Unexpectedly efficient activation of push-pull nitriles by a Pt II center toward dipolar cycloaddition of Z-nitrones

Article abstract of DOI:10.1039/c0dt01689f

Pt^II-coordinated NCNR′₂ species are so highly activated towards 1,3-dipolar cycloaddition (DCA) that they react smoothly with the acyclic nitrones ArCHN⁺(O^-)R′′ (Ar/R′′ = C₆H₄Me-p/Me; C₆H ₄OMe-p/CH₂Ph) in the Z-form. Competitive reactivity study of DCA between trans-[PtCl₂(NCR)₂] (R = Ph and NR′₂) species and the acyclic nitrone 4-MeC₆H ₄CHN⁺(O^-)Me demonstrates comparable reactivity of the coordinated NCPh and NCNR′₂, while alkylnitrile ligands do not react with the dipole. The reaction between trans-[PtCl ₂(NCNR′₂)₂] (R′₂ = Me₂, Et₂, C₅H₁₀) and the nitrones proceed as consecutive two-step intermolecular cycloaddition to give mono-(1a-d) and bis-2,3-dihydro-1,2,4-oxadiazole (2a-d) complexes (Ar/R′′ = p-tol/Me: R′₂ = Me₂a, R′₂ = Et ₂b, R′₂ = C₅H₁₀c; Ar/R′′ = p-MeOC₆H₄/CH₂Ph: R′₂ = Me₂d). All complexes were characterized by elemental analyses (C, H, N), high resolution ESI-MS, IR, ¹H and ¹³C{¹H} NMR spectroscopy. The structures of trans-1b, trans-2a, trans-2c, and trans-2d were determined by single-crystal X-ray diffraction. Metal-free 5-NR′₂-2,3-dihydro-1,2,4-oxadiazoles 3a-3d were liberated from the corresponding (dihydrooxadiazole) ₂Pt^II complexes by treatment with excess NaCN and the heterocycles were characterized by high resolution ESI⁺-MS, ¹H and ¹³C{¹H} spectroscopy.

Full text of DOI:10.1039/c0dt01689f

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PAPER
Unexpectedly efficient activation of push–pull nitriles by a PtII center toward
dipolar cycloaddition of Z-nitrones†
Andreii S. Kritchenkov,a Nadezhda A. Bokach,*a Matti Haukkab and Vadim Yu. Kukushkin*a,c
Received 2nd December 2010, Accepted 2nd February 2011
DOI: 10.1039/c0dt01689f
PtII-coordinated NCNR¢2 species are so highly activated towards 1,3-dipolar cycloaddition (DCA) that
they react smoothly with the acyclic nitrones ArCH N+(O-)R¢¢ (Ar/R¢¢ = C6H4Me-p/Me;
C6H4OMe-p/CH2Ph) in the Z-form. Competitive reactivity study of DCA between
trans-[PtCl2(NCR)2] (R = Ph and NR¢2) species and the acyclic nitrone 4-MeC6H4CH N+(O-)Me
demonstrates comparable reactivity of the coordinated NCPh and NCNR¢2, while alkylnitrile ligands
do not react with the dipole. The reaction between trans-[PtCl2(NCNR¢2)2] (R¢2 = Me2, Et2, C5H10) and
the nitrones proceed as consecutive two-step intermolecular cycloaddition to give mono-(1a–d) and
bis-2,3-dihydro-1,2,4-oxadiazole (2a–d) complexes (Ar/R¢¢ = p-tol/Me: R¢2 = Me2 a, R¢2 = Et2 b, R¢2 =
C5H10 c; Ar/R¢¢ = p-MeOC6H4/CH2Ph: R¢2 = Me2 d). All complexes were characterized by elemental
1
analyses (C, H, N), high resolution ESI-MS, IR, 1H and 13C{ H} NMR spectroscopy. The structures of
trans-1b, trans-2a, trans-2c, and trans-2d were determined by single-crystal X-ray diffraction. Metal-free
5-NR¢2-2,3-dihydro-1,2,4-oxadiazoles 3a–3d were liberated from the corresponding
(dihydrooxadiazole)2PtII complexes by treatment with excess NaCN and the heterocycles were
1
characterized by high resolution ESI+-MS, 1H and 13C{ H} spectroscopy.
Introduction
Nitriles can be strongly activated upon their ligation to a metal
center, which often results in promotion of nucleophilic1–3 and
electrophilic addition,2 or 1,3-dipolar cycloaddition (DCA);1,4–6
many of these processes are not feasible for the corresponding
metal-free RCN species. In particular, reactions of metal-activated
RCN ligands with 1,3-dipoles leads to nitrogen heterocycles, which
are stabilized by metal centers and some of them do not even exist
after liberation and they split to the starting reactants.7
Although a rather wide variety of allyl- [acyclic nitrones (A,
Fig. 1),8–13 cyclic nitrones such as pyrroline-N-oxide (B),14–16
imidazoline-N-oxides (C),7 oxazoline-N-oxides (D),17 and ni-
tronates (E)18] and propargyl/allenyl- [azides (F)6,19–22 and nitrile
oxides (G)23,24] anion type dipoles have been applied for metal-
mediated DCA to nitriles, the range of employed RCN substrates
is usually restricted to the conventional species such as alkyl- and
Fig. 1 1,3-Dipoles applied for DCA to nitriles.
aDepartment of Chemistry, St. Petersburg State University, Universitetsky
Pr. 26, 198504, Stary Petergof, Russian Federation. E-mail: bokach@
nb17701.spb.edu, kukushkin@vk2100.spb.edu; Fax: +7 812 4286939;
Tel: +7 812 4286890
arylnitriles. Application of DCA in another important category
of nitrile substrates, the so-called push–pull nitriles (a push–pull
system is highly polarized and it is characterized by an electron-
withdrawing substituent or electronegative atom on one side of the
multiple bond and an electron-donating substituent on the other
side),25–27 e.g. dialkylcyanamides NCNR¢2, so far have attracted
only little attention despite the fact that data gradually accumu-
lated in the literature indicates intriguing reactivity differences
between the conventional and the push–pull RCN species.27–31
bDepartment of Chemistry, University of Eastern Finland, P.O. Box 111,
FI-80101, Joensuu, Finland
cInstitute of Macromolecular Compounds of Russian Academy of Sciences,
V.O. Bolshoi Pr. 31, 199004, St. Petersburg, Russian Federation
† Electronic supplementary information (ESI) available: crystal cif file;
view of 2a and 2d with the atomic numbering scheme. CCDC reference
numbers 803620–803623. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01689f
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Indeed, to the best of our knowledge only one report32 is devoted
to metal-free DCA of the highly reactive cyclic E-configurated
indole-based nitrone (H, Fig. 1) to NCNMe2 performed in toluene
under rather harsh conditions (80 ◦C, 3.5 h). In addition, few
examples of DCA to push–pull nitrile ligand, i.e. PdII- and PtIV-
mediated DCA of nitrile oxides to NCNR¢223,24 and also reactions
of E-configurated imidazoline-N-oxides (D)7 or oxazoline-N-
oxides (C)17 with the dialkylcyanamide ligands at PtII centers, were
previously reported by our group.
