Buchwald-Hartwig C-N cross coupling reactions catalyzed by a pseudo-pincer Pd(II) compound

Article abstract of DOI:10.1016/j.ica.2010.01.023

The reaction of the imino compound [C₆H₄-1-(OH)-3-(CH{double bond, long}NC₆H₂-2,4,6-Me₃)] (1) with [Pd(COD)Cl₂] affords in good yields the cyclometalated product {A figure is presented} (2), both species being unequivocally identified by single crystal X-ray structure analysis. Careful analysis of both structures in the solid state reveals the presence of important hydrogen bond interactions, leading in the case of the Pd(II) derivative to the formation of a pseudo-pincer/non-covalent pincer compound {A figure is presented} (2). The catalytic activity of this species was examined in Buchwald-Hartwig C-N cross coupling of morpholine with a series of p-substituted bromo-benzenes.

Full text of DOI:10.1016/j.ica.2010.01.023

Inorganica Chimica Acta 363 (2010) 1262–1268
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Inorganica Chimica Acta
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Buchwald–Hartwig C–N cross coupling reactions catalyzed by a pseudo-pincer
Pd(II) compound
Alcives Avila-Sorrosa, Fabiola Estudiante-Negrete, Simón Hernández-Ortega, Rubén A. Toscano,
*
David Morales-Morales
Instituto de Química, Universidad Nacional Autónoma de México, Cd. Universitaria, Circuito Exterior. Coyoacán 04510, DF, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the imino compound [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1) with
Received 29 December 2009
Accepted 19 January 2010
Available online 25 January 2010
[Pd(COD)Cl2]
affords
in
good
yields
the
cyclometalated
product
(2), both species being unequivocally identified by single
crystal X-ray structure analysis. Careful analysis of both structures in the solid state reveals the presence
of important hydrogen bond interactions, leading in the case of the Pd(II) derivative to the formation of a
Dedicated to Jonathan R. Dilworth
pseudo-pincer/non-covalent pincer compound
(2). The
catalytic activity of this species was examined in Buchwald–Hartwig C–N cross coupling of morpholine
with a series of p-substituted bromo-benzenes.
Keywords:
C–N cross coupling reactions
Buchwald–Hartwig reaction
Palladium complexes
Palladacycles
Ó 2010 Elsevier B.V. All rights reserved.
Pincer compounds
Crystal structures
Catalysis
1. Introduction
the last decade and several reviews [7] have been written regard-
ing this chemistry, mainly on the design of new and every time
Supramolecular chemistry [1] and catalysis [2] have had a
renaissance in the last decade and this hand to hand progress
was quickly adverted by chemist around the world, thus bringing
together this two research areas for the creation of what is now
called supramolecular catalysis [3]. Part of the aim of this crescent
area of chemistry its been on the design and exploration of non-
covalent interactions (e.g. hydrogen bond, etc.) in the creation of
new catalysts, in principle more efficient for a given catalytic pro-
cess, thus reducing either the loads of catalyst or mild the reaction
conditions by reducing reaction times or temperatures. Taking
advantage of these non-covalent interactions also may lead in prin-
ciple to the easy creation and synthesis of a series of catalysts li-
braries for their exploration in different catalytic processes, by
reducing tedious procedures of synthesis. This efforts have been
well documented in different papers and a book dealing with this
very much interesting area has been recently published [4]. On the
other hand, the development of palladacycles [5] and particularly
pincer compounds [6] have also been developed side by side of
catalysis and both areas have advanced as a consequence of this
symbiosis, thus pincer chemistry has gained a lot of interest in
more robust and catalytically active compounds. Thus, given our
current interest in the chemistry of pincer compounds [8] and
the study of supramolecular interactions [9] we would like to re-
port in this paper the easy synthesis of a potentially interesting
and versatile non-covalent pincer frame and the preliminary
exploration of the catalytic activity of their palladium derivative
on Buchwald–Hartwig C–N cross coupling reactions.
2. Experimental
2.1. Material and methods
Unless stated otherwise, all reactions were carried out under an
atmosphere of dinitrogen using conventional Schlenk glassware,
solvents were dried using established procedures and distilled un-
der dinitrogen immediately prior to use. The 1H and 13C NMR spec-
tra were recorded on a JEOL GX300 spectrometer. Chemical shifts
are reported in ppm down field of TMS using the solvent (CDCl3)
as internal standard. Elemental analyzes were determined on a
Perkin Elmer 240. Positive-ion FAB mass spectra were recorded
on a JEOL JMS-SX102A mass spectrometer operated at an acceler-
ating voltage of 10 kV. Samples were desorbed from a nitrobenzyl
alcohol (NOBA) matrix using 3 keV xenon atoms. Mass
* Corresponding author. Tel.: +52 55 56224514; fax: +52 55 56162217.
E-mail address: damor@unam.mx (D. Morales-Morales).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.023

A. Avila-Sorrosa et al. / Inorganica Chimica Acta 363 (2010) 1262–1268
1263
measurements in FAB are performed at a resolution of 3000 using
magnetic field scans and the matrix ions as the reference material
or, alternatively, by electric field scans with the sample peak brack-
eted by two (polyethylene glycol or cesium iodide) reference ions.
Melting points were determined in a MEL-TEMP capillary melting
point apparatus and are reported without correction. GC–MS ana-
lyzes were performed on a Agilent 6890 N GC with a 30.0 m DB-
1MS capillary column coupled to an Agilent 5973 Inert Mass Selec-
tive detector. The PdCl2 was purchased from Pressure Chemical Co.,
and 3-hydroxy-benzaldehyde and 2,4,6-trimeyl-aniline were com-
mercially obtained from Aldrich Chemical Co. All compounds were
used as received without further purification. The starting material
[Pd(COD)Cl2] was prepared according to published procedures
[10].
was cooled to room temperature and the organic phase analyzed
by gas chromatography (GC–MS) by duplicate.
2.5. Mercury drop experiments
Following the above described procedures; additionally adding
two drops of elemental Hg° to the reaction mixture. After the pre-
scribed reaction times, a sample of the solution was analyzed by
GC–MS: no significant difference in conversion between these
experiments and those in the absence of mercury was observed,
indicating that heterogeneous Pd(0) is not involved. These experi-
ments were performed under the same condition for the experi-
ments with bromobenzene.
2.6. Data collection and refinement for [C6H4-1-(OH)-3-(CH@NC6H2-
2,4,6-Me3)] (1) and [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-
(CH@NC6H2-2,4,6-Me3)}] (2)
2.2. Synthesis of [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1)
To 2,4,6-trimethyl-aniline (1.85 mL, 13.0 mmol) was added
dropwise under stirring a solution of 3-hydroxy-benzaldehyde
(1.58 g, 12.95 mmol) in CH2Cl2 (100 ml). The resulting solution
was stirred for 10 min, after this time 30 g of activated molecular
sieves were added. The reaction was then allowed to proceed un-
der stirring for 24 h. After the prescribed reaction time, the mixture
was filtered and washed with brine, dried (Na2SO4), filtered and
evaporated under vacuum to afford ligand (1) (2.81 g, 11.78 mmol,
94%) as a microcrystalline white powder. m.p 106–107 °C. EI-MS:
239 (100, [M+]), 223 (20), 146 (70), 77 (10) m/z (%). 1H NMR
(300 MHz, CDCl3): d 8.08 (s, 1H, CHN), 6.89–7.35 (m, 6H, Ar-H),
2.11 (s, 6H, CH3), 2.26 (s, 3H, CH3). 13C{1H} NMR (75.5 MHz, CDCl3):
d 164.04, 156.59, 148.08, 137.06, 133.62, 130.20, 128.97, 127.51,
121.61, 119.37, 114.55, 20.78, 18.29. IR (KBr): 3231 2916, 2721,
2585, 1632, 1593, 1451, 1374, 1298, 1276, 1207, 1139, 856, 786,
679 cmꢀ1. Anal. Calc. for C16H17N1O1 (Mr = 239.31): C, 80.30; H,
7.16. Found: C, 80.16; H, 7.17%.
Crystalline colorless prisms of [C6H4-1-(OH)-3-(CH@NC6H2-
2,4,6-Me3)] (1) and yellow prisms of [PdCl(H2NC6H2-2,4,6-
Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2) were grown by
slow evaporation of CH2Cl2/n-heptane and CH2Cl2/iPrOH solvent
systems respectively, and mounted in random orientation on glass
fibers. In all cases, the X-ray intensity data were measured at
298 K on a Bruker SMART APEX CCD-based three-circle X-ray
diffractometer system using graphite mono-chromated Mo K
a
(k = 0.71073 Å) radiation. The detector was placed at a distance
of 4.837 cm from the crystals in all cases. A total of 1800 frames
were collected with a scan width of 0.3° in
x and an exposure time
of 10 s/frame. The frames were integrated with the Bruker SAINT
software package [11] using a narrow-frame integration algorithm.
