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

Supramolecular mixed metal complexes combining the trimetallic chromophore [{(bpy)2Ru(dpp)}2Ru(dpp)]6+ (Ru3) with [Rh(bpy)Cl2]+ or [RhCl2]+ catalytic fragments to form [{(bpy)2Ru(dpp)}2Ru(dpp)RhCl2(bpy)](PF6)7 (Ru3Rh) or [{(bpy)2Ru(dpp)}2Ru(dpp)]2RhCl2(PF6)13 (Ru3RhRu3) (bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) catalyze the photochemical reduction of protons to H2. This first example of a heptametallic Ru,Rh photocatalyst produces over 300 turnovers of H2 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 H2 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 3MLCT excited and cannot be formed by static electron transfer quenching of the 3MLCT state. Instead, a mechanism of subnanosecond charge separation from high lying states is proposed. Multiple reductions of Ru3 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 Ru3 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.