Syntheses and characterization of oxo-centered triruthenium compounds with orthometalated bipyridine

Reaction of oxo-centered triruthenium precursor compound Ru3O(OAc)6(py)2(CH3OH)](PF6) (1) with 1.3 equiv of bipyridine ligand at ambient temperature gave oxo-centered triruthenium derivatives Ru3O(OAc)5{micro-eta1(C),eta2(N,N)bipyridine}(py)2](PF6) (bipyridine = 4,4'-dibutyl-2,2'-bipyridine (dbbpy) (2),4,4'-dimethyl-2,2'-bipyridine (dmbpy) (3), 2,2'-bipyridine (bpy) (4), 5,5'-dibromo-2,2'-bipyridine (Br2bpy) (5), 1,10-phenanthroline (phen) (6)). Formation of compounds 2-6 involved substitution of the axial methanol and one of bridging acetates in the precursor compound 1 by an orthometalated bipyridine. Reduction of 2 and 4 by addition of excess hydrazine gave one-electron-reduced neutral products Ru3O(OAc)5(py)2{micro-eta1 (C),eta2(N,N)-bipyridine} (bipyridine = dbbpy (2a), bpy (4a)). As established in the structure of 3 by X-ray crystallography, the orthometalated 2,2'-bipyridine adopts a micro-eta1(C),eta2(N,N) bonding mode. In the 1H NMR spectra of 2-6, the protons of acetate, pyridine, and bipyridine show obvious paramagnetic shifts. Tentative assignments of these proton signals were carried out. Absorption spectra of the bipyridine triruthenium derivatives show characteristic intracluster charge transfer (IC) transitions in the visible to near-infrared region (600-1000 nm) and cluster-to-ligand charge transfer (CLCT) transitions at 320-450 nm. By comparison of the redox data for 2-6, it is concluded that introducing electron-donating substituents to the bipyridine favors stabilizing the RuIII3]+ and RuIII2RuII]0 states against disproportionation.