Metal-Bonded Redox-Active Triarylamines and Their Interactions: Synthesis, Structure, and Redox Properties of Paddle-Wheel Copper Complexes

Article abstract of DOI:10.1002/open.201800243

Four new triphenylamine ligands with different substituents in the para position and their corresponding copper(II) complexes are reported. This study includes their structural, spectroscopic, magnetic, and electrochemical properties. The complexes possess a dinuclear copper(II) paddle-wheel core, a building unit that is also common in metal-organic frameworks. Electrochemical measurements demonstrate that the triphenylamine ligands and the corresponding complexes are susceptible to oxidation, resulting in the formation of stable radical cations. The square-wave voltammograms observed for the complexes are similar to those of the ligands, except for a slight shift in potential. Square-wave voltammetry data show that, in the complexes, these oxidations can be described as individual one-electron processes centered on the coordinated ligands. Spectroelectrochemistry reveals that, during the oxidation of the complexes, no difference can be detected for the spectra of successively oxidized species. For the absorption bands of the oxidized species of the ligands and complexes, only a slight shift is observed. ESR spectra for the chemically oxidized complexes indicate ligand-centered radicals. The copper ions of the paddle-wheel core are strongly antiferromagnetic coupled. DFT calculations for the fully oxidized complexes indicate a very weak ferromagnetic coupling between the copper ions and the ligand radicals, whereas a very weak antiferromagnetic coupling is found among the ligand radicals.

Full text of DOI:10.1002/open.201800243

Full Papers
DOI: 10.1002/open.201800243
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Metal-Bonded Redox-Active Triarylamines and Their
Interactions: Synthesis, Structure, and Redox Properties of
Paddle-Wheel Copper Complexes
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Oluseun Akintola, Michael Böhme, Manfred Rudolph, Axel Buchholz, Helmar Görls, and
Winfried Plass*[a]
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Four new triphenylamine ligands with different substituents in
the para position and their corresponding copper(II) complexes
are reported. This study includes their structural, spectroscopic,
magnetic, and electrochemical properties. The complexes
possess a dinuclear copper(II) paddle-wheel core, a building
unit that is also common in metal-organic frameworks. Electro-
chemical measurements demonstrate that the triphenylamine
ligands and the corresponding complexes are susceptible to
oxidation, resulting in the formation of stable radical cations.
The square-wave voltammograms observed for the complexes
are similar to those of the ligands, except for a slight shift in
potential. Square-wave voltammetry data show that, in the
complexes, these oxidations can be described as individual
one-electron processes centered on the coordinated ligands.
Spectroelectrochemistry reveals that, during the oxidation of
the complexes, no difference can be detected for the spectra of
successively oxidized species. For the absorption bands of the
oxidized species of the ligands and complexes, only a slight
shift is observed. ESR spectra for the chemically oxidized
complexes indicate ligand-centered radicals. The copper ions of
the paddle-wheel core are strongly antiferromagnetic coupled.
DFT calculations for the fully oxidized complexes indicate a very
weak ferromagnetic coupling between the copper ions and the
ligand radicals, whereas a very weak antiferromagnetic coupling
is found among the ligand radicals.
1. Introduction
rings particularly at the para positions.[6] Moreover, the molec-
ular framework of triarylamines is also known to possess
structural flexibility, which when coupled together with bulky
substituents can lead to distortion or rotation within its
structure.[7]
Triphenylamine-based molecules and their derivatives are a
class of compounds widely studied due to their importance for
the design of electronic materials,[1] which is particularly related
to their pronounced hole-transport ability.[2] This fact coupled
with the relatively low ionization potentials of triarylamines has
led to their extensive application in electroluminescent devices
as well as in photovoltaic materials.[3] Triarylamines typically
undergo oxidation with ease resulting in the formation of a
stable triarylammonium radical cation,[4] the stability of which
depends on whether a para substitution at the aromatic rings is
present or not.[5] For the cases of triphenylamines with an
unsubstituted para position, these radicals are known to under-
go dimerization to give tetraphenylbenzidines in a well-known
oxidative coupling process. However, for the substituted
derivatives a reversible redox behavior is observed, which can
be tuned by varying the substituents attached to the aromatic
The unique electronic character of the triphenylamine
moiety has led to its incorporation into molecular frameworks.
This allows for interesting combinations of electronic and
photophysical properties leading to new materials with appeal-
ing characteristics.[8] In particular, transition metal complexes
with triphenylamine-based ligands have been used to inves-
tigate the charge-transfer properties of mixed valence sys-
tems.[9] In the latter cases, the well-known dinuclear paddle-
wheel motif M2(RCOO)4 was used in combination with carbox-
ylate-functionalized triphenylamine ligands. Moreover, this
paddle-wheel motif is a common building unit for microporous
coordination polymers, the so-called metal-organic frameworks
(MOFs).[10] The dinuclear paddle-wheel fragment found in
copper(II) acetate,[11] which can be regarded as an archetype for
such building units, has been extensively utilized in MOFs since
the seminal report on HKUST-1.[12] Such copper-based MOF
systems are attracting significant interest and have been
intensively investigated towards their electronic structure and
other properties as well as potential applications.[13] On the
other hand, 4,4’,4’’-nitrilotribenzoic acid (H3ntb), a carboxylate
derivative of triphenylamine, has found extensive use as linker
in the construction of MOFs.[14] The molecular properties of the
ntb3À linker result in characteristic redox- and photo-active
behavior of the corresponding MOFs which has led to specific
applications such as sensing, catalysis, and generation of
[a] Dr. O. Akintola, M. Böhme, Dr. M. Rudolph, Dr. A. Buchholz, Dr. H. Görls,
Prof. Dr. W. Plass
Institut für Anorganische und Analytische Chemie
Friedrich-Schiller-Universität Jena
Humboldtstr. 8, 07743 Jena, Germany
E-mail: sekr.plass@uni-jena.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.201800243
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This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited
and is not used for commercial purposes.
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nanoparticles for hydrogen storage.[15] In fact, MOFs containing
redox-active linkers of different type have been recently
addressed toward their electrochemical behavior[16] and it could
be shown that this can be used to control the properties of the
corresponding frameworks.[17] Moreover, the combination of
ntb3À with ditopic linkers has been employed in the coordina-
tion copolymerization approach leading to multicomponent
MOFs with interesting architectures including pillared-layer
frameworks.[18] In this context, it should also be mentioned that
extended triphenylamine-based linkers with two nitrogen
centers have been utilized in the construction of MOFs with
unusual architectures and properties.[19] In addition, also the
generation of MOFs based on a branched triphenylamine-
derived linker with even four nitrogen centers has been
described.[20]
In this contribution, we present molecular model systems
for the interaction of the copper paddlewheel moiety with
triphenylamine-based ligands. For this purpose, the synthesis
and characterization of four complexes of the general formula
[Cu2(L)4(dmf)2] with ligands derived from 4-(diphenylamino)
benzoic acid is reported. Of particular interest here is the redox
behavior of the triphenylamine moiety and its interaction with
the dinuclear core. This is investigated in view of the electronic
and steric variation of the substituent at the para positions of
the phenyl groups of the 4-(diphenylamino)benzoic acid ligand
(R=H, Me, t-Bu, and OMe). To this end, electrochemical,
structural, and magnetic properties are reported and comple-
mented by theoretical calculations.
amino)benzoic acid (Haba-Me, 2), 4-(bis(4-tert-butylphenyl)
amino)benzoic acid (Haba-tBu, 3), and 4-(bis(4-methoxyphenyl)
1
2
3
4
5
6
7
8
9
amino)benzoic acid (Haba-OMe, 4) were obtained via
a
BuchwaldÀ Hartwig palladium catalyzed coupling reaction. For
each ligand its bromophenyl precursor (4-bromotoluene, 4-tert-
butylbromobenzene, and 4-bromoanisole for 2, 3, and 4,
respectively) was combined with methyl 4-aminobenzoate
while using Cs2CO3 as base. The resultant ester was subse-
quently subjected to alkaline hydrolysis followed by acid-
ification with concentrated aqueous HCl (see Scheme 2).[23]
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Scheme 2. Preparation scheme for 2–4: a) Pd/P(t-Bu)3; Cs2CO3, toluene,
°
110 C, 5–7 d; b) KOH, MeOH, reflux; c) HCl.
The synthesis of the copper complex [Cu2(aba)4(dmf)2] (5)
was performed by a solvothermal process in dmf using copper
(II) nitrate. The use of acetonitrile was avoided due to its
reported tendency to aid oxidation of triphenylamines in the
presence of copper(II) ions, which in turn leads to the formation
of the benzidine derivative.[24] An alternative route to synthesize
5 is given by reacting the ligand and copper(II) nitrate in
ethanol. By layering the resultant solution with dmf it was
possible to obtain green crystals of 5 within a period of 2–3
weeks. Both routes led to good quality crystals suitable for X-
ray crystallographic studies. Similarly also the complex [Cu2(aba-
Me)4(dmf)2] (6) was prepared via the solvothermal route, which
yielded a light green solution. To reduce the solubility of the
product methanol was slowly layered on top of the obtained
solution, which was allowed to stand for a few hours, upon
which a light green precipitate formed. Additional material as
green block crystals was obtained from the filtrate after it was
left standing for slow evaporation. An alternative route to 6 was
established by heating a solution of both the ligand and copper
2. Results and Discussion
2.1. Synthesis and Characterization
The ligand 4-(diphenylamino)benzoic acid (Haba, 1) was
obtained in a two-step synthetic route. In the first step, the
formylation of triphenylamine was performed through the
VilsmeierÀ Haack reaction using phosphoryl chloride in dime-
thylformamide.[21] The resulting aldehyde was subsequently
oxidized with KMnO4 under alkaline conditions followed by
acidification of the resultant potassium salt using concentrated
aqueous HCl according to a modified published procedure for
the 4,4’-(phenylazanediyl)dibenzoic acid (see Scheme 1).[22]
On the other hand, the ligands with substituents at the para
position of the two phenyl rings, namely 4-(bis(4-methylphenyl)
°
(II) nitrate in dmf at 110 C for 30 min. Subsequent cooling of
the reaction solution and allowing it to stand for a few days
resulted in the formation of micro-crystalline product, which
was unfortunately not suitable for X-ray crystallography. This
latter route of simply refluxing a mixture of the metal salt and
ligand in dmf was successfully used in the synthesis of the
complex [Cu2(aba-tBu)4(dmf)2] (7), yielding crystalline material
also suitable for X-ray structure determination. For the complex
[Cu2(aba-OMe)4(dmf)2] (8) again the solvothermal route similar
to 5 and 6 was employed for the synthesis. However, this only
led to the formation of dark green micro crystals which were
not suitable for single crystal X-ray studies. Nevertheless,
crystals suitable for X-ray studies were obtained by stirring both
ligand and metal salt in dmf for about 20 min followed by
slowly layering methanol on top of this solution. Both solvents
Scheme 1. Synthesis of 4-(diphenylamino)benzoic acid (Haba, 1): a) POCl3,
°
dmf, 0 C; b) reflux under N2 for 22 h; c) K2CO3, KMnO4, acetone, reflux, 12 h;
d) HCl.
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were then allowed to slowly evaporate over 4 weeks to yield
green crystals of 8.
are displaced by about 18 pm from the basal [O4] plane towards
the dmf oxygen donor atom.
The phenyl rings of the triphenylamine moieties in the
complexes are twisted out of the plane of the central nitrogen
atom (cf. Figure S6). This is enforced by repulsion between the
phenyl rings and packing effects, leading to corresponding
1
2
3
4
5
6
7
8
9
The composition of the obtained bulk material for com-
pounds 5–8 was analyzed by elemental and thermogravimetric
analysis, which revealed the presence of varying amounts of
different solvent molecules of crystallization. The corresponding
data is given in Figure S1 and Table S1.
°
angles Φ within the range from 21 to 65 (see Table S4).
Moreover, also the planarity at the central nitrogen atom varies
considerably (average distance of the nitrogen atom from the
plane: 3.4, 5.9, 7.1, and 13.9 pm for 5, 6, 7, and 8, respectively;
cf. Table S4), which is most likely also indicative for electronic
effects of the substituents at the para position, similar to what
is observed for electrochemical properties and UV/vis spectra
(vide infra). In contrast, the dihedral angle between the
carboxylate group coordinating the central paddle-wheel unit
and the connected phenyl group (see Figure S7) shows a
considerably smaller variation within the range from 5.2 to
2.2. X-ray Crystal Structures
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The crystallographic characterization reveals that all four
�
complexes 5–8 crystallize in the triclinic space group P1 (see
Table S2 for details). The molecular structure of 5 depicted in
Figure 1 is representative of the other isostructural complexes
°
27.8 , consistent with an overall reduced influence of the
above-mentioned effects. As expected the four nitrogen centers
of the triphenylamine moieties show an almost rectangular
planar arrangement with N···N distances of about 1160 pm
along the edges with the largest variation observed for 8 (1088
and 1235 pm) while the diagonal distances are approximately
1647 pm (see Figure S8 and Table S5).[25]
Except for the crystal structure of complex 8 all the other
three complexes 5–7 contain additional solvent molecules of
crystallization. In the case of complex 6 one co-crystallized dmf
molecule was found that is disordered over two crystallographic
positions. The X-ray powder diffraction (XRPD) measurements
on bulk material of complexes 6 and 8 is in agreement with the
corresponding simulated patterns obtained from the single
crystal X-ray measurements (see Figure S9). However, the
situation is somewhat different for the complexes 5 and 7,
which both possess a rather large void space in the crystal
structure. The disordered solvent molecules crystallized in these
voids could not be located and have been treated using the
SQUEEZE routine (see Experimental Section). Consequently, for
the latter two complexes (5 and 7) differences between the
experimental and simulated XRPD patterns are observed (see
Figure S9). This can be attributed to corresponding structural
changes related to solvent loss upon drying the bulk material
or the possible presence of a second crystalline phase with the
same molecular composition. Moreover, differences in inten-
sities of individual reflections may also be due to orientation
effects of the crystallites.
Figure 1. Molecular structure of 5 with hydrogen atoms omitted for clarity.
6–8 and shows that a dimeric paddle-wheel arrangement is
adopted. As the center of the paddle-wheel dimers, for the
crystal structures of all complexes, is situated on a crystallo-
graphic inversion center, only half of the complex molecules are
within the asymmetric units. A comparison of the selected
bond lengths in all four complexes is summarized in Table S3
and additional representations of the molecular structures of 6–
8 including full labeling are given in Figures S2–S5.
The molecular structure consists of four bridging carbox-
ylate ligands and two axial coordinated dmf ligands at the two
copper(II) ions as shown in Figure 1. Each copper ion is
coordinated by four oxygen atoms from different carboxylate
groups in the equatorial plane with distances at around
196 pm. The carboxylate CÀ O bond lengths are close to
126 pm, while the Cu···Cu distances are at about 260 pm.
Together with the oxygen donor of the coordinating dmf
molecule in the axial position at a distance of about 216 pm
(Cu1–O1D, see Table S3) this leads to a square pyramidal
coordination geometry. For all complexes the copper centers
2.3. Magnetic Properties
The magnetic susceptibility data for the complexes 5–8 were
obtained in the temperature range from 4 to 300 K. Figure 2
shows the representative data for complex 5 as temperature-
dependent plot of χMT (for data 6–8 see Figure S10). The χMT
values at 300 K for the four complexes 5–8 are about
0.46 cm3 KmolÀ 1, which is only slightly above half the spin-only
value of 0.75 cm3 KmolÀ 1 expected for two independent copper
(II) ions.[26] Upon lowering the temperature the χMT value
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calculated coupling constants can be found (jJcalcd j =336–
351 cmÀ 1) indicating a negligible electronic effect of the differ-
ent substituents present in the ligand backbone. In fact, the
observed variation based on the structural disorder of the dmf
co-ligands in 7 is with 10 cmÀ 1 (7(A): À 336; 7(B): À 346 cmÀ 1) in
the same range as the observed variation over all complexes 5–
8. Spin density plots for the high-spin and broken-symmetry
state of the complexes 5–8 (see Figures S11–S14) show that the
spin density in all complexes is primarily localized on the
1
2
3
4
5
6
7
8
9
2
2
paddle-wheel core, i.e. in the dx À y magnetic orbitals of the
copper(II) ions. This further supports the idea of negligible
electronic effects of the para substituents at the triphenylamine
moiety.
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To further investigate the effect of the triphenylamine
moieties and their attached para substituents on the exchange
coupling within the dicopper core unit two additional structur-
ally reduced models have been used for BS-DFT calculations. In
the medium model (5M–8M) the para substituted phenyl groups
at the bridging aminobenzoates have been replaced by methyl
groups, whereas in the small model (5S–8S) the amine
substituent at the bridging benzoates have been completely
removed and replaced by hydrogen atoms. For both reduced
models the structural parameters are based on the crystal
structures of the complexes 5–8. The corresponding results
summarized in Tables S7 and S8 show that both reduced
models can reproduce the strong antiferromagnetic coupling
obtained from the calculations for the full complexes. Interest-
ingly, the results from both reduced models differ with a mean
value less than 3 cmÀ 1 from the values obtained for the
calculations based on the original structure of the complexes 5–
8, with two slightly larger deviations observed for 6S (6 cmÀ 1)
and 8S (4 cmÀ 1). This further indicates that neither the
substitution pattern at the triphenylamine moieties nor the
amino groups as such at the bridging benzoate groups have a
significant effect on the exchange coupling between the
copper(II) centers of the paddle-wheel unit.
Figure 2. Temperature dependence of the magnetic susceptibility χMT for
complex 5 measured at an applied field of 2 kOe. The solid red line
represents the best fit.
decreases to very small values in all cases at a temperature of
about 70 K indicating a diamagnetic ground state. Overall, this
behavior is consistent with antiferromagnetic exchange cou-
pling between the copper(II) centers of the paddle-wheel
moiety.[27] The experimental data were fitted using the program
PHI[28] applying the Heisenberg Hamiltonian H ¼ À JS1S2:The
b
b b
best fit is obtained for the parameters given in Table 1. The
Table 1. Parameters obtained from fitting the magnetic susceptibility data
of 5–8 with corresponding data obtained from theoretical calculations
(vide infra).
g
J
exp [cmÀ 1
]
J ]
calcd [cmÀ 1
5
6
7
8
2.14
2.12
2.18
2.23
À 321
À 295
À 330
À 320
À 345
À 351
À 336, À 346(a)
À 345
[a] The single-crystal structure of 7 contains a disorder for the dmf co-
ligands at the copper centers.
resulting exchange coupling constants of the four complexes
are virtually invariant with respect to the type of substituent
present at the para positions of the phenyl groups of the
triphenylamine ligand moieties.
2.4. Electrochemical Properties
The cyclic voltammogram of ligand 1 is indicative for an
irreversible process with a single peak in the anodic scan,
corresponding to the oxidation of monomeric species to radical
cations, which subsequently dimerize to form a tetraphenylben-
zidine (TPB) dication.[30] In the following cathodic scan two
reduction peaks were observed, which can be attributed to the
reduction of the TPB dication to the monocation and then
further to the neutral dimer (see Figure S15).[31] The same
behavior was likewise seen for complex 5 and is consistent with
reported triphenylamine systems that have an attached para
substituent only at one of the phenyl rings.[32] The observed
tendency to dimerize unfortunately hampered any further
detailed studies on these systems.
To corroborate the experimental data and to gain further
insight with respect to the coupling constants we have
performed broken-symmetry DFT (BS-DFT) calculations for the
complexes 5–8. The corresponding results are included in
Table 1 (for detailed data see Table S6). The BS-DFT calculations
confirm the strong antiferromagnetic coupling between the
copper centers of the paddle-wheel unit obtained by simulating
the experimental data. These strong antiferromagnetic ex-
change interactions are a result of the congruent alignment of
the square planar coordination environment of the two copper
(II) centers in the complexes which are bridged by four aryl
carboxylates. Nevertheless, the calculated coupling constants
slightly overestimate the experimental values,[29] where the
For the analogous para substituted ligands 2–4 square-
wave voltammetry measurements were carried out. The
absence of the oxidative coupling reaction due to the occupied
para positions allows for the observation of more detailed
largest deviation is found for 6 (Jexp =À 295 cmÀ 1; Jcalcd
=
À 351 cmÀ 1). Moreover, only
a small variation within the
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features related to the electron-transfer process in these cases.
The oxidation of the ligands 2–4 can be described in terms of
single quasi-reversible one-electron oxidation steps. The under-
lying mechanism for the simulation of the recorded square-
wave voltammetric data is given in Equations (1) and (2), where
1
2
3
4
5
6
7
8
°
E is the standard potential, ks the heterogeneous rate constant,
α the charge-transfer parameter, D the diffusion coefficient, and
kf the rate constant for the subsequent chemical reaction.
HLþ þ eÀ Ð HL ðE�, ks, a, DÞ
HLþ ! P ðkfÞ
ð1Þ
ð2Þ
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The data obtained from simulation are summarized in
Figure 3. Square-wave voltammograms of the ligand 4 (c=1.95 mM) in
dichloromethane solution at square-wave frequencies of 25, 50, 100, 200,
400, 600, and 750 Hz. Open circles represent simulated and lines
experimental data.
°
Table 2. The reported standard potentials E are assigned to the
Table 2. Experimental parameters from simulating square-wave voltammo-
grams of the ligands 2–4.
2
Me
3
t-Bu
4
OMe
R
D [10À 6 cm2 sÀ 1
]
8.5
6.9
7.8
°
E [mV]
550
0.13
�4
556
0.16
�4
388
0.14
�4
ks [cmsÀ 1
]
kf [10À 4 sÀ 1
]
half-wave potentials derived from square-wave voltammetry.
This assignment is justified by approximately equal diffusion
coefficients of the oxidized and reduced species,[33] which was
confirmed by cyclic voltammetric studies at a platinum disk
electrode (∅=10 μm). Moreover, the charge-transfer parameter
was assumed to be α=0.5, since varying this parameter over
the range 0.45�α�0.55 has only a negligible small effect on
the standard deviation between simulated and experimental
curves. It should be noted that the kf values reported in Table 2
not only minimize the difference between experimental and
simulated square-wave voltammograms, but also the difference
between experimental and simulated thin-layer cyclic voltam-
mograms measured in the course of the spectroelectrochemical
experiments (vide infra). A thorough analysis of the electro-
chemical data for the ligand systems 2–4 (see Figure S16)
Figure 4. Square-wave voltammograms of the complex 8 (c=1.18 mM) in
dichloromethane solution at square-wave frequencies of 25, 50, 100, 200,
400, 600, and 750 Hz. Open circles represent simulated and lines
experimental data.
The square-wave voltammograms recorded for the com-
plexes 6–8 show a rather similar behavior as those observed for
their corresponding ligands. In Figure 4 the square-wave
voltammograms of 8 are depicted as a representative example
(for 6 and 7 see Figure S18). The oxidation of the complexes 6–
8 can be described according the underlying mechanism given
°
reveals that an error of 1 mV in the standard potential E has
°
approximately the same effect on the standard deviation as
errors of about 0.3×10À 6 cm2 sÀ 1 in the diffusion coefficient D
and about 0.05 cmsÀ 1 in the heterogeneous rate constant ks,
respectively. In fact, with respect to the estimated errors the
heterogeneous rate constant is essentially the same for all
ligands, i.e. ks �0.15�0.05 cmsÀ 1.
in Equations (3)–(6), where Ei are the standard potentials for
the individual one-electron steps.
�
½Cu2ðaba-RÞ4�þ þ eÀ Ð ½Cu2ðaba-RÞ4� ðE1 Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
½Cu2ðaba-RÞ4�2þ þ eÀ Ð ½Cu2ðaba-RÞ4�þ ðE2 Þ
�
2þ
�
½Cu2ðaba-RÞ4�3þ þ eÀ Ð ½Cu2ðaba-RÞ4� ðE3 Þ
A comparison of the diffusion coefficients of the ligands 2–4
shows the expected trend with the bulky tert-butyl substituted
ligand exhibiting the slowest diffusion followed by the methoxy
and methyl analogues in that order. Similarly, the values of the
standard potentials match the expected electronic trend given
by the para substituents at the triphenylamine moiety.[34] The
representative square-wave voltammograms of ligand 4 are
displayed in Figure 3 and show good agreement between
simulated and experimental data (for 2 and 3 see Figure S17).
3þ
�
½Cu2ðaba-RÞ4�4þ þ eÀ Ð ½Cu2ðaba-RÞ4� ðE4 Þ
Consistent with the data presented for the ligands also for
the complexes the charge-transfer parameter α was supposed
to be 0.5 for all four charge-transfer reactions. Moreover, it was
further assumed that the diffusion coefficient D and the
heterogeneous rate constant ks are related to the particular
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complex system and are independent of the charge of the
individual species. The parameters obtained from simulation of
the experimental square-wave voltammograms are presented
in Table 3.
