A route to 1,2,4-oxadiazoles and their complexes via platinum-mediated 1,3-dipolar cycloaddition of nitrile oxides to organonitriles

Article abstract of DOI:10.1021/ic026103v

A significant activation of the C≡N group in organonitriles upon their coordination to a platinum(IV) center has been found in the reaction of [PtCl₄(RCN)₂] (R = Me, Et, CH₂Ph) with the nitrile oxides 2,4,6-R′3C6H2CNO (R′ = Me, OMe) to give the (1,2,4-oxadiazole)platinum(IV) complexes [PtCl₄{N=C(R)ON=CC₆H₂R′₃}] (R = Me, R′ = Me (1); R = Et, R′ = Me (2); R = Et, R′ = OMe (3); R = CH₂Ph, R′ = Me (4)); the [2 + 3] cycloaddition was performed under mild conditions (unless poor solubility of [PtCl₄(RCN)₂] precludes the reaction) starting even from complexed acetonitrile and propionitrile, which exhibit low reactivity in the free state. The reaction between complexes 2-4 and 1 equiv of Ph₃P=CHCO₂Me in CH₂Cl₂ leads to the appropriate platinum(II) complexes [PtCl₂{N=C(R)-ON=CC₆H₂R′₃}] (5-7); the reduction failed only in the case of 1 insofar as this complex is insoluble in the most common organic solvents. All the platinum compounds were characterized by elemental analyses, FAB mass spectrometry, and IR and ¹H, ¹³C{¹H}, and ¹⁹⁵Pt NMR spectroscopies, and three of them also by X-ray crystallography. The oxadiazoles formed in the course of the metal-mediated reaction were liberated almost quantitatively from their Pt(IV) complexes by reaction of the latter (complexes 2-4) with an excess of pyridine in chloroform, giving free 1,2,4-oxadiazoles and trans-[PtCl₄(pyridine)₂]; the sequence of the Pt(IV)-mediated [2 + 3] cycloaddition and the liberation opens up an alternative route for the preparation of this important class of heterocycles.

Full text of DOI:10.1021/ic026103v

Inorg. Chem. 2003, 42, 896−903
A Route to 1,2,4-Oxadiazoles and Their Complexes via
Platinum-Mediated 1,3-Dipolar Cycloaddition of Nitrile Oxides to
Organonitriles
,†
Nadezhda A. Bokach,†,‡ Anatolii V. Khripoun,† Vadim Yu. Kukushkin,* Matti Haukka,§ and
,‡
Armando J. L. Pombeiro*
St. Petersburg State UniVersity, 198904 Stary Petergof, Russian Federation, Centro de Qu´ımica
Estrutural, Complexo I, Instituto Superior Te´cnico, AV. RoVisco Pais, 1049-001 Lisbon, Portugal,
and Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FIN-80101, Joensuu, Finland
Received October 12, 2002
A significant activation of the CtN group in organonitriles upon their coordination to a platinum(IV) center has
been found in the reaction of [PtCl4(RCN)2] (R ) Me, Et, CH Ph) with the nitrile oxides 2,4,6-R′3C H CNO (R′ )
2
6 2
Me, OMe) to give the (1,2,4-oxadiazole)platinum(IV) complexes [PtCl4{NdC(R)ONdCC H R′3}] (R ) Me, R′ )
6
2
Me (1); R ) Et, R′ ) Me (2); R ) Et, R′ ) OMe (3); R ) CH Ph, R′ ) Me (4)); the [2 + 3] cycloaddition was
2
performed under mild conditions (unless poor solubility of [PtCl4(RCN)2] precludes the reaction) starting even from
complexed acetonitrile and propionitrile, which exhibit low reactivity in the free state. The reaction between complexes
2−4 and 1 equiv of Ph3PdCHCO Me in CH Cl2 leads to the appropriate platinum(II) complexes [PtCl2{NdC(R)-
2
2
ONdCC H R′3}] (5−7); the reduction failed only in the case of 1 insofar as this complex is insoluble in the most
6
2
common organic solvents. All the platinum compounds were characterized by elemental analyses, FAB mass
spectrometry, and IR and 1H, 13C{1H}, and 195Pt NMR spectroscopies, and three of them also by X-ray crystallography.
The oxadiazoles formed in the course of the metal-mediated reaction were liberated almost quantitatively from
their Pt(IV) complexes by reaction of the latter (complexes 2−4) with an excess of pyridine in chloroform, giving
free 1,2,4-oxadiazoles and trans-[PtCl4(pyridine)2]; the sequence of the Pt(IV)-mediated [2 + 3] cycloaddition and
the liberation opens up an alternative route for the preparation of this important class of heterocycles.
Introduction
concerned the biological activity of these compounds and
the basic concept behind many developments is that the 1,2,4-
oxadiazole ring is a hydrolysis-resisting bioisosteric alterna-
tive for an ester moiety. Some derivatives of 1,2,4-
oxadiazoles have intrinsic analgesic,7 antiinflammatory,8 and
antipicornaviral9 properties and show high efficacy as
agonists (e.g., for muscarinic,10 adrenergic,11 and 5-hydroxy-
tryptamine12) and antagonists (e.g., for angiotensin13 and
adhesion14) for different receptors. Other useful applications
of 1,2,4-oxadiazoles include plant protection,15 use as liquid
crystalline mesophases,16 dyeing or printing with 1,2,4-
1,2,4-Oxadiazoles represent an important class of five-
membered heterocycles, and their rich chemistry has been
repeatedly reviewed over the years.1-6 Most of these surveys
indicate that more than half of the original publications
* Authors to whom correspondence should be addressed. E-mail:
kukushkin@VK2100.spb.edu (V.Yu.K.); pombeiro@ist.utl.pt (A.J.L.P.).
† St. Petersburg State University.
‡ Instituto Superior Te´cnico.
§ University of Joensuu.
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896 Inorganic Chemistry, Vol. 42, No. 3, 2003
10.1021/ic026103v CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/14/2003

A Route to 1,2,4-Oxadiazoles and Their Complexes
oxadiazole azo dyes,17 and use as constituents of fluorescent
whiteners.18 Many of these applications have been patented.
Known synthetic approaches1,2,4 to 1,2,4-oxadiazoles in-
clude cyclization of O-acylamidooximes, N-acylamino ethers,
and nitro compounds, oxidation of dihydrooxadiazoles,
amidoximes, and oximes, and 1,3-dipolar cycloaddition of
nitrile oxides to nitriles. The latter route could be one of the
most advantageous owing to the easy synthetic access to
(and/or commercial availability of) the starting materials, but
the low activation of the nitrile triple bond renders the
reaction less favorable. Thus, the 1,3-dipolar cycloaddition
of nitrile oxides to nonactivated nitriles proceeds only under
harsh reaction conditions when the formation of 1,2,4-
oxadiazoles competes with dimerization4,19 of unstable nitrile
oxides to give furoxanes or 1,2,4-oxadiazole 4-oxides.
