Mn(II) complexes of monoanionic bidentate chelators: X-ray crystal structures of Mn(dha)2(CH3OH)2 (Hdha = dehydroacetic acid) and [Mn(ema)2(H2O)]2 · 2H2O (Hema = 2-ethyl-3-hydroxy-4-pyrone)

This report describes synthesis and characterization of bis-ligand Mn(II) complexes of bidentate chelators: maltol (3-hydroxy-2-methyl-4-pyrone), ethylmaltol (2-ethyl-3-hydroxy-4-pyrone), 1,2-dimethyl-3-hydroxy-4-pyridinone (DMHP) and dehydroacetic acid. All four Mn(II) complexes were characterized by elemental analysis, IR, UV/Vis, EPR, cyclic voltammetry, and X-ray crystallography in cases of Mn(dha)2(CH3OH)2 and [Mn(ema)2(H2O)]2 · 2H2O. The bidentate chelator plays a significant role in the solid state structure of its Mn(II) complex. For example, dha forms the monomeric complex Mn(dha)2(CH3OH)2 while ethylmaltol forms the dimeric complex [Mn(ema)2(H2O)]2. Because of smaller size, maltol ligands in Mn(ma)2 are able to bridge adjacent Mn(II) centers to give a polymeric structure in solid state. Despite of the difference in their solid state structures, both Mn(ema)2 and Mn(ma)2 exist in solution as monomeric Mn(II) species, Mn(ema)2(H2O)2 and Mn(ma)2(H2O)2. This assumption is supported by the similarity in their UV/Vis spectra, EPR data and electrochemical properties. Replacing maltol with DMHP results in a decrease (by ∼100 mV) in the redox potential for the Mn(II)/Mn(III) couple, suggesting that DMHP stabilizes Mn(III) better than maltol. Since Mn(DMHP)2(H2O)2 is readily oxidized to form the more stable Mn(III) complex Mn(DMHP)3, DMHP has the potential as a chelator for removal of excess Mn(II) from patients with chronic Mn toxicity.

Graphical abstractThis report describes synthesis and characterization of bis-ligand Mn(II) complexes of four bidentate chelators (maltol, ethylmaltol, DMHP, and dehydroacetic acid). The X-ray crystal structure and variable-temperature magnetic susceptibility data revealed a dimeric Mn(II) complex [Mn(ema)2(H2O)]2, in which the two Mn(II) centers are ferromagnetically coupled. DMHP forms the bis-ligand complex Mn(DMHP)2(H2O)2, which can be readily oxidized to form the much more stable complex Mn(DMHP)3. DMHP has the potential as a chelator to remove the excess Mn(II) and Mn(III) in patients with chronic Mn toxicity.Figure optionsDownload full-size imageDownload as PowerPoint slide