Catalytic carbon-hydrogen bond functionalization in an ionic liquid medium

Article abstract of DOI:10.1021/om700822a

The functionalization of unreactive C-H bonds of several hydrocarbons has been achieved at room temperature by the formal insertion of carbene/CHCO ₂Et units (from ethyl diazoacetate) into such bonds, using copper-based catalysts in a biphasic reaction medium (ionic liquid-hydrocarbon). Catalyst and product separation has been carried out easily, and the recycle of the catalyst for five cycles has been achieved without any loss of catalytic activity.

Full text of DOI:10.1021/om700822a

Organometallics 2007, 26, 6661–6668
6661
Catalytic Carbon-Hydrogen Bond Functionalization in an Ionic
Liquid Medium
Pilar Rodríguez,† Eleuterio Álvarez,‡ M. Carmen Nicasio,*,† and Pedro J. Pérez*,†
Laboratorio de Catálisis Homogénea, Departamento de Química y Ciencia de los Materiales, Unidad
Asociada al CSIC, Campus de El Carmen s/n, UniVersidad de HuelVa, 21007 HuelVa, Spain, and Instituto
de InVestigaciones Químicas, CSIC-UniVersidad de SeVilla, AVenida Américo Vespucio 49,
41092 SeVilla, Spain
ReceiVed August 11, 2007
The functionalization of unreactive C-H bonds of several hydrocarbons has been achieved at room
temperature by the formal insertion of carbene/CHCO2Et units (from ethyl diazoacetate) into such bonds,
using copper-based catalysts in a biphasic reaction medium (ionic liquid-hydrocarbon). Catalyst and product
separation has been carried out easily, and the recycle of the catalyst for five cycles has been achieved
without any loss of catalytic activity.
Scheme 1. Examples of the Catalytic Functionalization of
Hydrocarbons by Carbene Insertion from Diazo Compounds
1. Introduction
The catalytic decomposition of diazo compounds and the
subsequent insertion of a carbene group into a C-H bond
constitute one of the very few examples of effective catalytic
functionalization of hydrocarbons (eq 1).1,2 With alkanes as the
substrate, such a transformation has been achieved with
rhodium-,3–5 copper-,6,7 silver-,8,9 and gold-based10 catalysts
under mild conditions. For instance, this methodology has
allowed the direct conversion of n-alkanes such as pentane or
hexane into mixtures of esters (Scheme 1a) or benzene into ethyl
(1)
phenylacetate (Scheme 1b). The asymmetric reaction has also
been reported by Davies and co-workers (Scheme 1c), with
chiral rhodium catalysts. These systems work under homoge-
neous conditions; therefore, product separation and catalyst
recovery and recycling are not an easy operation. Immobilization
of catalysts for carbene transfer from diazo compounds has been
reported in a few cases, mainly for the olefin cyclopropanation
reaction, using inorganic solids,11 fluorous solvents,12 polymeric
materials,13,14 or ionic liquids.15 In a recent report from this
laboratory, we have described16 a catalytic system for the
transfer of :CHCO2Et units from ethyl diazoacetate (EDA) to
saturated and unsaturated substrates with the complex
[Tpm*Cu(NCMe)]PF6 (1; Tpm* ) HC(3,5-Me2pz)3) as the
* To whom correspondence should be addressed. E-mail: perez@
dqcm.uhu.es, mcnica@dqcm.uhu.es.
† Universidad de Huelva.
‡ CSIC-Universidad de Sevilla.
(1) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods
for Organic Synthesis with Diazo Compounds; John Wiley & Sons: New
York, 1998.
(2) Díaz-Requejo, M. M.; Belderraín, T. R.; Nicasio, M. C.; Pérez, P. J.
Dalton Trans. 2006, 5559.
(7) (a) Díaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.;
Trofimenko, S.; Pérez, P. J. J. Am. Chem. Soc. 2002, 124, 896. (b) Caballero,
A.; Díaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko,
S.; Pérez, P. J. J. Am. Chem. Soc. 2003, 125, 1446. (c) Caballero, A.; Díaz-
Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Pérez,
P. J. Organometallics 2003, 22, 4145. (d) Fructos, M. R.; de Frémont, P.;
Nolan, S. P.; Díaz-Requejo, M. M.; Pérez, P. J. Organometallics 2006, 25,
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10.1021/om700822a CCC: $37.00
2007 American Chemical Society
Publication on Web 11/21/2007

6662 Organometallics, Vol. 26, No. 26, 2007
Rodríguez et al.
Scheme 2. Catalytic Transfer of :CHCO2Et from EDA under Biphasic Conditions Using [bmim][PF6]
Scheme 3. Catalytic Cycle for C-H Bond Functionalization
catalyst and using the ionic liquid [bmim][PF6] (bmim ) 1-n-
butyl-3-methylimidazolium hexafluorophosphate) and hexane as
the biphasic reaction mixture. The insertion reactions into N-H
and O-H bonds were quantitative, and the catalyst was
recovered and recycled five times. However, when cyclohexane
was used as the substrate, only 35% conversions were observed
(Scheme 2). It is worth mentioning that, in spite of this low
yield, this is the unique example, to the best of our knowledge,
of the intermolecular catalytic functionalization of a C-H bond
by this methodology using an ionic liquid as a reaction medium.
