Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry: Zn–H exchange reactions and structural authentication of a dinuclear Au(i) complex with a NHC anion

Abstract

Merging two evolving areas in synthesis, namely cooperative bimetallics and N-heterocyclic carbenes (NHCs), this study reports the isolation of the first intermediates of alkali-metal-mediated zincation (AMMZn) of a free NHC and a Zn–NHC complex using sodium zincate [(TMEDA)NaZn(TMP)(tBu)₂] (1) as a metallating reagent. The structural authentication of (THF)₃Na[:C{[N(2,6-iPr₂C₆H₃)]₂CHCZn(tBu₂)}] (2) and [Na(THF)₆]⁺[tBu₂Zn:C{[N(2,6-iPr₂C₆H₃)]₂CHCZn(tBu₂)}]⁻ (4), resulting from the reactions of 1 with unsaturated free NHC IPr (IPr = 1,3-bis(2,6-di-isopropylphenylimidazole-2-ylidene) and NHC complex ZntBu₂IPr (3) respectively demonstrates that in both cases, this mixed-metal approach can easily facilitate the selective C4 zincation of the unsaturated backbone of the NHC ligand. Furthermore, the generation of anionic NHC fragments enables dual coordination through their normal (C2) and abnormal (C4) positions to the bimetallic system, stabilising the kinetic AMMZn intermediates which normally go undetected and provides new mechanistic insights in to how these mixed-metal reagents operate. In stark contrast to this bimetallic approach when NHC-complex 3 is reacted with a more conventional single-metal base such as tBuLi, the deprotonation of the coordinated carbene is inhibited, favouring instead, co-complexation to give NHC-stabilised [IPr·LiZntBu₃] (5). Showing the potential of 2 to act as a transfer agent of its anionic NHC unit to transition metal complexes, this intermediate reacts with two molar equivalents of [ClAu(PPh₃)] to afford the novel digold species [ClAu:C{[N(2,6-iPr₂C₆H₃)]₂CHCAu(PPh₃)}] (6) resulting from an unprecedented double transmetallation reaction which involves the simultaneous exchange of both cationic (Na⁺) and neutral (ZntBu₂) entities on the NHC framework.