Pyrazolylamidino ligands from coupling of acetonitrile and pyrazoles: A systematic study

Article abstract of DOI:10.1021/ic5018054

Mixed pyrazole-acetonitrile complexes, both neutral fac-[ReBr(CO)₃(NCMe)(pzH)] (pzH = pzH, pyrazole; dmpzH, 3,5-dimethylpyrazole; or indzH, indazole) and cationic fac-[Re(CO)₃(NCMe)(pzH)₂]A (A = BF₄, ClO₄, or OTf), are described. Their role as the only starting products to obtain final pyrazolylamidino complexes fac-[ReBr(CO)₃(NH=C(Me)pz-κ²N,N)] and fac-[Re(CO)₃(pzH)(NH=C(Me)pz-κ²N,N)]A, respectively, is examined. Other products involved in the processes, such as fac-[ReBr(CO)₃(pzH)₂], fac-[Re(CO)₃(NCMe)(NH=C(Me)pz-κ²N,N)]A, and fac-[Re(CO)₃(pzH)₂(OTf)] are also described. Warming CD₃CN solutions of fac-[Re(CO)₃(NCMe)(pzH)₂]A at 40 °C gives cleanly the pyrazolylamidino complexes [Re(CO)₃(pzH)(NH=C(Me)pz-κ²N,N)]A as the only products, pointing to an intramolecular process. This is confirmed by carrying out reactions in the presence of one equivalent of a pyrazole different from that coordinated, which affords complexes where the pyrazolylamidino ligand contains only the pyrazole previously coordinated. When the reactions lead to an equilibrium mixture of the final and starting products, the reverse reaction gives the same equilibrium mixture, which indicates that the coupling reaction of pyrazoles and nitriles to obtain pyrazolylamidino ligands is a reversible intramolecular process. A systematic study of the possible factors which may affect the reaction gives the following results: (a) the yields of the direct reactions are higher for lower temperatures; (b) the tendency of the pyrazoles to give pyrazolylamidino complexes follows the sequence indzH > pzH > dmpzH; and (c) the reaction rates do not depend on the nature of the anion even when a large excess is added. The presence of a small amount of aqueous solution of NaOH catalyzes the reaction. Thus, addition of 0.5-1% of NaOH (aq) to solutions of fac-[ReBr(CO)₃(NCMe)(pzH)] (in CD₃CN) or fac-[Re(CO)₃(NCMe)(pzH)₂]A (in CD₃CN, CD₃NO₂ or (CD₃)₂CO) allowed the syntheses of the corresponding pyrazolylamidino complexes [ReBr(CO)₃(NH=C(Me)pz-κ²N,N)] or [Re(CO)₃(pzH)(NH=C(Me)pz-κ²N,N)]A with better yields, more rapidly, and in milder conditions.

Full text of DOI:10.1021/ic5018054

Article
pubs.acs.org/IC
Pyrazolylamidino Ligands from Coupling of Acetonitrile and
Pyrazoles: A Systematic Study
Patricia Gomez-Iglesias,† Marta Arroyo,† Sonia Bajo,† Carsten Strohmann,‡ Daniel Miguel,†
́
and Fernando Villafane*,†
̃
†GIR MIOMeT-IU Cinquima-Química Inorgan
Valladolid, Spain
́
ica, Facultad de Ciencias, Campus Miguel Delibes, Universidad de Valladolid, 47011
‡Anorganische Chemie, Technische Universitat Dortmund, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany
̈
S
* Supporting Information
ABSTRACT: Mixed pyrazole−acetonitrile complexes, both neutral fac-[ReBr-
(CO)3(NCMe)(pz*H)] (pz*H = pzH, pyrazole; dmpzH, 3,5-dimethylpyrazole;
or indzH, indazole) and cationic fac-[Re(CO)3(NCMe)(pz*H)2]A (A = BF4, ClO4,
or OTf), are described. Their role as the only starting products to obtain final
pyrazolylamidino complexes fac-[ReBr(CO)3(NHC(Me)pz*-κ2N,N)] and fac-
[Re(CO)3(pz*H)(NHC(Me)pz*-κ2N,N)]A, respectively, is examined. Other
products involved in the processes, such as fac-[ReBr(CO)3(pz*H)2], fac-
[Re(CO)3(NCMe)(NHC(Me)pz*-κ2N,N)]A, and fac-[Re(CO)3(pz*H)2(OTf)]
are also described. Warming CD3CN solutions of fac-[Re(CO)3(NCMe)(pz*H)2]A
at 40 °C gives cleanly the pyrazolylamidino complexes [Re(CO)3(pz*H)(NHC(Me)pz*-κ2N,N)]A as the only products,
pointing to an intramolecular process. This is confirmed by carrying out reactions in the presence of one equivalent of a pyrazole
different from that coordinated, which affords complexes where the pyrazolylamidino ligand contains only the pyrazole
previously coordinated. When the reactions lead to an equilibrium mixture of the final and starting products, the reverse reaction
gives the same equilibrium mixture, which indicates that the coupling reaction of pyrazoles and nitriles to obtain
pyrazolylamidino ligands is a reversible intramolecular process. A systematic study of the possible factors which may affect the
reaction gives the following results: (a) the yields of the direct reactions are higher for lower temperatures; (b) the tendency of
the pyrazoles to give pyrazolylamidino complexes follows the sequence indzH > pzH > dmpzH; and (c) the reaction rates do not
depend on the nature of the anion even when a large excess is added. The presence of a small amount of aqueous solution of
NaOH catalyzes the reaction. Thus, addition of 0.5−1% of NaOH (aq) to solutions of fac-[ReBr(CO)3(NCMe)(pz*H)] (in
CD3CN) or fac-[Re(CO)3(NCMe)(pz*H)2]A (in CD3CN, CD3NO2 or (CD3)2CO) allowed the syntheses of the corresponding
pyrazolylamidino complexes [ReBr(CO)3(NHC(Me)pz*-κ2N,N)] or [Re(CO)3(pz*H)(NHC(Me)pz*-κ2N,N)]A with
better yields, more rapidly, and in milder conditions.
displaying physical properties of interest, or their role in catalysis
or materials science.4 In particular, pyrazolylamidino ligands
present several appealing features: (a) their synthesis in situ
(Scheme 1) allows one to obtain easily new bidentate chelating
ligands of distinct electronic and steric properties just using
different nitriles and pyrazoles, both readily available; (b) the
different properties of the two donor atoms and the electron
delocalization within the ligand makes them potentially
interesting for electron transfer processes and related physical
properties; and (c) the NH group may give rise to further
reactivity, as it may be involved in noncovalent interactions or
may be deprotonated.
INTRODUCTION
■
The electrophilic character of nitriles is enhanced by
coordination to a Lewis-acidic metal fragment, and therefore,
coordinated nitriles react with different nucleophiles to obtain a
wide variety of ligands.1 When the nucleophile is a pyrazole,
pyrazolylamidino complexes are obtained (Scheme 1).2,3
Pyrazolylamidino ligands belong to the family of pyrazole- or
pyrazolyl-containing chelating ligands. Complexes containing
these ligands are receiving the most attention because they are
involved in different and interdisciplinary aspects, such as
synthesis of supramolecular assemblies, design of molecules
Although the reaction displayed in Scheme 1 has been
described for several metals, the mechanism of this process
remains unclear. It is generally assumed to be an intramolecular
nucleophilic attack of the pyrazole to the nitrile, once both are
coordinated cis (Scheme 2).2e,j This suggestion is difficult to
Scheme 1. Coupling Reaction of a Coordinated Nitrile and a
Pyrazole To Obtain a Pyrazolylamidino Ligand
Received: July 25, 2014
Published: November 7, 2014
© 2014 American Chemical Society
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a
Scheme 2. Intramolecular (a) and Intermolecular (b)
Mechanisms for the Coupling Reaction of a Pyrazole and a
Coordinated Nitrile
Table 1. Complexes Used in This Study
pzH dmpzH indzH
fac-[ReBr(CO)3(NCMe)(pz*H)]
fac-[ReBr(CO)3(NHC(Me)pz*-κ2N,N)]
fac-[ReBr(CO)3(pz*H)2]
1a3c
2a3c
3a7
4a
1b3c
2b3c
3b7
4b3c
5b
1c
2c
3c
4c
5c
6c
7c
b
fac-[Re(CO)3(NCMe)(pz*H)2]+
c
fac-[Re(CO)3(pz*H)2(OTf)]
5a
fac-[Re(CO)3(pz*H)(NHC(Me)pz*-κ2N,N)]+ 6a3c
6b3c
7b3c
fac-[Re(CO)3(NCMe)(NHC(Me)pz*-
7a3c
κ2N,N)]+
a
Complexes 4, 6, and 7 have been isolated as BF4, ClO4, or OTf salts.
