Palladium(II) complexes of the first pincer (Se,N,Se) ligand, 2,6-Bis((phenylseleno)methyl)pyridine (L): Solvent-dependent formation of [PdCl(L)]Cl and Na[PdCl(L)][PdCl4] and high catalytic activity for the heck reaction

Article abstract of DOI:10.1021/om900570g

The reaction of PhSe- with 2,6-bis(chloromethyl)pyridine has resulted in the first pincer (Se,N,Se) ligand, 2,6-bis((phenylseleno)methyl)pyridine (L), which has formed two types of complexes, [PdCl(L)]Cl (1) and Na[PdCl(L)][PdCl₄P), when reacte

Full text of DOI:10.1021/om900570g

6054 Organometallics 2009, 28, 6054–6058
DOI: 10.1021/om900570g
Palladium(II) Complexes of the First Pincer (Se,N,Se) Ligand,
2,6-Bis((phenylseleno)methyl)pyridine (L): Solvent-Dependent Formation
of [PdCl(L)]Cl and Na[PdCl(L)][PdCl4] and High Catalytic Activity for
the Heck Reaction
Dipanwita Das, G. K. Rao,† and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India. †Contributed to
the crystallographic analysis.
Received July 2, 2009
The reaction of PhSe- with 2,6-bis(chloromethyl)pyridine has resulted in the first pincer (Se,N,Se)
ligand, 2,6-bis((phenylseleno)methyl)pyridine (L), which has formed two types of complexes,
[PdCl(L)]Cl (1) and Na[PdCl(L)][PdCl4](2), when reacted with Na2[PdCl4], depending upon the
solvent. The complex 2 is formed in methanolic medium and 1 in an acetone-water (2:1) mixture.
L, 1, and 2 have been characterized by proton, carbon-13, and selenium-77 NMR spectra and single-
˚
crystal X-ray crystallography. The Pd-Se bond length is between 2.3891(1) and 2.4313(7) A due to
the trans influence of two Se donor atoms on each other and the rigidity of L. The noncovalent
interactions (hydrogen bonding) present in the crystal of 1 make a ring structure comprising its six
molecules. In the case of complex 2, a dimeric unit is formed due to such interactions. The geometry
around Pd is nearly square planar for both the complexes. 1 and 2 are both efficient as catalysts for the
Heck coupling (C-C) reaction, as the yield and TON values are up to 97% and 97 000, respectively.
For complex 2 yields and TON values are higher in comparison to those obtained with 1.
Pincer complexes of Pd(II) and Pt(II) with a variety of
ligands of N-C-N, P-C-P, S-P-S, S-C-S, and
Se-C-Se types have been investigated in the recent past.1-10
Foremost among the reasons for the attention received by
them are their catalytic applications. A palladium(II) complex
of a 1,2,4-triazole-based pincer ligand has been reported as a
catalyst for the Heck reaction.11 Pincer Pt(II) and Pd(II)
complexes with 1,3-bis(20-imidazolinyl)benzene show catalytic
activity in the Friedel-Crafts alkylation reaction.3 Pincer
thioamide and thioimide Pd(II) complexes synthesized from
2,6-bis((butylaminothio)carbonyl)pyridine were found to be
efficient catalysts for Negishi coupling involving primary and
secondary alkylzinc reagents.1 A pincer complex of palladium-
(II) with a ligand of Se-C-Se type has been shown to catalyze
selenylation of propargyl, allyl, benzyl, and benzoyl halides
under mild reaction conditions employing trimethylstannyl
phenyl selenide as a selenylating agent.12 An efficient route to
functionalized allylboronic acids and potassium trifluoroallyl-
borates has been reported using a pincer Pd(II) complex of a
Se-C-Se type ligand.13 An efficient one-pot synthesis of
stereodefined R-amino acids and homoallyl alcohols was de-
signed by integration of the Pd(II) pincer S-C-S/Se-C-Se
ligand complex catalyzed borylation of allyl alcohols in the
Petasis borono-Mannich reaction and in the allylation of
aldehydes and ketones.6 An efficient palladium pincer P-C-P
ligand complex catalyzed reaction has been developed for
R-C-H bond functionalization of benzyl nitriles.7 Direct
boronation of allyl alcohols with diboronic acid using a
palladium pincer Se-C-Se ligand complex as catalyst occurs
under mild conditions.14 Highly regio- and stereoselective
coupling of allyl alcohols with aldehydes has been achieved
with 5 mol % of Se-C-Se pincer Pd complex catalyst and
p-toluenesulfonic acid in the presence of diboronic acid.15 The
coupling of allylboronic acids with iodobenzenes, which results
in selective formation of the branched allylic product in the
absence of directing groups, has been catalyzed by a Se-C-Se
*To whom correspondence should be addressed. E-mail: aksingh@
chemistry.iitd.ac.in, ajai57@hotmail.com.
