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.
Reduction of acids to molecular hydrogen as a means of storing energy is catalyzed by platinum, but its low abundance and high cost are problematic. Precisely controlled delivery of protons is ...critical in hydrogenase enzymes in nature that catalyze hydrogen (H(2)) production using earth-abundant metals (iron and nickel). Here, we report that a synthetic nickel complex, Ni(P(Ph)(2)N(Ph))(2)(BF(4))(2), (P(Ph)(2)N(Ph) = 1,3,6-triphenyl-1-aza-3,6-diphosphacycloheptane), catalyzes the production of H(2) using protonated dimethylformamide as the proton source, with turnover frequencies of 33,000 per second (s(-1)) in dry acetonitrile and 106,000 s(-1) in the presence of 1.2 M of water, at a potential of -1.13 volt (versus the ferrocenium/ferrocene couple). The mechanistic implications of these remarkably fast catalysts point to a key role of pendant amines that function as proton relays.
Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by FeFe-hydrogenases, which contain a pendant amine ...positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as “P2N2” ligands, have profound effects on the reactivity of many catalysts. The resulting Ni(PR 2NR′ 2)22+ complexes are electrocatalysts for both the oxidation and production of H2. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with Ni(PR 2NR′ 2)22+ complexes for the oxidation and production of H2, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.
Electrocatalysts are critically important for a secure energy future, as they facilitate the conversion between electrical and chemical energy. Molecular catalysts offer precise control of structure ...that enables understanding of structure–reactivity relationships, which can be difficult to achieve with heterogeneous catalysts. Molecular electrocatalysts can be immobilized on surfaces by covalent bonds or through non‐covalent interactions. Advantages of surface immobilization include the need for less catalyst, avoidance of bimolecular decomposition pathways, and easier determination of catalyst lifetime. This Minireview highlights surface immobilization of molecular electrocatalysts for reduction of O2, oxidation of H2O, production of H2, and reduction of CO2.
The conversion between electrical and chemical energy by electrocatalysts means that they are critically important for a secure energy future. This Minireview highlights surface immobilization of molecular electrocatalysts for reduction of O2, oxidation of H2O, production of H2, and reduction of CO2 (see scheme).
The increasing energy needs of society have led to a search for technologies that can tap carbon-neutral and sustainable energy sources, such as solar and wind. Using properly designed catalysts, ...such sources can also be used to create fuels such as hydrogen; however, a significant barrier to the use of hydrogen as an energy carrier is the need for an inexpensive and efficient catalyst for its oxidation. The oxidation of hydrogen is the process by which electricity is produced in low-temperature fuel cells, and the best catalyst for this is platinum-a precious metal of low abundance. Here we report a molecular complex of iron (an abundant and inexpensive metal) as a rationally designed electrocatalyst for the oxidation of H(2) at room temperature, with turnover frequencies of 0.66-2.0 s(-1) and low overpotentials of 160-220 mV. This iron complex, Cp(C(6)F(5))Fe(P((t)Bu)(2)N(Bn)(2))(H), has pendent amines in the diphosphine ligand that function as proton relays.
Iron- or cobalt-based catalysts have outperformed traditional precious metal catalysts in several hydrogenation reactions.
Also see Reports by
Jagadeesh
et al.
,
Friedfeld
et al.
, and
Zuo
et al.
...Homogeneous catalysts based on precious metals have enabled highly selective synthesis of organic compounds. Precious metal catalysts including ruthenium (Ru), rhodium (Rh), and platinum (Pt) have been superstars of both homogeneous and heterogeneous catalysis. In recent years, increasing efforts have been devoted to the design and discovery of homogeneous catalysts that incorporate base metals, such as iron (Fe) and cobalt (Co) (
1
–
3
) (see the figure). Catalytic hydrogenations are one of the extraordinary success stories of homogeneous catalysis, and three reports in this issue describe remarkable progress in the use of Earth-abundant metals as catalysts for hydrogenations. Hydrogenations are conceptually simple—H
2
is added across a C=C or C=O double bond—but mechanistic studies have revealed considerable complexity with respect to how the metal catalyzes hydrogenations (
4
). On page 1080, Zuo
et al.
(
5
) report iron catalysts for asymmetric hydrogenation of C=O bonds. On page 1076, cobalt catalysts for asymmetric hydrogenation of C=C bonds are described by Friedfeld
et al.
(
6
). On page 1073, Jagadeesh
et al.
(
7
) report on nanoscale iron catalysts for synthesis of functionalized anilines through hydrogenation of nitroarenes.
A large variety of molecular cobalt complexes are used as electrocatalysts for H2 production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their ...high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient CoIIIH and CoIIH intermediates of electrocatalytic H2 production by CoII(P tBu 2NPh 2)(CH3CN)32+ and CoII(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the CoII/I couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a CoIIIH intermediate to CoIIH, which is then protonated by acid to generate H2. A bridge-protonated CoI species was ruled out as a catalytic intermediate for CoII(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate CoIII–H and CoII–H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co–H bond strengths for the CoIIIH and CoIIH intermediates.
Electrocatalysts for efficient conversion between electricity and chemical bonds will play a vital role in future systems for storage and delivery of energy. Our research on functional models of ...hydrogenase enzymes uses nickel and cobalt, abundant and inexpensive metals, in contrast to platinum, a precious metal used in fuel cells. A key feature of our research is a focus on the use of pendant amines incorporated into diphosphane ligands. These pendant amines function as proton relays, lowering the barrier to proton transfers to and from the catalytically active metal site. The hydride acceptor ability of metal cations, along with the basicity of pendant amines, are key thermochemical values that determine the thermodynamics of addition of H2 to a metal complex with a pendant amine incorporated into its ligand. Nickel catalysts for oxidation of H2 have turnover frequencies up to 50 s–1 (at 1 atm H2 and room temperature). Nickel and cobalt catalysts for production of H2 by reduction of protons were studied; one of them has a turnover frequency over 1000 s–1.
Pendant amines function as proton relays in molecular electrocatalysts, facilitating transfer of protons to and from the metal. A series of NiII complexes with diphosphanes bearing pendant amines are catalysts for oxidation of H2, and a related series of NiII and CoII complexes catalyze the production of H2 by reduction of protons.
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