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.

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Treatment of the protic N-heterocyclic carbene (NHC) complex [Cp*RuCl(LH)] (2; LH = N-(2-pyridyl)benzimidazolin-2-ylidene-κ2 N,C) with AgNO2 gives the nitrosyl–imidazolyl complex [Cp*Ru(NO)(L)][OTf] (OTf = OSO2CF3), which undergoes reversible protonation to afford the NHC complex [Cp*Ru(NO)(LH)][OTf]2. The bifunctional complex 2 also catalyzes the dehydrative condensation of N-(2-pyridyl)benzimidazole and allyl alcohol, leading to the formation of trans- and cis-2-(1-propenyl)-N-(2-pyridyl)benzimidazole.

N-N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal-ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ(1)N-phenylhydrazine complex.

Cubane-type sulfido clusters containing noble metals are newcomers compared with the corresponding clusters of the first transition series metals and molybdenum, which have been extensively studied in relation to metalloenzymes and industrial hydrodesulfurization catalysts. This Account reviews the recent progress in studies on the synthesis and reactivities of these noble metal cubane-type clusters. One of the goals in this new area lies in development of the unique catalysis of the noble metals embedded in the robust and redox-active cubane-type cores. Rational synthetic approaches indispensable to the preparation of such effective cluster catalysts are discussed to a significant extent.