Stacking Structure of Quinolinium Hydrogensquarate

A new proton-transfer organic salt, quinolinium hydrogensquarate (C13H9NO4), has been synthesized and fully characterized by single crystal x-ray diffraction. The salt crystallizes in the monoclinic space group P21/n with the parameters: a = 3.8290(12) A, b = 20.960(6) A, c = 13.802(4) A, β = 95.452(5)°, V = 1102.7(6) A3, Z = 4 formula units. The structure consists of uncommon supramolecular neutral dimers which pile up parallel to [100] forming infinite sheets. These centrosymmetric dimers are held together by lateral hydrogen-bonds whereby two neighboring coplanar hydrogensquarate anions act as a bridge between two terminal quinolinium cations and C-H...O bridgings interlink next neighboring sheets. The bulk structure of this salt is consolidated by weak ππ interactions within the sheets which are neatly ordered side-by-side relative to one another.


Introduction
The strategy of self-assembly, which makes use of in-built molecular capabilities to direct stereoselectively the association of chemical building blocks of comparable dimensions, has been widely applied in current research to design and develop new materials that are needed to foster emerging technologies [1][2][3] . In this respect, the vanguard field of onedimensional metals typified by the TTF-TCNQ salt 4 (TTF is tetrathiafulvalene; TCNQ is Tetracyano-p-quinodimethane) and related organic conductors 5 has been at the focus of interest in contemporary research and development.
Our current research program aims to fabricate Heisenberg-type magnetic materials suitable for systematic studies of long-range magnetic interactions as a function of on-site spin densities. Recently, indeed, we isolated quinolinium trans-diaquabis(oxalato) chromate(III) 6 , an organic-inorganic hybrid salt that crystallizes in an interesting structure formed by segregated infinite stacks of diamagnetic organic cations and paramagnetic inorganic complex anions, with a regular spacing of 6.635(1) Å between next neighboring Cr III centers. Herein, we describe the title compound as a useful precursor for the provision of the flat ions needed in forthcoming metathetic syntheses.

Structural determination
Crystal data for the title compound were collected on a Bruker-AXS SMART APEX CCD diffractometer at 293(2) K using Mo Kα graphite monochromated radiation (λ = 0.71073 Ǻ) and an ω-2θ scan mode (1.77° < θ < 23.35°). Data reduction was performed with the use of the Bruker AXS SAINT 7 and SADABS 8 packages. The structure was solved by direct methods and refined by full-matrix least-squares calculations using SHELXS-97 9 , All nonhydrogen atoms were refined anisotropically on F 2 . Hydrogen atoms were calculated isotropically at their fixed positions and allowed to ride on their respective parent atoms. The contributions of these hydrogen atoms were included in structure calculations. Atomic scattering factors and anomalous dispersion corrections were taken from International Tables for x-ray Crystallography 10 . Crystal data and structure refinement for the title compound are summarized in Table 1.

Results and Discussion
An ORTEP 11 plot of the title compound is displayed in Figure 1. This purely organic protontransfer salt is readily soluble in water and it is considered a convenient source for the desirable planar ionic constituents. Selected bond lengths and angles are listed in Table 2. They differ slightly from the values reported for the quinolinium cation 3 and the dinegative squarate anion 12,13 , respectively.   (13) As shown in Figure 2, dimerized formula units are the actual building blocks that generate the lattice network of the title compound. These dimers arise as the result of efficient side-by-side hydrogen bonds (Table 3) interlinking the ions into faintly corrugated neutral supramolecular motifs, with a dihedral angle of 23.97° between the central core of two coplanar hydrogensquarate anions and the two terminal quinolinium cations on both ends. The dimerization process, therefore, may be understood as occurring in two consecutive stages. First the two hydrogensquarate ions are dimerized, forming a dinegative coplanar motif in such a way that the O-H group of one monomer undergoes a hydrogenbond with the deprotonated O atom of the other monomer and vice-versa. Second the N-H group of each quinolinium cation is hydrogen-bonded to one carbonyle O atom of each hydrogensquarate anion. These neutral dimers then stack on top of each other in nearly eclipsed configuration along the a axis with a regular centroid inter-dimer spacing of 3.829 Ǻ, thus forming infinite sheets that run parallel to [100]. From Figure 2, one easily recognizes that the main planes of these sheets are oriented along two crystallographic directions which enclose a dihedral angle of approximately 60°. The intra-sheet spacing of 3.829 Ǻ (a axis) between nearest dimers may be interpreted as resulting from weak π-π interactions between next neighboring dimers. It is worth noting that in this crystal structure, O-H…O bonding is effective only within and not between the dimeric motifs. The bulk crystal structure, however, is stabilized by C-H…O bridgings between the sheets and by the judicious space filling of these infinite sheets which are neatly arranged side-by-side in a manner reminiscent of the relative sheet ordering of the anions in the structure of the [NBu 4 ][Ni(bdt) 2 ] salt 14 , bdt 2being the dianion of benzene-1,2-dithiol.

Conclusion
Quinolinium hydrogensquarate, C 13 H 9 NO 4 , is a novel proton-transfer organic salt which crystallizes in the monoclinic space group P2 1 /n, with the flat ions hydrogen-bonded into centro-symmetric supramolecular dimers; these neutral dimers pile up on top of each other to form sheets running parallel to [100].

Supplementary material
All crystallographic data for this paper were deposited with the Cambridge Crystallographic Data Centre (CCDC-698315