Ligand discrimination in the reaction of nitrones with [PtCl2(PhCN)2]. Selective formation of mono-oxadiazoline and mixed bis-oxadiazoline complexes under thermal and microwave conditions

Article abstract of DOI:10.1039/b309847h

[2+3] Cycloaddition of nitrones to the nitrile ligands in the complexes cis- or trans-[PtCl₂(PhCN)₂] occurs under ligand differentiation and allows for selective synthesis of complexes of the type cis- or trans-[PtCl₂(oxadiazoline)-(PhCN)]. Microwave irradiation enhances the reaction rates of the cycloaddition considerably and further favours the selectivity towards the mono-cycloadduct with respect to thermal conditions, because the first cycloaddition is accelerated to a higher extent than the second one. Reaction of the trans-substituted mono-oxadiazoline complexes with a nitrone different from the one used for the first cycloaddition step gives access to mixed bis-oxadiazoline compounds of the composition trans-[PtCl₂(oxadiazoline-a)(oxadiazoline-b)]. The corresponding cis-configured complexes, however, do not undergo further cycloaddition. All reactions described occur without isomerisation of the stereochemistry around the platinum center, independently of whether thermal or microwave heating is applied.

Full text of DOI:10.1039/b309847h

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Ligand discrimination in the reaction of nitrones with
[PtCl2(PhCN)2]. Selective formation of mono-oxadiazoline and
mixed bis-oxadiazoline complexes under thermal and microwave
conditions
Bimbisar Desai,a Timothy N. Danksa,b and Gabriele Wagner*a
a Chemistry Division, SBMS, University of Surrey, Guildford, Surrey, UK GU2 7XH
b The Oratory School, Woodcote, Reading, Berkshire, UK RG8 0PJ
Received 15th August 2003, Accepted 31st October 2003
First published as an Advance Article on the web 18th November 2003
[2ϩ3] Cycloaddition of nitrones to the nitrile ligands in the complexes cis- or trans-[PtCl2(PhCN)2] occurs under
ligand differentiation and allows for selective synthesis of complexes of the type cis- or trans-[PtCl2(oxadiazoline)-
(PhCN)]. Microwave irradiation enhances the reaction rates of the cycloaddition considerably and further favours
the selectivity towards the mono-cycloadduct with respect to thermal conditions, because the first cycloaddition is
accelerated to a higher extent than the second one. Reaction of the trans-substituted mono-oxadiazoline complexes
with a nitrone different from the one used for the first cycloaddition step gives access to mixed bis-oxadiazoline
compounds of the composition trans-[PtCl2(oxadiazoline-a)(oxadiazoline-b)]. The corresponding cis-configured
complexes, however, do not undergo further cycloaddition. All reactions described occur without isomerisation of
the stereochemistry around the platinum center, independently of whether thermal or microwave heating is applied.
atised or displaced. Such complexes might offer interesting
Introduction
opportunities for bonding to biologically relevant molecules or
substrates in catalysis. Their synthesis might be achieved
straightforwardly by cycloaddition of a nitrone to only one of
the two equivalent nitriles in complexes of the type [PtCl2-
(nitrile)2] or [PtCl4(nitrile)2], if the ligands were sufficiently dif-
ferent in reactivity. In the cycloaddition of nitrones to trans-
[PtCl2(cinnamonitrile)2], we did indeed observe for the first time
that such discrimination takes place in the microwave-assisted
reaction, although the reaction under thermal conditions was
unselective.12 The ability of the nitrone to differentiate between
the two nitrile ligands was argued as either a microwave specific
effect, favouring the first cycloaddition to a higher extent due to
its more polar transition state, or a faster dissolution of the
poorly soluble starting material under microwave irradiation
and superheating of the solvent, leading to a homogenous reac-
tion medium and stoichiometric reaction conditions. However,
up to this stage, we were not able to distinguish between these
two hypotheses. It was also unclear how generally applicable
this mono-cycloaddition route would be if nitriles different
from cinnamonitrile were to be used.
