Double HCl elimination and configuration change in the square-planar palladium complex trans-[{(Ph2PC6H4CONH)2C6H4}PdCl2] under Suzuki conditions: Isolation and molecular structure of cis-[{(Ph2PC6H4CON)2C6H4}Pd]

Article abstract of DOI:10.1016/j.inoche.2006.07.007

The trans-configurated square-planar palladium complex trans-[{(Ph₂PC₆H₄CONH)₂C₆H₄}PdCl₂] (1), which catalyzes the Suzuki cross coupling of 4-bromotoluene with phenylboronic acid, was found to react with potassium carbonate in toluene at 90 °C (Suzuki conditions) to give cis-[{(Ph₂PC₆H₄CON)₂C₆H₄}Pd] (2). The single-crystal X-ray structure analysis of 2 reveals the square-planar palladium center to be in a cis configuration. The trans-cis configuration change at palladium is possible because of the elimination of two HCl equivalents in the conversion of 1 into 2. Both complexes 1 and 2 show approximately the same catalytic performance for Suzuki reactions, suggesting 2 to be the catalytically active species.

Full text of DOI:10.1016/j.inoche.2006.07.007

Inorganic Chemistry Communications 9 (2006) 1151–1154
www.elsevier.com/locate/inoche
Double HCl elimination and configuration change in the
square-planar palladium complex
trans-[{(Ph2PC6H4CONH)2C6H4}PdCl2] under Suzuki
conditions: Isolation and molecular structure
of cis-[{(Ph2PC6H4CON)2C6H4}Pd]
*
Ludovic Chahen, Lydia Karmazin-Brelot, Georg Suss-Fink
¨
Institut de Chimie, Universite´ de Neuchaˆtel, Case Postale 158, CH-2009 Neuchaˆtel, Switzerland
Received 22 June 2006; accepted 6 July 2006
Available online 22 July 2006
Dedicated to Professor Max Herberhold on the occasion of his 70th birthday
Abstract
The trans-configurated square-planar palladium complex trans-[{(Ph2PC6H4CONH)2C6H4}PdCl2] (1), which catalyzes the Suzuki
cross coupling of 4-bromotoluene with phenylboronic acid, was found to react with potassium carbonate in toluene at 90 °C (Suzuki
conditions) to give cis-[{(Ph2PC6H4CON)2C6H4}Pd] (2). The single-crystal X-ray structure analysis of 2 reveals the square-planar pal-
ladium center to be in a cis configuration. The trans–cis configuration change at palladium is possible because of the elimination of two
HCl equivalents in the conversion of 1 into 2. Both complexes 1 and 2 show approximately the same catalytic performance for Suzuki
reactions, suggesting 2 to be the catalytically active species.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Palladium complexes; Suzuki cross-coupling; Square-planar geometry; cis and trans configuration
The Suzuki cross-coupling (Suzuki–Miyaura reaction),
catalyzed by various palladium complexes, is among the
most powerful synthetic tools for the generation of
carbon–carbon bonds [1]. Although many different palla-
dium(II) or palladium(0) complexes catalyze this cou-
pling reaction, significant efforts have been put on the
design of suitable ligands that can increase the catalytic
activity of the palladium center. Some of the most active
Suzuki catalysts are palladium complexes susceptible to
ortho-metallation or palladacycle formation [2,3]. Still,
the best results for the Suzuki cross-coupling of the deac-
tivated and sterically hindered aryl bromide with aryl
boronic acid have been obtained by Buchwald and co-
workers [4] with ligands avoiding palladacycles forma-
tion. Recently, several palladium(II) complexes contain-
ing trans-spanning diphosphine ligands that contain a
rigid ligand backbone have been synthesized [5,6]. Thus,
trans-[PdCl2(SPANphos)] reported by van Leeuwen does
not show any tendency to cis-trans isomerization [5], the
complex trans-[PdCl2-(diphos)] (diphos = 2,6-bis(2-((diph-
enylphosphino)methyl)phenyl)benzene) reported by Prot-
asiewicz [6] has been successfully used as Heck and
Suzuki catalyst, in spite of the rigid trans-geometry [7].
