Interplay between nitrones and (nitrile)PdII complexes: Cycloaddition vs. complexation followed by cyclopalladation and deoxygenation reactions

Article abstract of DOI:10.1002/ejic.200500124

The reaction between the nitrone P-MeC₆H₄CH=N(Me)O and trans-[PdCl₂(RCN)₂] (R = Ph, Me) in the corresponding RCN (or of the nitrone in neat RCN in the presence of PdCl₂) proceeds at 45°C (R = Ph) or reflux (R = Me) for 1 d and allows the isolation of the Δ⁴-1,2,4-oxadiazoline complexes [PdCl₂{N ^a=C(R)ON(Me)C^bH(C₆H₄Me-p)} ₂(N^a-C^b)] (R = Ph 1; R = Me 2) in ca. 50 and ca. 15 % yields, respectively. The reaction time can be drastically reduced by focused microwave irradiation of the reaction mixture. In CH₂Cl ₂ or acetone, this reaction proceeds in another direction to achieve the unstable nitrone complex [PdCl₂{ON(Me)=CH(C₆H ₄Me-P)}₂] (3), which was characterized by electrospray mass spectrometry, IR and ¹H NMR spectroscopy. Complex 3 is the first representative of (nitrone)Pd^II compounds and is the key in-termediate in at least two further reactions, i.e. cyclopalladation to give the dimeric complex [Pd₂(μ-Cl)₂{ON(Me)=C(H) C ₆H₃Me-P}₂] (5; 30% isolated yield) and deoxygenation of the nitrone to furnish the imine compound [PdCl ₂{N(Me)=CH(C₆H₄Me-P)}₂] (4). The palladium complexes 1, 2, 4 and 5 were characterized by C, H, and N analyses, FAB-MS, IR, ¹H and ¹³C{¹H} spectroscopy, while 4 and 5 additionally by X-ray crystallography. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500124

FULL PAPER
Interplay between Nitrones and (Nitrile)PdII Complexes: Cycloaddition vs.
Complexation Followed by Cyclopalladation and Deoxygenation Reactions
Nadezhda A. Bokach,[a,b] Artem A. Krokhin,[b,c] Alexey A. Nazarov,[c]
Vadim Yu. Kukushkin*[b] Matti Haukka,[d] João J. R. Fraústo da Silva,[a] and
Armando J. L. Pombeiro*[a]
Keywords: Palladium / Nitrone complexes / Nitrile complexes / 1,3-Dipolar cycloaddition / Cyclometalation
The reaction between the nitrone p-MeC6H4CH=N(Me)O
and trans-[PdCl2(RCN)2] (R = Ph, Me) in the corresponding
RCN (or of the nitrone in neat RCN in the presence of PdCl2)
proceeds at 45 °C (R = Ph) or reflux (R = Me) for 1 d and
allows the isolation of the Δ4-1,2,4-oxadiazoline complexes
[PdCl2{Na=C(R)ON(Me)CbH(C6H4Me-p)}2(Na–Cb)] (R = Ph 1;
R = Me 2) in ca. 50 and ca. 15% yields, respectively. The
reaction time can be drastically reduced by focused micro-
wave irradiation of the reaction mixture. In CH2Cl2 or ace-
tone, this reaction proceeds in another direction to achieve
the unstable nitrone complex [PdCl2{ON(Me)=CH(C6H4Me-
p)}2] (3), which was characterized by electrospray mass spec-
trometry, IR and 1H NMR spectroscopy. Complex 3 is the first
representative of (nitrone)PdII compounds and is the key in-
termediate in at least two further reactions, i.e. cyclopallad-
ation to give the dimeric complex [Pd2(μ-Cl)2{ON(Me)=C(H)
C6H3Me-p}2] (5; 30% isolated yield) and deoxygenation of
the
nitrone
to
furnish
the
imine
compound
[PdCl2{N(Me)=CH(C6H4Me-p)}2] (4). The palladium com-
plexes 1, 2, 4 and 5 were characterized by C, H, and N analy-
1
ses, FAB-MS, IR, H and 13C{1H} spectroscopy, while 4 and 5
additionally by X-ray crystallography.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
type (i.e., nitrile oxide,[12,13] route ii) dipoles[1,2] can be
reached by their ligation to a PtIV center, while PtII displays
little effect[10] on the reaction. Subsequent liberation of the
ligands allowed the metal-mediated synthesis of these two
classes of heterocycles.
Introduction
In organic chemistry, 1,3-dipolar cycloadditions are reac-
tions of paramount importance for the synthesis of a great
variety of heterocycles.[1–5] One of the most promising sub-
directions of research in this area includes metal control of
the interplay between various dipolarophiles and dipoles.
Indeed, metal-mediated cycloadditions have been the object
of extensive investigations in the past decade and substan-
tial progress has been achieved in both stoichiometric and
catalytic approaches.[5–8] In the overwhelming majority of
these works, alkenes have been employed as dipolarophiles,
whereas application of organonitriles for these purposes was
investigated to an incomparably lesser degree.[6,7] Recently
we found that significant activation of nitriles−even typi-
cally unreactive ones bearing an electron-donor R group in
RCN−toward cycloadditions of both allyl anion (i.e., nitro-
nes,[8–11] route i in Scheme 1) and propargyl/allenyl anion
[a] Centro de Química Estrutural, Complexo I, Instituto Superior
Técnico,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
E-mail: pombeiro@ist.utl.pt
[b] Department of Chemistry, St. Petersburg State University,
198504, Stary Petergof, Russian Federation
E-mail: kukushkin@VK2100.spb.edu
[c] Institute of Inorganic Chemistry – Bioinorganic, Environmental
and Radiochemistry,
Scheme 1.
