Copper(I) complexes with trispyrazolylmethane ligands: Synthesis, characterization, and catalytic activity in cross-coupling reactions

Article abstract of DOI:10.1021/ic300843a

Three novel Cu(I) complexes bearing tris(pyrazolyl)methane ligands, Tpm^x, have been prepared from reactions of equimolar amounts of CuI and the ligands Tpm, (HC(pz)₃), Tpm, (HC(3,5-Me₂-pz) ₃), and Tpm^Ms, (HC(3-Ms-pz)₃). X-ray diffraction studies have shown that the Tpm and Tpm^Ms derivatives exhibit a 2:1 Cu:ligand ratio, whereas the Tpm* complex is a mononuclear species in nature. The latter has been employed as a precatalyst in the arylation of amides and aromatic thiols with good activity. The synthesis of a TpmCu(I)-phthalimidate, a feasible intermediate in this catalytic process, has also been performed. Low temperature ¹H NMR studies in CDCl ₃ have indicated that this complex exists in solution as a mixture of two, neutral and ionic forms. Conductivity measurements have reinforced this proposal, the ionic form predominating in a very polar solvent such as DMSO. The reaction of TpmCu(I)-phthalimidate with iodobenzene afforded the expected C-N coupling product in 76% yield accounting for its role as an intermediate in this transformation.

Full text of DOI:10.1021/ic300843a

Article
pubs.acs.org/IC
Copper(I) Complexes with Trispyrazolylmethane Ligands: Synthesis,
Characterization, and Catalytic Activity in Cross-Coupling Reactions
†
‡
,§
,†
́
Estela Haldon, Eleuterio Alvarez, M. Carmen Nicasio,* and Pedro J. Perez*
́
́
†Laboratorio de Catal
Investigacion en Química Sostenible (CIQSO), Campus de El Carmen s/n, Universidad de Huelva, 21007-Huelva, Spain
‡Instituto de Investigaciones Químicas Isla de la Cartuja, CSIC-Universidad de Sevilla, Avenida de Amer
ico Vespucio 49,
́ ́
isis Homogenea, Departamento de Química y Ciencias de los Materiales, Unidad Asociada al CSIC, Centro de
́
́
41092-Sevilla, Spain
§Departamento de Química Inorgan
́
ica, Universidad de Sevilla, Aptdo. 1203, 41071-Sevilla, Spain
S
* Supporting Information
ABSTRACT: Three novel Cu(I) complexes bearing tris(pyrazolyl)-
methane ligands, Tpmx, have been prepared from reactions of
equimolar amounts of CuI and the ligands Tpm, (HC(pz)3), Tpm*,
(HC(3,5-Me2-pz)3), and TpmMs, (HC(3-Ms-pz)3). X-ray diffraction
studies have shown that the Tpm and TpmMs derivatives exhibit a 2:1
Cu:ligand ratio, whereas the Tpm* complex is a mononuclear species in
nature. The latter has been employed as a precatalyst in the arylation of
amides and aromatic thiols with good activity. The synthesis of a
Tpm*Cu(I)-phthalimidate, a feasible intermediate in this catalytic
process, has also been performed. Low temperature 1H NMR studies in
CDCl3 have indicated that this complex exists in solution as a mixture
of two, neutral and ionic forms. Conductivity measurements have reinforced this proposal, the ionic form predominating in a
very polar solvent such as DMSO. The reaction of Tpm*Cu(I)-phthalimidate with iodobenzene afforded the expected C−N
coupling product in 76% yield accounting for its role as an intermediate in this transformation.
chelating nitrogen-donor ligand) and demonstrated its kinetic
and chemical competence in the Ullmann etherification of aryl
halides.7
INTRODUCTION
■
During the past few years we have witnessed a remarkable
progress in synthetic applications of copper-assisted Ullmann
and Goldberg condensations (Scheme 1).1 The origin of the
Very active catalytic systems based on the previous strategy
(bidentate ligand) have been already described,3−5 most of
them upon in situ generation of the catalyst precursor. In
contrast, examples describing the use of multidentate ancilliary
ligands, other than bidentates, in the copper-assisted Ullmann
and Goldberg condensations are yet scarce (Scheme 2). For
example, Taillefer and co-workers have examined the effect of a
range of polidentate oxyme type and pyridine-imine chelators
in arylation of N-heterocycles5d,e and cyanation of aryl
halides.8a These studies revealed that a combination of a
tetradentate Schiff base with a suitable copper(I) source in situ
generates a highly active catalyst for various type of cross-
coupling processes.5d,e,8 Recently, Zhou et al.9 have prepared a
tetra-coordinated Cu(II) complex bearing a tetradentatesulfo-
nato-salen ligand and described its catalytic activity in different
N-arylation reactions carried out in water. With regard to
tripodal chelators, catalysts systems based on 1:1 mixtures of
CuI and commercially available 1,1,1-tris(hydroxymethyl)-
ethane10 or tris(2-aminoethyl)amine11 tridentate ligands have
proven to be efficient in carbon-heteroatom bond forming
reactions. Of note, well-defined cationic trinuclear copper(I)
Scheme 1. Ullmann and Golberg Reactions
growing interest in this research area stems from the discovery
by Buchwald and co-workers, at the end of the 1990s, that the
use of certain chelating ligands produced an accelerating effect
in these copper-mediated organic transformations.2 Since then,
different types of neutral bidentate chelators based on
N,N-,2c,d,3 O′O-4, or mixed N,O5- donor atoms have been
mainly employed, along with copper, allowing these processes
to occur in a catalytic manner under mild conditions. Kinetic,
experimental, and theoretical studies carried out on the
mechanism of Cu(I)-catalyzed amidation of aryl iodides6 (the
Goldberg reaction) support the formation of a neutral,
tricoordinated CuI(amidate) intermediate stabilized by a single
molecule of the bidentate ligand. In this context, Hartwig et al.
