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

Abstract

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.

Synthesis, Spectral Characterization, and Structural Modelling of Di- and Trinuclear Iron(III) Monensinates with Different Bridging Patterns

“…In addition, the formation of 1 is accompanied by co-precipitation of the mono-complex 1a (Figure S2). The complete set of experimental data is significantly different from that previously reported for iron(III) monensinate and salinomycinate [41], a fact indicating that we have prepared new ferric coordination compounds of the polyether ionophore monensin. The isolated solids 1 and 2 are amorphous without any sign of crystallinity, making their precise characterization difficult.…”

“…In the presence of organic bases (Et 4 NOH or Et 3 N), monensic acid effectively deprotonates and acts as a carboxylate monoanion in complexation reactions with metal ions of different oxidation states [41][42][43][44][45]. Monensinate reaction with FeCl 3 or FeSO 4 leads to the formation of new complexes which exhibit spectral properties characteristic of di-(1) and trinuclear (2) coordination species of iron(III), respectively.…”

“…The following approaches and devices were utilized in the present study: infrared spectroscopy (IR) on a Nicolet 6700 FT-IR spectrometer (Thermo Scientific, Madison, WI, USA); thermogravimetry (TG-DTA, TG-MS) on a Setaram Labsys Evo 1600 (Caluire-et-Cuire, France); electron paramagnetic resonance (EPR) on a Bruker BioSpin EMXplus10/12 EPR spectrometer (Karlsruhe, Germany); nuclear magnetic resonance (NMR) on a Bruker NEO 600 spectrometer (Karlsruhe, Germany); 57 Fe Mössbauer measurements on a Wissel spectrometer (Wissenschaftliche Elektronik GmbH, Starnberg, Germany); microanalysis on a Vario MACRO cube Elementar analysensysteme Gmbh (Stuttgart, Germany) (C, H) and Perkin-Elmer SCIEX-ELAN DRC-e ICP-MS (Massachusetts, USA) (Fe). Details of the technical parameters of the spectrometers and the corresponding sample preparation procedures are described in [41,59].…”

“…The quantum chemical calculations were performed according to a previously published protocol [41]. Briefly, the geometries of all constructs were optimized with Becke's three-parameter hybrid-exchange functional combined with the Lee-Yang-Parr correlation functional [60], and with Grimme D3 correction [61] for the dispersion interactions and the 6-31G(d) basis set.…”

“…Monensic acid (MonH, Figure S1) was selected as the most widely applied antibiotic in veterinary medicine, and salinomycinic acid (SalH) was deemed as a promising anticancer agent. Our first findings led to the isolation and characterization of iron(III) monensinate and salinomycinate of composition [Fe 3 (µ 3 -O)L 3 (OH) 4 ] [41]. Then, under completely different reaction conditions, two new Fe(III) coordination compounds of monensin were obtained with the involvement of the antibiotic carboxylate function in the complex formation.…”