As can be inferred from the inspection of all these works,7,17,32
highly reactive dipoles in the E-form33 have so far been applied for
DCA, while reactivity of much less reactive33,34 acyclic nitrones in
the Z-configuration was not studied. It is worthwhile mentioning
that Hermkens et al.32 pointed out that the acyclic nitrone Z-
PhCH N+(O-)Me reacts only with RCN substrates bearing
electron withdrawing substituents (R = CCl3, Ph) and no examples
of DCA with nitriles having donor groups (R = Me, NMe2) were
observed.
As an amplification of our previous studies on metal-mediated
DCA,7,13,17,18,23 and in view of our general interest in reactiv-
ity of nitriles (for reviews see1,2,4,5,35,36 and for recent works
see7,17,18,27,30,31,37,38) and isonitriles (for recent works see ref. 39,40),
we studied DCA of the acyclic nitrones ArCH N+(O-)R¢¢
(Ar/R¢¢ = C6H4Me-p/Me; C6H4OMe-p/CH2Ph)41,42 in the Z-
configuration to the push–pull nitriles in the complexes
[PtCl2(NCNR¢2)2] (R¢2 = Me2, Et2, C5H10). The scenario of this
work was the following: (i) to discover the reactivity of NCNR¢2
ligands toward DCA of the acyclic nitrones Z-ArCH N+(O-)R¢¢
and to characterize the DCA products; (ii) to decoordinate the
5-NR¢2-2,3-dihydro-1,2,4-oxadiazoles and to characterize these
metal-free heterocycles; (iii) to perform a comparative kinetic
study on the reactivity of dialkylcyanamide ligands and the
conventional coordinated RCN species (i.e., PhCN and EtCN).
Scheme 1 Two-step PtII-mediated DCA and liberation of the oxadiazoles
(Ar/R¢¢ = p-tol/Me: R¢2 = Me2 a, R¢2 = Et2 b, R¢2 = C5H10 c; Ar/R¢¢ =
p-MeOC6H4/CH2Ph: R¢2 = Me2 d).
Thus, the reaction of Z-ArCH N+(O-)R¢¢ and trans-
[PtCl2(NCNR2)2] proceeds as a consecutive two-step DCA; the
first step occurs faster than the second one. These observations
are coherent with the previous report12 indicating that DCA of
the acyclic nitrone Z-PhCH N+(O-)Ph to the nitrile ligands in
trans-[PtCl2(PhCN)2] also occurs via two consecutive steps. The
significant difference in the reaction rates between the two steps of
the DCA could be accounted for by a higher electron donor ability
of the newly formed heterocycle, compared to the corresponding
push–pull nitrile ligand that leads to an increase in the LUMO
of the remaining trans-located NCNR¢2 ligand and deactivate the
dipolarophile towards DCA.
Complexes 1a–d and 2a–d were obtained as yellow solids and
characterized by elemental analyses (C, H, N), high resolution
1
ESI+-MS, IR, and 1H and 13C{ H} NMR spectroscopies, and
Results and discussion
also by X-ray diffraction (for 1b, 2a, 2c, and 2d). All complexes
gave satisfactory microanalyses. In the ESI+-MS, the typical ions
that were detected are [M]+ and [M + Na]+. A comparison
of the IR spectra of the products with those of the starting
trans-[PtCl2(RCN)2] indicated the absence of n(C N) stretching
vibrations at ca. 2300 cm-1 for 2a–d, while for 1a–d these stretches
emerge in the region from 2289 to 2299 cm-1. The presence of
intensive n(C N) vibrations in the range 1640–1670 cm-1 was
detected in the IR spectra of all complexes.
DCA of the acyclic nitrones to dialkylcyanamide ligands
The reaction between Z-ArCH N+(O-)R¢¢ (Ar/R¢¢ = C6H4Me-
p/Me; C6H4OMe-p/CH2Ph) and trans-[PtCl2(RCN)2] (R = NMe2,
NE◦t2, NC5H10) in a molar ratio 1 : 1.5 in chloroform at 20–
1
25 C was monitored by both TLC and H NMR. It leads to
1a–d (Scheme 1) for 30 h. These species are formed in almost
quantitative NMR spectroscopy yields and 70–75% isolated yields
after column chromatography; both yields are based on the
nitrone.
1
For 1a–d and 2a–d, signal integration in the H NMR spectra
gives evidence that the reaction between each of the coordinated
1
When DCA was performed at a 2 : 1 molar ratio at room
temperature, we observed generation of 1a–d followed by their
transformation to 2a–d (Scheme 1). However, under these con-
ditions the conversion is slow and DCA is accompanied with a
gradual degradation of the nitrone. Furthermore, upon heating at
50 ◦C, the reaction loses its selectivity providing a broad spectrum
of products originated from degradations of both nitrones and
DCA products. The performance of DCA in a molar ratio 15 : 1
proceeds much faster, more selective, and with the complete
conversion of trans-[PtCl2(RCN)2] to bis-CA species 2a–d (after
24 h NMR yields ca. 100% based on the starting complexes;
isolated yields 70–86%) via intermediate formation of 1a–d (NMR
yields ca. 95–97% after 2 h).
nitriles and the nitrone proceeds in a 1 : 1 ratio. The H NMR
spectra of 1a–d and 2a–d display the broad (owing to the
quadrupole effect of two 14N) singlets of the C3H protons in the
range 5.33–5.85 ppm. The signals from the C3H protons for 2a–d
are low-field shifted from those for the corresponding 1a–d. In the
1
13C{ H} NMR spectra, the peaks due to C5 N (157.4–160.4 ppm)
1
1
and C3 (89.8–93.4 ppm) were recognized. Both H and 13C{ H}
NMR spectra of 1a–d exhibit the signals from the oxadiazole and
the nitrile ligands.
