Alternative synthetic methods for PEPPSI-type palladium complexes

Article abstract of DOI:10.1002/ejic.201402317

Alternative procedures are reported for the preparation of PEPPSI-type palladium complexes (PEPPSI = pyridine-enhanced precatalyst preparation stabilization and initiation) in good to excellent yields. One method involves the reaction of [(NHC)Pd(acac)Cl]

Full text of DOI:10.1002/ejic.201402317

FULL PAPER
DOI:10.1002/ejic.201402317
Alternative Synthetic Methods for PEPPSI-Type
Palladium Complexes
Kerry-Ann Green,[a] Paul T. Maragh,[a] Kamaluddin Abdur-Rashid,[b]
Alan J. Lough,[c] and Tara P. Dasgupta*[a]
Keywords: Synthesis design / Carbene ligands / NHC ligands / Palladium
Alternative procedures are reported for the preparation of
PEPPSI-type palladium complexes (PEPPSI = pyridine-en-
hanced precatalyst preparation stabilization and initiation) in
good to excellent yields. One method involves the reaction
of [(NHC)Pd(acac)Cl] complexes (NHC = N-heterocyclic
carbene, acac = acetylacetonate) with hydrohalides (HX; X =
Cl, Br, I) of pyridine or 2,6-lutidine. Two other one-pot syn-
theses involve Pd(acac)2, the azolium salt and pyridine
hydrohalide (or 2,6-lutidine hydrohalide). The complexes
were obtained as moisture- and air-stable solids and were
characterized by 1H and 13C NMR spectroscopy, CHN analy-
sis, and IR spectroscopy. The various synthetic procedures
and the yields of the desired products are discussed. The
X-ray structure of [1-(2,6-diisopropylphenyl)-3-(2,4,6-tri-
methylphenyl)imidazolin-2-ylidene]Pd(acac)Cl (4c) is also
reported.
On the premise of this proposal, Organ[8,15] and his
group developed a class of palladium complexes consisting
Introduction
Since the isolation of the first stable N-heterocyclic carb- of a 3-chloropyridine sacrificial ligand trans to an NHC
ene (NHC) by Arduengo et al.[1] in 1991, there has been a ligand and in a plane with two halide ligands in a distorted
resurgence of interest in these systems, following the earlier square-planar geometry about the palladium centre (Fig-
work of pioneers such as Wanzlick and Öfele.[2,3] NHCs ure 1). Organ et al. later coined the term PEPPSI (pyridine-
have emerged as an important class of ancillary ligands for enhanced precatalyst preparation stabilization and initia-
several transition metal complexes that serve as catalysts for tion) for such compounds.
a variety of useful synthetic reactions. Important properties
of NHCs include: their tuneable steric and electronic fea-
tures, excellent σ-donating and π-accepting abilities, and
strong metal–carbon bonds,[4–7] and these properties make
these ligands desirable for a range of catalytic applications.
The use of preformed NHC-containing palladium(II)
complexes in catalysis is desirable owing to their high sta-
bility and ease of handling and activation under the reac-
tion conditions.[8–10] In cross-coupling reactions catalyzed
by palladium-NHC complexes, it has been proposed that
Figure 1. Structure of a typical PEPPSI complex.
The 3-chloropyridine ligand is believed to enhance the
the active form of the catalyst is a monoligated NHC–Pd0 formation and provide stabilization of the complex. It is
species.[10,11] This proposal also contends that “sacrificial” believed to be readily dissociated from the complex during
ligands are necessary to stabilize the precatalyst and reac- the formation of the active catalyst species in a catalytic
tive transition states, to direct activity and selectivity, and cycle.[16] PEPPSI complexes have exhibited high activities
to facilitate the efficient delivery of the metal centre into for several reactions including the Suzuki–Miyaura,
the catalytic cycle.[12–14]
Kumada–Tamao–Corriu, Buchwald–Hartwig and Negishi
coupling reactions.[8,16,17,18] The stability and potentially
broad applications of these systems have made them very
appealing for use in catalytic reactions.[19]
However, we have experienced some challenges with the
synthesis of some of these complexes by the established
method,[8,15] such as low yields and a propensity for the
formation of analogues of trans-dichlorobis(pyridine)palla-
dium(II). Other researchers[20,21] have also encountered sim-
ilar challenges. Our group has been investigating alternative
[a] Department of Chemistry, University of the West Indies,
Mona, Kingston 7, Jamaica, West Indies
E-mail: tara.dasgupta@gmail.com
http://www.mona.uwi.edu/
[b] Kamal Pharmachem Inc., 100 College Street, Banting Institute,
Toronto, Ontario M5G 1L5, Canada
[c] Department of Chemistry, University of Toronto,
Ontario M5S 3H6, Canada
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402317.
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means of preparation for palladium–NHC complexes with mode as evidenced by two distinct carbon resonances at δ
a primary focus on reactions that do not require an external ≈ 187 and 183 ppm in the complexes.[9,22,23] The resonance
base and that can be conducted in air and in reagent-grade of the metal-bound carbene carbon atom (NCN–Pd) was
solvents. We hereby report alternative modular and effective observed at δ = 185.8 ppm for 4c, whereas this resonance
methods to prepare PEPPSI-type palladium complexes was observed at 162.7, 158.1 and 159.4 ppm for 6d, 6e and
from a commercially available Pd(acac)2 (acac = acetyl- 6f, respectively. These values are consistent with those of
acetonate) precursor and involving (NHC)Pd(acac)Cl spe- similar complexes.[9,22,23,25] Compounds 4a, 4b, 5a and 5b
cies as intermediates.
have been previously prepared, and the NMR spectroscopic
data are consistent with those found in the litera-
ture.[9,22,23,26] To the best of our knowledge, 4c, 6d, 6e and
6f have not been previously reported.
