Unexpected C−H Activation of Ru(II)−Dithiomaltol Complexes upon Oxidation
datasetposted on 07.04.2008, 00:00 by Malin Backlund, Joseph Ziller, Patrick J. Farmer
Thione-substituted derivatives of maltol are of interest in several applications of metal-based drugs. In order to investigate the effect of the oxygenation on such thione chelates, Ru complexes of 3-hydroxy-2-methyl-4-thiopyrone (thiomaltol or Htma) and 3-hydroxy-2-methyl-4H-thiopyran-4-thione (dithiomaltol or Httma), [Ru(bpy)2(tma)]+, 1, and [Ru(bpy)2(ttma)]+, 2, were synthesized as diamagnetic PF6− salts. Peroxidation of 2 unexpectedly generated products of C−H activation at its pendant methyl group; an air-stable aldehyde [Ru(bpy)2(ttma-aldehyde)]+, 4, was the major product. In addition, an intermediate oxidation product [Ru(bpy)2(ttma-alcohol)](PF6), 3, was characterized. Both 3 and 4 are also formed by reaction of 2 with outersphere oxidants (e.g., Na2IrCl6) and by bulk electrolysis under anaerobic conditions. Similar oxidations of the analogous [Ru(bpy)2(ettma)]+, 2′, complex (3-hydroxy-2-ethyl-4H-thiopyran-4-thione; ethyl dithiomaltol or Hettma) formed the corresponding ketone, [Ru(bpy)2(ettma-ketone)](PF6), 4′, by oxidation at the same position adjacent to the conjugated ring. The structures of the aldehyde 4 and starting materials 1 and 2 have been confirmed by X-ray crystallography, and all complexes have been characterized by UV–vis, 1H NMR, and IR spectroscopies. Initial mechanistic investigations are discussed.