Doubly cyclometalated pyridazines: Contrasting behavior with palladium and platinum

Article abstract of DOI:10.1021/om010449g

The cyclometalation of 3,6-diphenylpyridazines with palladium and platinum has been studied. When palladium acetate is used as the metal source, two sequential cyclometalations are observed, with the first cyclometalation deactivating the system toward a second cyclometalation. Both monocyclopalladated and dicyclopalladated species have been isolated and characterized as their acetylacetonate derivatives. When potassium tetrachloroplatinate is used as the metal source, only double cycloplatination is observed: the first cycloplatination activates the system toward a second cycloplatination through a combination of nitrogen coordination and a chloride bridging two platinums. A doubly cycloplatinated compound has been isolated as its triphenylphosphine derivative, which has been fully characterized: the single-crystal X-ray structure shows that the two platinums are bridged by a chloride.

Full text of DOI:10.1021/om010449g

4418
Organometallics 2001, 20, 4418-4423
Dou bly Cyclom eta la ted P yr id a zin es: Con tr a stin g
Beh a vior w ith P a lla d iu m a n d P la tin u m
J onathan W. Slater, Donocadh P. Lydon, Nathaniel W. Alcock, and
J onathan P. Rourke*
Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K.
Received May 29, 2001
The cyclometalation of 3,6-diphenylpyridazines with palladium and platinum has been
studied. When palladium acetate is used as the metal source, two sequential cyclometalations
are observed, with the first cyclometalation deactivating the system toward a second
cyclometalation. Both monocyclopalladated and dicyclopalladated species have been isolated
and characterized as their acetylacetonate derivatives. When potassium tetrachloroplatinate
is used as the metal source, only double cycloplatination is observed: the first cycloplatination
activates the system toward a second cycloplatination through a combination of nitrogen
coordination and a chloride bridging two platinums. A doubly cycloplatinated compound
has been isolated as its triphenylphosphine derivative, which has been fully characterized:
the single-crystal X-ray structure shows that the two platinums are bridged by a chloride.
In tr od u ction
reaction has been to use a central ring incorporating
two ligating groups and to use this to bring about two
cyclometalations on separate ring systems.15 It is in this
last area that we have recently made considerable
progress, and we present the results of those investiga-
tions here.
Suitable ligands for bringing about two cyclometala-
tions on adjacent aromatic rings include 2,5-diphen-
ylpyrazines and 3,6-diphenylpyridazines. 3,6-Diphen-
ylpyridazines (1) are conveniently prepared by the
coupling of 2 equiv of an aryl methyl ketone, as outlined
in Scheme 1,17 and it is these ligands that we concern
ourselves with in this paper.
Cyclometalation has conventionally involved one
ligating group holding a metal center close to a C-H
bond and the subsequent closure of the ring via the
formation of a carbon to metal bond.1 While some have
adapted this reaction to activate bonds other than C-H
(for example, C-Si2 or C-C bonds3), we have been
investigating bringing about double cyclometalations
with a single metal (generating C∧N∧C tridentate
complexes).4,5 Others have followed Trofimenko’s pio-
neering work of 30 years ago6,7 using two ligating groups
to hold two metal centers close to a single benzene ring,
thus forming two metal to carbon bonds on the same
aromatic ring. Trofimenko’s original work concerned
systems where the two metals on the metalated ring
are ortho, meta, and para to each other, while we8 and
others9-12 have largely restricted our investigations to
systems where the two metals are opposite (para) across
a phenyl ring, though other arrangements have been
studied.13-16 Another variant on the cyclometalation
Resu lts a n d Discu ssion
Cyclop a lla d a tion . Pyridazines 1 react cleanly with
1 equiv of palladium acetate at 60 °C in acetic acid to
yield a cyclopalladated species in high yield (Scheme 2).
We presume this species is the dimer 2, as all the
characterization (NMR, mass spectrometry, elemental
analysis) we have is consistent with this formulation,
and an acetate-bridged dimer is a very common feature
of similar palladium complexes.18 We can then react this
dimer with sodium acetylacetonate (Na(acac)) to yield
the monomeric species 3. While we lack a single-crystal
X-ray structure of 3, all other data are consistent with
this formulation. The proton NMR of 3 is particularly
useful, indicating the inequivalence of the two protons
in the pyridazine ring, the presence of one metalated
* To whom correspondence should be addressed. E-mail: j.rourke@
warwick.ac.uk.
(1) Ryabov, A. D. Chem. Rev. 1990, 90, 403.
(2) Steenwinkel, P.; Gossage, R. A.; Maunula, T.; Grove, D. M.; van
Koten, G. Chem. Eur. J . 1998, 4, 763.
(3) Rybtchinski, R.; Vigalok, A.; Ben-David, Y.; Milstein, D. J . Am.
Chem. Soc. 1996, 118, 12406.
(4) Cave, G. W. V.; Alcock, N. W.; Rourke, J . P. Organometallics
1999, 18, 1801.
(5) Cave, G. W. V.; Fanizzi, F. P.; Deeth, R. J .; Errington, W.;
Rourke, J . P. Organometallics 2000, 19, 1355.
(6) Trofimenko, S. J . Am. Chem. Soc. 1971, 93, 1808.
(7) Trofimenko, S. Inorg. Chem. 1973, 12, 1215.
(8) Lydon, D. P.; Rourke, J . P. Chem. Commun. 1997, 1741.
(9) Steenwinkel, P.; J ames, S. L.; Grove, D. M.; Kooijman, H.; Spek,
A. L.; van Koten, G. Organometallics 1997, 16, 513.
(10) O’Keefe, B. J .; Steel, P. J . Organometallics 1998, 17, 3621.
(11) Hartshorn, C. M.; Steel, P. J . Organometallics 1998, 17, 3487.
(12) Ferna´ndez, A.; Ferna´ndez, J . J .; Lo´pez-Torres, M.; Sua´rez, A.;
Ortigueira, J . M.; Vila, J . M.; Adams, H. J . Organomet. Chem. 2000,
612, 85.
(14) Chakladar, S.; Paul, P.; Mukherjee, A. K.; Dutta, S. K.; Nanda,
K. K.; Podder, D.; Nag, K. J . Chem. Soc., Dalton Trans. 1992, 3119.
(15) de Geest, D. J .; O’Keefe, B. J .; Steel, P. J . J . Organomet. Chem.
1999, 579, 97.
(16) Doppiu, A.; Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Zucca,
A.; Manassero, M. Organometallics 2001, 20, 1148.
(17) Nakayama, J .; Konishi, T.; Ishii, A.; Hoshino, M. Bull. Chem.
Soc. J pn. 1989, 62, 2608.
(18) Navarro-Ranninger, C.; Lo´pez-Solera, I.; Alvrez-Valde´s, A.;
Rodr´ıguez-Ramos, J . H.; Masaguer, J . R.; Garc´ıa-Ruano, J . L.; Solans,
X. Organometallics 1993, 12, 4104.
(13) Chakladar, S.; Paul, P.; Venkatsubramanian, K.; Nag, K. J .
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10.1021/om010449g CCC: $20.00 © 2001 American Chemical Society
Publication on Web 09/11/2001

Doubly Cyclometalated Pyridazines
Organometallics, Vol. 20, No. 21, 2001 4419
Sch em e 1
Sch em e 2
and one nonmetalated phenyl ring, and one acac per
pyridazine core. If we use 2 equiv of palladium acetate
(or indeed, react our monometalated species 2 with a
second equivalent) and an extended reaction time, we
can bring about a second cyclometalation to give a new
species (4). We have been unable to characterize com-
pound 4 in detailssolution NMR spectra are too broad
to interpretsbut we believe the compound to be doubly
cyclopalladated. Whether 4 is polymeric with bridging
acetate groups or, by analogy with other work,10,19
a
simple dimer is of little relevance here. Subsequent
reaction of 4 with sodium acac yields a well-character-
ized compound (5) with the formulation depicted in
Scheme 2. In particular, the proton NMR of 5 is very
informative: the aromatic region shows only four reso-
nances of equal intensity (one singlet for the central
pyridazine ring and three multiplets for the metalated
ring, one a doublet of 8.5 Hz, one a doublet of 3 Hz, and
one a double doublet of 8.5 and 3 Hz), there is a singlet
for the central acac proton of the same intensity as the
aromatic resonances, and the resonances for the methyl
groups of the acacs are 6 times this intensity. While we
were unable to grow a crystal of 5 suitable for X-ray
diffraction, all other data fit with this formulation.
While we are confident that this is the correct
formulation for 5, it does raise the issue of the arrange-
ment of the acac groups. Simple modeling demonstrates
the impossibility of achieving the preferred planar
arrangement of the pyridazine core, both cyclometalated
rings, and both acac groupssindeed, this is one of the
reasons we wanted to make these compounds. Hence,
we suggest the helical twist on the structure of 5 shown
in Scheme 2. The practical effect of this twist is to
reduce the symmetry of the compound, leaving a C2 axis
as the only symmetry elementshence, the molecule
must be chiral. It follows that, if these molecules are
chiral, we must have a racemic mixture, as no attempt
was made to control any stereochemistry in the reaction.
The solution state proton and carbon NMRs of 5 do not
show any unusual features: the chemical shifts of
neither the aromatic nor acac resonances are much
different from those in the monocyclopalladated 3. The
only potentially significant feature of the solution NMR
is the equivalence of the chemical shifts of the methyl
groups of the acacsboth ends of the acac resonate as
singlets at 2.08 ppm in the proton NMR and at 27.4 ppm
in the carbon NMR. This equivalence might at first
suggest that the correct structure is one in which the
palladiums are tetrahedral (with the acac groups per-
(19) Ca´rdenas, D. J .; Echavarren, A. M.; Arellano, M. C. R. d.
Organometallics 1999, 18, 3337.

4420 Organometallics, Vol. 20, No. 21, 2001
Slater et al.
Sch em e 3
pendicular to the pyridazine core), but this would
require the palladiums to be paramagnetic, and the
sharp NMR precludes this possibility. Another inter-
pretation of the solution NMR data is that the molecule
is fluxional and the ends of the acacs are interchanging
rapidly on the NMR time scale, but cooling the sample
to -80 °C did not reveal any significant changes in the
NMR. Thus, we conclude that there is an accidental
equivalence of the methyl groups in the NMR and the
structure is indeed the helically twisted one depicted
in Scheme 2. Platinum derivatives of diphenylpyridine
with a similar sterically imposed helical twist have been
isolated in good yields,20-24 indicating that our proposed
structure is not unprecedented. It is not obvious whether
the enantiomers of our doubly cyclometalated acac
compound will be sufficiently configurationally stable
to ever allow their separation.
In any event, the situation we observe with pyrida-
zines and palladium is that a first cyclometalation is
facile and a subsequent cyclometalation less so. Whether
this is due to the steric hindrance presented by the first
metal or an electronic effect due to the nature of the
second pyridazine nitrogen has been touched upon,15
and we will return to this issue later in this paper. One
other point to note is the liquid crystallinity of com-
pound 3 (R ) Bu), where a smectic A phase was
unambiguously identified on the basis of its optical
texture when observed through the crossed polarizers
of a hot-stage microscope: this aspect of the behavior
of compounds 3 will be discussed in a later paper.25
Cyclop la tin a tion . Pyridazines 1 react cleanly with
either 1 or 2 equiv of potassium tetrachloroplatinate in
acetic acid to yield a single dimetalated species (leaving
unreacted ligand if only 1 equiv of platinum is used).
We believe the product to be 6, as depicted in Scheme
3, since all the data we have are consistent with this
formulation. 1H NMR data show the molecule to be
symmetric in solution (only four aromatic resonances);
elemental analysis and mass spectrometry are consis-
tent with three chlorides, two platinums, and one
potassium per pyridazine unit. Cycloplatinated com-
pounds do not react with sodium acetylacetonate in the
same way as cyclopalladated compounds; therefore, we
were unable to prepare the platinum analogue of the
helically twisted compound 5. However, subsequent
reaction of 6 with triphenylphosphine cleanly gave
compound 7, to which we can unambiguously assign the
illustrated structure. Solution NMR indicates only one
type of phosphine in 7 (the presence of satellites in the
31P NMR clearly showing the phosphine is bound to
platinum), and the symmetric nature of the molecule
is confirmed by the uncomplicated proton NMR, which
is qualitatively similar to that of 6. Both elemental
analysis and mass spectrometry are consistent with the
proposed formulation, which was confirmed by the
single-crystal X-ray structure.
Both compounds 6 and 7 are salts, but with a
fundamental difference between them. In compound 6
the large organometallic species is the anion with the
counterion being potassium, whereas in 7 the large
organometallic species is the cation, with the counterion
being chloride. This difference manifests itself as very
different solubilities for the two compounds: compound
6 is very soluble in acetone but insoluble in chloroform,
whereas 7 is insoluble in acetone but readily soluble in
chloroform. Acetone is well-known to be able to solubi-
lize smaller cations such as potassium, whereas chloro-
form is equally well able to solubilize smaller anions
such as chloride through hydrogen bonding.
The chloride bridge between the two cyclometalated
platinums is clearly a key feature of compounds 6 and
7. We believe it to be part of the driving force of the
second cyclometalation: if we react pyridazine with only
1 equiv of K2PtCl4 or use short reaction times, we only
isolate unreacted pyridazine and doubly metalated 6.
If we follow the reaction in deuterated acetic acid by
NMR, we do not see any indication of the monocyclo-
metalated species that must exist as an intermediate
(Scheme 3). One can therefore imagine a second plati-
num being held in close proximity to the relevant C-H
bond by a combination of nitrogen coordination and a
chloride bridge and this species reacting faster than free
pyridazine. Thus, the first cyclometalation activates the
(20) Cornioley-Deuschel, C.; Ward, T.; von Zelewsky, A. Helv. Chim.
Acta 1988, 71, 130.
(21) Deuschel-Cornioley, C.; Stoeckli-Evans, H.; von Zelewsky, A.
J . Chem. Soc., Chem. Commun. 1990, 121.
(22) J olliet, P.; Gianini, M.; von Zelewsky, A.; Bernardinelli, G.;
Stoeckli-Evans, H. Inorg. Chem. 1996, 35, 4883.
(23) Gianini, M.; Forster, A.; Haag, P.; von Zelewsky, A.; Stoeckli-
Evans, H. Inorg. Chem. 1996, 35, 4889.
(24) Gianini, M.; von Zelewsky, A.; Stoeckli-Evans, H. Inorg. Chem.
1997, 36, 6094.
(25) Slater, J . W.; Lydon, D. P.; Rourke, J . P. Manuscript in
preparation.

Doubly Cyclometalated Pyridazines
Organometallics, Vol. 20, No. 21, 2001 4421
Sch em e 4
number of broad humps from which no useful informa-
tion could be inferred. Subsequent workup of the experi-
ment with sodium acetylacetonate showed that both
monocyclometalation and dicyclometalation had oc-
curred. The experiment with platinum was slightly
more illuminating. The 2 equiv of potassium tetrachlo-
roplatinate used was not soluble in the reaction solvent
at 70 °C, and neither was the yellow precipitate of the
doubly cycloplatinated 6 that formed (subsequently
identified by NMR in acetone after filtration). The only
resonances observed were those of the unreacted pyri-
dazine, and at no point did we see any signals in the
baseline that might have risen from our postulated in-
termediate, any other intermediate, or the final product
6. Thus, we conclude that any intermediate that does
form must react on to form 6 more quickly that the
starting pyridazine. We did, however, observe a pro-
gressive deuteration of the diphenylpyridazine at the
2′-position of the phenyl rings (i.e., the position where
the platinum would be attached in a cyclometalated
compound). After about 1 day at 70 °C, when approxi-
mately one-third of the platinum source appeared to
have been consumed, the signal for the 2′-protons of the
phenyl rings was down to about half the intensity of
the 3′-protons. This clearly indicates the reversible
nature of the initial steps of the cycloplatination reaction
under the conditions we are using.
system to a second cyclometalation, in contrast to that
which we ourselves observe with palladium and in
complete contrast to all comparable systems reported
to date. For example, when Steel attempted to doubly
cyclopalladate a pyridazine he was only able to induce
a single cyclometalation, with the failure to bring about
the second cyclopalladation being ascribed to the steric
encumbrance of the second nitrogen donor on the
pyridazine.15
The activation of a pyridazine system toward a second
cyclometalation that we observe has parallels in Trofi-
menko’s original work: diamines 8 could doubly meta-
late to give either trans isomers 9 or cis isomers 10
(Scheme 4).7 The driving force for the formation of the
sterically more crowded cis isomer must be the presence
of the intramolecular chloride bridge. It perhaps should
be noted that both we26 and others27 have reported
systems where a platinum(II) salt brings about a C-H
activation without the prior coordination of a ligating
group.
NMR Stu d ies. We have attempted to follow the
course of both the cyclopalladation and the cycloplati-
nation of pyridazines by NMR. Thus, solutions of
pyridazine in deuterated acetic acid were treated with
palladium acetate or potassium tetrachloroplatinate and
proton NMR spectra recorded at regular intervals. The
experiment with palladium acetate at 60 °C proved to
be of little use: while the 2 equiv of palladium acetate
was soluble in the solvent, as were all products, neither
a resonance for the palladium acetate nor any reso-
nances for the acetate of any products could be distin-
guished (presumably the nondeuterated starting mate-
rial exchanged acetate groups with vast excess of
deuterated acetate groups from the solvent). In addition,
the aromatic resonances for the pyridazine quickly
disappeared (gone within 2-3 h), to be replaced by a
Cr ysta l Str u ctu r e. The single-crystal X-ray struc-
ture of the triphenylphosphine platinum compound 7
has been determined. It consists of the diplatinum
cation (Figure 1) and chloride anions solvated by
chloroform molecules via hydrogen bonds. A certain
amount of disorder exists in the solvated anions and in
the extended hydrocarbon chains attached to the pyri-
dazine, but the core of the molecule (the three aromatic
rings of the diphenylpyridazine, the two platinums, the
two triphenylphosphine ligands, and the bridging chlo-
ride) is clearly defined.
The coordination geometry of each platinum is a
slightly distorted square plane: the N-Pt-C angles are
81.3(4) and 81.4(4)°, the P-Pt-C angles are 96.0(3) and
95.1(3)°, the P-Pt-Cl angles are 91.3(2) and 91.4(2)°,
and the Cl-Pt-N angles are 91.9(1) and 92.1(1)°. Each
plane is flat to within an rms deviation of 0.1 Å with
the two planes inclined at 8.6° to each other. The angle
that the two Pt-Cl bonds make at the chloride is
105.39(11)°, very close to tetrahedral. The Pt-Cl bond
lengths at 2.388(3) and 2.371(3) Å are slightly longer
than those of 2.339(7) and 2.312(6) Å that we have
observed in a cycloplatinated diphenylpyridine with two
chlorides bridging two platinums, when the chlorides
are trans to the pyridine nitrogen, and somewhat
shorter than those of 2.468(8) and 2.460(7) Å observed
in the same compound when the chlorides are trans to
a cyclometalated carbon.5 The Pt-C distances at 2.027-
(10) and 2.023(11) Å are not significantly different from
the values of 1.98(3) and 1.96(2) Å observed in that same
compound. Overall, the backbone of the three aromatic
rings of the diphenylpyridazine is bowed toward the two
platinums while the whole core of the molecule is kept
effectively flat (Figure 2). The phenyl rings in the
diphenylpyridazine are undistorted: all the bond lengths
are within 0.03 Å of 1.40 Å and all internal angles
within 2° of 120°. The pyridazine ring is largely sym-
(26) Cave, G. W. V.; Hallett, A. J .; Errington, W.; Rourke, J . P.
Angew. Chem., Int. Ed. Engl. 1998, 37, 3270.
(27) Falvello, L. R.; Ferna´ndez, S.; Larraz, C.; Llusar, R.; Navarro,
R.; Urriolabeitia, E. P. Organometallics 2001, 20, 1424.

4422 Organometallics, Vol. 20, No. 21, 2001
Slater et al.
F igu r e 1. Crystal structure of 7.
F igu r e 2. Another view of the crystal structure of 7.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 7
propriate conditions and can conveniently be used to
bring about two cyclometalations to adjacent phenyl
rings. We do, however, observe contrasting behavior
with palladium and platinum metal reagents. An initial
cyclopalladation deactivates the system toward a second
cyclopalladation, an effect we believe to be largely steric
in origin. In contrast, an initial cycloplatination acti-
vates the system toward a second cycloplatination, so
much so in fact that it proved impossible to isolate single
cycloplatinated species. We believe this activation to be
the results of a combination of nitrogen coordination and
the presence of a chloride bridge.
Pt(1)-C(23)
Pt(1)-N(18)
Pt(1)-P(2)
Pt(1)-Cl(1)
Pt(2)-C(12)
2.027(10)
2.101(8)
2.251(3)
2.388(3)
2.023(11)
Pt(2)-N(17)
Pt(2)-P(1)
Pt(2)-Cl(1)
N(17)-N(18)
2.122(8)
2.249(3)
2.371(3)
1.314(11)
C(23)-Pt(1)-N(18) 81.3(4)
96.0(3)
N(18)-Pt(1)-P(2) 173.1(3)
C(23)-Pt(1)-Cl(1) 170.3(3)
N(17)-Pt(2)-Cl(1)
P(1)-Pt(2)-Cl(1)
Pt(2)-Cl(1)-Pt(1)
91.4(2)
92.11(10)
105.39(11)
C(23)-Pt(1)-P(2)
N(18)-N(17)-C(16) 122.5(9)
N(18)-N(17)-Pt(2) 125.1(6)
N(18)-Pt(1)-Cl(1)
P(2)-Pt(1)-Cl(1)
91.3(2)
91.95(10) C(16)-N(17)-Pt(2) 112.3(7)
C(12)-Pt(2)-N(17) 81.4(4)
N(17)-N(18)-C(19) 120.8(9)
N(17)-N(18)-Pt(1) 126.1(6)
C(19)-N(18)-Pt(1) 113.0(7)
C(12)-Pt(2)-P(1)
N(17)-Pt(2)-P(1) 176.5(2)
C(12)-Pt(2)-Cl(1) 170.1(3)
95.1(3)
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All chemicals were used as
supplied, unless noted otherwise. The pyridazines were syn-
thesized via the published route.17 All elemental analyses were
performed by Warwick Analytical Service.
metric with all internal angles within 1.5° of 120°; bond
lengths vary from 1.423(14) Å for the C-C bond opposite
the nitrogens through 1.344(12) and 1.342(13) Å for the
N-C bonds to 1.314(11) Å for the N-N bond. The
phenyl rings of the triphenylphosphine ligands appear
to be interleaved with no significant interactions. The
chloride counterion is hydrogen-bonded to three chlo-
roform molecules with a linear Cl-H-C arrangement.
A pyramidal arrangement of the three chloroforms
about the chloride results, with the distance of the
chloride to the carbons of the solvating chloroforms
being 3.30(1), 3.34(1), and 3.30(1) Å. Selected bond
lengths and angles are listed in Table 1, full data are
available as Supporting Information.
P r ep a r a tion of Bis(µ-a ceta to)bis[3,6-bis(4′-(bu tyloxy)-
ph en yl)pyr idazin e]dipalladiu m (2). Palladium acetate (0.059
g, 2.63 × 10-4 mol) was added to a solution of 3,6-bis(4′-
(butyloxy)phenyl) pyridazine (0.100 g, 2.63 × 10-4 mol) in
acetic acid (25 mL) and heated (60 °C, 6 h). The solvent
removed to yield a yellow solid. Yield 0.142 g (100%, 1.32 ×
1
10-4 mol). H NMR (δH; 400 MHz, CDCl3): 7.95 (4H, AA′XX′,
unmetalated phenyl, ortho to pyridazine), 7.45 (2H, d,3J ) 8.5
Hz, pyridazine), 7.05 (2H, d, 3J ) 8.5 Hz, pyridazine), 7.00
(4H, AA′XX′, unmetalated phenyl, ortho to OCH2), 6.75 (2H,
d, 3J ) 8.5 Hz, metalated phenyl, meta to Pd), 6.30 (2H, d,
4J ) 2.5 Hz, metalated phenyl, ortho to Pd), 6.25 (2H, dd,
4
3J ) 8.5 Hz, J ) 2.5 Hz, metalated phenyl, para to Pd), 4.00
(8H, t, OCH2), 2.35 (6H, s, acetate), 1.80 (8H, m, chain), 1.45
(8H, m, chain), 0.95 (12H, m, chain Me).
P r ep a r a tion of (Acetyla ceton a te)[3,6-bis(4-(bu tyloxy)-
p h en yl)p yr id a zin e)]p a lla d iu m (3). Sodium acetylacetonate
(0.032 g, 2.63 × 10-4 mol) was added to a solution of bis(µ-
Con clu sion s
Palladium acetate and potassium tetrachloroplatinate
will both smoothly metalate pyridazines under ap-

Doubly Cyclometalated Pyridazines
Organometallics, Vol. 20, No. 21, 2001 4423
3
acetato)bis[3,6-bis(4′-(butyloxy)phenyl)pyridazine]dipalladi-
um (2; 0.142 g, 1.32 × 10-4 mol) in acetone (30 mL). The
solution was stirred (room temperature, 18 h) and the solvent
removed to yield a yellow solid that was purified on a silica
column using a 90/10 chloroform/methanol eluent. The solvent
was removed to yield a crystalline yellow solid. Yield: 0.111 g
(73%, 1.92 × 10-4 mol). Anal. Found (calcd for C29H34N2O4-
Pd): C, 63.3 (63.3); H, 6.96 (6.93); N, 3.99 (4.21). 1H NMR (δH;
400 MHz, CDCl3): 8.05 (2H, AA′XX′, unmetalated phenyl,
4J ) 2.7 Hz, phenyl ring, para to Pt), 3.89 (4H, t, J ) 6.4 Hz,
OCH2), 1.85 (8H, m, chain), 1.25 (16H, m, chain), 0.85 (6H, t,
Me). FAB MS (NBA): m/z 983 (largest, M+ - K), 1021 (M+)
and 1058 (M+ + K).
P r ep a r a tion of Bis(tr ip h en ylp h osp h in e)(µ-ch lor o)[3,6-
bis(4′-(octyloxy)p h en yl)p yr id a zin e]d ip la tin u m Ch lor id e
(7). Triphenylphosphine (0.011 g, 4.10 × 10-5 mol) was added
to a stirred solution of 6 (0.021 g, 2.05 × 10-5 mol) in acetone
(2 mL), and the mixture was stirred for 5 min, whereupon a
color change from orange to yellow was observed. The solvent
was removed under reduced pressure to yield a yellow solid
which was purified by column chromatography using an 80:
20 mixture of chloroform and methanol as an eluent. Yield:
0.018 g (82%, 1.68 × 10-5 mol). 1H NMR (δH; 400 MHz,
CDCl3): 9.05 (2H, s, pyridazine ring), 7.68 (2H, d, 3J ) 8.5
Hz, metalated ring, meta to Pt), 7.50 (12H, m, PPh3), 7.35 (6H,
3
ortho to pyridazine), 7.90 (1H, d, J ) 9 Hz, pyridazine), 7.65
(1H, d, 3J ) 9 Hz, pyridazine), 7.35 (1H, d, 3J ) 8.5 Hz,
metalated phenyl, meta to Pd), 7.25 (1H, d, 4J ) 2.5 Hz,
metalated phenyl, ortho to Pd), 6.95 (2H, AA′XX′, unmetalated
phenyl, ortho to OCH2), 6.65 (1H, dd, 3J ) 8.5, 4J ) 2.5 Hz,
metalated phenyl, para to Pd), 5.45 (1H, s, central acac), 4.10
3
3
(2H, t, J ) 6 Hz, OCH2), 4.00 (2H, t, J ) 6 Hz, OCH2), 2.15
(3H, s, acac Me), 2.10 (3H, s, acac Me), 1.75 (4H, m, chain),
1.50 (8H, m, chain), 0.95 (6H, m, chain Me). Melting behav-
ior: crystal f smectic A, 198 °C; smectic A f isotropic, 276
°C. FAB MS (NBA): m/z 580 (largest, M+), 481 (M+ - acac).
P r epar ation of Bis(µ-acetato)[3,6-bis(4′-(bu tyloxy)ph en -
yl)p yr id a zin e]d ip a lla d iu m (4). Palladium acetate (0.239 g,
1.06 × 10-3 mol) was added to a solution of 3,6-bis(4-(butyloxy)-
phenyl)pyridazine (0.200 g, 5.31 × 10-4 mol) in acetic acid (100
mL). The reaction mixture was stirred (60 °C, 48 h), yielding
a dark red-orange solution. The solvent was removed under
reduced pressure, the solid dissolved in chloroform, and this
solution filtered through Celite to remove traces of palladium
black. The chloroform was removed under reduced pressure
to yield a dark red solid. Yield: 0.325 g (87%, 4.61 × 10-4 mol).
NMR: too broad to interpret.
3
4
tt, PPh3), 7.28 (12H, m, PPh3), 6.55 (2H, dd, J ) 9 Hz, J )
3 Hz, metalated ring, para to Pt), 5.90 (2H, t, 4J ) 3 Hz,
4J (PH) ) 3 Hz, J (PtH) ) 64 Hz, metalated ring, ortho to Pt),
3
3
3.80 (4H, t, J ) 7 Hz, OCH2), 1.25 (4H, m, chain), 1.10 (4H,
m, chain), 1.00 (16H, m, chain), 0.80 (6H, t, 3J ) 7 Hz, methyl).
1
31P NMR (δP; 121.4 MHz, CDCl3): 25.28 ppm (s, J (Pt-P) )
4373 Hz). FAB MS (NBA): m/z 1436 (M+ - Cl) and 944
(M+ - PtCl2PPh3)
X-r a y Cr ysta llogr a p h ic Stu d y of 7. Crystals suitable for
structural analysis were grown from chloroform/ether. An
orange prism (dimensions 0.60 × 0.06 × 0.04 mm) was
mounted with oil on a thin quartz fiber. Data were collected
at 180(2) K using a Siemens SMART CCD area-detector
diffractometer. Crystal data for 7: C68H72Cl2O2N2Pt2‚3.25CHCl3,
Mr ) 1860.75, monoclinic, space group P21/c, a ) 9.8492(5) Å,
b ) 25.009(1) Å, c ) 32.025(2) Å, â ) 91.263(5)°, V ) 7886.57-
(14) Å3, Z ) 4, D(calcd) ) 1.567 Mg/m3. Refinement was by
full-matrix least squares on F2 for 19 139 reflection positions
using SHELXL-9628 with additional light atoms found by
Fourier methods. The asymmetric unit includes three full-
occupancy molecules of solvent CHCl3 and one with occupancy
estimated at 0.25; the atoms in one of the hydrocarbon chains
have very high displacement parameters, probably indicating
partial disorder (though it was refined with 100% occupancy);
the alternative position lies in the same region as the partial-
occupancy solvent, and it could not be fully modeled. Hydrogen
atoms were added at calculated positions and refined using a
riding model with freely rotating methyl groups. Anisotropic
displacement parameters were used for all non-H atoms, apart
from the carbon of the partly occupied solvent and the outer
atoms of the disordered chain; H atoms were given isotropic
displacement parameters equal to 1.2 (or 1.5 for methyl
hydrogen atoms) times the equivalent isotropic displacement
parameter of the atom to which the H atom is attached. The
weighting scheme was calculated using w ) 1/[σ2(Fo2) +
P r ep a r a tion of Bis(a cetyla ceton a to)[3,6-bis(4-(bu tyl-
oxy)p h en yl)p yr id a zin e]d ip a lla d iu m (5). Sodium acetylac-
etonate (0.112 g, 9.22 × 10-4 mol) was added to a solution of
bis(µ-acetato)[3,6-bis(4′-butyloxyphenyl)pyridazine]dipalladi-
um (4; 0.325 g, 4.61 × 10-4 mol) in THF (125 mL). The solution
was stirred (room temperature, 4.5 h) and the solvent removed.
The resulting yellow-orange product was washed with hexane
(2 × 10 mL) and diethyl ether (2 × 10 mL) to yield a yellow
solid which was dried under vacuum. Yield: 0.304 g (84%,
3.87 × 10-4 mol). Anal. Found (calcd for C34H40N2O6Pd2): C,
1
51.5 (52.0); H, 4.9 (5.1); N, 3.6 (3.6). H NMR (δH; 400 MHz,
CDCl3): 7.80 (2H, s, pyridazine), 7.25 (2H, d, 3J ) 8.5 Hz,
4
phenyl, meta to Pd), 7.10 (2H, d, J ) 3 Hz, phenyl, ortho to
3
4
Pd), 6.65 (2H, dd, J ) 8.5 Hz, J ) 3 Hz, phenyl, para to Pd),
3
5.35 (2H, s, central acac), 4.00 (4H, t, J ) 6 Hz, OCH2), 2.08
(12H, s, acac Me), 1.80 (4H, m, chain), 1.30 (8H, m, chain),
0.95 (6H, t, chain Me). 13C NMR (δC; 100.6 MHz, CDCl3): 186.5
(acac C-O), 164.2 (phenyl, bonded to pyridazine), 159.4
(phenyl, bonded to OCH2), 156.8 (pyridazine, bonded to phen-
yl), 132.8 (phenyl, bonded to Pd), 125.1 (phenyl, meta to Pd,
meta to OCH2), 124.6 (pyridazine), 116.0 (phenyl, ortho to Pd,
ortho to OCH2), 111.8 (phenyl, para to Pd), 99.8 (central acac),
67.8 (OCH2), 29.3 (chain), 27.4 (acac Me), 22.6 (chain), 14.0
(chain Me).
(0.0580P)2 + 17.000P] where P ) (Fo + 2Fc2)/3. The goodness
2
of fit on F2 was 1.016; R1 ) 0.0771 for 8813 reflections with
I > 2σ(I), and wR2 ) 0.1741. The number of data/restraints/
parameters was 19 139/9/811. The largest difference Fourier
peak and hole were 1.103 and -1.308 e Å-3; the only large
peaks are in the solvent region and indicate additional disorder
that could not be fully modeled. The relatively high R value is
to be expected in view of the disorder.
P r ep a r a tion of P ota ssiu m Tr ich lor o[3,6-bis(4′-(octyl-
oxy)p h en yl)p yr id a zin e]d ip la tin u m (6). Potassium tetra-
chloroplatinate(II) (0.180 g, 4.35 × 10-4 mol) was added to a
solution of 3,6-bis(4-(octyloxy)phenyl)pyridazine) (0.100 g,
2.18 × 10-4 mol) in acetic acid (50 mL); the solution was
refluxed for 4 days, during which time an orange-brown
precipitate formed. The solvent was removed under reduced
pressure and the orange product washed with hexane, diethyl
ether, and chloroform. The product was then dissolved in
acetone and filtered. The solvent was removed under reduced
pressure to yield an orange product. Yield: 0.12 g (56%,
1.22 × 10-4 mol). Anal. Found (calcd for C32H42Cl3KN2O2Pt2):
C, 37.5 (37.6); H, 4.2 (4.1); N, 2.9 (2.7). 1H NMR (δH; 400 MHz,
Ack n ow led gm en t. We thank J ohnson-Matthey for
the loan of chemicals.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
parameters, bond distances, bond angles, and anisotropic
parameters for the structural analysis of 7 as well as crystal-
lographic files in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM010449G
4
CD3COCD3): 8.10 (2H, s, pyridazine ring), 7.58 (2H, d, J )
3
2.7 Hz, J (PtH) ) 52 Hz, phenyl ring, ortho to Pt), 7.28 (2H,
(28) Sheldrick, G. M. SHELXL-96: Program for Crystal Structure
Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1996.
d, 3J ) 9 Hz, phenyl ring, meta to Pt), 6.52 (2H, dd, 3J ) 9 Hz,