Actually, under biphasic conditions, we are only aware of two
examples, described by Maas et al.12a–c and Biffis et al.12d in
which rhodium(II) complexes containing perfluoro chains were
employed as the catalyst for the carbene transfer from methyl
diazoacetate to the C-H bonds of hexane.
by Carbene Insertion from Diazo Compounds
2. Results and Discussion
Synthesis and Characterization of Tris(3-mesitylpyrazol-
1-yl)methane (TpmMs) and Bis(3-mesitylpyrazol-1-yl)(5-
mesityl-pyrazol-1-yl)methane (TpmMs*) and Their Copper(I)
[TpmxCu(NCMe)]PF6 Complexes. As mentioned above, the
complex [Tpm*Cu(NCMe)]PF6 induced decomposition of EDA
and the subsequent transfer of the :CHCO2Et unit to several
substrates using an ionic liquid as the reaction medium.
However, cyclohexane was converted into ethyl cyclohexylac-
etate in low yield (35 %), with diethyl fumarate (DEF) and
maleate (DEM) being the major products in this case. This result
is due to the low nucleophilicity of the C-H bond of
cyclohexane; therefore, the electrophilic metallocarbene inter-
mediate presumably formed during this transformation,1 being
not reactive enough toward the hydrocarbon (Scheme 3). Two
factors affect the interaction of such an intermediate and the
C-H bond: the electrophilicity of the metallocarbene species
and the steric protection of the metal center. The low electronic
density at the metal center favors not only its reaction with the
diazo compound but also further interaction with the C-H bond.
In addition, the steric protection of the MdCR1R2 unit seems
On the basis of our preliminary results16 and of our experience
in the design of copper-based catalysts for C-H bond func-
tionalization with EDA,7 we decided to focus on the synthesis
of new catalysts capable of inducing such a transformation with
low reactive, unmodified alkanes and using an ionic liquid as
the reaction medium to favor catalyst separation and recycling.
In this paper, we report the synthesis and structural characteriza-
tion of two copper complexes that encompass that goal and that
can be recovered and reused several times without loss of
activity.
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A.; Maas, G. J. Organomet. Chem. 2002, 643, 174–644. (c) Endres, A.;
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Cadamuro, S.; Tubaro, C.; Basato, M. Org. Lett. 2005, 7, 1841. (e) Urbano,
J.; Izarra, R.; Gómez-Ariza, J. L.; Trofimenko, S.; Díaz-Requejo, M. M.;
Pérez, P. J. Chem. Commun. 2006, 1000.
(13) (a) Doyle, M. P.; Yan, M.; Gau, H.-M.; Blossey, E. C. Org. Lett.
2003, 5, 561. (b) Doyle, M. P.; Timmons, D. J.; Tumonis, J. S.; Gau, H.-
M.; Blossey, E. C. Organometallics 2002, 21, 1747. (c) Burguete, M. I.;
Fraile, J. M.; García, J. I.; García-Verdugo, E.; Luis, S. V.; Mayoral, J. A.
Org. Lett. 2000, 2, 3905.
(14) (a) Davies, H. M. L.; Walji, A. M. Org. Lett. 2005, 7, 2941. (b)
Davies, H. M. L.; Walji, A. M.; Nagashima, T. J. Am. Chem. Soc. 2004,
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Nagashima, T.; Davies, H. M. L. Org. Lett. 2002, 4, 1989.
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Pérez, P. J. Org. Lett. 2006, 8, 557.

Catalytic C-H Bond Functionalization
Organometallics, Vol. 26, No. 26, 2007 6663
sized, the TpMs* ligand is the only compound formed.21 It
requires heating at high temperature to induce isomerization to
TpMs. In our case, the formation of both compounds 2 and 3
occurs under the reaction conditions (70 °C, 24 h). However,
prolonged heating of solutions of pure samples of 2 and 3 at
100 °C did not show any conversion of one into the other.
The structural data in 2 and 3 are quite similar to those of
the 3-phenyl derivative, already reported by Reger and co-
workers.22 The structure of 2 consists of discrete molecules with
aromatic-aromatic intermolecular associations, π stacking, and
edge-to-face interactions. The orientation of the three pyrazolyl
rings is a propeller arrangement with only one of the donor N
atoms oriented toward the potential bonding “pocket” of the
ligand. This arrangement is in good agreement with published
results for comparable molecules.18 Therefore, the molecule does
not have threefold symmetry. In the case of 3, a similar structure
with aromatic-aromatic intermolecular associations, π stacking,
and edge-to-face interactions, as well as with the orientation of
the three pyrazolyl rings in a propeller arrangement, has been
determined.
The above ligands have been employed to prepare new
copper(I) complexes by their direct reaction with CuI in
acetonitrile as the solvent, at room temperature, followed by
the addition of AgPF6 to promote chloride extrusion and
acetonitrile coordination (eq 2). Crystalline materials of com-
position [TpmxCu(NCMe)]PF6 (Tpmx ) TpmMs, 4; TpmMs*, 5)
have been isolated in 65–70% yield. NMR data is in accordance
with the existence of a Tpmx ligand coordinated to the copper
center, with patterns of signals similar to those of the free
ligands, as well as those of a coordinated acetonitrile molecule.
These data are in accordance with a structure in which the Cu
ion is bonded to the three N donors of the Tpmx ligand and to
the N donor of the acetonitrile ligand, in a tetrahedral environ-
ment, similarly to other related cationic complexes.18,23 Figure
3 shows the structure of the molecules of 4 obtained by X-ray
diffraction studies, which corroborates this proposal. The CuI
ion is tetrahedrally coordinated by three N atoms of TpmMs and
one N atom of acetonitrile. The Cu-NCMe unit is slightly
deviated from linearity, as is inferred from the value of the
Cu1-N7-C38 angle of 165.5°, in contrast with other analogous
complexes that show an almost linear Cu-N-C unit
(176–179°).23a The Cu-N distance to the N atom of acetonitrile
1.877(2) Å is almost identical with that of analogously described
complexes.18,23 The pyrazolyl and mesityl rings are not in the
same plane, with the torsion angles between the rings being
122.5°, 113.8°, and 110.4°. This implies that the steric hindrance
at the position of the pyrazolyl ring located in the vicinity of
the metal center (Ms, tBu, iPr) does not significantly affect the
structures of the acetonitrile complexes.
