Novel Cerium(IV) Coordination Compounds of Monensin and Salinomycin

Unusual Caesium Monensinate A: Crystallographic Evidence for the Formation of Dinuclear Coordination Species in the Solid State

The polyether ionophore monensin A (MonH), applied as silver monensinate, reacts with caesium cations to form a dinuclear complex [Mon2Cs2] the structure of which has been solved by single-crystal X-ray diffraction. Two Cs+ ions are located in the hydrophilic cage of two ligand anions, achieving coordination number eight. In addition, the metal cations are bridged by two functional groups of monensinate A, completing the inner tenfold coordination sphere. NMR studies show that the dinuclear complex dissociates to its mononuclear counterparts in methanol solutions. Further molecular dynamics theoretical modelling of the interaction of monensinate A with alkali metal ions reveals the effect of solvent polarity on the zipping ability of the ligand. Thus, in methanol, used as an explicit solvent, potassium and rubidium cations fully occupy the cavity of the ligand, whereas the sodium monensinate exists in an "open" form, with Na+ ions still interacting with the monodentate carboxylate group. The replacement of methanol by the less polar chloroform induces the folding of monensinate A and the formation of "closed" structures with all group 1 metal cations. The obtained data explain the specifics in the behaviour of monensinate A caused by the environment, e. g., physical state or solvent.

Heteronuclear Complexes of Hg(II) and Zn(II) with Sodium Monensinate as a Ligand

The commercial veterinary antibiotic sodium monensinate (MonNa) binds mercury(II) or zinc(II) cations as thiocyanate [Hg(MonNa)2(SCN)2] (1) or isothiocyanate [Zn(MonNa)2(NCS)2] (2) neutral coordination compounds. The structure and physicochemical properties of 1 and 2 were evaluated by the methods of single crystal and/or powder X-ray diffraction, infrared, nuclear magnetic resonance, X-ray photoelectron spectroscopies, and electrospray-mass spectrometry. The primary cores of the two complexes comprise HgS2O2 (1) and ZnN2O2 (2) coordination motifs, respectively, due to the ambidentate binding modes of the SCN-ligands. The directly bound oxygen atoms originate from the carboxylate function of the parent antibiotic. Sodium cations remain in the hydrophilic cavity of monensin and cannot be replaced by the competing divalent metal ions. Zinc(II) binding does not influence the monensin efficacy in the case of Bacillus cereus and Staphylococcus aureus whereas the antimicrobial assay reveals the potential of complex 2 as a therapeutic candidate for the treatment of infections caused by Bacillus subtilis, Kocuria rhizophila, and Staphylococcus saprophyticus.

Experimental and DFT Study of Monensinate and Salinomycinate Complexes Containing {Fe3(µ3-O)}7+ Core

Two trinuclear oxo-centred iron(III) coordination compounds of monensic and salinomycinic acids (HL) were synthesized and their spectral properties were studied using physicochemical/thermal methods (FT-IR, TG-DTA, TG-MS, EPR, Mössbauer spectroscopy, powder XRD) and elemental analysis. The data suggested the formation of [Fe3(µ3-O)L3(OH)4] and the probable complex structures were modelled using the DFT method. The computed spectral parameters of the optimized constructs were compared to the experimentally measured ones. In each complex, three metal centres were joined together at the axial position by a μ3-O unit to form a {Fe3O}7+ core. The antibiotics monoanions served as bidentate ligands through the carboxylate and hydroxyl groups located at the termini. The carboxylate moieties played a dual role bridging each two metal centres. Hydroxide anions secured the overall neutral character of the coordination species. Mössbauer spectra displayed asymmetric quadrupole doublets that were consistent with the existence of two types of high-spin iron(III) sites with different environments-two Fe[O5] and one Fe[O6] centres. The solid-state EPR studies confirmed the +3 oxidation state of iron with a total spin St = 5/2 per trinuclear cluster. The studied complexes are the first iron(III) coordination compounds of monensin and salinomycin reported so far.