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(μ-OAc)]2[Hg 2Cl6]. Catalytic activity in the heck reaction

Article abstract of DOI:10.1021/om040102o

The synthesis of C-2 cyclopalladated derivatives of 1,3-bis(2-pyridyl)benzene (N-CH-N) has been achieved for the first time through a transmetalation reaction. C-2 mono- and dinuclear complexes have been obtained by reaction of Pd(OAc)₂ and the organomercury(II) compound [Hg(N-C-N)Cl] (1) as precursors. The new species 2-11 were characterized in solution and, in the case of [Pd(N-C-N)Cl] (2) and [Pd₂(N-C-N) ₂(OAc)]₂[Hg₂Cl₆] (11) as well as of [Hg(N-C-N)Cl] (1), in the solid state. The X-ray crystal structures unambiguously show the pincer behavior of the N-C-N ligand in the palladium species and the linear coordination C-Hg-Cl in the mercury precursor. Displacement of the coordinated halide gives cationic species such as [Pd(N-C-N)(H₂O)] [BF₄] (5) or, in the absence of donor solvents or anions, the dinuclear halide-bridged species [Pd ₂(N-C-N)₂(μ-X)][BAr′₄] (X = Cl (6), Br (7), I (8); Ar′ = 3,5-(CP₃)₂-C₆H ₃). For comparison purposes [Pd(N-C-N)(OAc)] (9) and [Pd ₂(N-C-N)₂(μ-OAc)]-[BAr′₄] (10) were also prepared. The mononuclear complexes 2, 4, 5, and 9 are high-performance catalysts in the Heck reaction of iodobenzene and methyl acrylate, even at moderate temperatures. Under microwave heating a turnover frequency as high as 280 000 h^-1 is observed with complex 2 at the nominal temperature of 135°C.

Full text of DOI:10.1021/om040102o

Organometallics 2005, 24, 53-61
53
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(µ-OAc)]2[Hg2Cl6]. Catalytic Activity in the
Heck Reaction
Barbara Soro,*,† Sergio Stoccoro,*,† Giovanni Minghetti,† Antonio Zucca,†
Maria Agostina Cinellu,† Serafino Gladiali,† Mario Manassero,*,‡ and
Mirella Sansoni‡
Dipartimento di Chimica, Universita` di Sassari, Via Vienna 2, I-07100 Sassari, Italy, and
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita` di Milano,
Centro CNR, Via Venezian 21, I-20133 Milano, Italy
Received August 2, 2004
The synthesis of C-2 cyclopalladated derivatives of 1,3-bis(2-pyridyl)benzene (N-CH-N)
has been achieved for the first time through a transmetalation reaction. C-2 mono- and
dinuclear complexes have been obtained by reaction of Pd(OAc)2 and the organomercury(II)
compound [Hg(N-C-N)Cl] (1) as precursors. The new species 2-11 were characterized in
solution and, in the case of [Pd(N-C-N)Cl] (2) and [Pd2(N-C-N)2(OAc)]2[Hg2Cl6] (11) as well
as of [Hg(N-C-N)Cl] (1), in the solid state. The X-ray crystal structures unambiguously show
the pincer behavior of the N-C-N ligand in the palladium species and the linear coordination
C-Hg-Cl in the mercury precursor. Displacement of the coordinated halide gives cationic
species such as [Pd(N-C-N)(H2O)][BF4] (5) or, in the absence of donor solvents or anions,
the dinuclear halide-bridged species [Pd2(N-C-N)2(µ-X)][BAr′4] (X ) Cl (6), Br (7), I (8); Ar′
) 3,5-(CF3)2-C6H3). For comparison purposes [Pd(N-C-N)(OAc)] (9) and [Pd2(N-C-N)2(µ-OAc)]-
[BAr′4] (10) were also prepared. The mononuclear complexes 2, 4, 5, and 9 are high-
performance catalysts in the Heck reaction of iodobenzene and methyl acrylate, even at
moderate temperatures. Under microwave heating a turnover frequency as high as 280 000
h-1 is observed with complex 2 at the nominal temperature of 135 °C.
Introduction
rare.2 The so-called pincer ligands, N-C-N, where C is
an anionic aryl carbon atom, have attracted great
attention:5 their derivatives are currently of paramount
interest, owing to the array of their potential applica-
tions.6 In the case of late transition metals the synthesis
of mononuclear pincer N-C-N species is not that straight-
forward, as in this case direct Caryl-H activation is the
exception instead of the rule. Alternative strategies,
which have been recently reviewed,6 comprise oxidative
addition of carbon-halogen bonds, transmetalation,
and, more recently, even transcyclometalation. A pe-
rusal of the literature shows that the behavior of these
types of ligands is often hardly predictable and that the
choice of the best-suited synthetic methodology is not
that trivial. This fact is well exemplified by the reactiv-
Palladium(II) and platinum(II) organometallic deriva-
tives with terdentate mono- or dianionic ligands with
pyridine nitrogen donors are well-known. Five- and six-
membered cyclometalated species with N-C-N,1 C-N-C,2
and N-N-C3 sequences of donor atoms have been re-
ported and sometimes compared4 with the N-N-N
coordination complexes of the isostructural 2,2′:6′,2"-
terpyridine. The N-N-C derivatives have been widely
investigated, and many examples have been reported,
with both metal-C(sp2) and metal-C(sp3) bonds. At
variance, species with the dianionic ligands C-N-C,
which imply a trans C-metal-C arrangement, are still
† Universita` di Sassari.
‡ Universita` di Milano.
(3) (a) Minghetti, G.; Cinellu, M. A.; Chelucci, G. A.; Gladiali, S.;
Demartin, F.; Manassero, M. J. Organomet. Chem. 1986, 679, 107. (b)
Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J. Chem.
Soc., Dalton Trans. 1990, 443. (c) Constable, E. C.; Henney, R. P. G.;
Leese, T.A.; Tocher, D. A. J. Chem. Soc., Chem. Commun. 1990, 513.
(d) Constable, E. C.; Henney, R. P. G.; Raithby, P. R.; Sousa, L. R. J.
Chem. Soc., Dalton Trans. 1992, 2251. (e) Minghetti, G.; Zucca, A.;
Stoccoro, S.; Cinellu, M. A.; Manassero, M. J. Chem. Soc., Dalton Trans.
1995, 777. (f) Zucca, A.; Cinellu, M. A.; Pinna, M. V.; Stoccoro, S.;
Minghetti, G.; Manassero, M.; Sansoni, M. Organometallics 2000, 19,
4295. (g) Zucca, A.; Stoccoro, S.; Cinellu, M. A.; Minghetti, G.;
Manassero, M.; Sansoni, M. Eur. J. Inorg. Chem. 2002, 12, 3336. (h)
Neve, F.; Crispini, A., Di Pietro, C.; Campagna, S. Organometallics
2002, 21, 3511. (i) Stoccoro, S.; Soro, B.; Minghetti, G.; Zucca, A.;
Cinellu, M. A. J. Organomet. Chem. 2003, 679, 1.
(1) (a) Canty, A. J.; Minchin, N. J.; Skelton, B. W.; White, A. H. J.
Chem. Soc., Dalton Trans. 1987, 1477. (b) Cardenas, D. J.; Echavarren,
A. M.; Ramırez de Arellano, C. M. Organometallics 1999, 18, 3337. (c)
Song, D.; Wu, Q.; Hook, A.; Korin, I.; Wang, S. Organometallics 2001,
20, 4683. (d) Sumby, C. J.; Steel, P. J. Organometallics 2003, 22, 2358.
(2) (a) Deuschel-Cornioley, C.; Ward, T.; von Zelewsky, A. Helv.
Chim. Acta 1988, 71, 130. (b) Cave, G. W. V.; Alcock, N. W.; Rourke,
J. P. Organometallics 1999, 18, 1801. (c) Cave, G. W. V.; Fanizzi, F.
P.; Deeth, R. J.; Eddington, W.; Rourke, J. P. Organometallics 2000,
19, 1355. (d) Lu, W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M.