The activation of the CtN group in nitriles RCNsand,
consequently, their reactivity toward the cycloaddition of
nitrile oxidesscan be enhanced by introducing strong
acceptor R groups to the nitrile carbon.20 Another mode of
the nitrile activation involves affecting the nitrile through
the opposite end, i.e., by coordination to a metal center via
the N atom.20,21 The latter activation type is the least explored
one, and until recently it was limited to the reaction of a
ligated nitrile with azides as dipoles.20 A few years ago,
within the framework of our continuous project on reactions
of metal-activated nitriles (this topic has been reviewed by
two of us20), it was found that the platinum(IV) center in
[PtCl4(RCN)2] complexes provides sufficiently strong activa-
tion of the nitriles to assist the facile [2 + 3] cycloaddition
between RCN ligands and various nitrones (R1)(R2)Cd(R3)-
N+O- to achieve the first examples of ∆4-1,2,4-oxadiazoline
complexes [PtCl4(∆4-1,2,4-oxadiazoline)2].22,23
In this work, we endeavored to extend the [2 + 3]
cycloaddition of complexed RCN species from nitrones to
nitrile oxides. The main goals of this work are the following
ones: (i) to determine whether the results disclosing the
enhanced reactivity of Pt(IV)-bound nitriles are specific for
dipoles of allyl anion type,24 e.g., nitrones, or the reactions
can be spread out to dipoles of the propargyl/allenyl anion
type,24 e.g., nitrile oxides; (ii) to develop a general route to
1,2,4-oxadiazoles which is based on the [2 + 3] cycloaddition
between the metal-activated nitriles and nitrile oxides and
to perform the synthesis under mild conditions, thus prevent-
ing the nitrile oxides from dimerizing. The achieved results,
showing a very high activation of nitriles upon their ligation
to the metal center, thus facilitating the [2 + 3] cycloaddition
of nitrile oxides, are reported in this paper.
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2001, 411, 115. Watson, J.; Selkirk, J. V.; Brown, A. M. J. Biomol.
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Colpaert, F. C. Br. J. Pharmacol. 1998, 123, 51.
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Melzer, G. PCT Int. Appl. 97 44,333, 1997; Chem. Abstr. 1997, 128,
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Experimental Section
Materials and Instrumentation. Solvents were obtained from
commercial sources and used as received. The complexes [PtCl4-
(RCN)2]25,26 and the nitrile oxides 2,4,6-Me3C6H2CNO and 2,4,6-
(MeO)3C6H2CNO27 were prepared as previously described. C, H,
and N elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Te´cnico. Melting points were
determined on a Kofler table. For TLC, Merck UV 254 SiO2 plates
were used. Positive-ion FAB mass spectra were obtained on a Trio
2000 instrument by bombarding 3-nitrobenzyl alcohol (NBA)
matrixes of the samples with 8 keV (ca. 1.28 × 1015 J) of Xe atoms.
Mass calibration for data system acquisition was achieved using
CsI. Infrared spectra (4000-400 cm-1) were recorded on a BIO-
1
RAD FTS 3000MX instrument in KBr pellets. H, 13C{1H}, and
195Pt NMR spectra in CDCl3 were measured on Varian UNITY
300 and Bruker AMX 300 spectrometers at ambient temperature.
195Pt chemical shifts are given relative to the peak of Na2[PtCl6]
(20) Kukushkin, V. Yu.; Pombeiro, A. J. L. Chem. ReV. 2002, 102, 1771
and references therein. Pombeiro, A. J. L.; Kukushkin, V. Yu.
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2003; Vol. 1; in press.
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H. Ger. Offen. DE 2,712,409, 1977; Chem. Abstr. 1978, 88, 51973.
Siegrist, A. E.; Kormany, G.; Kabas, G HelV. Chim. Acta 1976, 59,
2469-2491. Domerque, A. Ger. Offen. DE 2,503,439, 1975; Chem.
Abstr. 1975, 83, 165830.
(19) Torssell, K. B. G. Nitrile Oxides, Nitrones, and Nitronates in Organic
Synthesis. NoVel Strategies in Synthesis; VCH: Weinheim, Germany,
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L.; Kukushkin, V. Yu. Inorg. Chem. 2001, 40, 264.
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K. V.; Jørgensen, K. A. Chem. Commun. 2000, 1449.
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R.; Wagner, G.; Pombeiro, A. J. L. Inorg. Chem. 1998, 37, 6511.
(26) Luzyanin, K. V.; Haukka, M.; Bokach, N. A.; Kuznetsov, M. L.;
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Inorganic Chemistry, Vol. 42, No. 3, 2003 897

Bokach et al.
by using aqueous K2[PtCl4] (-1630 ppm) as a standard, and the
half-height line width is given in parentheses.
Ph), 22.6 (o-CH3Ar), 21.4 (p-CH3Ar). The solubility of the complex
is insufficient to measure the 195Pt NMR spectrum.
Synthetic Work. Cycloaddition of Nitrile Oxides to Ligated
Nitriles. (i) A suspension of trans-[PtCl4(MeCN)2] (42 mg, 0.1
mmol) and 2,4,6-Me3C6H2CNO (48 mg, 0.3 mmol) in MeCN (2
mL) is vigorously stirred at room temperature for 1 day, whereupon
the new precipitate formed is filtered off, washed with three 3 mL
portions of CH2Cl2, and dried in air at room temperature. A similar
reaction with 2,4,6-(MeO)3C6H2CNO results in a broad mixture of
yet unidentified species. (ii) A suspension of trans-[PtCl4(EtCN)2]
(45 mg, 0.1 mmol) and 2,4,6-R′3C6H2CNO (R′ ) Me, OMe) (0.4
mmol) in CH2Cl2 (2 mL) is stirred at room temperature for 1 day,
whereupon the new precipitate formed is filtered off, washed with
three 3 mL portions of CH2Cl2, and dried in air at room temperature.
(iii) A suspension of trans-[PtCl4(PhCH2CN)2] (57 mg, 0.1 mmol)
and 2,4,6-Me3C6H2CNO (0.6 mmol) in PhCH2CN (0.4 mL) is
stirred at room temperature for 1 day, whereupon the new precipitate
formed is filtered off, washed with one 2 mL portion of toluene
and two 3 mL portions of Et2O, and dried in air at room temperature.
A similar reaction with 2,4,6-(MeO)3C6H2CNO results in a broad
mixture of yet unidentified species.
Reduction of the (1,2,4-Oxadiazole)platinum(IV) Complexes.
The reduction was carried out with the carbonyl-stabilized phos-
phorus ylide Ph3PdCHCO2Me using the method previously
described,28 and the final platinum(II) complexes were purified by
column chromatography on SiO2. The very low solubility of [PtCl4-
{NdC(Me)ONdCC6H2Me3}2] in the most common organic sol-
vents, e.g., dichloromethane, does not allow the compound to be
selectively reduced.
[PtCl2{NdC(Et)ONdCC6H2Me3}2] (5). Yield: 60-80%. Anal.
Calcd for C26H32N4Cl2O2Pt: C, 44.70; H, 4.62; N, 8.02. Found:
C, 44.72; H, 4.61; N, 7.85. FAB+-MS: m/z 699 [M + H]+, 627
[M - 2Cl]+. IR, selected bands, cm-1: 2922 m-w and 2856 w
1
[ν(C-H)], 1612 m and 1576 s [ν(CdN + CdC)]. H NMR, two
sets of signals for each proton: δ 7.02 and 6.91 (s, 2H, m-Ar),
3.69 and 3.25 (quart, 7.5 Hz, 2H, Et), 2.50 and 2.39 (s, 3H,
p-CH3C6H2), 2.29 and 1.99 (s, 6H, o-CH3C6H2), 1.68 and 1.41 (t,
7.5 Hz, 3H, Et). 13C{1H} NMR: δ 182.0 (PtsNdCsO), 167.1
and 166.9 (NdCsN), 140.3 and 140.2 (p-Ar), 139.0 and 138.8
(o-Ar), 128.4 (m-Ar), 120.9 and 120.3 (Cipso), 21.9 and 21.3 (CH2
from Et), 21.5 and 21.3 (p-CH3C6H2), 20.8 and 20.4 (o-CH3C6H2),
9.9 and 10.0 (CH3 from Et). 195Pt NMR: δ -2224 (700 Hz).
[PtCl4{NdC(Me)ONdCC6H2Me3}2] (1). Yield: 10%. Anal.
Calcd for C24H28N4Cl4O2Pt: C, 38.88; H, 3.81; N, 7.56. Found:
C, 38.92; H, 3.79; N, 7.37. FAB+-MS: m/z 705 [M - Cl]+, 669
[M - 2Cl]+, 633 [M - 3Cl - H]+, 598 [M - 4Cl - H]+. IR,
selected bands, cm-1: 3080 w, 2918 w [ν(C-H)], 1611 and 1560
[PtCl2{NdC(Et)ONdCC6H2(OMe)3}2] (6). Yield: 50%. Anal.
Calcd for C36H36N4Cl2O8Pt: C, 39.30; H, 4.06; N, 7.05. Found:
C, 39.35; H, 4.09; N, 6.96. FAB+-MS: m/z 817 [M + Na]+, 794
[M + H]+, 759 [M - Cl - H]+, 722 [M - 2Cl - H]+. IR, selected
bands, cm-1: 2938 m-w and 2845 w [ν(C-H)], 1616 s-m and
1695 m [ν(CdN + CdC)]. 1H NMR: δ 3.86 (s, 3H, p-OMe) 3.76
(s, 6H, o-OMe), 3.22 (quart, 7.6 Hz, 2H, Et), 1.40 (t, 7.6 Hz, 3H,
Et), 6.17 (s, 2H, m-Ph). 13C{1H} NMR: δ 9.9 (Et), 21.2 (Et), 55.4
(o-CH3OC6H2), 55.8 (p-CH3OC6H2), 90.1 (m-Ar), 95.8 (Cipso), 160.5
(o-Ar), 163.4 (NdCsN), 164.4 (p-Ar), 180.9 (NdCsO). 195Pt
NMR: δ -2201 (850 Hz).
1
m [ν(CdN + CdC)]. H NMR: δ 6.84 (s, 2H, m-Ph), 3.38 (s,
3H, Me), 2.31 (s, 3H, p-CH3C6H2), 2.11 (s, 6H, o-CH3C6H2). The
solubility of the complex is insufficient to measure both 13C{1H}
and 195Pt NMR spectra.
[PtCl4{NdC(Et)ONdCC6H2Me3}2] (2). Yield: 80%. Anal.
Calcd for C26H32N4Cl4O2Pt: C, 40.58; H, 4.19; N, 7.28. Found:
C, 40.25; H, 4.12; N, 7.10. FAB+-MS: m/z 755 [M - HCl + Na]+,
732 [M - HCl]+, 721 [M - 2Cl + Na]+, 697 [M - HCl - Cl]+,
661 [M - 3Cl - 2H]+, 626 [M - 4Cl - H]+. IR, selected bands,
cm-1: 2983 w, 2922 m and 2855 w [ν(C-H)], 1611 and 1560 m
[PtCl2{NdC(CH2Ph)ONdCC6H2Me3}2]‚0.25CH2Cl2
(7).
1
[ν(CdN + CdC)]. H NMR: δ 6.63 (s, 2H, m-Ph), 3.87 (quart,
Yield: 70%. Anal. Calcd for C36H36N4Cl4O2Pt‚0.25CH2Cl2: C,
51.59; H, 4.36; N, 6.64. Found: C, 51.67; H, 4.46; N, 6.60. FAB+-
MS: m/z 823 [M]+, 750 [M - 2Cl - H]+. IR, selected bands,
cm-1: 2923 m-w and 2854 w [ν(C-H)], 1611 m and 1578 s-m
[ν(CdN + CdC)]. 1H NMR: δ 2.28 and 1.99 (s, 6H, o-Me), 2.50
and 2.24 (s, 3H, p-Me), 4.86 and 4.64 (s, 2H, CH2Ph), 6.93 (s, 2H,
m-Ar), 7.43 and 7.37 (m, 5H, CH2Ph). 13C{1H} NMR: δ 33.8 and
33.7 (CH2Ph), 20.8 and 20.4 (o-CH3C6H2), 21.5 and 21.2 (p-
CH3C6H2), 141.0 (o-Ar), 127.4 (m-Ar), 129.3, 129.1 and 129.0 (o-
and m-PhCH2), 128.1 (p-PhCH2), 128.4 and 128.3 (m-Ar), 131.4
and 130.9 (Cipso from CH2Ph), 120.6 and 120.3 (Cipso from Ar),
141.1 and 140.4 (p-Ar), 139.0 and 138.8 (o-Ar), 167.3 and 166.9
(NdCsN), 179.8 and 179.6 (NdCsO). 195Pt NMR: δ -2235 (700
Hz).
Attempted Cycloaddition to (Nitrile)platinum(II) Complexes.
Nitrile oxide (0.2 mmol) is added to a solution of trans-[PtCl2-
(EtCN)2] (20 mg; 0.05 mmol) in CHCl3 (5 mL). The mixture is
refluxed for 6-8 h, whereupon that solvent is evaporated and the
residue is analyzed by 1H NMR and FAB-MS. Apart from
unidentified products, the following ones are observed in FAB+-
7.5 Hz, 2H, Et), 1.45 (t, 7.5 Hz, 3H, Et), 2.30 (s, 3H, p-CH3Ph),
2.10 (s, 6H, o-CH3Ph). 13C{1H} NMR: δ 141.0 (o-Ar), 138.8 (p-
Ar), 127.4 (m-Ar), 120.7 (Cipso) 26.5 and 12.1 (Et), 22.7 (o-
CH3C6H2), 21.4 (p-CH3C6H2). 195Pt NMR: δ 18 (620 Hz).
[PtCl4{NdC(Et)ONdCC6H2Me3}2]‚CHCl3 (3). Yield: 60%.
Anal. Calcd for C26H32N4Cl4O8Pt‚CHCl3: C, 32.93; H, 3.38; N,
5.69. Found: C, 33.20; H, 3.60; N, 5.80. FAB+-MS: m/z 830 [M
- HCl]+, 795 [M - HCl - Cl]+, 757 [M - 3Cl - 2H]+, 722 [M
- 4Cl - H]. IR, selected bands, cm-1: 2940 m-w [ν(C-H)], 1610
1
s-m and 1586 m [ν(CdN + CdC)]. H NMR: δ 6.00 (s, 2H,
Ph), 3.84 (s and q, 5H, p-C6H2CH3 and Et), 3.68 (s, 6H, o-C6H2-
CH3), 1.45 (t, 7.5 Hz, 3H, Et). 13C{1H} NMR: δ 164.4 (p-Ar),
161.8 (o-Ar), 89.5 (m-Ar), 55.3 (OMe), 25.9 and 11.7 (Et). The
solubility of the complex is insufficient to measure the 195Pt NMR
spectrum.
[PtCl4{NdC(CH2Ph)ONdCC6H2Me3}2] (4). Yield: 75%. Anal.
Calcd for C36H36N4Cl4O2Pt: C, 48.39; H, 4.06; N, 6.27. Found:
C, 48.27; H, 4.21; N, 6.09. FAB+-MS: m/z 856 [M - Cl]+, 821
[M - 2Cl]+, 786 [M - 3Cl - 2H]+, 749 [M - 4Cl - 2H]+. IR,
selected bands, cm-1: 3026 w, 2923 m [ν(C-H)], 1611 and 1554
m [ν(CdN + CdC)]. 1H NMR: δ 7.35 (m, 5H, CH2Ph), 6.84 (s,
2H, m-C6H2), 5.22 (s, 2H, CH2Ph), 2.29 (s, 3H, p-CH3C6H2), 2.12
(s, 6H, o-CH3C6H2). 13C{1H} NMR: δ 141.0 (o-Ar), 127.4 (m-
Ar), 128.0, 129.6, and 128.9, (o-, m-, and p-PhCH2), 38.4 (CH2-
MS: [PtCl2(EtCN){NdC(Et)ONdCC6H2Me3}], m/z 536 [M -
H]+, 500 [M - HCl]+, 465 [M - 2Cl - H]+, 445 [M - HCl -
(28) Wagner, G.; Pakhomova, T. B.; Bokach, N. A.; Frau´sto da Silva, J. J.
R.; Vicente, J.; Pombeiro, A. J. L.; Kukushkin, V. Yu. Inorg. Chem.
2001, 40, 1683.
898 Inorganic Chemistry, Vol. 42, No. 3, 2003

A Route to 1,2,4-Oxadiazoles and Their Complexes
Table 1. Crystal Data for Compounds 2, 6, and 8
EtCN]+, 410 [M - 2Cl - EtCN - H]+; [PtCl2(EtCN){NdC(Et)-
2
5
8
ONdCC6H2(OMe)3}], m/z 608 [M - H + Na]+, 584 [M - H]+,
empirical formula
fw
Temp (K)
λ (Å)
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C26H36Cl4N4O2Pt C26H32Cl4N4O2Pt C26H32Cl2N4O8Pt
549 [M - Cl]+, 514 [M - 2Cl]+.
773.48
150(2)
0.71073
monoclinic
P21/c
769.45
120(2)
0.71073
triclinic
P1h
794.55
150(2)
Preparation of trans-[PtCl4{HNdC(Et)ONdC(H)C6H2Me3}2]
(8). This complex was obtained by the reaction of trans-[PtCl4-
(EtCN)2] and the aldoxime HONdCH(C6H2Me3) in accord with
the published method for related complexes.25 Yield: 92%. Anal.
Calcd for C26H34N4Cl4O2Pt: C, 40.37; H, 4.69; N, 7.24. Found:
C, 40.08; H, 4.73; N, 6.96. FAB+-MS: m/z 665 [M - 3Cl]+, 630
[M - 4HCl]+. IR, selected bands, cm-1: 3280 m [ν(N-H)], 2978
w, 2921 w [ν(C-H)], 1657 and 1642 [ν(CdN)]. 1H NMR: δ 8.87
(s, 1H, CH), 8.74 (s, br, 1H, NH), 6.94 (s, 2H, m-Ar), 3.26 (quart,
7.6 Hz, 2H, CH2 from Et), 2.49 (s, 6H, o-CH3C6H2), 2.31 (s, 3H,
p-CH3C6H2), 1.36 (t, 7.6 Hz, 3H, CH3 from Et). 13C{1H} NMR: δ
177.0 (CdN), 15.9 (CH), 142.4 (p-Ar), 139.9 (o-Ar), 130.3 (m-
Ar), 122.8 (Cipso), 24.9 (CH2 from Et), 22.2 (o-CH3C6H2), 21.3 (p-
CH3C6H2), 10.6 (CH3 from Et). 195Pt NMR: δ -171.7 (660 Hz).
Attempted Ligand Oxidation of 8. Reactions of 8 with Cl2 in
CH2Cl2 (20 min) or with H2O2 in CDCl3 (1 day), at room
temperature, afford a mixture of unidentified products.
