Thomas S. Teets

Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds

1. Setting the Scope of the Challenge 6474 1.1. The Need for Solar Energy Supply and Storage 6474 1.2. An Imperative for Discovery Research 6477 1.3. Scope of Review 6478 2. Large-Scale Centralized Energy Storage 6478 2.1. Pumped Hydroelectric Energy Storage (PHES) 6479 2.2. Compressed Air Energy Storage (CAES) 6480 3. Smaller Scale Grid and Distributed Energy Storage 6481 3.1. Flywheel Energy Storage (FES) 6481 3.2. Superconducting Magnetic Energy Storage 6482 4. Chemical Energy Storage: Electrochemical 6482 4.1. Batteries 6482 4.1.1. Lead-Acid Batteries 6483 4.1.2. Alkaline Batteries 6484 4.1.3. Lithium-Ion Batteries 6484 4.1.4. High-Temperature Sodium Batteries 6484 4.1.5. Liquid Flow Batteries 6485 4.1.6. Metal-Air Batteries 6485 4.2. Capacitors 6485 5. Chemical Energy Storage: Solar Fuels 6486 5.1. Solar Fuels in Nature 6486 5.2. Artificial Photosynthesis and General Considerations of Water Splitting 6486

Photocatalytic hydrogen production

The efficient storage of solar energy in chemical fuels, such as hydrogen, is essential for the large-scale utilisation of solar energy systems. Recent advances in the photocatalytic production of H(2) are highlighted. Two general approaches for the photocatalytic hydrogen generation by homogeneous catalysts are considered: HX (X = Cl, Br) splitting involving both proton reduction and halide oxidation via an inner-sphere mechanism with a single-component catalyst; and sensitized H(2) production, employing sacrificial electron donors to regenerate the active catalyst. Future directions and challenges in photocatalytic H(2) generation are enumerated.

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.

Oxygen reduction reactivity of cobalt(II) hangman porphyrins

Cobalt(II) hangman porphyrins are delivered from easily available starting materials, in two steps, in good yields, and with abbreviated reaction times. Selected compounds from a library of Co(II) hangman porphyrins immobilized on multiwall carbon nanotubes establish that the four-electron four-proton catalytic reduction of oxygen to water in aqueous solution can be achieved at the single cobalt center of the hangman platform. Reaction trends within the library reveal that the selective reduction of O2 to H2O occurs at electron deficient hangman porphyrin platforms possessing a distal group that is capable of proton transfer.

Halogen Photoreductive Elimination from Gold(III) Centers

Monomeric complexes of the type Au(III)(PR(3))X(3) and bimetallic complexes of the type Au(2)(I,III)[mu-CH(2)(R(2)P)(2)]X(4) and Au(2)(III,III)[mu-CH(2)(R(2)P)(2)]X(6) (R = Ph, Cy, X = Cl(-), Br(-)) undergo facile photoelimination of halogen. M-X bond activation and halogen elimination is achieved upon LMCT excitation of solutions of Au(III) complexes in the presence of olefin chemical traps. As opposed to the typical one-electron redox transformations of LMCT photochemistry, the LMCT photochemistry of the Au(III) centers allows for the unprecedented (i) two-electron photoelimination of X(2) from a monomeric center and (ii) four-electron photoelimination of X(2) from a bimetalllic center. The quantum yields for X(2) photoproduction, in general, are between 10% and 20% for all species, showing only minimal dependence on the identity of the ligands about gold, or the nuclearity of the complex. Efficient X(2) photoelimination is observed in the absence of a chemical trap, providing a rare example of authentic, trap-free halogen elimination from a transition metal center.

Homoleptic, Four-Coordinate Azadipyrromethene Complexes of d10 Zinc and Mercury

Tetraarylazadipyrromethenes, and especially their boron chelates, are a growing class of chromophores that are photoactive toward red light. The coordination chemistry of these ligands remains to be explored. Reported here are four-coordinate zinc(II) and mercury(II) complexes of tetraarylazadipyrromethene ligands. The new complexes contain two azadipyrromethenes bound per d(10) metal center and are characterized by (1)H NMR, optical absorption spectroscopy, X-ray diffraction crystallography, and elemental analysis. Solid-state structures show that these bis-chelate complexes distort significantly from idealized D2d symmetry. AM1 geometry optimizations indicate relaxation energies in the range of 6.8-15.2 kcal mol(-1); interligand pi-stacking provides an added energetic impetus for distortion. The absorption spectra show a marked increase in the absorption intensity in the red region and, in the case of the zinc(II) complexes, the development of a second distinct absorption band in this region, which is red-shifted by ca. 40-50 nm relative to the free ligand. Semiempirical INDO/S computations indicate that these low-energy optical absorptions derive from allowed excitations among ligand-based orbitals that derive from the highest occupied molecular orbital and lowest unoccupied molecular orbital of the free azadipyrromethene.

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.

Oxygen Reduction to Water Mediated by a Dirhodium Hydrido-Chloride Complex

The two-electron mixed-valence dirhodium complex Rh(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)Cl(2) (tfepma = CH(3)N[P(OCH(2)CF(3))(2)](2)) reacts with HCl to furnish two isomeric dirhodium hydrido-chloride complexes, Rh(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(3)H. In the presence of HCl, the hydride complex effects the reduction of 0.5 equiv of O(2) to 1 equiv of H(2)O, generating Rh(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(4), which can be prepared independently by chlorine oxidation of the Rh(2)(0,II) precursor. The starting Rh(2)(0,II) complex is regenerated photochemically to close an oxygen-to-water reduction photocycle.

Efficient Synthesis of Hangman Porphyrins

A two-step synthetic method has been designed to furnish hangman porphyrins in good yields from easily available starting materials. The use of the microwave irradiation technique has been found to be valuable for delivering the carboxylic acid hanging group in a much simplified and less time-consuming basic ester hydrolysis (4 h vs 7 days under harsh acidic conditions). The new route facilitates the synthesis of various hangman porphyrins that previously had limited or no access.

Halogen photoreductive elimination from metal-metal bonded iridium(II)-gold(II) heterobimetallic complexes.

Halogen oxidation of [Ir(I)Au(I)(dcpm)(2)(CO)X](PF(6)) (dcpm = bis(dicyclohexylphosphino)methane, X = Cl, Br) and [Ir(I)Au(I)(dppm)(2)(CN(t)Bu)(2)](PF(6))(2) (dppm = bis(diphenylphosphino)methane) furnishes the heretofore unknown class of d(7)-d(9) compounds comprising an Ir(II)Au(II) heterobimetallic core. A direct metal-metal bond is evident from a 0.2 A contraction in the intermetallic distance, as determined by X-ray crystallography. The photophysical consequence of iridium-gold bond formation, as elucidated by experimental and computational investigations, is an electronic structure dominated by a sigma --> sigma* transition that possesses significant ligand-to-metal charge transfer (LMCT) character. Accordingly, these compounds are non-emissive but photoreactive. Excitation of Ir(II)Au(II) complexes in the presence of a halogen trap prompts a net photoreductive elimination of halogen and the production of the two-electron reduced Ir(I)Au(I) species with about 10% quantum efficiency. The Ir(II)Au(II) complexes add to a growing library of d(7)-d(9) heterobimetallic species from which halogen elimination may be driven by a photon.