(Equation (6)) is close to the potential observed for the
corresponding ligand (see Tables 2 and 3).
1
2
3
4
5
2.5. Spectroelectrochemistry
6
7
8
9
Table 3. Experimental parameters obtained from simulating square-wave
voltammograms of the complexes 6–8.
To further explore the electrochemical oxidation of the
complexes 6–8 and to probe the electronic properties of
oxidized species spectroelectrochemical investigations have
been performed. In order to address this point we first
performed spectroelectrochemical measurement for the corre-
sponding ligands to examine their properties as a basis for the
understanding of the related complexes. It has to be noted here
that a general assumption is required for simulating electro-
chemical data recorded with an optically transparent thin-layer
electrochemical (OTTLE) cell as used in our experiments.
Generally, the theoretical model to simulate thin-layer cyclic
voltammograms assumes a cell geometry for which one of the
boundaries is a smooth optically transparent electrode, while
the other being an optically transparent insulator. However, the
experimental thin-layer cell consists of two optically transparent
6
Me
7
t-Bu
8
OMe
R
D [10À 6 cm2 sÀ 1
]
4.8
3.2
4.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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34
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
°
E1 [mV]
451
497
521
558
0.170
467
503
527
562
0.110
283
329
351
389
0.150
°
E2 [mV]
°
E3 [mV]
°
E4 [mV]
ks [cmsÀ 1
]
Qualitatively the diffusion coefficients of the complexes are
following the same trend as observed for the related ligands.
Nevertheless, the absolute values are as expected smaller than
those of the corresponding ligands. Moreover, the voltammo-
grams are only dependent on the ligand present, rather than
being indicative of a metal ion-centered process. The standard
potentials observed for the complexes 6–8 show a slight shift
which corresponds to the substitution pattern of the related
ligand. The individual oxidation steps given in Equations (3) to
(6) can be described as single one-electron processes despite
the presence of four oxidizable centers. Such a behavior is
theoretically expected if the interactions between the electro-
chemically active centers are negligibly small and the charge-
transfer process is virtually reversible.[35] Simulation of the
experimental data allowed determination of the separation
between the standard potential of the four oxidation steps
which are summarized in Table 4 and compared to the
corresponding values expected for a simple entropy effect with
ΔES.[36]
insulators having
a platinum net electrode in between.
Consequently, unlike in the case of the simulated cyclic
voltammograms, the shape of the experimental ones does not
only depend on diffusion processes occurring perpendicular to
the electrode, but also on processes occurring within the
meshes of the platinum net.
In Figure 5 the experimental and simulated thin-layer cyclic
voltammograms for ligand 3 and complex 7 are depicted as
representative examples (for ligands 2 and 4 as well as
complexes 6 and 8 see Figure S19), indicating a suitable
agreement and proving the validity of the assumption made to
simulate the data (also cf. Table 2). In fact, it is obvious that the
differences between both data sets are smaller for lower scan
rates. This is consistent with the limiting condition that the
shape of the thin-layer cyclic voltammograms becomes inde-
pendent of the diffusion processes, if the scan rate and/or the
cell thickness tends toward zero.
The UV/vis absorption spectra recorded for ligand 3 during
the electrochemical forward and back ward scan are depicted
as a representative example in Figure 6 (for 2 and 4 see
Figure S20). A factor analysis of the spectra obtained for the
three ligands during thin-layer cyclic voltammetry experiments
revealed that only a single optically active species is produced
in the forward scan, while in the backward scan this species is
completely consumed, which is consistent with the observed
quasi-reversible one-electron oxidation steps for the ligands in
the square-wave voltammetry measurements. Consequently, it
was possible to determine the UV/vis absorption spectra of the
corresponding oxidized ligands which are depicted in Figure 7.
The values of the molar extinction coefficients are based on the
assumption that the optical path length of the thin-layer cell is
exactly 0.2 mm. However, the uncertainty estimated for the
absolute value is about 10%, despite the fact, that the
reproducibility of the individual species spectra was distinctly
better in a series of independent experiments.
Table 4. Separations between the standard potential of individual one-
electron processes for complexes 6–8 as compared with value expected
from a solely entropy-related effect ΔES.
6
Me
7
t-Bu
8
OMe
ΔES
R
°
°
E2 –E1 [mV]
46
24
37
36
24
35
46
22
38
25
21
25
°
°
E3 –E2 [mV]
°
°
E4 –E3 [mV]
An important observation to note is that experimental
square-wave voltammograms are distinctly broader than ex-
pected from the pure entropy effect for a system containing
four chemically identical oxidizable ligands. Moreover, this
discrepancy also holds for the differences between the
individual standard potentials of the four oxidation steps, for
which significantly larger values are observed than those
estimated from the presence of a solely entropy effect of four
chemically equivalent oxidizable fragments in the system.
Remarkably, the standard potential of the fourth oxidation step
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Figure 5. Thin-layer cyclic voltammograms of ligand 3 (top, c=2.40 mM) and
complex 7 (bottom, c=0.59 mM) in dichloromethane solutions using scan
rates of 1.25, 2.5, and 5 mVsÀ 1. Open circles represent simulated and lines
experimental data.
Figure 6. UV/vis spectra recorded during the electrochemical forward (top,
blue to red line) and backward (bottom, red to blue line) scan for ligand 3
(c=2.40 mM). The difference in potential between successive spectra is
20 mV.
The maxima in the observed UV/vis spectra of the oxidized
ligands were found at 701, 711, and 784 nm for 2, 3, and 4,
respectively, which can be assigned as the HOMO!LUMO
transitions, with both orbitals having π character. For all three
oxidized ligands additional less intense bands at higher
energies are observed (2: 570; 3: 576; 4: 544 and 584 nm), which
is typical for triphenylamine systems with lower symmetry.[37]
This shows a clear trend for the ligands that the lowest energy
absorption is shifted toward higher wavelengths within the
series of methyl (2), tert-butyl (3), and methoxy (4) substitution,
which is consistent with the variation of the electronic proper-
ties of para substituents and also reflects the trend observed for
the standard potentials (cf. Table 2). Moreover, the observed
trend is also consistent with the values reported for the
corresponding symmetrically substituted triphenylamines.[4,38]
Interestingly, the extinction coefficient of the absorption
maximum for the methoxy substitute ligand 4 is significantly
larger than those observed for ligands 2 and 3.
Spectroelectrochemical investigations were also performed
for the complexes 6–8 by recording UV/vis absorption spectra
during the electrochemical forward and backward scan, which
are shown for 7 as a representative example in Figure 8 (for 6
and 8 see Figure S21). As in the case of the ligands factor
analysis of the spectra of the copper complexes reveals that
only a single optically active species is produced in the forward
scan of each thin-layer cyclic voltammetry experiment, which is
Figure 7. UV/vis spectra of the oxidized species for ligands 2 (black line), 3
(red line), and 4 (blue line) determined from spectroelectrochemical experi-
ments.
consumed during the following backward scan. Therefore, the
absorption spectra of the oxidized copper complexes can be
extracted from the data, again assuming an idealized optical
path length as in the ligand case. The obtained spectra of the
complexes 6–8 are depicted in Figure 9.
For the absorption maxima of the complexes a similar basic
trend as in the case of the corresponding ligands is observed,
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maximum. A comparison of the spectra obtained for the ligands
and complexes reveals an approximately four-fold increase in
the extinction coefficient (see Figure S22). Therefore, it is
tempting to assume that the complex spectra can be viewed as
a simple sum of four individual ligand spectra. This view is
consistent with the fact that only one optically active species is
generated during the oxidation of the complexes 6–8. In fact,
there is no indication for any difference in the spectra of the
oxidized species obtained by successive oxidations steps, which
means that all oxidized species [Cu2(aba-R)4]n+ independent of
their charge have the same spectra only related to the variation
of the para substituents. This is further evidenced by the
comparison of UV/vis spectra recorded at different potentials
during the spectroelectrochemical oxidation, which are related
to different molar fractions of the possible charged species. The
latter can be concluded from the fact that the difference in
standard potentials of the four oxidation processes cannot
solely be related to the pure entropy term (cf. Table 4). Indeed,
applying an appropriate scaling factor the spectra obtained for
intermediate potentials can be superimposed with the spec-
trum of the fully oxidized complex cation (see Figure S23 for
complex 6 as a representative case).
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2.6. Chemical Oxidation of Ligands and Complexes
Figure 8. UV/vis spectra recorded during the electrochemical forward (top,
blue to red line) and backward (bottom, red to blue line) scan for complex 7
(c=0.59 mM). The difference in potential between successive spectra is
20 mV.
Chemical oxidation of the ligands and complexes was carried
out with silver tetrafluoroborate as an oxidant in dichloro-
methane solution. It was observed that the methoxy derivatives
4 and 8 showed greatest ease in oxidation, for which it was
possible to obtain the oxidized species upon addition of
stoichiometric amounts of oxidant. This is consistent with the
appreciably lower standard potential observed for 4 and 8 with
respect to the other derivatives (see Tables 2 and 3). The
corresponding UV/vis absorption spectra recorded for the
chemically oxidized species of the ligand 4 and complex 8 are
depicted in Figure 10. However, due to their higher standard
potentials, the methyl (2 and 6) and tert-butyl (3 and 7)
counterparts required an excess of oxidant to obtain the radical
species. The corresponding UV/vis spectra of their chemically
oxidized species depicted in Figure S24 are in full agreement
with the data obtained from the spectroelectrochemical experi-
ments (vide supra). It is interesting to note here that the
oxidized species of all ligands and complexes did generally
show a remarkable stability, as their spectra did not change
appreciably over time for at least several days, particularly
when care was taken to prevent solvent evaporation. This even
holds for samples which were appropriately stored in contact to
ambient atmosphere.
Figure 9. UV/vis spectra of the oxidized species of the complexes 6 (black
line), 7 (red line), and 8 (blue line) determined from spectroelectrochemical
experiments.
where again the highest wavelength is found for the methoxy
derivative 8. However, comparing the absorption maxima of the
ligands and complexes a slight shift to lower wavelengths is
observed for the complexes with about 5, 6, and 10 nm for 6, 7,
and 8, respectively (cf. Figures 7 and 9), indicative for only
minor contributions of the copper paddle-wheel unit on the
optical properties of the oxidized ligand moieties in the
complexes. As in the case of the ligands the methoxy derivative
shows the largest extinction coefficient for the absorption
To further characterize the oxidized species derived from
the ligands 2–4 and complexes 6–8 room temperature X-band
ESR spectra were recorded. The radicals were obtained by
treating solutions of the ligands and complexes in dichloro-
methane with an excess of silver tetrafluoroborate, upon which
the solutions turned deep blue-violet to give the corresponding
radicals. The ESR spectra recorded for the chemically oxidized
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Figure 10. UV/vis spectra for the chemically oxidized species of ligand 4
(top) and complex 8 (bottom: different stoichiometric ratios of oxidant)
measured in dichloromethane.
Figure 11. X-band ESR spectra for the oxidized ligand 4 (top) and complex 8
(bottom) measured in dichloromethane solution at room temperature
(c=0.223 mM for 4 and 8; c �1 mM for the oxidant).
species of the ligand 4 and complex 8 are depicted in Figure 11
(for ESR spectra of 2, 3, 6, and 7 see Figure S25).
wheel core, BS-DFT calculations for the corresponding radical
cations of the complexes [Cu2(aba-R)4(dmf)2]4+ with fully
oxidized triphenylamine moieties have been performed based
on the molecular structures of the neutral complexes 5–8
derived from crystallography. The results of these BS-DFT
calculations are summarized in Table S9 and the corresponding
spin density distributions of the cationic radical species 54+–84+
are depicted in Figures S26–S29. The data confirms that the
unpaired spin density of the radicals is mainly localized at the
central nitrogen atoms of the triphenylamine ligand moieties,
which is in agreement with the observed hyperfine coupling in
the ESR experiments. The BS-DFT calculations reveal a rather
small ferromagnetic exchange coupling between the organic
radicals and the copper(II) ions of the core unit in the order of
2–3 cmÀ 1. This again is consistent with the observed ESR spectra
indicative of independent nitrogen-centered radicals. It should
be noted that the exchange coupling between the two copper
(II) ions within the paddle-wheel core is two orders of
magnitude larger than their coupling with the coordinated
radical cations, a situation which has also been observed for
dinuclear copper paddle-wheel complexes with coordinated
radical ligands on the basis of magnetic susceptibility measure-
ments.[40] This consequently leads to a situation where the
radical species can be regarded as virtually independent, which
is consistent with the observations from ESR spectroscopy as
well as electrochemistry.
The observed g factor for all compounds is close to the
expected value for the free electron at about 2.003 as expected
for triarylamine radicals. The ESR spectra for the oxidized radical
species of the methoxy substituted ligand 4 and complex 8
show a well-resolved hyperfine coupling of the electron spin to
the nuclear spin of the nitrogen atom (14N: I=1) of the
triphenylamine moiety of 22 MHz (0.79 mT). This is well within
the expected range usually reported for triarylamine
radicals.[4–5,39] A similar behavior is observed for the compounds
with tert-butyl substitution (3 and 7) for which the hyperfine
coupling is found to be 25 MHz (0.89 mT). For the methyl
derivatives the hyperfine coupling is not resolved in the spectra
(cf. Figure S25). Therefore, the hyperfine coupling can only be
estimated to about 22 MHz (0.79 mT). The observed values for
the hyperfine coupling constants are consistent with a nearly
planar geometry at the nitrogen atom of the triphenylamine
moiety as observed in the crystal structures.[38] Moreover, all
radicals exhibit an isotropic signal with g values typical for
triarylamine radicals. Comparing the ESR spectra and the fit
parameters obtained from simulation it is obvious that there is
very little variation between the ligands and their correspond-
ing complexes, indicative for an only minor influence of the
central dicopper unit on the coordinated triphenylamine
radicals.
To further elucidate the character of possible interactions
between the oxidized ligand backbone and the central paddle-
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Additional theoretical calculations to investigate the possi-
ble magnetic exchange between the organic radicals in the fully
oxidized species have been performed utilizing the model
systems [Zn2(aba-R)4(dmf)2]4+, where the copper ions have been
replaced by zinc(II) ions. Generally, due to the arrangement of
the four aryl carboxylates at the paddle-wheel core, two
different magnetic couplings between the organic radicals are
possible based on their relative position which can be cis or
trans (Jcis and Jtrans). The detailed results are summarized in
Table S10 and the corresponding spin density distributions are
depicted in Figures S30–S34. The BS-DFT calculations revealed
an almost negligible antiferromagnetic coupling between the
case of the presented complexes this leads to the observation
of virtually independent radical species in the fully oxidized
complex species [Cu2(aba-R)4(dmf)2]4+. This is most likely due to
the fact that the dinuclear paddle-wheel core represents a
strongly antiferromagnetically coupled unit with a diamagnetic
singlet ground state. Nevertheless, triphenylamine derivatives
as linkers can be regarded as promising candidates for
obtaining new coordination polymers and MOFs that can lead
to new magnetic materials via oxidation of the amine nitrogen
leading to the generation of extra spin centers within the
frameworks, which in turn can allow to transmit and trigger the
magnetic exchange interactions between the paramagnetic
metal ions of corresponding frameworks. However, this will
preferentially require frameworks that contain metal ion
building units, which are either based on mononuclear para-
magnetic metal ions or exhibit magnetic interactions within the
metal clusters that are in the same order of magnitude as those
with the bridging linkers.
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organic radicals for both possible type of interactions (Jcis
=
À 0.09 to À 0.22 cmÀ 1; Jtrans =0 to À 0.09 cmÀ 1).
3. Conclusions
In summary, we have reported on four new monocarboxyl
triphenylamine based ligands Haba-R (1–4) with varying
substitution at the remaining para postilions of the phenyl rings Experimental Section
and the corresponding dinuclear copper(II) complexes (5–8). All
complexes adopt the common paddle-wheel arrangement for
the dinuclear core, where the remaining axial coordination site
at the copper(II) ions is occupied by a dmf molecule. The four
complexes 5–8 have been fully characterized toward their
structural and magnetic properties. A strong antiferromagnetic
coupling between the copper(II) ions of the paddle-wheel core
was observed and could be confirmed by BS-DFT calculations.
Moreover, the variation of the substitution at the triphenyl-
amine moieties did not show a significant influence on the
observed magnetic exchange. Although triphenylamine deriva-
tives have been widely studied due to their electrical
conductivity, electroluminescence, and hole-transport proper-
ties, reports on their redox behavior and the interactions of
their corresponding radical species within metal bonded
systems are scarce. In the present work detailed electrochemical
and spectroelectrochemical investigations of the triphenyl-
amine-based ligands and complexes have been performed. For
the para substituted ligands (2–4) and complexes (6–8) electro-
chemistry demonstrates the expected behavior leading to
highly stable oxidized species for both ligands and complexes.
Spectroelectrochemical experiments allowed us to characterize
the properties of these oxidized radical species and to elucidate
their interactions with the central dicopper core. It was found
that the UV/vis and ESR spectroscopic properties do not
significantly vary between the oxidized radical species derived
from the ligands and the complexes, indicating that only small
interactions between the radical species and the copper(II) ions
can be present. This was further confirmed by BS-DFT
calculations which show that the exchange interactions of the
coordinated radicals with the copper(II) ions of the core unit are
weak ferromagnetically coupled, but about two orders of
magnitude smaller than the magnetic coupling within the
dinuclear paddle-wheel core. Moreover, BS-DFT calculations for
the fully oxidized complexes further indicate a very weak
antiferromagnetic coupling among the ligand radicals. In the
Materials
All starting chemicals are commercially available and were used
without further purification. The methyl 4-aminobenzoate was
prepared as reported in literature.[41] Also the BuchwaldÀ Hartwig
coupling procedure utilized in the syntheses of the ligands 2, 3,
and 4 was adapted from literature.[23] The syntheses of the
complexes 5, 6, and 8 were carried out in Teflon-lined acid
digestion vessels from Parr Instruments.
Physical Measurements
1H and 13C NMR spectra were recorded with Bruker Avance 400 and
600 MHz spectrometers. 1H NMR assignments and comparison of
the data for 1–4 are presented in Figure S35 and Table S11.
Thermogravimetric analysis (TGA) on powdered samples was
performed using
a
Netzsch STA409PC Luxx apparatus under
°
constant flow of air ranging from room temperature up to 1000 C
with a heating rate of 5 CminÀ 1. Mass spectra were measured on a
°
Bruker MAT SSQ 710 spectrometer. Elemental analyses were
determined on a Leco CHNS/932 and a VARIO EL III elemental
analyzer. The FT-IR spectra were measured using the Specac
Diamond ATR optional accessory on a VERTEX70 spectrometer by
Bruker Optics. The UV/vis spectra were obtained on a Varian
Cary5000 UV/vis/NIR spectrometer.
Syntheses
4-(Diphenylamino)benzaldehyde:[21]
Triphenylamine
(3.0 g,
°
12.2 mmol) was dissolved in dmf (150 mL) under stirring at 0 C.
The stirring was continued for 10 min and subsequently POCl3
(1.2 mL) was added dropwise. After complete addition, the mixture
was refluxed under N2 atmosphere for 22 h. The dark mixture was
then allowed to cool to room temperature and added into ice
water (100 mL). The mixture was then neutralized with aqueous
NaOH and extracted with CH2Cl2 (3×100 mL). The combined
organic layers were washed with water (2×100 mL) and dried with
anhydrous sodium sulphate. The product was then purified using
flash column chromatography (hexane/ethyl acetate 5:1). Yield:
1
°
2.4 g, 72%; H NMR (400 MHz, CDCl3, 25 C): δ=9.81 (s, 1H, CHO),
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7.68 (d, J=8.7 Hz, 2H), 7.34 (t, J=7.8 Hz, 2H), 7.15À 7.19 (m, 6H),
30%) and methanol (60 mL) was added and the mixture refluxed
overnight. The reaction mixture was then filtered hot over a celite
pad and subsequently acidified with concentrated aqueous HCl to
give a colorless precipitate, which was filtered off, washed, and
1
2
3
4
5
6
7
8
9
13
°
7.02 ppm (d, J=8.7 Hz, 2H): C NMR (100 MHz, CDCl3, 25 C): δ=
190.6, 153.5, 146.3, 131.5, 129.9, 129.3 126.5, 125.3, 119.5 ppm; IR
(ATR, cmÀ 1): u=3063 (w), 1675 (s), 1581 (s), 1564 (m), 1487 (s), 1328
~
(s), 1285 (vs), 1218 (s), 1155 (m).
dried overnight in an oven at 80 C. Yield: 0.97 g, 91%; 1H NMR
°
°
(400 MHz, dmso-d6, 25 C) δ=7.74 (d, J=8.9 Hz, 2H), 7.38 (d, J=
4-(Diphenylamino)benzoic acid (Haba, 1): To a solution of 4-
(diphenylamino)benzaldehyde (2.0 g, 7.3 mmol) in acetone (60 mL)
was added dropwise an aqueous solution (80 mL) containing K2CO3
(0.7 g, 5.1 mmol) and KMnO4 (4.56 g, 29.2 mmol). The mixture was
refluxed for 12 h overnight and subsequently filtered over a celite
pad while still hot. The filtrate was then acidified using concen-
trated aqueous HCl, upon which the solution turned cloudy. The
light yellow precipitate obtained was then filtered off, washed and
8.2 Hz, 4H), 7.05 (d, J=8.3 Hz, 4H), 6.78 (d, J=8.3 Hz, 2H), 1.27 ppm
13
°
(s, 18H, CMe3); C NMR (100 MHz, dmso-d6, 25 C) δ=167.5, 152.0,
147.7, 143.8, 131.3, 127.1, 125.1, 122.0, 118.3, 34.7, 31.6 ppm; IR
(ATR, cmÀ 1): u=3024 (w), 1667 (s), 1594 (s), 1505 (s), 1416 (m), 1315
~
(s), 1280 (vs), 1175 (s), 949 (w); UV/vis spectra see Figure S38; EI-MS:
m/z (%): 401 (85) [M]+; elemental analysis calcd (%) for C27H31NO2
(401.54 gmolÀ 1): C 80.76, H 7.78, N 3.49; found: C 80.96, H 7.89, N
3.50.
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dried overnight in an oven at 80 C. Yield: 1.56 g, 76%; 1H NMR
°
°
(600 MHz, dmso-d6, 25 C): δ=12.48 (s, 1H, COOH), 7.79 (d, J=
4-(Bis(4-methoxyphenyl)amino)benzoic acid (Haba-OMe, 4): The
procedure described for 2 was adopted for the coupling of 4-
bromoanisole (3 mL, 23.7 mmol) utilizing the similar reaction
conditions and compounds (Pd(OAc)2: 88 mg, 0.4 mmol; P(t-Bu)3:
0.48 mL, 1.2 mmol; methyl 4-aminobenzoate: 1.2 g, 7.9 mmol;
Cs2CO3: 6.6 g, 19.8 mmol) as well as solvent (toluene: 20 mL). The
reaction mixture was allowed to cool to room temperature and
poured into dichloromethane (150 mL). After filtration over a celite
pad the filtrate was loaded onto silica gel by adding the silica
(about 10 g) to the filtrate and evaporating all volatiles from the
mixture. The loaded silica gel was used for flash column
chromatography (chloroform/hexane 2:1) from which a colorless
oil was obtained. A mixture of aqueous KOH solution (30 mL, 30%)
and methanol (30 mL) was added and the mixture refluxed over-
night. The resulting mixture was filtered hot over a celite pad
followed by acidification with concentrated aqueous HCl to give a
8.8 Hz, 2H), 7.39 (t, J=7.9 Hz, 4H), 7.18 (t, J=7.4 Hz, 2H), 7.13 (d, J=
7.6 Hz, 4H), 6.88 ppm (d, J=8.8 Hz, 2H); 13C NMR (151 MHz, dmso-
°
d6, 25 C): δ=166.9, 151.3, 146.1, 130.8, 129.9, 125.7, 124.7, 122.4,
119.1 ppm; IR (ATR, cmÀ 1): u=3063 (w), 1665 (s), 1608 (m), 1583 (s),
~
1510 (m), 1488 (s), 1430 (m), 1415 (m), 1316 (s), 1271 (vs), 1177 (s),
1074 (m), 1028 (w), 949 (m); UV/vis spectra see Figure S36; EI-MS:
m/z (%): 289 (100) [M]+; elemental analysis calcd (%) for C19H15NO2
(289.33 gmolÀ 1): C 78.87 H 5.23, N 4.84; found: C 78.64, H 5.11, N
4.89.
4-(Bis(4-tolylphenyl)amino)benzoic acid (Haba-Me, 2): To a sol-
ution of Pd(OAc)2 (36 mg, 0.16 mmol) in degassed toluene (20 mL)
P(t-Bu)3 (0.19 mL, 0.48 mmol, 0.5 gmLÀ 1 in hexane) was added,
which was then stirred for 15 min at room temperature. Sub-
sequently 4-bromotoluene (1.57 g, 9.6 mmol), methyl 4-amino-
benzoate (0.48 g, 3.2 mmol), and Cs2CO3 (2.6 g, 8.0 mmol) were
added to this solution. The resulting mixture was refluxed for
5 days. After this period the reaction mixture was allowed to cool
to room temperature and poured into dichloromethane (60 mL).
After filtration over a celite pad the filtrate was loaded onto silica
gel by adding the silica (about 10 g) to the filtrate and evaporating
all volatiles from the mixture. The loaded silica gel was used for
flash column chromatography (chloroform/hexane 2:1) from which
a yellow oil was obtained. A mixture of aqueous KOH solution
(30 mL, 30%) and methanol (30 mL) was added and the reaction
mixture refluxed overnight. Subsequently, the resulting mixture
was filtered over a celite pad while still hot. The filtrate was
acidified with concentrated aqueous HCl to give a colorless
precipitate, which was filtered off, washed, and dried overnight in
light brown precipitate, which was filtered off, washed, and dried
1
°
overnight in an oven at 80 C. Yield: 1.22 g, 46%; H NMR (400 MHz,
°
CDCl3, 25 C) δ=7.84 (d, J=8.9 Hz, 2H), 7.12 (t, J=8.9 Hz, 4H), 6.88
(d, J=8.9 Hz, 4H), 6.81 (d, J=8.9 Hz, 2H), 3.81 ppm (s, 6H, OMe); 13C
°
NMR (100 MHz, CDCl3, 25 C) δ=171.1, 157.0, 153.4, 139.3, 131.6,
127.9, 119.0, 116.8, 115.0, 55.3 ppm; IR (ATR, cmÀ 1): u=3394 (w),
~
1666 (s), 1594 (m), 1502 (m), 1462 (m), 1317 (s), 1280 (vs), 1174 (s),
1126 (w), 1033 (w), 930 (w); UV/vis spectra see Figure S39; EI-MS: m/
z (%): 349 (100) [M]+; elemental analysis calcd (%) for C21H19NO4
(349.38 gmolÀ 1): C 72.19, H 5.48, N 4.01; found: C 72.02, H 5.38, N
4.09.
[Cu2(aba)4(dmf)2] (5): A mixture of the ligand Haba (1, 0.05 g,
0.173 mmol) and Cu(NO3)2 ·3H2O (0.08 g, 0.34 mmol) in dmf (4 mL)
was stirred for 1 h. The resulting green solution was transferred into
a 23 mL Teflon-walled Parr acid digestion bomb. The reaction
an oven at 80 C. Yield: 0.65 g, 64%; 1H NMR (400 MHz, dmso-d6,
°
°
25 C): δ=7.73 (d, J=8.9 Hz, 2H), 7.18 (d, J=8.2 Hz, 4H), 7.02 (t, J=
8.3 Hz, 4H), 6.77 (d, J=8.3 Hz, 2H), 2.28 ppm (s, 6H, Me); 13C NMR
container was placed in an oven and heated at 110 C for 24 h
°
°
(100 MHz, dmso-d6, 25 C): δ=167.04, 151.67, 143.53, 134.21,
under autogenous pressure followed by cooling at a constant rate
130.42, 126.04, 121.44, 117.75, 20.51 ppm; IR (ATR, cmÀ 1): u ¼3024
of 0.2 CminÀ 1. Allowing the resulting reaction solution to stand for
°
~
(w), 1667 (s), 1594 (s), 1505 (s), 1416 (m), 1315 (s), 1280 (vs), 1175
(s), 949 (w); UV/vis spectra see Figure S37; EI-MS: m/z (%): 317 (100)
[M]+; elemental analysis calcd (%) for C21H19NO2 (317.38 gmolÀ 1): C
79.47, H 6.03, N 4.41; found: C 79.53, H 5.99, N 4.43.
a few hours at room temperature led to the formation of green
crystals from the brown mother liquor. The crystals were filtered
off, washed with dmf until the wash fluid was no longer colored
and dried in vacuo. Yield: 48 mg, 66%; IR (ATR, cmÀ 1): u=1667 (m),
~
1608 (s), 1587(s), 1558 (w), 1488 (m), 1391 (vs), 1317 (m), 1274 (s),
1176 (m), 1100 (w), 1090 (w); UV/vis spectra see Figure S36;
elemental analysis calcd (%) for 5·4 dmf, C94H98Cu2N10O14
(1718.93 gmolÀ 1): C 65.68, H 5.75, N 8.15; found: C 66.10, H 5.74, N
8.05.
4-(Bis(4-tert-butylphenyl)amino)benzoic acid (Haba-tBu, 3): The
procedure described for 2 was adopted for the coupling of 4-tert-
butylbromobenzene (1.7 mL, 9.9 mmol) utilizing the similar reaction
conditions and compounds (Pd(OAc)2: 37 mg, 0.17 mmol; P(t-Bu)3:
0.20 mL, 0.5 mmol; methyl-4-aminobenzoate: 0.50 g, 3.3 mmol;
Cs2CO3: 2.7 g, 8.3 mmol) as well as solvent (toluene: 20 mL). The
final reaction mixture was allowed to cool to room temperature
and poured into dichloromethane (60 mL). After filtration over a
celite pad the filtrate was loaded onto silica gel by adding the silica
(about 10 g) to the filtrate and evaporating all volatiles from the
mixture. The loaded silica gel was used for flash column
chromatography (chloroform/hexane 2:1) from which a brown
solid was obtained. A mixture of aqueous KOH solution (30 mL,
[Cu2(aba-Me)4(dmf)2] (6): The ligand Haba-Me (2, 0.15 g,
0.476 mmol) and Cu(NO3)2 ·3H2O (0.23 g, 0.952 mmol) were dis-
solved in dmf (4 mL) and stirred overnight. The resulting green
solution was then transferred into a 23 mL Teflon-walled Parr acid
digestion bomb. The reaction vessel was placed in an oven to heat
°
at 110 C for 24 h under autogenous pressure followed by cooling
at a constant rate of 0.2 CminÀ 1. The insoluble products were
°
separated by centrifugation leading to a clear supernatant, which
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Full Papers
was separated and carefully layered by methanol (1 mL) on top.
However, the solvent layer separation disappeared within about
15 min leading to a clear solution from which a light green
precipitate was formed after a few hours. The precipitate was
filtered off and washed with methanol. Additional product could be
isolated from the filtrate as green block crystals after allowing it to
stand for a few days. The combined product was dried in vacuo.
Square-wave measurements: Square-wave voltammetric measure-
ments were conducted on the substituted ligand analogues 2–4
1
2
3
4
5
6
7
8
9
and their copper complexes 6–8, utilizing
a three-electrode
technique using the same potentiostat as above. The instrument
was controlled by the DigiElch 8 software (available from GAMRY).
This program provides not only routines for the digital simulation
of electrochemical experiments but also those for performing the
measurements in a consistent way making use of the GAMRY
Electrochemical Toolkit library. The square-wave voltammograms
were measured in dichloromethane (containing 0.25 M tetra-n-
hexylammoniumperchlorate) under a blanket of solvent-saturated
nitrogen gas using a square-wave signal with an amplitude of
25 mV and potential steps of 5 mV in all experiments. The ohmic
resistance, which had to be compensated for, was determined by
measuring the impedance of the system at potentials where the
faradaic current was negligibly small. Background correction was
accomplished by subtracting the current curves of the blank
electrolyte (containing the same concentration of supporting
electrolyte) from the experimental square-wave voltammograms.
The working electrode was an 1.6 mm carbon disk electrode (ALS
Japan). A Ag/AgCl electrode in acetonitrile containing 0.25 M tetra-
n-butylammonium chloride served as reference electrode. All
potentials reported in this paper refer to the ferrocenium/ferrocene
couple, which was always measured at the end of a series of
experiments.
Yield: 126 mg, 68%; IR (ATR, cmÀ 1): u=1667 (m), 1608 (s), 1599 (s),
~
1504 (s), 1394 (vs), 1316 (s), 1267 (s), 1169 (s); UV/vis spectra see
Figure S37; elemental analysis calcd (%) for 6·1.5H2O,
C90H89Cu2N6O11.5 (1565.79 gmolÀ 1) C 69.04, H 5.73, N 5.37 found: C
68.73, H 5.46, N 5.43.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
[Cu2(aba-tBu)4(dmf)2] (7): A mixture of the ligand Haba-tBu (3,
0.05 g, 0.124 mmol) and Cu(NO3)2 ·3H2O (0.03 g, 0.124 mmol) in dmf
°
(2 mL) was stirred for 1 h followed by heating at 110 C for 30 min.
