Ghumaan, Sandeep ; Sarkar, Biprajit ; Chanda, Nripen ; Sieger, Monika ; Fiedler, Jan ; Kaim, Wolfgang ; Lahiri, Goutam Kumar (2006) 2,2'-dipyridylketone (dpk) as ancillary acceptor and reporter ligand in complexes [(dpk)(Cl)Ru(μ -tppz)Ru(Cl)(dpk)]n+ where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine Inorganic Chemistry, 45 (19). pp. 7955-7961. ISSN 0020-1669
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Official URL: http://pubs.acs.org/doi/abs/10.1021/ic060887l
Related URL: http://dx.doi.org/10.1021/ic060887l
Abstract
The tppz-bridged diruthenium(II) complex [(dpk)(Cl)RuII(μ -tppz)RuII(Cl)(dpk)](ClO4)2, [2](ClO4)2, and mononuclear [(dpk)(Cl)RuII(tppz)](ClO4), [1](ClO4) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, dpk = 2,2'-dipyridylketone], have been synthesized. The 260 mV separation between successive one-electron oxidation couples in [2]2+ translates to a relatively small comproportionation constant, Kc, of 2.5 × 104 for the intermediate. It is shown how electrochemical data (Eox, Ered, Kc) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(μ -tppz)RuCl(L)]n, particularly the competition between L and tppz for electron density from the metal. According to EPR (g1 = 2.470, g2 = 2.195, and g3 = 1.873 at 4 κ) the intermediate [2]3+ is a mixed-valent RuIIRuIII species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (ε = 1500 M-1 cm-1). The width at half-height (Δν½ ) of 700 cm-1 of the IVCT band is much smaller than the calculated value of 3584 cm-1, obtained by using the Hush formula Δν½ = (2310Eop)½ (Eop = 5556 cm-1, energy of the IVCT transition) which would be applicable to localized (Class II) mixed-valent RuIIRuIII systems. Valence delocalization in [2]3+ is supported by the uniform shift of the νC=O band of the N,N'-coordinated dpk ligands from 1676 cm-1 in the RuIIRuII precursor to 1690 cm-1 in the Ru2.5Ru2.5 form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but π-conjugated organic carbonyl group. The apparent contradiction between the moderate value of Kc and the narrow IVCT band is being discussed considering "borderline" or "hybrid" "Class II-III" concepts of mixed-valency, as well as coordination aspects, i.e., the bis-tridentate nature of the π-acceptor bridging ligand. Altogether, the complex ions [1]+ and [2]2+ display four and five successive reduction processes, respectively, involving both tppz- and dpk-based unoccupied π orbitals. The one-electron reduced form [2]+ has been assigned as a tppz·- radical-anion-containing species which exhibits a free-radical-type EPR signal at 4κ (g|| = 2.002, g⊥ = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (ε = 1100 M-1 cm-1).
Item Type: | Article |
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Source: | Copyright of this article belongs to American Chemical Society. |
ID Code: | 88386 |
Deposited On: | 29 Mar 2012 04:43 |
Last Modified: | 29 Mar 2012 04:43 |
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