0.71073
monoclinic
P21/n
16.3688(3)
7.6474(2)
22.9269(6)
90
91.6665(7)
90
2868.75(12)
4
10.0441(2)
16.6046(3)
9.2969(2)
90
100.4701(8)
90
6.7156(4)
10.0296(6)
11.9766(8)
65.932(3)
82.563(3)
76.640(4)
716.05(8)
1
1524.70(5)
2
Z
F
calcd (Mg/m3)
1.685
4.982
1.784
5.303
1.840
5.133
0.0321
0.0692
µ(Mo KR) (mm-1
R1a (I g 2σ)
)
0.0250
0.0566
0.0357
0.0765
wR2b (I g 2σ)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2
.
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Structures 2
and 6a
2
6(A)
6(B)
Pt(1)-Cl(1)
Pt(1)-Cl(2)
Pt(1)-N(1)
N(1)-C(1)
C(1)-O(1)
O(1)-N(2)
N(2)-C(4)
2.3176(12)
2.3150(13)
2.078(4)
1.320(7)
1.329(6)
1.422(6)
1.304(7)
2.2955(14)
2.3057(13)
2.015(4)
1.320(6)
1.333(6)
1.439(6)
1.292(7)
Liberation of 1,2,4-Oxadiazoles. One drop of pyridine is added
to a suspension of 2-4 (20 mg) in CHCl3 (0.5-1 mL) and the
mixture left to stand at 40 °C for 2 days, whereupon the yellow
precipitate formed is filtered off and 1,2,4-oxadiazole from the
filtrate is purified by column chromatography on SiO2 (silica gel
70-230 mesh, 60 Å, Aldrich; eluent CH2Cl2, first fraction).
2.021(4)
1.311(6)
1.344(6)
1.420(5)
1.288(7)
Cl(1)-Pt(1)-Cl(2)
Cl(1)-Pt(1)-N(1)
Cl(2)-Pt(1)-N(1)
N(1)-C(1)-O(1)
C(1)-O(1)-N(2)
O(1)-N(2)-C(4)
N(2)-C(4)-N(1)
85.55(5)
88.58(12)
91.96(12)
110.2(4)
108.4(4)
104.5(4)
111.3(5)
179.13(5)
89.25(12)
91.00(12)
110.8(5)
107.5(4)
103.3(4)
113.7(5)
NdC(Et)ONdCC6H2Me3 (9). EI-MS: m/z 216 [M]+, 161 [M
- EtCN]+. IR, selected bands, cm-1: 2961 w, 2922 m-w and
2854 w [ν(C-H)], 1612 m and 1577 s-m [ν(CdN + CdC)]. 1H
NMR: δ 1.44 (t, 7.8 Hz, 3H, Et), 2.98 (quart, 7.8 Hz, 2H, Et),
6.91 (s, 2H, m-Ar), 2.29 (s, 3H, p-CH3C6H2), 2.15 (s, 6H,
o-CH3C6H2). 13C{1H} NMR: δ 180.4 (OsCdN), 168.2 (NsCd
N), 139.6 (p-Ar), 137.7 (o-Ar), 21.2 (p-MeC6H2), 20.0 (o-MeC6H2),
20.0 and 10.8 (Et).
88.99(12)
90.79(12)
110.5(5)
107.6(4)
104.3(4)
112.8(5)
a The unequivalent halves of molecule 6 are denoted as 6(A) and 6(B).
interface.30-32 Structure 2 was solved by the Patterson method using
the DIRDIF-99 program.33 A multiscan absorption correction based
on equivalent reflections (XPREP in SHELXTL v. 5.1)34 was
applied to all data (Tmin/Tmax values were 0.21680/0.31796, 0.11832/
0.16567, and 0.22291/0.30662 for 2, 6, and 8, respectively).
Structural refinements were carried out with SHELXL97.35 In
structure 8, NH hydrogens were located from the difference Fourier
map. All other hydrogens were placed in idealized positions. The
NH hydrogen in 8 was refined isotropically, while other hydrogens
were constrained to ride on their parent atom. The crystallographic
data are summarized in Table 1. Selected bond lengths and angles
are shown in Table 2 and in Figure 1.
NdC(Et)ONdCC6H2(OMe)3 (10). EI-MS: m/z 264 [M]+, 233
[M - MeO]+, 235 [M - Et]+, 207 [M - EtCN]+, 209 [M -
EtCO]+. IR, selected bands, cm-1: 2961 m-w [ν(C-H)], 1620
s-m and 1583 s-m [ν(CdN + CdC)]. 1H NMR: δ 6.15 (s, 2H,
m-Ar), 3.83 (s, 3H, p-CH3C6H2), 3.74 (s, 6H, o-CH3C6H2), 2.96
(quart, 7.6 Hz, 2H, Et), 1.43 (t, 7.6 Hz, 3H, Et). 13C{1H} NMR: δ
10.6 (Et), 20.3 (Et), 55.9 (o-CH3O), 55.4 (p-CH3O), 90.6 (m-Ar),
163.1 (p-Ar), 160.1 (o-Ar), 163.9 (NdCsN), 180.0 (NdCsO).
NdC(CH2Ph)ONdCC6H2Me3 (11). EI-MS: m/z 278 [M]+, 263
[M - Me]+, 248 [M - 2Me]+, 187 [M - CH2Ph]+, 160 [M -
PhCH2CN]+, 161 [M - PhCH2CN - H]+, 91 [PhCH2]+. IR,
selected bands, cm-1: 2961 w, 2922 m-w and 2854 w [ν(C-H)],
Results and Discussion
1
1612 m and 1577 s-m [ν(CdN + CdC)]. H NMR: δ 7.3 (m,
Cycloaddition Reaction. Despite the versatile organic
chemistry of nitrile oxides, as reflected in a large number of
CH2Ph), 6.90 (s, m-Ar), 4.30 (s, CH2Ph), 2.28 (s, p-CH3C6H2), 2.13
(s, o-CH3C6H2). 13C{1H} NMR: δ 20.0 (o-Ar), 21.1 (p-Ar), 33.0
(CH2Ph), 128.8 and 128.9 (CH2Ph), 137.7 (o-Ar).
X-ray Structure Determinations. The X-ray diffraction data
were collected with a Nonius KappaCCD diffractometer using Mo
KR radiation (λ ) 0.71073 Å). The Denzo-Scalepack29 program
package was used for cell refinements and data reduction. The
structures 6 and 8 were solved by direct methods using the
SHELXS97 and SIR97 programs and the WinGX graphical user
(30) Sheldrick, G. M. SHELXS97, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(31) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(32) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(33) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.;
Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF-99 program
system; Crystallography Laboratory, University of Nijmegen: Nijmegen,
The Netherlands, 1999.
