Complexes bearing protic N-heterocyclic carbenes (protic NHCs, pNHCs), defined as cyclic carbenes stabilized by two heteroatoms including at least one NH group, have been less explored in comparison to the conventional N,N′-disubstituted NHCs. The small and reactive NH group of the pNHCs differentiates this class of compounds from the classical NR,NR-NHCs with regard to their properties and reactivity. While the free pNHCs have so far eluded isolation due to isomerization to the azole tautomer, a significant number of transition-metal pNHC complexes have by now been prepared by a variety of methods. This article reviews the coordination chemistry of the pNHCs. Synthetic approaches toward complexes of pNHCs are first described, followed by a discussion of the properties and reactivity of pNHC complexes with emphasis on the Brønsted-acidic nature of the NH wingtip. Involvement of the pNHC ligands in catalytically active intermediates and in cooperative catalysis is also discussed.

中文翻译：

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带有质子N-杂环碳原子配体的配合物

与常规的N，N′-二取代的NHC相比，带有质子N-杂环卡宾（质子NHCs，pNHC）的复合物，被定义为由包括至少一个NH基团的两个杂原子稳定的环状卡宾。pNHC的小分子和反应性NH基团在性质和反应性方面与经典的NR，NR-NHC区别开来。到目前为止，由于异构化为吡咯互变异构体，游离的pNHC尚未被分离，但到目前为止，已经通过多种方法制备了大量的过渡金属pNHC复合物。本文综述了pNHC的配位化学。首先介绍了合成pNHC的合成方法，随后讨论了pNHC配合物的性质和反应性，重点是NH翼尖的布朗斯台德酸性。还讨论了pNHC配体在催化活性中间体中以及在协同催化中的参与。