Design of a Mesoionic Janus-type Dicarbene

A versatile synthetic strategy for the preparation of homo- and heterobimetallic complexes bearing an unprecedented mesoionic Janus-type diNHC is presented. Moreover, its electronic property is evaluated, and a preliminary catalytic application in the direct diarylation of 1-methylpyrrole is demonstrated.

Unexpected structural preference with metallophilic Ag-Au contacts in silver(I)-N-heterocyclic carbene cluster; experimental and theoretical approach

A novel synthetic donor-atom-selective approach has been adopted for the synthesis of a heterobimetallic cluster of a new NCN-pincer, 1,3-bis-(1-methyl-1H-benzo[d]imidazol-2-yl-methyl)-1H-imidazol-3-ium hexafluorophosphate (1·HPF6). The complex [Ag3(1)3][PF6]3 (2) has been prepared via the Ag2O route; which undergoes transmetallation to yield a cluster that seems to be the first example of the heterobimetallic trinuclear system [Au-Ag2(1)2Cl][PF6]2, 3. Finally, the trinuclear cluster geometries of 2 and 3 were confirmed via SCXRD studies. Interestingly, Au(I) binds preferentially with soft donor Ccarbene, which transmetallated from the cluster of 2. In both the cyclic trinuclear clusters, the M-M interactions were further inspected using gauge independent atomic orbital (GIAO) computations. Both 2 and 3 are luminescent and possess σ-aromaticity; the NICS values indicate that 3 is more aromatic than 2.

A naphthalene-based heterobimetallic triazolylidene IrIII/PdII complex: regioselective to regiospecific C-H activation, tandem catalysis and a copper-free Sonogashira reaction

Heterobimetallic complexes featuring mesoionic carbene (MIC) donor ligands are gaining enormous popularity in tandem catalysis owing to the combined action of two different metal centers during catalysis. A rare version of the heterobimetallic Pd^II/Ir^III complex possessing a cyclometalated mesoionic carbene (MIC) ligand is presented along with the analogous homodinuclear Pd^II complex. A sterically controlled regiospecific cyclometalation towards the formation of a six-membered ring complex over a five-membered ring complex has been performed using a naphthalene-based bis-MIC ligand platform. The interplay between regioselective vs. regiospecific C-H bond activation for the synthesis of cyclometalated Ir^III complexes has also been demonstrated using the corresponding naphthyl-derived mono-imidazolylidene ligand. Both homodinuclear Pd^II and heterobimetallic Pd^II/Ir^III complexes have been characterized using standard spectroscopic techniques including ¹H, ¹³C{¹H}, 2D correlation NMR spectroscopy and ESI mass spectrometry. The structure of the cyclometalated heterobimetallic complex has been established by single crystal XRD. The heterobimetallic complex has been employed as a pre-catalyst in the tandem Suzuki-Miyaura/transfer hydrogenation reaction and the homobimetallic Pd^II complex has been successfully employed as a catalyst in both the Sonogashira coupling and α-arylation of 1-methyl-2-oxindole.

Synthesis of N-Heterocyclic Carbenes and Their Complexes by Chloronium Ion Abstraction from 2-Chloroazolium Salts Using Electron-Rich Phosphines

N‐Heterocyclic carbenes (NHCs) are commonly prepared by deprotonation of azolium salts using strong anionic bases. This reaction is often unselective, yielding alkali metal NHC complexes or dimerized NHCs. Alternatively, free NHCs are obtained by the dechlorination of 2‐chloroazolium salts using electron‐rich phosphines. PPh₃, PCy₃, and PtBu₃ are unsuitable for Cl⁺ abstraction, while the sterically encumbered tris(1,3‐tert‐butylimidazolidin‐2‐ylidenamino)phosphine 1 selectively removes Cl⁺ from 2‐chloroazolium salts. Since bulky 1 does not bind to metal complexes, it was used for the preparation of NHC complexes via in situ Cl⁺ abstraction from 2‐chloroazolium salts. The dechlorination was employed for the site‐selective monometallation with Ir^I, Ir^III, Rh^I, Rh^III, and Ru^II of a bis‐NHC precursor composed of a 2‐chlorobenzimidazolium and a 2‐chlorobenzimidazole group, followed by the preparation of the heterobimetallic Ir^III/Pd^II complex [18](BF₄)₂ by a dechlorination/oxidative addition reaction sequence.

Janus-type homo-, hetero- and mixed valence-bimetallic complexes with one metal encapsulated in a cyclodextrin

Bis-azolium salts with one azolium capping a perbenzylated α-cyclodextrin have been designed to generate Janus-type bimetallic complexes with various combinations of copper, silver, gold or palladium salts. Encapsulation of one metal center inside the cavity allowed (trans)metalation and oxidation reactions to be controlled at selected positions. In particular, it was possible to oxidize Au^I into Au^III selectively on the position outside the cavity of the cyclodextrin on the bis-Au^I Janus complex.

Synthesis, conformational analysis and antibacterial activity of Au(i)–Ag(i) and Au(i)–Hg(ii) heterobimetallic N-heterocyclic carbene complexes

A family heterobimetallic Au(i)–Ag(i) and Au(i)–Hg(ii) complexes of bis-N-heterocyclic carbene ligands been prepared and their antibacterial properties evaluated.

Imidazolium-benzimidazolates as convenient sources of donor-functionalised normal and abnormal N-heterocyclic carbenes

Imidazolium-benzimidazolates are readily available conjugated mesomeric betaines, whose carbenic tautomers can form C,N_imine-metal chelates containing amino-functionalised normal or abnormal NHC ligands, depending on the degree of steric congestion. Deprotonation of the amino moiety gives rise to imine-functionalised C,N_amido-metal chelates, whose N_imine atom can be easily utilised for the construction of heteronuclear complexes.

Possible Synthetic Approaches for Heterobimetallic Complexes by Using nNHC/tzNHC Heteroditopic Carbene Ligands

The synthesis of heterobimetallic complexes remains a synthetic challenge in the field of organometallic chemistry. A possible approach in this regard might be the use of a bidentate heteroditopic bis(carbene) ligand that combines an imidazol-2-ylidene (nNHC) with a 1,2,3-triazol-5-ylidene (tzNHC) connected by an organic spacer. The optimized strategy to heterobimetallic complexes with this type of ligand involves a 3-step procedure: (i) Coordination of the nNHC, functionalized with a 1,2,3-triazole ring, to a metal center; (ii) formation of the triazolium ring by alkylation of the triazole N-3; (iii) deprotonation of the tzNHC precursor and coordination of the second metal center. Following this procedure, a novel Au(I)-Ag(I) dinuclear complex was isolated and its properties were compared to the analogous homobimetallic Ag(I)-Ag(I) and Au(I)-Au(I) complexes. The study was completed by the determination of the molecular structures of some synthetic intermediates.