Preparation and characterization of the first pyrazole-based remote N-heterocyclic carbene complexes of palladium(II)

Article abstract of DOI:10.1039/b615441g

The first pyrazolin-4-ylidene complexes of palladium(ii) have been synthesized by oxidative addition of 4-iodopyrazolium salts to Pd ₂(dba)₃/PPh₃ and were fully characterized by multinuclear NMR spectroscopies, ESI mass sp

Full text of DOI:10.1039/b615441g

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Preparation and characterization of the first pyrazole-based remote
N-heterocyclic carbene complexes of palladium(II)
Yuan Han and Han Vinh Huynh*
Received (in Cambridge, UK) 24th October 2006, Accepted 29th November 2006
First published as an Advance Article on the web 5th January 2007
DOI: 10.1039/b615441g
commercially available.9 This straightforward, modular synthetic
The first pyrazolin-4-ylidene complexes of palladium(II) have
protocol allows facile fine-tuning of steric and electronic properties
of the resulting rNHC ligands by changing various R substituents.
The formation of 3a/b was confirmed by a base peak in the
positive ESI mass spectra at m/z = 327 (3a) and 265 (3b),
respectively, corresponding to the [M 2 I]+ fragments. In addition,
the C4 carbon atoms resonate at 72.6 ppm (3a, CDCl3) and
69.3 ppm (3b, d6-DMSO), which are shifted downfield upon
N-ethylation as compared to the analogous carbon resonances
for their precursors (2a/b) consistent with the formation of
aromatic cations.
been synthesized by oxidative addition of 4-iodopyrazolium
salts to Pd2(dba)3/PPh3 and were fully characterized by multi-
nuclear NMR spectroscopies, ESI mass spectrometry and
X-ray diffraction studies.
N-heterocyclic carbenes (NHCs) and their complexes are currently
the focus of intense research in organometallic chemistry and
catalysis due to their unique properties.1 As an extension of the
NHC concept, complexes bearing N-heterocyclic carbene ligands
with a remote heteroatom (rNHC) have been reported recently.
Such examples include complexes containing rNHC with a six-
membered heterocyclic ring derived from pyridine2 or quinoline.2–4
Computational studies have shown that these remote carbenes are
even stronger s donors than their well-known counterparts.2,3
Furthermore, preliminary catalytic studies have shown that
palladium(II) complexes with rNHC ligands are more active in
certain C–C coupling reactions than well-established standard
NHC-containing precatalysts.3 Despite these promising properties,
rNHCs have not attracted the same degree of attention as
standard NHCs and only few complexes with rNHC ligands
are known.
Treatment of Pd2(dba)3 with two equivalents of 3a/b and two
equivalents of triphenylphosphine in refluxing dichloromethane
leads to the formation of the first palladium(II) pyrazolin-4-ylidene
complexes 4a/b (Scheme 2). Slow evaporation of a concentrated
CH2Cl2 solution gave analytically pure 4a/b as yellow crystals in
yields of 60% and 45%, respectively. Complexes 4a/b are stable to
air and moisture. They are both soluble in polar solvents such as
dichloromethane, acetonitrile or DMSO, but insoluble in pentane,
diethyl ether or THF.
The formation of the complexes was confirmed by ESI mass
spectrometry with positive mode spectra showing base peaks at
m/z = 695 (4a) and 633 (4b) for the [M 2 I]+ fragments. The
phosphine donors in both complexes resonate at similar positions
in 31P NMR spectra with chemical shifts of 29.4 ppm (4a) and
29.3 ppm (4b), respectively. The 1H NMR spectrum of complex 4a
shows two doublets of quartets for the two inequivalent CH2
protons of the ethyl substituent centered at 3.78 and 3.63 ppm with
Herein, we present the first pyrazole-derived remote carbene
complexes of palladium(II) obtained by oxidative addition of
4-iodopyrazolium salts to Pd2(dba)3/PPh3.
