On the Determination of Halogen Atom Reduction Potentials with Photoredox Catalysts.

The standard one-electron reduction potentials of halogen atoms, °'(X), and many other radical or unstable species, are not accessible through standard electrochemical methods. Here, we report the use of two Ir(III) photoredox catalysts to initiate chloride, bromide, and iodide oxidation in organic solvents. The kinetic rate constants were critically analyzed through a derived diffusional model with Marcus theory to estimate °'(X) in propylene carbonate, acetonitrile, butyronitrile, and dichloromethane.

Photophysical characterization of new osmium (II) photocatalysts for hydrohalic acid splitting.

Two osmium(II) photocatalysts bearing a dicationic 4,4'-bis-(trimethylaminomethyl)-2,2'-bipyridine (tmam) ligand and 2,2'-bipyridine {[Os(bpy)(tmam)]} or 4,4'-(CF)-2,2'-bipyridine {[Os((CF)bpy)(tmam)]} ancillary ligands were synthesized and characterized for application in HX splitting. Iodide titration studies in acetone solutions provided evidence for an in situ formed terionic complex with two iodide ions as evidenced by H NMR and UV-visible absorption spectroscopies, as well as by density functional theory calculations and natural bond order analysis. The photocatalyst [Os(bpy)(tmam)] was shown to be inefficient in iodide oxidation.

Tuning the excited-state deactivation pathways of dinuclear ruthenium(ii) 2,2'-bipyridine complexes through bridging ligand design.

A detailed photophysical investigation of two dinuclear ruthenium(ii) complexes is reported. The two metallic centers were coordinated to a bis-2,2'-bipyridine bridging ligand, connected either through the para (Lp, Dp) or the meta position (Lm, Dm). The results obtained herein were compared to the prototypical [Ru(bpy)3]2+ parent compound.

Photostable Polynuclear Ruthenium(II) Photosensitizers Competent for Dehalogenation Photoredox Catalysis at 590 nm.

Higher nuclearity photosensitizers produced dehalogenation yields greater than 90% in the reported [Ru(bpy)]-mediated dehalogenation of 4-bromobenzyl-2-chloro-2-phenylacetate to 4-bromobenzyl-2-phenylacetate with orange light in 7 h, whereas after 72 h yields of 49% were obtained with [Ru(bpy)]. Dinuclear (), trinuclear (), and quadrinuclear () ruthenium(II) 2,2'-bipyridine based photosensitizers were synthesized, characterized, and investigated for their photoreactivity. Three main factors were shown to lead to increased yields (i) the red-shifted absorbance of polynuclear photosensitizers, (ii) the more favorable driving force for electron transfer, characterized by more positive (Ru), and (iii) the smaller population of the MC state (<0.

Improved Visible Light Absorption of Potent Iridium(III) Photo-oxidants for Excited-State Electron Transfer Chemistry.

Three iridium photosensitizers, [Ir(dCFppy)(N-N)], where N-N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino[2,3-]phenazine (pzph), or benzo[]pyrazino[2,3-]phenazine (bpph) and dCFppy is 2-(3,5-bis(trifluoromethyl-phenyl)pyridine), were found to be remarkably strong photo-oxidants with enhanced light absorption in the visible region. In particular, judicious ligand design provided access to , with a molar absorption coefficient, ε = 9800 M cm, at 450 nm and an excited-state reduction potential, (Ir) = 1.76 V vs NHE.

Halide Photoredox Chemistry.

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X-X bond-breaking and -forming reactivity.

Photophysical Properties of Tetracationic Ruthenium Complexes and Their Ter-Ionic Assemblies with Chloride.

The synthesis of seven ruthenium(II) polypyridyl complexes bearing one dicationic bis-4,4'-(trimethylaminomethyl)-2,2'-bipyridine (tmam) ligand is reported. The ancillary ligands of each complex were 2,2'-bipyrazine (bpz), 2,2'-bipyridine (bpy), 4,4'- tert-butyl-2,2'-bipyridine (dtb), 4,4'-dimethyl-2,2'-bipyridine (4,4'-dmb), 5,5'-dimethyl-2,2'-bipyridine (5,5'-dmb), 4,4'-nonyl-2,2'-bipyridine (nonyl), and 4,4'-methoxy-2,2'-bipyridine (MeO). The metal-to-ligand charge transfer excited state was localized on the tmam ligand in all instances with the exception of [Ru(bpz)(tmam)], where it was localized on the bpz ligand.

Ter-Ionic Complex that Forms a Bond Upon Visible Light Absorption.

A "ter-ionic complex" composed of a tetracationic Ru(II) complex and two iodide ions was found to yield a covalent I-I bond upon visible light excitation in acetone solution. H NMR, visible absorption and DFT studies revealed that one iodide was associated with a ligand while the other was closer to the Ru metal center. Standard Stern-Volmer quenching of the excited state by iodide revealed upward curvature with a novel saturation at high concentrations.

Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation.

The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)) were determined in the 800-1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000-2000 GM at 811.8 nm.

Chloride Oxidation by Ruthenium Excited-States in Solution.

Photodriven HCl splitting to produce solar fuels is an important goal that requires strong photo-oxidants capable of chloride oxidation. In a molecular approach toward this goal, three ruthenium compounds with 2,2'-bipyrazine backbones were found to oxidize chloride ions in acetone solution. Nanosecond transient absorption measurements provide compelling evidence for excited-state electron transfer from chloride to the Ru metal center with rate constants in excess of 10 M s.

Importance of the Active Site "Canopy" Residues in an O-Tolerant [NiFe]-Hydrogenase.

The active site of Hyd-1, an oxygen-tolerant membrane-bound [NiFe]-hydrogenase from Escherichia coli, contains four highly conserved residues that form a "canopy" above the bimetallic center, closest to the site at which exogenous agents CO and O interact, substrate H binds, and a hydrido intermediate is stabilized. Genetic modification of the Hyd-1 canopy has allowed the first systematic and detailed kinetic and structural investigation of the influence of the immediate outer coordination shell on H activation. The central canopy residue, arginine 509, suspends a guanidine/guanidinium side chain at close range above the open coordination site lying between the Ni and Fe atoms (N-metal distance of 4.

Hydrogen activation by [NiFe]-hydrogenases.

Hydrogenase-1 (Hyd-1) from Escherichia coli is a membrane-bound enzyme that catalyses the reversible oxidation of molecular H2 The active site contains one Fe and one Ni atom and several conserved amino acids including an arginine (Arg(509)), which interacts with two conserved aspartate residues (Asp(118) and Asp(574)) forming an outer shell canopy over the metals. There is also a highly conserved glutamate (Glu(28)) positioned on the opposite side of the active site to the canopy. The mechanism of hydrogen activation has been dissected by site-directed mutagenesis to identify the catalytic base responsible for splitting molecular hydrogen and possible proton transfer pathways to/from the active site.

Mechanism of hydrogen activation by [NiFe] hydrogenases.

The active site of [NiFe] hydrogenases contains a strictly conserved arginine that suspends a guanidine nitrogen atom <4.5 Å above the nickel and iron atoms. The guanidine headgroup interacts with the side chains of two conserved aspartic acid residues to complete an outer-shell canopy that has thus far proved intractable to investigation by site-directed mutagenesis.