Palladium(II) chloride complexes with 1, 2, 4-triazolo[1, 5-a]-pyrimidines: X-ray,15N-1H NMR and15N CP MAS studies

Article abstract of DOI:10.1039/a908469j

Palladium(ii) complexes of formula [Pd(tp)2Cl2] 1, [Pd(dmtp)2Cl2]-H2O 2, [Pd(dptp)2ClJ-H2O 3, [Pd(dbtp)2Cl2] 4a, 4b, where tp = l, 2, 4-triazolo[l, 5-a]pyrimidine, dmtp = 5, 7-dimethyl-l, 2, 4-triazolo[l, 5-a]pyrimidine, dptp = 5, 7diphenyl-l, 2, 4-triazolo[l, 5-a]pyrimidine, dbtp = 5, 7-di-/e/7-butyl-l, 2, 4-triazolo[l, 5-a]pyrimidine, were prepared. The crystal structure of [Pd(dbtp)2ClJ-0.5 C2HSOH 4c was resolved by X-ray diffraction analysis, exhibiting monomeric, nearly square-planar cis geometry and N(3) co-ordination. A small tetrahedral distortion from the co-ordination plane was observed. The Pd-Cl distances are 2.276(1) and 2.283(1) A, and Pd-N 2.042(3) and 2.040(3) A. Spectroscopic measurements (UV-VIS, IR, NMR) suggested analogous structures for 1, 2 and 3. Compounds 4a and 4b are most likely rotational isomers in solution and distortional isomers in the solid state. I5N-'H heteronuclear correlation NMR was measured for 2, 4a and 4b whereas 1 and 3 were characterized with 13C and N CP MAS. Significant shielding of the co-ordinated N(3) and adjacent C(2) or C(3a) nuclei was observed. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a908469j

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Palladium(II) chloride complexes with 1,2,4-triazolo[1,5-a]-
15
15
pyrimidines: X-ray, N–1H NMR and N CP MAS studies
Edward Szłyk,*a Antoni Grodzicki,a Leszek Pazderski,a Andrzej Wojtczak,a Janusz Chatłas,a
Grzegorz Wrzeszcz,a Jerzy Sitkowskib and Bohdan Kamien´skic
a Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, Torun´, Poland.
E-mail: eszlyk@dove.boa.uni.torun.pl
b Drug Institute, Chełmska 30/34, Warsaw, Poland
c Institute of Organic Chemistry, Kasprzaka 44/52, Warsaw, Poland
Received 25th October 1999, Accepted 24th January 2000
Palladium() complexes of formula [Pd(tp)2Cl2] 1, [Pd(dmtp)2Cl2]ؒH2O 2, [Pd(dptp)2Cl2]ؒH2O 3, [Pd(dbtp)2Cl2] 4a,
4b, where tp = 1,2,4-triazolo[1,5-a]pyrimidine, dmtp = 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine, dptp = 5,7-
diphenyl-1,2,4-triazolo[1,5-a]pyrimidine, dbtp = 5,7-di-tert-butyl-1,2,4-triazolo[1,5-a]pyrimidine, were prepared.
The crystal structure of [Pd(dbtp)2Cl2]ؒ0.5 C2H5OH 4c was resolved by X-ray diffraction analysis, exhibiting
monomeric, nearly square-planar cis geometry and N(3) co-ordination. A small tetrahedral distortion from the
co-ordination plane was observed. The Pd–Cl distances are 2.276(1) and 2.283(1) Å, and Pd–N 2.042(3) and 2.040(3)
Å. Spectroscopic measurements (UV-VIS, IR, NMR) suggested analogous structures for 1, 2 and 3. Compounds 4a
and 4b are most likely rotational isomers in solution and distortional isomers in the solid state. 15N–1H heteronuclear
correlation NMR was measured for 2, 4a and 4b whereas 1 and 3 were characterized with 13C and 15N CP MAS.
Significant shielding of the co-ordinated N(3) and adjacent C(2) or C(3a) nuclei was observed.
ligands are 5,7-disubstituted with bulky groups revealing strong
π-acceptor (phenyl) or σ-donor (tert-butyl) properties and
Introduction
It has been known for years that cis-dichloro complexes of PtII
therefore some conformational changes in the co-ordination
have anti-tumor properties.1,2 However, most of them are not
sphere of PdII could be expected.
active against all types of cancer and have significant side
effects. This is why platinum() chloride complexes with other
N-donors, among them 1,2,4-triazolo[1,5-a]pyrimidine (tp) and
Experimental
its derivatives, are intensively studied. These heterocycles have a
structure similar to that of purine and adenine, their fused ring
system differing in having the pyrimidine nitrogen atom in a
bridgehead position.3–5
Materials
PdCl2 was purchased from POCh Gliwice (Poland), tp, dmtp,
3-amino-1,2,4-triazole, 1,3-diphenylpropane-1,3-dione and
2,2,6,6-tetramethylheptane-3,5-dione of 98–99% purity from
Aldrich. dptp was synthesized from 3-amino-1,2,4-triazole
and 1,3-diphenylpropane-1,3-dione, dbtp from 3-amino-1,2,4-
triazole and 2,2,6,6-tetramethylheptane-3,5-dione.17
Syntheses
The complexes were prepared in a general way as follows: 1
mmol of PdCl2 was dissolved in 10 ml of a mixture of concen-
trated HCl and ethanol (1:10) and added to 2 mmol of an
organic ligand diluted in 40 ml of boiling ethanol. The details
of particular syntheses are given below.
Recently anti-tumor activity of cis-[PtII(Hmtpo-N3)2Cl2]ؒ
2H2O where HmtpO = 4,7-H-5-methyl-7-oxo-1,2,4-triazolo-
[1,5-a]pyrimidine was reported.6 Considering that also pal-
ladium() co-ordination compounds are promising in cancer
therapy,7 we have decided to study a class of cis-PdL2Cl2 type
complexes (L = tp or its derivative). The known co-ordination
compounds of PdII are mono- or poly-meric species of
square-planar geometry around the central ion due to the d8
configuration and dsp2 hybridization.7 This geometry was
found for complexes with such heterocyclic ligands as pyridine,
[Pd(py)2Cl2], imidazole, [Pd(Him)2Cl2], adenine, [Pd(Had)2Cl2],
[{Pd(Had)Cl2}n], adenosine, [Pd(ado)2Cl2], [{Pd(ado)Cl2}n],8–15
and 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp), trans-
[PdII(dmtp-N3)2Br2]ؒCH3OH.16 In this paper we present the
crystal and molecular structure of a new co-ordination
compound with a recently synthesized tp derivative, 5,7-di-
tert-butyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp).17 A series of
palladium() chloride complexes with tp, dmtp, dbtp and
5,7-diphenyl-1,2,4-triazolo[1,5-a]pyrimidine (dptp)17 has been
obtained and the spectral characteristics reported. The last two
cis-[Pd(tp)2Cl2] 1. The reaction mixture was heated with stir-
ring at 75 ЊC, for 15 min. The precipitate was filtered off, washed
with ethanol and dried in air (Found: C, 29.3; H, 1.9; N, 26.4;
Pd, 25.4. C10H8Cl2N8Pd requires C, 28.8; H, 1.9; N, 26.8; Pd,
25.5%). Yield 0.40 g (95%).
cis-[Pd(dmtp)2Cl2]ؒH2O 2. The reaction mixture was heated
with stirring at 75 ЊC, for 1 hour, cooled to 5 ЊC and left
overnight. The yellow precipitate was filtered off, washed with
ethanol and dried in air (Found: C, 33.6; H, 3.5; N, 22.1; Pd,
21.4; H2O, 3.4. C14H18Cl2N8OPd requires C, 34.2; H, 3.7; N,
22.8; Pd, 21.6; H2O, 3.7%). Yield 0.37 g (75%).
cis-[Pd(dptp)2Cl2]ؒH2O 3. The reaction mixture was heated
with stirring at 75 ЊC, for 5 min. The yellow precipitate was
filtered directly from hot solution, washed with boiling ethanol
DOI: 10.1039/a908469j
J. Chem. Soc., Dalton Trans., 2000, 867–872
This journal is © The Royal Society of Chemistry 2000
867

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Table 1 Crystallographic data for complex 4c
and dried in air (Found: C, 55.3; H, 3.5; N, 15.2; Pd, 14.0; H2O,
2.5. C34H26Cl2N8OPd requires C, 55.2; H, 3.5; N, 15.1; Pd, 14.4;
H2O, 2.4%). Yield 0.59 g (80%).
Empirical formula
C26H40Cl2N8Pdؒ0.5 C2H6O
665.02
M
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Triclinic
P1
cis-[Pd(dbtp)2Cl2] 4a, 4b. The reaction mixture was heated
with stirring at 75 ЊC, for 15 min. The yellow precipitate (4a)
was filtered off, washed with ethanol and dried in air. The fil-
trate was heated again at 75 ЊC for 2 h. The yellow-green pre-
cipitate (4b) was filtered off, washed with ethanol and dried in
air (Found for 4a: C, 48.8; H, 7.0; N, 17.1; Pd, 16.7; Found for
4b: C, 48.5; H, 6.7; N, 17.3; Pd, 16.6. C26H40Cl2N8Pd requires
C, 48.6; H, 6.3; N, 17.5; Pd, 16.6%). Yield 0.27 g (42%) for 4a,
0.24 g (37%) for 4b.
¯
10.7427(8)
11.0143(5)
14.1340(8)
75.327(4)
81.426(5)
79.660(4)
1582.10(16)
2
γ/Њ
V/Å3
Z
µ/mmϪ1
6.563
Reflections collected
Reflections unique
Data/parameters
Final R1, wR2 indices [I > 2σ(I)]
(all data)
6874
6498
6498/366
0.0403, 0.0980
0.0583, 0.1082
cis-[Pd(dbtp)2Cl2]ؒ0.5 C2H5OH (4c). This compound was
isolated by slow evaporation of the filtrate left after precipit-
ation of 4a and 4b. Its formula and structure were determined
by X-ray analysis (see below).
Measurements
1H and 13C NMR spectra were recorded with a Varian 200 XL
spectrometer operating at typical frequencies. 15N NMR meas-
urements were performed with a Bruker DRX 500 spectrometer
by 15N–1H heteronuclear correlation (HETCOR) techniques:
for ligands by Gradient Heteronuclear Multiple-Quantum
1
Coherence (50.66 MHz for 15N, 499.80 MHz for H); for com-
plexes by Gradient Heteronuclear Single-Quantum Coherence
(50.68 MHz for 15N and 500.13 MHz for 1H). The solvent was
CDCl3, the concentration of samples 0.05 M, the temperature
295 K, the reference standard TMS for 1H, 13C and CH3NO2 for
15N. 13C and 15N CP MAS spectra were recorded with a Bruker
DRX 500 spectrometer in 4 mm SB-MAS probeheads, using a
standard technique (all nuclei detected) or Short Cross-
Polarization Contact (protonated atoms only). The reference
standard was glycine and chemical shifts were recalculated to
TMS for 13C {δ(CH2 in glycine) 43.3} or to CH3NO2 for 15N
{δ(NH2 in glycine) Ϫ347.6}. UV-VIS reflectance spectra (300–
800 nm, MgO) were performed with a Specord M40 Carl Zeiss
Jena spectrophotometer. IR spectra were measured with a
Perkin-Elmer Spectrum 2000 FT IR spectrometer using
KBr (4000–400 cmϪ1) and polyethylene discs (400–100 cmϪ1).
Kinetic runs were studied with a Hewlett-Packard 8453
diode-array spectrophotometer, equipped with a Peltier type
thermostat. C, H, N were determined by elemental semi-
microanalysis, Pd gravimetrically.18 The amount of water in the
hydrated complexes 2 and 3 was calculated from thermogravi-
metric analysis, performed with a MOM OD-102 derivatograph
(Hungary) (samples 50 mg, heating range 300–1300 K, nitrogen
atmosphere, reference material α-Al2O3).
Fig. 1 The ORTEP22 drawing of the [Pd(dbtp)2Cl2]ؒ0.5 C2H5OH 4c
molecule.
view of the molecule, with its numbering scheme, is shown in
Fig. 1. The asymmetric part of the unit cell contains one mole-
cule of cis-[Pd(dbtp)2Cl2] and half of a molecule of ethanol.
