Divergent Behavior of Palladium(II) and Platinum(II) in the Metalation of 1,3-Di(2-pyridyl)henzene

Article abstract of DOI:10.1021/om990125g

1,3-Di(2-pyridyl)benzene undergoes regioselective orthometalation with Pd(OAc)₂ on the central aryl ring, giving rise to a dimeric complex in which four acetate units act as bridges between two doubly metalated ligands and each pyridine ligand coordinates to a different metal atom. The reaction with chloride-containing Pd(II) complexes does not lead to C-H activation but to coordination complexes without Pd-C bonds. Cycloplatination of 1,3-di-(2-pyridyl)benzene takes place at C-2 of the benzene ring, generating a terdentate ligand. This behavior is the same as that previously observed for Ru and Os. Both orthometalated complexes have been structurally characterized by X-ray diffraction.

Full text of DOI:10.1021/om990125g

Organometallics 1999, 18, 3337-3341
3337
Diver gen t Beh a vior of P a lla d iu m (II) a n d P la tin u m (II) in
th e Meta la tion of 1,3-Di(2-p yr id yl)ben zen e
Diego J . Ca´rdenas* and Antonio M. Echavarren*
Departamento de Quı´mica Orga´nica, Universidad Auto´noma de Madrid,
Cantoblanco, 28049-Madrid, Spain
M. Carmen Ram´ırez de Arellano†
Departamento de Quı´mica Inorga´nica, Universidad de Murcia, 30071-Murcia, Spain
Received February 22, 1999
1,3-Di(2-pyridyl)benzene undergoes regioselective orthometalation with Pd(OAc)2 on the
central aryl ring, giving rise to a dimeric complex in which four acetate units act as bridges
between two doubly metalated ligands and each pyridine ligand coordinates to a different
metal atom. The reaction with chloride-containing Pd(II) complexes does not lead to C-H
activation but to coordination complexes without Pd-C bonds. Cycloplatination of 1,3-di-
(2-pyridyl)benzene takes place at C-2 of the benzene ring, generating a terdentate ligand.
This behavior is the same as that previously observed for Ru and Os. Both orthometalated
complexes have been structurally characterized by X-ray diffraction.
In tr od u ction
with Pd(II) and Pt(II).12 In this study, we have found
that ligand 1 undergoes regioselective monometalation
at the more hindered position (C-2) with Pt(II), while
Pd(II) leads to dimetalation of the central ring at C-4
and C-6.
There has been an increasing interest in the prepara-
tion and study of complexes containing terdentate
ligands which include one metal-carbon bond, espe-
cially for the platinum metals.1 The variety of coordi-
nating atoms (X) that can be used along with the
possibility of having two types of coordinating struc-
tures, i.e., X-C-X and C-X-X, allow one to obtain
complexes with different properties. For instance, pho-
toinduced electron transfer has been observed in Ru and
Os complexes derived from pyridyl-substituted arenes,2
and certain Pd complexes display nematic phases.3 The
development of new catalysts is of course of main
interest, and in this sense Pd complexes have been
shown to be the most active and versatile. Complexes
with terdentate ligands containing a Pd-C bond have
shown catalytic activity in some C-C forming reactions
in which the intermediacy of Pd(IV) intermediates has
been invoked.4 In addition, certain Pd(II) metallacycles
have been demonstrated to catalyze Heck reactions,
cross-couplings, and related processes.5-8
Resu lts a n d Discu ssion
Cyclom eta la tion Stu d ies. Ligand 113 was best
prepared by using an approach recently developed for
the preparation of pyridine-based ligands based on the
Stille coupling reaction.14 Thus, reaction between (2-
pyridyl)trialkylstannanes 2 or 3 and 1,4-dibromoben-
(5) (a) Herrmann, W. A.; Brossmer, C.; Reisinger, C.-P.; Riermeier,
T. H.; O¨ fele, K.; Beller, M. Chem. Eur. J . 1997, 3, 1357. (b) Beller, M.;
Riermeier, T. H.; Haber, S.; Kleiner, H.-J .; Herrmann, W. A. Chem.
Ber. 1996, 129, 1259. (c) Herrmann, W. A.; Brossmer, C.; O¨ fele, K.;
Reisinger, C.-P.; Riermeier, T. H.; Beller, M.; Fischer, H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1844.
(6) For synthetic applications of the Herrmann palladacycle: (a)
Beller, M.; Fischer, H.; Herrmann, W. A.; O¨ fele, K.; Brossmer, C.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1848. (b) Louie, J .; Hartwig, J .
F. Angew. Chem., Int. Ed. Engl. 1996, 35, 2359. (c) Herrmann, W. A.;
Reisinger, C.-P.; O¨ fele, K.; Brossmer, C.; Beller, M.; Fischer, H. J . Mol.
Catal. A: Chem. 1996, 108, 51. (d) Tietze, L. F.; No¨bel, T. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2259. (e) Beller, M.; Riermeier, T. H.
