Next Generation of Guanidine Quinoline Copper Complexes for Highly Controlled ATRP: Influence of Backbone Substitution on Redox Chemistry and Solubility

Article abstract of DOI:10.1002/ejic.201800511

Ligands DMEG6etqu, TMG6etqu, DMEG6buqu, and TMG6buqu were developed on the basis of guanidine quinoline (GUAqu) ligands 1,3-dimethyl-N-(quinolin-8-yl)imidazolidin-2-imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)guanidine (TMGqu). These ligands feature an alkyl substituent at the C6 of the quinoline backbone. The synthetic strategy developed here enables inexpensive syntheses of any kind of C6-substituted GUAqu ligands. On one hand, the alkylation increases the solubility of corresponding copper complexes in apolar atom transfer radical polymerization (ATRP) monomers like styrene. On the other hand, it has a significant electronic influence and thus an effect on the donor properties of the new ligands. Seven Cu^I and Cu^II complexes of DMEG6etqu and TMG6etqu have been crystallized and were studied with regard to their structural and electrochemical properties. Cu^I and Cu^II complexes of DMEG6buqu and TMG6buqu turned out to be perfectly soluble in pure styrene even at room temperature, which makes them excellent catalysts in the ATRP of apolar monomers. The key characteristics of the ATRP equilibrium, K_ATRP and k_act, were determined for the new complexes. In addition, we used our recently developed DFT methodology, NBO analysis, and isodesmic reactions to predict the influence of the introduced alkyl substituents. It turned out that high conformational freedom in the complex structures leads to a significant uncertainty in prediction of the thermodynamic properties.

Full text of DOI:10.1002/ejic.201800511

A Journal of
Accepted Article
Title: Next Generation of Guanidine Quinoline Copper Complexes for
Highly Controlled ATRP: Influence of Backbone Substitution on
Redox Chemistry and Solubility
Authors: Sonja Herres-Pawlis, Thomas Rösener, and Alexander
Hoffmann
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800511
Link to VoR: http://dx.doi.org/10.1002/ejic.201800511

10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
Next Generation of Guanidine Quinoline Copper Complexes for
Highly Controlled ATRP: Influence of Backbone Substitution on
Redox Chemistry and Solubility
Thomas Rösener,[a] Alexander Hoffmann[a] and Sonja Herres-Pawlis*[a]
Abstract: Based on the guanidine quinoline (GUAqu) ligands 1,3-
dimethyl-N-(quinolin-8-yl)-imidazolidin-2-imine
(DMEGqu)
and
1,1,3,3-tetramethyl-2-(quinolin-8-yl)-guanidine (TMGqu) the four
ligands DMEG6etqu, TMG6etqu, DMEG6buqu and TMG6buqu were
developed. These ligands feature an alkyl substitutent at C6 of the
quinoline backbone. The synthetic strategy developed here allows
inexpensive syntheses of any kind of C6 substituted GUAqu ligands.
The alkylation on one hand increases the solubility of corresponding
copper complexes in apolar ATRP monomers like styrene. On the
other hand it has a significant electronic influence and thus an effect
on the donor properties of the new ligands. Seven CuI and CuII
complexes of DMEG6etqu and TMG6etqu have been crystallised and
were studied with regards to their structural and electrochemical
properties. CuI and CuII complexes of DMEG6buqu and TMG6buqu
turned out to be perfectly soluble in pure styrene even at room
temperature which makes them excellent catalysts in ATRP of apolar
monomers. The key characteristics of the ATRP equilibrium KATRP and
kact were determined for the new complexes. In addition we used our
recently developed DFT methodology, NBO analysis and isodesmic
reactions to predict the influence of the introduced alkyl substituents.
It turned out, that high conformational freedom in the complex
structures leads to a significant uncertainty in prediction of the
thermodynamic properties.
Scheme 1. The ATRP equilibrium and substitution at known ATRP ligands[3,4]
depends mainly on the ligand environment. Thus, in copper
ATRP a large variety of different N-donor ligands has been
evaluated and structure reactivity relationships were derived. The
activity of a copper complex in ATRP is influenced by the denticity
of the ligand, the nature of the N-donor and its electron donating
ability.[3] The analysis of different 4,4’-substituted 2,2’-bipyridine
(top left of Scheme 1) ligands furthermore exhibited the effect of
ligand substitution on the catalyst activity.[4] The most active
ATRP catalyst so far features the tetrapodal ligand TMPANMe2 (top
middle of Scheme 1, R = NMe2). Copper complexes of TPMANMe2
show KATRP values of about 1.[5]
Introduction
Atom transfer radical polymerisation (ATRP) has become one of
the most versatile reversible-deactivation radical polymerisation
(RDRP) methods since its invention in 1995.[1] In ATRP transition
metal complexes mediate an equilibrium between dormant and
active radical chains (Scheme 1, bottom). The transition metal
complex reversibly caps active radical chains with a halogen
atom (usually Br· or Cl·). Due to this equilibrium few active radical
species coexist and thus chain termination reactions can be
effectively suppressed. RDRP techniques based on ATRP allow
the application of air stable catalyst precursors, the drastic
reduction of transition metal concentration and an even higher
controllability.[2] Amongst other factors the polymerisation rate in
ATRP highly depends on the nature of the catalyst, which itself
In the past, we focused on the implementation of a new and
promising ligand class in ATRP, the guanidine ligands.[6–11] Since
guanidines represent a class of strong and neutral N-donors, they
have great potential in ATRP. Guanidine ligands already found
broad application in the field of homogenous catalysis. Recent
publications describe the application in the ring opening
polymerisation (ROP) of lactide[12] or oxygen activation.[13]
Nevertheless only few examples of guanidine complexes in
ATRP are known.[6–11,14]
Recently, we showed that copper complexes of the bidentate
guanidine quinoline (GUAqu) ligands DMEGqu and TMGqu (top
right of Scheme 1, R = H) possess redox potentials that are
comparable to the tridentate ligand PMDETA. ATRP reactions
could be successfully conducted.[6] These ligands also recently
attracted attention in the fields of copper photochemistry[15] and
[a]
T. Rösener, Dr. A. Hoffmann, Prof. Dr. S. Herres-Pawlis
Institut für Anorganische Chemie
RWTH Aachen University
as entatic state models for electron transfer proteins.[16]
A
Landoltweg 1, 52074 Aachen (Germany)
sonja.herres-pawlis@ac.rwth-aachen.de
http://www.bioac.ac.rwth-aachen.de/
targeted improvement of GUAqu ligands thus is the next logical
step.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
A major drawback of GUAqu copper halogenido complexes with
regard to ATRP was their bad solubility in apolar monomers like
styrene. So we target herein the improvement of the GUAqu
ligands for a better solubility of corresponding complexes. We
present a new synthetic pathway for the synthesis of C6-
substituted GUAqu ligands and will herein focus on the synthesis
of the alkyl substituted ligands DMEG6etqu, TMG6etqu,
DMEG6buqu and TMG6buqu. The corresponding copper halide
complexes were investigated towards their molecular structures,
redox potential and properties in ATRP.