General routes to prepare palladium(II) NHC complexes include
(a) in situ deprotonation of azolium salts with Pd(OAc)2 and
(b) Ag–carbene transfer method.5 However, initial attempts to
synthesize a palladium(II) pyrazolin-4-ylidene complex from a
1,2,3,5-tetrasubstituted pyrazolium salt using these methods
proved unsuccessful. Apparently, the proton attached to C4 in
such carbene precursors is less acidic than the C2 proton in
conventional imidazolium or benzimidazolium salts. The former
experiences comparatively weaker inductive effects from the
electron-withdrawing N atoms, which are located 2 bonds away
from the pre-carbene carbon.6
Due to the difficulty in deprotonating pyrazolium salts, the
oxidative addition method using a low-valent metal precursor was
explored.7 Suitable ligand precursors (3a/b) were readily prepared
in moderate yields of 33–50% by heating 1,3,5-trisubstituted-4-
iodopyrazoles (2a/b) in neat iodoethane under reflux (Scheme 1).
The former were obtained by iodination of the corresponding
1,3,5-trisubstituted pyrazoles (1a/b),8 which can be generated via
condensation of diones with monosubstituted hydrazines if not
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, 117543, Singapore, Singapore. E-mail: chmhhv@nus.edu.sg;
Fax: +65 67791691; Tel: +65 65162670
Scheme 1 Synthetic pathway for the preparation of ligand precursors.
Chem. Commun., 2007, 1089–1091 | 1089
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Scheme 2 Synthetic pathway for the preparation of palladium(II) pyrazolin-4-ylidene complexes.
2
a geminal coupling constant of J(HH) = 15.4 Hz. This can
In order to improve the solubilities of the new palladium(II)
be attributed to a hindered rotation of the ethyl group in
close proximity of the sterically bulky N-phenyl substituent.
Correspondingly, the 13C NMR spectrum of complex 4a exhibits
six distinct signals in the range of 132.3–127.7 ppm revealing a
hindered rotation for the N-phenyl substituent as well. In contrast
to 4a, the CH2 protons of the ethyl substituent in 4b appear as one
quartet in the 1H NMR spectrum indicating a free rotation of the
ethyl group neighboring to the less bulky methyl group. In
addition, the carbon atoms C3 and C5 adjacent to the carbene
pyrazolin-4-ylidene complexes, which would help in identifying the
carbene signals by 13C NMR spectroscopy, we substituted the iodo
ligands of complexes 4a/b by trifluoroacetato ligands according
to a literature method (Scheme 2).10 The dicarboxylato complexes
5a/b were obtained as colorless crystals in yields of 70% and 68%,
respectively, by slow evaporation of concentrated CH2Cl2–hexane
solutions. They are well soluble in chlorinated solvents, aceto-
nitrile, and DMSO, but insoluble in nonpolar solvents such as
hexane, toluene and diethyl ether. Under aerobic conditions and
upon prolonged standing, solutions of both complexes 5a/b slowly
deposit palladium black. A similar behavior has been observed for
mixed dicarboxylato–NHC complexes derived from imidazole
carbon of both complexes resonate in the range of 145.4–
3
147.2 ppm as doublets with coupling constants of J(CP)
=
2.7–4.6 Hz, respectively. The carbenoid signals, on the other hand,
were not observed due to insufficient solubilities. However, the
identity of the palladium(II) pyrazolin-4-ylidene complexes 4a/b
was further confirmed by single crystal X-ray diffraction studies.
