Electrochemical Exploration of Active Cu-Based Atom Transfer Radical Polymerization Catalysis through Ligand Modification

Article abstract of DOI:10.1021/acs.inorgchem.1c01001

The intersection between Cu-catalyzed atom transfer radical polymerization (ATRP) and organometallic mediated radical polymerization (OMRP) has been recently shown to be a result of competition between the CuI and CuII complexes of polyamine ligands for the same organic free radical. The tetradentate ligands N,N′-bis-2′-pyridylmethyl-ethane-1,2-diamine (L1) and N,N′-dimethyl-N,N′-bis-2′-pyridylmethyl-ethane-1,2-diamine (L2) form stable Cu complexes which, depending on their oxidation state, can either liberate or complex organic radicals. Herein, we show that this process may be affected by subtle changes to the ligand system. Switching from a tertiary amine (L2) to a secondary amine (L1) retains ATRP and OMRP activity through a series of cyclic voltammetry measurements in the presence of the initiator bromoacetonitrile.

Full text of DOI:10.1021/acs.inorgchem.1c01001

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Article
Electrochemical Exploration of Active Cu-Based Atom Transfer
Radical Polymerization Catalysis through Ligand Modification
Jamie N. Melville and Paul V. Bernhardt*
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ABSTRACT: The intersection between Cu-catalyzed atom trans-
fer radical polymerization (ATRP) and organometallic mediated
radical polymerization (OMRP) has been recently shown to be a
result of competition between the CuI and CuII complexes of
polyamine ligands for the same organic free radical. The
tetradentate ligands N,N′-bis-2′-pyridylmethyl-ethane-1,2-diamine
(L1) and N,N′-dimethyl-N,N′-bis-2′-pyridylmethyl-ethane-1,2-dia-
mine (L2) form stable Cu complexes which, depending on their
oxidation state, can either liberate or complex organic radicals.
Herein, we show that this process may be affected by subtle
changes to the ligand system. Switching from a tertiary amine (L2)
to a secondary amine (L1) retains ATRP and OMRP activity
through a series of cyclic voltammetry measurements in the
presence of the initiator bromoacetonitrile.
appropriate rate.3 Manipulation of these values is possible by
varying the Cu catalyst, initiator, solvent, monomer, and/or
halide involved in this equilibrium.
INTRODUCTION
■
Atom transfer radical polymerization (ATRP) is a controlled
radical polymerization technique first demonstrated in 1995 in
which the radical is generated via a reversible redox process
that is mediated by a transition metal complex, in most cases
containing copper, which cycles between its mono- and
divalent forms.1,2 As illustrated in Scheme 1, the initiating
Copper is well suited to its role in ATRP catalysis. Upon
oxidation from CuI to CuII, the coordination number rises
from 4 to 5 (common for most CuII complexes).4 This is
necessary in ATRP systems, as halogen atom transfer to CuI
requires an increase in coordination number. Copper
complexes of multidentate N-donor ligands provide the most
effective ATRP catalysts in terms of activity and stability of the
complex in both oxidation states compared with ligands
bearing O-, P-, or S-donors.5 Essentially all of the Cu
complexes used in ATRP comprise tertiary amine or pyridyl
N-donors. The choice of ligand also governs the CuII/I redox
potential, and there is an empirical correlation between log
Scheme 1. Cu-Catalyzed ATRP
K
ATRP and the CuII/I redox potential (more negative potentials
being associated with larger log KATRP).6,7 Lower redox
potentials also signal increasing stability of the CuII form of
the catalyst relative to its CuI form.
catalyst [CuIL]+ reacts reversibly with an alkyl halide (R−X)
initiator, concurrently abstracting a halogen atom (X)
(captured as [CuIILX]+) and releasing a radical (R•). The
radical adds to a monomer (M), and the polymer chain
propagates (kp) by consecutive reaction with other monomers.
The radical activation process exists in dynamic equilibrium
with the halide-terminated growing polymer chain.
The ATRP equilibrium constant, KATRP (≪ 1), limits the
concentration of radicals to minimize radical−radical coupling
(termination, kt) reactions, and as such kact must be smaller
than kdeact by several orders of magnitude. However, kact must
still be large enough that the radical activation occurs at an
Organic synthesis can be used to introduce inductive effects
to the polyamine ligand that lower the CuII/I redox potential.
In 2018, Ribelli et al. synthesized a copper ATRP catalyst
Received: April 1, 2021
Published: June 18, 2021
https://doi.org/10.1021/acs.inorgchem.1c01001
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based on tris-(2-pyridylmethyl)amine (TPMA), namely its 4-
dimethylamino-substituted analog TPMANMe2.8 The addition
of electron-donating −NMe2 groups to the pyridine rings of
TPMA lowered the CuII/I redox potential by more than 300
mV (from −240 mV to −554 mV vs SCE). This led to an
increase of KATRP from ∼10−5 (for [Cu(TPMA)Br]+) to ∼10−1
(for [Cu(TPMANMe2)Br]+) in MeCN, in the presence of the
initiator methyl 2-bromoproprionate (MBrP).
Compared to tertiary polyamine ligands, which dominate
ATRP Cu catalysis, their secondary amine analogs have
received little attention.9−11 This is important as a change to
the N-donor (from a tertiary to a secondary amine) will lower
the CuII/I redox potential significantly as the Cu−N bonds
shorten through relieved steric crowding.12 This opens up the
possibility for ATRP catalysts of even higher activity without
the need for synthetic elaboration of the ligand system.
