Bis(2-methyl-4-nitroanilinium) tetrachloridomercurate(II)

The title compound, (C7H9N2O2)2[HgCl4], self-assembles into cationic organic bilayers containing the 2-methyl-4-nitroanilinium cations, sandwiched between anionic inorganic layers built up by the distorted tetrahedral [HgCl4]2− groups. The organic sheets are interlinked through weak C—H⋯O hydrogen bonds, while they interact with the anionic part via strong charge-assisted N+—H⋯Cl—Hg hydrogen bonds. The [HgCl4]2− anions are bisected by a mirror plane passing through the metal and two of the chloride ions.

The molecular geometry and atomic numbering scheme of (I) are illustrated in Fig. 1. The asymmetric unit contains one 2-methyl-4-nitroanilinium cation and a HgCl 4 2anion, halved by a mirror plane (x, 1/2-y, z) passing through the metal and two of the chlorine ions. The structure consists of alternating, non-interdigitated organic bilayers containing the 2-methyl-4-nitroanilium cations, and inorganic layers containing the isolated (HgCl 4 ) 2anions ( Fig. 2.).
In the organic bilayers the nitro groups pack in the centre of the layer, in a tail-to-tail arrangement, and the aromatic ring plane (C1->C6) forms an angle of 86.3° to the inorganic layer plane. It has been reported by Sharma and Desiraju (1994) that weak C-H···O interactions, with the nitro group as a hydrogen bond acceptor occurs in many unsaturated compounds, despite the fact that the nitro group is not very basic, and it is precisely this type of interaction the one which links both organic layers in (I): atom C3 on the aromatic ring at symmetry position (1/2 -x, 1/2 + y, z -1/2) acts as proton donor while the O1 of nitro group at symmetry position (x, 1/2 -y, z) acts as acceptor, with an H···O distance of 2.52 Å.
The organic and inorganic layers are linked through charge assisted N + -H···Cl-Hg hydrogen bonds, with the hydrogen bonding interactions listed in Table 1. N1 is the only hydrogen bond donor with all three hydrogen atoms involved in hydrogen bonding. Atom H6 is shared by two chlorine atoms (Cl2 at symmetry position: (x, y, z) and Cl3 at symmetry position: (x -1/2, 1/2 -y, 1/2 -z)) and thus forms a bifurcated interaction. Two approximately linear hydrogen bonds are formed through atoms H4 and H5 with Cl1 at symmetry positions (x -1, y, z) and (x -1/2, y, 1/2 -z), respectively. All four chloro ligands on the HgCl 4 2anion act as hydrogen bond acceptors.

Experimental
Compound (I) was prepared by the addition of 0.097 g (0.357 mmol) of HgCl 2 (Aldrich) and 0.102 g (0.333 mmol) of 2-methyl-4-nitroaniline (Aldrich) to 6 ml of 33% HCl. Complete dissolution was obtained after refluxing at 90°C for 12 h in an oil bath. Slow cooling in oil bath over 48 h produced the crystals. A colourless crystal of 0.42 x 1/4x 0.16 mm was used for X-ray data collection.
supplementary materials sup-2 Refinement H atoms were placed geometrically and refined in idealized positions in the riding-model approximation, with C-H 0.95 (ArH) and 0.98 Å (CH 3 ) and N-H = 0.91 Å; U iso (H) = 1.5U eq (N), 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms. The highest residual peaks in the final ΔF syntheses lie at 0.90 Å from Cl3. Fig. 1. Atom labelling scheme of (I) with thermal ellipsoids drawn at the 50% probability level. The Cl atom marked with a prime (') is at symmetry position (1/2 + x, 1/2 -y, 1/2 -z).

Special details
Experimental. Numerical integration absorption corrections based on indexed crystal faces were applied using the XPREP routine 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 Rfactors(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.