Only one of them is described for each complex in the Experimental
Section, since the spectroscopic data are esentially the same.
b
Previously described with the anion BAr′4 (Ar′ = 3,5-bis-
c
(trifluoromethyl)phenyl). Complexes with coordinated perchlorate
gave similar spectroscopic data.
Syntheses and Characterization of the Complexes.
Reactions between fac-[ReBr(CO)3(MeCN)2] and indzH occur
similarly to those previously reported for pzH or dmpzH,3c,7
giving the substitution (1c, 3c) or the insertion (2c) products
depending on the ratio and temperature used (eqs 1−3). Thus, 0
°C and a 1/1 ratio allow one to obtain the mixed acetonitrile−
indazole complex fac-[ReBr(CO)3(NCMe)(indzH)] (1c),
whereas a 1/2 ratio at room temperature gives the disubstituted
product, fac-[ReBr(CO)3(indzH)2], 3c. A 1/1 ratio, but at 80 °C,
leads to the pyrazolylamidino ligand resulting from the coupling
of indazole and acetonitrile, that is, fac-[ReBr(CO)3(NH
C(Me)indz-κ2N,N)], 2c.
reconcile with the lack of nucleophilicity of the uncoordinated
nitrogen of the pyrazole, similar to a pyrrole. A possible solution
to this problem has been proposed by considering the previous
deprotonation of the NH group (Scheme 2, path a).5
The group of Kukushkin and Pombeiro reported both
intramolecular or intermolecular paths depending on the
electronic configuration of the metal center.2b The intermo-
lecular mechanism, based on the attack of a uncoordinated
pyrazole, is depicted in Scheme 2, path b. The more recent
contribution to this problem is the report of a mechanism driven
by anion-mediated/hydrogen-bonding proton transfer.2a
No direct evidence supporting either intra- or intermolecular
coupling has been given so far. Path a in Scheme 2 shows that
complexes containing both pyrazoles and nitriles can be
considered as the only precursors of pyrazolylamidino complexes
if the mechanism is intramolecular. There are very few reports of
this type of complex.2a,e,6 During the course of our studies on
pyrazolylamidino rhenium complexes we were able to obtain
mixed acetonitrile−pyrazole complexes such as fac-[ReBr-
(CO)3(NCMe)(pzH)] (pzH = pyrazole), fac-[ReBr-
(CO)3(NCMe)(dmpzH)] (dmpzH = 3,5-dimethylpyrazole),
and cationic fac-[Re(CO)3(NCMe)(dmpzH)2]+.3c Therefore,
we decided to explore their reactivity as well as the processes
where they are involved in order to support or discard the
intramolecular mechanism. On the other hand, no previous
research has been carried out on the ideal conditions to obtain
pyrazolylamidino complexes. Herein, a systematic study of the
factors which may affect the process is reported.
fac‐[ReBr(CO)3(MeCN)2] + indzH
1/1,0°C
⎯⎯⎯⎯⎯⎯⎯→ fac‐[ReBr(CO)3(NCMe)(indzH)](1c)
(1)
−MeCN
fac‐[ReBr(CO)3(MeCN)2] + indzH
1/1,80°C
⎯⎯⎯⎯⎯⎯⎯⎯→⎯ fac‐[ReBr(CO)3(NHC(Me)(indz‐κ2N, N)]
−MeCN
(2c)
(2)
fac‐[ReBr(CO)3(MeCN)2] + 2indzH
1/2,r.t.
⎯⎯⎯⎯⎯⎯⎯⎯→⎯ fac‐[ReBr(CO)3(indzH)2](3c)
(3)
−2MeCN
Their spectroscopic data are collected in the Experimental
Section. 2c, 3b,8 and 3c were subjected to crystallographic
studies, shown in Figure 1 (2c), 2 (3c), and S1 (3b), Supporting
Information. The distances and angles (available from the CIF in
the Supporting Information) are similar to those found in other
pyrazolylamidino- and bis(pyrazole)rhenium complexes.3,7 As
observed in the structures of similar manganese complexes,3d
dimethylpyrazole ligands in 3b are tilted in the same sense
around the Re−N bonds in order to reduce the steric hindrance
of the methyl groups, in this case 34° and 39° with respect to the
“ideal” orientation, perpendicular to the ReN2 plane. This is not
observed in the structure of 3c, where less hindered indazoles are
coordinated perpendicular to the ReN2 plane.
Cationic acetonitrile−pyrazole mixed complexes fac-[Re-
(CO)3(NCMe)(pz*H)2]+, 4, are easily obtained from com-
plexes 3 by extracting the bromido ligand with the appropriate
silver salt in THF and ulterior addition of an excess of MeCN (eq
4). Synthesis and characterization of the pyrazole and indazole
complexes 4a and 4c are herein described, whereas the
RESULTS AND DISCUSSION
■
All complexes involved in this study are collected in Table 1 and
Chart 1. In addition to the mixed acetonitrile−pyrazole
complexes fac-[ReBr(CO)3(NCMe)(pz*H)] (1) (pz*H =
pzH, pyrazole; dmpzH, 3,5-dimethylpyrazole; or indzH,
indazole) and cationic fac-[Re(CO)3(NCMe)(pz*H)2]+ (4),
which play the leading role in this work as starting materials, we
also describe herein other complexes which are either byproducts
(3, 5) or final products of the processes studied (2, 6, and 7).
Most of the pzH and dmpzH complexes had been previously
reported by the group of Ardizzoia and Masciocchi7 or by us.3c
We decided to synthesize a similar family of complexes with a
third pyrazole, i.e., indazole, in order to have a wider range of
complexes to carry out in the study. The syntheses and
characterization of the species involved in these processes are
described next, before discussing the coupling reaction.
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Chart 1. Complexes Used in This Study
Figure 1. Perspective view of fac-[ReBr(CO)3(NHC(Me)indz-
κ2N,N)], 2c, showing the atom numbering. Ellipsoids are drawn at
30% probability.
Figure 3. Perspective view showing the atom numbering (top, ellipsoids
are drawn at 30% probability) and hydrogen bonds (bottom) of fac-
[Re(CO)3(NCMe)(pzH)2]+, 4a-ClO4.
molecule as well as the hydrogen bonds detected in the solid state
structure: the N-bound hydrogens of each pyrazole and the
oxygens of the perchlorate form a chain structure. The distances
and angles detected (H(2)···O(92) 1.974(10) Å, N(2)···O(92)
2.953(12) Å, N(2)−H(2)···O(92) 157.9(5)°, and H(4)···
O(94), 1.869(7) Å; N(4)···O(94) 2.893(10) Å, N(4)−H(4)···
O(94) 171.9(5)°) may be considered as “moderate” hydrogen
bonds.9
Figure 2. Perspective view of fac-[ReBr(CO)3(indzH)2], 3c, showing
the atom numbering. Ellipsoids are drawn at 30% probability.
