Patrick L. Holland

Robust Photogeneration of H2 in Water Using Semiconductor Nanocrystals and a Nickel Catalyst

Robust Reduction A major challenge in the design of artificial photosynthesis catalysts has been their instability under the reaction conditions—a problem that plants and other autotrophs address by perpetually reproducing their biochemical machinery. Han et al. (p. 1321, published online 8 November) now demonstrate a system for photoreductive hydrogen generation in water that manifests undiminished activity for weeks at a time. Semiconductor nanoparticles for light absorption were combined with a soluble nickel complex for the catalytic chemistry. The system currently requires a sacrificial electron donor, but its robustness shows promise for future pairing with an integrated oxidation catalyst. A photoreduction system combining nanoparticulate light absorbers with a soluble molecular catalyst proves stable for weeks. Homogeneous systems for light-driven reduction of protons to H2 typically suffer from short lifetimes because of decomposition of the light-absorbing molecule. We report a robust and highly active system for solar hydrogen generation in water that uses CdSe nanocrystals capped with dihydrolipoic acid (DHLA) as the light absorber and a soluble Ni2+-DHLA catalyst for proton reduction with ascorbic acid as an electron donor at pH = 4.5, which gives >600,000 turnovers. Under appropriate conditions, the precious-metal–free system has undiminished activity for at least 360 hours under illumination at 520 nanometers and achieves quantum yields in water of over 36%.

N2 Reduction and Hydrogenation to Ammonia by a Molecular Iron-Potassium Complex

A molecular iron complex offers insights into the industrial iron catalyst used to split nitrogen to make ammonia. The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N2) to ammonia (NH3) is an iron surface promoted with potassium cations (K+), but soluble iron complexes have neither reduced the N-N bond of N2 to nitride (N3–) nor produced large amounts of NH3 from N2. We report a molecular iron complex that reacts with N2 and a potassium reductant to give a complex with two nitrides, which are bound to iron and potassium cations. The product has a Fe3N2 core, implying that three iron atoms cooperate to break the N-N triple bond through a six-electron reduction. The nitride complex reacts with acid and with H2 to give substantial yields of N2-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N2 cleavage and N-H bond-forming reactions of iron.

A cobalt-dithiolene complex for the photocatalytic and electrocatalytic reduction of protons.

The complex [Co(bdt)(2)](-) (where bdt = 1,2-benzenedithiolate) is an active catalyst for the visible light driven reduction of protons from water when employed with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial electron donor. At pH 4.0, the system exhibits very high activity, achieving >2700 turnovers with respect to catalyst and an initial turnover rate of 880 mol H(2)/mol catalyst/h. The same complex is also an active electrocatalyst for proton reduction in 1:1 CH(3)CN/H(2)O in the presence of weak acids, with the onset of a catalytic wave at the reversible redox couple of -1.01 V vs Fc(+)/Fc. The cobalt-dithiolene complex [Co(bdt)(2)](-) thus represents a highly active catalyst for both the electrocatalytic and photocatalytic reduction of protons in aqueous solutions.

Electronic structure and reactivity of three-coordinate iron complexes.

[Reaction: see text]. The identity and oxidation state of the metal in a coordination compound are typically thought to be the most important determinants of its reactivity. However, the coordination number (the number of bonds to the metal) can be equally influential. This Account describes iron complexes with a coordination number of only three, which differ greatly from iron complexes with octahedral (six-coordinate) geometries with respect to their magnetism, electronic structure, preference for ligands, and reactivity. Three-coordinate complexes with a trigonal-planar geometry are accessible using bulky, anionic, bidentate ligands (beta-diketiminates) that steer a monodentate ligand into the plane of their two nitrogen donors. This strategy has led to a variety of three-coordinate iron complexes in which iron is in the +1, +2, and +3 oxidation states. Systematic studies on the electronic structures of these complexes have been useful in interpreting their properties. The iron ions are generally high spin, with singly occupied orbitals available for pi interactions with ligands. Trends in sigma-bonding show that iron(II) complexes favor electronegative ligands (O, N donors) over electropositive ligands (hydride). The combination of electrostatic sigma-bonding and the availability of pi-interactions stabilizes iron(II) fluoride and oxo complexes. The same factors destabilize iron(II) hydride complexes, which are reactive enough to add the hydrogen atom to unsaturated organic molecules and to take part in radical reactions. Iron(I) complexes use strong pi-backbonding to transfer charge from iron into coordinated alkynes and N 2, whereas iron(III) accepts charge from a pi-donating imido ligand. Though the imidoiron(III) complex is stabilized by pi-bonding in the trigonal-planar geometry, addition of pyridine as a fourth donor weakens the pi-bonding, which enables abstraction of H atoms from hydrocarbons. The unusual bonding and reactivity patterns of three-coordinate iron compounds may lead to new catalysts for oxidation and reduction reactions and may be used by nature in transient intermediates of nitrogenase enzymes.