Complexes 1b (Fig. 2), 2a (Fig. S1, supplementary†), 2c (Fig. 3),
and 2d (Fig. S2, supplementary†) were characterized by single-
crystal X-ray diffraction. In 1b, the coordination polyhedron of the
Pt atom is a slightly distorted square plane with the hetetocyclic
4176 | Dalton Trans., 2011, 40, 4175–4182
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N
C double bond,44 while the N(1)–C(1), N(2)–C(1), N(4)–C(3),
and N(5)–C(3) bond lengths in 2a (1.475(4), 1.480(5), 1.469(4)
˚
and 1.480(4) A), and in 2c (1.485(4), 1.482(4), 1.482(4), and
˚
1.486(4) A) are specific for the N–C single bonds. In 2a and 2c, both
asymmetric atoms C3 in the heterocyclic ligands exhibit the same
configuration (RR/SS), while in 2d the configuration of the C3
atoms in two heterocyclic ligands is different (RS). It is worthwhile
mentioning that in 2a, 2d, and 1b, the alkyl (Me for 2a and 1b;
CH2Ph for 2d) and aryl (p-tol for 2a and 1b; p-MeOC6H4 for
2d) groups of the heterocyclic ligands adopt the trans-orientation,
while in 2c one of the heterocyclic ligands has the trans-orientation
of Me and p-tol and another one has Me and p-tol from the nitrone
situated in the cis-position.
Fig. 2 View of 1b with the atomic numbering scheme. Thermal ellipsoids
are drawn at the 50% probability level. Selected bond lengths (A)
˚
The previous studies9 demonstrated that PtIV centers activate
nitrile substrates towards DCA of acyclic nitrones in a signif-
icantly more efficient way than PtII centers. Having an aim to
enhance the cycloaddition and to achieve higher yields of (2,3-
dihydro-1,2,4-oxadiazole)Pt species, we attempted DCA of the
nitrones to the dialkylcyanamides at a PtIV center. The reac-
tion between the nitrone Z-MeC6H4CH N+(O-)Me and trans-
[PtCl4(NCNMe2)2]29—performed under the same conditions as
for PtII-mediated DCA—does not accomplish the corresponding
oxadiazole complexes. Instead, a wide range of products (up to
seven spots on the TLC; additionally a signal corresponding to
the (oxadiazole)2PtIV complex was detected in ESI+-MS: found
798.4552, calcd. [PtCl4(2,3-dihydro-1,2,4-oxadiazole)2] 798.4445)
was formed after a few minutes. After two weeks at 20–25 ◦C we
were able to identify in the solution several additional species, i.e.
the platinum(II) complex 2a and the aldehyde MeC6H4CHO. The
formation of 2a can be accounted for by the reduction of the
PtIV complex with the nitrone and/or products of its degradation
(i.e. hydroxylamine and the aldehyde formed upon hydrolysis)
and/or DCA to the PtII nitrile complex also generated in situ
upon reduction. The orange precipitate released from the reaction
mixture, based on IR spectroscopy, ESI mass-spectrometry and
13C CP-MAS NMRdata, was formulated as [NH2Me2]2[PtCl6].
This complex was previously observed in another PtIV-mediated
reaction of dialkylcyanamide, i.e. when [PtCl4(MeCN)2] was
kept in the neat undried NCNMe2 and this process afforded
the metal-mediated C–N bond cleavage of NCNMe2 to furnish
[NH2Me2]2[PtCl6].29 Thus, the PtIV center activates NCNR2 ligands
so greatly, that DCA loses its selectivity and generates a broad
mixture of products.
and angles (◦): Pt(1)–N(4) 1.960(3), Pt(1)–N(1) 2.003(3), Pt(1)–Cl(1)
2.2983(9), Pt(1)–Cl(2) 2.3002(10), N(1)–C(2) 1.299(4), N(1)–C(1) 1.480(4),
O(1)–N(2) 1.474(4), O(1)–C(2) 1.364(4), N(3)–C(2) 1.328(5), N(4)–C(3)
1.141(5), N(4)–Pt(1)–N(1) 177.65(13), Cl(1)–Pt(1)–Cl(2) 178.13(3).
Fig. 3 View of 2c with the atomic numbering scheme. Thermal ellipsoids
˚
are drawn at the 50% probability level. Selected bond lengths (A) and
angles (◦): Pt(1)–N(4) 2.006(2), Pt(1)–N(1) 2.016(2), Pt(1)–Cl(1) 2.3087(8),
Pt(1)–Cl(2) 2.3108(7), N(1)–C(2) 1.316(4), N(1)–C(1) 1.485(4), N(2)–C(1)
1.482(4), N(4)–C(4) 1.302(4), N(4)–C(3) 1.482(4), N(5)–C(3) 1.486(4),
N(4)–Pt(1)–N(1) 177.74(9), Cl(1)–Pt(1)–Cl(2) 177.81(3).
and nitrile ligands in the trans-position (Fig. 3). The N(4)C(3)
˚
bond (1.141(5) A) is the typical C N triple bond in the coordi-
nated dialkylcyanamide and nitrile ligands.43 The distance N(1)–
C(2) (1.299(4) A) is typical for the N C double bond,44 the N(1)–
˚
Liberation of 2,3-dihydro-1,2,4-oxadiazoles from 2a–d
C(1), and N(2)–C(1), are characteristic for the N–C (1.480(4) and
˚
1.478(5) A) single bonds. The NEt2 fragment of the nitrile ligand
In the past few years, several methods for liberation of the strongly
bound imines and nitrogen heterocycles from their PtII complexes
have been developed and they are based on displacement with
an excess of a diphosphine,37,45–48 or monodentate and bidentate
amines bearing two sp3-N-donor centres.11,17,49 We observed that
in 2a–d the newly formed heterocyclic ligands are so strongly
bound to the platinum(II) center that the decoordination cannot
be achieved even with 1,2-bis-(diphenylphosphino)ethane (6-fold
excess, 35–40 ◦C, 2 d) and ethane-1,2-diamine (10-fold excess, 35–
40 ◦C, 1 d). However, it was previously reported that the alkali
metal cyanides could be applied for the decoordination of some
chelated phosphine ligands, and this observation indicates that the
cyanide species are highly potent reactants for the displacement
is strongly disordered and this prevents the comparison of the
geometrical parameters. To the best of our knowledge, the mixed
(nitrile)(2,3-dihydro-1,2,4-oxadiazole)PtII complex 1b represents
the first example of a structurally characterized platinum(II)
species of this type.
