Impact of Pendent Ammonium Groups on Solubility and Cycling Charge Carrier Performance in Nonaqueous Redox Flow Batteries.

The synthesis, characterization, electrochemical performance, and theoretical modeling of two base-metal charge carrier complexes incorporating a pendent quaternary ammonium group, [Ni(bppn-Me3)][BF4], 3', and [Fe(PyTRENMe)][OTf]3, 4', are described. Both complexes were produced in high yield and fully characterized using NMR, IR, and UV-vis spectroscopies as well as elemental analysis and single-crystal X-ray crystallography. The solubility of 3' in acetonitrile showed a 283% improvement over its neutral precursor, whereas the solubility of complex 4' was effectively unchanged. Cyclic voltammetry indicates an ∼0.1 V positive shift for all waves, with some changes in reversibility depending on the wave. Bulk electrochemical cycling demonstrates that both 3' and 4' can utilize the second more negative wave to a degree, whereas 4' ceases to have a reversible positive wave. Flow cell testing of 3' and 4' with Fc as the posolyte reveals little improvement to the cycling performance of 3' compared with its parent complex, whereas 4' exhibits reductions in capacity decay when cycling either negative wave. Postcycling CVs indicate that crossover is the likely source of capacity loss in complexes 3, 3', and 4' because there is little change in the CV trace. Density functional theory calculations indicate that the ammonium group lowers the HOMO energy in 3' and 4', which may impart stability to cycling negative waves while making positive waves less accessible. Overall, the incorporation of a positively charged species can improve solubility, stored electron density, and capacity decay depending on the complex, features critical to high energy density redox flow battery performance.

Role of Electrostatics in Influencing the Pathway by Which the Excited State of [Ru(bpy)3]2+ Is Deactivated by Ferrocene Derivatives.

Excited states of tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)3]2+, can be deactivated by a wide range of ferrocene derivatives. The pathway by which deactivation takes place, either energy transfer (EnT) or electron transfer (ET), depends on several factors inherent to each specific donor-acceptor (D···A) system. In this work, we provide mechanistic insight into bimolecular quenching between [Ru(bpy)3]2+* and several ferrocene (Fc) derivatives in a variety of solvents. By introducing various functional groups onto the cyclopentadienyl ring of ferrocene, the chemical properties of the organometallic complexes were altered by tuning the oxidation potentials and charge of the iron complexes, and the manner in which the [Ru(bpy)3]2+ excited state is quenched by each ferrocene complex in solvents of various dielectric constants, including anhydrous acetonitrile (ACN), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and water (pH 10), was assessed. Through the use of transient absorption (TA) spectroscopy, the mechanism of [Ru(bpy)3]2+* quenching by each of five ferrocene derivatives (i.e., either EnT or ET) in the aforementioned solvents was evaluated. On the basis of these studies, electrostatic factors relating to the charge on the ferrocene moiety were found to influence the quenching pathway(s) for the [Ru(bpy)3]2+···Fc systems under interrogation. When the ferrocene moiety is positively charged, the [Ru(bpy)3]2+ excited state is quenched by EnT to Fc, while when the ferrocene moiety is neutral or negatively charged, the [Ru(bpy)3]2+ excited state is quenched via reductive ET.

pH-Driven Mechanistic Switching from Electron Transfer to Energy Transfer between [Ru(bpy)3]2+ and Ferrocene Derivatives.

The metal-to-ligand charge transfer excited states of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) may be deactivated via energy transfer or electron transfer with ferrocene derivatives in aqueous conditions. Stern-Volmer quenching analysis revealed that the rate constant for [Ru(bpy)3]2+ excited-state quenching depends on solution pH when a ferrocenyl-amidinium derivative (Fc-am) containing a proton-responsive functionality tethered to the ferrocene center was present. By contrast, the rate constant with which the [Ru(bpy)3]2+ excited state is quenched by an analogous ferrocene derivative (ferrocenyl-trimethylammonium, Fc-mam) that lacks a protonic group does not depend on pH. These results show that the presence (or absence) of a readily transferrable proton modulates quenching rate constants in bimolecular events involving [Ru(bpy)3]2+ and ferrocene. More surprisingly, transient absorption spectroscopy reveals that the mechanism by which the [Ru(bpy)3]2+ excited state is quenched by Fc-am appears to be modulated by solution proton availability, switching from energy transfer at low pH when Fc-am is protonated, to electron transfer at high pH when Fc-am is deprotonated. The mechanistic switching that is observed for this system cannot be aptly explained using a simple driving force dependence argument, suggesting that more subtle factors dictate the pathway by which the [Ru(bpy)3]2+ excited state is deactivated by ferrocene in aqueous solutions.

Deconvoluting the Innocent vs. Non-innocent Behavior of N,N-diethylphenylazothioformamide Ligands with Copper Sources.

Redox-active ligands lead to ambiguity in often clearly defined oxidation states of both the metal centre and the ligand. The arylazothioformamide (ATF) ligand class represents a redox-active ligand with three possible redox states (neutral, singly reduced, and doubly reduced). ATF-metal interactions result in strong colorimetric transitions allowing for the use of ATFs in metal detection and/or separations. While previous reports have discussed dissolution of zerovalent metals, the resulting oxidation states of coordination complexes have proved difficult to interpret through X-ray crystallographic analysis alone. This report describes the X-ray crystallographic analysis combined with computational modelling of the ATF ligand and metal complexes to deconvolute the metal and ligand oxidation state of metal-ATF complexes. Metal(ATF)2 complexes that originated from zerovalent metals were found to exist as dicationic metal centers containing two singly reduced ATF ligands. When employing Cu(I) salts instead of Cu(0) to generate copper-ATF complexes, the resulting complexes remained Cu(I) and the ATF ligand remained "innocent", existing in its neutral state. Although the use of CuX (where X = Br or I) or [Cu(NCMe)4]Y (where Y = BF4 or PF6) generated species of the type: [(ATF)Cu(µ-X)]2 and [Cu(ATF)2]Y, respectively, the ATF ligand remained in its neutral state for each species type.

