Synthesis of (1,2,4-oxadiazole)palladium(II) complexes by [2 + 3] cycloaddition of nitrile oxides to organonitriles in the presence of PdCl 2

Article abstract of DOI:10.1002/ejic.200400580

The reaction between the nitrile oxides 2,4,6-R′₃C ₆H₂CNO (R′ = Me, OMe) and trans-[PdCl ₂(RCN)₂], or RCN (R = Me, Et, CH₂CN, NMe ₂, Ph) in the presence of PdCl₂, proc

Full text of DOI:10.1002/ejic.200400580

FULL PAPER
Synthesis of (1,2,4-Oxadiazole)palladium(II) Complexes by [2 + 3]
Cycloaddition of Nitrile Oxides to Organonitriles in the Presence of PdCl2
Nadezhda A. Bokach,[a,b] Vadim Yu. Kukushkin,*[b] Matti Haukka,[c] and
Armando J. L. Pombeiro*[a]
Keywords: Palladium / 1,2,4-oxadiazole complexes / Nitrile complexes / Nitrile oxide complexes / Cycloaddition
The reaction between the nitrile oxides 2,4,6-RЈ3C6H2CNO
(RЈ = Me, OMe) and trans-[PdCl2(RCN)2], or RCN (R = Me,
Et, CH2CN, NMe2, Ph) in the presence of PdCl2, proceeded
smoothly under mild conditions and allowed the isolation of
the 1,2,4-oxadiazole complexes trans-[PdCl2{Na=C(R)-
ON=Cb(C6H2RЈ3)(Na–Cb)}2] (1–8) in 40–85% yields. In
CH2Cl2, the reaction between 2,4,6-RЈ3C6H2CNO and
[PdCl2(MeCN)2] furnishes [PdCl2(ONCC6H2RЈ3)2] (9 and 10),
which are the first representatives of metal compounds
where nitrile oxides act as ligands. The 1,2,4-oxadiazole
complexes 1–8 were characterized by elemental analysis,
FAB mass spectrometry, and IR, 1H and 13C{1H} NMR spec-
troscopy, while 2, 3, 7, and 8 were additionally characterized
by X-ray crystallography. The liberation of the heterocyclic
species from 1–8 was successfully performed by substitution
reaction either with 1,2-bis(diphenylphosphanyl)ethane or
with an excess amount of Na2S·7H2O in MeOH; the liberated
1,2,4-oxadiazoles (11–18) were characterized by positive-ion
FAB mass spectrometry and 1H and 13C{1H} NMR spec-
troscopy.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
ligands exhibit important luminescent,[14] electrolumines-
cent,[15] liquid crystal,[16] and magnetic properties.[17]
As a continuation of our project on [2 + 3] cycload-
ditions[18,19] of dipoles to metal-activated nitriles[20,21] to
give heterocyclic systems, we have studied a [2 + 3] cycload-
dition of nitrile oxides to nitriles in the presence of a PdII
center and isolated a family of previously unknown (1,2,4-
oxadiazole)palladium(ii) species. We also observed that the
reaction between the nitrile oxides and [PdCl2(MeCN)2] in
CH2Cl2 (instead of MeCN) led to the first representatives
of nitrile oxide metal compounds. All these results are re-
ported in this work.
Introduction
1,2,4-Oxadiazoles are an important class of ring systems
containing one O and two N atoms.[1] These compounds
have widespread applications in biology and medicine inso-
far as some of them exhibit intrinsic analgesic,[2] anti-in-
flammatory,[3] and anti-picornaviral[4] effects and show high
efficacy as agonists[5] and antagonists[6] for different recep-
tors. Other areas of research in the chemistry of 1,2,4-oxa-
diazoles include plant protection,[7] their use as liquid-crys-
talline mesophases,[8] dyeing or printing with 1,2,4-oxadi-
azole azo dyes,[9] and they are also constituents of fluores-
cent whiteners.[10] Despite the wealth of chemistry associ-
ated with 1,2,4-oxadiazoles in general, the coordination
chemistry of these heterocycles has so far been little ex-
plored,[11] although the opposite holds true for the isomeric
1,3,4-oxadiazoles. Complexes of the latter have been inten-
sively investigated as angular bridging ligands that give un-
usual coordination polymers[12] and binuclear complexes.[13]
In addition, metal complexes with 1,3,4-oxadiazole-derived
Results and Discussion
Preparation of (1,2,4-Oxadiazole)palladium(II) Complexes
Recently we have studied the reaction of [PtCl4(RCN)2]
with the nitrile oxides 2,4,6-RЈ3C6H2CNO which, after
[2 + 3] cycloaddition, gives the (1,2,4-oxadiazole)plati-
num(iv) complexes [PtCl4{Na=C(R)ON=CbC6H2RЈ3(Na–
Cb)}2].[19] This reaction opens up an easy route to 1,2,4-
oxadiazole metal complexes and also allows the preparation
of the heterocycles, after liberation of the ligands, under
mild conditions. This method to achieve 1,2,4-oxadiazole
complexes has been applied here to PdII systems to obtain a
family of previously unknown 1,2,4-oxadiazole complexes.
As dipolarophiles for this study we used the nitrile
and dialkylcyanamide palladium(ii) complexes trans-
[PdCl2(RCN)2] because it has been shown[20,21] that the PdII
[a] Centro de Química Estrutural, Complexo I, Instituto Superior
Técnico,
Av. Rovisco Pais, 1049–001 Lisboa, Portugal
Fax: +351-21-8464-455/7
E-mail: pombeiro@ist.utl.pt
[b] Department of Chemistry, St.Petersburg State University,
198504, Stary Petergof, Russian Federation
Fax: +7-812-428-6939
E-mail: kukushkin@VK2100.spb.edu
[c] Department of Chemistry, University of Joensuu,
P. O. Box 111, 80101, Joensuu, Finland
Fax: +358-13-2513390
E-mail: matti.haukka@joensuu.fi
Eur. J. Inorg. Chem. 2005, 845–853
DOI: 10.1002/ejic.200400580
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N. A. Bokach, V. Y. Kukushkin, M. Haukka, A. J. L. Pombeiro
FULL PAPER
Scheme 1
center strongly activates the ligated nitriles toward the ad- 8 starting directly from PdCl2 and omitting the unnecessary
dition of various nucleophiles and this type of reactivity can step of the isolation of trans-[PdCl2(RCN)2].
