Dimerization of a mixed‐carbene PdII dibromide complex by elemental iodine

A monomeric Pd^II complex bearing a mixed carbocyclic/N‐heterocyclic carbene ligand and two bromides was reacted with an excess of elemental iodine, which resulted in the surprising removal of one bromide ligand and dimerization of the mixed‐carbene complex to form di‐μ‐bromido‐bis{[1‐(cyclohepta‐2,4,6‐trien‐2‐yl‐1‐ylidene‐κC ¹)‐3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]palladium(II)} bis(pentaiodide) dichloromethane monosolvate, [Pd₂Br₂(C₂₂H₂₄N₂)₂](I₅)₂·CH₂Cl₂. The dimeric complex features a slightly distorted square‐planar core of two Pd^II centres bridged by two bromide ligands, which lie in the same plane as the seven‐ and five‐membered rings of the bidentate carbene ligand. The counter‐ions in the single crystal were found to be pentaiodide monoanions featuring their typical V‐shape, whereas for the bulk material, a mixture of Br/I interhalides is proposed.     

Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)–NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)–N and C(acyl)–O bond cleavage reactions.     

Synthesis,chemical properties,and CO2 electroreduction of π-conjugated polymers bearing imidazolium and imidazolinium rings in the polymer backbone

π-Conjugated polymers (CPs) bearing imidazolium and imidazolinium rings in the polymer backbone were prepared from a previously reported monomer, 1,3-bis(2,3-dimethyl-4-bromophenyl)imidazolium chloride, and a newly synthesized monomer, 1,3-bis(2,3-dimethyl-4-bromophenyl)imidazolinium chloride, via Sonogashira and Suzuki–Miyaura coupling reactions. The corresponding model compounds were also synthesized via the same reactions and monomers. The absorption maxima and onset position of the UV–vis spectra of the polymers were observed at longer wavelengths than those of the model compounds, thus revealing the presence of extended π-conjugation along the polymer backbone chain. Both polymers and model compounds were photoluminescent in solution and exhibited solvatochromism. The cast films of the polymers on the Pt and Ag electrodes were demonstrated to be effective catalysts for CO₂ electroreduction.     

A computational quest for the effects of fused rings on the stability of Hammick carbenes type remote N‐heterocyclic carbenes

Substituent effects of fused six, and five‐membered aromatic rings are investigated on the stability, aromaticity, charge distribution, nucleophilic (N), and electrophilic (ω) characters of 20 singlet (s) and triplet (t) Hammick carbenes, at B3LYP/AUG‐cc‐pVTZ and M06‐2X/AUG‐cc‐pVTZ. Results display: (a) The higher thermodynamic and kinetic stability is revealed by carbenes situated between two nitrogen and/or two oxygen heteroatoms of two substituted rings, in a “W” arrangement toward the carbenic center; (b) Regardless of the arrangement, the order of thermodynamical and kinetic stabilization for fused rings is pyrrole > furan > thiophene > phosphole. (c) The substituted Hammick carbenes with two fused heterocyclics, in a given arrangement, show more stability than unsubstituted Hammick carbene; (d) While two five‐membered heterocyclic rings stabilize their corresponding substituted carbenes, two benzene rings destabilize Hammick carbene; (e) In all structures, s species emerges as ground state, exhibiting more stability than its t state; (f) The scrutinized s carbenes show higher N and lower ω than their corresponding t states.     

Mesoionic Carbene Cobalt Complexes as Multipurpose Catalyst Precursors for Hydrosilylation and Dihydropyrimidinone Synthesis

Metalation of CH₂OH-substituted triazolium salts with CoCl₂ under basic conditions affords C,O-bidentate chelating carbene Co(III) complexes ( 3a , 3b ), while analogous phenyl-substituted triazolium salts produce monodentate carbene Co(II) complexes ( 3c , 3d ). The distinct substituent-induced properties of the metal centers were demonstrated by electrochemical measurements and catalytic activities in two specific processes. The complexes showed appreciable activity in the reduction of C=O bonds through hydrosilylation, with methoxybenzene-functionalized triazolylidene Co(III) complex 3a achieving a high selectivity towards aldehydes vs. ketones with turnover frequencies (TOFs) up to 200 h⁻¹. The C,O-chelate systems were also active catalysts in the Biginelli process, a one-step three-component reaction for efficient dihydropyrimidinone synthesis. Optimization of reaction conditions provides high activity with complex 3a , reaching TOFs of 800 h⁻¹, the highest activity known for cobalt NHC complexes to date.     