Matyjasewski et al. established a linear correlation of the redox
potential and lg(KATRP). By solely knowing the redox potential of
the activator/deactivator couple, the value for KATRP and
consequently the activity of the catalyst can be estimated.[17] The
evaluation of new ATRP catalysts with respect to their activity
thus became much easier. In addition, efforts have been made to
calculate E1/2 of certain compounds via DFT. Unfortunately, the
uncertainty of these methods is too high when it comes down to
small differences.[18] As we showed earlier, systematic errors in
calculation can be overcome with the concept of the isodesmic
reaction and, as long as there is structural data available, correct
predictions can be made.[6,19]
Table 1. NBO charges of NGUA and Nqu in different substituted GUAnetqu-
ligands and the unsubstituted ligands DMEGqu and TMGqu. Highest
absolute sum of charges highlighted (TPSSh/def2-TZVP; GD3BJ; SMD:
MeCN).
NBO charges [e-]
DMEGnetqu
Nqu
TMGnetqu
Nqu
Position
unsub.
C2
NGUA
Sum
NGUA
Sum
-0.595
-0.597
-0.593
-0.593
-0.596
-0.441
-0.455
-0.434
-0.447
-0.442
-1.036
-1.052
-1.027
-1.041
-1.038
-0.595
-0.593
-0.593
-0.594
-0.596
-0.431
-0.444
-0.420
-0.439
-0.434
-1.026
-1.036
-1.013
-1.033
-1.030
C3
C4
C5
C6
C7
-0.599
-0.594
-0.441
-0.444
-1.040
-1.038
-0.597
-0.592
-0.434
-0.438
-1.031
-1.030
Since the alkyl substituent has an influence on the donor
properties of the ligands, we tried to predict the thermodynamic
properties of the new catalyst with our computational method. We
investigated whether the influence of the alkyl substituents can
be predicted correctly even when there is no structural data
available and whether DFT might be a tool for the targeted
syntheses of new ligands in ATRP.
methodology includes the basis set def2-TZVP,[20] the hybrid
functional TPSSh[21] and empirical dispersion with Becke-
Johnson damping GD3BJ.[22–24] Since acetonitrile was used as
solvent in most of our reactions, all calculations were performed
with a SMD model for acetonitrile.[25] The structural optimisations
were followed by NBO calculations. NBO charges give hints to
the donor properties of the ligands.[6] Considering that the ethyl-
substituent has an influence on both the guanidine and quinoline
donor, the NBO charges of these donors were added up. A more
negative sum of charges suggests overall better donor properties
of the ligand. The results for differently substituted DMEGnetqu
and TMGnetqu are summarised in Table 1.
Results and Discussion
Synthesis of the ligands
An ethyl substituent in position C3 seems to decrease the overall
charge in comparison with the unsubstituted ligand and was
therefore ruled out as possible substitution position. Substitution
in positions C2, C4, and C6 on the other hand has the largest
influence on the donor atoms in increasing the charge.
Substitution on C2 might have a significant steric effect on the
coordination chemistry and was thus also ruled out. Substitution
at position C6 is synthetically much easier in comparison with the
C4 and thus became our position of choice to modify the original
ligand.
Development of Alkylated Guanidine Quinoline Ligands
In order to improve the solubility of copper guanidine quinoline
complexes in apolar monomers, the quinoline backbone of the
ligands was alkylated. Beside its effect on the solubility, such an
alkyl substituent has a positive electronic effect on the donor
properties of the ligands. With the help of DFT and subsequent
NBO calculations we estimated, at which position the alkyl
substituent has its maximal influence. All possible substitution
positions are depicted in Scheme 2.
Synthetic Strategy
The synthetic strategy to obtain the target ligands requires no late
transition metal cross-coupling reactions and hence, is easy
scalable and rather inexpensive. The fundamental steps for the
synthesis of the ligand precursor NH26alkqu are shown in
Scheme 3. Different alkylated guanidine quinoline ligands can be
synthesised starting from commercially available 4-alkylated
anilines. Thus, with our synthetic approach and appropriate
anilines at hands, a whole library of differently C6 substituted
guanidine quinoline ligands can be synthesised.
Scheme 2. Possible substitution positions (in red) on GUAqu.
Starting geometries of substituted DMEGnalkqu and TMGnalkqu
(where n is the position of substitution) ligands and the
unsubstituted Ligand TMGqu were generated from the molecular
structure of DMEGqu. To evaluate the electronic influence of an
ethyl-substituent at the different positions, the starting geometries
were first optimised using our methodology in Ref.[6]. This DFT
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
The raw ligands appear as yellow brownish solids (L1) or highly
viscous liquids (L2-L4), which had to be purified for their
application in in situ KATRP or kact measurements and
polymerisation kinetics. L1 was purified by sublimation, whereas
L2-L4 were purified by column chromatography. The pure ligands
appear as yellow crystalline solids (L1, L3) or highly viscous
liquids (L2, L4). Experimental details for synthesis and
purification are summarised in the SI. Crystals of L1 and L3 were
suitable for X-ray crystallography. The molecular structures are
depicted in Figure S1, key structural parameters are summarised
in Table S1 in the SI.
Scheme 3. Synthetic access to NH26alkqu. R = different alkyl-substituents.
Synthesis and structural characterisation of the complexes
Herein, the ligand precursors NH26etqu (with an ethyl substituent
at 6-position of qu) and NH26buqu (with a n-butyl substituent at
6-position of qu) were synthesised as starting materials for the
four new ligands.
The reaction of DMEG6etqu and TMG6etqu with various
anhydrous CuI and CuII halide salts resulted in crystals suitable
for X-ray crystallography, whereas complexes of DMEG6buqu
and TMG6buqu could not be crystallised. Thus with CuIBr, CuICl,
For NH26etqu, first nitration was accomplished following a slightly
modified literature procedure by Buszek et al..[26] Instead of
acetamide, trifluoroacetamide was used as protecting group for
the amine function. The modified protection step runs at lower
temperatures and thus, hazardous dichloroethane could be
replaced by dichloromethane as solvent. In the next step,
NO26etqu was synthesised according to a protocol by Wielgosz-
Collin et al.[27] In this modified Skraup synthesis, the nitrated
aniline was transformed into the corresponding quinoline.
