Crystal Structure of 2-(Pyridin-2-ylamino)pyridinium Trifluoromethanesulfonate

The compound 2-(pyridin-2-ylamino)pyridine, known as di-2pyridylamine (Hdpa), has been widely used as a bidentate ligand with various transition metal ions. The first crystal structure of a complex with the Hdpa ligand, which was a Cu(II)-Hdpa complex, [Cu(Hdpa)2](ClO4)2, was reported by J. E. Jonson, T. A. Beineke, and R. A. Jacobson in 1971.1 They then also determined the crystal structure of uncoordinated Hdpa.2 Up to now many chemists have revealed many crystal structures of transition metal complexes with Hdpa,1,3,4 and a few crystal polymorphisms of uncoordinated Hdpa.5,6 Moreover, about eighty crystal structures of compounds containing a protonated Hdpa cation, H2dpa, 2-(pyridin-2-ylamino)pyridinium, are reported, such as (H2dpa)2[CoCl4] by F. A. Cotton and co-workers in 1998,3 and (H2dpa)(NCS)·CH3CN by B. Schmitt and co-workers in 2011,7 as well as we know. We determined the crystal structure of a 2-(pyridin-2-ylamino)pyridinium trifluoromethanesulfonate, H2dpa(OTf ) (OTf– = CF3SO3, Fig. 1). Single crystals of H2dpa(OTf ) were obtained from a thermal reaction of an aqueous solution of cis-[Co(CO3)(Hdpa)2](OTf ) in the presence of an excess amount of (HOTf )aq. Although we expected the reaction to afford cis-[Co(Hdpa)2(OH2)2](OTf )3, an unexpected decomposition reaction occurred to afford free Hdpa, of which the reaction with excess HOTf afforded H2dpa(OTf ). Colorless crystals of H2dpa(OTf ) suitable for X-ray crystallography were obtained by allowing the reaction mixture to evaporate under reduce pressure. The crystals of H2dpa(OTf ) were also obtained from the mixture of an ethanol solution of Hdpa with HOTf at 278 K without stirring for several days. X-ray crystallography was performed at 173 K on a Rigaku Mercury70 diffractometer using graphite-monochromated MoKα radiation. The structure was solved by direct methods (SIR2014), and refined by full-matrix least-squares techniques with anisotropic thermal parameters for all non-hydrogen atoms. The C–H hydrogen atoms and the N–H hydrogen atom of an amino group were located in the difference Fourier map, and their coordinate and isotropic thermal parameters were refined. The H7a and H7b hydrogen atoms associated with N2 and N3, 2021 © The Japan Society for Analytical Chemistry