Synthesis and characterization of metallodendritic palladium-biscarbene complexes derived from 1,1′-methylenebis(1,2,4-triazole)

Article abstract of DOI:10.1039/c0dt01535k

The syntheses of first generation dendritic compounds bearing 1,1′-alkane-1,1-diylbis(4-butyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene) palladium(ii) dibromide on the periphery are described. The metallabiscarbene moieties have also been studied separately from the dendrimer. These compounds display a non-symmetric boat-to-boat conformational equilibrium that has axial and equatorial arrangements. The predominance of the axial conformer in the equilibrium is supported by DFT calculations. The X-ray solid state structures of axial conformers of 3-hydroxy- and 3-mesyloxy-1,1′-propane-1,1- diylbis(4-butyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene)palladium(ii) dibromide are reported. These complexes display modest catalytic activity in the Heck reaction with activated aryl bromides and the dendritic catalysts were more active than the corresponding non-dendritic mononuclear species, a finding indicative of a cooperative effect.

Full text of DOI:10.1039/c0dt01535k

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PAPER
Synthesis and characterization of metallodendritic palladium-biscarbene
complexes derived from 1,1¢-methylenebis(1,2,4-triazole)†
Valent´ın Hornillos,‡a Javier Guerra,‡a,c Abel de Co´zar,a Pilar Prieto,a Sonia Merino,a Miguel A. Maestro,b
Enrique D´ıez-Barra*a and Juan Tejeda*a
Received 4th November 2010, Accepted 2nd February 2011
DOI: 10.1039/c0dt01535k
The syntheses of first generation dendritic compounds bearing
1,1¢-alkane-1,1-diylbis(4-butyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene)palladium(II) dibromide on the
periphery are described. The metallabiscarbene moieties have also been studied separately from the
dendrimer. These compounds display a non-symmetric boat-to-boat conformational equilibrium that
has axial and equatorial arrangements. The predominance of the axial conformer in the equilibrium is
supported by DFT calculations. The X-ray solid state structures of axial conformers of 3-hydroxy- and
3-mesyloxy-1,1¢-propane-1,1-diylbis(4-butyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene)palladium(II)
dibromide are reported. These complexes display modest catalytic activity in the Heck reaction with
activated aryl bromides and the dendritic catalysts were more active than the corresponding
non-dendritic mononuclear species, a finding indicative of a cooperative effect.
Immobilization of the catalysts onto polymers has commonly
been used as a method to recover the catalysts. However, very
few examples of immobilized catalysts based on NHCs have
been described. The main strategy that has been reported in the
Introduction
The chemistry of stable N-heterocyclic carbenes (NHCs) has
experienced remarkable growth since their initial synthesis as
1
2
¨
literature concerns NHCs that are linked to a polymer or silica.8
On the other hand, dendritic macromolecules can be separated
from the reaction media by precipitation or ultra-centrifugation
because of their large size. Since the work published by van
Koten in 1994,9 where nickel pincer complexes were present on the
periphery of a carbosilane dendrimer, an increase in research in
this area has taken place and very useful reviews were published.10
Metallodendrimers combine advantages from homogeneous and
heterogeneous catalysis. On the homogeneous side, the fact that
they are macromolecules with a very low polydispersity (1.0 or near
to 1.0) facilitates the study of their role in reaction mechanisms.10a
These complexes show activity and selectivity similar to those
of conventional homogeneous catalysts. Moreover, tuning the
complex periphery through a rational synthetic design can affect
the properties of the metal centre. On the heterogeneous side,
recovery and recyclability are very likely to be successful on using
the aforementioned techniques. However, examples of dendrimers
bearing NHCs in the core are scarce.11 Some NHCs complexes
were non-covalently attached to dendrimers by either ionic or
hydrophobic interactions.12 Complexes bound to the periphery
of a dendrimer through a ligand in the organometallic complex
other than the NHC system have also been described.13 Our group
reported an NHC system covalently attached to a dendritic struc-
ture that is able to chelate three palladium atoms (Fig. 1).14 The
significant electrophilic character of the benzyl methyne makes the
synthesis of 1 difficult as the loss of triazole rings is observed.15
transition metal complexes by Wanzlick and Ofele and as free-
carbenes by Arduengo.3 The synthesis of NHCs has contributed
to the development of new organometallic complexes that have
mainly been employed as powerful catalysts. Within this topic,
the tunability of NHCs gives rise to a wide variety of ligands,
comparable to the range shown by phosphines.4 NHCs derived
from imidazole are the most widely studied carbenes although
some NHC systems derived from 1,2,4-triazole have been prepared
and used as catalysts.5,6
Recovery and re-use of the catalysts are amongst the main
goals in organometallic chemistry due to the increased interest
in the design of efficient and environmentally benign synthetic
methods, which is one of the main principles of Green Chemistry.7
aDepartamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, Facultad
de Qu´ımica, Universidad de Castilla-La Mancha, Avda. Camilo Jose´ Cela,
10, E-13071, Ciudad Real, Spain. E-mail: Juan.Tejeda@uclm.es; Fax: +34
926295318; Tel: +34 926295300
bDepartamento de Qu´ımica Fundamental, Universidade da Corun˜a, Campus
de A Zapateira, E-15071, A Corun˜a, Spain
cNanoDrugs, S.L., Paseo de la Innovacio´n 1, Campus Universitario, Albacete,
Spain 02006
† Electronic supplementary information (ESI) available: X-ray data,
CCDC reference numbers 699228 (4) and 699229 (7). Total electronic and
Z-matrix of the stationary points calculated. Cyclic voltammogram and
absorption spectra of compounds 4 and 5. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0dt01535k
‡ VH and JG contributed equally to this work
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Table 1 Bond lengths (A) and angles (deg) for 4 and 7
Bond
4
Bond
7
Pd(1)–C(1)
Pd(1)–C(5)
N(1)–C(1)
N(3)–C(1)
N(4)–C(5)
N(6)–C(5)
1.972(4)
1.989(5)
1.353(6)
1.338(5)
1.322(6)
1.355(6)
Pd(1)–C(2)
Pd(1)–C(5)
N(3)–C(2)
N(1)–C(2)
N(4)–C(5)
N(6)–C(5)
1.975(4)
1.983(4)
1.344(5)
1.336(5)
1.333(5)
1.357(5)
Fig. 1 Dendritic G0 palladabiscarbene complex.
In this paper, we report the synthesis and catalytic activity in a
standard Heck reaction of dendritic palladabiscarbene complexes
bearing chelating NHCs derived from 1,1¢-methylenebistriazole.
Angles
4
Angles
7
C(1)–Pd(1)–C(5)
C(5)–Pd(1)–Br(1)
C(1)–Pd(1)–Br(2)
84.25(18)
169.60(14)
176.83(13)
C(2)–Pd(1)–C(5)
C(5)–Pd(1)–Br(1)
C(2)–Pd(1)–Br(2)
84.52(15)
172.06(11)
175.87(11)
Results and discussion
Mononuclear dicarbene structures
In an effort to synthesize dendrimers bearing N-heterocyclic
carbene complexes on the periphery, we focused our at-
tention on 1,1¢-alkane-1,1-diylbis(4-butyl-4,5-dihydro-1H-1,2,4-
triazol-5-ylidene)palladium(II) diiodide and dibromides 4–7
(Scheme 1). These compounds were previously synthesized by our
group16 and have a structure that allows them to be incorporated
within different dendritic architectures. We proposed that the
palladium atoms have square-planar geometries and that the
metallacycles show a boat-to-boat inversion at room temperature.
NMR studies showed two conformers for these systems in a
10 : 1 ratio and, based on bibliographical data, we proposed
that the main conformers are those in which the alkyl chains
are placed in the axial position and the minor ones in the
equatorial. Before embarking on the synthesis of the dendritic
molecules, we wished to gain an insight into the structure and
the catalytic activity of these non-dendritic complexes. In order
to increase our knowledge of these compounds, we report here
the solid-state structure of the axial conformers of compounds
4 and 7, as determined by X-ray diffraction. Monocrystals were
grown from solutions in dichloromethane/diethyl ether. ORTEP
diagrams are shown in Fig. 2 and selected bond lengths and angles
are shown in Table 1. The figure shows disorder in one of the
butyl substituents as well as in the OH group of 4 and in one
of the butyl groups of 7. However, the correlation factors are
good and the structures were determined unequivocally. In both
compounds two carbene carbons are bound to one palladium
atom, which has a quasi square planar geometry with the two-
carbene carbons in cis positions and two bromine atoms in the
other two positions. Both complexes contain a six-membered
metallacycle, which adopts boat-like conformations with the alkyl
chain in the axial position. The main distortion is detected in
Fig. 2 ORTEP diagrams of 4 and 7. Hydrogen atoms have been omitted
for clarity.
the angles between the two carbenes and the palladium atoms,
84.25◦ for 4 and 84.52◦ for 7. These angles are less than 90◦
due to a chelate effect. These values are comparable to those
of related imidazole-derived compounds.17 The Pd–C bonds are
slightly shorter than those in a similar iodide complex,6 thus
reflecting the lower trans influence of bromide versus iodide, and
are comparable with related dibromo-biscarbene palladium com-
plexes derived from 1,1¢-methylenebis(imidazole).17b Endocyclic
nitrogen–carbene carbon bonds are similar to those in other
related iodide complexes5a and they are of the same order or
slightly larger than the distances observed in unsubstituted 1H-
1,2,4-triazole,18 showing an NCN delocalization. These complexes
are reminiscent of the heteroscorpionates derived from pyrazole.19
It is very likely that metals other than palladium that display
an octahedral geometry could show a similar arrangement to
heteroscorpionates.
Scheme 1 i) n-BuOTs, 100 ◦C, 24 h. ii) Pd(OAc)2, KBr, THF, DMSO, r. t. 1 h. iii) Pd(OAc)2, KI, THF, 0 ◦C, 24 h. iv) CBr4, PPh3, CH3CN, r. t. 14 h.
v)MsCl, Et3N, CH2Cl2, r. t. 2 h.