Organometallics 2001, 20, 2477. (e) Newman, C. P.; Cave, G. W. V.;
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10.1021/om040102o CCC: $30.25 © 2005 American Chemical Society
Publication on Web 12/03/2004

54 Organometallics, Vol. 24, No. 1, 2005
Soro et al.
ity of bis(pyridine) N-C-N arylpincer ligands. Although
these derivatives have been less investigated by far than
the analogous diamino species, their peculiar behavior
is readily apparent, even from the very few reports
published thus far. With 1,3-bis(2-pyridyl)benzene direct
C-H activation has been successfully achieved in the
case of platinum(II):1b regioselective monometalation
takes place at C-2 of the benzene ring, as previously
observed for ruthenium and osmium,7 giving [Pt(N-C-
N)Cl] in fairly good yields. Despite numerous attempts
under different conditions with several precursors, this
was not the case for palladium: no C-2 metalation was
achieved and either adducts, [Pd(N-CH-N)Cl2]2, or
dimeric 4,6-bis-palladated derivatives were isolated.1b,8
In sharp contrast, introduction of a spacer, e.g. -C(O)-
or -CH(Me)-, between the central aryl ring and the
pyridine substituents promotes ready palladation at C-2
to give [Pd(N-C-N)X] (X ) OAc, Cl) derivatives involving
two six-membered palladacyclic rings, as reported by
Canty et al. several years ago.1a,9
benzene is reacted with Li2[PdCl4] or with other Pd(II)
chloride species but that C-H activation readily occurs
in the reaction with [Pd(OAc)2]. Rather surprisingly, this
does not result in the formation of a C-2 metal bond,
but instead it leads to the isolation of dimeric species
with bridging acetato ions between two doubly 4,6-
metalated units.1b In view of the potential interest of a
C-2 palladated derivative of this N-CH-N ligand, e.g.
as a catalytic precursor,12 we deemed it worthwhile to
pay some more effort to its preparation through trans-
metalation. Transmetalation of aryllithium species has
been widely applied, but lithiation of N-C(H)-N ligands
is not straightforward, as it is frequently not regiose-
lective. Hence, since mercury(II) salts, particularly the
acetate, are known to easily attack aromatic com-
pounds,13 we decided to synthesize the mercury deriva-
tive and try a metal-exchange reaction. Indeed, reaction
of the ligand with [Hg(OAc)2] in methanol, after treat-
ment with LiCl, affords [Hg(N-C-N)Cl] (1), albeit in
moderate yield (ca. 60%).
Here we report the synthesis of the first C-2 cyclo-
palladated derivatives of 1,3-bis(2-pyridyl)benzene ob-
tained by transmetalation of the corresponding aryl-
mercury derivative. Also described are the X-ray crystal
structures of the mercury derivative [Hg(N-C-N)Cl] (1),
that of [Pd(N-C-N)Cl] (2), and that of a more complex
species, [Pd2(N-C-N)2(OAc)]2[Hg2Cl6] (11), isolated in the
course of the transmetalation reaction. Preliminary
results on the catalytic activity of a range of these new
complexes in the Heck reaction, between iodobenzene
and methyl acrylate, are reported as well.
1
The H NMR spectrum of 1 confirms the occurrence
of C-metalation: of the six resonances observed, two,
at δ 7.98 (d, 2H) and 7.52 (t, 1H), are easily assigned to
H4 + H6 and H5, respectively, of the aryl ring. The
former resonance is flanked by satellites due to coupling
to 199Hg, 4J(Hg-H) ) 69 Hz,14 proving a covalent C(2)-
Hg bond. The four resonances of the pyridyl rings are
almost unchanged with respect to the free ligand: in
particular, the most deshielded resonance, ca. δ 8.7,
assigned to the proton close to the nitrogen atom, is
almost coincident in the two spectra, suggesting unco-
ordinated pyridine rings. On the whole, however, the
NMR spectrum does not allow us to assign the exact
structure of complex 1.
The X-ray diffraction study of a single crystal, ob-
tained by slow diffusion of isopropyl ether into a
dichloromethane solution of 1, confirms that the ligand
does not behave as a pincer and that coordination
around the mercury ion is linear, as often is the case
for R-Hg-Cl species.15 A view of 1 is given in Figure
1, and principal metric parameters are reported in the
figure caption. Hg-Cl and Hg-C(2) bonds and the Cl-
Hg-C(2) angle are normal.16 The Hg-N(1) and Hg-
N(2) distances of 2.747(3) and 2.722(3) Å, respectively,
are in our opinion nonbonding, because the nitrogen
lone pairs do not point at the metal atom,17 but it is
Results and Discussion
The ligand 1,3-bis(2-pyridyl)benzene, N-CH-N, was
prepared from 1,3-dicyanobenzene and acetylene, in the
presence of a cobalt catalyst according to a literature
procedure.11
Cardenas et al. have previously shown that carbon
palladation does not take place when 1,3-bis(2-pyridyl)-
(4) Hofman, A.; Dahlenburg, L.; van Eldik, R. Inorg. Chem. 2003,
42, 6528.
(5) (a) van Koten, G. Pure Appl. Chem. 1989, 61, 1681. (b) Rietvield,
M. P. H.; Grove, D. M.; van Koten, G. New J. Chem. 1997, 21, 751. (c)
Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem. Eur. J. 1998, 4,
759. (d) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. Acc. Chem.
Res. 1998, 31, 423. (e) Albrecht, M. A.; van Koten, G. Angew. Chem.,
Int. Ed. 2001, 40, 5000. (f) Slagt, M. Q.; Klein Gebbink, R. J. M.; Lutz,
M.; Spek, A. L.; van Koten, G. Dalton 2002, 2591. (g) Slagt, M. Q.;
Dijkstra, H. P.; McDonald, A.; Klein Gebbing, R. J. M.; Lutz, M.; Ellis,
D. D.; Mills, A. M.; Spek, A. L.; van Koten, G. Organometallics 2003,
22, 27.
(6) Albrecht, M. A.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750.
(7) Beley, M.; Collin, J.-P.; Sauvage, J.-P. Inorg. Chem. 1993, 32,
4539.
(8) Several doubly cyclopalladated species with different N-C-N
systems are known: (a) Trofimenko, S. J. Am. Chem. Soc. 1971, 93,
1808. (b) Trofimenko, S. Inorg. Chem. 1973, 12, 1215. (c) Chakladar,
S.; Paul, P.; Nag, K. Polyhedron 1991, 10, 1513. (d) Chakladar, S.; Paul,
P.; Venkatsubramanian, K.; Nag, K. J. Chem. Soc., Dalton Trans. 1991,
2669. (e) Hartshorn, C. M.; Steel, P. J. Organometallics 1998, 17, 3487.
(f) Meijer, M. D.; Kleij, A. W.; Lutz, M.; Spek, A. L.; van Koten, G. J.
Organomet. Chem. 2001, 640, 166. A triply palladated derivative has
been obtained very recently.1d
(12) (a) Stark, M. A.; Jones, G.; Richards, C. J. Organometallics
2000, 19, 1282. (b) Dijkstra, H. P.; Meijer, M. D.; Patel, J.; Kreiter, R.;
van Klink, G. P. M.; Lutz, M.; Spek, A. L.; Canty, A. J.; van Koten, G.
Organometallics 2001, 20, 3159. (c) Rodriguez, G.; Lutz, M.; Spek. A.
L.; van Koten, G. Chem. Eur. J. 2002, 8, 45.
(9) Other [6,6]-cyclopalladated derivatives with N-C-N ligands
obtained by direct C-H activation have been reported: (a) Reference
1c. (b) Reference 8e. (c) Diez-Barra, E.; Guerra, J.; Lopez-Solera, I.;
Merino, S.; Rodriguez-Lopez, J.; Sanchez-Verdu, P.; Tejeda, J. Orga-
nometallics 2003, 22, 541. (d) Diez-Barra, E.; Guerra, J.; Hornillos,
V.; Merino, S.; Tejeda, J. Organometallics 2003, 22, 4610.
(10) Jung, I. G.; Son, S. U.; Park, K. H.; Chung, K.-C.; Lee, J. W.;
Chung, Y. K. Organometallics 2003, 22, 4715.
(13) Wardell, J. L. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
U.K., 1982; Vol. 2, p 874.
(14) PhHgO3SCF3, 4J(Hg-H) ) 69 Hz; see: Peringer, P.; Winkler,
P.-P. J. Organomet. Chem. 1980, 195, 249.