have recently isolated a closely related LCuI(OPh) (L =
Received: April 25, 2012
Published: July 25, 2012
© 2012 American Chemical Society
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Scheme 2. Previously Employed Tri- and Tetradentate
Ligands in Goldberg/Ullmann Reactions and Tpmx Ligands
Employed in This Work
RESULTS AND DISCUSSION
Synthesis and Molecular Structures of Metal Com-
■
plexes. The reaction of copper(I) iodide in acetonitrile with
the Tpmx ligands illustrated in Scheme 3 afforded several CuI-
Scheme 3. Synthesis of Tpmx-Copper(I) Complexes 1−3
complexes containing two anionic triscarbene ligands have been
successfully tested as catalysts in arylation of N-, O-, and C-
centered nucleophiles,12 a rare example of the use of NHC-
carbene ligands in copper-catalyzed cross-coupling reactions.
Noteworthy, the use of stable preformed copper(I)
complexes as catalysts in copper-mediated C−C and C-
heteroatom cross-coupling reactions is not a common
practice,3b,13 despite this methodology can overcome the
drawbacks of the catalyst in situ generation strategy, such as
the formation of metal species with different catalytic activities,
the decrease of yield due to side reactions,2e,14 or the
contamination of products with the typically employed excess
of ligand. In addition, such use of well-defined precatalysts
could shed some light on the influence of the catalyst structure
on the mechanism of the reaction.
We are interested in the preparation and characterization of
stable copper(I) complexes that can serve as catalysts in cross-
coupling processes, particularly those involving the formation
of C-heteroatom bonds. In this context, we have recently
reported the synthesis of two well-defined, isomeric,
dinuclearCu(I) complexes [Cu2I2L2] (L = bis(azaindolyl)-
methane) containing one molecule of the bidentate ligand per
copper atom. Both complexes efficiently catalyzed the N-
arylation of 2-pyrrolidinone and S-arylation of thiols with aryl
iodides.15 In that work, we briefly mentioned that the complex
[Tpm*Cu(NCMe)]BF4 induced, at a moderate extent, the N-
arylation reaction of pyrrolidone. Given the tridentate mode of
this ligand, we wondered if other related complexes bearing
these Tpmx ligands (Scheme 2) could catalyze these trans-
formations effectively. Herein, we describe the preparation and
full characterization as well as the catalytic activity in C-
heteroatom bond-forming reactions of new Cu(I) derivatives
bearing Tpmx ligands. In order to compare this system with the
well-established CuI(amidate) bearing bidentate ligands, we
have prepared the complex Tpm*Cu(I)-phthalimidate, a
potential intermediate in this transformation, for which NMR
and chemical reactivity studies have also been carried out.
Tpmx complexes with different Cu/ligand stoichiometry. Thus,
when equimolar amounts of CuI and the parent ligand Tpm
were reacted at room temperature, a rapid precipitation of the
tetranuclear complex [Tpm2Cu4I4(NCMe)2] (1) was observed.
Despite the initial CuI/Tpm molar ratio of 1:1, we only isolated
complex 1. Subsequently, this compound was prepared in 87%
yield by using a 2:1 stoichiometric mixture of CuI and Tpm,
respectively. Conversely, the use of the dimethylpyrazolyl-
containing Tpm* ligand yielded the expected mononuclear
derivative [Tpm*CuI] (2) from a 1:1 initial ratio of reagents.
The bulkier TpmMs induced a somewhat distinct reactivity,
since its addition to an acetonitrile solution containing one
equiv of CuI resulted in the formation of {[TpmMsCu-
(MeCN)]+}2{[Cu2I4]2‑}·MeCN (3), in which the empirical
ratio of CuI to TpmMs was 2:1. Not surprisingly, a high yield
preparation of this complex (84%) was achieved from an initial
CuI:TpmMs, 2:1 ratio.
All complexes were stable to air both in solution and in the
solid state. Compounds 1 and 3 were only sparingly soluble in
MeCN and DMSO, but complex 2 was well soluble in those
solvents and also in CHCl3. The coordination of Tpmx ligands
to the copper(I) center in the molecules of 1−3 was confirmed
by a characteristic shift of resonances for the ligands to the
downfield region in their 1H NMR spectra recorded in DMSO-
d6 (see the Experimental Section). In addition, the IR spectra of
1 and 3 in Nujol mull showed weak stretching bands at 2133
and 2141 cm−1, respectively, due to the presence of coordinated
MeCN in these compounds. Nevertheless, spectroscopic and
analytical data obtained for these copper derivatives were
insufficient for unequivocally proposing a formulation for each
of them. Therefore, the molecular structures of 1−3 were
determined by X-ray diffraction studies carried out with single
crystals of the complexes obtained from acetonitrile solutions.
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The crystal structure of 1 (Figure 1) consists of two Tpm
ligands, two acetonitrile molecules, and a step-Cu4I4 core which
Figure 2. Molecular structure of 2. Thermal ellipsoids are drawn at the
50% probability level. H atoms have been omitted for clarity. Selected
bond lengths (Å) and bond angles (deg) for 2: Cu(2)−N(1) =
2.0994(5), Cu(2)−N(3) = 2.1416(13), Cu(2)−N(5) = 2.0863(13),
Cu(2)−I(1) = 2.4667(2), N(5)−Cu(2)N(1) = 84.86(5), N(5)−
Cu(2)−N(3) = 87.49(5), N(1)−Cu(2)−N(3) = 87.72(5), N(5)−
Cu(2)−I(1) = 128.88(4), N(1)−Cu(2)−I(1) = 129.49(3), N(3)−
Cu(2)−I(1) = 124.38(3).
Figure 1. Molecular structure of 1. Thermal ellipsoids are drawn at the
50% probability level. H atoms have been omitted for clarity. Selected
bond lengths (Å) and bond angles (deg) for 1: Cu(1)−N(1) =
2.037(4), Cu(1)−I(2) = 2.6469(7), Cu(1)−N(1) = 2.037(4), Cu(1)−
I(2A) = 2.7040(7), Cu(2)−N(3) = 2.063(4), Cu(2)−N(7) =
1.996(4), Cu(2)−I(2) = 2.6694(7), Cu(2)−I(1A) = 2.5720(7),
with the three Cu ions in the +1 oxidation state. The Cu(I)
cation shows a distorted tetrahedron environment with a N4
donor set, one from the coordinated MeCN molecule and the
other three N atoms from the tridentate ligand TpmMs (Figure
3). In the [Cu2I4]2‑counteranion, both Cu(I) are symmetrically
bridged by two I atoms and coordinated by a terminal iodide
anion, leading to a trigonal planar geometry around each
copper center. The structure of the cation has already been
N(1)−Cu(1)−I(2A)
= 104.76(11), N(1)−Cu(1)−I(2) =
112.00(11), I(1)−Cu(1)−I(2) = 114.67(2), I(1)−Cu(1)−I(2A) =
116.40(2), N(7)−Cu(2)−N(3) = 101.96(16), N(3)−Cu(2)−I(1A) =
117.37(10), N(7)−Cu(2)−I(2) = 103.61(12), I(1A)−Cu(2)−I(2) =
118.87(2).
reported by our group as a PF6 salt,17d and the dianion
−
contains two types of I atoms (μ2-I and μ3-I) and two types of
Cu(I) centers, both in tetrahedral environments. Cu(1) is
bonded to three iodine atoms and to a nitrogen atom from one
of the pyrazolyl rings of a Tpm ligand, whereas Cu(2) is
coordinated by two iodine atoms and two nitrogen atoms, one
from a pyrazolyl ring of the same Tpm moiety and one from
the acetonitrile molecule: each Tpm ligand bridges between
both types of Cu(I) centers in a kappa2NN-coordination mode,
the third pyrazolyl ring remaining uncoordinated. The bond
angles around Cu(1) ranging from 104.76(11) to 116.40(2)°
(see the caption of Figure 1) are normal for a tetrahedral
coordination, although Cu(2) exists in a slightly more distorted
tetrahedral arrangement with angles around Cu(2) being in the
interval 101.96(16)−118.87(2)°. The Cu−N and Cu−I bond
lengths are comparable with those of other tetrameric
clusters.16
Single-crystal analyses of 2 revealed that it is a mononuclear
complex (Figure 2). The Cu(I) ion is coordinated by the iodide
anion and the three N atoms of the Tpm* ligand in a trigonally
distorted tetrahedral geometry, with angles varying from
84.86(5) to 129.49(3). The Cu−N bond distances for the
tridentate ligand (2.0863(13)−2.1416(13) Å) are similar to
those previously described for other tris(pyrazolyl)methane
copper derivatives,17 with a shorter Cu−I distance (2.4667(2)
Å) compared to tetrahedral L3CuI complexes.18
[Cu2I4]2‑ has also been crystallographically characterized.18b,19
Bond distances and angles are in good agreement with those
already published.
NMR features of complexes 1−3 showed the equivalence of
the three pyrazolyl rings of the Tpmx ligands in their molecules.
This observation is in agreement with the solid-state structures
being maintained in solution for 2 and 3 but not for 1. In the
latter case, it is likely that the tetramer dissociates somehow in
the presence of the solvent DMSO-d6. As shown above,
molecular structures of these three TpmxCu(I) complexes in
the solid state are quite different. Despite the distinct electronic
nature of the pyrazolyl substituents (i.e. H, Me, and Ms), we
believe that it is the steric hindrance at the 3 position of the
pyrazolyl rings that determines the structure adopted by
complexes 1−3. In this regard, both Tpm* and TpmMs favor
the formation of N3-mononuclear complexes, but the larger size
of the Ms group in the latter prevents the coordination of the
iodine atom to the Cu(I) center, yielding the acetonitrile
adduct.
N-Arylation and S-Arylation Reactions Catalyzed by
1−3. Although we were initially seeking for complexes with a
1:1 metal to ligand ratio for catalytic purposes, and only 2 met
such a requirement, we decided to examine the catalytic
properties of all these TpmxCu compounds in the aryl
amidation reaction2a,5e,10,13j,14b,20 of iodobenzene with 2-
pyrrolidinone (Table 1). The experiments were performed
with 5 mol % based on copper in the presence of K3PO4 as the
base in dioxane at 110 °C for 24 h. Under these conditions
The solid-state structure of 3 has shown its ionic nature. This
compound contains two independent [TpmMsCu-
(MeCN)]+cations and one [Cu2I4]2‑ as the counteranion,
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Table 1. N-Arylation of 2-Pyrrolidinone Catalyzed by
a
TpmxCu(I) Complexes
bc
,
entry
precatalyst
solvent
yield (%)
1
1
2
3
2
1
2
3
1
3
dioxane
dioxane
dioxane
toluene
MeCN
MeCN
MeCN
DMSO
DMSO
dioxane
dioxane
dioxane
dioxane
dioxane
63
80
80
86
80
80
73
93
88
2
3
4
5
6
7
8
9
d
10
11
12
13
14
CuI + Tpm (1:1)
CuI + Tpm* (1:1)
CuI + TpmMs (1:1)
1 + Tpm (1:2)
CuI
95
d
84
d
73
65
58
a
Reaction conditions: iodobenzene (1 mmol), 2-pyrrolidinone (1.2
mmol), K3PO4 (2 mmol), copper complex (5 mol % of copper, 0.0125
mmol for 1 and 3, 0.05 mmol for 2 and CuI), solvent (1 mL).
b
c
Isolated yields (average of two runs). Unreactive iodobenzene
d
accounted for the overall mass balance. The reaction was carried out
in the presence of CuI (5 mol %) and Tpmx (5 mol %) as the catalyst.
Figure 3. Molecular structure of 3. Thermal ellipsoids are drawn at the
50% probability level. H atoms have been omitted for clarity. Selected
bond lengths (Å) and bond angles (deg) for 3: Cu(1)−N(5) =
2.034(2), Cu(1)−N(1) = 2.117(2), Cu(1)−N(3) = 2.1177(19),
Cu(I)−N(7) = 1.874(2), Cu(2)−I(1) = 2.5011(4), Cu(2)−I(2) =
2.5666(4), Cu(2A)−I(2) = 2.5611(4), Cu(2)−I(2A) = 2.5611(4),
Cu(2)−Cu(2A) = 2.6535(7), N(7)−Cu(1)−N(5) = 146.52(9),
N(7)−Cu(1)−N(1) = 117.35(9), N(7)−Cu(1)−N(3) = 114.25(9),
N(5)−Cu(1)−N(1) = 87.19(8), N(5)−Cu(1)−N(3) = 87.98(8),
N(1)−Cu(1)−N(3) = 86.73(8), I(1)−Cu(2)−I(2A) = 116.568(14),
I(1)−Cu(2)−I(2) = 125.754(15), I(2A)−Cu(2)−I(2) = 117.672(14),
Cu(2A)−I(2)−Cu(2) = 62.328.
experiment performed under ligandless conditions,21 it was
found that the yield only reached 58% after 24 h. Thus, it seems
that there is no effect of the use of well-defined or in situ
generated catalyst precursors with the Tpm* or TpmMs
derivatives from the point of view of activity. But for the
parent Tpm, the in situ strategy led to substantially higher
yields. We believe that this is the result of a readily formation of
a 1:1 TpmCu(I) adduct that would act as an effective catalyst
precursor. Although the use of yields values to assess trends in
catalytic activities must be considered with caution, we believe
that the overall picture indicates that in all cases the species
responsible for catalysis is the TpmxCu core, showing that
coordination of these ligands may exert a positive effect in
catalysis.
Next, we decided to study the scope of this Goldberg-type
condensation using only precatalyst 2, on the basis of its well-
defined 1:1 Cu/ligand stoichiometry. The coupling reactions of
pyrrolidinone with different aryl iodides were carried out using
5 mol % of 2 and K3PO4 as the base in dioxane at 110 °C
(Scheme 4). Electron-deficient and electron-rich para-sub-
stituted aryl iodides were suitable for this reaction, providing N-
arylated products in high yields. 2-Methoxy-1-iodobenzene
afforded lower yield of the coupling product (62%), but the
reaction yield could be significantly improved (up to 85%) by
increasing the amount of catalyst employed (10 mol %). In
addition, under the present reaction conditions the system was
chemoselective toward iodides, providing a complete replace-
ment of the iodo-group by the nucleophile in the presence of a
bromo substituent on the aryl ring.
complexes 1−3 were active (Table 1, entries 1−3), although
with variable efficiency. Taking into account the above-
mentioned insolubility of complexes 1 and 3 in most organic
solvents, we studied the effect of the solvent in this cross-
coupling process. Thus, in toluene only 2 induced the reaction,
affording the expected coupling product in 86% yield (entry 4).
In more polar solvents such as MeCN and DMSO, the catalytic
activity of 3 remained unchanged but that of 1 was increased as
the result of the solubility enhancement of the latter in these
polar solvents (entries 5−9). Next, we decided to compare the
catalytic performance of well-defined precatalysts 1−3 with that
of in situ prepared catalysts. For that purpose, the coupling
reactions were performed in dioxane in the presence of 5 mol %
of CuI and 5 mol % of the ligand. It was found that in the case
of Tpm* and TpmMs ligands similar yields of the coupling
product were obtained under both protocols (entries 2, 3 vs 11,
12). On the contrary, the in situ made catalyst with the Tpm
ligand resulted to be more effective than the preformed
complex 1, yielding the product in quantitative yield (entries 1
vs 10). Moreover, an experiment carried out using complex 1
and two additional equivalents of the ligand Tpm to match the
overall 1:1 Cu:Tpm ratio, afforded the coupling product in 65%
yield (entry 13), almost identical to that found when 1 was
employed as the precatalyst (see entry 1). In a blank
A comparison between the results illustrated in Scheme 4
and those reported by Chen et al.10 using the in situ prepared
CuI/1,1,1-tris(hydroxymethyl)ethane system revealed that our
well-defined system behaves similarly. Thus, the coupling
reaction of iodobenzene and pyrrolidinone in dioxane afforded
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Scheme 4. Coupling of ArI with 2-Pyrrolidinone in the
Presence of 2
Table 2. Coupling of ArI with Thiophenol in the Presence of
2
a b
,
a
a
Reaction conditions (as in Table 1): ArI (1 mmol), 2-pyrrolidinone
(1.2 mmol), K3PO4 (2 mmol), precatalyst 2 (5 mol %), dioxane (1
b
c
mL), 110 °C, 24 h. Isolated yields (average of two runs). Reaction
carried out with using 10 mol % of 2.
80% yield of the coupling product, whereas the CuI/1,1,1-
tris(hydroximethyl)ethane led to a 81% yield of the
corresponding product under the same reaction conditions
(K3PO4 as the base in dioxane at 110 °C for 24 h) but using 10
mol % catalyst loading. But, the Tpm*CuI precatalyst is
considerably less active than Buchwald’s catalytic system based
on the use of CuI and excess of a diamine ligand.2c Under
similar reaction conditions (temperature and time) the latter
afforded quantitative yields of the amidation products by using
1 mol % of the copper source. However, our protocol avoids
the removal of free ligand in the reaction workup and also
prevents the formation of different CuLn species that could
affect the concentration of the active catalytic species in
solution. In any case, our aim in this work is to demonstrate
that tridentate Tpmx ligands are beneficial to this reaction, in
order to provide a new window for future catalyst development.
We have also extended this protocol to C−S bond-forming
reactions22 by examining the arylation of thiols with aryl iodides
with complex 2 as the catalyst precursor. The coupling
reactions were carried out under the optimal conditions
found for the N-arylation reactions but with LiOtBu (2
equiv) as the base. In order to minimize the formation of
undesired phenyl disulfide (C6H5−S−S−C6H5) the experi-
ments were done with a slight excess of the aryl iodide. The
desired thioethers were prepared in good to excellent yields as
shown in Table 2. We found that aryl iodides bearing electron-
withdrawing groups at the para position were more prone to
undergo this transformation (entries 3−5) than those bearing
electron-donating substituents on that position (entries 6 and
7). In this regard, the coupling reaction of thiophenol and 4-
methoxy-1-iodobenzene furnished quantitative yield of the S-
arylated product in the presence of 10 mol % of precatalyst 2.
Like in the N-arylation of pyrrolidinone (see above), full
selectivity toward iodo derivatives was highlighted on the
coupling of 1-bromo or 1-chloro-4-iodobenzene with thio-
phenol affording the corresponding 4-bromo- or 4-chlorophen-
yl phenyl thioether as the only product (entries 3 and 4). In
contrast to this result, the system developed by the group of
Shingare11a based on the use of CuI and the ligand tris(2-
aminoethyl)amine has been reported to be active in the
coupling of thiols with both aryl iodides and aryl bromides. An
increase in the catalyst loading was required with a sterically
hindered substrate such as 2-iodoanisol (entry 8).
a
Reaction conditions: ArI (1.2 mmol), thiophenol (1.0 mmol),
LiOtBu (2 mmol), precatalyst 2 (5 mol %), dioxane (1 mL), 110 °C,
24 h. bIsolated yields (average of two runs). cReaction carried out with
using 10 mol % of 2.
However, up to date it is not possible to make a unique
mechanistic proposal that satisfactorily explains the entire
catalytic cycle. Experimental evidence6a−c,7,23 and theoretical
calculations6d,24support the formation of a ligated Cu(I)-
nucleophile intermediate in the initial step of the reaction
(Scheme 5). In this regard, complexes of the type LCu(I)-
Scheme 5. Simplified Catalytic Cycle for Copper-Catalyzed
Ullmann and Goldberg Reactions
nucleophile (nucleophile = amidate,6a−c imidate,6a−c amido,6a−c
phenoxide,7 and pyrazolate23d) containing a bidentate o
monodetate ligand have been prepared, characterized, and
shown to react with aryl halides to form the corresponding
coupling products. But how the activation of the aryl halide by
the Cu(I)-nucleophile proceeds is still a matter of controversy.
For example, experiments performed in the presence of a
radical clock argued in favor of an oxidative addition pathway to
form a Cu(III)-aryl(nucleophile) intermediate.6,7,23a Such
species have not been yet detected nor isolated from the
reaction mixture, although well-defined Cu(III)-aryl complexes
have been reported recently to react with N-and O-base
nucleophiles to afford C−N and C−O coupling products.25 On
the other hand, very recently Buchwald et al.26 have examined
the selectivity in Ullmann-type reactions by DFT calculations
Synthesis of Cu(I)−Imidate Complex and Reactivity
with Iodobenzene. In the past few years, different studies
have been carried out with the aim to elucidate the mechanism
of copper-catalyzed Goldberg and Ullmann-type reactions.
