Syntheses, Structures, and Redox Properties of Dimeric Triruthenium Clusters Bridged by Bis(diphenylphosphino)acetylene and -ethylene

Reactions of oxo-centered triruthenium acetate complexes [Ru3O(OAc)6(py)2(CH3OH)](PF6) (py = pyridine, OAc =
CH3COO-) (1) with nearly equimolar amounts of dppa {bis(diphenylphosphino)acetylene} or dppen {trans-1,2-bis(diphenylphosphino)ethylene} gave [Ru3O(OAc)6(py)2(L)](PF6) (L = dppa, 2; dppen, 3). With 2.4 equiv of 1, the
reactions provided diphosphine-linked triruthenium dimers, [{Ru3O(OAc)6(py)2}2(L)](PF6)2 (L = dppa, 4; L = dppen,
5), respectively. Similarly, the reactions of [Ru3O(OAc)6(L‘)2(MeOH)]+ {L‘ = dmap (4-(dimethylamino)pyridine), 1a;
L‘ = abco (1-azabicyclo[2.2.2]octane), 1b} with dppen gave dppen-linked dimers, [{Ru3O(OAc)6(dmap)2}2(dppen)](SbF6)2 (6) and [{Ru3O(OAc)6(abco)2}2(dppen)](BF4)2 (7), respectively. The chemical reduction of 2, 4, and 5 by
hydrazine afforded one- or two-electron-reduced, neutral products, Ru3O(OAc)6(py)2(dppa) (2a), {Ru3O(OAc)6(py)2}2(dppa) (4a), and {Ru3O(OAc)6(py)2}2(dppen) (5a), respectively. The complexes were characterized by elemental
analyses, ES-MS, UV−vis, IR, and 31P NMR spectroscopies, and cyclic and differential-pulse voltammetries. The
molecular structures of compounds 2, 4, 5, 5a, 6, and 7 were determined by single-crystal X-ray diffraction. In 0.1
M (Bu4N)PF6−acetone, the monomers and dimers of triruthenium clusters show reversible and multistep redox
responses. The two triruthenium cluster centers in dimers undergo stepwise reductions and oxidations due to the
identical redox processes of the individual Ru3O cluster cores, suggesting the presence of electronic communications
between them through the conjugated diphosphine spacer. The redox wave splitting mediated by dppa containing
an ethynyl group (C⋮C) is found to be more extensive than that by dppen containing an ethenyl (CC) one. It
appears that the redox wave splitting is enhanced by the introduction of electron-donating substituents on the
auxiliary pyridine rings.