(15) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochman, M.
Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; p 619.
(16) Constable, E. C.; Leese, T. A.; Tocher, D. A. J. Chem. Soc.,
Chem. Commun. 1989, 570.
(11) Bonnemman, H. Angew. Chem., Int. Ed Engl. 1978, 17, 505.

Pd Derivatives of 1,3-Bis(2-pyridyl)benzene
Organometallics, Vol. 24, No. 1, 2005 55
Figure 1. ORTEP view of 1. Ellipsoids are drawn at the
30% probability level. Principal bond parameters: Hg-Cl
) 2.324(1) Å, Hg-C(2) ) 2.073(3) Å, Cl-Hg-C(2) ) 179.3-
(1)°.
Figure 2. ORTEP view of 2. Ellipsoids are as in Figure
1.
worth mentioning that in other cases similar values
have been considered weakly bonding (compare for
instance Hg-N ) 2.70(2) Å in ref 18).18,19 The three
aromatic rings are strictly planar. The dihedral angles
between their best planes are C(1)/C(6)-N(1)/C(11) )
26.7(2)°, C(1)/C(6)-N(2)/C(16) ) 24.7(2)°, and N(1)/
C(11)-N(2)/C(16) ) 16.7(3)°.
Reaction of the mercury derivative 1 with palladium-
(II) acetate (molar ratio 1:1) affords an orange-red
species (see below) which after exchange with the
appropriate lithium halide, LiCl, LiBr, or LiI, cleanly
gives the cyclometalated complexes [Pd(N-C-N)X] (X )
Cl (2), Br (3), I (4)). All of them display a high thermal
stability.
Table 1. Selected Distances (Å) and Angles (deg)
in 2 with Estimated Standard Deviations (Esd’s)
on the Last Figure in Parentheses
Pd-Cl
Pd-N(2)
2.427(1)
2.063(1)
Pd-N(1)
Pd-C(2)
2.065(1)
1.910(2)
Cl-Pd-N(1)
Cl-Pd-C(2)
N(1)-Pd-C(2)
99.6(1)
179.2(1)
80.0(1)
Cl-Pd-N(2)
N(1)-Pd-N(2)
N(2)-Pd-C(2)
100.4(1)
160.1(1)
80.1(1)
being +0.007(1) and -0.011(1) Å for N(1) and C(2),
respectively. All bond lengths and angles are as ex-
pected. The whole molecule can be considered es-
sentially planar, maximum distances from the least-
squares plane being +0.053(2) and -0.034(2) Å for C(15)
and C(5), respectively.
From the neutral species 2-4, cationic derivatives can
be obtained by abstraction of the halide ion. Reaction
of compound 2 with Ag[BF4] in dichloromethane/acetone
affords the solvento cationic complex [Pd(N-C-N)(H2O)]-
[BF4] (5). Reaction of 2-4 with Na[BAr′4] (Ar′ ) 3,5-
(CF3)2-C6H3), in dichloromethane under strictly anhy-
drous conditions, leads to the dinuclear halide-bridged
species [Pd2(N-C-N)2(µ-X)][BAr′4] (X ) Cl (6), Br (7), I
(8)). The reaction, likely driven to completeness by the
The 1H NMR spectra of complexes 2-4 are unexcep-
tional: the pattern observed, six sharp resonances in
the aromatic region, is consistent with an anionic N-C-N
ligand. The resonances at δ 8.95 (ddd, 2H; 2), δ 9.24 (d
br, 2H; 3), δ 9.50 (broad, 2H; 4), assigned to H6′ and
H6′′, are deshielded with respect to the free ligand and
shifted more and more downfield on going from 2 to 3
and to 4. This trend, previously reported for analogous
systems,20 supports the N-C-N pincer-like coordination
of the ligand. The X-ray structure determination of 2
(crystals suitable for the X-ray analysis were obtained
by slow diffusion of isopropyl ether into a dichlo-
romethane solution of 2) proves the mononuclear nature
of these complexes and the terdentate behavior of the
ligand. The structure closely reminds that of the related
platinum derivative:1b an ORTEP diagram of 2 is shown
in Figure 2. Principal bond lengths and angles are
reported in Table 1. The Pd atom displays a square-
planar coordination with a very slight tetrahedral
distortion, maximum distances from the best plane
insolubility of NaX in dichloromethane, gives com-
pounds 6-8 even with an excess of Na[BAr′4]. The
formation of the nontrivial monobridged complexes 6-8
indicates the high reactivity of the coordinative unsat-
urated fragment [Pd(N-C-N)]+: in the absence of other
(17) Berger, A.; de Cian, A.; Djukic, J.-P.; Fischer, J.; Pfeffer, M.
Organometallics 2001, 20, 3230.
(18) Ali, M.; McWhinnie, W. R.; Hamor, T. A. J. Organomet. Chem.
1989, 371, C37-C39.
(19) Kuz’mina, L. G.; Struchkov, Y. T. Croat. Chem. Acta 1984, 701.
(20) Byers, P. K.; Canty, A. J. Organometallics 1990, 9, 210.

56 Organometallics, Vol. 24, No. 1, 2005
Soro et al.
donors, it hooks the neutral reagent [Pd(N-C-N)Cl], to
attain saturation.21
We investigated also the first product of the reaction
of [Pd(OAc)2] with the mercury precursor [Hg(N-C-N)-
Cl] (1) in a 1:1 molar ratio. The reaction was carried
out in ethanol/dichloromethane solution at reflux tem-
perature. After removal of the solvents, the crude
product was taken up with dichloromethane to give,
after filtration, a yellow solution. From the solution an
orange-red solid was obtained by evaporation of the
solvent. Recrystallization from dichloromethane/diethyl
ether gave samples which were repeatedly submitted
to elemental analyses: the C, H, and N values were
found to be poorly reproducible, suggesting that more
1
species are formed. On the other hand, the H NMR
spectra show in the aromatic region only one set of
signals indicative of a [Pd(N-C-N)] unit. Accordingly, the
samples were thought to be mixtures of [Pd(N-C-N)-
(OAc)], the expected product, and of other species,
possibly an ionic isomer, [Pd2(N-C-N)2(µ-OAc)][OAc], or
other salts (e.g. chloride, chloromercurates, ...) of the
same cation. Indeed, the IR spectra show a rather
complex pattern in the region of the acetate absorp-
tions22 and in the 1H NMR spectrum the integral ratio
between the aromatic protons of the ligand and the
methyl protons of the acetate is somewhat erratic. To
get a better insight into the nature of the reaction
products, the mononuclear acetate [Pd(N-C-N)(OAc)] (9)
was prepared through an exchange reaction:
Figure 3. ORTEP view of a monomeric cation in 11‚EtOH.
Ellipsoids are as in Figure 1.
