Characterization of copper complexes with derivatives of the ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and their reactivity towards oxygen

Article abstract of DOI:10.1016/j.jinorgbio.2021.111544

A series of copper(I) complexes with ligands derived from the tripodal ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) have been structurally characterized and their redox chemistry analyzed by cyclic voltammetry. While the redox potentials of m

Full text of DOI:10.1016/j.jinorgbio.2021.111544

Journal of Inorganic Biochemistry 223 (2021) 111544
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Characterization of copper complexes with derivatives of the ligand
(2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and their reactivity
towards oxygen
Tim Brückmanna, Jonathan Beckera, Christian Würtelea, Marcel Thomas Seufferta,
,
*
Dominik Heulera, Klaus Müller-Buschbauma, Morten Weiß b, Siegfried Schindlera
a
¨
Institut für Anorganische und Analytische Chemie, Justus-Liebig-Universitat, Heinrich-Buff-Ring 17, 35392 Gießen, Germany
b
¨
¨
¨
Fakultat für Biologie, Chemie und Geowissenschaften, Universitat Bayreuth, Universitatsstrasse 30, 95447 Bayreuth, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of copper(I) complexes with ligands derived from the tripodal ligand (2-aminoethyl)bis(2-pyridylmethyl)
amine (uns-penp) have been structurally characterized and their redox chemistry analyzed by cyclic voltam-
metry. While the redox potentials of most of the complexes were similar their reactivity towards dioxygen was
quite different. While the complex with a ferrocene derived ligand of uns-penp reacted in solution at low tem-
peratures in a two-step reaction from the preliminary formed mononuclear end-on superoxido complex to a quite
stable dinuclear peroxido complex it did not react with dioxygen in the solid state. Other complexes also did not
react with dioxygen in the solid state while some showed a reversible formation to a green compound, indicating
formation of an end-on superoxido complex that unfortunately so far could not be characterized. In contrast,
copper complexes with the Me2uns-penp and Et-iProp-uns-penp formed dinuclear peroxido complexes in a solid-
state reaction. While the reaction of dioxygen with the [Cu(Me2uns-penp]BPh4 was quite slow an instant reaction
took place for [Cu(Et-iProp-uns-penp]BPh. Very unusual, it turned out that crystals of the copper(I) complex that
could be structurally characterized still were crystalline when reacted with dioxygen. Therefore, it was possible
to solve the structure of the corresponding dinuclear peroxido complex directly from the same batch of crystals.
The crystalline structures of the copper(I) and copper(II) complex revealed that the reason for this is the fact, that
the copper(I) complex is kind of preorganized for the uptake of dioxygen and does not really change in its overall
structure when being oxidized.
Copper complex
Tripodal ligands
Dioxygen activation
Peroxido complex
Superoxido complex
Stopped-flow
1. Introduction
species, an end-on
η
1-superoxido (a) and a side-on η2
:
η
2-peroxido copper
complex (e), have been detected in biological systems, this does not
Copper plays an important role in organic life. Due to its accessible I/
II redox pair and biological availability it is mostly present in enzymatic
processes of electron transfer, dioxygen transport and oxidation/
oxygenation of organic substrates [1]. During the last decades, quite
some understanding had been gained for the reactions of copper(I)
complexes with dioxygen and a large number of different so called
“dioxygen adduct” complexes could be identified. Some structurally
characterized examples of these complexes are presented in Fig. 1. Some
of these complexes are found in the active site of copper enzymes,
mainly monooxygenases, such as e. g. a (Fig. 1) in peptidylglycine
mean that the other complexes in Fig. 1 are less interesting [2]. For
example, bis(μ-oxido) copper complexes f (which in some cases can be in
an equilibrium with the side-on η2
:
η
2-peroxido copper complex e) proved
to be quite useful in selective hydroxylation reactions (only two selected
examples are given in the references) [3,4]. Furthermore, we as well as
Karlin and co-workers had observed that trans-μ-1,2-peroxido com-
plexes b are capable to catalytically oxidize toluene selectively to
benzaldehyde [5,6]. There is high interest to transfer this type of reac-
tion from the lab into industry where such oxygenations are usually
done at higher temperatures, sometimes in the presence of quite toxic
chemicals and often accompanied by undesired side products. In
contrast, copper complexes that model the reactivity of the natural
α
-hydroxylating monooxygenase (PHM) and dopamine-β-mono‑ox-
ygenase (DβM) or e in tyrosinase. While actually, so far, only these two
* Corresponding author.
E-mail address: siegfried.schindler@anorg.chemie.uni-giessen.de (S. Schindler).
https://doi.org/10.1016/j.jinorgbio.2021.111544
Received 17 April 2021; Received in revised form 5 July 2021; Accepted 12 July 2021
Available online 16 July 2021
0162-0134/© 2021 Elsevier Inc. All rights reserved.

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
corresponding copper(I) complexes with dioxygen.
With regard to these findings we have started a thorough investi-
gation on copper complexes with derivatives of the ligand (2-amino-
ethyl)bis(2-pyridylmethyl)amine (uns-penp, Fig. 3), that can be viewed
as a combination of the ligands tmpa and tren. The advantage of this
ligand is the possibility to easily modify the amine arm to test for the
influence of different substituents or to prepare it for an immobilization
[16]. To get a better picture of the influence of the amine arm modifi-
cation on the reactivity of the corresponding copper(I) complexes with
dioxygen we in here now present a systematic study with the ligands
presented in Fig. 3.
Fig. 1. Examples of copper “dioxygen adduct” species: (a) end-on superoxido,
(b) trans-
μ
-1,2-peroxido, (c)
η
1-hydroperoxido, (d) side-on superoxido, (e) η2
μ- :
2. Experimental
η
2-peroxido, (f) bis(
μ-oxido).
2.1. Materials and methods
enzymes should catalyze these reactions under ambient conditions with
dioxygen from air as the sole oxidant, with high yields and with fewer
side products.
All chemicals were purchased from commercial suppliers. Extra dry
solvents were refluxed under argon over a drying agent prior to taking
them into the glove box. [Cu(MeCN)4]+ salts were synthesized accord-
ing to literature methods [17]. Storage and preparation of air sensitive
compounds was carried out in a glove box under an argon atmosphere
(Braun, Garching, Germany; water and dioxygen <0.1 ppm). NMR
measurements were performed with a Bruker Avance II 400 MHz (AV II
400) and Bruker Avance III HD 400 MHz (AV III 400). Electrospray
ionization-MS (ESI-MS) measurements were performed on a Bruker
microTOF mass spectrometer. UV–Vis investigations in solution were
performed with an Agilent 8453 spectrometer, equipped with a cryostat
CoolSpeK UV USP-203-B (Unisoku Co., Ltd., Osaka, Japan). Solid state
UV–Vis spectra were recorded using a Perkin Elmer Lambda 750
UV–Vis-NIR spectrometer equipped with a Harrick praying mantis. All
shown spectra were converted into absorption by the Kubelka–Munk
equation [18]. A Hi-Tech SF-61SX2 low-temperature stopped-flow unit
equipped with a diode array spectrophotometer (Hi-Tech, Salisbury,
UK) was used for kinetic measurements in solution. A two-syringe setup
was applied to perform the experiments. Detailed descriptions on the
setup and the procedure have been reported previously [19]. Raman
spectroscopy was executed on a SENTERRA dispersive Raman micro-
scope (Bruker optics) equipped with a Nd:YAG laser (λexc = 532 nm).
Cyclic voltammetry measurements (CV) were performed with an e-
corder 410 High Resolution Laboratory Data Recorder, connected with
an EA163 Potentiostat (eDAQ, Denistone East, Australia). Powder X-ray
diffraction (PXRD) was carried out on a STOE STADI P diffractometer
(Darmstadt, Germany), equipped with a DECTRIS MYTHEN 1 K detector
and a Ge(111) monochromator. Cu Kα1 radiation (λ = 1.5406 Å) was
used in Debyeꢀ Scherrer geometry. Samples were filled under an argon
The first structurally characterized “dioxygen adduct” complex,
[(tmpa)Cu(O2)Cu(tmpa)](PF6)2 (tmpa = tris(2-methylpyridyl)-amine,
Fig. 2), a b system was reported by Karlin and co-workers [7]. The
formation of this complex had been investigated in great detail and low
temperature stopped-flow measurements allowed the brief observation
of a short-lived end-on superoxido complex (a) in a first reaction step
according to the Eq. (1) [8,9]. Due to the consecutive reaction to the
dinuclear peroxido complex this labile intermediate could only be
spectroscopically (UV–vis data) characterized.
(1)
Following up on these results it was possible, with the copper(I)
complex and the tripodal ligand tris(2-dimethylaminoethyl)amine
(Me6tren, Fig. 2), to observe the same reactivity (Eq. (1)), however
here it was possible to obtain a resonance Raman spectrum of the in-
termediate with a characteristic peak at 1122 cmꢀ 1 for the vibration
–
frequency of 16
O
16O) [10]..
With the tren system it seemed easy to modify the ligand system
accordingly to finally stabilize an end-on superoxido copper enough for a
full characterization. Instead, this turned out to be a journey for many
years with some of the efforts by us only been published recently [11].
However, by applying the tren derivative tris(tetramethylguanidino)
tren (TMG3tren, Fig. 2) as a ligand the corresponding end-on superoxido
complex, [(TMG3tren)Cu(O2)]SbF6, was formed in a reversible reaction
and was stabilized to the point that it could finally be structurally
characterized [12,13]. While this complex so far is still the only example
of a structurally characterized end-on superoxido copper complex and
can be regarded as a decent model complex for PHM and DβM, it lacks
their reactivity (due to the increased stability). However, most recently
in a cooperation with Karlin and co-workers it could be shown that a
copper(I) complex with a sulfur derivative of TMG3tren can model this
reactivity [14,15].
From all these findings over the last 20 years it became clear that
slight ligand modifications of the tripodal ligand system (chelate ring
size, donor atoms, alkyl groups) in combination with solvent and
selected anions can have a huge influence on the reaction of the
Fig. 3. Ligands L, derivatives of uns-penp derivatives and abbrevations (Me =
methyl; Et = ethyl; Prop = propyl; iProp = isopropyl).
Fig. 2. Tripodal ligands tmpa, Me6tren and TMG3tren.
2

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
atmosphere into glass capillaries (Hilgenberg ∅ = 0.5 mm) and sealed
prior to the measurements. The WinXPOW program package (V3.05,
STOE & Cie GmbH 2011, Darmstadt, Germany) was used for data
collection. Details on single crystal characterization are reported in the
Supporting Information.
sodium triacetoxyborohydride (STAB) (64,4 mmol) were added. The
solution was stirred overnight under argon. Then the reaction suspen-
sion was quenched with 60 mL of 2 M NaOH solution and extracted three
times with DCM. The organic layers were combined, washed with brine,
dried over MgSO4, filtrated and the solvent was removed under vacuum.
The resulting brown oil was treated with 100 mL half concentrated
hydrochloric acid (intense gas formation was observed). The brown
solution was heated to 130 ◦C and kept under reflux overnight. The
solution was made basic by adding dropwise 6 M NaOH solution while
simultaneously cooling the solution in an ice bath. The solution obtained
was extracted three times with DCM, washed with brine, dried over
MgSO4, filtrated and the solvent was removed under vacuum. Distilla-
tion via Kugelrohr lead to a pure product which was stored under an
argon atmosphere (3.11 g, 56%). Overall yield 45% (respectively 2.0 g
N-methylethylenediamine). 1HNMR (400 MHz, CDCl3, 24 ◦C, δ [ppm]):
8.53 (d, 2H, py-H), 7.65 (td, 2H, py-H); 7.48 (d, 2H, py-H), 7.14 (t, 2H,
py-H), 3.85 (s, 4H, py-CH2), 2.75 (t, 2H, NCH2CH2N(CH2py)2), 2.67 (t,
2H, NCH2CH2N(CH2py)2), 2.31 (s, 2H, NCH3) 13CNMR (101 MHz,
CDCl3, 24 ◦C, δ [ppm]): 159.6, 149.0, 136.4, 122.9, 121.9, 64.4, 60.8,
53.9, 49.4, 36.3 ESI-MS (m/z): [M + H]+: 257.18; [M + Na]+: 279.16
2.2. Reactivity towards oxygen
The reactions of the complexes with dioxygen were carried out either
in solution or in the solid state. For reactions in solution, the complexes
were dissolved in dry and dioxygen free acetone. For time resolved
stopped-flow UV–Vis measurements the solutions were reacted at low
temperatures with dioxygen saturated acetone. The solvent was satu-
rated by passing dioxygen for 15 min through a syringe filled with dry
acetone [20]. For other UV–Vis measurements the complex solutions
were cooled to low temperatures using a cryo unit. The solutions were
subsequently flushed with dioxygen. For solid state reactions, the crys-
talline complexes were treated with gaseous dioxygen.
2.3. Cyclic voltammetry measurements
2.4.2. (2-ethylaminoethyl)bis(2-pyridylmethyl)amine (Et-uns-penp)
2.0 g of N-ethylethylenediamine (22.7 mmol) were dissolved in 50
mL DCM. The solution was cooled to 0 ◦C with an ice bath and a solution
of 3.22 g ethyltrifluoroacetate (22.7 mmol) in 50 mL DCM was added
dropwise over a period of 40 min under argon. After the addition was
complete, the mixture obtained was stirred for another 60 min at room
temperature. The solvent, as well as the ethanol from the reaction were
removed using a rotary evaporator. The derived colorless oil crystallized
as a white colored solid briefly afterwards. The product was dissolved in
100 mL of DCM. 4.95 g of Di-tert-butyldicarbonate in 5 mL DCM were
added to the product solution at 0 ◦C under argon. The reaction mixture
was stirred for 90 min at room temperature and then quenched with 30
mL of saturated Na2CO3 solution and 30 mL of ethyl acetate. After the
gas formation had stopped, the aqueous phase was extracted three times
with ethyl acetate. The combined organic layers were washed with
brine, dried over MgSO4, filtrated and concentrated using a rotary
evaporator. The oil obtained crystallized as a white solid after a few
moments. The product was placed into a vessel with 75 mL methanol
and 25 mL of a 2–3 M NaOH solution and stirred at room temperature for
two hours. Finally, methanol was removed and the aqueous layer was
extracted three times with ethyl acetate. The combined organic layers
were washed with brine, dried over MgSO4, filtrated and the solvent was
removed under vacuum. The obtained crude product was a pale yellow
colored oil (3,97 g, 21.1 mmol, 93%). The product was used for the
following syntheses without further purification and characterization.
N,N-Boc-ethylethyelenediamine was dissolved in 60 mL DCE. 4.52 g 2-
pyrdinecarboxaldehyde (42.2 mmol) and 13.41 STAB (63.3 mmol)
were added. The solution was stirred overnight under argon. Then the
reaction suspension was quenched with 60 mL of 2 M NaOH solution and
extracted three times with DCM. The organic layers were combined,
washed with brine, dried over MgSO4, and after filtration the solvent
was removed under vacuum. The obtained brown colored oil was
treated with 100 mL half concentrated hydrochloric acid (intense gas
formation was observed). The brown solution was heated to 130 ◦C and
refluxed overnight. The solution was made basic by adding dropwise 6
M NaOH solution by simultaneous cooling it in an ice bath. The resulting
solution was extracted three times with DCM, washed with brine, dried
over MgSO, filtrated and the solvent was removed under vacuum.
Distillation via Kugelrohr lead to a pure product which was stored under
an argon atmosphere (3.49 g, 61%). Overall yield 57% (in respect to
The cyclic voltammetry measurements were carried out in a con-
ventional, three-electrode setup (working, counter, and reference elec-
trode) [21]. As working electrode, a glassy carbon electrode and as
counter electrode a Pt/Ti wire were used. As reference served an Ag/
AgCl electrode. The cell was prepared under an inert atmosphere. 0.1
mol/L dried tetrabutylammonium tetrafluoroborate solution in dry
acetonitrile was used as electrolyte; the scanning rate was 50 mV/S.
Complex concentrations of 1 mmol/L was used. During the measure-
ments the vessel was flushed continuously with dinitrogen through a
syringe to avoid air exposure.
2.4. Syntheses
Syntheses were performed according to procedures reported in the
literature for the asymmetric uns-penp derivatives and furthermore for
the basic ligand uns-penp and its derivatives [22,23].
2.4.1. (2-methylaminoethyl)bis(2-pyridylmethyl)amine (Me-uns-penp)
2.0 g N-methylethylenediamine (27.7 mmol) were dissolved in 50
mL dichloromethane (DCM). The solution was cooled to 0 ◦C with an ice
bath and a solution of 3.83 g ethyltrifluoroacetate (27.7 mmol) in 50 mL
DCM was added dropwise over a period of 40 min under argon. After the
addition was complete, the resulting mixture was stirred for another 60
min at room temperature. The solvent, as well as the ethanol from the
reaction were removed using a rotary evaporator. The obtained colorless
oil crystallized as a white solid briefly afterwards. To attach a butox-
ycarbonyl (Boc) protecting group the product was dissolved in 100 mL of
DCM. 5.89 g of di-tert-butyldicarbonate (Boc2O) in 10 mL DCM were
added to the product solution at 0 ◦C under argon. The reaction mixture
was stirred for 90 min at room temperature and then quenched with 30
mL of saturated Na2CO3 solution and 30 mL of ethyl acetate. After gas
formation stopped the aqueous phase was extracted three times with
ethyl acetate. The combined organic layers were washed with brine,
dried over MgSO4, filtrated and concentrated using a rotary evaporator.
The resulting oil crystallized as a white solid after a few minutes. The
product was placed into a vessel with 75 mL methanol and 25 mL of a
2–3 M NaOH solution and stirred at room temperature for two hours.
Finally, methanol was removed and the aqueous layer was extracted
three times with ethyl acetate. The combined organic layers were
washed with brine, dried over MgSO4, filtrated and the solvent was
removed under vacuum. The crude product was a pale, yellow colored
oil (3,74 g, 21.5 mmol, 79%). The product was used for the following
syntheses without further purification and characterization. N,N-Boc-
methylethylenediamine was dissolved in 60 mL 1,2-dichloroethane
(DCE). 4.82 g 2-pyrdinecarboxaldehyde (45.1 mmol) and 13.63
1
starting material N-ethylethylenediamine). HNMR (400 MHz, CDCl3,
23 ◦C, δ [ppm]): 8.48 (d, 2H, py-H), 7.60 (td, 2H, py-H), 7.43 (d, 2H, py-
H), 7.09 (t, 2H, py-H), 3.81 (s, 4H, py-CH2), 2.75–2.62 (m, 4H,
NCH2CH2N), 2.45 (q, 2H, NCH2CH3), 1.00 (t, 3H, NCH2CH3). 13CNMR
(101 MHz, CDCl3, 24 ◦C, δ [ppm]): δ 159.7, 149.0, 148.5, 136.5, 136.4,
3

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
122.9, 122.1, 121.9, 120.4, 64.3, 60.8, 54.2, 47.1, 44.0, 15.3. ESI-MS
(m/z): [M + H]+: 271.19; [M + Na]+: 293.17
50 mL DCE. 2-pyridincarboxaldehyde (1.48 g; 13.8 mmol) and STAB
(4.41 g; 20.8 mmol) were added. The foamy mixture was stirred at room
temperature overnight under argon. The reaction was quenched with 50
mL of 2 M NaOH solution and extracted three times with DCM. The
combined organic layers were washed with brine and dried over MgSO4.
The crude product was obtained by removing the solvent. The pure
product was obtained by purification via Kugelrohr distillation. The
light brown colored oil was stored under an argon atmosphere (1.63 g,
72%). 1H NMR (400 MHz, CDCl3, 23 ◦C, δ [ppm]): 8.50 (d, 2H, py-H),
7.62 (td, 2H, py-H), 7.56 (d, 2H, py-H), 7.11 (t, 2H, py-H), 3.84 (s,
4H, py-CH2), 2.88 (m, 4H, NCH(CH3)2), 2.55 (s, 4H, NCH2CH2N), 0.89
(d, 12H, NCH(CH3)2. 13C NMR (101 MHz, CDCl3, 24 ◦C, δ [ppm]):
160.2, 149.0, 136.3, 122.7, 121.8, 61.1, 56.4, 49.1, 43.6, 20.6. ESI-MS
(m/z): [M + H]+: 327.26; [M + Na]+: 349.24.
2.4.2.1. Symmetric uns-penp derivatives. The preparation of symmetric
uns-penp derivatives Me2uns-penp and Et2uns-penp were reported pre-
viously [24,25]. Furthermore, the synthesis of the ligand iProp2uns-
penp was performed according to these descriptions. The ligand Pro-
p2uns-penp was synthesized from uns-penp. The terminal primary amine
was subsequently modified via an SN2 reaction [23,26].
2.4.3. Me2uns-penp (1)
N,N-dimethylethylenediamine (2.0 g, 22.7 mmol) was dissolved in
30 mL of DCE. 2-pyridincarboxaldehyde (4.86 g; 45.4 mmol) and STAB
(14.43 g; 68.1 mmol) were added. The foamy mixture was stirred at
room temperature overnight under an argon atmosphere. The reaction
was quenched with 30 mL of 2 M NaOH solution and extracted three
times with DCM. The combined organic layers were washed with brine
and dried over MgSO4. The crude product was obtained by removing the
solvent. The pure product was obtained by purification via Kugelrohr
distillation. The light brown colored oil was stored under an argon at-
mosphere (4,83 g, 79%). 1H NMR (400 MHz, CDCl3, 26 ◦C, δ [ppm]):
8.48 (d, 2H, py-H), 7.60 (td, 2H, py-H), 7.49 (d, 2H, py-H), 7.09 (t, 2H,
py-H), 3.82 (s, 4H, py-CH2), 2.69–2.63 (t, 2H, NCH2CH2N(CH2py)2),
2.45 (t, 2H, NCH2CH2N(CH2py)2), 2.13 (s, 6H, N(CH3)2). 13C NMR (101
MHz, CDCl3, 28 ◦C, δ [ppm]): 149.8, 149.2, 149.0, 136.9, 136.7, 136.3,
122.9, 122.4, 122.3, 122.2, 121.9, 120.4, 60.53, 45.8, 45.61, 5.5, 21.8.
ESI-MS (m/z): [M + H]+: 270.18.
2.4.6.1. Asymmetric uns-penp derivatives. The asymmetric ligands are
based on the priorly synthesized precursor ligands with a secondary
amine at the terminal position. The second alkyl residue was added via a
simple SN2 reaction in an established procedure [26]. However, the
Ferrocene residue was connected via reductive alkylation [23].
2.4.7. Me-Et-uns-penp (5)
Me-uns-penp (1.0 g, 3.9 mmol) was dissolved in 20 mL acetonitrile.
Bromoethane (0.74 g; 6.8 mmol) and K2CO3 (2.5 g; 18.1 mmol) were
added. The solution was stirred at 50 ◦C under argon for three days. The
solvent was removed. The residue was treated with diethyl ether, and
the solvent was removed after filtration. Kugelrohr distillation led to a
pure product that was stored under an argon atmosphere (0.60 g; 54%).
1H NMR (400 MHz, CDCl3, 23 ◦C, δ [ppm]): 8.49 (d, 2H, py-H), 7.61 (td,
2H, py-H), 7.52 (d, 2H, py-H), 7.10 (t, 2H, py-H), 3.83 (s, 4H, py-CH2),
2.68 (t, 2H, NCH2CH2N(CH2py)2), 2.52 (t, 2H, NCH2CH2N(CH2py)2),
2.4.4. Et2uns-penp (2)
N,N-diethylethylenediamine (2.0 g, 17.2 mmol) was dissolved in 30
mL of DCE. 2-pyridincarboxaldehyde (3.69 g; 34.4 mmol) and STAB
(10.94 g; 51.6 mmol) were added. The foamy mixture was stirred at
room temperature overnight under argon. The reaction was quenched
with 30 mL of 2 M NaOH solution and extracted three times with DCM.
The combined organic layers were washed with brine and dried over
MgSO4. The crude product was obtained by removing the solvent. The
pure product was obtained by purification via Kugelrohr distillation.
The light brown colored oil was stored under an argon atmosphere (3.04
g, 59%). 1H NMR (400 MHz, CDCl3, 26 ◦C, δ [ppm]): 8.50 (d, 2H, py-H),
7.62 (td, 2H, py-H), 7.53 (d, 2H, py-H), 7.11 (t, 2H, py-H), 3.85 (s, 4H,
py-CH2), 2.67 (t, 2H, NCH2CH2N(CH2py)2), 2.60 (t, 2H, NCH2CH2N
(CH2py)2), 2.44 (q, 4H, N(CH2CH3)2), 0.93 (t, 6H, N(CH2CH3)2). 13C
2.33 (q, 2H, NCH2CH3), 2.12 (s, 3H, NCH3), 0.97 (t, 3H, NCH2CH3). 13
C
◦
NMR (101 MHz, CDCl3, 25 C, δ [ppm]): 160.0, 149.1, 136.4, 123.0,
122.0, 61.0, 55.2, 52.3, 51.8, 42.1, 12.3. ESI-MS (m/z): [M + H]+:
285.21; [M + Na]+: 307.19.
2.4.8. Me-Prop-uns-penp (6)
Me-uns-penp (1.0 g, 3.9 mmol) was dissolved in 20 mL acetonitrile.
1-iodopropane (0.70 g; 4.1 mmol) and K2CO3 (2.5 g; 18.1 mmol) were
added. The solution was stirred at 50 ◦C under argon for two days. The
solvent was removed. The residue was treated with diethyl ether, fil-
trated and the solvent was removed. Kugelrohr distillation led to a pure
product that was stored under an argon atmosphere (0.65 g; 56%). 1H
NMR (400 MHz, CDCl3, 23 ◦C, δ [ppm]): 8.49 (d, 2H, py-H), 7.61 (td,
2H, py-H), 7.52 (d, 2H, py-H), 7.10 (t, 2H, py-H), 3.83 (s, 4H, py-CH2),
2.67 (t, 2H, NCH2CH2N(CH2py)2), 2.52 (t, 2H, NCH2CH2N(CH2py)2),
2.21 (t, 2H, NCH2CH2CH3), 2.14 (s, 3H, NCH3), 1.40 (h, 2H,
NCH2CH2CH3), 0.80 (t, 2H, NCH2CH2CH3). 13C NMR (101 MHz, CDCl3,
24 ◦C, δ [ppm]): 160.0, 149.1, 136.41, 122.9, 122.0, 61.0, 60.2, 55.7,
52.3, 42.7, 20.5, 12.0. ESI-MS (m/z): [M + H]+: 299.22.
◦
NMR (101 MHz, CDCl3, 26 C, δ [ppm]): 160.0, 149.0, 136.3, 122.8,
121.9, 61.0, 52.3, 50.9, 47.5, 47.4, 11.8. ESI-MS (m/z): [M + H]+:
298.22.
2.4.5. Prop2uns-penp (3)
The precursor ligand uns-penp was synthesized according to the
literature and purified via Kugelrohr distillation [23]. The such purfied
uns-penp (1.0 g; 4.1 mmol) was dissolved in 20 mL acetonitrile. K2CO3
(4.1 g; 30.0 mmol) and 1-iodopropane (1.50 g; 8.8 mmol) were added to
the solution. The mixture was stirred overnight at 50 ◦C under an argon
atmosphere. Afterwards the solvent was removed. The residue was
treated with diethylether, filtrated and the solvent was removed.
Kugelrohr distillation led to a pure product that was stored under an
2.4.9. Et-Prop-uns-penp (7)
Et-uns-penp (1.0 g, 3.7 mmol) was dissolved in 20 mL acetonitrile. 1-
iodopropane (0.87 g; 5.1 mmol) and K2CO3 (2.5 g; 18.1 mmol) were
added. The solution was stirred at 50 ◦C under an argon atmosphere for
seven days. The solvent was removed. The residue was treated with
diethyl ether, filtrated and the solvent was removed. Kugelrohr distil-
lation led to a pure product that was stored under an argon atmosphere
(0.75 g; 65%). 1H NMR (400 MHz, CDCl3, 23 ◦C, δ [ppm]): 8.49 (d, 2H,
py-H), 7.62 (td, 2H, py-H), 7.53 (d, 2H, py-H), 7.10 (t, 2H, py-H), 3.84 (s,
4H, py-CH2), 2.62 (m, 4H, NCH2CH2N), 2.42 (q, 2H, NCH2CH3), 2.28 (t,
2H, NCH2CH2CH3), 1.36 (h, 2H, NCH2CH2CH3), 0.80 (t, 2H,
1
◦
argon atmosphere (0.90 g; 67%). H NMR (400 MHz, CDCl3, 24 C, δ
[ppm]): 8.48 (d, 2H, py-H), 7.61 (td, 2H, py-H), 7.52 (d, 2H, py-H), 7.10
(t, 2H, py-H), 3.83 (s, 4H, py-CH2), 2.61 (m, 4H, NCH2CH2N), 2.27 (t,
4H, N(CH2CH2CH3)2), 1.34 (m, 4H, N(CH2CH2CH3)2), 0.77 (t, 6H, N
13
◦
(CH2CH2CH3)2). C NMR (101 MHz, CDCl3, 27 C, δ [ppm]): 160.1,
149.1, 136.4, 122.9, 121.9, 61.1, 56.8, 52.4, 20.4, 12.0. ESI-MS (m/z):
[M + H]+: 327.26; [M + Na]+: 349.24.
NCH2CH2CH3)0.0.92 (t, 3H, NCH2CH3), 0.78 (t, 3H, NCH2CH2CH3). 13
C
◦
2.4.6. iProp2uns-penp (4)
NMR (101 MHz, CDCl3, 26 C, δ [ppm]): 160.1, 149.1, 136.4, 122.9,
121.9, 61.1, 56.13, 52.4, 51.7, 48.1, 20.4, 12.0, 11.9. ESI-MS (m/z): [M
N,N-diisopropylethylenediamine (1.0 g, 6.9 mmol) was dissolved in
4

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
+ H]+: 313.24, [M + Na]+: 335.22.
2.4.14. [Cu(Ligand)Cl]ClO4
2.4.10. Me-iProp-uns-penp (8)
70 mg of the ligand was dissolved in 2 mL of methanol. A solution of
0.5 eq of copper(II)chloride dehydrate and 0.5 eq copper(II)perchlorate
hexahydrate were dissolved in 4 mL of methanol. The blue solution
containing the copper salts was added to the ligand solution. Slow
evaporation yielded crystals suitable for X-ray analysis.
Me-uns-penp (0.71 g, 2.8 mmol) was dissolved in 20 mL acetonitrile.
2-iodopropane (0.75 g; 4.4 mmol) and K2CO3 (2.5 g; 18.1 mmol) were
added. The solution was stirred at 50 ◦C under argon for four days. The
solvent was removed. The residue was treated with diethylether, fil-
trated and the solvent was removed. Kugelrohr distillation led to a pure
product that was stored under an argon atmosphere (0.51 g; 62%). 1H
NMR (400 MHz, CDCl3, 25 ◦C, δ [ppm]): 8.49 (d, 2H, py-H), 7.61 (td,
2H, py-H), 7.53 (d, 2H, py-H), 7.09 (t, 2H, py-H), 3.84 (s, 4H, py-CH2),
2.72 (h, 1H, NCH(CH3)2))2.65 (t, 2H, NCH2CH2N(CH2py)2), 2.53 (t, 2H,
NCH2CH2N(CH2py)2), 2.10 (s, 3H, NCH3), 0.91 (d, 2H, NCH(CH3)2)).
13C NMR (101 MHz, CDCl3, 26 ◦C, δ [ppm]): 160.1, 149.1, 136.4, 122.9,
121.9, 61.0, 53.8, 53.1, 51.3, 37.5, 18.0. ESI-MS (m/z): [M + H]+:
299.22, [M + Na]+: 321.21.
3. Results and discussion
3.1. Syntheses and characterization
The ligands presented in Fig. 3 were synthesized without too many
problems in acceptable yields according to published procedures.
Symmetrical ligands Me2uns-penp, Et2uns-penp and iProp2uns-penp
could be obtained from commercially available chemicals in a one step-
reaction. Only for the synthesis of ligand Prop2uns-penp, uns-penp was
needed as a precursor. The addition of the second alkyl residue for the
unsymmetric ligands was performed under basic conditions in a SN2
reaction. The reaction duration of the synthesis of Et-iProp-uns-penp
was striking. A reason for this might be the strong steric hindrance
around the fully substituted nitrogen atom. This effect further explains
the lack of an oversubstitution which was detected within all other SN2
reactions performed. All ligands needed to be purified by Kugelrohr
distillation. Ferrocene aldehyde was attached to the Me-uns-penp ligand
by reductive alkylation. Here purification via Kugelrohr distillation was
not possible due to similar sublimation temperatures of the final ligand
and the reactants. However, with column chromatography separation
was achieved and after a final treatment with base under inert condi-
tions the pure ligand was obtained.
2.4.11. Et-iProp-uns-penp (9)
Et-uns-penp (1.0 g, 3.7 mmol) was dissolved in 20 mL acetonitrile. 2-
iodopropane (1.74 g; 10.2 mmol) and K2CO3 (5.0 g; 36.2 mmol) were
added. The solution was refluxed under argon for five days. The solvent
was removed. The residue was treated with diethyl ether, filtrated and
the solvent was removed. Kugelrohr distillation led to a pure product
that was stored under an argon atmosphere (0.77 g; 67%). 1H NMR
(400 MHz, CDCl3, 23 ◦C, δ [ppm]): 8.49 (d, 2H, py-H), 7.62 (td, 2H, py-
H), 7.54 (d, 2H, py-H), 7.10 (t, 2H, py-H), 3.84 (s, 4H, py-CH2), 2.84 (h,
1H, NCH(CH3)2)), 2.61 (t, 2H, NCH2CH2N(Py)2), 2.54 (t, 2H,
NCH2CH2N(Py)2), 2.38 (q, 2H, NCH2CH3), 0.90 (m, 9H, NCH2CH3 and
NCH(CH3)2). 13C NMR (101 MHz, CDCl3, 23 ◦C, δ [ppm]): 160.1, 149.3,
149.1, 136.4, 122.9, 121.9, 61.1, 54.4, 50.8, 47.6, 44.9, 18.4, 14.1. ESI-
MS (m/z): [M + H]+: 313.24, [M + Na]+: 335.22.
The copper(I) complexes had to be synthesized under inert condi-
tions in a glove box (argon) due to their high sensitivity towards air and
moisture. However, it was possible to determine the molecular struc-
tures by SCXRD of the copper(I) complexes with all ligands in Fig. 3 with
the exception of the copper(I) complex with iProp2uns-penp as a ligand.
Therefore, [Cu(4)]BPh4 was analyzed by ESI-MS instead. While molec-
ular structures of [Cu(1)]BPh4 and [Cu(6)]BPh4 have been reported
previously crystallographic data for [Cu(1)]BPh4 have not been pub-
lished yet (crystallographic data are reported in the Supporting Infor-
mation, Tables SI-5 – SI-9) [16,27]. As an example for a copper(I)
complex with a symmetric ligand arm, Fig. 4 shows the molecular
structure of the cation of [Cu(1)]BPh4 with its symmetry equivalent. As
an example for a copper(I) complex with an asymmetric ligand arm, the
molecular structure of the cation of [Cu(9)]BPh4 (one complex cation
and its symmetry equivalent to show their relative orientation) is
2.4.12. Ferrocene-Me-uns-penp (10)
Me-uns-penp (0.50 g; 2.0 mmol) was dissolved in 20 mL DCE.
Ferrocene aldehyde (0.46 g; 2.1 mmol) and STAB (0.94 g; 4.4 mmol)
were added. The reaction mixture was stirred at room temperature
under argon atmosphere overnight. The reaction was quenched with 30
mL of 2 M NaOH and extracted with DCM. The combined organic layers
were washed with brine, dried over MgSO4 and the solvent was
removed. The product was purified by column chromatography on silica
as stationary phase eluted with DCM/MeOH (95: 5 v/v, + 0,2 vol%
DIPEA). The solvent of the collected fractions was removed and the
obtained oil was transferred into a glove box. Subsequently the crude
product was dissolved in dry diethylether and treated with a small
amount of DIPEA. After stirring for a few minutes, the solution was
separated via a syringe filter (0.2 μm) and the solvent and the residual
DIPEA as well were removed under vacuum. ESI-MS (m/z): [M + H]+:
455.19.
Copper(I) complexes: All copper(I) complexes with tetraphenylbo-
rate as anion were obtained under inert conditions according to a gen-
eral procedure described in the literature [5]. In case of [Cu(10)]BPh4,
the copper(I) salt [Cu(MeCN)4]SO3CF3 was used to prevent
disproportionation.
2.4.13. [Cu(Ligand)]BPh4
200 mg of the ligand were dissolved in a small amount of dry
acetone. A solution of 0.95 eq [Cu(MeCN)4]PF6 was dissolved in a small
amount of dry acetone and added dropwise to the ligand solution while
stirring. Afterwards, 1.05 eq NaBPh4 was added. The complex was
precipitated by dropwise addition into dry diethyl ether. The yellow
solid was filtrated, washed with diethyl ether and dried under vacuum.
Diffusion of diethyl ether led to crystals which were suitable for X-ray
analysis (except for [Cu(iProp2uns-penp)]BPh4, detection via ESI-MS).
Copper(II) complexes: All copper(II) complexes with coordinating
chloride and non-coordinating perchlorate as anion were obtained ac-
cording to a general procedure described in the literature [23].
Fig. 4. Molecular structure of the cation of [Cu(1)]BPh4 complex and its
symmetry equivalent (ꢀ x, 1-y, 1-z). Ellipsoids set at 50% probability; the an-
ions, H atoms, and solvent molecules are omitted for clarity. Carbon: black;
nitrogen: blue; copper: brown. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
5

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
presented in Fig. 5. Furthermore, the molecular structure of the cation of
the somewhat a bit more special copper(I) complex with the redox active
ferrocene derivative as ligand, [Cu(10)]BPh4 is shown in Fig. 6. The
molecular structures of all copper(I) complexes as well as their crystal-
lographic data are reported in the Supporting Information (Figs. SI-8 –
SI-15 and Tables SI-5 - SI-44). These complexes are perfectly in line with
the data already reported for related copper(I) complexes with tripodal
ligands [19,28,29].
Due to the fact that usually “dioxygen adduct” copper complexes are
only spectroscopically observed as reactive intermediates, molecular
structures of these compounds are rare. However, to get an idea for the
structure of these complexes, it is a good strategy to structurally char-
acterize the corresponding copper(II) complexes with chloride anions as
additional ligands (representing/modelling the binding of the oxygen
atom) [28–30]. Copper(II) complexes with chloride could be structurally
characterized and molecular structures of the cations of [Cu(2)Cl]ClO4,
[Cu(3)Cl]ClO4, [Cu(5)Cl]ClO4, [Cu(7)Cl]ClO4, [Cu(8)Cl]ClO4 and [Cu
(9)Cl]ClO4 as well as crystallographic data are reported in the Sup-
porting Information (Figs. SI-16 – SI-22 and Tables SI-45 – SI-81). The
molecular structure of [Cu(6)Cl]ClO4 has been reported previously [16].
No crystals suitable for structural characterization of the copper(II)
complex with the Ferrocene-Me-uns-penp (10) were obtained. Further-
more, efforts to crystallize and structurally characterize [Cu(4)Cl]ClO4
were not successful. Instead, crystallization of the product from a solu-
tion of this complex showed that the (iProp)2-amine group obviously got
detached from the ligand and that the remaining ligand part was
oxidized. The molecular structure of this complex [Cu(11)Cl] (11 =
dipicolinamide anion) is presented in the Supporting Information
(Fig. SI-1). The molecular structure has already been reported previously
[31,32]. While the mechanism of this reaction is not completely clear
yet, it is more than likely that it follows a similar mechanism proposed
for related copper complexes [33,34]. The reactivity of copper chloride
towards organic substrates is beyond the scope of this work and was not
investigated any further herein.
Fig. 6. Molecular structure of the cation of [Cu(10)]BPh4 complex. Ellipsoids
set at 50% probability; the anions, H atoms, and solvent molecules are omitted
for clarity. Carbon: black; nitrogen: blue; iron: orange; copper: brown. (For
interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
3.2. Cyclic voltammetry
Fig. 7. Cyclic voltammograms of the copper(I) complexes with ligands 1, 2, 3
and 4 (symmetric uns-penp derivatives) in acetonitrile at room temperature.
[complex] = 1 mmol/L; [electrolyte] ([NBu4]BF4) = 0.1 mol/L.
To gain more information on the influence of the alternating residues
at the terminal amine with regard to their redox potentials, cyclic vol-
tammetry was performed in acetonitrile. The cyclic voltammograms for
symmetric and asymmetric substituted ligands are shown in Figs. 7 and
8. The redox potentials are reported in Table 1. All complexes show a
reversible Cu(I)/Cu(II) redox reaction and the potentials of nearly all
copper(I) complexes are quite similar with a value of E1/2 close to 0.1 V.
Fig. 8. Cyclic voltammograms of the copper(I) complexes with ligands 5, 6, 7,
8 and 9 (asymmetric uns-penp derivatives) in acetonitrile at room temperature.
[complex] = 1 mmol/L; [electrolyte] ([NBu4]BF4) = 0.1 mol/L.
Fig. 5. Molecular structure of the cation of [Cu(9)]BPh4 complex and its
symmetry equivalent (2-x, 2-y, 1-z). Ellipsoids set at 50% probability; the an-
ions, H atoms, and solvent molecules are omitted for clarity. Carbon: black;
nitrogen: blue; copper: brown. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
6

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
in the introduction these complexes form quite stable trans-μ-1,2,-per-
Table 1
Potentials of the copper(I) complexes and ferrocene in comparison.
oxido copper complexes at low temperatures (Eq. (1)). The same
behavior was observed for the copper complexes with ligands 2, 3 and
5–10. The UV–vis spectra of [Cu(1)]BPh4, [Cu(4)]BPh4 and [Cu(9)]
BPh4 prior and after their reaction with dioxygen in acetone at ꢀ 85 ◦C
are presented in Fig. 10. The typical spectra of the dinuclear copper
peroxido complexes with an absorbance maximum at around 530 nm
and a shoulder around 600 nm are observed [37,38]. The UV–vis spectra
of the other copper dioxygen “adduct” complexes are reported in the
Supporting Information (Figs. SI-2 and SI-3).
Ep red[V]
Ep ox[V]
E1/2 [V]
ΔE [mV]
[Cu(1)]BPh4
[Cu(2)]BPh4
[Cu(3)]BPh4
[Cu(4)]BPh4
[Cu(5)]BPh4
[Cu(6)]BPh4
[Cu(7)]BPh4
[Cu(8)]BPh4
[Cu(9)]BPh4
[Cu(10)]BPh4
0.034
0.055
0.023
0.254
0.048
0.043
0.066
0.059
0.100
0.014
0.848
0.127
0.147
0.115
0.348
0.140
0.132
0.163
0.152
0.189
0.130
0.930
1.270
0.686
0.081
0.101
0.069
0.301
0.094
0.088
0.115
0.106
0.145
0.072
0.889
93
92
92
94
92
89
97
93
89
116
82
Again, [Cu(4)]BPh4 shows different reactivity, as can be seen in
Fig. 10. Only a small and very broad maximum was observed that could
be assigned to the corresponding trans-μ-1,2,-peroxido copper complex,
indicating an already decaying oxygen “adduct complex”. Time resolved
ferrocene
0.613
0.650
73
◦
UV–Vis spectra obtained in a stopped-flow measurement at ꢀ 83 C in
acetone (Fig. 11) revealed a fast formation of the peroxido complex
within 1.7 s at 550 nm according to Eq. (1) (briefly the superoxido in-
termediate can be seen as well with an absorbance maximum at 420
nm). Despite the low temperature, the peroxido complex is not stable
and decomposes over two minutes (time trace shown in the inset in
Fig. 11). This furthermore explains the observation from Fig. 10 and
gives a hint towards the decomposition and oxidation of 4 described
above.
Not unexpected, [Cu(4)]BPh4 has a much higher potential with E1/2
= 0.301 V. Furthermore, the potential E1/2 = 0.145 V of [Cu(9)]BPh4 is
somewhat higher with regard to the other complexes. This can be easily
explained by the known fact that the N-donor atom can dissociate from
the metal ion in the tripodal complex if the R groups become sterically
too demanding. Acetonitrile that has been used for the CV measurements
is a strongly competing ligand that additionally will enforce such a
dissociation. So the CV of [Cu(4)]BPh4 most likely shows the redox
potential of the complex [Cu(4)(CH3CN)x]BPh4 with a detached amine
arm and therefore is quite different in comparison with the other
complexes.
Quite surprising was the fact that the copper(I) complex with the
ferrocene unit, [Cu(10)]BPh4, also reacted according to Eq. (1) and
formed a quite stable peroxido complex at low temperatures. Stopped-
flow measurements at ꢀ 81 ◦C are shown in Fig. 12 and nicely show
the formation of this complex in the time resolved UV–vis spectra
together with the decrease of the absorbance maxima of the superoxido
complex at 410 nm (inset in Fig. 12).
Complex [Cu(10)]BPh4 was synthesized to integrate an internal
redox indicator. It is treated separately because the modification of the
uns-penp ligand is much bigger in comparison with the other ligands
applied. The CV of this complex is presented in Fig. 9 and potentials are
reported in Table 1. The redox potential of the copper complex is well in
line with the other complexes and does not seem to be influenced by the
directly bonded ferrocene unit. The potential of the ferrocene unit with
0.89 V is shifted to a more positive value compared with 0.65 V of the
free ferrocene under these conditions. However, this again is expected
because ferrocene derivatives such as ferrocene aldehyde show similar
shifts in comparison with ferrocene [35].
3.4. Solid state reactions
As has been observed previously, by utilizing the anion tetraphe-
nylborate, copper peroxido complexes with some tripodal ligands could
be stabilized in the solid state to such an extent that it was possible to
even heat them up to around a 100 ◦C and furthermore to use them for
catalytic oxygenation of toluene [5,16]. To investigate the influence of
the substitution at the terminal nitrogen atom, all solid copper(I) com-
plexes with tetraphenylborate as anion were treated with dioxygen at
room temperature. The results of the oxygen exposure of the complexes
3.3. Reactivity towards dioxygen
The reactivity of the copper(I) complexes with uns-penp and Me2uns-
penp (1) towards dioxygen have been studied in great detail in the same
way as with the ligands tmpa and Me6tren [9,10,19,23,36]. As described
Fig. 10. UV–Vis of symmetric complexes [Cu(1)]BPh4 and [Cu(4)]BPh4 and
the asymmetric complex [Cu(9)]BPh4 in acetone at ꢀ 85 ◦C. Spectra are shown
before and after treatment with dioxygen.
Fig. 9. Cyclic voltammogram of [Cu(10)]BPh4 in acetonitrile at room tem-
perature. [complex] = 1 mmol/L; [electrolyte] ([NBu4]BF4) = 0.1 mol/L.