(29) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology, Volume
276, Macromolecular Crystallography, part A; Carter, C. W., Jr.,
Sweet, R. M., Eds.; Academic Press: New York, 1997; pp 307-326.
(34) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker Analytical X-ray
Systems, Bruker AXS, Inc.: Madison, WI, 1998.
(35) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 42, No. 3, 2003 899

Bokach et al.
Scheme 1. 1,3-Dipolar Cycloaddition of Nitrile Oxides
As dipolarophiles for this study we addressed the platinum-
(IV) complexes [PtCl4(RCN)2] (R ) Me, Et, CH2Ph) because
it has been proven by our previous works that the RCN
ligands are highly activated toward the addition of nucleo-
philes such as oximes,25,50,51 dione monoximes,52 Vic-di-
oximes,53 hydroxylamines,54 hydroxamic acids,55 alcohols,56
and imines and sulfimides57 and they are also quite reactive
in 1,3-dipolar cycloaddition of nitrones.22,23 Two relatively
stable aryl nitrile oxides, i.e., 2,4,6-Me3C6H2CNO and 2,4,6-
(MeO)3C6H2CNO,27 have been chosen as dipoles. The
reaction between [PtCl4(EtCN)2], exhibiting moderate solu-
bility in CH2Cl2, and the nitrile oxides in a molar ratio of
1:4 proceeds smoothly in a suspension at room temperature
and is complete after 1 day (Scheme 1), and the final products
were isolated in good yields. The reaction with the two other
nitrile complexes [PtCl4(RCN)2] (R ) Me, PhCH2) is
hampered by their very low solubility even in the appropriate
neat nitriles, and we were able to obtain (1,2,4-oxadiazole)-
platinum complexes 1 and 4 only with the more stable nitrile
oxide, i.e., 2,4,6-Me3C6H2CNO, while in the case of 2,4,6-
(MeO)3C6H2CNO gradual decomposition of the latter is faster
than the overall process comprising the dissolution of the
starting material and the cycloaddition.
Figure 1. Molecular structure of 8. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (Å) and angles (deg): Pt-
(1)-N(1) 2.023(3), Pt(1)-Cl(2) 2.3162(9), Pt(1)-Cl(1) 2.3198(8), N(1)-
C(1) 1.273(4), C(1)-C(2) 1.482(5), C(1)-O(1) 1.357(4), O(1)-N(2)
1.449(4), N(2)-C(4) 1.268(4); Cl(1)-Pt(1)-Cl(2) 89.22(4), Cl(1)-Pt(1)-
N(1) 93.66(9), Cl(2)-Pt(1)-N(1) 86.26(8), N(1)-C(1)-C(2) 128.9(3),
N(1)-C(1)-O(1) 120.1(3), C(1)-O(1)-N(2) 112.3(2), O(1)-N(2)-C(4)
108.7(3). Hydrogen bond parameters: N(1)-H(1) 0.89(4), H(1)‚‚‚N(2) 2.07,
N(1)‚‚‚N(2) 2.557(4), N(1)-H(1)‚‚‚N(2) 114(3).
reviews36-38 and books,19 their coordination chemistry is still
poor, and known examples include deoxygenation of nitrile
39
oxides with Fe(CO)5 or Pt(0)-bound phosphines40 and the
reaction of isocyanide cobalt complexes with a nitrile oxide,
which leads to the formation of carbamoyl cobalt com-
pounds.41 In addition, a few publications concern the dipolar
cycloaddition of nitrile oxides to coordinated ligands by
reactions with metal carbonyls42,43 and multiple bonds in
[W]sPdC44 and also the cycloaddition to remote CdC and
CtC bonds which are not ligated to a metal center.45-49 To
the best of our knowledge, 1,3-dipolar cycloaddition of nitrile
oxides to coordinated nitriles has not yet been investigated.
To get an estimate of the effect of the metal center on the
cycloaddition, the reaction between 2,4,6-Me3C6H2CNO
(0.01 mmol) and [PtCl4(EtCN)2] (0.005 mmol) or EtCN (0.01
mmol) has been investigated in CDCl3 solution (1.5 mL) at
50 °C. It was found that signals from the cycloaddition
products appear in the spectra after 20 min and 18 h,
correspondingly. These data explicitly show the activation
effect of the metal center and indicate that the cycloaddition
(36) Karlsson, S.; Hogberg, H.-E. Org. Prep. Proced. Int. 2001, 33, 103.
(37) Kotyatkina, A. I.; Zhabinsky, V. N.; Khripach, V. A. Russ. Chem.
ReV. 2001, 70, 641.
(50) Ferreira, C. M. P.; Guedes da Silva, M. F. C.; Frau´sto da Silva, J. J.
R.; Pombeiro, A. J. L.; Kukushkin, V. Yu.; Michelin, R. A. Inorg.
Chem. 2001, 40, 1134.
(51) Kopylovich, M. N.; Kukushkin, V. Yu.; Frau´sto da Silva, J. J. R.;
Pombeiro, A. J. L. Inorg. Chem. 2002, 41, 4798.
(52) Makarycheva-Mikhailova, A. V.; Haukka, M.; Bokach, N. A.; Gar-
novskii, D. A.; Galanski, M.; Keppler, B. K.; Pombeiro, A. J. L.;
Kukushkin, V. Yu. New J. Chem. 2002, 26, 1085.
(38) Litvinovskaya, R. P.; Khripach, V. A. Russ. Chem. ReV. 2001, 70,
405.
(39) Genco, N. A.; Partis, R. A.; Alper, H. J. Org. Chem. 1973, 38, 4365.
(40) Beck, W.; Keubler, M.; Leidl, E.; Nagel, U.; Schaal, M.; Cenini, S.;
Del Buttero, P.; Licandro, E.; Maiorana, S.; Villa, A. C. Chem.
Commun. 1981, 446.
(41) Strecker, B.; Werner, H. Z. Anorg. Allg. Chem. 1991, 605, 59.
(42) Chetcuti, P. A.; Walker, J. A.; Knobler, C. B.; Hawthorne, M. F
Organometallics 1988, 7, 641.
(43) Werner, H.; Brekau, U.; Nuernberg, O.; Zeier, B. J. Organomet. Chem.
1992, 440, 389.
(44) Maerkl, G.; Beckh, H. J. Tetrahedron Lett. 1987, 28, 3475.
(45) Baldoli, C.; Del Buttero, P.; Maiorana, S.; Zecchi, G.; Moret, M.
