Synthesis and characterization of trigonal monopyramidal complexes of divalent manganese

Bio-inspired studies of manganese coordination chemistry often seek to replicate features within the secondary coordination sphere of manganese-bearing metalloenzymes through judicious ligand design. While polar or charged secondary-sphere motifs are frequently employed, the intentional orientation of hydrophobic motifs within proximity to the metal center is rarely undertaken. We have prepared and fully characterized two new manganese(II) complexes in the unusual trigonal monopyramidal coordination geometry using related tris(amidate)amine ligands. One such ligand houses a narrow arene-lined cavity within the secondary sphere, and its rigid confines guard access to the open axial coordination site. Both complexes have been characterized crystallographically, spectroscopically, and electrochemically. While the Mn^III/II couple is seen to be electrochemically irreversible in the absence of axial coordination, cyclic voltammograms of the macrocycle-free congener in the coordinating solvent DMF show a quasi-reversible oxidation feature, suggesting that ligand binding can stabilize the oxidized manganese(III) species.