model,[22] to some extent, the [2 + 3] cycloaddition of di-
In order to get a rough estimate of the effect of the PdII
poles. Two relatively stable aryl nitrile oxides, i.e. 2,4,6- center on the cycloaddition rate, the reaction between PdCl2
Me3C6H2CNO (I) and 2,4,6-(MeO)3C6H2CNO (II),[23] were (1 equiv.) in EtCN (3 mL) and I or II (2 equiv.) at 40 °C
chosen as dipoles.
was compared with the reaction of I or II and EtCN under
In agreement with our expectations, the cycloaddition of the same experimental conditions; only an approximate 1.5-
I or II to nitriles in trans-[PdCl2(RCN)2] to achieve 1,2,4- fold acceleration in the metal-mediated vs. metal-free syn-
oxadiazole metal species proceeded smoothly and gave the thesis was observed. It appears that PdII acts mostly as a
complexes trans-[PdCl2{Na=C(R)ON=Cb(C6H2RЈ3)(Na– trapping center, thus allowing a facile isolation and charac-
Cb)}2] (1–8) in 40–85% yields. The reaction was performed terization of 1,2,4-oxadiazoles as metal-bound species.
by two rather similar, albeit technically distinct, routes
(Scheme 1). In the first route (A), both PdCl2 and the nitrile
oxide I or II (3–4 equiv.) were reacted in a suspension of
neat nitrile RCN (R = Me, Et, NMe2, Ph) or, in the case of
the solid malonodinitrile (R = CH2CN) (sixfold molar ex-
Binding of the Nitrile Oxides by a PdII Center
The interaction between [PdCl2(MeCN)2] and the nitrile
cess vs. PdCl2), in a CH2Cl2 solution. In the second route oxides I and II proceeds in another direction when dichlo-
(B), the nitrile complexes trans-[PdCl2(RCN)2] (R = Me, Et, romethane is used as solvent instead of MeCN: the reaction
Ph), dissolved in the corresponding nitrile, were treated gives (nitrile oxide)palladium(ii) complexes 9 and 10
with the nitrile oxides I or II (3–4 equiv.). In both routes (Scheme 1, route C). In the electrospray mass spectra of
the reaction was performed at 40 °C for 12–18 h and the both complexes, we observed the [M + H]+ ion with the ex-
isolated yields for every particular compound, prepared by pected isotopic pattern. Complex 10 gave satisfactory C, H,
the two methods, are almost the same. The isolation of the and N microanalyses, whereas we failed to get reproducible
(1,2,4-oxadiazole)palladium(ii) products failed only in the data for the rather unstable 9. In the IR spectra of both
case of treatment of the less-reactive (towards the cycload- complexes, strong (9) or medium (10) bands of ν(CϵN) are
dition, but more reactive towards the substitution; see be- shifted toward lower frequencies (15 cm–1 for 9 and 56 cm–1
1
low) nitrile oxide II with the reactive (but weak ligand) ni- for 10) upon coordination. The H NMR spectra display
triles PhCN and NCCH2CN at the PdII center; this reaction all required resonances with their expected integration. In
gives a mixture of products.
CDCl3, the complexes exist as a number of conformers
Reaction of PdCl2 with neat RCN is well known and is probably because of the hindered rotation of the bulky li-
commonly applied for the preparation of the complexes gands.
trans-[PdCl2(RCN)2].[24] Hence, it is not surprising that the
Although, to the best of our knowledge, metal complexes
reaction rate and the isolated yields are similar for both with nitrile oxides as ligands are unknown, there are a few
routes, because route A conceivably includes the formation reports of 1,3-dipolar cycloadditions where the formation
of [PdCl2(RCN)2], upon dissolution of PdCl2 in RCN, fol- of complexes between Lewis acids and nitrile oxides was
lowed by the cycloaddition of a nitrile oxide. Despite the assumed, from indirect evidence, to occur.[25,26] The major
similarity of routes A and B, the obvious advantage of the argument supporting the idea of the formation of (nitrile
first method is that it allows the synthesis of complexes 1– oxide)[M] complexes is based on the inhibition of the 1,3-
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Synthesis of (1,2,4-Oxadiazole)palladium(ii) Complexes
FULL PAPER
dipolar cycloaddition (sometimes with an increase of selec- and their structures were determined by X-ray crystallogra-
tivity of the reaction[25–29]) in the presence of metal centers, phy.
which, in agreement with the authors,[25,26,30] blocks the
oxygen atom of the nitrile oxide by coordination. The latter
hypothesis was supported in the framework of this study by
the lack of formation of (oxadiazole)palladium(ii) com-
plexes when 9 or 10 were treated with the corresponding
nitriles (route D, Scheme 1).
Characterization of the (1,2,4-Oxadiazole)palladium(II
)
Complexes
Complexes 1–8 were characterized by C, H, and N analy-
ses. In their FAB spectra they display the quasi-ion
[M + Na]+ (1, 2) and/or the fragmentation [M – H]+ and/
or [M – Cl]+ ions (1, 2, 5, 7, and 8); for 3, 4, and 6 only
deep fragmentation was observed. In their IR spectra, the
oxadiazole complexes 1–8 exhibit two relatively strong
bands in the range of 1537–1658 cm–1. The first one lies in a
relatively narrow interval, i.e. 1607–1614 cm–1, for all these
complexes, and can be attributed to a ν(C=N) stretching
vibration in the oxadiazole ligand. The position of the sec-
ond band varies and depends on the type of the complex.
Thus, medium-to-strong and strong bands at 1537–
1585 cm–1 (1, 2, 4 derived from I) and 1598 cm–1 (6 and
7 derived from II) can be attributed to the other ν(C=N)
stretching vibration of the heterocycle and/or ν(C=C)
stretch of the aromatic ring. In contrast to the alkyl and
aryl derivatives of oxadiazoles, the dialkylcyanamide ones
(4 and 8) do not display strong peaks in the area 1537–
1585 cm–1 but exhibit a strong band in the range 1657–
1658 cm–1. This shift is probably the consequence of a dif-
ferent electron distribution in the dialkylcyanamide-substi-
tuted oxadiazole ligands (see below).
Figure 1. Thermal ellipsoid view of [PdCl2{Na=C(Et)ONCb(C6H2-
Me3)(Na–Cb)}2] (2)with atomic numbering scheme; thermal ellip-
soids are drawn with 50% probability
1
In the H and 13C{1H} NMR spectra of [PdCl2(1,2,4-
oxadiazole)2], two (1–4 and 6–8) or five (5) sets of each
signal (or a group of signals) were detected; this is probably
due to the hindered rotation of the bulky oxadiazole ligands
around the Pd–N bond, which leads to inequivalence of the
protons and carbon atoms. In the 13C{1H} NMR spectra,
two carbons from the N=C–N and N=C–O moieties were
detected at δ = 164–171 and 178–190 ppm, respectively.
These assignments are based upon the data for similar plati-
num(iv) and platinum(ii) oxadiazole complexes, i.e.
[PtCl4{Na=C(R)ON=CbC6H2RЈ3)(Na–Cb)}2] and [PtCl2-
{Na=C(R)ON=CbC6H2RЈ3)(Na–Cb)}2], where the signals of
C from the NCN and NCO groups can be distinguished by
Figure 2. Thermal ellipsoid view of [PdCl2{Na=C(CH2CN)
ONCb(C6H2Me3)(Na–Cb)}2] (3)with atomic numbering scheme;
thermal ellipsoids are drawn with 50% probability
1
using the long-range selective INEPT technique. In the H
NMR spectra of both 4 and 8, the signal from Me in NMe2
is broad, apparently because of the hindered rotation of the
NMe2 group (see later).
The 1,2,4-oxadiazole ligands in all these complexes are
coordinated in mutual trans positions. In 2, 7, and 8 the
heterocycles have an antiparallel orientation [angles be-
tween N(1)–C(1)–O(1)–N(2)–C(4) planes are 0°, 57.63(9)°,
10.16(14)°, and 0° for 2, 3, 7, and 8, respectively], while in
3, they are turned in one direction and the torsion angle is
Crystal Structure Determinations of (1,2,4-Oxadiazole)
palladium(II) Complexes
Crystals of 2 (Figure 1), 3 (Figure 2), 7 (Figure 3), and 8 approximately 60°. The geometrical parameters of the het-
(Figure 4) were obtained directly from the reaction mixtures erocycles in 2, 3, and 7 are the same within 3σ and are
Eur. J. Inorg. Chem. 2005, 845–853
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N. A. Bokach, V. Y. Kukushkin, M. Haukka, A. J. L. Pombeiro
FULL PAPER
Figure 5. Possible mesomeric forms of the heterocycle in complex
8
This observation is coherent with the unusual IR data
(strong band at 1658 cm–1, see above) and NMR spectro-
scopic data in solution, where the NMe2 signal is broad
probably due to hindered rotation around the partially
double CN bond of the CNMe2 functionality.
Figure 3. Thermal ellipsoid view of [PdCl2{Na=C(Et)ONCb(C6H2-
(OMe)3(Na–Cb)}2] (7) with atomic numbering scheme; thermal el-
lipsoids are drawn with 50% probability
Liberation of the 1,2,4-Oxadiazoles
Although the activation of nitriles by PdII toward the
cycloaddition is not advantageous from synthetic view-
point, we still developed a method for liberation of 1,2,4-
oxadiazoles from their complexes with the hope of further
progress in this area which can be associated with either the
application of cationic PdII complexes or the use of sup-
porting ligands with pronounced π-acceptor properties.[20]
Complexes 1–8 are stable and remain intact in an excess
of nitrile even at 40 °C for 24 h, thus preventing the reaction
from being catalytic. However, the liberation of the oxadia-
zoles from 1–8 was successfully performed by substitution
following two routes. In the first one, a solution of 1–8 in
CDCl3 was treated with two equivalents of 1,2-bis(diphen-
ylphosphanyl)ethane (dppe) and, after 3 h, when the pre-
cipitation of the colorless complex [Pd(dppe)2]Cl2 [31P{1H}
(CDCl3): δ = 53.15 ppm (ref.[37] δ = 56.7 ppm)] was com-
plete, the quantitative formation of the free oxadiazoles 11–
18 was confirmed by 1H NMR spectroscopy. In the prepar-
ative experiment, the precipitate of [Pd(dppe)2]Cl2 can eas-
ily be separated by filtration, and subsequent evaporation
of the filtrate gives the 1,2,4,-oxadiazoles in the almost
quantitative yield.
Figure 4. Thermal ellipsoid view of [PdCl2{Na=C(NMe2)
ONCb(C6H2(OMe)3)(Na–Cb)}2] (8)with atomic numbering scheme;
thermal ellipsoids are drawn with 50% probability
similar to those in the previously described (1,2,4-oxadia-
In the second route, a solution of 1–8 in CH2Cl2 was
zole)platinum(iv) and -platinum(ii) complexes,[19] in 1,2,4- treated with an excess of Na2S·7H2O in MeOH to immedi-
oxadiazole complexes of other metals,[31] and even in the ately give a precipitate of PdS; the free oxadiazoles 11–18
uncomplexed heterocycles.[32–34] The Pd–N and Pd–Cl bond remain in solution. After separation of PdS by filtration,
lengths and angles around the Pd center are similar to those the solvent was evaporated from the filtrate and the 1,2,4-
given in the literature for [PdCl2(N-coordinated heterocy- oxadiazoles were extracted almost quantitatively from the
cle)2]-type complexes.[35]
residue with CH2Cl2. The second method is preferable be-
The C=N bond length in the Pd–N=C–N fragment of 8 cause Na2S is much cheaper than dppe and the formation
[1.377(5) Å] is larger than that for the corresponding bond of PdS proceeds substantially faster than that of [Pd-
in 2, 3, and 7 [1.306(3)–1.314(3) Å] and also larger than (dppe)2]Cl2.
