DECANUCLEAR HOMOMETALLIC AND HETEROMETALLIC POLYPYRIDINE COMPLEXES - SYNTHESES, ABSORPTION-SPECTRA, LUMINESCENCE, ELECTROCHEMICAL OXIDATION, AND INTERCOMPONENT ENERGY-TRANSFER

Decanuclear homo- and heterometallic complexes of the general formula M(c){(BL)M(i)[(BL)M(p)(L)2]2}3(20+) have been prepared as PF6- salts. The central (M(c)), intermediate (M(i)), and peripheral (M(p)) metal ions are Ru2+ or Os2+, BL is the bridging ligand 2,3-bis(2-pyridyl)pyrazine (2,3-dpp), and L is either 2,2'-bipyridine (bpy) or 2,2'-biquinoline (biq). The decanuclear complexes have been synthesized by the reaction of appropriate building blocks, namely M(c)(BL)3(2+) cores, which contain three free chelating sites, with M(i)[(BL)M(p)(L)2]2Cl2(4+) species, which contain easily replaceable Cl- ligands. The six compounds prepared contain the following metals (M) and ligands (L) (BL is 2,3-dpp in all cases): 1, M(c) = M(i) = M(p) = Ru2+, L = bpy; 2 M(c) = M(i) = M(p) = Ru2+, L = biq; 3, M(c) = Os2+, M(i) = M(p) = Ru2+, L = bpy; 4, M(c) = Os2+, M(i) = M(p) = Ru2+, L = biq; 5, M(c) = M(p) = Os2+, M(i) = Ru2+, L = bpy; 6, M(c) = M(i) = Ru2+, M(p) = Os2+, L = bpy. 1-6 display extremely intense L- and BL-centered absorption bands in the UV region (epsilon up to approximately 6 X 10(5) M-1 cm-1) and intense M --> L and M --> BL charge transfer (CT) bands in the visible region (lambda(max) around 550 nm, epsilon about 1.3 X 10(5) M-1 cm-1). Cyclic voltammetry and differential pulse voltammetry measurements show that in 1 and 2 the first oxidation wave (E1/2 = +1.43 and +1.62 V, respectively) represents a six-electron process, as expected for independent reversible one-electron transfer, at the same potential, of the six peripheral RU2+ ions, while oxidation of the central and of the three intermediates RU2+ ions cannot be observed in the potential window examined (< +1.8 V vs SCE). For 3 and 4, two reversible waves are observed at +1.17 and +1.50 V (for 3) and at +1.24 and +1.59 V (for 4) with 1:6 intensity ratio, as expected for oxidation of the central Os2+ ion followed by oxidation of the six peripheral Ru2+ ions. For 5, a reverse intensity pattern is observed (waves at +1.05 and +1.39 V, with 6:1 intensity ratio), as expected for oxidation of the six peripheral Os2+ ions followed by oxidation of the central Os2+ ion. For 6, a six-electron process with E1/2 = +1.00 V is observed, assigned to the oxidation of the six peripheral Os2+ ions. Compounds 1-4 exhibit luminescence in deaerated acetonitrile solution (lambda(max) = 809, 789, 808, and 789 nm; tau = 55, 130, 65, and 125 ns, respectively), while no luminescence can be observed (below the instrumental limit of our equipment, 880 nm) for 5 and 6. Corrected excitation spectra, luminescence lifetimes, and comparison with the known properties of analogous complexes of lower nuclearity show that (i) in 1 and 2 100% efficient energy transfer occurs from the central and intermediate components to the peripheral components, where the lowest energy luminescent 3CT level is localized; (ii) in 3 and 4, the broad luminescence band can be resolved into a weak emission resulting from the central Os-based component, which contains the lowest energy 3CT level, and a stronger emission originating from the peripheral Ru-based components; (iii) in 5 and 6, the potentially luminescent M(c)- and M(i)-based components are quenched by the nonluminescent (at lambda < 880 nm) peripheral Os-based components. Supramolecular species like 1-6 are interesting because they contain many chromophoric and redox centers and, at least in four cases, display luminescence from relatively long-lived excited states.