7

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
Fig. 13. Solid copper(I) complexes with symmetric uns-penp ligands after
exposure with dioxygen; (a) [Cu(1)]BPh4; (b) [Cu(2)]BPh4; (c) [Cu(3)]BPh4.;
(d) [Cu(4)]BPh4.
Fig. 11. Time-resolved spectra of the reaction of [Cu(4)]BPh4 with dioxygen in
acetone at ꢀ 83 ◦C ([complex] = 2.0 × 10ꢀ 4 M, [O2] = 4.4 × 10ꢀ 3 M, total time:
150 s). At 420 nm the spectra shows the decreasing intensity of the superoxido
intermediate. At 550 nm the increasing intensity of the labile peroxido complex
is visible. The time trace at the upper right shows the fast decomposition of the
peroxido species.
Fig. 14. Solid copper(I) complexes with asymmetric uns-penp ligands after
exposure with dioxygen; (a) [Cu(5)]BPh4; (b) [Cu(6)]BPh4; (c) [Cu(7)]BPh4;
(d) [Cu(8)]BPh4; (e) [Cu(9)]BPh4. [Cu(10)]BPh4 did not show any reactivity in
the solid state and was not considered.
5–9 exhibited a visible reaction with dioxygen. The yellow solids shown
in Fig. 14 a-d reacted to a green solid (a bit difficult to see in the picture
but very clear in the direct observation in the experiment). It is notice-
able that these products kept their color and no further reaction to a
purple/blue peroxido complex was observed. Reactions with the com-
parable complexes with tmpa and Me2uns-penp also develop a green
color shortly after exposure to dioxygen, which however, after some
time, changed to a dark blue/purple color typical for a binuclear per-
oxido complex. The green color meanwhile is a hint for copper η1
-
superoxido complex which is formed in a reversible reaction and is
stable at room temperature [12,13,39,40]. Storing the [Cu(6)]BPh4
complex under argon atmosphere for a couple of days brought back the
former yellow color. Treating the sample with oxygen led to the green
colored compound again. Same results were obtained by heating the
green solid up to 50 ◦C under an argon atmosphere. This effect was
described previously by us within an investigation of the immobilization
of copper complexes (with ligands derived from uns-penp) on silica
surfaces [16].
Fig. 12. Time resolved spectra of the reaction of [Cu(10)]BPh4 with dioxygen
in acetone at ꢀ 81 ◦C ([complex] = 2.0 × 10ꢀ 4 M, [O2] = 4.4 × 10ꢀ 3 M, total
time: 500 s). At 410 nm the spectra show the decreasing intensity of the
superoxido intermediate. At 528 nm the increasing intensity of the final per-
oxido complex is visible. The time trace at the upper right shows the formation
of the peroxido species with the parallel decomposition of the super-
oxido complex.
[Cu(9)]BPh4 (Fig. 14e) showed a different behavior, similar to the
symmetric system with Me2uns-penp. Here as well a binuclear peroxido
complex was formed. Solid UV–Vis measurements supported this
observation (see Supporting Information Fig. SI-4). But the formation
works much faster than in the symmetric system. The first contact with
dioxygen led immediately to the shown blue/purple color (video of this
reaction is provided in the Supporting Information).
with symmetric uns-penp derivatives are shown in Fig. 13, the results
with the asymmetric ligands in Fig. 14.
These results initiated a closer investigation to elucidate the differ-
ences between the symmetric and the asymmetric derivatizations of the
ligands. Therefore the compounds [Cu(1)]BPh4, [Cu(3)]BPh4, [Cu(6)]
BPh4 and [Cu(9)]BPh4 were investigated further.
As was observed previously, the yellow colored powder of [Cu(1)]
BPh4 turned slowly into the intensively purple colored peroxido com-
plex (Fig. 13 a) when reacted with dioxygen [5,16]. A diffuse reflectance
UV–vis spectrum of the peroxido complex [(1)CuO2Cu(1)](BPh4)2 is
reported in the Supporting Information (Fig. SI-4). In contrast no reac-
tion was observed for complexes with ligands 2, 3 and 4. Furthermore,
solid [Cu(10)]BPh4 did not react with dioxygen under these conditions
(not shown in the pictures).
Raman measurements of the oxygen treated copper(I) complexes are
presented in Fig. 15. Especially the distinct band at 830 cmꢀ 1 at the
lower spectra of [Cu(1)]BPh4 demonstrates the formation of the trans-
μ
-1,2-peroxido complex [5]. Also the spectra of [Cu(9)]BPh4 contains
this feature but with a relative small intensity. More striking is the
In contrast all solid copper(I) complexes with asymmetric ligands
presence of that band in the spectra of [Cu(6)]BPh4 which does not show
8

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
Fig. 16. Powder XRD patterns of [Cu(1)]BPh4 measured before (green) and
after the treatment with dioxygen (purple) in comparison to powder diffraction
patterns of [Cu(1)]BPh4 simulated from single crystal data (blue), NaBPh4
(black) [42], and [Cu(MeCN)4]PF6 (red) [43]. (For interpretation of the refer-
ences to color in this figure legend, the reader is referred to the web version of
this article.)
Fig. 15. Raman measurement of the complexes [Cu(1)]BPh4, [Cu(3)]BPh4, [Cu
(6)]BPh4 and [Cu(9)]BPh4 after treatment with oxygen. The band around 830
cmꢀ 1 indicates a peroxide species. The Inset shows the spectrum of [Cu(1)]BPh4
magnified between 900 and 1750 cmꢀ 1
.
an obvious conversion into a blue product. An explanation for this
finding might be that a small amount of the batch is converted to the
peroxido complex which cannot be seen by looking at the sample. The
spectrum of the [Cu(3)]BPh4 complex did not show any sign of a reac-
tion with dioxygen. This correlates with the visual findings. The absor-
bance around 1020 cmꢀ 1 is common for all complexes investigated. It is
Comparable results were obtained from the investigation of the
complex [Cu(6)]BPh4 with the asymmetric ligand (see Supporting In-
formation Fig. SI-6). Although there has been a change in color detected
for the product, there is no observable change in the powder pattern and
thus meaning no change in crystal structure and lattice. Due to these
findings, an insertion of a dioxygen molecule would be unlikely, too, by
means of the result of PXRD. Nonetheless, the single crystal measure-
ment of [Cu(6)]BPh4 indicates the structural possibility of a reaction
with dioxygen. A change in color is observable upon dioxygen treat-
ment, which indicates a suchlike insertion. A possible explanation for
the discrepancy to powder XRD data is an insertion of dioxygen in low
quantity only and not for the bulk material. Thus, in the measured
powder diffraction pattern, only the majority phase is observable, which
is the un-reacted complex [Cu(6)]BPh4, and still the color change can
apply.
–
assigned as a ring and C H stretching mode of the coordinated pyridine
residues [41]. The inset shows a magnified region in the spectrum of
complex [Cu(1)]BPh4. Due to the strong resonance of the peroxido
ligand the intensities of the remaining bands almost disappear. Only by
magnification at the corresponding wavelengths made visible.
To gain information about the change in the crystal structure of the
solid compounds, powder XRD measurements under inert conditions
before and after exposure with dioxygen were performed. Recorded
diffractograms of [Cu(3)]BPh4, [Cu(6)]BPh4 and [Cu(9)]BPh4
compared with the corresponding diffraction patterns of the educts of
the complex synthesis simulated from single crystal data are given in the
Supporting Information (Fig. SI-5 – SI-7) to verify phase purity.
The diffractograms of the complex [Cu(1)]BPh4 before and after the
treatment with dioxygen as well as a comparison with a simulated
pattern from single crystal data as well as patterns of the reagents are
shown in Fig. 16. Small deviations of the simulated from the experi-
mental diffraction pattern can be explained by different measurement
temperatures of single crystal XRD and powder XRD. By comparing the
patterns before and after the treatment with dioxygen, it is obvious that
[Cu(1)]BPh4 undergoes a transformation upon exposure to dioxygen, as
the PXRD after the treatment strongly differs from the pattern before and
also does not coincide with the simulated pattern anymore.