Tetrahedron Lett. 1996, 37, 6535. (f) Prinz, P.; Lansky, A.; Haumann,
T.; Boese, R.; Noltemeyer, M.; Knieriem, B.; de Meijere, A. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1289. (g) Beller, M.; Riermeier, T. H.;
Reisinger, C.-P.; Herrmann, W. A. Tetrahedron Lett. 1997, 38, 2073.
(h) Hennings, D. D.; Iwasa, S.; Rawal, V. H. J . Org. Chem. 1997, 62,
2. (i) Rossi, R.; Bellina, F.; Bechini, C.; Mannina, L.; Vergamini, P.
Tetrahedron 1998, 54, 135. (j) Dyker, G.; Kellner, A. J . Organomet.
Chem. 1998, 555, 141. (k) Beller, M.; Riermeier, T. H. Eur. J . Inorg.
Chem. 1998, 29.
(7) Orthopalladated triaryl phosphites as catalysts: (a) Albisson,
D. A.; Bedford, R. B.; Lawrence, S. E.; Scully, P. N. Chem. Commun.
1998, 2095. (b) Albisson, D. A.; Bedford, R. B.; Scully, P. N. Tetrahedron
Lett. 1998, 39, 9793. Peripalladated tri(1-naphthyl)phosphine cata-
lysts: (c) Spescha, M.; Shaw, B. L.; Perera, S. D.; Staley, E. A. Chem.
Commun. 1998, 1361.
(8) However, it has been shown that under the reaction conditions
reduction of the precatalyst forms a catalytically active Pd(0) species.6b,k
(9) Review on cyclometalation: Ryabov, A. D. Chem. Rev. 1990, 90,
403.
Metalation reactions are key steps in some metal-
catalyzed reactions.9,10 Therefore, studies on regiose-
lectivity could shed light on the mechanism of these
important processes.11 As part of a project directed
toward the synthesis of new catalysts, we decided to
examine the metalation of 1,3-di(2-pyridyl)benzene (1)
† Present address: Departamento de Qu´ımica Orga´nica, Facultad
de Farmacia, Universidad de Valencia, 46100 Burjassot, Valencia,
Spain.
(1) (a) Lagunas, M.-C.; Gossage, R. A.; Smeets, W. J . J .; Spek, A.
L.; van Koten, G. Eur. J . Inorg. Chem. 1998, 163. (b) Longmire, J . M.;
Zhang, X.; Shang, M. Organometallics 1998, 17, 4374.
(2) Beley, M.; Collin, J .-P.; Re´my, L.; Metz, B.; Sauvage, J .-P. J . Am.
Chem. Soc. 1991, 113, 8521. Beley, M.; Collin, J .-P.; Sauvage, J .-P.
Inorg. Chem. 1993, 32, 4539.
(3) Lydon D. P.; Rourke, J . P. Chem. Commun. 1997, 1741.
(4) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J . Am.
Chem. Soc. 1997, 119, 11687.
10.1021/om990125g CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/20/1999

3338 Organometallics, Vol. 18, No. 17, 1999
Ca´rdenas et al.
Sch em e 1
F igu r e 1. ORTEP plot of 4 with the labeling scheme (50%
probability level).
zene in toluene in the presence of Pd(PPh3)2Cl2 (ca. 8%)
and LiCl afforded 1 in 83% yield (eq 1).
Reaction of 1 with Pd(OAc)2 in HOAc led cleanly to
the dimeric dipalladated complex 4 in 54% yield (Scheme
1). The existence of the doubly metalated ligand was
evidenced by the presence of two broad singlets at 7.61
and 7.27 ppm in the 1H NMR spectrum (DMSO-d6)
which correspond to the 1,2,4,5-tetrasubsituted aromatic
ring. The IR spectrum showed two strong bands at 1573
and 1410 cm-1 typical of bridging acetate ligands. Two
different signals in the 13C{1H} NMR spectrum at
180.31 and 179.03 ppm for the carboxylate, as well as
methyl groups at 25.05 and 24.58 ppm, evidenced the
F igu r e 2. View of 4 showing the open book structure.
existence of inequivalent acetates. X-ray diffraction of
a single crystal obtained by slow diffusion of EtOH into
a DMSO solution of 4 confirmed the existence of two
monomeric dipalladated units bonded by four bridging
acetate ligands (Scheme 1, Figures1 and 2). Only a few
examples of this type of double cyclopalladation on an
arene ring have been reported.4,15 Usually, a 1,3-
difunctionalized benzene ring with coordinating sub-
stituents yields monometalated complexes in position
2.1,2,16,17
(10) Reviews on metalation by late transition metals: (a) Spencer,
J .; Pfeffer, M. In Advances in Metal-Organic Chemistry; Liebeskind,
L. S., Ed.; J AI Press: Stamford, 1998; Vol. 6, p 103. (b) Shilov, A. E.;
Shul’pin, G. B. Chem. Rev. 1985, 85, 245. (c) Sen, A. Acc. Chem. Res.