fac‐[ReBr(CO)3(pz*H)2](3)
−
(1)+Ag+A ,AgBr
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ fac‐[Re(CO)3(NCMe)(pz*H)2]+A (4)
−
dimethylpyrazole complex 4b has been previously reported.3c As
indicated in Table 1, they can be isolated as BF4, ClO4, or OTf
salts, although only one of them (BF4 salts) is described in the
Experimental Section, since the spectroscopic data are esentially
the same. Their spectroscopic and analytical data support the
proposed structure, which was confirmed by an X-ray diffraction
study for 4a-ClO4. Figure 3 shows a perspective view of the
(2)+MeCN
(4)
Substitution of the acetonitrile ligand present in complexes 4
by the corresponding counterion was observed during their
reactivity studies. Therefore, we decided to isolate and
characterize the neutral complexes containing the coordinated
anion. Their synthesis was carried out by extracting the bromido
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ligand of complexes 3 with the appropriate silver salt in the
absence of MeCN (see eq 5 for OTf complexes). As expected,
tetrafluoroborate complexes were shown to be very unstable,
which precluded their characterization. Both perchlorate and
triflate could be isolated, giving almost coincident spectroscopic
data, and again only one of them (containing coordinated OTf)
is described in the Experimental Section. The crystal structure of
the dimethylpyrazole complex 5b containing either triflato or
perchlorato ligands could be obtained. They are shown in Figures
4 and S2, Supporting Information, respectively. Distances and
(NHC(Me)indz-κ2N,N)]+, 7c, were obtained as previously
described for similar pyrazolilamidino complexes with pyrazole
or dimethylpyrazole. Alternatively, 6c could also be obtained by
substituting the coordinated anion in 5 by MeCN and
subsequent coupling (eqs 6−8). Their spectroscopic and analytic
data (Experimental Section) are as expected given this analogy.3c
As discussed below, the reaction conditions are milder for
pyrazolilamidino complexes derived from indazole with respect
to pyrazole or dimethylpyrazole. The crystal structure of 7c-ClO4
Figure 5. Perspective view of fac-[Re(CO)3(NCMe)(NHC(Me)-
indz-κ2N,N)]ClO4, 7c-ClO4, showing the atom numbering. Ellipsoids
are drawn at 30% probability.
is depicted in Figure 5, and distances and angles are available
from the CIF in the Supporting Information.
fac‐[Re(CO)3(NCMe)(indzH)2]OTf (4c‐OTf) →
fac‐[Re(CO)3(indzH)(NHC(Me)indz‐κ2N, N)]OTf (6c‐OTf)
(6)
+MeCN
Figure 4. Perspective view of fac-[Re(CO)3(dmpzH)2(OTf)], 5b-OTf
(above), and hydrogen bonds (below), showing atom numbering.
Ellipsoids are drawn at 30% probability.
fac‐[Re(CO)3(indzH)2](OTf) (5c‐OTf) ⎯⎯⎯⎯⎯⎯⎯→⎯
fac‐[Re(CO)3(indzH)(NHC(Me)indz‐κ2N, N)]OTf (6c‐OTf)
(7)
1)+AgOTf,−AgBr
(
fac‐[ReBr(CO)3(NHC(Me)indz‐κ2N, N)] (2c) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→
(2)+MeCN
angles, which are available from the CIF in the Supporting
Information, show the analogy between these structures and that
of 3b.
fac‐[Re(CO)3(NCMe)(NHC(Me)indz‐κ2N, N)]OTf (7c‐OTf)
(8)
Kinetic Studies of the Coupling Processes. The first
attempt to obtain pyrazolylamidino complexes from mixed
acetonitrile−pyrazole complexes as the only starting material was
carried out using neutral bromido complexes 1b or 1c in CD3CN
solution.10 Heating any of these complexes at 80 °C leads to a
mixture of complexes 2 and 3, unreacted 1, as well as other very
minor products, which were identified mainly as a mixture of 6
and 7. Figure 6 shows the concentrations of the species present in
solution when 1b is heated in CD3CN at 80 °C. Similar mixtures
were obtained when using 1c as starting material or solutions of
different concentrations. When the processes were attempted at
lower temperatures, the reactions were too slow. On the other
hand, the reactions were repeated with a noncoordinating
solvent, but heating 1 at 80 °C in CD3NO2 led to residual
amounts of the pyrazolylamidino complexes 2b or 2c, with
bis(pyrazole) complexes 3 being the main products (ca. 50%).
This expected result shows that conditions favoring uncoordi-
nation of acetonitrile preclude the coupling process.
fac‐[ReBr(CO)3(pz*H)2] (3)
+AgOTf
⎯⎯⎯⎯⎯⎯⎯→⎯ fac‐[Re(CO)3(pz*H)2(OTf)]( 5‐OTf)
−AgBr
(5)
Opposite to 4a, which forms anion−cations hydrogen-bonded
chains (Figure 3, below), the solid crystal structure of 5b
indicates that in these cases hydrogen bonds generate discrete
dimer ionic pairs (Figure 4, below). Uncoordinated oxygen
atoms of the coordinated triflate are hydrogen bonded with N-
bound hydrogens of dimethylpyrazoles. The distances and angles
detected (H(2)···O(56) 1.938(6) Å, N(2)···O(56) 2.933(9) Å,
N(2)−H(2)···O(56) 161.5(4)°, and H(4)···O(5) 1.903(5) Å;
N(4)···O(5) 2.862(8) Å; N(4)−H(4)···O(5) 153.4(5)° for 5b-
OTf) indicate that these may be considered again as “moderate”
hydrogen bonds.9
Cationic pyrazolylamidino complexes fac-[Re(CO)3(indzH)-
(NHC(Me)indz-κ2N,N)]+, 6c, and fac-[Re(CO)3(NCMe)-
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Figure 6. Representation of the concentrations of 1b, 2b, 3b, and the mixture 6b + 7b vs time when 1b is heated in CD3CN at 80 °C. Final ratio: 1b/2b/
3b/(6b + 7b) = 25/50/10/5.13
The most striking feature about the processes carried out in
CD3CN at 80 °C was the presence of small amounts of the
cationic complexes 6 and 7. In a cationic complex, the
coordinated acetonitrile should be more activated than when
coordinated to a neutral complex, and therefore, the coupling
process to form the pyrazolylamidino ligand should be easier.
This led us to consider the possibility of the presence of cationic
complexes as intermediates for the process, as depicted in
Scheme 3. It is well known that MeCN may coordinate to Mn
solution for reaction of 4b-BF4. Although this result may seem
disappointing, close study of the first minutes of the process
reveals that conversion of 4b to 6b is clean (Scheme 4, framed
reaction). Only after ca. 30 min, the concentration of 6b reaches a
maximum and starts to decrease whereas the concentations of
equimolar amounts of 7b and free pyrazole increase. This
indicates that the pyrazole coordinated in 6b is being substituted
by acetonitrile from the solvent to give 7b and free pyrazole
(Scheme 4, right). This is a somehow surprising result, because
pyrazoles are better ligands than nitriles, but in this case the high
temperature might favor this entropy-driven process. As this
secondary process is favored by higher temperatures and the
coupling process should be easier for cationic complexes, the
reaction was repeated at 40 °C.
Scheme 3. Proposal of the Coupling Process of Neutral
Complexes through Cationic Intermediates
In these conditions, conversion of 4 into 6 is clean (Scheme 4,
framed reaction), as shown in Figure 8 for 4b-OTf (see Figure
S3, Supporting Information, for 4a-ClO4 and 4c-ClO4), which
supports that the coupling reaction of pyrazoles and nitriles to
obtain pyrazolylamidino ligands is an intramolecular process.
However, mild temperatures are crucial to obtain this result, as at
60 °C or higher temperatures substitution of the coordinated
pyrazole in 6 by CD3CN from the solvent to get 7 and free
pyrazole is favored (Scheme 4, right). Another process observed
when the reaction is carried out above 40 °C is the scrambling of
the coordinated CH3CN with the solvent, CD3CN. This
scrambling process is detected by examining the methyl group
signal in the 1H NMR spectra, which integrates at lower values
than expected. In fact, the framed reaction leads to a mixture of
6b and “deuterated” 6b in a ca. 40/60 ratio when the reaction was
carried out at 80 °C but to neat 6b when the reaction ocurs at 40
°C. However, this scrambling at higher temperatures should
occur before the coupling process and does not affect the
intramolecular nature of the reaction. Therefore, all data support
that the methyl group in the pyrazolylamidino ligand, either
deuterated or not, is present in the coordinated acetonitrile.
When the same reaction was attempted using CD3NO2 as
solvent, conversion of 4 into 6 was not so clean as in CD3CN,
because the final pyrazolylamidino complexes 6 were usually
mixed with neutral complexes 5 (Scheme 4, left, and Figure S4,
Supporting Information). However, addition of a slight excess of
MeCN to complexes 5 did not afford back 4 in the reaction
conditions.
and Re complexes by substituting the halide ligand.11 This
process would give rise to cationic complexes, where the
electrophilic character of the nitrile should be enhanced, and
thus, the coupling process should be favored. Therefore, we
decided to focus our work on the cationic acetonitrile−pyrazole
complexes and investigate whether they could be intermediates
in the coupling process to obtain cationic pyrazolylamidino
complexes.
