Characterization of infrared vibrational activity in specific totally symmetric bridging modes of mixed-valence systems near the localized-to-delocalized transition

The presence or absence of infrared (IR) activity in totally symmetric bridging vibrations of ligand-bridged mixed-valence molecular systems has often been taken, on the basis of electronic (a)symmetry arguments, as markers for electronic localization (class II) or delocalization (class III) on the vibrational time scale. However, because IR absorption intensity in such nominally-forbidden modes may be anomalously enhanced due to vibronic coupling effects, especially for cases in the localized-to-delocalized (class II/III) transition regime, a direct and continuous correlation between the extent of (de)localization and IR activity is challenged. In order to experimentally illustrate such phenomena, here we provide a detailed comparison of relevant IR signatures across a series of structurally related, symmetric dinuclear complexes of the type [(bpy)2(Cl)M-pz-M(Cl)(bpy)2]n+ (bpy = 2,2′-bipyridine, pz = pyrazine, M = RuII,III or OsII,III; n = 2, 3, or 4), in which the “mixed valency” (II,III; n = 3) has been characterized as largely localized for M = Ru and approaching the near-delocalized behavior for M = Os. To improve analytical accuracy and facilitate standardization in the comparison of results spanning the various isovalent and mixed-valent redox species, the IR spectroscopic features were probed via an in situ spectroelectrochemical (SEC) approach, in identical medium (solution) conditions. The parent monomers, i.e. [(bpy)2(Cl)M-pz]n+ (M = RuII,III or OsII,III; n = 1 or 2), which represent the extreme examples of structural and electronic symmetry breaking in the series, were also investigated and compared with the dimers. In all cases reported here, the spectroscopic assignments of vibrational modes were supported by isotopic modification (deuteration) of the bridging pyrazine ligand.