The Chemistry of Acylgermanes: Triacylgermenolates Represent Valuable Building Blocks for the Synthesis of a Variety of Germanium-Based Photoinitiators

Article abstract of DOI:10.1021/acs.inorgchem.0c02181

The formation of a stable triacylgermenolate 2 as a decisive intermediate was achieved by using three pathways. The first two methods involve the reaction of KOtBu or alternatively potassium with tetraacylgermane 1 yielding 2 via one electron transfer. The mechanism involves the formation of radical anions (shown by EPR). This reaction is highly efficient and selective. The third method is a classical salt metathesis reaction toward 2 in nearly quantitative yield. The formation of 2 was confirmed by NMR spectroscopy, UV-vis measurements, and X-ray crystallography. Germenolate 2 serves as a starting point for a wide variety of organo-germanium compounds. We demonstrate the potential of this intermediate by introducing new types of Ge-based photoinitiators 4b-4f. The UV-vis absorption spectra of 4b-4f show considerably increased band intensities due to the presence of eight or more chromophores. Moreover, compounds 4d-4f show absorption tailing up to 525 nm. The performance of these photoinitiators is demonstrated by spectroscopy (time-resolved EPR, laser flash photolysis (LFP), photobleaching (UV-vis)) and photopolymerization experiments (photo-DSC measurements).

Full text of DOI:10.1021/acs.inorgchem.0c02181

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Article
The Chemistry of Acylgermanes: Triacylgermenolates Represent
Valuable Building Blocks for the Synthesis of a Variety of
Germanium-Based Photoinitiators
̈ ̈
Philipp Fruhwirt, Andreas Knoechl, Michael Pillinger, Stefanie M. Muller, Perry T. Wasdin,
Roland C. Fischer, Judith Radebner, Ana Torvisco, Norbert Moszner, Anne-Marie Kelterer,
Thomas Griesser, Georg Gescheidt, and Michael Haas*
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sı Supporting Information
ABSTRACT: The formation of a stable triacylgermenolate 2 as a
decisive intermediate was achieved by using three pathways. The
first two methods involve the reaction of KOtBu or alternatively
potassium with tetraacylgermane 1 yielding 2 via one electron
transfer. The mechanism involves the formation of radical anions
(
shown by EPR). This reaction is highly efficient and selective.
The third method is a classical salt metathesis reaction toward 2 in
nearly quantitative yield. The formation of 2 was confirmed by
NMR spectroscopy, UV−vis measurements, and X-ray crystallog-
raphy. Germenolate 2 serves as a starting point for a wide variety of
organo-germanium compounds. We demonstrate the potential of this intermediate by introducing new types of Ge-based
photoinitiators 4b−4f. The UV−vis absorption spectra of 4b−4f show considerably increased band intensities due to the presence of
eight or more chromophores. Moreover, compounds 4d−4f show absorption tailing up to 525 nm. The performance of these
photoinitiators is demonstrated by spectroscopy (time-resolved EPR, laser flash photolysis (LFP), photobleaching (UV−vis)) and
photopolymerization experiments (photo-DSC measurements).
INTRODUCTION
and methyl radicals, which were trapped by unreacted
■
51
benzophenone.
Benzophenone and related derivatives are frequently applied
In organic as well as in inorganic chemistry, alkoxides of alkali
metals are widely used as powerful bases. Recently, these
alkoxides were also found to be involved in a wide variety of
reactions where single electron transfer reactions take
52
as photoinitiators for UV-curing of inks and coatings. In this
context, the keto-derivatives of main group IV organo-
metalloids (mainly germanium-based) have also attracted
considerable attention as visible light induced photoinitia-
1
−50
51
place.
In his landmark work, Murphy et al. explained
the controversial electron transfer from KOtBu to benzophe-
none (compare Scheme 1). They showed that KOtBu and
5
3−58
tors.
To date, di- and tetraacylgermanes are well
established (see Chart 1), although both compound classes
Scheme 1. Photochemical Induced SET of Benzophenone
have significant drawbacks.
with KOtBu
The synthetic pathway for all commercially available
diacylgermanes (e.g., Ivocerin) relies on a multistep synthesis
(
based on the Corey−Seebach reactions), which results in a
complex purification process. Tetraacylgermanes, on the other
hand, have a low solubility in various monomers, which
excludes them from several applications. Therefore, these
systems cannot fully meet the requirements for photoinitiators
Received: July 21, 2020
benzophenone form a metastable complex, which absorbs light
above 400 nm. The light-induced activation results in an
electron transfer from the alkoxide to benzophenone, which
leads to the formation of a radical anion. Subsequently, the
tert-butoxy radicals undergo a fragmentation to form acetone
©
XXXX American Chemical Society
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A
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Chart 1. State-of-the-Art Germanium-Based Photoinitiators
assignment are given in the Experimental Section, together
with experimental details.
59
The reduction potential of 1 (−1.57 V vs SCE in DMF )
falls well in line with the reduction potentials observed for
other tetraacylgermanes, for which radical anions could be
59
detected by EPR spectroscopy. Therefore, we investigated a
THF solution of 1 after exposure to metallic potassium by EPR
spectroscopy and could obtain an EPR spectrum (SI, Figure
S14). The unresolved EPR signal (g = 2.0039) is likely caused
by the formation of tight ion pairs between the ketyl-type
in high-throughput polymer synthesis. To overcome the above-
mentioned restrictions, we further studied reactivities of
acylgermanes to develop new pathways toward oligoacylger-
manes.
Recently, we investigated the redox chemistry of acylger-
manes and discovered their “dark side.” Upon reduction with
metallic potassium under inert conditions, the corresponding
acylgermane radical anions are formed (see Scheme 2), which
39
radical anions and the magnetically active K nuclei (I = 3/2;
60
natural abundance 93.3%) in ethereal solvents (like THF).
39
Generally, the interaction of a K nucleus in an ion pair
between a radical anion and a K counterion leads to four
+
equidistant lines (1:1:1:1) with a very small hyperfine coupling
constant. Accordingly, these lines only rarely appear resolved
in the EPR spectra and, in most cases, lead to line broadening.
This is in accordance with observations on the related radical
anions of TPO ((2,4,6-trimethylbenzoyl)diphenylphosphane
oxide) and BAPO (2,4,6-bis(trimethylbenzoyl)-
phenylphosphane oxide). Whereas electrochemically (in
CH CN with Bu NClO as supporting electrolyte) generated
Scheme 2. Alkali Metal Reduction of Acylgermanes to the
Corresponding Radical Anions
3
4
4
+
radical anions where the Bu N counterions have no close
4
interaction with the radical anion revealed well-resolved
spectra, reduction with K metal in THF leads to severely
+
61
broadened lines caused by tight ion-pair formation with K .
Electron Transfer Reaction Mechanism. On the basis of
the above-mentioned observations, Scheme 3 presents a
were characterized by EPR spectroscopy in combination with
59
0
DFT calculations. While monoacylgermanes display similar
reduction potentials as benzaldehyde derivatives, the di- and
tetraacylgermanes can be reduced at less negative potentials
due to the presence of more electron-withdrawing acyl
Scheme 3. SET Reaction of 1 with K (Method 1)
5
9
groups.
On the basis of the accessibility of acylgermanium based
radical anions and taking into account the seminal work of
Murphy and co-workers, merging the chemistry of strong bases
and electron transfer, we now explore whether the reduction of
acylgermanes provides synthetically attractive reaction path-
ways.
Our results show a novel aspect of acylgermane chemistry.
The aim of this work is to demonstrate that triacylgermeno-
lates can serve as new building blocks for the formation of high
performance group-14-based photoinitiators. Additionally, this
pathway significantly expands the number of available
acylgermanes. Finally, the performance of these photoinitiators
was evaluated by spectroscopy (time-resolved EPR), laser flash
photolysis (LFP), photobleaching (UV−vis), and photo-
polymerization experiments (photo-DSC measurements).
plausible single electron transfer (SET) mechanism for the
formation of 2 with the use of potassium as a reduction agent.
In the initial reaction step, the radical anion is formed via an
SET reaction. This radical anion undergoes an acyloin
condensation and after elimination of the diketone, the
germenolate 2 is formed. A second SET reaction probably
leads to the formation of a diradical, which undergoes a
complex and uncharacterized degradation sequence.
In order to prove our mechanism, a trapping experiment
with MeI was performed. The reaction solution was quenched
with an excess of MeI. The expected triacylgermane 3 was
obtained in a very selective fashion alongside uncharacterisable
degradation products deriving from the diketone intermediate
(Scheme 4).