Heating cationic complexes 4 in the same conditions, CD3CN
at 80 °C, gave a mixture of the pyrazolylamidino cationic
complexes 6 and 7 as well as unreacted 4 and free pyrazole.
Figure 7 shows the concentrations of the species detected in
Clean conversion of 4 into 6 (Figure 8 and Scheme 4, framed
reaction) clearly demonstates that the coupling reaction occurs
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Figure 7. Representation of the observed concentrations of 4b-BF4, 6b-BF4, 7b-BF4, and dmpzH vs time when 4b is heated in CD3CN at 80 °C. Final
ratio 4b-BF4/6b-BF4/7b-BF4 = 22/34/44.
Scheme 4. Conversions of 4 Into 6 in CD3CN (framed
reaction) and Byproducts Detected When the Process Is
Carried out at Higher Temperatures (right) and in CD3NO2
(left)
Figure 8. Representation of the observed concentrations of 4b-OTf and
6b-OTf when 4b-OTf is heated in CD3CN at 40 °C. Final ratio: 4b-
OTf/6b-OTf = 21/79.
A 100% conversion is not usually achieved in all these
processes where mixed acetonitrile−pyrazole cationic complexes
convert into the corresponding pyrazolylamidino ligands;
instead, a constant ratio mixture of species is commonly obtained
when both the nitrile and the pyrazole are coordinated, and
therefore, it is an intramolecular process. In order to confirm this
proposal, the coupling process was carried out in the presence of
a pyrazole different from that coordinated. Thus, a solution of 4b-
OTf and an equimolar amount of indzH in CD3CN was heated at
40 °C, giving only 6b-OTf and unreacted indzH (Scheme 5,
above). The results were the same after exchanging the role of the
pyrazoles, and reaction of 4c-ClO4 and a equimolar amount of
dmpzH in CD3CN at room temperature again gave only 6c-ClO4
and unreacted dmpzH (Scheme 5, below). These results confirm
that the pyrazole which undergoes the coupling process is that
previously coordinated, thus supporting definitively a intra-
molecular mechanism for this reaction.
(for example, 79/21 for the process depicted in Figure 8,
although 100/0 for that shown in Figure S3, Supporting
Information). According to the principle of microscopic
reversibility, if the reaction were reversible the same mixture
should be obtained by the reverse reaction, which is using the
final product as starting material. In order to confirm this,
different experiments were designed in order to carry out the
inverse experiments to those described in the framed reaction of
Scheme 4. Thus, heating 6b-OTf at 40 °C in CD3CN afforded a
mixture of 6b-OTf and 4b-OTf in a 80/20 ratio, that is, similar to
that obtained in the direct process described in Figure 8.
On the other hand, in a separate experiment, reaction of 4b-
OTf in CD3CN at 40 °C was allowed to reach to the final
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demonstrate that yields are higher for low temperatures,
indicating that the reverse reaction (formation of the
pyrazole−nitrile mixed complex from the pyrazolylamidino
complex) is more favorable at higher temperatures. This result
might be explained considering that the presence of a chelating
ligand in the pyrazolylamidino complexes 6 represents a decrease
in the entropy, and therefore, increasing temperatures prevents
its formation.
Scheme 5. Reactions Supporting That the Coupling Process
Proceeds via an Intramolecular Mechanism
(b) Pyrazole. Considering the temperatures and the time
needed to achieve the reaction, the tendency of the pyrazoles
used in this study to give pyrazolylamidino complexes follow the
sequence indzH > pzH > dmpzH (Figure 9, Table S2,
equilibrium, giving the expected mixture of 79/21 of 6b-OTf/4b-
OTf (Figure 8), and then 5 mg of 4b-OTf was added. When the
reaction was heated again to 40 °C, the final ratio was the same
(Figure S5, Supporting Information), thus supporting that the
coupling reaction of pyrazoles and nitriles to obtain pyrazoly-
lamidino ligands is a reversible intramolecular process.
In order to obtain more information about the mechanism of
the coupling process, a systematic study was carried out
considering the possible factors which may affect the reaction,
that is, temperature, pyrazole, anion, and solvent. As shown
below, this study allowed us to find a catalyst for this reaction.
All of the following experiments were carried out with cationic
complexes 4, since conversion of neutral bromido complexes 1 to
the corresponding pyrazolylamidino complexes 2 is not clean nor
reversible. The proposal suggested in Scheme 3 for the neutral
bromido complexes is confirmed by the experiments just
described, carried out with cationic complexes 4. Removal of
the bromido ligand in 1 and substitution by acetonitrile would
afford cationic complexes of the type fac-[Re-
(CO)3(NCMe)2(pz*H)]+,12 which would give the pyrazolyla-
midino complexes 7 by the reversible intramolecular process
explained above. Substitution of the labile acetonitrile in 7 by the
bromide present in solution would afford the neutral
pyrazolylamidino complexes 2 (Scheme 3). However, the high
temperatures needed to remove the bromido ligand and thus to
start the coupling process may give rise to other secondary
reactions, such as substitution of nitriles by pyrazoles to afford
bis(pyrazole) complexes 3.13 However, the proposed path
through cationic complexes might not be the only way to obtain
pyrazolylamidino neutral ligands, because they are also formed,
although in lower yield, when the process is carried out with an
Figure 9. Representation of the observed concentrations vs time of
complexes 6 when 4 are heated in CD3CN at 40 °C.
Supporting Information). This is in accordance with the acidity
of the pyrazoles used, which has been experimentally
demonstrated to be that theorically expected, indzH > pzH >
dmpzH.14 Therefore, the more acidic pyrazoles facilitate proton
transfer from the pyrazole to the donor nitrogen atom of the
pyrazolylamidino ligand. This is in accordance with a coupling
process with concomitant deprotonation of the pyrazole, as
mentioned in the Introduction (Scheme 2, path a).
(c) Anion. As indicated also in the Introduction, the influence
of the anion in the coupling process of a coordinated nitrile and
pyrazole to give pyrazolylamidino complexes has been already
reported, and a transition state for the metal-activated pyrazole−
nitrile coupling reaction was then proposed.2a However, in our
−
−
case, use of different anions (BF4 , ClO4 , OTf−) did not allow
us to establish clear differences between them. Therefore,
conversion reactions of 4 to 6 were repeated using an excess of
sodium salts (usually 3/1, as a larger excess causes precipitation
of the salts). Surprisingly, addition of a sodium salt of the anion
present in the complex has two effects on the process: reducing
both the reaction time and the temperature needed to observe
the coupling process. On the other hand, the product/substrate
final ratio does not change compared to that obtained without an
excess of anion. These experiments allowed us to propose the
following sequence for the accelerating role of the anions used in
n
excess of Bu4NBr, which should prevent formation of cationic
complexes. Nevertheless, drawing conclusions from the latter
experiment is difficult, since new species (probably polybromide
anionic complexes) are present and contaminate the whole
process.
(a) Temperature. As indicated above, clean conversion of 4 to
6 occurs at 40 °C in CD3CN, whereas at 80 °C substitution of the
pyrazole by CD3CN is observed (Scheme 4). In order to know
the optimal temperature for this process, we determined the
yields for the synthesis of 6c-OTf from 4c-OTf at different
temperatures (from 20 to 60 °C) in CD3CN. In this range of
temperatures the results (Table S1, Supporting Information)
this reaction: OTf− > ClO4 > BF4 . However, this sequence
does not coincide with the strength of the acids involved, neither
in the gas phase nor in MeCN.15 Therefore, there must be
something else accelerating the reaction, and it might be an
−
−
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impurity present in the sodium salts used, which points to a
catalyst, given the small amount present and its decisive effect on
the reaction.
The review of Kukushkin and Pombeiro on additions to metal-
activated organonitriles described that some of them are base
catalyzed,1c although this had not been previously reported for
additions of pyrazoles to metal-activated organonitriles. This led
us to conclude that the sodium salts used might be mixed with
small amounts of a base. Then we measured the pH of 1 M
solutions in H2O of the sodium salts used, and the amount of
impure base is directly related to the observed sequence (pH of 1
M solutions in H2O): OTf− (11.1) > ClO4− (9.3) > BF4− (2.6)).