The structure of 2d is strongly disordered, however the X-ray
data confirm its formulation. In 2a (Fig. S1, supplementary†),
2c (Fig. 3), and 2d (Fig. S2, supplementary†), the oxadiazole
ligands are located in the trans position. The Pt(1)–N(1) and Pt(1)–
N(4) bond lengths in each complex are typical for (imine)PtII
species.44 N(1)–C(2) and N(4)–C(4) (1.305(5) A, 1.303(5) A in
2a and 1.316(4) A, 1.302(4) A in 2c) are characteristic for the
˚
˚
˚
˚
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of strongly bound ligands at PtII centers.50 In accord with this
assumption, we observed that treatment of 2a–d with NaCN
(6 equivs., 35–40 ◦C, 1 d) leads to almost quantitative formation
of the metal-free oxadiazoles (Scheme 1). These species were
characterized by high resolution ESI+ mass spectrometry and 1H
1
and 13C{ H} NMR spectroscopy. In the ESI+-MS, the observed
peaks were attributed to the quasi-ions [M + H]+ and [M +
1
1
Na]+. The H and 13C{ H} NMR spectra of the uncomplexed
oxadiazoles demonstrate all signals specific for these ring systems.
Thus, the heterocycles exhibit signals from the C3a (88.9–92.3 ppm)
1
and the C N (159.4–161.2 ppm) carbons in the 13C{ H} spectra,
the former signal is high field shifted (ca. 1.0–1.5 ppm) and the
latter is low field shifted (1–3 ppm) relative to the corresponding
signals in the (oxadiazole)2PtII species, although chemical shifts
are not directly comparable because the spectra of the complexed
and metal-free oxadiazoles were measured in different solvents.
Competitive reactivity study
Scheme 2 Competitive reactivity study of DCA to PhCN and NCNR¢2
ligands.
The synthetic data of the current work and those previously
obtained for DCA of nitrones to platinum complexes bearing the
conventional nitrile ligands (for discussion see ref. 4,9) explicitly
indicate that the alteration of the substituent of R of RCN in
(nitrile)2PtII complexes plays a dramatic role on the reactivity.
In general, the reactivity toward DCA of nitrones increases
with the increase of electron-deficiency of RCN species. The
most intriguing issue of the current work is the observation of
unexpectedly high reactivity of the push–pull dialkylcyanamide
ligands NCNR¢2 bearing the strong donor substituents NAlk2
(as confirmed by considering their Pickett PL and Lever EL
parameters51,52) toward DCA. As it was revealed in the preparative
experiments, the reactivity of the nitriles with NAlk2 is comparable
with that of a moderate acceptor group Ph. To get a quantitative
estimate of the reactivity dependence on the nature of the R group,
we conducted a competitive reactivity study of DCA to NCNR¢2
(R¢2 = Me2, Et2, NC5H10) and PhCN coordinated species. The
method employed includes DCA between the equimolar mixture
of two complexes and one dipole, i.e. 4-MeC6H4CH N+(O-)Me,
with a 2 : 1 overall molar ratio between the metal-containing
species and the dipole (see Experimental); under these conditions
the first step of DCA was monitored (Scheme 2).
In the context of the similar reactivity of NCNR¢2 and NCPh
ligands, attention should be drawn to the work by Hermkens
et al.32 who found (based purely on synthetic experiments) that the
uncomplexed nitriles RCN react with the highly reactive indole-
derived cyclic nitrone in the E-form (H, Fig. 1) and the reactivity
in DCA generally increases with an increase in the electron-
withdrawing ability of the substituents R in the following order
Me ꢀ Ph < NMe2 < CO2Et < CCl3. Position of the strong
electron-donating group NMe2 between the moderate (Ph) and
strong (CO2Et) accepting groups in the metal-free experiment32
and the similar reactivity of the dialkylcyanamides and PhCN,
verified in our kinetic study, apparently could be explained by the
crossover in FMO control of DCA, specific for dialkylcyanamides.
chemistry of the push–pull nitriles such as, e.g. dialkylcyanamides
R¢2NCN, has so far been little explored, although data gradually
accumulated in the literature indicates that the R¢2NCN ligands
might exhibit some exciting reactivity modes unknown for alkyl-
or arylnitrile ligands.27,29,30,53–55
DCA of the rather unreactive acyclic Z-nitrones to dialkyl-
cyanamide ligands, conducted under mild conditions and de-
scribed above, represents a novel example of reactivity for metal-
activated push–pull nitriles. These reactions constitute a facile
route to the coordinated and metal-free (after the liberation) 5-
NR¢2-2,3-dihydro-1,2,4-oxadiazoles. In this context, it is worth
mentioning that platinum(II) 5-Ph-2,3-dihydro-1,2,4-oxadiazole
complexes exhibit significant antitumor activity56,57 and we believe
that the antitumor activity study of more water soluble (5-
NR¢2-2,3-dihydro-1,2,4-oxadiazoles)2PtII complexes could be an
interesting further task.
The competitive reactivity study of DCA between
[PtCl2(RCN)2] (R = Ph, NMe2, NEt2, NC5H10) and the acyclic
nitrone demonstrates comparable reactivity of coordinated NCPh
and NCNR¢2. Unexpectedly high activation of the NCNR¢2
species due to the coordination in contrast to alkylnitrile ligands
(see results and discussion) requires additional investigation.
Further works on the reactivity differences between the push–pull
and the conventional nitrile species ligated to various metal
centers are under way in our group.