Synthesis and structure of palladium(II) complexes supported by bis-NHC pincer ligands for the electrochemical activation of CO2.

A series of bis-NHC pincer complexes of palladium(II) have been prepared and characterized. These pyridyl-spaced dicarbene complexes ([(PDCR)Pd(MeCN)](PF6)2 ) were synthesized with substituents of varying steric bulk at the wingtip positions, which include R = methyl, ethyl, isopropyl, cyclohexyl, mesityl and 2,6-diisopropylphenyl. The synthesis of this library of complexes was accomplished either by direct metallation of the prerequisite pyridyl-spaced bis-imidazolium proligands with Pd(OAc)2 or via treatment with Ag2O to afford the corresponding silver carbenes, which were then transmetallated onto palladium. Solid-state structures for each of the [(PDCR)Pd(MeCN)](PF6)2 derivatives were obtained via X-ray crystallography and allowed for the steric properties of each PDCR ligand to be evaluated by two methods. These analyses, which included calculation of the percent buried volume (%VBur) and solid angles of the PDCR ligands, served to characterize the steric environment around the palladium center in each of the complexes that was prepared. Finally, voltammetry and controlled potential electrolysis studies were performed to characterize the redox chemistry of the [(PDCR)Pd(MeCN)](PF6)2 derivatives and assess if they could electrocatalyze the reduction of CO2. The influence of the steric properties of the PDCR ligand on the electrochemistry of the resulting complexes [(PDCR)Pd(MeCN)](PF6)2 is also discussed.

Rapid Bioorthogonal Chemistry Turn-on through Enzymatic or Long Wavelength Photocatalytic Activation of Tetrazine Ligation.

Rapid bioorthogonal reactivity can be induced by controllable, catalytic stimuli using air as the oxidant. Methylene blue (4 µM) irradiated with red light (660 nm) catalyzes the rapid oxidation of a dihydrotetrazine to a tetrazine thereby turning on reactivity toward trans-cyclooctene dienophiles. Alternately, the aerial oxidation of dihydrotetrazines can be efficiently catalyzed by nanomolar levels of horseradish peroxidase under peroxide-free conditions. Selection of dihydrotetrazine/tetrazine pairs of sufficient kinetic stability in aerobic aqueous solutions is key to the success of these approaches. In this work, polymer fibers carrying latent dihydrotetrazines were catalytically activated and covalently modified by trans-cyclooctene conjugates of small molecules, peptides, and proteins. In addition to visualization with fluorophores, fibers conjugated to a cell adhesive peptide exhibited a dramatically increased ability to mediate contact guidance of cells.

A tris(pyrazol-1-yl)methane bearing carboxylic acid groups at position 4: {1-[bis(4-carboxy-3,5-dimethyl-1H-pyrazol-1-yl)methyl]-3,5-dimethyl-1H-pyrazole-4-carboxylato}sodium dihydrate.

A low-resolution X-ray molecular structure of the title hydrated salt, [Na(C19H21N6O6)]·2H2O, displays scorpionate character and resolves apparent ambiguities between solution and solid-state NMR spectroscopies. The (13)C NMR CPMAS spectrum is consistent with this structure showing some splittings, which have been rationalized using GIAO/B3LYP/6-311++G(d,p) theoretical calculations.

Linked Picolinamide Nickel Complexes as Redox Carriers for Nonaqueous Flow Batteries.

The use of nickel complexes utilizing non-innocent ligands based on picolinamide to function as redox carriers in flow batteries was explored. The picolinamide moiety was linked together with -CH2 CH2 - (bpen), -CH2 CH2 CH2 - (bppn), and -C6 H4 - (bpb) moieties, resulting in two, three, and four quasi-reversible waves, respectively, for the nickel complexes and >3 V difference between the outermost positive and negative waves. The redox events were theoretically modelled for each complex, showing excellent agreement (<0.3 V difference) between the experimental and modelled potentials. Bulk cycling of the most soluble complex, Ni(bppn), indicated only one of the three waves was reversible. Therefore, Ni(bppn) has the ability to act as a negative charge redox carrier in flow cells.

Photocatalytic Conversion of CO2 to CO using Rhenium Bipyridine Platforms Containing Ancillary Phenyl or BODIPY Moieties.

Harnessing of solar energy to drive the reduction of carbon dioxide to fuels requires the development of efficient catalysts that absorb sunlight. In this work, we detail the synthesis, electrochemistry and photophysical properties of a set of homologous fac-ReI(CO)3 complexes containing either an ancillary phenyl (8) or BODIPY (12) substituent. These studies demonstrate that both the electronic properties of the rhenium center and BODIPY chromophore are maintained for these complexes. Photolysis studies demonstrate that both assemblies 8 and 12 are competent catalysts for the photochemical reduction of CO2 to CO in DMF using triethanolamine (TEOA) as a sacrificial reductant. Both compounds 8 and 12 display TOFs for photocatalytic CO production upon irradiation with light (λex ≥ 400 nm) of ~5 hr-1 with TON values of approximately 20. Although structural and photophysical measurements demonstrate that electronic coupling between the BODIPY and fac-ReI(CO)3 units is limited for complex 12, this work clearly shows that the photoactive BODIPY moiety is tolerated during catalysis and does not interfere with the observed photochemistry. When taken together, these results provide a clear roadmap for the development of advanced rhenium bipyridine complexes bearing ancillary BODIPY groups for the efficient photocatalytic reduction of CO2 using visible light.