Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.
Four new Pd(II) complexes containing sulfonate-functionalized N-heterocyclic carbene ligands have been synthesized. All new complexes are palladium bis-NHCs, in which the ligands adopt a monodentate, bis-chelating, and pincer coordination form, so that a good comparison between their catalytic activities can be performed. The complexes have been used in the Suzuki−Miyaura cross-coupling reaction between aryl halides and phenylboronic acid in water and in
i
PrOH/water. The bis-NHC-palladium complex 1, in which the two NHC ligands are in a relative cis configuration, affords the best catalytic outcomes, with high TON values of 105 for 4-bromoacetophenone and 3.7 × 104 for 4-chloroacetophenone.
An exceptionally efficient ruthenium-based catalyst for olefin oxidation has been designed by exploiting N,N'bis(pyridylidene)oxalamide (bisPYA) as a donor-flexible ligand. The dynamic donor ability of the bisPYA ligand, imparted by variable zwitterionic and neutral resonance structure contributions, paired with the redox activity of ruthenium provided catalytic activity for Lemieux-Johnsontype oxidative cleavage of olefins to efficiently prepare ketones and aldehydes. The ruthenium bisPYA complex significantly outperforms state-of-the-art systems and displays extraordinary catalytic activity in this oxidation, reaching turnover frequencies of 650 000 h À1 and turnover numbers of several millions. Scheme 1. Limiting resonance structures of the PYA ligand.
A ruthenium cymene complex bearing a bidentate ligand composed of the N-mesoionic donor N-[1-methylpyridin-4(1H)-ylidene]-amide and the C-mesoionic donor 1,2,3-triazolylidene was prepared. Spectroscopic analyses including UV−vis, electrochemical, and NMR methods demonstrate that the pyridylideneamide ligand adapts to its environment and switches, depending on the solvent, between a formally anionic and a neutral donor. A mesoionic pyridinium-amidate structure predominates in polar solvents, whereas a neutral pyridylidene imine structure prevails in apolar solvents. The implications of these solventdependent electronic characteristics have been exploited in redox catalysis involving alcohol dehydrogenation and transfer dehydrogenation. The results indicate that the ligand resonance flexibility provides a new approach to enhance catalytic performance.
Expect the unexpected: The reactions of a series of imidazolium pyridinium salts with [{IrCp*Cl2}2] and [{RhCp*Cl2}2] afford a series of complexes. Together with the expected bis(NHC) complexes, some species resulting from CC coupling between the pyridylidene and Cp* ligands were observed (see figure; Cp*=pentamethylcyclopentadienyl).
Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.
Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.
Thermolysis of Rh(PPh 3 ) 4 H in the presence of the sixmembered N-heterocyclic carbene 1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine (6-i Pr) gave the monocarbene complex Rh(6-i Pr)-(PPh 3 ) 2 H as a 1:2 mixture of the cis-and trans-phosphine isomers 1a and 1b. This same isomeric mixture was formed as the ultimate product from treating Rh(PPh 3 ) 3 (CO)H with 6-i Pr at room temperature, although pathways involving both CO and PPh 3 loss were observed at initial times. Treatment of 1a/1b with Et 3 N•3HF generated the bifluoride complex cis-Rh(6-i Pr)(PPh 3 ) 2 (FHF) (2a), which upon stirring with anhydrous Me 4 NF was converted to the rhodium fluoride complex cis-Rh(6-i Pr)(PPh 3 ) 2 F (3a). Thermolysis of 1a/1b with C 6 F 6 resulted in C−F bond activation to afford a mixture of 3a and the pentafluorophenyl complex trans-Rh(6-i Pr)(PPh 3 ) 2 (C 6 F 5 ) (5b). Complexes 1b, 2a, 3a, and 5b were structurally characterized.
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