Tetra- and Heptametallic Ru(II),Rh(III) Supramolecular Hydrogen Production Photocatalysts

Supramolecular mixed metal complexes combining the trimetallic chromophore [{(bpy)(2)Ru(dpp)}(2)Ru(dpp)](6+) (Ru-3) with [Rh(bpy)Cl-2](+) or [RhCl2](+) catalytic fragments to form [{(bpy)(2)Ru(dpp)}(2)Ru(dpp)RhCl2(bpy)] (PF6)(7) (Ru3Rh) or [{(bpy)(2)Ru(dpp)}(2)Ru(dpp)](2)RhCl2(PF6)(13) (Ru3RhRu3) (bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) catalyze the photochemical reduction of protons to H-2. This first example of a heptametallic Ru,Rh photocatalyst produces over 300 turnovers of H-2 upon photolysis of a solution of acetonitrile, water, triflic acid, and N,N-dimethylaniline as an electron donor. In contrast, the tetrametallic Ru3Rh produces only 40 turnovers of H-2 due to differences in the excited state properties and nature of the catalysts upon reduction as ascertained from electrochemical data, transient absorption spectroscopy, and flash-quench experiments. While the lowest unoccupied molecular orbital of Ru3Rh is localized on a bridging ligand, it is Rh-centered in Ru3RhRu3 facilitating electron collection at Rh in the excited state and reductively quenched state. The Ru -> Rh charge separated state of Ru3RhRu3 is endergonic with respect to the emissive Ru -> dpp (MLCT)-M-3 excited and cannot be formed by static electron transfer quenching of the (MLCT)-M-3 state. Instead, a mechanism of subnanosecond charge separation from high lying states is proposed. Multiple reductions of Ru-3 and Ru3Rh using sodium amalgam were carried out to compare UV-vis absorption spectra of reduced species and to evaluate the stability of highly reduced complexes. The Ru-3 and Ru3Rh can be reduced by 10 and 13 electrons, respectively, to final states with all bridging ligands doubly reduced and all bpy ligands, singly reduced.