Herein, we study the Cu complexes of a homologous series
of tetradentate N-donor ligands (Figure 1) based on the parent
structure, the radical, and the rate of deactivation all influence
this balance, and this will be investigated in the present work
by comparisons between the three homologues [CuL1]2+,
[CuL2]2+, and [CuL3]2+.
EXPERIMENTAL SECTION
■
Synthesis. Safety Note. Perchlorate salts are potentially explosive.
Although no problems were encountered when handling the
compounds in this work, they should not be heated in their solid
state or scraped from sintered glass frits.
Reagents. Most solvents and reagents were obtained commercially
and used without further purification. MeCN was distilled and dried
over 3−4 Å molecular sieves prior to use.
[CuL1](ClO4)2. A synthesis for L1 has previously been reported
(Figure 2), and this was followed with minor modifications.18
A
solution of pyridine-2-carbaldehyde (2.45 mL, 25.8 mmol) in MeOH
(15 mL) and a solution of ethylenediamine (0.7 mL, 10 mmol) in
MeOH (5 mL) were combined and refluxed, with stirring, for 3.5 h.
After cooling to room temperature, NaBH4 (1.6 g, 42 mmol) was
added and the solution was stirred overnight. The solvent was
removed via rotary evaporation to give a dark yellow oil. This was re-
dissolved in water and extracted with four 50 mL aliquots of CHCl3.
The solvent was removed via rotary evaporation to give crude L1 (2.6
g, approximately 10.7 mmol) as a brown oil. Some of this product
(1.08 g, approximately 4.46 mmol) was dissolved in EtOH (10 mL).
A solution of Cu(ClO4)2·6H2O (2.01 g, 5.42 mmol) in EtOH (15
mL) was added slowly with stirring. Water (15 mL) was added to
prevent premature precipitation. The resulting mixture was stirred on
gentle heat for 1 h before being purified on a Sephadex SP-25 (Na+
form) column. A pale purple band eluted first with 0.3 M NaClO4,
followed by a light, bright blue band, and finally a major dark, vivid
blue band of the desired compound. The dark blue solution was
concentrated via rotary evaporation to one-quarter of its volume.
Some purple precipitate formed on the sides of the flask. This was
filtered off, and the filtrate was allowed to slowly concentrate at room
temperature. After 10 d, deep blue/purple crystals formed (336 mg,
15%). Further crops could be obtained from the filtrate if desired.
Anal. Calcd for C14H20Cl2CuN4O9 (monohydrate 522.78 g mol−1): C,
32.17; H, 3.86; N, 10.72. Found: C, 32.2; H, 3.62; N, 10.7. IR (cm−1):
3249 (m, N−H, str), 3078 (w, ar. C−H str), 2969 (w, sat. C−H str),
1615 (m, CC), 1452 (m, CH2 bend), 1076 (s, Cl−O str). Visible
absorption spectra λmax/nm (ε/M−1 cm−1): MeCN 599 (144);
DMSO 621 (186). X-ray diffraction of the crystals matched the
previously published structure of rac-[CuL1](ClO4)2 (CCDC
reference code IDUMEZ).15
Figure 1. Three complexes studied in this work.
N,N′-bis(2-pyridylmethyl)ethylenediamine (L1), and inclusive
of its tertiary amine (L2) and diimine (L3) analogs. All three
Cu complexes are known compounds, and their EPR and
electronic spectroscopic properties have been investi-
gated.13−17 Crystal structures of two of these complexes,
[CuL1]2+ and [CuL2]2+, are known,15,18,19 and [CuL2]2+ has
already found favor as an ATRP catalyst where it exhibits one
of the highest activities of all Cu complexes.20
As many of the compounds relevant to ATRP are unstable
(including the intermediate radicals and CuI complexes
(Scheme 1)), mechanistic studies are not straightforward and
we have developed novel electrochemical methods to generate
these highly reactive intermediates in situ and understand their
chemistry using combinations of time- and concentration-
dependent voltammetry.21−25 These studies have offered new
ways to access kinetic information relevant to the use of these
compounds in polymer chemistry.26,27
Recent research has revealed that, in some copper-mediated
ATRP systems, organocopper(II) complexes may be formed
by C-atom (radical) capture (as opposed to halogen atom
transfer) to the CuI catalyst.24,25,28 This intersects with
organometallic mediated radical polymerization (OMRP)
(Scheme 2),29 a different controlled radical polymerization
approach, and introduces a new layer of complexity that must
be understood if fine control over activity is to be achieved in
both ATRP and OMRP. Factors such as multidentate ligand
[CuL2](ClO4)2. A previously reported synthetic method for
methylation of L1 was followed with some modifications.30 Crude
L1 (1.36 g, 5.61 mmol, prepared above) was dissolved in water (15
mL) and combined with CH2O solution (8.0 mL, 37%) and HCOOH
(7 mL, 99%). The mixture was refluxed, with stirring, for 3 d.
Aqueous NaOH was added until the pH was approximately 13. The
solution was extracted with four 40 mL aliquots of CHCl3. The
solvent was removed via rotary evaporation to give crude L2 (1.4 g,
5.2 mmol). This product was dissolved in a minimal amount of
MeOH (10 mL). A solution of Cu(ClO4)2·6H2O (2.01 g, 5.4 mmol)
in MeOH (15 mL) was added slowly with stirring. The resulting
mixture was stirred with warming for 3 h. Half of this mixture was
purified on a Sephadex SP-25 (Na+ form) column with 0.3 M NaClO4
as the eluent. A minor pale purple band eluted first and was discarded.
The desired product followed as an intensely blue band. This solution
was concentrated via rotary evaporation to one-quarter of its volume.
After 24 h, bright blue crystals (185 mg, 0.346 mmol) were collected.
The remaining half of the reaction mixture (not purified by column
chromatography) was left to concentrate for 3 d, which also afforded
dark blue crystals identical to the fraction obtained after column
chromatography (278 mg, 0.525 mmol). Overall, 463 mg (0.871
mmol, 17%) of [CuL2](ClO4)2 were collected and further crops could
be obtained from the filtrate if desired. Anal. Calcd for
C16H22Cl2CuN4O8 (532.82 g mol−1): C, 36.07; H, 4.16; N, 10.52.
Scheme 2. Competition between the CuI and CuII Forms of
the Catalyst for the Radical R•
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Figure 2. Synthetic summary of the compounds in this work.
Found: C, 35.9; H, 4.2; N, 10.2. IR (cm−1, ATR): 2914 (w, sat. C−H
str), 1614 (m, CN str), 1447 (m, CH2 bend), 1073 (s, Cl−O str).
Visible absorption spectra λmax/nm (ε/M−1 cm−1): MeCN 623 (270);
DMSO 642 (272). X-ray diffraction identified the complex as the
known compound meso-[CuL2](ClO4)2 (CCDC reference code
IFIXUR).19
[CuLn]2+ (n = 1, 2, 3) in the same solvent. The complex solution also
contained 0.1 M (Et4N)ClO4 as the supporting electrolyte to mirror
the conditions of subsequent cyclic voltammetry experiments.
Aliquots of bromide solution were added past the stoichiometric
end point (1:1) complex to ensure complete formation of the
bromido complex. Once the spectra exhibited no further changes with
successive bromide additions, the titration was finished. Global
analysis of the data collected was undertaken using ReactLab
EQUILIBRIA32 to obtain the requisite bromide binding constants
(KII,Br) (Scheme 3).
[CuL3](ClO4)2. A solution of pyridine-2-carbaldehyde (4.42 g, 41.2
mmol) in MeOH (15 mL) and a solution of ethylenediamine (1.20 g,
20.0 mmol) in MeOH (5 mL) were combined and refluxed, with
stirring, for 5.5 h. The solvent was removed via rotary evaporation to
give a brown oil. This product was left under vacuum overnight to
give crude L3 (5.02 g, approximately 10 mmol). This product was
dissolved in MeOH (35 mL), and a solution of Cu(ClO4)2·6H2O
(7.41 g, 20.0 mmol) dissolved in methanol (20 mL) was added
dropwise. The resulting solution was stirred with gentle warming.
After standing for 7 d at room temperature, small dark blue crystals of
[CuL3](ClO4)2 formed which were suitable for X-ray diffraction (5.81
g, 11.6 mmol, 58%). Anal. Calcd for C14H14Cl2CuN4O8 (500.74 g
mol−1): C, 33.58; H, 2.82; N, 11.19. Found: C, 33.5; H, 3.0; N, 10.9.
IR (cm−1, ATR): 3074 and 3044 (ar. C−H str), 2969 and 2930 (w,
sat. C−H str) 1663 (m, CN str), 1604 (m, CC str), 1046 (s, Cl−
O str), 778 (s, ar. C−H bend). Visible absorption spectra λmax/nm
(ε/M−1 cm−1): MeCN 612 (193); DMSO 628 (155).
Scheme 3. Model Describing Cu-Catalyzed Electrochemical
Radical Activation and Deactivation by Atom Transfer
PHYSICAL METHODS
■
UV−vis spectra were measured using a Shimadzu UV−2600 UV−
visible spectrophotometer over the range 300−900 nm. Solid state IR
spectroscopy was performed on a PerkinElmer Frontier FTIR
spectrometer from 4000 to 550 cm−1 in Attenuated Total Reflectance
mode. Elemental microanalyses were acquired on a FLASH 2000
CHNS/O Analyzer.
X-ray Crystallography. X-ray crystallographic data were collected
at 190 K on an Oxford Diffraction Gemini diffractometer operating
within the range 2 < 2θ < 50° with Mo Kα radiation (λ = 0.71073 Å).
The structure was solved and refined with SHELX.33 Calculations
were all undertaken within the WinGX program.34 Images of all
structures were created in Mercury.35 Data in CIF format have been
deposited with the CCDC (2074222).
Cyclic voltammetry was performed using a BAS100B/W
potentiostat, a glassy carbon working electrode (surface area =
0.055 cm2), platinum counter electrode, and non-aqueous Ag/Ag+
reference electrode in either DMSO or MeCN with 0.1 M
(Et4N)ClO4 supporting electrolyte. Ferrocene was used as an external
standard, and all potentials were cited versus Fc+/0. All solutions were
purged with nitrogen prior to each measurement. Following each
measurement, the working electrode was polished using 0.05 μm
alumina nanoparticles, rinsed with water and acetone, and dried.
Voltammetry data were simulated with the program Digisim,31 and
the simulation parameters (rate coefficients, redox potentials,
diffusion coefficients, etc.) are summarized in Table 2 and Table S1.
UV−visible Spectrophotometric Titrations. In a standard
titration, 0.1 equiv of bromide (as 200 mM (Et4N)Br solutions in
either DMSO or MeCN) was added to 2 mL of a 3.0 mM solution of
RESULTS AND DISCUSSION
■
Structural, Spectroscopic, and Electrochemical Char-
acterization. Structural Considerations. Crystalline samples
of [CuL1](ClO4)2, [CuL2](ClO4)2, and [CuL3](ClO4)2 were
obtained and used in all experiments. The structure of
[CuL3](ClO4)2 is new. The complex occupies a crystallo-
graphic two-fold axis (Figure 3). The copper has a distinctively
flat CuN4 plane as part of a tetragonally elongated octahedral
geometry, enforced by the rigidity of the numerous double
bonds within the ligand structure. This forces the hydrogens
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properties (Table 1) due to stereochemical changes at the
ethylenediamine/diimine N-donors (N2 in Figure 3). Without
the CN(2) double bond present in [CuL3]2+, the ligands in
[CuL1](ClO4)2 and [CuL2](ClO4)2 are able to fold away from
planarity, relieving the torsional strain on the central C−C
bond. This also introduces stereogenic centers at each NH/
NMe donor, meaning that two N-based isomers of these
complexes may exist; rac, with the hydrogen/methyl
substituents on N2 on opposite sides of the CuN4 plane,
and meso, where they occupy the same side. Crystalline
[CuL1](ClO4)2 prepared herein (Figure 4A) corresponds to
the rac isomer.15 Conversely, crystals of [CuL2](ClO4)2 are the
meso isomer (Figure 4B).19
The geometrical distortion from CuN4 planarity is most
pronounced in the structure of [CuL2](ClO4)2. This distortion
is enforced by the addition of a methyl group to each N which
causes crowding of the CuN4 plane. If one of the weakly bound
perchlorate ions is treated as a fifth ligand, as Pandiyan et al.19
have in their analysis, the divergence of the CuN4 group from
planarity can be quantified using the τ5 parameter (eq 1) for 5-
coordinate metal complexes:45
Figure 3. View of the crystal structure of [CuL3](ClO4)2 (30%
probability ellipsoids). Perchlorate disorder omitted for clarity.
Selected bond lengths and angles: Cu1−N1 2.007(4), Cu1−N2
1.953(4), Cu1−O1 2.517(3) Å; N2−Cu1−N1 82.0(2), N2−Cu1−
N2′ 82.0(2)°.
β − α
τ5 =
60o
(1)
where β > α are the two largest coordinate angles. The τ5
parameter was intended to fall between 0 and 1, with 0 being
ideal square pyramidal and 1 perfect trigonal bipyramidal,
although it has been pointed out that τ5 may be greater than
1.46,47 The τ5 values for each of the three complexes are shown
in Figure 4. This illustrates the significant structural change
enacted upon the coordination center by hydrogenation and
then methylation of N2 as well as the stereochemistry at the
amine N-donor.
on the two methylene carbon atoms into an eclipsed
formation, introducing a source of torsional strain to the C−
C bond. The weakly bound axial perchlorate ligands are a
consequence of an underlying Jahn−Teller distortion of the
2
otherwise degenerate Eg electronic ground state. Any ligands
bound in the equatorial plane will exhibit shorter bond lengths
2
to the metal than ligands aligned with the dz orbital.
Few perfectly trigonal bipyramidal or square pyramidal CuII
structures are known. Indeed, crystal structures of several Cu
complexes of L1 and L2 with various coligands are known and
all show some degree of distortion.48−54 The Supporting
Information shows a selection of these (Figure S1), to enable a
better understanding of the extent to which divergences from
ideal 5-coordinate geometries (trigonal bipyramidal and square
pyramidal) are common among complexes of this type. The
rac-[CuL1]2+ isomer is dominant for structures of this complex,
but for [CuL2]2+ there are approximately equal numbers of
meso and rac isomers. It is apparent that the rac isomers of
[CuL2]2+ adopt distinctly folded (non-planar) conformations,
and the two pyridyl groups occupy trans coordination sites in
some extreme cases. The only CuI structure of these ligands is
rac-[CuL2]BF4 (CCDC reference code AXOKUV48) which
There are several known crystal structures of L3 bound as a
tetradentate ligand to transition metals,36 lanthanoid ions,37
and main group metals,38 but, interestingly, a large proportion
of them involve the ligand binding to two metals in a bridging
bis-bidentate mode.39−41 This includes the dicopper(I)
complexes [Cu2(L3)2]2+ (a double helicate structure)42 as
well as the non-helical [(Ph3P)ICu(L3)CuI(PPh3)]43 and
[(Ph3P)2Cu(L3)Cu(PPh3)2]2+ complexes.44
Crystallographic analysis of [CuL1](ClO4)2 and [CuL2]-
(ClO4)2 revealed them to be identical to their previously
published structures,15,18,19 and the crystal structures of
[CuL1](ClO4)2, [CuL2](ClO4)2, and [CuL3](ClO4)2 are
compared in Figure 4. Despite their apparent similarity, they
exhibit quite different coordination geometries and physical
Figure 4. Crystal structures of (A) rac-[CuL1](ClO4)2 (IDUMEZ15), (B) meso-[CuL2](ClO4)2 (IFIXUR19), and (C) [CuL3](ClO4)2 (this work).
−
Only one ClO4 shown for comparison and C−H atoms are omitted for clarity. The distortion parameter τ5 is shown for each structure.
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Table 1. Spectroscopic, Thermodynamic, and Electrochemical Data for [CuLn]2+ (n = 1, 2, 3) and Their Bromido Complexes
in MeCN and DMSO
MeCN
DMSO
λmax/nm (ε/M−1 cm−1
)
log KIIBr
E(CuII/I)/mV vs Fc+/0
λmax/nm (ε/M−1 cm−1
)
log KIIBr
E(CuII/I)/mV vs Fc+/0
[CuL1]2+
[CuL1Br]+
[CuL2]2+
[CuL2Br]+
[CuL3]2+
[CuL3Br]+
599 (144)
688 (190)
623 (270)
784 (272)
612 (193)
751 (269)
−
−634
−784
−547
−697
−129
−352
621 (186)
688 (240)
642 (166)
737 (209)
628 (155)
695 (174)
−
−764
−814
−649
−699
−243
−260
5.05(5)
−
>5
−
3.109(1)
−
3.291(1)
−
3.756(5)
3.381(8)
Figure 5. Visible absorption spectra upon sequential addition of bromide to (A) [CuL1]2+, (B) [CuL2]2+, and (C) [CuL3]2+ in DMSO (left) and
MeCN (right).
also shows a folded conformation of the 4-coordinate complex
as would be expected for an ideally tetrahedral monovalent Cu
compound.
Bromide Affinity. As the product of ATRP radical
activation, the halide (in this case bromide) binding constant
of the resulting CuII complex is an important component of
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any prospective catalyst. UV−vis spectrophotometric titrations
of [CuLn]2+ (n = 1, 2, 3) with bromide were carried out in both
MeCN and DMSO. The data from these titrations were
analyzed using ReactLab EQUILIBRIA32 to determine the
bromide binding constants (KII,Br) in each solvent. Addition-
ally, analysis of the resultant spectra gives valuable information
about the structure of these complexes in solution, in an
environment as close as possible to that in which the ATRP
reaction would take place. The data are summarized in Table
1.
constants in DMSO to [CuL1]2+ and [CuL2]2+, but a much
lower formation constant in MeCN.
Cyclic Voltammetry. The redox potentials of all compo-
nents within Scheme 3 were obtained from their CVs (Figure
6), and their values appear in Table 1. The CuII/I redox
The spectral changes upon bromide addition for the three
complexes in MeCN and DMSO (Figure 5) are rather similar.
Clear isosbestic points are seen which indicates that only two
absorbing species are ever present. The perchlorate anion is a
notoriously poor ligand and in solution does not compete with
the solvent for coordination sites on labile metal ions such as
CuII. Therefore, the initial complexes shown as [CuLn]2+ in
Figure 5 are best represented as [CuLn(solv)]2+ (solv = MeCN
or DMSO), which explains the bathochromic shifts in peak
maxima going from MeCN (an N-donor) to DMSO (an O-
donor).
The electronic maxima (Table 1) are all of d−d origin for
the d9 electronic configuration, and this peak varied according
to the donor strength of the fifth ligand (solvent or bromide).
In all cases, the absorption peaks exhibited a bathochromic and
hyperchromic shift over the course of the bromide titration
(see Table 1). This is consistent with coordination of the weak
field bromide ion in place of the previously N- or O-bound
solvent. Typically, this change was accompanied by broadening
and increased asymmetry of the peak which is indicative of a
less symmetric ligand field. Notably, the crystal structure of
rac-[CuL1Cl]+ finds the chlorido ligand tightly bound in the
equatorial CuN3 plane while one pyridyl ligand is weakly
bound in the axial site (CCDC reference code CPAZHC,
Figure S1).50 Folding of the ligand results in a τ5 value of 0.14,
compared with rac-[CuL1](ClO4)2 (0.02). Also, this conforma-
tional change may occur with retention of the rac N-based
isomeric configuration as the pyridyl group switches from
equatorial to axial.
Figure 6. CVs of the complexes in this work with and without
bromide (A) [CuL1]2+, (B) [CuL2]2+, and (C) [CuL3]2+. All
complexes 1 mM with 0.1 M (Et4N)ClO4 and the scan rate is 50
mV s−1. The systems in DMSO are on the left and those in MeCN are
on the right.
On the basis of existing crystallographic data, it appears that
the meso-[CuL2]2+ isomer (identified here as meso-[CuL2]-
(ClO4)2) prefers to accommodate additional coligands in an
axial coordination site as seen in meso-[CuL2(NCMe)]2+ and
meso-[CuL2Cl]+ (Supporting Information Figure S1).48,51 On
the other hand, rac-[CuL1]2+ may fold without N-inversion
and the incoming ligand can bind in the equatorial plane as in
rac-[CuL1Cl]+.50
The bromide binding constant for each complex (KII,Br) was
determined, or estimated, in both MeCN and DMSO by global
analysis of the spectrophotometric bromide titration using
ReactLab EQUILIBRIA.32 Although the bathochromic shifts in
peak maxima in Figure 5 are similar, a more in-depth analysis
of these spectral changes as a function of concentration
(Supporting Information Figure S2) reveals that the bromide
titrations of [CuL1]2+ and [CuL2]2+ in MeCN are associated
with sharp end points corresponding to a stoichiometric
equivalent of bromide. This means that their calculated
bromide constants are only lower bounds (log KII,Br > 5)
while those obtained in DMSO, where spectral changes
approach a plateau asymptotically, lead to lower and better-
defined bromide binding constants. The outlier is
[CuL3(solv)]2+, which showed similar bromide binding
potentials of the [CuLn]2+ (n = 1, 2, 3) complexes (EL) were of
the species [CuLn(solv)]2+ (solv = MeCN or DMSO).