(1) Wang, H.; Liu, J; Deng, Y.; Min, T.; Yu, G.; Wu, X.; Yang, Z.;
Lei, A. Chem. Eur. J. 2009, 15, 1499.
(2) Kruithof, C. A.; Berger, A.; Dijkstra, H. P.; Soulimani, F.; Visser,
T.; Lutz, M.; Spek, A. L.; Klein Gebbink, R. J. M.; van Koten, G. Dalton
Trans. 2009, 3306.
(3) Wu, L.-Y.; Hao, X.-Q.; Xu, Y.-X.; Jia, M.-Q.; Wang, Y.-N.;
Gong, J.-F.; Song, M.-P. Organometallics 2009, 28, 3369.
(4) Li, J.; Minnaard, A. J.; Klein Gebbink, R. J. M.; van Koten, G.
Tetrahedron Lett. 2009, 50, 2232.
(5) Liu, J.; Wang, H.; Zhang, H.; Wu, X.; Zhang, H.; Deng, Y.; Yang,
Z.; Lei, A. A. Chem. Eur. J. 2009, 15, 4437.
(6) Selander, N.; Kipke, A.; Sebelius, S.; Szab, K. J. J. Am. Chem.
Soc. 2007, 129, 13723.
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(7) Aydin, J.; Conrad, C. S.; Szabo, K. J. Org. Lett. 2008, 10, 5175.
(8) Selander, N.; Szab, K. J. Adv. Synth. Catal. 2008, 350, 2045.
(9) Selander, N.; Szab, K. J. Chem. Commun. 2008, 3420.
(10) Bedford, R. B.; Betham, M.; Charmant, J. P. H.; Haddow, M. F.;
Orpen, A. G.; Pilarski, L T.; Coles, S. J.; Hursthouse, M. B. Organo-
metallics 2007, 26, 6346.
(12) Wallner, O A.; Szab, K. J. J. Org. Chem. 2005, 70, 9215.
(13) Sebelius, S.; Olsson, V. J.; Szab, K. J. J. Am. Chem. Soc. 2005,
127, 10478.
(14) Olsson, V. J.; Sebelius, S.; Selander, N.; Szab, K. J. J. Am. Chem.
Soc. 2006, 128, 4588.
(15) Selander, N.; Sebelius, S.; Estay, C.; Szabo, K. J. Eur. J. Org.
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(11) Dez-Barra, E.; Guerra, J.; Hornillos, V.; Merino, S.; Tejeda, J.
Organometallics 2003, 22, 4610.
Chem. 2006, 4085.