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-1
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Table 2 Calculated main distances (A) and energy differences (kcal mol )
In both cases the axial conformers were crystallized and,
although this is consistent with the proposal that the main
conformers in the equilibria have axial arrangements, we can not
unequivocally rule out the possibility that the minor conformers
are the axial ones and they were obtained in the solid state due
to crystal packing effects. In order to ascertain whether the axial
conformers are the main compounds in the equilibria, we esti-
mated the energy differences between these two conformations by
DFT20 calculations. The non-dendritic mononuclear compounds
4–7 were used as models to provide an explanation for the location
of the alkyl chain. The two possible conformers a (axial) and b
(equatorial) were located at the B3LYP/6-31G*&LANL2DZ21–23
level of theory using the GAUSSIAN 03 programme.24 First
of all, compounds 4a and 4b were fully optimized. Structural
simplifications were also introduced with the aim of minimizing
the computational cost of the calculations; i.e. the two butyl
groups (compounds 4a and 4b) were changed to methyl groups
(compounds 4¢a and 4¢b). The main geometrical features of the
compounds are depicted in Fig. 3. The outcomes showed that
the geometrical parameters are quite similar in each pair of
compounds (4a/4¢a and 4b/4¢b) and we therefore optimized the
rest of the carbene complexes with two methyl groups instead of
butyl groups (compounds 5¢, 6¢ and 7¢, Fig. 4). Geometrical data
and formation energies of the complexes are collected in Table 2.
The formation energy differences between equatorial and axial
conformers were determined by taking away the formation energy
of the axial conformers from that of the equatorial ones. It can
be seen that the axial conformers are more stable (1.34–2.73 kcal
mol-1) than the equatorial ones and therefore in the boat-to-boat
equilibrium the axial conformers must be the major compounds.
In order to understand the origin of these energy differences, we
analysed the optimized geometry of these compounds. We noticed
that three critical distances could be responsible for possible steric
interactions: firstly, the distances between Pd and H(10) in the
axial conformers, secondly the distances between Pd and H(3)
in the equatorial conformers and finally the distances between
N(2)/N(5) and C(10) in both axial and equatorial conformers.
The first and second distances are similar to each other and larger
than the sum of the van der Waals radii—therefore these steric
interactions are negligible and they can not be responsible for the
difference in the energy values. On the other hand, in the third case,
the distances between N(2) and C(10) or N(5) and C(10) are in the
Comp. N(5)–C(10)a,c Pd–H(10)d Comp. N(5)–C(10)a,c Pd–H(3)d DEb
4a
3.40
3.42
3.43
3.40
3.41
2.92
2.92
2.85
2.90
2.86
4b
2.81
2.80
2.84
2.81
2.80
3.02
3.03
3.02
2.99
3.09
2.64
1.90
1.34
1.72
2.73
4¢a
5¢a
6¢a
7¢a
4¢b
5¢b
6¢b
7¢b
a Distances N(5)–C(10) and N(2)–C(10) are identical. b DE = (formation
energy of conformer b) - (formation energy of conformer a). c Sum of the
van der Waals radii [N–C] = 3.25 A. d Sum of the van der Waals radii
˚
˚
[Pd–H] = 2.83 A.
Fig. 3 Fully optimized compounds 4a, 4b, 4¢a and 4¢b calculated at the
B3LYP/6-31G*&LANL2DZ level.
˚
˚
order of 3.40 A in the axial conformers, which is around 0.60 A
˚
larger than in the equatorial conformers (2.80 A). In addition, the
sums of the van der Waals radii are shorter than the distances in
the axial conformers (N(2)/N(5)–C(10)) and larger than the same
distance in the equatorial conformers. These analyses led us to
conclude that the steric interactions between N(2)/N(5) and C(10)
in the equatorial compounds have to be greater than in the axial
ones. These interactions are reminiscent of the allylic 1,3-strain25
between the C(3)–H(3) bond and sp2 orbitals of N(5) and N(2),
which are minimized in the axial conformers. In our opinion, all
of these steric repulsions are likely to be responsible for the higher
energy level of the equatorial isomers and, as a consequence, the
axial conformers should be the main products in the boat-to-boat
equilibrium.
Fig. 4 Non-dendritic mononuclear theoretical models.
it is highly desirable to study the catalytic performance of
the compounds that have a palladium-diiodide or palladium-
dibromide moiety, focusing on the right choice of palladium
derivative to incorporate within the dendritic framework. We
selected a standard Heck reaction as the appropriate tool to
study the catalytic activity because of our previous experience
with this C–C coupling. We used compounds 4 and 5 to catalyze
the coupling of 4-bromobenzaldehyde and n-butyl acrylate. This
reaction is appropriate for the determination of the activity in the
Heck reaction and it was the subject of catalytic studies performed
by Herrmann et al.26 We previously used this methodology to
elucidate the catalytic activity of 114 and other related palladium-
pincer complexes.27 We used 1.40 equiv. of butyl acrylate per mol of
A main drawback within the dendrimer synthesis concerns
the time-consuming process and low yields associated with the
synthesis of a multicarbenic dendrimeric entity. For this reason,
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Table 3 Heck reaction between 4-bromobenzaldehyde and n-butyl acry-
late using compounds 4 and 5 as catalysts
better catalytic activity and leads to a higher reaction rate in all
cases, both with and without TBAB. Therefore the reduction of
compounds 4 and 5 should not be the rate-limiting step. The data
are consistent with oxidative addition being the slowest step34 and
this depends on the nature and concentration of the catalytic
species and on the nature of the halogen atom bonded to the
aryl group. As halogen atoms bonded to palladium in complexes
4 and 5 are lost in the reduction step,29 catalytic cycles should
be identical in all cases. Therefore the observed results could be
tentatively explained based on the differences in the concentration
of the catalytic species.