With the diphosphine Xantphos, developed by van Leeu-
wen [8] and used as a ligand for Suzuki reaction, palla-
dium complexes with trans-standing alkyl and chloro
ligands [9] and with trans-standing aryl and bromo ligands
[10] have been isolated and characterized by X-ray crystal-
lography. Since we recently synthesized a square planar
*
Corresponding author. Tel.: +41 32 718 2405; fax: +41 32 718 2511.
E-mail address: georg.suess-fink@unine.ch (G. Suss-Fink).
¨
1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.07.007

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L. Chahen et al. / Inorganic Chemistry Communications 9 (2006) 1151–1154
Table 1
We studied the Suzuki cross-coupling of phenylboronic
acid with 4-bromotoluene catalyzed by complex 1 at differ-
ent temperatures and in different catalyst/substrate ratios.
The results compiled in Table 1 show that complex 1 has
a medium catalytic performance with respect to known
Suzuki catalysts [1–4], the highest catalytic turnover num-
ber (TON) being 61500 (catalyst/substrate ratio of
1:100000 at 90 °C) for the cross-coupling with the deacti-
vated substrate 4-bromotoluene.
Catalytic turnover numbers (TON), indicating the moles of product
formed per mol of catalyst used after 18 h, for the Suzuki cross-coupling
of phenylboronic acid with the substrate 4-bromotoluene catalyzed by 1
and 2 in toluene in the presence of K2CO3 (2 equivalents with respect to
substrate) [12]
Br (HO) B
+
2
T (°C)
Catalyst/substrate
1
2
As we observed a color change from yellow to red dur-
ing the catalytic reaction, we decided to study the reactivity
of 1 towards a base such as K2CO3 in toluene under the
same conditions (90 °C) as those for the Suzuki cross-cou-
pling reaction but without substrate and phenylboronic
acid [13]. Under these conditions we observed the same
color change from yellow to red, and from the resulting
solution we isolated the complex cis-[{(Ph2PC6H4CON)2-
C6H4}Pd] (2). The conversion of 1 into 2 implies the loss
of two molecules of HCl, caused by the action of the base
(Scheme 1). The double HCl elimination from 1 is the same
reaction already observed for the platinum analogue trans-
[{(Ph2PC6H4CONH)2C6H4}PtCl2] which, however, works
30
30
30
60
60
60
90
90
90
1:1000
1:10000
1:100000
1:1000
1:10000
1:100000
1:1000
280
1350
0
1000
5100
33000
1000
7350
450
2000
0
1000
7400
49000
1000
8800
1:10000
1:100000
61500
71000
trans-palladium complex containing a rigid diphosphine
ligand backbone, trans- [{(Ph2PC6H4CONH)2C6H4}PdCl2]
(1) [11], we tested it as a catalyst for Suzuki cross-cou-
pling reactions.
Scheme 1.
˚
Fig. 1. ORTEP view of 2 with thermal ellipsoids at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (A): Pd(1)–
P(1) 2.2528(10), Pd(1)–P(2) 2.2481(11), Pd(1)–N(1) 2.032(4), Pd(1)–N(2) 2.069(3), C(7)–O(1) 1.229(5), C(14)–O(2) 1.235(5), C(7)–N(1) 1.356(5), C(14)–
N(2) 1.353(5), C(9)–P(1) 1.817(4), C(20)–P(2) 1.823(4). Selected bond angles (°): P(1)–Pd(1)–P(2) 105.44(4), N(1)–Pd(1)-N(2) 82.19(13), P(1)–Pd(1)–N(1)
82.29(10), P(1)–Pd(1)–N(2) 161.74(10), P(2)–Pd(1)-N(1) 172.01(10), P(2)–Pd(1)–N(2) 89.85(10), O(1)–C(7)–N(1) 124.5(4), O(2)–C(14)-N(2) 122.9(4).