Their molecular structures depicted in Fig. 1 and 2, show the
expected square planar arrangement around the palladium center
with the rNHC and the phosphine cis to each other. The
1
and benzimidazole as well.10 The H and 31P NMR spectra of
complexes 5a/b show little changes in their chemical shifts
compared to those for their precursor complexes 4a/b. However,
19F NMR (external standard: CF3CO2H) spectroscopy on 5a/b
reveals two singlets in a narrow range of 0.70 to 20.05 ppm,
pointing to a cis arrangement of the carboxylato ligands in
solution. In addtition, the 13C carbene signals of 5a and 5b arise as
doublets at 115.1 ppm with 2J(CP) = 10.4 Hz and at 113.8 ppm with
2J(CP) = 9.9 Hz, respectively. It is worth mentioning, that these
carbene signals are significantly shifted to lower field by 42.5 and
44.5 ppm with reference to the analogous C4 carbon resonances
in their ligand precursors 3a/b. Finally, two broad quartets are
˚
˚
Pd–Ccarbene bond lengths of 2.012(8) A in 4a and 1.996(7) A in 4b
are comparable with those reported for standard NHC complexes
or other types of rNHC complexes. Furthermore, the Pd–I1 bonds
˚
˚
trans to the rNHC (2.6691(10) A in 4a and 2.6730(7) A in 4b) are
slightly longer than the Pd–I2 bonds trans to the phosphine
˚
˚
(2.6564(10) A in 4a and 2.6458(7) A in 4b) confirming a slightly
stronger trans influence of the rNHCs. The carbene ring plane of
the rNHC is oriented almost perpendicular to the PdI2CP
coordination plane with a torsion angle of 89.40u (4a) or 85.98u
(4b) to minimize interligand interactions.
2
observed at 160.5 ppm with J(CF) = 31.8 Hz and at 116.3 ppm
1
with J(CF) = 273.9 Hz for the CO and CF3 carbon atoms of
complex 5a, respectively. Similar signals for the trifluoroacetato
ligands are also observed for complex 5b.
Fig. 1 Molecular structure of 4a showing 50% probability ellipsoids.
˚
Fig. 2 Molecular structure of 4b showing 50% probability ellipsoids.
˚
Selected bond lengths[A] and angles [u]: Pd1–C1 2.012(8), Pd1–P1 2.266(3),
Selected bond lengths[A] and angles [u]: Pd1–C1 1.996(7), Pd1–P1
Pd1–I1 2.6691(10), Pd1–I2 2.6564(10), C1–C2 1.381(13), C1–C3 1.373(13),
N1–C2 1.358(12), N2–C3 1.366(12), N1–N2 1.358(11); C1–Pd1–P1
90.6(3), C1–Pd1–I2 86.1(3), P1–Pd1–I1 91.67(7), I2–Pd1–I1 91.69(3),
C3–C1–C2 106.0(8).
2.2765(16), Pd1–I1 2.6730(7), Pd1–I2 2.6458(7), C1–C2 1.377(9), C1–C3
1.388(9), N1–C2 1.330(9), N2–C3 1.348(9), N1–N2 1.359(8); C1–Pd1–P1
89.79(19), C1–Pd1–I2 86.44(18), P1–Pd1–I1 90.69(4), I2–Pd1–I1 93.35(2),
C3–C1–C2 104.3(6).
1090 | Chem. Commun., 2007, 1089–1091
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1J(P,C) = 48.6 Hz, Ar–C), 130.2 (d, J(P,C) = 1.8 Hz, Ar–C), 127.7 (d,
4
In conclusion, we have presented a straightforward synthesis of
unprecedented palladium(II) pyrazolin-4-ylidene complexes by
oxidative addition of 4-iodopyrazolium salts to Pd2(dba)3/PPh3.
Studies to explore the catalytic activities of these complexes for
C–C coupling reactions and to gain a better understanding of these
unique ligands are ongoing.
