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 metallocenebased 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.

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.

Orthometalation at Ir(III) centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C-H activation sites, we present here mono- and dinuclear Ir(III) complexes with "click"-derived 1,2,3-triazole and 1,2,3-triazol-5-ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir(III) centers. Structural characterization of the complexes reveal a piano stool-type of coordination around the metal centers with the "click"-derived ligands bound either with C^N or C^C donor sets to the Ir(III) centers. Furthermore, whereas bond localization is observed within the 1,2,3-triazole ligands, a more delocalized situation is found in their 1,2,3-triazol-5-ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer-pure, dinuclear cyclometalated Ir(III) complexes with poly-mesoionic-carbene ligands.

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)2]2 (M = Rh(III), Ir(III)) to yield heterobimetallic complexes [3] (M = Rh) and [4] (M = Ir), in which the Pd(II) ion is chelated by two ortho N-heterocyclic carbene (NHC) donors while the third NHC donor coordinates to the M(III) center, which orthometalates the central phenyl ring.

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.