Electrochemistry of Ruthenium Bis(imino)pyridine Compounds: Evidence for an ECE Mechanism and Isolation of Mono and Dicationic Complexes

Ruthenium-Catalyzed Dehydrogenation Through an Intermolecular Hydrogen Atom Transfer Mechanism

The direct dehydrogenation of alkanes is among the most efficient ways to access valuable alkene products. Although several catalysts have been designed to promote this transformation, they have unfortunately found limited applications in fine chemical synthesis. Here, we report a conceptually novel strategy for the catalytic, intermolecular dehydrogenation of alkanes using a ruthenium catalyst. The combination of a redox-active ligand and a sterically hindered aryl radical intermediate has unleashed this novel strategy. Importantly, mechanistic investigations have been performed to provide a conceptual framework for the further development of this new catalytic dehydrogenation system.

N,N'-Ethylene-Bridged Bis-2-Aryl-Pyrrolinium Cations to E-Diaminoalkenes: Non-Identical Stepwise Reversible Double-Redox Coupled Bond Activation Reactions

This work presents a stepwise reversible two‐electron transfer induced hydrogen shift leading to the conversion of a bis‐pyrrolinium cation to an E‐diaminoalkene and vice versa. Remarkably, the forward and the reverse reaction, which are both reversible, follow two completely different reaction pathways. Establishing such unprecedented property in this type of processes was possible by developing a novel synthetic route towards the starting dication. All intermediates involved in both the forward and the backward reactions were comprehensively characterized by a combination of spectroscopic, crystallographic, electrochemical, spectroelectrochemical, and theoretical methods. The presented synthetic route opens up new possibilities for the generation of multi‐pyrrolinium cation scaffold‐based organic redox systems, which constitute decidedly sought‐after molecules in contemporary chemistry.

Crystal structure of a new 2,6-bis-(imino)-pyridine derivative: (1E,1'E)-1,1'-(pyridine-2,6-di-yl)bis-[N-(4-chloro-phen-yl)ethan-1-imine]

The asymmetric unit of the title compound, C21H17Cl2N3, contains two crystallographically independent mol-ecules (A and B). Both mol-ecules have E configurations for both imine double bonds with regard to the aryl and pyridine groups. The conformations of the two mol-ecules differ with the 4-chloro-phenyl rings being inclined to the central pyridine ring by 77.64 (6) and 86.18 (6)° in mol-ecule A, and 80.02 (5) and 43.41 (6)° in mol-ecule B. In the crystal, mol-ecules are linked by a number of C-H⋯π inter-actions, forming layers parallel to the bc plane.

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer

In this work, the effect of the electronically different ancillary ligands on the overall properties of the Ru^IIL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic complexes of general formula [RuLQCl]^0/+ was investigated. Four different ancillary ligands (Q) with different electronic effects were used to prepare the heteroleptic compounds from the precursor complex, [RuL(CH₃CN)Cl₂] (1); Q = pcp: 2-(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]⁺; bpy: 2,2'-bipyridyl (moderate π-acceptor), [3]⁺; acac^-: acetylacetonate (strong σ-donor), 4; and DTBCat^2-: 3,5-di- tert-butyl catecholate (strong π-donor), 5. The complexes [2]⁺, [3]⁺, 4, and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic paramagnetic resonance, UV-vis-NIR spectroscopy, and density functional theory. The studies revealed significant effects of the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic effect of the coligand. Accordingly, the complex [2]⁺ (containing a strong π-acceptor coligand, pcp) brings about oxidation efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating coligand) furnished only 15-20% of benzaldehyde under identical reaction conditions. Investigations of the reaction mechanism suggest that an unstable Ru-H species is formed, which is transformed to a Ru-hydrazo intermediate by H-walking as reported by Hall et al. ( J. Am. Chem. Soc., 2015, 137, 12330). Aerial O₂ regenerates the catalyst via oxidation of the hydrazo intermediate.