RESULTS AND DISCUSSION
Reaction of 1 with K (Method 1). Our first task was to
■
0
explore the reactivity of 1 with potassium. Therefore, 1 was
dissolved in THF, and solid freshly cleaned potassium was
added. The surface of the potassium became immediately
reddish, which indicates the beginning of the reaction. After
approximately 5 min, the yellow reaction solution became
orange. The full consumption of the potassium marks the end
of the reaction, which results in the formation of hitherto
unknown triacylgermenolate 2. This germenolate was obtained
with remarkable selectivity. The obtained THF solution could
be stored for prolonged periods (usually days). Analytical and
spectroscopic data that strongly support the structural
Reaction of 1 with KOtBu (Method 2). Following the
work of Murphy et al., we explored the reactivity of 1 with
KOtBu. On the one hand KOtBu is, in comparison to
potassium, significantly easier to handle and, on the other
hand, a more selective reducing reagent than potassium.
Therefore, 1 was dissolved in C D in the presence of [18]-
6
6
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Scheme 4. Reaction of 2 with MeI
Scheme 6. Direct Approach toward 2 (Method 3)
crown-6, and solid KOtBu was added. The reaction solution
became immediately reddish, which indicates the beginning of
the reaction. After approximately 60 min, the mixture became
clear, which indicates the full consumption of the KOtBu. This
marks the end of the reaction and again the selective formation
of 2 alongside the corresponding ester. Also, this obtained
solution could be stored for prolonged periods (usually days).
Analytical and spectroscopic data that strongly support the
structural assignment are given in the Experimental Section,
together with experimental details.
X-ray Crystallography of 2. The structure of 2 was
confirmed by single-crystal X-ray diffraction analysis (compare
Figure 1 and 2). Figure 1 depicts the standard structure of a
We also obtained a well-resolved EPR spectrum for the
mixture of 1, [18]-crown-6, and KOtBu in C D at room
6
6
temperature (g = 2.0047), which corroborates the formation of
•
tBuO radicals (see SI, Figure S15 and the corresponding
discussion below the caption). On the basis of the results from
the EPR investigation, we propose a single electron transfer
reaction, which allows straightforward access to a radical anion
and a tert-butoxy radical. Subsequently, these two radicals
undergo a hemiketal formation. After elimination of the tert-
butyl-2,4,6-trimethylbenzoate, the germenolate 2 is formed in
excellent yields (see Scheme 5).
Scheme 5. SET Reaction of 1 with KOtBu (Method 2)
Figure 1. ORTEP representation for compound 2-18-c-6 (1:1
adducts with [18]crown-6). Thermal ellipsoids are depicted at the
5
0% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and bond angles (deg) with estimated
standard deviations: ∑αGe(1) 310.34, Ge(1)−C(1) 2.0137 (12),
Ge(1)−C(2) 2.0161 (12), Ge(1)−C(3) 2.0279 (12), C(1)−O(1)
1
.2311 (15), C(2)−O(2) 1.2298 (15), C(3)−O(3) 1.2259 (15),
K(1)−O(1) 2.7562 (10), K(1)−O(2) 2.7052 (10), K(1)−O(3)
6
.289 (10), K(1)−Ge(1) 3.615 (4).
heavier group 14 enolate as a 1:1 adduct of [18]-crown-6.
Compound 2-18-c-6 crystallized in the triclinic space group
Direct Approach toward 2 (Method 3). As published
previously, KOtBu is also of central importance for the
̅
P1, and the unit cell contains two molecules. Figure 2 shows
the first undistorted structure of a heavier group 14 enolate.
Compound 2 crystallized in the monoclinic space group C2/c,
and the unit cell contains eight molecules. As for all other
54
formation of tetraacylgermane 1. During the synthesis of 1,
this strong base cleaves all silicon−germanium bonds in order
to form the desired product 1. On the basis of our new
observations that KOtBu also reacts with 1 to form 2, we
reinvestigated the formation of 1. Indeed, we found that during
the synthetic protocol toward 1 (reaction of 4K with 4 molar
62−64
structural characterized germenolates,
the central Ge
atoms of 2-18-c-6 and 2 are pyramidal and have elongated
Ge−C single bonds. Particularly striking are two structural
features in the structures of 2 with respect to the relative
orientation of the three carbonyl groups as well as the distance
between the anionic and cationic centers in the molecule. In
the case of the 2-18-c-6 derivative, the crown ether is
successfully saturating the metal coordination site of the
potassium in the equatorial plane, forcing two carbonyl groups
to approach the metal center from the axial position. However,
in the case of the smaller dimethoxyethane, which is bidentate
and considerably less sterically encumbering than the crown
ether, the potassium metal is less coordinative saturated and
essentially all the carbonyl groups orient themselves in order to
54
equiv of mesitoylfluoride) and prolonged stirring of this
mixture (more than 2 days at room temperature), the
formation of small amounts of 2 was observed. A complete
conversion of 1 to 2 was not detected. Therefore, we set out
and reacted the potassium germanide 4K with 3.0 molar equiv
of the mesitoylfluoride, which gave rise to the quantitative
formation of 2 within 60 min (see Scheme 6). This direct
approach presents a facile synthesis toward 2. Due to the
quantitative formation of 2, the reaction solution can be
directly derivatized with electrophiles.
C
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Figure 2. ORTEP representation for compound 2. Thermal ellipsoids
are depicted at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and bond angles (deg) with
estimated standard deviations: ∑αGe(1) 309.13, Ge(1)−C(1) 2.017
(
3), Ge(1)−C(11) 2.016 (3), Ge(1)−C(21) 2.011 (3), C(1)−O(1)
1
.234 (3), C(11)−O(2) 1.229 (4), C(21)−O(3) 1.232 (3), K(1)−
O(1) 2.605 (2), K(1)−O(2) 2.616 (2), K(1)−O(3) 2.648 (2),
K(1)−Ge(1) 4.397 (7).
satisfy the coordination sphere of potassium. While the
distance between the central germanium atom and the
potassium atom in 2-18-c-6 is well in line with all other
62,63
structural characterized germenolates,
the distance of the
corresponding atoms in 2 is significantly longer (compare
Figures 1 and 2). We assume that this is due to the
coordination sphere described above.
Stability of 2. Compound 2 can be stored in the absence of
air at room temperature for several months without detection
of any degradation products. Furthermore, 2 can be completely
vacuum-dried. This is in stark contrast to the vacuum
64
instability of all other known heavier group 14 enolates,
which undergo degradation reactions during this process. We
assume that the remaining acyl groups significantly stabilize the
negative charge due to hyperconjugation.
UV−vis Spectroscopy of 2. THF was used as a solvent to
−
4
Figure 3. Measured UV−vis spectra of 2 in THF (5 × 10 mol/L;
top) and calculated absorption spectrum for compound 2 with two
determine the charge transfer behavior for the longest
65
wavelength absorption bands. Figure 3 depicts the measured
and calculated UV−vis spectra in THF together with their
calculated frontier Kohn−Sham orbitals for the HOMO and
LUMO orbitals. The germenolate 2 exhibits two intense
absorption bands with λmax = 427 nm (band I) and 353 nm
+
K ’s as counterions. The vertical transitions are marked as vertical
lines with their respective oscillator strengths (right axis). The orbitals
involved in the first bands are presented.
(
band II). Qualitative agreement between calculated and
experimental absorption maxima could be achieved for both
bands. In band I, consisting of the first two excitations, the S1
transition is assigned mainly to the HOMO−LUMO
examples of electrophiles. As suitable electrophiles, we choose
aryl- as well as alkyl-substituted di- and triacyl chlorides and
two bromo-alkanes. All these reactions resulted in the clean
formation of a variety of new oligoacylgermanes in good to
excellent yields (compare Scheme 7). These new compounds
represent promising high performance photoinitiators. Ana-
lytical and spectroscopic data obtained for 4a−4f are
consistent with the proposed structures. NMR spectra and
detailed assignments are provided in the Experimental Section
and in the Supporting Information. All derivatives show very
similar C chemical shifts for the carbonyl C atom between δ
= 219.5 and 237.6 ppm, which is characteristic for carbonyl
groups directly linked to a germanium atom. As method 3
circumvents the usage of [18]-crown-6 and furthermore gives
excitation. The HOMO mainly corresponds to the p orbital
z
of the germanium atom with little variation in shape and
energy. Upon excitation, electron density is displaced into the
π* orbital of the carbonyl moieties (LUMO). The next
transitions (S2−S5) consist of carbonyl π−π* excitations from
HOMO−2, HOMO−1, and HOMO into LUMO, LUMO+1,
LUMO+2 orbitals with different mixing (see the transition
density, the detailed orbital pictures and the interpretation of
the spectrum in the Supporting Information).
1
3
Reactivity of 2 with Selected Electrophiles. In order to
test our new precursor system, we reacted 2 with selected
D
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Scheme 7. Reaction of 2 with Selected Electrophiles
Figure 4. ORTEP representation for compound 4d. Thermal
ellipsoids are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å):
dGe−C(mean) = 2.028, dC−O(mean) = 1.211.