The triazolopyrimidine ligands are bonded to PdII via their
N(3) atoms, the Pd(1)–N(3) distance being 2.042(3) Å and
Pd(1)–N(3Ј) 2.040(3) Å. The chlorine atoms are in terminal
positions, the Pd(1)–Cl(1) distance being 2.276(1) Å and Pd(1)–
Cl(2) 2.283(1) Å. The lengths of Pd–N and Pd–Cl bonds are
similar to those found in palladium() chloride complexes with
such N-donors as pyridine, imidazole, 2,2Ј-bipyridine and 1,10–
phenanthroline (2.011–2.063 Å for Pd–N and 2.276–2.319 Å for
Pd–Cl).21
X-Ray crystallography
X-Ray data for compound 4c were collected with a Kuma
KM-4 diffractometer using Cu-Kα radiation, λ = 1.54178 Å at
293(2) K, by the ω–2θ method. The structure was solved by the
Patterson method and refined with full-matrix least squares
against F2 using SHELX 97,19 giving R = 0.040, wR2 = 0.098
and goodness of fit on F2 = 1.058 [I > 2σ(I)]. The atomic scat-
tering factors were taken from ref. 20. The geometry of 4c was
compared to those of other complexes using the Cambridge
Structural Database.21
The N and Cl atoms in the co-ordination sphere are not
exactly coplanar, their rms deviation from the Cl2N2 best
plane being 0.014 Å. It is a result of a small tetrahedral dis-
tortion, described by the Cl(1)–N(3)–N(3Ј)–Cl(2) torsion angle
Ϫ1.27(9)Њ. This type of deformation, analogous to the one
observed in palladium() chelates,23–25 may be caused by the
steric repulsion of bulky tert-butyl groups in the dbtp mole-
cules. For comparison, the central PdII is positioned in the
Cl2N2 best plane, its deviation being only 0.002(1) Å.
The dbtp rings are planar, the rms deviations from the best
planes for the N(1) to N(8) and N(1Ј) to N(8Ј) moieties being
0.012 and 0.010 Å, respectively. The PdII is displaced from these
planes, its respective deviations being Ϫ0.054(4) and Ϫ0.049(4)
Å for the N(1) to N(8) and N(1Ј) to N(8Ј) moieties. It indicates
that the planar geometry of the co-ordinated N(3) atom (sp2
hybridization) is slightly modified towards pyramidal. The
dihedral angles between the Cl2N2 best plane and the ligand
planes are 39.32(7) and 38.86(7)Њ for the N(1) to N(8) and N(1Ј)
CCDC reference number 186/1819.
See http://www.rsc.org/suppdata/dt/a9/a908469j/ for crystal-
lographic files in .cif format.
Results and discussion
The crystal structure of compound 4c
The crystal structure data of compound 4c are listed in Table 1,
selected bond lengths and angles in Table 2. The perspective
868
J. Chem. Soc., Dalton Trans., 2000, 867–872

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Table 2 Selected bond lengths [Å] and angles [Њ] for complex 4c
Table 3 1H NMR chemical shifts (δ) of dmtp, dbtp and their
palladium() chloride complexes, in CDCl3 (co-ordination shifts in
parentheses)
Pd(1)–N(3)
Pd(1)–Cl(1)
N(8)–N(1)
N(1)–C(2)
C(2)–N(3)
N(3)–C(3a)
C(3a)–N(8)
C(3a)–N(4)
N(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–N(8)
2.042(3)
2.276(1)
1.370(4)
1.304(5)
1.352(4)
1.333(4)
1.380(3)
1.330(4)
1.322(4)
1.427(4)
1.357(4)
1.384(4)
Pd(1)–N(3Ј)
Pd(1)–Cl(2)
N(8Ј)–N(1Ј)
N(1Ј)–C(2Ј)
C(2Ј)–N(3Ј)
N(3Ј)–C(3aЈ)
C(3aЈ)–N(8Ј)
C(3aЈ)–N(4Ј)
N(4Ј)–C(5Ј)
C(5Ј)–C(6Ј)
C(6Ј)–C(7Ј)
C(7Ј)–N(8Ј)
2.040(3)
2.283(1)
1.373(4)
1.310(4)
1.352(4)
1.334(4)
1.372(4)
1.342(4)
1.310(4)
1.426(4)
1.364(4)
1.378(4)
Compound
H(2)
H(6)
Ra
dmtp
2
dbtp
4a
4b
8.37
8.71 (ϩ0.34)
8.43
9.04 (ϩ0.61)
8.65 (ϩ0.22)
6.79
6.98 (ϩ0.19)
7.00
6.87 (Ϫ0.13)
7.11 (ϩ0.11)
2.62; 2.75
2.77; 2.83
1.44; 1.61
0.94; 1.52
1.53; 1.57
a R = CH3 for dmtp, C(CH3)3 for dbtp.