1985, 28, 154. (d) Murai, S.; Kakiuchi, F.; Sekine, S., Tanaka, Y.;
Kakitani, A.; Sonoda, M.; Chatani, N. Pure Appl. Chem. 1994, 66, 1527.
(11) Lead references on the mechanism of the metalation of are-
nes: (a) Vigalok, A.; Uzan, O.; Shimon, L. J . W.; Ben-David, Y.; Martin,
J . M. L.; Milstein, D. J . Am. Chem. Soc. 1998, 120, 12539. (b)
Matsubara, T.; Koga, N.; Musaev, D.; Morokuma, K. J . Am. Chem.
Soc. 1998, 120, 12692.
(12) Previous work of this group demonstrated that small variations
on the metal complex could lead to complete regiocontrol in the
palladation reactions: (a) Ca´rdenas, D. J .; Echavarren, A. M.; Vegas,
A. Organometallics 1994, 13, 882. (b) Ca´rdenas, D. J .; Echavarren, A.
M. Organometallics 1995, 14, 4427. See also: (c) Constable, A. G.;
McDonald, W. S.; Sawkins, L. C.; Shaw, B. L. J . Chem. Soc., Chem.
Commun. 1978, 1061. (d) Constable, A. G.; McDonald, W. S.; Sawkins,
L. C.; Shaw, B. L. J . Chem. Soc., Dalton Trans. 1980, 1992. (e) Galli,
B.; Gasparrini, F.; Maresca, L.; Natile, G.; Palmieri, G. J . Chem. Soc.,
Dalton Trans. 1983, 1483, and references therein.
When ligand 1 was allowed to react with [Pd(MeCN)4]-
(BF4)2 in MeCN at 45 °C, a new complex was obtained
1
whose H NMR spectrum (DMSO-d6) was very compli-
cated probably due the coexistence of different species
formed by coordination with the solvent. The addition
of NaOAc to the DMSO-d6 solution gave rise to a
spectrum superimposable to that of 4, indicating that
[Pd(MeCN)4](BF4)2 also yields the double palladation
(15) (a) Chakladar, S.; Paul, P.; Venkatsubramanian, K.; Nag, K.
J . Chem. Soc., Dalton Trans. 1991, 2669. (b) Carina, R. F.; Williams,
A. F.; Bernardinelli, G. J . Organomet. Chem. 1997, 548, 45. (c) Fuchita,
Y.; Akiyama, M.; Arimoto, Y.; Matsumoto, N.; Okawa, H. Inorg. Chim.
Acta 1993, 205, 185. (d) Trofimenko, S. J . Am. Chem. Soc. 1971, 93,
1808. (e) Trofimenko, S. Inorg. Chem. 1973, 12, 1215. (f) Phillips, I.
G.; Steel, P. J . J . Organomet. Chem. 1991, 410, 247.
(13) Previous synthesis from 1,3-dicyanobenzene and acetylene
using a cobalt catalyst: Bo¨nnemann, H. Angew. Chem., Int. Ed. Engl.
1978, 17, 505.
(14) (a) Ca´rdenas, D. J .; Sauvage, J .-P. Synlett 1996, 916. (b) Fujita,
M.; Oka, H.; Ogura, K. Tetrahedron Lett. 1995, 36, 5247. (c) Benaglia,
M.; Toyota, S.; Woods, C. R.; Siegel, J . S. Tetrahedron Lett. 1997, 38,
4737.
(16) A short review on cyclometallation of meta-substituted aryl
ligands by platinum group metal complexes: Steenwinkel, P.; Gossage,
R. A.; van Koten, G. Chem. Eur. J . 1998, 4, 759.
(17) A number of examples can be found in the following review:
Canty, A. J . In Comprehensive Organometallic Chemistry II; Abel, E.
W., Stone, F. G. A., Wilkinson, G., Eds.; Puddephat, R. J ., Eds.; Vol.
9, Chapter 5, pp 225-290.

Divergent Behavior of Pd(II) and Pt(II) in Metalation
Organometallics, Vol. 18, No. 17, 1999 3339
compound. On the other hand, treatment of ligand 1
with a HOAc solution of Li2PdCl4 gave a coordination
complex corresponding to the formula Pd(1)Cl2 (5),
isolated in almost quantitative yield. The 13C{1H} NMR
spectrum (DMSO-d6) of 5 showed nine signals, which
indicates that the complex has two equivalent pyridine
rings. The highest shift observed in the 1H NMR
spectrum corresponds to the pyridine H-5 hydrogen
(0.36 ppm downfield) due to the coordination with Pd-
(II). The FAB-MS spectrum obtained from crystals
isolated from a DMSO solution shows peaks corre-
sponding to the fragments [Pd(1)] and [Pd(1)Cl] as well
as the higher fragments [Pd2(1)2Cl] and [Pd(1)2H]. A
peak at m/z 489 may correspond to Pd(1)(DMSO)Cl2.
These data and the fact that if the central aryl ring is
not metalated both pyridine moieties of one ligand
cannot coordinate to a single Pd atom support the
structure 5 shown in Scheme 1.