L. Chahen et al. / Inorganic Chemistry Communications 9 (2006) 1151–1154
1153
[4] (a) T.E. Barder, S.D. Walker, J.R. Martinelli, S.L. Buchwald, J. Am.
Chem. Soc. 127 (2005) 4685–4696;
already with triethylamine as a base [16], while the palla-
dium complex 1 requires a stronger base such as K2CO3.
Complex 2 was characterized by correct NMR (1H, 13C,
31P) and mass-spectroscopic data as well as by satisfactory
elemental analysis data [13]. Moreover, red crystals of
2ÆCH2 Cl2, suitable for single-crystal X-ray structure anal-
ysis, were obtained by leaving a concentrated dichloro-
methane solution of the complex standing several days at
room temperature [14]. The molecular structure of 2 shows
the palladium atom to be in a distorted square-planar
geometry, surrounded by two chlorine atoms and two
nitrogen atoms in a cis coordination geometry, see Fig. 1.
The formation of five- and six-membered chelate rings
imposes a considerable distortion around the palladium
atom. The N–Pd–N angle [82.19(13)°] is acute, whereas
the P–Pd–P angle is obtuse by more than 15° [105.44(4)°].
The atoms Pd(1), P(1), P(2), N(1) and N(2) are almost
(b) J. Yin, M.P. Rainka, X. Zhang, S.L. Buchwald, J. Am. Chem.
Soc. 124 (2002) 1162–1163;
(c) J.P. Wolfe, R.A. Singer, B.H. Yang, S.L. Buchwald, J. Am. Chem.
Soc. 121 (1999) 9550–9561.
[5] Z. Freixa, M.S. Beentjes, G.D. Batema, C.B. Dieleman, G.P.F. van
Strijdonck, J.N.H. Reek, P.C.J. Kamer, J. Fraanje, K. Goubitz,
P.W.N.M. van Leeuwen, Angew. Chem., Int. Ed. 42 (2003) 1284–
1287.
[6] R.C. Smith, J.D. Protasiewicz, Organometallics 23 (2004) 4215–
4222.
[7] R.C. Smith, C.R. Bodner, M.J. Earl, N.C. Sears, N.E. Hill, L.M.
Bishop, N. Sizemore, D.T. Hehemann, J.J. Bohn, J.D. Protasiewicz,
J. Organomet. Chem. 690 (2005) 477–481.
[8] M. Kranenburg, Y.E.M. van der Burgt, Organometallics 14 (1995)
3081–3089.
[9] P.C.J. Kramer, P.W.N.M. van Leeuwen, Acc. Chem. Res. 34 (2001)
895–904.
[10] J. Yin, S.L. Buchwald, J. Am. Chem. Soc. 124 (2002) 6043–6048.
[11] S. Burger, B. Therrien, G. Suss-Fink, Eur. J. Inorg. Chem. (2003)
¨
˚
coplanar, with an average deviation of 0.0813 A; the metal
3099–3103.
˚
lies out of the plane by 0.1038(13) A. The Pd–P distances of
[12] In a Schlenk tube, the catalyst was added (in the molar ratio given in
Table 1) to a solution of 138 mg (0.5 mmol) of K2CO3 Æ1.5 H2O,
91 mg (0.75 mmol) of phenylboronic acid and 0.5 mmol of the aryl
bromide in 5 mL of toluene. Then, the mixture was heated to the
desired temperature (Table 1) and stirred for 18 h. After cooling, the
solution was filtrated over a small silica gel column, then the silica gel
was eluted with ether (20 mL). The filtrate was combined with the
ether washings, and the solution obtained was analyzed by GC.