Platinum compounds are used for medical purposes,1 besides
other applications as photochemical devices,2 in molecular
architecture,3 or other fields of chemistry such as catalysis or
metal-mediated organic synthesis.4 Platinum complexes are
typically prepared by ligand exchange, making use of the trans-
effect5 to overcome selectivity issues. Thus, the cancer drug
cis-[PtCl2(NH3)2] is prepared from tetraiodoplatinate rather
than tetrachloroplatinate to selectively introduce two ammonia
ligands in cis-position to each other. Subsequent exchange of
the iodo ligands in cis-[PtI2(NH3)2] requires the presence of a
silver salt to produce the intermediate aquo complex, from
which the aquo ligands are displaced by chloride.6 Compared to
the frequent application of ligand exchange reactions, surpris-
ingly little use is made of selective ligand modification in the
coordination sphere of the metal.7 In particular, reactions lead-
ing to a differentiation between initially equivalent ligands are
quite neglected up to now, although such effects are known to
occur, as for example in the selective nucleophilic addition of
hydroxide to Pt() bis-nitrile complexes to give mixed amide
nitrile complexes.8
Previously, we showed that nitriles coordinated to Pt() cen-
ters undergo [2ϩ3] cycloaddition with aromatic and aliphatic
nitrones under very mild conditions, and with this, the first
∆4-1,2,4-oxadiazoline complexes were prepared.9 The nitriles in
the corresponding Pt() complexes exhibit a slightly lower
reactivity, indicating that the Lewis acidity of the metal plays an
important role.10 When the cycloaddition is performed with
nitriles bound to suitable chiral platinum compounds, the reac-
tion is diastereoselective. Release of the ∆4-1,2,4-oxadiazoline
ligand from the resulting complexes allowed for the first
enantioselective synthesis of this class of heterocycles.11 Up to
now, the platinum compounds were exhaustively reacted with
nitrones, i.e. both ligands in bis-nitrile complexes of the type
[PtClx(RCN)2] (x = 2, 4) were transformed into the oxadiazoline
simultaneously. The resulting products are chemically stable
and kinetically quite inert, and therefore, their suitability for
medicinal purposes or any chemical application remains
doubtful.
Results and discussion
In the present study, we addressed these questions by using cis-
and trans-[PtCl2(PhCN)2] as starting materials for reactions
with the nitrones PhCH᎐N(Me)O (2a) and p-MeC H CH᎐
᎐
᎐
6
4
N(Me)O (2b). The trans-isomer is well soluble in unpolar sol-
vents such as CHCl3 and thus allows for homogenous reactions
and favourable conditions for a 1 : 1 stoichiometric reaction.
Once the first cycloaddition has taken place, the second nitrile
ligand still present in the molecule is expected to be different in
reactivity because of the electronic interaction with the oxa-
diazoline in trans-position. Steric effects are not expected to
play an important role because the ligands in question are at a
maximum distance from each other. On the other hand, the cis-
complex is less soluble in chloroform, although its reactivity
can be assumed to be similar to the trans-compound. With
ligands in a cis-arrangement, the second cycloaddition is not
much influenced electronically by the first oxadiazoline ligand,
but the approach of the nitrone might now suffer from steric
repulsion. A comparision of the selectivity of these reactions
with respect to mono-cycloaddition might thus allow a rational-
In the present work, we concentrated on the development of
new routes for the synthesis of platinum complexes bearing
only one oxadiazoline and another potentially labile and
reactive ligand, e.g. a nitrile, which can be further deriv-
D a l t o n T r a n s . , 2 0 0 4 , 1 6 6 – 1 7 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
166

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Scheme 1 a: R1, R2 =Ph, b: R1, R2 =p-MeC6H4.
isation of conditions under which a discrimination between two
cycloaddition is used (e.g. reaction of 3a ϩ 2b or 3b ϩ 2a), the
equivalent ligands can take place efficiently.
mixed unsymmetric bis-oxadiazoline complex 4ab is obtained.
This second cycloaddition step was again completed at room
temperature overnight, indicating that the difference in reactiv-
ity for the two cycloadditions is not drastically high, although
sufficient to achieve a high selectivity.
Reactions of trans-[PtCl2(PhCN)2]
The starting trans-[PtCl2(PhCN)2] complex was prepared from
PtCl2 and benzonitrile using Kharasch’s method and purified
by chromatography prior to use.13,14 Cycloadditions were
carried out on a 0.1 mmol scale with equimolar amounts of
trans-[PtCl2(PhCN)2] 1 and nitrone 2a or 2b, in CHCl3 (1 ml) as
a solvent. The reaction mixtures were left at room temperature
overnight. NMR of the crude mixture showed the presence of a
new compound 3a or 3b, together with a small amount (2–4%)
of the known diastereomeric bis-oxadiazoline complexes 4aa or
4bb as the only bypoducts. The new products 3a and 3b were
isolated by chromatography and obtained as pale yellow solids
in 68 and 72% yield. Characterisation of the products by ele-
mental analysis, mass spectrometry, IR, 1H, 13C and 195Pt NMR
spectroscopy showed that the compounds contained both an
oxadiazoline and a nitrile ligand, selective mono-cycloaddition
had thus taken place. The FAB-MS spectra display a character-
The product was isolated as a pale yellow powder in 76%
1
yield. All signals in IR, H, 13C and 195Pt NMR are in their
expected positions and highly similar to the ones obtained for
the symmetric complexes 4aa and 4bb.10 NMR data also show
that the complex is formed as a 1 : 1 diastereomeric mixture, as
expected. The aromatic groups of the oxadiazoline ligand facili-
tate the detection of these diastereoisomers because of their
magnetic interaction with the ligand in trans-position on the
metal. Still, their steric influence is not strong enough to induce
any stereoselection in the cycloaddition to the trans-positioned
ligand.
