N‐Heterocyclic Carbenes Carrying Weakly Coordinating Anions

Abstract

In this Concept article we provide a brief overview of the design and preparation of N-heterocyclic carbenes carrying weakly coordinating anions (WCA-NHCs). The anionic charge in these ligand systems is located on an exocyclic group, for example, B(C 6 F 5 ) 3 , tethered to the backbone of the imidazole ring, thus resembling a weakly coordinating moiety.With the general guiding principle behind the application of WCA-NHCs being the conversion of otherwise cationic NHC complexes into their overall neutral congeners, numerous transition metal as well as main group element complexes were isolated during the last decade, which are summarized herein.

Adaptive carbonyl umpolung involving a carbanionic carbene Breslow intermediate: an alternative mechanism for NHC-mediated organocatalysis

“…Therefore, the high thermodynamic stability of II-B can be attributed to its anionic carbene structure, which may be more stable than the cationic, zwitterionic, or mesoionic counterparts. 14,23,54…”

“…Despite the significantly lower acidity of the hydrogen atoms at C4/5–H (p K a > 33) 10 than that at C2–H (p K a = 18–25), 11–13 the nucleophilic reactivity of the “abnormal” C4/5 positions in NHC towards carbonyl substrates can be enhanced by relatively weak bases. 14 This “abnormal” reactivity of NHC has been widely employed for synthesizing C4/5-substituted neutral, anionic or mesoionic NHC-derived species and transition-metal complexes with a NHC or dc NHC ligands, 15–24 as well as in organocatalysis. 25–29 However, the underlying mechanism behind this unconventional reactivity of NHC-based species remains poorly understood.…”

Synthesis and a combined experimental/theoretical structural study of a comprehensive set of Pd/NHC complexes witho-,m-, andp-halogen-substituted aryl groups (X = F, Cl, Br, CF3)

“…Pd/NHC complexes (NHC – N -heterocyclic carbene) have proven to play important roles in various fields of chemistry, such as cross-coupling reactions, polymerization reactions, and other catalytic processes. 1–11 Their main advantages include a tunable electron-transfer ability to metal centers, as well as the ability to control the electronic and steric effects for each specific task by changing the substituents at nitrogen atoms. 12–17 There are a large number of studies devoted to the variation of steric bulk and electronic parameters of M/NHC complexes.…”

Synthesis of N-heterocyclic carbene gold(i) complexes from the marine betaine 1,3-dimethylimidazolium-4-carboxylate

“…9 The latter approach corresponds in principle to the construction of anionic N-heterocyclic carbenes with a fluoroborate unit in the 4-position, which have found widespread application in transition metal and main group element chemistry as well in homogeneous catalysis. 10…”