When the experiment was repeated with pure salts (pH of 1 M
solutions in H2O = 7) the excess of anions did not have any
evident effect on the reaction time nor on the reaction yield
(Figure S6, Supporting Information).
(d) Catalyst for the reaction. Therefore, we decided to repeat
the coupling process in the presence of a small amount of
aqueous solution of NaOH. Formation of pyrazolylamidino
complexes 6 after addition of a 0.5% NaOH (as ca. 0.02 M
aqueous solution) to CD3CN solutions of 4 was immediate,
occurring at room temperature and quantitatively (for pzH or
indzH) or with very high yield (for dmpzH),
Figure 10. Representation of the observed concentrations of 6a when
(a) 4a−ClO4 is heated at 40 °C and when 4a−BF4, is stirred at room
temperature with (b) 6.2 μL of 0.5% of NaOH (aq), (c) 6.2 μL of 0.5%
of HBF4 (aq), or (d) 6.2 μL of H2O are added. All reactions are carried
out in CD3CN; 0.05 M is 100% yield.
As described above, formation of pyrazolylamidino neutral
bromo complexes 2 from 1 was not clean (Figure 6). However,
when these processes were repeated in the presence of 1% of
NaOH (again as ca. 0.02 M aqueous solution), formation of 2
occurred in milder conditions and with higher yields with respect
to those reactions carried out without base (see the synthesis of
2c in the Experimental Section, carried out by both methods, and
Table S3, Supporting Information). As indicated above, reactions
are not so favored for neutral complexes as for cationic
complexes, since the nitrile is less activated. These experiments
lead us to conclude that the coupling reaction of pyrazoles and
acetonitrile to give pyrazolylamidino ligands is base catalyzed.
As we are using an aqueous solution of NaOH, we decided to
explore the role in the reaction of (a) water and (b) acids (since
the NaBF4 first used was contaminated with acid). Figure 10
shows the concentration vs time of 6a, when the parent complex
4a was heated at 40 °C in dry CD3CN or treated with either 0.5%
of NaOH (aq), 0.5% of HBF4 (aq), or the same amount of
neutral H2O in CD3CN at room temperature. Figure 10 shows
that the reaction is immediate when NaOH (aq) is added (the
reaction had already concluded when the first NMR spectrum is
obtained), whereas the reaction carried out in the presence of the
same amount of H2O is slower. The reaction rate of the latter is
almost identical to that carried out when of 0.5% of HBF4 is
added; therefore, strong acids have no effect on the reaction rate.
The concentration of the final product when the reaction is
carried out in dry CD3CN is shown at the bottom, but these data
correspond to the reaction at 40 °C, because it is too slow in dry
CD3CN at room temperature.
only possible by considering the previous deprotonation of the
NH group (Scheme 2, path a). Therefore, the actual role of the
base as catalyst for this process may be facilitating this
deprotonation. The fact that small amounts of water (either
neutral or acidic) also accelerate the process (although less
efectively compared to the aqueous base solution, Figure 10)
might indicate that autoprotolysis occurring in aqueous media
may also help this deprotonation and therefore the coupling
process. Furthermore, a clear dependence of the basicity is
established, as the effect of weaker bases is feebler than that of
NaOH.
(e). Solvent. As indicated above, all kinetic studies were carried
out in CD3CN because the attempts to carry out the reactions in
CD3NO2 led to total or partial substitution of the acetonitrile
ligand. However, as the reactions base catalyzed are immediate
(starting from cationic complexes 4) or very fast (for neutral
complexes 1), we decided to explore the use of other solvents in
this case. For the latter, the processes could be carried out only in
CD3CN, since use of other solvents led again to mixtures of
products. However, for cationic complexes 4, the results of the
base-catalyzed coupling processes in CD3NO2 or (CD3)2CO are
similar to those obtained in CD3CN, that is, the reactions are
quantitative and immediate and occurred at room temperature.
CONCLUSIONS
■
Synthesis of different rhenium complexes containing both
pyrazole and nitrile ligands has allowed us to study the
mechanism of the coupling reaction between these two ligands
to afford pyrazolylamidino complexes. The study was carried out
on cationic complexes because the nitrile is more electrophilic
than in neutral complexes, and thus, for the latter higher
temperatures are needed, which facilitates formation of different
byproducts. All of the data obtained starting from pyrazole−
nitrile mixed cationic complexes support that the coupling
process to give pyrazolylamidino ligands occurs by a reversible
intramolecular mechanism. The systematic study carried out on
this process revealed that the coupling process is favored: (a) at
We also explored the behavior of weaker bases than NaOH.
Figure S7, Supporting Information, shows how addition of
weaker bases than NaOH (aq), such as acetate or fluoride,
produces the expected results considering their pH, which is a
faster process than those with neutral or acid water but slower
than those with the same amount of NaOH (aq). Surprisingly,
addition of cyanide gives rise to an even slower process.
Unfortunately, so far we have no explanation for this surprising
behavior of the cyanide ion.
As indicated in the Introduction, intramolecular nucleophilic
attack of the pyrazole to the coordinated nitrile coordinated cis is
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1
506 w, 492 w, 471 m. H NMR (399.8 MHz, Me2CO-d6): 3.28 (s,
NCCH3, 3 H), 7.59 (t, J = 7.5 Hz, H6 indz, 1 H), 7.82 (t, J = 8.0 Hz, H5
indz, 1 H), 8.11 (d, J = 8.0 Hz, H7 indz, 1 H), 8.19 (d, J = 9.0 Hz, H4 indz,
1 H), 9.07 (s, H3 indz, 1 H), 10.92 (s, NH, 1 H). 13C{1H} NMR (100.5
MHz, Me2CO-d6): 20.3 (s, NCCH3), 112.2 (s, C4 indz), 122.6 (s, C7
indz), 125.1 (s, C6 indz), 131.3 (s, C5 indz), 143.8 (s, C3 indz), 163.6 (s,
NCCH3), C3a (indz), C7a (indz) and CO were not observed due to the
low solubility of the compound. Anal. Calcd for C12H9BrN3O3Re: C,
28.30; H, 1.78; N, 8.25. Found: C, 28.36; H, 1.73; N, 8.28.
moderate temperatures (and unfavored as the temperature
increases), opposite to what was commonly accepted, since
pyrazolylamidino complexes had been traditionally synthesized
by refluxing or heating pyrazole complexes in nitriles as solvents,
and (b) with the acidity of the pyrazole. We found that the
coupling process is base catalyzed, and the reactions to obtain
cationic pyrazolylamidino complexes are quantitative and
immediate and occurred at room temperature for different
solvents when ≤1% of NaOH (aq) is added to the parent
complex solution. Neutral bromido pyrazolylamidino complexes
are obtained rapidly and at room temperature when 1% of NaOH
(aq) is added to the parent complex CD3CN solution. We believe
that these results open a broad range of synthetic possibilities to
obtain new pyrazolylamidino complexes in the future.
fac-[ReBr(CO)3(indzH)2], 3c. Indazole (0.071 g, 0.60 mmol) was
added to a solution of fac-[ReBr(CO)3(MeCN)2] (0.130 g, 0.30 mmol)
in THF (20 mL). The solution was stirred for 1 h at room temperature.