r
pubs.acs.org/Organometallics
Published on Web 10/02/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 20, 2009 6055
pincer Pd complex,16 which has been found to be a highly
active catalyst for the Heck reaction17 and has also been used
in the catalytic system for the single-pot synthesis of homo-
allyl alcohols.9 The palladium bis(pyrroloimidazolidinone)
(N-C-N pincer) complex exhibits extraordinarily high cata-
lytic activity in the Heck arylation reaction.18 A Pd(II) complex
of an S-P-S pincer ligand featuring a metalated ylidic P atom
efficiently catalyzes the coupling reaction between pinacolbor-
ane and various aryl iodides to yield the corresponding aryl-
boronic esters (TON between 5100 and 76 500).19 Pincer sul-
fato palladium complexes (pincer=[C6H3(CH2E)2-2,6]-; E=
SMe, SBu-t, NMe2) show promising catalytic activity for the
Suzuki-Miyaura C-C cross-coupling reaction of 3-iodoben-
zoic acid and sodium tetraphenylborate in water.2 A P-C-P
pincer palladium complex catalyzed allylation of N,N-di-
methylsulfamoyl-protected aldimines with allyltributylstan-
nane has been found suitable for the synthesis of N-homo-
allylic sulfamides.4 The complex of palladium(II) with 2,6-
bis(2-pyridyl)-4-tert-butylbenzene (N-C-N type pincer)
shows activity for the Mizoroki-Heck reaction.20 Recently,
carborane-based pincers of SeBSe and SBS type and their
Pd(II) complexes have been reported.21 We are unaware of
any report on a pincer ligand of the Se-N-Se type and its
ligand chemistry. Attempts to synthesize such a ligand of
Te-N-Te type did not succeed earlier.22 Therefore, 2,6-bis-
((phenylseleno)methyl)pyridine (L) has been synthesized. Its
ligation with Pd(II) has been studied and the novel complex
Na[Pd(L)Cl][PdCl4] synthesized. The Pd(II) complexes have
been found promising for the catalytic Heck reaction. The
results are reported in the present note.
Table 1. Conversions (%) in Heck Reactions of Aryl Halides with
Styrene/n-Butyl Acrylatea
a In the body of the table, values for n-butyl acrylate are indicated with
an asterisk.
group, structure determination, and refinements.23,24 Hydrogen
atoms were included in idealized positions with isotropic ther-
mal parameters set at 1.2 times that of the carbon atom to which
they were attached in all cases. The melting points determined in
an open capillary are reported as such.
Synthesis of Ligand L. A solution of diphenyl diselenide
(0.32 g, 1 mmol) in 30 mL of EtOH was stirred under a nitrogen
atmosphere. Sodium borohydride (0.076 g, 2 mmol) dissolved
in 5 mL of aqueous NaOH (5%) was added to it dropwise until
it became colorless due to the formation of PhSeNa. 2,6-Bis-
(chloromethyl)pyridine (0.176 g, 1 mmol) dissolved in 10 mL of
ethanol was added to the colorless solution with constant
stirring and the mixture stirred further for 3 h. It was then
poured into cold water (30 mL). The ligand L was extracted with
chloroform (4 ꢀ 25 mL) from the aqueous layer. The extract was
washed with water (3 ꢀ 40 mL) and dried over anhydrous
sodium sulfate. The solvent was evaporated off under reduced
pressure on a rotary evaporator to result in a viscous oil, which
on mixing with methanol (5-7 mL) and placing in a refrigerator
(5 °C) gave single crystals of L. Yield: 0.62 g (∼75%). Anal.
Found: C, 53.98; H, 3.95; N, 3.76. Calcd for C19H17NSe2: C,
54.71; H, 4.11; N, 3.36. Mp: 45 °C. 1H NMR (CDCl3, 25 °C vs
Experimental Section
Diphenyl diselenide, 2,6-bis(chloromethyl)pyridine, and dis-
odium tetrachloropalladate procured from Sigma-Aldrich
(USA) were used as received. All the solvents were dried and
distilled before use by well-known standard procedures. The
C, H, and N analyses were carried out with a Perkin-Elmer 2400
Series II C, H, N analyzer. The 1H, 13C{1H}, and 77Se{1H}
NMR spectra were recorded on a Bruker Spectrospin DPX-300
NMR spectrometer at 300.13, 75.47, and 57.24 MHz, respec-
tively. IR spectra in the range 4000-250 cm-1 were recorded on
TMS; δ, ppm): 4.18 (s, 2JSe,H=6.3 Hz, 4H, H5), 6.89 (d, 3JH-H
7.8 Hz, 2H, H7), 7.21-7.25 (m, 6H, H1 þ H2), 7.35 (t, 3JH-H
=
=
ꢀ
a Nicolet Protege 460 FT-IR spectrometer as KBr pellets.
7.8 Hz, 1H, H8), 7.46-7.49 (m, 4H, H3). 13C{1H} NMR
(DMSO-d6, 25 °C vs TMS; δ, ppm): 32.2 (C5), 121.0
(C7), 126.7 (C1), 129.1 (C2), 130.5 (C4), 131.6 (C3), 137.1 (C8),
158.0 (C6). 77Se{1H} NMR (DMSO-d6, 25 °C vs Me2Se; δ, ppm):
351.2.
Single-crystal structure data were collected with a Bruker AXS
˚
SMART Apex CCD diffractometer using Mo KR (0.710 73 A)
radiation at 298(2) K. The software SADABS was used for
absorption correction (if needed) and SHELXTL for space
ꢀ
(16) Sebelius, S.; Olsson, V. J.; Wallner, O. A.; Szabo, K. J. J. Am.
Chem. Soc. 2006, 128, 8150.
(17) Yao, Q.; Kinney, E. P.; Zheng, C. Org. Lett. 2004, 6, 2997.
(18) Takenaka, K.; Uozumi, Y. Adv. Synth. Catal. 2004, 346, 1693.
(19) Doux, M.; Mezailles, N.; Melaimi, M.; Ricard, L.; Le Floch, P.
Chem. Commun. 2002, 1566.
Synthesis of [PdCl(L)]Cl H2O (1). A solution (in 5 mL of
3
water) of Na2[PdCl4] (0.029 g, 0.1 mmol) was mixed with a
solution of L (0.042 g, 0.1 mmol) in acetone (10 mL) with
vigorous stirring. The orange precipitate of 1 obtained instan-
taneously was filtered, washed with water, and dried. Single
crystals of 1 were grown from a 1:1 mixture of acetonitrile and
methanol. Yield: 0.37 g (∼79%). Anal. Found: C, 37.68; H, 2.95;
(20) Hirano, Y.; Saiki, Y.; Taji, H.; Matsukawa, S.; Yamamoto, Y.
Heterocycles 2008, 76, 1585.
(21) Spokoyny, A. M.; Reuter, M. G.; Stern, C. L.; Ratner, M. A.;
Seideman, T.; Mirkin, C. A. J. Am. Chem. Soc. 2009, 131, 9482.
(22) Khalid, A.; Singh, A. K. Polyhedron 1997, 16, 33.
(23) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(24) Sheldrick, G. M. SHELXL-NT Version 6.12; University of
Gottingen, Gottingen, Germany, 2000.
N 2.02. Calcd for C19H17Cl2NPdSe2 H2O: C, 37.26; H, 3.13;
3
1
N, 2.29. Mp: 217 °C. H NMR (DMSO-d6, 25 °C vs TMS; δ,
ppm): 5.22 (s, 4H, H5), 7.46-7.52 (m, 6H, H1 þ H2), 7.69 (d,
3JH-H=7.8 Hz, 2H, H7), 7.94 (d, 3JH-H=7.5 Hz, 4H, H3), 8.07
(t, 3JH-H=7.8 Hz, 1H, H8). 13C{1H} NMR (DMSO-d6, 25 °C vs

6056 Organometallics, Vol. 28, No. 20, 2009
Das et al.
Scheme 1
Figure 1. ORTEP diagram of L with 50% probability ellipsoids;
˚
H atoms are not shown for clarity. Bond distances (A): Se(2)-
C(14), 1.920(5); Se(2)-C(13), 1.953(5); Se(1)-C(6), 1.927(5);
Se(1)-C(7), 1.967(5). Bond angles (deg): C(14)-Se(2)-C(13),
101.3(2); C(6)-Se(1)-C(7), 98.1(2).
TMS; δ, ppm): 42.6 (C5), 124.7 (C7), 127.5 (C1), 130.8 (C2), 130.9
(C4), 132.6 (C3), 140.6 (C8), 164.8 (C6). 77Se{1H} NMR (DMSO-
d6, 25 °C vs Me2Se; δ, ppm): 390.4.
