Metal-NHC Dynamics in Organometallic Chemistry and Catalysis
Valentine P. Ananikov
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences
Leninsky Prospekt 47, Moscow, Russia; http://AnanikovLab.ru
N-heterocyclic carbenes (NHCs) are superior ligands for binding with transition metals and forming unique structures. High stability on air, stable metal-ligand framework and outstanding possibility to tune electronic and steric properties (by varying types of NHC rings and substituents) has led to a large variety of metal complexes with numerous applications in chemistry and catalysis. Applications of Metal/NHC (M/NHC) complexes in homogeneous catalysis are or particular importance, where a paramount progress was demonstrated in recent decades .
Detailed mechanistic studies have revealed a unique picture with dual opportunities in catalysis: i) homogeneous catalysis with molecular M/NHC complexes, and ii) nanoparticle catalysis with NHC-stabilized metal clusters. The interchange between two types of catalysis was first described by our group upon studying a well-known Mizoroki-Heck reaction . A new mode of catalysis, which takes an advantage of chemical lability of the M-NHC bond (rather than stability, as it was previously assumed), was revealed in the experiment . Facile R-NHC coupling was shown to be a general process for various metal complexes and organic groups R ,,,, thus opening excellent possibilities for in situ generation of stabilized metal clusters and nanoparticles from M/NHC precursors. A possibility of the reverse process (R-NHC oxidative addition) and NHC-mediated leaching of metal species from the surface of nanoparticles  give rise to construction of dynamic catalytic systems.
Figure 1. Universal range of catalytic systems accessible from M/NHC complexes (image from ref. , (C) The Royal Society of Chemistry, CC-BY-NC license).
Detailed analysis of the literature has shown that M-NHC dynamics indeed may be present in a number of systems for various metals and NHC ligands . Using a single M/NHC complex three types of catalytic systems may be generated: i) molecular catalysis, ii) nanoparticle catalysis, and iii) ionic catalysis (Figure 1) . It appears that many M/NHC-catalyzed reactions may involve dynamic and cocktail-type systems, with dynamic interconversion of active species during the reaction.
Interestingly, NHC ligands continue to play an important role in the stabilization of metal centers after breakage of the M-NHC bond. Stabilization of metal clusters and nanoparticles via azolium species or stabilization of molecular ionic complexes via counterion interactions were considered (Figure 1) .
Figure 2. Single M/NHC complex as a universal precursor for a number of tuned catalytic systems (image from ref. , (C) The Royal Society of Chemistry, CC-BY-NC license).
Intrinsic dynamic behavior recalls for more studies on the synthesis, structure, stability and reactivity of M/NHC complexes in organometallic chemistry. New wave of dynamic and cocktail-type catalysts design based on the M/NHC complexes is anticipated in the near future for development of a new generation of catalytic systems. Formulation of the concept of “NHC-connected” and “NHC-disconnected” modes of catalysis (Figure 2)  is of much importance to develop efficient and recyclable catalytic systems. Thus, understanding the role of NHC ligands and controlling the nature of catalytic centers becomes the topic of key importance.
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