Faculty of Chemistry, University of Vienna,
Währinger Str. 42, 1090 Vienna, Austria
[d] Department of Chemistry, University of Joensuu,
P. O. Box 111, 80101 Joensuu, Finland
As a continuation of our project on reactions of metal-
activated nitriles in general[6,14] and 1,3-cycloadditions[8–13]
in particular, we have chosen (nitrile)PdII complexes as di-
3042
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200500124
Eur. J. Inorg. Chem. 2005, 3042–3048

Interplay between Nitrones and (Nitrile)PdII Complexes
FULL PAPER
Scheme 2.
polarophiles and observed a palladium-mediated [2 + 3] cy- trile, was treated with the nitrone. In both routes, the reac-
cloaddition of a nitrone to achieve (Δ4-1,2,4-oxadiazoline)- tion proceeds at 45 °C for 1 d (or for 4 h under 20 W micro-
PdII complexes. A recent brief report[15] on the occurrence wave irradiation) and the isolated yield of the Δ4-1,2,4-ox-
of this type of reaction for the benzonitrile complex adiazoline complex [PdCl2{Na=C(Ph)ON(Me)CbH(C6H4-
[PdCl2(PhCN)2], including an indication that the analogous Me-p)}2(Na–Cb)] (1), prepared by the two methods, is al-
acetonitrile compound is unreactive, prompted us to display most the same (ca. 50%) irrespective of whether conven-
our own results in this field. In contrast to that work, we tional or microwave conditions are applied.
have observed not only that the [2 + 3] cycloaddition pro-
ceeds for [PdCl2(MeCN)2] but also that other types of reac-
tions, i.e. cyclopalladation and nitrone deoxygenation, take
place in these systems. The occurrence of the latter two pro-
cesses, which were not previously observed in the chemistry
of nitrones, explains the lack of selectivity of the cycload-
dition and provides further insight into understanding the
effect of the metal center upon the cycloaddition.
The reaction of PdCl2 with neat PhCN is well known
and it constitutes a method for the preparation of trans-
[PdCl2(PhCN)2].[16] Hence, it is not surprising that the reac-
tion rates and the isolated yields are similar for both routes,
because the first route conceivably includes the formation
of [PdCl2(PhCN)2] (upon dissolution of PdCl2 in PhCN)
followed by the cycloaddition of the nitrone. Despite the
similarity of the synthetic approaches A and B, the obvious
advantage of the former method is that it allows us to ob-
tain complex 1 starting directly from PdCl2 and omitting
the unnecessary step of the isolation of trans-[PdCl2-
(PhCN)2].
The complex 1 gave satisfactory microanalysis for the
PhCN solvate and the expected fragmentation/isotopic
pattern in FAB-MS+ ([M – Cl]+ and [M – 2 HCl]+). In the
IR spectrum, the complex exhibits a strong band at
1649 cm–1 from ν(C=N) of the heterocycle and 2231 cm–1
from ν(CϵN) of the solvate. In the 1H NMR spectrum, the
Results and Discussions
We chose to study the reaction of palladium(ii) com-
plexes trans-[PdCl2(RCN)2], bearing nitriles with acceptor
(R = Ph) and donor (R = Me) substituents, with a stable
nitrone, p-MeC6H4CH=N(Me)O, as the dipole/ligand.
Palladium(II)-Mediated 1,3-Dipolar Cycloaddition
The reaction between the nitrone p-MeC6H4CH=N(Me)O Δ4-1,2,4-oxadiazoline ring displays signals in the aromatic
and PhCN mediated by a PdII center was performed by range from 8.62 to 7.32 ppm, two Me groups appear at 2.90
two similar, although technically distinct, routes (Scheme 2, and 2.43, and the CH signal emerges at δ = 5.82 ppm. Ex-
routes A and B).
cess of Ph’s from the solvated PhCN were detected in the
In the first route (A), both PdCl2 and the nitrone p- spectrum by integration. All our data agree well with the
MeC6H4CH=N(Me)O were reacted in a suspension of neat previously reported formulation.[15] In addition, we per-
PhCN, while in the second route (B) the nitrile complex formed an X-ray analysis of 1 and it is indicated that the
trans-[PdCl2(PhCN)2],[16] dissolved in the corresponding ni- R,R- and S,S-Δ4-1,2,4-oxadiazoline ligands are in the mutu-
Eur. J. Inorg. Chem. 2005, 3042–3048
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3043

V. Yu. Kukushkin, A. J. L. Pombeiro et al.
FULL PAPER
ally trans-positions (see Figure 1 and deposited data). Un- reactions other than the cycloaddition and to understand
fortunately, the rather poor quality of crystals precluded a the lack of selectivity, indicated in the previous section, we
detailed comparison of the geometrical parameters.
studied the reaction between [PdCl2(RCN)2] complexes and
the nitrone p-MeC6H4CH=N(O)Me in dichloromethane
and acetone. Indeed, in these experiments we did not
detect complexes
1
and
2
but the complex
[PdCl2{ON(Me)=CH(C6H4Me-p)}2] (3) (Scheme 2, route
C), which originates from the substitution, was isolated. Al-
though 3 is unstable even in the solid state and undergoes
further reactions in solutions (see later) we succeeded to
characterize this product by electrospray mass spectrometry
([M – HCl]+ and [M – 2HCl]+ ions were observed). In the
IR spectrum of 3, the ν(C=N) stretching vibration is shifted
to a higher frequency (by, 14 cm–1), while the ν(N–O)
stretch is shifted to a lower frequency (by 10 cm–1) vs. those
signals in the free nitrone and these observations favour O-
coordination of the nitrone to a PdII center.
