Mechanistic Insight into the Electronic Influences Imposed by Substituent Variation in Polyazine‐Bridged Ruthenium(II)/Rhodium(III) Supramolecules

Abstract

What is the most significant result of this study?The work presented in this paper uncovers new redox pathways for metal complexes. Electron-transfer or redox reactions are critical to many areas of chemical research, as well as to the functioning of biological systems; thus, uncovering new redox pathways has a wide ranging impact. Unprecedented redox properties of Rh complexes are discovered in this study. The Ru II /Rh III complexes undergo a number of different, concurrent electrochemical reduction mechanisms. The competing reduction pathways give rise to peak currents that do not correspond to integer values for the number of electrons transferred, presenting a very unusual result for single molecular systems not reacting with a substrate. In addition, Rh II complexes are prepared with unusual stability in marked contrast to the wide array of Rh II complexes prone to rapid disproportionation. What aspects of this project do you find most exciting?Uncovering unexpected reactivity of Ru II /Rh III complexes after several decades of studying related metal complexes highlights the challenges that still exist in molecular design. Development of molecular machines for the conversion of light or electrical energy into chemical energy is a challenging and exciting field. The systems we study apply these molecular devices to solar energy conversion applicable in water splitting and artificial photosynthesis. The assembly of supramolecules built from individual subunits that each performs a simple task leading to the overall molecular device accomplishing a complex function is a promising field of study with limitless applications and much new chemistry to explore. To think about molecules as atomic scale machines is an important view of chemistry. The new reactivity of reduced Rh complexes uncovered in this study will provide insight into the mechanism of water reduction by metal complexes. Uncovering the factors that impact partitioning of the analyte along the competing reductive pathways will allow for control of catalyst speciation in the catalytic cycle and development of more active catalysts for solar energy harvesting and conversion. What other topics are you working on at the moment?The Brewer group at Virginia Tech is exploring other areas of research including other areas of solar energy conversion, NSF sponsored research in the development of structurally diverse supramolecules with unusual reactivity with biomolecules, the development of new drugs to treat cancer, polymer-supported supramolecules for light to energy conversion, and light cycled materials that are self-sterilizing. We have had a number of outstanding collaborations including with Prof. Invited for the cover of this issue are researchers from the Department of Chemistry at Virginia Tech featuring work on electrochemical properties of photochemical molecular devices for solar hydrogen production. The image highlights the complexity of the redox chemistry of the studied supramolecules, detailing speciation following reduction. Mr. Skye King a...

Increased photocatalytic activity in Ru(II),Rh(III) supramolecular bimetallic complexes with terminal ligand substitution

“…Cyclic voltammograms (CVs) of the title Ru(II),Rh(III) complexes (Fig. 3) display similar redox-activity and features as those for previously reported polyazine-bridged Ru(II),Rh(III) bimetallic complexes that possess a cis-Rh III X 2 center [29][30][31][32][33]46]. Anodic scans display a reversible, one-electron Ru II/III oxidation at E ½ = +1.60 V vs. Ag/AgCl for all three bimetallic complexes, indicative of minimal-to-negligible electronic communication between the Ru(II) metal center and the terminal ligand coordinated to Rh(III).…”

“…3). This first cathodic couple is labeled as an ECEC process consisting of a Rh III/II Cl 2 reduction followed by chloride dissociation and subsequent Rh II/I Cl reduction prior to cleavage of the second Rh-Cl bond [32]. Varied reversibility of the first cathodic couple is attributed to differing rates of chloride dissociation following the first reduction.…”

“…Simulations provide an estimated k ÀCl value of 0.2 s À1 for Ru-Rh(Me 2 bpy) and Ru-Rh(bpy) and 0.7 s À1 for Ru-Rh(dmeb). The second cathodic couple is labeled an EECC process whereby a portion of the one-electron reduced species, Rh II Cl 2 , does not dissociate chloride rapidly and undergoes further reduction prior to dissociation of both chloride ligands [32]. The second cathodic couple shifts from E p c = À0.82 V vs. Ag/AgCl for Ru-Rh(Me 2 bpy) and E p c = À0.80 V vs. Ag/AgCl for Ru-Rh(bpy) to E p c = À0.68 V vs. Ag/AgCl for Ru-Rh(dmeb) which supports more rapid chloride dissociation with the methyl estersubstituted TL.…”

“…Cathodic scans present somewhat complicated electrochemistry given the nearly isoenergetic nature of the dpp(p ⁄ ) and Rh (dr ⁄ ) unoccupied MOs and the competitive electrochemical mechanisms upon reduction [32]. The bimetallic complexes display a reversible reduction at À0.41 V vs. Ag/AgCl for Ru-Rh(Me 2 bpy) and Ru-Rh(bpy) and a quasi-reversible reduction at À0.38 V vs. Ag/AgCl for Ru-Rh(dmeb) (Fig.…”

“…The judicious choice of molecular components throughout the Ru(II),Rh(III) bimetallic architecture dictates photocatalytic activity towards H 2 O reduction through a careful balance of sterics, electronics, light absorption, and ion pairing throughout the photocatalytic cycle [30][31][32][33][34]. The bimetallic complexes [(Ph 2 phen) 2 Ru(dpp)RhX 2 (Ph 2 phen)] 3+ (X = Cl À or Br À ) represent a systematic study undertaken to determine the influence that the r-donating ability and degree of ion pairing imparted by the monodentate halide ligands have on the photocatalytic H 2 O reduction activity [30].…”