Article

Mixed-Valent Metals Bridged by a Radical Ligand: Fact or Fiction Based on Structure-Oxidation State Correlations

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Journal of the American Chemical Society, 130 (11): 3532--3542 (2008)
DOI: 10.1021/ja077676f

Abstract

Electron-rich Ru(acac)2 binds to the p electron-deficient bis-chelate ligands L (L = 2,2'-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp)), with considerable transfer of neg. charge. The compds. studied, (abpy)Ru(acac)2 (1), meso-(m-abpy)Ru(acac)22 (2), rac-(m-abpy)Ru(acac)22 (3), and (m-abcp)Ru(acac)22 (4), were calcd. by DFT to assess the degree of this metal-to-ligand electron shift. The calcd. and exptl. structures of 2 and 3 both yield $\sim$1.35 \AA for the length of the central N-N bond which suggests a monoanion character of the bridging ligand. The NBO anal. confirms this interpretation, and TD-DFT calcns. reproduce the obsd. intense long-wavelength absorptions. While mononuclear 1 is calcd. with a lower net Ru-to-abpy charge shift as illustrated by the computed 1.30 \AA for d(N-N), compd. 4 with the stronger p accepting abcp bridge is calcd. with a slightly lengthened N-N distance relative to that of 2. The formulation of the dinuclear systems with monoanionic bridging ligands implies a valence-averaged RuIIIRuII mixed-valent state for the neutral mols. Mixed valency in conjunction with an anion radical bridging ligand had been discussed before in the discussion of MLCT excited states of sym. dinuclear coordination compds. Whereas 1 still exhibits a conventional electrochem. and spectroelectrochem. behavior with metal centered oxidn. and two ligand-based 1-electron redn. waves, the two 1-electron oxidn. and two 1-electron redn. processes for each of the dinuclear compds. Ru2.5(L.bul.-)Ru2.5 reveal more unusual features via EPR and UV-visible-NIR spectroelectrochem. In spite of intense near-IR absorptions, the EPR results show that the 1st redn. leads to RuII(L.bul.-)RuII species, with an increased metal contribution for system 4.bul.-. The 2nd redn. to RuII(L2-)RuII causes the disappearance of the NIR band. One-electron oxidn. of the Ru2.5(L.bul.-)Ru2.5 species produces a metal-centered spin for which the alternatives RuIII(L0)RuII or RuIII(L.bul.-)RuIII can be formulated. The absence of NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors the 2nd alternative. The 2nd 1-electron oxidn. is likely to produce a dication with RuIII(L0)RuIII formulation. The usefulness and limitations of the increasingly popular structure/oxidn. state correlations for complexes with noninnocent ligands is being discussed. on SciFinder(R)

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