Table 2. Selected Distances (Å) and Angles (deg)
in 11‚EtOH with Estimated Standard Deviations
(Esd’s) on the Last Figure in Parentheses
Hg-Cl(1)
Hg-Cl(2)
2.637(2)
2.430(3)
Hg-Cl(1)′
Hg-Cl(3)
2.591(2)
2.429(3)
[Pd(N-C-N)Cl] + Ag(OAc) f
[Pd(N-C-N)(OAc)] + AgClV
Pd(1)-Pd(1)′
Pd(1)-O(1)
Pd(1)-N(2)
Pd(2)-O(2)
Pd(2)-N(4)
3.253(1)
2.163(4)
2.063(4)
2.149(3)
2.060(4)
Pd(1)-Pd(2)
Pd(1)-N(1)
Pd(1)-C(2)
Pd(2)-N(3)
Pd(2)-C(18)
3.069(1)
2.065(4)
1.905(5)
2.052(4)
1.908(5)
The yellow complex 9 reacts with Na[BAr′4] in dichlo-
romethane to afford the dinuclear compound [Pd2(N-C-
N)2(µ-OAc)][BAr′4] (10), analogous to the halide-bridged
species 6-8. Comparison of the 1H NMR spectra of
compounds 9 and 10 with that of the orange-red
material, particularly with regard to the most deshield-
ed resonances, rules out the presence of the mono-
nuclear species 9 (in the orange-red material) and points
to a mixture of different salts of the unique dinuclear
cation [Pd2(N-C-N)2(µ-OAc)]+. Final evidence for the
dinuclear cation [Pd2(N-C-N)2(µ-OAc)]+ was attained
when a crystal, serendipitously grown from an ethanolic
solution, was found suitable for X-ray analysis, 11. The
structure consists of the packing of [Pd2(N-C-N)2(µ-
OAc)]+ cations, [Hg2Cl6]2- anions, and EtOH molecules
in the molar ratio 2:1:1. An ORTEP view of the cation
is shown in Figure 3. Principal distances and angles are
reported in Table 2. To the best of our knowledge, the
structure of the present cation is the first example of a
palladium derivative where two equal complex moieties
are bridged by a single acetato ligand. In the [Hg2Cl6]2-
dianion the metal atoms are in a distorted-tetrahedral
coordination, each of them being bonded to two terminal
Cl(1)-Hg-Cl(1)′
Cl(1)-Hg-Cl(3)
Cl(1)′-Hg-Cl(3)
Hg-Cl(1)-Hg′
88.8(1) Cl(1)-Hg-Cl(2)
108.8(1) Cl(1)′-Hg-Cl(2)
111.5(1) Cl(2)-Hg-Cl(3)
91.2(1)
113.9(1)
109.0(1)
120.4(1)
O(1)-Pd(1)-N(1)
O(1)-Pd(1)-C(2)
N(1)-Pd(1)-C(2)
O(2)-Pd(2)-N(3)
O(2)-Pd(2)-C(18)
N(3)-Pd(2)-C(18)
96.2(1) O(1)-Pd(1)-N(2)
173.8(2) N(1)-Pd(1)-N(2)
80.0(2) N(2)-Pd(1)-C(2)
100.4(2) O(2)-Pd(2)-N(4)
177.8(1) N(3)-Pd(2)-N(4)
80.1(2) N(4)-Pd(2)-C(18)
103.9(1)
159.7(2)
80.1(2)
99.3(2)
160.1(2)
80.3(2)
Pd(2)-Pd(1)-Pd(1)′ 164.7(1)
and two double-bridging chloride ligands.23 The dianion
lies around a crystallographic 2-fold axis and displays
normal bond lengths and angles. In the cation the Pd-
(1)-Pd(2) distance is 3.069(1) Å. If this interaction is
not considered (but see below), both Pd atoms are in a
square-planar coordination with a slight tetrahedral
distortion. Maximum distances from the best plane of
Pd(1) are +0.084(4) and -0.064(4) Å for C(2) and N(1),
respectively. For Pd(2), maximum distances from the
best plane are +0.049(5) Å for N(4) and -0.056(4) Å for
C(18). The dihedral angle for these two least-squares
planes is 18.5(2)°: i.e., the two planes are clearly
nonparallel. The [Pd2(N-C-N)2(µ-OAc)]+ cations are
packed in stacked dimers, as shown in Figure 4. The
Pd(1)-Pd(1′) distance is 3.253(1) Å, not much longer
than the Pd(1)-Pd(2) distance mentioned above. The
Pd(2)-Pd(1)-Pd(1′) angle is 164.7(1)°, and the metal
(21) The influence of the anion on the stabilization of similar mono-
vs dinuclear cations has been recently thoroughly discussed: van de
Broeke, J.; Heeringa, J. J. H.; Chuchuryukin, A. V.; Kooijman, H.;
Mills, A. M.; Spek, A. L.; van Lenthe, J. H.; Ruttink, P. J. A.; Deelman,
B. J.; van Koten, G. Organometallics 2004, 23, 2287 and references
therein.
(22) (a) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 5th ed.; Wiley: New York, 1997; Part B
(Applications in Coordination, Organometallic, and Bioinorganic Chem-
istry), pp 59-60. (b) Deacon, G. B.; Philipps, R. J. Coord. Chem. Rev.
1980, 33, 227.
(23) Bonnardel, P. A.; Parish, R. V.; Pritchard, R. G. J. Chem. Soc.,
Dalton Trans. 1996, 3185.

Pd Derivatives of 1,3-Bis(2-pyridyl)benzene
Organometallics, Vol. 24, No. 1, 2005 57
probably due to the π stacking between the four
neighboring planes.
Catalytic Activity. The Heck reaction provides for
the coupling of a suitable aryl halide to an olefin through
the formation of a new C-C bond between two sp2
carbons.24
Due its wide scope and its enormous synthetic poten-
tial, this reaction is one of the most frequently applied
among the synthetic methodologies reliant on transi-
tion-metal catalysis. Palladium is the metal of choice
for this process, and a large variety of Pd(II) complexes
have been tested and found catalytically active in this
coupling.
Aryl pincer palladium complexes have been poorly
utilized until recently.25 Both phenyl-PCP phosphino26
and phenyl-SCS pincer sulfide27 complexes 12a and 13
are active catalysts in promoting the coupling of phenyl
iodide with acrylic acid esters at 110-140 °C. The
Figure 4. ORTEP view of a cationic dimeric couple in 11.
Ellipsoids are as in Figure 1. Intermetallic distances and
angles: Pd(1)-Pd(2) ) 3.069(1) Å, Pd(1)-Pd(1′) ) 3.253-
(1) Å, Pd(2)-Pd(1)-Pd(1)′ ) 164.7(1)°.
catalytic activity of these complexes is not tremendously
impressive, as it ranges between 150 and 1500 h-1
(TOF) with the SCS and the PCP ligands, respectively.
This notwithstanding, given the exceptional thermal
stability of the tridentate system, extremely high turn-
over numbers (TON) can be attained if long reaction
times are applied (over 500 000 in 350 h with the PCP
phosphino species).26
Both the introduction of electron-releasing substitu-
ents in the para position of the phenyl-SCS derivative
and the interposition of an sp3 carbon between the
phenyl ring and the metal of the PCP complex lead to
catalysts of improved performances, probably because
of the increased electron density at the metal. For
instance, the palladium benzyl PCP-pincer ligand 12b
is 1 order of magnitude more active (13 000 h-1 at 140
°C) than the related phenyl derivative and is also
capable of activating phenyl bromide toward the cou-
pling with acrylate.26
Table 3a
T
yield
TOF
d
pincer com
t (h) (°C) (%)b TONc
(h-1
)
[Pd(N-C-N)Cl] (2)
14 110 75 75 000
5 357
[Pd(N-C-N)Cl] (2)e
[Pd(N-C-N)Cl] (2)
[Pd(N-C-N)I] (4)
0.33 135 93 93 000 281 818
14
14
135 98
110 66
110 84
110
98 000
65 500
83 500
7 036
4 679
5 964
[Pd(N-C-N)(OH2)][BF4]‚H2O (5) 14
[Pd2(N-C-N)2(µ-Cl)][BAr′4] (6)
[Pd(N-C-N)(OAc)] (9)
[Pd(N-C-N)(OAc)] (9)
[Pd(N-C-N)(OAc)] (9)
[Pd2(N-C-N)2(µ-OAc)][BAr′4] (10) 14
14
14
5
110 99 100 000
7 143
110 97
110 93
100
97 000 19 400
93 000 46 500
2
a Reaction conditions: PhI, 2 mmol; methyl acrylate, 3 mmol;
NEt3, 3 mmol; amount of catalyst (mol %), 0.001; DMF, 10 mL.
b Isolated yield. c TON ) turnover number, in units of mol of
product/mol of catalyst. d TOF ) turnover frequency, in units of
mol of product/((mol of catalyst) h). e Reaction carried out in a
microwave oven.