2/3
J
= 11.0 Hz, Ar–C), 41.6 (s, CH2), 33.2 (s, NCH3), 15.0, 14.9 (s,
(P,C)
CCH3), 14.7 (s, CH2CH3). 5a: 1H NMR (300 MHz, CD2Cl2): d 7.74–6.94
(m, 20 H, Ar–H), 3.82 (m, 2J(H,H) = 15.0 Hz, 3J(H,H) = 7.2 Hz, 1 H, CH2),
3.69 (m, 2J(H,H) = 15.0 Hz, 3J(H,H) = 7.2 Hz, 1 H, CH2), 2.42 (s, 3 H, CH3),
2.04 (s, 3 H, CH3), 0.95 (m, J(H,H) = 7.2 Hz, 3 H, CH2CH3). 31P NMR
3
(121 MHz, CD2Cl2): 29.8 (s, 1 P, PPh3). 19F NMR (282 MHz, CD2Cl2):
0.58, 20.04 (s, CF3). 13C{1H} NMR (75.47 MHz, CD2Cl2): 160.5 (m,
3J(F,C) = 31.8 Hz, CO), 148.9 (d, 3J(P,C) = 2.7 Hz, CCH3), 148.4 (d, 3J(P,C)
=
We thank the National University of Singapore (NUS) for
financial support. Technical support from staff at the CMMAC of
our department is appreciated. Y.H. thanks NUS for a research
scholarship.{{
2.7 Hz, CCH3), 134.3 (d, 2/3J(P,C) = 11.5 Hz, Ar–C), 132.0, 131.9 (s, Ar–C),
131.1 (d, 4J(P,C) = 2.7 Hz, Ar–C), 130.7, 130.3 (s, Ar–C), 129.1 (d, 1J(P,C)
55.4 Hz, Ar–C), 128.4 (s, Ar–C), 128.3 (d, 2/3J(P,C) = 11.5 Hz, Ar–C), 127.7
=
1
2
(s, Ar–C), 116.3 (m, J(F,C) = 273.9 Hz, CF3), 115.1 (d, J(P,C) = 10.4 Hz,
carbene), 42.0 (s, CH2), 14.4, 14.2 (s, CCH3), 13.9 (s, CH2CH3). 5b: 1H
C
NMR (300 MHz, CD2Cl2): d 7.66–7.38 (m, 15 H, Ar–H), 3.99 (m, 2 H,
CH2), 3.55 (s, 3 H, NCH3), 2.27 (s, 3 H, CH3), 2.22 (s, 3 H, CH3), 1.17 (m,
3J(H,H) = 7.2 Hz, 3 H, CH2CH3). 31P NMR (121 MHz, CD2Cl2): 29.4 (s,
1 P, PPh3). 19F NMR (282 MHz, CD2Cl2): 0.65, 20.04 (s, CF3). 13C{1H}
Notes and references
{ Crystal data: 4a?0.5toluene: C34.5H35I2N2PPd, M = 868.82, monoclinic,
˚
a = 9.3123(4), b = 16.8404(7), c = 21.6379(9) A, b = 95.719(1), U =
3
3
3376.4(2) A , T = 295(2) K, space group P2(1)/c, Z = 4, Dc = 1.709 g cm
21
˚
NMR (75.47 MHz, CD2Cl2): 160.4 (m, J(F,C) = 34.4 Hz, CO), 147.7 (d,
3
,
3J(P,C) = 3.3 Hz, CCH3), 146.8 (d, J(P,C) = 2.7 Hz, CCH3), 134.3 (d,
m (Mo-Ka) = 2.451 mm21, 19717 reflections measured, 5935 unique (Rint
=
2/3
4
= 11.0 Hz, Ar–C), 131.0 (d, J(P,C) = 2.2 Hz, Ar–C), 128.9 (d,
0.0657) which were used in all calculations. The final wR2 was 0.1548 (all
data).
J
(P,C)
1J(P,C)= 54.9 Hz, Ar–C), 128.1 (d,
J
(P,C)
= 11.0 Hz, Ar–C), 116.3 (m,
2/3
1J(F,C) = 290.9 Hz, CF3), 113.8 (d, 2J(P,C) = 9.9 Hz, Ccarbene), 41.5 (s, CH2),
33.1 (s, NCH3), 14.2, 13.9 (s, CCH3), 13.6 (s, CH2CH3).
4b?0.5CH2Cl2: C26.50H30ClI2N2PPd,
a = 9.3019(5), b = 16.8771(9), c = 18.9728(9) A, b = 94.530(1), U =
M = 803.14, monoclinic,
˚
3
2969.2(3) A , T = 295(2) K, space group P2(1)/c, Z = 4, Dc = 1.797 g cm
21
,
=
˚
m (Mo-Ka) = 2.865 mm21, 20857 reflections measured, 6797 unique (Rint
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0.0357) which were used in all calculations. The final wR2 was 0.1573 (all
data).
