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Electrification to reduce or eliminate greenhouse gas emissions is essential to mitigate climate change. However, a substantial portion of our manufacturing and transportation infrastructure will be difficult to electrify and/or will continue to use carbon as a key component, including areas in aviation, heavy-duty and marine transportation, and the chemical industry. In this Roadmap, we explore how multidisciplinary approaches will enable us to close the carbon cycle and create a circular economy by defossilizing these difficult-to-electrify areas and those that will continue to need carbon. We discuss two approaches for this: developing carbon alternatives and improving our ability to reuse carbon, enabled by separations. Furthermore, we posit that co-design and use-driven fundamental science are essential to reach aggressive greenhouse gas reduction targets.

Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis. We highlight the key physical properties of abundant metals that distinguish them from precious metals, and we look to nature to understand how the inherent attributes of abundant metals can be embraced to produce highly efficient catalysts for reactions crucial to the sustainable production and transformation of fuels and chemicals.

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.

Benchmarking is a community-based and (preferably) community-driven activity involving consensus-based decisions on how to make reproducible, fair, and relevant assessments. In catalysis science, important catalyst performance metrics include activity, selectivity, and the deactivation profile, which enable comparisons between new and standard catalysts. Benchmarking also requires careful documentation, archiving, and sharing of methods and measurements, to ensure that the full value of research data can be realized. Beyond these goals, benchmarking presents unique opportunities to advance and accelerate understanding of complex reaction systems by combining and comparing experimental information from multiple, in situ and operando techniques with theoretical insights derived from calculations characterizing model systems. This Perspective describes the origins and uses of benchmarking and its applications in computational catalysis, heterogeneous catalysis, molecular catalysis, and electrocatalysis. It also discusses opportunities and challenges for future developments in these fields.

Mechanistic studies of substrate insertion into dimeric [(NHC)CuH]2 (NHC=N‐heterocyclic carbene) complexes with two bridging hydrides have been shown to require dimer dissociation to generate transient, highly reactive (NHC)Cu−H monomers in solution. Using single‐crystal to single‐crystal (SC‐SC) transformations, we discovered a new pathway of stepwise insertion of CO2 into [(NHC)CuH]2 without complete dissociation of the dimer. The first CO2 insertion into dimeric [(IPr*OMe)CuH]2 (IPr*OMe=N,N′‐bis(2,6‐bis(diphenylmethyl)‐4‐methoxy‐phenyl)imidazole‐2‐ylidene) produced a dicopper formate hydride [(IPr*OMe)Cu]2(μ‐1,3‐O2CH)(μ‐H). A second CO2 insertion produced a dicopper bis(formate), [(IPr*OMe)Cu]2(μ‐1,3‐O2CH)(μ‐1,1‐O2CH), containing two different bonding modes of the bridging formate. These dicopper formate complexes are inaccessible from solution reactions since the dicopper core cleanly ruptures to monomeric complexes when dissolved in a solvent. [ABSTRACT FROM AUTHOR]

The synthesis and structure of CpMo(CO)(dppe)H (dppe = [Ph2PCH2CH2PPh2), its electrochemical oxidation, its reaction with Ph3[C.sup.+] and its protonation by acid to give a dihydride complex is reported. The results reveal that the reaction of these metal hydride complex with Ph2[C.sup.+]B [F.sub.4.sup.-] proceeds by an initial electron-transfer reaction rather than single-step hydride transfer.

Kinetic isotope effects on the reaction of a number of metal hydrides with Ph3C+ have kinetic deuterium isotope effects of k(sub MH)/K(sub MD) = 1.7-1.9. For a series of tungsten hydrides with systematically varied substituted cyclopentadienyl ligands, the values of k(sub MH)/K(sub MD) steadily decreased with decreasing rate constants of hydride transfer. This indicates progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction decelerates in progressing from more electron-donating Cp ligands to less electron-rich Cp ligands.

The reaction of acetals with the transition metal hydride Cp(CO)3WH and the strong acid HOTf affords cationic tungsten complexes containing an ether ligand. However, the bonding of the ether to tungsten may differ. This was demonstrated in the reactions of PhCH(OCH3)2 and the vinyl acetal CH2=CHCH(OEt)2 with both Cp(CO)3WH and HOTf. While the product of PhCH(OCH3)2 had an oxygen-bonded ether, the vinyl ether was bonded to tungsten through the carbon bonds of the vinyl group.

A novel ionic hydrogenation reaction for alkynes is described. The method uses triflic acid (CF3SO3H) as the proton donor and a transition metal hydride (Cp CO3 WH) as the hydride donor. The reaction is characterized by the formation of vinyl triflates and geminal ditriflates as key intermediates. These intermediates are converted to the hydrogenation products with the donation of hydride by the transition metal hydride in the presence of strong acids.

Triflic acid and a hydride donor help hydrogenate sterically hindered olefins at low temperatures in a medium of dichloromethane. Mechanistic studies reveal that the vital step in the hydrogenation reaction is the transfer of the hydride from a neutral metal hydride to a carbenium ion, which is generated by the protonation of the olefin by a strong acid. Olefins yielding tertiary carbenium ions upon protonation produce high yield of hydrogenated products.

X-ray diffraction is used to establish the structure of (MoH(eta-C5Me5)(Co)3). Orientational irregularity in the C5Me5 group is evident, but comparatively precise geometric parameters have resulted. A comparison of the title complex with associated d4 CpMo(CO)3L complexes (L = formally anionic, monodentate, eta ligand) indicates that instead of electronic effects, steric interactions between the ligand, L and the Cp ring chiefly influence the structure of these complexes.

Analysis of the photoelectron spectra of the CpFe(CO)2CO(triple bond)CR compounds revealed that the interaction between the filled metal d(pi) orbitals and the acetylide pi orbitals was characterized by the predominance of acetylide ligands. Similarly, examination of the Cp ionizations verified the interactions of the acetylide pi orbitals with the occupied metal d(pi) orbitals. The results showed that the form of substitution affected the electron mixing of the acetylide with the metal unit.

A single-crystal neutron diffraction study of (CO)5Re(mu-H)Mn(CO)4Mn(CO)5 was undertaken to probe the geometry of the Re(mu-H)Mn bridge. The complex is characterized by a relativelyweak Re-Mn bond and a hydride ligand asymmetrically positioned between the two metal atoms. (CO)5Re(mu-H)Mn(CO)4Re(CO)5 was found to have cocrystallized with the complex. The presence of this impurity was signified by an additional singlet at delta -19.24 in the 1H NMR spectrum assigned to the bridging hydrideof the major product.