Pd(0) nanoparticles that can be stabilized by the presence
of TBAB, are often invoked to explain the Heck reaction
mechanism.34,35 We observed that the presence of TBAB improved
the results in this study. This is consistent with the formation
of TBAB-stabilized nanoparticles. On the other hand, we also
observed darkening of the reaction crudes; this is indicative of
the formation of insoluble and less reactive palladium black in
some range. Soluble palladium clusters are stable at low Pd-
loading, but as catalyst concentration increases, Pd black is rapidly
formed.34,35b As 5 is more easily reduced than 4, we propose that
the former gives rise to a higher concentration of Pd(0) atoms
and this could lead to formation of a higher amount of palladium
black. This aggregation phenomenon is less likely to take place
with compound 4 that should release Pd(0) atoms more slowly,
giving rise to a higher concentration of nanoparticles and therefore
leading to a higher catalytic activity.
Entry
Cat. (0.5% mol)
TBAB (% mol)
t (h)
Conversion (%)a
1
2
3
4
5
6
7
8
4
4
4
4
5
5
5
5
0
0
100
100
0
0
100
100
6
20
2
8
6
20
6
20
50
67
77
99b
19
22
50
69
a Conversions of the reactions were estimated by 1H NMR. b Isolated
product yield was higher than 90%.
p-bromobenzaldehyde, 1.10 equiv. of sodium acetate, 0.5 mol% of
catalyst and tetrabutylammonium bromide (TBAB).28 The results
are shown in Table 3. Conversions of the reactions were estimated
1
by H NMR spectroscopy because starting materials and final
products were the only materials detected by this technique.26
However, in order to verify the suitability of this method to
evaluate the conversions, on several occasions we isolated the final
product obtained under the best conditions described in entry 4.
In the initial study the conversions obtained for the mononu-
clear compounds 4 and 5 were compared. The differences between
these conversion levels could be a consequence of the nature of the
halogen atoms in the coordination sphere of the palladium(II). It
can be seen that the presence of TBAB was essential to obtain
good conversions (compare entries 1 and 3 or 4 for 4, and 6
and 8 for 5). The second conclusion that can be drawn is that
compound 4 is a more active catalyst than 5 (compare entries
7 and 8 with entries 3 and 4). Higher conversions and shorter
reaction times were observed for Pd–Br in comparison to Pd–I
carbene complexes.
The addition of mercury to the reaction mixture (300 eq. of
metallic mercury relative to the organometallic complex) under the
conditions described for entry 4 was carried out at the beginning of
the reaction, at room temperature 110 and 125 ◦C. In all cases, the
C–C coupling reaction was stopped after the addition of mercury.
This reinforces the hypothesised presence of Pd nanoparticles,
which are poisoned by the mercury.35a
Synthesis of dendritic structures bearing palladium-biscarbene
complexes
In the search for an explanation for the differences in the
catalytic activities displayed by the mononuclear biscarbene
species 4 (X = Br) and 5 (X = I), we analysed their electrochemical
properties. Cyclic voltammograms are included in the Supporting
Information.† Both compounds present an irreversible reduction
peak at -1.8 and -1.0 V, respectively, versus silver wire (See
Supporting Information†) at a scan rate of 0.1 V s-1. These values
are consistent with the trend reported by Jutand et al.29 Compound
5 has a lower reduction potential than 4. This means that the
former is electronically poorer and is more easily reduced. The UV-
Visible spectra of compounds 4 and 5 lead to a similar conclusion
(See Supporting Information†). The absorption spectra of both
compounds contain two bands, with the most intense band
tentatively attributed to a high-energy ligand-to-metal charge
transfer transition (LMCT)30–32 where the N-heterocyclic carbenes
act as strong donor and poor p-acceptor ligands. The other band
In the previous section we showed that dibromopalladium deriva-
tive 4 is more active in the C–C coupling reaction than diiodo
counterpart 5. With this knowledge, we decided to synthesize
dendrimers bearing dibromopalladium-biscarbene moieties. We
identified two possible approaches to the metallacarbene den-
drimers, one of which involved coupling the pre-formed complex
onto the periphery of a dendrimer. Note that compounds 4, 6 and
7 have focal groups that could be linked to appropriate functional
groups. The second approach consists of coupling the precursor of
the metallacarbenes to a dendritic structure to form the complex
in the final step. Firstly, we tried to bind a previously formed
biscarbene complex to 3,5-dihydroxybenzyl alcohol. However, all
attempts to couple 4, 6 or 7 to the core were unsuccessful. For
these reasons, the introduction of the metal in the final step was
the best choice. We decided to use 3,5-bis[3,3-bis(1H-1,2,4-triazol-
1-yl)propoxy]benzyl alcohol (9), which was previously synthesized
by our group36 by the reaction of a slight excess of mesylate
8 (obtained by a standard mesylation reaction of 2) with 3,5-
dihydroxybenzyl alcohol in the presence of potassium carbonate
and 18-crown-6 (Scheme 2). Quaternization of N5 in all 1,2,4-
triazole rings was performed by reaction with a large excess of
(300–400 nm) is assigned to a halogen→Pd LMCT transition.33
A
bathochromic shift in this band was observed when iodides were
present in the metal coordination sphere in comparison to the
complex (NHC)2PdBr2. This finding indicates that the palladium
atom in 5 is electronically poorer and would therefore be more
easily reduced than that in 4. Despite this fact, compound 4 shows
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Scheme 2 Synthesis of dinuclear tetracarbene complex 11.
