NCN palladium pincer via transmercuration. Synthesis of [2-(2-oxazoliny)-6-(2-pyridyl)] phenylpalladium(II) chloride and its catalytic activity in Suzuki coupling

Article abstract of DOI:10.1016/j.inoche.2013.03.012

The unsymmetrical NCN ligand 1-(2-oxazolinyl)-3-(2-pyridyl)benzene 6 was synthesized starting from m-bromotoluene after a series of transformations. The reaction of 6 and mercury (II) acetate resulted in mercurial derivative 7 which was transformed into the corresponding NCN palladium pincer 8 via transmercuration. The ultraviolet spectra of 6-8 in acetonitrile were also studied. The structures of 6 and 8 were further confirmed by X-ray single crystal diffraction. The carbon-carbon cross coupling reactions between aryl halides and phenylboronic acid catalyzed by 8 were investigated. The results indicate that this palladium pincer is more highly active to the coupling of aryl bromides than aryl chlorides.

Full text of DOI:10.1016/j.inoche.2013.03.012

Inorganic Chemistry Communications 32 (2013) 78–81
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
NCN palladium pincer via transmercuration. Synthesis of
[2-(2-oxazoliny)-6-(2-pyridyl)] phenylpalladium(II)
chloride and its catalytic activity in Suzuki coupling
⁎
Ping Li, Hai-Feng Zhou, Fang Liu, Zhao-Xia Hu, Hong-Xing Wang
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The unsymmetrical NCN ligand 1-(2-oxazolinyl)-3-(2-pyridyl)benzene 6 was synthesized starting from
m-bromotoluene after a series of transformations. The reaction of 6 and mercury (II) acetate resulted in
mercurial derivative 7 which was transformed into the corresponding NCN palladium pincer 8 via
transmercuration. The ultraviolet spectra of 6–8 in acetonitrile were also studied. The structures of 6 and
8 were further confirmed by X-ray single crystal diffraction. The carbon–carbon cross coupling reactions
between aryl halides and phenylboronic acid catalyzed by 8 were investigated. The results indicate that
this palladium pincer is more highly active to the coupling of aryl bromides than aryl chlorides.
© 2013 Elsevier B.V. All rights reserved.
Received 5 January 2013
Accepted 8 March 2013
Available online 26 March 2013
Keywords:
Palladium
Mercury
Pincer
Transmetalation
Catalytic activity
As an important class of pincers, palladium complexes have been
widely used in organic synthesis and material sciences [1,2]. Their syn-
thesis normally includes C–H bond activation, oxidative addition and
transmetalation. C–H bond activation is very useful for preparation of
PCP, SCS and some NCN pincers bearing the side arms, because without
this requirement, such activation for NCN ligands will occur at the 4-,
6-positions [3]. The latter two methods have no such limitation, but
both oxidative addition and most of transmetalation reactions via
organostannanes and organolithiums require the comparatively expen-
sive 2-halo substituted starting material and low reaction temperature
(−78 °C) [4]. Though there are also reports on the synthesis of palladi-
um pincers via organomercurials, the examples in this aspect are still
rare [5]. As we know, mercury (II) salts are cheap and easily available,
and react normally under mild reaction. Moreover, their good func-
tions on site-directing and transmetalation are documented very
well in literatures [6]. Thus, to prepare the palladium pincers via
mercurials will become an attractive choice. Herein, we present an
example of unsymmetrical NCN palladium pincer, i.e. [2-(2-oxazolinyl)-
6-(2-pyridyl)]phenylpalladium (II) chloride via transmercuration. In ad-
dition, the photophysical properties of the ligand, its mercurial and the
corresponding palladium pincer and catalytic activity of this pincer in
Suzuki coupling between aryl halides and phenylboronic acid were also
investigated.