Complexes with O-bound nitrones as ligands are known
although scarce. These species can be formed (i) in the di-
rect reaction of a nitrone and a metal complex at various
metal centers such as hard (UVI,[17] ZnII,[18,19] SnIV,[20]
TiIV,[15,21] ZrIV),[15] soft (AuI,[22] AuIII [22]
,
AgI)[19] and bor-
and RuII-
derline (NiII,[23] CuII, MnII, CoII, FeII, FeIII [24,25]
,
and OsII-[26] porphyrines) Lewis acids and (ii) in a metal-
mediated process, e.g. by reaction of a carbene ligand with
nitrosoarenes at an FeII center[27] or by addition of 2-
methyl-2-nitrosopropane to coordinated ethylene[28] at a
PtII center. To the best of our knowledge, the complex 3 is
the first representative of a (nitrone)PdII compound.
Figure 1. ORTEP view of 1.
We believe that the nitrone complex 3 can be the key
Previously, it has been reported that the acetonitrile com- intermediate in at least two further reactions, which are ob-
plex [PdCl2(MeCN)2] is unreactive toward the nitrone p- served in this work, i.e. cyclopalladation to give the
MeC6H4CH=N(O)Me even under relatively harsh condi- dimeric complex [Pd2(μ-Cl)2{ON(Me)=C(H)C6H3Me-p}2]
tions (80 °C, 1 week).[15] In contrast to that report, in our 5 (Scheme 2, route D) and deoxygenation of the nitrone to
hands,
an
interplay
between
the
nitrone
p- furnish the imine compound [PdCl2{N(Me)=CH(C6H4Me-
MeC6H4CH=N(O)Me and [PdCl2(MeCN)2] (or with p)}2] 4 (Scheme 2, route E). Thus, 5 is released as crystals
PdCl2) in MeCN (Scheme 2, A and B) upon reflux for 1 d (yield is 30%) when the reaction mixture is kept in CH2Cl2
(or, 20 min at ca. 80 °C under 70 W microwave irradiation) for 3 d at room temperature (Figure 2).
results in the formation of a broad mixture of products,
The dimer 5 was characterized by X-ray crystallography.
where the complex 2, derived from the 1,3-dipolar cycload- Both palladium atoms form a square-planar arrangement
1
dition is formed in 10–15% H NMR yield [integration is with two mutually trans cyclometalated nitrones. The devia-
based on the CH(R) proton of the ring which usually tion of the metallacycle planes Pd(1)C(1)C(7)C(8)N(1)O(1)
emerges in a very specific region from 5.0 to 6.0 ppm]. The from the Pd(1)Cl(1)Pd(1A)Cl(1A) plane is 22.36°. The
complex
[PdCl2{Na=C(Me)ON(Me)CbH(C6H4Me-p)}2- bond lengths Pd–Cl (trans to O) [2.3234(6) Å] and Pd–Cl
(Na–Cb)] (2) was isolated as a solid and characterized as the (trans to C) [2.4712(6) Å] are normal and agree well with
bis-hydrate (see Exp. Sect.). Microwave irradiation of the other palladium(ii) halogen-bridged complexes.[29] The dif-
reaction mixture did not improve the selectivity of the ference in the Pd–Cl bond lengths trans to O or trans to C
cycloaddition but allowed the dramatic reduction of the are due to a strong ground state trans-influence of C-
reaction time.
bonded ligands.[30] The bond length C(8)–N(1) [1.289(3) Å]
is a double one[31] and the bond length N(1)–O(1)
[1.331(2) Å] has a value that falls in the range of typical N–
O bond lengths in N-oxides.[31]
Coordination of the Nitrone and Subsequent Reactions
In general, dissociation of RCN from (nitrile)PdII com-
The data on stability of nitrone metal complexes are
plexes in non-nitrile solutions (vs. nitrile ones), similar to quite limited in the literature and the only relevant infor-
that described for weak electrolytes by the Ostwald dilution mation so far reported is for the iron complexes [Cp(CO)2-
law, increases and consequently the amount of ligated RCN Fe{ON(Ar)=CH(ArЈ)}]+ which are light- and moisture-
decreases thus favouring the substitution via the expected sensitive and decompose to the imine ArN=CH(ArЈ) (by
dissociative mechanism. Thus, to increase the fraction of deoxygenation; the reducing agent was not verified[27]) and
3044
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 3042–3048

Interplay between Nitrones and (Nitrile)PdII Complexes
FULL PAPER
reductive systems, or TiCl3.[36] In the first four cases, these
systems are believed to involve initial activation of the N–
O group of nitrone by an electrophilic reagent followed by
the reduction. In the latter case, TiCl3 plays the role of both
activator and reducing reagent. Our palladium(ii) systems
could, in principle, reduce nitrones in a similar way insofar
as they contain an activating center, i.e. PdII (Scheme 2, C),
and
a reducing reagent, e.g. the aldehyde O=(H)-
C(C6H4Me-p), detected in the reaction mixture by NMR
method, formed upon metal-mediated hydrolysis of the
nitrone.
Final Remarks
Figure 2. Thermal ellipsoid view of complex 4 with atomic number-
ing scheme. Thermal ellipsoids are drawn with 50% probability.