n-butyl tosylate at 100 ◦C to render the tetrasalt 10 in good yield
(Scheme 2). The structure of 10 was confirmed by the presence of
the 1H NMR signals for the n-butyl and tosyl protons and by the
shift of the triazole H5 signal from 8.41 ppm to 10.62 ppm. The
presence of a unique set of 1,2,4-triazole signals indicates that all
N5 atoms were quaternized.
spectrum shows a peak at 2504.1 D corresponding to [M - Br]+.
1H NMR signals corresponding to H5 of the triazolium rings are
absent and a 13C NMR signal is observed at 162.3 ppm due to the
carbene carbons bound to palladium. These data are consistent
with the formation of a multicarbene complex.
Our aim was to highlight any influence that the dendritic nature
of the system may have on the catalytic activity of these complexes.
In order to compare the reactivity of the dendritic structures 11
and 14 with the non-dendritic complex 4, we turned our attention
to the Heck reaction shown in Table 3. The quantities of catalyst
were 0.5 mol% of 11 and 0.063 and 0.006 mol% of 14. The results
are shown in Table 4. One important finding is that the dendritic
complexes show a higher activity than the non-dendritic ones (cf.
entries 3 and 4, Table 3, with 1 and 2, Table 4). A detailed analysis
of the experimental data shows that 4 has half the amount of
palladium as 11, but the former required 8 h for completion of
the reaction whereas only 1 h was required with the latter. This
indicates that 11 is a better catalyst than 4. On the other hand, in
0.063 mol% of 14 there is half the amount of palladium as in 0.5
mol% of 4 and the conversions are similar but in 8 h for 4 and
1.5 h for 14. Another interesting observation is that 0.063 mol%
of 14 has a quarter of the amount of palladium as in 0.5 mol%
of 11, however the times to complete the reactions were not very
different, with the former requiring 1.5 h to complete the reaction
and the latter 1 h. These observations indicate a cooperative effect
of the palladium atoms in the dendritic system. These results agree
with those previously reported by Astruc37 and de Jesu´s et al.38
All reactions were quenched on the addition of metallic
mercury (300 eq.). This observation is consistent with the possible
formation of Pd(0) nanoparticles.35a
Finally, carbene-dendron 11 was synthesized by reaction of the
salt 10 with a slight excess of both palladium acetate and potassium
bromide with a yield of 67%. The 1H NMR spectrum shows
patterns consistent with palladium-biscarbene moieties. The most
1
important feature observed was the loss of the H NMR signal
corresponding to H5 and the appearance of a 13C NMR signal at
163.4 ppm, which corresponds to the carbene carbon bound to
palladium.
In a similar way to monometallic complexes,16 we propose that
the carbene metallacycle moieties must have boat-like structures
with two main conformations in a non-equimolar boat-to-boat
equilibrium. The alkyl chains bound to the methyne bridges
of the metallacyclic rings will interchange their axial–equatorial
arrangements.
We proceeded to turn our attention to the synthesis of
larger structures and, to this end, we attempted to couple the
metallodendron 11 to several cores. However, all attempts to
form ether or ester linkages were unsuccessful, meaning that
formation of the metallacarbene in the final step is the preferred
strategy once again. Dendritic system 3,5-bis[3,3-bis(1H-1,2,4-
triazol-1-yl)propoxy]benzyl terephthalate (12) was obtained by
the Scho¨tten–Baumann reaction of dendron 9 with terephthaloyl
chloride (Scheme 3). After work-up, the pure product was isolated
1
in 45% yield. A new signal was observed at 8.14 ppm in the H
NMR spectrum that integrates for 4H and this is due to the core
resonance. The MS (MALDI-TOF) shows two peaks at 1115.3
and 1137.3 due to (M + H)+ and (M + Na)+, as expected for the
target structure.
The C–C coupling reactions of n-butyl acrylate and less reactive
halides were investigated with 4-chlorobenzaldehyde and bro-
mobenzene using 14 as catalyst. In these cases, higher quantities
of catalyst (0.12% mol) and longer reaction times (40–47 h) were
necessary to obtain moderate yields (~30%).
Quaternization of all N5 atoms of 12 was carried out by reaction
with a large excess of n-butyl tosylate at 100 ◦C (Scheme 3). A long
reaction time (5 days) was required to obtain the octa-salt 13. This
is a result of the low solubility of the partially quaternized species.
The 1H NMR signals due to the butyl and tosyl groups in 13 are
similar to those for the equivalent groups in compound 10 and
the triazolium H5 signals are shifted to 10.64 ppm. The latter fact
Table 4 Heck reaction between 4-bromobenzaldehyde and n-butyl acry-
late using compounds 11 and 14 as catalysts
Entrya
Cat.
