Carbon is a ubiquitous element in society as a critical component of materials, medicines, commodity chemicals, and fuels. A primary reason for the broad utility of carbon-containing compounds is the redox and chemical versatility of carbon. Remarkably, a carbon atom can span 9 oxidation states (+4 to −4), with fuels typically consisting of highly reduced carbon-based compounds (−1 to −4), (1) biological metabolites concentrating carbon in intermediate oxidation states (−1 to 2), (2) and plastics such as polycarbonates featuring some carbon atoms in their highest oxidation state (+4). (3) The anthropogenic use of carbon-containing compounds to generate energy typically relies on combustion, resulting in the accumulation of CO₂, the most oxidized form of carbon. In contrast, both biotic and abiotic anoxic processes use oxidized forms of carbon as an electron sink, forming CH₄, the most reduced form of carbon, as the ultimate product. While CO₂ and CH₄ span the extreme ends of the viable oxidation states of carbon, they share two important chemical attributes: (i) they have high global warming potentials and contribute substantially to climate change (4) and (ii) they have thermodynamically strong C–O or C–H bonds, respectively, and the kinetic barriers to convert these abundant gases into other compounds are typically large. As a result, despite their abundance and low cost, neither CO₂ nor CH₄ is currently used as a carbon feedstock at scale. (4) This special issue of Accounts of Chemical Research is centered on research that addresses the global challenge of CO₂ and CH₄ upgrading. Specifically, it focuses on using homogeneous molecular catalysts or enzymes to valorize these under utilized C₁ gases. For CO₂, these studies are motivated by the opportunity to replace the fossil-fuel-based carbon feedstocks that are currently used in fuels, chemicals, and materials with a nonfossil source, with global ramifications for climate, the environment, and the economy. (5) For CH₄, these studies are motivated by the opportunity to limit flaring and instead use it to generate chemicals, which will also impact the global climate. (6) Accounts articles that describe research on the development of heterogeneous catalysts for CO₂ reduction, which is a complementary approach to the molecular-based systems described in this special issue, are already available, (7) as well as a collection that focuses on photo- and electrochemical approaches. (8) Chemists have an expansive toolbox for finding systems for the conversion of CO₂ and CH₄. Biology offers important templates based on billions of years of evolution selecting for the transformation of inert carbon compounds into value-added products. The methods described in this issue include the use of enzymes that activate CH₄ and CO₂ with high efficiency and selectivity. For example, Allen and co-workers describe the factors that enable methyl-coenzyme M reductase to anaerobically oxidize methane and also study related enzymatic methane formation. (9) They use calculations to show that an enzyme which optimizes the electric field is more active for methane formation than an enzyme which does not maximize electric field effects. The end goal of their work is the development of enzymes for microbial biomanufacturing, and they discuss the strengths and weaknesses of current systems for methane oxidation in this context. In related work on enzymatic systems, Wagner and co-workers discuss the highly processive formylmethanofuran dehydrogenases, which contain interconnected active sites and directional substrate channels. (10) One can envision leveraging nature’s pathway for forming specific C–H and C–N bonds through sequential CO₂ reduction and formate condensation to drive atom-by-atom construction of precise organic building blocks. These studies complement several other studies on valuable enzymatic reactivity that have recently been published in Accounts of Chemical Research, including an article describing the biochemical upconversion of C₁ precursors to energy-conserving thioester compounds (11) and another on the selective activation of alkanes to alcohols that connects recently elucidated structural information to extensive functional and mechanistic studies. (12) The active sites of enzymes and their mechanistic details also serve as a source of inspiration in the design of more efficient and/or selective synthetic molecular catalysts. For example, Ishizuka and Kojima describe the development of iron catalysts for the selective oxidation of methane to methanol that feature hydrophobic pockets similar to those found in metalloenzymes. (13) They also discuss their work using Lewis acids to improve the selectivity for CO₂ reduction to CO in nickel catalysts akin to the mode of action of carbon monoxide dehydrogenase. Similarly, Dutta and co-workers describe how the enzyme formate dehydrogenase has inspired molecular systems for CO₂ electroreduction to formate and discuss the challenges that remain to develop large-scale electrolyzers for this process. (14) Enzymes are effective in redox processes in part because they are tuned for controlling the rate of electron transfer. Machan and Moberg describe the optimization of novel chromium-based catalysts for CO₂ reduction through the use of redox-active ligands and redox mediators, which allow precise control over the movement of both electrons and protons. (15) The careful control of the secondary and outer coordination environment is another important feature of natural enzymes. Warren describes his group’s research exploring the impact of the number and position of hydrogen bonding groups on the periphery of iron tetraarylporphyrin complexes for CO₂ electroreduction. (16) Surprisingly, even the interactions between the solvent and the hydrogen bonding group can impact the catalytic activity. Although the secondary ligand environment is important, the primary coordination sphere around the metal center is even more crucial in the design of catalysts for CO₂ reduction. Choudhury et al. demonstrate that the use of strongly donating N-heterocyclic carbene (NHC) ligands provide metal complexes with increased stability in CO₂ hydrogenation reactions and show that metal hydrides supported by NHC ligands are more likely to transfer a hydride. (17) Their studies on metal hydrides form the basis for the discovery of an organic hydride for catalytic CO₂ electroreduction to formate. In more fundamental stoichiometric studies, Lee and co-workers demonstrate that an unusual acridine-based pincer ligand is crucial for enabling a nickel complex to break a C–O bond in CO₂. (18) This motif provides a new strategy for activating CO₂. In general, the two-electron reduction of CO₂ to either CO or formate is easier to achieve than six-electron reduction to methanol. Onishi and Himeda carefully design an iridium complex to facilitate H₂ heterolysis and then show that by performing a reaction between the solid complex and gaseous CO₂ they can generate methanol. (19) This work is an example of homogeneous studies guiding the development of a heterogeneous system. Whereas the focus of most of the studies in this issue is the generation of commodity chemicals, Yu and co-workers use electrocatalysis to facilitate the coupling of organic molecules such as aryl and alkyl halides with CO₂ to generate carboxylic acids. This new synthetic methodology may enable the late-stage incorporation of carboxylic acids in complex molecules. (20) Finally, in many transition-metal-mediated transformations of CO₂, the insertion of CO₂ into a metal hydride or metal alkyl is a crucial