Tuning the Copper(II)/Copper(I) Redox Potential for More Robust Copper-Catalyzed C–N Bond Forming Reactions

Article abstract of DOI:10.1002/ejic.201901269

Complexes of copper and 1,10-phenanthroline have been utilized for organic transformations over the last 50 years. In many cases these systems are impacted by reaction conditions and perform best under an inert atmosphere. Here we explore the role the 1,1

Full text of DOI:10.1002/ejic.201901269

A Journal of
Accepted Article
Title: Tuning the copper(II)/copper(I) redox potential for more robust
copper-catalyzed C—N bond forming reactions
Authors: James D. Cope, Henry U. Valle, Ruby S. Hall, Kathleen M.
Riley, Ekta Goel, Saborni Biswas, Michael P. Hendrich, David
O. Wipf, Sean L. Stokes, and Joseph Emerson
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901269
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
Key Topic : Copper(II) catalysis
TOC Graphic
TOC Text:
Complexes of copper(II) and 1,10-phenanthroline (and phen derivatives) have been utilized for
organic transformations over the last 50 years. Here we explore the role phen and phen-based
ligands plays on the electronic structure and redox properties of copper coordination
complexes, and what benefits these related ligands may provide to enhance copper-based C—
N coupling reactions. X-ray crystallographic data show these related ligands subtly impacted
the coordination geometry of the copper(II) ion in these complexes. The diazafluorenone
containing complex showed a drastically different redox potential, which affords the potential
to stabilize a copper(I) complex. The increased stability of the copper(I) species correlates to
higher performing CEL cross couplings reactions.
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
Tuning the copper(II)/copper(I) redox potential for more robust
copper-catalyzed C—N bond forming reactions
James D. Cope,1 Dr. Henry U. Valle,1Ruby S. Hall,1 Kathleen M. Riley1 Dr. Ekta Goel,1
Saborni Biswas,1 Dr. Michael P. Hendrich,2 Dr. David O. Wipf,1 Dr. Sean L. Stokes,1 and Dr.
Joseph P. Emerson1,*
1Department of Chemistry, Mississippi State University, Mississippi State, Mississippi,
39762, United States.
2Department of Chemistry, Carnegie Mellon University, Pittsburg, PA 15213, United States.
Corresponding Author: jemerson@chemistry.msstate.edu
Abstract
Complexes of copper and 1,10-phenanthroline have been utilized for organic transformations
over the last 50 years. In many cases these systems are impacted by reaction conditions and
perform best under an inert atmosphere. Here we explore the role the 1,10-phenanthroline
ligand plays on the electronic structure and redox properties of copper coordination complexes,
and what benefit related ligands may provide to enhance copper-based coupling reactions.
Copper(II) triflate complexes bearing 1,10-phenanthroline (phen), ([Cu(phen)2(OTf)]OTf, 1)
and oxidized derivatives of phen including [Cu(edhp)2](OTf)2 (2), [Cu(pdo)2](OTf)2 (3),
[Cu(dafo)2](OTf)2 (4) were prepared and characterized. X-ray crystallographic data show these
related ligands subtly impacted the coordination geometry of the copper(II) ion. Complexes 1-
3 had only incremental changes to the redox properties of the copper ions, complex 4 showed
a drastically different redox potential affording a remarkably air stable copper(I) complex.
These complexes 1-4 were then used to catalyze the C—N bond forming cross coupling
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European Journal of Inorganic Chemistry
between imidazole and various boronic acid substrates, where the increased stability of the
copper(I) species in complex 4 appears to better support these CEL cross couplings.
Introduction
Copper catalyzed coupling reactions have become a valuable synthetic strategy for the
construction of C—C or C–heteroatom bonds.1 Several different catalytic systems have been
developed based on the Ullman reaction with a common mechanistic feature that all of these
systems are thought to proceed via a copper(I)/copper(III) catalytic cycle.2 Stability of the
copper(I) complex in solution is often the key aspect for reactivity, yet at the same time these
species are highly sensitive to atmospheric oxidation. To circumvent this issue, reductants can
be added to the reaction to generate a copper(I) species in situ,3 or new ligands can be designed
that stabilize low valent metal ions. One of the most common ligands used to support copper
catalyzed transformations is 1,10-phenanthroline (phen) due its ability to generate high stable
complexes and its relative cost.4 Several review articles exist focusing on the use of these
ligands as catalysts since they have been used to support both copper(I) and copper(II) based
catalysis.5,6 Yet copper-phen complexes still suffer from a relatively low redox potential (~200
mV vs NHE), which then requires anaerobic conditions for these catalysts to turnover
efficiently. However the distal C=C bond between C5 and C6 in phen can be easily oxidized
generating an electron deficient phen derivative.7 From this, several derivatives can be
synthesized which include 5,6-epoxyphen (L2), 5,6-phendione (L3) and 4,5-diazafluorenone
(L4). These derivatives are redox active and thought of as non-innocent,8 which may contribute
to changes in the electronic nature of the metal center and contribute to their reactivity. Each
of these have been used as ligands for copper complexes and have been extensively
characterized, via crystallography,9 electrochemistry,10 and their DNA binding affinity.11 The
catalytic capabilities of these ligands support have only been briefly measured,12 where no
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
attempt to support C—N bond forming reactions have been made, such as the Chan-Evans-
Lam (CEL) coupling. Here we attempt to remedy this situation, where we report a series of
new copper complexes made from 5,6-epoxyphen, phen-5,6-dione, and 4,5-diazafluorenone
with triflate counter-ions, then then demonstrate their value as a class of new ligands for copper
complexes to support C—N bond forming reactions.
Results and Discussion
Previously, we have reported a discrete Cu2+ phen complex, namely [Cu(phen)2OTf]OTf
(1),14 which shows a good reactivity performing C- and N-atom transfer reactions.15 However,
the phen ligand can be easily oxidized to functionalize the central ring of this ligand system.
The C=C bond between C5 and C6 positions can be oxidized to form the 5,6-epoxyphen (L2)16
and 5,6,-dione (L3).17 L3 can be further reacted to form the 4,5-diazafluorenone (L4) by the
loss of CO.18 These ligands (L2-L4) coordinate copper(II) triflate in a bidentate manner in
EtOH to generate copper(II) complexes (2-4), as shown in Scheme 1.
Scheme 1. Synthesis of copper(II) complexes 1–4
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European Journal of Inorganic Chemistry
The X-ray structure of [Cu(phen)2OTf]OTf has been previously reported, where the
copper(II) center adopted a distorted square pyramidal structure that fits between a square
pyramidal and a trigonal bipyramidal coordination mode (τ5 = 0.47).14 The coordinating triflate
ion is bound weakly to the metal ion (Cu—O bond is ~2.5 Å), which is likely a highly labile
bonding interaction in solution leaving an open coordination site that can be used to initiate
targeted reactivity. X-ray quality crystals of complexes 2-4 were obtained and ORTEP
diagrams of these complexes are reported in Figure 1. Complex (2 adopts a different 5-
coordinate geometry (Fig. 1A), where the coordination mode surrounding the Cu2+ ion is much
closer to a pure trigonal bipyramidal mode (τ5 = 0.84).19 A molecule of water is bound to the
copper center instead of triflate in an equatorial position which has a Cu1—O1 bond distance
of 2.086 Å, which is in between the Cu—O bond lengths of related [Cu(phen)2](NO3)2 and
[Cu(phen)2]SO4.20,21 The Owater—Otriflate distance is ~ 2.70 and 2.73 Å, indicating the water is
hydrogen bonded to the two triflate counter-ions.22
Complex 3 was crystalized with a bound chloride ion, which it likely extracted from the
solvent and thus this is unlikely the structure of the complex formed in the reaction. Compared
to previous structures, 3 a adopts distorted square-pyramidal (τ5 = 0.28), which has been
observed in 5,6 dione complexes previously,23 where one of the phen Cu—N bonds is in the
axial position as it is more elongated than the other (2.164 vs. 1.984 Å), while the second phen
has almost equivalent C—N bonds (2.070 and 2.000 Å). We can see that the copper is bound
by a chloride atom in the axial position, with a bond distance of 2.269 Å. This differs from the
2.250 Å Cu—Cl bond observed in [Cu(pda)2Cl]PF6 and the 2.294 Å Cu—Cl bond in
[Cu(phen)2Cl]PF6.23
Complex 4 formed an octahedral complex with two water molecules bound to the center with
each triflate interreacting with two different neighboring complexes, forming a large hydrogen
bonded network with Owater—Otriflate distances of ~ 2.72 and 2.76 Å. This phenomena has been
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
-
-
observed with the NO3 and ClO4 ions with similar complexes.24 The molecule is highly
centrosymmetric with a center of inversion located on the copper giving only one triflate
counter-ion in the asymmetric unit. The Cu—O bond distances of ~ 1.98 Å in this complex
suggest these are stronger bonding interactions than those in complex (2. Two distinct C—N
bond lengths were observed in 4 (1.992 and 2.568 Å), which are consistent with the
coordination of modes in other bis(diazafluorenone) complexes.24-25 This asymmetry is tied to
the improved overlap of the nitrogen lone pair and the metal’s d-orbitals resulting in
unsymmetrical bond lengths.26
Figure 1. X-ray molecular structure of copper complexes. Thermal ellipsoids are shown at
50% probability. Selected bond lengths (Å) and angles (°). (2) Cu1-O1W: 2.086(2); Cu1-N3:
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European Journal of Inorganic Chemistry
2.059 (2); Cu1-N4: 1.994(2); Cu1-N2: 1.990(2); Cu1-N1: 2.082(2); N1-Cu1-N3: 127.56(7);
N2-Cu1-N4: 177.70(7); N2-Cu1-O1 (α): 90.24(7); N3-Cu1-O1 (β): 118.30(7). (3); Cu1-Cl1:
2.269(2); Cu1-N3: 1.984(4); Cu1-N4: 2.164(4); Cu1-N2: 2.000(5); Cu1-N1: 2.070(5); N1-
Cu1-N3: 93.7(2); N2-Cu1-N4: 98.8(2); N2-Cu1-Cl (α): 90.6(1); N4-Cu1-Cl (β): 106.8(1). (4);
Cu1-O2: 1.980; Cu1-N1: 1.992; Cu1-N2: 2.568; N1-Cu1-O: 89.45; N4-Cu1-O: 90.55; N1-
Cu1-N2: 79.34
There is a wealth of information regarding the redox potential of copper complexes.27 In
order to characterize the impact of the phen derivatives on the redox behavior of these
copper(II) complexes, we have turned to cyclic voltammetry experiements.28 The complexes
were scanned from between -1.5 to +1.5 V beginning at the open circuit potential. The redox
potentials of all four complexes are shown in Figure 2 and summarized in Table 1, where the
potentials are reported versus the normal hydrogen electrode (NHE).
Figure 2. Cyclic voltammograms of complexes 1–4 vs NHE showing the Cu2+/+ redox couple
at concentrations of 1 mM with a scan rate of 100 mV/s. 1, 2, 3, and 4 are shown in black,
blue, orange, and red, respectively.
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European Journal of Inorganic Chemistry
Table 1: Spectroscopic and electrochemical data for
copper complexes 1–4 in aqueous solution.
λmax
ε*
E
Epc
Epa
½
Complex
(nm)
(M-1cm-1) (mV) (mV) (mV)
[Cu(phen)2]2+
740
690
685
700
200
510
400
150
265
285
343
190
220
191
340
350
494
[Cu(edhp)2]2+
[Cu(pdo)2]2+
[Cu(dafo)2]2+
1165 1050 1280
*Estimated molar absorptivity for the Cu2+ species, based on spin
quantitation of samples
The unbound phen ligand has been known to lower the cathodic region with one electron
quasi-reversible waves.29 The Cu2+/Cu+ redox wave for phenanthroline complexes has been
shown to occur between 200 and 1200 mV (vs NHE) depending on the substitution present on
the ligand.30 For 1 the reduction wave associated with the Cu2+ to Cu+ transition appears at 190
mV, while its oxidation wave occurs at 340 mV. These processes are consistent with a fast,
metal ion centered, quasi-reversible electrochemical process.31 The formation of Cu+ is
followed by the appearance of second smaller and broader reduction peaks starting at -260 mV
and extending up to the cathodic limit of the complex. This indicates a kinetically slower
electron exchange process of the reduced complex and its ligands. The shape of the wave at
the cathodic limit is consistent with the decomposition of the complex leading to formation of
free ligands and Cu metal. The reverse oxidation wave has slower kinetics as well. The
oxidation wave starts to appear at -580 mV and the oxidation current increases until a distinct
peak at –40 mV appears. This has previously ascribed to the oxidation of phen32 followed by
the Cu+ oxidation at 340 mV, which is in range of other reported [Cu(Phen)2]2+ complexes (see
Figures S1-S4 for further information).33 Complex (2 shows two reduction waves at 220 mV
and -1060 mV. The first reduction wave at 220 mV is quasi-reversible generating Cu+ from the
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European Journal of Inorganic Chemistry
Cu2+ complex. The second reduction wave is irreversible and occurs at -1060 mV. This peak
is typically described as a ligand conjugate reduction,34 where in both ligands undergo
reduction simultaneously giving rise to a high cathodic current. John et al.34 observed these
peaks when they scanned the cathodic regions with individual ligands. In the reverse scan, the
one or both the ligands oxidize in a slow electron transfer process indicated by wider peaks.
The oxidation wave associated with the conversion of Cu+ to Cu2+ is seen at 350 mV. Complex
3 shows that the Cu2+ complex undergoes reduction at 191 mV vs NHE. Upon oxidation of
+
[Cu(pdo)]2 at 494 mV, the complex decomposes to yield free ligand and copper ions. The
decomposition of phendione has been reported by Goss et al.35 and was confirmed by matching
the potentials of free ligand and the copper complex. The cyclic voltammogram in Figure S2
shows similar data that is consistent with ligand decomposition followed by an oxidation event.
Scanning in the negative potential region showed a decrease in the oxidation currents, which
is indicative of copper deposition on the glassy carbon electrode. Complex 4 showed two
reduction waves. The first wave appearing at 1050 mV which we have assigned as the Cu2+/+
reduction wave. Immediately after the reduction, a sharp increase in current is observed
indicating copper deposition on the glassy carbon electrode. When the scan is stopped before
the oxidative stripping of Cu, the electrode surface showed a coating consistent with Cu is
being electrodeposited at -240 mV from Cu+ complex. The plating of copper continues as the
cathodic potential is increased. Correspondingly, during the reverse oxidative stripping, the
oxidative current is higher indicating significant metal deposition. Nonetheless, the Cu2+/+
couple is always electrochemically quasi-reversible. The oxidation potential for the
conversation of the Cu+ complex to its +2 oxidation state is 1279 mV. It is interesting to note
that the presence of the dafone ligand in these complexes shifts the Cu2+/+ redox potential by at
32
least 900 mV. Kulkarni et al. determined that the Cu(dafo)2Cl2 complex was observed to
have Cu2+/+ redox couple of around 800 mV vs NHE, which strongly suggests that out of all
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
the ligands investigated, complex 4 is going to be the most efficient at stabilizing copper(I)
under atmospheric conditions.
Electroparamagnetic resonance (EPR) spectroscopy was performed to characterize the
electronic structure of complexes 1-4 shown in Figure 3. Complex 1 shows a distinctive Cu2+
signal with a prototypical four-line hyperfine signal g|| = 2.27 (A|| = 157 G) and g⊥ 2.04, which
=
is consistent with the nuclear spin associated with Cu (I = 3/2). This signal is axial and since
g||> g⊥ the unpaired electron lies in the dx2-y2 orbital.10 Complexes 2-4 show a very similar EPR
,
spectra with g⊥ 2.04, however the four hyperfine coupling signals for these complexes are not
=
as distinct. Complexes 3 and 4 show some additional hyperfine structure that could be
consistent with at least 2 electronically different copper(II) species contributing to the EPR
spectra collected with g|| = 2.27 (A|| = 159 G) and g|| = 2.32 (A|| = 159 G). These data are
consistent with EPR data collected for hydrated copper(II) complexes.12 Interestingly, the
spectra shown in Figure 3 all come from samples with similar copper concentrations, which
are confirmed by atomic absorption spectroscopy. Spin quantification of these samples indicate
the percentage of copper(II) in samples 1-4 is only 58%, 14%, 22%, and 17%, respectively.
Presumably the remainder of the sample are in an EPR silent, copper(I) state. Generally, these
samples are made using copper(II) precursors and there are no obvious reducing agents added
to these copper complexes. However, during complex formation L1, L2, L3, and L4 are added
to copper(II) salts in sight excess to generate complexes 1-4 in moderate yields, some of these
ligands, which as described above have a propensity to oxidize, may contribute the formation
of copper(I) character of these samples.
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European Journal of Inorganic Chemistry
Figure 3. EPR of complexes 1 – 4 collected in water (A) and MeCN (B). Complexes 1, 2, 3,
and 4 are shown in black, blue, orange, and red.
Subtle coordination changes in transition metal complexes have an impact on their electronic
structure. The UV-vis spectra of complexes 1-4 are shown in Figure 4. Each complex has
broad d-to-d transitions between 500 – 900 nm. Complex 1 had the most red-shifted absorbance
maxima in this region, showing a broad absorption transition with a λmax of 735 nm. Complexes
2, 3 and 4 have similar absorbance maxima of 690 nm, 685 nm and 700 nm, respectively. Only
complex 3 shows a second distinctive electron transition in the visible region (450 nm).
Complex 4 has considerably lower intensity in this region than the others, which is likely to be
due to the larger copper(I) content in this sample.
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European Journal of Inorganic Chemistry
Figure 4. UV-Vis spectra of complexes 1-4 collected in water. 1, 2, 3, and 4 are shown in
black, blue, orange, and red, respectively.
Complexes 1-4 show similar spectroscopic data, which suggests the metal centers in these
complexes have similar electronic structure in their Cu2+ oxidation state. Using density
functional theory, we have optimized models of these complexes to examine the electron
structures of these complexes in hopes of better understanding the dramatic shift in redox
potential for [Cu(dafo)2OH]+ compared to the other complexes described herein. Complexes
in both the copper(I) and copper(II) states were explored (see Figure S6-S13 for further details).
The [Cu(phen)2OTf]+ DFT model complex optimized using pBP/DN** to show a distorted
trigonal pyramidal coordination geometry, much like the X-ray structural data for the same
complex. However, in the gas phase calculation, some of the bonds and bonding angles show
more favorable bond lengths and bond angles compared to its X-ray resolved coordination
complex. For example, the crystallographically characterized complex 1 has a long 2.3 Å Cu—
O bond, whereas the DFT model complex show a stronger 2.06 Å interaction between Cu and
the triflate ion. The reduced 4-coordinate [Cu(phen)2]+ complex was also optimized to yield a
copper complex with a distorted tetrahedral coordination mode. We omitted non-coordinating
counterions when modeling this 18-electron complex. In both DFT optimized structures, the
HOMO have distinctive metal based character, where the LUMO are primarily ligand-based
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European Journal of Inorganic Chemistry
delocalized π*-type orbitals (Figure 5.) Similar efforts were performed for complexes 2, 3, and
4, along with their 18-electron Cu+ forms as well. Interestingly, the [Cu(dafo)2OH]+ complex
DFT optimized to demonstrate a distorted trigonal bipyrmidal structure over the X-ray
crystallized octahedral complex observed in Figure 1. Although the DFT models seem to be
some consistency in the electronic structure of the copper(II) complexes and their reduced
counterparts in terms of the nature of the molecular orbitals and their energies, there is little
insight offered from the DFT calculations which accounts for the drastic shift in redox potential
for complex 4.
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European Journal of Inorganic Chemistry
Figure 5. Computed HOMO-LUMO orbital diagrams using DFT optimization using
pBP/DN**basis sets. The frontier orbitals of complex 1 and 4 are shown for different oxidation
states of these complexes. Specifically A shows LUMO and HOMO orbitals associated with
[Cu(phen)2]+ B shows related orbitals for [Cu(phen)2OTf]+, C shows LUMO and HOMO
,
orbitals calculated for [Cu(dafo)2]+, and D shows the frontier orbitals for [Cu(dafo)2OH]+
In order to access these complexes as catalysts, we chose to investigate their potential
application for C—C and C—heteroatom cross coupling reactions, such as the Chan- Evans-
Lam coupling reaction. This CEL coupling is the only reported type of copper-dependent
coupling reaction that is proposes to be initiated through a copper(II) species and uses O2 as an
oxidant within the cycle to regenerate the copper(II) state during turnover.36 These coupling
reactions occur with a boronic acid as the coupling agent to form C—C or C—heteroatom
bonds and have begun to be extensively studied with most of the research focus on using copper
salts or copper salts with a supporting ligand as the catalyst,37 where very few discrete
complexes have been reported.38 CEL coupling reactions have also been reported to be more
active under protic conditions when using copper(I) sources,39 where the current consensus
mechanism for this process involves copper(I), copper(II), and copper(III) states.40 Here we
apply complexes 1-4 to support C—N bond formation by coupling boronic acids to indole and
other amines, where we aim to connect observed reactivity the structure or redox potential of
these complexes.
The results and optimization for these reactions are shown in Table 2. Initially the four
complexes were investigated as potential catalysts for the coupling of boronic acid with
imidazole in methanol under ambient conditions (room temperature and aerobic environment)
over a 24 hour time period. These reactions have been previously reported to proceed with high
yields for copper(II) triflate species.39 It should be noted that negligible reactivity was observed
without catalyst, under any of the reported conditions (Table 2, entry 1). Complexes 1-3
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European Journal of Inorganic Chemistry
showed trace levels of the coupling product by gas chromatography mass spectrometry (GC-
MS), while complex 4 showed 38% conversion of substrate to product (Table 2, entries 2-5).
Heating the reactions to 50 °C increased the GC yields of complexes 1 and 2 to each around
30%, with complex 3 rising to 45% and complex 4 showing yields of ~72% (entries 6-9). The
addition of base also improved yields further, where complex 4 was able to achieve yields of
92 % (entries 10-13). A similar observation has been reported for other CEL coupling
reactions.41 Anisole, a side product, was also generated under these conditions with a yield of
25-30% by GC-MS. Although anisole is also observed in trace amounts in some other
reactions, in relatively high concentrations it creates complicates the isolation of the cross-
coupling product. This reaction was attempted in other polar solvents (including water, DMF,
DMSO, and acetonitrile (Table 2, entries 14-18), however only limited reactivity was observed
in other solvents. Reaction time and temperature were also investigated, cooling the reaction
to 0 °C lowered the conversion to less than 10 %. Longer reaction times also increased yield,
but full conversion was still not observed after 72 hrs at 25 °C.
Complexes 1-4 all showed reactivity for CEL reaction, where complex 4 demonstrating good
yields and the other complexes showing at best, fair yields. Overall, these complexes show
reactivity for the CEL process along the following order: complex 4 > complex 3 > complex 2
≈ complex 1, which is consistent with the increasing redox potential of these complexes.
Additionally, complex 4 showed good consistency across different reactants, where several
alternative substrates were investigated with both the boronic acid and the amine were
examined (Figure 6.) The reactivity of complex 4 is affected when using benzimidazole or a
substituted imidazole or boronic acid. However, no reactivity was observed for either 1,2 4,
triazole or pyrrole, the cause of this is currently unknown but is being further explored.
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European Journal of Inorganic Chemistry
Table 2. Chen-Lam-Evans coupling of imidazole and boronic acid using catalysts 1-4
Entry
Catalyst
Base
Solvent
Temp
Time
Yield(%)a
1
2
-
-
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
DMF
25°C
25°C
25°C
25°C
25°C
50°C
50°C
50°C
50°C
25°C
25°C
25°C
25°C
25°C
25°C
25°C
25°C
0°C
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
24 hr
36 hr
ND
ND
1
2
3
4
1
2
3
4
1
2
3
4
4
4
4
4
4
4
-
3
-
ND
4
-
<5%
38%
32%
25%
45%
75% (62%)b
9%
5
-
6
-
7
-
8
-
9
-
10
11
12
13
14
15
16
17
18
19
K2CO3
K2CO3
22%
41%
92%
<5%
<5%
<5%
<5%
<5%
72%
K2CO3
K2CO3
-
-
-
-
-
-
DMSO
H2O
MeCN
MeOH
MeOH
25°C
a. Yield by GC-MS b. Isolated Yield
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European Journal of Inorganic Chemistry
Figure 6. Substrate scope for Chan-Evans-Lam coupling of complex 4.
A series of copper complexes bound to phen and phen-derivatives have been synthesized and
characterized. these new coordination complexes represent an air-stable option for copper-
dependent organic transformations. The potential utility of these compounds was demonstrated
using the CEL coupling reaction. In our hands, complex 4 performed well in supporting these
copper-dependent C—N bond forming reactions under partially optimized conditions.
Interestingly, complexes 1-4 all have similar visible and EPR spectra that suggest that the
electronic states of the Cu2+ ions in these complexes are in similar ligand fields, yet all show
some copper(I) content as well. Complexes 1-3 even share similar coordination geometries,
where their X-ray structures show 5 coordinate Cu complexes with distorted square pyramidal
to trigonal bipyramidal geometries. The [Cu(dafo)2]2+ complex, however, is structurally very
different. Complex 4 was characterized in an octahedral coordination geometry with 2 solvent
molecules in a trans orientation to each other. Interestingly the [Cu(dafo)2]2+ was also measured
to have a uniquely high redox potential (+1.1 V v. NHE) compared to the other complexes
described here that range from 265-343 mV v NHE for complexes 1-3, respectively. These
potentials can easily be converted into ΔG using the relationship of ΔG = -nFE, where
complexes 1-3 have ΔG associated with the one electron reduction to their Cu+ states of -6.1, -
6.5, and -7.9 kcal/mol, respectively. The [Cu(dafo)2]2+ complex has a -26.9 kcal/mol ΔG
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European Journal of Inorganic Chemistry
associated with the reduction to its Cu+ species. One rational for this difference is that the
structurally similar complexes 1-3 undergo dramatic reorganization upon reduction, which
impacts the favorability of this process. The structurally distinctive [Cu(dafo)2]2+ complex must
not require an impactful reorganization of the copper’s coordination geometry. This notion is
partially supported by our DFT optimized structures for complexes 1-3 (c.f. the SI section),
which show similar structure to the divalent X-ray crystallized complexes show in the literature
and Figure 2.14 DFT optimized models of the related copper(I) complexes all show 4-
coordinate, 18-electron copper(I) complexes with their copper(I) ions coordinated in distorted
tetrahedral geometries. On the other hand, we had no success generating a DFT optimized
model of the [Cu(dafo)2]2+ complex. By removing one of the axial ligands, this structure
eventually converged to a structure consistent with complexes 1-3 as shown in Figure 5, but it
remains to be seen if this model complex reflects the solution structure of these [Cu(dafo)2]2+
complexes. The reality is, at present it is unclear how the ligand architecture and coordination
mode of these complexes impacts the Cu2+/+ redox potential in these complexes, but it is
reasonably clear that the more oxidized (i.e. electron deficient) the phen derivative is the more
it stabilizes the copper(I) oxidation state. The source of electrons that afford these copper(II)
species to spontaneously reduce to their copper(I) states remains unclear. The most likely
suspect capable of giving up electrons are the ligands themselves, but currently we have no
specific proof supporting this notion.
Traditionally CEL coupling reactions are run with 50-150 mol% copper to achieve good
yields of C—N coupled products.41-42 However the complexes reported here can catalyze CEL
coupling reactions to generate similarly good yields, but with a fraction of the copper. These
catalytic data are on par with reactivity reported for other N-donating copper complexes that
have been reported.43 To date, Schaper’s catalysts for CEL coupling still show the best catalytic
turnover, but have significantly different donor abiilities.44 However within the context of this
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
work, it appears that the more electron deficient phen derivatives support the CEL coupling
reaction better than those that are more electron rich, where complex 4 generated the highest
yields for C—N bond forming reactions. At this time, we have no data that indicates how the
electron deficient ligands to support this higher CEL reactivity. However, if we consider the
most recent CEL mechanistic studies proposed by Watson,51 there appear to be several crucial
steps that are highly depend on the oxidative power of the copper catalyst, where this oxidizing
power is directly dependent on the reduction potentials associated with these complexes. We
have adapted this mechanism to our work below, (Scheme 2) though further electrochemical
and spectroscopic work is ongoing to help elucidate the validity of this mechanism on our
system. The most obvious step that is redox dependent is the disproportionation of copper(II)
into Cu+ and Cu3+ species (Scheme 2, reaction B.) In this case, the Cu2+/+ potential provides a
wealth of energy that can be coupled to the generation of the catalytically important copper(III)
species. Where we expect the Cu3+/2+ potential for these complexes to follow the same trend as
the Cu2+/+ potentials measured above. This suggests the copper(III) species generated as 4
proceeds in the CEL reaction will be highly unstable leading to a rapid decay proceed through
a reductive elimination to generate the C—N bond linked product (Scheme 2, reaction C.)
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
Scheme 2. Suggested mechanism of catalyzed CEL coupling.
The increased stability of the resulting copper(I) state leads to increased formation of the
phenolic based side products,45 which was also observed for complex 4 in this study.
Alternately, these copper(I) species are thought to re-oxidized in air to generate the catalytically
active starting copper(II) state (Scheme 2, reaction D), which implies that the redox potential
associated with these Cu2+/+ have to be sufficiently low to react with dioxygen too.
Alternatively, the Stahl group and others have used 4,5-diazafluoren-9-one (L4) as an ancillary
ligand for many palladium-catalyzed reactions.46 Stahl has proposed that the geometric
properties (or “bite angle”) of the 4,5-diazafluoren-9-one ligand helps support catalytic activity
around the metal center,47 This geometric aspect could also enhanced catalytic activity of the
copper(II)-dafo (4) complex for CEL coupling over the more prototypical copper-phen
complexes 1, 2, and 3.)
Conclusion
It is our contention that there is a direct correlation between redox tuning of copper and the
oxidized nature of the ligand geometric constraints that are key to developing better catalysts
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
copper catalysts for CEL coupling. Currently, we are working to connect these individual steps
in the CEL mechanism to the copper complexes and their reduction potentials.
Experimental
General Considerations
All solvents and reagents were obtained from commercial sources and were used as received. NMR
spectra were obtained using a Bruker AVANCE III 500 MHz spectrometer at room temperature. 1H and
13C chemical shifts are reported vs TMS and are referenced to the residual solvent peaks. Mass spectra
were obtained using a Bruker UHPLC microTOF-Q II High Resolution MS system operating in ESI
ionization mode. Single crystal X-ray Crystallography was obtained using a Bruker Venture D8 with
an IμS microfocus source. UV-Vis spectroscopy was obtained using an Olis14 UV/VIS/NIR
Spectrophotometer.
Synthesis of [Cu(edhp)2(OH2)](OTf)2 (2).
Cu(OTf)2 (0.723g, 2 mmol) in a 100 mL RBF was treated with EtOH (25 mL) and stirred for
15 min to allow for complete dissolution. To this was added 5,6-epoxy-5,6-dihydro-
[1,10]phenanthroline (0.728 g, 4 mmol) dissolved in EtOH (25 mL), causing an immediate color change
and precipitation of 2 to occur. The mixture was for 18 hrs and filtered and the blue solid collected and
dried under vacuum to yield 1.02 g (1.32 mmol, 66%). Crystals suitable for X-ray diffraction were
grown via vapor diffusion of Et2O into an MeCN solution of 2. ESI–MS (m/z): observed, 455.0512 for
[M – 2OTf]+; calcd, 455.0569 for [C24H16CuN4O2]+ ; 227.5249 for [M – 2OTf]2+; calcd, 227.5279 for
[C24H16CuN4O2]2+ Elem. Anal. Calcd for C26H18CuF6N4O9S2: C, 40.45; H, 2.35; N, 7.26 Found C,
40.19; H 2.13; N, 7.93
Synthesis of [Cu(pdo)2(OH2)2][(OTf)2] (3).
