X-ray, spectroscopic and antibacterial activity studies of the 1:1 complex of lasalocid acid with 1,1,3,3-tetramethylguanidine

The crystal structure of the 1:1 complex between lasalocid acid (LAS) and 1,1,3,3-tetramethylguanidine (TMG) with one inclusion acetone molecule is studied by X-ray diffraction, FT-IR spectroscopy, 1H and 13C NMR. The complex is stabilized by three intra- and two inter-molecular hydrogen bonds formed between LAS anion and protonated TMG molecule. The NH2+ protons of the protonated TMG molecule are hydrogen bonded with the etheric oxygen atom O(6) and the hydroxyl oxygen atom O(8) of the LAS anion. The intermolecular NH⋯O hydrogen bonds are relatively long (2.933(4) Å and 2.903(4) Å). One oxygen atom of the carboxylate group is involved in a relatively strong intramolecular quasi-aromatic O(1)–H⋯O(3) hydrogen bond of 2.428(4) Å length, and the second oxygen atom in the bifurcated intramolecular relatively weak O(4)–H⋯O(2) of 2.803(4) Å and O(8)–H⋯O(2) of 2.805(4) Å hydrogen bonds. The O(4)–H⋯O(2) and O(8)–H⋯O(2) hydrogen bonds bind the ends of the LAS anion forming a pseudo-cyclic structure. The FT-IR spectra of the complex in the solid state and in the solution are comparable, thus the structures observed in the both states are also comparable. The in vitro biological tests of LAS–TMG show its good activity towards some strains of Gram-positive bacteria but this activity is lower than that of lasalocid acid.