Copper(II) as a Platform for Probing the Steric Demand of Bulky β-Diketonates

Article abstract of DOI:10.1021/acs.inorgchem.9b02721

The synthesis and characterization of a series of copper bis(β-diketonate) complexes, functionalized with sterically hindered o-biphenyl and m-terphenyl functional groups, are reported. X-ray structural analysis reveals that the ligands exhibit several mo

Full text of DOI:10.1021/acs.inorgchem.9b02721

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Copper(II) as a Platform for Probing the Steric Demand of Bulky
β‑Diketonates
†
†
Alec T. Larson, Aaron S. Crossman, Sebastian M. Krajewski, and Michael P. Marshak*
Department of Chemistry, University of Colorado at Boulder (CU Boulder), Boulder, Colorado 80303, United States
*
S Supporting Information
ABSTRACT: The synthesis and characterization of a series
of copper bis(β-diketonate) complexes, functionalized with
sterically hindered o-biphenyl and m-terphenyl functional
groups, are reported. X-ray structural analysis reveals that the
ligands exhibit several modes of flexibility in order to
accommodate the steric demand. Increased steric bulk of
II/I
the ligands influences the Cu electrochemical reduction,
which is likely due to inhibited ligand rotation. Chemical
II
I
0
reduction of Cu forms Cu , which disproportionates to Cu
II
I
and Cu . The Cu species could be quantitatively trapped
using triphenylphosphine to form Cu(β-diketonate)(PPh3)2
(
7), which is also characterized. The catalytic ability of these
complexes, along with several common precatalysts, was determined for the reaction of bromobenzene and 2-naphthol, an
Ullmann-type C−O bond coupling reaction. Control experiments in toluene show no catalytic ability in the absence of β-
diketonates, suggesting involvement of the ligand in catalytic turnover.
2
8
INTRODUCTION
increasingly bulky β-diketone ligands. A consistent metal
chelate was sought in order to compare the effect of ligand
bulk on coordination.
■
Copper β-diketonates catalyze Ullmann-type coupling reac-
tions to form C−X bonds (X = C, N, O, S, P).
implicated in these catalytic mechanisms but undergoes
disproportionation to Cu and Cu in the absence of strong-
field supporting ligands such as phosphines.
1
−6
I
Cu is often
Copper(II) β-diketonates have a particularly regular
structure across dozens of crystallographic examples.
Complexes exhibit nearly identical, coplanar chelate rings
irrespective of substituents. They are reliably prepared from
alcohol/water mixtures, are indefinitely shelf stable, and are
2
9
0
II
7
−11
Although
these represent general, base-metal-catalyzed transformations,
mechanistic aspects remain poorly understood, garnering much
6
30−33
research effort. Stabilization and appraisal of the putative
often formed and hydrolyzed to purify β-diketones.
Owing to their ease of synthesis and crystallinity, a small
1
2
intermediates have yet to be accomplished, although progress
8
,13
has been made toward their in situ generation
and
The development of such base-
metal catalysts is an important research goal for the chemical
library of Cu(acac) -type crystal structures with differing
2
14−18
29,34
computational study.
ligands are available for comparison.
Preliminary inves-
tigations among the first-row late transition metals showed
responses to similar sterics different from those enforced by
the 2,6-dimesitylbenzoyl pinacolone (L , esac) ligand, with
Cu being the most indifferent to steric bulk. Given the ease
of preparation and consistency of copper β-diketonate
complexes, Cu presents a compelling platform for investigat-
1
9,20
industry.
One common approach to inhibiting bimolecular reactions
such as disproportionation involves the use of sterically bulky
ancillary ligands to protect the reactive catalytic intermediates.
For example, β-diketiminates (i.e., NacNac ligands) have been
2
II
26
II
I
used to stabilize Cu intermediates for further reactivity
ing the coordination chemistry of sterically hindered β-
diketones in a planar environment.
2
1
studies. Unfortunately, direct heteroatom substitution is
infeasible with β-diketonates. Over the past few decades, m-
terphenyls have emerged as popular functional groups for
ancillary ligands because of their ability to direct steric bulk
Here, we report the synthesis, characterization, and catalytic
behavior of a series of bulky copper(II) bis(β-diketonate)
complexes. In particular, the crystallography, spectroscopy,
electrochemistry, chemical reactivity, and catalytic behavior
were investigated in order to understand the effect of steric
encumbrance on the coordination and facilitation of Ullmann-
22−25
back toward a metal center.
functionalized β-diketone prevented the formation of the more
stable tris(β-diketonate) metal complexes among the early and
late metals of the first transition series.
The use of an m-terphenyl-
26,27
In an effort to
further probe the steric influence of β-diketonates in catalysis,
we recently reported a modified Claisen condensation reaction
employing acid chlorides that enabled the synthesis of
Received: September 10, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
type reactions. These results are discussed in context with
other common ligand platforms used in catalysis.
relative to the four known copper(II) β-diketonates by a factor
of 2.5. Additionally, one of the two o-biphenyl groups rotates
away from the metal center, dramatically reducing its steric
influence on coordination. The remaining ligands contain an
m-terphenyl group, which shows two predominant flexing
modes. 2 arranges the m-terphenyl group nearly perpendicular
to the chelate plane with the most extreme aryl chelate
dihedral angle among all examples, although the o-aryl dihedral
angle is close to an ideal 120° angle between the mesityl rings.
This angle contracts to 115° in 3 to benefit from π stacking
with the additional mesityl ring, and the ligand enjoys
considerable folding. Substituent angle deviations notwith-
standing, 1−3 have very similar chelate bond lengths and all
are square-planar.