Synthesis of Na[PdCl(L)][PdCl4 ] (2). A solution of Na2-
[PdCl4] (0.029 g, 0.1 mmol) in 5 mL of methanol was mixed
with a solution of ligand L (0.042 g, 0.1 mmol) also in methanol
(10 mL) with vigorous stirring for ∼2 h. The resulting orange
solution was warmed to a mild level (∼45 °C) for 5-10 min
and kept aside for crystallization. Orange needle-shaped fine
single crystals of 2 suitable for X-ray diffraction were obtained.
Anal. Found: C, 28.13; H, 2.42; N 2.02. Calcd for C19H17Cl5-
NaNPd2Se2: C, 27.49; H, 2.06; N, 1.69. Mp: 195 °C. 1H NMR
(DMSO-d6, 25 °C vs TMS; δ, ppm): 5.17 (s, 4H, H5), 7.42-7.53
3
(m, 6H, H1 þ H2), 7.80 (d, JH-H=6.6 Hz, 2H, H7), 7.90 (d,
3
3JH-H = 7.5 Hz, 4H, H3), 7.99 (t, JH-H = 8.1 Hz, 1H, H8).
Figure 2. ORTEP diagram of 1 with 50% probability ellipsoids.
˚
H atoms are not shown for clarity. Selected bond lengths (A):
13C{1H} NMR (DMSO-d6, 25 °C vs TMS; δ, ppm): 41.4 (C5),
123.8 (C7), 125.8 (C1), 129.6 (C2), 13.8 (C4), 131.2 (C3), 139.4
(C8), 163.1 (C6). 77Se{1H} NMR (DMSO-d6, 25 °C vs Me2Se;
δ, ppm): 390.1.
Pd(1)-N(1), 1.985(3); Pd(1)-Cl(1), 2.2309(12); Pd(1)-Se(1),
2.3891(7); Pd(1)-Se(2), 2.4313(7). Selected bond angles (deg):
N(1)-Pd(1)-Cl(1), 177.99(11); N(1)-Pd(1)-Se(1), 90.87(11);
Cl(1)-Pd(1)-Se(1), 88.86(5); N(1)-Pd(1)-Se(2), 85.15(11);
Cl(1)-Pd(1)-Se(2), 94.91(5); Se(1)-Pd(1)-Se(2), 172.65(2).
Procedure for Catalytic Heck Reaction. A mixture of styrene/
n-butyl acrylate (2 mmol), aryl halide (1 mmol), n-butylamine
(0.146 g, 2.0 mmol), p-xylene (∼3 mL), and complex 1 or 2
(100 μL of a 10-4 M solution in p-xylene; ∼0.001 mol %) was
stirred for 24 h at 100-110 °C in an oil bath. It was cooled to
room temperature, treated with chloroform (40 mL), and fil-
tered. The filtrate was washed with acidified water and dried
over anhydrous Na2SO4 and its solvent was evaporated on a
rotary evaporator to obtain the product, which was purified
with flash column chromatography (using a silica gel column
and chloroform/hexane (1:1) as solvent). The products were
authenticaed by proton and carbon-13 NMR spectra. Yields
(%) and TON values are given in Table 1.
It is interesting to note that a change in solvent of the
complexation reaction from acetone to methanol results in
a different species. Probably the reason for this is the poor
solubility of [PdClL]Cl in an acetone/water mixture in
comparison to that in methanol. Thus, in methanol the
association of [PdClL]þ with the counteranion [PdCl4]2-
,
available from the unreacted Na2[PdCl4], results in the
formation of complex 2.
NMR Spectra. The NMR (1H, 13C{1H}, and 77Se{1H})
spectra of L as well as both of its complexes were found to be
generally in agreement with their molecular structures. The
77Se{1H} spectra of all three are given in the Supporting
Information. There is one signal in the spectrum of each,
implying the equivalence of the two Se atoms in solution. The
presence of one signal in the 77Se NMR spectrum of 1 or 2
shifted to high frequency (39.2 ppm) with respect to that of
free L implies that both Se donor sites of L are engaged in
coordination with Pd in an almost equivalent manner. This is
corroborated by 1H and 13C NMR spectra of L, 1, and 2. The
signal of proton H5 (which belongs to the CH2 group
Results and Discussion
The syntheses of L and its complexes 1 and 2 are summar-
ized in Scheme 1. L, which is stable under ambient condi-
tions, can be stored in the refrigerator for several months. Its
complexes 1 and 2 are also stable under ambient conditions.
