Oxygen quenching of structurally characterized [5,10,15,20-tetrakis(4-fluoro-2,6-dimethylphenyl)porphyrinato]platinum(II).

The compound [5,10,15,20-tetrakis(4-fluoro-2,6-dimethylphenyl)porphyrinato]platinum(II), [Pt(C52H40F4N4)] or Pt(II)TFP, has been synthesized and structurally characterized by single-crystal X-ray crystallography. The Pt porphyrin exhibits a long-lived phosphorescent excited state (τ0 = 66 µs), which has been characterized by transient absorption and emission spectroscopy. The phosphorescence is extremely sensitive to oxygen, as reflected by a quenching rate constant of 5.0 × 108 M-1 s-1, and as measured by Stern-Volmer quenching analysis.

Crystal structure of a methyl benzoate quadruple-bonded dimolybdenum complex.

Quadruple-bond dimolybdenum complexes provide invaluable insight into the two-electron bond, with structural chemistry providing a foundation for examination of bond properties. The synthesis and solid-state structure of the quadruple-bonded dimolybdenum(II) complex tetra-kis-(µ-4-methyl-benzoato-κ 2 O:O')bis[(tetra-hydro-furan-κO)molybdenum(II)] tetra-hydro-furan disolvate, [Mo2(C8H7O2)4(C4H8O)2]·2C4H8O, are presented. This complex crystallizes in a triclinic cell with low-symmetry space group P . The dimolybdenum paddlewheel structure comprises four methyl-benzoate ligands and two axial THF ligands. The dimolybdenum bond distance of 2.1012 (4) Šis exemplary of this class of compounds.

Syntheses and Properties of Metalated Tetradehydrocorrins.

The monoanionic tetrapyrrolic macrocycle B,C-tetradehydrocorrin (TDC) resides chemically between corroles and corrins. This chemical space remains largely unexplored due to a lack of reliable synthetic strategies. We now report the preparation and characterization of Co(II)- and Ni(II)-metalated TDC derivatives ([Co-TDC]+ and [Ni-TDC]+, respectively) with a combination of crystallographic, electrochemical, computational, and spectroscopic techniques. [Ni-TDC]+ was found to undergo primarily ligand-centered electrochemical reduction, leading to hydrogenation of the macrocycle under cathodic electrolysis in the presence of acid. Transient absorption (TA) spectroscopy reveals that [Ni-TDC]+ and the two-electron-reduced [Ni-TDC]- possess long-lived excited states, whereas the excited state of singly reduced [Ni-TDC] exhibits picosecond dynamics. The Co(I) compound [Co-TDC] is air stable, highlighting the notable property of the TDC ligand to stabilize low-valent metal centers in contradistinction to other tetrapyrroles such as corroles, which typically stabilize metals in higher oxidation states.

Probing the Halide Effect in the δ-Bond with One- and Two-Photon Spectroscopy.

Two electrons in two orbitals give rise to four states. When the orbitals are weakly coupled as in the case for the dxy orbitals of quadruple bond species, two of the states are diradical in character with electrons residing in separate orbitals and two of the states are zwitterionic with electrons paired in one orbital or the other. By measuring one-and two-photon spectra, the one-electron (ΔW) and two-electron (K) energies may be calculated, which are the determinants of the state energies of the four-state model for the two-electron bond. The K energy is thus especially sensitive to the size of the orbital as K is dependent on the distance between electrons. To this end, one- and two-photon spectra of Mo2X4(PMe3)4 are sensitive to secondary bonding interactions of the δ-orbital manifold with the halide orbitals, as reflected in decreasing K energies along the series Cl > Br > I. Additionally, the calculated one-electron energies have been verified with the spectroelectrochemical preparation of the Mo2X4(PMe3)4+ complexes, where the δ bond is a one-electron bond, and K is thus absent. The δ → δ* transition shifts over 10,000 cm-1 upon oxidation of Mo2X4(PMe3)4 to Mo2X4(PMe3)4+, establishing that transitions within the two-electron δ bond are heavily governed by the two-electron exchange energy.

