Estimation of Electron Distribution over Dinuclear Organometallic Molecular Wires by “IR Tag” Analysis of Ancillary Acyl-Cp Ligands

[Image: see text] Understanding the details of electronic properties of mixed-valence (MV) states of organometallic molecular wires is essential to gain insights into electron transfer phenomena. Although the field of MV chemistry is matured, there remain issues to be solved, which cannot be accessed by the conventional analytical methods. Here, we describe the synthesis and properties of diruthenium bridging (diethynylbenzene)diyl wires, (μ-p and m-C≡C–C(6)H(4)–C≡C){RuCp(R)(dppe)}(2)1, with the acyl-substituted cyclopentadienyl rings [Cp(R): Cp(e); η(5)-C(5)H(4)COOMe (a-series: ester derivatives), Cp(a); η(5)-C(5)H(4)COMe (b-series: acetyl derivatives)], which are installed as IR-tags to estimate electron densities at the metal centers in the MV species. The electrochemical and IR/near IR-spectroelectrochemical studies reveal that the two metal centers in the para-isomers p-1a,b(+) interact with each other more strongly than those in the meta-isomers m-1a,b(+). Electron-spin resonance study also supports the radicals being delocalized over the Ru-(p-C≡C–C(6)H(4)–C≡C)–Ru moieties in p-1a,b(+). The spectroelectrochemical IR study shows significant higher-energy shifts of the ν(C=O) vibrations brought about upon 1e-oxidation. Spectral simulation on the basis of the Bloch equations allows us to determine the electron transfer rate constants (k(et)) between the two metal centers being in the orders of 10(12) s(–1) (p-1(+)) and 10(9) s(–1) (m-1(+)). The shifts of the ν(C=O) bands reveal that the charge densities on the para-isomers p-1a,b(+) are widely delocalized over the Ru-(p-C≡C–C(6)H(4)–C≡C)–Ru linkages in contrast to the meta-isomers m-1a,b(+), where the electron densities are mainly localized on the metal fragments, as supported by the density functional theory and time-dependent density functional theory studies as well as comparison with the reference mononuclear acetylide complexes, C(6)H(5)–C≡C–RuCp(R)(dppe) 2. We have successfully demonstrated that the carbonyl groups (>C=O) in the ancillary Cp ligands also work as IR-tags to report detailed information on the electron densities at the metal centers and the electron distribution over MV complexes as well.