Results and Discussion
The molecular structure of 4c is depicted in Figure 2. Se-
lected bond lengths and bond angles are listed in Table 1.
The molecular structure confirms the η2-O,O binding mode
of the acac ligand. The palladium centre adopts a slightly
distorted square-planar geometry, in which the C(1)–Pd(1)–
O(1) bond angle is 179.29°, and the O(2)–Pd(1)–Cl(1) bond
angle is 178.64°. The five-membered heterocycle is also
nonplanar owing to the sp3-hybridized carbon atoms in the
backbone of the NHC ligand. The acetylacetonate ligand
lies out of plane with the bound 1-(2,6-diisopropylphenyl)-
3-(2,4,6-trimethylphenyl)imidazolin-2-ylidene. The Pd–
Ccarbenic bond length (1.9661 Å) is comparable to those of
similar complexes.[9,13a,27] A comparison of the Pd1–O1
(2.0573 Å) and Pd1–O2 (2.0192 Å) bond lengths shows that
the former is longer, as is consistent with the trans influence
exhibited by the strongly electron-donating NHC ligand.
A variety of (NHC)Pd(acac)Cl complexes containing
imidazol-2-ylidenes, imidazolin-2-ylidenes and benzimid-
azol-2-ylidenes were prepared by following a previously re-
ported procedure.[9,22,23] This involved the reaction of
Pd(acac)2 and the relevant azolium chloride under reflux
(Scheme 1).
Scheme 1. Preparation of (NHC)Pd(acac)Cl complexes.
This method of synthesis of the complexes is based on a
principle commonly found in the literature: protonation
and subsequent substitution of a ligand in the metal precur-
sor by a stronger nucleophile.[24] The method takes advan-
tage of the basic nature of the coordinated acac ligand and,
thereby, negates the need for an external base. The acac
ligand facilitates the in situ deprotonation of the azolium
chloride to generate the free carbene, which coordinates to
the metal centre along with the chloride ligand to form the
desired air- and moisture-stable (NHC)Pd(acac)Cl com-
plexes as pale yellow to golden yellow crystalline solids.
Figure 2. An ORTEP representation of 4c·CHCl3. Displacement
ellipsoids are represented at the 30% probability level. Hydrogen
atoms and the chloroform molecule are omitted for clarity.
The (NHC)Pd(acac)Cl complexes were used to prepare a
series of PEPPSI-type complexes. The method involves di-
The yields of the (NHC)Pd(acac)Cl complexes generally rectly reacting the (NHC)Pd(acac)Cl complexes with the
ranged from good to excellent, except for those of 6e and hydrohalides of pyridine or 2,6-lutidine in air and in the
6f. The greater steric bulk of the adamantyl and tert-butyl desired solvent [1,4-dioxane, tetrahydrofuran (THF) or
substituents, respectively, coupled with the congestion at the chloroform] at room temperature (Scheme 2). The PEPPSI
bridge carbon atoms of the benzimidazolylidenes relative to complexes were purified either by filtration through a short
those of the imidazolylidenes and imidazolinylidenes may pad of silica or by simple flash column chromatography.
contribute to these low yields. The successful synthesis of The complexes were isolated as air- and moisture-stable
the (NHC)Pd(acac)Cl complexes was confirmed by NMR compounds in good-to-excellent yields (74–99%). Com-
spectroscopy. The 13C NMR spectroscopic data indicate plexes 7a, 7b, 7c, 8a, 8b, 8h and 9f have been previously
that the acetylacetonate ligand adopts the η2-O,O binding prepared, and their NMR spectroscopic data are consistent
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Table 1. Selected bond lengths [Å] and angles [°] for 4c.
(NHC)Pd(acac)Cl species could act as in situ generated in-
termediates. One such procedure designated as a “one-pot,
one-step” method [Scheme 3 (i)] involves the direct reaction
of Pd(acac)2, the azolium salt and the pyridine hydrohalide
or 2,6-lutidine hydrohalide in 1,4-dioxane at reflux for 18 h.
Another procedure described as a “one-pot, two-step”
method [Scheme 3 (ii) and (iii)] involves the initial reflux of
the Pd(acac)2 and azolium chloride solution for 18 h, fol-
lowed by the addition of the solid pyridine hydrohalide or
2,6-lutidine hydrohalide with subsequent stirring at room
temperature for 18–24 h.
Bond Lengths
Pd(1)–C(1)
Pd(1)–O(2)
Pd(1)–O(1)
Pd(1)–Cl(1)
1.9661(18)
2.0192(14)
2.0573(13)
2.2911(5)
Bond Angles
C(1)–Pd(1)–O(2)
C(1)–Pd(1)–O(1)
O(2)–Pd(1)–O(1)
C(1)–Pd(1)–Cl(1)
O(2)–Pd(1)–Cl(1)
O(1)–Pd(1)–Cl(1)
86.99(6)
179.29(7)
92.32(5)
92.42(5)
178.64(4)
88.27(4)
with those found in the literature.[17a,28–30] To the best of
our knowledge, complexes 7g, 8g, 9d, 9i, 10a, 10g, 11a, 11g
and 12a have not been previously reported.
Scheme 2. Preparation of PEPPSI-type complexes from (NHC)-
Pd(acac)Cl and pyridine hydrohalides or 2,6-lutidine hydrohalides.
