 group (r.m.s. mean deviation for the -C(NH 2 )═NCN group is 0.016 Å), while Cl2 is almost perpendicular to this plane; thus Cl1 deviates by 0.65 and Cl3 by 0.84 Å from the plane. The parameter Δ CN = d(C-N) -d(C═N) has been found to range between 0 and 0.178 Å for many amidines for which the structures are known (Boeré et al., 1998). For (I), Δ CN = 0.008 Å, which is very small for a monomeric amidine with such unsymmetrical substitution. The N2-C3-N3 angle is almost linear, at 172.16 (13)°. There is a network of N-H···N hydrogen bonds (Table 1) (Ishida et al., 1989) and (E)-1,2-bis(1-amino-1-(cyanoimino)-2-methylprop-2-yl)diazene-1,2-dioxide, (IV) (Tretyakov et al., 2006), which all bear the NH 2 group in addition to the nitrile on N'. Each of these structures shares the high degree of planarity of the -C(NH 2 )═NCN group (r.m.s. deviations for (II) -(IV) are 0.008, 0.025 and 0.069 Å, respectively.) Of these three examples, (II) is E while (III) and (IV) are both Z; note that (II) and (III) differ only in methylation at a very remote amino group. There is only one acyclic trichloromethyl amidine with a crystal structure reported in the literature, viz. N-(4-amino-3-furanzanyl)-2,2,2-trichloro-N-methoxyacetamidine, (V) (George & Gilardi, 1986) and this is the Z isomer. The structure of (IV), which is arguably the most similar structure, electronically and chemically, to (I) also shows a very similar pattern of hydrogen bonding where centrosymetric dimers are linked in ribbons within the crystal structure by additional hydrogen bonds.
Key geometrical parameters for structures (I) -(V) are compared in Table 2, which includes values for Δ CN , all of which fall within the known range. However, (II) and (III) are highly unusual in having the wrong sign for this parameter. That is, the imino bond is actually longer than the amino. We are not aware of other instances of this occurrence; the locations of the NH 2 hydrogen atoms in both structures were corroborated by expected hydrogen bonding. It is likely that this powerful hydrogen bonding is responsible for the inversion in expected bond distances, perhaps augmented by the strong electron-withdrawing cyano subsituent on N'. Preparation of methyl trichloroacetimidate: 50 ml of dried methanol and 21.66 g (150 mmol) of trichloroacetonitrile were added to 0.50 g (10 mmol) of sodium methoxide. After stirring for 48 h at room temperature, the solution was saturated with CO 2 (s) to eliminate remaining sodium methoxide. Methanol was then distilled off at 335-7 K, whereafter the liquid methyl trichloroacetimidate was distilled at 415 K at a reduced pressure. Yield 19.59 g (110 mmol, 74%).
Preparation of 2,2,2-trichloro-N'-cyano-acetamidine: 0.42 g (10 mmol) of cyanamide was dissolved in 5 ml of anhydrous methanol. With stirring, 1.76 g (10 mmol) of methyl trichloroacetimidate was added dropwise. An ice bath may be required to maintain temperature during addition of methyl trichloroacetimidate. The solution was stirred for 3 h at RT. Methanol was removed by rotary evaporation followed by high vacuum. The solid residue was dissolved in a minimum volume (3.5 ml) of hot CH 3 CN, cooled to room temperature, and placed within the 238 K freezer. The colourless crystals produced were filtered and vacuum dried yielding 0.121 g (0.649 mmol, 6.51% yield), mp 433-7 K (Huffman & Schaefer, 1963).

Refinement
Both H atoms were located in a difference Fourier map. They were refined using a riding model and U iso (H) was set equal to 1.2U eq (N1). The highest residual peak has a fraction of the electron density of a single H atom and is located 0.76 Å from Cl1. Fig. 1. A view of (I), plotted with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius.