THE PEROXIDE-DEPENDENT MU-2-O BOND FORMATION OF [MN(IV)SALPN(O)]2

A variety of Mn(III)SALPN complexes as monomeric and dimeric derivatives have been prepared. Acetonitrile solutions of tetragonal complexes such as [Mn(III)(SALPN)(CH3OH)2]ClO4, 1, are unreactive with hydrogen peroxide unless base is added. In contrast, [Mn(III)(SALPN)(AcAc)], 2, [Mn(III)(SALPN)(OCH3)]2, 3, and Mn(III)2(SALPN)3, 4, internally carry an equivalent of base and react rapidly to form the oxidized dimer [Mn(IV)(SALPN)(O)]2, 5. The base is required to deprotonate hydrogen peroxide prior to reaction with 1-4. Alkyl peroxides such as tert-butyl hydroperoxide also require base for reactivity but appear to follow a different mechanistic pathway consistent with the slow loss of tert-butoxide leading to Mn(V)OSALPN, 19. The conversion of 2, 3, or 4 to 5 has been examined with use of isotopically labeled hydrogen peroxide and ring-substituted ligand derivatives. These data demonstrate that the first intermediate in the reaction is probably a monomeric hydroperoxide adduct [Mn(III)(SALPN)(O2H)], 16. The monomer 16 then reacts with another equivalent of Mn(III)(SALPN)(X) to form a peroxo-bridged dimer (SALPN)Mn(III)(O2)Mn(III)(SALPN), 17, which undergoes internal electron transfer forming 5. This proposed mechanism is consistent with X-ray structural characterization of the precursors (Mn(III)(SALPN)(AcAc), 2, [Mn(III)(5-Cl-SALPN)(CH3OH)2]ClO4, 7, [Mn(III)(3,5-diCl-SALPN)(CH3O)]2, 12, and Mn(III)2(SALPN)3, 4) and product (5) complexes in the solid state and H-1 NMR spectra of the Mn(III) species in methylene-d2 chloride/5% methanol-d4 at 22-degrees-C. Electrochemistry and isotope labeling experiments show that monomeric compounds such as [Mn(IV)(SALPN)(AcAc)]PF6, 6, Mn(IV)O(SALPN), or [Mn(V)(O)SALPN)]ClO4 cannot be involved in the hydrogen peroxide-dependent transformation to form 5. In contrast, intermediates such as 19 are implicated in the tert-butyl peroxide oxidations. X-ray parameters for 2, C22H23N2O4Mn1, 434 g/mol, crystal system, monoclinic (P2(1)/c), a = 8.004 (3) angstrom, b = 14.035 (8) angstrom, c = 18.134 (6) angstrom, beta = 83.30 (3)degrees, V = 2023 (2) angstrom3, Z = 4, 2666 data collected with 3-degrees < 2-theta < 45-degrees, 1624 data with I > 3-sigma(I), R = 0.0503, R(W) = 0.0503; 4, Mn2N7O6C56H53, 1029 g/mL, crystal system triclinic (P1BAR), a = 11.011 (4) angstrom, b = 14.103 (5) angstrom, c = 17.174 (7) angstrom, alpha = 74.59 (3)degrees, beta = 83.82 (3)degrees, gamma = 82.28 (3)degrees; V = 2540 (2) angstrom3, Z = 2, 6104 data collected with 3-degrees < 2-theta < 45-degrees, 4416 data with I > 3-sigma(I), R = 0.0475, R(w) = 0.0360; 7, C19H22N2O8MnCl3, 566 g/mol, crystal system, monoclinic (P2(1)/c), a = 10.579 (4) angstrom, b = 17.451 (11) angstrom, c = 13.651 (6) angstrom, beta = 112.75 (3)degrees, V = 2324 (2) angstrom3, Z = 4, 2178 data collected with 3-degrees < 2-theta < 45-degrees, 1693 data with I > 3-sigma(I), R = 0.0524, R(w) = 0.0457; 12, Mn2Cl8N4O7C37H33, 1039 g/mL, crystal system monoclinic (P2(1)/n), a = 9.213 (5) angstrom, b = 26.024 (16) angstrom, c = 17.992 (7) angstrom, beta = 91.87 (4)degrees, V = 4312 (4) angstrom3, Z = 2, 5639 data collected with 3-degrees < 2-theta < 45-degrees, 3197 data with I > 3-sigma(I), R = 0.0519, R(w) = 0.0470.