Figure 1. Proposed geometries for the molecules of 2 and 3.
to disfavor the formation of the coupling products DEF and
DEM. A few years ago, we applied this strategy with the
complex TpMsCu,7a which catalyzed the insertion of :CHCO2Et
(from EDA) into the C-H bonds of cycloalkanes or cyclic
ethers. With the idea of preparing a similar catalyst that could
operate under biphasic conditions in a manner similar to that
found with [Tpm*Cu(NCMe)]PF6, we decided to synthesize the
related TpmMs ligand HC(3-Ms-pz)3, which could later be
employed for the preparation of the complex analogue to 1.
The synthesis of the TpmMs ligand was performed following
the method described by Reger and co-workers18 for related
ligands. 3-Mesitylpyrazole was deprotonated and further reacted
with chloroform at 70 °C for 24 h. After workup, NMR studies
carried out with the reaction crude revealed the existence of
two compounds. One of them showed a set of signals for three
equivalent pyrazolyl rings, whereas a second, major species
showed all of the pyrazolyl resonances split into a 2:1 ratio.
Separation of both compounds was achieved by column
chromatography, affording the new compounds with analytical
purity. On the basis of the previous literature,18,19 it is reasonable
to assume that the compound with the three equivalent pyrazolyl
groups corresponds to the targeted TpmMs (2), with the other
having a less symmetric structure with two pyrazolyl rings
oriented in the opposite direction from the third one (Figure 1).
By analogy with the trispyrazolylborate case, the latter has been
named as TpmMs* (3). The proposal of the bulkier groups
pointing away from the methinic C-H bond is also a resem-
blance of the Tpx ligands. However, very recently we have
described a series of typical Tpx ligands20 in which the bulkier
groups were located in the vicinity of the B-H bond. Thus, to
unambiguously establish the structure of 2 and 3, X-ray
diffraction studies have been performed with single crystals of
both, leading to the establishment of the actual orientation of
the pyrazolyl rings. Figure 2 displays the structures of 2 and 3,
where the actual orientation of the pyrazolyl rings is demon-
strated. In the case of TpmMs, the three 3-mesitylpyrazolyl
groups are oriented with the mesityl group in the opposite
direction of the unique C-H bond, whereas for TpmMs* one of
the pyrazolyl rings displays a reverse direction. It is worth
mentioning that although these compounds are similar to their
related trispyrazolylborate ligands TpMs and TpMs* (Table 1),
there is a significant difference in both systems. When synthe-
(2)
Catalytic Functionalization of Alkane C-H Bonds. As
mentioned above, previous work16 from this laboratory has
(21) Trofimenko, S. Scorpionates, The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, 1999.
(22) Reger, D. L.; Grattan, T. C.; Brown, K. J.; Little, C. A.; Lamba,
J. J. S.; Rheingold, A. L.; Sommer, R. D. J. Organomet. Chem. 2000, 607,
120.
(17) Davies, H. M. L.; Beckwith, R. E. J. Chem. ReV. 2003, 103, 2861.
(18) Reger, D. L.; Collins, J. E.; Rheingold, A. L.; Liable-Sands, L. M.
Organometallics 1996, 15, 2029.
(19) (a) Trofimenko, S. J. Am. Chem. Soc. 1970, 92, 5118. (b) Julia, S.;
del Mazo, J. M.; Ávila, L.; Elguero, J. Org. Prep. Proced. Int. 1984, 16,
299.
(23) (a) Fujisawa, K.; Ono, T.; Ishikawa, Y.; Amir, N.; Miyashita, Y.;
Okamoto, K.; Lehnert, N. Inorg. Chem. 2006, 45, 1698. (b) Cvetkovic,
M.; Batten, S. R.; Moubaraki, B.; Murray, K. S.; Spiccia, L. Inorg. Chim.
Acta 2001, 324, 131.
(20) Yap, G. P. A.; Jove, F.; Urbano, J.; Álvarez, E.; Trofimenko, S.;
Díaz-Requejo, M. M.; Pérez, P. J. Inorg. Chem. 2007, 46, 780.

6664 Organometallics, Vol. 26, No. 26, 2007
Rodríguez et al.
Figure 2. Molecular structures of (left) tris(3-mesitylpyrazol-1-yl)methane (TpmMs, 2) and (right) bis(3-mesitylpyrazol-1-yl)(5-mesityl-
1pyrazol-1-yl)methane (TpmMs*, 3). Thermal ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity.
Table 1. Trispyrazolylmethane and Trispyrazolylborate Ligands Relevant to This Work
a Ms ) 2,4,6-Me3C6H2.
liquids. Only in the case of its insertion into the C-H bonds of
cyclohexane were the conversions low (35%), whereas nearly
no insertion was observed in the case of other alkanes such as
n-hexane or 2,3-dimethylbutane. Once the complexes 4 and 5
were prepared, we screened their catalytic potential toward this
transformation using cyclohexane as a model substrate (eq 3).
The results were quite stimulating because conversions of ca.
75% were observed in both cases under homogeneous conditions
(a cyclohexane/dichloromethane mixture as the solvent) using
0.05 mmol as the catalyst loading and 1 mmol of EDA (see the
Experimental Section). These conversions were quite higher
compared with those obtained with 1, and the cationic nature
of 4 and 5 allowed the extension of the study to the ionic liquid
[bmim]PF6 as the reaction medium.
