Syntheses, Crystal Structures, and Electron Communication Properties of a Series of Class II–III Mixed-Valence Complexes

The Class II–III mixed-valence species have attracted great attention because of the borderline behavior in the Class II to Class III transition. To further investigate those species, a series of binuclear and trinuclear complexes containing cyanide as a bridge, [CpMex(dppp)RuCNRu(dmap)5][PF6]n (x = 0, n = 2 and 3, 1n+; x = 5, n = 2 and 3, 2n+; CpMex = polymethylcyclopentadienyl; dppp = 1,3-bis(diphenyphosphino)propane); dmap = 4-dimethylamino-pyridine) and [CpMex(dppp)Ru–CNRu(dmap)4NCRu(dppp)CpMex][PF6]n (x = 0, n = 2 and 3, 3n+; x = 5, n = 2 and 3, 4n+), were synthesized and characterized using electrochemical and spectroscopic methods as well as single-crystal analysis. For all neutral species, the first oxidation occurs on the N-coordinated Ru entity of the cyanide according to IR spectra and crystallographic data. The mixed-valence complexes 13+–43+ show different degrees of electron delocalization and can be classified as Class II–III mixed-valence complexes, as evidenced by the NIR spectra, supported by (TD)DFT calculations. Moreover, variable-temperature electron paramagnetic resonance (EPR) spectra of the mixed-valence species show that 13+ and 23+ are localized and 33+ is temperature-dependent electron delocalized; however, 43+ is fully delocalized on EPR time scale. The energy of the intervalence charge transfer (IVCT) band in the NIR region decreases when [CpMexRu] fragments are more electron donating, which is also truly predicted by theoretical calculations.