Under microwave irradiation in a sealed tube at a set temper-
ature of 60 ЊC both cycloaddition steps were accelerated con-
siderably. The first cycloaddition was complete within 20 min,
providing similar yields and selectivities of the mono-cyclo-
adducts 3a/b as observed in the thermal reaction. Also the
second cycloaddition was accelerated significantly and brought
to completeness within 2.5 h. This is in agreement with results
found by other groups and also by us that many cycloadditions,
purely organic15,16 and Lewis acid catalysed17 or mediated
ones,12 benefit from microwave heating with respect to reaction
rates.
istic fragmentation pattern of loss of two Cl atoms and a PhCN
᎐
ligand. In the IR spectra, both C᎐N and C᎐N stretching bands
᎐
᎐
᎐
are present. The C᎐N vibration appears in the same range of
᎐
wavenumbers as observed for the starting material 1 (2286 cmϪ1
for 3a, 2283 cmϪ1 for 3b, vs. 2285 cmϪ1 for 1). The C᎐N
᎐
vibrations are quite comparable with those of the bis-oxadiazo-
line complexes (1631 cmϪ1 for 3a, 1630 cmϪ1 for 3b, vs. 1645 and
1623 cmϪ1 for 4aa, 1641 and 1627 cmϪ1 for 4bb).
1H and 13C NMR spectra show the expected signals of the
oxadiazoline and benzonitrile ligands. Compared to the bis-
Reactions of cis-[PtCl2(PhCN)2]
oxadiazoline complexes 4aa and 4bb, the C᎐NPh ortho protons
Pure cis-[PtCl2(PhCN)2] was obtained from reaction of benzo-
nitrile with aqueous tetrachloroplatinate18,14 and subsequent
chromatographic purification of the product. The cyclo-
addition reactions were performed as described in the
previous section for the corresponding trans-complex. How-
ever, due to the lower solubility of the cis-isomer, the reaction
mixture is now a suspension with an unfavourable stoichio-
metry of the reagents present in the solution. Moreover, the
oxadiazoline formed in the first step is now in cis-position
to the remaining benzonitrile ligand. The two ligands are
thus not able to communicate with each other electronically,
and conversion of one nitrile into an oxadiazoline is not
expected to change the reactivity of the second nitrile. If the
reaction still distinguishes between the two nitriles, steric
reasons should be made responsible for the result rather than
electronic ones.
᎐
are at significantly higher chemical shift, thus influenced by the
Pt center more strongly. Also the C᎐N in 13C NMR appears at
᎐
higher chemical shift, reflecting the higher Lewis acidity of the
Pt center. The signals of the benzonitrile ligand are practically
unchanged, as compared to the starting material. The 195Pt
NMR resonance appears at Ϫ2236 to Ϫ2237 ppm, in a similar
position as in the corresponding mono-oxadiazoline complex
derived from cinnamonitrile,12 and in between the resonances
of the starting material trans-[PtCl2(PhCN)2] (Ϫ2350 ppm) and
the trans-configured bis-oxadiazoline compounds (Ϫ2104 to
Ϫ2121 ppm).
The mono-oxadiazoline complexes 3a and 3b can be con-
verted into the known symmetric bis-oxadiazoline complexes
4aa and 4bb by reaction with a second equivalent of nitrone 2a
or 2b. If a nitrone different from the one applied in the first
D a l t o n T r a n s . , 2 0 0 4 , 1 6 6 – 1 7 1
167

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Scheme 2 a: R1, R2 =Ph, b: R1, R2 =p-MeC6H4.
The experiment showed that the cis-configured mono-
cycloadduct formed selectively, in spite of the unfavourable
conditions. The products 6a and 6b were isolated by crystallis-
ation from CHCl3 and diethyl ether. Mass spectra and ele-
(a) Thermal reaction of trans-[PtCl2(PhCN)2]. For the
thermal reaction, a solution of 0.1 mmol of the platinum com-
plex 1 and 0.1 mmol of the nitrone 2a in 1 ml CDCl3 was placed
into an NMR tube and the reaction was followed at 40 ЊC by
1H NMR spectroscopy. The composition of the mixture was
evaluated from the integrals of the signals of the ortho-protons
mental analysis are almost identical to those found for the
᎐
trans-complexes 3a/b. In the IR spectra, both C᎐N and C᎐N
᎐
᎐
stretching vibrations appear at slightly lower wavenumbers,
compared to the analogous signals of the trans-compounds.
