Structural Investigation of High-Valent Manganese–Salen Complexes by UV/Vis, Raman, XANES, and EXAFS Spectroscopy
✍ Scribed by Martin P. Feth; Carsten Bolm; Jens P. Hildebrand; Manuela Köhler; Oliver Beckmann; Matthias Bauer; Rivo Ramamonjisoa; Helmut Bertagnolli
- Publisher
- John Wiley and Sons
- Year
- 2003
- Tongue
- English
- Weight
- 243 KB
- Volume
- 9
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
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‐cyclohexanediaminomanganese(III) chloride ([(salen)Mn^III^Cl], 1) and (S,S)‐(+)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediaminomanganese(III) bromide ([(salen)Mn^III^Br], 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 ≈1.90 Å and two additional, axially bonded oxygen atoms of the 4‐PPNO at 2.25 Å. The oxidation state of this complex was determined as ≈+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 λ~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(V) or a dimeric manganese(IV) species was not detected.