Preparation, thermal behavior, luminescent properties, and crystal structures of aminoguanidinium 2,n-pyridine- dicarboxylate (n 5 3, 4, 5, and 6) salts S. Packiaraj • A. Pushpaveni • C. Senthil • S. Govindarajan • J. M. Rawson Received: 30 April 2014 / Accepted: 15 August 2014 / Published online: 11 September 2014 Ó Akade ´miai Kiado ´, Budapest, Hungary 2014 Abstract The reaction of 2,n-pyridinedicarboxylic acids (H 2 pydc) (n = 3, 4, 5, and 6) with the base guanylhydr- azine bicarbonate [(H 2 N) 2 C=N(H)NH 2 ][HCO 3 ] in different mole ratios has yielded three types of salts, viz. the 1:1 salt, [(H 2 N) 2 C=N(H)NH 2 ][Hpydc] (1), the 2:1 salt [(H 2 N) 2- C=N(H)NH 2 ] 2 [pydc] (2), and the 1:3 complex [(H 2 N) 2- C=N(H)NH 2 ][Hpydc].2[H 2 pydc] (3) in which the 1:1 salt co-crystallizes with the acid. These salts have been char- acterized by IR, thermal analysis, and luminescence prop- erties, and their antioxidant activity has been investigated. The structures of 1 (n = 3 and 6) have been determined by X-ray crystallography. The quinolinate salt [(H 2 N) 2- C=N(H)NH 2 ][Hpydc] (n = 3) crystallizes as a monohy- drate whereas the dipicolinate salt [(H 2 N) 2 C=N(H)NH 2 ] [Hpydc] (n = 6) crystallizes without lattice water. The structure of the quinolinate derivative reveals a conven- tional mono-deprotonated dicarboxylic acid, NC 5 H 3 (COOH)(COO - ), whereas the dipicolinate derivative exhibits an unusual zwitterionic anion, HNC 5 H 3 (COO - ) 2 . These structures are stabilized through extensive hydrogen bonding and p–p stacking. Both the parent acids and their salts display strong fluorescent emission at room temperature. Keywords 2,n-Pyridinedicarboxylic acid Á Guanyl hydrazine Á Thermal behavior Á Crystal structure Á Luminescence property Á Antioxidant activity Introduction Aminoguanidine, Agun, also known as guanylhydrazine exhibits a versatile basic character. It is dibasic in a similar fashion to hydrazine, forming both ?1 and ?2 cationic salts, [AgunH] ? and [AgunH 2 ] 2? . The aminoguanidinium(?1) salts [AgunH] ? of various anions such as Cl - , NO À 3 , SO 2À 3 , and H 2 PO À 4 have been prepared and their crystal structures have been reported [1–5]. In these structures, the imine nitrogen appears as the most basic site (Scheme 1). The crystal structures of dicationic species of aminoguanidine such as aminoguanidinium(?2)-sulfate and di-nitrate salts have been well characterized [6–8]. In this latter series, not only the imine nitrogen, but also the hydrazinic nitrogen becomes protonated (Scheme 1). A structural study on the commercially available aminoguanidine bicarbonate was found to be a zwitterionic 2-guanidinium-1-amino-carbox- ylate hydrate [9]. While there are a number of preparative and computational studies on the syntheses and energetics of aminoguanidinium salts with inorganic acids [10], a few aminoguanidinium salts of organic acids have been repor- ted. These include a handful of organic salts [10–15] viz. aminoguanidinium hemi-oxalate [11], hydrogen-L-tartrate monohydrate [12], and squarate [13]. These studies reveal that the aminoguanidinium ions in these salts exist in the monoprotonated form, AgunH ? . However, there are no reports on salts of aromatic carboxylic acids in general and Electronic supplementary material The online version of this article (doi:10.1007/s10973-014-4098-3) contains supplementary material, which is available to authorized users. S. Packiaraj Á A. Pushpaveni Á C. Senthil Á S. Govindarajan (&) Department of Chemistry, Bharathiar University, Coimbatore 641 046, India e-mail: drsgovind@yahoo.co.in S. Packiaraj e-mail: srajguru1987@gmail.com J. M. Rawson Department of Chemistry and Biochemistry, The University of Windsor, Windsor, ON N9B3P4, Canada 123 J Therm Anal Calorim (2015) 119:15–25 DOI 10.1007/s10973-014-4098-3