The solubility of ligand L in common organic solvents, viz.
chloroform, methanol, dichloromethane, and acetone, has
been found to be good. In contrast, both of the complexes
(1 and 2) are moderately soluble in chloroform dichloro-
methane, benzene, and xylene. In methanol complex 1 is
sparingly soluble but 2 exhibits somewhat better solubility. L
and both of its complexes are soluble in DMSO, but their
solutions show signs of decomposition after 16-17 h.
1
attached to the donor site Se) in the H NMR spectra of 1
and 2 appears at a higher frequency (∼1 ppm) relative to that
of free L. Similarly, the signal of C5 in the 13C NMR spectra

Article
Organometallics, Vol. 28, No. 20, 2009 6057
˚
Figure 3. Hydrogen bonds (A) in 1: Cl(1)-H(3), 2.733(1); Cl(2)-H(7B), 2.824(2); Cl(2)-H(10), 2.707(2).
28-31
˚
2.4 A.
of 1 and 2 has been observed at a higher frequency (10.3 and
9.1 ppm respectively) with respect to that of free L. These
Some examples followed by their Pd-Se bond
˚
lengths (A) are as follows: Pd complex of Se-C-Se pincer,
2.430(2);17 [PdCl2{(C4H3S)SeCH3}2], 2.439(2);28 Pd(II) com-
plex of 1,6-[CH3SeCH2CH2NHC(dO)]2C5H3N, 2.425(1),
2.422(1);29 (Me3SiCH2SeMe)2PdCl2, 2.429;30 trans-dichlorobis-
(phenyl 2,4,6-trimethylbenzyl selenido)palladium(II), 2.4299(6).31
In the Pd complex of 3H-1,4,5,7-tetrahydro-2,6-benzodiseleno-
nine, which has Se donor atoms cis to each other, the Pd-Se
1
shifts in H5 and C5 signals in H and 13C NMR spectra,
respectively, support the formation of a Pd-Se bond in 1 and
2, as inferred by 77Se NMR spectra. The signals of C6, C7,
and C8 in the 13C NMR spectra of 1 and 2 appear at higher
frequency (3.5-6.8 and 2.3-5.1 ppm, respectively) in com-
parison to those of free L, most probably due to the forma-
tion of a Pd-N(py) bond, which is further supported by
the shifts (to high frequency) in the signals of H7 (0.80-
0.90 ppm) and H8 (0.64-0.70 ppm) in the 1H NMR spectra
of 1 and 2 relative to those of free L.
bond length is around 2.381(2) A.32 In the crystal of 1, there
˚
are noncovalent interactions (hydrogen bonds) (2.707(2)-
2.733(1) A) between Cl bonded to Pd and a hydrogen of the
phenyl group and between free Cl and a hydrogen of CH2
˚
˚
(2.824(2) A) (see Figure S.2 in the Supporting Information).
Crystal Structures. The crystal structures of L, 1, and 2 have
been solved. The crystal data and refinement parameters are
given in the Supporting Information (see Table S.1). The
molecular structure of L with selected bond lengths and angles
is shown in Figure 1. More bond angles and distances are given
in Table S.1 of the Supporting Information. The Se-C(aryl)
bond is somewhat shorter than Se-C(alkyl). The Se-C-Se
bond angles (98.1(2) and 101.3(2)°) are as expected. The
hydrogen of the CH2 group of L is engaged in a C-H- - -π
The two together make a ring structure comprised of six
molecules of 1 (Figure 3). Each ring is connected with two
other such rings through noncovalent interactions (hydrogen
bonds) formed by two Cl atoms bonded to Pd. The molecular
structure of 2 is shown in Figure 4 with selected bond distances
and angles (see Table S.1 in the Supporting Information for
more values). The geometry around Pd in 2 is nearly square
planar. The Pd-Se bond distances of 1 and 2 are not much
different. All Pd-Cl bond distances in 2 are similar,
˚
interaction (2.933(8) A) (see the Supporting Information,
˚
˚
2.295(2)-2.310 A, except for one which is 2.243(2) A. This
shortening appears to be due to crystal packing effects. The
Pd-N bond distance of 2 is consistent with that of 1. A dimeric
unit (see Figures S.3 and S.4 in the Supporting Information)
results in 2 due to noncovalent interactions (hydrogen bonds)
Figure S.1). The molecular structure of 1 is shown in Figure 2
with selected bond lengths and angles (more values are given in
Table S.1). The geometry of palladium is nearly square planar.