Tetrahedron Lett. 1993, 34, 2529.
(46) Le Gall, T.; Lellouche, J. P.; Toupet, L.; Beaucourt, J. P. Tetrahedron
Lett. 1989, 30, 6517.
(47) Malisch, W.; Zoeller, J.; Schwarz, M.; Jaeger, V.; Arif, A. M. Chem.
Ber. 1994, 127, 1243.
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1996, 37, 4341.
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Kuznetsov, M. L.; Galanski, M.; Pombeiro, A. J. L. Inorg. Chem.
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D. A.; Haukka, M.; Pombeiro, A. J. L. Inorg. Chem. 2002, 41, 2981.
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D. A.; Natile, G.; Pombeiro, A. J. L. Inorg. Chem. 2002, 41, 2041.
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900 Inorganic Chemistry, Vol. 42, No. 3, 2003

A Route to 1,2,4-Oxadiazoles and Their Complexes
can be performed under mild conditions starting even from
complexed acetonitrile and propionitrile, which exhibit low
reactivity in the free state.
We also tried to perform the cycloaddition of 2,4,6-
Me3C6H2CNO to nitriles in the platinum(II) complex [PtCl2-
(EtCN)2] upon prolonged reflux of these reagents in CHCl3
solution, whereupon the solvent was evaporated and the
residue was analyzed by 1H NMR and FAB-MS. New signals
were observed in 1H NMR spectra, but their intensity is much
lower than that of the signals from starting Pt(II) complex
and products from the degradation of nitrile oxide, and they
are close to the detection limit. However, peaks from
products of the cycloaddition to only one ligated nitrile, i.e.,
[PtCl2(EtCN){NdC(Et)ONdCC6H2Me3}] and [PtCl2(EtCN)-
{NdC(Et)ONdCC6H2(OMe)3}], were observed in FAB+
mass spectra.
We also attempted to develop another route to the (1,2,4-
oxadiazole)platinum(IV) complexes as follows: (i) coupling
between EtCN in trans-[PtCl4(EtCN)2] and the oxime HONd
C(H)C6H2Me3 to give 8 (for the X-ray structure see Figure
1, and for characterization see the Experimental Section),
(ii) the Piloty reaction58 of 8 with Cl2, hoping to furnish
[PtCl4{HNdC(Et)ONdC(Cl)C6H2Me3}2] followed by de-
hydrochlorination (similar to that observed in the case of
halo alcohols59) to yield [PtCl4(1,2,4-oxadiazole)2]. However,
the chlorination of 8 brings about overall degradation of the
complex, and the Piloty product was not isolated.
Reduction of the (1,2,4-Oxadiazole)platinum(IV) Com-
plexes to the Corresponding (1,2,4-Oxadiazole)platinum-
(II) Complexes. The reaction between complexes 2-4 and
1 equiv of Ph3PdCHCO2Me proceeds under mild conditions
(50 °C, 24 h) in CH2Cl2 to give the reduction product along
with various (partially identified28) phosphorus-containing
species; the latter are retained on SiO2 upon purification by
column chromatography, while the former come out in the
first fraction. The reduction failed only in the case of 1
insofar as this complex is insoluble in the most common
organic solvents. The Pt(II) compounds 5-7 were character-
ized by elemental analyses, FAB mass spectrometry, and IR
and 1H, 13C{1H}, and 195Pt NMR spectroscopies, and 6 was
also characterized by X-ray crystallography (see later).
General features of the IR and 1H and 13C{1H} NMR spectra
of the corresponding Pt(II) and Pt(IV) complexes are similar.
The most significant difference between the two types of
complexes was detected for 2 and 5 in their 195Pt NMR
Figure 2. Molecular structure of 2. Thermal ellipsoids are drawn at the
50% probability level.
spectra, which show the shift of the Pt(IV) signal from 18
(2) to -2224 (5) ppm. Thus, the (1,2,4-oxadiazole)platinum-
(II) complexes which could not be prepared by the cycload-
dition between [PtCl2(RCN)2] and nitrile oxides were
prepared by the alternative route via reduction of the Pt(IV)
precursors. The obtained [PtCl2(1,2,4-oxadiazole)2] com-
plexes were employed in attempted liberation of the hetero-
cycles (see later).
Characterization of the (1,2,4-Oxadiazole)platinum
Complexes. The structures of the platinum(IV) 2 (Figure 2)
and the platinum(II) 6 complexes (Figure 3) were determined
by X-ray diffraction. The coordination polyhedra of the
complexes are a slightly distorted octahedron and a square
plane, correspondingly. The Pt-Cl bond lengths are normal.25
The crystallographic data indicate the presence of two
1,2,4-oxadiazole ligands bound to the Pt atom via their N4
atoms. To the best of our knowledge, despite the wealth of
structural chemistry of uncoordinated 1,2,4-oxadiazoles,60
only three structures of complexed species are known, i.e.,
Sn(IV),61 Fe(II),62 and Ni(II) complexes.62 The structures
(60) See, e.g.: Golic, L.; Leban, I.; Stanovnik, B.; Tisler, M. Acta
Crystallogr., Sect. B 1979, 35, 2256. Albinati, A.; Bruckner, S. Acta
Crystallogr., Sect. B 1978, 34, 3390. Press, J. B.; Eudy, N. H.; Lovell,
F. M.; Morton, G. O.; Siegel, M. M. J. Am. Chem. Soc. 1982, 104,
4013. Horvath, K.; Korbonits, D.; Naray-Szabo, G.; Simon, K. J. Mol.
Struct. 1986, 136, 215. Eberson, L.; Calvert, J. L.; Harishom, M. P.;
Robinson, W. T. Acta Chem. Scand. 1994, 48, 347. Baker, R.; Showell,
G. A.; Street, L. J.; Saunders, J.; Hoogsteen, K.; Freedman, S. B.;
Hargreaves, R. J. Chem. Commun. 1992, 817. Batista, H.; Carpenter,
G. B.; Srivastava, R. M. J. Chem. Crystallogr. 2000, 30, 131. Hewlins,
S. A.; Murphy, J. A.; Lin, J.; Hibbs, D. E.; Hursthouse, M. B. J. Chem
Soc., Perkin. Trans. 1 1997, 1559.
(58) Kukushkin, Yu. N.; Krylov, V. K.; Kaplan, S. F.; Calligaris, M.;
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Inorganic Chemistry, Vol. 42, No. 3, 2003 901

Bokach et al.