% Cat.
t (h)
Conversion (%)b
1
confirms the structure of 13. In the H NMR spectrum there is
1
2
3
11
14
14
0.5
0.063
0.006
1
1.5
20
99
96
18
only one set of signals for the triazole rings. This indicates that all
heterocycles have been alkylated. Finally, the dendritic carbene 14
was obtained by reaction of the salt 13 with palladium acetate and
potassium bromide as described for 11. The MS and NMR data
confirm the formation of the carbenes. The MS-MALDI-TOF
a Reaction conditions are the same as used in Table 3. b Conversions of the
reactions were estimated by 1H NMR.
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Scheme 3 Synthesis of the dendritic octacarbene structure 14.
instrument at the Servicio Interdepartamental de Investigacio´n,
Universidad Auto´noma de Madrid. NMR spectra were recorded in
CDCl3 or DMSO-d6 on a Varian Inova-500 instrument with TMS
or the solvent carbon signal as standards, operating at 500 MHz for
1H and 125 MHz for 13C. Chemical shifts are expressed in parts per
million. The signals were assigned with the aid of NOESY-1D. X-
ray diffraction experiments were run using a Bruker SMART-
CDD diffractometer at the Universidad de la Corun˜a. Monocrys-
tals were grown from solutions in dichloromethane/diethyl ether
(see Table 5 for crystallography data). UV-vis absorbance spectra
were obtained using a Hewlett–Packard HP8453 spectrometer and
quartz cuvettes with a path length of 1 cm. Solutions of biscarbenes
4 and 5 in THF (spectrophotometric grade, Sigma–Aldrich, Inc.)
were prepared at a concentration of 3 mM. THF was used to obtain
background spectra. The syntheses of products 2–9 have been
described previously.16,36 Anhydrous dimethylformamide was ob-
tained from Fisher (Fair Lawn, NJ) and transferred into a helium
atmosphere drybox (Vacuum Atmospheres Corp., Hawthorne,
CA). Electrochemical grade tetra-n-butylammonium phosphate
was obtained from Fluka and transferred into the drybox. The
electrochemical cell for cyclic voltammetry (CV) consisted of a
1.5 mm Pt disc inlaid in glass as the working electrode, a coiled Pt
wire as the counter electrode, and a Ag wire as a quasireference
electrode. Prior to each experiment, the working electrode was
polished with 0.3 mm alumina (Buehler, Ltd., Lake Bluff, IL),
sonicated in water, and then sonicated in ethanol for 1 min and
rinsed with acetone before transferring into the glovebox. Cyclic
voltammograms were recorded using a CH Instruments model
Conclusion
The following conclusions can be drawn from the work described
here. 3-Substituted-propane-1,1-diylbis(4-butyl-4,5-dihydro-1H-
1,2,4-triazol-5-ylidene)palladium(II) dibromide can be incorpo-
rated into dendritic structures to give first generation dendritic
metallacarbenes. These complexes display a modest catalytic
activity in the Heck reaction. Mononuclear palladabiscarbene
4 (bearing bromides) shows higher activity than the iodide-
containing counterpart 5. The catalytic activity increases as the
quantity of palladium increases. Tetranuclear dendritic compound
14 is more active than the dinuclear derivative 11 and the latter
is more active than the non-dendritic mononuclear compound 4.
This trend is consistent with a cooperative effect of the palladium
atoms in the dendritic system.
Experimental section
Solvents were purified by distillation from appropriate drying
agents before use. All reagents were used as received and without
further purification. When necessary, work was carried out
using standard Schlenk techniques under an atmosphere of dry
argon. Melting points were determined in capillary tubes on a
Gallenkamp apparatus and are uncorrected. Elemental analyses
were performed on a Perkin–Elmer 2400 CHN microanalyser.