UV/vis Spectroscopy of 4a−4f. To further elucidate
substituent effects on the absorption behavior of these new
oligoacylgermanes, UV−vis absorption spectra of 4a−4f were
recorded (compare Figure 5). As previously shown for this
compound class, the absorption behavior is strongly influenced
higher yields, this method was used for our reactions with
electrophiles.
Crystals suitable for single-crystal X-ray diffraction analysis
were obtained for compounds 4b, 4d, and 4e. As a
representative example, the molecular structure of 4d is
depicted in Figures 4. All other structures are included in the
Supporting Information. Structural data are in accordance with
54,62,63,66
literature values of other acyl germanium compounds.
A structural feature, which significantly influences the optical
properties, is the torsion angle between the CO groups and
the aromatic ring planes. As a representative example, we
discuss the structure of 4d, which has two substituted carbonyl
groups. The six mesityl substituted carbonyl groups have a
torsion angle of 57.70°. The two 1,4-phenyl substituted
carbonyl groups adopt an angle of 20.94°. The smaller the
angle, the more pronounced the conjugation between the
aromatic ring and the carbonyl group that is possible, and this
Figure 5. (a) UV/vis absorption spectra of 4a−4f; dichloromethane
−
4
solution, c = 5 × 10 mol/L, and (b) expanded tails of n/σ−π*
results in a bathochromic shift of λmax
.
transition bands.
E
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by the substitution pattern at the aromatic ring system. In
54
comparison to the tetraacylgermane 1 and the triacylgermane
4a, all oligoacylgermanes 4b−4f showed a significant higher
extinction coefficient, which can be attributed to the higher
number of chromophores in the molecule. Moreover, the
oligoacylgermanes 4d−4f, where the two triacylgermyl
moieties are connected via a dicarbonyl−phenyl bridge, display
a substantial red-shift of the absorption edge. The para-
substitution of compound 4d induces the bathochromic shifted
absorption edge, which shows absorption up to 525 nm (see
Table 1). In contrast, the traditional phosphorus-based
photoinitiators TPO and BAPO do not absorb above 420
66
and 440 nm, respectively.
Table 1. Experimental Wavelength Absorption Maxima λ
−
1
−1
[
4
nm], Extinction Coefficients ε [L·mol ·cm ] for 1 and
a−4f (Dichloromethane)
λmax,exp [nm] (n/σ−π*(CO/aryl))
375, 408
ε [L·mol−1·cm−1
]
1
4
4
4
4
4
4
1485, 938
1303, 938
2628, 1982
3464, 1960
5252, 1645
4328, 3642
5142, 1776
a
b
c
d
e
f
382, 402
382, 400
363, 400
377, 434sh
379, 394sh
381, 434sh
Characterization of Oligoacylgermane Photoinitia-
tors. In the following paragraphs, the photochemical perform-
ance of the newly synthesized oligoacylgermanes is inves-
tigated. Compound 4a has been already characterized in a
66
previous publication, and the low solubility of compound 4c
prevents the further characterization of this compound.
Detection of Primary Radicals. Laser-flash photolysis (λ
=
355 nm) of the compounds 4b and 4d−4f (4 mM in
toluene) yields TR-EPR (time-resolved electron paramagnetic
resonance) spectra, which are compared to the TR-EPR
spectrum of tetramesitoylgermane 1. In all five spectra, the
Figure 6. (a) TR-EPR spectrum of 4b (4 mM in toluene) in the time
span of 500 to 1300 ns (black). Red: simulation of the spectrum. The
black bars indicate the splitting due to the protons marked in red. (b)
Full time profile of the spectrum.
•
typical unresolved signal from the mesitoyl radical M (g =
2
.000) is present (see Figure 6 and Supporting Information).
The TR-EPR spectrum of 4b shows additional peaks, which
can be attributed to the germyl radical featuring a CH group
2
adjacent to the Ge center (triplet (1:2:1), a = 0.53 mT, g =
in the area around 350 nm and also between 400 and 500 nm.
This latter absorption is characteristic for germyl radi-
2
.003) as shown in Figure 6. This value is in good agreement
with the hyperfine coupling constants observed for the related
germyl radicals originating from 4a (triplet, a = 0.44 mT) and
bis(p-methoxybenzoyl)diethylgermane (BMDG, Ivocerin),
53,66,68
cals.
Absorptions of the mesitoyl radical are centered
in the UV range and therefore do not interfere with the germyl
69,70
66
radicals.
which shows a quintet (two CH groups, a = 0.5 mT).
2
•
Following an exponential fitting to the decay curves of the
transient optical absorptions (at the maxima), the pseudo-first-
order rate constants kexp were obtained. The second-order
addition rate constant kmonomer was then determined by using
Most of the spin density in the germyl radical Ge(4b) is found
at the Ge center, as DFT calculations show (see Figure S32
and the corresponding discussion in Supporting Information).
In the case of 1 and 4d−4f with no protons adjacent to the
germanium center, relatively narrow and unresolved peaks are
expected for the germyl radicals. Accordingly, the intensive
EPR signal detected upon photolysis of 1 and 4d−4f
represents a superposition of the benzoyl and germyl radical.
Kinetics of the Addition to Double Bonds. Laser-flash
the following relation, where k is the rate constant for the
0
53,71
decay in the absence of a monomer:
kexp = k + kmonomer·c
0
monomer
(1)
The linear fits are displayed in the Supporting Information and
the obtained values for kmonomer are shown in a bar chart in
Figure 7. The numerical values are listed in Table S2
(Supporting Information). For 1, which serves as a reference,
the obtained value in this work (in toluene) is lower than the
6
7
photolysis (λ = 355 nm) was used for observing the short-
lived germyl radicals and determining the kinetics of their
addition to double bonds (of styrene and butyl acrylate) using
a pseudo-first-order analysis. At first, the transient optical
absorption spectra were acquired for 4b and 4d−4f (see
Supporting Information). The spectra show absorption bands
66
value in acetonitrile. This is in agreement with observations
53
made by Neshchadin et al. who found that the addition rate
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bisacylgermane BMDG (Ivocerin) in a mixture of toluene
and methyl methacrylate (MMA) were photolyzed with LEDs.
The use of MMA and the additional deoxygenation of the
samples (with argon) have the purpose of suppressing the
formation of undesired (colored) photoproducts, which
6
6
interfere with the UV−vis measurements.
First, we photolyzed the solutions with an LED emitting at
85 nm (LED385, see Experimental Section for details), since
3
all the studied compounds absorb strongly at this wavelength
see Figure 5). To ensure good comparability, the concen-
(
trations of the compounds were adjusted so that A385 ≈ 0.7.
The results (normalized absorbance as a function of time) are
shown in Figure 8a. All compounds show nearly identical time
Figure 7. Bar graph of the second-order addition rate constants for
•
•
•
the germyl radicals Ge(1) , Ge(4b) , and Ge(4d−4f) to the
monomers styrene (green bars) and butyl acrylate (red bars) and their
respective structures.
•
for Et BzGe to butyl acrylate is slightly higher in acetonitrile
2
than in toluene.
The rate constants for the addition of the germyl radicals to
the electron-poor double bond of butyl acrylate decrease in the
•
•
•
•
following order: Ge(4b) ≈ Ge(4d) > Ge(4e) > Ge(1) >
•
Ge(4f) . This is in agreement with observations made in a
66
previous publications by our groups. The more sterically
congested the Ge center, the lower is the reactivity toward
double bonds (e.g., in Table S2 in the Supporting Information,
•
•
compare Ge(1) bearing three mesitoyl groups with Ge(4a)
featuring two mesitoyl groups and one ethyl group). Ge(4b)•
comprises an alkyl substituent (or bridge) at the Ge center
•
(
comparable to Ge(4a) ) resulting in an increased reactivity
•
compared to Ge(1) . Replacing one of the mesitoyl groups
with a para-dicarbonyl benzene bridge (Ge(4d) ) also leads to
•
a higher addition rate constant, since the bridge is sterically less
demanding than a mesitoyl group. This effect is only slightly
•
pronounced in Ge(4e) , where the carbonyl groups in the
bridge are in a meta position to each other. The lowest rate is
•
seen for Ge(4f) , which is the heaviest and bulkiest germyl
radical in this row.
