(S)-2-Methylpiperazinediium dichloride 0.42-hydrate

The cations and anions of the chiral title compound, C5H14N2 2+·2Cl−·0.42H2O, are linked by N—H⋯Cl hydrogen bonds into chains propagating in [100], which contain R 4 2(14) loops.


Experimental
I thank the EPSRC National Crystallography Service (University of Southampton) for the data collection.

Comment
The title compound, (I), is a chiral molecular salt, in which the organic species has accepted two protons from the hydrochloric acid. The geometrical parameters of the C 5 H 14 N 2 2+ dication ( Fig. 1) are similar to those of the same speies in other structures (Muller et al., 2005;Tuel et al., 2002) and its six-membered ring is a typical chair. The C4 stereogenic centre has S configuration and the pendant C5 methyl group occupies an equatorial position with respect to the ring.
In the crystal of (I), the cations and anions are linked by N-H···Cl hydrogen bonds (Table 1)

into chains propagating in
[100], with two chloride ions bridging each dication, as shown in Fig 2. In terms of graph theory (Bernstein et al., 1995), R 4 2 (14) loops arise from this connectivity.
The O1 water molecule is partially occupied in the crystal of (I), although there is no obvious crystallographic reason (e.g. symmetry generated close contacts) as to why this should be the case. Based on short O···Cl contacts of less than 3.5 Å, the water molecule probably participates in O-H···Cl hydrogen bonds thereby helping to crosslink the [100] chains, but the water H atoms could not be found or placed unambiguously in the present study.

Experimental
Equimolar quantities of 0.1 M aqueous (S)-2-methylpiperazine and 0.1 M aqueous hydrochloric acid were mixed, leading to a clear solution. Colourless plates of (I) grew as the water slowly evaporated.

Refinement
When refined with full occpancy, atom O1 showed an excessively large U iso value of 0.15 Å 2 . Its fractional site occupancy was refined and rapidly converged to 0.420 (11) with a more reasonable U iso value and improvement in fit. Its U ij values were subsequently refined and converged without difficulty. Its presumed attached H atoms could not be located from difference maps in the present study. Attempts at geometrical placement were ambiguous, as there are several possible O···Cl contacts that might correspond to O-H···Cl hydrogen bonds.
The other hydrogen atoms were geometrically placed (C-H = 0.95-0.99 Å, N-H = 0.92 Å) and refined as riding with U iso (H) = 1.2U eq (C,N) or 1.5U eq (methyl C). The methyl group was allowed to rotate, but not tip, to best fit the electron density.
The highest difference peak is 0.73Å from H1. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (