Synthesis, structural characterization, reactivity, and catalytic properties of copper(I) complexes with a series of tetradentate tripodal tris(pyrazolylmethyl)amine ligands

Article abstract of DOI:10.1021/ic500311y

Novel tris(pyrazolylmethyl)amine ligands Tpa^Me3, Tpa*^,Br, and Tpa^Br3 have been synthesized and structurally characterized. The coordination chemistries of these three new tetradentate tripodal ligands and the already known Tpa and Tpa* have been explored using different copper(I) salts as starting materials. Cationic copper(I) complexes [Tpa^xCu]PF₆ (1-4) have been isolated from the reaction of [Cu(NCMe)₄]PF₆ and 1 equiv of the ligand. Complexes 2 (Tpa^x = Tpa*) and 3 (Tpa^x = Tpa^Me3) have been characterized by X-ray studies. The former is a 1D helical coordination polymer, and the latter is a tetranuclear helicate. In both structures, the Tpa^x ligand adopts a μ²: κ²:κ¹-coordination mode. However, in solution, all of the four complexes form fluxional species. When CuI is used as the copper(I) source, neutral compounds 5-8 have been obtained. Complexes 6-8 exhibit a 1:1 metal-to-ligand ratio, whereas 5 presents 2:1 stoichiometry. Its solid-state structure has been determined by X-ray diffraction, revealing its 3D polymeric nature. The polymer is composed by the assembly of [Tpa ₂Cu₄I₄] units, in which Cu₄I ₄ presents a step-stair structure. The Tpa ligands bridge the Cu ₄I₄ clusters, adopting also a μ²: κ²:κ¹-coordination mode. As observed for the cationic derivatives, the NMR spectra of 5-8 show the equivalence of the three pyrazolyl arms of the ligands in these complexes. The reactivities of cationic copper(I) derivatives 1-4 with PPh₃ and CO have been explored. In all cases, 1:1 adducts [Tpa^xCuL]PF₆ [L = PPh₃ (9-11), CO (12-15)] have been isolated. The crystal structure of [Tpa*Cu(PPh₃)]PF₆ (9) has been obtained, showing that the coordination geometry around copper(I) is trigonal-pyramidal with the apical position occupied by the tertiary amine N atom. The Tpa* ligand binds the Cu center to three of its four N atoms, with one pyrazolyl arm remaining uncoordinated. In solution, the carbonyl adducts 13-15 exist as a mixture of two isomers; the four- and five-coordinate species can be distinguished by means of their IR ν_CO stretching bands. Finally, the catalytic activities of complexes 1-4 have been demonstrated in carbene- and nitrene-transfer reactions.

Full text of DOI:10.1021/ic500311y

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Article
pubs.acs.org/IC
Synthesis, Structural Characterization, Reactivity, and Catalytic
Properties of Copper(I) Complexes with a Series of Tetradentate
Tripodal Tris(pyrazolylmethyl)amine Ligands
†
†
‡
§
§
́
Estela Haldon, Manuela Delgado-Rebollo, Auxiliadora Prieto, Eleuterio Alvarez, Celia Maya,
́
M. Carmen Nicasio,*,⊥ and Pedro J. Perez*,†
́
†Laboratorio de Catal
́
isis Homogen
Departamento de Química y Ciencias de los Materiales, Universidad de Huelva, Campus de El Carmen s/n, 21007 Huelva, Spain
‡Laboratorio de Catal
isis Homogenea, Unidad Asociada al CSIC, CIQSO-Centro de Investigacion en Química Sostenible and
Departamento de Ingeniería Química, Química Física y Química Organica, Universidad de Huelva, Campus de El Carmen s/n, 21007
Huelva, Spain
§Instituto de Investigaciones Químicas Isla de la Cartuja, CSIC-Universidad de Sevilla, Avenida de Amer
́ ́
ea, Unidad Asociada al CSIC, CIQSO-Centro de Investigacion en Química Sostenible and
́
́
́
́
́
ico Vespucio 49, 41092
Sevilla, Spain
⊥Departamento de Química Inorgan
́
ica, Universidad de Sevilla, Apartado 1203, 41071 Sevilla, Spain
S
* Supporting Information
ABSTRACT: Novel tris(pyrazolylmethyl)amine ligands TpaMe3, Tpa*,Br, and
TpaBr3 have been synthesized and structurally characterized. The coordination
chemistries of these three new tetradentate tripodal ligands and the already known
Tpa and Tpa* have been explored using different copper(I) salts as starting
materials. Cationic copper(I) complexes [TpaxCu]PF6 (1−4) have been isolated
from the reaction of [Cu(NCMe)4]PF6 and 1 equiv of the ligand. Complexes 2
(Tpax = Tpa*) and 3 (Tpax = TpaMe3) have been characterized by X-ray studies.
The former is a 1D helical coordination polymer, and the latter is a tetranuclear
helicate. In both structures, the Tpax ligand adopts a μ2:κ2:κ1-coordination mode.
However, in solution, all of the four complexes form fluxional species. When CuI
is used as the copper(I) source, neutral compounds 5−8 have been obtained.
Complexes 6−8 exhibit a 1:1 metal-to-ligand ratio, whereas 5 presents 2:1
stoichiometry. Its solid-state structure has been determined by X-ray diffraction,
revealing its 3D polymeric nature. The polymer is composed by the assembly of
[Tpa2Cu4I4] units, in which Cu4I4 presents a step-stair structure. The Tpa ligands bridge the Cu4I4 clusters, adopting also a
μ2:κ2:κ1-coordination mode. As observed for the cationic derivatives, the NMR spectra of 5−8 show the equivalence of the three
pyrazolyl arms of the ligands in these complexes. The reactivities of cationic copper(I) derivatives 1−4 with PPh3 and CO have
been explored. In all cases, 1:1 adducts [TpaxCuL]PF6 [L = PPh3 (9−11), CO (12−15)] have been isolated. The crystal
structure of [Tpa*Cu(PPh3)]PF6 (9) has been obtained, showing that the coordination geometry around copper(I) is trigonal-
pyramidal with the apical position occupied by the tertiary amine N atom. The Tpa* ligand binds the Cu center to three of its
four N atoms, with one pyrazolyl arm remaining uncoordinated. In solution, the carbonyl adducts 13−15 exist as a mixture of
two isomers; the four- and five-coordinate species can be distinguished by means of their IR νCO stretching bands. Finally, the
catalytic activities of complexes 1−4 have been demonstrated in carbene- and nitrene-transfer reactions.
because they have been used as models for reaction centers in
proteins such as those involved in oxygen activation.5
INTRODUCTION
■
Copper(I) complexes containing polydentate nitrogen ligands1
display a rich structural chemistry as a result of its d10 electronic
configuration, which enables it to adopt a great variety of
coordination numbers and geometries. This flexible coordina-
tion ability has allowed the assembly of complex supra-
molecular architectures2 with interesting photoluminescent
properties3 and potential applications in material science.4 In
addition, copper(I) complexes with tripod nitrogen ligands
have enjoyed considerable success in bioinorganic chemistry
Another important field in which these coordination
compounds have been largely applied is catalysis. Within this
context, our research group has been investigating, for the last
few decades, the catalytic properties of families of copper(I)
complexes with tris(pyrazolyl)borate6 (Tpx) and tris-
(pyrazolyl)methane7 (Tpmx) ligands (Figure 1) in an array of
Received: February 7, 2014
Published: April 3, 2014
© 2014 American Chemical Society
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Scheme 1
Figure 1. Ligands frequently used for our research group.
catalytic processes that span from carbene- and nitrene-transfer
reactions,8−10 oxidation reactions,11 atom-transfer radical
addition12 and atom-transfer radical polymerization13 reactions,
Ullmann and Golberg type condensations,14 and copper-
catalyzed azide−alkyne cycloaddition (CuAAC) reactions.15
While exploring the activity of the precatalyst [Tpm*,BrCu-
containing a bromo substituent on the pyrazolyl ring, their
synthesis required heating of the corresponding hydroxyme-
thylpyrazolyl precursors with ammonium acetate at 70 °C for 3
(Tpa*,Br) and 6 days (TpaBr3), respectively.
16
(MeCN)]BF4 in the reaction of N-carbonylazides and
The three new ligands were characterized by elemental
1
-alkynes, we discovered a novel transformation that led to the
formation of 2,5-disubstituted oxazoles rather than the expected
1,4-disubstituted N-carbonyltriazoles.17 In order to improve
both the activity and selectivity of the catalyst, different Tpmx
ligands were tested with no success. At this point, we
considered the preparation of a novel copper(I) precatalyst,
focusing our attention on a family of tetradentate tripodal
ligands, the tris(pyrazolylmethyl)amine (Tpax) ligands18 (Chart
1) structurally related with tripod Tpx and Tpmx ligands. The
analyses and NMR spectroscopy. Their H NMR spectra in
CD3CN displayed a single resonance in the range of 4.85−5.50
ppm assigned to methylene protons, whereas the correspond-
ing 13C{1H} NMR signals were found in the 63.1−68.2 ppm
range. The structures of Tpa*,Br and TpaBr3 were also
determined by X-ray crystallography (Figure 2a,b). In both
structures, the geometry around the tertiary amine N atom is
almost tetrahedral. The mean bond distances and angles are
near identical in both ligands and in agreement with those
18c
reported for Tpa*18c and TpaiPr2
.
As noted from crystallo-
Chart 1
graphic data, substituents on the pyrazolyl ring do not exert
much influence on the ligand structure.
Tpax + [Cu(NCMe)4]PF ⎯M⎯⎯e⎯O⎯⎯→H [TpaxCu]PF
6
6
1,Tpax=Tpa
2,Tpax=Tpa*
rt
3,Tpax=TpaMe3
x
,Br
*
4,Tpa =Tpa
(1)
Synthesis of Complexes. Cationic copper complexes 1−4
were prepared following the procedure reported18c for the
synthesis of [Tpa*Cu]PF6 (2), i.e., by reacting [Cu(NCMe)4]-
PF6 with equimolar amounts of the Tpax ligands in methanol
(MeOH) at room temperature (eq 1). All attempts to
synthesize the TpaBr3-CuI derivative resulted in the unsuccessful
recovery of only unreacted starting materials at the end of the
reaction. This could be attributed to the poor donating
properties of the TpaBr3 ligand due to the presence of nine Br
atoms in this ligand.
choice of this type of ligand was dictated primarily by the need
to keep a coordination environment similar to that found for
the Tpm*Cu derivative but also by the acceleration effect
exerted by tris(heterocyclemethyl)amine ligands in CuAAC
reactions.19 Furthermore, the electronic and steric properties of
the ligand could be easily tuned by changing the nature of the
substituents on the pyrazole rings. Unlike Tpx and Tpmx
ligands, the coordination chemistry of Tpax ligands with
transition metals has been much less explored.20 However,
some copper(II),18c,21 and to a lesser extent copper(I),18c,21a
coordination complexes are known, but only the crystal
structures of two mononuclear Tpa-CuI species have been
reported to date.18c,21a
Complexes 1−4 precipitated from MeOH solutions and were
isolated as white solids in high yields. They were only soluble in
acetonitrile and dimethyl sulfoxide (DMSO) and could be
1
exposed to air while in the solid state but not in solution. H
We describe here the synthesis of a series of tris-
(pyrazolylmethyl)amine ligands (Chart 1) and the preparation
and spectroscopic characterization of several cationic and
neutral Tpax-CuI complexes. The solid-state crystal structures
of some of these copper(I) derivatives have been determined,
revealing their multinuclear nature. The reactivity of the new
complexes toward CO and PPh3 has also been studied. Finally,
a survey on the catalytic activity of Tpax-CuI complexes in
standard carbene- and nitrene-transfer reactions has been
undertaken.
NMR spectra of 1, 3, and 4 in CD3CN evidenced coordination
of the ligands to the Cu ion, as inferred from the slight
downfield shifts of the resonances of pyrazolyl rings with
respect to those observed for the free ligands. Moreover, the
observation of a unique set of signals for all three pyrazolyl
arms of the Tpax ligands in their 1H and 13C{1H} NMR spectra
indicated the existence of a fluxional process in solution (see
the Experimental Section). No other resonances were observed,
in spite of the use of [Cu(NCMe)4]PF6 as the copper source.
In good agreement with this, IR spectra of 1, 3, and 4 did not
exhibit any absorption in the region due to νCN bands, and
elemental analyses also supported this observation.
RESULTS AND DISCUSSION
■
Synthesis of Ligands. Tpax ligands were prepared in good
yields by the same procedure as that described for the synthesis
of Tpa18b and Tpa*18c derivatives (Scheme 1). For those
Fujisawa et al. proposed a tetradentate κ4-coordination mode
of the Tpa* ligand in 2 based on its analytical and spectroscopic
features. Considering that such data matched those found for
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Figure 2. Molecular structures of Tpa*,Br (a) and TpaBr3 (b). 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 Tpa*,Br: C1−N1, 1.446(5); N1−C1#1, 1.446(5); N1−C1#2, 1.446(5); C1−N1−
C1#1, 113.2(3); C1−N1−C1#2, 113.2(3); C1#1−N1−C1#2, 113.2(3). Selected bond lengths (Å) and bond angles (deg) for TpaBr3: N1−C1,
1.421(8); N1−C5, 1.473(8); N1−C9, 1.449(7); C1−N1−C9, 116.9(5); C1−N1−C5, 115.9(5); C9−N1−C5, 113.2(5).
Figure 3. (a) ORTEP representation of the coordination environment around the CuI centers in 2. H atoms have been omitted for clarity. Selected
bond lenghts (Å) and angles (deg) for 2: Cu1−N3, 1.989(3); Cu1−N5, 2.004(4); Cu1−N7#, 1.964(3); N7#−Cu1−N3, 128.11(15); N7#−Cu1−
N5, 118.79(14); N3−Cu1−N5, 112.70(14). (b) Structure of the 1D helices in 2 that run along the b axes.
complexes 1, 3, and 4, it was reasonable to suggest that Tpax
ligands in these complexes coordinated the CuI ion in the same
fashion. To further confirm this proposal, we decided to
determine the solid-state structures of 1−4. Unfortunately, only
single crystals of 2 and 3 suitable for X-ray diffraction studies
could be obtained so far. To our surprise, the two copper
complexes were not mononuclear in nature but polynuclear
copper(I) derivatives. The crystal structure of 2 (Figure 3)
shows that this material is an infinite one-dimensional (1D)
helical coordination polymer of composition {[Tpa*Cu]-
PF6}∞.2a,e,4c,22 Each CuI ion is coordinated to two pyrazole N
atoms of one Tpa* ligand and a pyrazole N atom of another
ligand, forming a distorted trigonal-planar CuN3 arrangement,
with the N−Cu−N angles varying from 112.7 to 128.1° (Figure
3). The Cu−N distances range from 1.964 to 2.004 Å (average
1.986 Å) and are similar to those reported for mononuclear
Tpa-CuI species.23 The apical amine N of each Tpa* ligand
remains unbound, resulting in a μ2:κ2:κ1-coordination mode for
the Tpa* moiety. The coordination scheme described above
affords 1D helical chains, with an equal amount of each
enantiomer in the crystal. The Cu−Cu separation between
metals linked by the same bridging Tpa* ligand is 6.840 Å
excluding any interaction between the two metal ions.
The X-ray crystal structure of 3 (Figure 4) discloses the
compound to be a tetramer with a tetranuclear circular helical
architecture.24 As in complex 2, each CuI ion is coordinated to
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However, the solid-state structures found for 2 and 3 were
not maintained in solution, and NMR spectra of both polymer
and tetranuclear compounds in CD3CN at room temperature
showed the equivalence of all three pyrazolyl arms of the
ligands. The observed dynamic behavior could result from a fast
equilibrium in solution between species with different
nuclearities or from a fluxional process involving association
or dissociation of the ligand in the polynuclear complexes. The
insolubility of 2 and 3 prevented the study of the behavior of
these compounds in other deuterated solvents and also
performance of variable-temperature NMR experiments.
The coordination chemistry of the Tpax ligands was also
explored using CuI as the copper(I) source. The reaction of
CuI with 1 equiv of Tpax ligands in dichloromethane afforded
colorless solutions, from which complexes 5−8 were obtained
(Scheme 2) as white solids by precipitation (5) or by
Scheme 2
Figure 4. ORTEP drawing of 3. H atoms have been omitted for clarity.
Selected bond lenghts (Å) and angles (deg) for 3: Cu1−N5, 1.965(5);
Cu1−N3, 1.989(5); Cu1−N28, 1.968(5); Cu2−N7, 1.966(6); Cu2−
N10, 1.960(6); Cu2−N12, 2.017(5); Cu3−N14, 1.963(5); Cu3−N17,
1.967(6); Cu3−N19, 2.007(6); Cu4−N21, 1.996(6); Cu4−N24,
1.958(6); Cu4−N26, 1.990(6); N5−Cu1−N28, 124.3(2); N5−
Cu1−N3, 118.0(2); N28−Cu1−N3, 117.0(2); N10−Cu2−N7,
128.4(2); N10−Cu2−N12, 118.6(2); N7−Cu2−N12, 113.0(2);
N14−Cu3−N17, 120.4(2); N14−Cu3−N19, 120.3(2); N17−Cu3−
N19, 119.3(2); N24−Cu4−N21, 122.2; N24−Cu4−N26, 119.5(2);
N26−Cu4−N21, 118.3(2).
evaporation of the solvent (6−8). Elemental analysis obtained
for 5 indicated a 2:1 copper/ligand ratio, whereas those
obtained for 6−8 were in accordance with formulations as 1:1
adducts. Presumably, the steric hindrance of the substituent
(methyl group) at the third position of the pyrazolyl groups in
complexes 6−8 favored the formation of 1:1 adducts. A similar
behavior has been observed in the reactions of CuI with the
tris(pyrazolyl)methane ligands Tpm and Tpm*.14
three pyrazole N atoms provided by two different TpaMe3 units,
adopting a distorted trigonal-planar geometry. The TpaMe3
ligand exhibits coordination behavior identical with that
observed for the Tpm* ligand in compound 2. Cu−N bond
lengths (average 1.979 Å) and bond angles (average 120.2°)
and the intermetallic Cu−Cu separation (6.921−7.157 Å) are
similar to those found for 2. A hexafluoro phosphate anion
resides inside the cavity formed by the Cu ions and the pyrazole
rings, with the remaining anions being packed around the
tetrameric units. The anion that occupies the cavity shows short
contacts with the methyl and methylene protons of the TpaMe3
ligands [see the Supporting Information (SI) for details]. A
similar encapsulation of the anions has been described for other
copper-containing coordination polymers.25
The most relevant information that could be inferred from
1H and 13C{1H} NMR spectra of 5−8 in DMSO-d6 or CDCl3
solutions was the equivalence of all three pyrazolyl arms of the
ligands in these molecules. In view of the different structural
behavior found for derivatives 2 and 3 in solution and in the
solid state and considering the well-known tendency of
copper(I) halides with chelating ligands to form multinuclear
systems with diverse structural complexity,2d,26 it was not
possible to propose a formulation for complexes 5−8 on the
basis of their analytical and spectroscopic data. Efforts to grow
crystals of X-ray quality were fruitless, leading in most cases to
decomposition of the starting compounds except for 5, whose
solid-state structure could be resolved. X-ray structural analysis
reveals a three-dimensional (3D) polymeric network con-
structed with {(Tpa)2Cu4I4} units (Figure 5). Each [Cu4I4]
core is formed by two planar rhomboid [Cu2I2] clusters
constituted by two μ2-I atoms linking two CuI atoms. They are
connected through two Cu−I interactions [2.780(1) Å], giving
rise to a “step stair” [Cu4I4] fragment (Figure 5). These
fragments are cross-linked through bridging Tpa ligands,
forming the infinite network. Each of the Tpa molecules acts
as a tridentate ligand (μ2:κ2:κ1-coordination mode) binding to
two Cu atoms of different [Cu4I4] cores. Both Cu1 and Cu2
atoms can be considered in a four-coordinate distorted
tetrahedral geometry; however, their coordination environ-
ments are different from each other. The Cu1 atoms binds to
two I atoms (I1 and I2) and two pyrazole N atoms (N1 and
N3) of the same Tpa ligand, while the Cu2 atoms coordinate to
three I atoms (one I1 and two I2) and one N atom (N5) from
a neighboring [Cu4I4(Tpa)2] fragment. The Cu−N and Cu−I
bond lengths are comparable to those found for similar steplike
As shown above, the Tpa* and TpaMe3 ligands do not bind
the CuI center using the three pyrazole N atoms (κ3-
coordination mode) or their full denticity in a tetradentate
fashion (κ4-coordination mode). Conversely, they act as
bridging ligands between two metal ions, chelating one of
them through two of the pyrazolyl arms and using the third arm
to bridge the other metal. As a result, each Cu center is in a
three-coordinate distorted trigonal-planar environment. The
lower coordination geometry displayed by the CuI ion in these
crystal structures may reflect the difficulty of the Tpa* and
TpaMe3 ligands to fulfill the strong preference of this metal ion
for tetrahedral coordination presumably because of steric
constraints. In line with this, the bulky derivative tris[3-(4-
methoxyphenyl)-5-methylpyrazolylmethyl]amine has been
found to coordinate to CuI ions, adopting a κ4-coordination
mode.21a The steric bulk of the substitutent at the 3 position of
the pyrazolyl rings in this particular ligand seems to reduce the
strain, allowing bonding of the four N atoms to the metal ion.
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