Crystal structure of 2,6-diaminopyridinium chloride

The crystal structure of the organic–inorganic hybrid title salt is held together by N—H⋯Cl hydrogen bonds.


Chemical context
Pincer compounds are an important class of chelating ligands, and their metal complexes have attracted tremendous interest due to their high stability, activity, variability and applicability in organic synthesis and catalysis (Szabo & Wendt, 2014). Whereas a plethora of (mostly) precious transition-metal pincer complexes has been reported, information on group 6 pincer complexes is rather scarce. During a project aimed at the preparation and characterization of group 6 PNP pincer compounds (Ö ztopcu et al., 2013;de Aguiar et al., 2014;Mastalir et al., 2016), crystals of the title salt, C 5 H 8 N 3 + ÁCl À , were obtained accidentally through hydrolysis of the employed ligand N,N'-bis(diisopropylphosphino)-2,6-diaminopyridine in the presence of CrCl 3 Á6H 2 O. Here we report on the crystal structure of this salt.

Structural commentary
The cation of the title structure is protonated at the pyridine N atom (Fig. 1). The asymmetric unit comprises half a molecule of the 2,6-diaminopyridinium cation, with a mirror plane running through the pyridinium group (N1-H1N1) and the para-C-H group (C3-H1C3); the Cl À anion is also located on the mirror plane. In agreement with other 2,6-diamino-pyridinium cations, the C-N(H) + -C angle involving the pyridinium group is enlarged [C1-N1-C1 i = 123.37 (8) ; symmetry code: (i) x, Ày, z] whereas the angle between the pyridinium N atom and the C atom in the ortho position (bearing the amino group) and in the meta position is reduced [N1-C1-C2 = 118.83 (6) ]. This situation is reversed in 2,6diaminopyridine due to the non-protonated ring N atom in this structure (Schwalbe et al., 1987). A common feature of the non-protonated 2,6-diaminopyridine molecule and the 2,6diaminopyridinium cation is a significant pyramidalization of the amino N atom. In the title structure, the bond angle sum at this atom (N2) deviates with 349.0 clearly from the expected 360 for an ideal trigonal-planar group. The pyramidalization is also reflected by the dihedral angle of 30.4 (14) between the least-squares planes of the amino group and the non-H atoms of the 2,6-diaminopyridinium moiety.

Supramolecular features
The pyridinium N1-H1N1 group is the donor of a nearly linear hydrogen bond to the Cl À counter anion ( Table 1). The amino group also participates in the formation of N-HÁ Á ÁCl hydrogen bonds, albeit of explicit weaker nature. One hydrogen atom (H2N2) is clearly involved in hydrogen bonding with an H2N2Á Á ÁCl1 distance of 2.63 Å and an N2-H2N2Á Á ÁCl1 angle of 157 . Although the DÁ Á ÁA contact involving the second hydrogen atom, H2N2, is 0.04 Å shorter than that of the other hydrogen bond of this group, the comparatively long H1N2Á Á ÁCl distance of 2.88 Å and the very small N2-H1N2Á Á ÁCl1 angle of 117 give room for interpretation whether or not this is a real hydrogen bond.

Figure 2
Crystal packing of the organic and inorganic components in the title structure in a projection along [001]. N-HÁ Á ÁCl hydrogen bonds involving the pyridinium group are shown as magenta dotted lines and those involving the amino group are shown as orange dotted lines.

Figure 3
Crystal packing of the organic and inorganic components in the title structure in a projection along [100]. The colour code of the intermolecular interactions is as in Fig. 2.

Figure 1
The molecular structure of the cation and the inorganic anion in the title structure. Displacement ellipsoids are drawn at the 50% probability level.

Database survey
A search in the CSD (Groom & Allen, 2014; CSD Version 5.31) revealed 87 different salts containing the 2,6-diaminopyridinium cation, with the majority of cases in the form of organic anions (46 representatives), followed by complex metal anions (31 representatives Kristiansson, 2000). It should be noted that the chemically most related anhydrous Br À salt crystallizes in space group I42d and hence shows no isotypism with the title Cl À salt.

Synthesis and crystallization
N,N'-bis(diisopropylphosphino)-2,6-diaminopyridine (0.2 g, 0.53 mmol) was dissolved in dry tetrahydrofuran (5 ml) under argon atmosphere. CrCl 3 Á6H 2 O (0.134 g, 0.51 mmol) was added and the resulting mixture stirred for 4 h at room temperature. The formed purple solid was filtered off, washed with dry diethyl ether and dried. The solid was redissolved in acetonitrile for crystallization initiated by solvent diffusion with diethyl ether. The title compound grew in the form of yellow crystals as the only solid product. We assume that the Lewis acid CrCl 3 in combination with water is able to cleave the P-N bond of the pincer compound accompanied by an in situ formation of HCl which eventually yields the title compound.