The resulting green solution was allowed to stand leading to green
crystals being formed after a few days. The crystalline material was
separated, washed with dmf, and dried in vacuo for 4 h to obtain
the dried material. Yield: 39 mg, 16%; IR (ATR, cmÀ 1): u=2962 (m),
~
1671 (s), 1699 (s), 1506 (s), 1485 (s) 1397 (vs), 1321 (s), 1267 (m),
1176 (s), 1146 (w), 1063 (m); UV/vis spectra see Figure S38;
elemental analysis calcd (%) for 7·dmf·H2O, C117H143Cu2N7O12
(1966.51 gmolÀ 1): C 71.46, H 7.33, N 4.99; found: C 71.52, H 7.11, N
4.95.
[Cu2(aba-OMe)4(dmf)2] (8): Solutions of the ligand Haba-OMe (4,
0.04 g, 0.114 mmol) in dmf (1.5 mL) and Cu(NO3)2 ·3H2O (0.014 g,
0.057 mmol) in dmf (1.5 mL) were mixed in a 23 mL Teflon-walled
Parr acid digestion bomb and stirred for 1 h at room temperature.
Subsequently, the mixture was heated under autogenous pressure
Spectroelectrochemistry
Spectroelectrochemical experiments were performed using the
Reference 600 potentiostat in combination with an AvaSpec 2048
spectrometer and an AvaLight-DHc light-source (both from
Avantes, Apeldoorn, The Netherlands). The synchronization be-
tween potentiostat and spectrometer as well as the simultaneous
recording of spectra and current curve was accomplished by the
SPELCH software module included in DigiElch 8. A commercially
available optically transparent thin-layer electrochemical (OTTLE)
cell (University of Reading) with an optical path length of about
0.2 mm was used in all experiments. The cell consists of two
platinum net electrodes serving as working and counter electrode.
The (pseudo-)reference electrode is a silver wire. All spectra are
difference spectra with respect to the spectrum of the starting
material, which was taken as reference spectrum.
°
at 110 C for 24 h followed by cooling at a constant rate of
0.2 CminÀ 1. From the mother liquor which was allowed to stand
°
undisturbed dark green micro-crystals were obtained. The micro-
crystalline material was filtered off, washed with dmf until the wash
fluid was no longer colored, and dried for 1 h in vacuo. Crystals
suitable for X-ray studies were obtained by stirring a mixture of
ligand and metal salt in dmf for about 20 min followed by slowly
layering methanol on top of the mixture. Both solvents were
allowed to slowly evaporate over 4 weeks to yield green crystals of
8, which were dried in vacuo. Yield: 61 mg, 32%; IR (ATR, cmÀ 1): u=
~
1660 (m), 1600 (s), 1557 (s), 1391 (vs), 1319 (s), 1274 (m), 1239 (vs),
1182 (s), 1144 (w), 1088 (m); UV/vis spectra see Figure S39;
elemental analysis calcd (%) for C90H86Cu2N6O18 (1666.77 gmolÀ 1): C
64.85, H 5.20, N 5.04; found: C 64.95, H 5.16, N 5.37.
Magnetic Measurements and ESR Spectroscopy
The magnetic susceptibility was measured on bulk vacuum dried
materials in the 4–300 K temperature range with a Quantum Design
MPMS-5 superconducting SQUID magnetometer. The measured
data were corrected for diamagnetism of the capsules used and the
intrinsic diamagnetism of the constituent atoms using Pascal
constants. The ESR spectra were recorded at room temperature
using an X-Band ELEXSYS E500 spectrometer from Bruker equipped
with a SHQE resonator. The simulation of the experimental data
was performed with EasySpin.[42]
Synthesis of Radical Cations
Excess of silver tetrafluoroborate (2 mM in CH2Cl2) was added into
dilute solutions of the compounds in dichloromethane (�10À 4 M)
leading to deep blue-violet solutions which were centrifuged. The
clear supernatant was decanted off and used for collection of ESR
as well as UV/vis spectroscopic data. The obtained solutions of the
oxidized radical cations were observed to be stable for several days,
as no spectral changes could be detected within this period.
Voltammetry
X-ray Diffraction
CV measurements: Cyclic voltammetry measurements were carried
out at room temperature using a Reference 600 potentiostat
(GAMRY Instruments). For the measurements of the ligand Haba (1)
and its copper complex 5 in dichloromethane solution a Pt
electrode was employed with 0.1 M tetrabutylammonium
perchlorate (TBAP) as co-electrolyte. A scan rate of 1 VsÀ 1 was used
and the reference electrode was a Ag/AgCl electrode.
The single crystal X-ray data for the compounds 5–8 were collected
on a Nonius KappaCCD diffractometer, using graphite-monochro-
mated MoÀ K radiation (λ=71.073 pm). Data were corrected for
Lorentz and polarization effects, absorption was taken into account
on a semi-empirical basis using multiple scans.[43] The structures
were solved by direct methods (SHELXS[44]) and refined by full-
matrix least squares techniques against F2o (SHELXL-2014[44]). All
hydrogen atoms were included at calculated positions with fixed
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Full Papers
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thermal parameters. All non-disordered, non-hydrogen atoms were
refined anisotropically.[44] The crystals of 5 and 7 contain large
voids, filled with disordered solvent molecules, with a size of 821
1
2
3
4
5
6
7
8
9
and 554×106 pm3/unit cell for
5 and 7, respectively. Their
contribution to the structure factors of 5 and 7 were secured by
back-Fourier transformation using the SQUEEZE routine of the
program PLATON[45] resulting in 225 and 151 electrons/unit cell,
respectively. Crystallographic data as well as structure solution and
refinement details are summarized in Table S2. Diamond 4.2.2,[46]
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graphic data for complexes 5–8. These data are provided free of
charge by the Cambridge Crystallographic Data Centre (http://
www.ccdc.cam.ac.uk). The powder measurements were performed
on a Stoe Powder Diffractometer with a Mythen 1 K detector at
room temperature. Measurements were done using capillary tubes
while the DebyeÀ Scherrer Scan Mode was applied with a 2θ scan
type. The X-ray tube was a Cu-long fine focus tube. The measure-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
°
°
ment was carried out between 2 and 50 with steps of 2.1 per 20
seconds.
Computational Details
The structures used for calculations are based on the single-crystal
structure data of 5–8 as all atomic positions of non-hydrogen atoms
are concerned. The positions of all hydrogen atoms were optimized
in the high-spin state (S=1) at RI-DFT[49]/PBE[50]/def2-SVP[51] level of
theory utilizing the TURBOMOLE 6.6 package of programs.[52] For
broken-symmetry DFT (BS-DFT) calculations the PBE0 hybrid func-
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triple-ζ def2-TZVPP basis sets.[51] The coupling constants were
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[54]
b
b b
Heisenberg Hamiltonian (H ¼ À JS1S2: ).
2ðEBS À EHSÞ
J ¼
ð7Þ
hS2HSi À hS2BSi
Acknowledgements
O.A. thanks the Evangelisches Studienwerk Villigst and the
Graduierten Akademie of the Friedrich-Schiller-Universität Jena for
scholarships. We also thank Mr. Reinhardt for the measurement of
the thermogravimetric and magnetic data and Mrs. Wermann for
measuring the powder diffraction data. Finally, we want to thank
the URZ Jena for providing additional computational resources.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: copper · molecular electrochemistry · magnetic
properties · carboxylate ligands · density functional calculations
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FULL PAPERS
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Spinning the wheel: Triphenylamine
ligands at a copper paddle-wheel
core are studied as models for the
properties of such building units in
polymeric structures with particular
emphasis on the electronic structure
upon oxidation of the triphenylamine
ligand framework. The interaction of
the oxidized radical species of the tri-
phenylamine ligands in these
Dr. O. Akintola, M. Böhme, Dr. M.
Rudolph, Dr. A. Buchholz, Dr. H. Görls,
Prof. Dr. W. Plass*
1 – 15
Metal-Bonded Redox-Active Triaryl-
amines and Their Interactions:
Synthesis, Structure, and Redox
Properties of Paddle-Wheel Copper
Complexes
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complexes is investigated by using
electrochemical, spectroelectrochem-
ical, and computational methods.