The dichloro complexes are the more common PEPPSI-
type compounds prepared and employed as potential cata-
lysts. However, we discovered that the mixed-halide, di-
bromo and diiodo PEPPSI-type systems are also accessible
by modification of the ratios of reactants and the reaction
time in this new method. For the dibromo and diiodo prod-
ucts, a large excess of the pyridine hydrohalide or the 2,6-
Scheme 3. One-pot synthesis of PEPPSI-type complexes: (i) pyr-
idine HCl or 2,6-lutidine HCl, 1,4-dioxane, reflux, 24 h; (ii) 1,4-
dioxane, reflux, 24 h; (iii) pyridine HCl or 2,6-lutidine HCl, room
temp, 24 h.
Both routes afforded the desired complexes (7g, 8a and
lutidine hydrohalide (10–20 equiv.) is required (X = I or Br). 8g) following purification by column chromatography.
However, for the dichloro and mixed-halide products, 1– However, the two-step pathway gave better yields than the
3 equiv. of the desired hydrohalides are required.
one-step method, as the latter method also formed the cor-
Despite the ease of the preparation by this new protocol, responding Pd(pyridine)2Cl2 or Pd(2,6-lutidine)2Cl2 as by-
it was necessary to prepare and isolate the precursor products. The yields of 8a and 8g by the one-pot two-step
(NHC)Pd(acac)Cl complex. To further simplify the proto- method were 79 and 66%, respectively. However, the one-
col and make it more convenient and practical, “one-pot” pot, one-step pathway gave a yield of 53% for 7g. The yields
syntheses of the PEPPSI-type complexes from the Pd- of the prepared PEPPSI-type complexes and the corre-
(acac)2 precursor were explored with the premise that the sponding reaction conditions are summarized in Table 2.
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Table 2. Summary of yields of PEPPSI-type complexes.
(kinetic products) to the dibromo and diiodo complexes
(thermodynamic products), because the mixed-halide 10g
was isolated rather than the dibromo 11g after 9 h. This
was confirmed by NMR spectroscopy and CHN analyses.
Extending the reaction time to one week led to the isolation
of the dibromo analogue 11g as the only product. However,
further substitution of the chloride ligand in the (SIMes)-
Pd(Pyridine)XCl complexes occurs more readily to yield the
dibromo or diiodo complexes after several hours. It appears
that the steric bulk of the pyridine, 2,6-lutidine and halide
ligands significantly influence the extent of substitution at
the palladium centre.
Entry
Complex
Yield [%]
1[a]
7a
7b
92
97
79
97
53
98
79
95
92
66
99
89
90
84
79
74
86
75
76
2[a]
3[a]
7c
4[a]
7g
5[b]
7g
6[a]
8a
7[c]
8a
8[a]
8b
9[a]
8g
10[c]
11[a]
12[a]
13[a]
14[a]
15[a]
16[a]
17[a]
18[a]
19[a]
8g
8h
9d
In CDCl3 and at ambient temperature, it is apparent that
the mixed-halide (SIMes)Pd(pyridine)XCl species (X = I or
Br) undergo halide exchange, and an equilibrium is estab-
lished between all three species (Scheme 4). This was evi-
dent from the 13C NMR spectra for these species, wherein
a mixture of the mixed-halide (XCl; X = Br, I), the dichloro
(Cl2) and the diiodo or the dibromo (X2) species was ob-
9f
9i
10a
10g
11a
11g
12a
[a] Conditions: (NHC)Pd(acac)Cl, pyridine HX or 2,6-lutidine HX,
solvent, room temp., 9–24 h. [b] Conditions: Pd(acac)2, 1a, 2,6-
lutidine HCl, 1,4-dioxane, reflux, 24 h. [c] Conditions: (i) Pd-
(acac)2, 2a, 1,4-dioxane, reflux, 24 h; (ii) pyridine HCl or 2,6-
lutidine HCl, room temp, 24 h.
1
served. The H NMR spectroscopic data were less useful
for indication of this because of the similarity of the spectra
for all three species.
The successful syntheses of the PEPPSI-type complexes
1
were confirmed by H and 13C NMR spectroscopy. For the
pyridine-based homodihalides, the carbene carbon atom of
the bound 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylid-
ene (SIMes) ligand resonated at δ = 184.5 and 185.3 ppm
in 11a and 12a, respectively. However, for the benzimidazol-
2-ylidene complex 9d, the carbene carbon (C-2) atom reso-
nated at δ = 161.0 ppm. These values are comparable to
those found in the literature for other related Pd–NHC
complexes.[8,28]
The C-2 atom of the SIMes ligand of the lutidine-based
homodihalide complexes resonated at δ = 187.6 and
187.8 ppm for 7g and 11g, respectively. However, the carb-
ene carbon atom of the benzimidazolylidene complex 9i res-
onated at δ = 163.8 ppm. The carbene carbon atom of the
only imidazolylidene complex 8g prepared resonated at δ =
Scheme 4. Halide exchange between mixed-halide PEPPSI com-
plexes.
Conclusions
(NHC)Pd(acac)Cl complexes are useful precursors for
156.2 ppm. The C-2 resonance was not observed for the the synthesis of PEPPSI-type palladium complexes. The re-
pyridine mixed dihalide complex 10a. However, the reso- action conditions are mild, and the desired products are
nance of this atom was observed at δ = 187.7 ppm for the obtained in high yields and purity. There are no unwanted
lutidine counterpart 10g. It is apparent that the 13C NMR byproducts or palladium black formation to contend with,
resonances of the metal-bound carbene atoms of the com- as heating and the use of an external base are not required.
plexes of the more nucleophilic 2,6-lutidine[31] occurred fur- A variety of solvents may be employed, contingent on the
ther downfield relative to those of pyridine derivatives, solubility of the reactants. The method allows easy modifi-
possibly owing to the ortho-methyl groups. These observa- cation of the ligands in the palladium coordination sphere,
tions are consistent with reported results for other Pd– and there is also the option to prepare either the homodi-
NHC complexes, wherein, the resonance of the carbene car- halide or mixed-halide complexes.