In the 1H NMR spectra, the oxadiazoline N–CH–N and the
of the N᎐CPh group of the cycloaddition products 3a and 4aa,
᎐
and the phenyl group of the nitrone 2a. These signals were
selected because they were well separated from each other
and not affected by overlap by other signals. The starting
[PtCl2(PhCN)2] complex could not be monitored since no
undisturbed signals were present.
ortho-protons of the N᎐CPh moiety appear at higher chemical
᎐
shift than in the corresponding trans-compound. In contrast,
the signals of the meta- and para-hydrogens of the CHPh group
in 6a emerge at lower chemical shifts (7.31 and 7.36 ppm, as
compared to 7.45 ppm in 3a). Accordingly in 6b, the methyl
group and the ortho-protons of the p-MeC6H4CH moiety are
shifted similarly (2.19 and 7.14 ppm, as compared to 2.38 and
7.27 ppm in 3b). An exceptionally low chemical shift is also
found for the ortho-protons of the coordinated benzonitrile. In
the cis-complexes, these protons appear at 7.17 ppm whereas
the corresponding protons in the trans-complexes resonate at
Fig. 1 shows the progress of the reaction with time. The
nitrone is consumed at the same rate that the mono-cyclo-
addition product is formed. After about 400 min the ratio
2a : 3a was practically constant over a period of 1 h, indicating
that the reaction was complete. In the final mixture, 80% of the
mono-cycloaddition product 3a was present, and only 2.7% of
the bis-cycloadduct 4aa was formed. Thus, the thermal reaction
is highly selective. The sample was further used to study the rate
of the second cycloaddition of 2a to the mono-cycloaddition
product 3a to give 4aa. Another equivalent of the nitrone was
added, and the reaction was again followed by 1H NMR. Fig. 1
shows that now the nitrone and the mono-cycloadduct are
consumed simultaneously, and the bis-cycloadduct is formed.
However, the reaction is slower than in the first cycloaddition
step. In terms of half-life times, formation of 50% of the mono-
cycloaddition product requires approximately 75 min, whereas
formation of 50% of the bis-cycloadduct takes approximately
450 min. The half life times thus differ by a factor of about 6.
7.68 ppm. In the 13C NMR spectra, both C᎐N and C᎐N are at
᎐
᎐
᎐
lower chemical shift than in the analogous trans-complex.
As expected, the 195Pt signal of the cis-complexes 6 is shifted
downfield with respect to the trans-complexes 3, as observed for
many other cis/trans-isomeric Pt() compounds.19 However, the
effect is exceptionally small (∆δ ≈ 10 ppm). Compared to the
195Pt chemical shift of the starting material cis-[PtCl2(PhCN)2]
of Ϫ2288 ppm, the resonance of the mono-oxadiazoline com-
plex appears at higher ppm values (Ϫ2224 to Ϫ2228 ppm).
The cis-configured mono-oxadiazoline complex does not
react to the bis-oxadiazoline complex upon addition of a
second equivalent of a nitrone, when similar conditions as in
the synthesis of the corresponding trans-complexes are applied.
We attribute this to steric hindrance rather than to electronic
effects. Prolonged heating of the reaction mixture (60 ЊC, up to
6 days) leads to decomposition of 6a/b to form a number of
uncharacterised products. Isomerisation of cis-6a/b to trans-
3a/b seems to play a minor role under the reaction conditions
applied, since no formation of 3a/b or the known trans-
configured bis-oxadiazoline complexes 4 was detected.
Kinetic studies of the thermal and microwave-assisted reactions
In our previous study, the accelerating effect of microwave
irradiation on the cycloaddition of nitrones to platinum-co-
ordinated nitriles was observed but only discussed qualitatively
because of solubility problems encountered with the cinnamo-
nitrile complex used as starting material.12 The reactions
of the benzonitrile complexes described in the present work do
not suffer from this problem to such an extent, and a more
detailed investigation of the progress of the reaction, under
thermal and microwave conditions, was thus possible. Still, rate
constants on physical grounds were not determined because
experimental error (e.g. in the temperature measurement in
microwave experiments using an IR sensor) and differences in
the reaction conditions (e.g. no possibility for online monitor-
ing in the microwave reaction) would not allow numerically
meaningful results to be produced.
Fig. 1 Reaction of trans-[PtCl2(PhCN)2] 1 and 2a under thermal
conditions.
(b) Thermal reaction of cis-[PtCl2(PhCN)2]. The complex
cis-[PtCl2(PhCN)2] converts selectively into the corresponding
cis-configured mono-cycloaddition product 6a upon reaction
with one equivalent of the nitrone 2a, as shown in Fig. 2. Under
the same experimental conditions as described for the analo-
gous reaction of the trans-complex, approximately 300 min are
required to reach 50% conversion. The reaction is thus a factor
of 4 slower than in the case of the trans-complex. Taking into
D a l t o n T r a n s . , 2 0 0 4 , 1 6 6 – 1 7 1
168

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respect to thermal conditions (see Fig. 1). The mono-cycloaddi-
tion achieves a 50% conversion after 3 min already, the reaction
is thus a factor 25 faster than the thermal reaction. Subsequent
production of 50% of the bis-cycloadduct requires approxi-
mately 65 min under microwave heating, the second cycloaddi-
tion step is thus accelerated by only a factor of 7 relative to the
thermal reaction. Comparing these values and the reaction
profiles of the thermal and microwave-assisted reaction, it
becomes evident that the difference in reaction rates of first and
second cycloaddition is much more pronounced in microwave
conditions. This supports our initial assumption that micro-
wave irradiation has a much stronger effect on the first step of
the reaction.
It is commonly accepted that reactions occuring via more
polar transition states are more susceptible to microwaves.20
Our previous quantum chemical calculations showed this might
indeed apply to the reactions described here. It was found that
coordination of the nitrile to a Lewis acid changes the reaction
mechanism from a concerted one involving an unpolar transi-
tion state to a two-step reaction, in which transition states and
the intermediate are zwitterionic, highly polar species.21 Thus,
the stronger the Lewis acid, the more polar the transition states
would be. In the initial bis-nitrile complex, all ligands are elec-
tron withdrawing and with this, the platinum center is expected
to be more Lewis-acidic than in the mono-oxadiazoline com-
plex, in which the oxadiazoline ligand rather acts as an electron
donating ligand.
Fig. 2 Reaction of cis-[PtCl2(PhCN)2] 5 and 2a under thermal
conditions.
account that the starting material is not completely dissolved at
the beginning of the reaction, the difference in reaction rate
may be partially attributed to concentration effects rather than
a reduced reactivity of the coordinated nitrile.
Formation of the mono-cycloadduct 6a passes a maximum
yield of about 52% after 380 min. At more prolonged reaction
times, degradation of 6a to a number of uncharacterised com-
1
pounds becomes noticeable. From the H NMR spectra, no
evidence was found for an isomerisation of cis-6a to trans-3a.
Addition of a second equivalent of the nitrone 2a and con-
tinuation of the reaction produces an increasing amount of
unidentified products. No signals at the expected positions
Concluding remarks
The results of this work indicate that ligand differentiation
might have a much wider application range in the selective
synthesis of coordination compounds than initially assumed,
and both electronic and steric effects can be used to induce a
selective reaction. Thus, the nitrile ligands in the soluble
complex trans-[PtCl2(PhCN)2] are different in reactivity
towards cycloaddition with nitrones, under both thermal
and microwave conditions. This can be explained by a
higher Lewis acidity of the metal center in the starting bis-
nitrile complex with respect to the mono-oxadiazoline complex.
A rather subtle differentiation between the initially equivalent
ligands is thus possible, on the grounds of electronic com-
munication of the ligands in trans-position to each other.
Cycloaddition of nitrones with cis-[PtCl2(PhCN)2] equally
occurs under ligand discrimination, to produce the mono-
cycloadduct as the sole product. The selectivity of this
reaction can be attributed to steric effects rather than electronic
ones.
Microwave irradiation generally enhances the reaction rates
of the cycloadditions and also renders the reaction with the
symmetric transition metal complex trans-[PtCl2(nitrile)2] sig-
nificantly more selective, because the first cycloaddition appears
to be accelerated to a higher extent than the second one.
Microwave heating is therefore advantageous for the prepar-
ation of the unsymetrically substituted complexes in cases
where ligand differentiation under thermal conditions is not
much pronounced.
1
for a bis-oxadiazoline complex were detected in the H NMR
spectra.
(c) Microwave reaction of trans-[PtCl2(PhCN)2]. The
microwave-assisted reaction was performed in CH2Cl2 because
of its higher microwave-susceptibility as compared to CHCl3,
under otherwise similar conditions. Pulsed microwave irradi-
ation of 100 W was applied over a period of 1 min, giving rise
to an increase of temperature from room temperature to 55 ЊC.
Every minute, a 25 µl sample was taken from the reaction mix-
ture, the solvent was evaporated in a stream of nitrogen, and
the residual oil was analysed immediately by 1H NMR in
CDCl3 solution. Meanwhile, the bulk sample was stored on ice
to prevent thermal reaction. Once the formation of the mono-
cycloadduct was found complete, a second equivalent of the
nitrone was added and microwave irradiation at a set temper-
ature of 60 ЊC was continued. Every 10 min, a sample was taken
and analysed by 1H NMR as described above.
The progress of the reaction with time is shown in Fig. 3.
Both cycloaddition steps were significantly accelerated with
The complexes [PtCl2(oxadiazoline)(PhCN)] obtained in this
study can be used for further chemistry due to the presence of
one labile and reactive nitrile ligand. Thus, reaction with a
nitrone different from the one used in the first cycloaddition
gives easy access to complexes bearing two different oxadiazo-
lines. Other promising applications, as for example in medicinal
chemistry, can be envisaged since the labile nitrile ligand present
in the molecule might allow the platinum center to interact with
biological environments. Additionally, such complexes are suit-
able for use in combinatorial chemistry to generate platinum
libraries, or for anchoring platinum compounds to biologically
relevant carrier molecules. Further studies in these directions
are ongoing in our group.
Fig. 3 Reaction of trans-[PtCl2(PhCN)2] 1 and 2a under microwave
irradiation.
