Cyclometallated nitrogen heterocycles

Article abstract of DOI:10.1016/j.jorganchem.2003.08.038

The cyclometallation of four different classes of nitrogen heterocycle (pyridazine, tetrazines, pyrazines and pyrimidines) by palladium and platinum has been studied. Pyridazines readily form singly and doubly cyclometallated compounds, whilst pyrazines only metallate once. Both tetrazines and pyrimidines readily metallate once and doubly metallated species have been identified in solution. The use of trans -cyclometallating reagents based on benzylamine and Schiff's base ligands was also studied: considerable promise was shown by a benzylamine compound.

Full text of DOI:10.1016/j.jorganchem.2003.08.038

Journal of Organometallic Chemistry 688 (2003) 112ꢀ120
/
www.elsevier.com/locate/jorganchem
Cyclometallated nitrogen heterocycles
Jonathan W. Slater, Jonathan P. Rourke *
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK
Received 23 July 2003; received in revised form 29 August 2003; accepted 29 August 2003
Abstract
The cyclometallation of four different classes of nitrogen heterocycle (pyridazine, tetrazines, pyrazines and pyrimidines) by
palladium and platinum has been studied. Pyridazines readily form singly and doubly cyclometallated compounds, whilst pyrazines
only metallate once. Both tetrazines and pyrimidines readily metallate once and doubly metallated species have been identified in
solution. The use of trans-cyclometallating reagents based on benzylamine and Schiff’s base ligands was also studied: considerable
promise was shown by a benzylamine compound.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Cyclometallation; Nitrogen heterocycles; Bridging ligands
1. Introduction
metals are opposite (para) across a phenyl ring, though
other arrangements have been studied [13ꢀ16]. A recent
/
Cyclometallation has proved to be a fertile area of
research, both for mechanistic studies, and as a route
into stable compounds. Conventionally, cyclometalla-
tion has involved one ligating group holding a metal
development has been the design of a ligand capable of
undergoing a triple cyclopalladation, and the successful
realisation of this aim [17]. We too have been investigat-
ing a number of different nitrogen containing hetero-
cycles that are capable of multiple cyclometallation
reactions, and it is the results concerning four of these
systems that we report in this paper.
centre close to a CꢀH bond and the subsequent closure
/
of the ring via the formation of a carbon to metal bond
[1]. Whilst some have adapted this reaction to activate
bonds other than Cꢀ
/
H (for example Cꢀ
/
Si [2] or CꢀC
/
bonds [3]), we have been investigating bringing about
double cyclometallations with a single metal (generating
ˆ ˆ
CNC tridentate complexes) [4,5]. Others have followed
2. Results and discussion
Trofimenko’s pioneering work of thirty years ago [6,7]
using two ligating groups to hold two metal centres 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 metallated ring are ortho, meta and para to each
2.1. Heterocycles
We chose to study four nitrogen containing hetero-
cycles that would be capable of undergoing multiple
cyclometallations (Fig. 1): 3,6-bis-(4-alkyloxyphe-
nyl)pyridazines (1), 3,6-bis(4-alkyloxyphenyl)[1,2,4,5]te-
trazines (2), 2,5-bis-(4-alkyloxyphenyl)pyrazines (3) and
1,4-bis-(5-alkyloxypyrimidin-2-yl)benzenes (4). A num-
ber of different alkyloxy groups were used, and there
were no differences in the reactivity of the homologues.
We have reported previously some of our results
concerning pyridazines [18,19]. The synthesis of the
other, whilst we [8] and others [9ꢀ/12] have largely
restricted our investigations to systems where the two
* Corresponding author. Tel.: ꢁ
7652-4112.
E-mail address: j.rourke@warwick.ac.uk (J.P. Rourke).
/
44-024-7652-3263; fax: ꢁ44-024-
/
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.08.038

J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ
/120
113
analogy with the work of others [10,28,29] these
compounds are assumed to be dimers. These dimers
were characterised by solution NMR and then cleaved
by reaction with sodium acetylacetonate in tetrahydro-
furan to give the monomeric compound 12. Compounds
(12) have been fully characterised. In contrast to the
derivatives of the pyridazines [19] none of the tetrazine
derivatives showed any mesomorphic behaviour, instead
they decompose at temperatures in excess of 200 8C
without melting. While the co-ordination compounds of
a number of 3,6-substituted-[1,2,4,5]tetrazines have been
reported [23] this would appear to be the first time such
a cyclometallated tetrazine compound has been re-
ported. In an attempt to doubly cyclometallate the
tetrazines, they were reacted with two equivalents of
palladium acetate in acetic acid, at elevated tempera-
tures and for an extended time. No second cyclometalla-
tion was observed to take place.
We also tried to doubly cyclopalladate the tetrazines
to form compounds 13 and 14, using potassium tetra-
chloropalladate. While this did yield a small amount of a
new compound, as identified by NMR, it was not clear
which isomer was formed, either the cis metallated (13)
or the trans metallated (14). The yield was very small,
even when the reaction was carried out under a number
of different reaction conditions; the other identified
product being the monometallated chloride equivalent
of 11. The product had a distinctive NMR: only three
peaks in the aromatic region, two doublets and a
doublet of doublets, with both alkoxy chains being
equivalent (all coupling constants and integrations
appropriate for either structure). Even when vast
excesses of palladium source was used the yield was
never good, nor was triply or tetra cyclometallated
product observed.
Fig. 1.
heterocycles has previously been reported though the
22].
yields might leave a little to be desired [20ꢀ
/
2.2. Pyridazines (1)
We have discussed the cyclometallation of pyridazines
(1) previously [18], but it is salient to summarise our
findings here (Scheme 1). Pyridazines are cleanly cyclo-
palladated once by palladium acetate in acetic acid,
when one equivalent of palladium is used. Extended
reaction temperatures and reaction times are necessary
to bring about a second cyclopalladation with palladium
acetate, but the compounds can be isolated. In contrast,
the use of potassium tetrachloroplatinate results in
exclusive double cycloplatination, even with one equiva-
lent of platinum. We ascribed this difference to the
presence of halide bridges which bring a second metal
close to the organic framework, resulting in second
cyclometallation that is quicker than the first.
Cycloplatination reactions were also carried with
tetrazines: one equivalent of potassium tetrachloropla-
tinate in acetic acid yielded an insoluble mono metal-
lated derivative, assumed to be 15, again this is in
contrast to the case with pyridazines where no mono
platination was observed. Subsequent reaction with
triphenylphosphine gave the orange monomeric species
16, whose structure was confirmed by good quality
solution state NMR. The configuration of 16, with the
triphenylphosphine trans to the nitrogen of the tetrazine
was inferred from the platinum satellite coupling con-
stants in the 31P-NMR of the compound: a coupling of
2.3. Tetrazines (2)
Tetrazines have the potential to be mono, di, tri and
tetra-dentate, although by comparison with similar co-
ordination
complexes
of
3,6-bis(2-pyrimidyl)-
[1,2,4,5]tetrazine the formation of either tri- or tetra-
nuclear compounds is highly unlikely [23]. While the
mesomorphic properties of 3,6-disubstituted tetrazines
have been studied [24,25] and the use of tetrazines as
1
guestꢀ
/
host dyes for use in guestꢀ
/
host liquid crystal
4336 Hz was observed which is typical of J coupling
displays has previously been described [26,27], the
mesomorphic properties of the tetrazines discussed
here does not seem to have been reported in the
literature before, we include full details here in the
supporting information.
trans to nitrogen. Attempts at double cycloplatination
using two equivalents potassium tetrachloro platinate
proved unsuccessful with only the monometallated
species being formed.
The reason why these tetrazines will not readily
undergo a second cyclometallation is, perhaps at first,
not clear. The fact that pyridazines (1) will undergo a cis
double cyclometallation rules out the possibility of it
Tetrazines (2) react with one equivalent of palladiu-
m(II) acetate in acetic acid to give the brownꢀ
/orange
mono cyclopalladated compound, (11) (Scheme 2). By

114
J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ
/120
Scheme 1.
being a totally steric effect. Some light is shed on the
situation if we consider the following points:
product due to the presence of p-backbonding, it does
hinder the formation of the di-metallated product. In
addition, the central tetrazine ring is very electron
withdrawing, this results in a lowering of the electron
density in the phenyl rings in the 2 and 6 positions
making them less susceptible to electrophilic attack and
therefore cyclometallation. With the less active chloride
based palladation source K2PdCl4, tetrazines were
observed to undergo a second cyclometallation, but
with unimpressive yields. Presumably this is similar to
the effect we observed with pyridazines, where a chloride
based platinum source resulted in high yields of a
doubly metalled compound [18]. This would also imply
The first cyclometallation of the tetrazines takes place
as with the pyridazines, but at a substantially lower rate.
