Unique example of flexible phenol coordination in mononuclear manganese compounds

Abstract

The synthesis and characterization of six novel mononuclear Mn^II and Mn^III complexes are presented. The tripodal ligands 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (HL1), 2-[[((6-methylpyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino]methyl]-4-nitrophenol (HL2), (2-pyridylmethyl)(6-methyl-2-pyridylmethyl)(2-hydroxybenzyl)amine (HL3) and 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-bromophenol were used. All ligands provide an N₃O donor set. The compounds [Mn^II(HL1)Cl₂]·CH₃OH (1), [Mn^III(L1)Cl₂] (2), [Mn^II(HL2)(EtOH)Cl₂] (3), [Mn^II(HL3)Cl₂]·CH₃OH (4), [Mn^III(HL4)Br₂] (5) and [Mn^III(L1)(tcc)] (6), with tcc = tetrachlorocatecholate dianion, were synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR and UV-vis spectroscopy, cyclic voltammetry, and elemental analysis. Compound 1 crystallizes in the triclinic space group P, compounds 2, 3 and 4 were solved in the monoclinic space group P2₁/c, whereas the structure determination of 5 and 6 succeeded in the orthorhombic space groups Pbca and P2₁2₁2₁, respectively. Notably, the crystal structures of 1 and 3 are the first Mn^II complexes featuring a non-coordinating phenol moiety. Compound 2 oxidizes 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations with a turnover number of k_cat = 173 h⁻¹. The electronic influence of different substituents in para position of the phenol group is lined out.