Proton-coupled electron transfer of macrocyclic ring hydrogenation: The chlorinphlorin.

SignificanceThe chemical reduction of unsaturated bonds occurs by hydrogenation with H2 as the reductant. Conversely, in biology, the unavailability of H2 engenders the typical reduction of unsaturated bonds with electrons and protons from different cofactors, requiring olefin hydrogenation to occur by proton-coupled electron transfer (PCET). Moreover, the redox noninnocence of tetrapyrrole macrocycles furnishes unusual PCET intermediates, including the phlorin, which is an intermediate in tetrapyrrole ring reductions. Whereas the phlorin of a porphyrin is well established, the phlorin of a chlorin is enigmatic. By controlling the PCET reactivity of a chlorin, including the use of a hangman functionality to manage the proton transfer, the formation of a chlorinphlorin by PCET is realized, and the mechanism for its formation is defined.

Synthesis, Characterization, and Hydrogen Evolution Activity of Metallo-meso-(4-fluoro-2,6-dimethylphenyl)porphyrin Derivatives.

Zn(II), Cu(II), and Ni(II) 5,10,15,20-tetrakis(4-fluoro-2,6-dimethylphenyl)porphyrins (TFPs) have been synthesized and characterized. The electronic spectroscopy and cyclic voltammetry of these compounds, along with the free-base macrocycle (2H-TFP), have been determined; 2H-TFP was also structurally characterized by X-ray crystallography. The Cu(II)TFP exhibits catalytic activity for the hydrogen evolution reaction (HER). The analysis of linear sweep voltammograms shows that the HER reaction of Cu(II)TFP with benzoic acid is first-order in proton concentration with an average apparent rate constant for HER catalysis of k app = 5.79 ± 0.47 × 103 M-1 s-1.

Crystal structure of a tri-fluoro-methyl benzoato quadruple-bonded dimolybdenum complex.

The study of quadruple bonds between transition metals, in particular those of dimolybdenum, has revealed much about the two-electron bond. The solid-state structure of the quadruple-bonded dimolybdenum(II) complex tetra-kis-[µ-4-(tri-fluoro-methyl)-benzoato-κ2 O:O']bis[(tetra-hydro-furan-κO)molybdenum(II)] 0.762-pentane 0.238-tetra-hydro-furan solvate, [Mo2(p-O2CC6H4CF3)4·2THF]·0.762C5H12·0.238C4H8O or [Mo2(C8H4F3O2)4(C4H8O)2]·0.762C5H12·0.238C4H8O is reported. The complex crystallizes within a triclinic cell and low symmetry (P ) results from the inter-calated penta-ne/THF solvent mol-ecules. The paddlewheel structure at 100 K has inversion symmetry and comprises four bridging carboxyl-ate ligands encases the Mo2(II,II) core that is characterized by two axially coordinated THF mol-ecules and an Mo-Mo distance of 2.1098 (7) Å.

Syntheses and solid-state structures of two cofacial (bis)dipyrrin dichromium complexes in different charge states.

The dichromium Pacman complex (tBudmx)Cr2Cl2·C4H10O (1) [(tBudmx)H2 is a dimethylxanthene-bridged cofacial (bis)dipyrrin, C49H58N4O] was synthesized by salt metathesis using anhydrous CrCl2 and previously reported (tBudmx)K2. Treatment of 1 with two equivalents of the reductant potassium graphite afforded K2(tBudmx)Cr2Cl2(thf)3·0.5C4H10O·0.5C4H8O (thf is tetrahydrofuran, C4H8O) (2), with both potassium ions intercalated between the pyrrolic subunits. Comparison of the solid-state structures for 1 and 2 reveals minimal changes in the primary coordination sphere of each Cr ion, with notable elongation of the dipyrrin C-C and C-N bonds upon reduction, consistent with computational support for a ligand-based reduction.

Crystal structure of the RuPhos ligand.

