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.

A Co(III) complex with a mesoionic pyridylcarbene ligand is presented. This complex is an efficient electrocatalyst for H2 production at very low overpotential and high turnovers when using a (glassy carbon) GC electrode. The corresponding triazole complexes display no catalytic activity whatsoever under identical conditions. The remarkable robustness of the Co-C(carbene) bond towards acids is likely responsible for the high efficiency of this catalyst. The present results thus open new avenues for carbene-based ligands for generating functional models for hydrogenases.

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.

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.

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.