Reaction of Benzopinacol with Non-ionic Bases: Reversing the Pinacol Coupling

The reaction of benzopinacol with the non-ionic bases butyllithium and phosphazene P₄ leads to the formation of the corresponding ketyl radical anions, which have been characterized by EPR/ENDOR spectroscopy. This conversion has a high efficiency. Such a reversed pinacol reaction can be used for a controlled release of ketyl radicals. Moreover, the nature of the base has a marked effect on the association of the ketyl radical anion and the counterions. This illustrates the importance of ion pairing for reductive coupling

Monotrimethylene-Bridged Bis‑<i>p</i>‑phenylenediamine Radical Cations and Dications: Spin States, Conformations, and Dynamics

The properties of <i>p</i>-phenylenediamine- (PD-) based systems substantially depend on the molecular topology. The singly bridged PD analogues HMPD and OMPD in which the PD rings are connected by a flexible linker reveal particular electronic properties in their radical cations and dications. The EPR and UV–vis spectra of HMPD^2+•• were found to be exceptionally temperature-sensitive, following a change from the extended conformation (doublet–doublet state) predominant at room temperature to the π-stacked conformation (singlet state) prevailing at dry-ice temperature. Changing the single bridge from (CH₂)₃ to dimethylated CH₂CMe₂CH₂ in OMPD^2+•• causes considerably less of the π-stacked conformation to be present at low temperature as a result of the steric interactions with the methyl groups of the bridge. In contrast to HMPD^2+•• and OMPD^2+••, in which the positive charges are localized separately in each PD^+• ring, in the extended conformation, exchange of the electron (“hole hopping”) between the two PD units (fast at the time scale of EPR experiments) was observed for HMPD^+• and OMPD^+•. This process slows in a reversible manner with decreasing temperature, thus forming the radical cation with the unpaired electron spin density predominantly on one PD core, at low temperatures. Accordingly, a subtle balance between conformational changes, electron delocalization, and spin states could be established

In Situ EPR Study of the Redox Properties of CuO–CeO₂ Catalysts for Preferential CO Oxidation (PROX)

Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO–CeO₂ catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a <i>synergetic mechanism</i> that involves the redox pair Ce⁴⁺/Ce³⁺ during oxidation of Cu⁰/Cu⁺ species to Cu²⁺ and (ii) a <i>direct mechanism</i> that bypasses the redox pair Ce⁴⁺/Ce³⁺. In addition, EPR experiments with isotopically enriched ¹⁷O₂ established the <i>synergetic mechanism</i> as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. With the guidance of these results, an optimized CuO–CeO₂ catalyst could be designed. A rather wide temperature operation window of 11 K (from 377 to 388 K), with 99% conversion efficiency and 99% selectivity, was achieved for the preferential oxidation of CO in a H₂ feed

Helicene Quinones: Redox-Triggered Chiroptical Switching and Chiral Recognition of the Semiquinone Radical Anion Lithium Salt by Electron Nuclear Double Resonance Spectroscopy

We present the synthesis and characterization of enantiomerically pure [6]­helicene <i>o</i>-quinones (<i>P</i>)-(+)-<b>1</b> and (<i>M</i>)-(−)-<b>1</b> and their application to chiroptical switching and chiral recognition. (<i>P</i>)-(+)-<b>1</b> and (<i>M</i>)-(−)-<b>1</b> each show a reversible one-electron reduction process in their cyclic voltammogram, which leads to the formation of the semiquinone radical anions (<i>P</i>)-(+)-<b>1</b>^•– and (<i>M</i>)-(−)-<b>1</b>^•–, respectively. Spectroelectrochemical ECD measurements give evidence of the reversible switching between the two redox states, which is associated with large differences of the Cotton effects [Δ­(Δε)] in the UV and visible regions. The reduction of (±)-<b>1</b> by lithium metal provides [Li⁺{(±)-<b>1</b>^•–}], which was studied by EPR and ENDOR spectroscopy to reveal substantial delocalization of the spin density over the helicene backbone. DFT calculations demonstrate that the lithium hyperfine coupling <i>A</i>(⁷Li) in [Li⁺{(±)-<b>1</b>^•–}] is very sensitive to the position of the lithium cation. On the basis of this observation, chiral recognition by ENDOR spectroscopy was achieved by complexation of [Li⁺{(<i>P</i>)-(+)-<b>1</b>^•–}] and [Li⁺{(<i>M</i>)-(−)-<b>1</b>^•–}] with an enantiomerically pure phosphine oxide ligand