Hexane (ca. 20 mL) was added, and the solution was concentrated and
cooled to −20 °C, giving a colorless microcrystalline solid, which was
decanted, washed with hexane (3 × 3 mL approximately), and dried in
vacuo, yielding 0.143 g (81%). IR (THF, cm−1): 2026 vs, 1924 vs, 1895
vs. IR (KBr, cm−1): 3311 s, 2020 vs, 1910 vs, 1891 vs, 1629 m, 1511 w,
1356 m, 1244 m, 1002 w, 961 w, 832 m, 752 m, 744 m, 673 w, 660 m, 648
m, 542 m, 430 w. 1H NMR (399.8 MHz, Me2CO-d6): 7.26 (t, J = 7.5 Hz,
H6 indzH, 2 H), 7.52 (t, J = 7.5 Hz, H5 indzH, 2 H), 7.69 (d, J = 8.5 Hz,
H4 indzH, 2 H), 7.85 (d, J = 8.0 Hz, H7 indzH, 2 H), 8.56 (s, H3 indzH, 2
H), 12.63 (s, NH, 2 H). 13C{1H} NMR (100.5 MHz, Me2CO-d6): 112.3
(s, C4 indzH), 122.7 (s, C7 indzH), 124.2 (s, C6 indzH), 124.8 (s, C3a
indzH), 130.8 (s, C5 indzH), 141.6 (s, C3 indzH), 142.7 (s, C7a indzH),
191.9 (br, 1 CO), 197.4 (br, 2 CO). Anal. Calcd for C17H12BrN4O3Re:
C, 34.82; H, 2.06; N, 9.55. Found: C, 34.75; H, 1.86; N, 9.09.
fac-[Re(CO)3(NCMe)(pzH)2]BF4, 4a-BF4. Silver tetrafluoroborate
(0.054 g, 0.28 mmol) was added to a solution of 3a (0.122 g, 0.25
mmol) in THF (20 mL), and the mixture was stirred for 1 h at 40 °C in
the absence of light. The reaction mixture was dried in vacuo, and the
residue was extracted with CH2Cl2 (20 mL), filtered, and dried in vacuo.
The white residue was dissolved in CHCl3/MeCN (5 mL/5 mL), and
the solution was stirred for 4 h at 0 °C. The solvent was then removed in
vacuo to give a white solid, which was recrystallized from CHCl3/hexane
at −20 °C, giving a colorless microcrystalline solid, which was decanted,
washed with hexane (3 × 3 mL approximately), and dried in vacuo,
yielding 87 mg (60%). IR (THF, cm−1): 2034 vs, 1934 vs, 1922 vs. IR
(KBr, cm−1): 3222 m, 3160 m, 2040 vs, 1950 vs, 1918 vs, 1290 s, 1257 s,
1244 s, 1224 m, 1182 s, 1131 m, 1070 w, 1055 m, 1026 s, 950 w, 878 w,
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were performed under N2
atmosphere following conventional Schlenk techniques. Solvents were
purified according to standard procedures.16 fac-[ReBr-
(CO)3(NCMe)2] was obtained as previously described.17 Table 1
collects references for the preparation and characterization of some of
the complexes herein used. All other reagents were obtained from the
usual commercial suppliers and used as received. Caution! Although no
difficulties were experienced with the perchlorate complexes described herein,
all perchlorate species should be treated as potentially explosive and handled
with care. Infrared spectra were recorded in a PerkinElmer FT-IR
spectrum BX apparatus using 0.2 mm CaF2 cells for solutions or on KBr
pellets for solid samples. NMR spectra were recorded in Bruker AV-400
or Varian MR500 instruments at room temperature and are referred to
the internal residual solvent peak for 1H and 13C{1H} NMR. Assignment
of the 13C{1H} NMR data was supported by 2D heteronuclear
experiments and relative intensities of the resonance signals. Elemental
analyses were performed on a PerkinElmer 2400B microanalyzer.
fac-[ReBr(CO)3(NCMe)(indzH)], 1c. Indazole (0.059 g, 0.50 mmol)
was added to a solution of fac-[ReBr(CO)3(NCMe)2] (0.216 g, 0.50
mmol) in THF (15 mL). The solution was stirred at 0 °C for 10 min.
Hexane (ca. 25 mL) was added, and the solution was concentrated in
vacuo and cooled to −20 °C, giving a colorless microcrystalline solid,
which was decanted, washed with hexane (3 × 3 mL approximately), and
dried in vacuo, yielding 0.154 g (61%). IR (THF, cm−1): 2030 vs, 1930
vs, 1900 vs. IR (KBr, cm−1): 3378 m, 2028 vs, 1903 vs br, 1628 w, 1560
w, 1508 w, 1438 w, 1358 m, 1273 w, 1246 w, 1150 w, 1096 m, 1029 w,
964 w, 749 m, 640 w, 529 w, 487 w. 1H NMR (399.8 MHz, Me2CO-d6):
2.58 (s, NCCH3, 3 H), 7.30 (t, J = 7.5 Hz, H6 indzH, 1 H), 7.56 (t, J = 7.5
Hz, H5 indzH, 1 H), 7.76 (d, J = 8.5 Hz, H4 indzH, 1 H), 7.91 (d, J = 8.0
Hz, H7 indzH, 1 H), 8.68 (s, H3 indzH, 1 H), 12.64 (s, NH, 1 H).
13C{1H} NMR (100.5 MHz, Me2CO-d6): 112.1 (s, C4 indzH), 122.1 (s,
NCCH3), 122.4 (s, C7H indzH), 123.9 (s, C6 indzH), 124.5 (s, C3a
indzH), 130.6 (s, C5 indzH), 141.2 (s, C3 indzH), 142.4 (s, C7a indzH),
163.6 (s, NCCH3), CO not observed. Anal. Calcd for C12H9BrN3O3Re:
C, 28.30; H, 1.78; N, 8.25. Found: C, 27.98; H, 2.00; N, 8.58.
1
777 m, 652 w, 637 m, 606 w, 575 w, 519 w, 494 w, 411w. H NMR
(399.8 MHz, CDCl3): 2.67 (s, NCCH3, 3 H), 6.39 (s, H4 pzH, 2 H), 7.71
(s, H3,5 pzH, 2 H), 7.77 (s, H5,3 pzH, 2 H), 12.19 (s, NH, 2 H). 13C{1H}
NMR (100.5 MHz, CDCl3): 5.8 (s, NCCH3), 107.7 (s, C4 pzH), 125.3
(s, NCCH3), 133.5 (s, C5,3 pzH), 145.3 (s, C3,5 pzH), 191.7 (s, 3 CO).
19F NMR (376.2 MHz, CDCl3): −149.8 (s, 10BF4, 4 F), −149,9 (s,
11BF4, 4 F). Anal. Calcd for C11H11BF4N5O3Re: C, 24.67; H, 2.07; N,
13.08. Found: C, 24.44; H, 1.71; N, 12.78.
fac-[Re(CO)3(NCMe)(indzH)2]BF4, 4c-BF4. Silver tetrafluoroborate
(0.107 g, 0.55 mmol) was added to a solution of 3c (0.293 g, 0.50 mmol)
in THF (30 mL). Working up as for 4a-BF4 gave 173 mg (53%) of 4c-
BF4 as a colorless microcrystalline solid. IR (THF, cm−1): 2036 vs, 1929
vs. IR (KBr cm−1): 3338 m, 3138 m, 2291 w, 2041 vs, 1943 vs, 1925 vs,
1630 m, 1515 m, 1388 m, 1377 m, 1360 m, 1285 w, 1247 w, 1226 w,
1084 vs, 1062 vs, 1031 s, 902 w, 840 w, 782 w, 753 m, 647 w, 534 w, 482
w, 432 w. 1H NMR (499.7 MHz, CDCl3): 2.82 (s, NCCH3, 3 H), 7.24
(partially overlapped by the solvent signal, H6 indzH, 2 H), 7.51 (t, J =
8.0 Hz, H5 indzH, 2 H), 7.69 (d, J = 7.5 Hz, H7 indzH, 2 H), 7.82 (d, J =
8.5 Hz, H4 indzH, 2 H), 8.32 (s, H3 indzH, 2 H), 11.74 (s, br, NH, 2 H).
13C{1H} NMR (499.7 MHz, CDCl3): 5.9 (s, NCCH3), 112.1 (s, C4
indzH), 120.4 (s, C7 indzH), 122.8 (s, C3a indzH), 123.5 (s, C6 indzH),
126.4 (s, NCCH3), 130.2 (s, C5 indzH), 141.0 (s, C3 indzH), 141.8 (s,
C7a indzH), 191.3 (s, 2 CO), 192.0 (s, CO). 19F NMR (470.2 MHz,
CDCl3): −149.97 (s, 10BF4, 4 F), −150.2 (s, 11BF4, 4 F). Anal. Calcd for
C19H15BF4N5O3Re: C, 35.90; H, 2.38; N, 11.02. Found: C, 35.62 ; H,
2.11 ; N, 10.77.
