First solid state alkaline-earth complexes of monensic acid A (MonH): crystal structure of [M(Mon)₂(H₂O)₂] (M = Mg, Ca), spectral properties and cytotoxicity against aerobic Gram-positive bacteria

Alkaline-earth metal complexes of the monoanionic form of the polyether ionophore monensin A were isolated for the first time in solid state and were structurally characterized using various spectroscopic methods (IR, NMR, FAB-MS). The stoichiometric reaction of monensic acid (MonH) with M²⁺ (M = Mg, Ca) in the presence of an organic base leads to the formation of mononuclear complexes of composition [M(Mon)₂(H₂O)₂]. The structures of magnesium (1) and calcium (2) monensin complexes in the solid state were established by single crystal X-ray crystallography. The complexes crystallize as [Mg(Mon)₂(H₂O)₂]·5MeCN (1) and [Ca(Mon)₂(H₂O)₂]·H₂O·5MeCN (2) in the monoclinic P21 space group. The alkaline-earth metal ion is placed in a distorted octahedral environment, defined by two monensin anions acting as bidentate ligands in the equatorial plane of the complex as well as by two water molecules occupying the axial positions of the inner coordination sphere. The bactericidal activity of 1 and 2 was evaluated against aerobic Gram-positive microorganisms applying the double layer agar hole diffusion method.