The catalytic activity of the aryl pincer SCS Pd(II)
complex is substantially preserved, even when the
ligand is attached to a soluble polymer,27a and is
strongly reduced but not suppressed when sterically
demanding groups such as tert-butyl are positioned on
the sulfur donors.27b Quite recently the fluorous SCS
pincer palladium complex 13 (R ) C6H4-p-C6F13) has
been synthesized and shown to be catalytically active
under both microwave and thermal heating. This com-
plex has the advantage that at the end of the reaction
coordination planes of the two dinuclear units are
rotated ca. 90° with respect to each other, as can be seen
in Figure 4. In light of this packing, the aforementioned
Pd-Pd distance can be considered weakly bonding.3g
Pd-C and Pd-N bond lengths are very similar to those
found in 2 (see Tables 2 and 3). The Pd(1)-O(1) and
Pd(2)-O(2) bond lengths, 2.163(4) and 2.149(3) Å,
respectively, are both elongated by the trans influence
of the aryl carbon atoms. The acetato ligand is strictly
planar, and the dihedral angles between its plane and
the best planes of Pd(1) and Pd(2) are 97.3(1) and 92.3-
(1)°, respectively. At variance with 2, the two whole Pd-
(N2C16)O moieties in 11 show large deviations from
planarity, in the range +0.263(5) and -0.308(5) Å for
Pd(1) and +0.232(5) and -0.159(6) Å for Pd(2). This is
(24) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009.
(25) Singleton, J. T. Tetrahedron 2003, 59, 1837.
(26) Ohff, M.; Ohff, A.; van der Boom, M.; Milstein, D. J. Am. Chem.
Soc. 1997, 119, 11687.
(27) (a) Bergbreiter, D. E.; Osburn, P. L.; Liu, Y.-S. J. Am. Chem.
Soc. 1999, 121, 9531. (b) Gruber, A. S.; Zim, D.; Ebeling, G.; Monteiro,
A. L.; Dupont, J. Org. Lett. 2000, 2, 1287.

58 Organometallics, Vol. 24, No. 1, 2005
Soro et al.
it can be recovered by solid fluorous phase extraction
and reused in the same or in different Heck couplings.28
The catalytic activity of pincer PCP complexes in-
creases significantly on increasing the π-acceptor char-
acter of the P-donor. While PCP phosphino catalysts are
totally inactive toward aryl chloride, the corresponding
PCP phosphinito derivatives such as 14 have been found
to be highly active in promoting the Heck coupling of
aryl chloride.29 An even more active catalyst is obtained
average TOF of 350 000 h-1 has been attained in 12 h
with a homeopathic amount of Pd complex (100 ppb of
Pd/mmol of iodide). These values are over 1 order of
magnitude higher than those recorded with the more
simple NCN pincer complex 17b without any additional
N donor. It is claimed that the observed increase in
activity and stability is related to the dynamic behavior
of the complex 17a, which follows from the presence of
additional coordination sites.10
We have tested our complexes in the benchmark Heck
reaction of iodobenzene with methyl acrylate (Table 3).
The experiments have been run for 14 h at 100-135 °C
at a substrate to Pd ratio of 104, using DMF as solvent
and triethylamine as base. In all cases the reaction is
completely diastereoselective and methyl trans-cin-
namate is obtained as the exclusive reaction product.
No apparent precipitation of palladium was noticed
during the reaction, and the solutions at the end of the
experiments were always clear and transparent.
when the pincer Pd complex is supported by a phosphite
donor, as in the case of 15 (Ar ) p-anisyl). In the
presence of hydroquinone as an additive, an incredibly
high TON (8 900 000!) has been achieved with this
catalyst in the coupling of iodobenzene with butyl
acrylate at 180 °C.30
As to the utilization of NCN pincer palladium cata-
lysts in the Heck reaction, to the best of our knowledge
there are no more than two precedents in the recent
literature which are pertinent to our case. In the first
one the cationic Pd(II) complex 16, featuring a triden-
As a general trend, the mononuclear complexes 2, 4,
and 5 are quite close in activity, while at variance the
dinuclear species 6 and 10 are completely inactive under
the same conditions. At 110 °C the TOF’s improve
slightly on moving from the iodo derivative 4 (4700 h-1
)
to the cationic aquo fluoborate 5 (6.000 h-1) with the
chloro complex 2 lying between (5400 h-1). Application
of microwave heating to the nominal temperature of 135
°C causes an exceptional increase of the catalytic
activity of the chloro complex 2, with a TOF of over
280 000 h-1 being reached. Notably, this figure overrides
by far the value attained at the same temperature but
with thermal heating (7000 h-1).
The best catalyst in the range of complexes screened
is the acetato derivative 9, which is able to provide
nearly quantitative conversions in 2-3 h even at 110
°C with very high TOF’s in the early times of the
reaction (46 500 h-1 after 2 h). This is one of the highest
activities ever reported for this reaction at this temper-
ature.
Conclusions
tate carbene ligand flanked by two 2-picolyl fragments
capable of trans coordination to the metal, has shown a
good catalytic activity in the coupling of 4-bromoace-
tophenone with n-butyl acrylate, where a TOF slightly
higher than 5000 h-1 has been recorded at 120 °C.31
The second report describes an unusual NCN pincer
ligand featuring additional N donors, whose presence
is assumed to provide a less rigid tridentate coordination
to the metal.10 The relevant Pd complex 17a performs
well in the coupling of para-substituted aryl halides to
methyl acrylate, where exceptionally high peaks of
catalytic activity have been sometimes reached. For
instance, in the reaction of 4-tolyl iodide at 110 °C an
impressive TON value of 4.3 × 106 corresponding to an
The results obtained in this study point out that aryl
NCN pincer pyridine Pd(II) complexes are catalysts of
high activity in the Heck reaction. This allows us to
obtain high reaction rates (TOF > 45 000 h-1) even at
fairly low temperatures (110 °C) and to run the reaction
at high substrate loading. Although apparently the
scope of the reaction is limited to aryl iodides, the
catalytic activities observed are comparably much higher
than those recorded with the corresponding pincer PCP
and SCS complexes and indicate a great potential for
complexes of similar design in the asymmetric version
of this reaction.
Experimental Section
(28) Curran, D. P.; Fischer, K.; Moura-Letts, G. Synlett 2004, 1379.
(29) (a) Morales-Morales, D.; Redo`n, R.; Yung, C.; Jensen, C. M.
Chem. Commun. 2000, 1619. (b) Morales-Morales, D.; Grause, C.;
Kasaoka, K.; Redo`n, R.; Cramer, R. E.; Jensen, C. M. Inorg. Chim.
Acta 2000, 300-302, 958.
(30) Miyazaki, F.; Yamaguchi, K.; Shibasaki, M. Tetrahedron Lett.
1999, 40, 7379.
(31) Magill, A. M.; McGuiness, D. S.; Cavell, K. J.; Britovsek, G. J.
P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; White, A. H.; Skelton,
B. W. J. Organomet. Chem. 2001, 617-618, 546.
General Considerations. All reactions for the preparation
of novel compounds were carried out under nitrogen using
standard Schlenk-type flasks. Test reactions for the catalytic
activity of catalysts in the Heck reaction were carried out in
air. Workup procedures were done in air. Reagents were
purchased from Aldrich Chemical Co. and used as received.
[Pd(OAc)2] was obtained from Engelhard. The salt Na[BAr′4]

Pd Derivatives of 1,3-Bis(2-pyridyl)benzene
Organometallics, Vol. 24, No. 1, 2005 59
(sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was
synthesized by the literature method.32
Typically, to a suspension of the orange-red solid in metha-
nol (50 mL) was added, with stirring, an excess of lithium
halide (Cl, Br, or I). The mixture was stirred for 1 h at room
temperature. The pale yellow solid that formed was filtered
off, washed with methanol and diethyl ether, and finally dried
in vacuo. Analytical samples of compounds 2-4 were obtained
by recrystallization from dichloromethane-diethyl ether.
[Pd(N-C-N)Cl] (2). Yield: 56.3 mg, 58% (on Pd). Mp: stable
up to 290 °C. Anal. Calcd for C16H11ClN2Pd: C, 51.46; H, 2.95;
N, 7.50. Found: C, 50.82; H, 2.86; N, 7.22. 1H NMR (400 MHz,
Solvents were distilled and purified prior to use according
to standard methods. 1H and 13C NMR spectra were recorded
with a Varian VXR300 and a Varian Mercury VX spectrometer
(400 MHz). Chemical shifts are given in ppm relative to
internal TMS (1H and 13C). Flash chromatography was per-
formed using Merck silica gel 60 (230-400 mesh). Elemental
analyses (C, H, N) were performed with a Perkin-Elmer 240B
elemental analyzer by Mr. A. Canu (Dipartimento di Chimica,
Universita` di Sassari). Infrared spectra were recorded with a
Jasco FT-IR 480P using Nujol mulls.