fac-[ReBr(CO)3(NHC(Me)indz-κ2N,N)], 2c. Method A. Indazole
(0.059 g, 0.50 mmol) was added to a solution of fac-[ReBr-
(CO)3(NCMe)2] (0.216 g, 0.50 mmol) in CH3CN (15 mL). The
solution was stirred at 80 °C for 17 h. Volatiles were removed in vacuo,
and the yellow residue was crystallized in acetone/hexane at −20 °C,
giving a yellow microcrystalline solid, which was decanted, washed with
hexane (3 × 3 mL approximately), and dried in vacuo, yielding 0.172g
(67%). Method B. Indazole (0.035 g, 0.30 mmol) was added to a solution
of fac-[ReBr(CO)3(NCMe)2] (0.130 g, 0.30 mmol) in CH3CN (15
mL), and the solution was stirred for 20 min at 40 °C. Then 0.17 mL of
NaOH aqueous solution (0.018 M, 0.003 mmol) was added, and the
solution was stirred for another 30 min at 40 °C. Volatiles were removed
in vacuo, and the residue was crystallized in acetone/hexane at −20 °C,
yielding 135 mg (89%). IR (THF, cm−1): 2021 vs, 1923 vs, 1894 vs. IR
(KBr, cm−1): 3183 s, 2020 vs, 1925 vs, 1889 vs, 1632 m, 1509 m, 1474 m,
1449 m, 1355 m, 1280 m, 1218 w, 1201 m, 1160 w, 1102 w, 1063 m,
1016 w, 984 w, 913 w, 883 w, 839 w, 788 w, 768 m, 646 w, 623 w, 532m,
fac-[Re(CO)3(pzH)2(OTf)], 5a. Silver triflate (0.084 g, 0.33 mmol) was
added to a solution of 3a (0.146 g, 0.3 mmol) in THF (15 mL), and the
mixture was stirred for 1 h at room temperature in the absence of light.
The reaction mixture was filtered, and hexane was added to the filtrate.
The solution was concentrated and cooled to −20 °C, giving a colorless
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microcrystalline solid, which was decanted, washed with hexane (3 × 3
mL approximately), and dried in vacuo, yielding 0.145 g (87%). IR
(THF, cm−1): 2037 vs, 1932 vs, 1905 vs. IR (KBr, cm−1): 3291 s, 3270 s,
2036 vs, 1930 vs, 1917 vs, 1654 w,1560 w, 1540 w, 1491 w, 1412 w, 1364
w, 1314 s, 1236 s, 1212 s, 1191 m, 1139 m, 1070 w, 1057 m, 1031 m, 952
w, 912 w, 863 w, 777 m, 659 w, 633 m, 606 w, 573 w, 537 w. 1H NMR
(399.8 MHz, CDCl3): 6.45 (s, H4 pzH, 1 H), 7.58 (s, H3,5 pzH, 1 H),
7.71 (s, H5,3 pzH, 1 H), 11,42 (s, NH, 1 H). 13C{1H} NMR (100.5 MHz,
CDCl3): 107.8 (s, C4 pzH), 119.1 (s, CF3SO3), 131.5 (s, C5,3 pzH), 143.1
(s, C3,5 pzH), 192.3 (s, 1 CO), 193.0 (s, 2 CO). 19F NMR (376.2 MHz,
CDCl3): −75.5 (s, CF3SO3). Anal. Calcd for C10H8F3N4O6ReS: C,
21.58; H, 1.45; N, 10.07. Found: C, 21.77; H, 1.36 ; N, 9.90.
fac-[Re(CO)3(NCMe)(NHC(Me)indz-κ2N,N)]OTf, 7c-OTf. Silver
triflate (0.085 g, 0.33 mmol) was added to a solution of 2c (0.152 mg,
0.30 mmol) in MeCN (30 mL). The mixture was refluxed for 2 h in the
absence of light. The reaction mixture was then filtered and dried in
vacuo. The yellow residue was crystallized in acetone/hexane at −20 °C,
giving a yellow microcrystalline solid, which was decanted, washed with
hexane (3 × 3 mL approximately), and dried in vacuo, yielding 0.158 g
(85%). IR (THF, cm−1): 2035 vs, 1939 vs, 1924 vs. IR (KBr, cm−1):
3194 w, 2027 vs, 1902 vs br, 1638 m, 1512 w, 1484w, 1424 m, 1355 m,
1276 s, 1253s, 1222s, 1149 s, 1091 w, 1030 w, 876 w, 800 w, 758 s, 668 w,
637 s, 572 w, 517 m, 485 m, 431 w. 1H NMR (376.2 MHz, Me2CO-d6):
2.38 (s, NCCH3, 3 H), 3.30 (s, NHCCH3, 3 H), 7.64 (t, J = 8.0 Hz, H6
indz, 1 H), 7.91 (t, J = 8.0 Hz, H5 indz, 1 H), 8.17 (d, J = 8.0 Hz, H7 indz,
1 H), 8.22 (d, J = 9.0 Hz, H4 indz, 1 H), 9.29 (s, H3 indz, 1 H), 10.87 (s,
br, NH, 1 H). 13C{1H} NMR (125.7 MHz, Me2CO-d6): 3.2 (s,
NCCH3), 21.4 (s, NHCCH3), 113.7 (s, C4 indz), 123.41 (s, C7 indz),
126.5 (s, C6 indz), 127.2 (s, C3a indz), 133.4 (s, C5 indz), 140.5 (s, C7a
indz), 147.4 (s, C3 indz), 167.8 (s, NCCH3), 190.2 (s, CO), 194.1 (s, 2
CO). 19F NMR (470.2 MHz, Me2CO-d6): −73.69 (s, CF3SO3). Anal.
Calcd for C15H12F3N4O6ReS: C, 29.03; H, 1.95; N, 9.03. Found: C,
28.74; H, 1.71; N, 8.88.
fac-[Re(CO)3(dmpzH)2(OTf)], 5b. Silver triflate (0.056 g, 0.22 mmol)
was added to a solution of 3b (0.108 g, 0.2 mmol) in THF (15 mL), and
the mixture was stirred for 2 h at room temperature in the absence of
light. Working up as for 5a gave 0.103 g (84%) of 5b as a colorless solid.
IR (THF, cm−1): 2034 s, 1928 vs, 1901 vs. IR (KBr, cm−1): 3446 m,
3349 m, 3288 m, 2036 vs, 1923 vs, 1917 vs, 1654 w, 1636 w, 1578 w,
1419 w, 1379 w, 1316 m, 1294 m, 1261 m, 1233 m, 1205 m, 1186 m,
1053 w, 1026 m, 819 w, 800 w, 661 w, 636 m, 518 w, 458 w, 419 w. 1H
NMR (399.8 MHz, CDCl3): 2.11 (s, C3H3 dmpzH, 6 H), 2.21 (s, C5H3
dmpzH, 6 H), 5.93 (s, H4 dmpzH, 2 H), 10.63 (s, NH, 2 H). 13C{1H}
NMR (100.5 MHz, CDCl3): 10.9 (s, C5H3 dmpzH), 14.5 (s, C3H3
Kinetic Studies. Reactions were monitored by 1H NMR using 0.05
M solutions prepared under Ar atmosphere. Pure NaBF4, NaClO4, and
NaOTf were prepared by adding NaOH solutions to the corresponding
acid solutions until pH = 7 (measured with a pH meter) and drying to
dryness. Studies with excess of NaOH (aq) were carried out by mixing
0.025 M solutions of the complex and a recently prepared 0.02 M
aqueous solution of NaOH. The NMR probe temperature was
calibrated using an ethylene glycol standard before and after the
experiment.
−
dmpzH), 106.7 (s, C4H dmpzH), 119.2 (q, J = 318.5 Hz, CF3SO3 ),
143.2 (s, CCH3 dmpzH), 153.7 (s, CCH3 dmpzH), 193.4 (s, CO), 193.5
(s, 2 CO). 19F NMR (376.2 MHz, CDCl3): −75.6 (s, CF3SO3). Anal.
Calcd for C14H16F3N4O6ReS: C, 27.49; H, 2.64; N, 9.16. Found: C,
27.24 ; H, 2.36 ; N, 8.99.
fac-[Re(CO)3(indzH)2(OTf)], 5c. Silver triflate (0.056 g, 0.22 mmol)
was added to a solution of 3c (0.117 g, 0.2 mmol) in THF (15 mL), and
the mixture was stirred for 2 h at room temperature in the absence of
light. Working up as for 5a gave 0.123 g (94%) of 5c as a colorless solid.