Cu(OTf)2 (0.723 g, 2 mmol) in a 100 mL RBF was treated with EtOH (25 mL) and stirred for 15min to
allow for complete dissolution. To this was added 1,10-Phenanthroline-5,6-dione (0.825 g, 4 mmol)
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
dissolved in EtOH (12 mL), causing an immediate color change and precipitation of 3 to occur. The
mixture was for 18 hrs and filtered and the blue solid collected and dried under vacuum. 1.18 g (1.44
mmol, 72%). Crystals suitable for x-ray diffraction were grown via vapor diffusion of hexanes into a
chloroethanol solution of 3. ESI–MS (m/z): observed, 483.0126 for [M – 2OTf]+; calcd, 483.0144 for
[C24H12CuN4O4]+ ; 241.5053 for [M – 2OTf]2+; calcd, 241.5072 for [C24H12CuN4O4]2+ Elem. Anal.
Calcd for C26H16CuF6N4O12S2: 38.17; H, 1.97; N, 6.85 Found C, 38.07; H 1.89 N, 6.76
Synthesis of [Cu(dafo)2(OH2)2][(OTf)2] (4).
Cu(OTf)2 (0.173 g, 0.48 mmol) in a 100 mL RBF was treated with EtOH (12 mL) and stirred for 15min
to allow for complete dissolution. To this was added 4,5-Diazafluoren-9-one (0.175 g, 0.96 mmol)
dissolved in EtOH (12 mL), causing an immediate color change and precipitation of 4 to occur. The
mixture was for 18 hrs and filtered and the blue solid collected and dried under vacuum. 0.272 g (0.36
mmol, 74%). Crystals suitable for x-ray diffraction were grown via vapor diffusion of Et2O into an
MeCN solution of 4. ESI–MS (m/z): observed, 427.0215 for [M – 2OTf]+; calcd, 427.0245 for
[C22H12CuN4O2]+ ; 213.5102 for [M – 2OTf]2+; calcd, 213.5123 for [C24H12CuN4O4]2+ Elem. Anal.
Calcd for C24H16CuF6N4O10S2: C, 37.83; H, 2.12; N, 7.35 Found C, 37.18; H 2.00 N, 7.02
General procedure for copper catalysed coupling.
A 4 mL scintillation vial with a magnetic stirring bar was charged with imidazole (0.1 mmol), boronic
acid (0.2 mmol), and copper complex (8 mol%). Methanol (0.5 mL) was added and stirred for the
desired time and temperature. Once reaction was completed, water was added (0.5 mL) and the aqueous
layer was extracted. This was repeated and the organic layer was dried with Na2SO4, filtered, and
concentrated.
Electrochemistry
All electrochemical measurements were performed on Solartron SI1287 potentiostat. The supporting
electrolyte was 0.1 M tetraethylammounium hexafluorophosphate (TEAPF6) in acetonitrile (MeCN) as
the solvent. The working electrode was a 3 mm glassy carbon disk with platinum wire counter and
Ag/Ag+ ion reference. The analyte concentrations were 1 mM and all solutions were maintained oxygen
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10.1002/ejic.201901269
European Journal of Inorganic Chemistry
free by degassing in Argon for 15 minutes before measuring the current response. All potentials were
converted to NHE based on the conversion: ENHE = EAg/Ag+ + 0.54V.
EPR
X-band EPR spectra were recorded on a Bruker ELEXSYS spectrometer equipped with an
Oxford ESR-910 liquid helium cryostat. The quantification of all signals was relative to a Cu2+-
EDTA spin standard prepared from a copper atomic absorption standard (Sigma-Aldrich). The
microwave frequency was calibrated with a frequency counter and the magnetic field with a
gaussmeter. The sample temperature was calibrated with a cernox sensor (Lakeshore CX-
1050) mounted inside an EPR tube. The modulation amplitude and frequency were 1 mT and
100 kHz for all spectra. Species concentrations were determined with the software SpinCount
developed by M. P. Hendrich.13
Accession Codes
CCDC 1952035 – 1952037 contain 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 Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Acknowledgement
This research was supported by NIH R01 GM077387 (M.P.H.), with funding for the EPR
spectrometer supported by the National Science Foundation (NSF CHE1126268). The authors
are grateful to Mr. Bruno Donnadieu for his assistance with X-ray crystallography data
analysis.
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