The integration of the data was done using a monoclinic and tri-
clinic unit cells to yield a total of 22 831 and 12 959 reflections
for 1 and 2, respectively, to a maximum 2h angle of 50.00°
(0.93 Å resolution), of which 5173 (1) and 4270 (2) were indepen-
dent. Analysis of the data showed in all cases negligible decays
during data collections. The structures were solved by Patterson
method using SHELXS-97 [12] program. The remaining atoms were
located via a few cycles of least-squares refinements and difference
2.3. Synthesis of [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-
(CH@NC6H2-2,4,6-Me3)}] (2)
A solution of [Pd(COD)Cl2] (143 mg, 0.5 mmol) in 10 mL of tol-
uene was slowly added at room temperature to a stirred suspen-
sion of (1) (0.120 g, 0.5 mmol) and Na2CO3 (0.060 g, 0.5 mmol) in
toluene (10 mL). The resulting reaction mixture was set to reflux
for 24 h. After this time, the reaction was filtered and evaporated
under vacuum to give complex (2) as a microcrystalline yellow so-
lid (0.052 g, 62% with respect to the ligand). m.p 211 °C (decomp).
Crystals suitable for X-ray analysis were obtained form a CH2Cl2/i-
PrOH solvent system. 1H NMR (300 MHz, CDCl3): d 6.65–7.05 (m,
7H, Ar-H), 7.73 (s, 1H, CHN), 2.17 (s, 6H, CH3), 2.19 (s, 6H, CH3),
2.25 (s, 3H, CH3) 2.27 (s, 3H, CH3) 3.39 (s, 2H, NH), 13C{1H} NMR
(75.5 MHz, CDCl3): d 179.60, 161.89, 148.16, 143.34, 135.97,
132.42, 130.61, 128.30, 128.14, 126.08, 121.51, 119.04, 20.47,
ꢀ
Fourier maps, using P21/n and P1 space groups for complexes 1 and
2, respectively, with Z = 8 and 2 for compounds 1 and 2, respec-
tively. Hydrogen atoms were input at calculated positions, and al-
lowed to ride on the atoms to which they are attached. Thermal
parameters were refined for hydrogen atoms on the phenyl groups
with Uiso(H) = 1.2 Ueq of the parent atom in all cases. For all com-
plexes, the final cycle of refinement was carried out on all non-zero
data using SHELXL-97 [13] and anisotropic thermal parameters for all
non-hydrogen atoms. The details of the structure determinations
are given in Table 1. The numbering of the atoms is shown in Figs.
1 and 3, respectively (ORTEP) [14]. Geometric calculations were
done using PLATON [15].
20.03, 17.99, 17.67. IR (KBr):
m 3324, 3259, 2909, 1683, 1598,
1433, 1356, 1288, 1143, 1034, 851, 781,705, 682, 630, cmꢀ1. Anal.
Calc. for C25H29Cl1N2O1Pd1 (Mr = 515.38): C, 58.26; H, 5.67. Found:
C, 58.21; H, 5.65%.
3. Results and discussion
The reaction of 3-hydroxybenzaldehyde with 2,4,6-trimethyl
aniline at room temperature under anhydrous conditions (Scheme
1) affords ligand [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1) as a
colorless microcrystalline product in high yields.