Experimental
Materials and instrumentation
Solvents were obtained from commercial sources and used as re-
ceived. Complexes [PtCl2(RCN)2] (R = NEt2, NMe2, NC5H10) were
synthesized in accordance with the published procedures.29 The
nitrones ArCH N+(O-)R¢¢ (Ar/R¢¢ = C6H4Me-p/Me; C6H4OMe-
p/CH2Ph) were obtained by the known protocol based on con-
densation of the aldehydes p-MeC6H4CHO and p-MeOC6H4CHO
with R¢¢NHOH·HCl (R¢¢ = Me, CH2Ph).41,42 C, H, and N elemental
analyses were carried out by the Department of Organic Chemistry
Final remarks
Despite the wealth of chemistry associated with reactions of metal-
bound conventional nitriles RCN (R = alkyl, aryl), the coordination
4178 | Dalton Trans., 2011, 40, 4175–4182
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Table 1 Crystal data
1b
2a
2c
2d
Empirical formula
Fw
C19H31Cl2N5OPt
611.48
100(2)
C24H34Cl2N6O2Pt
704.56
100(2)
0.71073
Monoclinic
P21/n
16.0916(4)
9.3706(2)
18.3895(3)
90
91.5729(13)
90
2771.87(10)
4
1.688
5.287
C30H42Cl2N6O2Pt
784.69
100(2)
0.71073
Monoclinic
P21/n
9.6768(2)
12.1702(2)
26.9509(3)
90
96.1540(11)
90
3155.68(9)
4
1.652
4.653
C36H42Cl2N6O4Pt
888.75
100(2)
0.71073
Monoclinic
P21/n
8.4287(7)
11.1631(8)
19.8266(14)
90
97.669(4)
90
1848.8(2)
2
1.596
3.986
11105
Temp (K)
˚
l (A)
0.71073
Cryst syst
Triclinic
¯
Space group
P1
˚
a (A)
8.2227(2)
11.0745(3)
13.7740(4)
102.4393(13)
99.8321(14)
102.6972(13)
1163.15(5)
2
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
Z
rcalc (Mg m-3)
m (Mo Ka) (mm-1)
No. reflns.
Unique reflns.
GOOF (F2)
Rint
1.746
6.280
16522
5325
1.080
0.0274
0.0250
0.0558
45783
6354
1.069
0.0533
0.0283
0.0534
44178
9211
1.037
0.0556
0.0313
0.0558
3320
1.105
0.0465
0.0515
0.1025
R1a (I ≥ 2s)
wR2b (I ≥ 2s)
a R1 = RꢁFo| - |Fcꢁ/R|Fo|. b wR2 = [R[w(Fo - Fc2)2]/R[w(Fo2)2]]1/2
.
2
of St. Petersburg State University on a 185B Carbon Hydrogen
Nitrogen Analyzer Hewlett Packard. Electrospray ionization mass
spectra were obtained on a Bruker micrOTOF spectrometer
equipped with an electrospray ionization (ESI) source. The
instrument was operated both in positive and negative ion mode
using a m/z range of 50–3000. The capillary voltage of the ion
source was set at -4500 V (ESI+-MS) and the capillary exit at
(70–150) V. The nebulizer gas flow was 0.4 bar and the drying
gas flow 4.0 L min-1. For ESI, species were dissolved in MeOH
or MeCN, NaBF4 was used as an addition ionization agent. In
the isotopic pattern, the most intensive peak is reported for both
the complexes and the free heterocycles. TLC was performed on
Merck 60 F254 SiO2 plates. Infrared spectra (4000–400 cm-1) were
recorded on a Shimadzu FTIR-8400S instrument in KBr pellets.
ligand were restrained to be similar. The hydrogen atoms were
positioned geometrically and constrained to ride on their parent
˚
carbon atoms, with C–H = 0.95–1.00 A, Uiso = 1.2–1.5 Ueq (parent
atom). The crystallographic details are summarized in Table 1.
Competitive reactivity study
The reactions were performed in a CDCl3 solution (0.5 mL
CDCl3, 0.5 mmol of each of two studied complexes, 0.5 mmol
of the nitrone) at 25 ◦C and the reactions were monitored by
1H NMR spectroscopy. Hexamethyldisiloxane was used as an
internal standard. The spectra were registered immediately after
the addition of the dipole to the reaction mixture and then after
0.25, 0.75, 2, 4, 9, and 20 h; after 36 h the nitrone was not detected
in the 1H NMR spectra.
1
1H and 13C{ H} NMR spectra were measured in CDCl3 on Bruker
k1
DPX-300 and AMX-400 spectrometers at ambient temperature.
The ratio
was obtained in accord with the formula
k2
X-ray crystal structure determination
k1 S(X )t
=
,
The crystals of 2a, 2c, 2d, and 1b and were immersed in cryo-
oil, mounted in a nylon loop, and measured at a temperature of
100 K. The X-ray diffraction data were collected on a Nonius Kap-
k2 S(Y i )t
where S(X)t and S(Yi)t are integral intensities of signals of the
C3H proton of the oxadiazole ring from X and Yi (Scheme 2). The
mean value was calculated based on the timepoints for 0.25, 0.75,
k1
˚
paCCD diffractometer using Mo Ka radiation (l = 0.71073 A).
The Denzo-Scalepack58,59 program package was used for cell
refinements and data reductions. The structures were solved by
direct methods using SIR200459 or SHELXS-9760 programs with
a WinGX61 graphical user interface. A semi-empirical (SADABS)
method62 was applied to all of the data. Structural refinements were
carried out using SHELXL-97.60 In 1b, the N,N -diethylcyanamide
ligand was heavily disordered. The N(5), C(6), and C(7) atoms
were disordered over two sites with occupancies 0.9 and 0.1.
The C(4) and C(5) carbon atoms were disordered over three sites
with occupancies 0.7, 0.2, and 0.1. The anisotropic displacement
parameters of the disordered atoms were constrained to be similar.
Also, the C–C and N–C distances in the N,N -diethylcyanamide
2 and 4 h since the constant ratio
is stable within the specified
k2
time period.
