Pyrylenes: A New Class of Tunable, Redox-Switchable, Photoexcitable Pyrylium-Carbene Hybrids with Three Stable Redox-States

Article abstract of DOI:10.1021/jacs.8b08545

A new synthetic and modular access to a large family of redox-switchable molecules based upon the combination of pyrylium salts and carbenes is presented. The redox-properties of this new molecule class correlate very well with the π-accepting properties of the corresponding carbenes. While the pyrylium moiety acts as a chromophore, the carbene moiety can tune the redox-properties and stabilize the corresponding radicals. This leads to the isolation of the first monomeric pyranyl-radical in the solid-state. The three stable oxidation states could be cleanly accessed by chemical oxidation, characterized by NMR, EPR, UV-vis, and X-ray diffraction and supported by (TD)-DFT-calculations. The new hybrid class can be utilized as an electrochemically triggered switch and as a powerful photoexcited reductant. Importantly, the pyrylenes can be used as novel photocatalysts for the reductive activation of aryl halides and sulfonamides by consecutive visible light induced electron transfer processes.

Full text of DOI:10.1021/jacs.8b08545

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Article
Pyrylenes: A New Class of Tuneable, Redox-Switchable, Photo-
Excitable Pyrylium-Carbene Hybrids with Three Stable Redox-States
Patrick W Antoni, and Max M Hansmann
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08545 • Publication Date (Web): 10 Oct 2018
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Pyrylenes: A New Class of Tuneable, Redox-Switchable, Photo-
Excitable Pyrylium-Carbene Hybrids with Three Stable Redox-States
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Patrick W. Antoni and Max M. Hansmann*
Georg-August Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany.
ABSTRACT: A new synthetic and modular access to a large family of redox-switchable molecules based upon the combination of
pyrylium salts and carbenes is presented. The redox-properties of this new molecule class correlate very well with the π-accepting
properties of the corresponding carbenes. While the pyrylium moiety acts as a chromophore, the carbene moiety can tune the redox-
properties and stabilize the corresponding radicals. This leads to the isolation of the first monomeric pyranyl-radical in the solid-
state. The three stable oxidation states could be cleanly accessed by chemical oxidation, characterized by NMR, EPR, UV-VIS, X-
ray diffraction and supported by (TD)-DFT-calculations. The new hybrid class can be utilized as electrochemically triggered switch
and as powerful photo-excited reductant. Importantly, the pyrylenes can be used as novel photo-catalysts for the reductive activa-
tion of aryl halides and sulfonamides by consecutive visible light induced electron transfer processes.
or 4,4’pyrylogenes (IV).20,21 As a result of the dicationic nature of
IV, these sensitizers generate upon PET mutually repulsive radi-
cal-cation/radical-cation pairs leading to increased rate of diffu-
sive separation and therefore disfavoring BET processes. Note,
that the mono and/or bisreduced form of 4,2’- or 4,4’pyrylogenes
(IV) could not be isolated,22 and that IV is synthesized in several
steps by an oxidative cyclization of a 1,5-dione as the key step.
Furthermore, bispyrylium salts such as V have found various
applications such as organic field-effect transistors (OFETs)23 and
in organic solar cells.24 In the following we demonstrate that
carbene/pyrylium hybrids can be straightforwardly accessed in
one step, the oxidation state can easily be tuned, and that the
properties combine the advantages in redox-chemistry from car-
benes with the photo-physical advantages of pyrylium salts.
INTRODUCTION
Organic molecules featuring multiple stable redox-states offer
various applications ranging from data1 or energy2 storage to logic
operations suitable for quantum information science.3 Further-
more, organic radicals generated by an external trigger such as
light or electric fields, offer great potential for switchable electro-
conductive4 and magnetic materials.5 While heteroatom-centered
radicals have been quite well studied, carbon centered radicals are
much less well developed,6 presumably a result of their usually
high intrinsic reactivity. Nevertheless, they offer several applica-
tions ranging from photovoltaics, redox-flow batteries to spin-
memory devices.7 Electron-rich olefins are an interesting class of
organic molecules as they feature a small HOMO−LUMO gap
and can easily be oxidized to radical cations and dications, there-
by acting as an organic reductant.8 Tetraazafulvalenes (I) and
tetrathiafulvalenes (II) are important representatives of this sub-
stance class, which have received tremendous attention as super-
electron donors9 or advanced materials, respectively.10 While I
and II can be considered as homodimers of 1,3-diaza and 1,3-
dithiol-2-ylidenes, we reasoned that the hybrid of stable carbenes
with kinetically unstable pyran-4-ylidenes should lead to a new
class of electron-rich alkenes (III; Scheme 1a). Importantly, one
or two electron oxidation should give access to pyranyl radicals
and pyrylium dications, respectively. While pyranyl radicals have
been studied as transient, unstable intermediates by EPR spectros-
copy,11 their main reaction pathway is dimerization.12 As carbenes
have been shown to be suitable for stabilizing main-group radi-
cals,13 we were curious if a carbene substituent could lead to
stable, isolable pyranyl radicals. Pyrylium salts have found vari-
ous applications,14 such as in laser dyes, Q-switches,15 sensors,16
nonlinear optical (NLO) materials,17 or phototherapeutic agents;18
most extensively they have been utilized as organic photosensitiz-
ers to undergo photo induced electron transfer (PET) processes
(Figure 1b).19 The singlet and triplet excited state of 2,4,6-
triphenylpyrylium tetrafluoroborate (TPT+) is a strong oxidant
[Ered* =(3TPT*+/TPT·) = ca. +1.9V],19b easily generating the pyra-
nyl radical by electron uptake from an electron donor such as a
THF solvent molecule. One of the undesired reaction pathways of
excited state pyrylium cations is the energy wasting back electron
transfer (BET), which could be significantly disfavored by 4,2’-
Scheme 1. (a) Electron-rich olefins and the partition into the corre-
sponding carbenes. (b) TPT+ as photoredox-catalyst and derived
pyrylium salts (bottom).
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unlikely due to the anti-aromaticity of the resulting 8π-electron
RESULTS AND DISCUSSION
pyrylium core. However, natural resonance theory (NRT) calcula-
tions on structurally simplified 2a predict a 36% contribution of
the zwitter-ionic structure (see SI). Importantly, as a result of the
electron donating properties of carbenes, the polarized alkenes are
strongly electron rich, therefore susceptible to one or two electron
oxidation to give radical cations (3) or dications 4, respectively
(Scheme3). In the reverse sense, upon reduction of the dication 4
to the radical cation 3, the pyrylium core can act as a chromo-
phore enabling PET processes, while the carbene moiety can
stabilize the resulting radicals, a property which might be interest-
ing for applications in photo-redox catalysis (vide infra).