bon atom are shifted downfield as the donor ability of the
Two one-pot methods (one-pot, one-step and one-pot,
ligand trans to the NHC increases.[32] The binding of the two-steps) with Pd(acac)2 and the azolium salts as precur-
pyridine ligand was evident from the 13C NMR spectro- sors also gave moderate-to-good yields of the PEPPSI-type
scopic data, wherein the resonances of the carbon atoms of palladium complexes and obviate the need for the isolation
this group were slightly shifted relative to those of unbound of the intermediate (NHC)Pd(acac)Cl complex. The one-
pyridine or the precursor hydrohalide salt.
pot, two-step method was the more successful and practical
It was apparent during the synthesis of 10a, 11a, 10g, of the two procedures. This provides a very effective means
11g and 12a that the mixed-halide species are precursors of preparing PEPPSI-type palladium catalysts from com-
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raphy with hexanes/CH2Cl2/EtOAc (8:2:0.1) as eluent. Complex 6d
was isolated as a yellow crystalline solid, yield 201 mg (51%). H
mercially available Pd(acac)2 under base-free conditions
without the isolation of the free carbene ligand.
1
NMR (200 MHz, CDCl3, 293 K): δ = 7.66 (m, 2 H, meta-H), 7.22
(m, 2 H, ortho-H), 5.97 (m, 2 H, NCH), 5.37 (s, 1 H, acac CH),
2.38–1.25 (m, 26 H, CH2 cyclohexyl and CH3 acac) ppm. 13C NMR
(200 MHz, CDCl3, 293 K): δ = 188.1, 184.2, 162.7 (Pd–C), 133.6,
122.3, 112.9, 100.0, 61.7, 31.5, 31.0, 27.4, 26.0, 25.9, 25.7, 25.5, 25.3
Experimental Section
General: The azolium salts 1,3-bis(1-adamantyl)benzimidazolium
chloride, 1,3-dicyclohexylbenzimidazolium chloride, 1,3-bis(1-tert-
ppm. IR: ν = 2928, 2854, 1578, 1513, 1474, 1386, 1361, 1302, 1266,
˜
1241, 1051, 1022, 932, 896, 744, 621 cm–1. C24H33ClN2O2Pd
(523.39): calcd. C 55.07, H 6.35, N 5.35; found C 54.78, H 6.43, N
5.20.
butyl)benzimidazolium
chloride,
1-(2,6-diisopropylphenyl)-3-
(2,4,6-trimethylphenyl)-4,5-dihydroimidazolium chloride and pyr-
idine hydroiodide were obtained from Kamal Pharmachem Inc. All
other reagents were purchased from Sigma–Aldrich and BDH and
used without further purification. Solvents were dried (by using
standard procedures),[33] freshly distilled and deoxygenated prior to
use when necessary. All other chemicals were used as received. All
other imidazoli(ni)um salts were prepared according to literature
procedures.[34] The pyridine hydrohalides and 2,6-lutidine hydrohal-
ides were prepared by using a modified literature procedure.[35] The
1H and 13C NMR spectra were recorded with a Bruker ACE
200 MHz Fourier transform (FT) spectrometer and referenced to
the residual protons in the incompletely deuterated solvents. Chem-
ical shifts are reported in parts per million (ppm) relative to tet-
ramethylsilane (TMS). IR spectra were obtained with Bruker Vec-
tor 22 and Tensor 37 spectrometers. Elemental analyses were per-
formed with a PE 2400 CHNS/O Analyzer at the UWI, Mona,
Jamaica.
(BzImAd)Pd(acac)Cl (6e): To a dry reaction tube equipped with a
stirring bar was added 3e (240 mg, 0.57 mmol) and palladium(II)
acetylacetonate (172 mg, 0.57 mmol). The vessel was purged with
argon, and dry 1,4-dioxane (5 mL) was added by syringe. The mix-
ture was heated to reflux for 24 h, and significant Pd black forma-
tion was observed. The solvent was then evaporated in vacuo. The
remaining residue was dissolved in CH2Cl2, the solution was fil-
tered, and the product was purified by column chromatography
with hexanes/CH2Cl2/EtOAc (8:2:0.1) as eluent. Complex 6e was
1
isolated as a yellow crystalline solid, yield 36 mg (10%). H NMR
(200 MHz, CDCl3, 293 K): δ = 7.93 (m, 2 H, meta-H), 7.17 (m, 2
3
H, ortho-H), 5.31 (s, 1 H, acac CH), 3.57–3.51 (br d, JH,H
=
3
12.0 Hz, 6 H, CH2 adamantyl), 3.12–3.06 (br d, JH,H = 12.0 Hz,
6 H, CH2 adamantyl), 2.42 (br s, 6 H, CH adamantyl), 2.04–1.79
(m, 18 H, CH2 adamantyl and CH3 acac) ppm. 13C NMR
(200 MHz, CDCl3, 293 K): δ = 188.0, 183.7, 158.1 (Pd–C), 134.4,
Preparation of Complexes: The procedures for the alternative syn-
theses of the PEPPSI-type complexes are found below. The prepa-
rations of the (NHC)Pd(acac)Cl compounds were performed in
sealed glass reaction tubes on a carousel stirring hotplate. All other
reactions were conducted in air in round-bottomed flasks.