2.4. Buchwald–Hartwig cross coupling reactions of aryl bromides;
general procedure
Analysis of this compound by infrared spectroscopy reveals a
A toluene solution (3 ml) of 1.2 mmol of halobenzene, 1.4 mmol
of morpholine, and the prescribed amount of catalyst (0.1% mmol)
was introduced into a Schlenk tube under nitrogen. The tube was
charged with a magnetic stir bar and a slightly excess of base
(tBuOK, 1.70 mmol), sealed and then fully immersed in a 110 °C sil-
icon oil bath. After the prescribed reaction time (12 h), the mixture
strong band at
further, a broad signal about
m
1632 cmꢀ1 due to the presence of the imino group,
m
2916 cmꢀ1 exhibits the presence of
the hydroxy group of the phenol. Analysis by 1H NMR results more
illustrative exhibiting signals between d 2.11 and 2.26 ppm due to
the methyl groups of the CH@NC6H2-2,4,6-Me3 fragment. Addition-
ally signals between d 6.89–7.35 ppm correspond to the protons on

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Table 1
Crystal data and structure parameters for [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1) and [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2).
Identification code
(1)
(2)
Empirical formula
Formula weight
C16H17N1O1
239.31
C25H29Cl1N2O1Pd1
515.35
Temperature (K)
298(2)
0.71073
298(2)
0.71073
0
Wavelength (ÅA)
Crystal system
Space group
monoclinic
P21/n
triclinic
ꢀ
P1
Unit cell dimensions
0
11.0503(14)
10.0949(13)
25.447(3)
7.9411(9)
a (AÅ)
0
b (AÅ)
0
11.3634(12)
13.7500(15)
c (AÅ)
a
(°)
90
75.157(2)
b (°)
92.911(2)
82.027(2)
c
(°)
90
2835.0(6)
79.409(2)
1173.4(2)
V (Å3)
Z
8
2
Dcalc (Mg/m3)
1.121
0.070
1024
1.459
0.923
528
Absorption coefficient (mmꢀ1
F(0 0 0)
)
Crystal size (mm)
h (°)
0.38 ꢁ 0.20 ꢁ 0.20
0.274 ꢁ 0.186 ꢁ 0.098
1.97–25.34
1.88–25.35
Index ranges
Reflections collected
ꢀ13 6 h 6 13, ꢀ12 6 k 6 12, ꢀ30 6 l 6 30
ꢀ9 6 h 6 9, ꢀ13 6 k 6 13, ꢀ16 6 l 6 16
12 959
22 831
Independent reflections (Rint
Absorption correction
Maximum, minimum transmission
Refinement method
)
5173 (0.0543)
none
N/A
4270 (0.0291)
semi-empirical from equivalents
0.9187, 0.7941
full-matrix least-squares on F2
5173/0/337
full-matrix least-squares on F2
4270/2/286
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
0.882
1.039
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0374, wR2 = 0.0843
R1 = 0.0702, wR2 = 0.0925
0.117 and ꢀ0.123
R1 = 0.0326, wR2 = 0.0743
R1 = 0.0383, wR2 = 0.0768
0.525 and ꢀ0.220
Largest difference peak and hole (eÅꢀ3
)
Fig. 1. An ORTEP representation of the structure of the ligand [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1) at 50% of probability showing the atom labeling scheme. Selected
Bond lengths (Å): O(1)–C(1) 1.3576(17), O(1)–H(1) 0.913(16), N(1)–C(7) 1.2688(17), N(1)–C(8) 1.4358(18), O(2)–C(17) 1.3565(17), O(2)–H(2A) 0.898(19), N(2)–C(23)
1.2640(17), N(2)–C(24) 1.4384(18). Selected bond angles (°): C(1)–O(1)–H(1) 108.8(10), C(7)–N(1)–C(8) 116.54(13), C(17)–O(2)–H(2A) 113.1(11), C(23)–N(2)–C(24)
120.39(13).
the aromatic rings of the molecule. However, most significantly re-
sults the presence of a singlet at d 8.08 ppm, this signal being indic-
ative of the presence of the iminic proton CH@NC6H2-2,4,6-Me3.
Relevant from the analysis by 13C{1H} NMR is the signal at d
164.04 ppm due to the iminic carbon CH@NC6H2-2,4,6-Me3, other
signals due to the aromatic carbons are observed at d 156.59,
148.08, 137.06, 133.62, 130.20, 128.97, 127.51, 121.61, 119.37,
114.55 ppm, while signals at d 20.78 and 18.29 ppm are con-
sistent with the presence of the methyl substituents. EI-Mass
spectroscopy spectrum shows a peak at [M+] = 239 m/z (100%)

A. Avila-Sorrosa et al. / Inorganica Chimica Acta 363 (2010) 1262–1268
1265
CH2Cl2
RT
+
HO
HO
O
N
NH2
(1)
Scheme 1. Synthesis of the ligand [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1).