IR (THF, cm−1): 2038 vs, 1937 vs, 1910 vs. IR (KBr, cm−1): 3415 m,
3134 m, 3055 m, 2974 m, 2880 m, 2036 vs, 1924 vs, 1906 vs, 1630m,
1516 w, 1360m, 1338m, 1234 s, 1199 s, 1174 s, 1150 m, 1084 w, 1050 m,
1014 s, 886 w, 839 w, 751 m, 658 w, 631 m, 572 w, 526 w, 482 w, 433 w.
1H NMR (399.8 MHz, CDCl3): 7.26 (m, H6 indzH 2 H), 7.51−7.53 (m,
H5 and H7 indzH, 4 H), 7.71 (d, J = 8.5 Hz, H4 indzH, 2H), 8.15 (s, H3
indzH, 2 H), 11.39 (s, NH indzH, 2 H). 13C{1H} NMR (100.5 MHz,
Crystal Structure Determination for Compounds 2c, 3b, 3c,
4a-ClO4, 5b-OTf, 5b-ClO4, and 7c-ClO4. Crystals were grown by slow
diffusion of hexane into concentrated solutions of the complexes in
chloroform (for 3b, 3c, 4a-ClO4, 5b-OTf) or acetone (for 2c, 7c-ClO4)
at −20 °C. Relevant crystallographic details can be found in the CIF. For
all complexes except 2c, a crystal was attached to a glass fiber and
transferred to a Bruker AXS SMART 1000 diffractometer with graphite
monochromatized Mo Kα X-radiation and a CCD area detector. Raw
frame data were integrated with the SAINT program.19 The structure
was solved by direct methods with SHELXTL.20 A semiempirical
absorption correction was applied with the program SADABS.21 All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were set in calculated positions and refined as riding atoms, with a
common thermal parameter. All calculations and graphics were made
with SHELXTL. Distances and angles of hydrogen bonds were
calculated with PARST22 (normalized values).23 For 2c, crystal structure
determination was accomplished on an Oxford Diffraction Xcalibur S
diffractometer: data collection, CrysAlis CCD;24 cell refinement,
CrysAlis RED;24 data reduction, CrysAlis RED; absorption correction,
multiscan.24 Crystal structure determinations were effected at −100 °C
(Mo Kα radiation, α = 0.71073 Å). The structure was solved by applying
direct and Fourier methods using SHELXS97 and SHELXL-97.20 All
nonhydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in geometrically calculated positions, and each was assigned a
fixed isotropic displacement parameter based on a riding model.
−
CDCl3): 110.5 (s, C7 indzH), 119.1 (q, J = 314.0 Hz, CF3SO3 ), 120.1
(s, C4 indzH), 122.6 (s, C3a indzH), 123.2 (s, C6 indzH), 129.9 (s, C5
indzH), 138.8 (s, C3 indzH), 140.6 (s, C7a indzH), 192.4 (br, 1 CO),
192.9 (br, 2 CO). 19F NMR (376.2 MHz, CDCl3): −75.4 (s, CF3SO3).
Anal. Calcd for C18H12F3N4O6ReS: C, 32.98; H, 1.84; N, 8.55. Found:
C, 32.68; H, 1.76; N, 8.49.
fac-[Re(CO)3(indzH)(NHC(Me)indz-κ2N,N)]OTf, 6c-OTf.18 A sol-
ution of 5c-OTf (0.197 g, 0.3 mmol) in MeCN (15 mL) was stirred for
10 h at room temperature. Volatiles were removed in vacuo to give a
yellow solid, which was recrystallized from THF/hexane at −20 °C,
giving a yellow microcrystalline solid, which was decanted, washed with
hexane (3 × 3 mL approximately), and dried in vacuo, yielding 0.186 g
(89%). IR (THF, cm−1): 2035 vs, 1939 vs, 1924 vs. IR (KBr, cm−1):
3434 m, 3125 m, 2976 m, 2874 w, 2031 vs, 1916 vs br, 1631 m, 1514 w,
1480 m, 1463 w, 1425 m, 1356 m, 1281 w, 1194 w, 1117 s, 1086 s, 1044
m, 915 w, 884 w, 845 w, 795 w, 753 m, 624 m, 535 w, 521 w, 475w. 1H
NMR (399.8 MHz, Me2CO-d6): 3.32 (s, NHCCH3, 3 H), 7.19 (t, J =
7.5 Hz, H5 indzH, 1 H), 7.47 (t, J = 7.5 Hz, H6 indzH, 1 H), 7.55 (d, J =
8.5 Hz, H7 indzH, 1 H), 7.59 (t, J = 7.5 Hz, H5 indz, 1 H), 7.71 (d, J = 8.5
Hz, H4 indzH, 1 H), 7.82 (t, J = 8.0, H6 indz 1 H), 8.11 (d, J = 9.0 Hz, H7
indz, 1 H), 8.14 (d, J = 9.0 Hz, H4 indz, 1 H), 8.36 (s, H3 indzH, 1 H),
9.47 (s, H3 indz, 1 H), 10.86 (s, NHCCH3, 1 H), 12.81 (s, NH indH, 1
H). 13C{1H} NMR (100.5 MHz, Me2CO-d6): 21.6 (s, NHCCH3),
111.2 (s, C7 indzH), 113.7 (s, C7 indz), 121.8 (s, C4 indzH), 123.5 (s, C5
indzH), 124.4 (s, C4 indz), 126.6 (s, C5 indz), 127.2 (C3a, the other C3a
was not observed), 130.3 (s, C6 indzH), 133.4 (s, C3 indzH), 140.6 (C7a),
142.1 (C7a), 148.0 (s, C3 indz), 168.3 (s, NHCCH3), 191.6 (s, CO),
195.2 (s, CO), 195.4 (s, CO). 19F NMR (376.2 MHz, Me2CO-d6):
−73.7 (s, CF3SO3). Anal. Calcd for C20H15F3N5O6ReS: C, 34.43; H,
2.17; N, 10.04. Found: C, 34.79; H, 2.29; N, 9.86.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data for compounds 2c, 3b, 3c, 4a-ClO4,
5b-OTf, 5b-ClO4, and 7c-ClO4 in CIF format; figures of the
crystal structures of fac-[ReBr(CO)3(dmpzH)2], 3b, [Re-
(CO)3(dmpzH)2(OClO3)], 5b-ClO4, representations and tables
of kinetic experiments. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Inorganic Chemistry
Article
(n) Huang, C.-Y.; Kuan, K.-Y.; Liu, Y.-H.; Peng, S.-M.; Liu, S.-T.
AUTHOR INFORMATION
Corresponding Author
*Phone: 34 983 184620. Fax: 34 983 423013. E-mail: fervilla@qi.
uva.es.
■
Organometallics 2014, 33, 2831−2836. (o) Muller, K.; Sun, Y.;
Heimermann, A.; Menges, F.; Niedner-Schatteburg, G.; van Wullen,
̈
C.; Thiel, W. R. Chem.Eur. J. 2013, 19, 7825−7834. (p) Ferrer, I.;
Rich, J.; Fontrodona, X.; Rodríguez, M.; Romero, I. Dalton Trans. 2013,
42, 13461−13469. (q) Ojwach, S. O.; Okemwa, T. T.; Attandoh, N. W.;
Omondi, B. Dalton Trans. 2013, 42, 10735−10745. (r) Milek, M.;
Heinemann, F. W.; Khusniyarov, M. M. Inorg. Chem. 2013, 52, 11585−
11592. (s) Shavaleev, N. M.; Monti, F.; Scopelliti, R.; Baschieri, A.;
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Spanish Ministerio de Ciencia e
■
Sambri, L.; Armaroli, N.; Gratzel, M.; Nazeeruddin, M. K. Organo-
̈
metallics 2013, 32, 460−467.
Innovacion
́
(CTQ2009-12111) and the Junta de Castilla y
(5) Reisner, E.; Arion, V. B.; Rufiska, A.; Chiorescu, I.; Schmid, W. F.;
Keppler, B. K. Dalton Trans. 2005, 2355−2364.
Leon
́
(VA070A08 and GR Excelencia 125) for financial support.
M.A. thanks the MEC (FPI Program) and P. G.-I. the UVa for
their grants.
́
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