121.0, 116.0, 99.8; 63.0, 42.7, 36.2, 30.5, 27.4, 26.5 ppm. IR: ν =
˜
2908, 2851, 1580, 1511, 1452, 1390, 1351, 1337, 1296, 1263, 1105,
1060, 1022, 983, 931, 842, 745, 699, 615 cm–1. C32H41ClN2O2Pd
(627.54): calcd. C 61.24, H 6.59, N 4.46; found C 61.21, H 6.50, N
3.84.
[1-(2,6-Diisopropylphenyl)-3-(2,4,6-trimethylphenyl)imidazolin-2-ylid-
ene]Pd(acac)Cl (4c): To a dry reaction tube equipped with a stirring
bar was added 1c (346 mg, 0.90 mmol) and palladium(II) acetylace-
tonate (274 mg, 0.90 mmol). The vessel was purged with argon, and
dry 1,4-dioxane (5 mL) was added with a syringe. The mixture was
heated to reflux for 24 h, and the solvent was evaporated in vacuo.
The residue was dissolved in CH2Cl2, the solution was filtered, and
the product was purified by column chromatography with hexanes/
EtOAc/CH2Cl2 (4:1:0.5) as eluent. Complex 4c was isolated as a
(BzImtBu)Pd(acac)Cl (6f): To a dry reaction tube equipped with a
stirring bar was added 3f (265 mg, 0.99 mmol) and palladium(II)
acetylacetonate (304 mg, 0.10 mmol). The vessel was purged with
argon, and dry 1,4-dioxane (5 mL) was added with a syringe. The
mixture was heated to reflux for 24 h, and significant Pd black
formation was observed. The solvent was then evaporated in vacuo.
The residue was dissolved in CH2Cl2, the solution was filtered, and
the product was purified by column chromatography with hexanes/
CH2Cl2/EtOAc (8:2:0.1) as eluent. Complex 6f was isolated as a
1
yellow crystalline solid, yield 494 mg (93%). H NMR (200 MHz,
1
yellow crystalline solid, yield 161 mg (35%). H NMR (200 MHz,
CDCl3, 293 K): δ = 7.43 (m, 1 H, para-H), 7.30 (m, 2 H, meta-H),
7.06 (s, 1 H, meta-H), 6.92 (s, 1 H, meta-H), 5.09 9s, 1 H, acac
CH), 4.01 (m, 4 H, im 4-H, 5-H], 3.34 (m, 2 H, iPr CH), 2.65 (br
s, 3 H, ortho-CH3), 2.42 (br s, 3 H, ortho-CH3), 2.35 (s, 3 H, para-
CH3), 1.85 (s, 3 H, acac CH3), 1.80 (s, 3 H, acac CH3), 1.51–1.48
CDCl3, 293 K): δ = 7.79 (m, 2 H, meta-H), 7.20 (m, 2 H, ortho-H),
5.32 (s, 1 H, acac CH), 2.45 (br s, br, 18 H, CH3 tBu), 2.03 (s, 3
H, acac CH3), 1.88 (s, 3 H, acac CH3) ppm. 13C NMR (200 MHz,
CDCl3, 293 K): δ = 188.6, 183.5, 159.4 (Pd–C), 135.0, 121.8, 115.6,
3
100.1, 61.2, 32.2, 27.5, 26.0 ppm. IR: ν = 2980, 2920, 2080, 1581,
˜
(d, JH,H = 6.0 Hz, 3 H, iPr CH3), 1.26 (m, 9 H, iPr CH3) ppm.
13C NMR (200 MHz, CDCl3, 293 K): δ = 186.9, 185.8 (Pd–C),
183.1, 147.9, 146.7, 138.4, 137.4, 136.4, 135.0, 134.8, 130.2, 129.4,
128.7, 124.9, 124.9, 99.4, 53.9, 50.5, 28.6, 28.6, 27.1, 26.7, 25.8,
1510, 1478, 1387, 1371, 1345, 1262, 1228, 1183, 1021, 931, 785,
746, 613 cm–1. C20H29ClN2O2Pd (471.31): calcd. C 50.97, H 6.20,
N 5.94; found C 50.63, H 6.11, N 6.18.
23.7, 21.1, 19.2, 18.3 ppm. IR: ν = 2962, 2924, 2867, 1581, 1515,
˜
trans-(SIMes)Pd(2,6-Lut)Cl2 (7g): To a round-bottomed flask
equipped with a stirring bar was added 4a (23 mg, 0.042 mmol),
2,6-lutidine hydrochloride (7 mg, 0.049 mmol) and 1,4-dioxane
(3 mL). The flask was stoppered, and the mixture was stirred at
room temperature overnight. The solvent was then removed in
vacuo to reveal a yellow residue, which was purified by flash col-
umn chromatography with hexanes/EtOAc (3:1) as eluent. Complex
7g was isolated as a yellow crystalline solid, yield 24 mg (97%). 1H
NMR (200 MHz, CDCl3, 293 K): δ = 7.25 [m, 1 H, para-H (2,6-
Lut)], 7.03 [s, 4 H, meta-H (SIMes)], 6.77–6.72 [d, 3JH,H = 10.0 Hz,
2 H, meta-H (2,6-Lut)], 4.06 (s, 4 H, im 4-H, 5-H), 2.58 [s, 12 H,
ortho-CH3 (SIMes)], 2.50 [s, 6 H, ortho-CH3 (2,6-Lut)], 2.33 (s, 6
1485, 1456, 1337, 1269, 1019, 932, 851, 806, 762, 613 cm–1.
C29H39ClN2O2Pd (589.49): calcd. C 59.09, H 6.67, N 4.75; found
C 58.97, H 6.62, N 4.85. Single crystals were obtained by slow
evaporation of a solution of the compound in chloroform.