Electron Impact (EI) (working at 70 V and 200 C), Fast Atom
Bombardment (FAB) mass spectrometry (using m-NBA as matrix)
and MALDI-TOF experiments were performed on a VG Autospec
◦
4100 | Dalton Trans., 2011, 40, 4095–4103
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Table 5 Crystallography data for compounds 4 and 7
Compound
4
7
Empirical formula
Formula weight
T (K)
C15H26Br2N6OPd
572.64
298(2)
C16H28Br2N6O3PdS
650.72
298(2)
˚
Wavelength (A)
0.71073
0.71073
Crystal system
Rhombohedral
R3
20.1373(7)
20.1373(7)
20.1373(7)
117.27
117.27
117.27
3442.3(2)
6
1.657
Monoclinic
P21/n
¯
Space group
˚
a (A)
10.4862(6)
15.5606(9)
14.7375(8)
90◦
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
100.5700(10)◦
90◦
3
˚
Volume (A )
2363.9(2)
4
1.828
4.282
1288
Z
Density (calculated) (mg m-3)
Absorption Coefficient (mm-1)
F(000)
4.304
1692
Crystal size (mm)
Index ranges
0.55 ¥ 0.18 ¥ 0.17
-26 £ h £ 26
-26 £ k £ 25
-18 £ l £ 26
0.0358
0.50 ¥ 0.48 ¥ 0.30
-12 £ h £ 13
-19 £ k £ 11
-18 £ l £ 15
0.0233
R(int)
Refinement method
Full-matrix least squares on F2
5576/0/264
1.043
Full-matrix least squares on F2
4836/0/272
1.054
Data/restraints/parameters
Goodness-of-fit on F2
Final R1 values [I > 2s(I)]
Final wR2 values[I > 2s(I)]
0.0365
0.0973
1.128 and -1.083
0.0337
0.0885
0.625 and -0.943
-3
˚
Largest diff. peak and hole (e·A )
660 Electrochemical Workstation (Austin, TX) and are collected
in the Supporting Information.†
(1H, s, Cpara), 6.39 (2H, s, Cortho), 7.11, 7.47 (16H, AA¢BB¢ system,
J 8 Hz, p-CH3C6H4SO3), 7.75 (2H, t, J 6.8 Hz, CHCH2CH2O),
9.45 (4H, s, 3-Htriazole), 10.62 (4H, s, 5-Htriazole); dC(d6-DMSO) 13.2,
18.8, 20.8, 30.5, 47.8, 62.5, 69.4, 71.4, 99.3, 99.9, 105.0, 106.0,
125.4, 128.1, 137.8, 144.6, 145.3, 145.4, 158.7; m/z (ESI) 1233 [M
- TsO]+.
Computational methods
All calculations reported in this paper were performed within the
Density Functional Theory,20 using the hybrid three-parameter
functional customarily denoted as B3LYP.21 The standard 6-
31G*22 basis set, as implemented in the GAUSSIAN 0324 suite of
programmes, was used to describe hydrogen, carbon, nitrogen and
oxygen atoms. Palladium atoms were described by the Hay–Wadt
effective core potential.23 This computational treatment is denoted
as B3LYP/6-31G*&LANL2DZ. The reported differences in en-
ergy include zero-point vibration energy corrections, denoted as
DZPVE. Harmonic analysis showed that all conformers had only
one imaginary frequency. Geometric features of all compounds
are collected in the Supporting Information.†
Synthesis of the palladium-tetracarbene 11
Compound 10 (0.150 g, 0.105 mmol), DMSO (2 mL), KBr (58 mg,
0.480 mmol) and Pd(OAc)2 (56 mg, 0.250 mmol) were added, in
this order, to THF (100 mL) in a 500 mL two-necked round-
bottomed flask. The resulting brown suspension was stirred for
5 min. The THF was removed under reduced pressure and the
◦
DMSO was removed in a bulb-to-bulb oven at 60 C, 2 ¥ 10-1
mmHg. The residue was washed with carbon tetrachloride (3 ¥
50 mL) and the product was extracted with THF (3 ¥ 80 mL).
The THF was evaporated and the product was purified by column
chromatography (silica gel, CH2Cl2/acetone 16 : 1 to 8 : 1) to give
an orange solid. Yield 67%; mp: 164 ◦C; dH(d6-DMSO) 0.92
(12H, t, J 7.3 Hz, NCH2CH2CH2CH3), 1.32 (8H, sxt, J 7.4 Hz,
NCH2CH2CH2CH3), 1.87 (8H, m, J 7.2 Hz, NCH2CH2CH2CH3),
3.47 (4H, m, J 6.5 Hz, CHCH2CH2O), 4.05 (4H, t, J 7.1 Hz,
CHCH2CH2O), 4.28 (4H, dt, J 13.2 and 6.8 Hz, one H for each
NCH2CH2CH2CH3), 4.39 (2H, s, ArCH2OH), 5.07 (4H, dt, J 13.2
and 6.8 Hz, one H for each NCH2CH2CH2CH3), 6.40 (1H, t, J
2.1 Hz, Cpara), 6.53 (2H, d, J 1.7 Hz, Cortho), 6.85 (2H, t, J 7.2 Hz,
CHCH2CH2O), 8.29 (4H, s, 3-Htriazole); dC(d6-DMSO) 13.6, 19.6,
32.9, 39.2, 49.6, 62.7, 70.6, 72.5, 101.0, 107.2, 141.6, 143.6, 159.3,
163.4; m/z (FAB+) 1225.1 [M - H2O]+.
Synthesis of the tetra-salt 10
A mixture of 9 (0.200 g, 0.406 mmol) and n-butyl tosylate (15 mL)
was heated and stirred at 100 ◦C in a 25 mL round-bottomed flask
for 72 h. The crude material was washed with ethyl acetate and
the product was obtained as a colourless solid after crystallization
◦
from ethanol/diethyl ether. Yield: 85%; mp: 158–162 C; dH(d6-
DMSO) 0.86 (12H, t, J 7.3 Hz, NCH2CH2CH2CH3), 1.27 (8H,
sxt, J 7.3 Hz, NCH2CH2CH2CH3), 1.77 (8H, q, J 7.3 Hz,
NCH2CH2CH2CH3), 2.28 (12H, s, p-CH3C6H4SO3), 3.14 (4H, q,
J 6.8 Hz, CHCH2CH2O), 4.10 (2H, t, J 5.4 Hz, ArCH2OH), 4.26
(4H, t, J 7.3 Hz, CHCH2CH2O), 5.23 (1H, t, J 5.4 Hz, OH), 6.26
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3,5-Bis[3,3-bis(1H-1,2,4-triazol-1-yl)propoxy]benzyl terephthalate
(12)
32.8, 39.4, 49.0, 63.4, 67.1, 76.7, 101.1, 107.3, 130.5, 134.3, 138.7,
145.5, 160.0, 162.3, 165.5. m/z (MALDI-TOF) 2540.1 [M - Br]+.
