title: Synthesis, characterization, and solution properties of a novel cross-bridged cyclam manganese(IV) complex having two terminal hydroxo ligands.
abstract: A novel monomeric tetravalent manganese complex with the cross-bridged cyclam ligand 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (Me2EBC), [Mn(IV)(Me2EBC)(OH)2](PF6)2, was synthesized by oxidation of Mn(II)(Me2EBC)Cl2 with H2O2 in the presence of NH4PF6)in aqueous solution. The X-ray crystal structure determination of this manganese(IV) compound revealed that it contains two rare terminal hydroxo ligands. EPR studies in dry acetonitrile at 77 K show two broad resonances at g = 1.96 and 3.41, indicating that the manganese(IV) exists as a high-spin d3 species. Resonance Raman (rR) spectra of this manganese(IV) species reveal that the dihydroxy moiety, Mn(IV)(OH)2, is also the dominant species in aqueous solution (pH < 7). pH titration provides two pK(a) values, 6.86(4) and 10.0(1), associated with stepwise removal of the last two oxygen-bound protons from [Mn(IV)(Me2EBC)(OH)2](2+). The cyclic voltammetry of this manganese(IV) complex in dry acetonitrile at 298 K demonstrates two reversible redox processes at +0.756 and -0.696 V (versus SHE) for the Mn4+/Mn3+ and Mn3+/Mn2+ couples, respectively. This manganese(IV) complex is relatively stable in weak acidic aqueous solution but easily degrades in basic solution to manganese(III) derivatives with an 88 +/- 1% yield.
authors.affiliation: Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
authors.forename: Guochuan
authors.initials: G
authors.lastname: Yin
authors.forename: James M
authors.initials: JM
authors.lastname: McCormick
authors.forename: Maria
authors.initials: M
authors.lastname: Buchalova
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Danby
authors.forename: Kent
authors.initials: K
authors.lastname: Rodgers
authors.forename: Victor W
authors.initials: VW
authors.lastname: Day
authors.forename: Kevyn
authors.initials: K
authors.lastname: Smith
authors.forename: Chris M
authors.initials: CM
authors.lastname: Perkins
authors.forename: David
authors.initials: D
authors.lastname: Kitko
authors.forename: John D
authors.initials: JD
authors.lastname: Carter
authors.forename: William M
authors.initials: WM
authors.lastname: Scheper
authors.forename: Daryle H
authors.initials: DH
authors.lastname: Busch