Unexpected C−H Activation of Ru(II)−Dithiomaltol Complexes upon Oxidation

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.