RESULTS
■
Synthesis and Crystallography. We have previously
reported that the reaction of mesitylcopper(I) with H(L )
gives 2, presumably via the intermediacy of Cu (L ). This
experiment was replicated using the very hindered L , which
2
I
2
26
6
was difficult to metalate using typical conditions. Treatment of
I
the protonated ligand with 2 equiv of Cu Mes in a sealed tube
(
toluene, 140 °C) for several days afforded elemental copper
and a green solution that contained 6 in 60% yield. Metalation
of ligands L −L was accomplished by salt metathesis with
CuCl , resulting in synthesis of the corresponding Cu(L )
1
5
n
2
2
4
is unique among the examples because of its considerable
complexes 1−5. These complexes could be recrystallized to
give microcrystalline materials of various colors suitable for
crystal structure determination. The synthesis and ligands for
new compounds are collected in Scheme 1.
departure from the square-planar geometry. 4 adopts a
pseudotetrahedral geometry by twisting its chelate rings 26°
relative to each other. This distortion is very rarely observed in
this ligand class, occurring in only one irreproducible example
29
II
with an MLL′-type complex. Because of the breakdown of C2
symmetry, both ligands have been included in the table as
separate entries. One shows a more vertical orientation of the
m-terphenyl group with a relaxed fold angle, and vice versa.
Both ligands present the greatest o-aryl dihedral angle of 133°.
Although the average M−O bond length is very similar to
those of smaller complexes, one bond contracts, while the
other extends (Figure 2).
Scheme 1. Metalation of β-Diketones To Give Cu Chelates
On the other hand, 5 presents as two alternating,
centrosymmetric coordination geometries that have also been
recorded as two rows in Table 1. In one complex, the fold
angle is exaggerated, while in the other, the o-aryl dihedral
angle is contracted like that of 3. One of the conformers
presents a significant lengthening of the M−O bond closer to
the o-biphenyl group by about 0.026 Å. 6 appears to be the
most distorted with high fold and o-aryl dihedral angles and the
longest M−O bonds by almost 0.02 Å. Close inspection of 6
shows that this lengthening affects the o-biphenyl side entirely
(
1.94 vs 1.90 Å). This lengthening of one M−O bond
corresponds with the ramping of steric encumbrance, with a
notable divergence in these lengths from 1−6. Both 5 and 6
feature the hindering di-tert-butyl-o-biphenyl groups and are
once again square-planar.
II
These new complexes highlight the ability of Cu to
organize the coordination sphere in a preferred square-planar
geometry. However, in the case of 4, the copper atom adopts a
pseudotetrahedral geometry, possibly because of steric
mismatch between the two chelate substituents. Parameters
for describing the degree of intermediacy between two
coordination geometries (i.e., geometry indices) have been
Among the four known complexes, Cu(acac)2 has the
longest M−O bonds, while Cu(dpm) has the shortest, on
2
average, in Table 1. This length also decreases as the methyl
groups are exchanged for phenyl, as indicated by Cu(bzac)2
and Cu(dbm) . Another trend with substitution can be found
2
39
in the chelate fold angles, decreasing similarly. The C−C and
described in the literature. There are two parameters for
four-coordinate systems, τ and τ′ , with the latter being
modified to be better discriminate tetrahedral and seesaw
C−O bond lengths are indifferent to substitution, although
4
4
Cu(dbm) has a notable difference between its C−O bonds
2
40,41
because the aryl substituents enjoy conjugation to the
geometries.
Both values have been determined and can be
diketonate. In the case of Cu(bzac)2 and Cu(dbm) , a
found in Table 1. The values for most of the complexes are
2
coplanar arrangement of the aryl and chelate rings is preferred
to enjoy conjugation; however, the phenyl groups show a slight
deviation from coplanarity. All four complexes are square-
planar. The chelate rings are highly symmetric, with many pairs
of bond lengths within experimental error of each other.
Most of the coordination complexes 1−6 (Figure 1) manage
null, indicating rigorously square-planar geometry. However, 4
is 26% distorted from square planar per τ . These parameters
4
II
show a clear preference of Cu for a square-planar O4
environment, which is quite difficult to alter by steric
embellishment of the β-diketonate ligands.
I
7 was obtained by trapping of a Cu intermediate during the
to obtain a planar MO coordination environment, although
reduction of 2 (vide infra). Single-crystal X-ray diffraction
(XRD) analysis reveals alternating complex geometries similar
to that of 5. The fold and aryl chelate dihedral angles are
4
the rest of the ligand bulk show considerable flexibility in order
to permit this. For example, 1 shows an increased fold angle
B
DOI: 10.1021/acs.inorgchem.9b02721
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Inorganic Chemistry
Article
Table 1. Selected Bond Angles and Lengths As Determined from X-ray Crystallography for New and Common Complexes
geometric
indices
a
angles (deg)
lengths (Å)
o-aryl
complex
fold
dihedral
aryl chelate dihedral
M−O
1.9268(16),
C−O
C−C
τ4
τ′4
3
5
Cu(acac)2
6.21(11)
1.276(3), 1.275(3)
1.403(3), 1.402(3)
0
0
1
.9227(16)
36
Cu(bzac)2
Cu(dbm)2
Cu(dpm)2
5.00(6)
3.85(2)
1.61(14)
15.14(16)
2.75(9)
14.79(18)
10.56(5), 3.81(5)
1.924(3), 1.918(3)
1.907(4), 1.910(5)
1.902(2), 1.891(2)
1.270(5), 1.269(5)
1.285(3), 1.260(2)
1.273(3), 1.276(4)
1.277(4), 1.282(4)
1.271(2), 1.272(2)
1.400(6), 1.405(6)
1.397(3), 1.393(4)
1.376(5), 1.395(5)
1.394(4), 1.394(5)
1.390(3), 1.398(3)
0
0
0
0
0
0
0
0
0
0
37
38
1
2
46.52(12), 37.86(12) 1.906(2), 1.898(2)
121.34(7)
115.13(7)
133.20(13)
133.79(5)
115.76(6)
122.04(3)
78.54(7)
66.39(9)
70.78(6)
66.31(6)
63.49(6)
61.73(6)
1.9074(13),
.8972(15)
1
3
4
10.85(10)
2.10(9)
1.9093(13),
.8962(14)
1.271(2), 1.275(2)
1.398(3), 1.395(2)
0
0
1
1.8917(9),
.9062(10)
1.2816(15),
1.2643(16)
1.3877(18),
1.4063(19)
1.3837(17),
1.4048(18)
1.393(2), 1.405(2)
1.395(2), 1.400(2)
1.389(5), 1.400(5)
0.26
0.25
1
6
.55(10)
2.16(7)
.72(8)
1.8874(9), 1.9156(9) 1.2776(15),
.2699(16)
1
5
1.8953(11),
.9216(11)
1.2742(19),
1.2727(19)
1.2752(19),
1.2777(19)
1.282(4), 1.269(4)
1.265(12), 1.255(11) 1.390(14), 1.435(14)
1.290(12), 1.270(12) 1.364(14), 1.393(14)
0
0
0
0
1
3
1.8841(11),
.9207(11)
1
6
7
10.71(17)
6.7(5)
.7(6)
128.7(13)
128.2(4)
132.5(5)
66.50(13)
67.3(5)
60.9(5)
1.897(3), 1.939(3)
2.058(7), 2.033(8)
2.036(8), 2.037(7)
0
0.89
0.87
0
0.88
0.84
3
a
The first bond lengths correspond to the side of the chelate with the larger substituent, for asymmetrically substituted ligands.
Figure 1. Crystal structures for the new copper β-diketonates described. Thermal ellipsoids (50%) are shown for the chelate rings and phosphorus
atoms. Hydrogen atoms and solvent molecules have been removed for clarity. Unabridged structures can be found in Figures S38−S43.
exhibits the shortest C−O and longest C−C bonds, while the
other is exactly the reverse. This suggests that the chelate ring
I
is particularly more flexible when coordinated to Cu , while
II
Cu enforces a more regular chelate geometry. The phosphine-
Figure 2. Visualizations of the various angles and lengths included in
Table 1.
supported 7 is pseudotetrahedral in character with a τ′ value
4
40,41
of about 0.86.
Spectroscopy and Electrochemistry. Spectroscopic and
magnetic data for the copper complexes are collected in Table
. The magnetic moment of each complex was determined by
similar to those of the CuII examples, although the o-aryl
dihedral angle is again extended. The M−O bonds are longer
by 0.13 Å than 1−6, consistent with an increased ionic radius
due to the lower oxidation state of copper. One conformer
2
Evans’ method, and all are consistent with a single unpaired
9
electron, as expected for a d electronic configuration. UV−vis
C
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Table 2. Spectroscopic and Magnetic Details of 1−6
λmax(DCM), nm (ε, M− cm−1
1
)
CO stretch (cm−1
)
μeff (BM)
1
2
3
4
5
6
1.45(15)
1.23(15)
1.22(2)
1.72(17)
1.65(15)
1.71(25)
524 (82.7), 618 (81.6)
543 (45.1), 663 (51.3)
543 (50.3), 655 (57.1)
557 (52.3), 696 (64.3)
533 (70.3), 668 (75.9)
530 (70.7), 622 (79.1)
1599, 1573
1610, 1565
1614, 1581
1611, 1577
1580, 1577
1599, 1572
spectra collected in dichloromethane (DCM) exhibit two
maxima in the visible region, similar to known copper(II) β-
4
2,43
diketonates.
However, 4 has an absorbance 30 nm greater
than those of the others. This lowering of the energy is
consistent with a decrease in symmetry of the planar MO4
coordination environment, which usually enjoys conjugation
44
through the metal center, and this suggests that twisting of
the chelate rings is also present in solution. The carbonyl
stretching frequencies were determined by IR spectroscopy,
and all fall in a similar range and suggest little sensitivity to
sterics or electronics.
Four oxidation states for copper catalysis have been
0
III
1
implicated in the literature: Cu to Cu . We were therefore
interested in the accessibility of these oxidation states by cyclic
voltammetry. A recent electrochemical survey of copper(II) β-
n
44
Figure 3. Onset voltages for reduction and reoxidation of Cu(L ) in
THF, reported versus Fc/Fc .
diketonates was conducted in acetonitrile (MeCN), but 1−6
were insoluble in the solvent. Therefore, copper(II) complexes
of acetylacetone (acac), benzoylacetone (bzac), dibenzoyl-
methane (dbm), and dipivaloylmethane (dpm) were also
studied electrochemically in tetrahydrofuran (THF; Figures
S34−S37). A comparison of the complexes in commonly
2
+
Table 3. Numerical Values for the Voltages of Onset and
Peak Reductions (E ) and Oxidations (E ) versus Fc/Fc
+
C
A
peak E
(V)
onset E
(V)
onset EA
(V)
peak E
A
(V)
Δ onset
C
C
studied MeCN (CuL , where L = acac, bzac, and dbm)
(V)
2
indicates a similar trend of increasing reduction potential as the
methyl groups are replaced with phenyl rings. The reductions
were about 300 mV more negative in THF because of the
known solvent dependence on the electrochemical re-
acac
bzac
dbm
dpm
1
3
5
2
4
−2.18
−1.86
−1.68
−2.26
−1.81
−1.92
−1.94
−2.20
−2.21
−2.29
−1.55
−1.43
−1.34
−1.89
−1.52
−1.63
−1.60
−1.78
−1.84
−2.01
−1.35
−1.25
−1.16
−1.06
−0.42
−0.52
−0.48
−0.49
−0.43
−0.23
−0.93
−0.81
−0.60
−0.49
−0.09
−0.30
−0.12
−0.20
−0.04
−0.09
0.20
0.18
0.18
0.83
1.10
1.11
1.12
1.29
1.41
1.78
4
5
sponses. There is a different response in the subsequent
reoxidation of the CuL anion, presumably because of the
2
I
46
different abilities of the solvents to support Cu species.
Similarly, these compounds show about half a volt difference in
the onset potentials for reduction and reoxidation in MeCN,
whereas the windows are less than half as wide in THF.
Encouraged by these results, the cyclic voltammetry of more
hindered complexes was studied in THF. Each experiment
began by sweeping oxidatively to confirm no reduced species
are present and reversing before the solvent limit. None of the
metal complexes could be oxidized in THF, suggesting that
6
at about −0.5 V, which is about half a volt more positive than
those of the smaller complexes. Furthermore, the window of
onset voltages is notably wider for 6 by about 600 mV than
otherwise expected based on similarly substituted examples,
which shows a considerable steric influence. The absence of
overpotentials needed to drive the smaller copper(II)
complexes suggests a much faster ligand reorganization
event. The electrochemical irreversibility of the larger
complexes 1−6 is due to the slow electron-transfer rate and
reorganization of a planar-to-tetrahedral shift in the coordina-
tion geometry, resulting in a large difference in the electrode
response potential between oxidation and reduction.
Reactivity and Catalysis. As predicted by electrochemical
analysis, 2 could not be oxidized by the strong oxidant (4-Br-
Ph) N SbCl and is unreactive with atmospheric oxygen.
3 6
However, 2 is slowly reduced by a sodium/mercury (Na/Hg)
amalgam to Cu and Na(L ). The reduction of 2 with excess
Na/Hg in the presence of PPh (2 equiv) resulted in the
formation of Cu(L )(PPh ) (7) and Na(L ) in quantitative
III
Cu might not be easily accessed with this ionic ligand
platform. A reduction is observed in each complex around
−
1.5 V, after which the CuI species may undergo
disproportionation, generating free ligand anions and copper
metal. Several new oxidations can be observed on the second
I
n
scan. The first oxidation was assigned to that of the Cu (L )
anion, followed by oxidation of the free ligand anion and Cu
2
0
oxidative stripping from the electrode. The reduction of the
free ligand anion occurs at potentials of around −2.5 V, as
2
corroborated by separate analysis of K(L ) (Figure S9).
The ranges between the onset voltages for Cu /Cu are
II
I
+
−
displayed in Figure 3 and Table 3. Complexes 1−6 could be
+
reduced between −1.6 and −2.2 V (vs Fc/Fc ). The onset of
0
2
reduction seems most affected by electron-donating groups like
47
tert-butyl, as has been noted in other metal β-diketonates.
Thereafter, an oxidation can be observed with an onset voltage
3
2
2
3
2
D
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Inorganic Chemistry
Article
yield. No coordination of phosphine and Cu(L2)2 was
observed by UV−vis upon mixing without a reductant (Figure
portionation to Cu and CuBr was observed in the nonpolar
0
2
0
reaction medium. Therefore, using toluene as the solvent, Cu ,
CuBr, and CuBr are catalytically incompetent in the absence
I
2
S6). The resulting 1:1 ratio of Cu to Na(L ) in the product
2
I
and quantitative chemical trapping of Cu as its bis(phosphine)
of supporting ligands because of poor stability and/or
solubility. This indicates that the catalytic performance
observed is specifically due to a copper species stabilized by
β-diketonates. 7 was also utilized and showed trace conversion
to products presumably because of a lack of available
coordination sites and poor dissociation of the supporting
I
adduct suggest stepwise reduction and relative stability of Cu
intermediates, such that they may be intercepted by supporting
0
II
n
phosphines prior to disproportionation to Cu and Cu (L ) .
2
On the other hand, a higher ratio of sodium to copper would
be expected if the CuI species underwent immediate
decomposition.
I
2
phosphine ligands, preventing activation. Both Cu Br/K(L )
2
In order to observe the effect of sterics on catalysis, a
challenging Ullmann-type diaryl ether reaction was then
and Cu(L )(PPh ) registered nonzero yields but lagged all
3
2
other examples in comparison.
These data compellingly suggest that the coupling reaction is
facilitated by a copper β-diketonate catalytic species, which
lacks strong-field donor ligands such as PPh . The fact that
proposed catalytic intermediate copper(I) β-diketonates are
unstable relative to copper metal and Cu could help to explain
why catalyst loadings remain orders of magnitude greater than
the precious metals.
sterics indicates that disproportionation can be slowed relative
to the catalyst turnover frequency, allowing the hindered 2 to
surpass the commonly used precatalysts’ yields with extended
reaction time.
3
chosen as a model catalytic system. Control experiments
indicated that the reaction proceeded in good-to-excellent
yield using polar solvents (N,N-dimethylformamide, DMF) at
elevated temperatures (120 °C), as is known. However, we
sought a more discriminating reaction condition to identify
subtle differences in the catalytic behavior. It was found that
reactions carried out in toluene at 90 °C allow for such a
comparison and are also similar to those at typical palladium-
catalyzed coupling temperatures. After a brief screening of the
catalytic conditions using 2 as the baseline precatalyst (Table
3
II
8
,11,26
Promisingly, the increase of ligand
2
5
S32), many of the new complexes (L −L ) were benchmarked
against some commonly employed copper β-diketonate
precatalysts (Scheme 2). Additionally, CuBr was used as a
DISCUSSION
■
Possible Mechanistic Pathways. One goal of this work
was to understand the impact of sterics on the course of the
Ullmann-type coupling reactions, which are research subjects
of renewed interest. The β-diketonate ligand serves to
solubilize and support the copper species, while it facilitates
the bond rearrangements. Increasing steric bulk seems to
Scheme 2. Screen of Copper(II) β-Diketonate Precatalysts
in an Ullmann-Type Diaryl Ether Preparation
I
improve this support, per Cu trapping and catalytic experi-
ments. In toluene, the hindered complex 2 showed an
improved endurance in solution relative to the common
precatalysts, while the unchelated control was ineffective.
Reactions in DMF support a variety of solubilized copper
compounds that might be catalytically active; however,
discerning them at high temperatures is difficult, and it is
desirable to phase out this solvent because of toxicity and
engineering concerns. However, rational progress in catalyst
development requires a more precise knowledge of the
catalytic cycle. Several mechanistic proposals could be
envisioned (Figure 4).
I/III
A Cu
redox cycle is often invoked to parallel the
I
monometallic precious metal catalytic cycles. Cu can be
generated via comproportionation between Cu and Cu-
acac) , although this equilibrium strongly disfavors Cu based
0
I
(
2
on reduction potentials. Alternatively, arylcopper(II) com-
I
pounds were shown to form biaryl and Cu products a century
48
I
control both with and without added ligand. Compound 7 was
also tested as an example of a copper(I) β-diketonate
supported by strong-field ligands.
ago. In any case, Cu is thought to undergo oxidative
addition, occurring before or after coordination of the
15−18
I
nucleophile.
Cu has also been proposed to react via a
Notably, the yields of the smaller complexes were higher, as
might be expected because of the sterics about the metal
center. Although the reactions were concluded at 24 h, yields
improved over subsequent days for reactions catalyzed by m-
terphenyl groups (Table S32), reaching up to 31%. The
increased size of the ligand improves the persistence of the
active species in solution but at the cost of the turnover
frequency. Additionally, the m-terphenyl complexes performed
equally as well as Cu(dpm) , suggesting that the reactivity is
single-electron-transfer mechanism with aryl halides to form an
II
organic radical, which then rebounds with the Cu
1
4
intermediate. In any event, once the aryl group and
III
nucleophile are coordinated to Cu , reductive elimination
I
affords the product and regenerates the monomeric Cu active
species.
48,49
Alternatively, a dimeric mechanism remains a possibility.
Many of the first-row late transition metals have been shown to
exist as dimers or higher oligomers with acetylacetonate
2,
5
0−53
influenced by the least bulky substituent. Control experiments
using CuBr showed no product formation. Instead, dispro-
ligands.
A dimeric mechanism instead supposes a
I/II
bimetallic Cu redox cycle to facilitate the net two-electron
E
DOI: 10.1021/acs.inorgchem.9b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. Summary of possible mechanistic proposals for the diaryl ether Ullmann coupling reaction.
5
4
chemistry. Exchange of the nucleophile for β-diketonate and
are directed 180° and 120° away from the metal center,
respectively. This is less problematic in other well-known
ligands such as N-heterocyclic carbenes (NHCs), m-terphenyl
groups, and NacNac ligands, all of which possess hindering
groups directed about 60° away from the coordinated metal
dimerization then permits homocoupling and formation of the
required Cu dimers. These dimers would offer an increased
reducing ability relative to equivalent monomeric species and
can subsequently reduce the organic radical to form bridged
Cu intermediates, which are well-known. Exchange of
halogen for the nucleophile then prepares for reductive
I
II
55
21,57−60
I
atom.
Correspondingly, stabilization of a Cu species
has been accomplished with a variety of ligands, such as
I
61
62,63
64
elimination to reform the Cu active species. This mechanistic
phosphines, NHCs,
and m-terphenyl groups, among
proposal also obviates the CuIII intermediates, which seem
inaccessible in even moderately coordinating solvents (i.e.,
THF) by cyclic voltammetry.
other mostly strong-field ligands. In a striking example, a
NacNac ligand was used to stabilize a monomeric
pentafluorophenylcopper(II) adduct, which could stoichio-
I
21
Lastly, σ-bond metathesis avoids even Cu catalytic
metrically effect C−O bond formation. This result bodes
14
intermediates, which remain exceedingly unstable. In this
mechanistic manifold, an organocopper(II) β-diketonate active
species coordinates an aryl halide to form a four-membered
metallacycle. Rearrangement of the electrons results in the
formation of a product and a halocopper(II) intermediate,
perhaps construed as copper-catalyzed S Ar. This complex
can then transmetalate to regenerate the active species. The
well for β-diketonates because of similarity in the bonding
modes, although we note that electronic acac and NacNac
ligands are different because of the electronegativity differences
26
between oxygen- and nitrogen-donor atoms. The difficulty in
6
metalating L mirrors reports of other hindered ligands, such as
56
65
bis(iminopyridines).
N
I
Contrary to other ligand platforms’ abilities to support Cu
II
III
commonly isolated Cu dimers might represent the resting
and/or Cu compounds, β-diketonates are shown to support
54
state of the active species, which dissociates into 2 equiv of
active catalyst under the influence of Lewis bases such as
solvent molecules or aryl halides. Nonetheless, the catalyst still
slowly decomposes to copper metal, suggesting that the other
mechanisms are competitive.
Taking a step back, the attenuated catalytic ability of the
hindered complexes 1−6 could be either a result of slowed
catalytic turnover by the active species or due to inhibited
precatalyst activation or both. Each of the proposed
mechanisms involves monochelated copper catalysts, although
monomer/dimer speciation is unclear at this time. Ideally, the
isolated mono(β-diketonate) of each oxidation state, with
varying coligands, would be prepared and considered
separately in stoichiometric and catalytic experiments in
order to directly probe the catalytic cycle.
these oxidation states only fleetingly, if at all. Oxygen forms
more polarized chelate bonds and perturbs the metal’s
electronic structure minimally (i.e., weak field). In the weak-
field case, the chelate bonds have considerable ionic character,
conferring innocence to the π system and deriving much of the
bond strength from electrostatics. Additionally, oxygen-bound
ligands benefit from the increased oxophilicity of the first
transition series. This reliance on electrostatics rationalizes the
highly regular bond lengths at the expense of incredible ligand
flexibility. Yet, hindered and unhindered copper(II) β-
diketonates effectively promote Ullmann cross-coupling
2
5
reactions. Qualitatively, L −L are effective supporting ligands
for homogeneous diaryl ether synthesis, as shown by the
results comparable to those of commonly used precatalysts,
whereas the unchelated systems immediately disproportionate
because of poor solubility and stability in the nonpolar reaction
medium.
Stabilization of Low-Coordinate Copper β-Diketo-
nates. Another goal of this work is gaining steric control over
the Cu coordination geometry using the β-diketonate ligand
platform. Enforcing steric influence using the acac framework
is difficult on geometric grounds because the α and β positions
II
Because the stabilization of low-coordinate or low-oxidation
metal acetylacetonates lacking strong-field supporting ligands
remains synthetically elusive, it is unclear to what extent steric
F
DOI: 10.1021/acs.inorgchem.9b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
II
bulk needs to be applied to the ligand platform to achieve
results similar to those of other popular ligands. The past
several decades of ligand development have highlighted several
fragments that effectively promote steric encumbrance. By a
comparison of structurally varied ligand sets using a single
metal platform, the dominance of the chelate bonds and ligand
flexibility became apparent. Ligand bulk will twist and bend to
permit the inclusion of a second ligand. Even if directly
formed, the disproportionation of copper(I) β-diketonates to
form Cu and Cu is rapid. This is particularly relevant in the
context of catalyst deactivation in Ullmann-type coupling
reactions. Nonetheless, signs of strain can be found by the
careful observer. The notable deviation from square planar in 4
and the asymmetric M−O bond lengthening in 6 indicate that
accumulated steric strain can affect the dominant parameters of
chelation in the crystal phase. Likewise, electrochemical
experiments and the trapping of 7 suggest that the increased
steric bulk of the ligands has consequences for solution-phase
chemistry.
has yet to be achieved with Cu , although this approach may
offer significant insight and guide improvements in base metal
catalytic reactions of industrial and medicinal relevance.
EXPERIMENTAL METHODS
■
Syntheses were performed in a nitrogen-filled glovebox or in a fume
hood using standard air-free Schlenk techniques. Metal halides were
purchased from Strem Chemical and used as received. Deuterated
solvents were purchased from Cambridge Isotope Laboratories and
stored over 4 Å molecular sieves. Acetylacetone (acac), dibenzoyl-
methane (dbm), and benzoylacetone (bzac) were purchased from
Alfa Aesar and used as received. All solvents were purchased in
anhydrous form from Sigma-Aldrich and further dried with and stored
over 4 Å molecular sieves. dpm and L −L were synthesized
according to literature procedures. Copper(II) β-diketonate
complexes containing acac, dpm, dbm, and bzac ligands were
synthesized according to literature procedures. NMR spectra were
recorded on a Bruker Avance-III 400 NMR spectrometer (400 MHz),
IR spectra were recorded on an Agilent Cary 630 Fourier transform
infrared (FTIR) spectrometer with an ATR attachment, and UV−vis
spectra were recorded on an Agilent Cary 5000 UV−vis−near-IR
spectrophotometer. Magnetic data were collected in chloroform
solutions at room temperature using Evans’ method (average of three
0
II
1
6
2
8
66
Our earlier work with cobalt has shown that preclusion of
trischelation by steric encumbrance prevents disproportiona-
III
26
tion to form Co compounds. Similar preclusion of copper
6
7
I
samples).
diamagnetic contributions for the core diamagnetism of each sample
estimated using Pascal’s constants). Elemental analyses were
Magnetic susceptibility data were corrected for
bis(β-diketonates) is anticipated to afford stable Cu
compounds and various Cu intermediates using a suitably
hindered ligand. Such kinetically stabilized Cu complexes are
II
(
I
performed by Midwest Microlab LLC. All cyclic voltammetry
experiments were conducted in a 0.1 M tetrabutylammonium
hexafluorophosphate/THF solution using a platinum working
electrode at 17 °C. The analyte concentration was fixed at 5 mM
(or saturated) with a scan rate of 200 mV s (unless otherwise
specified). The initial blank scans were conducted by starting at 600
of interest as reactive intermediates in Ullmann-type reactions.
Thus, a ligand topology that commands steric control of the
reactive metal center is still required within the β-diketonate
platform. Promisingly, monovalent metals form complexes
with these hindered ligands to form dimers, typified by
−
1
+
2
+
I
mV versus Fc/Fc and sweeping toward a negative potential. Scans of
the analyte were conducted by starting at 600 mV versus Fc/Fc and
[
K(L )] (Figure S44). Assuming K is a good model for Cu ,
2
+
this analogue would represent the metastable complex just
0
sweeping toward a positive potential. X-ray crystal structures were
collected using a Bruker D8 Quest Eco three-circle goniometer
platform equipped with a Bruker APEX-II charge-coupled detector. A
graphite monochromator was employed for wavelength selection of
the Mo Kα radiation (λ = 0.71073 Å). The data were processed using
APEX III software provided by Bruker. Except as noted below, the
structures were solved by intrinsic phasing in SHELXT and refined
by standard difference Fourier techniques with SHELXL within the
OLEX2 software package. Hydrogen atoms were placed in
calculated positions using the standard riding model and refined
isotropically, except as noted below.
Synthesis of 1. L (1.00 g, 1.66 mmol), KH (0.133 g, 3.33 mmol),
and THF (12 mL) were mixed and allowed to stir for 30 min at
ambient. The excess base was removed by filtration, and the filtrate
before disproportionation and deposition of Cu .
CONCLUSIONS
■
The synthesis, characterization, and reactivity of a series of
sterically encumbered copper(II) β-diketonates are reported.
The preponderance of physical data indicates largely typical
68
69
n
II
Cu(L ) -type compounds. However, it was noted that Cu will
2
70
induce several modes of ligand flexibility to permit the
chelation of two ligands. m-Terphenyl groups effectively
project sterics, deforming mainly via out-of-plane rotation
1
I
and aryl group flexibility. Cu remains metastable with respect
0
to Cu and copper(II) β-diketonates. This can be exploited
II
synthetically to prepare extremely hindered Cu coordination
was combined with anhydrous CuCl (0.112 g, 0.83 mmol). The
2
I
mixture was allowed to stir at 60 °C for 24 h, filtered to remove salts,
and diluted with an equal portion of methanol (MeOH). Water was
added dropwise until a precipitate formed, and then the mixture was
placed into a −20 °C freezer for 18 h. The crude precipitate was then
recrystallized by diffusing MeOH into a saturated THF solution to
yield 1 as a green microcrystalline powder (0.69 g, 0.55 mmol, 66%).
H NMR (CDCl , 25 °C, 400 MHz): δ 9.52, 7.59, 6.45, 2.00, 1.55. IR
compounds. Alternatively, the Cu compounds can be trapped
in excellent yield with PPh , suggesting a certain persistence of
3
I
Cu in solution. The evidence suggests that realizing steric
control of the metal center is difficultbut not impossible
using the β-diketonate platform.
The efficacy of these hindered precatalysts in an Ullmann-
1
3
−1
II
type reaction was determined. Sterically hindered Cu
(
neat solid, cm ): 2955, 2907, 2866, 1599, 1573, 1540, 1521, 1503,
precatalysts are more effective, albeit slower, than the smaller
systems as expected. It remains unclear if this is due to slowed
activation or turnover events. Hindered precatalysts showed
improved persistence under reaction conditions, eventually
1428, 1423, 1391, 1365, 1312, 1249, 1204, 1103, 1060, 883, 887, 809,
−
1
−1
783, 758, 749, 717, 693. UV−vis [DCM; λmax, nm (ε, M cm )]:
524 (82.7), 618 (81.6). Evans’ method (CDCl , μ , 25 °C): μeff
.45 ± 0.15. Anal. Calcd: C, 81.77; H, 8.14. Found: C, 81.53; H, 8.06.
Synthesis of 3. The method used to synthesize 1 was ineffective
=
3
B
1
II
surpassing the yields of the screened common Cu precataysts.
3
in the preparation of 3. L (0.500 g, 1.00 mmol), CuCl ·2H O (0.085
2
2
Nonchelated and phosphine-supported precatalysts were
ineffective in the reaction medium, asserting the necessity of
β-diketonates. The identity of the active species remains
elusive, and the isolation of low-coordinate and low-valent
copper β-diketonates would help to determine the likelihood
of either. Steric inhibition of bischelation using β-diketonates
g, 0.50 mmol), and 190 proof ethanol (EtOH; 25 mL) were
combined and allowed to reflux at 90 °C for 30 min. NaOAc·3H O
2
(
0.150 g, 1.10 mmol) was added, and the mixture was allowed to
reflux at 100 °C for 18 h. The product was collected by filtration,
washed with 190 proof EtOH, water, hexanes, and dried in vacuo to
yield 3 as a yellow-green powder (0.44 g, 0.41 mmol, 82%), insoluble
G
DOI: 10.1021/acs.inorgchem.9b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
in most solvents. H NMR (CDCl , 25 °C, 400 MHz): δ 9.75, 7.71,
vacuo, and the crude solids were triturated with PhMe. The resulting
suspension was filtered and concentrated in vacuo to yield 2 as a gray
powder.
3
6
.86, 5.37, 5.23, 4.03, 3.11, 2.29, 2.23, 2.03, 1.81, 1.70, 1.30. IR (neat
−
1
solid, cm ): 2918, 2858, 1614, 1581, 1529, 1450, 1396, 1369, 1304,
163, 1081, 1051, 1028, 851, 816, 786, 776, 739, 687. UV−vis
1
[
General Catalytic Conditions (2-Phenoxynapthalene). 2-
DCM; λ , nm (ε, M− cm )]: 543 (50.3), 655 (57.1). Evans’
1
−1
Napthol (0.144 g, 1 mmol), K CO (0.553 g, 4 mmol), copper(II) β-
max
2
3
method (CDCl , μ , 25 °C): μ = 1.22 ± 0.02. Anal. Calcd: C, 81.05;
diketonate (0.1 mmol, 10 mol %), and PhMe (5 mL) were mixed.
Following the addition of toluene, PhBr (0.21 mL, 2 mmol) was
added. The mixture was then heated at 90 °C for 24 h, then cooled,
and filtered through a pad of silica with a 1:1 EtOAc/DCM (50 mL)
eluent. The filtrate was concentrated to yield a crude solid, which was
then dried in vacuo. The crude solids were analyzed via H NMR to
determine the conversion.
3
B
eff
H, 6.99. Found: C, 80.82; H, 6.95.
Synthesis of 4. Using the same method that was used for 1, 4 was
synthesized to yield forest-green crystals (0.624 g, 0.488 mmol, 60%).
1
H NMR (CDCl , 25 °C, 400 MHz): δ 10.59, 7.78, 5.94, 4.48, 3.54,
.75, 0.96. IR (neat solid, cm ): 2959, 2929, 2869, 1611, 1577, 1544,
3
−
1
1
1
1
1
6
521, 1498, 1462, 1395, 1365, 1315, 1289, 1237, 1222, 1167, 1108,
054, 1021. UV−vis [DCM; λ , nm (ε, M− cm )]: 557 (52.3),
1
−1
max
96 (64.3). Evans’ method (CDCl , μ , 25 °C): μ = 1.72 ± 0.17.
3
B
eff
ASSOCIATED CONTENT
■
Anal. Calcd: C, 80.73; H, 9.30. Found: C, 79.84; H, 8.97.
*
S
Supporting Information
Synthesis of 5. Using the same method that was used for 1, 5 was
synthesized to yield a dark-green microcrystalline powder (0.612 g,
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02721.