6
6
Previous investigations indicate that germyl radicals
derived from mono- to tetraacylgermanes prefer electron-rich
double bonds (e.g., higher rate constants for the addition to
methyl methacrylate compared to butyl acrylate). To inspect
whether this trend is also evident for the oligoacylgermanes, we
choose the electron-rich double bond of styrene as a second
reaction partner. Indeed, the addition rate constants increase
•
Figure 8. Steady-state photolysis of BMDG and 1, 4b, and 4d−4f
with (a) LED385, (b) LED470 in toluene/MMA (1:1 v/v). The
absorbance traces are normalized to the initial absorptions at the
observation wavelengths (maxima of n/σ−π* transitions; BMDG,
for all investigated germyl radicals. In the case of Ge(4d) , a
multiplication by a factor of 7 is observed, compared to the
other germyl radicals, where the factor is between 3.5 and 4.
The higher addition rate constant toward styrene suggests that
the germyl radicals show electrophilic behaviora feature
which they share with phosphanoyl radicals derived from
4
12.0 nm; 1, 378.0 nm; 4b, 384.0 nm; 4d, 379.5 nm; 4e, 380.5 nm;
4f, 381.0 nm).
72
bis(acyl)phosphane oxide photoinitiators. Accordingly, their
addition rate constants toward butyl acrylate are on the same
order of magnitude as those of TPO and BAPO (see Table S2
traces when irradiating with LED385, except for the para-
substituted digermane 4d, whose absorbance decays much
slower than the absorbances of the other compounds, so that a
slight yellow color is still visible after 600 s.
71
in the Supporting Information).
Photobleaching and Quantum Yields. To avoid colored
polymers and reach high curing depths, efficient photo-
bleaching of the photoinitiator is desirable. Solutions of
compounds 1, 4b, 4d−4f and the well-characterized
Since compounds 4d−4f feature absorbance peaks with
considerable tailings up to 500 nm, we also photolyzed
solutions of the investigated compounds with an LED emitting
66
G
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Article
at 470 nm (LED470; mimicking the longest wavelength
emission of dental lamps, see Experimental Section for
details). The results are shown in Figure 8b4e and 4f
bleach most efficiently, followed by BMDG and 4d.
Compounds 1 and 4b do not show substantial absorbance at
enthalpy ΔH (peak area), providing that the theoretical heat of
polymerization (ΔH ) is known.
0
,p
7
3
Photo-DSC has been chosen to evaluate the performance of
the synthesized photoinitiators in 1,6-hexanediol diacrylate
(HDDA) as model monomer system. The previously reported
tetraacylgermane 1 was investigated as a reference com-
4
70 nm, hence a slow decay is observed. The UV−vis spectra,
54
which were acquired while irradiating the samples with the
LEDs, are found in the Supporting Information.
pound.
The PI performance was compared at equal molar PI
concentrations (0.30 mol %) as well as equal amounts of
photocleavable groups (0.15 mol % for 1; 0.2 mol % for 4a; 0.1
mol % for 4b; 0.075 mol % for 4d and 4e; 0.05 mol % for 4f;
see Figure 9). All investigated PIs show nearly identical
From the data obtained in the photolysis experiments with
LED385, the decomposition quantum yields were determined
74
following the procedure by Stadler et al. For each compound,
the absorbance trace obtained during irradiation with LED385
was converted to a concentration trace using the extinction
coefficients given in Table 2 and then fitted with a
Table 2. Wavelength of n/σ−π* Absorption Maxima and
Extinction Coefficients at λ
(in Toluene/MMA 1/1 (v/
max,exp
v)), and Determined Quantum Yields for BMDG, 1, 4b, and
4d−4f
1
λmax,exp[nm]
ε [M− cm−1] at λmax,exp
Φ (385 nm)
0.86 ± 0.02
0.83 ± 0.01 )
BMDG
412.0
1001
a
(
1
378.0
384.0
379.5
380.5
381.0
1839
2280
3879
3209
5035
0.38 ± 0.01
0.27 ± 0.01
0.08 ± 0.01
0.23 ± 0.01
0.13 ± 0.01
4b
4d
4e
4f
a
66
value in acetonitrile/MMA 1/1 (v/v) from ref.
monoexponential function (y = y + A ekfitx) as shown in the
Figure 9. Photo-DSC (left) and conversion plots (right) for the
photopolymerization of HDDA with 0.3 mol % PI (top) and PI
concentration with equimolar amounts of photocleavable groups
(bottom).
0
Supporting Information. The product of the time constant k
fit
−1
(
in s ) and the initial concentration of the compound c (in
0
−
1
mol L ) are then divided by the product of the photon flux I
0
−
5
−1 −1
(
LED385 operated at 45 mA; I = 1.7 × 10 mol L s ) and
0
−A
i
the factor (1−10 ′), where A ′ is the absorbance at the peak
i
behavior for both measurement series. This indicates that the
use of lower molar concentrations is possible without negative
effects on the observed photoreactivity. Only at very low
concentrations, we see a reduction in conversion speed for 1,
wavelength of the LED (387 nm, see Experimental Section), to
74
obtain the quantum yield:
k c
fit 0
I0(1 − 10−
ϕ =
Ai′
4a, and 4b as shown in Table S4. Compound 4d exhibits a
)
(2)
slightly lower DBC in both measurement series, which is
consistent with the reduced photobleaching ability and low
quantum yield (see Figure 8a and Table 2).
The calculated quantum yields are shown in Table 2. For
BMDG, the obtained quantum yield is almost identical to the
66
value in acetonitrile/MMA. The second highest quantum
CONCLUSION
yield is found for 1, which is well in line with values found for
■
66
other tetraacylgermane compounds (ϕ = 0.34−0.44; in
acetonitrile/MMA). For the oligoacylgermanes 4b and 4d−4f,
the determined quantum yields range from 0.08 (4d) to 0.27
In summary, we investigated the single electron transfer
reactions of tetraacylgermanes induced by potassium or
KOtBu. We detected the formation of radicals (by EPR),
which act as intermediates for the formation of a stable
triacylgermenolate 2. The one-pot synthetic protocol gives 2 at
an excellent yield, as confirmed by NMR spectroscopy, UV−
vis measurements, and X-ray crystallography. Furthermore, we
could determine the crystal structure of 2, which represents the
first undistorted structure of a heavier group 14 enolate. The
efficiency of 2 to serve as new germanium centered precursor
system is demonstrated by the reactions with a variety of
electrophiles. In all cases, the salt metathesis reaction gave rise
to novel acylgermanes in excellent yields. These new
acylgermanes were examined for application as high perform-
ance photoinitiators. In comparison to 1, 4b−4f show a
significant bathochromic shift of the absorption edge.
(4b). Compared to BMDG, the quantum yields of the
oligoacylgermanes are comparably smaller; however, the high
extinction coefficients of the n/σ−π* transitions compensate
for the low quantum yields giving rise to an effective
photobleaching.
Photo-DSC measurements. Photo-DSC is a versatile
method for the fast and accurate evaluation of the performance
of PIs in polymerizable resins. Various characteristic kinetic
parameters can be obtained with one single measurement
including the time to reach the maximum heat flow (tmax),
maximum rate of polymerization (Rp,max), and time to reach
9
5% of final conversion (t95%). Furthermore, the double bond
conversion (DBC) can be calculated from the overall reaction
H
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Article
1
Furthermore, 4b−4f also outperform 1 in terms of extinctions
coefficients, resulting in a possible lower consumption of these
high priced photoinitiators with the same performance. The
data obtained from TR-EPR spectroscopy clearly show that the
newly synthesized oligoacylgermanes 4b and 4d−4f undergo
bond cleavage upon irradiation with UV light (λ = 355 nm)
forming mesitoyl and germyl radicals. The germyl radicals
C); 70.30 (−CH
2
−CH
2
−O−) ; 21.31, 20.45 (Aryl-CH ). H NMR
6 3
(
C D , TMS, ppm): 6.61 (s, 6H, Mes-H); 3.23 (s, 24H, (−CH −
6
6
2
13
CH −O−) ); 2.49 (s, 18H, Mes−CH ); 2.19 (s, 9H, Mes−CH ).
C
2
6
3
3
NMR (THF-d , TMS, ppm): 263.14 (CO); 148.62, 135.41, 131.77,
8
1
1
28.60 (Aryl-C); 21.44, 20.46 (Aryl-CH ). H NMR (THF-d , TMS,
3
8
ppm): 6.41 (s, 6H, Mes-H), 2.16 (s, 9H, Mes−CH ), 2.05 (s, 18H,
Mes−CH ). UV−vis, λ [nm] (ε [L mol cm ]): 427 (3454), 353
3
−1
−1
3
(3030). IR (neat), ν(CO): 1604, 1590, 1555, 1535.
•
•
•
0
Ge(4b) , Ge(4d) , and Ge(4e) show higher rate constants
Reaction of 1 with K and Subsequent Quenching with MeI.
A vial was charged with 240 mg (0.363 mmol) of 1a and 29.8 mg
(0.762 mmol) of K . A total of 4 mL of THF was added. The reaction
mixture was stirred until the solid K was completely consumed.
