supplementary materials

Squaric acid, H₂Sq, is a very strong dibasic acid and has been studied for potential application to xerographic photoreceptors, organic solar cells and optical recording (Seitz & Imming, 1992; Liebeskind et al., 1993). It is also a useful tool for constructing crystalline architecture because of its rigid and flat four-membered ring framework (Reetz et al., 1994). Squaric acid (H₂Sq) can be found in three forms, viz. (a) as uncharged H₂Sq, (b) as the HSq⁻ monoanion and (c) as Sq²⁻ dianions on deprotonation by amines. These forms have been observed to crystallize with various types of hydrogen bonding, as summarized by Bertolasi et al. (2001). Our ongoing research on metallic and organic salts of squaric acid (Uçar et al., 2004; Uçar et al., 2005; Köroglu et al., 2005), we have synthesized the title compounds, (I), in which the dianion form of squaric acid is observed.

Each squaric acid molecule donates both its H atoms to the pyridine N atom of two 2,2'dipyridylamine molecules, forming the bis(2,2'dipyridiniumamine) squarate dihydrate salt. The C₄O₄²⁻ anion is centrosymmetric. (Fig. 1). The most noteworthy aspect of the structure of (I) concerns the hydrogen bonding (Table 1). Intermolecular hydrogen bonding is conventional (in terms of geometry), with each of the H atoms being donated. However, the N—H···N intramolecular hydrogen bond associated with the pyridine nitrogen atoms has much more unusual behaviour. The freely refined N—H bond length of 1.28 (2)Å is very long for an N—H covalent bond, which would be expected to be around 0.87Å in an X-ray crystallographic analysis. Consequently, the H···N distance is 1.42 (2) Å, whic is rather short. Given that the overall N···N distance is relatively short at 2.586 (2) Å, these values indicate a strong hydrogen bond, which nevertheles display unusual disorder or thermal motion. A difference Fourier map (Fig. 2; Farrugia, 1999) of the electron density associated with this H atom shows this to be smeared out between the dipyridinium N atoms, with the maximum lying closer to the N3 atom. These type hydrogen bonds have been called ^"positive charge assisted hydrogen bonds (N⁺—H···N or N···H—N⁺)^" by Gilli and co-workers [Gilli et al., 1994; Bertolasi et al., 1996; Gilli & Gilli, 2000;] and found in organic amines and carboxylic acids [Steiner et al., 2000; Mathew et al., 2002].

In the crystal of (I), both intermolecular H bonds and van der Waals interactions combine to stabilize the extended structure (Fig. 3). The crystal water molecule and amine nitrogen atom link cations to dianions, acting as hydrogen-bond donors to the squarate oxygen atoms. These interactions mediate the formation of chains in the ac plane (Fig. 3) and adjacent chains are linked by van der Waals interactions.