2,6-Diaminopyridinium tetraphenylborate–1,2-bis(5,7-dimethyl-1,8-naphthyridin-2-yl)diazene (1/1)

In the title compound, C5H8N3 +·C24H20B−·C20H18N6, the 1,2-bis(5,7-dimethyl-1,8-naphthyridin-2-yl)diazene molecule is essentially planar (r.m.s. deviation = 0.0045 Å) and aligned in nearly coplanar manner with the 2,6-diaminopyridinium ion, making a dihedral angle of 5.19 (5)°. The diaminopyridine molecule is protonated on the central pyridine N atom and the B atom bears the counter-charge. The amine groups of the diamino pyridinium cation form intramolecular N—H⋯N hydrogen bonds, resulting in linear and bent interactions with the naphthyridine ring system.


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We thank the University of Western Ontario and NSERC for their generous support of this research.

Comment
In the context of utility, 1,8-Naphthyridine derivatives are found to be valuable drugs and with a wide variety of pharmacological applications. They are effective fungicides and known for their antimycobacterial activity. Recent studies have revealed their ability in treatments of diabetes and related disorders. Herein, we report the crystal structure of the title compound C 49 H 46 BN 9 that has almost coplanar naphthyridine and pyridinium moieties and an almost perfect tetrahedral borate ion.
The main forces of attraction here are hydrogen bonding between the acceptor atoms, N of naphthyridine unit and the donor N-H atoms of the diaminopyridinium ion. There is also a π-π stacking interaction between adjacent parallel naphthyridyl rings. The extensions due to these interactions form the three dimensional π-stacked network structure as shown in figure 2.
Within the bis-naphthyridine molecule, the plane of the naphthyridine ring system consisting of N1 and N2 nitrogen atoms is slightly deviated from that of the second naphthyridine ring system consisting of N5 and N6 nitrogen atoms by an angle of 5.300 (4). The torsion angle between the N2, C9, N3 and N4 atoms is -176.514 (280)° indicative of an anti conformation and the torsion angle between the N5, C11, N4 and N3 is 4.165 (441)° indicating the syn conformation of the azo function with each naphthyridine ring system. The diaminopyridinium cation is complexed to the bis (1,8-naphthyridine) in an unsymmetrical fashion via hydrogen bonding and ion-dipole bonding. The hydrogen bonding in the complex displays head-on and bent geometries. The hydrogen bond distances are N1···N9 = 2.887 (4) Å, N2···N8 = 3.175 (4) Å, N3···N7 = 3.084 (4) Å and N6···N7 = 3.304 (4) Å with NH···N bond angles 169.989 (211)°, 177.236 (184)°, 176.497 (194)° and 150.139 (188)° respectively. The hydrogen bond distances range from 2.887 (4) Å to 3.304 (4) Å within the complex. Apart from hydrogen bonding, the naphthyridine moieties interact with the diamino pyridinium cations of adjacent complexes by π-π interactions of their terminal rings. The distance between the centroid of the N1, N2 naphthyridine ring to the plane of the N5, N6 naphthyridine ring and the distance between the centroid of the diamino pyridinium cation to C17 atom which are 3.447 (1) Å and 3.412 (3) Å respectively, strongly indicate π-π interactions. The complexes are set in a columnar arrangement with a distance of 15.531 (1) Å along a axis and 20.350 (1) Å along b axis between the centroids of the columns. The interstices of the columns are occupied by the tetraphenylborate anions. Four phenyl rings complete the slightly distorted tetrahedral geometry around each boron atom and appears non-interactive with the rest of the complex.
supplementary materials sup-2 Synthesis of 2,6-Diaminopyridinium tetrakisphenylborate: The synthesis of (C 5 H 8 N 3 + [BPh 4 -]) was carried out by adding solution of sodium tetrakisphenylborate (1.65 g, 3.45 mmol, 5 eq.) in water (5 ml) to a a solution of 2,6-di aminopyridine hydrochloride (100 mg, 0.69 mmol) in water (5 ml). After stirring the solution at room temperature for 20 minutes, the resulting precipitate was filtered and washed with small aliquots of water (4x 3 mL) and dried to yield the pure title complex.