MANGANESE COMPLEXES OF ALPHA-HYDROXY ACIDS

Abstract

Manganese complexes of α-hydroxy acids (H2L) have been implicated in the basidomycete oxidative degradation pathway of lignin. In this contribution, we evaluate the chemistry of manganese(III) and manganese(IV) α-hydroxy acid compounds using lactic acid (H2Lc), 2-hydroxy-2-ethylbutyric acid (H2HEB), and 2-hydroxyisobutyric acid (H2HIB). Both oxidative and reductive methodologies to give compounds of stoichiometry (Cat)3[(Cat)MnIV2L6], (Cat)3[MnIII2(L)4HL] and (Cat)2[MnIII(L)2HL] have been explored. The X-ray structure of 1, Na3[NaMnIV2(HIB)6]·4MeOH, which was prepared by the reaction of MnCl2, H2HIB, and NaOCH3 in methanol, reveals a mixed-metal trinuclear cluster composed of two Mn(IV) octahedra that are bridged by a sodium ion. The face-shared geometry leads to a very short Mn-Na distance (2.98 A). 1 is a unique Mn(IV) complex with all oxygen ligation and containing redox-innocent ligands. The O(3)O(3′) coordination shell is trigonally compressed along the face containing the three alkoxide ligands. The X-band EPR spectrum of 1 at 100 K is consistent with an isolated S = 3/2 spin system. The Mn(III) dimer (TMA)3[MnIII2(Lc)4HLc]·H 2O (5) can be prepared by the reaction of tetramethylammonium permanganate ((TMA)MnO4), H2Lc, and (TMA)OH in methanol. An excess of ligand allows for the reduction of the Mn(VII) to Mn(III). The X-ray structure of 5 provides the first example of an Mn(III) dimer with axial compression. Two MnIIILc2 units, having a propeller configuration, are bridged by the carboxylate of an HLc-, having a protonated hydroxyl oxygen atom, and weak bridges from coordinated alkoxides of the adjacent MnIIILc2 unit. The Mn-Mn separation is 3.09 A. A third compound of composition (Cat)2[MnIII(L)2HL] has also been prepared by both reductive and oxidative methodologies and is most likely an infinite chain structure that has a trans-MnIIIL2 plane that is linked by the anti-anti coordination of carboxylate oxygens of an HL-. The relevance of these manganese α-hydroxy acid complexes to structural proposals for the oxygen-evolving complex and diffusible intermediates of the manganese lignin peroxidase is discussed. X-ray parameters for Na3[NaMnIV2(HIB)6]·4MeOH (1) Na4Mn2C28H52O22: 942.6 g/mol, triclinic crystal system, P1, a = 9.549 (6) A, b = 9.843 (4) A, c = 13.966 (8) A, α = 108.12 (4)°, β = 76.44 (5)°, γ = 119.61 (4)°, V = 1079 (1) A3, Z = 1, 2845 data collected with 3° < 2θ < 45°, 2216 data with I > 3σ(I), R = 0.057, Rw = 0.058. X-ray parameters for (TMA)3[MnIII2(Lc)4HLc]·H 2O (5), Mn2C27H59N3O16: 791 g/mol, monoclinic crystal system, P21, a = 11.425 (6) A, b = 15.214 (10) A, c = 12.108 (4) A, β = 106.45 (3)°, V = 2018 (2) A3, Z = 2, 2767 data collected with 3° < 2θ < 45°, 1719 data with I > 3σ(I), R = 0.057, Rw = 0.058.