Donor-Substituted Octacyano[4]dendralenes: Investigation of π‑Electron Delocalization in Their Radical Ions

Symmetrically and unsymmetrically electron-donor-substituted octacyano[4]­dendralenes were synthesized and their opto-electronic properties investigated by UV/vis spectroscopy, electrochemical measurements (cyclic voltammetry (CV) and rotating disk voltammetry (RDV)), and electron paramagnetic resonance (EPR) spectroscopy. These nonplanar push–pull chromophores are potent electron acceptors, featuring potentials for first reversible electron uptake around at −0.1 V (vs Fc⁺/Fc, in CH₂Cl₂ + 0.1 M <i>n</i>-Bu₄NPF₆) and, in one case, a remarkably small HOMO–LUMO gap (Δ<i>E</i> = 0.68 V). EPR measurements gave well-resolved spectra after one-electron reduction of the octacyano[4]­dendralenes, whereas the one-electron oxidized species could not be detected in all cases. Investigations of the radical anions of related donor-substituted 1,1,4,4-tetracyanobuta-1,3-diene derivatives revealed electron localization at one 1,1-dicyanovinyl (DCV) moiety, in contrast to predictions by density functional theory (DFT) calculations. The particular factors leading to the charge distribution in the electron-accepting domains of the tetracyano and octacyano chromophores are discussed

Stable Radical Trianions from Reversibly Formed Sigma-Dimers of Selenadiazoloquinolones Studied by In Situ EPR/UV–vis Spectroelectrochemistry and Quantum Chemical Calculations

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro­[1,2,5]­selenadiazolo­[3,4-<i>h</i>]­quinolines and 8-substituted 9-oxo-6,9-dihydro­[1,2,5]­selenadiazolo­[3,4-<i>f</i>]­quinolines with R₇, R₈ = H, COOC₂H₅, COOCH₃, COOH, COCH₃, and CN has been studied by in situ EPR and EPR/UV–vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from <i>N</i>-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV–vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations

Donor-Substituted Octacyano[4]dendralenes: Investigation of π‑Electron Delocalization in Their Radical Ions

Symmetrically and unsymmetrically electron-donor-substituted octacyano[4]­dendralenes were synthesized and their opto-electronic properties investigated by UV/vis spectroscopy, electrochemical measurements (cyclic voltammetry (CV) and rotating disk voltammetry (RDV)), and electron paramagnetic resonance (EPR) spectroscopy. These nonplanar push–pull chromophores are potent electron acceptors, featuring potentials for first reversible electron uptake around at −0.1 V (vs Fc⁺/Fc, in CH₂Cl₂ + 0.1 M <i>n</i>-Bu₄NPF₆) and, in one case, a remarkably small HOMO–LUMO gap (Δ<i>E</i> = 0.68 V). EPR measurements gave well-resolved spectra after one-electron reduction of the octacyano[4]­dendralenes, whereas the one-electron oxidized species could not be detected in all cases. Investigations of the radical anions of related donor-substituted 1,1,4,4-tetracyanobuta-1,3-diene derivatives revealed electron localization at one 1,1-dicyanovinyl (DCV) moiety, in contrast to predictions by density functional theory (DFT) calculations. The particular factors leading to the charge distribution in the electron-accepting domains of the tetracyano and octacyano chromophores are discussed

6,6-Dicyanopentafulvenes: Electronic Structure and Regioselectivity in [2 + 2] Cycloaddition–Retroelectrocyclization Reactions

We present an investigation of the electronic properties and reactivity behavior of electron-accepting 6,6-dicyanopentafulvenes (DCFs). The electron paramagnetic resonance (EPR) spectra of the radical anion of a tetrakis­(silylalkynyl) DCF, generated by Na metal reduction, show delocalization of both the charge and unpaired electron to the nitrogens of the cyano moieties and also, notably, to the silicon atoms of the four alkynyl moieties. By contrast, in the radical anion of the previously reported tetraphenyl DCF, coupling to the four phenyl rings is strongly attenuated. The data provide physical evidence for the different conjugation between the DCF core and the substituents in both systems. We also report the preparation of new fulvene-based push–pull chromophores via formal [2 + 2] cycloaddition–retroelectrocyclization reaction of DCFs with electron-rich alkynes. Alkynylated and phenylated DCFs show opposite regioselectivity of the cycloaddition, which can be explained by the differences in electronic communication between substituents and the DCF core as revealed in the EPR spectra of the radical anions