During this process, the color of the reaction mixture changed from
yellow to red. At this time, reaction control by NMR spectroscopy
showed the conversion of the starting material to the triacylgerme-
nolate 2. Subsequently, the reaction solution was quenched with an
excess of MeI. The reaction solution became immediately yellow.
After an aqueous work up with 10 mL of a saturated ammonium
chloride solution, the organic layer was separated and dried over
Na SO , and the solvents were stripped off with a rotary evaporator.
The reaction mixture was then chromatographed on silica gel using
ethyl acetate/n-heptane 1:20 as a mobile phase to give 157 mg (82%)
of the triacylgermane 3 as yellow oil.
Compound 3. Anal. Calcd for C H GeO : C, 70.35; H, 9.07%.
Found: C, 70.42; H, 9.12%. C NMR (CDCl , TMS, ppm): 237.14
for the addition to butyl acrylate when compared to the
•
0
“
lighter relative” Ge(1) . When using styrene with its electron-
0
rich double bond as a reaction partner, the rate constants are
•
•
increased even more. Especially, Ge(4d) as well as Ge(4b)
•
are more reactive toward styrene than Ge(1) . Photolysis
experiments show that all compounds except 4d, which is
characterized by a very small quantum yield of decomposition
(
ϕ = 0.08), bleach efficiently when irradiated with UV light
from an LED (emission maximum 387 nm). Due to the
substantial absorption around 500 nm, 4e and 4f show
considerable photobleaching upon illumination with an LED
emitting at about 470 nm. The reaction rates and DBC for 4a,
2
4
4
b, 4e, and 4f are comparable to those of 1. Notably, 4e and 4f
3
1
36
3
show consistently high photoreactivity even at very low PI
concentrations. On the basis of the photochemical character-
ization of the compounds, we consider 4e and 4f as the most
promising candidates for the use as photoinitiators. Further
studies to probe the scope of this new synthetic pathway are
currently in progress. In addition, we are currently testing these
new type I photoinitiators for visible light induced photo-
polymerization because they have the substantial advantage of
bleaching, yielding colorless products.
1
3
3
(
CO); 141.74, 139.74 132.53, 128,85 (Aryl-C); 21.25, 19.08 (Aryl-
1
CH ); −1.57 (Ge-CH ). H NMR (CDCl , TMS, ppm): 6.71 (s, 6H,
3
3
3
Mes-H); 2.25 (s, 9H, Mes−CH ); 2.06 (s, 18H, Mes−CH ); 0.66 (s,
3
3
−
1
−1
3
H, Ge−CH ). UV−vis, λ [nm] (ε [L mol cm ]): 382 (1290), 400
3
(970). IR (neat) ν(CO): 1671, 1639, 1602, 1599, 1574.
Reaction of 1 with KOtBu. A vial was charged with 120 mg
(0.181 mmol) of 1, 22.4 mg (0.200 mmol) of KOtBu, and 52.7 mg
(
0.200 mmol) of [18]-crown-6. Subsequently, 3 mL of C D was
added. During addition of the C D , the color of the reaction mixture
6 6
6
6
changed from yellow to red. Stirring of the red solution was continued
for approximately 1 h. At this time, reaction control by NMR
spectroscopy showed the conversion of the starting material to
triacylgermenolate 2 and tert-butyl 2,4,6-trimethylbenzoate. In order
to obtain crystals for 2, the solvent was stripped off under a vacuum.
Recrystallization from DME afforded 134 mg (90%) of analytically
pure 2 as orange powder. After removal of the volatile components on
a rotary evaporator, the reaction mixture was chromatographed on a
precoated TLC plate SIL G-200 UV254 to give 33.2 mg (83%) of tert-
butyl-2,4,6-trimethylbenzoate as colorless oil.
EXPERIMENTAL SECTION
■
All experiments were performed under a nitrogen atmosphere using
standard Schlenk techniques. Solvents were dried using a column
75
solvent purification system. Me SiCl (95%), GeCl (>98%), KOtBu
3
4
(
(
(
>98%), ClCOMes (99%), ClCO(oTol) (98%), C H -1-4−(COCl)
6
4
2
>99%), C H -1-3−(COCl) (>99%), Br(CH ) Br (99%), MeI
6
4
2
2 4
>99%) and [18]-crown-6 (99%), toluene (≥99%), toluene-d (99
8
atom % D), THF-d (99.5 atom % D), CDCl (99.8 atom % D), butyl
8
3
acrylate (≥99%), styrene (≥99%), and methyl methacrylate (99%)
tert-Butyl-2,4,6-trimethylbenzoate. Anal. Calcd for C H O : C,
were used without any further purification. Tetrakis(trimethylsilyl)-
14 20
2
76
13
7
6.33; H, 9.15%. Found: C, 76.63; H, 9.33%. C NMR (CDCl3,
TMS, ppm): 169.49 (CO); 138.67, 134.43 132.66, 128.33 (Aryl-
C); 81.46 (−C(CH ) ); 28.31 (−C(CH ) ); 21.14, 19.54 (Aryl-CH ).
germane, tetraacylgermane 1 and FCOMes were prepared according
5
4
to published procedures.
1
H, C, and 29Si NMR spectra were recorded on either a Varian
13
3
3
3
3
3
1
INOVA 300 spectrometer in C D , THF-d , or CDCl solutions and
H NMR (CDCl
Mes−CH ); 2.30 (s, 3H, Mes−CH
(neat), ν(CO): 1717, 1612.
3
, TMS, ppm): 6.87 (s, 2H, Mes-H); 2.35 (s, 6H,
6
6
8
3
2
referenced versus TMS using the internal H-lock signal of the
solvent. HRMS spectra were run on a Kratos Profile mass
spectrometer. Infrared spectra were obtained on a Bruker Alpha-P
Diamond ATR Spectrometer from the solid sample. Melting points
were determined using a Stuart SMP50 apparatus and are
uncorrected. Elemental analyses were carried out on a Hanau Vario
Elementar EL apparatus. UV absorption spectra were recorded on a
PerkinElmer Lambda 5 spectrometer.
3
3
); 1.63 (s, 9H, −C(CH ). IR
)
3 3
Direct Approach to 2. A flask was charged with 500.0 mg (1.37
mmol) of tetrakis(trimethylsilyl)germane and 153.5 mg (1.37 mmol)
of KOtBu. A total of 10 mL of DME was added. The reaction mixture
was stirred for about 1 h. Subsequently, 227.4 mg (1.37 mmol; 1,0
equiv) of mesitoylfluoride was added to this solution and stirred for
another 10 min. This addition was repeated two times with the exact
same amount of mesitoylfluoride (in total 454.9 mg, 2.0 equiv). After
complete addition, the reaction was stirred for another 30 min. At this
time, the orange precipitate was filtered off from the reaction solution,
which was identified by NMR spectroscopy as triacylgermenolate 2
(note: 2 contains 0.5 molecules of DME). Yield: 862.9 mg (98%) of
analytically pure 2 as orange powder.
0
Reaction of 1 with K . A vial was charged with 120 mg (0.181
0
mmol) of 1 and 14.9 mg (0.381 mmol) of K . A total of 1 mL of
0
THF-d was added. The reaction mixture was stirred until the solid K
8
was completely consumed. During this process, the color of the
reaction mixture changed from yellow to red. At this time, reaction
control by NMR spectroscopy showed the conversion of the starting
material to the triacylgermenolate 2. In order to obtain crystals, 48 mg
Compound 2. mp: 154−156 °C. Anal. Calcd for C H GeKO : C,
3
2
38
4
1
3
(
0.381 mmol) of [18]-crown-6 was added. Subsequently, the solvent
64.23; H, 6.40%. Found: C, 64.32; H, 6.32%. C NMR (THF-d ,
8
was stripped off under a vacuum. Recrystallization from DME
afforded 119 mg (80%) of analytically pure 2 as orange powder.
Compound 2. mp: 165−168 °C. Anal. Calcd for C H GeKO :
TMS, ppm): 262.77 (CO); 148.62, 135.17, 131.63, 128.44 (Aryl-
C); 72.76 [CH −O−(CH )−] ; 58.95 [CH −O−(CH )−] ; 21.30,
3
2
2
3
2
2
1
20.33 (Aryl-CH ). H NMR (THF-d , TMS, ppm): 6.39 (s, 6H, Mes-
4
2
57
9
3
8
1
3
C, 61.70; H, 7.03%. Found: C, 61.82; H, 7.01%. C NMR (C D ,
H); 3.43 (s, 2H, [CH −O−(CH )−] ); 3.27 (s, 3H, [CH -O−
6
6
3
2
2
3
TMS, ppm): 261.30 (CO); 148.45, 134.94 131.48, 128.38 (Aryl-
(CH )−] ); 2.15 (s, 9H, Mes−CH ); 2.03 (s, 18H, Mes−CH ).
