Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular ...synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.
The field of multimetallic catalysis is rapidly developing and some multimetallic complexes catalyze organic transformations to yield the desired products in more efficient ways owing to the combined ...action of different metals in a cooperative fashion. This Concept article describes the recent advances of cooperative catalysis playing in multimetallic systems such as homo‐multimetallic complexes with short metal‐metal distances, homo‐multimetallic complexes with long metal‐metal distances, hetero‐multimetallic complexes and metallocene‐based multimetallic complexes with special attention towards redox‐switchable catalysis. Examples are illustrated in which the use of multimetallic complexes show clear enhancement of catalytic outcomes when compared with the sum of their corresponding mononuclear counterparts. Furthermore, several examples are discussed showing the effects of electronic communication in cooperative systems.
Teamwork: In this Concept article the authors discuss selected examples of cooperativity in multimetallic systems. Homobimetallic systems with short and long metal‐metal distances are considered, and the interplay of the different metals in several heterometallic complexes is reviewed. Cooperativity in metallocene‐based systems and their utilization in redox‐switchable catalysis is also discussed.
The unsymmetrical tris(imidazolium) salt H3-1(Br)3, featuring a 1,2,4-substitution pattern of the central phenyl ring, after triple imidazolium C2 deprotonation reacts in a one-pot reaction with ...Pd(OAc)2 and M(Cp*)(Cl)22 (M = RhIII, IrIII) to yield heterobimetallic complexes 3 (M = Rh) and 4 (M = Ir), in which the PdII ion is chelated by two ortho N-heterocyclic carbene (NHC) donors while the third NHC donor coordinates to the MIII center, which orthometalates the central phenyl ring.
The field of asymmetric catalysis is rapidly developing and the chiral ligands play a key role in enantioselective transition‐metal catalysis. The electron‐rich chiral N‐heterocyclic carbenes (NHCs) ...have established themselves as a popular class of stereodirecting ancillary ligands to catalyze enantioselective organic transformations in more efficient ways. Several novel transition‐metal complexes in combination with tailored ligand design have emerged during last few decades in asymmetric catalysis. The tailor‐made NHCs can easily be accessed due to the modular synthesis of their parent azolium salt precursors. Their donor capability and the molecular shape can easily be tuned by changing substituent at N‐atom or by changing the cyclic backbone framework. This review article aims to describe the recent advances in this rapidly evolving research area of enantioselective catalysis using well‐defined transition‐metal complexes possessing chiral NHC donor ligands.
The field of asymmetric catalysis is rapidly developing, and the electron‐rich chiral NHC ligands play a key role as stereodirecting ancillary ligands in enantioselective transition‐metal catalysis. Several well‐defined novel palladium(II), iridium(I)/(III), and rhodium(I)/(III) complexes possessing easily accessible tailor‐made chiral NHC ligands for asymmetric catalysis have been described.
The cyclometalated complexes with imidazolylidene ligands have been presented. A competitive nature of the cyclometalation towards the formation of five- and six-membered complexes has been ...discussed. All the complexes have been utilized as active pre-catalyst for the transfer hydrogenation reactions of aldehyde and ketone.
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•Synthesis of iridium(III) carbene complexes has been described.•Carbene ligand has additionally cyclometalated to iridium(III) center.•A competitive nature of the cyclometalation towards the formation of five- and six-membered complexes has been discussed.•All the complexes have been utilized as active pre-catalyst for the transfer hydrogenation reactions.
Cyclometalated N-heterocyclic carbene (NHC) complexes possess fascinating electronic and catalytic properties. A series of cyclometalated iridium(III) complexes have been synthesized from the corresponding imidazolium salts via the aryl sp2 CH bond activation at the N1-phenyl/N3-benzyl wingtips of the imidazolylidene moiety. The electronic tuning towards the competitive formation of cyclometalated complexes (sp2 phenyl CH activation vs sp2 benzyl CH activation) has been discussed. These complexes have been utilized for the transfer hydrogenation (TH) reaction of 4-bromobenzaldehyde and 4-bromoacetophenone. These complexes showed excellent yields of the aryl-alcohols from 4-bromobenzaldehyde or 4-bromoacetophenone.
A heterobimetallic IrIII/PdII complex from a dicarbene donor ligand featuring cyclometalated IrIII and PEPPSI type PdII is presented along with a homodinuclear PEPPSI type PdII complex starting from ...the same bis‐imidazolium salt. All the PdII complexes are active precatalyst in both α‐arylation and Suzuki–Miyaura coupling reactions. The heterobimetallic IrIII/PdII complex shows much higher yields in tandem C–C coupling/transfer hydrogenation reactions compared to the equimolar mixture of their mononuclear PdII and IrIII counterparts.
A heterobimetallic IrIII/PdII complex from a dicarbene donor ligand featuring cyclometalated IrIII and PEPPSI type PdII is presented along with a homodinuclear PEPPSI type PdII complex starting from the same bis‐imidazolium salt. The heterobimetallic IrIII/PdII complex appears as active precatalyst in tandem catalytic transformations of 4‐bromoabenzaldehyde to 4‐biphenylmethanol derivatives.
Synthesis of a ligand platform to generate di- and tri-mesoionic carbenes is reported together with their multinuclear Pd(II) complexes. Complete structural characterization and preliminary ...electrochemical data are presented.
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