Isoselective Ring-opening Polymerization of Racemic Lactide Catalyzed by N-heterocyclic Olefin/(Thio)urea Organocatalysts

The isoselective ring-opening polymerization of racemic lactide was achieved by combining N-heterocyclic olefin(NHO) with mono(thio)ureas or bis(thio)ureas as catalytic systems. The polymerization process shows high stereoselectivity, controllability and reactivity,delivering multi-block isotactic polylactides with high chain-end fidelity and narrow molecular weight distributions. The enhancement of catalytic performance was observed in the following order: bisthiourea(DTU) monothiourea(TU) bisurea(DU) urea(U). The highest Pm(probability of forming a meso dyad) = 0.91 was observed at-70 °C when using NHO/U1 catalytic system and the high stereoselectivity was attributed to chain-end control mechanism.     

Redox-Switchable Complexes Based on Nanographene-NHCs

A series of rhodium and iridium complexes with a N-heterocyclic carbene (NHC) ligand decorated with a perylene-diimide-pyrene moiety are described. Electrochemical studies reveal that the complexes can undergo two successive one-electron reduction events, associated to the reduction of the PDI moiety attached to the NHC ligand. The reduction of the ligand produces a significant increase on its electron-donating character, as observed from the infrared spectroelectrochemical studies. The rhodium complex was tested in the [3+2] cycloaddition of diphenylcyclopropenone and methylphenylacetylene, where it displayed a redox-switchable behavior. The neutral complex showed moderate activity, which was suppressed when the catalyst was reduced.     

Reactions of a Four-Membered Borete with Carbon,Silicon, and Gallium Donor Ligands: Fused and Spiro-Type Boracycles

Donor-acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four-membered borete cyclo-iPr₂N-BC₁₀H₆ reacted with the carbon donor ligands 2,6-xylylisonitrile and the carbene IMes :C(NMesCH)₂ with ring expansion and ring fusion, respectively. In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)₂ and :Ga(AmIm) with a two-coordinate donor atom afford spiro-type bicyclic compounds, which display four-coordinate geometry at silicon or gallium. (TD-)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.     

Phosphorescent Bimetallic C^C* Platinum(ii) Complexes with Bridging Substituted Diphenylformamidinates

A series of phosphorescent bimetallic platinum(II) complexes is presented, which were synthesized by the combination of bidentate cyclometalated N-heterocyclic carbene ligands and different bridging diphenylformamidinates. The complexes were characterized by standard techniques and additionally two solid-state structures could be obtained. Photoluminescence measurements revealed the strong emissive behavior of the compounds with quantum yields of up to 90 % and emission lifetimes of approx. 2 μs. The effect of the substitution pattern in the bridging ligands on the structural and photophysical properties of the complexes was examined in detail and rationalized by density functional theory calculations (PBE0/6-311G*).     

Catalytic activity of chiral chelating N-heterocyclic carbene palladium complexes towards asymmetric allylic alkylation

Abstract

The catalytic activity of a series of chiral heteroaryl coordinated chelating N-heterocyclic carbene (NHC) palladium complexes towards asymmetric allylic alkylation (AAA) were presented here. The effects of different N-substituents, NHC backbones and chelate rings on the catalytic activity and the enantioselectivity of the alkylation of (E)-1,3-diarylallyl acetates with dialkyl malonate were investigated. The results showed that, under the optimized conditions, complexes 3a, 3b, and 3i carrying the pyridinyl-coordinated five-membered chelate ring showed high catalytic activity and chiral induction efficiency. The corresponding alkylated products were obtained in high yields with moderate ee. Furthermore, it was found that the substituents of (E)-1,3-diarylallyl acetates and the type of the nucleophile affect the results as well.