Palladium 2-di-cyclo-hexyl-phosphanyl-2',6'-diisopropoxybiphenyl (Pd-RuPhos) catalysts demonstrate high catalytic activity for Negishi cross-couplings of sterically hindered aryl halides, for Suzuki-Miyaura cross-couplings of tosyl-ated olefins, and for Buchwald-Hartwig amination of sterically hindered amines. The solid-state structure of the free RuPhos ligand, C30H43O2P, is reported herein for the first time. RuPhos crystallizes in a triclinic cell containing two independent mol-ecules of the phosphine without any lattice solvent. Pertinent bond metrics and comparisons to other phosphine ligands are presented. The structure of RuPhos will be of assistance in the use of this ligand in the design of cross-coupling catalysts.

Synthesis of Hangman Chlorins.

Two complementary rational synthetic routes have been developed in order to synthesize hangman chlorins, which differ with regard to the order of the installation (pre- and post-formation of the chlorin macrocycle) and position of the xanthene backbone about the chlorin periphery. The versatility of the synthetic method is demonstrated with the preparation of ten new hangman chlorins bearing a xanthene backbone and a pendant carboxylic acid. Cyclic voltammograms of hangman chlorins exhibit a hangman effect derived from intermolecular proton transfer. This hangman effect is manifested in catalytic hydrogen evolution production.

Artificial Photosynthesis at Efficiencies Greatly Exceeding That of Natural Photosynthesis.

Sunlight is an abundant energy source for a sustainable society. Indeed, photosynthetic organisms harness solar radiation to build the world around us by synthesizing energy-rich compounds from water and CO2. However, numerous energy conversion bottlenecks in the natural system limits the overall efficiency of photosynthesis; the most efficient plants do not exceed solar storage efficiencies of 1%. Artificial photosynthetic solar-to-fuels cycles may occur at higher intrinsic efficiencies, but they typically terminate at hydrogen, with no process installed to complete the cycle for carbon fixation. This limitation may be overcome by interfacing solar-driven water splitting to H2-oxidizing microorganisms. To this end, hybrid biological-inorganic constructs have been created to use sunlight, air, and water as the only starting materials to accomplish carbon fixation in the form of biomass and liquid fuels. This artificial photosynthetic cycle begins with the Artificial Leaf, which accomplishes the solar process of natural photosynthesis-the splitting of water to hydrogen and oxygen using sunlight-under ambient conditions. To create the Artificial Leaf, an oxygen evolving complex of Photosystem II was mimicked, the most important property of which was the self-healing nature of the catalyst. Self-healing catalysts permit water splitting to be accomplished using any water source, which is the critical development for (1) the Artificial Leaf, as it allows for the facile interfacing of water splitting catalysis to materials such as silicon, and (2) the hybrid biological-inorganic construct, called the Bionic Leaf, as it allows for the facile interfacing of water splitting catalysis to bioorganisms. Hydrogenases in the bioorganism allow the hydrogen to be coupled to NADPH and ATP production, thus allowing the solar energy from water splitting to be converted into cellular energy to drive cellular biosynthesis. In the design of the hybrid system, water splitting catalysts must be designed that support hydrogen generation at low applied potential to ensure a high energy efficiency while avoiding reactive oxygen species. Using the tools of synthetic biology, a bioengineered bacterium, Ralstonia eutropha, converts carbon dioxide from air, along with the hydrogen produced from such catalysts of the Artificial Leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. The Bionic Leaf operates at solar-to-biomass and solar-to-liquid fuels efficiencies that greatly exceed the highest solar-to-biomass efficiencies of natural photosynthesis.

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism.

The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. Herein nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C-H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H2. Support for the theoretically predicted mechanism is provided by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins.

The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism. Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction, in a stepwise proton transfer-electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron is transferred to a molecular orbital on the porphyrin ring.

Cationic copper(II) porphyrins intercalate into domains of double-stranded RNA.