Figure 3. Molecular structure of 4. Thermal ellipsoids are drawn
at the 30% probability level. The H atoms, the PF6 anion, and the
three MeCN solvation molecules have been omitted for clarity.
(3)
shown that the complex 1 catalyzed the decomposition of EDA
and the transfer of the :CHCO2Et unit to several unsaturated
and saturated substrates under biphasic conditions in ionic
As a general procedure, a solution of the copper catalyst in
the ionic liquid was prepared and the hydrocarbon (cyclohexane,

Catalytic C-H Bond Functionalization
Organometallics, Vol. 26, No. 26, 2007 6665
Table 2. Carbene Transfer from EDA to Alkane C-H Bonds Using 4 and 5 as Catalysts, under Homogeneous or Biphasic (Ionic Liquid)
Conditionsa
a Reactions performed at room temperature with 0.05 mmol of the catalyst, 5% with respect to EDA. b Percentage of products, based on EDA
(average of two runs), determined by 1H NMR with trimethylvinylsilane as the internal standard. DEF and DEM accounted for the remaining EDA.
c C2:C3 regioselectivity, determined by gas chromatography.
n-hexane, or 2,3-dimethylbutane) was added as a second layer.
The diazo compound dissolved in the hydrocarbon was added
with the aid of a syringe pump. After the addition, the organic
layer was investigated by NMR using an internal standard. The
results are shown in Table 2. It is clearly observable that the
catalytic activity shown in the homogeneous phase (the hydro-
carbon as the sole solvent) was nearly identical with that under
biphasic conditions. It is well-known that one of the main
drawbacks of homogenous catalysis is the problem of catalyst
and product separation, and immobilization of homogeneous
catalysts is a common strategy to surpass such a problem.
However, in many cases, such immobilization supposes a certain
loss of the degree of the activity and/or selectivity of the catalyst.
In our case, the transition from homogeneous to biphasic
conditions does not alter either the activity or the selectivity.
Data in Table 2 indicate that, as is usually observed in these
types of transformations, cyclohexane C-H bonds are readily
functionalized in comparison with those of other less constrained
alkanes. The low reactivity of n-hexane precluded higher
conversions, although only very few examples in the literature
have reported moderate to high yields with this substrate. No
discrimination about the use of 4 or 5 for the functionalization
of cyclohexane or n-hexane seems to exists, but when 2,3-
dimethylbutane was employed, a substantial difference was
observed. This alkane was modified in 40-45% yield (homo-
geneous and biphasic, respectively) with 5 as the catalyst,
whereas the use of 4 only provided ca. 10%. Because the only
reactive site is the tertiary C-H bond, i.e., the most sterically
hindered, a perceptible effect of the size of the catalytic pocket
must exist to account for this behavior. The existence of one
“inverted” pyrazolyl ring in the molecules of 5, with one of the
mesityl groups in the methine side and far away from the metal,
should enlarge the catalytic pocket, facilitating the functional-
ization of the tertiary sites of this substrate. This could also
explain the regioselectivity observed in the case of n-hexane,
where the C2:C3 functionalization ratio is slightly higher with
4 as the catalyst.
was added, and EDA was introduced in the reaction mixture.
After five cycles, the conversions of 75, 72, 71, 73, and 68%
could be considered to be constant, within experimental error.
It is worth mentioning that after each cycle the mass balance
was verified by NMR in the organic layer; therefore, all of the
initial EDA was converted into the functionalized alkane and a
mixture of DEF and DEM. In addition, in order to verify a
potential leaching of the copper complex into the organic layer,
we charged the latter with more EDA. After 24 h, all of the
added EDA remained unreacted, indicating the inexistence of
appreciable amounts of active copper species in the extract. We
can conclude at this stage that these compounds operate under
biphasic conditions in a manner similar to that under homoge-
neous conditions and with the advantage of being separated and
recycled several times with no loss of activity. To the best of
our knowledge, these results find no precedent in the catalytic
functionalization of C-H bonds by carbenoid insertion. This
is the first example of efficient catalyst recovery and recycle in
an ionic liquid. The already mentioned work by Endres and
Maas,12a-c the unique reported example of this transformation
under biphasic conditions (a fluorous medium), gave a ca. 40%
conversion with n-hexane in the first cycle and 10% in the
second, with no more consecutive runs being described.
IR and Kinetic Studies: Insights for Further Catalyst
Development. As shown in Scheme 3, the main requirement
for a transition metal to catalyze these transformations is to
display a certain electrophilic character at the metal center.1,2,17
Such nature would enhance its interaction with the diazo
compound and, once the metallocarbene intermediate is formed,
also would enhance the electrophilia at the carbene C atom,
therefore favoring the reaction with the nucleophile, the C-H
bond. Because the latter is a poor nucleophile, increasing the
electrophilia at the metal center is the main strategy to improve
catalytic activities. Only a few systems have been reported to
induce these transformations, some of them being based in
copper or silver complexes of type TpxM containing trispyra-
zolylborate ligands (Tpx).7a–c,8,9 In this case, a useful tool to
estimate the electron density at the metal center is to evaluate
the ν(CO) of the carbonyl adducts TpxCu(CO). A correlation
between ν(CO) and the catalytic activity is usually observed,
in such a way that the higher the ν(CO) values, the higher the
catalytic activity. Therefore, we have applied this methodol-
ogy to complexes 4 and 5, upon bubbling CO throughout
Once the catalytic capabilities of these complexes for the
functionalization of C-H bonds of unreactive alkanes were
demonstrated, the separation and recycling of the catalyst was
tested for five cycles. Thus, once the reaction was finished, the
organic layer was separated, the ionic liquid was washed twice
with the same hydrocarbon before the next loading of alkane

6666 Organometallics, Vol. 26, No. 26, 2007
Rodríguez et al.