1
0
7
.450 mmol, 58%). H NMR (CDCl , 25 °C, 400 MHz): δ 10.19,
3
.65, 6.21, 3.79, 3.77, 3.76, 2.92, 2.02, 1.88, 1.88, 1.87, 1.27. IR (neat
solid, cm ): 3059, 2951, 2921, 2866, 1610, 1591, 1577, 1540, 1521,
−
1
Spectroscopic, crystallographic, electrochemical, and
other additional referenced data (PDF)
1
428, 1479,1446, 1423, 1395, 1376, 1305, 1246, 1204, 1181, 1041,
9
M
43, 877, 851, 820, 771, 762, 747, 715. UV−vis [DCM; λmax, nm (ε,
−
1
−1
Accession Codes
cm )]: 533 (70.3), 668 (75.9). Evans’ method (CDCl , μ , 25
3 B
°
C): μeff = 1.65 ± 0.15. Anal. Calcd: C, 83.05; H, 7.56. Found: C,
CCDC 1952058−1952064 contain the supplementary crys-
tallographic 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.
82.65; H, 7.58.
Synthesis of 6. The methods used to synthesize 1 and 3 were
6
ineffective in the preparation of 6. L (0.10 g, 0.12 mmol), CuMes
(
walled glass bomb reactor. The mixture was heated to 140 °C for 72
h. The reaction mixture was filtered to remove insolubles and then
evaporated to dryness in vacuo. The crude solids were recrystallized
first by diffusing MeOH into a THF solution and second out of hot
CHCl to yield 6 as a dark-gray microcrystalline powder (0.06 g, 0.04
mmol, 60%). H NMR (CDCl , 25 °C, 400 MHz): δ 8.73, 7.35, 6.59,
6
2
1
7
0.04g, 0.25 mmol), and PhMe (10 mL) were combined in a thick-
AUTHOR INFORMATION
■
3
Corresponding Author
Email: michael.marshak@colorado.edu.
1
3
*
−
1
.38, 6.14, 3.45, 2.36, 1.92, 0.87, 0.07. IR (neat solid, cm ): 2962,
929, 2869, 1599, 1506, 1551, 1521, 1476, 1461, 1383, 1365, 1305,
246, 1207, 1163, 1073, 1069, 1039, 943, 877, 805, 767, 756, 742,
ORCID
Michael P. Marshak: 0000-0002-8027-2705
Author Contributions
15. UV−vis [DCM; λ , nm (ε, M− cm )]: 530 (70.7), 622
1
−1
max
(
79.1). Evans’ method (CDCl , μ , 25 °C): μ = 1.71 ± 0.25. Anal.
3 B eff
†
These authors contributed equally. Experimental work was
Calcd: C, 78.89; H, 8.15. Found: C, 78.73; H, 8.38.
conducted by A.T.L. and A.S.C. XRD data were collected by
S.M.K. The manuscript was prepared by A.S.C., with assistance
from A.T.L. and M.P.M. The work was supervised by M.P.M.
All authors have given approval to the final version of the
manuscript.
Synthesis of 7. CuCl (0.52 mmol, 0.052 g), PPh (1.04 mmol,
3
0
.273 g), and DCM (5 mL) were combined and allowed to stir at
2
room temperature for 30 min. The mixture was treated with K(L )
0.52 mmol, 0.250 g), allowed to stir for 1 h at room temperature, and
(
then filtered through a pad of Celite. The filtrate was concentrated to
dryness in vacuo, and the solids were recrystallized by diffusing
Funding
MeCN into a concentrated DCM solution to yield 7 as bright-yellow
1
crystals (0.336 g, 0.327 mmol, 63%). H NMR (CDCl , 25 °C, 400
This work was funded by laboratory startup funds provided by
the University of Colorado, Boulder, CO. A.S.C. gratefully
acknowledges a Sharrah fellowship award from CU Boulder.
S.M.K. acknowledges a summer UROP award from CU
Boulder.
3
MHz): δ 7.32−7.24 (m, 7H), 7.22−7.11 (m, 24H), 6.96 (d, 2H), 6.36
s, 4H), 5.00 (s, 1H), 1.97 (s, 6H), 1.94 (s, 12H), 0.68 (s, 9H). C
1
3
(
NMR (CDCl , 25 °C, 101 MHz): δ 197.49, 186.23, 143.48, 139.23,
3
1
1
4
−
1
39.00, 135.91, 134.94, 134.69, 134.46, 134.06, 133.90, 128.87,
28.50, 128.15, 128.07, 127.70, 127.09, 95.12, 77.34, 77.02, 76.70,
3
1
Notes
0.45, 27.81, 20.89, 20.86. P NMR (CDCl , 25 °C, 162 MHz): δ
3
−1
5.60. IR (dispersion in oil, cm ): 3048, 1585, 1566, 1503, 1436,
The authors declare the following competing financial
interest(s): M.P.M. is an inventor on U.S. Patent 9,676,693,
which comprises various bulky diketones, other related ligands,
and their metal complexes.
402, 1361, 1167, 1100, 1030, 851, 743, 695. Anal. Calcd (for O2
adduct): C, 75.94; H, 6.18. Found: C, 75.71; H, 6.24.
Reaction of 2 with PPh and Na/Hg Amalgam. 2 (0.050 g,
3
0
.05 mmol), PPh (0.026 g, 0.1 mmol), and THF (3 mL) were
3
combined. A bead of Na/Hg amalgam (0.497 w/w %, 0.276 g, 0.06
mmol of Na) was added to the solution and allowed to stir for 1 h at
room temperature. The supernatant was removed from the Na/Hg
ACKNOWLEDGMENTS
■
The authors acknowledge the sesquicentennial of the first
bead and filtered to ensure complete removal of the amalgam. The
copper coupling reaction: the Glaser reaction, a homocoupling
1
71
filtrate was then concentrated and dried in vacuo. H NMR in CDCl
3
of phenylacetylene.
2
of the solids revealed a 1:1 ratio of Na(L ) and 7.
Reaction of 2 with [(4-BrC H ) N]SbCl . 2 (0.100 g, 0.11
6
4 3
6
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unreacted [(4-BrC H ) N]SbCl . The filtrate was concentrated in
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DOI: 10.1021/acs.inorgchem.9b02721
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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