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We started our investigation by reacting two equivalents of free
unsaturated N-heterocyclic carbene a (imidazol-2-ylidene) with
2,6-disubstituted pyrylium salt 1tBu. The free carbene adds selec-
tively to the 4-position of the pyrylium core,25 while the second
equivalent a deprotonates the addition intermediate I to give 4-
carbene-substituted pyran 2atBu. Note, the isolation of the product
is straightforward as the only byproduct (protonated carbene salt)
is easily separated by extraction. This approach is broadly appli-
cable and can not only be transferred from di-alkyl-pyrylium
(2atBu) to di-aryl-pyrylium (2aPh), but most importantly can be
performed with a series of different stable carbenes (Scheme 2).
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Scheme 2. Synthesis of carbene-pyran hybrids.
Figure 1. X-ray solid-state structures of alkenes 2atBu-2dtBu. Hydro-
gen atoms and solvent molecules omitted for clarity. Ellipsoids shown
with 50% probability. Selected bond parameters in [Å] and [°]: 2atBu
(top left): N1−C1 1.411(8), C1−C2 1.371(8), C2−C3 1.456(4), C3−C4
1.338(8), N1−C1−C2−C3 5.2 (as mean over 6 independent molecules
in the unit cell) 2btBu (top right): N1−C1 1.403(1), C1−C2 1.359(1),
C2−C3 1.458(1), C3−C4 1.331(1), N1−C1−C2−C3 167.4; 2ctBu (bot-
tom left): N1−C1 1.4035(8), C1−C2 1.3729(9), C2−C3 1.4651(9),
C3−C4 1.3400(9), N1−C1−C2−C3 179.9; 2dtBu (bottom right):
N1−C1 1.439(2), C1−C2 1.370(2), C2−C3 1.463(2), C3−C4 1.338(2),
N1−C1−C2−C3 167.5.
Following the outlined procedure, the saturated N-heterocyclic
carbene b leads to 2btBu, while cyclic (alkyl)amino carbene
(CAAC)26 c gives access to 2ctBu in 72 and 82% yield, respective-
ly. In case of diamido carbene (DAC)27 d, the carbene can be
generated in situ in THF at -40°C,28 and reacts cleanly with the
corresponding pyrylium salt to give 2dtBu/2dPh. Strikingly, the
stability of the pyranes increases with the electron deficiency of
the carbene; while DAC derived 2dtBu/2dPh can be purified by
column chromatography in air, 2atBu/2aPh is only stable under
inert atmosphere. Interestingly, the electron rich systems
(2atBu/2aPh/2bPh) show very broad 1H NMR resonances which
sharpen upon cooling (see Figure S1−S2). This dynamic behavior
might be explained by a partial diradical character allowing the
rotation of the central C−C bond (vide infra)29 or by pyramidaliza-
tion of the nitrogen atoms and fluctuation of the N-aryl-groups.
Indeed, single X-ray crystallography shows a strong twisting of
Scheme 3. Redox-states of pyrylenes.
the N-aryl groups out of plane [2atBu: ∢ CAr−N1−C1−C2 31.3°;
2btBu: ∢ CAr−N1−C1−C2 42.1°, 2bPh: ∢ N1−C1−C2−C3 31.4°],
while the central C−C bond is only slightly bend out of the car-
bene/pyrylium planes (up to 28°) and the C−C distances [1.359-
1.373Å] are nearly independent of the carbene moiety (Figure 1;
for the X-ray structures of 2bPh and 2cPh see SI). Computational
data at the B3LYP-D3BJ/def2-TZVP level of theory are in very
good agreement with the experimental data (see Table S13−S16).
Interestingly, while the neutral carbene-pyranes 2a-2d can be
described as electron rich alkenes, there is also the possibility to
formulate diradical (I) and zwitter-ionic (II) mesomeric structures
(Scheme 3 top). Preliminary multi-reference CAS-SCF/NEVPT2
calculations with a (10,10) active space of 2atBu indicate a low
population of the antibonding lowest unoccupied molecular or-
bital (LUMO) by 0.11 electrons indicating a diradical (I) charac-
ter of 11% (see SI). The charge-separated zwitter-ionic descrip-
tion II, typical for example for N-heterocyclic olefins,30 seems
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In order to evaluate the oxidation potentials we measured cyclic
er with the here developed electrochemical correlation, similar π-
accepting properties of 4-pyridylidenes compared to stable
CAACs.36
voltammograms for all pyranes (Figure 2). Indeed, all compounds
feature two distinctively separated quasi-reversible redox-waves
with the first oxidation spanning a range of 1.19 V [E10 (2atBu) = -
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1.22 V to E1 (2dtBu) = -0.03 V] and the second of 0.55 V [E2
(2atBu) = -0.34 V to E2 (2dtBu) = +0.21V; all potentials against
0
Fc/Fc+]. Pyran 2atBu shows a strongly reducing character with
0
both redox-events separated by 0.88 V (E2 = -0.34 V). Note, the
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electron donation ability of carbene a in combination with the
gain of aromaticity upon oxidation, leads to a reduction potential
of 2atBu in the same range with zinc or the strong organic reduct-
ant tetrakis(dimethylamine)ethylene [TDAE; E = -0.78/-0.61 vs.
SCE or E = -1.18/-1.01 V vs. Fc/Fc+ in CH3CN].8,9,31 When mov-
ing from dialkyl-substituted pyranes (2atBu-2dtBu) to diaryl-
substituted (2aPh-2dPh) the redox-potential systematically shifts
by ca. 100-200 mV to more positive redox potentials, while a
more π-accepting carbene significantly shifts the oxidation waves
to more positive potentials. Note, the reduction potential of 2b is
in a similar range to 2,4,6-triarylpyrylium tetrafluoroborate [E0 = -
0.83V in CH2Cl2 vs. Fc/Fc+ (see SI)].
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Figure 3. Correlation of the redox-potentials E1 with the 77Se
NMR of the corresponding carbene-Se adducts.
We performed spectroelectrochemistry experiments on all
carbene/pyrylium hybrids to study the photochemical properties
(Figure 4; for 2btBu-2dtBu see SI). Each compound and oxidation
state features characteristic UV-vis transitions (for a summary of
all transitions of each oxidation state with isobestic points and
exctinction coeficients see Table S11/12). While in general the
alkyl substituted series 2a-4atBu shows transitions at the edge to
the UV-spectrum λ ~ 300-400nm (ε ~ 5000-15000 cm-1M-1), the
aryl-subsituted compounds (2a-4aPh) show additional strong
transitions for all oxidation states in the VIS-area (λ ~ 400-600
nm; ε ~ 20000-60000 cm-1M-1; Figure 4 A, B). A general trend of
λmax (neutral) < λmax (dication) < λmax (radical) was detectable.
