\title{Proton-transfer and non-transfer in compounds of quinoline and quinaldic acid with L-tartaric acid}

\author{Graham Smith \\ Urs D. Wermuth \\ Jonathan M. White}

\begin{abstract}
The structures of two compounds of L-tartaric acid with quinoline, viz. the proton-transfer compound quinolinium hydrogen (2R,3R)-tartrate monohydrate, C$_9$H$_8$N$^+\cdot$C$_4$H$_5$O$_6^-$\,$\cdot$H$_2$O, (I), and the anhydrous non-proton-transfer adduct with quinaldic acid, bis(quinolinium-2-carboxylate) (2R,3R)-tartaric acid, 2C$_{10}$H$_7$NO$_2\cdot$C$_4$H$_6$O$_6$, (II), have been determined at 130 K. Compound (I) has a three-dimensional honeycomb substructure formed from head-to-tail hydrogen-bonded hydrogen tartrate anions and water molecules. The stacks of $\pi$-bonded quinolinium cations are accommodated within the channels and are hydrogen bonded to it peripherally. Compound (II) has a two-dimensional network structure based on pseudo-centrosymmetric head-to-tail hydrogen-bonded cyclic dimers comprising zwitterionic quinaldic acid species which are interlinked by tartaric acid molecules.
\end{abstract}

\section{Comment}
Tartaric acid is a relatively strong diprotic chiral $\alpha$-hydroxy acid ($pK_{a1}=2.93$ and $pK_{a2}=4.23$) and, therefore, is potentially capable of forming both 1:1 and 1:2 proton-transfer salts with most Lewis bases. However, with stoichiometric control it is possible to selectively form 1:1 hydrogen tartrates, and the crystal structures of a large number of these 1:1 salts have been reported, particularly since these compounds usually have good crystal morphology, allowing structure determination by single-crystal X-ray analysis, which is often not possible with the parent Lewis base. Applications have been with drugs such as epinephrine \cite{Carlstrom_1973}, dextromoramide \cite{Bye_1975}, amsulolal \cite{Furuya_1989}, alprenolol \cite{Glowka_Codding_1989}, phendimetrazine \cite{Glaser_1994} and tolterodine \cite{Kosutic-Hulita_Zegarec_2005}, as well as natural products such as alkaloids, e.g. strychnine \cite{Gould_1987}, brucine \cite{Smith_Wermuth_White_2006}, quinine \cite{Ryttersgaard_Larsen_2004}, cinchonine \cite{Puliti_2001}, cinchonidine \cite{Ryttersgaard_Larsen_2003,Zhang_2003} and quincoridine \cite{Kania_2004}, and amino acids, e.g. L-alanine \cite{Rajagopal_2002}, L-proline \cite{Subha_Nandhini_2001}, D-, L- and DL-histidine \cite{Marchewska_2003,Rajagopal_2003,Johnson_Feeder_2004a,Johnson_Feeder_2004b,Johnson_Feeder_2004c}, and L-lysine \cite{Debrus_2005}.

Because of the ready availability of L-(+)-tartaric acid, which has the confirmed 2R,3R absolute configuration, it has been very useful for both resolution and the crystallographic determination of absolute configuration in chiral molecular species, e.g. with the anticholinergic agent R(-)-1,1-diphenyl-3-piperidinobutan-1-ol \cite{Schjelderup_1990}. More recently, its utility as an agent for the introduction of chirality in achiral organic compounds for the generation of crystalline materials with potentially useful non-linear optical properties has been explored \cite{Aakeroy_1992,Fuller_1995,Marchewska_2003,Debrus_2005,Manivannan_2006}.

We have found that 1:1 stoichiometric interactions of the relatively strong carboxylic acids 5-sulfosalicylic acid (5-SSA) \cite{Smith_2004} and 3,5-dinitrosalicylic acid (DNSA) \cite{Smith_Wermuth_Healy_White_2006} with a series of bicyclic heteroaromatic amines, including quinoline, tetrahydroquinoline, 8-hydroxyquinoline, 8-aminoquinoline and

\begin{figure}
\centering
\includegraphics{figure_1.pdf}
\caption{The molecular configuration and atom-numbering scheme for the quinolinium cation, the hydrogen L-tartrate anion and the solvent water molecule in (I). Non-H atoms are shown as 50\% probability displacement ellipsoids.}
\label{figure_1}
\end{figure}