Sarkar, Biprajit ; Patra, Srikanta ; Fiedler, Jan ; Sunoj, Raghavan B. ; Janardanan, Deepa ; Lahiri, Goutam Kumar ; Kaim, Wolfgang (2008) Mixed-valent metals bridged by a radical ligand: fact or fiction based on structure-oxidation state correlations Journal of The American Chemical Society, 130 (11). pp. 3532-3542. ISSN 0002-7863
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Official URL: http://pubs.acs.org/doi/abs/10.1021/ja077676f
Related URL: http://dx.doi.org/10.1021/ja077676f
Abstract
Electron-rich Ru(acac)2 (acac- = 2,4-pentanedionato) binds to the p 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-(μ-abpy)[Ru(acac)2]2 (2), rac-(μ-abpy)[Ru(acac)2]2 (3), and (μ-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 Å 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 Å for d(N-N), compound 4 with the stronger p 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 RuIIIRuII 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 Ru2.5(L·-)Ru2.5 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 RuII(L·-)RuII species, with an increased metal contribution for system 4·-. The second reduction to RuII(L2-)RuII causes the disappearance of the NIR band. One-electron oxidation of the Ru2.5(L·-)Ru2.5 species produces a metal-centered spin for which the alternatives RuIII(L0)RuII or RuIII(L·-)RuIII 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 RuIII(L0)RuIII formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations for complexes with noninnocent ligands is being discussed.
| Item Type: | Article |
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| Source: | Copyright of this article belongs to American Chemical Society. |
| ID Code: | 88379 |
| Deposited On: | 29 Mar 2012 04:44 |
| Last Modified: | 29 Mar 2012 04:44 |
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