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 new iron age: The stabilization of an iron(III) complex with mesoionic carbenes has been shown to lead to record‐breaking excited‐state lifetimes and an unprecedented spin‐allowed radiative decay ...from a 2LMCT state. These results set the stage for a sustainable future in the new “iron age”.
Even though the existence of 1,2,3‐triazoles has been known for more than a century, the recent discovery of a copper(I) catalyzed version of this reaction has attributed unprecedented importance to ...these compounds. Coordination and organometallic chemists have benefited from this modular synthetic route, and have accessed ligands based on both the triazoles as well as the triazolylidenes. The wide variation of steric and electronic properties that can be achieved for this ligand class has made them useful for generating metal complexes with various applications. Examples include, among others: magnetically switchable molecules, electro‐ and photo‐active molecules, molecules for light‐emitting electrochemical cells, dyes for dye‐sensitized solar cells and a host of homogeneous catalytic processes. In this contribution, we present recent developments in this field with examples of some selected ligands. The focus is on systems developed in our group over the past few years.
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.
Single-molecule magnets display magnetic bistability of molecular origin, which may one day be exploited in magnetic data storage devices. Recently it was realised that increasing the magnetic moment ...of polynuclear molecules does not automatically lead to a substantial increase in magnetic bistability. Attention has thus increasingly focussed on ions with large magnetic anisotropies, especially lanthanides. In spite of large effective energy barriers towards relaxation of the magnetic moment, this has so far not led to a big increase in magnetic bistability. Here we present a comprehensive study of a mononuclear, tetrahedrally coordinated cobalt(II) single-molecule magnet, which has a very high effective energy barrier and displays pronounced magnetic bistability. The combined experimental-theoretical approach enables an in-depth understanding of the origin of these favourable properties, which are shown to arise from a strong ligand field in combination with axial distortion. Our findings allow formulation of clear design principles for improved materials.
Catalysis with gold(I) complexes is a useful route for synthesizing a variety of important heterocycles. Often, silver(I) additives are necessary to increase the Lewis acidity at the gold(I) ...center and to make them catalytically active. We present here a concept in redox-switchable gold(I) catalysis that is based on the use of redox-active mesoionic carbenes, and of electron transfer steps for increasing the Lewis acidity at the gold(I) center. A gold(I) complex with a mesoionic carbene containing a ferrocenyl backbone is presented. Investigations on the corresponding iridium(I)–CO complex show that the donor properties of such carbenes can be tuned via electron transfer steps to make these seemingly electron rich mesoionic carbenes relatively electron poor. A combined crystallographic, electrochemical, UV–vis–near-IR/IR spectroelectrochemical investigation together with DFT calculations is used to decipher the geometric and the electronic structures of these complexes in their various redox states. The gold(I) mesoionic carbene complexes can be used as redox-switchable catalysts, and we have used this concept for the synthesis of important heterocycles: oxazoline, furan and phenol. Our approach shows that a simple electron transfer step, without the need of any silver additives, can be used as a trigger in gold catalysis. This report is thus the first instance where redox-switchable (as opposed to only redox-induced) catalysis has been observed with gold(I) complexes.
The metal–metal interaction in ligand-bridged diruthenium(II,III) complex intermediates is commonly characterized by using the redox potential difference, usually converted to the comproportionation ...constant
K
c, and by analyzing intervalence charge transfer (IVCT) absorption features and deriving electronic coupling parameters. Although these two kinds of information may yield very different estimates of the extent of metal–metal interaction in any given system, the absolute values have often been used interchangeably to quantify the strength of electronic “coupling”. Using examples from the literature with substantially different coordination arrangements in terms of denticity and donor/acceptor character of the π conjugated bridging ligand we can show that close correlations between electrochemically and spectroscopically determined coupling are only valid within structurally related classes of compounds with qualitatively similar interaction at the metal–ligand interface. Most strikingly, situations with high
K
c values but weak, low energy IVCT absorptions have been identified for complexes with bis- and tris-bidentate acceptor bridges whereas intermediates with small comproportionation constants but intense, narrow IVCT bands are typical with bridging bis-tridentate acceptors.
Metal complexes of mesoionic carbenes (MICs) of the triazolylidene type and their derivatives have gained increasing attention in the fields of electrocatalysis and photochemistry. The redox activity ...of these metal complexes is critical for their applications in both the aforementioned fields. Easy accessibility and modular synthesis open a wide field for the design of ligands, such as bidentate ligands. The combination of an MIC with a pyridyl unit in a bidentate ligand setup increases the π acceptor properties of the ligands while retaining their strong σ donor properties. The analogy with the well-established 2,2′-bipyridine ligand allows conclusions to be drawn about the influence of the mesoionic carbene (MIC) moiety in tetracarbonyl group 6 complexes in cyclic voltammetry and (spectro)electrochemistry (SEC). However, the effects of the different connectivity in pyridyl-MIC ligands remain underexplored. Based on our previous studies, we present a thorough investigation of the influence of the two different pyridyl-MIC constitutional isomers on the electrochemical and the UV-vis-NIR/IR/EPR spectroelectrochemical properties of group 6 carbonyl complexes. Moreover, the presented complexes were investigated for the electrochemical conversion of CO2 using two different working electrodes, providing a fundamental understanding of the influence of the electrode material in the precatalytic activation.
A convenient access to a triad of triazoles with ferrocenyl and cobaltoceniumyl substituents is reported. N‐Alkylation, deprotonation and metalation with CuI/AgI/AuI synthons affords the heteroleptic ...triazolylidene complexes. Due to the combination of neutral, electron‐donating ferrocenyl substituents and cationic, strongly electron‐withdrawing cobaltocenium substituents, the mesoionic carbene (MIC) ligands of these complexes are electronically interesting “push–pull”, “pull–push” and “pull–pull” metalloligands with further switchable redox states based on their fully reversible FeII/FeIII, (ferrocene/ferrocenium) and CoIII/CoII, (cobaltocenium/cobaltocene) redox couples. These are the first examples of metal complexes of (di)cationic NHC ligands based on cobaltoceniumyl substituents. DFT calculated Tolman electronic parameter (TEP) of the new MIC ligands, show these metalloligands to be extremely electron‐poor NHCs with properties unmatched in other carbene chemistry. Utilization of these multimetallic electronically tunable compounds in catalytic oxazoline synthesis and in antitumor studies are presented. Remarkably, 1 mol % of the AuI complex with the dicationic MIC ligand displays full catalytic conversion, without the need for any other additives, in less than 2 hours at ambient temperatures. These results thus firmly establish these new classes of cobaltoceniumyl based (di)cationic MIC ligands as prominent players in several branches of chemistry.
Cationic mesoionic carbene ligands with cobaltoceniumyl substituents are presented. These are the first examples of NHC ligands with such substituents. The cationic MIC ligands are extremely electron‐poor displaying unprecedented high TEP values for NHC ligands. This fact was used in catalysis with their AuI complex that functions without the need of any additive.
A gold(III) complex with the hitherto most electron poor mesoionic carbene ligand is presented. Aqua regia was the oxidizing agent of choice for the synthesis of this unusual organometallic ...compound. The AuIII complex is redox-rich and also acts as a catalyst for oxazole formation, delivering selectively a completely different isomer in comparison to its AuI congener.
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