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coming to the conclusion that a Cu(I)/Cu(II) radical pathway
is energetically favored over the formation of a Cu(III)-aryl
intermediate.
range of 7.4−8.0 ppm (Figure 4a). One group of these
resonances could be assigned to the anionic species [Cu-
Since examples of LCu(I) amidate and imidate compounds
containing bidentate and monodentate ligands are known, we
decided to prepare similar complexes with the tripod Tpm*
ligand. The synthesis was carried out following a two-step
procedure similar to that reported for the preparation of (1,2-
diamine)Cu(I)-pyrrolidinonate.6a Initially, we attempted to
synthesize the Tpm*Cu-pyrrolidonate, but it decomposed
when trying to isolate it. Thus, we decided to prepare the
phtalimidate derivative instead. First, Cu(I)-phthalimidate, 4,
was obtained from the reaction of mesitylcopper27 and
phthalimide as a pale yellow solid in 90% yield (Scheme 6).
Scheme 6. Synthesis of the Tpm*−Cu(I) Phthalimidate
Complex
Figure 4. 1H NMR spectra of a solution of 5 in CDCl3 at (top) room
temperature and (bottom) −40 °C.
The room temperature 1H NMR spectrum of 4 in C6D6
consists of two well-resolved doublet of doublets centered at
7.34 and 6.79 ppm, in contrast to the spectrum of the related
Cu(I)-pyrrolidinonate for which the broadening of resonances
are attributed to a rapid exchange between oligomeric species in
solution.6a In addition, elemental analysis of 4 is in agreement
with a 1:1 Cu/phthalimidate ratio. Then, the Tpm* ligand was
added to a THF solution of 4, a slightly cloudiness being
observed within 15 min of reaction. Evaporation of solvent
yielded compound 5 as a yellow solid of analytical purity that
reflected a 1:1 ratio of the Tpm* ligand and the phthalimidate
(phth) moiety. Any attempt to crystallize the complex led to
decomposition preventing the elucidation of its molecular
structure in the solid state. It has been described that in
solution copper(I) amidate and imidate complexes ligated by
bidentate chelators form ion pairs consisting on a L2Cu+cation
1
(phth)2]− by the direct comparison with the H NMR of the
complex [NBu4][Cu(phth)2] recorded in CDCl3 (two
multiplets centered at δ 7.63 and 7.49). Cooling the sample
from 20 °C to −40 °C did not produce a significant change in
the resonances of the phth ligands but led to the emergence in
the spectrum of two patterns of signals for the methyl protons
of the Tpm* ligand in a 2:1 ratio (Figure 4b). This observation
together with the appearance of a new broad signal in the low
field region of the spectrum (ca. 8.7 ppm) suggested the
existence of Tpm* ligands in a different chemical environment.
The pattern of Tpm* signals of greater intensity was very
similar to that of the free ligand, whereas the minor Tpm*
species seemed to be related by integration with the phth ligand
that appeared at lower field. The insolubility of 5 in a more
and a CuX2 anion.6b To check such possible behavior for
appropriate solvent to record lower temperature H NMR
−
1
compound 5, conductivity measurements were undertaken.
Thus, the molar conductivity of a 1.0 mM solution of 5 in
DMSO was 38.0 Ω−1 cm2 mol−1, a value higher than those
measured for ferrocene (1.39 Ω−1 cm2 mol−1) or [NBu4]-
[BPPh4] (22.9 Ω−1 cm2 mol−1) as standards, and similar to that
of the complex [NBu4][Cu(phth)2]6b (40.6 Ω−1 cm2 mol−1).
We interpret these data as evidence that complex 5
predominantly forms ion pairs in polar solvents. Unfortunately,
we could not gain any information about the molecular
structure of 5 in a less polar solvent, such as THF due to its
poor solubility in this medium.
spectra precluded collecting more information about the
structure of all the species presented in solution. However,
the data available so far allow proposing that CDCl3 solutions
of 5 consist of a mixture of a neutral form, Tpm*Cu(phth), and
an ionic form. The latter contains [Cu(phth)2]− as the anion,
but the cationic part of the ion pair is not formed by the
coordination of two molecules of the tripod Tpm* ligand to the
metal atom, since Cu(I) does not favor an octahedral
environment. Instead, we believe the cationic moiety contains
only one molecule of the Tpm* ligand coordinated to the
copper(I) center. The observation of the free Tpm* ligand in
solution seems to support this proposal.
Once the solution structure of complex 5 was established, we
examined its reactivity toward iodobenzene. The reaction of a
dioxane solution of 5 with 1 equiv of iodobenzene afforded the
coupling product in 76% yield (isolated) after 24 h at 110 °C
(eq 1). Interestingly, the coupling of phthalimide with
iodobenzene catalyzed by 5 mol % of Tpm*CuI under the
same conditions (temperature and reaction time) produced the
coupling product with a very low yield. These contrasting
NMR experiments were conducted to get a better knowledge
about the identity of the species in solution. The 1H NMR of 5
in DMSO-d6 did not provide any useful information since
resonances for both the anionic (phth) and the Tpm* ligands
were very broad indicating that exchange processes were taking
1
place. More informative was the room temperature H NMR
spectrum of 5 recorded in CDCl3. Despite the broadness of the
signals, two sets of resonance patterns for two different anionic
phth ligands in ca. 1:1 ratio were clearly distinguished in the
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Inorganic Chemistry
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Synthesis of Tpm*CuI (2). Following the same procedure
1
complex 2 was obtained as a white solid (0.49 g, 99%). H NMR
(400 MHz, DMSO-d6): δ 7.64 (s, 1H), 5.85 (s, 3H), 2.39 (s, 18H).