Afterwards, the nitro-quinoline was reduced to NH26etqu.
NH26buqu was made analogously from 4-n-butylaniline.
Experimental details for all steps are summarised in the SI.
CuIIBr2
and
CuIICl2
eight
complexes,
namely
[Cu(DMEG6etqu)2]Br, [Cu(DMEG6etqu)2]Cl, [Cu(TMG6etqu)2]Br,
[Cu(TMG6etqu)2]Cl,
[Cu(DMEG6etqu)2Br]Br,
[Cu(TMG6etqu)2Br]Br,
[Cu(DMEG6etqu)2Cl]Cl
[Cu(TMG6etqu)2Cl]Cl could be structurally characterised. The
crystallographic data of [Cu(TMG6etqu)2Br]Br was insufficient for
publication and therefore is not discussed herein. Molecular
structures of the chlorido complexes are exemplarily shown in
Figure 1. Molecular structures of the bromido complexes are
depicted in figure S2.
Table 2 summarises key bond lengths and angles of the copper
GUA6etqu complexes. All CuI complexes comprise a cationic unit
containing a copper centre coordinated by two GUA6etqu ligands
in a distorted tetrahedron. A closer look on the geometrical
properties in the [Cu(DMEG6etqu)2]Br and [Cu(DMEG6etqu)2]Cl
complexes reveals significant but small differences between both
structures although they only differ in their non-coordinating
counter-ion. We ascribe this to packing effects in solid state, and
this effect was also found in crystal structures of [Cu(TMGqu)2]+
with the counterions Cl-, Br- and ClO4-.[6,30] The differences in key
bond lengths in the complexes [Cu(TMG6etqu)2]Br and
[Cu(TMG6etqu)2]Cl on the other hand are insignificant, whereas
the angles around the central metal also vary notably. NGUA-Cu
bond lengths in [Cu(DMEG6etqu)2]Br are significantly prolonged
in comparison with [Cu(TMG6etqu)2]Br. As noticed earlier the
TMG moiety is the stronger donor compared to the DMEG
moiety.[6] This is expressed in shorter NGUA-Cu bond lengths in
CuI complexes featuring TMG6etqu. In CuII complexes the metal
Synthesis of the ligands
Starting from the ligand precursors NH26etqu and NH26buqu the
four new ligands DMEG6etqu (L1), TMG6etqu (L2), DMEG6buqu
(L3) and TMG6buqu (L4) were synthesised according to a
general procedure (Scheme 4).[28,29]
centre is supplemented by
a coordinating halide anion.
Comparing [Cu(DMEG6etqu)2Cl]Cl and [Cu(TMG6etqu)2Cl]Cl it
can be seen that the Cu-Cl bond length in the complex featuring
TMG6etqu is significantly elongated. In comparison with the
unsubstituted ligands one can see, that the introduction of an
ethyl substituent in C6 position of the ligands leads to a
shortening of the Cu-X bond in the case of DMEGqu and to an
elongation of the same bond in the case of TMGqu.[6] To prove
that complexes of the GUA6buqu ligands also form bischelate
Scheme 4. Synthesis of differently substituted DMEG6alkqu and TMG6alkqu
ligands.
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
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Table 2. Key bond lengths and angles of the CuI complexes [Cu(L)2]X and the CuII complexes [Cu(L)2X]X. L = DMEG6etqu or TMG6etqu and
X = Cl or Br.
Bond lengths[Å]
CuI
CuII
Ligand/ Cu-salt
DMEG6etqu/
CuBr
DMEG6etqu/
CuCl
TMG6etqu/
CuBr
TMG6etqu/
CuCl
DMEG6etqu
/ CuBr2
DMEG6etqu/
CuCl2
TMG6etqu/
CuCl2
Cu-X (Cl/Br)
Cu-NGUA (1)
Cu-NGUA (2)
Cu-Nqu (1)
-
-
-
-
2.493(1)
2.043(4)
2.152(3)
1.989(4)
1.985(3)
2.319(1)
2.022(3)
2.211(3)
1.983(3)
1.999(3)
2.428(7)
2.074(2)
2.101(2)
1.982(2)
1.982(2)
2.083(3)
2.139(3)
1.985(3)
1.970(3)
2.154(3)
2.154(3)
1.969(3)
1.969(3)
2.059(4)
2.081(4)
2.002(4)
2.009(4)
2.067(6)
2.088(6)
1.980(6)
1.963(6)
Cu-Nqu (2)
Bond angles [°]
Nqu (1)-Cu-Nqu (2)
NGUA (1)-Cu-X
NGUA (1)-Cu-NGUA (2)
Structure factors

145.2(2)
-
149.0(2)
-
146.2(2)
-
142.1(3)
-
178.2(2)
137.7(1)
115.2(2)
174.8(2)
149.7(1)
110.1(1)
178.3(1)
125.9(2)
126.7(1)
126.2(2)
131.5(2)
128.9(2)
116.2(2)
[a]
[b]
4
5
0.63
0.97
0.56
0.97
0.60
0.98
0.72
0.96
0.68
0.99
0.42
0.99
0.87
1.00
[c]
[a] 휏4
휏5
=
360°−(훼+훽). A 4 value of 1 is found in ideal tetrahedral complexes were a 4 value of 0 is found in ideal square planar complexes.[14] [b]
141
=
(훼−훽). A 5 value of 1 is found in ideal trigonal bipyramidal complexes were a 5 value of 0 is found in ideal square-based pyramidal
60
2푎
complexes. [15] [c] 휌 =
) with a = d(CGUA- NGUA) and b and c = d(CGUA- Namine).[16] Average -value of both guanidine moieties.
(
푏+푐
complexes in solution, a NMR experiment was carried out. The
exemplary complex [Cu(TMG6buqu)2]Br was formed in situ in
deuterated acetonitrile. Ligand and copper salt were used in a
ratio of 2:1. Only one set of NMR signals could be detected in
both 1H- and 13C-NMR spectra. This is an evidence that the
bischelate complex is formed exclusively. Otherwise a second or
even more sets of NMR signals should be found. The NMR
spectra are depicted in Figures S36 and S37.
Figure 1. Molecular structures of the complex cations of the chlorido complexes. Key atoms are exemplarily marked in one complex. H atoms are omitted for
clarity.
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
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Electrochemistry
influence of the alkyl substituent seems to be more pronounced
in ligands including the TMG moiety.