A mixture of compound 9 (0.740 g, 1.50 mmol) and anhydrous
freshly distilled triethylamine (200 mL, 1.430 mmol) was dissolved
in anhydrous THF (20 mL) under Ar in a 100 mL Schlenk tube.
A solution of terephthaloyl chloride (0.120 g, 0.590 mmol) in
anhydrous THF (10 mL) was added dropwise and the reaction
mixture was heated under reflux under an inert atmosphere for
16 h. After this period of time, the solvent was evaporated. The
residue was dissolved in dichloromethane (60 mL) and washed
with water (3 ¥ 20 mL). The organic layer was dried over
anhydrous Na2SO4. The product was finally washed with cold
methanol. The pure compound was obtained as colourless crystals.
Further purification was achieved by column chromatography
(silica gel, ethyl acetate/methanol 3 : 1). Yield: 45%; mp: 197–
200 ◦C; dH(CDCl3) 3.09 (8H, br s, CHCH2CH2O), 3.88 (8H, br s,
CHCH2CH2O), 5.27 (4H, s, ArCH2O), 6.26 (2H, s, Cpara), 6.53
(4H, s, Cortho), 6.97 (4H, t, J 7.1 Hz, CHCH2CH2O), 7.99 (8H, s,
3-Htriazole), 8.14 (4H, s, core), 8.40 (8H, s, 5-Htriazole); dC(d6-DMSO)
33.3, 62.5, 66.6, 68.6, 101.2, 107.3, 129.8, 133.9, 138.6, 143.1, 152.7,
159.3, 165.4; m/z (MALDI-TOF) 1137.3 [M + Na]+, 1115.3 [M +
H]+.
General method for the Heck reactions
A dimethylacetamide (5 mL) suspension of the corresponding
aryl halide (2 mmol), sodium acetate (2.2 mmol), the appropriate
NHC-Pd catalyst (0.5%, 0.063% or 0.006% mol depending on the
case) and, where appropriate, tetra-n-butylammonium bromide
(2 mmol) was degassed, and then n-butyl acrylate (2.8 mmol)
was added. The resulting suspension was stirred and heated to
125 ◦C. For analysis of the reaction, aliquots of 0.4 mL were taken
and dissolved in 3.5 mL of dichloromethane. Then 5 mL of 5% HCl
was added and the mixture stirred for 15 min. The organic layer
was then separated and successively washed with water (15 mL),
dried (Na2SO4), and evaporated under vacuum. Conversions of
the reactions were estimated by 1H NMR spectroscopy. To isolate
the product, the reaction mixture was worked-up in a similar way
to the aliquots but using 35 mL of CH2Cl2, 50 mL of 5% HCl and
150 mL of water. The pure product was obtained by bulb-to-bulb
distillation at 188 ◦C and 2 mmHg.
Acknowledgements
J. G. acknowledges the Junta de Comunidades de Castilla-La
Mancha for a predoctoral grant and a postdoctoral fellowship. The
authors are indebted to Alec Nepomnyashchii (University of Texas
at Austin) for his help and advice in performing the electrochemical
measurements. We also acknowledge L. A. Mart´ınez and M. Es-
cribano for their help in the syntheses of these compounds.
Synthesis of the octa-salt 13
A mixture of 12 (0.225 g, 0.195 mmol) and n-butyl tosylate (15
mL) was heated and stirred at 100 ◦C in a 25 mL round-bottomed
flask for 5 d. The crude material was washed with hot ethyl acetate
and the product was obtained as a colourless solid. Yield: 58%;
mp: 168–173 ◦C (decomposition); dH(d6-DMSO) 0.84 (24H, br s,
NCH2CH2CH2CH3), 1.24 (16H, br s, NCH2CH2CH2CH3), 1.75
(16H, br s, NCH2CH2CH2CH3), 2.27 (24H, s, p-CH3C6H4SO3),
3.16 (8H, br s, CHCH2CH2O), 4.14 (8H, br s, CHCH2CH2O),
4.24 (16H, br s, NCH2CH2CH2CH3), 5.26 (4H, s, ArCH2O), 6.45
(2H, s, Cpara), 6.57 (4H, s, Cortho), 7.10 and 7.47 (32H, AA¢BB¢
system, J 6.5 Hz, p-CH3C6H4SO3), 7.78 (4H, br s, CHCH2CH2O),
8.1 (4H, s, core), 9.45 (8H, s, 3-Htriazole), 10.64 (8H, s, 5-Htriazole).
dC(d6-DMSO) 13.2, 18.7, 20.7, 30.5, 47.7, 62.6, 66.5, 71.3, 100.7,
107.3, 125.4, 128.1, 129.7, 133.5, 137.9, 144.7, 145.3, 145.4, 159.0,
164.8.
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