Selected bond lengths [Å] and angles [°]: Pd(1)–N(1) 2.024(5),
Pd(1)–Cl(1) 2.3058(16), N(1)–C(1) 1.250(8), N(1)–C(9) 1.469(8),
C(1)–C(2) 1.473(10), N(1)–Pd(1)–Cl(1) 90.53(15), Pd(1)–N(1)–C(1)
126.7(5), Pd(1)–N(1)–C(9) 113.3(4), N(1)–C(1)–C(2) 126.8(6).
The results obtained in this work could be considered
from at least two perspectives. From the viewpoint of 1,3-
dipolar cycloaddition it is worthwhile to note that (i) even
the rather electron-deficient nitrile PhCN in the free state
does not react with the nitrone (allyl anion type dipole[2])
under the reaction conditions described above. Further-
more, a structurally similar nitrone, i.e. PhCH=N(O)Me, re-
acts with PhCN under harsh conditions (110 °C, 10 d) giv-
ing the corresponding oxadiazoline in 57% yield;[37] (ii) in
the synthetic experiments we did not observe a significant
difference in reactivities toward the cycloaddition between
the corresponding palladium(ii)- (this work) and plati-
num(ii) (ref.[9]) complexes [MCl2(PhCN)2], albeit conditions
are not strictly similar (1 d, 40 °C, in PhCN for the PdII
complex and 1 d, 20–25 °C, in CH2Cl2 for the PtII com-
plex). The opposite holds true for the cycloaddition of ni-
trile oxides (a propargyl/allenyl anion type dipole[2]) to ni-
triles in [MCl2(RCN)2] complexes where the activation of
RCN species, toward the cycloaddition, is higher at a PdII
center. A similar trend has been observed by Lippert et
al.[38] for nucleophilic addition of water to nitriles ligated to
[M(en)2]2+ moieties; the rate of the hydrolysis found for M
= PdII was substantially larger than for M = PtII. This dif-
ference in activation between the two metal centers toward
nitrile oxides and similarity toward nitrones deserves fur-
ther investigations directed to the understanding of such
relative behaviors and the development of a strategy for the
metal-mediated 1,3-dipolar cycloaddition.
to aldehyde O=CH(ArЈ) (by hydration) along with some un-
identified iron-containing products.[27] In accord with this
report, we isolated (by mechanical separation of crystals
obtained from evaporation of a CH2Cl2/Et2O solution of
the residue of the reaction mixture PdCl2–p-
MeC6H4CH=N(O)Me–PhCN) the aldimine compound
[PdCl2{N(Me)=CH(C6H4Me-p)}2] (4) (Scheme 2, route E;
Figure 3) which was characterized by X-ray diffraction
study. In complex 4, the palladium atom is in a square-
planar arrangement and two aldimine ligands are mutually
trans. The value of the C(1)–N(1) [1.250(8) Å] bond length
is in the typical double bond range[31] and the Pd–Cl
[2.3057(16) Å] bonds are normal.[31]
From the viewpoint of coordination of nitrones, we ob-
served that the more labile PdII center (vs. the kinetically
inert PtII and PtIV centers) reacts to a comparable degree
with both dipolarophile and dipole and the latter interac-
tion, i.e. ligation of the nitrone to a PdII center (observed
in this work for the first time), inhibits the 1,3-dipolar cy-
cloaddition by coordination of the oxygen atom of the di-
pole.
Figure 3. Thermal ellipsoid view of complex 5 with atomic number-
ing scheme. Thermal ellipsoids are drawn with 50% probability.
Selected bond lengths [Å] and angles [°]: Pd(1)–C(1) 1.973(4),
Pd(1)–O(1) 1.996(3), Pd(1)–Cl(1) 2.4696(12), N(1)–O(1) 1.333(4),
C(1)–Pd(1)–O(1) 92.21(15), C(1)–Pd(1)–Cl(1A) 94.65(13), O(1)–
Pd(1)–Cl(1) 87.82(8), Pd(1)–Cl(1)–Pd(1A) 94.73(4).
In general, the directing of the reaction of nitriles with
A possible rationale for the formation of 4 is deoxygen- nitrones to a cycloaddition or a complexation route de-
ation of the parent nitrone, whereupon the appropriate aldi- pends on a delicate balance between donor/acceptor prop-
mine MeN=CH(C6H4Me-p) coordinates to a PdII center. erties of substituents at both dipolarophile and dipole, acti-
Indeed, the deoxygenation of nitrones involving metal cen- vating power of the metal center toward cycloaddition and
ters is known and includes CoCl2·6H2O/Sm,[32] its substitution inertness/lability toward interaction with the
AlCl3·6H2O–THF/Zn,[33] TiCl4/NaI,[34] (CF3CO)2O/NaI[35] reactants. From this perspective the PdII center occupies an
Eur. J. Inorg. Chem. 2005, 3042–3048
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3045

V. Yu. Kukushkin, A. J. L. Pombeiro et al.
FULL PAPER
CCDC-262709 to -262711 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
intermediate position between such a substitutionally inert
strong RCN activator as a PtIV center (where only efficient
cycloaddition of both nitrones and nitrile oxides[8] was ob-
served) and kinetically labile (nitrile)TiIV and (nitrile)ZrIV
systems (where only displacement of nitriles by nitrones was
reported[15]).
Synthetic Work and Characterization
Reaction of PdCl2 with p-MeC6H4CH=N(O)Me in PhCN: Nitrile
(0.5 mL) was added to a mixture of PdCl2 (12 mg, 0.068 mmol) and
the nitrone p-MeC6H4CH=N(O)Me (22.4 mg, 0.150 mmol) and the
suspension formed left to stand for 1 d at 45 °C. During this time
the solid PdCl2 was completely dissolved to give a yellow solution.