Since electrochemical properties provide information about the
activity of copper complexes in ATRP, we performed
cyclovoltammetric measurements (CV) to determine the redox
potentials of our new catalysts. The measurements were done in
acetonitrile starting from crystalline bischelate CuII complexes
(GUA6etqu complexes) or bischelate CuII complexes generated
in situ (GUA6buqu complexes). All measurements were carried
out at room temperature at different sweep rates to proof
reversible behavior of the redox process. E1/2 was determined
against the Fc/Fc+ couple. Figure 2 shows cyclic voltammograms
of the [CuI(DMEG6etqu)2Br]/[CuII(DMEG6etqu)2Br]+ couple at
different sweep rates. For better comparability with literature data
the potentials were recalculated against SCE.[31]
Figure 3. Redox potentials of various [CuIL2Br]/[CuIIL2Br]+ couples with L =
GUAqu, GUA6etqu or GUA6buqu.
Based on the CV data we expected TMG6alkqu complexes with
CuBr to be more active in ATRP in comparison with their
unsubstituted counterpart [Cu(TMGqu)2Br]Br. The activities of
DMEG6alkqu complexes with CuBr should resemble the activity
of [Cu(DMEGqu)2]Br. For all further ATRP studies, we focused on
the bromido complexes due to better comparability to other ATRP
studies.[17,32]
Atom Transfer Radical Polymerisation
Determination of KATRP
KATRP is the central equilibrium constant of the ATRP equilibrium
and represents the ratio of the rate constants of activation kact and
deactivation kdeact. Polymerisation velocity and control highly
Figure 2.
Cyclic
voltammograms
of
the
[CuI(DMEG6etqu)2Br]/[CuII(DMEG6etqu)2Br]+ couple at different sweep rates
starting from [Cu(DMEG6etqu)2Br]Br.
depend on KATRP
.
Here, KATRP was determined by reacting CuBr complexes of the
different GUA6alkqu ligands with the ATRP initiator ethyl α-
bromoisobutyrate (EBrib) and following the evolution of the CuII
species via UV/Vis spectroscopy. KATRP measurements for the
previously reported systems featuring the ligands DMEGqu und
TMGqu were repeated with the improved method to guarantee
consistent conditions for all measurements.[6] Details about the
procedure are summarised in the experimental section. For the
UV/Vis spectroscopic determination of KATRP we followed the
characteristic d-d transition band of the CuII complexes around
900-950 nm. Since the extinction coefficient ε of this UV/Vis
absorption is essential for the determination of KATRP by the
methods of Fisher and Fukuda and the method of Matyjaszewski,
ε had to be determined precisely. Values for ε (Table S3) were
determined from crystalline CuII complexes if possible. In the
case of [Cu(DMEG6buqu)2Br]Br and [Cu(TMG6buqu)2Br]Br the
complexes were generated in situ.
KATRP values were examined for all CuBr complexes of the new
GUA6alkqu ligands at 22 °C in acetonitrile with EBrib as initiator.
The measurements were repeated at least two times and the
KATPR values arithmetically averaged.
Sample Matyjaszewski plots for all complexes are shown in
Figures S3-S8. KATRP was calculated by the method developed
by Matyjaszewski.[33] Values are summarised in Table 3.
Cyclovoltammetric measurements were performed starting from
both CuI and CuII complexes of the ligands L1 – L4. Values for
E1/2 are listed in Tables S2 and S3. Starting from CuI or CuII
complexes has only a negligible effect on the determined redox
potentials. In relevant literature mostly redox potentials measured
starting from CuII complexes are discussed, therefore in the
following redox potentials measured this way are used for
discussion. To illustrate the difference in redox potentials
between all complexes, the half-wave potentials are depicted in
Figure 3.
With introduction of electron donating alkyl substituents at C6 of
the quinoline backbone we expected lower redox potentials for all
new complexes, as better donating ligands stabilise the CuII
complexes. In case of [Cu(DMEG6buqu)2Br]Br and
[Cu(DMEG6etqu)2Br]Br almost no change in the redox potential
could be observed which might be related to the weaker donor
capability of the DMEG unit.[6] CuBr2 complexes of TMG6alkqu
ligands in contrast show redox potentials reduced by about
20 mV. CuCl2 complexes of all GUA6alkqu ligands exhibit more
negative redox potentials. The reduction of the redox potential is
more pronounced in CuCl2 complexes. While unsubstituted
DMEGqu complexes are more reducing than analogous TMGqu
complexes, this ratio is inverted in alkylated ligands. The
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Determination of kact and kdeact
:
Table 3. KATRP values for [Cu(L)2]Br. L = GUAqu, GUA6etqu, GUA6buqu.
Besides KATRP, the rate constant of deactivation kdeact is another
important constant for the characterisation of newly developed
ATRP catalysts. For the establishment of functional ARGET
ATRP reactions the knowledge of kdeact is indispensable.
Unfortunately, the direct measurement of kdeact is rather
complicated.[34] The rate constant of activation kact on the other
hand can be easily determined via time-dependent UV/Vis
spectroscopy. For the measurement a protocol published by
Matyjaszewski et al. was applied.[35] The respective activator
complex is reacted with a tenfold excess of the initiator (EBrib) to
guarantee pseudo-first-order conditions. Addition of a trapping
agent (2,2,6,6,-tetramethylpiperidinyl-1-oxyl, TEMPO) makes the
activation step irreversible. Here, a tenfold excess with respect to
the CuI complex was used as well. The formation of the CuII
(deactivator) species was followed by UV/Vis spectroscopy. The
same d-d absorption band (Table S4) as within the KATRP
determination was used. The time-dependent evolution of the CuII
complex can be fitted by the monoexponential equation 퐴 =
[Cu(L)2]Br; L =
KATRP (Matyjaszewski)
DMEGqu
9.0±0.7x10-8
8.6±0.5x10-8
7.9±0.5x10-8
1.6±0.2x10-7
7.8±0.7x10-8
1.34±0.04x10-7
TMGqu
DMEG6etqu
TMG6etqu
DMEG6buqu
TMG6buqu
In Figure 4 the logarithmic values of KATRP (calculated by the
method of Matyjaszewski) are plotted against E1/2 determined via
CV. All values roughly follow the correlation published by
Matyjaszewski et al.[32]
−푘
표푏푠
푡
(
)
+ 퐶.
퐴0 1 − 푒
The activation rate constant is subsequently calculated via the
equation 푘푎푐푡 = 푘표푏푠/[퐼]0. [I]0 is the initiator concentration at the
beginning of the reaction which can be approximated to stay
constant under pseudo-first-order conditions. Sample raw data
and fit for all complexes are depicted in Figure S9-S14 in the SI.
Values for kact and kdeact are summarised in Table 4.
Table 4. kact and kdeact for [Cu(L)2]Br. L = GUAqu, GUA6etqu, GUA6buqu.