2
2
3
3
I
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Article
Synthesis of 4a. Compound 2 was prepared according to method
of toluene at −30 °C via a syringe. After complete addition, the
reaction was allowed to warm up to room temperature. At this time,
reaction control by NMR spectroscopy showed the conversion of the
starting material to 4d. After aqueous workup with 50 mL of saturated
NH Cl, the organic layer was separated and dried over Na SO and
3
with 3.00 g (8.21 mmol) of (Me Si) Ge, 1.01 g of KOtBu (9.03
3 4
mmol; 1.1 equiv), and 4.09 g (24.63 mmol; 3,0 equiv) of
mesitoylfluoride. Subsequently, this solution was added to 0.98 g
9.03 mmol, 1.10 equiv) of ethyl bromide dissolved in 60 mL of
(
4
2
4
toluene at −30 °C via a syringe. After complete addition, the reaction
was allowed to warm up to room temperature. At this time, reaction
control by NMR spectroscopy showed the conversion of the starting
filtered and the volatile components removed in vacuo. The crude
product was recrystallized from acetone yielding 3.91 g (82%) of
analytical pure 4d as yellow crystals.
material to 4a. After aqueous workup with 50 mL of saturated NH Cl,
Compound 4d. mp: 245−247 °C. Anal. Calcd for C H Ge O :
4
68 70
2
8
1
3
the organic layer was separated and dried over Na SO and filtered
C, 70.38; H, 6.08%. Found: C, 70.42; H, 6.15%. C NMR (CDCl ,
2
4
3
and the volatile components removed in vacuo. The crude product
was recrystallized from acetone, yielding 4.19 g (94%) of analytically
pure 4a as yellow crystals.
TMS, ppm): 231.61, 222.24 (CO), 141.94, 141.33, 140.12, 133.11,
1
128.87, 128.61 (Aryl-C), 21.23, 19.35 (Aryl-CH ). H NMR (CDCl ,
3
3
TMS, ppm): 7.52 (s, 4H, Aryl-H), 6.56 (s, 12H, Aryl-H), 2.15 (s,
18H, Aryl−CH ), 2.11 (s, 36H, Aryl−CH ). UV−vis, λ [nm] (ε [L
Compound 4a. mp: 100−103 °C. Anal. Calcd for C H GeO : C,
3
2
38
3
3
3
1
3
−1
−1
7
0.75; H, 7.05%. Found: C, 70.73; H, 7.07%. C NMR (CDCl ,
mol cm ]): 434sh (1645), 377 (5252). IR (neat), ν(CO): 1658,
3
TMS, ppm): 237.58 (CO); 142.22, 139.21, 132.51, 128.80 (Aryl-
1643, 1632, 1619, 1607.
C); 21.24, 19.07 (Aryl-CH ); 10.42 (GeCH −CH ), 8.76 (GeCH −
Synthesis of 4e. Compound 2 was prepared according to method
3 with 3.00 g (8.21 mmol) of (Me Si) Ge, 1.01 g of KOtBu (9.03
3
2
3
2
1
CH ). H NMR (CDCl , TMS, ppm): 6.68 (s, 6H, Mes-H); 2.25 (s,
3
3
3
4
9
H, Mes−CH ); 2.07 (s, 18H, Mes−CH ); 1.38 (q, 2H, Ge−CH −
mmol; 1.1 equiv), and 4.09 g (24.63 mmol; 3,0 equiv) of
mesitoylfluoride. Subsequently, this solution was added to 0.92 g
(4.51 mmol, 0.55 equiv) of isophthaloyl chloride dissolved in 60 mL
of toluene at −30 °C via a syringe. After complete addition, the
reaction was allowed to warm up to room temperature. At this time,
reaction control by NMR spectroscopy showed the conversion of the
starting material to 4e. After aqueous workup with 50 mL of saturated
NH Cl, the organic layer was separated and dried over Na SO and
3
3
2
−
1
CH ), 1.00 (t, 3H, Ge−CH −CH ). UV−vis, λ [nm] (ε [L mol
3
2
3
−
1
cm ]): 382 (1303), 402 (938). IR (neat), ν(CO): 1650, 1628,
1
608.
Synthesis of 4b. Compound 2 was prepared according to method
with 3.00 g (8.21 mmol) of (Me Si) Ge, 1.01 g of KOtBu (9.03
3
3
4
mmol; 1.1 equiv), and 4.09 g (24.63 mmol; 3.0 equiv) of
mesitoylfluoride. Subsequently, this solution was added to 0.98 g
4
2
4
(
4.51 mmol, 0.55 equiv) of 1,4-dibromobutane dissolved in 60 mL of
filtered and the volatile components removed in vacuo. The crude
product was recrystallized from acetone, yielding 4.05 g (85%) of
analytical pure 4e as yellow crystals.
toluene at −30 °C via a syringe. After complete addition, the reaction
was allowed to warm up to room temperature. At this time, reaction
control by NMR spectroscopy showed the conversion of the starting
Compound 4e. mp: 207−208 °C. Anal. Calcd for C H Ge O :
6
8
70
2
8
1
3
material to 4b. After aqueous workup with 50 mL of saturated NH Cl,
C, 70.38; H, 6.08%. Found: C, 70.50; H, 6.16%. C NMR (CDCl ,
4
3
the organic layer was separated and dried over Na SO and filtered
TMS, ppm): 231.80, 220.55 (CO), 141.36, 140.21, 139.96, 134.69,
2
4
1
and the volatile components removed in vacuo. The crude product
was recrystallized from n-pentane, yielding 3.34 g (75%) of analytical
pure 4b as yellow crystals.
133.11, 128.85, 125.41 (Aryl-C), 21.25, 19.37 (Aryl-CH ). H NMR
3
(CDCl , TMS, ppm): 7.90 (t, 1H, Aryl-H), 7.79−7.76 (dd, 2H, Aryl-
3
H), 7.05 (t, 1H, Aryl-H), 6.56 (s, 12H, Aryl-H), 2.17 (s, 18H, Aryl−
−
1
Compound 4b. mp: 180−181 °C. Anal. Calcd for C H Ge O :
CH ), 2.12 (s, 36H, Aryl−CH ). UV−vis, λ [nm] (ε [L mol
6
4
74
2
6
3
3
1
3
−1
C, 70.88; H, 6.88%. Found: C, 70.92; H, 6.87%. C NMR (CDCl ,
cm ]): 394sh (3642), 379 (4328). IR (neat), ν(CO): 1656, 1641,
3
TMS, ppm): 237.45 (CO), 142.16, 139.30, 132.49, 128.83, (Aryl-
1606, 1577.
1
C), 27.52, 16.75 (−(−CH ) −), 21.26, 19.10 (Aryl-CH ). H NMR
Synthesis of 4f. Compound 2 was prepared according to method
3 with 3.00 g (8.21 mmol) of (Me Si) Ge, 1.01 g of KOtBu (9.03
2
4
3
(
CDCl , TMS, ppm): 6.68 (s, 12H, Aryl-H), 2.26 (s, 18H, Mes−
3
3
4
CH ), 2.04 (s, 36H, Mes-CH ), 1.13 (bs, 8H, −(CH ) −). UV−vis, λ
mmol; 1.1 equiv), and 4.09 g (24.63 mmol; 3,0 equiv) of
mesitoylfluoride. Subsequently, this solution was added to 0.72 g
(2.71 mmol, 0.33 equiv) of benzene-1,3,5-tricarbonyl trichloride
dissolved in 60 mL of toluene at −30 °C via a syringe. After complete
addition, the reaction was allowed to warm up to room temperature.
At this time, reaction control by NMR spectroscopy showed the
conversion of the starting material to 4f. After an aqueous workup
3
3
2 4
−
1
−1
[
nm] (ε [L mol cm ]): 400 (1982), 382 (2628). IR (neat), ν(C
O): 1650, 1632, 1627, 1606.
Synthesis of 4c. Compound 2 was prepared according to method
3
with 3.00 g (8.21 mmol) of (Me Si) Ge, 1.01 g of KOtBu (9.03
3 4
mmol; 1.1 equiv) and 4.09 g (24.63 mmol; 3,0 equiv) of
mesitoylfluoride. Subsequently, this solution was added to 0.83 g
(
4.51 mmol, 0.55 equiv) of adipoyl chloride dissolved in 60 mL of
with 50 mL of saturated NH Cl, the organic layer was separated and
4
toluene at −30 °C via a syringe. After complete addition, the reaction
was allowed to warm up to room temperature. At this time, reaction
control by NMR spectroscopy showed the conversion of the starting
material to 4c. After an aqueous workup with 50 mL of saturated
NH Cl, the organic layer was separated and dried over Na SO and
dried over Na SO and filtered and the volatile components removed
2
4
in vacuo. The crude product was recrystallized from acetone, yielding
2.86g (62%) of analytical pure 4f as yellow crystals.