{ Spectroscopic data: 3a: 1H NMR (300 MHz, CDCl3): d 7.81–7.65 (m, 5 H,
Ar–H), 4.39 (m, J(H,H) = 7.3 Hz, 2 H, CH2), 2.70 (s, 3 H, CH3), 2.25 (s,
3
3
3 H, CH3), 1.22 (t, J(H,H) = 7.3 Hz, 3 H, CH2CH3). 13C{1H} NMR
(75.47 MHz, CDCl3): 149.9, 149.8 (s, CCH3), 133.6, 131.8, 131.6, 129.8 (s,
Ar–C), 72.6 (s, CI), 46.7 (s, CH2), 15.4 (s, CCH3), 15.0 (s, CH2CH3). 3b: 1H
NMR (300 MHz, d6-DMSO): d 4.54 (m, 3J(H,H) = 7.2 Hz, 2 H, CH2), 4.03
(s, NCH3), 2.49 (s, 3 H, CH3), 2.45 (s, 3 H, CH3), 1.30 (t, 3J(H,H) = 7.2 Hz,
3 H, CH2CH3). 13C{1H} NMR (75.47 MHz, d6-DMSO): 147.9, 146.8 (s,
CCH3), 69.3 (s, CI), 43.2 (s, CH2), 35.1 (s, NCH3), 13.7, 13.2 (s, CCH3),
12.8 (s, CH2CH3). 4a: 1H NMR (300 MHz, CD2Cl2): d 7.78–6.87 (m, 20 H,
2
3
Ar–H), 3.78 (m, J(H,H) = 15.4 Hz, J(H,H) = 7.2 Hz, 1 H, CH2), 3.63 (m,
2J(H,H) = 15.4 Hz, 3J(H,H) = 7.2 Hz, 1 H, CH2), 2.36 (s, 3 H, CH3), 1.99 (s,
3 H, CH3), 0.95 (m, 3J(H,H) = 7.2 Hz, 3 H, CH2CH3). 31P NMR (121 MHz,
CD2Cl2): 29.4 (s, 1 P, PPh3). 13C{1H} NMR (75.47 MHz, CD2Cl2):
147.2 (d, 3J(P,C) = 3.8 Hz, CCH3), 146.9 (d, 3J(P,C) = 2.7 Hz, CCH3), 134.9
2/3
1
(d,
J
(P,C)
= 11.0 Hz, Ar–C), 132.9 (d, J(P,C) = 48.3 Hz, Ar–C), 132.3,
131.6, 130.5 (s, Ar–C), 130.2 (d, 4J(P,C) = 2.2 Hz, Ar–C), 130.1, 128.7, 127.8
(s, Ar–C), 127.7 (d, 2/3J(P,C) = 11.0 Hz, Ar–C), 41.9 (s, CH2), 15.3, 15.1 (s,
CCH3), 14.7 (s, CH2CH3). 4b: 1H NMR (300 MHz, CD2Cl2): d 7.68–7.30
3
(m, 15 H, Ar–H), 3.86 (m, J(H,H) = 7.3 Hz, 2 H, CH2), 3.43 (s, 3 H,
NCH3), 2.21 (s, 3 H, CH3), 2.13 (s, 3 H, CH3), 1.15 (t, 3J(H,H) = 7.3 Hz, 3 H,
CH2CH3). 31P NMR (121 MHz, CD2Cl2): 29.3 (s, 1 P, PPh3). 13C{1H}
NMR (125.76 MHz, CD2Cl2): 145.9 (d, 3J(P,C) = 4.6 Hz, CCH3), 145.4 (d,
3J(P,C) = 3.7 Hz, CCH3), 135.0 (d,
= 11.0 Hz, Ar–C), 132.7 (d,
2/3
J
(P,C)
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Chem. Commun., 2007, 1089–1091 | 1091