Time dependent (TD)-DFT B3LYP/def2-TZVPP//B3LYP/def2-
TZVP calculations of the neutral, alkyl series (2atBu-2dtBu) predict
strong π-π* transitions for the central carbene/pyran moiety in
good agreement with the experimental data (see SI). In case of
aryl substitution (2aPh-2dPh) an additional strong UV-VIS band
arises from the charge transfer from the central carbene/pyran
moiety onto the aryl groups (see Figures S87-S91). Interestingly,
in case of pyrylium salts, Balaban et al. introduced an X,Y-band
notation based on two perpendicular chromophores polarized in
the X and Y directions.14,37,38 Note, 2,4,6-triphenylpyrylium tetra-
fluoroborate (TPT+) shows two transitions at 355 nm and 401nm,
which have been assigned as Y and X-band, respectively.20 This
assignment is also valid for the dications (4aPh-4dPh) and is sup-
ported by TD-DFT B3LYP/def2-TZVPP calculations (see SI). In
the dications the X-band arises from the charge transfer from the
aryl-groups to the pyrylium core, which is as expected, nearly
independent of the carbene (480-494 nm) (Figure 4, bottom right).
Interestingly, the radicals (3aPh-3dPh) also feature the characteris-
tics of the X and Y-bands, both however shifted to significantly
higher wavelengths.
Figure 2. Cyclic voltammograms of 2atBu-2dtBu [nBu4NPF6
(0.1 M), Pt/C-graphite 100 mV s−1, versus Fc/Fc+, in THF]. Re-
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dox-potentials E1 /E2 in [V] 2atBu: -1.22/-0.34; 2aPh: -1.03/-0.17;
2btBu: -0.89/-0.14; 2bPh: -0.76/-0.07; 2ctBu: -0.55/0.08; 2cPh: -
0.42/0.04; 2dtBu: -0.03/0.19; 2dPh 0.10.
Interestingly, the oxidation potentials of the carbene/pyrylium
hybrids do not correlate with the overall donor ability of the cor-
responding carbenes. For example CAAC c is a much stronger
overall donor ligand with a Tolman electronic parameter (TEP) of
~2042 cm-1 compared to NHC a with ~ 2051 cm-1, however 2a is
more readily oxidized than 2c. The 77Se-NMR shifts of carbene-
selenium adducts is a well-established scale for π-accepting prop-
erties of carbenes.32 Very interestingly, the redox potentials of the
first oxidation to give 3a-d show a linear correlation with the π-
accepting properties of the corresponding carbenes based on 77Se
NMR shifts (Figure 3).33 This correlation is reasonable since the
oxidation step reduces electron density in the π and not σ-system
of the pyrylium/carbene hybrids and paves the way for an electro-
chemical derived π-accepting scale.34 Furthermore, it allows to
predict the right choice of carbene for a specific application with
the pyrylium/carbene hybrids. Additionally, there is also a good
While the carbene moiety can significantly alter the electro-
chemical properties, we were also interested in tuning the photo-
chemical properties. Furthermore, in order to validate the modu-
larity of our outlined synthesis, we reacted CAAC (c) and 2,6-
bis(4-methoxyphenyl)pyrylium tetrafluoroborate to give cleanly
hybrid 2cPh-OMe (see SI). While the UV-vis spectra of 2cPh and
2cPh-OMe on the neutral oxidation state are fairly similar, the dona-
tion of electron density from the p-OMe groups in the mono-
cationic and dicationic oxidation states leads to pronounced bath-
ochromically shifted UV-Vis transitions (Figure 5). Note, the
introduction of p-OMe groups results in a relative small oxidation
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linear correlation between E(E1 /E2 ) with the 77Se-NMR shifts
(see Figure S22). Considering IV (Scheme 1) as a formal twice
oxidized adduct of two unknown pyran-4-ylidene and 4-
pyridylidenes,35 it is possible to predict, based on the reported
0
0
redox-potential difference of IV (E(E1 /E2 ) = 0.52V),20 togeth-
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Figure 4. Spectroelectrochemistry of 2atBu (A) 2aPh (B), 2aPh (C), 2cPh (D), 2dPh (E) [in nBu4NPF6 (0.1 M) in DCM; 2.5 mV/s, against
Ag/AgNO3 (0.01M)] and visualization (TD-DFT B3LYP/def2-TZVPP) of representative X,Y-transitions for the dications 4atBu and 4dPh.
potential shift by ca. 100 mV to more negative values [E10 -0.53V
and E20 -0.11 V (2cPh-OMe)] (Figure S15/16).
hours in air, and the UV-VIS spectra of the isolated species match
the corresponding spectra obtained by spectroelectrochemistry
(see SI). The EPR spectra feature in each case 14N hyperfine
The large electrochemical separation of first and second
oxidation, together with the spectroelectrochemical data indicated
the potential of isolating stable radicals. Indeed, chemical
oxidation of all pyranes 2a-2d with one equivalent AgSbF6 gives
the pyranyl radicals 3a-3d, which could be characterized by their
UV-VIS and EPR spectra (Figure 6; EPR spectra of 3bPh-3dPh,
see SI). Note, the very electron rich alkenes 2a/b are prone to be
oxidized by air to give the radicals; for example the UV-VIS
spectrum of neutral 2aPh changes in seconds upon exposure to air
to the UV-VIS spectrum of radical 3aPh.
couplings (hfcs) of the carbene moiety [3atBu: aN = 2.63 G; 3btBu
:
aN = 1.66 G; 3ctBu: aN = 5.42 G; 3dtBu: aN = 2.63 G], but also hfcs
to the meta-pyrylium H-atoms. Comparison of EPR spectra of
3atBu and 3aPh (Figure 6a/b) clearly indicates hfcs also into the
aromatic groups in 2,6-position demonstrating spin delocalization
across the π-system (see also SOMOs Figure 7 A/B). All EPR
parameters were rationalized at the B3LYP-D3BJ/def2-
TZVPP//B3LYP-D3BJ/def2-TZVP level of theory and show very
good agreement with the experimental data (see Figures S82-
S86).
Interestingly, all radicals can be fully characterized, are inde-
finitively stable under inert conditions, even stable for minutes to
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Figure 5. Comparison of the UV-VIS spectra of 2-4cPh and 2-
4cPh-OMe
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Figure 7. SOMOs, Mulliken spin-densities (in %) and EPR-hfcs
[G] calculated at the B3LYP-D3BJ/def2-TZVPP//B3LYP-
D3BJ/def2-TZVP level of theory for 3atBu (A), 3aPh (B), 3btBu
(C), 3ctBu (D), 3dtBu (E).
Importantly, while pyranyl radicals, such as derived from 2,4,6-
triarylpyrylium, are known to dimerize,11,12 which represents a
serious deactivation pathway in photo-redox catalysis,39 we were
able to obtain single crystals of the monomeric pyranyl radical
3ctBu (Figure 8). As far as we know this is the first structurally
characterized pyranyl radical. In the solid-state structure the
N1−C1 bond [1.339(2)] and C1−C2 [1.433(2)] distances are
positioned between the neutral [1.4035(8); 1.3729(9); 2ctBu] and
dicationic [1.291(2); 1.485(2); 4ctBu vide infra] bond lengths, in
very good agreement with the calculated data (see Table S15).