13C{1H} NMR (100 MHz, DMSO-d6): δ 150.5, 140.8, 107.1, 68.7,
14.2, 11.0. Anal. Calcd. for C16H22N6CuI: C, 39.32; H, 4.54; N, 17.19.
Found: C, 39.00; H, 4.54; N, 17.70
Synthesis of TpmMs2Cu4I4(NCMe)3 (3). Following the same
procedure, complex 3 was obtained as a white solid (0.42 g, 84%).1H
NMR (400 MHz, DMSO-d6): δ 9.59 (s, 1H), 8.37 (s, 3H), 6.89 (s,
6H), 6.50 (s, 3H), 2.20 (s, 9H), 2.05 (s, 3H), 1.88 (s, 18H). 13C{1H}
NMR (100 MHz, DMSO-d6): δ 153.2, 138.6, 137.1, 134.1, 128.9,
128.4, 118.1, 108.0, 67.2, 21.1, 20.2, 1.6. Anal. Calcd. for
C84H95N17Cu4I4: C, 47.95; H, 4.54; N, 11.31. Found: C, 47.94; H,
4.55; N, 11.31.
results can be explained if we assume that the stoichiometric
reaction shown in eq 1 corresponds to a step beyond the rate
determining step in the catalytic process, which complex 5 can
be proposed as an intermediate in this transformation.
It is worth mentioning that the reaction shown in eq 1
slightly differs from the catalytic reaction. The Cu-amidate
species, during catalysis, is formed upon the action of the base.
In this stoichiometric reaction, the Cu-amidate species has
already been formed, and therefore we have no base in the
reaction mixture. This experiment resembles those by Hartwig
and Buchwald,6 with bidentate ligands. As mentioned above, we
have tried to isolate the corresponding copper-pyrrolidonate
compound but failed to characterize it due to its instability.
However, upon generation and fast workup (as for the phth
analog), reaction with iodobenzene was carried out, verifying
the formation of the cross coupling product in a shorter
reaction time than in the catalytic reaction. We believe that
these experiments support the intermediacy of these amidate
species in the catalytic cycle.
Synthesis of Copper(I)-Phthalimidate (4). To a toluene
solution (5 mL) of mesitylcopper (115 mg, 0.633 mmol) was added
phthalimide (102 mg, 0.7 mmol). The colorless mixture was stirred for
30 min before volatiles were removed under vacuum. The residue was
washed with THF (20 mL) and dried under vacuum to afford complex
1
4 (128 mg, 90%) as an air-sensitive pale yellow solid. H NMR (400
MHz, C6D6): δ 7.62 7.34 (dd, J = 5.4, 3.1 Hz, 2H), 6.79 (dd, J = 5.5,
3.0 Hz, 2H). 13C{1H}NMR (100 MHz, DMSO-d6): δ 179.8, 136.5,
132.8, 121.6. Anal. Calcd. for C8H4CuNO2·0.2THF: C, 47.16; H, 2.50;
N, 6.25. Found: C, 47.09; H, 2.89; N, 5.93.
Preparation of Tpm*Cu-(phthalimidate) (5). Inside a glovebox,
a solution of copper(I)-phthalimidate (0.05 mmol) in THF (0.5 mL)
was placed into a vial equipped with a magnetic stirbar. A THF
solution of ligand (0.05 mmol) was then added. After 15 min, a fine
precipitated was formed. The reaction mixture was stirred for 2 h at
ambient temperature. The solvent was evaporated, and complex 5 was
CONCLUSION
■
1
obtained as a bright yellow solid in quantitative yield. H NMR (400
In summary, we have synthesized and fully characterized new
Cu(I) complexes containing tripod ligands of type tris-
(pyrazolyl)methane, Tpmx. The Cu/ligand stoichiometry
encountered for these complexes markedly depends on the
steric properties of the Tpmx ligand. Only in the case of Tpm*,
a neutral, tetrahedral compound Tpm*CuI, with a 1:1 metal to
ligand ratio, was obtained. This well-defined complex was used
as the precatalyst in the arylation of amides and aromatic thiols,
allowing the efficient coupling of substrates under conditions
similar to those described for CuI/tridentate ligand mixtures.
Tpm*Cu-phthalimidate complex has also been prepared and
been shown to be the active species in the arylation of amides
with Tpm*CuI precatalyst. The design of special Tpmx ligands
that could provide stability to those copper-amidates is in
progress in our laboratory.
MHz, CDCl3): δ 7.93 (s, 1H), 7.85 (s, 1H), 7.75 (s, 1H), 7.68 (s, 1H),
7.47 (s, 1H), 5.86 (s, 3H), 2.23 (s, 9H), 2.13 (s, 9H). 13C{1H}NMR
(100 MHz, CDCl3): δ 168.0, 134.7, 133.2, 131.3, 124.2, 121.2, 107.3,
67.2, 13.7, 11.4. Anal. Calcd. for C24H26CuN7O2: C, 56.75; H, 5.12; N,
19.31. Found: C, 56.64; H, 5.68; N, 18.61.
General Catalytic Procedure for the N-Arylation of
Pyrrolidinone with Aryl Iodides. The catalyst (0.05 mmol) was
dissolved in dioxane (1 mL) in an ampule. The aryl iodide (1.0 mmol),
2-pyrrolidinone (1.2 mmol), and the base, K3PO4 (2 mmol), were
added under a nitrogen atmosphere. The reaction was stirred at 110
°C for 24 h in an oil bath. The reaction mixture was allowed to cool to
room temperature, diluted with ethyl acetate (15 mL), and centrifuged
for 5 min. The clean solution was evaporated to dryness, and the
residue was purified by flash chromatography on silica gel.
