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|Title:||Mixed-valent metals bridged by a radical ligand: Fact or fiction based on structure-oxidation state correlations|
|Publisher:||AMER CHEMICAL SOC|
|Citation:||JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 130(11), 3532-3542|
|Abstract:||Electron-rich Ru(acaC)(2) (acac(-) = 2,4-pentanedionato) binds to the pi electron-deficient bis-chelate ligands L, L = 2,2'-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp), with considerable transfer of negative charge. The compounds studied, (abpy)Ru(acac)(2) (1), meso-(mu-abpy)[Ru(acaC)(2)](2) (2), rac-(mu-abpy)[Ru(acac)(2)](2) (3), and (mu-abcp)[Ru(acac)(2)](2) (4), were calculated by DFT to assess the degree of this metal-to-ligand electron shift. The calculated and experimental structures of 2 and 3 both yield about 1.35 angstrom for the length of the central N-N bond which suggests a monoanion character of the bridging ligand. The NBO analysis confirms this interpretation, and TD-DFT calculations reproduce the observed intense long-wavelength absorptions. While mononuclear 1 is calculated with a lower net ruthenium-to-abpy charge shift as illustrated by the computed 1.30 angstrom for d(N-N), compound 4 with the stronger pi accepting abcp bridge is calculated with a slightly lengthened N-N distance relative to that of 2. The formulation of the dinuclear systems with monoanionic bridging ligands implies an obviously valence-averaged (RuRuII)-Ru-III mixed-valent state for the neutral molecules. Mixed valency in conjunction with an anion radical bridging ligand had been discussed before in the discussion of MLCT excited states of symmetrically dinuclear coordination compounds. Whereas 1 still exhibits a conventional electrochemical and spectroelectrochemical behavior with metal centered oxidation and two ligand-based one-electron reduction waves, the two one-electron oxidation and two one-electron reduction processes for each of the dinuclear compounds Ru-2.5(L*(-))Ru-2.5 reveal more unusual features via EPR and UV-vis-NIR spectroelectrochemistry. In spite of intense near-infrared absorptions, the EPR results show that the first reduction leads to Ru-II(L*(-))Ru-II species, with an increased metal contribution for system 4*(-). The second reduction to Ru-II(L2-)Ru-II causes the disappearance of the NIR band. One-electron oxidation of the Ru-2.5(L*(-)) Ru-2.5 species produces a metal-centered spin for which the alternatives Ru-III(L-0)Ru-II or Ru-III(L*(-))Ru-III can be formulated: The absence of NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors the second alternative. The second one-electron oxidation is likely to produce a dication with Ru-III(L-0)Ru-III formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations for complexes with noninnocent ligands is being discussed.|
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