[13] A solution of complex 1 (0.050 g, 0.058 mmol) and K2CO3 (1.2 g,
8.56 mmol) in toluene (10 mL) was stirred at 90 °C, until the yellow
solution turned dark red (ꢀ3 h). Then the solvent was evaporated in
vacuo, and the product was purified by column chromatography on
silica gel using acetone as an eluant (red fraction, yield 50%). 1H
NMR (400 MHz, CDCl3): d = 8.68–8.65 (m, 2H), 7.75–7.72 (m, 2H),
7.57(t, J = 7.5 Hz, 2H), 7.43–7.40 (m, 4H), 7.24 (t, J = 7.5 Hz, 2H),
7.21–7.13 (m, 16H), 6.82–6.73 (m, 4H). 31P NMR (81 MHz, CDCl3):
d = 26.77(s). 13C NMR (100 MHz, CDCl3): d = 164.86, 146.53,
133.88, 133.82, 133.76, 132.19, 131.58, 129.31, 129.26, 129.20,
128.78, 128.24, 124.87, 122.43. Masse: ESI-m/z = 811 [M + Na]+.
Anal. Calcd.: C, 66.97; H, 4.09; N, 3.55 Found: C, 66.75; H, 4.23; N,
3.41.
the cis complex 2 are shorter than the Pd–P distances in the
trans complex 1 [11] (see Fig. 1).
The isolation of the red complex 2, formed from the yel-
low complex 1 under Suzuki conditions, suggests 2 to be
the active species in Suzuki cross-coupling reactions cata-
lyzed by 1, given the fact that the color of the reaction solu-
tion changes from yellow to red during the Suzuki reaction
with 1 as a catalyst precursor. We therefore compared the
catalytic activities of 1 and 2 for the Suzuki cross-coupling
of phenylboronic acid with 4-bromotoluene under the same
conditions. The results compiled in Table 1 show that the
catalytic turnover numbers of both complexes are compa-
rable, 2 being always slightly more active. This is reason-
able, if one assumes that 1 is only a catalyst precursor
which has to be transformed under catalytic conditions
into the active species 2 by double HCl elimination. As this
reaction implies a trans–cis configuration change at the pal-
ladium center, it appears that the cis configuration is the
more adequate one for Suzuki activity.
[14] 2ÆCH2Cl2: C45H34Cl2N2O2P2Pd, M = 873.98 g molÀ1, trigonal, R3,
ꢀ
3
˚
˚
a = 44.266(2), c = 11.6026(4) A, V = 19689.1 (15) A , T = 153(2) K,
Z = 18, Dc = 1.327 g cmÀ3, l(Mo- Ka) = 0.656 mmÀ1, 35624 reflec-
tions measured, 8040 unique (Rint = 0.0855) which were used in all
calculations. R1 [I > 2r(I)] = 0.0513, wR2 (F2) = 0.1061 (all data). The
Acknowledgements
intensity data were collected on
a Stoe Mark II-Image Plate
Diffraction System [15] equipped with a two-circle goniometer and
using Mo-Ka graphite monochromated radiation (k = 0.71073 A).
Image plate distance 135 mm, x rotation scans 0–146° at / 0°, step
Financial support of the Fond National Suisse de la
Recherche Scientifique is gratefully acknowledged (Grant
No. 200020-105’132). We thank Professor H. Stoeckli-
˚
Dx = 1.0°, exposures of 9 min per image, 2h range 1.70–51.55°, dmin
–
´
ˆ
Evans (Universite de Neuchatel, Switzerland) for helpful
˚
dmax = 23.995–0.817 A. The structure was solved by direct methods
using the program SHELXS-97 [16] and refined by full matrix least
squares on F2 with SHELXL-97 [17]. The hydrogen atoms were
included in calculated positions and treated as riding atoms using
SHELXL-97 default parameters. All non-hydrogen atoms were
refined anisotropically, using weighted full-matrix least-squares on
F2. A semi-empirical absorption correction was applied using the
MULscanABS routine in PLATON [18]; transmission factors:
discussions.
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T
min/Tmax = 0.80681/0.93560. All attempts to refine the disordered
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¨
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˚
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´
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L. Chahen et al. / Inorganic Chemistry Communications 9 (2006) 1151–1154
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