This reduction in rate can be attributed to the presence
of four electronegative nitrogens in a conjugated six
member ring which results in the basicity of each
individual N donor centre being very small with
pKaB0: therefore they are poor nucleophiles. Although
/
compared to the pyridazine the tetrazine ring is a poor
s-donor it is an excellent p acceptor due to the presence
of an unusually low p*-orbital. Whilst this may result in
a more thermodynamically stable mono cyclometallated

J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ
/120
115
that the compound we produce here is the cis isomer 13
and not the trans isomer 14. The absence of any sign of
multiple cycloplatinations could just be due to the fact
that platinations are that much less easy than pallada-
tions.
elevated temperatures and other palladium sources did
not lead to any doubly cyclometallated species. The lack
of a second cyclometallation taking place must result
entirely from electronic effects and not sterics, since
there is no possibility of a sterically disfavoured cis
dimetallation taking place. Although there is not the
option for a cis dimetallation there is also no possibility
of the first metallation holding a second metal atom
close to the second co-ordination site via a bridging
acetate or chlorine, thus promoting a second cyclome-
tallation, as we proposed with the metallation of the
pyridazines (1). Once again, the first co-ordination
results in electron density being withdrawn from the
pyrazine ring, resulting in the second nitrogen being a
poorer nucleophile.
2.4. Pyrazines (3)
Pyrazines (3) are only capable of undergoing a trans
dicyclometallation; when they were reacted with one
equivalent of palladium acetate in acetic acid they
yielded the monometallated dimer (17) (Scheme 3).
These monometallated compounds were unambiguously
1
characterised by H-NMR, with the diagnostic doublet,
doublet and doublet of doublets splitting pattern for the
metallated ring and the protons of the central pyrazine
ring rendered inequivalent, thus coupling to each other.
The dimeric compound was cleaved by reaction with
sodium acetylacetonate to give compound 18, which was
characterised by 1H-NMR. Extended reaction times and
2.5. Pyrimidines (4)
Reaction of pyrimidines (4) with palladium acetate
gave unexpected results. Rather than reacting cleanly
Scheme 2.

116
J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ
/120
Scheme 3.
with one equivalent of palladium acetate to give only
one compound with an unambiguous NMR spectrum
comprising a singlet for the unmetallated ring, a pair of
doublets for the metallated pyrimidine ring and the
highly characteristic set of two doublets and a doublet of
doublet for the central ring, the NMR spectra were in
sium tetrachloro palladate and palladium acetate.
Reaction with palladium chloride sources simply yielded
the monometallated compound. This is presumably due
to the lack of solubility of the monometallated deriva-
tive hindering the formation of a second metallocycle
(this was the only case where a lack of solubility was an
issue). When the more soluble acac complex 20 was
reacted with a second equivalent of potassium tetra-
chloro palladate in acetic acid, rather than a second
metallation occurring, the chloride (19) was simply
reformed.
In an attempt to force a dimetallation reaction to take
place we investigated the possibility of carrying out
trans-cyclometallation [30] reactions with pyrimidines
(4). Two palladium sources were synthesised as potential
transmetallating agents. When the pyrimidines were
mixed with one equivalent of the metallated Schiff’s
base compound 21 no reaction took place. If, however,
the metallated N,N-dimethylbenzylamine (22) was re-
acted with the pyrimidine ligands then a small amount
of what we believe to be a dimetallated pyrimidine
compound 23 or 24 was identified by 1H-NMR (Scheme
5). It is not clear which of the two possible isomers is
formed, as the NMR spectrum for both will be very
similar. In the aromatic region the NMR spectrum will
essentially only comprise two doublets with small
1
fact far more complicated. The H-NMR showed a set
of four singlets of equal intensity equally spaced and
centred about 6.70 ppm, and five distinct sets of
multiplets situated up field of this. Reaction of this
mixture with sodium acetylacetonate, in an attempt to
isolate some of the monometalated acetylacetonate
derivatives proved unsuccessful. This line of synthesis
was not pursued any further. However, pyrimidines do
react cleanly with one equivalent of potassium tetra-
chloropalladate in acetic acid to give a yellow insoluble
species, again by analogy with our work and others this
is assumed to be the dimer (Scheme 4). Due to the
insolubility of this material, no NMR data could be
recorded. Subsequent reaction with acetylacetonate in
tetrahydrofuran yields a monomeric species (20) which
was fully characterised. Cyclometallation was clearly
shown to have occurred at only one of the four possible
positions.
Attempts at synthesising the di- tri- and tetra-metal-
lated compounds proved unsuccessful both with potas-
Scheme 4.

J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ
/120
117
Scheme 5.
coupling constants and a singlet; since the acetate
The pyridazines were the only series of ligands to
successfully undergo a double cyclometallation with
isolable compounds in high yields. It might be expected
that with pyridazines a second co-ordination would be
less favourable than the first as, following the formation
of the first s-bond between a lone pair on nitrogen and
the metal, the remaining nitrogen in the heterocycle is a
less good s-donor. In part this is simply due to the fact
that the first co-ordination reduces the electron density
in the donor ring, reducing its ability to co-ordinate, but
pyridazines have an enhanced nucleophilicity of the first
nitrogen as a result of the a-effect: two adjacent lone
pairs on the nitrogens raise the HOMO of the ligand,
thus making it a better nucleophile. With the pyrida-
zines, we believe another factor that affects the second
cyclometallation is the participation of the first metal
with a bridging ligand holding a second metal close to
the ligand thus assisting the second cyclometallation.
The extreme manifestation of this effect is that a single
cycloplatinated pyridazine derivative could not be iso-
lated, and only doubly metallated derivatives were
identified. When palladium acetate is used as the source
of palladium the bridging group will be an acetate,
resulting in a seven membered ring. Whilst this arrange-
ment might assist in the second cyclometallation, the
groups are fluxional on the NMR timescale no differ-
ence will be seen in that region. By analogy with our
previous work we might expect the cis (23) isomer to be
favoured over the trans (24).
Attempts at reacting 23/24 further with either sodium
acetylacetonate or triphenylphosphine gave untractable
products.
2.6. Comparative reactivities
All of the ligands we have used underwent a single
cyclometallation with monomeric compounds isolated
as their acetylacetonate or PPh3 derivatives. Differences
in reactivity were observed in this cyclometallation
reaction. In the case of pyridazines, a reaction of one
equivalent of ligand with palladium acetate consumed
all the ligand within 4 h. Pyrazines and pyrimidines
reacted slower with all the ligand having been consumed
within 24 h. In the case of cyclometallation of tetrazines
the rate was even slower and there was still unreacted
ligand present after reaction times of 24 h. However,
these differences were not as pronounced as those in the
reactivity when we attempted to doubly metallate the
ligand.

118
J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ120
/
steric and electronic requirements means that this
second cyclometallation is kinetically disfavoured over
ligands. Additional work with a trans-cyclometallation
reagent shows considerable promise.
another monocyclometallation on a different ligand*
/
hence we can isolate 100% monocyclopalladated mate-
rial from one equivalent of palladium acetate and
pyridazine.
4. Experimental
With pyrimidines and tetrazines there is also the
potential to form the same sort of bridge as with
pyridazines, but no second cyclometallation was ob-
served with palladium acetate. The first cyclometallation
of a pyrimidine results in an insoluble compound thus
precluding the formation of any multiply cyclometal-
lated species. The tetrazine ligands are very poor s-
donors and as such the formation of complexes is
presumably unfavourable, even with the presence of a
bridging ligand. As with the other ligands here, co-
ordination of the first metal would reduce the capability
of the remaining nitrogens in the tetrazine ring to form
s-bonds due to the reduction of electron density in the
ring, thus it is not surprising that multiple cyclometalla-
tion is not favoured.
4.1. General
All chemicals were used as supplied, unless noted
otherwise. All heterocycles were synthesised via the
published routes [20ꢀ22]. All elemental analyses were
/
performed by Warwick Analytical Service.
4.2. Preparation of palladium(acetato)-3,6-bis(4-
pentoxphenyl)[1,2,4,5]tetrazine (11)
Palladium acetate (0.025 g, 1.10ꢂ
added to a solution of 3,6-bis-(4-pentoxy-phenyl)-
[1,2,4,5]tetrazine (0.051 g, 1.10ꢂ
10ꢀ ꢃ4 mol) in acetic
acid (15 ml) at 60 8C. The solution was stirred for 16 h
and the solvent removed. The compound was washed
with hexane, dissolved in chloroform and filtered
through Celite. The chloroform was removed and the
/
10ꢀ ꢃ4 mol) was
With pyrazines the second metallation will be less
likely to take place than the first one because of the
reduced s-donor ability of the ligand after the co-
ordination of the first metal, and also because there is no
opportunity for any bridging ligands to hold a metal in
close proximity to the second nitrogen.
/
brown solid dried. Yield 0.04 g (4.02ꢂ
/
10ꢃ3 mol, 73%).