Treatment of complex 5 with different bases com-
monly used to induce C-H activation (MeOH-NaOAc,
DMF-K2CO3, DMF-NaOAc) did not give rise to any
cyclopalladated complex. A possible explanation for this
lack of reactivity toward C-H activation is that the
necessary Cl dissociation from Pd to permit the ortho-
palladation does not take place. The change in the
reactivity of Pd complexes due to Cl coordination has
been observed in the cyclopalladation of 2-aldimines
derived from 8-methylquinolines.18 We tried to perform
the palladation reaction with other Pd(II)-chloride
complexes which contained a lesser proportion of Cl as
compared to Li2PdCl4. Thus, the palladation of 1 with
Pd(MeCN)2Cl2, Pd(COD)Cl2, and 1:1 mixtures of these
complexes with Pd(OAc)2 was assayed. However, in
these experiments, complex mixtures of different species
were obtained as shown by 1H NMR, and cyclopalla-
dated complexes could not be isolated.
F igu r e 3. ORTEP plot of 6 with the labeling scheme (50%
probability level).
reversible when performed in HOAc.20 Thus, these
results suggest that such a complex with a single Pd-C
bond is not the most stable species under the mentioned
conditions. On the other hand, when the reaction
between Pd(OAc)2 and 1 was performed in deuterated
acetic acid (DOAc), no incorporation of deuterium into
the aryl ring took place, which demonstrates that
palladation at C-2 of the central benzene does not
precede the formation of 4. Complex 4 is the single
species observed under all the conditions assayed. In
the case of Pt (as well as for Os and Ru)2 the activation
of the more hindered position may arise from the need
for having both N atoms coordinated to the metal before
the C-H activation takes place. The different behavior
of Pd(II) and Pt(II) in this case contrasts with the results
obtained for the metalation of other N- or P-containing
1,3-disubstituted benzene ligands in which C-H activa-
tion takes place at C-2 for a series of metal atoms.1b,15
X-r a y Cr ysta l Str u ctu r e of th e Dou ble P a lla d a -
tion Com p lex 4‚EtOH. An ORTEP diagram of 4 is
shown in Figure 1. Crystallographic data are listed in
Table 1, and significant bond distances and angles are
in Table 2. The molecular structure of complex 4‚EtOH
contains two dipalladated ligands 1 bridged by four
acetate molecules. The acetate-bridged dimer shows the
usual open book or cleft structure where the angle
between both dimetalated ligand planes is 26.3° (see
Figure 3). The palladium coordination sites are es-
sentially square-planar for the palladium atoms Pd(1),
Pd(3), and Pd(4). However, a distorted square-planar
coordination geometry has been found for the palladium
atom Pd(2): the angle between the planes O(5)-Pd(2)-
O(7) and N(2)-Pd(2)-C(3) is 8.4°. The Pd(1)-Pd(3)
and Pd(2)-Pd(4) distances are significantly different
[2.8460(5) and 2.9548(6) Å, respectively] the angle
between the distances Pd(1)-Pd(2) and Pd(3)-Pd(4)
being 3.2°. The Pd-C bond lengths [mean value
1.959(5) Å] are in the range observed for related
dipalladated complexes (1.94-1.98 Å).15a,b The Pd-N
distances are similar to those observed for Pd-terpy-
ridine complexes21 and for structurally related palla-
dacycles with N-sp2 bonded ligands.14a,b The Pd-O
distances trans to carbon bond lengths [mean value
On the other hand, treatment of 1 with K2PtCl4 in
HOAc under reflux conditions for 3 days afforded the
cycloplatinated complex 6 in 78% yield (Scheme 1).19
The structure of 6 was determined on the basis of its
1H and 13C{1H} NMR spectra (DMSO-d6) which show
the absence of H-2 of the benzene ring. Elemental
analysis and the FAB mass spectrum also supported the
monomeric structure 6. A single crystal suitable for
X-ray diffraction was grown by diffusion of EtOH into
a DMSO solution, confirming the proposed platinated
structure (Figure 3), analogous to that observed for the
metalation of ligand 1 with Ru(II) and Os(II).2
The formation of the double palladation complex 4
must take place through a first C-H activation directed
by coordination of only one of the pyridine rings to Pd.
This would give rise to a more electron-rich aryl ring,
which would be more reactive toward a second palla-
dation.14 Complex 4 remained unchanged when it was
treated with 1 equiv of ligand 1 in refluxing HOAc to
try to obtain an analogue of complex 6. If such a complex
were thermodynamically more stable, it could have been
formed since orthopalladation has been shown to be
(18) (a) Deeming, A. J .; Rothwell, I. P. J . Chem. Soc., Chem.
Commun. 1978, 344. (b) Deeming, A. J .; Rothwell, I. P.; Hursthouse,
M. B.; Malik, K. M. A. J . Chem. Soc., Dalton Trans. 1979, 1899. (c)
Deeming, A. J .; Rothwell, I. P. J . Organomet. Chem. 1984, 268, 91.
(19) Platination of 2,6-diarylpyridines can take place at C-4 of the
pyriding ring: Cave, G. W. V.; Hallett, A. J .; Errington, W.; Rourke, J .
P. Angew. Chem., Int. Ed. Engl. 1998, 37, 3270.
(20) (a) Ryabov, A. D.; Sakodinskaya, I. K.; Yatsimirsky, A. K. J .
Chem. Soc., Dalton Trans. 1985, 2629. (b) Ryabov, A. D. Inorg. Chem.
1987, 26, 1252.
(21) Ramdeehul, S.; Barloy, L.; Osborn, J . A. Organometallics 1996,
15, 5442.

3340 Organometallics, Vol. 18, No. 17, 1999
Ca´rdenas et al.
Ta ble 1. Cr ysta l Da ta for Com p lexes 4‚EtOH a n d 6
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 6
4‚EtOH
6
Pt-C(1)
Pt-N(2)
1.907(8)
2.033(6)
Pt-N(1)
Pt-Cl
2.041(6)
2.417(2)
molecular formula
source
C
42H38N4O9Pd4
C16H11ClN2Pt
EtOH/DMSO
vapor diffusion
bright yellow needles
monoclinic
8.8985(4)
16.7545(9)
9.0451(7)
92.154(5)
1347.58(14)
4
Et2O/DMSO
vapor diffusion
orange prisms
orthorhombic
19.716(2)
N(1)-Pt-Cl
N(1)-Pt-C(1)
C(1)-Pt-Cl
Pt-N(1)-C(7)
Pt-C(1)-C(2)
C(1)-C(2)-C(7)
99.8(2)
80.9(3)
179.0(2)
114.6(5)
118.5(6)
111.9(7)
N(2)-Pt-Cl
99.1(2)
crystal habit
crystal system
a, Å
b, Å
c, Å
N(2)-Pt-C(1)
N(1)-Pt-N(2)
Pt-N(2)-C(12)
Pt-C(1)-C(6)
C(1)-C(6)-C(12)
80.1(3)
161.1(2)
114.5(5)
119.8(6)
112.2(7)
15.347(1)
26.779(2)
â, deg
V, Å3
8103.3(10)
8
X-ray diffraction studies confirmed the proposed mono-
metalated structure with both nitrogen atoms coordi-
nated to platinum. The platinum center is four-
coordinated in an approximately square-planar NCNCl
environment, all the atoms of the molecule being es-
sentially coplanar (mean deviation 0.028 Å). At the
platinum center the chloride ligand is trans to the
carbon atom. The Pt-C(1) bond distance [1.907(8) Å]
lies in the range observed for Pt complexes of related
N, C, N terdentate ligands (1.90-1.94 Å), which have
been shown to exhibit shorter distances than those
found in other orthoplatination complexes.22 The Pt-
N(1) and Pt-N(2) bond distances [2.041(6) and 2.033(6)
Å] are shorter than those observed in related complexes
containing N(sp3)-C-N(sp3) ligands. The main devia-
tions from ideal sp2 geometry in the N, C, N chelate
system are the angles C(1)-C(2)-C(7) and C(1)-C(6)-
C(12) [111.9(7)°, 112.2(7)°]; the small “bite” angle of the
ligand [C(1)-Pt-N(1) 80.9(3)° and C(1)-Pt-N(2)
80.1(3)°] could explain these features. The C(1)-Pt-Cl
axis is almost linear [179.0(2)°], whereas N(1)-Pt-N(2)
is more distorted at 161.1(2)°, probably as a consequence
of chelate ring strain.
Z
λ (Å)
0.71073
173(2)
Mo KR
graphite
Pbca
1.54178
296(2)
Cu KR
graphite
T (K)
radiation used
monochromator
space group
crystal size, mm
µ, mm-1
P21/n
0.40 × 0.32 × 0.30 0.40 × 0.10 × 0.10
1.808
2.1203
abs correction
max/min
ψ scans
0.75/0.69
transmission, %
diffractometer
scan method
2θ range, deg
reciprocal lattice
segment
Siemens P4
ω scans
6.1-50.0
-h, -k, -l
Siemens P4
ω scans
5.3-57.2
-h, -k, ( l
no. of rflns measd
no. of indep rflns
Rint
8822
7130
0.0281
0.0366
0.0975
2341
1785
0.0630
0.0343
0.0943
R1a
wR2b
a
b
R1 ) ∑||Fo| - |Fc||/∑|Fo| for reflections with I > 2σI. wR2 )
[∑[w(Fo - Fc2)2]/∑[w(Fo2)2]]0.5 for all reflections; w-1 ) σ2(F2) +
2
(aP)2 + bP, where P ) (2Fc + Fo2)/3 and a and b are constants
2
set by the program.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 4‚EtOH
Pd(1)-C(1)
Pd(1)-O(3)
Pd(2)-N(2)
Pd(2)-O(7)
Pd(3)-C(21)
Pd(3)-O(4)
Pd(4)-C(23)
Pd(4)-O(8)
O(1)-C(17)
O(3)-C(19)
O(5)-C(37)
O(7)-C(39)
1.959(5)
2.063(4)
2.004(4)
2.054(3)
1.960(5)
2.047(4)
1.963(5)
2.057(3)
1.265(6)
1.248(6)
1.256(6)
1.254(6)
Pd(1)-N(1)
Pd(1)-O(1)
Pd(2)-C(3)
Pd(2)-O(5)
Pd(3)-N(3)
Pd(3)-O(2)
Pd(4)-N(4)
Pd(4)-O(6)
O(2)-C(17)
O(4)-C(19)
O(6)-C(37)
O(8)-C(39)
2.007(4)
2.182(3)
1.956(5)
2.159(4)
1.997(4)
2.138(3)
2.002(4)
2.137(3)
1.243(6)
1.264(6)
1.251(6)
1.263(6)
Con clu sion
The orthometalation reaction of 1,3-di(2-pyridyl)-
benzene with Pd(OAc)2 or K2PtCl4 yields structurally
different complexes. In the case of Pd, a double meta-
lation takes place on the aryl ring, giving rise to a
dimeric complex in which four acetate units act as
bridges between two doubly metalated ligands. Each N
coordinates to a different metal atom. The reaction with
chloride-containing Pd(II) complexes does not lead to
C-H activation but to coordination complexes without
Pd-C bonds. On the other hand, cycloplatination takes
place in position 2 of the benzene ring, generating a
terdentate ligand. This behavior is the same as that
previously observed for Ru and Os. The structure of both
orthometalation complexes has been determined by
X-ray diffraction. The details depending on the nature
of the metal that control the mono- or dimetalation
deserve further studies.
C(1)-Pd(1)-N(1)
N(1)-Pd(1)-O(1)
C(3)-Pd(2)-N(2)
N(2)-Pd(2)-O(5)
C(21)-Pd(3)-N(3)
N(3)-Pd(3)-O(2)
C(23)-Pd(4)-N(4)
N(4)-Pd(4)-O(6)
C(7)-N(1)-Pd(1)
81.2(2) C(1)-Pd(1)-O(3)
97.4(2) O(3)-Pd(1)-O(1)
82.0(2) C(3)-Pd(2)-O(7)
94.2(2) O(7)-Pd(2)-O(5)
82.1(2) C(21)-Pd(3)-O(4)
94.9(2) O(4)Pd(3)-O(2)
81.1(2) C(23)-Pd(4)-O(8)
93.3(2) O(8)-Pd(4)-O(6)
96.2(2)
85.2(2)
94.4(2)
89.87(14)
95.9(2)
87.22(14)
95.3(2)
90.23(14)
116.2(3) C(12)-N(2)-Pd(2) 115.8(3)
C(27)-N(3)-Pd(3) 116.1(3) C(32)-N(4)-Pd(4) 116.7(3)
C(6)-C(1)-Pd(1) 115.1(4) C(4)-C(3)-Pd(2) 113.6(3)
C(26)-C(21)-Pd(3) 112.9(3) C(24)-C(23)-Pd(4) 114.2(3)
O(2)-C(17)-O(1)
O(6)-C(37)-O(5)
125.2(5) O(3)-C(19)-O(4)
126.9(5) O(7)-C(39)-O(8)
126.3(5)
126.7(4)
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions (but not the subsequent
workup) were carried out under an atmosphere of Ar. MeCN
and CH2Cl2 were dried by standard methods. HOAc (synthetic
grade) was used as received. Pd(COD)Cl2 was prepared from
PdCl2,23 and [Pd(MeCN)4](BF4)2 was synthesized by reaction
2.154(4) Å] are longer than the Pd-O distances trans
to nitrogen bond lengths [mean value 2.055(4) Å], as
usually observed, as a consequence of a stronger trans
influence of the aryl group.15b
X-r a y Cr ysta l Str u ctu r e of th e Cyclop la tin a tion
Com p lex 6. An ORTEP diagram of 6 is shown in Figure
2. Crystallographic data are listed in Table 1, and
significant bond distances and angles are in Table 3.
24
of Pd(MeCN)2Cl2 with AgBF4 in MeCN.25
(22) Terheijden, J .; van Koten, G.; van Beek, J . A. M. Organome-
tallics 1987, 6, 89, and references therein.

Divergent Behavior of Pd(II) and Pt(II) in Metalation
Organometallics, Vol. 18, No. 17, 1999 3341
1,3-Di(2-p yr id yl)ben zen e (1). A mixture of 1,3-dibromopy-
ridine (1.07 g, 4.52 mmol), (2-pyridyl)tri-n-butylstannane (2)
or (2-pyridyl)trimethylstannane (3) (13.6 mmol), Pd(PPh3)2Cl2
(254 mg, 0.36 mmol), and LiCl (1.92 g, 45.2 mmol) in toluene
was heated under reflux conditions for 3 days. After cooling
to 23 °C a saturated KF solution (15 mL) was added, and the
mixture was stirred for 30 min. The solid residue formed was
filtered off. CH2Cl2 (200 mL) and a 5% NaHCO3 solution (150
mL) were added to the filtrate. The organic phase was
separated and dried (Na2SO4), and the solvent was evaporated.