(BzImCy)Pd(acac)Cl (6d): To a dry reaction tube equipped with a
stirring bar was added 3d (249 mg, 0.78 mmol) and palladium(II)
acetylacetonate (238 mg, 0.78 mmol). The vessel was purged with
argon, and dry 1,4-dioxane (5 mL) was added with a syringe. The
mixture was heated to reflux for 24 h, and the solvent was evapo-
rated in vacuo. The residue was dissolved in CH2Cl2, the solution
was filtered, and the product was purified by column chromatog-
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H, para-CH3) ppm. 13C NMR (200 MHz, CDCl3, 293 K): δ = 187.6
(Pd–C), 158.8, 138.3, 137.7, 137.3, 134.9, 129.1, 122.2, 50.8, 24.6,
hydrobromide (18 mg, 0.16 mmol) and THF (3 mL). The flask was
stoppered, and the mixture was stirred at room temperature for
18 h. The solvent was then removed in vacuo to reveal a yellow
residue, which was purified by flash column chromatography with
hexanes/EtOAc (3:0.5) as eluent. Complex 10a was isolated as a
pale yellow crystalline solid, yield 27 mg (79%). 1H NMR
(200 MHz, CDCl3, 293 K): δ = 8.36 (m, 2 H, ortho-H)], 7.45 (m, 1
H, para-H), 7.03 [m, 2 H, meta-H (Pyr)], 6.94 [s, 4 H, meta-H
(SIMes)], 3.94 (s, 4 H, im 4-H, 5-H), 2.53 [2ϫs (overlapped), 12
H, ortho-CH3], 2.27 (s, 6 H, para-CH3) ppm. 13C NMR (200 MHz,
CDCl3, 293 K): δ = 152.0, 138.3, 137.3, 137.2, 137.0 129.5, 123.9,
51.2, 21.1, 20.0, 19.5 ppm. Note: The resonance of the carbene
21.0, 19.3 ppm. IR: ν = 3002, 2968, 2951, 2915, 2854, 1604, 1491,
˜
1456, 1373, 1304, 1264, 1168, 1106, 1017, 885, 775, 739, 738,
641 cm–1. C28H35Cl2N3Pd (590.91): calcd. C 56.91, H 5.97, N 7.11;
found C 56.66, H 5.68, N 6.88. Single crystals were obtained from
the slow evaporation of a solution of the complex in chloroform.
trans-(IMes)Pd(2,6-Lut)Cl2 (8g): To
a round-bottomed flask
equipped with a stirring bar was added 5a (80 mg, 0.15 mmol), 2,6-
lutidine hydrochloride (28 mg, 0.20 mmol) and 1,4-dioxane (3 mL).
The flask was stoppered, and the mixture was stirred at room tem-
perature overnight. The solvent was then removed in vacuo to re-
veal a yellow residue. This was extracted in CHCl3 and purified by
passage through a short pad of silica. Complex 8g was isolated as
carbon atom was not observed. IR: ν = 2954, 2921, 2854, 1605,
˜
1486, 1448, 1377, 1302, 1266, 1216, 1075, 1033, 1018, 855, 755,
691 cm–1. C26H31BrClN3Pd (607.31): calcd. C 51.42, H 5.14, N
6.92; found C 51.47, H 5.10, N 6.94.
1
a yellow crystalline solid, yield 80 mg (92%). H NMR (200 MHz,
CDCl3, 293 K): δ = 7.31 [m, 1 H, para-H (2,6-Lut)], 7.09 [m, 6 H,
3
meta-H (2,6-Lut) and meta-H (IMes)], 6.79–6.75 (d, JH,H
=
(SIMes)Pd(2,6-Lut)ClBr (10g): To
a round-bottomed flask
8.0 Hz, 2 H, im 4-H, 5-H), 2.64 [s, 6 H, ortho-CH3 (2,6-Lut)], 2.39
[s, 6 H, para-CH3], 2.35 [s, 12 H, ortho-CH3 (IMes)] ppm. 13C
NMR (200 MHz, CDCl3, 293 K): δ = 158.9, 156.2 (Pd–C), 139.0,
137.4, 136.7, 136.2, 135.4, 128.9, 124.0, 122.2, 24.8, 21.1, 19.1 ppm.
equipped with a stirring bar was added 4a (23 mg, 0.042 mmol),
2,6-lutidine hydrobromide (159 mg, 0.85 mmol) and CHCl3 (3 mL).
The flask was stoppered, and the mixture was stirred at room tem-
perature for 9 h. The solvent was then removed in vacuo to reveal
a yellow residue, which was purified by flash column chromatog-
raphy with hexanes/EtOAc (3:0.5) as eluent. Complex 10g was iso-
lated as a pale yellow crystalline solid, yield 20 mg (74%). 1H NMR
(200 MHz, CDCl3, 293 K): δ = 7.26 (m, 1 H, para-H), 7.02 [s, 4 H,
IR: ν = 3150, 3114, 3086, 2952, 2919, 2860, 1603, 1584, 1487, 1467,
˜
1412, 1375, 1338, 1286, 1237, 1160, 1095, 1032, 927, 864, 842, 785,
749, 706 cm–1. C28H33Cl2N3Pd (588.89): calcd. C 57.11, H 5.65, N
7.14; found C 57.48, H 5.53, N 6.94.