[Cu(L)2]Br; L =
DMEGqu
kact [L mol-1 s-1
7.6±0.2x10-1
8.3±0.3x10-1
1.2±0.1x100
7.6±0.2x10-1
8.3±0.9x10-1
9±1x10-1
]
kdeact (= kact/KATRP) [L mol-1 s-1
8.4±0.2x10+6
]
TMGqu
9.7±0.3x10+6
DMEG6etqu
TMG6etqu
DMEG6buqu
TMG6buqu
1.5±0.1x10+7
4.8±0.2x10+6
1.1±0.9x10+7
Figure 4. Correlation of [CuIL2Br]/[CuIIL2Br]+ redox potentials with KATRP values
(measured with EBrib at 22 °C in MeCN). Black squares: Values published by
Matyjaszewski et al..[32] Green circles: Values for GUA6Rqu complexes.
7±1x10+6
Complexes of the bidentate GUAqu ligands exhibit redox
potentials and values for KATRP comparable to complexes of the
tridentate ligand PMDETA. Complexes of the substituted ligands
TMG6etqu and TMG6buqu reveal the highest values for KATRP in
comparison with all systems analysed herein. The ethyl/butyl
substituent leads to an increase of electron density at the N-donor
atoms and consequently stabilise CuII better than the original
TMGqu ligand. On the other hand, ethyl/butyl substitution at C6
of DMEGqu leads to no significant change of KATRP. Based on
these results we expected TMG6buqu complexes to be more
active in ATRP.
The values for kact and kdeact for all six complexes are within the
same order of magnitude. Only kact of [Cu(DMEG6etqu)2]Br,
which exhibits a rather low value for KATRP seems to be
extraordinarily high. The exceptional low solubility of
[Cu(DMEG6etqu)2Br]Br in acetonitrile might influence the
outcome of the kact measurement.
All complexes exhibit sufficient values of KATRP and kdeact to find
application in standard ATRP.[17]
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
Polymerisation kinetics:
Polymerisation kinetics with CuBr complexes of GUA6etqu and
GUA6buqu were performed in bulk. We expected the complexes
of alkyl substituted GUAqu ligands to be better soluble in styrene
than the parent systems. It turned out, that the solubility of
[Cu(DMEG6etqu)Br]Br in styrene is even worse in comparison to
the complexes of the unsubstituted ligands. The poor solubility of
the deactivator complex leads to termination of the ATRP reaction.
Semilogarithmic kinetic plots and Mn and PD developments of
ATRP
reactions
with
[Cu(DMEG6etqu)2]Br
and
[Cu(TMG6etqu)2]Br are depicted in Figure S15 and S16 in the SI.
On the contrary CuI and CuII complexes of GUA6buqu ligands are
remarkably well soluble in pure styrene, even at room
temperature. Figure 5 depicts semilogarithmic kinetic plots for
styrene polymerisations mediated by [Cu(DMEG6buqu)2]Br and
[Cu(TMG6buqu)2]Br.
Figure 6. Mn and PD vs. conversion for styrene polymerisations in bulk
mediated by [Cu(DMEG6buqu)2]Br and [Cu(TMG6buqu)2]Br. Conditions:
110 °C; Ratio: monomer (styrene)/catalyst/initiator (EBrib) = 100/1/1.
Both systems reveal linear development of the molecular masses.
The measured values for Mn are in accordance with expected
theoretical ones. The values of PD are around 1.05 – 1.10
throughout the whole course of reaction but slightly increasing at
higher conversion as the viscosity of the mixture rises. Table 5
summarises kobs of the newly developed catalysts in comparison
with kobs of the parent complexes.[6] [Cu(TMG6buqu)2]Br reveals
a higher kobs in comparison to [Cu(DMEG6buqu)2]Br. This is in
accordance with the results of the KATRP determination, since
[Cu(TMG6buqu)2]Br exhibits
a higher value for KATRP. In
comparison with the parent catalysts, complexes with
DMEG6buqu and TMG6buqu show lower values for kobs. Redox
potentials and values of KATRP gave rise to the assumption, that
kobs of the new catalyst might be higher in comparison with the
parent catalyst. But it has to be taken into account, that
determinations of E1/2 and KATRP are normally conducted in polar
acetonitrile,[32] whereas polymerisations took place in apolar
styrene. As shown by Matyjaszewski et al. KATRP is highly
dependent on the nature of the solvent and KATRP values are
usually smaller in apolar solvents.[32] The correlation of KATRP and
kobs might have been better, if the KATRP measurements were
carried out in a more apolar solvent. Due to comparability with
literature data, we restraint from performing KATRP measurements
in a more apolar solvent. In comparison, the GUAbuqu systems
allow a better controlled ATRP than their parent systems.
Figure 5. Semilogarithmic kinetic plots for styrene polymerisations in bulk
mediated by [Cu(DMEG6buqu)2]Br and [Cu(TMG6buqu)2]Br. Conditions:
110 °C; Ratio: monomer (styrene)/catalyst/ initiator (EBrib) = 100/1/1. Data
points marked in red were not used for the calculation of kobs
.
At higher conversion, the viscosity of the reaction mixtures
increases. Simultaneously acceleration of the polymerisation can
be observed. This effect can be drawn back on auto-acceleration
of the polymerisation reaction since the deactivation reaction
becomes diffusion controlled in viscous media.[36]
Prediction of Redox Properties and ATRP Activities
Nevertheless, as can be seen in Figure 6, the ATRP proceeds
very controlled, even at higher conversions of 60-70%.
In a previous publication we showed, that with density functional
theory, a suitable methodology and theoretical isodesmic
reactions it is possible to predict the influence of even small
differences in ligands on thermodynamic properties.[6] Here, we
tested whether this concept could be used in future ligand design.
In the case of the ethyl-substituted ligands, molecular structures
of the CuI (activator) and CuII (deactivator) complexes could be
obtained via X-ray crystallography. These served as starting
geometries in DFT. In the case of butyl-substituted ligands no
Table
5.
kobs
for
[Cu(DMEG6buqu)2]Br,
[Cu(TMG6buqu)2]Br,
[Cu(DMEGqu)2]Br and [Cu(TMGqu)2]Br. Styrene ATPR in bulk. Conditions:
110 °C for Br systems. Ratio: M/cat./init. = 100/1/1.
L=
DMEG6buqu
3.4±0.2x10-5
TMG6buqu
4.7±0.3x10-5
DMEGqu[6]
5.4±0.1x10-4
TMGqu[6]
kobs[s-1]
2.3±0.1x10-4
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
crystallographic data was available. Starting geometries for
GUA6buqu complexes were generated both from the molecular
structures of GUAqu and GUA6etqu complexes.