Addition of excess (Et4N)Br produced the [CuLnBr]+
complexes in situ (on the basis of their bromide binding
constants, Table 1) and allowed their redox potentials (ELBr) to
be determined similarly. For [CuL1]2+ and [CuL2]2+ (and their
bromido complexes, Figure 6A and 6B) the CuII/I CV
responses were symmetrical and quasi-reversible in MeCN
and DMSO, which suggests minimal rearrangement of their
coordination spheres upon reduction (apart from the expected
change in coordination number from 5 (CuII) to 4 (CuI)
which can be accommodated by reversible dissociation of one
donor atom).4
For [CuL3]2+ and [CuL3Br]+ (Figure 6C) the CVs were
asymmetric and irreversible, and the estimated redox potentials
were more than 300 mV positive of the [CuL1]2+ and [CuL2]2+
analogs in both MeCN and DMSO. With increasing scan rates,
the CV responses become more distorted (Figure S2) which is
indicative of slow heterogeneous electron transfer. Rigidity of
the tetradentate diimine chelating ligand and its preference for
a planar geometry are evidently also unsuitable for CuI, and
significant structural rearrangement occurs after reduction of
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Table 2. Values of kact, kdeact, KATRP, ka(OMRP), kd(OMRP), and KOMRP for [CuL1Br]+ and [CuL2Br]+ in MeCN and DMSO, with
either BrACN or EBriB Initiators
[CuL3]2+ to produce a CuI complex that is reoxidized at a
different potential.
equal), which can either actuate or preclude coupled chemical
reactions depending on their rate constant. Reproducing the
experimental CV data by simulation, using the model in
Scheme 3, allows the key rate constants in this system to be
obtained.
A typical cyclic voltammetry experiment consisted of the
sequential addition of 1 equiv aliquots of alkyl halide initiator
to a 5.0 mL DMSO or MeCN solution containing 1.0 mM
[CuLn]2+ (n = 1, 2, 3), 2.0−4.0 mM (Et4N)Br, 0.1 M
(Et4N)ClO4 and, in half of the experiments, 0.05 M TEMPO
as a radical scavenger. Sweep−rate dependent voltammetry (50
mV s−1, 100 mV s−1, 200 mV s−1, 500 mV s−1, and 1000 mV
s−1) was measured for each initiator concentration and for the
starting solution.
Upon electrochemical reduction of the starting complex
[CuIILBr]+ (eq 3), reaction with the initiator RBr (radical
activation, eq 4 rate constant kact) is rate-limiting and
irreversible in the presence of the nitroxyl radical trap
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, eq 5). The
combination of eqs 3 and 4 constitutes an ECcat mechanism
where the coupled radical activation reaction regenerates the
initial complex [CuIILBr]+ and a catalytically enhanced
cathodic current emerges.
The complete set of parameters describing the voltammetry
of [CuLn]2+ and [CuLnBr]+ (n = 1,2) in both MeCN and
DMSO is assembled in the Supporting Information (Table
S1). The redox potentials of the bromido complexes were
lower than the solvent complexes, typically by 50−150 mV.
This is expected following the addition of the anionic bromide.
The redox potentials of the CuL1/Br complexes were 60 to 100
mV more negative than the Cu/L2/Br analogs depending on
the solvent. This is expected for copper complexes containing
secondary amines instead of tertiary amines12 and attributable
to steric effects that elongate the Cu−N bonds of the tertiary
amine complex and stabilize the larger CuI ion.
The negative shift in CuII/I redox potential upon bromide
0
replacing the solvent ligand (Esol − EBr0) for both [CuL1]2+
and [CuL2]2+ was markedly greater in MeCN than in DMSO
(see Figure 6A and 6B). The shape of the CV waveform was
unchanged between the solvent and bromido complexes, with
very similar peak to peak separations and comparable anodic to
cathodic current ratios. The magnitude of this shift in redox
potential upon bromide coordination is related to the
corresponding bromide binding constants of the CuII and
CuI complexes by eq 2:
[CuIILBr]+ + e− → [CuIL]+ + Br−
(3)
KII,Br
Es0ol − EB0r = 59.2 log
·
[CuIL]+ + RBr ⥃ [CuIILBr]+ + R kact/kdeact
(4)
(5)
KI,Br
(2)
·
·
R + TEMPO → R‐TEMPO
Equation 2 then enables the CuI bromide formation constants
to be calculated from the CV data and the spectrophotometric
bromide titration data on the CuII analogs (Table 1). A key
point is that the smaller shift of potential in DMSO leads to a
stronger CuI bromido complex relative to MeCN, where the
CuI−Br bond is destabilized to a greater degree.
Kinetics of Radical Activation and Deactivation. The
methodology that we have developed for using cyclic
voltammetry to extract kinetic information from ATRP
activation and deactivation has been described else-
where.21,23,24 A key advantage of the technique is that stable
CuII precursor complexes may be prepared, which are then
converted electrochemically into their active CuI form in situ.