3
trans-(BzImCy)Pd(Pyr)Cl2 (9d): To
a round-bottomed flask
meta-H (SIMes)], 6.77–6.73 [d, JH,H = 8.0 Hz, 2 H, meta-H (2,6-
equipped with a stirring bar was added 6d (50 mg, 0.0955 mmol),
pyridine hydrochloride (18 mg, 0.156 mmol) and THF (3 mL). The
flask was stoppered, and the mixture was stirred at room tempera-
ture overnight. The solvent was then removed in vacuo to reveal a
yellow residue, which was purified by flash column chromatog-
raphy with hexanes/EtOAc (3:1) as eluent. Complex 9d was isolated
as a yellow crystalline solid, yield 46 mg (89%). 1H NMR
Lut)], 4.10 (s, 4 H, im 4-H, 5-H), 2.59 [2ϫs (overlapped), 12 H,
ortho-CH3 (SIMes)], 2.50 [s, 6 H, ortho-CH3 (2,6-Lut)], 2.34 (s, 6
H, para-CH3) ppm. 13C NMR (200 MHz, CDCl3, 293 K): δ = 187.7
(Pd–C), 158.9, 138.3, 137.8, 137.5, 137.3, 134.9, 129.2, 122.2, 50.9,
25.0, 21.0, 20.0, 19.5 ppm. IR: ν = 3005, 2981, 2952, 2920, 2863,
˜
2844, 1605, 1490, 1471, 1452, 1407, 1393, 1301, 1269, 1032, 851,
776, 634 cm–1. C28H35BrClN3Pd (635.36): calcd. C 52.93, H 5.55,
N 6.61; found C 52.90, H 5.45, N 6.58.
3
(200 MHz, CDCl3, 293 K): δ = 9.09–9.06 [d, JH,H = 6.0 Hz, 2 H,
3
ortho-H (Pyr)], 7.83–7.76 [t, JH,H = 7.6 Hz, 1 H, para-H (Pyr)],
3
trans-(SIMes)Pd(Pyr)Br2 (11a): To
a round-bottomed flask
7.64 [m, 2 H, meta-H (BzImCy)], 7.42–7.35 [t, JH,H = 14 Hz, 2 H,
equipped with a stirring bar was added 4a (24 mg, 0.044 mmol),
pyridine hydrobromide (140 mg, 0.88 mmol) and CHCl3 (3 mL).
The flask was stoppered, and the mixture was stirred for 24 h. The
solvent was then removed in vacuo to reveal a yellow residue, which
was purified by flash column chromatography with hexanes/EtOAc
(3:0.5) as eluent. Complex 11a was isolated as a pale yellow crystal-
line solid, yield 25 mg (86%). 1H NMR (200 MHz, CDCl3, 293 K):
δ = 8.41 (m, 2 H, ortho-H), 7.51 (m, 1 H, para-H), 7.06 [m, 6 H,
meta-H (Pyr) and meta-H (SIMes)], 4.00 (s, 4 H, im 4-H, 5-H),
2.61 (s, 12 H, ortho-CH3), 2.34 (s, 6 H, para-CH3) ppm. 13C NMR
(200 MHz, CDCl3, 293 K): δ = 184.5 (Pd–C), 152.4, 138.3, 137.1,
meta-H (Pyr)], 7.21 [m, 2 H, ortho-H (BzImCy)], 5.98 (m, 2 H,
NCH), 2.53–1.26 (m, 20 H, CH2) ppm. 13C NMR (200 MHz,
CDCl3, 293 K): δ = 161.0 (Pd–C), 151.3, 138.0, 133.8, 124.5, 122.2,
112.8, 62.1, 30.9, 26.0, 25.5 ppm. IR: ν = 2936, 2855, 1603, 1475,
˜
1450, 1413, 1365, 1267, 1242, 1216, 1072, 1053, 898, 825, 757, 742,
694 cm–1. C24H31Cl2N3Pd·CHCl3 (658.2): calcd. C 45.62, H 4.90,
N 6.38; found C 45.80, H 4.80, N 6.34.
trans-(BzImCy)Pd(2,6-Lut)Cl2 (9i): To a round-bottomed flask
equipped with a stirring bar was added 6d (31 mg, 0.059 mmol),
2,6-lutidine hydrochloride (14 mg, 0.098 mmol) and THF (3 mL).
The flask was stoppered, and the mixture was stirred at room tem-
perature overnight. The solvent was then removed in vacuo to re-
veal a yellow residue, which was purified by flash column
chromatography with hexanes/EtOAc (3:1) as eluent. Complex 9i
was isolated as a yellow crystalline solid, yield 28 mg (84%). 1H
NMR (200 MHz, CDCl3, 293 K): δ = 7.64 [m, 2 H, meta-H
(BzImCy)], 7.55 [m, 1 H, para-H (2,6-Lut)], 7.21 (m, 2 H, ortho-
137.1, 135.1, 129.5, 123.9, 51.3, 21.1, 20.3 ppm. IR: ν = 3023, 3002,
˜
2954, 2917, 2852, 1605, 1486, 1445, 1375, 1269, 1214, 1187, 1072,
1018, 850, 754, 691, 634 cm–1. C26H31Br2N3Pd (651.76): calcd. C
47.91, H 4.79, N 6.45; found C 48.60, H 4.81, N 6.35.
trans-(SIMes)Pd(2,6-Lut)Br2 (11g): To a round-bottomed flask
equipped with a stirring bar was added 4a (18 mg, 0.033 mmol),
2,6-lutidine hydrobromide (126 mg, 0.67 mmol) and CHCl3 (3 mL).
The flask was stoppered, and the mixture was stirred at room tem-
perature for 1 week. The solvent was then removed in vacuo to
reveal a yellow residue, which was purified by flash column
chromatography with hexanes/EtOAc (3:0.5) as eluent. Complex
11g was isolated as a pale yellow crystalline solid, yield 17 mg
3
H), 7.13–7.09 [d, JH,H = 8.0 Hz, 2 H, meta-H (2,6-Lut)], 6.27 (m,
2 H, NCH), 3.47 (s, 6 H, CH3), 2.34–1.25 (m, 20 H, CH2) ppm.
13C NMR (200 MHz, CDCl3, 293 K): δ = 163.8 (Pd–C), 158.8,
138.1, 133.6, 122.9, 122.1, 112.8, 62.0, 30.8, 26.3, 25.6, 26.3 ppm.
