(2R,2′S)-2,2′-Bipiperidine-1,1′-diium dibromide

The title compound, C10H22N2 2+·2Br−, was synthesized via reduction of 2,2′-dipyridyl with Ni–Al alloy/KOH, followed by separation of diastereoisomers (meso and rac) by recrystallization from ethanol. Although the two bridging C atoms are optically active, these two chiral centers adopt an (S,R) configuration; thus, the title compound contains an achiral meso form of 2,2′-bipiperidine. Both of the piperidinium rings adopt chair conformations, and the two N atoms are trans to each other; an inversion center is located in the mid-point of the central C—C bond. The conformation of the organic moiety resembles that of 1,1′-bi(cyclohexane). The organic diammonium cations are linked to each other through hydrogen bonding with bromide counter-ions, each of which forms two hydrogen bonds (N—H⋯Br) with two adjacent organic cations, thus linking the latter together in sheets parallel to (100).


Comment
N-substituted 2-picolyl derivatives of the title compound, and related tetradentate aminopyridine ligands (Mikhalyova et al., 2012;Lyakin et al., 2012), in the form of Fe(II) and Mn(II) complexes, are good catalysts for selective olefin epoxidation, as well as aromatic or aliphatic hydroxylation reactions, with peroxides or peracids as oxidants. Rigid cyclic diamine scaffolds increase thermodynamic and kinetic stability of the catalysts and orient picolyl groups in the pseudooctahedral complexes which also contain two labile solvent-occupied sites essential for hydrogen peroxide activation (Mikhalyova et al., 2012). Understanding the molecular structure of these diamine scaffolds is important for catalyst design.
The title compound, which consists of organic diammonium cations and bromide counteranions (Figure 1), crystallizes in the monoclinic space group C2/c. The structure of the organic moiety resembles that of 1,1′-bi(cyclohexane), with both piperidinium rings adopting chair conformations. Two nitrogen atoms are trans to each other. The least square planes passing through the heavy (non-hydrogen) atoms of the two piperidinium rings in the centrosymmetric molecule are coplanar. This conformation differs from the twisted orientation of the piperidinium rings in the racemic isomer of the title compound (the angle between their least-square planes was found to be 77 °) (Laars et al., 2011). Two stereocenters (bridging carbon atoms, C1 and C1′) adopt different configurations (R and S, respectively) and are symmetry-related. The centrosymmetric meso form of C 10 H 22 N 2 Br 2 described herein is achiral.
The organic diammonium cations are linked to each other through hydrogen bonding with bromide counter ions, each of which forms two hydrogen bonds (N-H···Br) with two adjacent organic cations, thus linking the latter together in sheets parallel to the (100) plane ( Figure 2).

Experimental
The synthesis of the title compound followed the procedure published by Herrmann et al. (2006). 2,2′-Dipyridyl (8.25g, 0.053mol, in 100ml methanol) was mixed with 3M aqueous solution of KOH (16.8g, 0.3mol); Raney type Ni/Al alloy (1:1, v/v, 30g) was then added in portions and this mixture was stirred and refluxed for 4 days. The solution was separated from nickel mud, concentrated and extracted with 15ml dichloromethane (DCM) 3 times. The combined DCM extracts were evaporated to a faint yellow oil, which was then dissolved in 20ml ethanol. 9 ml of concentrated hydrobromic acid were added dropwise into the ethanolic solution; clear colorless hexagonal plate crystals of meso-C 10 H 22 N 2 Br 2 were formed upon slow evaporation of the solution (one of these crystals was selected for X-Ray diffraction analysis).

Refinement
Except where noted, hydrogen atoms were placed at calculated geometries and allowed to ride on the position of the parent atom. C-H distances were constrained to 1.00 Å for tertiary CH groups and to 0.99 Å for methylene groups (CH 2 ).
Isotropic thermal parameters were set to 1.2× the equivalent isotropic thermal parameter of the parent atom. Hydrogens H1A and H1B, bound to N1, were located in difference density Fourier synthesis maps. The positions of these two H atoms were freely refined. Isotropic thermal parameters for H1A and H1B were set to 1.2× the equivalent isotropic U of N1.

Figure 1
A view of the title compound, with displacement ellipsoids shown at the 50% probability level. Symmetry transformations used to generate equivalent atoms: (i) -x + 1/2, -y + 3/2, -z + 1.  A fragment of the packing diagram of the title compound, with displacement ellipsoids shown at the 50% probability level (H atoms, except H atoms attached to N1 atom, are omitted for clarity). Symmetry transformations used to generate equivalent atoms: (i) -x + 1, -y + 1, -z; (ii) x, -y + 1, z + 1/2; (iii) -x + 1/2, y, -z + 1/2. Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.