Cl(1)–Pd(1)–Cl(2)
Cl(1)–Pd(1)–N(3)
Cl(1)–Pd(1)–N(3Ј) 178.5(1)
90.7(1)
87.9(1)
N(3)–Pd(1)–N(3Ј)
Cl(2)–Pd(1)–N(3Ј)
Cl(2)–Pd(1)–N(3)
C(2Ј)–N(3Ј)–Pd(1)
C(3aЈ)–N(3Ј)–Pd(1)
N(8Ј)–N(1Ј)–C(2Ј)
N(1Ј)–C(2Ј)–N(3Ј)
C(2Ј)–N(3Ј)–C(3aЈ)
N(3Ј)–C(3aЈ)–N(8Ј)
C(3aЈ)–N(8Ј)–N(1Ј)
C(3aЈ)–N(4Ј)–C(5Ј)
N(4Ј)–C(5Ј)–C(6Ј)
C(5Ј)–C(6Ј)–C(7Ј)
C(6Ј)–C(7Ј)–N(8Ј)
C(7Ј)–N(8Ј)–C(3aЈ)
N(8Ј)–C(3aЈ)–N(4Ј)
93.3(1)
88.2(1)
178.4(1)
125.2(2)
130.9(2)
102.0(3)
116.0(3)
103.9(3)
108.1(3)
110.0(3)
116.4(3)
122.0(3)
122.1(3)
113.8(3)
122.3(3)
123.3(3)
C(2)–N(3)–Pd(1)
C(3a)–N(3)–Pd(1)
N(8)–N(1)–C(2)
N(1)–C(2)–N(3)
C(2)–N(3)–C(3a)
N(3)–C(3a)–N(8)
C(3a)–N(8)–N(1)
C(3a)–N(4)–C(5)
N(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(6)–C(7)–N(8)
C(7)–N(8)–C(3a)
N(8)–C(3a)–N(4)
125.5(2)
130.0(2)
102.1(3)
115.9(3)
104.3(3)
107.4(3)
110.2(3)
116.5(3)
121.5(3)
122.5(3)
114.1(3)
121.6(3)
123.9(3)
to N(8Ј) moieties, respectively. The dihedral angle between the
ring systems of both triazolopyrimidine ligands is 51.56(8)Њ.
The displacement of ternary carbons of the tert-butyl groups
from the ligand best planes is more pronounced for C(51) or
C(51Ј) (Ϫ0.111(3) and 0.121(5) Å, respectively) than for C(71)
or C(71Ј) (0.030(6) and Ϫ0.046(5) Å respectively). It is also
interesting that the C–C bond distances and C–C–C angles
inside these substituents are in a relatively broad range, vary-
ing from 1.512(4) to 1.540(5) Å and from 107.5(3) to 111.4(3)Њ,
respectively. These observations confirm the occurrence of
steric effects associated with the presence of tert-butyl groups.
The ethanol molecule lies outside the co-ordination sphere
and is positioned near the center of symmetry [1/2,0,0] that
relates its two carbon atoms. Such positioning results in dis-
order. The only polar intermolecular interactions found in the
crystal packing are those formed by the ethanol O(10)H group
(site occupancy factor of 50%) with the Cl(2) [x,y,z] ions, the
respective distances being 2.519 Å for the H ؒ ؒ ؒ Cl(2) bonding
and 3.277 Å for the O(10) ؒ ؒ ؒ Cl(2). The O–H ؒ ؒ ؒ Cl(2) angle
is 154.2Њ.
Fig. 2 HETCOR 15N–1H NMR spectrum of dmtp.
collected in Tables 3–5. For comparison, we have measured also
the 13C and 15N CP MAS spectra for the ligands tp and dptp
(Tables 4, 5).