Figure 4. Plots of the evolution of N2 and of ln [EDA] vs time for the EDA decomposition using 1 (2) and 4 (0) as the catalysts.
Scheme 4. Catalytic Cycle for the EDA Decomposition
their solutions in CH2Cl2 and registering their IR spectra in
solution. A sharp absorption was observed in both cases
Reaction in the Presence of 1 or 4 as the Catalyst
centered at 2100 and 2113 cm-1 for [TpmMsCu(CO)]PF6 and
[TpmMs*Cu(CO)]PF6, respectively. Interestingly, the carbonyl
adduct of Tpm*, already reported by Reger and co-workers,18
also displayed ν(CO) at 2113 cm-1. This behavior is in contrast
with the aforementioned trispyrazolylborate-related complexes.
Thus, the values of ν(CO) for Tp*Cu(CO) and TpMsCu(CO)
are respectively 2056 and 2079 cm-1, which correlate with a
moderate activity of the latter and a nearly nonactivity of the
former toward the functionalization of C-H bonds by this
methodology. Back to the trispyrazolylmethane systems, both
TpmMs- and TpmMs*-containing complexes 4 and 5 display
higher activities than 1, the Tpm*-based catalyst. A very recent,
elegant work23a by Fujisawa and co-workers has demonstrated,
on the basis of experimental and theoretical calculations, that
Tpmx ligands are weaker donors than Tpx ligands when bonded
to CuI. They also propose that ν(CO) values can be employed
rate copper catalysts, for which we have already reported kinetic
data. On the basis of this previous work, we propose a pathway
similar to that depicted in Scheme 4. Starting from the
acetonitrile adducts 1 or 4, the first step would involve the
dissociation of the acetonitrile ligand to give an unsaturated
species A, which would react with EDA to afford the
copper-carbene intermediate (B), similar to those already
independently reported by Straub and Hofman26 and Dai and
Warren.27 Further reaction with a second molecule of EDA
would lead to the formation of the alkenes and the release of A
to restart the catalytic cycle. The aforementioned observation
of first-order kinetics supports this proposal. Assuming that B
is under steady-state conditions, the following rate law for this
transformation can be deduced:28
as a probe for the measurement of the electron density at the
metal center. We believe that this can be applied with the
exception of aryl substituents in the C3 position of the pyrazolyl
ring. Dias and Kim described the [TpmPhCu(CO)]+ complex24
to show a 2104 cm-1 band, and our mesityl-containing ligands
have provided cationic copper(I) complexes with absorptions
similar or lower than that with Tpm*. In no case did these
complexes provide ν(CO) absorptions higher than 2113 cm-1
.
The question that arises from these results is about the electron
density at the metal center with these ligands and whether or
not the replacement of Tpm* by TpmMs or TpmMs* provides
the expected decrease in the electron density, a crucial feature
for the improvement of the catalytic activity.
To validate the proposal of the electron density at the metal
center, we have performed kinetic studies using the complexes
1 and 4 as the catalyst for the sole decomposition of EDA into
DEF and DEM. Figure 4 shows the evolution of N2 with time
as well as the plot of ln [EDA] vs time. It is clearly observable
that the complex with the TpmMs ligand decomposes EDA faster
than that with Tpm*; i.e., 4 is more active for the diazo
compound decomposition. The observation of first-order kinetics
is in accordance with the formation of the metallocarbene
intermediate as the rate-determining step,25 in a fashion similar
to that of previous studies, including the related trispyrazolylbo-
2k1KL[Cu]T
-d[EDA]
)
[EDA] ) kobs[EDA]
dt
KL + [MeCN]
2k1KL[Cu]T
kobs
)
KL + [MeCN]
where [Cu]T is the total amount of the copper catalyst added
initially. Data from kinetic studies show kobs values of 5.2 ×
10-2 and 1.8 × 10-2 s-1 and [Cu]T values of 3.12 × 10-5 and
6.25 × 10-5 M for 4 and 1 as the catalysts, respectively. On
(24) Dias, H. V. R.; Kim, H.-J. Organometallics 1996, 15, 5374.
(25) (a) Díaz-Requejo, M. M.; Caballero, A.; Belderrain, T. R.; Nicasio,
M. C.; Trofimenko, S.; Pérez, P. J. J. Am. Chem. Soc. 2002, 124, 978. (b)
Díaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Prieto, F.; Pérez,
P. J. Organometallics 1999, 18, 2601.
(26) Straub, B. F.; Hofmann, P. Angew. Chem., Int. Ed. 2001, 40, 1288.
(27) Dai, X. L.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085.
(28) Derivation of kinetic equations is provided in the Supporting
Information.

Catalytic C-H Bond Functionalization
Organometallics, Vol. 26, No. 26, 2007 6667
chromatograph with a flame ionization detector. Elemental analyses
were performed in the Microanalytical Laboratory of the Instituto
de Investigaciones Químicas Isla de la Cartuja.
the basis of X-ray data that show nearly identical Cu-NCMe
distances for both complexes 1 and 4, we can assume that the
coordination-decoordination equilibria would not differ much
from one to another, with the values of KL also being similar.
