Hydrogen Bonding of O-Ethylxanthate Compounds and Neutron Structural Determination of C-H•••S Interactions

A range of ethylxanthate (EtXn) salts, containing either protic or aprotic cations (guanidinium (1), methylammonium (2), dimethylammonium (3), trimethylammonium (4), tetramethylammonium (5), tetraethylammonium (6), and tetrapropylammonium (7)), have been synthesised and structurally characterised. The cations in these compounds differ in their degree of hydrogen-bonding ability, i.e. the number of donor groups, with significant structural consequences. Compounds 1-4 contain cations that are able to form N-H center dot center dot center dot S hydrogen bonds, with six, three, two, and one donor groups in 1-4 respectively. The number of donor atoms affects greatly the dimensionality of the hydrogen-bonding networks in the solid state. The structure of 1 has a 3-D hydrogen-bonding network, 2 and 3 form 2-D sheets and 1-D chains respectively, whereas the lone NH donor group in 4 has strong hydrogen bonding only within a discrete cation-anion pair. The tetraalkylammonium salts 5-7 have no strong hydrogen bonding, with only C-H center dot center dot center dot S and C-H center dot center dot center dot O interactions possible. To determine unambiguously the presence of such interactions, single-crystal Laue neutron diffraction data were obtained for compound 5, providing a fully anisotropic model, which can be used to rationalise potential close interactions in the other structures. The neutron structure of 5 confirms the existence of C-H center dot center dot center dot S hydrogen bonds, with the H center dot center dot center dot S distance falling well within the sum of the van der Waals radii of the atoms. The close-packing in 5-7 is mediated solely through these weak interactions, with the size of the cations influencing the structures.