Beyond the Tolman electronic parameter: The 31P NMR chemical shifts of easily synthesized carbene–phenylphosphinidene adducts allow the determination of the relative π‐acceptor properties of carbenes. In combination with the Tolman electronic parameter value, the relative pure σ‐donation of carbenes can also be evaluated.

The discovery in 1900 by Gomberg that the trityl radical (Ph(3)C(.)) exists at room temperature is often considered to be the beginning of radical chemistry. Since then, persistent and even room-temperature stable radicals based on second-row and heavier elements have been synthesized. However, few of them have been characterized crystallographically, because they are either too reactive or dimerize in the solid state. Here, we show that a P(2) fragment, capped with two bulky, strongly electron-releasing singlet carbenes (dicoordinate carbon compounds with only six valence electrons), can undergo one-electron oxidation, giving rise to room-temperature stable radical cations. Moreover, when N-heterocyclic carbenes are used, two-electron oxidation can also be performed, producing the corresponding stable dicationic diphosphene, which has to be regarded as a P(2)(2+) fragment coordinated by two carbenes. These results reveal a new application of stable singlet carbenes, the stabilization of paramagnetic species and electron-poor fragments.

Transition metals are well-known for activating small molecules and for stabilizing highly reactive species. A recent trend in the accomplishment of either of these goals is the use of nonmetals, [1] especially stable singlet carbenes. Robinson and co-workers have reported that N-heterocyclic carbenes (NHCs) give rise to stable adducts with HBBH [2] and Si 2 , [3] which are otherwise not isolable. [4] We have shown that cyclic (alkyl)(amino)carbenes (CAACs) can activate CO, [5] H 2 , [6] and even NH 3 , [7] which is difficult when attempted with transition-metal centers. [7, 8] White phosphorus (P 4 ) is a small molecule that is of industrial interest as it is the classical starting material for the large-scale preparation of organophosphorus derivatives.[9]The reactivity of P 4 with transition metals has been widely studied, [10][11][12] and is therefore an excellent model to further test if carbenes can undergo reactions in the same manner as transition metals. Our research group has already shown that the bulky rigid CAAC 1 opens P 4 and simultaneously stabilizes the resulting acyclic P 4 species (A, [13a] Scheme 1), which is otherwise highly reactive. Moreover, when a bulky NHC 2 was used, the NHC-stabilized P 12 cluster B was isolated in high yield.[13b] Therefore, singlet carbenes can activate and induce the aggregation of white phosphorus and stabilize the resulting species, in a similar manner to transition metals. However, the most synthetically useful organophosphorus derivatives contain only one or two phosphorus atoms, and therefore it is of primary importance to induce the fragmentation of P 4 , which can be accomplished by using transition metals.[10] Cummins et al. have even shown that the resulting P 1 -and P 2 -niobium complexes can be used as phosphorus transfer agents.[14]Herein we report that carbenes can induce the fragmentation of white phosphorus in the same manner as transition metals. Depending on the nature of the activator, carbenestabilized P 2 and P 1 species can be isolated. Preliminary mechanistic studies of the reaction that leads to A and B, in particular trapping experiments, showed that both carbenes 1 and 2 first formed an unstable monocarbene adduct of type C. A second carbene molecule subsequently induces a ringopening reaction and the formation of bis(carbene) adducts of type A.[13] The different outcomes of the reactions shown in Scheme 1 can be rationalized by the different electronic properties of CAACs and NHCs. CAACs are more electrophilic (p acceptor) and strengthen the PC bonds of A, [5,6] while NHCs are less basic and are therefore better leaving groups, and favor the formation of clusters such as B. This analysis indicates that strongly basic but electrophilic carbenes should be the best candidates to induce the fragmentation of white phosphorus, provided that they are small enough to further attack the P 4 fragments of adducts of type A or C.According to our previous investigations, the acyclic (alkyl)(amino)carbene 3[15] (Scheme 2) is one of the most electro...

A variety of transition metal-nitrido complexes (metallonitrenes) have been isolated and studied in the context of modeling intermediates in biological nitrogen fixation by the nitrogenase enzymes and the industrial Haber-Bosch hydrogenation of nitrogen gas into ammonia. In contrast, nonmetallic nitrenes have so far only been spectroscopically observed at low temperatures, despite their intermediacy in a range of organic reactions. Here, we report the synthesis of a bis(imidazolidin-2-iminato)phosphinonitrene, which is stable at room temperature in solution and can even be isolated in the solid state. The bonding between phosphorus and nitrogen is analogous to that observed for metallonitrenes. We also show that this nitrido phosphorus derivative can be used to transfer a nitrogen atom to organic fragments, a difficult task for transition metal-nitrido complexes.

One-electron oxidation of a readily available phosphaalkene derived from a cyclic (alkyl)(amino)carbene affords a phosphorus-centered radical cation that is indefinitely stable both in solution and in the solid state, allowing a single X-ray diffraction study to be performed. This species can be regarded as a phosphinyl radical bearing a cationic substituent or, alternatively, as a carbene-stabilized phospheniumyl radical (carbene-RP(+*)).