Bis(6-methoxy-1-methyl-2,3,4,9-tetrahydro-1H-β-carbolin-2-ium) tetrachloridozincate(II) dihydrate

The asymmetric unit of the title compound, (C13H17N2O)2[ZnCl4]·2H2O, contains two tetrahydroharmine cations, one tetrachlorozincate(II) anion and two water molecules. In the cations, the two 1H-indole ring systems are essentially planar, with maximum deviations of 0.016 (2) and 0.018 (2) Å, and both tetrahydropyridinium rings show a half-chair conformation. The ZnII complex anion has a distorted tetrahedral geometry. In the crystal, intermolecular N—H⋯O, N—H⋯Cl, O—H⋯O, O—H⋯Cl and C—H⋯O hydrogen bonds link the components into a three-dimensional network. A π–π interaction with a centroid–centroid distance of 3.542 (14) Å is also observed.

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009  The metal complexes of the 6-methoxy-1-methyl-4,9-dihydro-3H-β-carboline and other carboline alkaloids was previously reported to have biological activity (Al-Allaf et al., 1990). It is now well established that these classes of beta carboline alkaloids may occur under mild conditions in foods from a Pictet Spengler condensation of indoleamines such as L-tryptophan and short aliphatic aldehydes (Herraiz et al., 2003). Our present work intend to synthesize the titled compound and prepare it in salt form to investigate its safety and antiproliferative efficacy in cancer cell line.

Experimental
2M solution of hydrochloric acid in methanol was prepared by transferring 1.68 ml of 37% concentrated HCl acid into a 10 ml volumetric flask and was topped up to volume using methanol. The ZnCl 2 (1 mmol, 136 mg) was weighed and 0.50 ml of 2M hydrochloric acid in methanol was added. Sonication was performed to aid dissolution. 6-methoxy-1methyl-4,9-dihydro-3H-β-carboline (2.5 mmol, 540 mg) was weighed and 0.50 ml HPLC grade methanol was added. The solution was heated on water bath to facilitate dissolution. 0.50 ml of the ZnCl 2 solution in 2M hydrochloric acid was then added to the 6-methoxy-1-methyl-4,9-dihydro-3H-β-carboline solution in methanol dropwise in a glass bottle. The side of the glass bottle was scratched with a small spatula and the bottle was kept in fridge at 4 °C for 5-7 days before yielding the colourless crystals of Bis(6-methoxy-1-methyl-4,9-dihydro-3H-β-carbolinium) tetrachloridozincatedihydrate which were filtered off, washed twice with acetone and air-dried. Crystals of the title compound, suitable to X-ray diffraction analysis, were selected directly from the samples as prepared.

Refinement
O and N bound H atoms are located from a difference Fourier map and refined using a riding model. The remaining H atoms were positioned geometrically and refined using a riding model with C-H = 0.95-1.00 Å and U iso (H) = 1.2 or 1.5U eq (C-methyl). A rotating group model was applied to the methyl groups.

Figure 1
The molecular structure of the title compound, showing 50% probability displacement ellipsoids. Hydrogen atoms are shown as spheres of arbitrary radius.

Bis(6-methoxy-1-methyl-2,3,4,9-tetrahydro-1H-β-carbolin-2-ium) tetrachloridozincate(II) dihydrate
Crystal data (C 13 (Cosier & Glazer, 1986) operating at 100.0 (1) K. 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.