Then the solvent was removed under vacuum at 50 °C and the oily
residue was crystallized under Et2O (five 2-mL portions) and dried
under a stream of nitrogen at room temperature. The reaction can
be performed starting from [PdCl2(PhCN)2] under the same reac-
tion conditions. The reaction between PdCl2 and p-
MeC6H4CH=N(O)Me in PhCN is complete after 4 h under micro-
wave irradiation (20 W, 50 °C).
Experimental Section
Materials and Instrumentation: Solvents and the nitriles were ob-
tained from commercial sources and used as received. The com-
plexes [PdCl2(RCN)2] (R = Me,[39] Ph[16]) and the nitrone[40] were
prepared as described previously. Positive-ion FAB mass spectra
were obtained on a Trio 2000 instrument by bombarding 3-ni-
trobenzyl alcohol (NBA) matrices of the samples with 8 keV (ca.
1.281015 J) Xe atoms. Mass calibration for data system acquisition
was achieved using CsI. Electrospray ionization mass spectra were
recorded with a Bruker Esquire 3000 in positive ion mode. Infrared
spectra (4000–400 cm–1) were recorded with a JASCO FT/IR-430
instrument in KBr pellets. 1H and 13C{1H} NMR spectra were
measured on Bruker DPX 300 and Varian UNITY 300 spectrome-
ters at ambient temperature.
[PdCl2{Na=C(Ph)ON(Me)CbH(C6H4Me-p)}2(Na–Cb)] (1):
Yield
50%. C32H32Cl2N4O2Pd (682.0): calcd. C 56.36, H 4.73, N 8.22;
found C 58.75, H 5.03, N 8.37%. Calcd. for C32H32Cl2N4O2Pd·
2/3PhCN: C 58.68, H 4.74, N 8.71. FAB-MS: m/z = 647 [M –
Cl]+, 609 [M – 2HCl]+. FT-IR (KBr, selected bands): ν = 1649 (s)
˜
X-ray Crystal Structure Determinations: Crystals were immersed in
cryo-oil, mounted in a Nylon loop and measured at a temperature
of 100 K or 120 K. The X-ray diffraction data was collected by
a Nonius KappaCCD diffractometer using Mo-Kα radiation (λ =
0.71073 Å). The Denzo-Scalepack[41] program package was used
for cell refinements and data reduction. The structures were solved
by direct methods using the SHELXS-97 or SIR2000 pro-
grams.[42,43] A multiscan absorption correction based on equiv. re-
flections (XPREP in SHELXTL v. 6.14)[44] was applied to all of
the data (the Tmin./Tmax. values were 0.7887/0.9215, 0.10130/
0.16564, and 0.8169/0.9055 for 1, 4, and 5, respectively). Structural
refinements were carried out using SHELXL-97 with the WinGX
graphical user interface.[45,46] All of the hydrogens were placed in
idealized positions and constrained to ride on their parent atom.
The crystallographic data of 4 and 5 are summarized in Table 1.
The selected bond lengths and angles of 4 and 5 are shown in
Figure captions. The structure solution of 1 is not satisfactory and
therefore the crystallographic details are only given as supplemen-
tary material (for details see below).
ν(C=N) cm–1 (ref.[15] 1632 vs. in KBr). 1H NMR (CDCl3): δ = 8.62
(d, 2 H, m-Ar), 7.55 (m, 5 H, Ph), 7.32 (d, 2 H, o-Ar), 5.82 (br. s,
1 H, CH), 2.90 (br. s, 3 H, NMe), 2.43 (s, 3 H, p-Me) ppm. (ref.[15]
8.68, 7.63, 7.48, 7.40, 7.30, 5.87, 2.95, 2.44 ppm. CDCl3). 13C{1H}
NMR (CDCl3): δ = 164.74 (C=N), 133.43, 130.46, 129.40, 128.84
and 128.41 (Ph and Ar), 93.76 (CH), 45.75 (NMe), 21.45 (p-Me)
ppm (ref.[15] 164.3, 139.4, 133.0, 130.4, 129.3, 129.1, 128.7, 128.6,
128.1, 122.5, 93.8, 46.0, 21.5 ppm; CDCl3).
Among other compounds formed in the mixture we succeeded in
identifying the aldehyde p-MeC6H4C(H)=O [1H NMR (CDCl3): δ
= 9.94 (s), 7.76 (d), 7.30 (d), and 2.42 (s)]. In addition, crystals of
[PdCl2{N(Me)=CH(C6H4Me-p)}2] (4) suitable for an X-ray study
were obtained by slow evaporation of a CH2Cl2/Et2O solution of
the residue obtained from the reaction mixture PhCN–PdCl2–p-
MeC6H4CH=N(O)Me.
Reaction of PdCl2 (or [PdCl2(MeCN)2]) and p-MeC6H4CH=N(O)-
Me in MeCN: MeCN (5 mL) was added to a mixture of PdCl2
(12.4 mg, 0.07 mmol) (or [PdCl2(MeCN)2], 18.1 mg, 0.070 mmol)
and p-MeC6H4CH=N(O)Me (21.0 mg, 0.14 mmol) and it was re-
fluxed on stirring for 1 d, whereupon the solvent was removed in
vacuo at room temperature to give a brown powder. 1H NMR
monitoring of the latter shows that the cycloaddition product is
formed in 10–15% yield. The powder was treated with two 2-mL
portions of Et2O, the yellow solution was separated from the brown
powder by filtration and then the filtrate was evaporated under
vacuum at room temperature to give a yellow oily residue that was
rapidly washed with cold Et2O (0.5 mL) and dried again under vac-
uum at room temperature to give 2. The reaction can also be per-
formed under focused microwave irradiation (15–20 min, 100 W)
and the oxadiazoline complex is formed also in ca. 10–20% yield.
