Carbene–bridge–carbene ensembles: tunable diradicaloids for singlet fission?

We thank the Fonds der Chemischen Industrie (Liebig fellowship for D.M.), the German‐American Fulbright Commission (Fulbright‐Cottrell Award for D.M.) as well as the Bavarian Equal Opportunities Sponsorship – Realization of Equal Opportunities for Women in Research and Teaching (fellowship for A.G.) for financial support. We thank the RRZ Erlangen for computational resources.

We report on the convenient synthesis of a CNC pincer ligand com¬po-sed of car¬ba¬zole and two mesoionic carbenes, as well as the corresponding lithium- and magnesium complexes. Mono-deprotonation affords a...

Only sterics and aromaticity control whether radical olefin cations are stable.

Herein, we report the isolation and a reactivity study of the first example of an elusive palladium(II) terminal imido complex. This scaffold is an alleged key intermediate for various catalytic processes, including the amination of C−H bonds. We demonstrate facile nitrene transfer with H−H, C−H, N−H, and O−H bonds and elucidate its role in catalysis. The high reactivity is due to the population of the antibonding highest occupied molecular orbital (HOMO), which results in unique charge separation within the closed‐shell imido functionality. Hence, N atom transfer is not necessarily associated with the high valency of the metal (PdIII, PdIV) or the open‐shell character of a nitrene as commonly inferred.

The homolytic cleavage of O−H and N−H or weak C−H bonds is a key elementary step in redox catalysis, but is thought to be unfeasible for palladium. In stark contrast, reported here is the room temperature and reversible oxidative addition of water, isopropanol, hexafluoroisopropanol, phenol, and aniline to a palladium(0) complex with a cyclic (alkyl)(amino)carbene (CAAC) and a labile pyridino ligand, as is also the case in popular N‐heterocyclic carbene (NHC) palladium(II) precatalysts. The oxidative addition of protic solvents or adventitious water switches the chemoselectivity in catalysis with alkynes through activation of the terminal C−H bond. Most salient, the homolytic activation of alcohols and amines allows atom‐efficient, additive‐free cross‐coupling and transfer hydrogenation under mild reaction conditions with usually unreactive, yet desirable reagents, including esters and bis(pinacolato)diboron.

Whereas triplet-nitrene complexes of the late transition metals are isolable and key intermediates in catalysis, singlet-nitrene ligands remain elusive. Herein we communicate three such palladium terminal imido complexes with singlet ground states. UV–vis–NIR electronic spectroscopy with broad bands up to 1400 nm as well as high-level computations (DFT, STEOM-CCSD, CASSCF/NEVPT2, EOS analysis) and reactivity studies suggest significant palladium(0) singlet-nitrene character. Although the aliphatic nitrene complexes proved to be too reactive for isolation in analytically pure form as a result of elimination of isobutylene, the aryl congener could be characterized by SC-XRD, elemental analysis, IR-, NMR spectroscopy, and HRMS. The complexes’ distinguished ambiphilicity allows them to activate hexafluorobenzene, triphenylphosphine, and pinacol borane, catalytically dehydrogenate cyclohexene, and aminate ethylene via nitrene transfer at or below room temperature.

We recently reported the first example of a palladium(ii) terminal imido complex. We proposed that this complex features exceptional high nucleophilicity at the nitrogen atom and a peculiar zwitterionic electronic structure with an anti-bonding highest-occupied molecular orbital (HOMO). This complex swiftly activated moderately acidic CH, OH, and NH bonds and also reacted with dihydrogen. However, unambiguous nucleophilic reactivity with substrates not featuring a hydrogen atom could not be observed. Herein, we now show that this nucleophilic complex also reacts with CO2 to give a ring-strained four-membered palladium(ii) carbamate complex. Remarkably, the same product is obtained in the reaction of the related bisamido complex, albeit at a slower reaction rate. Density functional theory calculations indicate that the addition of CO2 does not proceed via initial 1,2-addition across the Pd–N bond, but instead through nucleophilic attack by the imido (amido respectively) nitrogen atom.