D a l t o n T r a n s . , 2 0 0 4 , 1 6 6 – 1 7 1
169

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Analytical data for cis-6a: Yield is 43%. Anal. Calcd for
C22H19Cl2N3OPt: C, 43.50; H, 3.15; N, 6.92. Found: C, 43.22; H,
3.01; N, 6.88. FABϩ-MS, m/z: 607 [M]ϩ, 571 [M – HCl]ϩ, 535
[M – 2 HCl]ϩ, 506 [M – PhCN]ϩ, 432 [M – 2 HCl – PhCN]ϩ. IR
Experimental section
Materials and instrumentation
Solvents were obtained from commercial sources and used as
spectrum (selected bands), cmϪ1: 2280 w ν(C᎐N), 1625 s ν(C᎐N).
᎐
received. trans-[PtCl2(PhCN)2],13,14 cis-[PtCl2(PhCN)2]14, and
18
᎐
᎐
1H NMR (500 MHz, CDCl3) δ(ppm) 3.08 (s, 3H, NMe), 6.13 (s,
1H, N–CH–N), 7.31 (t, 7.5 Hz, 1H), 7.36 (m, 2H) and 7.78 (d, 7.6
Hz, 2H)(CH–Ph), 7.60 (t, 7.6 Hz, 2H), 7.71 (t, 7.8Hz, 1H) and 9.07
nitrones 2a and 2b22 were prepared according to published
methods. Microwave experiments were performed in a Smith
Creator microwave reactor (Personal Chemistry, Uppsala) in
0.5 to 2 ml septum sealed pyrex glass tubes. An IR detector was
used for the measurement of the reaction temperature. C, H
and N elemental analyses were carried out on a Leeman CE 440
automatic analyser. Infrared spectra (4000–400 cmϪ1) were
recorded on Perkin Elmer 2000 FTIR and Nicolet Avatar 320
FT-IR spectrometers in KBr pellets. Positive FAB-MS spectra
of the samples in 3-nitrobenzyl alcohol (NBA) matrices were
(d, 7.9 Hz, 2H)(N᎐C–Ph), 7.16 (d, 8.2 Hz, 2H), 7.40 (t, 7.8 Hz,
᎐
2H) and 7.63 (m, 1H)(N᎐CPh). 13C NMR (125.7 MHz, CDCl3)
᎐
᎐
᎐
δ(ppm) 46.9 (NMe), 94.3 (N–CH–N), 109.3 (C , N᎐CPh), 114.3
᎐
q
᎐
(N᎐C), 121.8 (C , N᎐CPh), 127.8 (CH, CHPh), 128.9 (CH,
᎐
᎐
᎐
᎐
q
᎐
CHPh), 129.0 (CH, N᎐CPh), 129.1 (CH, N᎐CPh), 129.6 (CH,
CHPh), 130.0 (CH, N᎐CPh), 133.0 (CH, N᎐CPh), 134.4 (CH,
᎐
᎐
᎐
᎐
N᎐CPh), 134.7 (CH, N᎐CPh), 136.7 (C , CHPh), 163.6 (C᎐N).
᎐
᎐
᎐
q
195Pt NMR (107.3 MHz, CDCl3) δ(ppm) Ϫ2228 (650 Hz).
obtained on a Thermoquest MAT 95XL instrument. H, 13C
1
and 195Pt NMR experiments were acquired on a Bruker DRX
Analytical data for cis-6b: Yield is 48%. Anal. Calcd for
C23H21Cl2N3OPt: C, 44.45; H, 3.41; N, 6.76. Found: C, 44.15;
H, 3.37; N, 6.93. FABϩ-MS, m/z: 585 [M – Cl]ϩ, 549 [M – 2
HCl]ϩ, 520 [M – PhCN]ϩ, 446 [M – 2 HCl – PhCN]ϩ. IR spec-
500 spectrometer at ambient temperature. Signals in 1H and 13
C
were assigned with the help of COSY, NOESY and HMQC
spectra. 195Pt chemical shifts are given relative to aqueous
K2[PtCl4] = Ϫ1630 ppm, with half height line widths in
parentheses.
trum (selected bands), cmϪ1: 2276 w ν(C᎐N), 1627 s ν(C᎐N and
᎐
᎐
᎐
1
C᎐C). H NMR (500 MHz, CDCl ) δ(ppm) 2.19 (s, 3H, C H
᎐
3
6
4
Me), 3.07 (s, 3H, NMe), 6.08 (s, 1H, N–CH–N), 7.13 (d, 7.8 Hz,
2H) and 7.61 (d, 7.8 Hz, 2H)(C6 H4Me), 7.60 (m, 2H), 7.72 (t,
Preparation of complexes cis- and trans-[PtCl2(PhCN)-
(oxadiazoline)]
7.5 Hz, 1H) and 9.09 (d, 7.6 Hz, 2H)(N᎐C–Ph), 7.17 (d, 7.4 Hz,
᎐
2H), 7.40 (t, 7.9 Hz, 2H) and 7.61 (m, 1H)(N᎐CPh). 13C NMR
᎐
᎐
A solution of trans-[PtCl2(PhCN)2] (42 mg, 0.1 mmol) and
nitrone 2a or 2b (0.1 mmol) in CH2Cl2 (1 ml) was left at room
temperature overnight. The reaction mixture was filtered
through a plug of silica gel. Evaporation of the solvent pro-
vided the products 3a or 3b as pale yellow solids.