Table 1. Crystallographic data for complexes 4 and 5.
4
5
Empirical formula
fw
C18H22Cl2N2Pd
443.68
C18H20Cl2N2O2Pd2
580.06
Temp. [K]
λ [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
100(2)
120(2)
0.71073
monoclinic
P21/c
6.4998(8)
19.8669(16)
7.2611(8)
90
0.71073
monoclinic
P21/c
10.4223(6)
12.0034(12)
8.0925(11)
90
α [°]
[PdCl2{Na=C(Me)ON(Me)CbH(C6H4Me-p)}2(Na–Cb)]·2H2O (2):
Isolated yield is 12%. C22H32Cl2N4O4Pd (557.8): calcd. C 44.50, H
5.43, N 9.43; found C 44.60, H 5.30, N 9.71. FAB-MS: m/z = 521
β [°]
90.734(6)
90
937.56(17)
2
1.572
1.275
96.125(6)
90
1006.6(2)
2
1.914
2.066
γ [°]
V [Å3]
[M – HCl]+. FT-IR (KBr, selected bands): ν = 1650 (s) ν(C=N)
˜
Z
ρcalcd. [Mg/m3]
cm–1; water of crystallization was observed in the IR spectrum in
μ(Mo-Kα) [mm–1]
1
well-dried KBr (3460 br). H NMR (CDCl3): δ = 7.47 (d, 2 H, m-
[a]
R1 (I Ն 2σ)
0.0497
0.1097
0.0339
0.0636
Ar), 7.21 (d, 2 H, o-Ar), 5.62 (br. s, 1 H, CH), 2.82 (br. s, 3 H,
NMe), 2.45 (s, 3 H, Me), 2.40 (s, 3 H, p-Me) ppm. 13C{1H} NMR
(CDCl3): δ = 166.53 (C=N), 139.19 (p-), 130.08 (Cipso), 129.26 and
[b]
wR2 (I Ն 2 σ)
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(Fo – Fc2)2]/Σ[w(Fo ) ]}1/2
.
2
2 2
3046
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 3042–3048

Interplay between Nitrones and (Nitrile)PdII Complexes
FULL PAPER
[3]
[4]
a) S. Karlsson, H.-E. Hogberg, Org. Prep. Proced. Int. 2001,
33, 103; b) L. Raimondi, M. Benaglia, Eur. J. Org. Chem. 2001,
1033; c) P. De March, M. Figueredo, J. Font, Heterocycles
1999, 50, 1213.
a) W. Carruthers, Cycloaddition Reactions in Organic Synthesis,
Pergamon, Oxford, 1990; b) 1,3-Dipolar Cycloaddition Chemis-
try (Ed.: A. Padwa), Wiley-Interscience, New York, 1984, vol.
1–2.
128.14 (o- and m-), 91.51 (br., CH), 46.09 (NMe), 21.36 (p-Me),
13.78 (Me) ppm.
Reaction of [PdCl2(MeCN)2] with p-MeC6H4CH=N(O)Me in
CH2Cl2 and Acetone: The nitrone p-MeC6H4CH=N(O)Me
(0.21 mmol) was added to a solution of [PdCl2(MeCN)2] (26 mg,
0.10 mmol) in CH2Cl2 or acetone (1 mL) at room temperature. The
reaction mixture became dark orange within several min, then it
was allowed to stay for 2–3 h without stirring. A dark-red solution
was formed, the solvent was evaporated under a stream of N2 and
the dark residue washed with Et2O (two 2-mL portions) and dried
in air and then under vacuum at room temperature. In the NMR
spectrum of the residue, signals that can be attributed to the nitrone
complex [PdCl2{ON(Me)=C(H)C6H4Me-p}2] and the correspond-
ing aldehyde p-MeC6H4C(H)O were detected. No traces of acet-
amide or acetic acid, corresponding to MeCN, were detected. Satis-
factory elemental analyses can not be obtained because of hydro-
lytic and redox decomposition of the nitrone complex
[PdCl2{ON(Me)=C(H)C6H4Me-p}2]. If the reaction mixture is left
standing at room temperature for 3 d, the solution becomes slightly
lighter and orange-yellow crystals of [Pd2(μ-Cl)2{ON(Me)=C(H)
C6H3Me-p}2] are released on the top of the flask. The yield of the
cyclometalated product 5 is ca. 30%.
[5]
[6]
For a review on metal-mediated 1,3-dipolar cycloadditions see:
G. Broggini, G. Molteni, A. Terraneo, G. Zecchi, Heterocycles
2003, 59, 823.
a) V. Yu. Kukushkin, A. J. L. Pombeiro, Chem. Rev. 2002, 102,
1771–1802 and references cited therein; b) A. J. L. Pombeiro,
V. Yu. Kukushkin, in Comprehensive Coordination Chemistry,
2nd ed., Elsevier, Amsterdam, 2003, vol. 1, pp. 639–660.
a) F. Himo, Z. P. Demko, L. Noodleman, K. B. Sharpless, J.
Am. Chem. Soc. 2003, 125, 9983; b) F. Himo, Z. P. Demko, L.
