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

Abstract

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.