The crude product was chromatographed (silica gel; hexanes-
Et2O, 4:1 to neat Et2O) to give 1 as a light yellow oil (75-
83%):12 1H NMR (300 MHz, CDCl3) δ 8.71 (dm, J ) 4.8 Hz,
2H), 8.65 (t, J ) 1.8 Hz, 1H), 8.06 (dd, J ) 7.8, 1.8 Hz, 2H),
7.81 (br d, J ) 7.5 Hz, 2H), 7.74 (tm, J ) 7.5 Hz, 2H), 7.58 (t,
J ) 7.8 Hz, 1H), 7.23 (ddm, J ) 7.5, 4.8 Hz, 2H); 1H NMR
(200 MHz, DMSO-d6) δ 8.81 (t, J ) 1.8 Hz, 1H), 8.69 (br d, J
) 4.5 Hz, 2H), 8.13 (dd, J ) 7.7, 1.5 Hz, 2H), 8.06 (br d, J )
7.7 Hz, 2H), 7.90 (td, J ) 7.7, 1.5 Hz, 2H), 7.61 (t, J ) 7.7 Hz,
1H), 7.37 (ddd, J ) 7.2, 4.6, 1.0 Hz, 2H); 13C{1H} NMR (75
MHz, CDCl3; DEPT) δ 156.77 (C), 149.32 (CH), 139.55 (C),
136.48 (CH), 128.92 (CH), 127.14 (CH), 125.22 (CH), 121.98
(CH), 120.36 (CH).
K2PtCl4. After 3 days the starting Pt salt was completely
consumed and the mixture was allowed to cool to 23 °C. The
resulting bright yellow complex was filtered off and washed
with MeOH, H2O, EtOH, and Et2O (39 mg). The filtrate was
evaporated to dryness and suspended in EtOH (5 mL), giving
rise to an additional precipitate, which was filtered off and
washed with EtOH and Et2O (30 mg). Combined yield: 78%.
An analytical sample of the corresponding dihydrate was
prepared by diffusion of Et2O into a DMSO solution (the
solvents were not dry. H2O in the complex was detected by 1H
NMR): 1H NMR (300 MHz, DMSO-d6) δ 9.07 [dd, J ) 5.5, 0.9
Hz, J (1H-195Pt) ) 105.1 Hz, 2H], 8.17 (td, J ) 7.7, 1.6 Hz,
2H), 8.07 (br d, J ) 7.6 Hz, 2H), 7.70 (d, J ) 7.7 Hz, 2H), 7.52
(ddd, J ) 7.4, 5.8, 1.6 Hz, 2H), 7.23 (t, J ) 7.7 Hz, 1H); 13C-
{1H} NMR (75 MHz, DMSO-d6) δ 166.73 [J (13C-195Pt) ) 113
Hz], 161.19, 151.41, 140.65 [J (13C-195Pt) ) 83 Hz], 140.60,
125.09 [J (13C-195Pt) ) 36.4 Hz], 124.28 [J (13C-195Pt) ) 30.5
Hz], 123.28, 120.62 [J (13C-195Pt) ) 46.2 Hz]. FAB-MS m/z
461.9 (M+), 425.9 [(M - HCl)+], 886.9 [(2M - Cl)+]. Anal. Calcd
for C16H11N2ClPt‚2H2O: C, 38.60; H, 3.04; N, 5.63. Found: C,
38.27; H, 2.94; N, 5.50.
X-r a y Str u ctu r e Deter m in a tion s. A crystal of 4‚EtOH
was mounted in inert oil on a glass fiber and transferred to
the diffractometer (Siemens P4 with LT2 low-temperature
attachment) as summarized in Table 1. Unit cell parameters
were determined from a least-squares fit of 62 accurately
centered reflections (9.4° < 2θ < 24.8°). The structure was
solved by direct methods and refined anisotropically on F2
(program SHELXL 93).26 The methyl group of the ethanol
molecule is disordered over two sites (81 and 19% occupancy).
Hydrogen atoms were included using a riding model or as rigid
methyl groups. Hydrogen atoms for the disordered solvent
were not considered. The final R(F) [I > 2σ(I)] was 0.0366, for
526 parameters and 433 restraints to local symmetry, and U
components of neighboring light atoms. Maximum ∆/σ ) 0.002,
maximum ∆F ) 0.92 e Å-3. For compound 6 the determination
of the cell constants and the data collection were performed
on a Siemens P4 diffractometer (see Table 1). Unit cell
constants were determined using 73 accurately centered
reflections (5.28° < 2θ < 47.47°). The structure was solved by
the heavy atom method and refined anisotropically on F2
(program SHELXTL 93).25 Hydrogen atoms were included
using a riding model. The final R(F) [I > 2σ(I)] was 0.0343,
for 182 parameters. Maximum ∆/σ ) 0.001, maximum ∆F )
2.15 e Å-3 (the largest difference peak is at 1.34 Å from the
platinum atom).