(125.7 MHz, CDCl3) δ(ppm) 21.1 (C6H4 Me), 46.9 (NMe), 94.2
᎐
᎐
(N–CH–N), 109.3 (C , N᎐CPh), 114.2 (C᎐N), 121.8 (C , N᎐
᎐
᎐
᎐
q
q
CPh), 127.8 (CH, C H Me), 129.0 (CH, N᎐CPh), 129.1 (CH,
᎐
6
4
᎐
N᎐CPh), 129.6 (CH, C H Me), 129.9 (CH, N᎐CPh), 133.0
᎐
᎐
᎐
6
4
᎐
᎐
(CH, N᎐CPh), 133.9 (C , C H Me), 134.4 (CH, N᎐CPh), 134.6
᎐
q
6
4
Analytical data for trans-3a: Yield is 68%. Anal. Calcd
for C22H19Cl2N3OPt: C, 43.50; H, 3.15; N, 6.92. Found: C,
43.71; H, 3.30; N, 6.93. FABϩ-MS, m/z: 571 [M – HCl]ϩ, 535
[M – 2 HCl]ϩ, 506 [M – PhCN]ϩ, 432 [M – 2 HCl – PhCN]ϩ.
(CH, N᎐CPh), 139.2 (C , C H Me), 163.5 (C᎐N). 195Pt NMR
᎐ ᎐
q 6 4
(107.3 MHz, CDCl3) δ(ppm) Ϫ2224 (650 Hz).
Preparation of complexes trans-[PtCl2(oxadiazoline-a)-
(oxadiazoline-b)]
IR spectrum (selected bands), cmϪ1: 2286 w ν(C᎐N), 1631 s
᎐
᎐
ν(C᎐N). 1H NMR (500 MHz, CDCl3) δ(ppm) 3.06 (s, 3H,
᎐
A solution of the mono-oxadiazoline complex 3a or 3b (0.1
mmol) and nitrone 2a or 2b (0.1 mmol) in CH2Cl2 (1 ml) was left
at room temperature overnight. The reaction mixture was filtered
through a plug of silica gel. The products 4aa, 4ab or 4bb were
obtained as pale yellow solids after evaporation of the solvent.
Analytical data for 4aa and 4bb correspond to those given in
the literature.10
NMe), 6.00 (s, 1H, N–CH–N), 7.45 (m, 3H) and 7.73 (d, 7.4 Hz,
2H)(CH–Ph), 7.60 (t, 7.8 Hz, 2H), 7.66 (m, 1H) and 9.00 (d, 8.0
Hz, 2H)(N᎐C–Ph), 7.49 (t, 7.8 Hz, 2H) and 7.68 (m,
᎐
3H)(N᎐CPh). 13C NMR (125.7 MHz, CDCl3) δ(ppm) 46.1
᎐
᎐
᎐
᎐
(NMe), 94.5 (N–CH–N), 109.7 (C , N᎐CPh), 116.5 (N᎐C),
᎐
᎐
q
122.5 (C , N᎐CPh), 128.6 (CH, N᎐CPh), 128.6 (CH, CHPh),
᎐
᎐
q
᎐
128.7 (CH, CHPh), 129.2 (CH, N᎐CPh), 129.8 (CH, CHPh),
᎐
Analytical data for 4ab: two diastereoisomers in a ratio 1 : 1.
Yield is 76%. Anal. Calcd for C31H30Cl2N4O2Pt: C, 49.21; H,
4.00; N, 7.41. Found: C, 48.65; H, 3.90; N, 7.15. FABϩ-MS, m/z:
᎐
᎐
130.6 (CH, N᎐CPh), 133.4 (CH, N᎐CPh), 134.2 (CH, N᎐CPh),
᎐
᎐
᎐
134.7 (CH, N᎐CPh), 135.5 (C , CHPh), 164.6 (C᎐N). 195Pt
᎐
᎐
q
NMR (107.3 MHz, CDCl3) δ(ppm) Ϫ2237 (750 Hz).