Synthetic work
Synthesis of the (2,3-dihydro-1,2,3-oxadiazole)(nitrile)PtII com-
plexes (a general procedure). The solutions of each of the nitrones
(0.1 mmol) in CHCl3 (1 mL) were added to a solution of
the corresponding trans-[PtCl2(NCR)2] (0.15 mmol; R = NMe2,
NC5H10, NEt2) in CHCl3 (1 mL). The mixture was stirred at
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4175–4182 | 4179
©

View Online
room temperature for 30 h and the progress of the reaction
was monitored by TLC. The separation of 1a–d was achieved
by column chromatography on SiO2 (the first fraction; eluent
CHCl3/Me2CO, 10/1, v/v). The solvent was evaporated in vacuo
at 20–25 ◦C to give yellow oily residues. The residues were
crystallized under n-hexane to form the yellow powders of
1a–d. The complexes were dried in air at 20–25 ◦C. Yields 70–
75%.
ligand), 46.2 (N(O)Me and a-CH2 from NC5H10 of the 2,3-
dihydro-1,2,4-oxadiazole ligand), 50.0 (a-CH2 from NC5H10 of
the nitrile ligand), 93 (N–CH–N), 128.6, 129.4, 135.2, and 139
(Caromatic), 157.7 (C(O) N); the C N carbon was not detected.
1d (47 mg, 73%); Found: C, 39.04; H, 4.21; N, 10.82; (Calc.
for C21H27N5Cl2O2Pt: C, 39.00; H, 4.21; N, 10.84); m/z (high
resolution ESI+): 647.1254 ([M + H]+, requires 647.1267); Rf =
0.49 (eluent CHCl3/Me2CO, 15/1, v/v); nmax(KBr)/cm-1: 3475 m
(C–H), 2299 m (C N), 1670 s (C N); dH (300 MHz, CDCl3):
2.96 (6H, s, NMe2 of the nitrile ligand), 3.63 (6H, br s, NMe2 of
the 2,3-dihydro-1,2,4-oxadiazole ligand), 3.78 (3H, s, CH3 from p-
MeOC6H4), 4.29 (2H, br s, N(O)CH2Ph), 5.85 (1H, br s, CH), 6.87
(2H, br d, Haromatic), 7.34, 7.46 (7H, m and s, Haromatic); dC (75.5 MHz,
CDCl3): 40.4 (NMe2 of the 2,3-dihydro-1,2,4-oxadiazole ligand),
40.5 (NMe2 of the nitrile ligand), 55.6 (N(O)CH2Ph), 62.1 (CH3
from p-MeOC6H4), 89.8 (N–CH–N), 113.9, 128.4, 129.0, 129.6,
130.3, and 135.0 (Caromatic), 160.2 (C(O) N); the C N carbon
was not detected.
1a (42 mg, 75%); Found: C, 32.84; H, 4.36; N, 12.29 (Calc. for
C15H23N5Cl2OPt: C, 32.48; H, 4.18; N, 12.63); m/z (high resolution
ESI+) 556.0876 ([M + H]+, requires 556.1029); 578.0700 ([M +
Na]+, requires 578.0849); 594.0432 ([M + K]+, requires 594.0588);
Rf = 0.35 (eluent CHCl3/Me2CO, 50/1, v/v); nmax(KBr)/cm-1:
2927 m (C–H), 2289 m (C N), 1666 s (C N); dH (300 MHz,
CDCl3): 2.32 (3H, s, CH3 from p-tol), 2.89 (9H, br s, N(O)Me,
NMe2 of the nitrile ligand), 3.61 (6H, br s, NMe2 of the 2,3-
dihydro-1,2,4-oxadiazole ligand), 5.56 (1H, s, CH), 7.16 (2H, d,
m-H from p-tol), 7.45 (2H, d, o-H from p-tol); dC (75.5 MHz,
CDCl3): 21.8 (CH3 from p-tol), 40.3 (NMe2 of the 2,3-dihydro-
1,2,4-oxadiazole ligand), 40.4 (NMe2 of the nitrile ligand), 46.3
(N(O)Me), 93.2 (N–CH–N), 128.6, 129.4, 135.09, and 139.1
(Caromatic), 158.5 (C(O) N); the C N carbon was not detected.
1b (43 mg, 70%); Found: C, 37.21; H, 5.10; N, 11.53 (Calc. for
C19H31N5Cl2OPt: C, 37.36; H, 5.12; N, 11.47); m/z (high resolution
ESI+) 612.1488 ([M + H]+, requires 612.1658); 634.1311 ([M +
Na]+, requires 634.1475); Rf = 0.46 (eluent CHCl3/Me2CO, 60/1,
v/v); nmax(KBr)/cm-1: 2976, 2933 m (C–H), 2291 m (C N), 1653 s
(C N); dH (300 MHz, CDCl3): 1.25 (6H, t, J = 7 Hz, CH3 from
NEt2 of the nitrile ligand), 1.39 (6H, t, J = 7 Hz, CH3 from CNEt2
of the 2,3-dihydro-1,2,4-oxadiazole ligand), 2.31 (3H, s, CH3 from
p-tol), 2.94 (3H, br s, N(O)Me), 3.10 (4H, q, J = 7 Hz, CH2 from
NEt2 of the nitrile ligand, 4.10 (4H, br s, CH2 from NEt2 of the 2,3-
dihydro-1,2,4-oxadiazole ligand), 5.63 (1H, s, CH), 7.17 (2H, d, m-
H from p-tol), 7.37 (2H, d, o-H from p-tol); dC (75.5 MHz, CDCl3):
13.3 (CH3 from NEt2 of the nitrile ligand), 14.2 (CH3 from NEt2
of the 2,3-dihydro-1,2,4-oxadiazole ligand), 21.7 (CH3 from p-tol),
44.6 (CH2 from NEt2 of the nitrile ligand), 46.2 (CH2 from NEt2 of
the 2,3-dihydro-1,2,4-oxadiazole ligand), 46.7 (N(O)Me), 93.2 (N–
CH–N), 128.1, 129.3, 138.5, and 138.9 (Caromatic), 158.5 (C(O) N);
the C N carbon was not detected. The crystals suitable for X-
ray study were obtained by a slow evaporation of the solvent
(heptane/acetone = 1/1, v/v) at room temperature.
1c (45 mg, 71%); Found: C, 39.52; H, 4.91; N, 11.02 (Calc.
for C21H31N5Cl2OPt: C, 39.73; H, 4.92; N, 11.04); m/z (high
resolution ESI+) 636.1471 ([M + H]+, requires 636.1655); 658.1290
([M + Na]+, requires 658.1475); 674.1013 ([M + K]+, requires
674.1214); Rf = 0.47 (eluent CHCl3/n-C6H14/Me2CO, 4/5/1,
v/v/v); nmax(KBr)/cm-1: 2927 m (C–H), 2289 m (C N), 1666 s
(C N); dH (300 MHz, CDCl3): 1.60 (12H, br m; b- and g-
CH2 from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole and nitrile
ligands), 2.33 (3H, s, CH3 from p-tol), 2.90 (3H, br s, N(O)Me),
3.23 (4H, 7, J = 5 Hz, a-CH2 from NC5H10 of the nitrile ligand),
4.27 (4H, br s, a-CH2 from NC5H10 of the 2,3-dihydro-1,2,4-
oxadiazole ligand), 5.57 (1H, s, CH), 7.43 (2H, d, m-H from
p-tol), 7.19 (2H, d, o-H from p-tol); dC (75.5 MHz, CDCl3):
21.8 (CH3 from p-tol), 22.9 (g-CH2 from NC5H10 of the nitrile
ligand), 24.3 (g-CH2 from NC5H10 of the nitrile ligand), 24.9 (b-
CH2 from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole ligand),
25.9 (b-CH2 from NC5H10 of the 2,3-dihydro-1,2,4-oxadiazole
Synthesis of the (2,3-dihydro-1,2,3-oxadiazole)2PtII complexes
(a general procedure). The solutions of each of the nitrones
(1.5 mmol) in CHCl3 (1 mL) were added to a solution of
the corresponding trans-[PtCl2(NCR)2] (0.1 mmol; R = NMe2,
N(C5H10), NEt2) in CHCl3 (1 mL). The mixture was stirred at
room temperature for 24 h and the progress of the reaction
was monitored by TLC. The separation of 2a–d was achieved
by column chromatography on SiO2 (the first fraction; eluent
CHCl3/Me2CO, 15/1, v/v). The solvent was evaporated in vacuo
at 20–25 ◦C to give yellow oily residues. The residues were
crystallized under n-hexane to form the yellow powders of
2a–d. The complexes were dried in air at 20–25 ◦C. Yields 70–
86%.
2a (58 mg, 82%); Found: C, 41.42; H, 4.82; N, 11.80 (Calc.
for C24H34N6Cl2O2Pt: C, 40.95; H, 4.87; N, 11.94); m/z (high
resolution ESI+) 743.1330 ([M + K]+, requires 743.1429); Rf =
0.44 (eluent CHCl3/Me2CO, 40/1, v/v); nmax(KBr)/cm-1: 2933 m
(C–H), 1665 s (C N); dH (300 MHz, CDCl3): 2.34 (3H, s, CH3
from p-tol), 2.73 (3H, s, N(O)Me), 3.13 (6H, br s, NMe2), 5.33
(1H, br s, CH), 7.17 (2H, two d, m-H from p-tol), 7.44 (2H, br
d, o-H from p-tol); dC (75.5 MHz, CDCl3): 21.7 (CH3 from p-
tol), 39.6 (NMe2), 44.5 (N(O)Me), 93.4 (N–CH–N), 129.1, 130.2,
135.08, and 139.3 (Caromatic), 157.4 (C(O) N). The crystals suitable
for X-ray study were obtained by a slow evaporation of the solvent
(heptane/acetone = 1/1, v/v) at room temperature.
2b (65 mg, 80%); Found: C, 44.37; H, 5.59; N, 11.02 (Calc. for
C28H42N6Cl2O2Pt: C, 44.25; H, 5.57; N, 11.06); m/z (high resolu-
tion ESI+) 761.2265 ([M + H]+, requires 761.2418); 783.2131 ([M +
Na]+, requires 783.2316); 799.1843 ([M + K]+, requires 799.2055);
Rf = 0.56 (eluent CHCl3/Me2CO, 60/1, v/v); nmax(KBr)/cm-1:
2967, 2929 m (C–H), 1645 s (C N); dH (300 MHz, CDCl3): 1.19
(6H, br s, CH3 from NEt2), 2.33 (3H, s, CH3 from p-tol), 2.83
(3H, s, N(O)Me), 3.9 (4H, br s, CH2 from NEt2), 5.49 (1H, br s,
CH), 7.15 (2H, two d, m-H from p-tol), 7.35 (2H, br d, o-H from
p-tol); dC (75.5 MHz, CDCl3) 14.1 (CH3 from NEt2), 21.7 (CH3
from p-tol), 44.1 (CH2 from NEt2), 46 (N(O)Me), 93.4 (N–CH–N),
129.2, 128.57, 135.5, and 138.8 (Caromatic), 158 (C(O) N).
2c (55 mg, 70%); Found: C, 45.72; H, 5.43; N, 10.63 (Calc.
for C30H42N6Cl2O2Pt: C, 45.96; H, 5.40; N, 10.72); m/z (high
resolution ESI+) 783.2146 ([M – H]+, requires 783.2339); 807.2114
4180 | Dalton Trans., 2011, 40, 4175–4182
This journal is
The Royal Society of Chemistry 2011
©

View Online
([M + Na]+, requires 807.2315); 823.1834 ([M + K]+, requires
823.2055); Rf = 0.60 (eluent CHCl3/n-C6H14/Me2CO, 4/5/1,
v/v/v); nmax(KBr)/cm-1: 2924 and 2856 m (C–H), 1647 s (C N);
dH (300 MHz, CDCl3): 1.53 (6H, br s, b- and g-CH2 from NC5H10),
2.32 (3H, s, CH3 from p-tol), 2.76 (s, 3H, N(O)Me) 3.88 and
4.00 (4H, br d, a-CH2 from NC5H10), 5.39 (1H, br s, CH), 7.15
(2H, two d, m-H from p-tol), 7.38 (2H, two s, o-H from p-
tol); dC (75.5 MHz, CDCl3): 21.7 (CH3 from p-tol), 24.3 (g-CH2
from NC5H10), 25.9 (b-CH2 c from NC5H10), 48.6 (a-CH2 from
NC5H10), 45.3 (N(O)Me), 93.3 (N–CH–N), 129.2, 135.5, 138.8,
and 138 (Caromatic), 157 (C(O) N). The crystals suitable for X-
ray diffraction study were obtained by a slow evaporation of the
solvent (heptane/acetone = 1/1, v/v) at room temperature.
2d (73 mg, 82%); Found: C, 48.62; H, 4.75; N, 9.45; (Calc.
for C36H42N6Cl2O4Pt: C, 48.65; H, 4,76; N, 9.45); m/z (high
resolution ESI+) 911.2139 ([M + Na]+, requires 911.2214); Rf =
0.36 (eluent CHCl3: Me2CO, 40 : 1, v/v); nmax(KBr)/cm-1: 2929
m (C–H), 1662 s (C N); dH (300 MHz, CDCl3): 3.19, 3.29 (6H,
two s, NMe2), 3.82 (3H, s, CH3 from p-MeOC6H4), 4.10 (2H, br s,
N(O)CH2Ph), 5.69 (1H, br s, CH), 6.89 (2H, br d, Haromatic), 7.43
(7H, m, Haromatic); dC (75.5 MHz, CDCl3): 39.8 (NMe2), 55.7 (CH3
from p-MeOC6H4), 61.4 (N(O)CH2Ph), 90.4 (N–CH–N), 113.8,
128.2, 129.7, 130.5, 130.8, 131.2, 135.5 (Caromatic), 160.4 (C(O) N).
The crystals suitable for X-ray study were obtained by a slow
evaporation of the solvent (heptane/acetone = 1/1, v/v) at room
temperature.
N(O)Me), 3.33 (4H, br d, a-CH2 from NC5H10), 5.39 (1H, br s,
CH), 7.18 (2H, d, m-H from p-tol), 7.23 (2H, d, o-H from p-tol);
dC (75.5 MHz, CDCl3/CD3OD): 20.4 (CH3 from p-tol), 24.0 (g-
CH2 from NC5H10), 25.5 (b-CH2 from NC5H10), 45.8 (N(O)Me),
91.9 (N–CH–N), 124.1, 126.7, 130.9, and 138.3 (Caromatic), 160.0
(C(O) N).
3d; m/z (high resolution ESI+) 312.1685 ([M + H]+, requires
312.1711); dH (300 MHz, CDCl3/CD3OD): 3.32 (6H, br s, NMe2),
3.77 (3H, s, CH3 from p-MeOC6H4), 4.12, 4.27 (2H, two br d,
N(O)CH2Ph), 5.60 (1H, br s, CH), 6.86, (2H, br d, Haromatic),
7.14 (2H, br d, Haromatic), 7.36 (5H, br m, Haromatic); dC (75.5 MHz,
CDCl3/CD3OD): 37.4 (NMe2), 54.9 (CH3 from p-MeOC6H4), 62.8
(N(O)CH2Ph), 88.9 (N–CH–N), 113.7, 124.2, 127.9, 128.0, 128.5,
131.2, and 160.1 (Caromatic), 161.2 (C(O) N).
Acknowledgements
We thank the Federal Grant-in-Aid Program «Human Capital for
Science and Education in Innovative Russia» (Governmental Con-
tract P1294 from 09.06.2010), Russian Fund for Basic Research
(grant 11-03-00262), RAS Presidium Subprogram coordinated by
acad. N.T. Kuznetsov. The authors acknowledge Saint-Petersburg
State University for a research grant. NAB expresses gratitude
to Saint-Petersburg Government Committee for Science and
Higher Education (grant 2010) and to the Fundac¸a˜o para a
Cieˆncia e a Tecnologia (FCT), Portugal – Project PTDC/QUI-
QUI/098760/2008. Dr E.V. Eliseenkov is thanked for valuable
discussion of the competitive reactivity study.
Liberation of the oxadiazoles from 2a–d. An excess of NaCN
(8.2 mg, 0.168 mmol) in methanol-d4 (0.5 mL) was added to a
suspension of each of 2a–d (0.028 mmol) in CDCl3 (0.1 mL) and
the reaction mixture was left to stand for 1 d at 35–40 ◦C. During
this time, the initially pale yellow suspension turned to a colorless
solution. Completeness of the reaction was monitored by 1H NMR
and high resolution ESI-MS indicating that the conversion was
completed after 1d. The metal-free heterocycles were separated
from excess NaCN, and also from NaCl and Na2[Pt(CN)4], formed
in the reaction, by evaporation of the solvent at room temperature
and treating of the colorless oily residue with CHCl3. The liquid
phase was separated by filtration, whereupon evaporation of the
solvent afforded in almost quantitative yields 2a–d as colorless oily
residues.
3a; m/z (high resolution ESI+) 242.1262 ([M + Na]+, requires
242.1269); dH (300 MHz, CDCl3/CD3OD) 2.33 (3H, s, CH3 from
p-tol), 2.89 (3H, s, N(O)Me), 2.98 (6H, br s, NMe2), 5.39 (1H,
br s, CH), 7.18 (2H, br d, m-H from p-tol), 7.24 (2H, br d, o-H
from p-tol); dC (75.5 MHz, CDCl3/CD3OD): 20.5 (CH3 from p-
tol), 37.4, (NMe2), 45.9 (N(O)Me), 92.3 (N–CH–N), 124.2, 126.7,
129.1, and 138.3 (Caromatic), 160.8 (C(O) N).
3b; m/z (high resolution ESI+) 248.1750 ([M + H]+, requires
248.1762); dH (300 MHz, CDCl3/CD3OD): 1.20 (6H, t, J = 7 Hz,
CH3 from NEt2), 2.33 (3H, s, CH3 from p-tol), 2.90 (3H, s,
N(O)Me), 3.32 (4H, m, CH2 from NEt2), 5.37 (1H, br s, CH),
7.17 (2H, d, m-H from p-tol), 7.21 (2H, d, o-H from p-tol); dC
(75.5 MHz, CDCl3/CD3OD) 13.3 (CH3 in NEt2), 20.7 (CH3 in p-
tol), 43.5, (CH2 in NEt2), 45.9 (N(O)Me), 92.1 (N–CH–N), 126.6,
128.9, and 138.3 (Caromatic), 159.7 (C(O) N).
3c; m/z (high resolution ESI+) 260.1746 ([M + H]+, requires
260.1762); dH (300 MHz, CDCl3/CD3OD): 1.65 (6H, br s, b- and
g-CH2 from NC5H10), 2.34 (3H, s, CH3 from p-tol), 2.89 (s, 3H,
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