reagent derived from m-bromotoluene 1 and 2-bromopyridine cata-
lyzed by Ni(dppe)2Cl2 resulted in 2-m-tolylpyridine 2 in a good yield
(80%). An attempt to catalyze this coupling with Ni(acac)2Cl2 was
not successful [7]. The NCN ligand 6 was synthesized starting from
phenylpyridine 2 after a series of transformations including oxidation
[8], acylation/amination, chlorination as well as ring closure. Elemental
analysis and IR spectrum (ν 1647 cm−1, C=N) as well as the NMR (δ
149.24 ppm, O–C=N) of 6 confirmed its structure. The reaction of 6
with one equivalent of mercury (II) acetate was carried out in refluxing
methanol and followed by treatment with lithium chloride to give
out organomercurial 7 in 44% yield after purification. In 13C NMR of 7,
a signal at δ 167.51 ppm corresponds to the C1, implying that the
mercuration of ligand 6 occurs at the expected position. Transmetalation
reaction of 7 with palladium (II) acetate was carried out in the same
manner as the mentioned mercuration previously. The pure palladium
pincer 8 was isolated in the yield of 35%.
In order to obtain unambiguous characterizations of 6–8, the
single crystal X-ray diffraction study of 6 and 8 was undertaken
(Figs. 1 and 2). Fig. 1 clearly demonstrates that a dihedral angle
29.18° exists between pyridyl and phenyl rings in the ligand. After
the introduction of palladium atom, these two rings are closed to be
coplanar (dihedral angle: 0.2° in 8). In addition, it is also observed
that the moieties of O–C=N in both 6 and 8 are coplanar with their
corresponding phenyl rings (dihedral angle: 0.29° in 6, 0.3° in 8, re-
spectively). The C–Pd bond length (1.917(4) Å) and the average
length of N–Pd bond (2.061 Å) are the same as those already reported
NCN palladium pincers (Table 1) [9].
The synthetic route for the unsymmetrical NCN palladium pincer 8
was shown in Scheme 1. The coupling reaction between Grignard
UV–vis spectra. The UV–vis spectra of 6–8 in acetonitrile were
recorded (Fig. 3) and the data were tabulated in Table 2. From Table 2,
⁎
Corresponding author. Tel./fax: +86 22 27403475.
E-mail address: hongxing_wang@tju.edu.cn (H.-X. Wang).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.03.012

P. Li et al. / Inorganic Chemistry Communications 32 (2013) 78–81
79
Scheme 1. Synthesis of the unsymmetrical NCN palladium pincer 8.
Fig. 1. Crystal structure of 6 (H atoms were omitted for clarity). Selected bond lengths (Å) and angles (°): O1–C12 = 1.3584(2), O1–C14 = 1.4560(1), N2–C12 = 1.2734(2), N2–C13 =
1.4713(2), C12–O1–C14 = 105.82(9), C12–N2–C13 = 106.54(1), C9–C10–C12 = 119.54(1), C11–C10–C12 = 120.53(1).
Fig. 2. Crystal structure of 8 (H atoms were omitted for clarity). Selected bond lengths (Å) and angles (°): N1–Pd1 = 2.066(3), N2–Pd1 = 2.055(3), C11–Pd1 = 1.917(4), Cl1–Pd1 =
2.4178(1), C11–Pd1–N2 = 79.36(1), C11–Pd1–N1 = 79.65(1), N2–Pd1–N1 = 159.01(1), C11–Pd1 –Cl1 = 177.82(1).

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P. Li et al. / Inorganic Chemistry Communications 32 (2013) 78–81
Table 1
Crystal data and structure refinement parameters for 6 and 8.