The transcyclometallation reactions outlined in
Scheme 5 show considerable promise, and allow us to
propose an adaptation to van Koten’s theory, where the
driving force for the reaction is attributed to the
differences in Pd-donor atom bond strength [30]. Our
Schiff’s base reagent 21 does not show any reactivity,
whereas the benzylamine reagent 22 does: we propose
that an additional factor that may tip the balance is the
aromaticity of the metallocycles that are formed or
destroyed. Thus, the five-membered aromatic pallado-
cycle [31] in the Schiff’s base reagent would be replaced
by another five membered aromatic palladocycle in the
product (energetically neutral) whereas the five mem-
bered non-aromatic palladocycle in the benzylamine
reagent would be replaced by five membered aromatic
palladocycle in the product (energetically favourable).
dH (400 MHz, CDCl3): 8.27 (2H, AA?XX?, unmetal-
3
lated ring), 7.55 (1H, d, J(HH)
7.00 (2H, AA?XX?, unmetallated ring), 6.40 (1H, dd,
3Jꢄ8 Hz, 4Jꢄ4 Hz, metallated ring), 6.35 (1H, d, 4Jꢄ
4 Hz, metallated ring), 4.05 (2H, t, Jꢄ
ꢄ8 Hz, metallated ring),
/
/
/
/
3
/7 Hz, OCH2),
3
3.45 (2H, t, Jꢄ
/7 Hz, OCH2), 2.35 (3H, s, OAc), 2.10
(3H, s, OAc), 1.9 (12H, m, CH2), 0.90 (6H, t, Me).
4.3. Preparation of palladium(acac)-[3,6-bis(4-
pentoxphenyl)[1,2,4,5] tetrazine] (12)
Sodium acetylacetonate (0.005 g, 4.05ꢂ
/
10ꢃ5 mol)
was added to a solution of palladium(m-acetato)-3,6-
bis(4-pentoxphenyl)[1,2,4,5]tetrazine (0.02 g, 1.84ꢂ
/
10ꢃ5 mol) in THF (25 ml). The mixture was stirred at
room temperature (r.t.) for 4 h and the solvent removed
under reduced pressure. The resulting solid was washed
with hexane (5 ml) and diethyl ether (5 ml) and dried to
3. Conclusions
The four different nitrogen containing heterocycles
that we studied here show very different reactivities
towards the cyclopalladation and cycloplatination reac-
tions. To a large extent these differences can be
attributed to the differing ground state structures of
the heterocycle, but there are additional factors at work
here. In particular we have identified further examples
of a first cyclometallated metal assisting a second
cyclometallation, through the assistance of bridging
yield an orange solid. Yield 0.015 g (2.57ꢂ
70%).
dH (400 MHz, CDCl3): 8.45 (2H, AA?XX?, unmetal-
/
10ꢃ5 mol,
3
lated ring), 7.90 (1H, d, J(HH)
4
7.20 (1H, d, J(HH)
ꢄ
/
7 Hz, metallated ring),
ꢄ
/
3 Hz, metallated ring), 7.00 (2H,
3
AA?XX?, unmetallated ring), 6.75 (1H, dd, J(HH)
ꢄ7
/
4
Hz, J(HH)
ꢄ3 Hz, metallated ring), 5.40 (1H, s, central
/
acac), 4.05 (4H, m, OCH2), 2.15 (3H, s, acac Me), 2.13
(3H, s, acac Me), 1.45 (12H, m, CH2), 0.90 (6H, m, Me).

J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ
/120
119
4.4. Preparation of di-palladium(chloro)-3,6-bis(4-
pentoxphenyl)[1,2,4,5]tetrazine (13)/(14)
4.7. Preparation of palladium(m-acetato)-2,5-bis(4-
hexyloxyphenyl)pyrazine (17)
10ꢃ4
Palladium acetate (0.05 g, 2.22ꢂ
added to a solution of 2,5-bis(4-hexyloxyphenyl)pyra-
zine (0.100 g, 2.17ꢂ
10ꢃ4 mol) in acetic acid (25 ml) and
/
10ꢃ4 mol) was
Potassium tetrachloropalladate (0.080 g, 2.46ꢂ
mol) was added to a solution of 3,6-bis-(4-pentoxy-
phenyl)-[1,2,4,5]tetrazine (0.050 g, 1.23ꢂ
10ꢃ4 mol) in
/
/
/
stirred overnight at 60 8C. The solvent was removed
under reduced pressure and the yellow residue washed
with hexane (5 ml) and diethyl ether (5 ml). Yield 0.136 g
acetic acid (15 ml) at 60 8C. The solution was stirred for
16 h and the solvent removed. The compound was
washed with water, acetone and then dissolved in
chloroform and filtered through Celite. The chloroform
was removed and the brown solid dried under vacuum.
(7.81ꢂ
/
10ꢃ5 mol, 72%).
