Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of RuII polypyridyl complexes of the structural design [RuII(R−tpy)(NN)(CH3CN)]2+ (R−tpy=2,2′:6′,2′′-terpyridine (R=H) or 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine (R=tBu); NN=2,2′-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO2 to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes’ reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [RuII(tpy)(6-mbpy)(CH3CN)]2+ (trans-[3]2+; 6-mbpy=6-methyl-2,2′-bipyridine) and [RuII(tBu−tpy)(6-mbpy)(CH3CN)]2+ (trans-[4]2+), in which the methyl group of the 6-mbpy ligand is trans to the CH3CN ligand, show electrocatalytic CO2 reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer–chemical reaction–electron transfer), and is a direct result of steric interactions that facilitate CH3CN ligand dissociation, CO2 coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations

Tetra-μ-benzoato-κ4 O:O′;κ3 O:O,O′;κ3 O,O′:O′-bis­[(benzoato-κ2 O,O′)(1,10-phenanthroline-κ2 N,N′)neodymium(III)]

The complete mol­ecule of the title compound, [Nd2(C7H5O2)6(C12H8N2)2], is generated by a crystallographic inversion center. The two NdIII ions are linked by four bridging benzoate ions, with an Nd⋯Nd separation of 4.0360 (2) Å. As well as the bridging ligands, each NdIII ion is coordinated by one N,N′-bidentate phenanthroline ligand and an O,O′-bidentate benzoate ion. The resulting irregular nine-coordinated geometry of the NdIII ion is completed by seven O and two N atoms. The mol­ecular structure is stabilized by intra­molecular C—H⋯O hydrogen bonds. In the crystal structure, mol­ecules are linked into infinite chains along the c axis by inter­molecular C—H⋯O hydrogen bonds. The crystal structure is consolidated by weak inter­molecular C—H⋯π inter­actions

Tuning the photophysical properties of BODIPY dyes through extended aromatic pyrroles

A series of naphthyl and fluorantho-fused BODIPY dyes have been synthesized by a simple two-step process. These dyes display high molar absorptivities in the far visible region of the spectrum with emission quantum efficiencies at or near unity

Enhanced Electrocatalytic Reduction of Oxygen at Electrodes Coated with a Multi-Metallic Co(II)/Pt(II) Porphyrin

Edge plane pyrolytic graphite (EPG) electrodes coated with the Co(II)/Pt(II)2 analog of 5,15-bis-(4-pyridyl)-10,20-bis-(3-methoxy-4-hydroxyphenyl)porphyrin undergo an electrochemical-chemical-electrochemical (ECE) reaction when anodically scanned in 1.0 M sulfuric acid. The new redox couple formed from this anodic conditioning of the coated electrode is dependent on the pH of the solution. Roughened EPG electrodes coated with the Co(II)/Pt(II)2 trimetallic porphyrin show a catalytic shift of 400 mV for the reduction of O2 when compared to the reduction of O2 at a bare EPG electrode. An additional catalytic shift of ca. 150 mV is observed for O2 reduction at an EPG electrode coated with the Co(II)/Pt(II)2 porphyrin which has been oxidized in 1.0 M sulfuric acid. In addition to the added electrocatalysis a significant percentage of O2 reduced at the oxidized Co(II)/Pt(II)2 EPG electrode is converted to H2O as determined by rotating disk electrode measurements.</jats:p

Development of Transition Metal Complexes As Electrocatalysts for Small Molecules

In this presentation I will describe the work performed in my laboratory over the past decade (including our most recent work) involving undergraduate researchers. We have synthesized a number of transition metal complexes and have characterized them by spectroscopic and electrochemical techniques. In addition many of these complexes have been adsorbed onto graphite electrodes and studied by rotating disc electrochemistry for the reduction of oxygen and the evolution of hydrogen.</jats:p

A Stream-Lined Approach to Evaluating New Ru(II)-Bodipy Complexes for Photodynamic Therapy

Photodynamic therapy is a more targeted treatment modality; utilizing light, oxygen, and a photosensitizer (prodrug) to induce cell damage and ultimately cell death. Current photosensitizers have many limitations leading to extensive studies over the past decade to develop the next generation of this prodrug. Two promising paths have focused on boron dipyrromethenes (BODIPY) and ruthenium(II) polypyridyl complexes; however, BODIPY dyes do not exhibit efficient singlet to triplet intersystem crossing (necessary for generating reactive oxygen species) and ruthenium(II) polypyridyl complexes have weak or no absorption within the PDT window (600-850 nm). By coordinatively combining Ru(II) polypyridyl complexes with pi-extended BODIPY dyes we have shown the ability to generate singlet oxygen within the PDT window and thus have generated a new class of photosensitizer. </jats:p

(Invited) Electropolymerized Metalloporphyrins as Catalysts for the Reduction of Oxygen

Abstract not Available.</jats:p