Thus, we can write the previous equation as follows:
Tris(3-mesitylpyrazol-1-yl)methane and Bis(3-mesitylpyrazol-
1-yl)(5-mesityl1pyrazol-1-yl)methane. To a mixture of 3-mesi-
tylpyrazole (2 g, 0.011 mol) and tetra-n-butylammonium bromide
(0.21 g, 0.67 mmol) was added distilled water (20 mL). With
vigorous stirring, sodium carbonate (7.2 g, 0.07 mol) was added
slowly to the reaction mixture. After the mixture was cooled to
room temperature, chloroform (10 mL) was added, and the reaction
mixture was heated at 70 °C for 24 h. A dark-orange suspension
was obtained, which was allowed to cool to room temperature and
filtered to remove the excess of base. The organic layer was
separated from the aqueous layer. The aqueous layer was extracted
with diethyl ether (3 × 10 mL), the combined organic fractions
were washed with distilled water (20 mL) and dried (MgSO4), and
the solvent was evaporated under vacuum, yielding a yellow solid.
1H NMR analysis of the reaction crude in CDCl3 showed the
presence of the desired isomer and the isomer with two pyrazole
rings with the mesityl group at the 3 position and the third ring
with the mesityl group at the 5 position in a 1:2 ratio. The isomers
could be separated by flash chromatography on silica gel (3 × 15
cm; petroleum ether/ether diethyl 4:1; 10 mL fractions). The major
isomer was eluted first, and evaporation of the solvent gave 0.62 g
of HC(3-Ms-pz)2(5-Ms-pz) as a pale-yellow solid. The desired
compound was eluted next, and evaporation of the solvent afforded
0.33 g of HC(3-Ms-pz)3 as a cream-colored solid. Yield: 0.95 g,
45% (based on 3-mesitylpyrazole).
2KL
kobs ) k′k1[Cu]T where k′ )
KL + [MeCN]
Under the previous assumption and from data in Figure 4,
we can calculate the ratio k1(4)/k1(1) as 5.7, demonstrating that
the incorporation of the mesityl group into the pyrazolyl rings
induced a considerable enhancement of the catalytic activity of
the corresponding copper complex. It is worth returning to the
already mentioned ν(CO) values for these complexes: in spite
of the frequency trend observed for [TpmxCu(CO)]PF6, Tpm*
) TpmMs* > TpmMs, the Tpm*-based catalyst is less active
than 4 (or 5). Therefore, the use of IR data in this family of
complexes, when including aromatic substituents in the pyra-
zolyl rings, as an estimation of the electron density at the metal
center should be taken with precautions.
Therefore, the above results indicate that the replacement of
3,5-dimethylpyrazole by mesitylpyrazole in the Tpmx ligands
exerts a decrease in the electron density at the metal center,
which cannot be envisaged using the traditional method based
in IR studies of carbonyl adducts. Such a decrease affects the
catalytic activity of those complexes toward the decomposition
of EDA and the subsequent insertion of the :CHCO2Et unit into
the C-H bonds of alkanes. On the basis of this knowledge, the
design of more electrophilic copper complexes containing novel
Tpmx ligands that could promote higher degrees of activation
of C-H bonds of alkanes is currently underway in our
laboratory.
HC(3-Ms-pz)3 (TpmMs, 2). 1H NMR (400 MHz, CDCl3): δ 8.47
(s, 1H, HC), 7.69 (d, 3H, CH(pz), JHH ) 2.4 Hz), 6.91 (s, 6H,
CH(Ar)), 6.28 (d, 3H, CH(pz), JHH ) 2.4 Hz), 2.30 (s, 9H,
CH3(Ar)), 2.08 (s, 18H, CH3(Ar)). 13C{1H} NMR (100 MHz,
CDCl3): δ 153.4 (C(pz)), 138.1 (C(Ar)), 137.6 (CH(pz)), 130.4,
130.3 (C(Ar)), 128.4 (CH(Ar)), 108.3 (CH(pz)), 83.7 (HC), 21.4,
20.7 (Me(Ar)). Anal. Calcd for C37H40N6: C, 78.17; H, 7.04; N,
14.79. Found: C, 77.90; H, 7.32; N, 14.65.
3. Conclusion
HC(3-Ms-pz)2(5-Ms-pz) (TpmMs*, 3). 1H NMR (400 MHz,
CDCl3): δ 7.97 (s, 1H, HC), 7.83 (d, 1H, CH(pz), JHH ) 1.8 Hz),
7.79 (d, 2H, CH(pz), JHH ) 2.6 Hz), 6.92 (s, 2H, CH(Ar)), 6.90 (s,
4H, CH(Ar)), 6.29 (d, 1H, CH(pz), JHH ) 1.8 Hz), 6.27 (d, 2H,
CH(pz), JHH ) 2.6 Hz), 2.31 (s, 9H, CH3(Ar)), 2.06 (s, 12H,
CH3(Ar)), 1.94 (s, 6H, CH3(Ar)). 13C{1H} NMR (100 MHz,
CDCl3): δ 152.4, 143.3 (C(pz)), 142.0 (CH(pz)), 140.1, 138.9,
137.8, 137.7, 130.6 (C(Ar)), 129.6 (CH(pz)), 128.8, 128.4 (CH(Ar)),
125.1 (C(Ar)), 108.5 (CH(pz)), 107.4 (CH(pz)), 80.4 (HC), 21.5,
21.4, 20.5, 20.2 (Me(Ar)). Anal. Calcd for C37H40N6: C, 78.17; H,
7.04; N, 14.79. Found: C, 77.86; H, 7.08; N, 14.70.
The functionalization of unreactive C-H bonds of several
hydrocarbons has been achieved at room temperature by the
formal insertion of carbene/CHCO2Et units, from EDA, into
such bonds, using cationic copper-based catalysts in a biphasic
reaction medium of an ionic liquid and hydrocarbon. Separation
of products is readily performed, and the recycle of the catalyst
for five cycles has been achieved without any loss of the
catalytic activity. These novel results serve as a starting point
for the development of more active and selective catalysts for
this transformation, with the goal of its application to a larger
array of alkanes.