3
CD2Cl2): δ 7.13, (dd, J(H-H) ) 8.0, 7.6 Hz, 1H (H5)); 7.21
(ddd,, J(H-H) ) 7.8, 5.8, 1.6 Hz, 2H (H5′, H5′′)); 7.36 (d, 3J(H-
H) ) 7.8 Hz, 2H (H4, H6)); 7.62 (ddd, J(H-H) ) 7.8, 1.6, 0.8
Hz, 2H (H3′, H3′′)); 7.84 (td, 3J(H-H) ) 7.8, 1.6, 2H (H4′,
H4′′)); 8.95 (ddd, J(H-H) ) 5.8, 1.6, 0.8 Hz, 2H (H6′, H6′′)).
[Pd(N-C-N)Br] (3). Yield: 62.9 mg, 58%. Mp: stable up to
290 °C. Anal. Calcd for C16H11BrN2Pd: C, 46.02; H, 2.66; N,
Synthesis of the Ligand 1,3-Bis(2-pyridyl)benzene (N-
CH-N). A solution of 1,3-dicyanobenzene (10.00 g, 0.078 mol)
and (η5-cyclopentadienyl)cobalt 1,5-cyclooctadiene (500.0 mg)
in degassed anhydrous toluene (100 mL) was introduced, by
suction, into a 0.2 L autoclave, evacuated from the air (0.1
Torr). The reaction vessel was pressurized to 13 bar with
acetylene and then rocked and heated to 120 °C for 20 h. After
cooling and release of the residual gas, the solvent was
evaporated and the brown residue treated with HCl (5%) and
NaOH (5%), then extracted with Et2O, and dried on Na2SO4
to give the crude 1,3-bis(pyridyl)benzene, as a pale brown oil.
Column chromatography on silica gel 60, using benzene/
acetone (10:1) as eluent, gave 16.48 g (0.071 mol, yield 91%)
of N-CH-N as a light yellow oil.
1
6.71 Found: C, 45.43; H, 2.79; N, 6.46. H NMR (300 MHz,
CD2Cl2): δ 7.26, (dd, 3J(H-H) ) 8.1, 7.2, 1H (H5)); 7.31 (ddd,
J(H-H) ) 8.1, 5.7, 1.5, 2H (H5′, H5′′)); 7.48 (d, 3J(H-H) )
7.5 Hz, 2H (H4, H6)); 7.73 (ddd, J(H-H) ) 8.1, 1.5 Hz, 2H
(H3′, H3′′)); 7.94 (td, J(H-H) ) 8.1, 1.8 Hz, 2H (H4′, H4′′));
3
9.24 (d br, J(H-H) ) 5.7 Hz, 2H (H6′, H6′′)).
[Pd(N-C-N)I] (4). Yield: 66.4 mg, 55%. Mp: stable up to
290 °C. Anal. Calcd for C16H11IN2Pd: C, 41.36; H, 2.39; N, 6.03.
1
Found: C, 41.58; H, 2.42; N, 6.22. H NMR (300 MHz, CD2-
3
1H NMR (300 MHz, CD2Cl2): δ 7.30 (ddd, J(H-H) ) 7.5,
4.8, 1.5 Hz, 2H (H5′, H5′′)); 7.62 (t, J(H-H) ) 7.8 Hz, 1H (H5));
Cl2): δ 7.25-7.30 (m, 3H (H5′, H5′′ + H5)); 7.47 (d, J(H-H)
) 7.8 Hz, 2H (H4, H6)); 7.73 (dd, 3J(H-H) ) 8.1 Hz, 2H (H3′,
H3′′)); 7.92 (td, J(H-H) ) 8.1, 1.8 Hz, 2H (H4′, H4′′)); 9.50
(broad, 2H (H6′, H6′′)).
3
7.82 (tm, J(H-H) ) 7.5 Hz, 2H (H4′, H4′′)); 7.90 (dm, J(H-
H) ) 7.5 Hz, 2H (H3′, H3′′)); 8.11 (dd 3J(H-H) ) 7.8 Hz, 4J(H-
H) ) 2.1 Hz, 2H (H4, H6)); 8.74 (m, overlapping 1H + 2H (H2
Synthesis of [Pd(N-C-N)(H2O)][BF4]‚H2O (5). To a solu-
tion of 2 (49.7 mg, 0.13 mmol) in 10 mL of dichloromethane
was added with stirring at room temperature 0.13 mmol of
Ag[BF4] (25.9 mg) dissolved in 15 mL of acetone. The yellow
color of the solution immediately faded, and AgCl was formed.
After it was stirred for 1 h, the mixture was filtered and the
resulting solution was evaporated to dryness. The crude
product was recrystallized from acetone/diethyl ether to give
the analytical sample of compound 5.
1
+ H6′, H6′′)). H NMR (400 MHz, CDCl3): δ 7.24 (ddd, J(H-
3
H) ) 7.5, 4.8, 1.5 Hz, 2H (H5′, H5")); 7.58 (t, J(H-H) ) 7.8
Hz, 1H (H5)); 7.76 (td, J(H-H) ) 7.5, 1.8 Hz, 2H (H4′, H4"));
3
7.84 (dm, 3J(H-H) ) 7.8 Hz, 2H (H3′, H3′′)); 8.05 (dd, J(H-
4
H) ) 7.8 Hz, 4J(H-H) ) 2.1 Hz, 2H (H4, H6)); 8.62 (t, J(H-
H) ) 2.1 Hz, 1H (H2)); 8.71 (m, 2H (H6′, H6′′)). 13C{1H} NMR
(75 MHz, CDCl3, APT): δ 156.93 (C); 149.43 (CH); 139.66 (C);
136.56 (CH); 129.01 (CH); 127.25 (CH); 125.33 (CH); 122.06
(CH); 120.48 (CH).
Yield: 56.4 mg, 94%. Mp: stable up to 290 °C. Anal. Calcd
for C16H13BF4N2OPd2‚H2O: C, 41.69; H, 3.25; N, 6.08. Found:
Synthesis of [Hg(N-C-N)Cl] (1). A mixture of N-CH-N
(239.7 mg, 1.03 mmol) and mercury(II) acetate (320.4 mg, 1.00
mmol) in absolute ethanol (20 mL) was heated under reflux
for 24 h. Afterward a solution of lithium chloride (94 mg) in
methanol (15 mL) was added and the mixture was heated for
15 min. The solution was poured into distilled water (100
mL): the white precipitate that formed was filtered off, washed
with water and ice-cold methanol, and dried to give [Hg(N-C-
N)Cl] (1).
Yield: 270 mg, 57%. Mp: 203-204 °C. Anal. Calcd for
C16H11ClHgN2: C, 41.08; H, 2.35; N, 5.99. Found: C, 41.04;
H, 2.29; N, 5.92. 1H NMR (300 MHz, CDCl3,): δ 7.35 (ddd,
J(H-H) ) 7.5, 5.1, 1.2 Hz, 2H (H5′, H5")); 7.52 (t, 3J(H-H) )
7.5 Hz, 1H (H5)); 7.81 (ddd, J(H-H) ) 7.8, 7.5, 2.1 Hz, 2H
(H4′, H4′′)); 7.87 (dm, 3J(H-H) ) 7.8 Hz, 2H (H3′, H3′′)); 7.98
(d, 3J(H-H) ) 7.5 Hz, 4J(Hg-H) 69 Hz, 2H (H4, H6)); 8.73
(ddd, J(H-H) ) 5.1 Hz, 2.1 Hz; 1.0 Hz, 2H (H6′, H6′′)).
Transmetalation Reactions. Synthesis of [Pd(N-C-
N)X] (X ) Cl (2), Br (3), I (4)). A suspension of compound 1
(120.0 mg, 0.26 mmol) in ethanol (20 mL) was added to a
solution of palladium(II) acetate (57.7 mg, 0.26 mmol) in
dichloromethane (8 mL), and the mixture was heated under
reflux for 4 h. The deep yellow solution was filtered through
Celite, and the resulting solution was evaporated to dryness.
The solid residue was washed with diethyl ether, filtered off,
and dried in vacuo to yield a bright orange-red solid. Yield:
160.3 mg. IR (Nujol; νmax/cm-1): 1578 w, 1541 m, 1410 w, 1317
m.