The co-ordination of triazolopyrimidine molecules by PdII
1
1
results in the shift of H and 13C signals (Tables 3, 4). The H
resonances have been shifted in both directions of the fre-
quency scale (maximum 0.6 ppm). For 13C NMR a clear pattern
of the co-ordination shifts has been observed for the complexes
of dmtp and dbtp (2, 4a–4c), for which the measurements were
performed in CDCl3. The C(2) and C(3a) signals have been
shifted upfield (maximum 3.6 ppm), C(5), C(6) and C(7) down-
field (maximum 3.6 ppm). This fact indicates that these two
ligands co-ordinate via N(3). Analogous results were found for
palladium() and platinum() chloride complexes with pyrim-
idine derivatives.38–42 For the complexes of tp and dptp (1, 3),
the results of 13C NMR measurements in the solid phase are not
so apparent and do not allow one unambiguously to determine
the metallation sites. The shifts of signals are large (maximum
6.8 ppm) but there is no correlation between their direction or
magnitude and the position of a carbon atom in the hetero-
cyclic ring. Most likely, the local magnetic fields are strongly
influenced by the orientation or packing of the molecules in the
crystal lattice.
NMR spectroscopy
1
The H and 13C NMR spectra of tp and dmtp were reported
by many authors.26–31 The 15N NMR spectrum of tp was also
published but the assignment of resonances was the subject
of literature controversies.32–36 Recently we have assigned
those signals using the 15N–1H HETCOR method.37 Selective
decoupling techniques for 1H, 13C and 15N NMR measurements
of dmtp, dptp and dbtp were also applied.17 During this work
we have measured 15N–1H HETCOR spectra for all discussed tp
derivatives, assigning 15N resonances (for dmtp, Fig. 2) as pre-
viously:17 N(1) and N(3) are coupled with H(2), N(4) with the
CH3 group at C(5) and N(8) with H(2), H(6) and the CH3 group
at C(7). The N(1) and N(3) signals can be identified on the basis
of their chemical shifts. It has been demonstrated by many
authors that in triazolopyrimidine ring systems the N(3)
nucleus is much more shielded than the other pyridine-type
nitrogens, i.e. N(1) and N(4).32–36
In the 15N NMR spectra of the examined complexes a large
upfield shift (78–90 ppm) of one of the resonances was
observed (Table 5). For 2, 4a and 4b this signal was unambigu-
ously assigned to N(3) by 15N–1H HETCOR measurements. In
the case of 1 and 3, for which the CP MAS spectra were
recorded and compared to those of tp and dptp, it could be
related to N(3) or N(8). However, the latter possibility can
be excluded, because N(8) is a pyrrole-type nitrogen which
is unable to co-ordinate with PdII and co-ordination occurs via
N(3).
The discussed palladium() complexes decompose in d6-
dmso, therefore we have performed NMR measurements in
CDCl3 (2, 4a–4c) or in the solid phase (1, 3), the data being
The observed upfield shift of the N(3) signal is comparable to
the one resulting upon protonation of this nitrogen atom in tp
J. Chem. Soc., Dalton Trans., 2000, 867–872
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Table 4 13C NMR chemical shifts (δ) of 1,2,4-triazolo[1,5-a]pyrimidines and their palladium() chloride complexes, in CDCl3 or solid phase
(co-ordination shifts in parentheses)
Compound
C(2)
C(3a)
C(5)
C(6)
C(7)
Ra
tpsolid b
154.7
159.0 (ϩ4.3)
155.1
154.0 (Ϫ1.1)
158.0
153.5 (Ϫ4.5)
154.3
154.1
151.1 (Ϫ3.0)
155.0
152.1 (Ϫ2.9)