Synthesis of [TpmMsCu(NCMe)]PF6 (4). To a stirred solution
of CuI (0.1g, 0.53 mmol) in acetonitrile (20 mL) was added a
suspension of the ligand HC(3-Ms-pz)3 (0.30 g, 0.53 mmol) in
acetonitrile (10 mL). A pale-yellow solution was obtained. The
reaction mixture was stirred for 2 h, AgPF6 (0.13 g, 0.53 mmol)
was added to the mixture, and precipitation of AgI was observed.
The resulting suspension was filtered, and the solvent was removed
under vacuum. The crude product was recrystallized from aceto-
4. Experimental Section
General Methods. All reactions and manipulations were carried
out under an oxygen-free nitrogen atmosphere with standard
Schlenk techniques or in a glovebox. All substrates were
purchased from Aldrich. Substrates and solvents were dried and
degassed before use. The complex [Tpm*Cu(NCMe)]BF4,18 the
ionic liquid [bmim]PF6,29 and 3-mesitylpyrazole21 were prepared
according to literature procedures. NMR spectra were recorded on
a Varian Mercury 400 MHz spectrometer. 1H NMR chemical shifts
were measured relative to deuterated solvent peaks but are reported
relative to tetramethylsilane. IR data were collected in a Varian
Scimitar 1000 Fourier transform IR spectrophotometer. Gas chro-
matography (GC) data were collected with a Varian GC-3900
1
nitrile to give 0.28 g (65% yield) of 4 as white crystals. H NMR
(400 MHz, CDCl3): δ 8.99 (s, 1 H, HC), 8.34 (d, 3H, CH(pz), JHH
) 2.5 Hz), 6.84 (s, 6H, CH(Ar)), 6.26 (d, 3H, CH(pz), JHH ) 2.5
Hz), 2.27 (s, 9H, CH3(Ar)), 1.91 (s, 18H, CH3(Ar)), 1.49 (s, 3H,
CH3CN). 13C{1H} NMR (100 MHz, CDCl3): δ 153.3 (C(pz)), 138.4
(C(Ar)), 137.6 (CH(pz)), 132.8, 128.8 (C(Ar)), 128.1 (CH(Ar)),
108.0 (CH(pz)), 76.6 (HC), 21.3, 20.4 (Me(Ar)), 1.8 (MeCN).
31P{1H} NMR (161 MHz, CDCl3): δ -142.5 (sept, PF6, JPF
)
713 Hz). Anal. Calcd for C39H43N7CuPF6 · NCMe: C, 57.31; H,
5.36; N, 13.05. Found: C, 57.48; H, 5.43; N, 13.42.
(29) (a) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg.
Chem. 1982, 21, 1263. (b) Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.;
de Souza, R. F.; Dupont, J. Polyhedron 1996, 15, 1217.
Synthesis of [TpmMs*Cu(NCMe)]PF6 (5). The procedure above
using HC(3-Ms-pz)2(5-Ms-pz) as the ligand afforded white crystals

6668 Organometallics, Vol. 26, No. 26, 2007
Rodríguez et al.
of 5 (yield: 0.30 g, 69%). 1H NMR (400 MHz, CDCl3): δ 8.06 (br
s, 2H, CH(pz)), 7.97 (s, 1H, HC), 7.71 (br s, 1H, CH(pz)), 7.12 (s,
2H, CH(Ar)), 6.89 (s, 4H, CH(Ar)), 6.26 (br s, 1H, CH(pz)), 6.22
(br s, 2H, CH(pz)), 2.43 (s, 3H, CH3(Ar)), 2.29 (s, 6H, CH3(Ar)),
1.94 (s, 12H, CH3(Ar)), 1.87 (s, 3H, CH3CN), 1.85 (s, 6H, CH3(Ar)).
13C{1H} NMR (100 MHz, CDCl3): δ 153.4, 143.9 (C(pz)), 142.0
(C(Ar)), 141.7 (CH(pz)), 138.5, 137.7, 137.6 (C(pz)), 132.9
(CH(pz)), 129.7 (CH(Ar)), 128.8 (C(Ar)), 128.2 (CH(Ar)), 122.5
(C(Ar)), 108.0, 107.6 (CH(pz)), 74.5 (HC), 21.5, 21.3, 20.4, 19.8
(Me(Ar)), 2.2 (MeCN). 31P{1H} NMR (161 MHz, CDCl3): δ
-143.4 (sept, PF6, JPF ) 712 Hz). Anal. Calcd for C39H43N7CuBF4:
C, 61.63; H, 5.75; N, 13.11. Found: C, 62.04; H, 5.93; N, 13.19.