5
dH (400 MHz, CDCl3): 8.35 (1H, d J(HH)
ꢄ
/
1.5 Hz,
1.5 Hz, central ring),
7.60 (2H, AA’XX’, unmetallated ring), 6.90 (2H,
5
Yield 0.0056 g (8.24ꢂ
dH (400 MHz, CDCl3): 8.42 (2H, d, J(HH)
metallated ring), 7.45 (2H, d, 4Jꢄ
2 Hz, metallated
ring), 7.03 (2H, dd, 3Jꢄ7 Hz, 4Jꢄ
2 Hz, metallated
ring), 4.02 (4H, t, 3Jꢄ
7 Hz, OCH2), 1.72 (4H, m, CH2),
/
10ꢃ6 mol, 7%).
central ring), 8.15 (1H, d J(HH)
ꢄ
/
3
ꢄ7 Hz,
/
3
AA?XX?, unmetallated ring), 6.80 (1H, d J(HH)
ꢄ9
/
/
4
Hz, metallated ring), 6.25 (1H, d J(HH)
ꢄ4Hz, metal-
/
/
/
3
4
lated ring), 6.05 (1H, dd J(HH)
metallated ring), 3.95 (4H, m, OCH2), 1.75 (4H, m,
CH2), 1.50 (6H, s, OAc), 1.45ꢀ1.10 (16H, m, CH2), 0.85
(6H, m, Me).
ꢄ
/
9 Hz J(HH)
ꢄ
/
4 Hz,
/
1.3 (8H, m, CH2), 0.91 (6H, t, Me).
/
4.5. Preparation of platinum(chloro)-3,6-bis(4-
pentoxphenyl)[1,2,4,5]tetrazine (15)
4.8. Preparation of [palladium(acac){2,5-bis(4-
hexyloxyphenyl)pyrazine] (18)
10ꢃ4 mol) was added a
Potassium tetrachloroplatinate (0.10 g, 2.50ꢂ
mol) was added to a solution of 3,6-bis-(4-pentoxy-
phenyl)-[1,2,4,5]tetrazine (0.10 g, 2.46ꢂ
10ꢃ4 mol) in
/
10ꢃ4
Sodium acac (0.0211 g, 7.6ꢂ
solution of palladium(m-acetato)-2,5-bis(4-hexyloxyphe-
nyl)pyrazine (17) (0.1g, 8.00ꢂ
10ꢃ5 mol) in acetone (15
/
/
/
acetic acid (25 ml). The mixture was stirred under reflux
for 48 h and the solvent removed to yield a brown solid,
which was washed with hexane (5 ml) and diethyl ether
(5 ml). The product proved insoluble in common NMR
solvents and was used in the next step without further
ml) and stirred overnight at room temperature. The
solvent was removed and the resulting yellow compound
purified by column chromatography on silica, using
CHCl3:MeOH, 90:10 as an elutant. Yield 0.086 g
(1.30ꢂ
10ꢃ4 mol, 81%).
dH (400 MHz, CDCl3): 8.85 (1H, s, central ring), 8.65
/
purification (0.052 g, 1.25ꢂ
10ꢃ4 mol, 41%).
/
(1H, s, central ring), 7.85 (2H, AA?XX?, unmetallated
3
ring), 7.30 (1H, d J(HH)
ꢄ8Hz, metallated ring), 6.95
/
4
(2H, AA?XX?, unmetallated ring), 6.95 (1H, d J(HH)
ꢄ
/
4.6. Preparation of [platinum
(triphenylphosphine)(chloro){3,6-(4-pentyloxyphenyl)
[1,2,4,5]tetrazine}] (16)
4
3Hz, metallated ring), 6.55 (1H, dd J(HH)
ꢄ3Hz
/
3J(HH)
ꢄ
/
8 Hz, metallated ring), 5.30 (1H, s, central
acac), 3.95 (4H, m, OCH2), 2.00 (3H, s, acac Me), 2.01
(3H, s, acac Me), 1.70 (2H, m, CH2), 1.3 (12H, m, CH2),
0.90 (6H, m, Me).
Triphenylphosphine (0.063 g, 2.4ꢂ
/
10ꢃ4 mol) was
added to 15 (0.041 g, 6ꢂ
/
10ꢃ5 mol) in acetone (5 ml)
and stirred for 10 min. The acetone was removed and
the residue dissolved in chloroform:methanol 99:1 and
filtered through a short column of silica. The solvent
was removed yielding a very dark yellow solid (0.035 g,
4.9. Preparation of [palladium(dichloro) {3,6-bis(4-
octyloxyphenyl)pyrimidine}] (19)
Potassium tetrachloropalladate (0.071 g, 2.16ꢂ
mol) was added to a stirred solution of 3,-6-bis(4-
octyloxyphenyl)pyrimidine (0.10 g, 2.16ꢂ
10ꢃ4 mol) in
/
10ꢃ4
6.08ꢂ
10ꢃ5 mol, 77%).