Noodleman, K. B. Sharpless, J. Am. Chem. Soc. 2002, 124,
12210; c) Z. P. Demko, K. B. Sharpless, Org. Lett. 2001, 3,
4091; d) Z. P. Demko, K. B. Sharpless, J. Org. Chem. 2001, 66,
7945.
G. Wagner, A. J. L. Pombeiro, V. Yu. Kukushkin, J. Am. Chem.
Soc. 2000, 122, 3106.
G. Wagner, M. Haukka, J. J. R. Fraústo da Silva, A. J. L. Pom-
beiro, V. Yu. Kukushkin, Inorg. Chem. 2001, 40, 264.
M. A. J. Charmier, V. Yu. Kukushkin, A. J. L. Pombeiro, J.
Chem. Soc., Dalton Trans. 2003, 2540.
[7]
[8]
[9]
[PdCl2{ON(Me)=CH(C6H4Me-p)}2] (3): MS (ESI+): m/z = 441
[10]
[11]
[12]
[13]
[14]
[M – Cl], 403 [M – 2Cl – H]. FT-IR (KBr, selected bands): ν =
˜
3012, 2944 (w) ν(C–H), 1601 (s) ν(C=N), 1159 (m-s) ν(N–O) cm–1;
M. L. Kuznetsov, V. Yu. Kukushkin, A. I. DementЈev, A. J. L.
Pombeiro, J. Phys. Chem. A 2003, 107, 6108.
N. A. Bokach, A. V. Khripoun, V. Yu. Kukushkin, M. Haukka,
A. J. L. Pombeiro, Inorg. Chem. 2003, 42, 896.
N. A. Bokach, V. Yu. Kukushkin, M. Haukka, A. J. L. Pom-
beiro, Eur. J. Inorg. Chem. 2005, 845.
1
in the free nitrone 1587 (s) ν(C=N), 1167 (m-s) ν(N–O) cm–1. H
NMR ([D6]acetone): δ = 8.20 (d, 7.8 Hz, 2 H, m-Ar), 8.08 (s, 1 H,
CH), 7.33 (d, 7.8 Hz, 2 H, o-Ar), 3.99 (s, 3 H, NMe), 2.39 (s, 3 H,
p-Me) ppm. Complex 3 was also detected, by 1H NMR method, in
the reaction between the nitrone and [PdCl2(PhCN)2].
V. Yu. Kukushkin, A. J. L. Pombeiro, Inorg. Chim. Acta 2005,
358, 1.
[Pd2(μ-Cl)2{ON(Me)=C(H)C6H3Me-p}2] (5): C18H20Cl2N2O2Pd2
[15]
[16]
[17]
[18]
G. Wagner, Inorg. Chim. Acta 2004, 357, 1320.
G. A. Anderson, M. Lin, Inorg. Synth. 1990, 28, 60.
S. Kannan, A. Usman, H. K. Fun, Polyhedron 2002, 21, 2403.
P. Merino, S. Anoro, E. Cerrada, M. Laguna, A. Moreno, T.
Tejero, Molecules 2001, 6, 208.
P. Merino, T. Tejero, M. Laguna, E. Cerrada, A. Moreno, J. A.
Lopez, Org. Biomol. Chem. 2003, 1, 2336.
T. A. K. Alallaf, A. Abdulrahman, Synth. React. Inorg. Metal-
Org. Comp. 1997, 27, 985.
W. W. Ellis, A. Gavrilova, L. Liable-Sands, A. L. Rheingold, B.
Bosnich, Organometallics 1999, 18, 332.
A. Ade, E. Cerrada, M. Contel, M. Laguna, P. Merino, T. Te-
jero, J. Organomet. Chem. 2004, 689, 1788.
E. G. Petkova, R. D. Lampeka, M. V. Gorichko, K. V. Doma-
sevitch, Polyhedron 2001, 20, 747.
F. A. Villamena, R. D. Crist, J. Chem. Soc., Dalton Trans. 1998,
4055.
F. A. Villamena, M.-H. Dickman, D. R. Crist, Inorg. Chem.
1998, 37, 1446.
J. Lee, B. Twamley, G. B. Richter-Addo, Chem. Commun. 2002,
380.
(580.1): calcd. C 37.27, H 3.48, N 4.83; found C 37.48, H 3.60, N
4.61. FT-IR (KBr, selected bands): ν = 1628 (m) ν(C=N), 1582 (s)
˜
ν(C=C), 1155 (m-s) ν(N–O) cm–1; in the free nitrone 1587 (s)
ν(C=N), 1167 (m-s) ν(N–O) cm–1. The solid complex is insoluble
in the most common deuterated solvents and it reacts with [D6]
DMSO to give a solution with the following spectrum, i.e. 1H
NMR ([D6]DMSO): δ = 8.39 (s, 1 H, CH); 7.75 (d, m-Ar); 7.14 (d,
o-Ar); 6.97 (s, m-Ar); 3.81 (s, 3 H, NMe); 2.21 (s, 3 H, p-Me) ppm.
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
Acknowledgments
This work has been supported by the Fundação para a Ciência e
a Tecnologia (FCT), Portugal, and its POCTI program (POCTI/
QUI/43415/2001 project) (FEDER funded) and the Russian Fund
for Basic Research (grants 03–03–32363 and 05–03–32140). N.A.B.
and V.Yu.K. express gratitude to the POCTI program, Portugal for
the grant SFRH/BPD/11448/2002 (N.A.B.) and a grant for Cienti-
sta Convidado/Professor at the Instituto Superior Técnico
(V.Yu.K.). N.A.B., M.H., and V.Yu.K. would like to thank the
Academy of Finland for financial support. V.Yu.K. is very much
obliged to ISSEP for the Soros Professorship (2001–2004) and the
Royal Society of Chemistry for a Grant for International Authors.
