Mix and (Mis)match: Further studies of the electronic structure and mixed-valence characteristics of 1,4-diethynylbenzene-bridged bimetallic complexes

The 1,4-diethynylbenzene motif is commonly employed as a bridging ligand in bimetallic molecular systems intended to show pronounced intramolecular electronic interactions, delocalized electronic structures and 'wire-like' properties between the metal fragments at the ligand termini. In contrast to these expectations, the donor-acceptor compounds [{Cp′(CO)xM′}(μ-CCC6H4CC){M(PP)Cp′}]n+ [n = 0, 1; M′(CO)xCp′ = Fe(CO)2Cp, W(CO)3Cp∗; M(PP)Cp′ = Fe(dppe)Cp, Fe(dppe)Cp∗, Ru(PPh3)2Cp, Ru(dppe)Cp, Ru(dppe)Cp∗] display remarkably little bridge-mediated electronic interaction between the electron-rich {M(PP)Cp′} and electron-poor {M′(CO)xCp′} fragments in the ground state. However, a relatively high-energy (26 000-30 000 cm-1) M-to-M′ charge transfer can be identified. One-electron oxidation is largely localized on the {M(CCR)(PP)Cp′} fragment and gives rise to a new charge transfer band with bridging-ligand-to-{M(PP)Cp′}+ (M′(CO)xCp′ = Fe(CO)2Cp) or M′-to-M(+) (M(CO)xCp′ = W(CO)3Cp∗) character. The localized electronic ground state of these complexes is better revealed through analysis of the IR spectra, taking advantage of the well-resolved ν(CC) and ν(CO) bands and IR spectroelectrochemical methods, than through the more classical analysis based on the concepts of Marcus-Hush theory and analysis of the putative IVCT electronic transition. The conclusions are supported by DFT calculations using the BLYP35 functional.