Figure 6. Experimental (top) and simulated (bottom) room tem-
perature X-band EPR spectra of 3atBu-3dtBu and 3aPh in THF;
fitting parameters (a) 3atBu 2xN: 2.63 G, 2xH: 1.06 G, 2xH: 0.50
G; (b) 3aPh 2xN: 2.31 G, 2xH: 1.83G, 2xH: 0.92 G, 2xH: 1.1 G,
4xH: 0.32 G; (c) 3btBu 2xN: 1.66 G, 4xH: 2.56 G; 2xH: 0.13G; (d)
3ctBu 1xN: 5.42 G, 2xH: 0.61 G; (e) 3dtBu 2xN: 1.00 G, 2xH: 1.11
G.
Localized Mulliken spin densities and the plots of the singly
occupied molecular orbitals (SOMOs) indicate that the spin-
density being located on the carbene moiety is moderate but
increases in the order unsaturated NHC ~ saturated NHC (ca. 26-
27%) < CAAC (35%) < DAC (42%) [This distribution is nearly
independent on the pyrylium substitution (tBu vs. Ar), see SI].
Note, in case of NHCs there is nearly no spin-density located on
the carbene carbon atom (1-8%) in contrast to other main-group
NHC-radicals,13 but instead smeared over the NHC moiety. In the
case of CAAC the carbene spin-density is partially localized onto
the N (23%) and C(12%) atoms, while in case of DAC the car-
bene C-atom spin-density increases to 22% with 10% spin-density
localized onto the oxygen-atoms of the backbone (Figure 7).
Figure 8. X-ray solid-state structure of radical cation 3ctBu. Hy-
drogen atoms omitted for clarity. Ellipsoids shown with 50%
probability. Selected bond parameters in [Å] and [°]: C1−N1
1.339(2), C1−C2 1.433(2), C2−C3 1.432(2), C3−C4 1.356(3),
N1−C1−C2−C3 172.0.
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Interestingly, two electron oxidation of 2 with either two equiv-
Following the above established synthesis, 2ePh can be isolated
alents of AgSbF6 in CH2Cl2 or two equivalents of NOSbF6 in a
CH2Cl2/CH3CN solvent mixture gives clean access to all dications
4. H NMR spectroscopy indicates a clear shift of the pyran al-
as intense red solid in 52% yield in a single-step. Interestingly, in
the X-ray solid-state structure the menthyl moiety, which is usual-
ly used as sterically confined bulky group,40 is widened away
from the pyran moiety, as a result of the repulsion between the H-
atom in the 3-position with the menthyl moiety. CV measure-
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kene protons from δ(1H) = 4.21 ppm (2atBu) to the aromatic
pyrylium shift δ(1H) = 8.26 ppm (4atBu). In case of 4ctBu and 4dPh
we were able to obtain single crystals suitable for X-ray diffrac-
tion (Figure 9). Most strikingly, in the solid-state the pyrylium
moiety is twisted perpendicular to the carbene plane (4ctBu: N1-
C1-C2-C3 96.1°; 4dPh: N1-C1-C2-C3 69.3°) while the C-C bond
is significantly elongated to 1.485(2) (4ctBu) and 1.510(7) Å
(4dtBu). This rotation motion might be interesting in respect to an
application as electrochemical switch (vide infra). Importantly,
the neutral compounds 2 and dications 4 not only feature strong
UV-vis absorptions, but also show fluorescence properties, ren-
dering them suitable for PET processes (see SI). The emission
wavelengths for the dications are in a range of 458 nm (4dPh) to
565 nm (4bPh) (see SI). Note, 2,4,6-triphenylpyrylium
tetrafluoroborate shows an emission band at 472 nm.
0
ments (E10 = -0.41 V; E2 = 0.03 V against Fc/Fc+ see Figure S20)
give similar data compared to diethyl-CAAC (2cPh). 2ePh can be
conveniently oxidized with one equivalent of AgSbF6 to afford
the stable, enantiomerically pure, red colored radical cation 3ePh
or oxidized with two equivalents of AgSbF6 to obtain the chiral
yellow colored dication 4ePh.
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In a next step, we investigated preliminary applications for this
new hybrid class. First we demonstrated the use as electrochemi-
cal switch with color being an output function. By applying a time
dependent current [cycles of 300s at each +0.2 V/-0.8V and +0.2
V/+0.9V, see SI for more details] to solutions of 2cPh we could
selectively and reversibly switch between three different colors
correlating to the neutral, radical and dicationic species (Fig-
ure 10).41 Importantly, while photo switches typically can only
vary between two different colors (on or off), we can selectively
address three different colors depending on the potential applied.
Moreover, switching between neutral and dicationic form also
goes in hand with a fixed and flexible rotation of the central C-C
bond, which translates into a non-directed motion. Obviously, in
case of chiral 4ePh it would be interesting to transfer this motion
into a directed pathway.
Figure 9. X-ray solid-state structures of the dications 4ctBu (left)
-
and 4dPh (right). Counter anions (SbF6 ) as well as hydrogen
atoms omitted for clarity. Ellipsoids shown with 50% probability.
Selected bond parameters in [Å] and [°]: 4ctBu: N1−C1 1.291(2),
C1−C2 1.485(2), C2−C3 1.392(2), C3−C4 1.380(2),
N1−C1−C2−C3 96.1; 4dPh: N1−C1 1.337(6), C1−C2 1.510(7),
C2−C3 1.380(7), C3−C4 1.375(7), N1−C1−C2−C3 69.3.
Additionally,
we
generated
enantiomerically
pure
pyrylium/carbene hybrids. For this purpose we chose the combi-
nation of chiral menthyl substituted cyclic (alkyl)aminocarbene
MenthCAAC,26 with 2,6-diphenylpyrylium 1Ph (Scheme 4).
Scheme 4. Synthesis of chiral 2ePh and its oxidation-states. (inset
top right: X-ray solid-state structure of 2ePh; inset bottom left: X-
band EPR spectrum of 3ePh (simulated parameters: a(N): 4.81 G;
a(H): 1.22 G).
Figure 10. Electrochemical switching of 2cPh between neutral
(black) – radical (red) and radical (red) – dication (blue).
Finally, we focused on the application of the hybrid molecules
as photo-excited oxidants or reductants, as both the neutral (2Ph)
and dicationic redox-states (4Ph) feature strong UV-vis absorp-
tions and fluorescence. Importantly, as the ground-state reduction
potentials of the dications [E2 = +0.33 (4aPh) to +0.6 (2dPh) vs.