˚
The two Pd-Se bond lengths of 1, 2.3891(12) and 2.4313(7) A,
differ from each other and are longer than the values 2.3543(16)-
(2.739(5)-2.774(8) A) between Cl of [PdCl4]2- and hydrogens
˚
˚
2.3669(11) A reported for Pd(II) complexes of tridentate
selenated Schiff bases.25-27 The elongation of the Pd-Se bond
distance in 1 may be attributed partly to the trans influence of
selenium. The greater rigidity of the pincer ligand L in compari-
son to Schiff bases may also be a contributing factor to the
of the CH2 and Ph groups.
Catalytic Applications. The Heck reaction is a powerful
tool for C-C bond formation in organic synthesis.33-36
ꢀ
(28) Oilunkaniemi, R.; Komulainen, J.; Laitinen, R. S.; Ahlgren, M.;
˚
elongation. The Pd-Cl bond distance of 1, 2.2309(12) A, is
Pursiainen, J. J. Organomet. Chem. 1998, 571, 129.
(29) Kumar, N.; Milton, M. D.; Singh, J. D.; Upreti, S.; Butcher, R. J.
Tetrahedron Lett. 2006, 47, 885.
(30) Chadha, R. K.; Chehayber, J. M.; Drake, J. E. Inorg. Chem.
1986, 25, 611.
(31) Yao, Q.-W.; Kinney, E. P.; Zheng, C.; Li, S.-J. Z. Kristall.: New
Cryst. Struct. 2004, 219, 295.
(32) Booth, D. G.; Levason, W.; Quirk, J. J.; Reid, G.; Smith, S. M.
˚
shorter than the value ∼2.31 A reported in the case of Pd
complexes of tridentate selenated Schiff bases.25-27 However,
˚
the Pd-N bond length 1.985(3) A is consistent with the value
˚
∼2.0 A reported for Pd complexes of tridentate selenated Schiff
bases.25-27 The Pd-Se bond length in complexes in which two
Se donor atoms are trans to each other is generally around
J. Chem. Soc., Dalton Trans. 1997, 3493.
(33) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic
Press: London, 1985.
(34) Tsuji, J. Palladium Reagents and Catalysts; Wiley: Chichester, U.K.,
1995.
(25) Kumar, A; Agarwal, M.; Singh, A. K. Polyhedron 2008, 27, 485.
(26) Kumar, A; Agarwal, M.; Singh, A. K. J. Organomet. Chem.
2008, 693, 3533.
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6058 Organometallics, Vol. 28, No. 20, 2009
Das et al.
Thus, their comparisons are not straightforward. However,
in comparison to Pd complexes of (Se,C,Se) pincer ligand,
which are excellent catalysts,17 our TON values of 24 h
reactions are very promising. For coupling of 4-bromoto-
luene and 4-bromoanisole with styrene (in 24 h) the use of 1
and 2 gives better TON values in comparison to Pd com-
plexes of the (Se,C,Se) pincer ligand, known to be among the
best catalysts.17 The performance of a palladium complex of
salicylaldehyde thiosemicarbazone48 as a catalyst for the
Heck reaction is not as efficient as that of 1 or 2 for many
Ar groups. In comparison to palladium(II) complexes of
tridentate selenated Schiff bases25-27 the catalytic perfor-
mance of 1 and 2 is far superior.
Figure 4. ORTEP diagram of 2 with 50% probability ellipsoids.