Structure parameters
6
C
8
Empirical formula
Formula weight
14H12N2O
C14H11ClN2OPd
365.10
224.26
F(000)
472
360
Crystal size/mm
Wavelength
0.18 × 0.12 × 0.10
0.71073 Å
0.08 × 0.06 × 0.04
0.71073 Å
Crystal system, space group
a/(Å), b/(Å), c/(Å)
α/(°), β/(°), γ/(°)
V/(Å3)
Monoclinic, P2(1)/c
5.7024(11), 9.4926(19), 20.010(4)
90, 92.23(3), 90
1082.3(4)
Triclinic, P1
6.7703(1), 9.762(2), 9.864(2)
103.47(3), 90.55(3), 100.41(3)
622.6(2)
Z
4
2
Calculated density/Mg·m−3
θ range for date collection
Limiting indices
1.376
1.947
1.13° ≤ θ ≤ 25.01°
−5 ≤ h ≤ 6, −10 ≤ k ≤ 11, −23 ≤ l ≤ 23
6957/1889 [R(int) = 0.0311]
1889/0/154
2.13° ≤ θ ≤ 25.01°
−8 ≤ h ≤ 8, −11 ≤ k ≤ 10, −10 ≤ l ≤ 11
4506/2156 [R(int) = 0.0238]
2156/0/173
Reflections collected/unique
Data/restraints/parameters
R indices (all data)
Final R1, wR2[I > 2σ(I)]
Largest diff. hole and peak
R1 = 0.0398, wR2 = 0.1015
0.0361, 0.0980
R1 = 0.0345, wR2 = 0.0741
0.0307, 0.0715
0.189, −0.195e. Å−3
1.973, −0.353e. Å−3
Fig. 3. UV−vis spectra of 6–8. Substance concentration: 1.27 × 10−4 mol/L(6), 1.15 × 10−4 mol/L(7), 1.77 × 10−4 mol/L(8). Solvent: CH3CN.
Table 2
UV absorption data of 6–8.
mercury atom, and this phenomenon is in consistence with other
nitrogen-containing organomercurials [6,10].
Suzuki coupling. A variety of pincer palladium complexes were
emerging as a new class of efficient catalysts for the carbon–carbon
bond formation reactions. In order to test the catalytic activity of 8,
the Suzuki coupling between aryl halides with phenylboronic acid
were assessed (Scheme 2). All the experiments were carried out
under the same optimized conditions (1,4-dioxane, 60 °C for 10 h,
K2CO3 as the base, catalyst loading: 1 mmol%) and the results were
summarized in Table 3. From Table 3, a good catalytic activity of 8
was observed, and as expected, aryl bromides (Entries 1–8) were
more reactive than their chlorides congeners (Entries 9–15). It was
also found that the couplings of aryl bromides bearing electron-
withdrawing groups (Entries 4–8) were easier than those bearing
electron-donating groups (Entries 1–3) and a similar phenomenon
was observed in the couplings of aryl chlorides (Entries 11–14 vs
Entry 9).
Complex
6
7
8
λmax (nm)
242, 273
243, 280
244, 273, 336, 350
it can be seen that the absorption bands of 6–8 at λmax ~ 243 and
273 nm, respectively are ascribed to the π → π* electron transition
from the O–C=N moiety of the oxazolinyl ring to the phenyl ring and
these results are in good agreement with the fact as discussed in
X-ray crystal analysis. The absorption bands at λmax = 336, and
350 nm in 8 are most likely caused by coplanarity between pyridyl
and phenyl rings. It is worth noting that a small red shift from λmax
273 nm (6) to 280 nm (7) indicates that a weak N–Hg interaction
might exist in 7 between nitrogen atom in the O–C=N moiety and
Scheme 2. Suzuki coupling between aryl halides and phenylboronic acid.

P. Li et al. / Inorganic Chemistry Communications 32 (2013) 78–81
81
molecular structure of the novel tetranuclear compound [Pd2{1,3-[C(H)=
NCH2C4H7O]2C6H2}-(l-Cl)(Cl) (PPh3)]2, New J. Chem. 6 (2002) 895–901.