IR: ν = 2911, 2853, 1612, 1583, 1476, 1411, 1366, 1345, 1302, 1242,
˜
1134, 1054, 1032, 897, 827, 722, 742, 664 cm–1. C26H35Cl2N3Pd
(566.89): calcd. C 55.09, H 6.22, N 7.41; found C 55.32, H 6.17, N
7.13.
(75%). 1H NMR (200 MHz, CDCl3, 293 K): δ = 7.31 (m, 1 H,
3
para-H), 7.02 [s, 4 H, meta-H (SIMes)], 6.78–6.74 [d, JH,H
=
(SIMes)Pd(Pyr)BrCl (10a): To a round-bottomed flask equipped
with a stirring bar was added 4a (30 mg, 0.055 mmol), pyridine
8.0 Hz, 2 H, meta-H (2,6-Lut)], 4.05 (s, 4 H, im 4-H, 5-H), 2.61 [s,
12 H, ortho-CH3 (SIMes)], 2.49 [s, 6 H, ortho-CH3 (2,6-Lut)], 2.34
Eur. J. Inorg. Chem. 2014, 3600–3607
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FULL PAPER
(s, 6 H, para-CH3) ppm. 13C NMR (200 MHz, CDCl3, 293 K): δ = Supporting Information (see footnote on the first page of this arti-
1
187.8 (Pd–C), 159.0, 138.3, 137.5, 137.3, 135.0, 129.2, 122.3, 51.0,
cle): 13C and H NMR spectroscopic data for 4c, 6d, 6e, 6f, 7g, 8g,
9d, 9i, 10a, 10g, 11a, 11g, 12a and mixtures of (i) 7a,10a and 11a
and (ii) 7a, 12a and mixed-halide (chloro/iodo) PEPPSI complex.
25.3, 21.0, 20.2 ppm. IR: ν = 3008, 2980, 2952, 2921, 2854, 1729,
˜
1604, 1583, 1489, 1470, 1449, 1407, 1373, 1299, 1269, 1188, 1163,
1110, 1032, 850, 807, 776, 730, 632, 616 cm–1. C28H35Br2N3Pd
(679.81): calcd. C 49.47, H 5.19, N 6.18; found C 49.83, H 5.27, N
5.65.
Acknowledgments
trans-(SIMes)Pd(Pyr)I2 (12a): To a round-bottomed flask equipped
with a stirring bar was added 4a (18 mg, 0.033 mmol), 2,6-pyridine
hydroiodide (70 mg, 0.34 mmol) and CHCl3 (3 mL). The flask was
stoppered, and the mixture was stirred at room temperature for
9 h. The solvent was then removed in vacuo to reveal a yellow resi-
due, which was purified by flash column chromatography with hex-
anes/EtOAc (3:0.5) as eluent. Complex 12a was isolated as an
orange-yellow crystalline solid, yield 19 mg (76%). 1H NMR
(200 MHz, CDCl3, 293 K): δ = 8.35 (m, 2 H, ortho-H), 7.50 (m, 1
H, para-H), 7.07 [m, 2 H, meta-H (Pyr)], 7.00 [s, 4 H, meta-H
(SIMes)], 3.98 (m, 4 H, im 4-H, 5-H), 2.66 (s, 12 H, ortho-CH3),
2.35 (s, 6 H, para-CH3) ppm. 13C NMR (200 MHz, CDCl3, 293 K):
δ = 185.3 (Pd–C), 153.3, 138.3, 136.9, 136.7, 135.7, 129.6, 123.9,
The authors thank the University of the West Indies, Mona, Ja-
maica for a research grant and technical assistance from the Chem-
istry Department. We are also grateful to Kamal Pharmachem Inc.
for providing some reagents.
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One-Pot One-Step Method
trans-(SIMes)Pd(2,6-Lut)Cl2 (7g): To a reaction tube equipped
with a stirring bar was added Pd(acac)2 (181 mg, 0.59 mmol), 1a
(204 mg, 0.60 mmol) and 2,6-lutidine hydrochloride (91 mg,
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(5 mL) was added by syringe. The mixture was heated to reflux for
24 h, after which the mixture was filtered, and the sample was puri-
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(4:2:1) as eluent. Complex 7g was isolated as a yellow crystalline
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palladium(II) chloride was also obtained: 16 mg (7%).
One-Pot Two-Step Method
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stirring bar was added Pd(acac)2 (49 mg, 0.16 mmol) and 2a
(54 mg, 0.16 mmol). The flask was purged with argon, and 1,4-
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room temperature for 24 h. The mixture was then filtered, and the
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a yellow crystalline solid, yield 70 mg (79%).
trans-(IMes)Pd(2,6-Lut)Cl2 (8g): To a reaction tube equipped with
a stirring bar was added Pd(acac)2 (64 mg, 0.21 mmol) and 2a
(71 mg, 0.21 mmol). The flask was purged with argon, and 1,4-
dioxane (5 mL) was added with a syringe. The mixture was heated
to reflux for 24 h, after which 2,6-lutidine hydrochloride (32 mg,
0.22 mmol) was added, and the resultant mixture was stirred at
room temperature for 24 h. The mixture was then filtered, and the
sample was purified by column chromatography with hexanes/
EtOAc/CH2Cl2 (4:2:1) as eluent. Complex 8g was isolated as a yel-
low crystalline solid, yield 80 mg (66%).
CCDC-996942 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: April 14, 2014
Published Online: June 25, 2014
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