Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.

Homogeneous systems for light-driven reduction of protons to H(2) 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 Ni(2+)-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%.

The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N2) to ammonia is an iron surface promoted with K+, but soluble iron complexes have neither reduced the N-N bond of N2 to nitride 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, though not yet catalytic, give structural and spectroscopic insight into N2 cleavage and N-H bond-forming reactions of iron.

Understanding the interaction of N2 with iron is relevant to the iron catalyst used in the Haber process and to possible roles of the FeMoco active site of nitrogenase. The work reported here uses synthetic compounds to evaluate the extent of NN weakening in low-coordinate iron complexes with an FeNNFe core. The steric effects, oxidation level, presence of alkali metals, and coordination number of the iron atoms are varied, to gain insight into the factors that weaken the NN bond. Diiron complexes with a bridgingFeCl]n under a dinitrogen atmosphere, and an iron(I) precursor of an N2 complex can be observed. X-ray crystallographic and resonance Raman data for L R FeNNFeL R show a reduction in the N-N bond order, and calculations (density functional and multireference) indicate that the bond weakening arises from cooperative backbonding into the N2 π* orbitals. Increasing the coordination number of iron from three to four through binding of pyridines gives compounds with comparable N-N weakening, and both are substantially weakened relative to five-coordinate iron-N2 complexes, even those with a lower oxidation state. Treatment of L R FeNNFeL R with KC8 gives K2L R FeNNFeL R , and calculations indicate that reduction of the iron and alkali metal coordination cooperatively weaken the N-N bond. The complexes L R FeNNFeL R react as iron(I) fragments, losing N2 to yield iron(I) phosphine, CO, and benzene complexes. They also reduce ketones and aldehydes to give the products of pinacol coupling. The K2L R FeNNFeL R compounds can be alkylated at iron, with loss of N2.

Mössbauer spectra of [LFe(II)X](0) (L = beta-diketiminate; X = Cl(-), CH(3)(-), NHTol(-), NHtBu(-)), 1.X, were recorded between 4.2 and 200 K in applied magnetic fields up to 8.0 T. A spin Hamiltonian analysis of these data revealed a spin S = 2 system with uniaxial magnetization properties, arising from a quasi-degenerate M(S) = +/-2 doublet that is separated from the next magnetic sublevels by very large zero-field splittings (3/D/ > 150 cm(-1)). The ground levels give rise to positive magnetic hyperfine fields of unprecedented magnitudes, B(int) = +82, +78, +72, and +62 T for 1.CH(3), 1.NHTol, 1.NHtBu, and 1.Cl, respectively. Parallel-mode EPR measurements at X-band gave effective g values that are considerably larger than the spin-only value 8, namely g(eff) = 10.9 (1.Cl) and 11.4 (1.CH(3)), suggesting the presence of unquenched orbital angular momenta. A qualitative crystal field analysis of g(eff) shows that these momenta originate from spin-orbit coupling between energetically closely spaced yz and z(2) 3d-orbital states at iron and that the spin of the M(S) = +/-2 doublet is quantized along x, where x is along the Fe-X vector and z is normal to the molecular plane. A quantitative analysis of g(eff) provides the magnitude of the crystal field splitting of the lowest two orbitals, /epsilon(yz) - epsilon(2)(z)/ = 452 (1.Cl) and 135 cm(-1) (1.CH(3)). A determination of the sign of the crystal field splitting was attempted by analyzing the electric field gradient (EFG) at the (57)Fe nuclei, taking into account explicitly the influence of spin-orbit coupling on the valence term and ligand contributions. This analysis, however, led to ambiguous results for the sign of epsilon(yz) - epsilon(2)(z). The ambiguity was resolved by analyzing the splitting Delta of the M(S) = +/-2 doublet; Delta = 0.3 cm(-1) for 1.Cl and Delta = 0.03 cm(-)(1) for 1.CH(3). This approach showed that z(2) is the ground state in both complexes and that epsilon(yz) - epsilon(2)(z) approximately 3500 cm(-1) for 1.Cl and 6000 cm(-1) for 1.CH(3). The crystal field states and energies were compared with the results obtained from time-dependent density functional theory (TD-DFT). The isomer shifts and electric field gradients in 1.X exhibit a remarkably strong dependence on ligand X. The ligand contributions to the EFG, denoted W, were expressed by assigning ligand-specific parameters: W(X) to ligands X and W(N) to the diketiminate nitrogens. The additivity and transferability hypotheses underlying this model were confirmed by DFT calculations. The analysis of the EFG data for 1.X yields the ordering W(N(diketiminate)) < W(Cl) < W(N'HR), W(CH(3)) and indicates that the diketiminate nitrogens perturb the iron wave function to a considerably lesser extent than the monodentate nitrogen donors do. Finally, our study of these synthetic model complexes suggests an explanation for the unusual values for the electric hyperfine parameters of the iron sites in the Fe-Mo cofactor of nitrogenase in the M(N) state.