156.4
152.8 (Ϫ3.6)
156.0
156.7
159.0 (ϩ2.3)
164.6
168.2 (ϩ3.6)
161.4
161.2 (Ϫ0.2)
175.6
109.9
116.7 (ϩ6.8)
110.5
112.9 (ϩ2.4)
105.8
105.2 (Ϫ0.6)
103.2
138.7
138.8 (ϩ0.1)
146.5
147.9 (ϩ1.4)
149.1
147.0 (Ϫ2.1)
157.4
—
—
1solid
dmtpCDCl
16.8; 24.8
16.5; 25.4
125–140
125–140
36.1; 38.6; 26.9; 29.5
36.4; 38.5; 26.9; 29.0
36.2; 39.3; 26.9; 29.5
3
2CDCl
3
dptpsolid
3solid
dbtpCDCl
3
4aCDCl
154.3 (0.0)
153.0 (Ϫ1.3)
152.4 (Ϫ3.6)
152.7 (Ϫ3.3)
177.6 (ϩ2.0)
178.3 (ϩ2.7)
104.4 (ϩ1.2)
105.2 (ϩ2.0)
158.9 (ϩ1.5)
158.3 (ϩ0.9)
3
4bCDCl
3
a R = H for tp, CH3 for dmtp, C6H5 for dptp, C(CH3)3 for dbtp. b Data from our earlier paper.37
Table 5 15N NMR chemical shifts (δ) of 1,2,4-triazolo[1,5-a]pyrimidines and their palladium() chloride complexes, in CDCl3 or solid phase
(co-ordination shifts in parentheses)
Compound
N(1)
N(3)
N(4)
N(8)
tpsolid a
Ϫ107.2
Ϫ106.0 (ϩ1.2)
Ϫ109.7
Ϫ109.8 (Ϫ0.1)
Ϫ108.3
Ϫ109.5 (Ϫ1.2)
Ϫ106.4
Ϫ154.7
Ϫ242.5 (Ϫ87.8)
Ϫ151.0
Ϫ240.2 (Ϫ89.2)
Ϫ151.2
Ϫ239.1 (Ϫ87.9)
Ϫ155.1
Ϫ107.2
Ϫ103.7 (ϩ3.5)
Ϫ111.7
Ϫ114.2 (Ϫ2.5)
Ϫ120.7
Ϫ125.0 (Ϫ4.3)
Ϫ114.2
Ϫ154.7
Ϫ155.4 (Ϫ0.7)
Ϫ152.8
Ϫ157.2 (Ϫ4.4)
Ϫ155.7
Ϫ163.8 (Ϫ8.1)
Ϫ157.5
1solid
dmtpCDCl
3
2CDCl
3
dptpsolid
3solid
dbtpCDCl
3
4aCDCl
Ϫ107.4 (Ϫ1.0)
Ϫ106.3 (ϩ0.1)
Ϫ233.3 (Ϫ78.2)
Ϫ242.6 (Ϫ87.5)
Ϫ120.1 (Ϫ5.9)
Ϫ113.9 (ϩ0.3)
Ϫ162.1 (Ϫ4.6)
Ϫ161.4 (Ϫ3.9)
3
4bCDCl
3
a N(1)/N(4) and N(3)/N(8) signals in the CPMAS spectrum of tp are not separated.37
Table 6 Selected IR bands/cmϪ1 for 1,2,4-triazolo[1,5-a]pyrimidines
and their palladium() chloride complexes (co-ordination shifts in
parentheses)
(78 ppm).35 This complexation shift is much more significant
than observed for zinc() or gold() triazolopyrimidine
chloride complexes (1–2 and 8–10 ppm respectively),37,43,44 but
smaller than for analogous platinum() compounds (92–96
ppm).44 Similar co-ordination shifts of 15N resonances were
already described for a series of palladium() aqua- and chloro-
complexes with ammonia (35–55 ppm).45
Compound
νtp
νpy
tp
1
dmtp
2
dptp
3
dbtp
4a
4b
4c
1621
1616 (Ϫ5)
1637
1640 (ϩ3)
1612
1615 (ϩ3)
1615
1617 (ϩ2)
1618 (ϩ3)
1616 (ϩ1)
1534; 1515
1538; 1519 (ϩ4; ϩ4)
1550
1552 (ϩ2)
1543
1553 (ϩ10)
1530
1537 (ϩ7); 1547 (ϩ17)
1536 (ϩ6); 1547 (ϩ17)
1538 (ϩ8); 1547 (ϩ17)
The shielding of atom N(3) (as well as of adjacent carbons
C(2) and C(3a)) is most likely caused by involvement of its lone-
pair electrons in the co-ordination bonding. This process is
accompanied by a slight modification of the trigonal geometry
of N(3) towards pyramidal (see 4c structure discussion),
which suggests that this nitrogen partially changes its sp2
hybridization and becomes closer to sp3. The lone-pair elec-
trons impact on the value of the shielding tensor can be illus-
trated by the comparison of pyridine-type nitrogens in azines
with much more shielded pyrrole-type atoms in azoles.46 The
phenomenon of the upfield shift of 15N resonances (50–150
ppm) was observed for many heteroaromatic ring systems upon
protonation, N-oxidation or platinum() complexation.47,48
The nitrogen co-ordination shift is sensitive to the presence of a
lone pair on nitrogen and π-electron delocalization. In transi-
tion metal complexes d-electron pair shift onto nitrogen is
also possible resulting in nitrogen shielding.49,50 The upfield
co-ordination shift of π-bonded nitrogen can be related to
the removal of the low energy n(N) → π* circulation and
stabilization by binding to the metal ion.
