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N-heterocyclic carbene (NHC) metal complexes are used widely as catalysts in organic reactions. Compared to other neutral type ligands, NHCs usually form stronger bonds with metals due to excellent s donating ability. The conjugation between the carbene carbon (C_carbene) p_p orbital and nitrogen lone pair electrons in the heterocycle further stabilizes the metal–C_carbene bonds. As a result, NHCs are often better at suppressing catalyst decompositions. The metal–C_carbene bonds are remarkably inert compared to other metal–carbon bonds at catalytic metal centers, which are prone to undergo various reactions such as migratory insertion and olefin metathesis. In contrast, only a small number of elementary organomellic reactions, mostly limited to C_carbene–C reductive elimination and ligand dissociation/displacement, are documented with the metal–C_carbene bonds.

Carbon-halogen (C–X) reductive elimination of metal–C_alkyl or metal–C_aryl bonds is an important elementary organometallic reaction, but no well-defined example is reported for C_carbene–X reductive elimination from NHC metal halide complexes, although such complexes are ubiquitous in NHC chemistry. Herein, we report facile formation of 2-halo-imidazoliums from NHC Cu(I) halide complexes at room temperature (RT) under oxidative conditions as well as computational evidence to support a mechanism involving C_carbene–X reductive elimination from NHC Cu(III) halide complexes.  

As a part of our efforts in the characterization of reactivity of copper(III) complexes, we postulated that NHC Cu(III) complexes might be isolable because of the relatively inert Cu–C_carbene bond. In that regard, oxidations of IPrCu^ICl (1_Cl, IPr: 1, 3-bis(2, 6-diisopropylphenyl)imidazol-2-ylidene) by various oxidants were investigated. No significant reaction was observed between 1_Cl and [Ph₂I]⁺PF₆^- or [Cp₂Fe]⁺PF₆^- (0.64 V vs NHE) in acetonitrile (MeCN). By contrast, mixing 1_Cl with Selectfluor^¨ (³ 1.5 eq.) or Cu^II(CF₃SO₃)₂ (³ 2.0 eq., 1.30 V vs NHE) in MeCN at RT rapidly and quantitatively forms 2-chloro-imidazolium 2_Cl (Scheme 1). Quantitative formation of 2-halo-imidazolium 2_Br or 2_I was realized for IPrCu^IBr (1_Br) or IPrCu^II (1_I) under similar conditions.

These reactions appear to occur through either an inner- or outer-sphere oxidation followed by C_carbene-X reductive elimination. An outer-sphere oxidation is supported by the ca. 80 % formation of 2_Cl by reacting 1_Cl with [(1, 10-phenanthroline)₃Fe^III]³⁺ (³ 2 eq., 1.22 V vs NHE) in MeCN at RT. However, all these reactions are fast even at low temperatures (t_1/2 ~ seconds at ca. -40 °C), and no intermediate species could be detected by UV-Vis spectroscopy on a timescale of seconds.

DFT calculations provide mechanistic insights into the oxidation by Selectfluor^¨. The calculated free-energy profiles corresponding to C_carbene-X reductive elimination from either a Cu(II) or Cu(III) species using a simplified model system (3) are shown in Scheme 2. The oxidation of 3 by Selectfluor^¨ to form 5, a three-coordinate Cu(III) species, is thermodynamically favorable. The Cu(III) species is further stabilized by coordination of an MeCN ligand to form 7. The activation barrier for C_carbene–Cl reductive elimination from 7 via TS₁ to form 2-chloro-imidazolium 10 is remarkably low at 3.5 kcal mol^-1. The overall reaction is favorable by -54.2 kcal mol^-1. Other possible Cu(III) species are less stable than 7 and lead to higher activation barriers. Alternatively, 7 could react with another equivalent of 3 to form Cu(II) intermediates 8 and 9, the two most stable Cu(II) species. Although no transition state from 9 to 10 could be located (the energy monotonically increases as the C_carbene–Cl separation shortens.), the lower limit of its activation barrier can be estimated by the corresponding reaction free energy, 44.1 kcal mol^-1, the least endothermic among various Cu(II) species. Therefore, the calculations on the simplified model system favor a mechanism of C_carbene–Cl reductive elimination from NHC Cu(III) chloride complexes for the oxidation of 1_Cl by Selectfluor^¨.

The thermodynamically unfavorable C_carbene–Cl reductive elimination from 9 is consistent with isolable NHC Cu(II) chloride complexes reported in the literature. Based on these results, we prefer a mechanism with two sequential inner- or outer-sphere 1e^- oxidations followed by C_carbene–Cl reductive elimination from NHC Cu(III) chloride complexes for the reaction with Cu(CF₃SO₃)₂, although the detailed mechanism is still unclear. Furthermore, the quantitative formation of 2_Cl instead of IPrCu(II) chloride complexes from the reaction of 1_Cl and Selectfluor^¨ suggests that C_carbene–Cl reductive elimination from IPrCu(III) chloride complexes is much faster than reactions of 1_Cl to form IPrCu(II) halide complexes. This reactivity is consistent with the calculated 3.5 kcal mol^-1 activation barrier from 7 to 10. Such a low barrier is probably a consequence of the electrophilic nature of a Cu(III) center that renders the NHC C_carbene susceptible to nucleophilic attack, as suggested by the remarkably close C_carbene–Cl contacts (ca. 2.7) in 5 and 7 as well as the interactions between the chloride lone pair electrons and the C_carbene p_p orbital in relevant molecular orbitals. A similar interaction has been invoked to rationalize the short C_carbene–Cl contact (ca. 2.85) in NHC V^V(O)Cl₃. In contrast, no evidence of such an interaction exists for 8.

In summary, we have demonstrated that C_carbene–halogen reductive eliminations readily occur from NHC copper halides at RT under oxidative conditions. These reactions provide new examples for the well-known oxidation-induced reductive eliminations. DFT calculations on a simplified model system suggest that the involvement of NHC Cu(III) halides is essential for these reactions and the reductive eliminations might be facilitated by the interaction between C_carbene and the halogen lone pair. Given the ubiquity of NHC metal halide complexes, the facile C_carbene–X reductive elimination reported here warrants consideration as a potential decomposition pathway in reactions involving NHC-supported high-valent metal complexes, especially with late transition metals.