those in the free oxadiazoles bearing NRRЈ substituents.[36]
The liberated 1,2,4-oxadiazoles (11–18) were charac-
In addition, the C(1)–N(3) bond length in the CNMe2 frag- terized by positive-ion FAB mass spectrometry − typically
ment [1.315(5) Å] is shorter than a normal C–N single bond the [M + H]+ ion is observed − and 1H and 13C{1H} NMR
1
and even shorter than the C=N bond in the oxadiazole spectroscopy. The chemical shifts in the H and 13C{1H}
ring; all these facts favor a significant double-bond charac- NMR spectra of 11–18 are similar to those for the corre-
ter in the CNMe2 moiety. A possible rationale for the latter sponding (1,2,4-oxadiazole)palladium(ii) complexes. How-
observation is the dominance of the right-hand structure ever, for 11–14 and 16–18 each proton or carbon has one
(Figure 5) in the mesomeric hybrid of the heterocycle.
set of signals, in contrast to the situation described pre-
848
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Synthesis of (1,2,4-Oxadiazole)palladium(ii) Complexes
FULL PAPER
ture was heated to 40 °C for 12–18 h. During this period the PdCl2
dissolved to give orange (for I) or dark-brown (for II) solutions. In
the case of complexes 1–3 and 6–8, orange crystals were formed at
the top of the vial and were removed by filtration (yields are 20–
40%). The orange solution (reaction with I,complexes 4 and 5) was
evaporated to dryness under vacuum and the orange oil formed
was crystallized under a layer of Et2O to furnish an additional
amount (yields are 30–50%) of the cycloaddition product (for 1–
3). All oxadiazole complexes (6–8) derived from II were crystallized
from the reaction mixture and separated by filtration. The dark
viously for the Pd complexes, i.e. the NMR inequivalence
of the oxadiazoles 11–14 and 16–18 disappears when they
are liberated from the corresponding palladium(ii) com-
plexes. However, in the case of 15, five sets of signals are
still displayed in the spectra for each proton and carbon
atom (in the temperature range from 20 to 50 °C) and we
believe that this is due to different conformations of the
oxadiazole 15 bearing the two bulky substituents in the ring
(Ph and C6H2Me3). For all free oxadiazoles 11–18, the
N=C–N and N=C–O carbons lie in the ranges δ = 164–168 reaction mixture formed in the case of nitrile oxide II and PhCN
and PhCH2CN contains a number of products and was not further
investigated.
and 168–180 ppm, respectively.
Method B: [PdCl2(RCN)2] (R = Me, Et, Ph; 15 mg) was suspended
with 3–4 equiv. of the nitrile oxides in the corresponding nitrile
RCN (R = Me, Et, Ph; 0.3 mL), and the reaction mixture was
heated to 40 °C for 12–18 h, whereupon an orange product formed.
The isolation procedures are similar to those described above. This
method was applied for the preparation of 1, 2, and 5–7.
Final Remarks
The results from this work can be considered from at
least two perspectives. From the first one, the developed
method, which is based on [2 + 3] cycloaddition of nitriles
and nitrile oxides in the presence of a trapping metal center,
is an excellent entry to 1,2,4-oxadiazole–metal complexes,
whose chemistry is almost unknown, although the opposite
holds true for metal compounds of the isomeric 1,3,4-oxa-
[PdCl2{Na=C(Me)ONCb(C6H2Me3)(Na–Cb)}2] (1): Yield: 32 mg
(80–85%). C24H28Cl2N4O2Pd (581.8): calcd. C 49.54, H 4.85, N
9.63; found C 49.95, H 5.15, N 9.68. FAB+-MS: m/z = 605 [M +
Na]+, 581 [M – H]+, 547 [M – Cl]+, 508 [M – 2HCl]+. IR (KBr,
diazole; the latter complexes exhibit a good number of use- selected bands): 2976 (w) and 2925 cm–1 (w) ν(C–H), 1610 (mw)
1
and 1585 (m) ν(C=N) and ν(C=C). H NMR (300 MHz, CDCl3;
two sets of signals in the approximate ratio3:2): δ = 2.00 (major)
and 2.29 (minor) (s, 6 H, o-CH3C6H2), 2.40 (minor) and 2.51
(major) (s, 3 H, p-CH3C6H2), 2.90 (minor) and 3.26 (major) (s, 3
H, N=CCH3), 6.93 (major) and 7.04 (minor) (s, 2 H, m-Ar) ppm.
13C{1H} NMR (75.4 MHz, CDCl3; two sets of signals): δ = 14.00
(minor) and 14.6 (major) (N=CCH3), 20.4 (major) and 20.8
(minor) (o-CH3C6H2), 21.4 (minor) and 21.5 (major) (p-CH3C6H2),
120.3 (major) (Cipso), 128.4 (minor) and 128.5 (major) (m-Ar),
138.8 (major) and 139.0 (minor) (o-Ar), 140.4 (major) and 141.2
ful properties. From the second perspective, we have found
the first example of nitrile oxide–metal complexes and fur-
ther development of chemistry of this species may shed light
on the mechanisms of the still poorly understood metal-
mediated 1,3-dipolar cycloaddition of propargyl/allenyl
anion type dipoles to nitriles.[19,38] We observed that the
more labile PdII center (as compared to a kinetically inert
PtIV center) reacts to a similar degree with both the di-
polarophile and the dipole, and the latter interaction, i.e.
ligation of nitrile oxide observed in this work for the first (minor) (p-Ar), 166.6 (two N=C–N), 178.0 (Pd–N=C–O) ppm.
time, inhibits the 1,3-dipolar cycloaddition by blocking the
oxygen center of the dipole. The latter points to the need
for the selective complexation of metal centers with nitrile
dipolarophiles (rather than with nitrile oxide dipoles) to
promote the cycloaddition.
[PdCl2{Na=C(Et)ONCbC6H2Me3)(Na–Cb)}2] (2): Yield: 32 mg (70–
80%). C26H32Cl2N4O2Pd (609.9): calcd. C 51.20, H 5.29, N 9.19;
found C 51.42, H 5.50, N 9.11. FAB+-MS: m/z = 633 [M + Na]+,
575 [M – Cl]+, 537 [M–Cl–HCl]+. IR (KBr, selected bands): 2982
(w) and 2921 cm–1 (w) ν(C–H), 1611 (mw) and 1573 (m) ν(C=N)
and ν(C=C). 1H NMR (300 MHz, CDCl3; two sets of signals in
the approximate ratio 1:1): δ = 1.46 and 1.72 (t, J = 7.5 Hz, 3 H,
CH2CH3), 1.99 and 2.28 (s, 6 H, o-CH3C6H2), 2.40 and 2.51 (s, 3
H, p-CH3C6H2), 3.25 and 3.71 (q, J = 7.5 Hz, 2 H, CH2CH3), 6.93
and 7.04 (s, 2 H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3;
two sets of signals): δ = 10.25 and 10.30 (CH2CH3), 20.5 and 20.9
(o-CH3C6H2), 21.4 and 21.5 (p-CH3C6H2), 21.7 and 22.4
(CH2CH3), 120.6 and 121.2 (Cipso), 128.4 and 128.6 (m-Ar), 138.9
and 139.2 (o-Ar), 141.1 and 140.4 (p-Ar), 166.4 and 166.6 (N=C–
N), 181.4 (Pd–N=C–O) ppm.
Experimental Section
Materials and Instrumentation: Solvents were obtained from com-
mercial sources and used as received. The complexes [PdCl2-(RCN)2]
(R = Me, Et, Ph[24]) and the nitrile oxides 2,4,6-Me3C6H2CNO and
2,4,6-(MeO)3C6H2CNO[23] were prepared as described previously.
C,H,N elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Técnico. Positive-ion FAB mass
spectra were obtained on a Trio 2000 instrument by bombarding
3-nitrobenzyl alcohol (NBA) matrices of the samples with 8 keV
(ca. 1.28×1015 J) Xe atoms. Mass calibration for data system ac-
quisition was achieved using CsI. Infrared spectra (4000–400 cm–1)
were recorded with a JASCO FT/IR-430 instrument in KBr pellets.
1H, 13C{1H}, and 31P{1H} spectra in CDCl3 were measured with
a Varian UNITY 300 spectrometer at ambient temperature.
[PdCl2{Na=C(CH2CN)ONCbC6H2Me3)(Na–Cb)}2]
(3):
Yield:
17 mg (40%). C26H26Cl2N6O2Pd·0.25CH2Cl2 (631.9): calcd. C
48.28, H 4.10, N 12.87; found C 48.68, H 4.26, N 12.82. In our
FAB experimental conditions, we were able to observe only deep
fragmentation and no [M]+ was found. IR (KBr, selected bands):
2966 (mw) and 2909 cm–1 (mw) ν(C–H), 2266 (vw)ν(CϵN), 1610
(mw) and 1585 (m) ν(C=N) and ν(C=C). 1H NMR (300 MHz,
Cycloaddition of Nitrile Oxides to Pd-Bound Nitriles. Method A: CDCl3; two sets of signals in the approximate ratio 1:6): δ = 1.98
PdCl2 (12 mg, 0.068 mmol, 1 equiv.) and 3–4 equiv. of I (32–44 mg,
0.200–0.271 mmol) or II (42–56 mg, 0.200–0.271 mmol) were sus-
pended in RCN (R = Me, Et, Ph, NMe2; 0.3 mL) or in a solution
of NCCH2CN (27 mg, 0.408 mmol) in CH2Cl2 (3 mL) and the mix-
(major) and 2.20 (minor) (s, 6 H, o-CH3C6H2), 2.34 (minor) and
2.52 (major) (s, 3 H, p-CH3C6H2), 3.61 (minor) and 4.89 (major)
(s, 2 H, CH2CN), 6.96 (minor) and 6.99 (major) (s, 2 H, m-Ar)
ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 19.3 (CH2), 20.4 (o-
Eur. J. Inorg. Chem. 2005, 845–853
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. A. Bokach, V. Y. Kukushkin, M. Haukka, A. J. L. Pombeiro
FULL PAPER
CH3C6H2), 21.5 (p-CH3C6H2), 111.4 (CϵN), 128.8 (m-Ar), 139.0
(o-Ar), 141.3 (p-Ar), 171.4 (N=C–N), 190.5 (Pd–N=C–O) ppm;
Cipso was not detected.
dance): δ = 3.45 (br. s, 3 H, Me), 3.78 (s, 6 H, o-CH3C6H2), 3.92
(s, 3 H, p-CH3OC6H2), 6.19 (s, 2 H, m-Ar) ppm. 13C{1H} NMR
(75.4 MHz, CDCl3): δ = 39.4 (NMe2), 55.7 (o-CH3OC6H2), 56.1
(p-CH3OC6H2), 90.1 (m-Ar) ppm; signals for other groups were not
detected due to poor solubility of compound.
[PdCl2{Na=C(NMe2)ONCbC6H2Me3)(Na–Cb)}2] (4): Yield: 17 mg
(40%). C26H34Cl2N6O2Pd (639.9): calcd. C 48.89, H 5.37, N 13.17;
found C 48.84, H 5.44, N 12.94. In our FAB experimental condi-
tions, we were able to observe only deep fragmentation and no[M]
(Nitrile oxide)palladium(II) Complexes: [PdCl2(MeCN)2] (20 mg,
0.08 mmol) and the nitrile oxides RЈCNO [0.20 mmol; RЈ = Me3-
C6H2I, (MeO)3C6H2II] were suspended in CH2Cl2 (3 mL) and the
mixture left to stand at room temperature for 2–4 h. During this
period the [PdCl2(MeCN)2] dissolved to give dark-brown solutions.
In case of II the brown solution remained unchanged after several
hours, therefore the solvent was evaporated and the dark oily resi-
due formed was crystallized under a layer of Et2O. In the case of I,
after several hours an orange precipitate begin to form and solution
become more pale and turned to orange-yellow. The precipitate
was removed by decantation and the solution was evaporated under
vacuum until dryness and crystallized under a layer of Et2O. Yield:
ca. 60% (9) and ca. 30% (10).
+
was found. IR (KBr, selected bands): 2923 cm–1) (mw) ν(C–H),
1658 (ms) and 1609 (m) ν(C=N) and ν(C=C). 1H NMR (300 MHz,
CDCl3; two sets of signals in the approximate ratio 1:2): δ = 2.29
(s, 3 H, p-CH3C6H2, minor), 2.33 (s, 6 H, o-CH3C6H2, major), 2.39
(s, 3 H, p-CH3C6H2, major), 3.39 (br. s, 6 H, NMe2), 6.88 (s, 2
H, m-Ar, minor), 6.98 (s, 2 H, m-Ar, major) ppm. 13C{1H} NMR
(75.4 MHz, CDCl3): δ = 20.8 (o-CH3C6H2), 21.3 (p-CH3C6H2),
40.1 (NMe2), 128.2 (m-Ar), 139.6 (o-Ar), 140.4 (p-Ar), 166.9
(N=C–N) ppm; signals of other groups were not detected.
[PdCl2{Na=C(Ph)ONCbC6H2Me3)(Na–Cb)}2] (5): Yield: 19 mg
(40%). C34H32Cl2N4O2Pd (706.0): calcd. C 57.84, H 4.57, N 7.94;
found C 57.37, H 4.38, N 7.68. FAB+-MS: m/z = 668 [M – HCl]+,
633 [M – 2Cl]+. IR (KBr, selected bands): 2918 cm–1 (mw) ν(C–H),
1607 (m) and 1557 (ms) ν(C=N) and ν(C=C). 1H NMR (300 MHz,
CDCl3; five sets for each signal in the spectrum): δ = 2.05–2.55
(several s, 9 H, o- and p-CH3C6H2), 6.92–7.09 (several s, 2 H, m-
Ar), 7.48–7.72 (several m, 3 H, m- and p-Ph from PhC=N), 8.20–
9.28 (several d, 2 H, o-Ph from PhC=N) ppm. 13C{1H} NMR
(75.4 MHz, CDCl3): δ = 20.4, 20.6, 20.8, 20.9, 21.5 and 21.6 (o-
and p-CH3C6H2), 128.5–129.3, 129.9–130.6, 132.1–134.5 and
138.3–140.9 (Ph and C6H2), 167.5 (C=N) ppm.
[PdCl2{Na=C(Me)ONCb(C6H2(OMe)3)(Na–Cb)}2] (6): Yield: 23 mg
(50%). C24H28Cl2N4O8Pd (677.8): calcd. C 42.53, H 4.16, N 8.27;
found C 42.00, H 4.27, N 8.10. In our FAB experimental condi-
tions, we were able to observe only deep fragmentation and no
[M]+ was found. IR (KBr, selected bands): 2942 cm–1 (mw) and
2842 (w) ν(C–H), 1614 (s) and 1598 (s) ν(C=N) and ν(C=C). 1H
NMR (300 MHz, CDCl3; two sets of signals in the approximate
ratio 2:1): δ = 2.90 (major) and 3.27 (minor) (s, 3 H, Me), 3.57
(minor) and 3.83 (major) (s, 6 H, o-CH3C6H2), 3.91 (major) and
3.98 (minor) (s, 3 H, p-CH3OC6H2), 6.14 (minor) and 6.25 (major)
(s, 2 H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 13.9
(Me), 55.8 (o-CH3OC6H2), 56.0 (p-CH3OC6H2), 90.3 (m-Ar), 160.7
(o-Ar) ppm.
[PdCl2(ONCC6H2Me3)2] (9): ES-MS+: m/z = 501 [M + H]+. IR
(KBr, selected bands): 2967 cm–1 (w) and 2922 (mw) ν(C–H), 2278
(s) ν(CϵN) 1792 (ms), 1735 (m), 1608 (s), 1456 (br. ms) [in the free
nitrile oxide: 2949 (w) and 2919 (mw) ν(C–H), 2293 (s) ν(CϵN),
1
1608 (m)]. H NMR (300 MHz, CDCl3): δ = 2.30–2.49 (set of s, 9
H, o- and p-CH3C6H2), 6.89–6.94 (set of s, 2 H, C6H2) ppm.
[PdCl2{ONCC6H2(OMe)3}2] (10): C20H22Cl2N2O8Pd (595.9):
calcd. C 40.32, H 3.72, N 4.70; found C 40.90, H 4.64, N 4.63. ES-
MS+: m/z = 597 [M + H]+. IR (KBr, selected bands): 2940 cm–1
(mw) ν(C–H), 2249 (m) ν(CϵN) 1606 (br. s), 1456 (br. m) [in the
free nitrile oxide: 2985 (w) and 2949 (mw) ν(C–H), 2305 (ms)
ν(CϵN), 1608 (s), 1582 (s), 1469 (ms)]. 1H NMR (300 MHz,
CDCl3): δ = 3.76–3.94 (set of s, 9 H, o- and p-CH3OC6H2), 6.02–
6.14 (set of s, 2 H, C6H2) ppm.
Liberation of the Oxadiazoles from Complexes 1–8: (i) dppe
(2 equiv.) was added to a solution of 1–8 (15 mg) in CDCl3
(0.5 mL) and the mixture was allowed to stand at 40 °C for 30 min,
whereupon colorless [Pd(dppe)2]Cl2 precipitated. The free oxadia-
zole was characterized in solution by 1H NMR spectroscopy. Evap-
oration of the solvent gave 11–18 (see later) in almost quantitative
yield.(ii) A solution of Na2S·7H2O (50 mg) in MeOH (1 mL) was
added dropwise to a solution of 1–8 (10 mg) in CH2Cl2 (0.5 mL).
The dark-brown PdS precipitated immediately after the addition of
ca. 0.15 mL of the sulfide solution (until the yellow color disap-
peared), whereupon the reaction mixture was evaporated to dryness
and the oxadiazole was extracted from the residue with CH2Cl2
(1 mL). Yields are almost quantitative.
The oxadiazoles Na=C(Et)ONCb(C6H2Me3)(Na–Cb) (12), and Na-
=C(Et)ONCb(C6H2(OMe)3)(Na–Cb) (17) have been characterized
previously.[10]
Na=C(Me)ONCb(C6H2Me3)(Na–Cb) (11): FAB+-MS: m/z = 203
[M + H]+. 1H NMR (300 MHz, CDCl3): δ = 2.18 (s, 6 H, o-
CH3C6H2), 2.33 (s, 3 H, p-CH3C6H2), 2.68 (s, 3 H, Me), 6.94 (s, 2
H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 12.4
(N=CMe), 20.0 (o-CH3C6H2), 21.2 (p-CH3C6H2), 128.5 (m-Ar),
137.7 (o-Ar), 139.6 (p-Ar) ppm.
[PdCl2{Na=C(Et)ONCb(C6H2(OMe)3)(Na–Cb)}2] (7): Yield: 24 mg
(50%). C26H32Cl2N4O8Pd (705.9): calcd. C 44.24, H 4.57, N 7.94;
found C 43.77, H 4.78, N 7.72. FAB+-MS: m/z = 671 [M – Cl]+.
IR (KBr, selected bands): 2941 cm–1 (mw) and 2845 (w) ν(C–H),
1
1614 (s) and 1589 (ms) ν(C=N) and ν(C=C). H NMR (300 MHz,
CDCl3; two sets of signals in the approximate ratio 3:1): δ = 1.50
(major) and 1.68 (minor) (t, J = 7.5 Hz, 3 H, Et), 3.29 (major) and
3.80 (minor) (q, J = 7.5 Hz, 2 H, Et), 3.57 (minor) and 3.81 (major)
(s, 6 H, o-CH3C6H2), 3.91 (major) and 3.99 (minor) (s, 3 H, p-
CH3OC6H2), 6.14 (minor) and 6.25 (major) (s, 2 H, m-Ar) ppm.
13C{1H} NMR (75.4 MHz, CDCl3): δ = 10.2 (CH3 from Et), 21.7
(CH2 from Et), 55.7 (o-CH3OC6H2), 55.9 (p-CH3OC6H2), 90.2 (m-
Ar), 160.7 (o-Ar), 162.7 (p-Ar), 164.5 (N=C–N), 180.3 (Pd–N=C–
O) ppm.
[PdCl2{Na=C(NMe2)ONCb(C6H2(OMe)3)(Na–Cb)}2] (8): Yield: Na=C(CH2CN)ONCb(C6H2Me3)(Na–Cb) (13): FAB+-MS: m/z =
1
20 mg (40%). C24H28Cl2N4O8Pd (735.9): calcd. C 42.43, H 4.66, N
11.42; found C 42.90, H 4.93, N 11.34. FAB+-MS: m/z = 700 [M –
Cl –H]+. IR (KBr, selected bands): 2935 cm–1 (mw) and 2845 (w)
228 [M + H]+. H NMR (300 MHz, CDCl3): δ = 2.20 (s, 6 H, o-
CH3C6H2), 2.34 (s, 3 H, p-CH3C6H2), 4.19 (s, 2 H, CH2), 6.96 (s,
2 H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 17.3
ν(C–H), 1657 (s) and 1614 (s) ν(C=N) and ν(C=C). 1H NMR (Me), 20.2 (o-CH3C6H2), 21.2 (p-CH3C6H2), 112.0 (CϵN), 122.6
(300 MHz, CDCl3; two sets of signals in the approximate ratio 11:1;
(Cipso), 128.7 (m-Ar), 137.8 (o-Ar), 140.3 (p-Ar), 168.1 (N=C–O),
signal sets for minor isomer are not given because of its low abun-
169.1 (N=C–N) ppm.
850
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 845–853

Synthesis of (1,2,4-Oxadiazole)palladium(ii) Complexes
FULL PAPER
Table 1. Crystal data for compounds 2, 3, 7, and 8
2
3
7
8
Empirical formula
Mol. mass
Temp [K]
λ [Å]
Cryst. syst.
Space group
a [Å]
C26H32Cl2N4O2Pd
609.86
110(2)
C26H22Cl2N6O2Pd
627.80
100(2)
C26H32Cl2N4O8Pd
705.86
110(2)
C26H34Cl2N6O8Pd
735.89
100(2)
0.71073
monoclinic
P21/c
8.03300(10)
6.7074(2)
24.7117(6)
92.7290(10)
1329.97(5)
2
1.523
0.929
0.0320
0.0733
0.71073
monoclinic
P21/c
13.5346(2)
16.0527(3)
13.7351(3)
117.1703(8)
2654.89(9)
4
1.571
0.936
0.0310
0.0665
0.71073
monoclinic
P21/n
16.3357(2)
7.62820(10)
22.8554(3)
91.8986(4)
2846.49(6)
4
1.647
0.896
0.0282
0.0653
0.71073
monoclinic
P21/c
11.1938(5)
8.1204(3)
16.5640(7)
94.045(2)
1501.89(11)
2
1.627
0.854
0.0456
0.1128
b [Å]
c [Å]
β [°]
V [Å3]
Z
ρcalc [Mg m–3]
µ(Mo-Kα) [mm–1]
R1[a] [I Ն 2σ(I)]
wR2[b] [I Ն 2σ(I)]
2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(Fo2 – Fc ) ]/Σ[w(Fo2)2]}1/2
.
Table 2. Selected bond lengths [Å] and angles [°] for 2, 3, 7, and 8
2
3
3b
7
7b
8
Ru(1)–Cl(1)
Ru(1)–N(1)
N(1)–C(1)
N(1)–C(4)
2.2899(8)
2.018(2)
1.307(4)
2.2947(6)
2.018(2)
1.307(3)
1.390(3)
1.483(4)
2.2927(6)
2.030(2)
1.305(3)
1.394(3)
1.484(4)
2.2940(6)
2.024(2)
1.314(3)
1.388(3)
1.474(3)
2.3082(6)
2.031(2)
1.306(3)
1.396(3)
1.481(3)
2.2952(10)
2.045(3)
1.337(5)
1.394(5)
C(1)–C(2)
1.476(4)
C(1)–N(3)
C(1)–O(1)
O(1)–N(2)
N(2)–C(4)
Cl(1)–Pd(1)–N(1)
Pd(1)–N(1)–C(1)
N(1)–C(1)–C(2)
N(1)–C(1)–N(3)
N(1)–C(1)–O(1)
C(1)–O(1)–N(2)
O(1)–N(2)–C(4)
N(2)–C(4)–N(1)
1.315(5)
1.341(5)
1.441(4)
1.287(5)
88.30(9)
129.9(3)
1.330(4)
1.425(3)
1.295(4)
91.83(7)
124.3(2)
128.3(3)
1.319(3)
1.433(3)
1.304(3)
89.61(6)
125.3(2)
129.5(2)
1.329(3)
1.429(3)
1.305(3)
90.23(6)
128.0(2)
130.2(2)
1.328(3)
1.430(3)
1.297(3)
88.95(5)
126.8(2)
129.4(2)
1.341(3)
1.425(2)
1.294(3)
90.58(5)
132.7(2)
132.4(2)
131.3(4)
110.9(3)
107.6(3)
103.3(3)
114.7(3)
111.0(2)
107.6(2)
104.0(2)
112.5(2)
112.3(2)
106.9(2)
104.0(2)
112.3(2)
112.0(2)
107.1(2)
103.8(2)
112.5(2)
111.0(2)
107.8(2)
103.5(2)
113.1(2)
111.1(2)
107.6(2)
103.7(2)
113.2(2)
Na=C(NMe2)ONCb(C6H2Me3)(Na–Cb) (14): FAB+-MS: m/z = 232
[M + H]+. 1H NMR (300 MHz, CDCl3): δ = 2.23 (s, 6 H, o-
CH3C6H2), 2.28 (s, 3 H, p-CH3C6H2), 3.20 (br. s, 6 H, NMe2), 6.89
(s, 2 H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 19.9
(o-CH3C6H2), 21.1 (p-CH3C6H2), 40.2 (NMe2), 128.2 (m-Ar), 137.5
(o-Ar), 139.1 (p-Ar) ppm; signals for other groups were not de-
tected.
(Cipso), 160.1 (o-Ar), 163.2 (p-Ar), 164.1 (N=C–N), 175.9 (N=C–
O) ppm.
Na=C(NMe2)ONCb(C6H2(OMe)3)(Na–Cb) (18): FAB+-MS: m/z =
280 [M + H]+. 1H NMR (300 MHz, CDCl3): δ = 3.18 (s, 3 H, Me),
3.77 (s, 6 H, o-CH3C6H2), 3.83 (s, 3 H, p-CH3OC6H2), 6.15 (s, 2
H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 40.4
(NMe2), 56.0 (o-CH3OC6H2), 55.4 (p-CH3OC6H2), 90.7 (m-Ar),
160.1 (o-Ar), 162.9 (p-Ar), 164.0 (N=C–N), 171.6 (N=C–O) ppm;
signals for other groups were not detected.
Na=C(Ph)ONCb(C6H2Me3)(Na–Cb) (15): FAB+-MS: m/z = 278
1
[M + Na]+, 265 [M + H]+. H NMR (300 MHz, CDCl3; three sets
of each signal in the approximate ratio 2:2:3): δ = 2.25, 2.30, and
2.34 (three s, 6 H, o-CH3C6H2), 2.05, 2.49, and 2.55 (three s, 3 H,
p-CH3C6H2), 6.92, 6.98, and 7.09 (three s, 2 H, m-Ar), 7.4–7,6 (m,
3 H, m- and p-Ph), 8.22, 8.71, and 8.98 (three d, 2 H, o-Ph) ppm.
13C{1H} NMR (75.4 MHz, CDCl3): δ = 20.1, 20.9 and 21.2 (o-
CH3C6H2), 20.6, 21.5 and 21.6 (p-CH3C6H2), 128.2, 128.6, 128.7,
128.9, 129.1, 129.9, 130.6, 132.7, 133.7, 134.0, 137.8, 139.8 and
140.9 (all Ar) ppm; signals from other groups were not detected.
X-ray Crystal Structure Determinations: The crystals were im-
mersed in perfluoropolyether, mounted in a cryo-loop and mea-
sured at 100 or 110 K. The X-ray diffraction data were collected
with a Nonius KappaCCD diffractometer using Mo-Kα radiation
(λ = 0.71073 Å). The Denzo-Scalepack program package[39] was
used for cell refinements and data reductions. Structures were
solved by direct methods using the SHELXS-97 or SIR-97 pro-
grams or by Patterson method using DIRDIF-99 with the WinGX
graphical user interface.[40–43] An empirical absorption correction
Na=C(Me)ONCb(C6H2(OMe)3)(Na–Cb) (16): FAB+-MS: m/z = 251
[M + H]+. 1H NMR (300 MHz, CDCl3): δ = 2.18 (s, 6 H, o-
CH3OC6H2), 2.33 (s, 3 H, p-CH3OC6H2), 2.68 (s, 3 H, Me), 6.94
(s, 2 H, m-Ar) ppm. 13C{1H} NMR (75.4 MHz, CDCl3): δ = 12.5
was applied to all data using XPREP in SHELXTL v.6.12[44] (Tmin
Tmax 0.12334/0.15440, 0.31972/0.35415, 0.12648/0.15208, and
0.13741/0.17395 for 2, 3, 7, and 8, respectively). Structural refine-
/
:
(Me), 55.4 (p-CH3OC6H2), 56.0 (o-CH3OC6H2), 90.7 (m-Ar), 97.6 ments were carried out with SHELXL-97.[41] The asymmetric unit
Eur. J. Inorg. Chem. 2005, 845–853
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
851

N. A. Bokach, V. Y. Kukushkin, M. Haukka, A. J. L. Pombeiro
FULL PAPER
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contained only half a molecule in 2 and 8, whereas in 3 and 7 the
asymmetric unit contained a whole molecule. All hydrogens were
placed in idealized positions and constrained to ride on their parent
atom. The crystallographic data are summarized in Table 1. Se-
lected bond lengths and angles are shown in Table 2.
[6]
[7]
CCDC-243489 to -243492 (for 2, 3, 7, and 8, respectively) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[8]
[9]
Acknowledgments
This work has been partially supported by the FCT (Foundation
for Science and Technology, Portugal) and its POCTI program
(POCTI/QUI/43415/2001 project; FEDER funded). N.A.B. ex-
presses gratitude to the POCTI program, Portugal (grant SFRH/
BPD/11448/2002) and the Center for Natural Sciences (Konkursnii
Tsentr Fundamentalnogo Estestvoznaniya), Russia (grant PD03-
1.3-28) and the Russian Fund for Basic Research (grant 05-03-
32140a) for financial support. M.H. would like to thank the Acad-
emy of Finland for financial support. V.Y.K. is very much obliged
to the Russian Fund for Basic Research for a grant (03-03-32363),
POCTI program (FEDER funded; Cientista Convidado/Professor
at the Instituto Superior Técnico, Portugal), the Royal Society of
Chemistry for a Grant for International Authors, and the Academy
of Finland for support of the cooperation with the University of
Joensuu. The authors thank NATO for the Collaborative Linkage
Grant PST.CLG.979289.
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