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.