The ensuing coupled chemical reactions with this CuI complex
are bimolecular, and their rates can be tuned by concentration-
dependent experiments. Within the electrochemical system,
the voltammetry scan rate is also a critical variable, as it allows
different time scales to be selected (all other things being
Accurate values of kact are obtained by simulating the CV
data taken at different sweep rates and RBr concentrations
using the program Digisim.31 The reverse reaction rate of eq 4
(kdeact, radical deactivation and see Scheme 3) can then be
determined in a similar manner when TEMPO is omitted as eq
5 vanishes and eq 4 becomes reversible. The decrease in
catalytic current (eq 3) is due to radical deactivation (kdeact).
In cases where OMRP activity (Scheme 3, left-hand side)
occurs24 concurrently with ATRP (Scheme 3, right-hand side),
a redox response for the organometallic species, [CuII/ILR]+/0
appears as a second wave (ER), typically at a lower potential
than EBr. Other parameters pertinent to the system (ka(OMRP)
d(OMRP)) are also found by simulation.24,25 Simulations across
,
,
k
the full range of initiator concentrations and sweep rates were
carried out to determine KATRP (= kact/kdeact), KI,Br, Esol, EBr,
and, where applicable, KOMRP, ka(OMRP), kd(OMRP), and ER. Note
that in Scheme 3 some of the equilibrium constants and redox
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Figure 7. CVs of the sequential addition of 1 equiv aliquots of EBriB to 1 mM [CuL1Br]+ in MeCN (A) with and (B) without 0.05 M TEMPO.
Both solutions contained 0.1 M (Et4N)ClO4 as supporting electrolyte. Scan rate: 50 mV s−1.
Figure 8. CV of the sequential addition of 1 equiv aliquots of EBriB to 5.0 mL of 1 mM [CuL2Br]+ in DMSO (A) with and (B) without (right)
0.05 M TEMPO. Both solutions contained 0.1 M (Et4N)ClO4 as supporting electrolyte. Scan rate: 50 mV s−1.
Figure 9. CV of the sequential addition of 1 equiv aliquots of EBriB to 5.0 mL of 1 mM [CuL2Br]+ in MeCN (A) with and (B) without 0.05 M
TEMPO. Both solutions contained 0.1 M (Et4N)ClO4 as supporting electrolyte. Scan rate: 50 mV s−1.
potentials are interdependent, as this is a closed thermody-
namic cycle (see eq 2).
Electrocatalytic Activity of [CuLnBr]+ with EBriB. An
example of catalytic radical activation of EBriB by [CuL1Br]+
(electrochemically reduced to [CuL1]+) is shown in Figure 7.
An enhancement of the current in the vicinity of the
[CuL1Br]+/0 couple is apparent upon sequential addition of
EBriB, along with a loss of the anodic peak as all [CuL1]+ is
consumed by EBriB before it can be electrochemically
reoxidized. The slightly diminished cathodic current in the
absence of TEMPO (Figure 7B) illustrates the effect of the
deactivation reaction on partially depleting [CuL1Br]+ at the
electrode surface.
Two alkyl bromide initiators, ethyl bromoisobutyrate
(EBriB) and bromoacetonitrile (BrACN), were titrated into
electrochemical solutions containing the divalent [CuLnBr]+ (n
= 1, 2) complexes and supporting electrolyte. All rate constants
determined by this method are assembled in Table 2. Figures
7−10 show examples of the catalytic voltammetry, and
representative experimental and simulated CVs are given in
the Supporting Information (Figures S4−S11). Briefly,
[CuL3Br]+ was found to be inactive as a catalyst in either
MeCN or DMSO, and additions of EBriB or BrACN elicited
no detectable changes in the CV. The inactivity of this complex
was likely due to its higher redox potentials and irreversible
electron transfer behavior.
Similar behavior is seen with [CuL2Br]+. A single catalytic
current response is seen at the [CuL2Br]+/0 potential, which is
ca. 100 mV more positive than [CuL1Br]+/0 (Figure 8, data in
DMSO).
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Figure 10. CVs of the sequential addition of 1 equiv aliquots of BrACN to 5.0 mL of 1 mM [CuL1Br]+ in MeCN (A) with and (B) without 0.05 M
TEMPO. Both solutions contained 0.1 M (Et4N)ClO4 as supporting electrolyte. Scan rate: 50 mV s−1.
An interesting feature of the [CuL2Br]+/EBriB voltammetry
in MeCN (Figure 9B), when compared with the same system
in DMSO (Figure 8B), was the relative insensitivity of the CV
to EBriB addition (in the absence of TEMPO). Increases in
the cathodic current were very small, and the anodic peak
current persisted with successive additions of EBriB. With
TEMPO present, this peculiarity abates, with a more sigmoidal
waveform found after 5 equiv of EBriB when TEMPO is
present (Figure 9A).
but rapid in all cases. Rapid radical capture is necessary if the
OMRP reaction is to be competitive with bimolecular radical
dimerization (termination), which is typically also very fast (kt
= 109 M−1 s−1). The [CuLn(CH2CN)]+/0 redox response is
reversible in both MeCN and DMSO which indicates the
organocopper complexes are quite stable on the CV time scale,
which is an emerging feature in the nascent chemistry of these
unusual complexes.24−26
The values of kact and kdeact show interesting dependences on
Cu complex and solvent. The most apparent contrast is that
Electrocatalytic Activity of [CuLnBr]+ with BrACN. When
BrACN was introduced to a solution of [CuL1Br]+ (in the
absence of TEMPO), CVs comprising two separate redox
responses were observed; one from [CuL1Br]+/0 at higher
potential and one from the organocopper(II) complex
[CuL1(CH2CN)]+/0 (Figure 10B). In the presence of
TEMPO (Figure 10A), only the higher potential catalytic
response is seen, which indicates that the lower potential
k
deact is about an order of magnitude greater in DMSO than in
MeCN, while kact does not change markedly. Overall the KATRP
values show that [CuL1]+, as yet untested, should be an equally
effective ATRP catalyst as [CuL2]+/EBriB. This is despite the
presence of NH groups as donor atoms, which have been
essentially neglected to date as ATRP catalysts. The lower
[CuL1Br]2+/+ redox potential compared to [CuL2Br]2+/+ does
not lead to a significantly larger value of log KATRP, as variations
to both kact and kdeact, measured independently, result in some
cancellation effects.