Table 6. ΔΔG values for theoretical isodesmic reactions with the ligands
GUAqu and GUA6etqu. Basis set: def2-TZVP; Functional: TPSSh;
Dispersion: GD3BJ.
Thus it should become possible to predict the thermodynamic
properties of substituted GUAqu ligands without having their
molecular structure at hands.
NBO Calculations
LA vs. LB / Transferred X
DMEGqu vs. TMGqu / Br[6]
DMEGqu vs. L1 / Br
DMEGqu vs. L1 / Cl
TMGqu vs. L2 / Br
TMGqu vs. L2 / Cl
ΔΔG[kJ/mol]
-4.8
Pred. correct?
Yes
To receive closer insights into the effects of the substituent on the
donor properties of the ligand, NBO calculations were performed
with optimised structures of the ligands and the complexes. In the
case of TMGqu, L2 and L4 no structural data was available, so
starting geometries were generated from the structure of the
corresponding DMEG ligands.
-10.0
-4.8
Yes
No
-3.1
No
NBO charges give an indication of the influence of the substituent
on the donor properties of the ligands.[37] Specific values are
summarised Tables S5 and S6. NBO calculations on the ligands
reveal, that an alkyl substitution at C6 increases the NBO charge
at the N-donors. This effect is more pronounced in TMG ligands.
Starting geometries for DMEG6etqu were generated both from
the molecular structure of DMEGqu and DMEG6etqu. It is worth
mentioning, that the NBO charges obtained by both methods
differ significantly. This has to be kept in mind when thinking
about in silico design of the new ligands.
+4.3
Yes
Yes
No
L1 vs. L2 / Br
+2.1
L1 vs. L2 / Cl
-0.9
generated from the molecular structures. Values for ΔΔG are
summarised in Table 6. Comparing substituted and unsubstituted
ligands 50 % of the isodesmic reactions are in accordance with
experimental data. In the case of TMGqu vs. TMG6etqu, where a
rather inactive catalyst is compared with one of the most active,
the relative activity is predicted wrong. Comprehensive
benchmarking on GUAqu copper complexes featuring non-
coordinating counter-ions showed the influence of even small
changes in the coordination angle on the relative energies.[38] If in
our case the absolute minimum of the optimisation is not reached
this might alter the outcome of the isodesmic reaction drastically
and lead to a wrong prediction.
Change from ethyl to butyl substitution does not alter the NBO
charge at the donor atoms in case of TMG ligands. In DMEG
ligands on the other hand the same substitution leads to a more
positive NBO charge at N-donors, which seems counterintuitive.
Looking at the NBO charges within the CuI complexes,
substituted DMEG and TMG ligands behave almost identically.
The introduction of the alkyl substituent leads to a more negative
charge at the metal centre, which underlines the improved donor
properties of the ligands. The length of the alkyl substituent does
not seem to have a significant influence.
Isodesmic reactions: GUA6buqu vs. GUAqu
In CuII bromido complexes similar trends are observable. With
introduction of the alkyl substituent the NBO charge at the metal
centre becomes more negative. In CuII chlorido complexes the
alkyl substituents do not alter the NBO charge (DMEG) or even
lead to a more positive NBO charge at the metal centre (TMG).
Generally it can be stated, that the introduction of an alkyl
substituent at C6 of the quinoline leads to enhanced donor
properties of the ligands. This is reflected in more negative NBO
charges at the donor atoms if considering the bare ligands and
(in most of the cases) more negative NBO charges at the metal
centre in CuI and CuII complexes.
Starting geometries for GUA6buqu complexes were generated
both from GUAqu and GUA6etqu complex structures. After
geometry optimisation and frequency calculation, the obtained
energies for the GUA6buqu complexes were compared to the
energies of the unsubstituted parent systems via isodesmic
reactions (illustrated in Table 7). In most of the cases the outcome
of the isodesmic reaction does not agree with the experimental
data or no sufficient statement can be made based on the present
data. Furthermore, it seems to have no significant influence
whether the starting geometries for the GUA6buqu complexes
originated from the molecular structures of GUAqu or GUA6etqu.
In Table 7 the difference between ΔΔG values obtained with
starting geometries from GUAqu or GUA6etqu structures is also
listed. It appeared, that the difference between both models lies
within the same order of magnitude than the actual values of ΔΔG.
A correct prediction of catalyst activities without structural data at
hands thus seems not possible. With the longer butyl substituent
even more structural conformers are possible in comparison with
the ethyl-bearing ligands. Thus, it is even more complicated to
find the absolute energy minimum.
Isodesmic reactions: GUA6etqu vs. GUAqu
Geometry optimisation was followed by frequency calculation.
Thermally corrected ΔG values were then inserted into the
isodesmic equation (illustrated in Table 6). The isodesmic
reaction provides ΔΔG, which gives relative information about the
activity of the analysed complexes. Similar catalysts can be
compared with regards to their thermodynamic properties. In
case of GUAqu and GUA6etqu, starting geometries were directly
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10.1002/ejic.201800511
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The redox potentials of copper complexes with GUA6alkqu
complexes were determined via CV. The alkyl substituents at 6-
position of the quinoline backbone yield reduced redox potentials
in comparison with complexes of GUAqu in most cases. In copper
bromido complexes featuring DMEG6etqu, the ethyl substituent
has only a minor effect, whereas in TMG6etqu complexes the
effect is more pronounced. This is also expressed in KATRP values.
The alkyl substituent in DMEG-bearing ligands only has a minor
activity of the corresponding complexes, whereas complexes of
TMG6alkqu show the highest values for KATRP within the series.
The values for kact and kdeact for all six complexes are within the
same order of magnitude. All complexes exhibit high values of
KATRP and kdeact for application in standard ATRP.
Table 7. ΔΔG values for theoretical isodesmic reactions with the ligands
GUAqu and GUA6buqu. Basis set: def2-TZVP; Functional: TPSSh;
Dispersion: GD3BJ; SMD: MeCN.
Origin:
GUAqu[a]
GUA6etqu[b]
LA vs. LB
Transferred X
/
ΔΔG
[kJ/mol]
Pred.
corr.?
ΔΔG
[kJ/mol
]
Pred.
corr.?
Diff.
[c]
DMEGqu vs. L3 / Br
DMEGqu vs. L3 / Cl
TMGqu vs. L4 / Br
TMGqu vs. L4 / Cl
L3 vs. L4 / Br
-0.1
-11.9
~0
?
