Labile Ligand Variation in Polyazine-Bridged Ruthenium/Rhodium Supramolecular Complexes Providing New Insight into Solar Hydrogen Production from Water

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Date
2015-12-15
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Virginia Tech
Abstract

Mixed-metal supramolecular complexes containing one or two RuII light absorbing subunits coupled through polyazine bridging ligands to a RhIII reactive metal center were prepared for use as photocatalysts for the production of solar H2 fuel from H2O. The electrochemical, photophysical, and photochemical properties upon variation of the monodentate, labile ligands coordinated to the Rh reactive metal center were investigated.

Bimetallic complexes [(Ph2phen)2Ru(dpp)RhX2(Ph2phen)]3+ (Ph2phen = 4,10-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine; X = Br- or Cl-) were prepared using a building block approach, allowing for selective component choice. The identity of the halide coordinated to Rh did not impact the light absorbing or excited state properties of the structural motif. However, the o-donating ability of the halides modulated the Rh-based cathodic electrochemistry and required the use of multiple pathways to explain the reduction of Rh by two electrons. Regardless of halide identity, the bimetallic complex possessed a Ru-based HOMO (highest occupied molecular orbital) and Rh-based LUMO (lowest unoccupied molecular orbital) important for photoinitiated electron collection at Rh. As a photocatalyst for H2 evolution, the X = Br- complex produced nearly 30% more H2 than the X = Cl- analogue. H2 production experiments with added halide suggested that ion pairing with halides played a major role in catalyst deactivation, which provided evidence for the importance of component selection for photocatalyst design.

New trimetallic complex {(bpy)2Ru(dpp)}2Ru(OH)25 (bpy = 2,2'-bipyridine) was prepared for comparison to halide analogues {(bpy)2Ru(dpp)}2RhX25 (X = Br- or Cl-). The synthesis of a halide-free supramolecule containing OH- ligands afforded an ideal system to further examine the impact of the ligands at the reactive metal center on H2 photocatalysis. Electrochemistry results revealed that while the identity of the ligands at Rh did modulate the Rh-based reduction potential, all three complexes possessed a Ru-based HOMO and Rh-based LUMO. The light absorbing properties were not impacted by the identity of the monodentate ligands at Rh; however, the excited state properties did vary upon changing the ligands at Rh. The hydroxo trimetallic complex functioned as a photocatalyst for H2 production in organic solvent, producing nearly double the amount of H2 as the highest performing Br-' trimetallic complex in DMF solvent. Interestingly, H2 production studies in high dielectric aqueous solvent revealed no discrepancies in H2 evolution upon variation of the ligands at Rh, which further supported the ion pairing phenomenon realized for the bimetallic motif.

Variation of the labile ligands coordinated to the Rh reactive metal center in RuIIRhIII multimetallic supramolecules provided important insight about the large impact of small structural variation on H2 photocatalysis. Electrochemical, photophysical, and photochemical studies of new RuIIRhIII complexes afforded a deeper understanding of the molecular processes important for the design of new complexes applicable to solar fuel production schemes.

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Keywords
inorganic chemistry, supramolecular complexes, photochemical molecular devices, electrochemistry, photophysics, photochemistry, ruthenium, rhodium, halides, hydroxide, ion pairing, photocatalysis, solar fuels, hydrogen production
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