General Catalytic Procedure for the S-Arylation of Thiols
with Aryl Iodides. The catalyst (0.05 mmol) was dissolved in
dioxane (1 mL) in an ampule. The aryl iodide (1.2 mmol), the thiol
(1.0 mmol), and the base, LiOtBu (2 mmol), were added under a
nitrogen atmosphere. The reaction was stirred at 110 °C for 24 h in an
oil bath. The reaction mixture was allowed to cool to room
temperature and treated with ethyl acetate (15 mL) and water (5
mL). The organic and aqueous layers were then separated, and the
organic layer was evaporated to dryness. The residue was purified by
flash chromatography on silica gel.
X-ray Crystal Determinations. Data associated with the crystal
structures of 1−3 are summarized in Tables S1, S7, and S13,
respectively (see the Supporting Information). A single crystal of 1, 2,
and 3 suitable for X-ray diffraction studies (obtained at −20 °C from
acetonitrile solutions), coated with dry perfluoropolyether, was
mounted on a glass fiber and fixed in a cold nitrogen stream [T =
173(2) K] to the goniometer head. Data collection29 was carried out
on a Bruker-Nonius X8 kappa APEX II CCD area-detector
diffractometer using graphite-monochromatic radiation λ(Mo Kα) =
0.71073 Å, by means of ω and φ scans with narrow frames. Data
reduction was performed using SAINT29 and corrected for Lorentz
polarization effects and absorption by a multiscan method applied by
SADABS.30 The structure was solved by direct methods (SIR-2002)31
and refined against all F2 data by full-matrix least-squares techniques
with SHELXTL.32 All non-hydrogen atoms were refined with
EXPERIMENTAL SECTION
■
General Methods. All reactions and manipulations were carried
out under an oxygen-free nitrogen atmosphere with standard Schlenk
techniques. All substrates were purchased from Aldrich. Solvents were
dried and degassed before use. Tris(pyrazolyl)methane ligands,28
mesitylcopper,27 and [NBu4][Cu(phth)2]6b (phth = phthalimidate)
were prepared according to the literature procedures. NMR spectra
were recorded on a Varian Mercury 400 MHz spectrometer. 1H NMR
shifts were measured relative to deuterated solvents peaks but are
reported relative to tetramethylsilane. Elemental analyses were
performed in Unidad de Anal
Huelva.
́
isis Elemental of the Universidad de
Synthesis of Tpm2Cu4I4(NCMe)2 (1). To a stirred solution of CuI
(0.190 g, 1 mmol) in acetonitrile (10 mL) was added a solution of the
Tpm ligand (0.248 g, 1 mmol) in acetonitrile (5 mL). Precipitation of
a white solid was immediately observed. The mixture was stirred for 2
h, and complex 1 was filtered, dried under vacuum, and obtained as a
white solid (0.28 g, 87%).1H NMR (400 MHz, DMSO-d6): δ 9.36 (s,
2H), 8.14 (s, 6H), 7.80 (s, 6H), 6.51 (s, 6H), 2.08 (s, 6H). 13C{1H}
NMR (100 MHz, DMSO-d6): δ 142.9, 132.7, 118.9, 107.7, 78.7, 1.8.
Anal. Calcd. for C24H26N14Cu4I4: C, 22.26; H, 2.06; N, 15.41. Found:
C, 22.65; H, 2.12; N, 15.32.
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Inorganic Chemistry
Article
anisotropic displacement parameters. The hydrogen atoms were
included from calculated positions and refined riding on their
respective carbon atoms with isotropic displacement parameters.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures, analytical and spectroscopic
data. and crystallographic information files in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: perez@dqcm.uhu.es.
Notes
The authors declare no competing financial interest.
(6) (a) Strieter, E. R.; Blackmond, D. G.; Buchwald, S. L. J. Am.
Chem. Soc. 2005, 127, 4120. (b) Tye, J. W.; Weng, Z.; Johns, A. M.;
Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 9971.
(c) Strieter, E. R.; Bhayana, B.; Buchwald, S. L J. Am. Chem. Soc. 2009,
131, 78. (d) Zhang, S.-L.; Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics
2007, 26, 4546.
ACKNOWLEDGMENTS
■
́
We thank Junta de Andalucia (Proyecto P07-FQM-02745) and
MICINN (Proyectos CTQ2011-24502 and CTQ2011-28942-
C02-01) for financial support. E.H. thanks MEC for a research
(7) Tye, J. W.; Weng, Z.; Giri, R.; Hartwig, J. F. Angew. Chem., Int. Ed.
2010, 49, 2185.
́
fellowship. Prof. Tomas R. Belderrain is gratefully acknowl-
edged for assistance with VT NMR studies. Thanks are also due
to Universidad de Huelva and Instituto de Investigaciones
(8) (a) Cristau, H.-J.; Ouali, A.; Spindler, J.-F.; Taillefer, M. Chem.
Eur. J. 2005, 11, 2483. (b) Ouali, A.; Spindler, J.-F.; Cristau, H.-J.;
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A.; Renard, B.; Spindler, J.-F. Chem.Eur. J. 2006, 12, 5301. (d) Ouali,
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J. Org. Chem. 2010, 1854. (c) Wu, Z.; Zhou, L.; Jiang, Z.; Wu, D.; Li,
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