P d 4(1-H2)2(OAc)4 (4). A mixture of 1 (108 mg, 0.46 mmol),
Pd(OAc)2 (95 mg, 0.42 mmol), and HOAc (7 mL) was heated
under reflux conditions for 4.5 h. After being cooled to 23 °C
the bright yellow precipitate was filtered and washed with
HOAc and Et2O to give 4 (64 mg, 54%). A lower yield (42%)
was obtained when the reaction was carried out at 50 °C for
15 h: 1H NMR (300 MHz, DMSO-d6) δ 7.92 (dd, J ) 5.7, 1.2
Hz, 4H), 7.69 (m, 4H), 7.61 (br d, J ) 7.4 Hz, 4H), 7.27 (s,
2H), 6.92 (ddd, J ) 7.2, 5.8, 1.4 Hz, 4H), 6.42 (s, 2H), 2.14 (s,
6H), 2.08 (s, 6H); 13C{1H} NMR (75 MHz, DMSO-d6; DEPT) δ
180.31 (C), 179.03 (C), 164.67 (C), 155.96 (C), 148.39 (CH),
140.40 (C), 138.71 (CH), 137.86 (CH), 121.69 (CH), 118.00
(CH), 117.31 (CH), 25.05 (CH3), 24.58 (CH3); IR (KBr) (selected
bands, cm-1) 1573, 1410; FAB-MS m/z 1122.0 (M+), 1063.0 [(M
- AcO)+], 2185.9 [(2M - AcO)+]. Anal. Calcd for C40H32N4O8-
Pd4: C, 42.80; H, 2.87; N, 4.99. Found: C, 42.45; H, 2.93; N,
4.72.
P d (1)Cl2 (5). Li2PdCl4 was prepared in situ by heating
under reflux a suspension of PdCl2 (79 mg, 0.45 mmol) and
LiCl (60 mg, 1.42 mmol) in MeOH (10 mL) for 1.5 h to give a
dark brown solution. The solvent was evaporated, and the
resulting oil was dissolved in HOAc (10 mL). Ligand 1 (130
mg, 0.56 mmol) was added, and a white precipitate im-
mediately appeared. The mixture was heated at 60 °C for 16
h. After being cooled to 23 °C, the light yellow precipitate was
filtered and washed with HOAc, MeOH, and Et2O (181 mg,
quantitative). An analytical sample was obtained after suspen-
sion of the complex in MeOH, sonication, and filtration: 1H
NMR (300 MHz, DMSO-d6) δ 8.80 (br d, J ) 5.5 Hz, 2H), 8.73
(t, J ) 2.0 Hz, 1H), 8.35-8.15 (m, 6H), 7.75 (t, J ) 7.8 Hz,
1H), 7.66 (td, J ) 7.0, 2.0 Hz, 2H); 13C{1H} NMR (75 MHz,
DMSO-d6; DEPT) δ 153.31 (C), 146.46 (CH), 142.10 (CH),
135.99 (C), 130.19 (CH), 129.50 (CH), 126.68 (CH), 124.69
(CH), 123.45 (CH); FAB-MS (DMSO) m/z 711 ([Pd2(1)2Cl]+),
569.1 ([Pd(12)-H]+), 489.0 ([Pd(1)(DMSO)Cl2]+) 373.0 ([Pd(1)-
Cl]+), 337.0 ([Pd(1)-H+]). Anal. Calcd for C16H12N2Cl2Pd‚
2H2O: C, 43.13; H, 3.62; N, 6.29. Found: C, 43.45; H, 3.23; N,
6.02.
Ack n ow led gm en t. We are grateful to the DGICYT
(Project PB97-0002-C01-01) for support of this research.
We acknowledge J ohnson Matthey PLC for a generous
loan of PdCl2. We also thank Ce´sar J . Pastor and Ivan
Shmytko (permanent address: Institute of Solid State
Physics, Chernogolovka Moscow distr. 149432, Russia)
of the SIdI (UAM) for the resolution of the crystal
structure of 6.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data and structure refinement details, positional and
thermal parameters, and bond distances and bond angles for
complexes 4‚EtOH and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
P t(1-H)Cl (6). A mixture of 1 (45 mg, 0.194 mmol), K2PtCl4
(80 mg, 0.193 mmol), and HOAc (5 mL) was heated at 110-
115 °C (reflux). The progress of the reaction was monitored
taking small aliquots and treating them with H2O. The
appearance of a red color indicated the presence of unreacted
OM990125G
(23) Chatt, J .; Vallarino, L.; Venanzi, M. J . Chem. Soc. 1957, 3413.
(24) Heck, R. F. Palladium Reagents in Organic Syntheses; Academic
Press: London, 1985; p 17.
(26) Sheldrick, G. M. SHELXTL-93; University of Go¨ttingen: Go¨t-
tingen, Germany.
(25) Thomas, R.; Sen, A. Inorg. Synth. 1989, 26, 128; 1990, 28, 63.