[4] (a) Q.L. Luo, J.P. Tan, Z.F. Li, et al., Novel bis(azole) pincer palladium complexes:
synthesis, structures and applications in Mizoroki–Heck reactions, Dalton
Trans. 40 (2011) 3601–3609;
Table 3
The Suzuki couplings of aryl halides and phenylboronic aicd catalyzed by 8.a
Entry
X
R
Yield (%)b
1
2
3
4
5
6
7
8
Br
4-MeO
4-Me
3-Me
4-O2N
4-Cl
4-COCH3
4-CO2CH3
2-Py
90.3
91.0
90.5
92.4
91.3
95.3
94.8
91.9
43.2
56.6
68.2
73.7
78.1
77.3
73.0
(b) B.M.J.M. Suijkerbuijk, G. van Koten, R.J.M.K. Gebbink, et al., Mono(NCN-pincer
palladium)metallo porphyrin catalysts: evidence for supramolecular bimetallic
catalysis, Dalton Trans. 39 (2010) 6198–6216;
(c) D.M. Grove, G. van Koten, J.N. Louwen, et al., Trans 2,6-bis[(dimethyamino)
methyl]phenyl-N, N’, C complexes of Pd(I1) and Pt(I1). Crystal structure of
[PtI(MeC6H3(CH2NMe2)2-o,o’)]BF4- : A cyclohexadienyl carbonium ion with a
α-bonded metal substituent, J. Am. Chem. Soc. 104 (1982) 6609–6616;
(d) P. Steenwinkel, J.T.B.H. Jastrzebski, G. van Koten, et al., Hypercoordinate
aryltrialkylsilanes and -stannanes and their use in the synthesis of
homodinuclear organometallic complexes with a 1,4-phenylene bridge, Organ-
ometallics 16 (1997) 5486–5498.
9
Cl
4-MeO
H
10
11
12
13
14
15
4-CH3CO
4-O2N
4-COCH3
4-CO2CH3
2-Py
[5] B. Soro, S. Stoccoro, M. Manassero, et al., Synthesis of the first C-2 cyclopalladated
derivatives of 1,3-bis(2-pyridyl)benzene. Crystal structures of [Hg(N-C-N)Cl],
[Pd(N-C-N)Cl], and [Pd2(N-C-N)2(í-OAc)]2 [Hg2Cl6]. Catalytic activity in the
Heck reaction, Organometallics 24 (2005) 53–61.
[6] (a) A. Beleaga, V.R. Bojan, A. Pollnitz, et al., Organomercury(II) and tellurium(II)
compounds with the “pincer” ligand 2,6-[O(CH2CH2)2 NCH2]2C6H3 stabilization
of an unusual organotellurium(II) cationic species, Dalton Trans. 40 (2011)
8830–8838;
a
Reagents: ArX (1.0 mmol), PhB(OH)2 (1.5 mmol), K2CO3 (2.0 mmol), and 1,4-dioxane
(10 ml).
b
Isolated yields, average two runs.
(b) Y.J. Wu, L.R. Yang, X.L. Cui, et al., Synthesis of novel chiral tellurium complexes by
redox reaction of planar chiral cyclomercurated ferrocenylimines with tellurium
powder and X-ray crystal structure of [TeCl{η5-C5H5FeC5H3C(CH3)-N-C6H4-4-
CH3}], Tetrahedron-Asymmetry 14 (2003) 1073–1077;
(c) P.-A. Bonnardel, R.V. Parish, Organomercury (II) and organotint (IV) compounds
with nitrogen-containing substituents, J. Organomet. Chem. 515 (1996) 221–232;
(d) J. Vicente, J.A. Abad, B. Rink, et al., Synthesis of mercury(II) and palladium(II) or-
ganometallic derivatives of terephthaldehyde by metalation, transmetalation,
oxidation, and condensation reactions. X-ray crystal structures of trans-[PdCl
Acknowledgments
We are grateful to Li-Ke Chemical Corporation limited (China) for
the financial support for this project.
