Interaction of I- and I3 - with a Redox-Stable Cr(III)-Based Structural Surrogate for Photo-Oxidized “N3 Dye”

A series of [Cr (L)2(NCS)2]X complexes, where L = 2,2‘-bipyridine (bpy) or 4,4‘-dimethyl-2,2‘-bipyridine (dmb) and X- is I-, I3 -, or NO3 -, have been prepared for study as redox-inactive surrogates for diisothiocyanatobis(2,2‘-bipyridine-4,4‘-dicarboxylic acid)ruthenium(III) (“N3+”). Dilute solution spectra in dichloromethane or acetonitrile of the I- and NO3 - salts are the same, but the I- salts show clear evidence of a charge-transfer interaction between the cation and anion in the solid state and in concentrated I- solutions in acetonitrile. X-ray structural determination of the dmb complex shows that this interaction is between the I- and one of the dmb ligands. X-ray structural determination of the bpy complex as the I3 - salt indicates a similar interaction between one of the bpy ligands and one of the terminal iodine atoms of the anion. These results have potential implications in the reduction mechanism of photo-oxidized N3 dye used as a sensitizer in nanocrystalline TiO2-based photogalvanic cells.