Synthesis, structure, and reactivity of new tetranuclear Ru-Hbpp-based water-oxidation catalysts

The preparation of three new octadentate tetranucleating ligands made out of two Ru-Hbpp-based units [where Hbpp is 3,5(bispyridyl)pyrazole], linked by a xylyl group attached at the pyrazolate moiety, of general formula (Hbpp) 2-u-xyl (u = p, m, or o) is reported, together with its dinucleating counterpart substituted at the same position with a benzyl group, Hbpp-bz. All of these ligands have been characterized with the usual analytical and spectroscopic techniques. The corresponding tetranuclear ruthenium complexes of general formula {[Ru2(trpy)2(L)]2(μ-(bpp) 2-u-xyl)}n+ [L = Cl or OAc, n = 4; L = (H 2O)2, n = 6] and their dinuclear homologues {[Ru 2(trpy)2(L)](μ-bpp-bz)}n+ [L = Cl or OAc, n = 2; L = (H2O)2, n = 3] have also been prepared and thoroughly characterized both in solution and in the solid state. In solution, all of the complexes have been characterized spectroscopically by UV-vis and NMR and their redox properties investigated by means of cyclic voltammetry techniques. In the solid state, monocrystal X-ray diffraction analysis has been carried out for two dinuclear complexes {[Ru2(trpy) 2(L)](μ-bpp-bz)}2+ (L = Cl and OAc) and for the tetranuclear complex {[Ru2(trpy)2(μ-OAc)] 2(μ-(bpp)2-m-xyl)}4+. The capacity of the tetranuclear aqua complexes {[Ru2(trpy)2(H 2O)2]2(μ-(bpp)2-u-xyl)} 6+ and the dinuclear homologue {[Ru2(trpy) 2(H2O)2](μ-bpp-bz)}3+ to act as water-oxidation catalysts has been evaluated using cerium(IV) as the chemical oxidant in pH = 1.0 triflic acid solutions. It is found that these complexes, besides generating significant amounts of dioxygen, also generate carbon dioxide. The relative ratio of [O2]/[CO2] is dependent not only on para, meta, or ortho substitution of the xylylic group but also on the concentration of the starting materials. With regard to the tetranuclear complexes, the one that contains the more sterically constrained ortho-substituted ligand generates the highest [O2]/[CO2] ratio. © 2011 American Chemical Society.