In possessing a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble transition metal centers and thus could potentially mimic their chemical behavior. Although singlet di(amino)carbenes are inert toward dihydrogen, it is shown that more nucleophilic and electrophilic (alkyl)(amino)carbenes can activate H2 under mild conditions, a reaction that has long been known for transition metals. However, in contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles, creating a hydride-like hydrogen, which then attacks the positively polarized carbon center. This nucleophilic behavior allows these carbenes to activate NH3 as well, a difficult task for transition metals because of the formation of Lewis acid-base adducts.

N-Heterocyclic carbene (NHC) rhodium(I) complexes of general formula Rh(COD)X(NHC) are synthesized and structurally characterized (spectroscopy, X-ray diffraction). The relative σ-donor/π-acceptor quality of various NHC ligands was examined and classified by means of IR spectroscopy at the corresponding Rh(CO)2I(NHC) complexes. The first single-crystal X-ray diffraction studies of rhodium pyrazolin- and tetrazolinylidene complexes are reported. A wide variety of different azolium salts was applied to obtain rhodium and iridium complexes with two and four carbene ligands.

A golden ticket to the synthesis of reactive nitrogen‐containing compounds, such as imines, enamines, and allyl amines, through the addition of NH3 to unsaturated bonds is the cationic cyclic (alkyl)(amino)carbene–gold(I) catalyst shown in blue (Dipp=diisopropylphenyl). An ideal initial step for the preparation of simple bulk chemicals, this reaction is also useful for the synthesis of more complex molecules (see examples).

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For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. [1,2] For example, stable singlet carbenes can activate CO, [3a] H 2 , [3b] and P 4 .[3c-e] Such reactions have long been known for transition metals. [4,5] However, stable singlet carbenes can also activate NH 3 ;[3b] a much more difficult task for transition metals. [6,7] The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds.[8]Herein, we report the first examples of the activation of E À H bonds (E = Si, B, P) at a single nonmetal center.On the basis of our successful results with H 2 , [3b] we began our study with the activation of SiÀH bonds. Indeed, silanes are similar to H 2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si À H s complexes, which undergo subsequent oxidative addition.[4] To test the possible activation of Si À H bonds with carbenes, we treated the cyclic (alkyl)-(amino)carbenes (CAACs) 1 a and 1 b [9] with primary, secondary, and tertiary silanes.The addition of phenylsilane to 1 a and 1 b occurred readily at room temperature, and the corresponding adducts 2 a,b were isolated in 91 and 83 % yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1 a, two diastereomers 2 a,a' were formed (in a 2:1 ratio), as shown by two singlets at d = À36.4 and À29.3 ppm in the 29 Si NMR spectrum. The 13 C NMR spectrum revealed the loss of the carbene signal and a new C À H peak at d = 63.2 (2 a) and 65.5 ppm (2 b). The 1 H NMR spectrum of the major isomer 2 a revealed a pseudotriplet at d = 4.78 ppm (SiCH) and two doublets at d = 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH 2 fragment. The structure of 2 a was confirmed by X-ray crystallography [10] (Figure 1, top), whereas the presence of a triplet at d = 4.53 ppm and a doublet at d = 4.08 ppm in the 1 H NMR spectrum confirmed the identity of adduct 2 b.CAACs 1 a,b also reacted with (EtO) 3 SiH to afford 3 a (d.r. 3:1) and 3 b in 64 and 73 % yield, respectively. However, when Ph 2 SiH 2 was used, only the less bulky carbene 1 b underwent insertion into the Si À H bond (to give 4 b in 65 % yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H 2 , [11] we found that imidazolidin-2-ylidene 5[12] also reacted at room temperature with phenylsilane to afford the Si À H insertion product 6 in 88 % yield (Figure 1, bottom).