SCE] are significantly more positive compared to the well-known
2,4,6-triphenylpyrylium salt [-0.32 V vs. SCE],19 the excited state
reduction potentials of the dicationic pyrylium salts (4c) are re-
markably high and clearly succeed the already very high oxidiz-
ing power of standard pyrylium salts. In a first approximation the
excited state reduction potential (Ered*) of the dications 4 can be
calculated according to Eq. I:
퐸푟∗푒푑[ퟒ∗/ퟑ] = 퐸푟푒푑[ퟒ/ퟑ] + ∆퐸푒푥푐푖푡[ퟑ/ퟒ∗]
(I)
While the first reduction potential Ered can be obtained by cyclic
voltammetry measurements (E2 in eV), the excitation energy
ΔEexcit can be obtained from the fluorescence spectra (more pre-
cisely from the crossing point of normalized absorption and fluo-
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Journal of the American Chemical Society
rescence). This leads to all dications 4 exhibiting an excited state
Scheme 6. Reductive photo-excited dehalogenation. Yields de-
termined by GC/MS analysis with calibration against an internal
standard.
reduction potential of Ered* > 2.5 V [~ 2.8 eV (4aPh), ~ 2.9 eV
(4bPh), ~ 3.09 (4cPh), ~ 2.5 eV (4dPh). In order to test the strong
photo-excited oxidants we selected a known pyrylium photo-
catalyzed Diels-Alder (D.-A.) reaction, in which the excited state
photo-catalyst oxidizes an alkene to its radical cation triggering an
electron-transfer mediated hetero-D.-A. reaction (Scheme 5).[42]
Upon irradiation of N-benzylideneaniline (5) and trans-anethole
(6) in the presence of catalytic amounts (10 mol%) 2dPh with a
blue light-emitting diode (LED, λexc. 390 nm), we could isolate
the desired product 7 in 60% yield as a 5:1 mixture of two dia-
stereomers (7/7’). However, importantly, the control experiment
without light at 60°C gave nearly identical results, indicating that
in the presence of 2dPh the reaction is not photo-chemical in-
duced, but instead 2dPh acts as a strong Lewis-acid triggering the
reaction. Note, the thermal control experiment utilizing 2,4,6-
triphenylpyrylium BF4 did not lead to the desired D.A. product,
highlighting the difference of mono vs. dicationic pyrylium salts.
This result clearly demonstrates the high Lewis acidity of the
dicationic ground-state of 4, which has to be considered develop-
ing photo-catalytic reactions with such systems.
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Scheme 5. Lewis-acid catalyzed hetero-D.A. reaction.
Recently, König et al. demonstrated a new concept in photo-
catalysis using the energy of two photons in one catalytic cy-
cle.[44] In this case, utilizing a compound featuring two intensely
colored oxidation states, an excited radical anion was generated
acting as strong electron donor. Inspired by this approach, we
reasoned that in our system radical cation 3 might also be photo-
excitable and in its excited state oxidize a sacrificial electron
donor such as Et3N to regenerate electron rich neutral 2
(Scheme 7).[45] In this case a full catalytic! cycle could be con-
structed which involves two single photon absorption processes
on two different oxidation states (Scheme 7). In contrast to Kö-
nig’s system, which proceeds through an excited radical anion,
the strong reducing power in our system would be harnessed from
a neutral excited state as a result of the electron rich ground state
of 2. Radical cation 3cPh shows an intense absorption at a similar
energy [λabs = 394 (ε = 18601 M-1cm-1)] compared to the neutral
species 2cPh and therefore is likely to be excitable under identical
experimental conditions at the same wavelength as neutral 2cPh
(see SI).
Importantly, classical pyrylium salts are exclusively excited
state oxidants, however, the existence of three stable oxidation
states in the pyrylium/carbene adducts, also allows to develop
excited state reductants. Moreover, as neutral 2a-2d can be widely
tuned in their ground state reducing properties (Figure 2), we
selected 2c as a moderate ground state reductant [E1 = + 0.12 V
vs. SCE]. In contrast to 2a and 2b, which both feature more nega-
tive redox potentials, 2c is stable in air and therefore more con-
venient to use. 2c shows two strong UV-vis absorptions centered
at λabs = 388 nm (ε = 49422 M-1cm-1) and 406 nm (ε = 53176 M-
1cm-1) (with fluorescence at λemis = 561 nm). In analogy to Eq. 1,
the excited state oxidation potential of 2 can be calculated accord-
ing to Eq. II:
Scheme 7. Mechanistic hypothesis for a catalytic cycle involving
two photon induced SET reactions.
퐸표∗푥[ퟐ∗/ퟑ] = 퐸표푥[ퟐ/ퟑ] − ∆퐸푒푥푐푖푡[ퟑ/ퟐ∗]
(II)
Applying Eq. II in case of 2cPh leads to Eox* ~ -2.4 eV, indicat-
ing 2cPh to be a very powerful excited state reductant. In order to
investigate the strong photo-induced oxidation potential we stud-
ied the stoichiometric dehalogenation of aryl halides, which is a
classic transformation for super-electron donors.9,43 Typically,
aryl iodides are utilized in photo-redox catalysis, as aryl bro-
mides, due to their more negative reduction potentials, remain
very challenging substrates. Importantly, upon 12 h irradiation of
ethyl 4-bromobenzoate (8) in the presence of one equivalent of
2cPh with a blue LED (λexc. 390 nm) in DMF, we observed the
formation of the reductive dehalogenation product ethyl benzoate
(9) in 78% yield (Scheme 6). Control experiments either under
exclusive thermal conditions (no light at 60°C), or without 2cPh,
showed no conversion and clearly demonstrates the high reducing
power of photo-excited 2cPh. As proof of concept, also the more
challenging 4-bromo-1,1’-biphenyl (10), bearing no electron-
withdrawing group, could be reduced with photo-excited 2cPh in
54% yield.
In order to investigate such a catalytic process as outlined in
Scheme 7, we performed the reaction of bromo ester 8 and a
catalytic amount 2cPh (10 mol%) in the presence of excess Et3N
(20 eq.) as sacrificial electron donor (Scheme 8). After 12 h irra-
diation with a LED (λexc. 390 nm) we could observe full conver-
sion to give dehalogenation product 9 in 87% yield. In a next step
we tried more challenging catalytic reduction reactions. The
reaction of 4-bromo-1,1’-biphenyl 10 under identical reaction
conditions, gave the desired dehalogenation product in 62% yield.
Remarkably, we were not only able to cleave the Ar-X bonds, but
also to reductively cleave a sulfonamide. Under the non-
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Page 8 of 12
optimized reaction conditions we could observe full conversion of
redox state. The later could be clearly demonstrated by stoichio-
indolesulfonamide to give indole in 85% yield. Note, sulfona-
mides have very negative reduction potentials, typically around -2
V vs. SCE and are traditionally cleaved by alkali metals.[46] It
should be highlighted that we can deprotect catalytically in the
presence of a sacrificial electron donor under mild reaction condi-
tions, while for the same deprotection reaction, even neutral
organic super-electron donors require super-stoichiometric (6
equivalents) reducing agents at forcing conditions (110°C).[46]
Furthermore, we could also trap the proposed transient aryl-
radical by addition of N-methyl pyrrole (20 eq.) to form the C-C
coupled 2-aryl pyrrole 14 in 71% yield. Interestingly, the reduc-
tive dehalogenation product 9 was formed in only minor quanti-
ties (6%) indicating a sufficient trapping by the pyrrole. Control
experiments showed no reaction of bromo ester 8 under identical
thermal conditions (60°C) but exclusion of light. Importantly,
irradiation without photo catalyst 2cPh under otherwise identical
conditions, also did not lead to any conversion, clearly proving
the necessity of photo catalyst 2cPh. We are currently investigat-
ing the mechanism and scope of these new photo catalyzed reac-
tions, not ruling out the option of even three oxidation states
being mechanistically involved.
metric photo-induced dehalogenation reactions. More important-
ly, the stability of the radical cation oxidation state allows subse-
quent excitation processes, leading to catalytic cycles involving
strong neutral photo-excited organic reductants. The simple syn-
thetic access and ability to tune the redox and photo-physical
properties of the carbene/pyrylium hybrids, should allow to spe-
cifically design novel photo-catalysts based on the here presented
approach. Furthermore, as pyrylium cations are also an important
synthetic precursor and easily transformed into pyridines, phos-
phinines etc., the dications offer a broad range of functionaliza-
tion strategies to new exciting molecules, which is under current
investigation.
