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.

Evidence for a Precursor Complex in CH Hydrogen Atom Transfer Reactions Mediated by a Manganese(IV) Oxo Complex

“…X-Ray diffraction structural analysis [12] shows that the metal ion adopts a distorted octahedral geometry. Mn À O bond lengths and the stoichiometry of the compound provide a direct indication that 2 has two hydroxide ligands, [13] in a cis arrangement relative to each other (see Figure 1).…”

“…Complex 2 is a rare example of a mononuclear Mn IV complex with terminal hydroxide ligands, the only precedent is [Mn IV (OH) 2 -(Me 2 EBC)](PF 6 ) 2 , which contains an ultrarigid, sterically demanding macrocyclic ligand. [13] Space-filling analysis ( Figure 1) of 2 shows that one of the N-methyl groups, and the pyridine ring provide steric isolation of the metal center, and this is presumably the key structural feature to prevent the formation of the more common oligomeric species.…”

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis

“…Busch and coworkers reported the first structurally characterized example of a mononuclear Mn (IV) complex with two terminal hydroxo ligands ( Fig. 2) (Yin et al, 2006). Using the pH titration of aqueous solutions of the complex, it revealed two acid-base equilibria with pK 1 = 6.86 and pK 2 = 10, the latter apparently being associated with dimer formation.…”

“…Using the pH titration of aqueous solutions of the complex, it revealed two acid-base equilibria with pK 1 = 6.86 and pK 2 = 10, the latter apparently being associated with dimer formation. The complex has shown as a catalyst in olefin epoxidation and hydrogen atom abstraction reactions (Yin et al, 2006). The Mn(V) = O are very important and it has been proposed as an intermediate in Natural water oxidation.…”

“…However, as considered by Pecoraro, these synthetic Mn(V) = O complexes are stabilized by special ligand(s) and the activity of Mn(V) = O could be completely different from Mn(V) = O of the WOC in PSII (Mullins & Pecoraro, 2008). (Miller et al, 1998) (a) and a Mn(IV) complex terminal hydroxo ligands (b) (Yin et al, 2006).…”