Structural investigation of high-valent manganese-salen complexes by UV/Vis, Raman, XANES, and EXAFS spectroscopy

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminoman- ganese((III)) chloride ([(salen)(MnCl)-Cl-III], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino- manganese((III)) bromide ([salen)-(MnBr)-Br-III], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn-III complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximate to 1.90 Angstrom and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 Angstrom. The oxidation state of this complex was determined as approximate to + (IV) by a comparative study of Mn-III and Mn-V reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of (IV) was deduced from the relative position of the pre-edge 1s --> 3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max) = 648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn-IV central atom. An oxomanganese((IV)) or a dimeric manganese((IV)) species was not detected.