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EXPERIMENTAL SECTION
General procedure for the synthesis of 2: To a suspension of
pyrylium tetrafluoroborate salt (1.0 eq.) in THF was added under
inert conditions at room temperature a solution of free carbene
(2.05 eq.). The solution was stirred for 16h, the solvent evapo-
rated under reduced pressure and the product extracted twice with
Et2O. Evaporation of the solvent gives the desired
pyrylium/carbene hybrid 2. Single crystals can be obtained from a
saturated pentane or Et2O solution upon cooling to -30°C. Note,
the remaining solid (protonated carbene can be recycled with base
to give the free carbene). General procedure for the synthesis of 3:
To a solution of neutral 2 (1.0 eq) in CH2Cl2 or THF was added at
room temperature AgSbF6 (1 eq). The solution was stirred for 1h,
then filtered and the solvent removed to give the corresponding
radicals 3. General procedure for the synthesis of 4: To neutral 2
(1.0 eq.) in CH2Cl2 was added at 0°C dropwise a solution of
NOSbF6 (2.0 eq.) dissolved in CH3CN. [Alternatively, AgSbF6
(2eq.) was added directly as solid]. The solution was warmed up
to room temperature, stirred for 1h and the solvent removed under
reduced pressure. The remaining solid was washed with Et2O and
dried under reduced pressure to give the desired product 4.
Synthesis and characterization data for selected compounds:
2atBu: To a suspension of 2,6-di-tert-butylpyrylium tetrafluorobo-
rate 1tBu (200 mg, 0.71 mmol, 1.0 eq.) in THF (15 mL) was added
free carbene a (434 mg, 1.43 mmol, 2.0 eq.). The yellow/orange
solution was stirred for 14h and the solvent evaporated under
reduced pressure. The obtained solid was extracted with pentane
(15 mL + 5 mL) and filtered. The solvent of the orange solution
was removed under reduced pressure to give 2atBu as orange solid
(275 mg, 0.55 mmol, 78%). Single crystals suitable for X-ray
diffraction can be obtained by slow evaporation from a saturated
pentane solution. m.p. 155°C; 1H-NMR (d8-THF, 400MHz,
208K): 6.97 (s, 4H), 6.15 (s, 2H), 4.21 (s, 2H), 2.27 (s, 6H), 2.24
(s, 12H), 0.63 (s, 18H); 13C-NMR (d8-THF, 100MHz, 298K):
159.4, 151.2, 138.0, 137.9, 131.3, 129.5, 116.6, 96.9, 93.9, 77.5,
35.1, 28.0, 21.3, 19.0; IR [cm-1]: ṽ = 2961, 2913, 2864, 1614,
1578, 1480, 1457, 1392, 1280, 1223, 1156, 1086, 1033, 961, 919,
851, 755; HR-MS-ESI(+) calc. C34H45N2O+ [M+H]+ 497.3526;
found 497.3527.
Scheme 8. Catalytic dehalogenation (i-ii), reductive cleavage of a
N-tosyl group (iii) and a C-C bond forming reaction (iv). Yields
determined by HPLC analysis (9, 11) and isolated yields (13, 14).
CONCLUSIONS
In summary we report an easy, fast and modular approach to-
wards a library of carbene/pyrylium hybrids which feature three
stable redox-states, rendering them suitable as novel class of
redox-switch. The choice of carbene can modulate the electro-
chemical properties by over 1 V, while the choice of pyrylium can
control the photo-physical properties proven by strong UV-VIS
transitions coupled with fluorescence. There is an excellent corre-
lation of the redox-properties with the π-accepting properties of
the carbene, which can be fitted by linear regression with the
77Se-NMR π-accepting scale. The one electron oxidation gives
access to stable radical cations from which we could structurally
characterize the first stable monomeric pyranyl radical. While
typically new main-group radicals are stabilized with only one
type of carbene, we present a comprehensive study on the influ-
ence of radical stabilization as a function of carbene. Furthermore,
we could show that these hybrid molecules can be excited by light
and represent either very strong oxidants in their dicationic oxida-
tion state, but also extraordinary strong reductants in their neutral
2cPh: To a suspension of 2,6-di-phenylpyrylium tetrafluoroborate
(200 mg, 0.62 mmol, 1.0 eq.) in THF (10 mL) was added the free
carbene EtCAAC (392 mg, 1.25 mmol, 2.0 eq.) and the intensely
orange/red solution stirred for 14h. The solvent was removed
under reduced pressure and the remaining solid extracted with
Et2O (20 mL + 5 mL). The solvent was removed under reduced
pressure to give 2cPh as an intense red solid (278 mg, 0.51 mmol,
82%). m.p. 200°C; 1H-NMR (C6D6, 400MHz, 298K): 7.80 (d, J =
7.4 Hz, 2H), 7.36-7.26 (m, 3H), 7.22-7.14 (m, 6H), 7.08-7.01 (m,
2H), 6.63 (d, J = 2.0 Hz, 1H), 5.46 (d, J = 2.0 Hz, 1H), 3.40
(sept., J = 6.8 Hz, 2H), 2.08-1.97 (m, 2H), 1.84-1.74 (m, 2H),
1.80 (s, 2H), 1.27 (d, J = 6.7 Hz, 6H), 1.23 (d, J = 6.7 Hz, 6H),
1.07 (s, 6H), 1.03 (t, J = 7.4 Hz, 6H); 13C-NMR (d8-THF, 100
MHz, 298K): 149.7, 145.1, 144.5, 141.6, 138.4, 136.0, 135.4,
128.7, 128.5, 127.3, 127.0, 125.2, 123.7, 123.7, 96.4, 63.9, 50.6,
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Journal of the American Chemical Society
47.4, 32.8, 30.1, 29.1, 25.1, 24.4, 9.2; IR [cm-1]: ṽ = 2962, 2868,
solvent was carefully evaporated under reduced pressure and the
sample redissolved in 1 mL EtOAc. The yield was determined by
integration of the resulting spectra in comparison to a previously
measured standard calibration curve.
1590, 1550, 1491, 1457, 1329, 1279, 1188, 1074, 1019, 930, 756;
UV-VIS [nm]: in THF: 261 (ε = 46372 M-1 cm-1), 388 (ε = 49422
M-1 cm-1), 406 (ε = 53176 M-1 cm-1), 461 (ε = 9085 M-1cm-1);
Emission [nm]: 562; HR-MS-ESI(+) calc. C39H47NO+ [M]+
545.3652; found 545.3653.
