Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes. Isolation, Characterization, and Catalytic Alcohol Oxidation

Reactivity of rhodium and iridium peroxido complexes towards hydrogen in the presence of B(C6F5)3 or [H(OEt2)2][B{3,5-(CF3)2C6H3}4]

The peroxido complexes trans-[M(4-C5F4N)(O2)(CNtBu)(PR3)2] (1: M = Rh, R = Et; 2a: M = Ir, R = iPr) can be used in the metal-mediated hydrogenation of O2. The reaction of trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] (1) with B(C6F5)3 and H2 gave trans-[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3), OPEt3 and (H2O)·B(C6F5)3, whereas treatment of [H(OEt2)2][B{3,5-(CF3)2C6H3}4] with 1 in the presence of H2 yielded trans-[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3) and H2O2. The reactivity of 2a towards B(C6F5)3 and BClCy2 was also studied and an intermediate was detected which is assigned to be trans-[Ir(4-C5F4N)(Cl)(OOBCy2)(CNtBu)(PiPr3)2] (4a).

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.

Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N-H/N-C Bond Homolysis Due to the Formation of Stable Ligand Radicals

The reactivity of a boron complex with a redox-active formazanate ligand, LBPh₂ [L = PhNNC(p-tol)NNPh], was studied. Two-electron reduction of this main-group complex generates the stable, nucleophilic dianion [LBPh₂]^2–, which reacts with the electrophiles BnBr and H₂O to form products that derive from ligand benzylation and protonation, respectively. The resulting complexes are anionic boron analogues of leucoverdazyls. N–C and N–H bond homolysis of these compounds was studied by exchange NMR spectroscopy and kinetic experiments. The weak N–C and N–H bonds in these systems derive from the stability of the resulting borataverdazyl radical, in which the unpaired electron is delocalized over the four N atoms in the ligand backbone. We thus demonstrate the ability of this system to take up two electrons and an electrophile (E⁺ = Bn⁺, H⁺) in a process that takes place on the organic ligand. In addition, we show that the [2e^–/E⁺] stored on the ligand can be converted to E^• radicals, reactivity that has implications in energy storage applications such as hydrogen evolution.

A boron complex with a redox-active formazanate ligand in its two-electron-reduced state is shown to react with electrophiles (BnBr and H⁺). The resulting “borataleucoverdazyl” products have weak N−C and N−H bonds; homolytic cleavage reactions lead to stable ligand-based radicals. Thus, the accumulation of [2e⁻/E⁺] on the formazanate ligand and conversion to E^• radicals are demonstrated, and their potential relevance in energy-related electrocatalysis (e.g., proton reduction) is discussed.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl₂ (1a) along with the two hydrogenated complexes of 1a, ZnH₂LCl₂ (1b), and ZnH₄LCl₂ (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O₂ and para-benzoquinone to H₂O₂ and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.