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Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atmosphere demands that holding atmospheric CO2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Powering the planet: Chemical challenges in solar energy utilization

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The site-selective formation of carbon-carbon bonds through direct functionalizations of otherwise unreactive carbon-hydrogen bonds constitutes an economically attractive strategy for an overall streamlining of sustainable syntheses. In recent decades, intensive research efforts have led to the development of various reaction conditions for challenging C-H bond functionalizations, among which transition-metal-catalyzed transformations arguably constitute thus far the most valuable tool. For instance, the use of inter alia palladium, ruthenium, rhodium, copper, or iron complexes set the stage for chemo-, site-, diastereo-, and/or enantioselective C-H bond functionalizations. Key to success was generally a detailed mechanistic understanding of the elementary C-H bond metalation step, which depending on the nature of the metal fragment can proceed via several distinct reaction pathways. Traditionally, three different modes of action were primarily considered for CH bond metalations, namely, (i) oxidative addition with electronrich late transition metals, (ii) σ-bond metathesis with early transition metals, and (iii) electrophilic activation with electrondeficient late transition metals (Scheme 1). However, more recent mechanistic studies indicated the existence of a continuum of electrophilic, ambiphilic, and nucleophilic interactions. Within this continuum, detailed experimental and computational analysis provided strong evidence for novel C-H bond metalationmechanisms relying on the assistance of a bifunctional ligand bearing an additional Lewis-basic heteroatom, such as that found in (heteroatom-substituted) secondary phosphine oxides or most prominently carboxylates (Scheme 1, iv). This novel insight into the nature of stoichiometric metalations has served as stimulus for the development of novel transformations based on cocatalytic amounts of carboxylates, which significantly broadened the scope of C-H bond functionalizations in recent years, with most remarkable progress being made in palladiumor ruthenium-catalyzed direct arylations and direct alkylations. These carboxylate-assisted C-H bond transformations were mostly proposed to proceed via a mechanism in which metalation takes place via a concerted base-assisted deprotonation. To mechanistically differentiate these intramolecular metalations new acronyms have recently been introduced into the literature, such as CMD (concerted metalationdeprotonation), IES (internal electrophilic substitution), or AMLA (ambiphilic metal ligand activation), which describe related mechanisms and will be used below where appropriate. This review summarizes the development and scope of carboxylates as cocatalysts in transition-metal-catalyzed C-H functionalizations until autumn 2010. Moreover, experimental and computational studies on stoichiometric metalation reactions being of relevance to the mechanism of these catalytic processes are discussed as well. Mechanistically related C-H bond cleavage reactions with ruthenium or iridium complexes bearing monodentate ligands are, however, only covered with respect to their working mode, and transformations with stoichiometric amounts of simple acetate bases are solely included when their mechanism was suggested to proceed by acetate-assisted metalation.

Carboxylate-assisted transition-metal-catalyzed C-H bond functionalizations: mechanism and scope.

Revisiting CO2 Reduction with NaBH4 under Aprotic Conditions: Synthesis and Characterization of Sodium Triformatoborohydride

The terminal nitride anion complex [Na][N≡Nb(N[tBu]Ar)3] ([Na][1], Ar = 3,5-Me2C6H3) reacts quantitatively with CO2 to give the carbamate complex [Na][O2CN≡Nb(N[tBu]Ar)3] ([Na][O2C-1]). The structure of [Na][O2C-1] as the bis-12-crown-4 solvate, as determined by X-ray crystallography, displays a unique N-bound carbamate ligand without any metal−oxygen interactions. When treated with organic acid anhydrides or acid chlorides, complex [Na][O2C-1] reacts via salt elimination to give the five-coordinate complexes (RC(O)O)(OCN)Nb(N[tBu]Ar)3 (R-2, R = Me, tBu, F3C). We show that complexes R-2 yield the starting complex [Na][1] with concomitant release of CO upon two-electron reduction. This series of reactions constitutes a closed cycle for the conversion of CO2 to CO mediated by a terminal nitride anion complex.

Ligand-Based Reduction of CO2 to CO Mediated by an Anionic Niobium Nitride Complex

Unprotected dibenzo-7λ3-phosphanorbornadiene derivatives RPA (A = C14H10 or anthracene; R = tBu, dbabh = NA, HMDS = (Me3Si)2N, iPr2N) are synthesized by treatment of the corresponding phosphorus dichloride RPCl2 with MgA·3THF, in cold THF (∼20% to 30% isolated yields). Anthracene and the corresponding cyclic phosphane (RP)n form as coproducts. Characteristic NMR features of the RPA derivatives include a doublet near 4 ppm in their 1H NMR spectra and a triplet peak in the 175–212 ppm region of the 31P NMR spectrum (2JPH ∼14 Hz). The X-ray structures of the AN–PA and (HMDS)PA derivatives are discussed. Thermolysis of RPA benzene-d6 solutions leads to anthracene extrusion. This process has a unimolecular kinetic profile for the iPr2NPA derivative. The 7-phosphanorbornene anti-iPr2NP(C6H8) could be synthesized (70% isolated yield) by thermolysis of iPr2NPA in 1,3-cyclohexadiene.

Facile synthesis of dibenzo-7λ3-phosphanorbornadiene derivatives using magnesium anthracene.

White phosphorus, generated in the legacy thermal process for phosphate rock upgrading, has long been the key industrial intermediate for the synthesis of phosphorus-containing chemicals, including herbicides, flame-retardants, catalyst ligands, battery electrolytes, pharmaceuticals, and detergents. In contrast, phosphate fertilizers are made on a much larger scale from phosphoric acid, obtained by treating phosphate rock with sulfuric acid. Dehydration of phosphoric acid using sodium chloride gives trimetaphosphate, and here we report that trichlorosilane, primarily used for the production of high-purity silicon, reduces trimetaphosphate to the previously unknown bis(trichlorosilyl)phosphide anion. This anion offers an entry point to value-added organophosphorus chemicals such as primary and secondary alkyl phosphines, and thus to organophosphinates, and can also be used to prepare phosphine gas and the hexafluorophosphate anion, all previously available only downstream from white phosphorus.

https://science.sciencemag.org/content/sci/359/6382/1383.full.pdf

Christopher C. Cummins

Papers

Revisiting CO2 Reduction with NaBH4 under Aprotic Conditions: Synthesis and Characterization of Sodium Triformatoborohydride

Ligand-Based Reduction of CO2 to CO Mediated by an Anionic Niobium Nitride Complex

Facile synthesis of dibenzo-7λ3-phosphanorbornadiene derivatives using magnesium anthracene.

Phosphoric acid as a precursor to chemicals traditionally synthesized from white phosphorus

Diorganophosphanylphosphinidenes as complexed ligands: synthesis via an anionic terminal phosphide of niobium.