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Photocatalytic deprotection reaction: In a 3mL screw cap vial,
to a solution of 2cPh (6.5 mg, 0.012 mmol, 10 mol%) in 1 mL
degassed DMF was added a solution of N-tosylindole (0.12
mmol, 1.0 eq.) in 2 mL degassed DMF. The reaction mixture was
irradiated at 390 nm. After 12 h the solvent was evaporated under
reduced pressure and the residue purified by flash column chro-
matography (SiO2; hexanes:EtOAc = 20:1) to give the desired
product 13 as a colorless solid (11.2 mg, 0.095 mmol, 85%).
Spectroscopic data are in good agreement with previous literature
reports.
3atBu: To a mixture of solids of 2atBu (16 mg, 0.03 mmol) and
AgSbF6 (11 mg, 0.03 mmol, 1.0 eq.) was added CH2Cl2 (5 mL) at
room temperature and stirred for 30min. The solution was filtered
and the solvent evaporated under reduced pressure to give 3aPh as
red solid (17 mg, 0.02 mmol, 72%). m.p. > 150°C; X-band EPR g
= 3359 G (2xN: 2.63G; 2xH: 1.06G; 2xH: 0.50G); IR [cm-1]: ṽ =
2961, 2924, 2868, 1677, 1616, 1559, 1484, 1461, 1365, 1259,
1230, 1083, 1015, 797; HR-MS-ESI(+) calc. C34H45N2O+ [M+H]+
497.3526; found 497.3521.
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Radical trapping experiment: To
a
solution of 4’-
3cPh: To a solution of 2cPh (100 mg, 0.18 mmol, 1.0 eq.) in
CH2Cl2 (5 mL) was added AgSbF6 (63 mg, 0.18 mmol, 1 eq.) and
the intense dark red solution stirred for 30 min. The solution was
filtered and the solvent evaporated to give 3cPh as dark red solid
(98 mg, 0.13 mmol, 70%). m.p. > 150°C; X-band EPR g = 3362
G (1xN: 5.10 G; 2xH 0.68G), IR [cm-1]: ṽ = 2974, 2934, 2874,
1632, 1463, 1446, 1388, 1348, 1249, 1153, 1130, 946, 904, 766,
654; UV-VIS [nm]: in CH2Cl2: 269 (ε = 19137 M-1cm-1), 394 (ε =
18601 M-1cm-1), 503 (17496); HR-MS-ESI(+) calc. C39H47NO+
[M]+ 545.3652; found 545.3650.
bromoethylbenzoate (27.5 mg, 120 µmol, 1.0 eq) and 2cPh
(6.6 mg, 0.012 mmol, 10 mol%) in 2.5 mL DMF was added tri-
ethylamine (334 µL, 2.4 mmol, 20 eq.) and N-methylpyrrole
(213 µL, 2.40 mmol, 20 eq.). After irradiating the reaction mix-
ture at 390 nm for 12 h a 200 µL sample was subjected to
GC/MS. The solvent was removed under reduced pressure and the
residue purified by flash column chromatography (SiO2; hex-
anes:EtOAc = 20:1) to give the desired product 14 as slightly
yellow solid (18.3 mg, 0.079 mmol, 71%). 1H-NMR (CDCl3,
400MHz, 298K): 8.06 (d, J =8.7 Hz, 2H), 7.47 (d, J =8.7 Hz, 2H),
6.77-6.75 (m, 1H), 6.33 (dd, J = 3.7 Hz, 1.8 Hz, 1H), 6.22 (dd, J =
3.7 Hz, 2.7 Hz, 1H), 4.39 (q, J = 7.1 Hz, 2H), 1.41 (t, J = 7.1 Hz,
3H); 13C-NMR (CDCl3, 100MHz, 298K): 166.6, 137.8, 133.7,
129.9, 128.5, 128.0, 125.2, 110.1, 108.4, 61.1, 35.5, 14.5. The
spectral data is in good agreement with the reported literature.
Electrochemical measurements: CVs were measured with a
Gamry Instruments Reference 600 and iR compensation using the
positive feedback method which is implemented in the PH200
software of Gamry. Samples were measured under inert atmos-
phere in a nitrogen glove box at room temperature in dry THF
solution containing tetrabutylammonium hexafluorophosphate
(0.1 M). The setup consist of a three-neck flask with a three elec-
trode setup containing a glassy carbon working electrode (GC:
CH Instruments, ALS Japan; A = 7.1 mm²), a platinum wire as a
counter electrode, and a Ag/AgNO3 reference electrode (0.01 M
AgNO3 in 0.1 M nBu4NPF6 in CH3CN) (for the setup see also
Figure S3). The reference electrode was freshly prepared by using
a fritted sample holder, which was activated by storing in a
CH3CN solution for one night, followed by diluted HCl (1M) for
one night, dried and stored in THF for at least one additional
night. To the fritted sample holder was added a freshly prepared
0.01 M AgNO3/ 0.1 M nBu4NPF6 solution in CH3CN and a silver
wire. The working electrodes were prepared according to the
following procedure before each scan: washed with water, pol-
ished with an alox-slurry (0.05 μm), washed with millipore water,
sonicated in millipore water for 3 minutes, rinsed with millipore
water, and dried. Initially a blank sample only containing electro-
lyte in THF (0.1M) was measured and the potential cycled for 3-5
scans until a stable and clean potential was reached. Then the 3-
neck cell was emptied and a specific amount of compound dis-
solved in 3mL THF added and the CV measured. The system was
furthermore referenced internally by addition of diacetyl ferro-
cene. The values obtained were corrected against ferrocene (diac-
etyl ferrocene to ferrocene ΔE = 430 mV in THF), which was
determined under identical measurement conditions by measuring
ferrocene against diacetylferrocene (see Figure S4).
4atBu: To a solution of 2atBu (47 mg, 0.09 mmol) in CH2Cl2 (10
mL) was added AgSbF6 (65 mg, 0.19 mmol, 2.0 eq.) and the dark
suspension stirred for 30 min under exclusion of light. The solu-
tion was filtered and the solid washed with CH2Cl2. The remain-
ing solid was extracted with CH3CN (10 mL) to give a yellow
solution. The solvent was removed under reduced pressure to give
4atBu as yellow solid (57 mg, 0.06 mmol, 65%). m.p. > 250°C;
1H-NMR (CD3CN, 400MHz, 298K): 8.26 (s, 2H), 7.27 (s, 4H),
7.03 (s, 2H), 2.40 (s, 6H), 2.10 (s, 12H), 1.22 (s, 18H); 13C-NMR
(CD3CN, 100MHz, 298K): 192.2, 145.0, 144.5, 135.9, 135.5,
131.7, 130.8, 130.5, 118.3, 40.8, 28.2, 21.1, 17.9; IR [cm-1]: ṽ =
3164, 3148, 2975, 1627, 1615, 1562, 1547, 1485, 1438, 1384,
1366, 1228, 1197, 1147, 1036, 869, 839; HR-MS-ESI(+) calc.