-6.4
-4.4
-0.1
-3.7
+1.5
-9.2
Yes
No
6.3
7.5
~0
4.9
3.3
6
Bulk ATRP experiments were conducted with the CuBr
complexes of all ligands. As targeted, CuI and CuII complexes of
GUA6buqu ligands are highly soluble in pure styrene. The
catalysts revealed excellent control throughout the ATRP. In
contrary to our expectations the deactivator species
[Cu(DMEG6etqu)2Br]Br was only barely soluble in styrene. The
butyl substituent in TMG6buqu leads to a lower redox potential, a
higher KATRP value, better solubility in styrene and better
polymerisation control.
No
[c]
[c]
?
?
1.2
Yes
No
No
+4.8
-3.2
No
L3 vs. L4 / Cl
Yes
Yes
[a]: Starting geometries for GUA6buqu complexes created from GUAqu
complex structures.
With DFT, NBO and isodesmic reactions the influence of the
substituent was further investigated. NBO showed that the alkyl
substituents lead to improved donor properties of the ligands. But
it also could be demonstrated that the result of the NBO
calculation depends on the origin of the applied starting geometry.
With isodesmic reactions and crystallographic data at hands (in
case of GUA6etqu) correct activity predictions could be made in
50 % of the cases. The flexibility of the ethyl substituent leads to
a higher conformational freedom and thus, the absolute minimum
of a geometry optimisation might not be found. Starting geo-
metries for GUA6buqu were generated both from complex
structures of GUAqu and GUA6etqu. We evaluated whether new
catalysts might be developed by simply generating them in silico
from known structures and compare them with known catalysts
via isodesmic reactions. It turned out, that the uncertainty of ΔΔG
is within the same order of magnitude as the actual values of ΔΔG.
Based on these results predictions of activities could not be made
with sufficient accuracy.
[b]: Starting geometries for GUA6buqu complexes created from
GUA6etqu complex structures.
[c]: No sufficient statement can be made based on the present data
As described above NBO calculations revealed high discrepancy
between data generated from actual molecular structures and
starting geometries that were designed in silico. Based on these
results, it seems not possible to use isodesmic reactions in
combination with in silico generated structures for the targeted
design of new ATRP catalysts. Even when crystallographic data
are available, the error rate is relatively high, especially when high
conformational freedom is relevant. Moreover, it has to be
remarked that the computed free energy differences lie close to
the significance limit of DFT. Herein, we have explored the limit
of the predictive power of the isodesmic equations.
Conclusion
To sum up, butylation of GUAqu leads to better solubility and
enhanced ATRP activity of certain copper complexes. Our newly
developed synthetic approach opens up an inexpensive way to
modify GUAqu-ligands with respect to desired properties. We
also showed up possibilities and limitations of our DFT method.
Improvement of this method might open up ways for the
development of tailor made ATRP catalysts
The synthesis of four substituted guanidine quinoline (GUAqu)
ligands was presented. The alkyl substituent in DMEG6etqu,
TMG6etqu, DMEG6buqu and TMG6buqu should primarily lead to
a better solubility of corresponding ATRP catalysts in apolar
monomers. With the help of DFT and NBO calculations, the
possible substitution positions were evaluated with respect to
their electronic influence. With substitution at C6 of the quinoline
backbone a compromise between electronic influence and
synthetic accessibility was chosen. Our synthetic approach
allows the synthesis of almost every 6-substituted GUAqu ligand
starting from para-substituted anilines.
Experimental Section
General:
We discussed the structures of seven GUA6etqu copper
complexes. Comparison of them reveals that the TMG moiety
represents the stronger N-donor. In comparison with the parent
ligands TMG6etqu shows better donor properties.
Ligand and complex syntheses were performed under inert conditions by
using Schlenk techniques and a glove box under nitrogen atmosphere.
Solvents were purified according to literature and kept under inert
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
conditions.[39] Chemicals for the synthesis of the ligands as well as CuII-
( = 0.71073 Å) was used and temperature control was achieved with an
Oxford Cryostream 700. Crystals were mounted with grease on glass
fibers and data were collected at 100 K in -scan mode. Data were
collected with SMART[42], integrated with SAINT[42] and corrected for
absorption by multi-scan methods with SADABS.[41] The structure was
solved by direct and conventional Fourier methods and all non-hydrogen
atoms were refined anisotropically with full-matrix least-squares based on
F² (XPREP[43], SHELXS-97[44] and ShelXle[45]). Hydrogen atoms were
derived from difference Fourier maps and placed at idealised positions,
riding on their parent C atoms, with isotropic displacement parameters
Uiso(H) = 1.2Ueq(C) and 1.5Ueq(C methyl). All methyl groups were allowed
to rotate but not to tip.
salts for complex syntheses
were all purchased from Grüssing,
AppliChem, Acros Organics or TCI and were used as received without
further purification. CuIBr, CuICl and the Vilsmeier salts N,N’-
dimethylethylenechloroformamidinium
chloride
(DMEG-VS)
and
N,N,N’,N’-tetramethylchloroformamidinium chloride (TMG-VS) were
synthesised as described in the literature.[28,40] The ligands DMEGqu and
TMGqu were synthesised according to the literature.[41]
General analytical methods:
IR: KBr IR spectra were measured with a ThermoFisher Avatar 360
(Resolution 2 cm-1). ATR IR spectra where measured with a Shimadzu
IRTracer 100 with CsI beamsplitter in combination with a Specac Quest
ATR unit (Resolution 2 cm-1).
In C2 it was not possible to model the disordered solvent molecules (THF)
in an adequate manner, and the data set was treated with the SQUEEZE
routine as implemented in PLATON.[46,47]
MS: EI mass spectra were obtained with a ThermoFisher Scientific
Finnigan MAT 95 mass spectrometer. FAB mass spectra were obtained
with a Thermo Finnigan MAT 95 or a Jeol MStation 700. Ionisation took
place in 2-nitrobenzyl alcohol or glycerol as matrix on a copper target with
8 kV xenon atoms. ESI mass spectra were obtained with a ThermoFisher
Scientific LTQ Orbitrap XL. The source voltage was 4.49 kV, the capillary
temperature amounted to 299.54 °C. The tube lens voltage lay between
110 and 130 V.
In C6, the space group was checked with the ADDSYM routine as
implemented in Platon[46,47] and ADDSYM suggested pseudo-translations
(A-face centered) or the space group C2/c. Also a B-alert of the checkcif-
routine occured. The data of this complex show no integral reflections, and
the most disagreeable reflections are also unobstrusive. Furthermore, in
P21/n only 2229 reflections from 87813 were rejected and in the centered
space-groups 50% of the reflections were rejected, e.g. in C2/c 45041
from 87813. Also the structure solution in the centered space-groups are
not suited. Hence, a pseudo-symmetry is present.
