2,2 '-Dipyridylketone (dpk) as ancillary acceptor and reporter ligand in complexes [(dpk)(Cl)Ru(mu-tppz)Ru(Cl)(dpk)](n+) where tppz=2,3,5,6-tetrakis(2-pyridyl)pyrazine
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The tppz-bridged diruthenium(II) complex [(dpk)(Cl) RuII(A-tppz) Ru-II(Cl)(dpk)](ClO4)(2), (ClO4)(2), and mononuclear [(dpk)(Cl) Ru-II(tppz)](ClO4), (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+) translates to a relatively small comproportionation constant, K-c, of 2.5 x 10(4) for the intermediate. It is shown how electrochemical data (E-ox, E-red, K-c) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(mu-tppz)-RuCl( L)](n), particularly the competition between L and tppz for electron density from the metal. According to EPR (g(1) = 2.470, g(2)) 2.195, and g(3)) 1.873 at 4K) the intermediate (3+) is a mixed-valent (RuRuIII)-Ru-II species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (epsilon = 1500 M-1 cm(-1)). The width at half-height (Delta(v1/2)) 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 Delta(v1/2) = (2310E(op))(1/2)(E-op = 5556 cm(-1), energy of the IVCT transition) which would be applicable to localized (Class II) mixed- valent (RuRuIII)-Ru-II systems. Valence delocalization in (3+) is supported by the uniform shift of the v(C=O) band of the N, N '-coordinated dpk ligands from 1676 cm(-1) in the (RuRuII)-Ru-II precursor to 1690 cm(-1) in the (RuRu2.5)-Ru-2.5 form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but pi-conjugated organic carbonyl group. The apparent contradiction between the moderate value of K c 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 pi-acceptor bridging ligand. Altogether, the complex ions (+) and (2+) display four and five successive reduction processes, respectively, involving both tppz- and dpk-based unoccupied pi* orbitals. The one-electron reduced form (+) has been assigned as a tppz center dot(-) radical-anion-containing species which exhibits a free-radical-type EPR signal at 4K (g(II) = 2.002, g = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (epsilon = 1100 M-1 cm(-1)).
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