Compound 4f. mp: 189−192 °C. Anal. Calcd for C H Ge O :
9
9
102
3
12
1
3
C, 69.87; H, 6.04%. Found: C, 69.97; H, 6.16%. C NMR (CDCl3,
4
2
4
filtered and the volatile components removed in vacuo. The crude
product was recrystallized from n-pentane, yielding 3.98 g (85%) of
analytical pure 4c as yellow crystals.
TMS, ppm): 231.05, 219.52 (CO), 141.47, 140.67, 139.78, 133.19,
1
132.66, 128.94 (Aryl-C), 21.26, 19.41 (Aryl-CH ). H NMR (CDCl ,
3
3
TMS, ppm): 8.05 (s, 3H, Aryl-H), 6.55 (s, 18H, Aryl-H), 2.14 (s,
27H, Aryl−CH ), 2.12 (s, 54H, Aryl−CH ). UV−vis, λ [nm] (ε [L
Compound 4c. mp: 289−293 °C dec. Anal. Calcd for
3
3
1
3
−1
−1
C H Ge O : C, 69.50; H, 6.54%. Found: C, 69.58; H, 6.60%.
C
mol cm ]): 434sh (1776), 381 (5142). IR (neat), ν(CO): 1654,
66
74
2
8
NMR (CDCl , TMS, ppm): 232.32, 229.56 (CO), 141.75, 139.90,
1639, 1606.
3
1
32.75, 128.91, (Aryl-C), 51.22, 2.80 (−(−CH ) −) 21.29, 19.17
X-ray Crystallography. All crystals suitable for single crystal X-
ray diffractometry were removed from a vial or a Schlenk and
immediately covered with a layer of silicone oil. A single crystal was
selected, mounted on a glass rod on a copper pin, and placed in the
2
4
1
(Aryl-CH ). H NMR (CDCl , TMS, ppm): 6.67 (s, 12H, Aryl-H),
3
3
2
.24 (s, 18H, Mes−CH ), 2.09 (s, 36H, Mes-CH ), 1.03 (bs, 4H,
3
3
−1
−
(CH ) −). 0.21 (s, 4H, −(CH ) −). UV−vis, λ [nm] (ε [L mol
2
4
2 4
−
1
cm ]): 400 (1960), 363 (3464). IR (neat), ν(CO): 1668, 1654,
cold N stream provided by an Oxford Cryosystems cryostream. XRD
2
1644, 1629, 1606.
data collection was performed for compounds 2-18-c-6, 2, 4b, 4d, and
4e, on a Bruker APEX II diffractometer with the use of an IμS
microsource (Incoatec microfocus), a sealed tube of Mo Kα radiation
(λ = 0.71073 Å), and a CCD area detector. Data integration was
Synthesis of 4d. Compound 2 was prepared according to method
3
with 3.00 g (8.21 mmol) of (Me Si) Ge, 1.01 g of KOtBu (9.03
3
4
mmol; 1.1 equiv), and 4.09 g (24.63 mmol; 3,0 equiv) of
mesitoylfluoride. Subsequently, this solution was added to 0.92 g
77
carried out using SAINT. Empirical absorption corrections were
78,79
(4.51 mmol, 0.55 equiv) of terephthaloyl chloride dissolved in 60 mL
applied using SADABS.
The structures were solved with the use
J
https://dx.doi.org/10.1021/acs.inorgchem.0c02181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
8
0
of the intrinsic phasing option in SHELXT and refined by the full-
to concentration traces and fitted with a monoexponential function.
The setup is described in more detail in ref 74.
8
0−84
matrix least-squares procedures in SHELXL
the program SHELXLE. The space group assignments and
structural solutions were evaluated using PLATON.
of crystallization for compound 4b was removed from the refinement
by using the “squeeze” option available in the PLATON program
as implemented in
8
5
Computational Methods. All computational studies were
86−88
91
The solvent
executed with the program ORCA4.0.1. Density functional theory
92,93
(DFT) was applied using the B3LYP functional
with inclusion of
the dispersion interaction including Becke-Johnson damping
88,89
94,95
suite.
Disorder was handled by modeling the occupancies of the
individual orientations using free variables to refine the respective
(D3BJ).
The def2-SVP basis set was used for optimization of
the geometries, and their harmonic frequencies were calculated to
90
96
occupancy of the affected fragments (PART). In some cases, the
similarity SAME restraint, the similar-ADP restraint SIMU, and the
rigid-bond restraint DELU and RIGU, as well as the constraints EXYZ
and EADP, were used in modeling disorder to make the ADP values
of the disordered atoms more reasonable. Disordered positions for the
germanium atom and one of the mesitoyl moieties in compound 4b
were refined using 55/45 split positions. CCDC 1992490−1992494
contain the supplementary crystallographic data for compounds 2-18-
c-6, 2, 4b, 4d, and 4e, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif. Table S1 contains crystallographic
data and details of measurements and refinement for compounds 2-
confirm the geometries as minima at the potential energy surface.
The basis set def2-TZVPP was used for the computation of the
vertical excitations by applying time-dependent density functional
97
theory (TD-DFT) calculations with 20 states. All calculations were
performed in the solvent THF in the continuum solvation model
98
CPCM. The geometry of 2 was optimized with one and two
potassium counterions by starting from the X-ray conformation. For
+
the UV data, the electronic distribution with two K agreed best with
the experimental data. The UV spectra were simulated with a half
width at half height of 18 nm, and the molecular orbitals were
generated with contour values of 0.05 au, both with the program
99
Gabedit. The hyperfine coupling constants were computed applying
18-c-6, 2, 4b, 4d, and 4e.
the def2-TZVPP basis set with FinalGrid = 7, and the spin density
TR-EPR. Continuous-wave time-resolved EPR (TR-EPR) experi-
ments were performed on a Bruker ESP 300E X-band spectrometer
unmodulated static magnetic field) equipped with a 125 MHz dual
channel digital oscilloscope (Le Croy 9400). As the light source, the
frequency tripled light of a Nd:YAG laser was used (InnoLas Spitlight
00, 355 nm, operating at 20 Hz, ∼10 mJ per pulse, 8 ns). The setup
is controlled by the fsc2 software developed by Dr. J. T. Torring
Berlin). Spectra were recorded by acquiring the accumulated time
responses to the incident laser pulses at each magnetic field value of
the chosen field range (field steps: 0.25 G, 50 accumulations). Argon-
saturated solutions in toluene were pumped through a flat quartz cell
positioned in the cavity of the EPR spectrometer using a flow system
97
map was drawn with Gabedit.
Photo-DSC Measurements. The photo-DSC measurements
were conducted using a Netzsch DSC 204 F1 with an autosampler.
The respective amounts of photoinitiator (0.3 and 0.15 mol % for 1;
(
0.3 and 0.2 mol % for 4a; 0.3 and 0.1 mol % for 4b; 0.3 and 0.075 mol
4
%
for 4d and 4e; 0.3 and 0.05 mol % for 4f) were added to the
̈
monomer HDDA and mixed in an ultrasonic bath for 30 min at room
temperature. A total of 10 ± 1 mg of each formulation was weighed
into an open aluminum pan, and all formulations were analyzed in
triplicate. The prepared formulations were measured at 25 °C under
(
−
1
an inert atmosphere (N flow = 20 mL min ). After an equilibration
2
−1
(
flow rate: 4−5 mL min ).
phase of 4 min, the samples were irradiated for 6 min with UV light
from a Lumen Dynamics OmniCure Series 2001-XLA. The light
LFP. The experiments were performed on a LKS80 Laser Flash
Photolysis Spectrometer (Applied Photophysics, UK). Samples were
excited with the frequency tripled light from a Spitlight Compact 100
−
1
intensity was set to 1 W cm , and the heat flow of the polymerization
reaction was recorded as a function of time. From the theoretical heat
−
1
(
InnoLas, Germany) solid state Nd:YAG laser at 355 nm (∼10 mJ per
of polymerization of the monomer HDDA (ΔH
= 761.92 J g
0
,HDDA
−
1
−1
54
pulse, 8 ns). Rate constants for the addition of the germyl radicals to
the monomer double bonds were determined in pseudo-first-order
experiments. To that end, solutions of the compounds in acetonitrile
containing monomer concentrations in the range of 0.01 to 0.1 M and
providing absorbance of ∼0.3 at 355 nm were prepared. Before the
measurements, the solutions were deoxygenated by bubbling with
argon. The decay of the germyl radicals was observed at the
absorption maxima in the transient absorption spectra.
or 172.4 kJ mol , determined from 86.2 kJ mol per acrylate unit),
the double bond conversion (DBC) was calculated by dividing the
measured heat of polymerization ΔH by ΔH0,HDDA. The maximum
rate of polymerization R
was calculated using eq 3, with h as the
p,max
−1
height of the exothermic polymerization signal in mW mg and ρ =
−1
1
010 g L as the density of HDDA at 25 °C.
h·ρ
ΔH0,HDDA
Rp,max
=
Steady-State Photolysis and Determination of Quantum
Yields. UV−vis spectra were acquired on a TIDAS UV−vis
spectrometer equipped with optical fibers and a 1024-pixel diode-
array detector (J&M Analytik AG, Essingen, Germany). For the
photobleaching experiments, two different LEDs (Roithner Laser-
technik GmbH, Vienna, Austria)LED385 (emission maximum 387
nm; 15 nm full width at half-maximum; 7.8 mW at 20 mA) and
LED470 (emission maximum at 463 nm; 30 nm full width at half-
maximum; 7.4 mW at 20 mA)were used. The output power of the
LEDs was determined by a spectrophotometer (GL Spectis, GL
Optics, Germany) equipped with an integrating sphere (Ulbricht
(3)
ASSOCIATED CONTENT
■
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02181.