1H- and 13C-NMR: 1H- and 13C-NMR spectra were measured on a Bruker
Avance III HD 400 or a Bruker Avance II 400 nuclear resonance
spectrometer. Measurements were performed in fully deuterated solvents.
The residual signal of the solvent served as an internal standard.
Full crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary no. CCDC – 1820775 for L1, CCDC – 1820776 for L3,
CCDC – 1820777 for C1, CCDC – 1820778 for C2, CCDC – 1820779 for
C3, CCDC – 1820780 for C4, CCDC – 1820781 for C5, CCDC – 1820782
for C6 and CCDC – 1820783 for C7. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Gel permeation chromatography:
The average molecular masses and the mass distributions of the obtained
polystyrene samples were determined by gel permeation chromatography
(GPC) in THF as mobile phase at a flow rate of 1 mL/min. The utilised
GPCmax VE-2001 from Viscotek is a combination of an HPLC pump, two
Malvern Viscotek T columns (porous styrene divinylbenzene copolymer)
with a maximum pore size of 500 and 5000 Å and a refractive index
detector (VE-3580) and a viscometer (Viscotek 270 Dual Detector).
Universal calibration was applied to evaluate the chromatographic results.
Computational details
Density functional theory (DFT) calculations were carried out with
Gaussian09 Revision D.01.[48] All calculations were performed using the
TPSSh [21] functional and def2-TZVP basis sets.[20] Starting geometries for
optimisation were generated from the crystal structures of DMEGqu,
UV/Vis setup for KATRP/kact determination:
DMEG6etqu,
DMEG6buqu,
[Cu(DMEGqu)2]Br,
[Cu(TMGqu)2]Br,
[Cu(DMEGqu)2Br]Br,
[Cu(TMGqu)2Cl]Cl
UV/Vis measurements were performed with an Avantes AvaSpec-
ULS2048 CCD-Spectrometer and an Avantes AvaLight-DH-S-BAL
lightsource. The measurements were done in Hellma QS-Screwcap-
Cuvettes with an optical pathlength of 10.00 mm.
[Cu(DMEGqu)2]Cl,
[Cu(TMGqu)2Br]Br,
[Cu(DMEG6etqu)2]Br,
[Cu(TMGqu)2]Cl,
[Cu(DMEGqu)2Cl]Cl,
[Cu(TMG6etqu)2]Br,
[Cu(DMEG6etqu)2]Cl,
[Cu(TMG6etqu)2]Cl, [Cu(DMEG6etqu)2Br]Br, [Cu(TMG6etqu)2Br]Br,
[Cu(DMEG6etqu)2Cl]Cl and [Cu(TMG6etqu)2Cl]Cl. To receive thermal
corrected values of ΔG, frequency calculations at room temperature
(25 °C) were performed. Calculations were conducted both in gas phase
and in solvent using the SMD model[25] with the implemented solvent
acetonitrile. All calculations were performed with Grimme dispersion and
Becke-Johnson damping GD3BJ.[22,23,49] NBO calculations were
performed with the NBO 6.0 software.[50]
CV measurements:
The measurements were performed at room temperature under inert
conditions with a Metrohm Autolab Potentiostat PGSTAT 101 using a
three electrode arrangement with a Pt disc working electrode (1 mm
diameter), a Pt wire as counter electrode and a Ag wire as reference
electrode (pseudo reference). The measurements were performed in
CH3CN / 0.1 mol L-1 NBu4PF6 with a sample concentration of 10 mM.
Ferrocene was added as an internal standard after the measurements of
the sample and all potentials are referenced relative to the Fc/Fc+ couple.
Cyclic voltammograms were measured with 200 mV/s, 100 mV/s, 50 mV/s
and 20 mV/s.
Polymerisation procedure:
Styrene (Acros Organics, 99 % stab.) and the initiator ethyl α-
bromoisobutyrate (EBrib, abcr, 98 %) have been freshly distilled over
CaH2. All polymerisations were performed with in situ generated catalysts.
First the copper salt (0.19 mmol, CuIBr: 27 mg) then ligand (0.38 mmol,
DMEG6etqu: 102 mg, TMG6etqu: 103 mg, DMEG6buqu: 113 mg,
TMG6buqu: 113 mg) were directly weighed into the polymerisation vessel
under inert conditions inside a glovebox. Outside the glove box the
prepared Schlenk tube was then connected to a Schlenk line. Styrene
X-ray diffraction analysis:
The single crystal diffraction data for L1, L3 and C1 to C7 are presented
in Tables S8-S10 . All data were collected on a Bruker D8 goniometer with
APEX CCD detector. An Incoatec microsource with Mo-K radiation
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10.1002/ejic.201800511
European Journal of Inorganic Chemistry
FULL PAPER
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(19 mmol, 2.2 mL) and finally the initiator (0.19 mmol, EBrib: 28 μL) were
added with gastight glass syringes.
After addition of the initiator, the mixture was heated (110 °C) under
vigorous stirring. The first sample was taken with a glass pipette under
inert conditions after 2.5 min. At this point of time the polymerisation
mixture reached its desired temperature and thus was chosen to be
starting point of the polymerisation. Further samples were taken in certain
time intervals. The samples were mixed with CDCl3 and the conversion
measured by 1H-NMR spectroscopy. Afterwards the polymer was
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KATRP determination:
All measurements were performed in oxygen free acetonitrile at 22 °C.
The acetonitrile has been degassed by three freeze-pump-thaw cycles.
Stock solutions of the complexes and the cuvettes were prepared in a
glovebox under inert conditions.
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2 mL of acetonitrile) were prepared. A screw cap cuvette containing a
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kact determination:
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Acknowledgements
Financial support by the Fonds der Chemischen Industrie (Fonds
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Keywords: Copper • ATRP • Guanidine • Molecular structure •
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Entry for the Table of Contents (Please choose one layout)
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A
synthetic approach towards C6
Thomas Rösener, Alexander Hoffmann
and Sonja Herres-Pawlis
substituted guanidine quinoline lig-
ands is presented. CuIX and CuIIX2 (X
= Br or Cl) complexes of alkylated
ligands DMEG6etqu, TMG-6etqu,
DMEG6buqu and TMG6buqu are
investigated with regard to their
structures, redox potential and activity
in ATRP. By DFT, NBO and isodesmic
reactions the influence of substituents
on the electrochemistry and ATRP
properties was further examined.
Page No. – Page No.
Next Generation of Guanidine
Quinoline Copper Complexes for
Highly Controlled ATRP: Influence of
Backbone Substitution on Redox
Chemistry and Solubility
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