Attempts to crystallize compound [C6H4-1-(OH)-3-(CH@NC6H2-
2,4,6-Me3)] (1) from a CH2Cl2/n-heptane solvent system yielded
crystals suitable for single crystal X-ray diffraction analysis. This
analysis confirmed unequivocally the proposed structure of ligand
(1) (Fig. 1).
Fig. 1 exhibits the structure of ligand (1) as determined by spec-
troscopic techniques. A close examination shows the unit cell to be
supported mainly by hydrogen bonds O–Hꢂ ꢂ ꢂN (Table 2) and aro-
matic p–p interactions (3.792 Å) (Fig. 2).
Initial interest in the synthesis of the above mentioned com-
pound raised from the idea of synthesizing asymmetric pincer
compounds, however we quickly realize that the use of the imine
compound (1) may well lead to the attaining of a new palladacycle.
Thus, with the ligand [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1)
on hand we decided to explore its reactivity with the palladium
starting material [Pd(COD)Cl2]. Thus the stoichiometric reaction
(Scheme 2) under reflux conditions in toluene afforded a yellow
microcrystalline product of [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-
(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2) in good yields. The resulting
species is both air and moisture stable and can be handled on air
without any apparent decomposition.
Fig. 2. A PLATON representation of the structure of ligand [C6H4-1-(OH)-3-
(CH@NC6H2-2,4,6-Me3)] (1) showing the hydrogen bond O–Hꢂ ꢂ ꢂN and
p–p inter-
actions in the unit cell.
Initial analysis of this product by infrared spectroscopy exhibits
absorptions at
the presence of both the amine ligand H2NC6H2-2,4,6-Me3 and the
hydroxy fragment of complex (2). An additional band at
m
3324 (w), 3259 (s), 2909 (w, br) cmꢀ1 indicative of
m
1683 cmꢀ1 reveals the presence of the imine moiety, the shift of
51 cmꢀ1 (vide supra) clearly illustrate the effect of the coordination
to the metal center. Analysis by 1H NMR provides further informa-
tion that supports the proposed structure. Thus, signals between d
2.17 and 2.19 ppm are due to the methyl groups in the 2 and 6
positions of both the amine and imine aromatic fragments in the
molecule, analogously signals between d 2.25 and 2.27 ppm are
Table 2
Hydrogen bonds for [C6H4-1-(OH)-3-(CH@NC6H2-2,4,6-Me3)] (1) [Å and °].
D–Hꢂ ꢂ ꢂA
d(D–H)
ꢃ(Hꢂ ꢂ ꢂA)
d(Dꢂ ꢂ ꢂA)
<(D–Hꢂ ꢂ ꢂA)
O(1)–H(1)ꢂ ꢂ ꢂN(2)#1
O(2)–H(2A)ꢂ ꢂ ꢂN(1)
0.913(16)
0.898(19)
1.911(17)
1.868(19)
2.8091(17)
2.7454(17)
167.2(16)
165.3(17)
Symmetry transformations used to generate equivalent atoms: #1
ꢀx + 1,ꢀy + 1,ꢀz + 1.
Fig. 3. An ORTEP representation of the structure of [PdCl(H2NC6H2-2,4,6-
Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2) at 50% of probability showing
the atom labeling scheme. Selected bond lengths (Å): Pd(1)–C(2) 2.007(3), Pd(1)–
N(1) 2.049(2), Pd(1)–N(2) 2.215(3), Pd(1)–Cl(1) 2.3293(8). Selected bond angles
(°):C(2)–Pd(1)–N(1) 80.69(10), C(2)–Pd(1)–N(2) 177.36(10), N(1)–Pd(1)–N(2)
99.08(9), C(2)–Pd(1)–Cl(1) 97.45(8), N(1)–Pd(1)–Cl(1) 177.64(6), N(2)–Pd(1)–Cl(1)
82.71(7).