{C6H3(CO2H)2-2,5}(PPh3)2]⋅Me2CO⋅2MeOH,
trans-[PdCl{C6H3(CHO)-2-
(CO2H)-5}(PPh3)2]⋅2CH2Cl2, and [Pd{C6H3(CHdNC6H4NH2-4)-2-(CHO)-5}(N,N,
N’,N’-tetramethylethylene- diamine)]CF3SO3. Molecular assemblies through
[Pd]-Cl⋅⋅⋅HO2C-Aryl and [Pd]-Cl⋅⋅⋅HOMeHO(O) C-aryl hydrogen bonds, Organ-
ometallics 16 (1997) 5269–5282;
Appendix A. Supplementary material
The syntheses and characterizations of 2–8 and catalytic experiment
of 8 can be found, in the online version, at http://dx.doi.org/10.1016/j.
inoche. CCDC 916904 and 916905 contain the supplementary crystallo-
graphic data for complexes 6 and 8. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
(e) A. Berger, A. de Cian, J.-P. Djukic, M. Pfeffer, et al., Syntheses of ortho-mercurated
and–palladated (η6-Arene)tricarbonylchromium complexes, Organometallics
20 (2001) 3230–3240;
(f) A. Berger, J.-P. Djukic, M. Pfeffer, Chloride-promoted synthesis of cis bis-chelated
palladium (II) complexes from ortho-mercurated tricarbonyl(η6-arene)chromium
complexes, Organometallics 22 (2003) 5243–5260.
[7] H. Hashizume, H. Ito, N. Kanaya, et al., Synthesis and biologic activities of new
HMG-COA synthase inhibitors: 2-oxetanones with a side chain containing biphenyl,
terphenyl or phenylpyridine, Heterocycles 38 (1994) 1551–1572.
[8] C. Hansch, S.D. Rockwell, P.Y.C. Jow, et al., Substituent constants for correlation
analysis, J. Med. Chem. 20 (1977) 304–306.
References
[9] Y. Motoyama, H. Kawakami, K. Shimozono, et al., Synthesis and X-ray crystal
structures of bis(oxazolinyl)phenyl-derived chiral palladium(II) and platinum(II)
and -(IV) complexes and their use in the catalytic asymmetric Aldol-type conden-
sation of isocyanides and aldehydes, Organometallics 21 (2002) 3408–3416.
[10] (a) J.D. Lee, S.J. Kim, J.J. Ko, et al., Synthesis and reactivity of intramolecularly stabi-
lized organotin compounds containing the C,N-chelating o-carboranylamino li-
gand [o-C2B10H10(CH2NMe2)-C,N]-(CabC,N). X-ray Structures of (CabC,N)SnR2X
(R) Me, X)Cl; R) Ph, X) Cl), (CabC,N)2Hg, and [(CabC,N)SnMe2]2, Organometal-
lics 19 (2000) 1695–1703;
[1] (a) N. Selander, K.J. Szabo, Catalysis by palladium pincer complexes, Chem. Rev.
111 (2011) 2048–2076;
(b) A. Bugarin, B.T. Connell, A highly active and selective palladium pincer cata-
lyst for the formation of α-aryl ketones via cross-coupling, Chem. Commun.
47 (2011) 7218–7220.
[2] B. Wieczorek, R.J.M.K. Gebbink, G. van Koten, et al., Incorporating ECE-pincer
metal complexes as functional building blocks in semisynthetic metalloenzymes,
supramolecular polypeptide hybrids, tamoxifen derivatives, biomarkers and sen-
sors, J. Organomet. Chem. 694 (2009) 812–822.
[3] (a) D.J. Cardenas, A.M. Echavarren, Divergent behavior of palladium(II) and
platinum(II) in the metalation of 1,3-di(2-pyridyl)benzene, Organometallics
18 (1999) 3337–3341;
(b) L. Ferro, M.P. Coles, J.R. Fulton, et al., Taking advantage of Hg-C bonds: Synthesis
of the first homoleptic bis-β-diketiminate complex bound through the
γ-carbons, Organometallics 29 (2010) 2911–2915.
(b) A. Fernandez, E. Pereira, J.M. Vila, et al., Sterically controlled reactivity
of palladium(II) tetranuclear cyclometallated complexes. Crystal and