C35H47N2O2+ [M+CH3O-]- 527.3632; found 527.3634.
4cPh: To a solution of 2cPh (42 mg, 0.077 mmol) in CH2Cl2 (5
mL) was added at 0°C a solution of NOSbF6 (41 mg, 0.15 mmol,
2 eq.) (the solution turns from bright red to dark red to yellow
upon dropwise addition of the NOSbF6 solution). The solution
was stirred for 10 min at 0°C, then 10 min at room temperature
and the solvent removed under reduced pressure. The resulting
solid was washed with Et2O (10 mL) followed by CH2Cl2/Et2O (5
mL/ 5mL). The remaining solid was dried under reduced pressure
1
to give 4cPh (67 mg, 0.066 mmol, 86%). m.p. > 200°C; H-NMR
(CD3CN, 400MHz, 298K): 8.37 (d, J = 7.8 Hz, 4H), 8.06 (s, 2H),
8.00-7.93 (m, 2H), 7.84-7.77 (m, 4H), 7.56-7.50 (m, 1H), 7.41 (d,
J = 7.8 Hz, 2H), 2.78 (sept., J = 6.4 Hz, 2H), 2.74 (s, 2H), 2.29-
2.11 (m, 4H), 1.64 (s, 6H), 1.45 (d, J = 6.3 Hz, 6H), 1.17 (t, J =
7.3 Hz, 6H), 1.07 (d, J = 6.4 Hz, 6H); 13C-NMR (CD3CN,
100MHz, 298K): 174.1, 152.5, 145.0, 138.9, 133.5, 131.6, 130.8,
128.3, 127.9, 116.6, 88.8, 61.6, 43.8, 30.9, 30.5, 29.5, 27.1, 24.6,
10.0; UV-VIS [nm]: in CH2Cl2: 471 (ε = 25051 M-1cm-1), 297 (ε
= 20933 M-1cm-1), 268 (ε = 24322 M-1cm-1); Emission [nm]: in
CH2Cl2: 506, 524; IR [cm-1]: ṽ = 3116, 2979, 2944, 2883, 1639,
1612, 1489, 1456, 1425, 1370, 1267, 1183, 1103, 998, 898, 870,
+
781; HR-MS-ESI(+) calc. C40H50NO2 [M+CH3O-]+ 576.3836;
found 576.3827.
Irradiation reactions were performed at 390 nm using a LED
photo-redox light Kessil PR 160 (390nm) with maximum 40W.
The light was placed typically 5-8cm in front of a vial.
Stoichiometric light-promoted dehalogenation reactions: A
3 mL screw cap vial was charged with solid 2cPh (21.8 mg, 0.04
mmol, 1.0 eq.) and a solution of the substrate (0.04 mmol, 1.0 eq.)
in 1 mL degassed DMF. The reaction mixture was irradiated at
390 nm for 12 h and a sample was taken for GC/MS analysis. The
Spectro-electrochemical measurements were recorded with a
Gamry Instruments Reference 600. The samples were measured
in CH2Cl2 starting from the neutral compounds, containing a three
electrode setup (platinum wire, platinum net and Ag/AgNO3
reference electrode) in a UV-VIS cell (1mm diameter from ALS
Co., Ltd; SEC-C spectroelectrochemical cell) under nitrogen. For
a picture of the setup see Figure S43. In order to guarantee a clean
oxygen free setup the measurement was performed in a nitrogen
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glove box. The reference electrode was freshly prepared by using
Göttingen/MPI are thanked and Prof. Dr. M. Alcarazo is
a fritted sample holder, which was activated by storing in a
CH3CN solution for one night, followed by diluted HCl (1M) for
one night, dried and stored in THF for at least one additional
night. To the fritted sample holder was added a freshly prepared
0.01 M AgNO3/ 0.1 M nBu4NPF6 solution in CH3CN and a silver
wire. The light source was a deuterium/tungsten light source. A
blank spectrum and a reference spectrum with just solvent was
taken in advance and was subtracted from the measured data. In
general, a UV-VIS spectrum was taken every 10 seconds, while
the electric current was scanned from negative potentials to posi-
tive potentials (usually around 1V) by a step size of 2.5 mV/s.
Electrochemical switch: For the application as an electrochemi-
cal switch the same setup was used as for the spectroelectrochem-
istry. An initial CV (40 mV/s) of a 55 μM solution in 0.7 mL
CH2Cl2 of 2cPh was recorded to determine the oxidation potentials
in the spectroelectrochemistry cell. The solution was replaced
with a fresh solution of 2cPh under identical conditions and a UV-
VIS spectrum taken every 20 s, while the potential was adjusted
to +0.2V for 300 s (radical cation 3cPh) then to -0.8V for 300 s
(neutral 2cPh); followed by +0.2V for 300 s (all potentials against
the Ag/AgNO3 reference). This procedure was repeated again and
then the potential changed (3 times) between +0.2V for 300 s and
+0.9V (dication 4cPh). In a first approximation the diffusion of
solution in the UV-VIS cell is fairly slow (> 10min). However,
there is very small diffusion between the volume of the light
beam/platinum net space to the solvent volume of the space in
which counter-electrode and reference electrode are positioned.
This leads to a slight decrease of the radical cation concentration
over time as neutral compound diffuses into the cell. In order to
minimize this effect the solution amount above the net was also
minimized, the only limitation that the platinum counter electrode
and the reference cell were still in contact with the solution. For
the analysis of the switch data, the absorption at three different
wavelengths was detected: 406 nm for neutral 2cPh, 503 nm for
radical cation 3cPh, and 471 nm for dication 4cPh (see UV-VIS
data). The raw data for the absorption is plotted in Figure S44.
The absorption was normalized from 0 to 1.
acknowledged for his continuous support.
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ABBREVIATIONS
NHC, N-heterocyclic carbene; CAAC, cyclic (alkyl)amino car-
bene; DAC, diamido carbene; PET photo-induced electron trans-
fer; BET back electron transfer; hfc, hyperfine couplings.
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AUTHOR INFORMATION
Corresponding Author
* mhansma@uni-goettingen.de
ORCHID
Max M. Hansmann: 0000-0003-3107-1445
Notes
The author declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the Fonds der chemischen Industrie
(Liebig scholarship to M.M.H) and DFG (INST 186/1237-1). Dr.
C. Golz, Prof. Dr. D. Stalke (X-ray analysis) and Dr. A. C. Stückl
(EPR) are greatly acknowledged. Prof. U. Diederichsen is
acknowledged for providing access to a fluorimeter. Prof. Dr. I.
Siewert and I. Fokin are thanked for their help with cyclic volt-
ammetry measurements Dr. D. Munz is thanked for the help with
the CAS-SCF calculations. Dr. A. Breder is thanked for fruitful
discussions on photo-catalysis and providing a 390nm LED light.
The high performance computing facilities of the University of
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