Topologically constrained manganese(III) and iron(III) complexes of two cross-bridged tetraazamacrocycles

A family of Mn3+ and Fe3+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo6.6.2]hexadecane (1) and 4,10-dimethyl-1,4,7,10-tetraazabicyclo5.5.2]tetradecane (2) has been prepared by the chemical oxidation of the divalent manganese and iron analogues. The ligands are ethylene cross-bridged tetraazamacrocycles derived from cylam and cyclen, respectively. The synthesis and characterization of these complexes, including X-ray crystal structure determinations, are described. The structural evidence demonstrates that the tetradentate ligands enforce distorted octahedral geometries on the metal ions, with two cis sites occupied by labile ligands. Magnetic measurements reveal that the complexes are high spin with typical magnetic moments. Cyclic voltammetry shows reversible redox processes for the Fe3+/Fe2+ couples of the iron(III) complexes, while Mn3+/Mn2+ and Mn4+/Mn3+ couples were observed for the complexes with manganese(III). The manganese chemistry of 1 was studied in depth. The dichloro manganese(III) cation of 1 undergoes facile ligand substitution reactions at the labile, monodentate sites, for example substituting azide for chloride ligands. Air oxidation of the dichloro complex of Mn (1)2+ in basic solution does not give the expected mu-oxo dimeric product common to manganese. Instead, an unusual manganese(III)-OH complex has been isolated from this reaction and structurally characterized. A similar reaction under slightly different conditions gives a putative MnIII(OH)2 complex that metathesizes to MnIII(OMe)2 upon recrystallization from methanol.