/
dH{31P} (400 MHz, CDCl3): 8.45 (2H, AA?XX?,
/
3
unmetallated ring), 8.05 (1H, d, J(HH)
ꢄ8 Hz, metal-
/
acetic acid (25 ml) at 60 8C and stirred overnight. The
solvent was removed to yield a bright yellow sparingly
lated ring), 7.60ꢀ
/
7.10 (15H, m, PPh3), 6.90 (2H,
3
soluble compound (0.083 g, 8.02ꢂ
10ꢃ5 mol, 74%).
/
AA?XX?, unmetallated ring), 6.60 (1H, dd J(HH)
ꢄ8
/
4
Hz J(HH)
4J(HH)
5 Hz J(PtH)
(2H, t, OCH2), 2.90 (2H, t, OCH2), 2.75 (4H, m, CH2),
ꢄ5 Hz, metallated ring), 6.20 (1H, d,
/
3
ꢄ
/
ꢄ61 Hz, metallated ring), 4.00
/
4.10. Preparation of [palladium(acac){3,6-bis(4-
octyloxyphenyl)pyrimidine}] (20)
1.50ꢀ1.0 (8H, m, CH2), 0.85 (3H, t, Me), 0.75 (3H, t,
/
Me). dP{1H} (161.92 MHz, CDCl3): 37.66 (s, J(PtꢀP)
4336 Hz)
ꢄ
/
1
Sodium acetylacetonate (0.013 g, 1.06ꢂ
/
10ꢃ4 mol)
was added to a solution of palladium(dichloro){3,6-

120
J.W. Slater, J.P. Rourke / Journal of Organometallic Chemistry 688 (2003) 112ꢀ120
/
bis(4-octyloxyphenyl)pyrimidine} (19) (0.05 g, 4.81ꢂ
/
References
10ꢃ5 mol) in THF (25 ml) and stirred overnight. The
solvent was removed under vacuum and the dark yellow
solid dissolved in chloroform and filtered through
Celite. The chloroform was removed under reduced
[1] A.D. Ryabov, Chem. Rev. 90 (1990) 403.
[2] P. Steenwinkel, R.A. Gossage, T. Maunula, D.M. Grove, G. v
Koten, Chem. Eur. J. 4 (1998) 763.
[3] R. Rybtchinski, A. Vigalok, Y. Ben-David, D. Milstein, J. Am.
Chem. Soc. 118 (1996) 12406.
pressure yielding (20) (0.048 g, 7.20ꢂ
4 Hz,
central ring), 8.45 (2H, s, non coordinating ring), 8.10
/
10ꢃ5 mol. 75%).
4
dH (400 MHz, CDCl3): 8.60 (1H, d J(HH)
ꢄ
/
[4] G.W.V. Cave, N.W. Alcock, J.P. Rourke, Organometallics 18
(1999) 1801.
3
(1H, dd J(HH)
4
8 Hz J(HH)
[5] G.W.V. Cave, F.P. Fanizzi, R.J. Deeth, W. Errington, J.P.
Rourke, Organometallics 19 (2000) 1355.
ꢄ
/
ꢄ4 Hz, central ring), 7.75
/
3
(1H, d J(HH)
ꢄ
/
8 Hz, central ring), 6.95 (2H, s,
coordinating ring), 5.01 (1H, s, central acac), 4.10 (2H,
[6] S. Trofimenko, J. Am. Chem. Soc. 93 (1971) 1808.
[7] S. Trofimenko, Inorg. Chem. 12 (1973) 1215.
[8] D.P. Lydon, J.P. Rourke, J. Chem. Soc. Chem. Commun. (1997)
1741.
3
t, J(HH)
3
7 Hz, OCH2), 3.75 (2H, t, J(HH)
ꢄ
/
ꢄ7 Hz,
/
OCH2), 2.25 (3H, s, acac Me), 2.20 (3H, s, acac Me),
1.85 (4H, m, CH2), 1.3 (16H, m, CH2), 0.90 (6H, m,
Me).
[9] P. Steenwinkel, S.L. James, D.M. Grove, H. Kooijman, A.L.
Spek, G. v Koten, Organometallics 16 (1997) 513.
[10] B.J. O’Keefe, P.J. Steel, Organometallics 17 (1998) 3621.
[11] C.M. Hartshorn, P.J. Steel, Organometallics 17 (1998) 3487.
[12] A. Ferna´ndez, J.J. Ferna´ndez, M. Lo´pez-Torres, A. Sua´rez, J.M.