A cationic, copper(II)-containing ligand, derived from bulky 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, Cu(T4), and two sterically friendlier forms, [trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(tD4), and [cis-5,10-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(cD4), bind to DNA and RNA hosts. Six hairpin-forming RNA 18-mer sequences and two previously studied DNA analogues serve as convenient binding platforms of programmable base composition. A crystal structure shows that the copper center of Cu(tD4) is four-coordinate, establishing compatibility with intercalative binding as well as susceptibility to solvent-induced emission quenching. From the hypochromic responses and the induced emission intensities obtained with all three porphyrins, it is clear that internalization into the RNA host occurs, irrespective of the base pair composition. Further analysis reveals that the porphyrins intercalate into the double-stranded stem domains. Subtle geometric and/or electronic aspects of the binding account for the signs of induced circular dichroic signals and splitting of the Soret band of Cu(tD4).

Hangman effect on hydrogen peroxide dismutation by Fe(III) corroles.

Hangman Fe(III) corroles catalyse H(2)O(2) disproportionation at a faster rate and display a more pronounced hangman effect than their one electron oxidized analogues owing to their ability to bypass high energy intermediates by redox-leveling derived from the use of the corrole as a non-innocent ligand.

Electocatalytic water oxidation by cobalt(III) hangman ß-octafluoro corroles.

Cobalt hangman corrole, bearing ß-octafluoro and meso-pentafluorophenyl substituents, is an active water splitting catalyst. When immobilized in Nafion films, the turnover frequencies for the 4e(-)/4H(+) process at the single cobalt center of the hangman platform approach 1 s(-1). The pH dependence of the water splitting reaction suggests a proton-coupled electron transfer (PCET) catalytic mechanism.

Hydrogen generation by hangman metalloporphyrins.

A cobalt(II) hangman porphyrin with a xanthene backbone and a carboxylic acid hanging group catalyzes the electrochemical production of hydrogen from benzoic and tosic acid in acetonitrile solutions. We show that Co(II)H is exclusively involved in the generation of H(2) from weak acids. In a stronger acid, a Co(III)H species is observed electrochemically, but it still needs to be further reduced to Co(II)H before H(2) generation occurs. Overpotentials for H(2) generation are lowered as a result of the hangman effect.

Xanthene-modified and hangman iron corroles.

Iron corroles modified with a xanthene scaffold are delivered from easily available starting materials in abbreviated reaction times. These new iron corroles have been spectroscopically examined with particular emphasis on defining the oxidation state of the metal center. Investigation of their electronic structure using (57)Fe Mössbauer spectroscopy in conjunction with density functional theory (DFT) calculations reveals the non-innocence of the corrole ligand. Although these iron corroles contain a formal Fe(IV) center, the deprotonated corrole macrocycle ligand is one electron oxidized. The electronic ground state of these complexes is best described as an intermediate spin S = 3/2 Fe(III) site strongly antiferromagnetically coupled to the S = 1/2 of the monoradical dianion corrole [Fe(III)Cl-corrole(+•)]. We show here that iron corroles as well as xanthene-modified and hangman xanthene iron corroles are redox active and catalyze the disproportionation of hydrogen peroxide via the catalase reaction, and that this activity scales with the oxidation potential. The meso position of corrole macrocycle is susceptible toward nucleophilic attack during catalase turnover. The reactivity of peroxide within the hangman cleft reported here adds to the emerging theme that corroles are good at catalyzing two-electron activation of the oxygen-oxygen bond in a variety of substrates.

Hangman corroles: efficient synthesis and oxygen reaction chemistry.

The construction of a new class of compounds--the hangman corroles--is provided efficiently by the modification of macrocyclic forming reactions from bilanes. Hangman cobalt corroles are furnished in good yields from a one-pot condensation of dipyrromethane with the aldehyde of a xanthene spacer followed by metal insertion using microwave irradiation. In high oxidation states, X-band EPR spectra and DFT calculations of cobalt corrole axially ligated by chloride are consistent with the description of a Co(III) center residing in the one-electron oxidized corrole macrocycle. These high oxidation states are likely accessed in the activation of O-O bonds. Along these lines, we show that the proton-donating group of the hangman platform works in concert with the redox properties of the corrole to enhance the catalytic activity of O-O bond activation. The hangman corroles show enhanced activity for the selective reduction of oxygen to water as compared to their unmodified counterparts. The oxygen adduct, prior to oxygen reduction, is characterized by EPR and absorption spectroscopy.