O
HO
[Pd(COD)Cl2]
H
N
toluene
Pd
N
Cl
reflux
NH2
(
1
)
(2)
corresponding to the molecular ion with the proper isotopic
distribution. Results from elemental analysis are also coherent
with the proposed formulation and structure.
Scheme 2. Synthesis of complex [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-
(CH@NC6H2-2,4,6-Me3)}] (2).

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due to the methyls in the position 4 of the same fragments. Addi-
tionally, a broad signal at d 3.34 ppm can be assigned to the pro-
tons on the amine ligand. Moreover, on the usual area for
aromatic protons a set of signals can be observed between d 6.65
and 7.05 ppm due to the protons on the aromatic rings of the mol-
ecule. Finally, as evidence for the presence of the imine group a
sharp singlet is observed at d 7.74 ppm due to the iminic proton
(HC@N–R), once again the shift of this signal to higher field clearly
evidences the coordination to the metal center.
Analysis by 13C{1H} NMR also shows some interesting features,
exhibiting signals at d 179.77, 162.71 and 147.27 ppm due to the
iminic carbon (HC@N–R), the phenolic carbon (HO–CAr) and the
carbon directly bonded to the CAr–NH2 group, respectively. Addi-
tionally a set of signals at d 143.34, 135.97, 132.42, 130.61,
128.30, 128.14, 126.08, 121.51, 119.04 ppm can be assigned to
the carbon atoms on the aromatic rings. Finally, signals between
d 20.47 and 17.67 ppm are due to the methyl groups in the aro-
matic rings of both the amine and imine fragments.
Although the molecular ion is not observed, further analysis by
FAB+-MS shows a peak at 497 (12%) m/z due to the fragment
[M+ꢀOH], other important fragments are observed at [M+ꢀCl] =
479 (12%) and [M+ꢀCl–(H2NC6H2-2,4,6-Me3)] = 344 (22%) m/z.
These results together with those of the elemental analyzes are
also coherent with the proposed formulation.
Recrystallization of [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-
(CH@NC6H2-2,4,6-Me3)}] (2) from a CH2Cl2/iPrOH solvent system
at room temperature, afforded crystals suitable for single crystal
X-ray diffraction analysis, revealing unequivocally the identity of
complex (2) (Fig. 3).
The structure shows the palladium center into a slightly dis-
torted square planar environment, two of the coordination sites
being occupied by the five membered C–N palladacycle and com-
pleting the coordination sphere the chloride and amine
(H2NC6H2-2,4,6-Me3) ligands (Fig. 3). Noteworthy is the fact that
a strong hydrogen bond O1–H1ꢂ ꢂ ꢂCl1 (2.14 Å) gives place to a six
member pseudo-palladacycle (Table 3), thus conforming a non-
covalent pincer compound for (2). Other important hydrogen
bonding interactions of the type N–Hꢂ ꢂ ꢂCl are also present in the
unit cell for this complex, similar interactions have been reported
previously by our group [16] (Fig. 4).
Fig. 4. A PLATON representation of the structure of complex [PdCl(H2NC6H2-2,4,6-
Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2) showing hydrogen bonds O–
Hꢂ ꢂ ꢂCl and N–Hꢂ ꢂ ꢂCl in the unit cell.
O
H
Cl
Pd
N
NH2
O
Br
H
N
N
+
tBuOK, Toluene
O
R
R
Scheme 3. Hartwig–Buchwald C–N cross coupling reactions mediated by
[PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2).
The advancement on the development of the cross coupling
reactions has been accompanied by the development of the palla-
dium chemistry, this being particularly true in the case of pallada-
cycles and more specifically in the case of pincer compounds. With
this in mind and the fact that complex [PdCl(H2NC6H2-2,4,6-
Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2) showed a good
thermal stability (m.p. 211 °C), similar to other genuine pincer
compounds and base on the experience our research group has
in the use of this sort of complexes in transition metal catalyzed
reactions [6–8], we decided to explore the reactivity of the pseu-
do-pincer compound (2) in still one of the most challenging cross
coupling processes, the C–N Buchwald–Hartwig reaction [17]
(Scheme 3).
more electro-withdrawing is the group the higher the conversion
to the amine product. This trend can be better observed when
the percentage of conversion is plotted against the Hammett
parameter [18] (Graphic 1) where a near linear behavior can be
clearly noted.