H atoms are not shown for clarity. Selected bond lengths
˚
(A): Cl(1)-Pd(1), 2.295(2); Cl(2)-Pd(2), 2.299(3); Cl(3)-
Pd(2), 2.243(2); Cl(4)-Pd(2), 2.296(3); Cl(5)-Pd(2), 2.310(3);
N(1)-Pd(1), 2.032(7); Pd(1)-Se(2), 2.3910(13); Pd(1)-Se(1),
2.4229(12). Selected bond angles (deg): N(1)-Pd(1)-Cl(1),
178.5(2); N(1)-Pd(1)-Se(2), 87.3(2); Cl(1)-Pd(1)-Se(2),
91.25(7); N(1)-Pd(1)-Se(1), 87.6(2); Cl(1)-Pd(1)-Se(1),
93.93(7); Se(2)-Pd(1)-Se(1), 165.94(4); Cl(3)-Pd(2)-Cl(4),
86.99(10); Cl(3)-Pd(2)-Cl(2), 92.62(9); Cl(4)-Pd(2)-Cl(5),
89.91(11); Cl(2)-Pd(2)-Cl(5), 90.46(11).
Conclusions
The pincer (Se,N,Se) ligand 2,6-bis(phenylselenomethyl)-
pyridine (L) has been prepared for the first time. It reacts
with Na2[PdCl4], giving complexes with the stoichio-
metries [PdCl(L)]Cl (1) and Na[PdCl(L)][PdCl4 ] (2), depend-
ing upon the solvent for the reaction: an acetone/water
mixture or methanol. Somewhat long Pd-Se bond lengths
Outstanding catalyst systems developed to date include those
that employ sterically bulky and electron-rich phosphine
ligands37-39 or phosphine mimics such as N-heterocyclic
nucleophilic carbenes (NHCs).40,41 Phosphorus-, nitrogen-,
and sulfur-containing palladacycles have also emerged re-
cently as powerful catalysts for this reaction.42-47 The strong
donating properties of organoselenides has made a Pd(II)
complex of an (Se,C,Se) pincer ligand an outstanding cata-
lyst system.17 This has motivated us to explore 1 and 2 as
catalysts for the Heck reaction at a concentration of 0.001
mol % using n-butylamine as base (see eq 1). Compound 2
has two palladium centers and, therefore, is expected to have
higher catalytic activity than 1, which is found to be experi-
mentally true (Table 1). Further, the data in Table 1 reveal
that complexes 1 and 2 both show very good catalytic activity
for the Heck reaction. TON and TOF values depend on
reaction conditions (solvent, base, temperature, time, etc.).
˚
(2.3891(1)-2.4313(7) A) are probably due to the combined
effect of the mutual trans influence of Se donor atoms on
each other and the rigidity of the (Se,N,Se) pincer. 1 and 2,
apart from showing noncovalent interactions (hydrogen
bonds) in their crystals (which result in extended structures),
show high catalytic activity for the Heck C-C coupling
reaction, as the percent yield and TON values are up to
97% and 9.7 ꢀ 104, respectively. 2 gives somewhat higher
values of percent yield and TON than 1, as a single molecule
has two Pd atoms. For some substrates these values are even
higher than those of an outstanding catalyst, Pd complex of
an (Se,C,Se) pincer ligand.17
Acknowledgment. We thank the Department of
Science and Technology (India) for a research project
grant, No. SR/S1/IC-23/06, and for partial financial
assistance given to establish the single -crystal X-ray
diffraction facility at IIT Delhi, New Delhi, India, under
its FIST program. D.D. thanks the University Grants
Commission (India) for an award of a Junior/Senior
Research Fellowship.
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Supporting Information Available: Tables giving crystal
data, bond lengths and angles, and IR data, figures shwoing
noncovalent interactions with bond lengths and 77Se NMR
spectra, and CIF files giving crystal data for L, 1, and 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Dupont, J. Org. Lett. 2000, 2, 1287.
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J. P.; Andreadaki, F. J.; Kostas, I. D. Tetrahedron Lett. 2004, 45, 2923.