Multiscale Photo-Based In-Situ and Operando Spectroscopies in Time and Energy Landscapes

Following catalytic reactions, in-situ and operando are now the focus of a number of dedicated experiments at light sources which have been developed to track the electronic and molecular structural dynamics of catalysts. The challenges for this goal are two-fold: first, the development of spectroscopic tools in the energy domain and time domain is required. The photocatalytic processes have early dynamics of tens of femtoseconds, while further reaction takes seconds, minutes, and even hours. Second, a combination of tools to probe processes not only in solids, but also in solutions and at interfaces, is now needed. In this special issue, we present recent developments at the synchrotron facility BESSY II using photon energy from the infrared and extreme ultraviolet up to the soft X-ray regime for in-situ and operando applications addressing these two major challenges. As this work is a result of contributions from several groups, each section will present the group's activities and related team members involved.     

Amphiphilic ESIPT benzoxazole derivatives as prospective fluorescent membrane probes

Fluorescent amphiphilic benzoxazole derivatives were synthesized and used to produce photoactive phosphatidylcholine (PC) liposomes by reserve-phase evaporation. The dyes absorbed in the UV region and were fluorescent in the blue-green region (determined by solvent polarity). The alkyl chain length seemed to play a fundamental role in the photophysics of the benzoxazole fluorophore in reverse liposomes, and despite the same ESIPT core and phospholipid building block, each amphiphilic dye had a particular emission profile related to the dye location in the liposome. The fluorescence emission spectra from dye 5 showed that its fluorophore experienced a polar environment, due to the single normal emission, while dyes 6–7 had (in part) a normal emission, and the main fluorescent band ascribed to the ESIPT emission indicated a more hydrophobic environment. Despite the complex fluorescent profiles, the benzoxazole derivatives could be successfully introduced into the reverse liposome structure due to the interaction between the alkyl chain and PC bilayer.     

Dinuclear Palladium Complexes with Two Ligand‐Centered Radicals and a Single Bridging Ligand: Subtle Tuning of Magnetic Properties

The facile and tunable preparation of unique dinuclear [(L^?)Pd?X?Pd(L^?)] complexes (X=Cl or N₃), bearing a ligand radical on each Pd, is disclosed, as well as their magnetochemistry in solution and solid state is reported. Chloride abstraction from [PdCl( NNO^ISQ )] ( NNO^ISQ =iminosemiquinonato) with TlPF₆ results in an unusual monochlorido‐bridged dinuclear open‐shell diradical species, [{Pd( NNO ^ISQ)}₂(μ‐Cl)]⁺, with an unusually small Pd‐Cl‐Pd angle (ca. 93°, determined by X‐ray). This suggests an intramolecular d⁸–d⁸ interaction, which is supported by DFT calculations. SQUID measurements indicate moderate antiferromagnetic spin exchange between the two ligand radicals and an overall singlet ground state in the solid state. VT EPR spectroscopy shows a transient signal corresponding to a triplet state between 20 and 60 K. Complex 2 reacts with PPh₃ to generate [Pd(NNO^ISQ)(PPh₃)]⁺ and one equivalent of [PdCl( NNO^ISQ )]. Reacting an 1:1 mixture of [PdCl( NNO^ISQ )] and [Pd(N₃)( NNO^{I SQ})] furnishes the 1,1‐azido‐bridged dinuclear diradical [{Pd( NNO ^ISQ)}₂(κ¹‐N;μ‐N₃]⁺, with a Pd‐N‐Pd angle close to 127° (X‐ray). Magnetic and EPR measurements indicate two independent S=1/2 spin carriers and no magnetic interaction in the solid state. The two diradical species both show no spin exchange in solution, likely because of unhindered rotation around the Pd?X?Pd core. This work demonstrates that a single bridging atom can induce subtle and tunable changes in structural and magnetic properties of novel dinuclear Pd complexes featuring two ligand‐based radicals.     

Ruthenium(II)‐Catalyzed CH Activation/Alkyne Annulation by Weak Coordination with O2 as the Sole Oxidant

Aerobic oxidative C? H functionalizations of weakly coordinating benzoic acids have been accomplished with versatile ruthenium(II) biscarboxylates under ambient oxygen or air. Mechanistic studies identified the key factors controlling the elementary step of the oxidation of the ruthenium(0) complex.     

Cluster-Cyclic Quivers with Three Vertices and the Markov Equation

Acyclic cluster algebras have an interpretation in terms of tilting objects in a Calabi-Yau category defined by some hereditary algebra. For a given quiver Q it is thus desirable to decide if the cluster algebra defined by Q is acyclic. We call Q cluster-acyclic in this case, otherwise cluster-cyclic. In this note we classify the cluster-cyclic quivers with three vertices using a Diophantine equation studied by Markov.     

Ruthenium(II)‐Catalyzed CH Acyloxylation of Phenols with Removable Auxiliary

Intermolecular C?H acyloxylations of phenols with removable directing groups were accomplished with a versatile ruthenium catalyst. Specifically, a cationic ruthenium(II) complex, formed in situ, enabled the chemoselective C?H oxygenations of a broad range of substrates. The catalyst proved tolerant of synthetically valuable functional groups, and the substrate scope included both (hetero)aromatic and, the more challenging, aliphatic carboxylic acids. The proposed reaction mechanism involves a reversible C?H ruthenation and an oxidatively induced C?O‐bond‐forming reductive elimination.