Structural Investigation of High-Valent Manganese-Salen Complexes by UV/Vis, Raman, XANES, and EXAFS Spectroscopy

摘要

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-butylsali-cylidene)-1,2-cyclohexanediaminoman-ganese(III) chloride ([(salen)Mn~IIICl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexanediamino-manganese(III) bromide ([(salen)-Mn~IIIBr], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxy-benzoic 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 oxygenitrogen atoms of the salen ligand at an average distance of approx = 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as and Mn~v reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated wiht 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. Thebroad UV/Vis band of this complex in acetonitrile with lambda_max=648 nm was consistent with a radical molecule was bound to the Mn~IV central atom. An oxomanganess(v) or a dimeric manganese(IV) species was not detected.