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A template‐controlled reaction cascade starting from [Pt(PMe3)4](CF3SO3)2 and 2‐azidophenyl isocyanide yields the tetracarbene complex [1](CF3SO3)2. The four NH,NH‐stabilized carbene ligands in [1]2+ can be bridged at the metal template by reaction with N,N‐dimethylformamide (DMF) and diphosgene to give the coordinated cyclic tetracarbene ligand with crown ether topology in complex [2](Cl)2.

The reaction of 2-azidophenyl isocyanide (7) with [M(CO)(5)(thf)] (M=Cr, W) yields the isocyanide complexes [M(CO)(5)(7)] (M=Cr 8, M=W 9). Complexes 8 and 9 react with tertiary phosphines such as triphenylphosphane at the azido function of the isocyanide ligand to give the 2-triphenylphosphiniminophenyl isocyanide complexes 10 (M=Cr) and 11 (M=W). The polar triphenylphosphiniminophenyl function in complexes 10 and 11 can be hydrolyzed with H(2)O/HBr to afford triphenylphosphane oxide and the complexes containing the unstable 2-aminophenyl isocyanide ligand. This ligand spontaneously cyclizes by intramolecular nucleophilic attack of the primary amine at the isocyanide carbon atom to yield the 2,3-dihydro-1H-benzimidazol-2-ylidene complexes 12 (M=Cr) and 13 (M=W). Double deprotonation of the cyclic NH,NH-carbene ligands in 12 and 13 with KOtBu and reaction with two equivalents of allyl bromide yields the N,N'-dialkylated benzannulated N-heterocyclic carbene complexes 14 (M=Cr) and 15 (M=W). The molecular structures of complexes 9 and 11-15 were confirmed by X-ray diffraction studies.

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.

Die Chemie der heterocyclischen Carbene hat sich in den letzten Jahren stürmisch entwickelt. Nach den Imidazolin‐2‐ylidenen wurden weitere cyclische Diaminocarbene mit unterschiedlichen Ringgrößen beschrieben. Neben den Diaminocarbenen sind auch P‐heterocyclische Carbene sowie Derivate mit nur einem oder gar keinem Heteroatom im Carbenring bekannt. Gleichzeitig sind zahlreiche neue Methoden zur Synthese von Komplexen heterocyclischer Carbene, wie die oxidative Addition oder die Cyclisierung von β‐funktionalisierten Isocyaniden an einem Metalltemplat, etabliert worden. Dieser Aufsatz fasst die Fortschritte bei der Synthese von heterocyclischen Carbenen und ihren Komplexen zusammen.

Although the coordination chemistry of N-heterocyclic carbenes (NHCs) with transition metals has been explored for half a century, only in the past ten years has the chemistry of metallosupramolecular assemblies based on poly-NHC ligands been studied more extensively. Remarkable discrete assemblies featuring poly-NHC ligands including two-dimensional metallacycles and three-dimensional metallaprisms/cages have since emerged. These assemblies are mostly obtained starting from various imidazolium or benzimidazolium salts. Driven by the increasing interest in new supramolecular architectures from carbon donor ligands, design, and construction of poly-NHC metal assemblies has become a rapidly growing area of research. The metal-carbene bond length is fixed to approximately 2.0 Å in linear NHC-M-NHC complexes. This allows the use of such complexes bearing olefin-substituted NHC ligands as templates for subsequent photochemical [2 + 2] cycloaddition reactions. The postassembly modification of such assemblies has been actively explored in recent years. In this review, we focus on the synthetic methods, characterization, structural features, and postassembly modifications of metallosupramolecular assemblies obtained from poly-NHC ligands.

Tungsten complex with a coordinated 2-azidoethyl isocyanide ligand reacts with PMe3 at the azido function to give a complex with a coordinated iminophosphorane which upon hydrolysis of the P=N bond yields a complex with an NH,NH-stabilized N-heterocyclic carbene ligand, 7; alkylation of the carbene ring nitrogen atoms gives a complex with an N,N'-dialkylated imidazolidin-2-ylidene ligand, 8 .