N-heterocyclic carbenes (NHCs) are a class of molecules with a lone pair of carbene electrons and thus, they have the ability to activate CO2 to form imidazolium carboxylates. The incorporation of activated, metal-free NHC moieties into metal-organic frameworks (MOFs) without the decomposition of metal-carboxylate coordination motifs is highly desired owing to the high CO2 affinity and versatile chemical functionalities in MOFs. Herein, we have summarized the recent in situ generation approaches to form metal-free NHC-functionalized MOFs, which are a unique class of CO2-conversion catalysts with high catalytic activity, selectivity and stability, superior to those of homogenous and other heterogeneous NHC analogues. The NHC-functionalized MOFs for catalytic CO2 reduction include reactions such as the hydroboration of CO2, hydrosilylation of CO2, N-methylation using CO2 and hydrogenation of CO2 to formic acid. Overall, the synthetic strategy of metal-free NHC-functionalized MOFs, the unique catalytic pathways of NHC-functionalized MOFs, and potentially new research directions of NHC-functionalized MOFs are discussed, which will guide researchers to attempt to design new NHC-MOFs and extend their catalytic applications in the chemical fixation of CO2.

中文翻译：

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N-杂环卡宾官能化的金属-有机骨架，用于化学固定CO2。

N-杂环卡宾（NHC）是一类带有一对卡宾电子的孤对分子，因此它们具有激活CO2形成咪唑鎓羧酸盐的能力。由于MOF中的高CO 2亲和力和广泛的化学功能性，非常需要将活化的，不含金属的NHC部分结合到金属有机骨架（MOF）中而不分解金属羧酸盐配位基序。本文中，我们概述了形成无金属NHC官能化MOF的最新原位生成方法，这是一类独特的CO2转化催化剂，具有高催化活性，选择性和稳定性，优于均相和其他非均相NHC类似物。用于催化还原CO2的NHC功能化MOF包括反应，例如CO2的硼氢化，CO2的硅氢化，使用CO2进行N-甲基化，然后将CO2加氢成甲酸。总体上，讨论了无金属NHC功能化MOF的合成策略，NHC功能化MOF的独特催化途径以及NHC功能化MOF的潜在新研究方向，这将指导研究人员尝试设计新的NHC-MOF。扩展了其在二氧化碳化学固定中的催化应用。