1
C, 41.99; H, 2.97; N, 5.90. H NMR (300 MHz, (CD3)2CO): δ
7.25 (t broad, 3J(H-H) ) 7.4 Hz, 1H (H5)); 7.44 (m, broad, 2H
(H5′, H5′′)); 7.57 (d, broad, 3J(H-H) ) 6.6 Hz, 2H (H4, H6 ));
8.01 (m, broad, 2H (H3′, H3′′)); 8.15 (m, broad, 4H (H4′, H4′′)
+ (H6′, H6′′)). 1H NMR (300 MHz, (CD3)2CO; after D2O
exchange): δ 7.21, (t, 3J(H-H) ) 7.7 Hz, 1H (H5)); 7.45 (ddd,
3
J(H-H) ) 7.8, 5.5, 1.3 Hz, 2H (H5′, H5′′)); 7.52 (d, J(H-H)
) 7.7 Hz, 2H (H4, H6)); 7.94 (d, broad, 3J(H-H) ) 7.8 Hz, 2H
(H3′, H3′′)); 8.11 (td, J(H-H) ) 7.8, 1.5 Hz, 2H (H4′, H4′′));
3
8.41 (d broad, J(H-H) ) 5.5 Hz, 2H (H6′, H6′′)).
Synthesis of [Pd2(N-C-N)2(µ-X)][BAr′4] (X ) Cl (6), Br
(7), I (8)). To a solution of Na[BAr′4] (36.3 mg, 0.041 mmol) in
10 mL of dichloromethane was added with stirring at room
temperature 0.082 mmol of [Pd(N-C-N)X] (30.6, 34.4, 38.1 mg
for X ) Cl, Br, I, respectively) dissolved in 20 mL of the same
solvent. The yellow color of the solution immediately faded,
and a NaX precipitate was formed. After it was stirred for 1
h, the mixture was filtered and the resulting solution evapo-
rated to dryness. Analytical samples of compounds 6-8 were
obtained by recrystallization from dichloromethane/diethyl
ether.
[Pd2(N-C-N)2(µ-Cl)][BAr′4] (6). Yield: 62.6 mg, 97%. Mp:
stable up to 290 °C. Anal. Calcd for C64H34BClF24N4Pd2: C,
1
48.83; H, 2.18; N 3.56. Found: C, 48.72; H, 1.88; N, 3.53. H
NMR (300 MHz, CD2Cl2): δ 7.24 (ddd, J(H-H) ) 7.8, 5.4, 1.2
Hz, 4H (H5′, H5′′)); 7.33 (dd, 3J(H-H) ) 7.2, 8.4 Hz, 2H (H5));
7.51 (d, 3J(H-H) ) 7.8 Hz, 4H (H4, H6)); 7.57 (s br, 4H (Hp));
7.73 (s broad, 8H (Ho)); 7.77 (dm, 3J(H-H) ) 7.8 Hz, 4H (H3′,
H3′′)); 7.96 (td, J(H-H) ) 7.8, 1.5 Hz, 4H (H4′, H4′′)); 9.08 (d
(32) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992,
11, 3920.
3
br, J(H-H) ) 5.4 Hz, 4H (H6′, H6′′)).

60 Organometallics, Vol. 24, No. 1, 2005
Soro et al.
Table 4. Crystallographic Data
1
2
11‚C2H5OH
formula
M
C
16H11ClHgN2
C
16H11ClN2Pd
C70H56Cl6Hg2N8O5Pd4
467.32
colorless
orthorhombic
P212121
7.207(1)
12.816(1)
15.167(1)
90
373.13
pale yellow
monoclinic
P21/n
2128.78
yellow
color
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
8.856(1)
16.798(1)
9.079(1)
90
24.846(2)
15.030(1)
19.479(2)
90
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
93.49(1)
90
105.39(1)
90
1400.9(2)
4
1348.1(2)
4
7013.3(11)
4
F(000)
872
736
4072
Dc (g cm-3
T (K)
)
2.216
1.838
2.016
296
296
296
cryst dimens (mm)
0.169 × 0.225 × 0.394
0.225 × 0.253 × 0.366
0.096 × 0.152 × 0.366
µ(Mo KR) (cm-1
)
111.7
15.47
56.39
min-max transmissn factors
scan mode
0.459-1.00
ω
0.875-1.00
ω
0.663-1.00
ω
frame width (deg)
0.30
0.30
0.30
time per frame (s)
15
20
20
no. of frames
3050
2475
2450
detector-sample dist (cm)
θ range (deg)
4.00
4.00
4.00
3.00-26.00
full sphere
28 480, 4259
0.0424
0.037, 0.048
0.020
3.00-26.00
full sphere
22 718, 4105
0.0231
0.031, 0.062
0.020
3.00-26.00
full sphere
45 737, 7686
0.0451
0.057, 0.083
0.033
reciprocal space explored
no. of rflns (total, indep)
Rint
final R2 and R2w indicesa (F2, all rflns)
conventional R1 index (I > 2σ(I))
no. of rflns with I > 2σ(I)
no. of variables
3709
3612
4561
181
181
442
goodness of fitb
0.943
1.101
1.052
2
2
2
2
a R2 ) [∑(|Fo2- kFc |)/∑Fo2], R2w ) [∑w(Fo2 - kFc )2/∑w(Fo2)2]1/2
.
b [∑w(Fo2 - kFc )2/(No - Nv)]1/2, where w ) 4Fo2/σ(Fo2)2, σ(Fo2) ) [σ2(Fo
)
+ (PFo2)2]1/2, No is the number of observations, Nv is the number of variables, and P, the ignorance factor, is 0.02 for 1, 0.04 for 2, and 0.03
for 11‚C2H5OH.
[Pd2(N-C-N)2(µ-Br)][BAr′4] (7). Yield: 63.7 mg, 96%. Mp:
240-242 °C. Anal. Calcd for C64H34BBrF24N4Pd2: C, 47.49; H,
2.12; N, 3.46. Found: C, 46.94; H, 2.06; N, 3.46. 1H NMR (300
MHz, CD2Cl2): δ 7.20 (ddd, J(H-H) ) 7.5, 5.4, 1.5 Hz, 4H
ature 0.13 mmol of [Pd(N-C-N)(OAc)] (9; 51.4 mg) dissolved
in 10 mL of the same solvent. The yellow solution immediately
became orange, and a precipitate was formed. After 2 h of
stirring at room temperature the orange solution was filtered
and evaporated to dryness. The crude product was recrystal-
lized from dichloromethane/diethyl ether to give the analytical
sample, compound 10.
Yield: 189.0 mg, 91%. Mp: 258-260 °C. Anal. Calcd for
C66H37BF24N4O2Pd2: C, 49.62; H, 2.33; N, 3.51. Found: C,
48.97; H, 2.93; N, 3.30. IR (Nujol; νmax/cm-1): 1545 m, 1355
m. 1H NMR (300 MHz, CD2Cl2): δ 2.46 (s, 3H (CH3)), 6.91 (ddd,
J(H-H) ) 7.8, 5.6, 1.5 Hz, 4H (H5′, H5′′)); 6.99 (m, 6H (H4,
H6 + H5)); 7.28 (d, 3J(H-H) ) 7.8 Hz, 4H (H3′, H3′′)); 7.58 (s,
4H (Hp)); 7.68 (td, 3J(H-H) ) 7.8, 1.7 Hz, 4H (H4′, H4′′)); 7.75
(s broad, 8H (Ho)); 7.99 (d, broad, 3J(H-H) ) 5.6 Hz, 4H (H6′,
H6′′)).
Method B. To a solution of the orange-red solid (see
Transmetalation Reactions) from which complex 11 was
obtained (270.5 mg in 20 mL of ethanol) was added, with
stirring at room temperature, 230.3 mg (0.26 mmol) of Na-
[BAr′4] dissolved in 10 mL of dichloromethane. The solution
was stirred for 2 h and then filtered and evaporated to dryness.
The crude product was recrystallized from dichloromethane/
diethyl ether to give compound 10 Yield: 184.7 mg (0.116
mmol).