Lorentz and polarization effects and absorption by a multiscan
method applied by SADABS.30,31 The structure was solved by direct
methods (SIR-2002)32 and refined against all F2 data by full-matrix
least-squares techniques (SHELXL97)33 converged to final R1 )
0.0532 [I > 2σ(I)] and wR2 ) 0.1481 for all data, with a GOF on
F2, and S ) 1.023 and 568 parameters. Crystal data for 2: C37H40N6,
Mw ) 568.75; a single crystal of suitable size, colorless needle
(0.51 × 0.15 × 0.14 mm3) crystallized from dichloromethane/
diethyl ether, coated with dry perfluoropolyether was mounted on
a glass fiber and fixed in a cold nitrogen stream, 100(2) K, to the
goniometer head; orthorhombic, space group Pca21 (No. 29), a )
17.5980(9) Å, b ) 17.1666(8) Å, c ) 10.4896(5) Å, V ) 3168.9(3)
Å3, Z ) 4, Fcalcd ) 1.192 g cm-3, λ(Mo KR1) ) 0.710 73 Å, F(000)
) 1216, µ ) 0.072 mm-1; 25 489 reflections were collected from
a Bruker-Nonius X8 Apex-II CCD diffractometer in the range of
6.64 < 2θ < 52.78°, and 3403 independent reflections [R(int) )
0.0557] were used in the structural analysis. Data reduction,
solution, and refinement were performed likewise for 4 converged
to final R1 ) 0.0369 [I > 2σ(I )] and to wR2 ) 0.0881 for all
data, with a GOF on F2, and S ) 1.067 and 398 parameters. Crystal
data for 3: C37H40N6, Mw ) 568.75; a single crystal of suitable
size, colorless prism (0.53 × 0.50 × 0.40 mm3) crystallized from
light petroleum containing diethyl ether, coated with dry perfluo-
ropolyether was mounted on a glass fiber and fixed in a cold
nitrogen stream, 100(2) K, to the goniometer head; triclinic, space
General Catalytic Procedure for Homogeneous Reactions.
The catalyst (0.05 mmol) was dissolved in a mixture of CH2Cl2 (2
mL) and the alkane (2 mL). Molecular sieves were added to absorb
the adventitious water (if any). A solution of EDA (1 mmol) in a
mixture of CH2Cl2 (1.5 mL) and the alkane (1.5 mL) was slowly
added (6 h) with the aid of a syringe pump; no remaining EDA
was detected by GC at the end of the addition time. The solution
was investigated by and the conversion determined by 1H NMR of
the reaction crude using trimethylvinylsilane (1 mmol) as the
internal standard.
General Catalytic Procedure for Biphasic Reactions. The
amount of catalyst and substrates as well as reaction procedures
and conditions are similar to those in the homogeneous reactions.
The catalyst and the alkane (2 mL) were added to [bmim]PF6 (2
mL). A solution of EDA (1 mmol) in the alkane (3 mL) was added
with the aid of a syringe pump for 6 h. No EDA was detected by
GC at the end of the addition time. The organic phase was separated,
and the ionic liquid was washed three times with the solvent (2
mL) that was employed during the reaction. The organic solution
was investigated by and the conversion determined by 1H NMR of
the reaction crude using trimethylvinylsilane (1 mmol) as the
internal standard.
j
group P1 (No. 2), a ) 8.2398(9) Å, b ) 11.7664(14) Å, c )
17.0887(18) Å, R ) 75.768(3)°, ꢀ ) 79.294(3)°, γ ) 85.418(3)°,
V ) 1577.0(3) Å3, Z ) 2, Fcalcd ) 1.198 g cm-3, λ(Mo KR1) )
0.710 73 Å, F(000) ) 608, µ ) 0.072 mm-1; 44 957 reflections
were collected from a Bruker-Nonius X8 Apex-II CCD diffracto-
meter in the range of 5.88 < 2θ < 59.66°, and 8770 independent
reflections [R(int) ) 0.0372] were used in the structural analysis.
Data reduction, solution, and refinement were performed likewise
for 4 converged to final R1 ) 0.0525 [I > 2σ(I )] and to wR2 )
0.1574 for all data, with a GOF on F2, and S ) 1.022 and 397
parameters.
Kinetic Studies. A solution of the catalyst (3.12 × 10-5 M for
4 and 6.25 × 10-5 M for 1) in methylene chloride (5 mL) was
placed into a 25 mL round-bottomed flask connected to a computer-
connected pressure gauge. EDA was added in one portion (0.5
mmol), and the evolution of nitrogen was monitored until the
pressure was maintained constant; i.e., all of the EDA was
consumed.
Acknowledgment. We thank MEC (Grant CTQ2005-
00324-BQU) and Junta de Andalucía for financial support.
Thanks are also due to Universidad de Huelva and the
Instituto de Investigaciones Químicas CSIC.
Crystal Structure Analyses. Crystal data for 4: C45H52CuF6N10P
(C39H43CuN7, F6P, and 3C2H3N), Mw ) 941.48; a single crystal of
suitable size, yellow prism (0.26 × 0.26 × 0.15 mm3) crystallized
from acetonitrile, coated with dry perfluoropolyether was mounted
on a glass fiber and fixed in a cold nitrogen stream, 100(2) K, to
the goniometer head; monoclinic, space group P21/c (No. 14), a )
17.6524(8) Å, b ) 19.7974(7) Å, c ) 14.4264(6) Å, ꢀ )
Supporting Information Available: Derivation of kinetic equa-
tions and crystallographic files in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM700822A
(30) Bruker. Apex 2, version 2.1; Bruker AXS Inc.: Madison, WI, 2004.
(31) Bruker. SAINT and SADABS; Bruker AXS Inc.: Madison, WI,
2001.
(32) Burla, C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giaco-
vazzo, C.; Polidori, G.; Spagna, R. SIR2002: the program. J. Appl.
Crystallogr. 2003, 36, 1103.
(33) Sheldrick, G. M. SHELXL97; University of Gottingen: Gottingen,
Germany, 1997.
112.1990(10)°, V ) 4667.9(3) Å3, Z ) 4, Fcalcd ) 1.340 g cm-3
,
λ(Mo KR1) ) 0.710 73 Å, F(000) ) 1960, µ ) 0.569 mm-1; 62 780
reflections were collected from a Bruker-Nonius X8 Apex-II CCD
diffractometer in the range of 5.66 < 2θ < 59.44°, and 13 136
independent reflections [R(int) ) 0.0559] were used in the structural
analysis. The data were reduced (BR SINT) and corrected for