Scheme 2. Reduction of the Platinum(IV) Oxadiazole Complexes to
the Platinum(II) Oxadiazole Complexes and Liberation of
1,2,4-Oxadiazoles from the Former Complexes
exhibit higher solubility, and signals of carbons from the
heteroaromatic rings were detected. For the three compounds,
the 13C signals of the group NdCsO lie in the range 179.6-
182.0 ppm and the signals of NdCsN in the range 163.4-
167.3 ppm. For the uncoordinated 1,2,4-oxadiazoles these
signals appear at 180.0 and 180.4 (NdCsO) and 163.9 and
168.2 (NdCsN) ppm, which are very close to the coordi-
nated ones. All complexes exhibit only one signal in the 195
Pt NMR spectra.
-
Liberation of 1,2,4-Oxadiazoles from the Platinum
Complexes. The heterocycles formed in the course of the
metal-mediated reaction were liberated almost quantitatively
from their Pt(IV) complexes. The method is based on the
reaction of the platinum(IV) complexes 2-4 with an excess
of pyridine in chloroform, giving free 1,2,4-oxadiazoles in
solution and a precipitate of the well-known trans-[PtCl4-
(pyridine)2]63 (Scheme 2).
The latter complex is separated from the reaction mixture
by filtration, whereupon the filtrate is evaporated to dryness
to give the pure 1,2,4-oxadiazoles 9-11. Compound 1
exhibits very low solubility in the most common solvents,
and the oxadiazole was not released even on prolonged
heating with pyridine. Moreover, the endeavor of the
liberation of the oxadiazoles from 2-4 with Ph2PCH2CH2-
PPh2 (dppe) was not successful from a synthetic viewpoint
since the phosphine reduces the Pt(IV) center to Pt(II)
without ligand liberation.
We also attempted to perform the liberation starting from
the platinum(II) complexes trans-[PtCl2(1,2,4-oxadiazole)2]
(5-7) with 2 equiv of dppe (Scheme 2). However, the
surprising stability of 5-7 toward the substitution for the
chelating ligand precluded this route. The replacement of
the 1,2,4-oxadiazole species from the platinum(II) complexes
with an excess of pyridine was not fully achieved, and the
liberated heterocycles were contaminated with unreacted
starting Pt(oxadiazole) complexes and with the [PtCl2-
(pyridine)2] product, which exhibits a good solubility. Hence,
this is not a useful method for that purpose.
Figure 3. Molecular structure of 5. Thermal ellipsoids are drawn at the
50% probability level.
reported here are the first examples of (1,2,4-oxadiazole)-
platinum compounds. The geometrical parameters of the ring
are almost unaffected by ligation of the heterocycles to the
metal centers or by a change of the oxidation state of the Pt
center (Table 2). Indeed, all bond lengths and angles in the
cycle of coordinated 1,2,4-oxadiazoles within 3σ correspond
to those in the purely organic species.60
In addition to the X-ray structural data, formulation of
complexes 1-4 was supported by satisfactory C, H, and N
elemental analyses and agreeable FAB mass spectrometry
data, and the complexes were also characterized by IR and
1H, 13C{1H}, and 195Pt NMR spectroscopies. The IR spectra
of the platinum(IV) complexes 1-4 and the platinum(II)
complexes 5-7 display two bands of medium intensity, ν-
(CdN and CdC from aromatic and heteroaromatic rings),
which emerge in the ranges of 1611-1616 and 1550-1580
cm-1, and these values for the coordinated heterocycles are
in good agreement with those for the corresponding free
1
1,2,4-oxadiazoles. In the H NMR spectra of 1-4, signals
of the alkyl group derived from the parent complexed nitrile
and of the aromatic and alkyl groups from the parent nitrile
oxide were detected in a 1:1 ratio, which corresponds to the
cycloaddition of the nitrile oxide to both the coordinated
1
nitriles. Comparison of the H NMR spectra of 1-4 with
the spectra of the starting nitrile complexes [PtCl4(RCN)2]
shows a lower field shift (ca. 0.4-0.5 ppm) of the alkyl group
R in the products. Two sets of signals for each proton and
carbon in both 1H and 13C{1H} NMR spectra were observed
for the two Pt(II) complexes, i.e., 5 and 7, and we attribute
this to nonequivalence of the bulky ligands. Because of the
low solubility of the Pt(IV) compounds 1-4 in the most
common organic solvents, we were unable to obtain their
complete spectra. However, the platinum(II) compounds 5-7
Final Remarks
This work demonstrates that nitriles can be activated, by
coordination, toward 1,3-dipolar cycloaddition of dipoles of
the propargyl/allenyl anion type24 and therefore are not
(63) Jørgensen, S. M. J. Prakt. Chem. 1886, 33, 489.
902 Inorganic Chemistry, Vol. 42, No. 3, 2003

A Route to 1,2,4-Oxadiazoles and Their Complexes
limited to dipoles of the allyl anion type.24 The unprecedented
resulting [2 + 3] cycloaddition, followed by liberation of
the 1,2,4-oxadiazoles formed, which has been achieved for
the first time in this work, represents a new and alternative
route, performed under mild conditions, to this important
class of heterocycles. The only limitation we see so far is
the poor solubility of the starting materials, making the
cycloaddition difficult, and/or the (1,2,4-oxadiazole)platinum-
(IV) complexes, which precludes the liberation. However,
we anticipate that this general barrier of platinum(IV)
chemistry can be overcome with the use of other activating
metal centers, and this project is under way in our group.
The metal enhancement of the activation of the ligated nitriles
is expected to promote other 1,3-dipolar cycloadditions, and
the generality of this strategy for the synthesis of heterocycles
should be further explored.
fellowship and also INTAS for Grant YSF 2002-93. N.A.B.
and A.V.K. are indebted to the Centro de Qu´ımica Estrutural,
Complexo I, Instituto Superior Te´cnico, Portugal, for a grant.
V.Yu.K. also thanks the International Science Foundation
(Soros Foundation) for a Soros Professorship, the Royal
Society of Chemistry for a Grant for International Authors,
and PRAXIS XXI and POCTI programs (Portugal) for Grant
BCC16428/98. A.J.L.P. and V.Yu.K. are grateful to the FCT
(Foundation for Science and Technology) (Portugal) and the
PRAXIS XXI and POCTI programs for financial support of
these studies. We thank Dr. S. I. Selivanov for measuring
some NMR spectra.
Supporting Information Available: Tables of crystallographic
data for 2, 6, and 8 (PDF). Crystallographic data for 2, 6, and 8 in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. N.A.B. express gratitude to the In-
ternational Science Foundation (Soros Foundation) for a
IC026103V
Inorganic Chemistry, Vol. 42, No. 3, 2003 903