Abstract. 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

Mesoionic carbenes (MICs) are currently hugely popular as ligands, and triazolylidenes are arguably the most prominent classes of such MICs. Mesoionic carbenes with ferrocenyl substituents are presented that can act as metalloligands for the generation of heteromultimetallic iridium(I) and gold(I) complexes. The ferrocenyl substituents allow for reversible oxidation of these heteromultimetallic complexes, and these oxidation steps have a strong influence on the donor properties of the MICs. Tolman electronic parameters (TEP) determined from analysis of the iridium-carbonyl complexes show that the neutral ferrocenyl-MIC ligands are stronger donors than the imidazolylidene based carbenes, the one-electron oxidized ferrocenyl MICs are in the range of the tricyclohexyl phosphines and the two-electron oxidized forms, which are electron-poor, lie in the range of triphenyl phosphines. Taking advantage of the generation of these electron-poor MICs, we show their gold(I) complexes are potent catalysts for the synthesis of oxazolines, with complexes of the oxidized MIC ligands, without any additional additive, outperforming their neutral counterparts by almost a factor of ten. These results thus present the first examples of MIC ligands that are reversibly electronically tunable, and show the potential of the oxidized MIC ligands in types of catalysis where electron-poor ligands are necessary. The potential of MICs for molecular electroactive materials is also shown.

Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other N-heterocyclic carbene counterparts. We present here seven rhenium(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through H andC NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV-vis-near-IR/IR/electron paramagnetic resonance spectroelectrochemistry and density functional theory calculations are used to address questions related to the electronic structures of the complexes in various redox states, their redox stability, and the understanding of chemical reactivity following electron transfer in these systems. The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components. Additionally, these results point to the fact that MIC-containing ligands confer remarkable redox stability to their fac-Re(CO)-containing metal complexes. These findings will probably be useful for fields such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry, where fac-Re(CO) complexes of triazoles/triazolylidenes are likely to find use.

The complexes [(Cym)Ru(L)Cl]PF6, 2–4, and [Cp*Ir(L)Cl]PF6, 6–8 (Cym = p-cymene, Cp* = pentamethylcyclopentadienyl), with L = “click”-derived pyridyl-triazol, bis-triazole, or bis-abnormal carbene, were synthesized and spectroscopically characterized. Structural elucidation of the complexes shows a half-sandwich, piano-stool type of coordination around the metal centers and a delocalized situation within the triazolylidene rings. All the complexes were tested for their catalytic efficiency in the transfer hydrogenation of nitrobenzenes, and the results were compared with their 2,2′-bipyridine (bpy) Ru counterpart 1 and Ir counterpart 5. Remarkably, the nature of the final catalytic product is strongly dependent on the chosen metal center, with aniline being preferentially formed with the Ru complexes and azobenzenes with the Ir complexes. Judicious selection of catalyst and reaction conditions also facilitates the isolation of azoxybenzene. To the best of our knowledge, this is a rare example of a homogeneous catalytic synthesis of azobenzene from nitrobenzene. The influence of ligand substitution, metal substitution, and temperature variation on catalytic activity and selectivity has been investigated, whereby a systematic variation of the ligands from bpy, to pyridyl-triazole, to bis-triazole, to bis-abnormal carbene has been carried out. We also present a mechanistic investigation for this transformation with the aim of understanding reaction behavior.

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.