As with so many other palladacycle examples, it is possible that
the reaction may proceed trough the formation of soluble palladium
nanoparticles [19]. Hence, in order to rule out this possibility a Mer-
cury drop experiment [20] (see Section 2) was performed noticing
no appreciable difference in the performance of the catalyst with
or without the presence of elemental mercury. There has been a
considerable debate in the literature about the oxidation states of
the species involved in the catalytic cycle with Pd(IV)/Pd(II) and
Pd(II)/Pd(0) both being proposed at various times [21]. However,
in this case we favor the Pd(IV)/Pd(II), although in this case the pres-
ence of the strong hydrogen bond and the imine group may led to
the behavior of the ligand as a hemilabile pincer compound in solu-
tion [22] (a possibility that can not be ruled out). However, it is more
plausible to think that the amine ligand will be lost in the process
generating a coordinative unsaturated species which in turn might
led to the coordination of the substrates and start the catalytic cycle.
Another possibility that can not be ruled out is the potential
enhancement in reactivity due to potential interactions in solution
due to the presence of the hydroxy group, this enhancement in reac-
Thus, the p-bromobenzenes-morpholine system was chosen as
a model reaction using potassium tert-butoxyde as base. The re-
sults attained are included in Table 4.
From this table a clear trend can be observed, which is depen-
dant upon the kind of substituent in the bromobenzene, thus the
Table 3
Hydrogen bonds for [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-(OH)-6-(CH@NC6H2-2,4,6-
Me3)}] (2) [Å and °].
D–Hꢂ ꢂ ꢂA
d(D–H)
d(Hꢂ ꢂ ꢂA)
d(Dꢂ ꢂ ꢂA)
<(D–Hꢂ ꢂ ꢂA)
O(1)–H(1)ꢂ ꢂ ꢂCl(1)
N(2)–H(2B)ꢂ ꢂ ꢂCl(1)
0.820(5)
0.856(19)
2.14(5)
2.755(18)
2.942(3)
3.506(3)
165.0(5)
147.0(2)

A. Avila-Sorrosa et al. / Inorganica Chimica Acta 363 (2010) 1262–1268
1267
Table 4
In summary, we have successfully synthesized a potentially
important example of a Pd(II) pseudo-pincer/non-covalent pincer
compound and tested its catalytic performance in C–N Buch-
wald–Hartwig cross coupling reactions exhibiting moderated to
good activities depending of the electron-withdrawing capabilities
of the para-substituent in the bromobenzene. The present system
is interesting given the number of primary amines commercially
available, thus a tuning of both sterics and electronics can be envi-
sioned in order to improve the catalytic performance of the pro-
posed complexes thus allowing the creation of a library of
catalyst. Another attractive characteristic of the present system is
the easy synthesis form cheap commercially available starting
materials, thus turning this system attractive for its potential
application in organic synthesis. Efforts aimed to achieve these
goals are currently under development in our laboratories.
Buchwald–Hartwig C–N cross couplings using [PdCl(H2NC6H2-2,4,6-Me3){C6H3-2-
(OH)-6-(CH@NC6H2-2,4,6-Me3)}] (2) as catalyst precursor.
O
H
Cl
Pd
N
NH
2
O
Br
H
N
N
+
tBuOK, Toluene
O
R
R
Entry
p-Bromobenzene
% Conversiona
1
O2N
Br
O2N
O
N
91.6
2
3
Acknowledgments
CN
O
Br
CN
O
N
O
80.50
72.60
We would like to thank Chem. Eng. Luis Velasco Ibarra, Dr. Fran-
cisco Javier Pérez Flores for their invaluable help in the running of
the FAB+-MS. The financial support of this research by CONACYT
(F58692) and DGAPA-UNAM (IN227008) is gratefully acknowl-
edged.
Br
Br
Br
N
O
O
4
O
H
O
H
N
56.40
Appendix A. Supplementary material
5
6
7
8
9
Cl
N
O
Cl
H
51.50
CCDC 759800 and 759801 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2010.01.023.
Br
H
N
O
37.00
H3C
O
N
O
H3C
O
Br
35.20
Br
N
O
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25.50
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