NMR spectra, X-ray data, additional graphs, kinetic data,
and DFT calculations (PDF)
−1
sphere). Both LEDs were operated at a photon flux of 0.05 μmol s
to ensure comparability of the results. Solutions of the investigated
compounds in a 1:1 (v/v) mixture of toluene and methyl
methacrylate (MMA) were filled into 1 cm × 1 cm quartz cuvettes
intended for fluorescence measurements and degassed by bubbling
with argon for 5 min. Samples were irradiated perpendicular to the
optical path of the spectrometer and stirred during the measurements
with a magnetic stirring bar (750 rpm). The concentrations of the
compounds were between 0.14 and 1.00 mM (within the linear range
of the spectrometer). The data acquired in the photobleaching
experiment with LED385 were used to determine the quantum yields
of decomposition. To that end, the absorbance traces were converted
Cartesian coordinates of calculated structures (XYZ)
Accession Codes
CCDC 1992490−1992494 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
K
https://dx.doi.org/10.1021/acs.inorgchem.0c02181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
DMEDA-catalyzed direct C-H arylation of unactivated benzene. J.
Am. Chem. Soc. 2010, 132, 16737−16740.
AUTHOR INFORMATION
Corresponding Author
Michael Haas − Institute of Inorganic Chemistry, Graz
University of Technology, A-8010 Graz, Austria; orcid.org/
■
(
5) Sun, C.-L.; Gu, Y.-F.; Wang, B.; Shi, Z.-J. Direct arylation of
alkenes with aryl iodides/bromides through an organocatalytic radical
process. Chem. - Eur. J. 2011, 17, 10844−10847.
0
000-0002-9213-940X; Email: michael.haas@tugraz.at
(6) Sun, C.-L.; Gu, Y.-F.; Huang, W.-P.; Shi, Z.-J. Neocuproine-
KOtBu promoted intramolecular cross coupling to approach fused
Authors
rings. Chem. Commun. 2011, 47, 9813−9815.
Philipp Fruhwirt − Institute of Physical and Theoretical
̈
(7) Shirakawa, E.; Zhang, X.; Hayashi, T. Mizoroki-heck-type
Chemistry, Graz University of Technology, A-8010 Graz,
Austria
reaction mediated by potassium tert-butoxide. Angew. Chem., Int. Ed.
011, 50, 4671−4674.
8) Rueping, M.; Leiendecker, M.; Das, A.; Poisson, T.; Bui, L.
2
(
Andreas Knoechl − Institute of Inorganic Chemistry, Graz
University of Technology, A-8010 Graz, Austria
Michael Pillinger − Institute of Inorganic Chemistry, Graz
University of Technology, A-8010 Graz, Austria
Potassium tert-butoxide mediated Heck-type cyclization/isomer-
ization-benzofurans from organocatalytic radical cross-coupling
reactions. Chem. Commun. 2011, 47, 10629−10631.
Stefanie M. Mu
̈
ller − Institute of Chemistry of Polymeric
(9) Qiu, Y.; Liu, Y.; Yang, K.; Hong, W.; Li, Z.; Wang, Z.; Yao, Z.;
Jiang, S. New ligands that promote cross-coupling reactions between
aryl halides and unactivated arenes. Org. Lett. 2011, 13, 3556−3559.
(10) Roman, D. S.; Takahashi, Y.; Charette, A. B. Potassium tert-
butoxide promoted intramolecular arylation via a radical pathway.
Org. Lett. 2011, 13, 3242−3245.
Materials, Montanuniversitaet Leoben, A-8700 Leoben, Austria
Perry T. Wasdin − Institute of Physical and Theoretical
Chemistry, Graz University of Technology, A-8010 Graz,
Austria
Roland C. Fischer − Institute of Inorganic Chemistry, Graz
University of Technology, A-8010 Graz, Austria; orcid.org/
(
11) Shirakawa, E.; Hayashi, T. Transition-metal-free Coupling
Reactions of Aryl Halides. Chem. Lett. 2012, 41, 130−134.
12) Liu, H.; Yin, B.; Gao, Z.; Li, Y.; Jiang, H. Transition-metal-free
0
000-0001-9523-5010
(
Judith Radebner − Institute of Inorganic Chemistry, Graz
University of Technology, A-8010 Graz, Austria
Ana Torvisco − Institute of Inorganic Chemistry, Graz
University of Technology, A-8010 Graz, Austria; orcid.org/
highly chemo- and regioselective arylation of unactivated arenes with
aryl halides over recyclable heterogeneous catalysts. Chem. Commun.
2
(
012, 48, 2033−2035.
13) Chen, W.-C.; Hsu, Y.-C.; Shih, W.-C.; Lee, C.-Y.; Chuang, W.-
0
000-0002-6203-7330
H.; Tsai, Y.-F.; Chen, P. P.-Y.; Ong, T.-G. Metal-free arylation of
benzene and pyridine promoted by amino-linked nitrogen hetero-
cyclic carbenes. Chem. Commun. 2012, 48, 6702−6704.
Norbert Moszner − Ivoclar Vivadent AG, FL-9494 Schaan,
Liechtenstein
Anne-Marie Kelterer − Institute of Physical and Theoretical
Chemistry, Graz University of Technology, A-8010 Graz,
Austria; orcid.org/0000-0001-8178-2670
Thomas Griesser − Institute of Chemistry of Polymeric
Materials, Montanuniversitaet Leoben, A-8700 Leoben,
Austria; orcid.org/0000-0002-9493-3770
Georg Gescheidt − Institute of Physical and Theoretical
Chemistry, Graz University of Technology, A-8010 Graz,
Austria; orcid.org/0000-0002-6827-4337
(14) De, S.; Ghosh, S.; Bhunia, S.; Sheikh, J. A.; Bisai, A.
Intramolecular direct dehydrohalide coupling promoted by KO(t)Bu:
total synthesis of Amaryllidaceae alkaloids anhydrolycorinone and
oxoassoanine. Org. Lett. 2012, 14, 4466−4469.
(15) Tanimoro, K.; Ueno, M.; Takeda, K.; Kirihata, M.; Tanimori, S.
Proline catalyzes direct C-H arylations of unactivated arenes. J. Org.
Chem. 2012, 77, 7844−7849.
(16) Wu, Y.; Wong, S. M.; Mao, F.; Chan, T. L.; Kwong, F. Y.
Intramolecular direct C-H bond arylation from aryl chlorides: a
transition-metal-free approach for facile access of phenanthridines.
Org. Lett. 2012, 14, 5306−5309.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02181
(17) Zhao, H.; Shen, J.; Guo, J.; Ye, R.; Zeng, H. A macrocyclic
aromatic pyridone pentamer as a highly efficient organocatalyst for
the direct arylations of unactivated arenes. Chem. Commun. 2013, 49,
Notes
The authors declare no competing financial interest.
2
(
323−2325.
18) Buden, M. E.; Guastavino, J. F.; Rossi, R. A. Room-temperature
́
ACKNOWLEDGMENTS
photoinduced direct C-H-arylation via base-promoted homolytic
■
We gratefully acknowledge financial support from Ivoclar
Vivadent AG, NAWI Graz, and FWF (Vienna, Austria; project
number P 32606-N).
aromatic substitution. Org. Lett. 2013, 15, 1174−1177.
(19) Liu, W.; Tian, F.; Wang, X.; Yu, H.; Bi, Y. Simple alcohols
promoted direct C-H arylation of unactivated arenes with aryl halides.
Chem. Commun. 2013, 49, 2983−2985.
(20) Kumar, A.; Bhakuni, B. S.; Prasad, C. D.; Kumar, S.; Kumar, S.
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