No abstract

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.

Like NHCs? Not quite: Cyclic (alkyl)(amino)carbenes (CAACs) are strong σ‐donors and have steric environments that differ dramatically from those of bulky, electron‐rich phosphines (A) and cyclic diaminocarbenes (B). These readily available carbenes are desirable ligands for catalysis, as illustrated by the first palladium‐mediated α‐arylation of ketones and aldehydes with non‐activated aryl chlorides at ambient temperature.

Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more–earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.

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.

No abstract

The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [Nheterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC.For decades, carbenes, which feature a neutral divalent carbon atom with two nonbonding electrons, were considered prototypical reactive intermediates (1). Today, thanks to the availability of stable carbenes (2,3), these molecules, especially the so-called N-heterocyclic carbenes (NHCs) (I) (4-6) ( Fig. 1, top left), are recognized as versatile ligands for transition metal-based catalysts (7-10) and as metal-free organic catalysts in their own right (11)(12)(13)(14). As expected, NHCs I usually bind metals via the carbene center (C2) to give η 1 complexes II. However, in 2001, Crabtree and co-workers discovered that 2-pyridylmethylimidazolium salts react with IrH 5 (PPh 3 ) 2 to give 1 with the imidazole ring bound the "wrong way," at C5 and not at C2 (15,16) (Fig. 1, center). Since that time, a few other complexes of type IV featuring the so-called abnormal NHCs (aNHCs) (III) (17-20) as ligands have been prepared (21-23) (Fig. 1, top right). Experimental and theoretical data suggest that aNHCs III are even stronger electron-donor ligands than are NHCs I. In line with these observations, initial catalytic screening of aNHC metal complexes IV reveals promising results for the activation of unreactive bonds such as C-H and H-H (24-26). As an example, an aNHC palladium complex has been reported to be an efficient catalyst in the Heck olefination of aryl bromides, whereas the corresponding NHC analog is virtually inactive under identical conditions (24).* To whom correspondence should be addressed. guy.bertrand@ucr.edu. Supporting Online Material NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptLassaletta and co-workers (27) have shown that the deprotonation of imidazo [1,5-a] pyridinium salts 2 leads to free NHC 3 that can be isolated ( Fig. 1, bottom). In contrast, using C2-substituted precursors, such as 4, Lassaletta et al. did not observe the corresponding free aNHC. However, by performing the deprotonation reaction in the presence of [Rh(COD)Cl] 2 , they were able to isolate the corresponding aNHC complex 5. Because calculations predicted that the parent aNHC III (where R is equal to H) is only about 17 kcal mol −1 ...

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π-accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main-group and transition-metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.