Mechanistic Insights into the Oxidative Coupling of N‐Heterocyclic Carbenes within the Coordination Sphere of Copper Complexes

Abstract

The behavior of N-heterocyclic carbene (NHC) ligands in organometallic chemistry is hugely important for catalysis, due to the effect of these ligands on catalytic pathways and their involvement in catalyst decomposition. In this report, a combined experimental and computational study is presented, which provides mechanistic understanding of the unprecedented oxidative coupling of NHCs at Cu. The presence of Cu(I) -, Cu(II) -, and Cu(III) -NHC complexes during the process is postulated, with the unusual Ccarbene -Ccarbene oxidative coupling reaction occurring under extremely mild reaction conditions. This process may represent a novel pathway for the decomposition of Cu-NHC complexes.

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“… 80 A similar C–C coupling leading to compound 10 occurs between two NHC ligands in the bis-NHC Cu III complex 9 that forms in situ from the silver complex 8 . 81 …”

“…Halogen–NHC coupling is also of great interest ( Scheme 6 ). 23 , 25 , 81 , 83 For example, treatment of Pd/NHC complex 17 with chlorine affords salt 18 , very likely by reductive elimination of Cl and NHC from the Pd IV intermediate similar to 4 . 83 The authors emphasize the critical influence of a slight change in the NHC bulkiness on the pathway of Pd IV /NHC intermediates decomposition ( Schemes 2 and 6 ).…”

“… 101 Br–NHC coupling was also observed in reaction of Ag/NHC complex 21 with excess of CuBr 2 ( Scheme 6 ). 81 The proposed mechanism of this reaction includes formation of Cu III /NHC complex 22 which suffers reductive elimination of Br and NHC ligands to give imidazolium salt 23 . This mechanism was supported by DFT calculations.…”

Electrochemical Generation of N‐Heterocyclic Carbenes for Use in Synthesis and Catalysis

“…In addition, transmetalation of the NHC from other metals such as copper and silver is possible (Scheme 8C). These methods typically produce stoichiometric by‐products from the metal precursor and the base, which can cause problems in downstream processing, and decomposition of the metal complex [37] . Depending on the reaction conditions, in particular the nature of the counterion, a neutral mono‐NHC or cationic bis‐NHC complex can be formed (only mono‐NHC shown in Scheme 8).…”

High-Oxidation-State 3d Metal (Ti–Cu) Complexes with N-Heterocyclic Carbene Ligation

“…373 In addition to the aforementioned studies, copper(III) species were also implicated as intermediates in the formation of bis(imidazolium) salts in the reactions of (1-pyridinylimidazol-2-ylidene)silver(I) bromide with copper(II) dibromide (Scheme 81). 374 Willans et al found that the equimolar reaction of (1-pyridinyl-3-allylimidazol-2-ylidene)silver(I) bromide with copper(II) dibromide produced the copper(II)− NHC complex 176 in good yield, whereas changing the molar ratio into 1:2 suppressed the isolation of 176 and resulted in the formation of the 2-bromo-1-pyridinyl-3-allylimidazolium salt (Scheme 81). More intriguingly, the reaction of a 3mesityl-substituted NHC silver(I) complex with 2 equiv of copper(II) bromide led to the formation of a bis(imidazolium) salt (Scheme 76).…”