Synthesis of [Pd2(N-C-N)2(µ-OAc)]2[Hg2Cl6]‚EtOH (11‚
EtOH). Crystals of 11‚EtOH suitable for X-ray analysis were
obtained as follows: the orange-red solid (see Transmetalation
Reactions) was dissolved by refluxing a suspension in EtOH.
When the solution was cooled to room temperature, two forms
of crystals (yellow and red) were obtained. The X-ray analysis
was successful unfortunately only in the case of the yellow
form, 11‚EtOH.
3
(H5′, H5′′)); 7.30 (dd, J(H-H) ) 8.4, 7.2 Hz, 2H (H5)); 7.46
3
(d, J(H-H) ) 7.8 Hz, 4H (H4, H6)); 7.57 (s, 4H (Hp)); 7.72-
7.74 (m, 4H + 8H (H3′, H3′ + Ho)); 7.93 (td, J(H-H) ) 7.5,
1.5 Hz, 4H (H4′, H4′′)); 9.17 (d br, 3J(H-H) ) 5.4 Hz, 4H (H6′,
H6′′)).
[Pd2(N-C-N)2(µ-I)][BAr′4] (8). Yield: 66.2 mg, 97%. Mp:
226-228 °C. Anal. Calcd for C64H34BF24IN4Pd2: C, 46.15; H,
2.06; N, 3.36. Found: C, 46.05; H, 2.01; N, 3.36. 1H NMR (CD2-
Cl2): δ 7.17 (m br, 4H (H5′, H5′′)); 7.30 (t, 3J(H-H) ) 7.5 Hz,
2H (H5)); 7.44 (d, 3J(H-H) ) 7.5 Hz, 4H (H4, H6)); 7.58 (s br,
4H (Hp)); 7.70-7.75 (m, 4H + 8H (H3′, H3′′ + Ho)); 7.91 (td,
3
J(H-H) ) 7.8, 1.5 Hz, 4H (H4′, H4′′)); 9.37 (d br, J(H-H) )
5.4 Hz, 4H (H6′, H6′′)).
Synthesis of [Pd(N-C-N)(OAc)] (9). To a solution of [Pd-
(N-C-N)Cl] (54.3 mg, 0.146 mmol) in 10 mL of a mixture of
tetrahydrofuran and dichloromethane (5:1 v/v) was added, with
stirring at room temperature, 24.4 mg (0.146 mmol) of Ag-
(OAc). The mixture was left to react for 3 h in the dark. The
AgCl precipitate was filtered off and the yellow solution
evaporated to dryness. The crude product was recrystallized
from dichloromethane to give the analytical sample, compound
9.
Yield: 52.2 mg, 93%. Mp: 218-220 °C. Anal. Calcd for
C18H14N2O2Pd‚H2O: C, 52.13; H, 3.89; N, 6.75. Found: C,
51.94; H, 3.69; N, 6.64. IR (Nujol; νmax/cm-1): 1578 m, 1338
m. 1H NMR (300 MHz, CD2Cl2): δ 2.11 (s, 3H, (CH3)), 7.22
3
(dd, J(H-H) ) 8.0, 7.3 Hz, 1H (H5)); 7.31 (ddd, J(H-H) )
3
7.8, 5.6, 1.5 Hz, 2H (H5′, H5′′)); 7.44 (d, J(H-H) ) 7.8 Hz,
3
2H (H4, H6)); 7.72 (d br, J(H-H) ) 7.8 Hz, 2H (H3′, H3′′));
7.93 (td, J(H-H) ) 7.8, 1.7 Hz, 2H (H4′, H4′′)); 8.63 (d, broad,
Catalytic Reactions. To a 25 mL Schlenk flask were added
catalyst (0.001%), triethylamine (0.42 mL, 3.0 mmol), iodo-
benzene (2.0 mmol), methyl acrylate (0.27 mL, 3 mmol), and
DMF (10 mL). The mixture was refluxed at 110 °C into an oil
bath. After 14 h (except where otherwise reported), the
3J(H-H) ) 5.6 Hz, 2H (H6′, H6′′)).
Synthesis of [Pd2(N-C-N)2(µ-OAc)][BAr′4] (10). Method
A. To a solution of Na[BAr′4] (57.4 mg, 0.065 mmol) in 10 mL
of dichloromethane was added with stirring at room temper-

Pd Derivatives of 1,3-Bis(2-pyridyl)benzene
Organometallics, Vol. 24, No. 1, 2005 61
reaction mixture was extracted with dichloromethane (5 mL)
and water (10 mL). The organic layer was washed four times
with 10 mL portions of water and dried with MgSO4. The
mixture was then filtered and the dichloromethane removed
in vacuo. A pure product was obtained by flash column
chromatography on silica gel with a mixture of hexane and
diethyl ether as eluent (12/1 v/v). The purity of the product
The structures were solved by direct methods (SHELXS)37 and
refined by full-matrix least squares using all reflections and
minimizing the function ∑w(Fo - kFc2)2 (refinement on F2).
2
In 11‚EtOH the ethanol molecule is disordered on a 2-fold axis.
All the non-hydrogen atoms were refined with anisotropic
thermal factors. The hydrogen atoms of the acetato ligand in
11‚EtOH were detected in the final Fourier maps and not
refined. The hydrogen atoms of the disordered EtOH molecule
were neglected. The remaining hydrogen atoms were placed
in their ideal positions (C-H ) 0.97 Å), with the thermal
parameter B 1.10 times that of the carbon atom to which they
are attached, and not refined. For noncentrosymmetric 1 full
refinement of the correct structure enantiomorph led to R2 )
0.037 and R2w ) 0.048, full refinement of the wrong one led to
R2 ) 0.069 and R2w ) 0.131. In the final Fourier maps the
maximum residuals were 1.76(29) e Å-3 at 0.85 Å from Hg,
0.26(66) e Å-3 at 1.02 Å from C(5), and 1.19(35) e Å-3 at 1.61
Å from Cl(3) for 1, 2, and 11‚EtOH, respectively.
1
was confirmed by comparison with H NMR of the authentic
sample.
Microwave reactions were conducted using a CEM Discover
Synthesis Unit (CEM Corp., Matthews, NC). The machine
consists of a continuous focused microwave power delivery
system with operator-selectable power output from 0 to 300
W. Reactions were performed in glass vessels (capacity 10 mL)
sealed with a septum. The pressure is controlled by a load cell
connected to the vessel via a 14 gauge needle, which penetrates
just below the septum surface. The temperature of the contents
of the vessel was monitored using a calibrated infrared
temperature control mounted under the reaction vessel. All
experiments were performed using a stirring option whereby
the contents of the vessel were stirred by means of a rotating
magnetic plate located below the floor of the microwave cavity
and a Teflon-coated magnetic stir bar in the vessel.
Acknowledgment. Financial support from the MIUR
(Grant No. PRIN-2003033857) and from the University
of Sassari is gratefully acknowledged.
X-ray Data Collection and Structure Determinations
of 1, 2, and 11‚EtOH. Crystal data are summarized in Table
4; other experimental details are listed in the Supporting
Information. The diffraction experiments were carried out on
a Bruker SMART CCD area-detector diffractometer at 296 K.
No crystal decay was observed, so that no time-decay correction
was needed. The collected frames were processed with the
software SAINT,33 and an empirical absorption correction was
applied (SADABS)34 to the collected reflections. The calcula-
tions were performed using the Personal Structure Determi-
nation Package35 and the physical constants tabulated therein.36
Supporting Information Available: Tables giving atomic
coordinates for non-hydrogen atoms, positional and thermal
parameters for hydrogen atoms, anisotropic thermal param-
eters (U’s) for non-hydrogen atoms, and all distances and
angles for compounds 1, 2 and 11‚EtOH; these data are also
available as CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM040102O
(35) Frenz, B. A. Comput. Phys. 1988, 2, 42.
(33) SAINT Reference Manual; Siemens Energy and Automation:
Madison, WI, 1994-1996.
(34) Sheldrick, G. M. SADABS, Empirical Absorption Correction
Program; University of Gottingen, Gottingen, Germany, 1997.
(36) Crystallographic Computing 5; Oxford University Press: Ox-
ford, U.K., 1991; Chapter 11, p 126.
(37) Sheldrick, G. M. SHELXS 8, Program for the Solution of Crystal
Structures; University of Gottingen, Gottingen, Germany, 1985.