Smart N-Heterocyclic Carbene Ligands in Catalysis

Abstract

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

Double dative bond between divalent carbon(0) and uranium

“…Usually, carbon uses all of its four valence electrons to form stable tetravalent carbon(IV) species. Since the first stable singlet carbene at room temperature was isolated by Bertrand and coworkers 3 , divalent carbon(II) chemistry, such as N-heterocyclic carbene (NHC) 6–9 and cyclic (alkyl)(amino)carbenes 10–13 , has flourished. Carbenes, CR 2 , which contain one lone electron pair, can coordinate to numerous main group atoms and d -block or even f -block elements forming stable species via a single dative bond.…”

“…Thus, the fundamental difference between a carbene CR 2 and a carbone CL 2 is the number of electron lone-pairs that may serve as donors: carbenes are two-electron (single lone-pair) donors while carbones are four-electron (double lone-pair) donors. Although the first divalent carbon(0) species 9 , carbodiphosphorane (CDP), was reported already in 1961, the actual bonding situation was not fully understood until a theoretical study appeared in 2006 by Frenking and coworkers 27–29 . They showed that CDP can be considered as two phosphine ligands coordinated to a carbon atom in the excited 1 D state with two lone electron pairs, which remain available for both σ- and π-donation simultaneously 13 .…”

Planar‐Chiral Ferrocene‐Based N‐Heterocyclic Carbene Ligands

“…As an interesting application, ferrocene‐based NHCs can be used as redox‐active ligands by changing the oxidation state of the Fe center . In a study to examine the redox chemistry of Rh complex 63 , which contains an achiral ferrocenyl phosphine‐NHC ligand, Labande and co‐workers reported in 2013 that they accidentally obtained planar‐chiral Rh complex 64 , which was probably formed by an intramolecular C−H activation of a ferrocene C−H bond on Rh triggered by oxidation of the Fe center (Scheme ) .…”

Immobilized Pd on a NHC functionalized metal–organic framework MIL‐101(Cr): an efficient heterogeneous catalyst in Suzuki−Miyaura coupling reaction in water

“…These facts have led to the development of a wide range of catalytic applications with metal−NHC complexes over recent decades. Along this line, NHC complexes of palladium have been widely considered to be used in Pd‐catalyzed reactions such as carbon–carbon and carbon−heteroatom coupling reactions due to the high performance and efficiency of these catalyst systems . Because both NHC ligands and Pd metal are valuable, the recycling and reuse of Pd−NHC catalysts is challenging.…”