step. Yang describes thermodynamic considerations for selective CO₂ insertion into metal hydrides. (21) Hazari describes fundamental kinetic studies exploring how the ancillary ligand, solvent, and additives such as Lewis acids influence the rate of CO₂ insertion. (22) Both of these studies provide guidelines for the design of catalysts for CO₂ valorization reactions. The articles described in this special issue highlight recent progress tackling CO₂ and CH₄ valorization to different products through a suite of creative approaches. However, these accounts also describe the challenges associated with these reactions that still need to be addressed. It is our hope that the compilation of these diverse strategies will provide context and inspiration for future work. The significance of the carbon management problem is increasing at an alarming rate, and effective large-scale strategies for activating carbon in both its highest and lowest oxidation states are essential for sustaining today’s societal demands. This article references 22 other publications. This article has not yet been cited by other publications.

中文翻译：

---

转化高度氧化和还原碳的原料：催化 CO2 和 CH4 价值化的策略


碳是社会中无处不在的元素，是材料、药物、商品化学品和燃料的关键组成部分。含碳化合物具有广泛用途的一个主要原因是碳的氧化还原和化学多功能性。值得注意的是，一个碳原子可以跨越 9 个氧化态（+4 到 -4），燃料通常由高度还原的碳基化合物（-1 到 -4）、（1） 将碳集中在中等氧化态（-1 到 2）的生物代谢物组成，（2） 以及聚碳酸酯等塑料，其中一些碳原子处于最高氧化态（+4）。（3） 人为利用含碳化合物产生能源通常依赖于燃烧，导致 CO₂ 的积累，这是碳中最容易氧化的形式。相比之下，生物和非生物缺氧过程都使用氧化形式的碳作为电子汇，形成 CH₄，即碳的还原性最强形式，作为最终产物。虽然 CO₂ 和 CH₄ 跨越了碳有效氧化态的极端，但它们具有两个重要的化学属性：（i） 它们具有很高的全球变暖潜能值，并对气候变化做出重大贡献 （4） 和 （ii） 它们分别具有热力学上强的 C-O 或 C-H 键，并且将这些丰富气体转化为其他化合物的动力学屏障通常很大。因此，尽管 CO₂ 和 CH₄ 储量丰富且成本低廉，但目前均未用作大规模碳原料。（4） 本期《化学研究账户》特刊以应对 CO₂ 和 CH₄ 升级的全球挑战的研究为中心。 具体来说，它侧重于使用均相分子催化剂或酶来评估这些未充分利用的 C₁ 气体。对于 CO₂，这些研究的动机是有机会用非化石来源取代目前用于燃料、化学品和材料的基于化石燃料的碳原料，从而对气候、环境和经济产生全球影响。（5） 对于 CH₄，这些研究的动机是有机会限制火炬，而是使用它来产生化学物质，这也将影响全球气候。（6） 已经有描述用于 CO₂ 还原的非均相催化剂开发的账户文章，这是本特刊中描述的基于分子的系统的一种补充方法，（7） 以及专注于光化学和电化学方法的集合。（8） 化学家有一个广泛的工具箱，用于寻找 CO₂ 和 CH₄ 的转化系统。生物学提供了基于数十亿年进化选择的重要模板，用于将惰性碳化合物转化为增值产品。本期介绍的方法包括使用高效、高选择性地激活 CH₄ 和 CO₂ 的酶。例如，Allen 及其同事描述了使甲基辅酶 M 还原酶能够厌氧氧化甲烷的因素，并研究了相关的酶甲烷形成。（9） 他们使用计算表明，优化电场的酶比不最大化电场效应的酶对甲烷形成更活跃。 他们工作的最终目标是开发用于微生物生物制造的酶，他们讨论了当前甲烷氧化系统在此背景下的优缺点。在酶系统的相关工作中，Wagner 及其同事讨论了高度合成的甲酰甲烷呋喃脱氢酶，其中包含相互连接的活性位点和定向底物通道。（10） 可以设想利用自然界的途径，通过连续的 CO₂ 还原和甲酸盐缩合形成特定的 C-H 和 C-N 键，以驱动精确有机构建单元的逐原子构建。这些研究补充了最近发表在《化学研究账户》上的其他几项关于有价值的酶反应性的研究，包括一篇描述 C₁ 前体生化上转化为守能硫酯化合物的文章 （11），以及另一篇关于烷烃向醇的选择性活化的文章，该文章将最近阐明的结构信息与广泛的功能和机理研究联系起来。（12） 酶的活性位点及其机理细节也是设计更高效和/或选择性的合成分子催化剂的灵感来源。例如，Ishizuka 和 Kojima 描述了用于将甲烷选择性氧化为甲醇的铁催化剂的开发，其特点是疏水口袋类似于金属酶中的疏水口袋。（13） 他们还讨论了他们使用 Lewis 酸来提高镍催化剂中 CO₂ 还原为 CO 的选择性的工作，类似于一氧化碳脱氢酶的作用方式。 同样，Dutta 及其同事描述了甲酸脱氢酶如何激发 CO₂ 电还原为甲酸盐的分子系统，并讨论了为这一过程开发大规模电解槽仍然存在的挑战。（14） 酶在氧化还原过程中有效，部分原因是它们被调整为控制电子转移速率。Machan 和 Moberg 描述了通过使用氧化还原活性配体和氧化还原介质来优化用于 CO₂ 还原的新型铬基催化剂，这些配体和氧化还原介质可以精确控制电子和质子的运动。（15） 对次级和外部协调环境的精心控制是天然酶的另一个重要特征。Warren 描述了他的小组的研究，探索氢键基团的数量和位置对 CO₂ 电还原的四芳基卟啉铁络合物外围的影响。（16） 令人惊讶的是，即使是溶剂和氢键基团之间的相互作用也会影响催化活性。尽管二级配体环境很重要，但金属中心周围的初级配位球在 CO₂ 还原催化剂的设计中更为重要。Choudhury 等人证明，使用强供体 N-杂环卡宾 （NHC） 配体可为金属络合物在 CO₂ 氢化反应中提供更高的稳定性，并表明由 NHC 配体支持的金属氢化物更有可能转移氢化物。（17） 他们对金属氢化物的研究构成了发现用于催化 CO₂ 电还原为甲酸盐的有机氢化物的基础。 在更基本的化学计量研究中，Lee 及其同事证明，一种不寻常的基于吖啶的钳形配体对于使镍络合物能够破坏 CO₂ 中的 C-O 键至关重要。（18） 该基序为激活 CO₂ 提供了一种新策略。一般来说，CO₂ 的双电子还原为 CO 或甲酸盐比六电子还原为甲醇更容易实现。Onishi 和 Himeda 精心设计了一种铱配合物以促进 H₂ 异质分解，然后证明通过进行固体配合物和气态 CO₂ 之间的反应，他们可以生成甲醇。（19） 这项工作是指导异质系统发展的同质研究的一个例子。虽然本期大多数研究的重点是商品化学品的产生，但 Yu 及其同事使用电催化来促进有机分子（如芳基和烷基卤化物）与 CO₂ 的偶联以生成羧酸。这种新的合成方法可能使羧酸在复杂分子中的后期掺入成为可能。（20） 最后，在许多过渡金属介导的 CO₂ 转化中，将 CO₂ 插入金属氢化物或金属烷基是关键步骤。Yang 描述了选择性 CO₂ 插入金属氢化物的热力学考虑因素。（21） Hazari 描述了探索辅助配体、溶剂和添加剂（如路易斯酸）如何影响 CO₂ 插入速率的基础动力学研究。（22） 这两项研究都为 CO₂ 增值反应的催化剂设计提供了指南。 本特刊中描述的文章重点介绍了通过一系列创造性方法解决 CO₂ 和 CH₄ 对不同产品的价值化问题的最新进展。然而，这些描述也描述了与这些反应相关的挑战，这些挑战仍然需要解决。我们希望这些不同策略的汇编将为未来的工作提供背景和灵感。碳管理问题的重要性正在以惊人的速度增加，在最高和最低氧化态下激活碳的有效大规模策略对于维持当今的社会需求至关重要。本文引用了其他 22 种出版物。本文尚未被其他出版物引用。