Crystal structure of hemikis(cyclohexane-1,4-diammonium) (pyridine-2-carboxylate), [C6H16N2]0.5[C6H4NO2]

Abstract [C6H16N2]0.5[C6H4NO2], triclinic, P1̄ (no. 2), a = 6.7956(2) Å, b = 7.2840(3) Å, c = 10.2615(4) Å, α = 70.483(3)°, β = 80.902(3)°, γ = 66.404(3)°, V = 438.55(3) Å3, Z = 2, Rgt(F) = 0.0359, wRref(F2) = 0.0988, T = 100(2) K.

CrysAlis PRO [1], SHELX [2,3], WinGX and ORTEP [4] The crystal structure is shown in the figure.Tables 1 and  2 contain details on crystal structure and measurement conditions and a list of the atoms including atomic coordinates and displacement parameters.

Source of material
The title salt was prepared through solvent-drop assisted grinding of 3-mercaptobenzoic acid

Experimental details
The C-bound H atoms were geometrically placed (C-H = 0.95−0.99Å) and refined as riding with U iso (H) = 1.2Ueq(C).
The N-bound H-atoms were located in difference Fourier maps

Discussion
The isomeric Schiff bases appended with pyridyl donors, N,N′-bis((pyridine-n-yl)methylene)cyclohexane-1,4-diamines for n = 2, 3 and 4, that is, of general formula (n-py)-CH=N-C 6 H 10 -N=CH-(n-py) for n = 2-4, were prepared and characterized sometime ago [5,6].Since the original syntheses [5], investigations have focused upon their utility in forming coordination polymers of cadmium [6], cobalt [7] and silver [8,9] salts.To the best of our knowledge, no reports on cocrystals of these Schiff base molecules exist.Recent studies of co-crystals designed to incorporate n-mercaptobenzoic acid as a conformer [10,11] resulted in unexpected outcomes, e.g.oxidation of the thiol, a well known phenomenon for n-mercaptobenzoic acids [12], and desulfurization [13], again observed previously [14].As a follow up to these studies, a co-crystallization experiment between an authenticated [5] sample of (2-py)-CH=N-C 6 H 10 -N=CH-(2-py) and 3-mercaptobenzoic acid was undertaken which lead to the isolation of the title 1:2 salt, that is, [H 3 NC 6 H 10 NH 3 ]: 2[C 6 H4NO 2 ].X-ray crystallography showed the composition of the product isolated from the co-crystallization experiment to comprise a cyclohexane-1,4-diammonium di-cation, located about a center of inversion, and a pyridine-2-carboxylate anion in a general position (Figure, 70% probability displacement ellipsoids with unlabelled atoms related by the symmetry operation − x, 2 − y, 2 − z).The confirmation of the presence of ammonium and carboxylate groups is seen in the pattern of supramolecular aggregation (see below).In addition, the carboxylate-C1-O1, O2 bond lengths are essentially equivalent, being 1.2595(16) and 1.2546(16) Å, respectively.The cation has a chair conformation indicating the 1,4-ammonium groups are anti.The pyridine-2-carboxylate anion is planar as seen in the magnitude of the O1-C1-C2-N1 torsion angle of 2.71(17)°.
In the crystal, significant charge-assisted hydrogen bonding is apparent.Thus, ammonium-N-H• • • O(carboxylate), N(pyridyl) hydrogen bonds cooperate to assemble ions into a supramolecular layer parallel to [0 0 1].The hydrogen bond donated by ammonium-N-H1n atoms is bifurcated, effectively bridging O(carboxylate) and N(pyridyl) atoms of the same anion forming a five-  The most closely related structure in the literature is that of the 2-isomeric structure, i.e. hemikis(cyclohexane-1,4diammonium) (pyridine-2-carboxylate) [15].It is interesting to note that supramolecular layers constructed by hydrogen bonds and connected by π• • • π stacking interactions are also apparent in this crystal.

Table 1 :
Data collection and handling.
but were refined with a distance restraint of N-H = 0.88 ± 0.01 Å, and with U iso (H) set to 1.2U equiv (N).Owing to poor agreement, two reflections, i.e. (1 1 4) and (2 2 5), were omitted from the final cycles of refinement.