Quite surprisingly, the small crystals of [Cu(9)]BPh4, which were
obtained by precipitation into diethylether during the common
complexation and anion exchange reactions retain an adequate crys-
tallinity after their reaction with dioxygen. The crystals were still suit-
able for single crystal X-ray analysis. Fig. 17 shows the molecular
structure of the binuclear peroxido complex [(9)Cu(O2)Cu(9)](BPh4)2
obtained this way. This complex is quite similar to the other three
structurally characterized end-on peroxido copper complexes with the
tripodal ligands tmpa, Bz3tren, Me6tren and a bit different to the copper
complex with the macrocyclic ligand tet b [5,7,44,45]. Our previous
efforts to accomplish the same solid state reaction with the copper
Me6tren system had failed. After the reaction with dioxygen the complex
had become amorph and the peroxido complex had to be crystallized in
a different way [5]. Crystallographic data of the complex [(9)Cu(O2)Cu
(9)](BPh4)2 are reported in the Supporting Information (Table SI-77-81)
Therefore, it was also possible to achieve the insertion of dioxygen
into the bulk substance of [Cu(9)]BPh4. The experimental powder dif-
fractograms of [Cu(9)]BPh4 before and after dioxygen treatment in
comparison to the powder diffraction patterns simulated from single
crystal data are depicted in Fig. 18. In both cases, the measured
diffraction patterns are in good accordance to the simulated patterns,
indicating the structures, single crystal structure analysis as bulk prod-
ucts, too. However, there is a distinct change in structure detectable
The results of the powder XRD measurements of [Cu(3)]BPh4 with
the symmetric ligand Prop2uns-penp are shown in Fig. SI-5. Comparing
the simulated diffraction pattern of [Cu(3)]BPh4 (black) with the
recorded pattern (red), a slight shift of the reflections of the experi-
mental diffractogram towards higher angles in 2θ can be noticed due to
the different temperatures. Different to [Cu(1)]BPh4, a comparison of
the experimental powder diffraction patterns before and after dioxygen
exposure reveals good accordance for [Cu(3)]BPh4. This indicates that
the lattice parameters do not differ significantly. In combination with
the findings of Figs. 13 and 15, a reaction with oxygen seems to be
unlikely.
9

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
Table 2
Cell parameters received from single crystal data of [Cu(9)]BPh4 complex before
and after oxygenation.
[Cu(9)]BPh4
◦
a / Å
b / Å
c / Å
11.3474(19)
13.161(2)
13.443(2)
α
/
94.310(5)
111.929(5)
101.989(6)
β / ◦
γ / ◦
[Cu(9)O]2(BPh4)2
◦
a / Å
b / Å
c / Å
11.392(3)
12.911(3)
13.538(3)
α
/
97.027(7)
109.247(7)
103.938(7)
β / ◦
γ / ◦
Relative differences
a / %
b / %
c / %
0.39%
1.94%
0.70%
α
/ %
2.80%
2.45%
1.88%
β / %
γ / %
degrees detectable. The complete structural parameters for the crystal
structure are reported in the Supporting Information. This single crystal
to single crystal transformation is most likely made possible due to the
orientation of the copper center towards its symmetry equivalent in the
crystal structure. Two copper ions are directly facing each other (Fig. 5).
The distance between them decreases after the formation of the peroxido
complex (5.591 Å to 4.475 Å), however, the relative orientation almost
remains. This special geometry is not observed in the complexes with the
other ligand derivatives. The metal centers in the crystal lattice of e. g.
[Cu(1)]BPh4 are orientated adversely towards each other (Fig. 4). The
copper‑copper distance is 12.230 Å. An end-on peroxide formation will
therefore lead to a complete loss of crystallinity.
Fig. 17. Molecular structure of the cation of binuclear [(9)Cu(O2)Cu(9)]
(BPh4)2. Ellipsoids set at 50% probability; the anions, H atoms, and solvent
molecules are omitted for clarity. Carbon: black; nitrogen: blue; oxygen: red;
copper: brown. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
4. Conclusion
Investigations on the reactivity of a series of copper(I) complexes [Cu
(L)]BPh4 towards dioxygen were performed to gain a better under-
standing of the steric influence of a tripodal ligand system. The ligand L,
R1-R2-uns-penp (uns-penp
=
(2-aminoethyl)bis(2-pyridylmethyl)
amine), was chosen because, while the bispicolylamine unit already
stabilized the copper(I) unit, the additional amine arm could be easily
modified. In combination with the same anion (tetraphenylborate is
essential for stabilization of the complexes in the solid state) it could be
shown that the redox potential Cu(I)/Cu(II) for nearly all complexes was
quite similar. Therefore, only the different alkyl groups bound to the
amine arm were responsible for different reactivities. While Me2uns-
penp as a ligand still supported quite slow formation of a solid peroxido
complex, symmetric substitution with two ethyl, propyl or isopropyl
groups completely suppressed reactivity of the corresponding copper(I)
complexes (in the solid state) towards dioxygen. Quite surprisingly,
however, was the observation that copper(I) complexes with ligands
with asymmetric substitution of the amine nitrogen, Me-Et-uns-penp
(5), Me-Prop-uns-penp (6), Et-Prop-uns-penp (7), Me-iProp-uns-penp
(8) and Et-iProp-uns-penp (9) clearly showed reactivity towards
dioxygen in the solid state. Copper(I) complexes with ligands 5–8
reversibly formed a green compound that indicated formation of a
mononuclear superoxido complex, however, so far and as described
previously, we did not find a way to really characterize these products
[16]. In contrast, [Cu(9)]BPh4 instantly reacted with dioxygen to form a
Fig. 18. Powder XRD patterns for [Cu(9)]BPh4 before (red) and after the
treatment with dioxygen (blue) compared with the powder diffraction patterns
of [Cu(9)]BPh4 and [Cu(9)O]2(BPh4)2 simulated from single crystal data
(black). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
upon dioxygen treatment, as it can be seen by differences in number,
intensities and positions of reflections between initial and [Cu(9)]BPh4
flushed with dioxygen. In contrast to the proposed reactivity towards
dioxygen of complex [Cu(6)]BPh4, a complete conversion of [Cu(9)]
BPh4 to the oxygenated product [Cu(9)O]2(BPh4)2 can be observed
corroborated by comparison of simulated with experimental patterns.
Table 2 contains the values of the unit cells before and after exposure
to oxygen. Comparison of the unit cells reveals only a slight expansion of
the unit cell of the peroxido complex. The relative change is less than
2%.
stable trans-μ-1,2-peroxido product complex that could be structurally
characterized. A look at the crystal structures of the copper(I) complexes
of all ligands clearly showed that [Cu(9)]BPh4 is kind of preformed for
the uptake of dioxygen and only has to undergo minor changes in the
overall geometry when reacted to the copper(II) peroxido complex.
Usually copper(I) complexes undergo a big change in coordination ge-
ometry/coordination number (bond lengths and angles) when oxidized
to the corresponding copper(II) complexes. However, this is slightly
different for copper complexes with tripodal ligands and thus allowed
with the modification of the uns-penp ligand to find a perfect system for
the fast uptake of dioxygen without a major structure change. Definitely
Furthermore, there is a small deformation between two and three
10

T. Brückmann et al.
Journal of Inorganic Biochemistry 223 (2021) 111544
monooxygenase (PHM), Z. Anorg. Allg. Chem. 639 (2013) 1504–1511, https://doi.
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we could prove that slight modifications of the tripodal ligand system
can have a large effect on the chemistry of the corresponding copper
complexes.
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Declaration of Competing Interest
[16] T. Brückmann, J. Becker, K. Turke, B. Smarsly, M. Weiß, R. Marschall, S. Schindler,
Immobilization of a copper complex based on the tripodal ligand (2-aminoethyl)bis
(2-pyridylmethyl)amine (uns-penp), Z. Anorg. Allg. Chem. 647 (2021) 560–571,
https://doi.org/10.1002/zaac.202100022.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Prof. Dr. Roland Marschall (Fakultat für Biologie, Chemie und Geo-
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