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H2). Harnessing solar energy to generate H2 from H+ is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 1∶1 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to . Subsequent reduction and reaction with H+ lead to H2 formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

Recent developments in the homogeneous reduction of dinitrogen by molybdenum and iron

The reduction of gaseous nitrogen is a challenge for industrial, biological and synthetic chemists. Major goals include understanding the formation of ammonia for agriculture, and forming N-C and N-Si bonds for the synthesis of fine chemicals. The iron-molybdenum active site of the enzyme nitrogenase has inspired chemists to explore iron and molybdenum complexes in transformations related to N2 reduction. This area of research has gained significant momentum, and the past two years have witnessed a number of significant advances in synthetic Fe-N2 and Mo-N2 chemistry. Furthermore, the identities of all atoms in the iron-molybdenum cofactor of nitrogenase have finally been elucidated, and the discovery of a carbide has generated new questions and targets for coordination chemists. This Perspective summarizes the recent work on iron and molydenum complexes, and highlights the opportunities for continued research.

The Reactivity Patterns of Low-Coordinate Iron−Hydride Complexes

We report a survey of the reactivity of the first isolable iron-hydride complexes with a coordination number less than 5. The high-spin iron(II) complexes [(beta-diketiminate)Fe(mu-H)] 2 react rapidly with representative cyanide, isocyanide, alkyne, N 2, alkene, diazene, azide, CO 2, carbodiimide, and Brønsted acid containing substrates. The reaction outcomes fall into three categories: (1) addition of Fe-H across a multiple bond of the substrate, (2) reductive elimination of H 2 to form iron(I) products, and (3) protonation of the hydride to form iron(II) products. The products include imide, isocyanide, vinyl, alkyl, azide, triazenido, benzo[ c]cinnoline, amidinate, formate, and hydroxo complexes. These results expand the range of known bond transformations at iron complexes. Additionally, they give insight into the elementary transformations that may be possible at the iron-molybdenum cofactor of nitrogenases, which may have hydride ligands on high-spin, low-coordinate metal atoms.

Impact of Ligand Exchange in Hydrogen Production from Cobaloxime-Containing Photocatalytic Systems

Ligand exchange on the Co(dmgH)(2)(py)Cl water reduction catalyst was explored under photocatalytic conditions. The photosensitizer fluorescein was connected to the catalyst through the axially coordinated pyridine. While this two-component complex produces H(2) from water under visible light irradiation in the presence of triethanolamine (TEOA), it is less active than a system containing separate fluorescein and [Co(III)(dmgH)(2)(py)Cl] components. NMR and photolysis experiments show that the Co catalyst undergoes pyridine exchange. Interestingly, glyoximate ligand exchange was also observed photocatalytically and by NMR spectroscopy, thereby showing that integrated systems in which the photosensitizer is linked directly to the Co(dmgH)(2)(py)Cl catalyst may not remain intact during H(2) photogeneration. These studies have also given rise to insights into the catalyst decomposition mechanism.

Metal–dioxygen and metal–dinitrogen complexes: where are the electrons?

Transition-metal complexes of O(2) and N(2) play an important role in the environment, chemical industry, and metalloenzymes. This Perspective compares and contrasts the binding modes, reduction levels, and electronic influences on the nature of the bound O(2) or N(2) group in these complexes. The charge distribution between the metal and the diatomic ligand is variable, and different models for describing the adducts have evolved. In some cases, single resonance structures (e.g. M-superoxide = M-O(2)(-)) are accurate descriptions of the adducts. Recent studies have shown that the magnetic coupling in certain N(2)(2-) complexes differs between resonance forms, and can be used to distinguish experimentally between resonance structures. On the other hand, many O(2) and N(2) complexes cannot be described well with a simple valence-bond model. Defining the situations where ambiguities occur is a fertile area for continued study.

Is the Bis(μ‐oxo)dicopper Core Capable of Hydroxylating an Arene?

Direct attack of the bis(μ-oxo)dicopper core on an arene appears feasible in tyrosinase and model complexes on the basis of studies of new [Cu(III) 2 (μ-O)2 ](2+) compounds supported by bidentate imine/amine ligands. In the first demonstration of such reactivity for a bis(μ-oxo)dicopper core, decomposition of these intermediates caused hydroxylation of a pendant phenyl ring [Eq. (a)] in a reaction analogous to that catalyzed by tyrosinase.