Ortigueira, J.M. Vila, H. Adams, J. Organomet. Chem. 612 (2000)
85.
FAB MS (NBA) m/zꢄ
/
695 [Mꢁ], 595 [largest, Mꢁ ꢃ
/
acac].
[13] S. Chakladar, P. Paul, K. Venkatsubramanian, K. Nag, J. Chem.
Soc. Dalton Trans. (1991) 2669.
4.11. Preparation of [bis-palladium-bis(m-acetato){1,4-
bis-(5-alkoxy-pyrimidin-2-yl)-benzene}] (23)/(24)
[14] S. Chakladar, P. Paul, A.K. Mukherjee, S.K. Dutta, K.K. Nanda,
D. Podder, K. Nag, J. Chem. Soc. Dalton Trans. (1992) 3119.
[15] D.J.D. Geest, B.J. O’Keefe, P.J. Steel, J. Organomet. Chem. 579
(1999) 97.
Cyclometallated benzylamine (22) (0.12 g, 2.04ꢂ
10ꢃ4 mol) was added to a solution of 1,4-bis-(5-
octyloxy-pyrimidin-2-yl)-benzene (0.10 g, 2.04ꢂ
10ꢃ4
/
[16] A. Doppiu, G. Minghetti, M.A. Cinellu, S. Stoccoro, A. Zucca,
M. Manassero, Organometallics 20 (2001) 1148.
[17] C.J. Sumby, P.J. Steel, Organometallics 22 (2003) 2358.
[18] J.W. Slater, D.P. Lydon, N.W. Alcock, J.P. Rourke, Organome-
tallics 20 (2001) 4418.
/
mol) in acetic acid. The mixture was stirred overnight at
60 8C and the solvent removed. Products were not
isolated, and the NMR spectrum run directly. The
peaks listed below were present corresponding to a yield
of about 20%, other expected peaks (the octyloxy chain)
were obscured by unreacted pyrimidine.
[19] J.W. Slater, D.P. Lydon, J.P. Rourke, J. Organomet. Chem. 645
(2002) 246.
[20] J. Nakayama, T. Konishi, A. Ishii, M. Hoshino, Bull. Chem. Soc.
Jpn. 62 (1989) 2608.
dH (400 MHz, CDCl3): 8.75 (2H, s, central ring), 7.10
[21] P. Yates, O. Meresz, L.S. Weiler, Tetrahedron Lett. 35 (1968)
3929.
4
(2H, d, J(HH)
4
3Hz), 6.90 (2H, d, J(HH)
7Hz, OCH2).
ꢄ
/
ꢄ3Hz), 3.90
/
3
(4H, t, J(HH)
[22] M.A. Mikhaleva, T.A. Kizner, N.I. Cherrnova, M.V. Loseva,
G.A. Kolesnichenko, A.Z. Rabinovich, A.E. Kuznetsova, V.P.
Mamaev, Khim. Geterotsiklichskikh Soedinenii (Eng. Trans.) 12
(1989) 1649.
ꢄ
/
[23] W. Kaim, Coord. Chem. Rev. 230 (2002) 127.
[24] C.M. Hudson, M.E. Neubert, A.M. Lackner, J.D. Margerum, E.
Sherman, Liq. Cryst. 19 (1995) 871.
5. Supplementary material
[25] T. Lifka, H. Meier, J. Prakt. Chem. 337 (1995) 641.
[26] A. Isenberg, B. Krucke, G. Pelzl, H. Zasheke, D. Demus, Cryst.
Res. Technol. 18 (1983) 1059.
Full thermal behaviour of the homologous tetrazines
and a discussion of this data is available as supporting
information and is available from the author on request.
[27] G. Pelzl, H. Zaschke, D. Demus, Displays 6 (1985) 141.
[28] D.J. Ca´rdenas, A.M. Echavarren, M.C.R. d Arellano, Organo-
metallics 18 (1999) 3337.
[29] C. Navarro-Ranninger, I. Lo´pez-Solera, A. Alvarez-Valde´s, J.H.
Rodr´ıguez-Ramos, J.R. Masaguer, J.L. Garc´ıa-Ruano, X. Solans,
Organometallics 12 (1993) 4104.
Acknowledgements
[30] M. Albrecht, P. Dani, M. Lutz, A.L. Spek, G. v Koten, J. Am.
Chem. Soc. 122 (2000) 11822.
[31] A Crispini, M Ghedini, J. Chem. Soc. Dalton Trans. (1997) 75.
The authors would like to thank Johnson-Matthey for
loan of chemicals.