756 [M]ϩ, 684 [M – 2 HCl]ϩ, 549 [M – 2 HCl – PhCH᎐
᎐
Analytical data for trans-3b: Yield is 72%. Anal. Calcd
for C23H21Cl2N3OPt: C, 44.45; H, 3.41; N, 6.76. Found: C,
45.27; H, 3.44; N, 6.66. FABϩ-MS, m/z: 585 [M – Cl]ϩ, 549
[M – 2 HCl]ϩ, 520 [M – PhCN]ϩ, 482 [M – Cl – PhCN]ϩ, 446
N(Me)O]ϩ, 535 [M – 2 HCl – PhCH᎐N(Me)O]ϩ. IR spectrum
᎐
(selected bands), cmϪ1: 1641 and 1625 s ν(C᎐N). 1H NMR (500
᎐
MHz, CDCl3) δ(ppm) 2.40 and 2.43 (s, 3H each, C6H4 Me), 2.94
(s, 9H) and 2.96 (s, 3H)(NMe), 5.83 (s, br., 1H), 5.87 (s, br., 2H)
and 5.92 (s, br., 1H)(N–CH–N), 7.27 (m, 4H), 7.46 (m, 2H) and
7.53 (m, 2H)(C6 H4Me), 7.33–7.38 (m, 8H), 7.56–7.61 (m, 4H),
[M – 2 HCl – PhCN]ϩ. IR spectrum (selected bands), cmϪ1
:
1
᎐
2283 w ν(C᎐N), 1630 s ν(C᎐N and C᎐C). H NMR (500 MHz,
᎐
᎐
᎐
CDCl3) δ(ppm) 2.38 (s, 3H, C6H4 Me), 3.04 (s, 3H, NMe), 5.95
(s, 1H, N–CH–N), 7.27 (d, 8.0 Hz, 2H) and 7.61 (d, 8.1 Hz,
2H)(C6 H4Me), 7.60 (t, 7.9 Hz, 2H), 7.66 (m, 1H) and 8.99 (d,
8.66 (m. 4H) and 8.88 (s, br., 4H)(N᎐C–Ph), 7.47 (m, 6H), 7.58
᎐
(m, 2H) and 7.61 (m, 2H)(CHPh). 13C NMR (125.7 MHz,
CDCl3) δ(ppm) 21.4 (C6H4 Me), 46.0, 46.1 and 46.3 (NMe),
7.5 Hz, 2H)(N᎐C–Ph), 7.48 (t, 7.9 Hz, 2H) and 7.68 (m,
᎐
94.7 (N–CH–N), 122.3, 122.4, 122.5 and 122.6 (C , N᎐CPh),
3H)(N᎐CPh). 13C NMR (125.7 MHz, CDCl3) δ(ppm) 21.8
᎐
q
᎐
᎐
128.2, 128.5, 128.58, 128.62 and 128.7 (2 CH C6H4Me, 4 CH
᎐
(C H Me), 46.0 (NMe), 94.6 (N–CH–N), 109.8 (C , N᎐CPh),
116.5 (C᎐N), 122.2 (C , N᎐CPh), 128.6 (CH, C H Me), 128.7
᎐
6
4
q
CHPh, 4 CH N᎐CPh), 129.2 and 129.3 (CH, C H Me), 129.6
᎐
6
4
᎐
᎐
᎐
q
6
4
and 129.7 (CH, CHPh), 130.4, 130.5, 130.6 and 130.7 (CH,
N᎐CPh), 133.0 (br., C , C H Me), 133.31, 133.36, 133.47 and
(CH, N᎐CPh), 129.1 (CH) and 129.3 (CH)(C H Me and
᎐
6
4
᎐
q
6
4
᎐
N᎐CPh), 130.4 (CH, N᎐CPh), 133.0 (C , C H Me), 133.5 (CH,
᎐
᎐
q
6
4
133.50 (CH, N᎐CPh), 136.1 (br., C , CHPh), 139.3 and 139.5
᎐
q
᎐
᎐
N᎐CPh), 134.0 (CH, N᎐CPh), 134.7 (CH, N᎐CPh), 139.8
᎐
᎐
᎐
(C , C H Me), 163.4 and 164.0 (C᎐N). 195Pt NMR (107.3 MHz,
(C , C H Me),164.3 (C᎐N). 195Pt NMR (107.3 MHz, CDCl3)
᎐
q
6
4
᎐
q
6
4
CDCl3) δ(ppm) Ϫ2111 (620 Hz) and Ϫ2121 (620 Hz).
δ(ppm) Ϫ2236 (750 Hz).
A suspension of cis-[PtCl2(PhCN)2] (42 mg, 0.1 mmol) and
nitrone 2a or 2b (0.1 mmol) in CH2Cl2 (1 ml) was left at room
temperature overnight. Evaporation of the solvent and re-
crystallisation from CHCl3/diethyl ether provided the products
cis-6a or cis-6b as pale yellow solids.
Acknowledgements
We are grateful to Personal Chemistry for the provision of a
Smith Creator microwave reactor, Johnson Matthey for a
D a l t o n T r a n s . , 2 0 0 4 , 1 6 6 – 1 7 1
170

View Article Online
10 G. Wagner, M. Haukka, J. J. R. Fraústo da Silva, A. J. L. Pombeiro
generous loan of Pt compounds, and the Department of
Chemistry of the University of Surrey for general support of
this work.
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D a l t o n T r a n s . , 2 0 0 4 , 1 6 6 – 1 7 1
171