Phototriggered S → O Isomerization of a Ruthenium-Bound Chelating Sulfoxide

We have prepared and characterized [Ru(bpy)2(OS)]+ (bpy = 2,2‘-bipyridine; OS = 2-methylthiobenzoate) and the chelating sulfoxide
S-[Ru(bpy)2(OSO)]+ (OSO = methylsulfonylbenzoate) by 1H NMR
(1-D and 2-D COSY), IR, UV−visible spectroscopy, electrochemistry, and X-ray crystallography. Many of the metrical and crystal
parameters are nearly identical between the two structures. The
sulfoxide is produced from m-cpba oxidation of the thioether.
Photolysis of S-[Ru(bpy)2(OSO)]+ results in a dramatic shift in the
metal-to-ligand charge-transfer (3MLCT) transition from 396 to 496
nm, with a new higher-energy 3MLCT transition appearing at 355
nm. Concomitant with this change, the Ru3+/2+ reduction potential
shifts from 1.25 V vs Ag/AgCl to 0.9 V vs Ag/AgCl. These changes
are ascribed to phototriggered excited-state isomerization of the
sulfoxide from S- to O-bonded. Examination of the 1H NMR spectra
in a CD3OD solvent before, during, and after irradiation shows
the presence of two O-bonded complexes that revert to the
structurally characterized S-bonded ground state. This represents
the first report of isomerization of a chelating sulfoxide in a
photochromic Ru complex.