8 research outputs found
Reaction of Benzopinacol with Non-ionic Bases: Reversing the Pinacol Coupling
The reaction of benzopinacol with the non-ionic bases butyllithium and phosphazene P<sub>4</sub> 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 Bisâ<i>p</i>âphenylenediamine 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 UVâvis spectra of
HMPD<sup>2+â˘â˘</sup> were found to be exceptionally temperature-sensitive,
following a change from the extended conformation (doubletâdoublet
state) predominant at room temperature to the Ď-stacked conformation
(singlet state) prevailing at dry-ice temperature. Changing the single
bridge from (CH<sub>2</sub>)<sub>3</sub> to dimethylated CH<sub>2</sub>CMe<sub>2</sub>CH<sub>2</sub> in OMPD<sup>2+â˘â˘</sup> causes considerably less of the Ď-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<sup>2+â˘â˘</sup> and OMPD<sup>2+â˘â˘</sup>, in which the positive charges
are localized separately in each PD<sup>+â˘</sup> ring, in the
extended conformation, exchange of the electron (âhole hoppingâ)
between the two PD units (fast at the time scale of EPR experiments)
was observed for HMPD<sup>+â˘</sup> and OMPD<sup>+â˘</sup>. 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 CuOâCeO<sub>2</sub> 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 CuOâCeO<sub>2</sub> 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 Ce<sup>4+</sup>/Ce<sup>3+</sup> during oxidation of Cu<sup>0</sup>/Cu<sup>+</sup> species
to Cu<sup>2+</sup> and (ii) a <i>direct mechanism</i> that
bypasses the redox pair Ce<sup>4+</sup>/Ce<sup>3+</sup>. In addition,
EPR experiments with isotopically enriched <sup>17</sup>O<sub>2</sub> 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 CuOâCeO<sub>2</sub> 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 H<sub>2</sub> 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]Âhelicene <i>o</i>-quinones (<i>P</i>)-(+)-<b>1</b> and (<i>M</i>)-(â)-<b>1</b> and their
application to chiroptical switching and chiral recognition. (<i>P</i>)-(+)-<b>1</b> and (<i>M</i>)-(â)-<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><sup>â˘â</sup> and (<i>M</i>)-(â)-<b>1</b><sup>â˘â</sup>, 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 [ÎÂ(ÎÎľ)]
in the UV and visible regions. The reduction of (Âą)-<b>1</b> by lithium metal provides [Li<sup>+</sup>{(Âą)-<b>1</b><sup>â˘â</sup>}], 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>(<sup>7</sup>Li) in [Li<sup>+</sup>{(Âą)-<b>1</b><sup>â˘â</sup>}] 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<sup>+</sup>{(<i>P</i>)-(+)-<b>1</b><sup>â˘â</sup>}] and [Li<sup>+</sup>{(<i>M</i>)-(â)-<b>1</b><sup>â˘â</sup>}]
with an enantiomerically pure phosphine oxide ligand
Donor-Substituted Octacyano[4]dendralenes: Investigation of ĎâElectron Delocalization in Their Radical Ions
Symmetrically and unsymmetrically electron-donor-substituted
octacyano[4]Âdendralenes
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 pushâpull chromophores
are potent electron acceptors, featuring potentials for first reversible
electron uptake around at â0.1 V (vs Fc<sup>+</sup>/Fc, in
CH<sub>2</sub>Cl<sub>2</sub> + 0.1 M <i>n</i>-Bu<sub>4</sub>NPF<sub>6</sub>) and, in one case, a remarkably small HOMOâLUMO
gap (Î<i>E</i> = 0.68 V). EPR measurements gave well-resolved
spectra after one-electron reduction of the octacyano[4]Âdendralenes,
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/UVâvis Spectroelectrochemistry and Quantum Chemical Calculations
The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydroÂ[1,2,5]ÂselenadiazoloÂ[3,4-<i>h</i>]Âquinolines and 8-substituted 9-oxo-6,9-dihydroÂ[1,2,5]ÂselenadiazoloÂ[3,4-<i>f</i>]Âquinolines with R<sub>7</sub>, R<sub>8</sub> = H, COOC<sub>2</sub>H<sub>5</sub>, COOCH<sub>3</sub>, COOH, COCH<sub>3</sub>,
and CN has been studied by in situ EPR and EPR/UVâvis 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 (Ď-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/UVâvis cyclovoltammetric
experiments. The site of the dimerization in the Ď-dimer and
the mapping of the unpaired spin density both for radical anions and
Ď-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 ĎâElectron Delocalization in Their Radical Ions
Symmetrically and unsymmetrically electron-donor-substituted
octacyano[4]Âdendralenes
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 pushâpull chromophores
are potent electron acceptors, featuring potentials for first reversible
electron uptake around at â0.1 V (vs Fc<sup>+</sup>/Fc, in
CH<sub>2</sub>Cl<sub>2</sub> + 0.1 M <i>n</i>-Bu<sub>4</sub>NPF<sub>6</sub>) and, in one case, a remarkably small HOMOâLUMO
gap (Î<i>E</i> = 0.68 V). EPR measurements gave well-resolved
spectra after one-electron reduction of the octacyano[4]Âdendralenes,
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] CycloadditionâRetroelectrocyclization 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 tetrakisÂ(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
pushâpull chromophores via formal [2 + 2] cycloadditionâretroelectrocyclization
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