The cathodic currents of both redox couples increased with
the concentration of BrACN in both MeCN and DMSO. The
magnitude of this increase was significantly greater for the
[CuLn(CH2CN)]+/0 response below −800 mV, indicating that
[CuLn(CH2CN)]+ was being formed at the expense of
[CuLnBr]+.
•
couple is a product of a reaction involving the radical NCCH2
that is otherwise quenched by TEMPO. This mirrors our
previous observations using BrACN as an initiator.24,25
Recently, Matyjaszewski et al. reported similar behavior with
different complexes using this methodology.26 The organo-
copper complex [CuL1(CH2CN)]+ forms by rapid capture of
the radical NCCH2 by [CuL1]+ within the reaction layer at
•
the electrode surface, which is a demonstration of the OMRP
deactivation reaction (kd(OMRP)). The Supporting Information
shows the corresponding CVs of [CuL1Br]+/BrACN in DMSO
(Figure S7), [CuL2Br]+/BrACN in MeCN (Figure S9), and
[CuL2Br]+/BrACN in DMSO (Figure S11), which are
qualitatively similar.
CONCLUSIONS
■
The effects of ligand structure, solvent, and alkyl halide
initiator on ATRP activity were examined. The three
complexes [CuL1Br]+, [CuL2Br]+, and [CuL3Br]+ were studied
in both MeCN and DMSO, using EBriB and BrACN as
initiators. [CuL1Br]+ and [CuL2Br]+ were both electrocatalyti-
cally active in generating radicals, while [CuL3Br]+ was not,
presumably due to high CuII/I redox potentials and irreversible
electron transfer which suggested significant rearrangement
upon reduction. Simulations of the CV data from [CuL1Br]+
and [CuL2Br]+ gave values for the equilibrium and rate
constants associated with radical activation/deactivation and
hence KATRP. In addition, the sterically unencumbered
Analysis of Kinetic Parameters. A full summary of the
values determined for the forward and reverse ATRP and
OMRP reactions and their associated equilibrium constants
appear in Table 2. The present study comprised two activator
complexes ([CuL1]+ and [CuL2]+), two initiators (EBriB and
BrACN), and two solvents (MeCN and DMSO). These 8
combinations showed that each of these three variables is
important, and any variations in KATRP (kact/kdeact) should be
carefully compared. The tertiary radical of EBriB
(Me2C•COOEt) was unable to form an organocopper(II)
complex, while [CuLn(CH2CN)]+ (n = 1, 2) complexes are
formed rapidly due to a relief of steric bulk with the sterically
NCCH2 radical was shown to react rapidly with [CuLn]+ to
•
generate the corresponding [CuLn(CH2CN)]+ complexes in
both MeCN and DMSO.
•
unencumbered primary radical NCCH2 . Similar features were
noted with computational chemistry of related [Cu-
(Me6tren)]+ systems.25 One striking feature is the solvent
dependence of the formation of [CuLn(CH2CN)]+ (n = 1, 2),
which was an order of magnitude faster in MeCN than DMSO,
To date, Cu complexes of exclusively tertiary amine and
pyridyl N-donors dominate ATRP catalysis. The present
finding that [CuL1Br]+, despite bearing two secondary amines,
exhibits comparable activity to the highly active tertiary amine
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[CuL2Br]+ is novel. As expected, this structural change lowered
the redox potential of [CuL1Br]+/0 by about 100 mV relative to
the tertiary amine analog [CuL2Br]+. The fact that radical
activation was still rapid and unaffected by the secondary
amine NH groups bodes well for the use of [CuL1Br]+ in
ATRP in future studies and indicates that the NH groups are
tolerant of radical formation.
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of Methyl Acrylate with Inexpensive Ligands and ppm Concentrations
of Catalyst. Macromolecules 2011, 44, 811−819.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01001.
■
sı
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T.; Serra, A. C.; Coelho, J. F. J. Thiourea Dioxide As a Green and
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Additional figures and tables (PDF)
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771.
CCDC 2074222 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(13) Sakurai, T.; Kimura, M.; Nakahara, A. A Facile Reduction of
Copper(II) Leading to Formation of Stable Copper(I) Complexes.
Redox Properties of Four- and Five-coordinate Copper Complexes.
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AUTHOR INFORMATION
Corresponding Author
Paul V. Bernhardt − School of Chemistry and Molecular
Biosciences, University of Queensland, Brisbane 4072,
Australia; orcid.org/0000-0001-6839-1763;
Email: p.bernhardt@uq.edu.au
■
Author
Jamie N. Melville − School of Chemistry and Molecular
Biosciences, University of Queensland, Brisbane 4072,
Australia
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01001
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from the
Australian Research Council (DP210102150).
■
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