Pincer‐Type Complexes for Catalytic (De)Hydrogenation and Transfer (De)Hydrogenation Reactions: Recent Progress

Pincer complexes are becoming increasingly important for organometallic chemistry and organic synthesis. Since numerous applications for such catalysts have been developed in recent decades, this Minireview covers progress in their use as catalysts for (de)hydrogenation and transfer (de)hydrogenation reactions during the last four years. Aside from noble‐metal‐based pincer complexes, the corresponding base metal complexes are also highlighted and their applications summarized.     

First Examples of de Vries‐like Smectic A to Smectic C Phase Transitions in Ionic Liquid Crystals

In ionic liquid crystals, the orthogonal smectic A phase is the most common phase whereas the tilted smectic C phase is rather rare. We present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core whereas the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature‐dependent layer spacing as well as their translational and orientational order parameters were studied by X‐ray diffraction. The X‐ray tilt angles derived from X‐ray studies of the layer contraction and the optically measured tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries‐like behavior with a layer shrinkage that is far less than that expected for conventional materials. These materials can thus be considered as the first de Vries‐type materials among ionic liquid crystals.     

Controlling Spin-Correlated Radical Pairs with Donor–Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

One-electron reduced metal complexes derived from photoactive ruthenium or iridium complexes are important intermediates for substrate activation steps in photoredox catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic efficiency problems resulting from rapid geminate recombination of radical pairs within the so-called solvent cage. In this study, we prepared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relying on a sequence of energy and electron transfer processes between a Ru complex and a covalently connected anthracene moiety. Our test reaction to establish the proof-of-concept is the photochemical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemical key step in the Ru-anthracene dyads is the reduction of a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spin-correlated radical pair whose (unproductive) recombination is strongly spin-forbidden. By carrying out detailed laser flash photolysis investigations, we provide clear evidence for the indirect reduced metal complex generation mechanism and show that this pathway can outperform the conventional direct metal complex photoreduction. The further optimization of our approach involving relatively simple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields.