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Synthesis, structure, and characterization of molybdenum(VI) imido complexes with N-salicylidene-2-aminothiophenol

Abstract:

The di-imido complex Mo(NAr)2Cl2(dme) (dme = 1,2-dimethoxyethane; Ar = 2,6-diisopropylphenyl) reacts with the sulfur ligand N-salicylidene-2-aminothiophenol (smaH2) in methanol in the presence of two equivalents of triethylamine to form Mo(NAr)2sma (1). The structure of this molybdenum(VI) complex has been determined by X-ray crystallography. The coordination sphere of the Mo center is approximately a trigonal bipyramid. The angles of the imido linkages (Mo–N–C) are 175° and 147°. The tridentate sma ligand binds with the imine nitrogen in the axial position trans to the 175° imido linkage. 1 can be reacted with bidentate aromatic ligands to form six-coordinate complexes of the type Mo(ArN)(sma)(L) (L = catecholate, 2; 2-mercaptophenolate, 3, 1,2-benzenedithiolate, 4; 2-amidophenolate, 5; 3-amidonaphtholate, 6; 1,2-amidonaphtholate, 7; 2-amidothiophenolate, 8). The structures of compounds 2–7 have been determined by X-ray crystallography. In 2, 3, and 4 the bidentate ligand, L, binds in two equatorial positions cis to the imido linkage. In 5, 6 and 7 L binds in an axial and an equatorial position where the amido nitrogen occupies the equatorial position. All complexes have been characterized by 1H NMR spectroscopy and cyclic voltammetry. Compounds 2, 3, and 4 can be reversibly reduced with one electron to EPR-active Mo(V) complexes. The EPR spectrum of 4 shows superhyperfine splitting from the imido nitrogen. This indicates that the unpaired electron is located in the dz2 orbital that interacts with the imido nitrogen and the imine nitrogen of the sma ligand and is not influenced by the bidentate aromatic ligand which binds in two equatorial positions. This explains the similar redox potentials of the three complexes. In complexes 5–8 the reduction is not reversible and the potential is influenced by the atom of the bidentate aromatic ligand trans to the imido group.