The authors also thank NATO for the Collaborative Linkage
Grant PST.CLG.979289.
C. Mukai, W. J. Cho, I. J. Kim, M. Hanaoka, Tetrahedron Lett.
1990, 31, 6893.
D. Mansuy, M. Dreme, J. C. Chottard, J. P. Girault, J. Gu-
ilhem, J. Am. Chem. Soc. 1980, 102, 844.
For recent works see: a) C. L. Chen, Y. H. Liu, S. M. Peng,
S. T. Liu, J. Organomet. Chem. 2004, 689, 1806; b) R. Zurawin-
ski, B. Donnadieu, M. Mikolajczyk, R. Chauvin, J. Organomet.
Chem. 2004, 689, 380; c) R. Ares, M. Lopez-Torres, A. Fernan-
dez, S. Castro-Juiz, A. Suarez, G. Alberdi, J. J. Fernandez, J. M.
Vila, Polyhedron 2002, 21, 2309.
a) A. D. Ryabov, G. M. Kazankov, A. K. Yatsimirsky, L. G.
Kuz’mina, O. Y. Burtseva, N. V. Dvortsova, V. A. Polyakov, In-
org. Chem. 1992, 31, 3083; b) A. D. Ryabov, R. Le Lagadec, H.
[1] S. Kobayashi, K. A. Jørgensen, Cycloaddition Reactions in Or-
ganic Synthesis, VCH, Weinheim, 2002.
[2] a) K. V. Gothelf, K. A. Jørgensen, Chem. Commun. 2000, 1449;
b) K. V. Gothelf, K. B. Jensen, K. A. Jørgensen, Sci. Prog.
1999, 82, 327; c) K. V. Gothelf, K. A. Jørgensen, Chem. Rev.
1998, 98, 863.
[30]
Eur. J. Inorg. Chem. 2005, 3042–3048
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3047

V. Yu. Kukushkin, A. J. L. Pombeiro et al.
FULL PAPER
Estevez, S. Hernandez, L. Alexandrova, V. S. Kurova, A. Fi-
scher, M. Pfeffer, Inorg. Chem. 2005, 44, 1626.
[31] F. A. Allen, O. Kennard, D. G. Watson, J. Chem. Soc., Perkin
Trans. 2 1987, S1.
[32] J. M. Zhang, Y. M. Zhang, Chinese Chem. Lett. 2001, 12, 945;
Chem. Abstr. 2001, 136:279154.
[33] D. K. Dutta, D. J. Konwar, Chem. Res., Synopses 1998, 266.
[34] R. Balicki, Chem. Ber. 1990, 123, 647.
[35] R. Balicki, Gazz. Chim. Ital. 1990, 120, 67.
[36] S. V. KuzЈmin, M. D. Mizhiritskii, L. M. Kogan, Zh. Org.
Khim. 1989, 25, 663; Chem. Abstr. 1989, 111:173182.
[37] a) Y. Yu, N. Watanabe, M. Ohno, S. Eguchi, J. Chem. Soc.,
Perkin Trans. 1 1995, 1417; b) Y. Yu, H. Fujita, M. Ohno, S.
Eguchi, Synthesis 1995, 498; c) P. H. H. Hermkens, J. H.
van Maarseveen, C. G. Kruse, H. W. Scheeren, Tetrahedron
1988, 44, 6491.
C. W. F. Cheng, T. J. Emge, K. P. Kelly, T. F. Koetzle, J. Am.
Chem. Soc. 1984, 106, 5913; d) B. B. Wayland, R. F. Schramm,
Inorg. Chem. 1969, 8, 971.
[40] Methoden der Organischen Chemie (Houben-Weyl), Georg Thi-
eme Verlag, Stuttgart, New York, 2001, 4th ed., vol. E 14b.
[41] Z. Otwinowski, W. Minor, “Processing of X-ray Diffraction
Data Collected in Oscillation Mode”, in Methods in Enzy-
mology, Macromolecular Crystallography, Part A (Eds.: C. W.
Carter, Jr., R. M. Sweet), Academic Press, New York, 1997,
vol. 276, pp. 307–326.
[42] G. M. Sheldrick, SHELXS-97, Program for Crystal Structure
Determination, University of Göttingen, 1997.
[43] M. C. Burla, M. Camalli, B. Carrozzini, G. L. Cascarano, C.
Giacovazzo, G. Polidori, R. J. Spagna, Appl. Cryst. 2003, 36,
1103.
[44] G. M. Sheldrick, SHELXTL Version 6.14, Bruker Analytical
X-ray Systems, Bruker AXS, Inc., Madison, Wisconsin2003.
[45] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Göttingen, 1997.
[38] A. Erxleben, I. Mutikainen, B. Lippert, J. Chem. Soc., Dalton
Trans. 1994, 3667.
[39] a) F. L. Wimmer, S. Wimmer, P. Castan, R. J. Puddephatt, In-
org. Synth. 1992, 29, 185; b) I. E. Bekk, E. V. Gusevskaya, V. A. [46] L. J. Farrugia, J. Appl. Cryst. 1999, 32, 837.
Likholobov, Yu. I. Ermakov, Reaction Kinetics and Catalysis
Letters 1987, 33, 209; c) M. A. Andrews, T. C. T. Chang,
Received: February 10, 2005
Published Online: June 22, 2005
3048
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 3042–3048