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.

Transition-metal complexes of O2 and N2 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 O2 or N2 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–O2−) are accurate descriptions of the adducts. Recent studies have shown that the magnetic coupling in certain N22− complexes differs between resonance forms, and can be used to distinguish experimentally between resonance structures. On the other hand, many O2 and N2 complexes cannot be described well with a simple valence-bond model. Defining the situations where ambiguities occur is a fertile area for continued study.

The splitting of water through artificial photosynthesis (AP) is a key transformation toward the conversion of solar energy into stored chemical potential in the form of fuel and oxidizer.[1] For water splitting, the reductive side of the reaction involves the light-driven conversion of aqueous protons into H 2 . To perform this half-reaction, photocatalytic systems typically consist of a catalyst, photosensitizer (PS), and sacrificial electron donor.[2] Recent studies on noblemetal-based [3] and noble-metal-free [4] homogeneous systems for light-driven hydrogen production have shown high activity. However, significant problems in the noble-metalfree molecularly based systems include relatively low catalyst turnover numbers (TON < 500 mole H 2 /mole catalyst) for hydrogen formation, and photodecomposition of the systems within a few hours. For most organic dye based systems that have recently been reported, the photochemical quenching step of the excited-state dye (PS*) is reductive, thus leading to unstable PS À radical anions that undergo decomposition.[4b]Thus, the development of more active catalysts, specifically ones that quench PS* oxidatively, would be of great value for obtaining long-lived homogeneous AP systems. Herein, we describe a new homogenous catalyst for H 2 production that has both high activity and the ability to oxidatively quench PS*, thus leading to a much longer system lifetime. Nickel cathodes are used in commercial electrolyzers, suggesting that nickel may be a worthwhile basis for homogeneous catalysts as well.[5] Nickel thiolate complexes have received special attention in recent years because sulfurligated nickel complexes mimic the [Fe-Ni]-hydrogenase active site, [6] and dimeric metal complexes based on nickel thiolate hydrides have been shown to be catalytically active for proton reduction.[7] DuBois and co-workers have also shown that mononuclear nickel(II) bis(diphosphine) complexes are effective catalysts for electrochemical hydrogen generation.[8] While photocatalytic hydrogen generation from the nickel-phosphine complexes is long-lived, the activity of the photocatalytic system with the nickel phosphine catalyst is low, with a turnover frequency (TOF) of approximately 20 equivalents of H 2 per hour.[9] Related nickel(II) complexes containing pyridine-2-thiolate ligands have been known for over two decades, [10] but their catalytic properties for proton reduction have not been reported. In the present study, the complex [Ni(pyS) 3 ] À (1 À ; pyS = pyridine-2-thiolate) is found to have impressive catalytic activity for the photocatalytic production of H 2 in a homogeneous system with fluorescein (Fl) as the PS and triethylamine (TEA) as the sacrificial electron donor.Photolysis of a solution of Fl and 1 À in EtOH/H 2 O (1:1) using a green-light-emitting diode (LED) (l = 520 nm, 0.12 W) at 15 8C results in H 2 generation which was monitored in real time by the pressure change in the reaction vessel, and quantified at the end of the photolysis by GC analysis of the headspace gase...