Table 7 Far-IR bands/cmϪ1 for palladium() chloride complexes with
1,2,4-triazolo[1,5-a]pyrimidines
Compound
ν(Pd–Cl)
ν(Pd–N)
1
360, 306
356, 338
352, 325
364, 341
356, 336
363, 341
285, 245
296, 280
286, 272
324, 300
307, 297
324, 299
2
3
4a
4b
4c
present in the ligands spectra, have been detected (Table 7). Two,
found in the range 306–364 cmϪ1, can be assigned to Pd–Cl
stretching vibrations as they are similar to those found for pal-
ladium() salts and co-ordination compounds such as PdCl2
(343 cmϪ1), PdCl42Ϫ (321 cmϪ1), trans-[PdCl2(NH3)2] (333 cmϪ1),
cis-[PdCl2(NH3)2] (306, 327 cmϪ1), trans-[Pd(py)2Cl2] (358
cmϪ1), cis-[Pd(py)2Cl2] (333, 342 cmϪ1), trans-[Pd(Him)2Cl2]
(373 cmϪ1) and cis-[Pd(Him)2Cl2] (335, 339 cmϪ1).8–10,51,52 The
two bands, found in the range 245–324 cmϪ1, can be allocated
to Pd–N stretching modes, as was reported for trans-
[Pd(py)2Cl2] (278 cmϪ1) and cis-[Pd(py)2Cl2] (266, 276 cmϪ1).8,9
Infrared spectroscopy
The two most characteristic bands in the IR spectra of tp and
its derivatives, νtp (1612–1637 cmϪ1) and νpy (1515–1550 cmϪ1),
were assigned to an overall triazolopyrimidine and the pyrim-
idine ring mode vibrations.37 The co-ordination by PdII results
in a shift of these bands towards higher frequencies (maximum
18 cmϪ1). This effect, similar to the one observed for anal-
ogous zinc() complexes,37,43 is more profound for νpy than for
νtp (Table 6).
In the range 100–400 cmϪ1 four new absorption bands, not
870
J. Chem. Soc., Dalton Trans., 2000, 867–872

View Article Online
Assuming the square-planar configuration around PdII,
one can expect from group theory calculations one Pd–Cl and
one Pd–N stretching vibration (IR-active) for trans isomers
(symmetry D2h) or two of each mode for cis forms (C2v).51
Therefore, the presence of four bands in the far-IR spectra of
all studied complexes is in favor of their C2v symmetry.
In the IR spectra of complexes 2 and 3 one can observe
broad absorption bands in the range 3450–3550 cmϪ1, deriving
from O–H stretching vibrations of the water molecules. Their
energy is about 200 cmϪ1 smaller than for free water (3755 and
3655 cmϪ1)53,54 and typical for lattice water.51 The deformation
vibrations of H–O–H, expected in the 1600–1650 cmϪ1 range,
cannot be observed as they are overlapped by intense νtp bands
of the triazolopyrimidine ligands. The molecules of H2O
are most likely hydrogen bonded to chlorine atoms, as for
ethanol in 4c or for lattice water in a ruthenium() complex,
[Ru(dmtp)2Cl3(H2O)]ؒH2O.55 However, they can also occupy the
axial positions in the co-ordination sphere of the central ion,
which is common in the chemistry of Pd and Pt.56 A similar
mode of binding of solvent molecules, by van der Waals forces,
was described for methanol in trans-[PdII(dmtp-N3)2Br2]ؒ
CH3OH.16
Fig. 3 The electronic spectra of isomers 4a (1), 4b (2) and their
equilibrium mixture (3); CHCl3, 25 ЊC, [PdII]T = 0.002 M.
Electronic spectroscopy
In the UV-VIS spectra of complexes 1 and 2 the absorption
bands have been observed at 340–350, 410–420 and 500–510
nm. Assuming C2v symmetry around PdII one can suggest that
1
they derive from the spin allowed d–d transitions: A1g →
1
1
1A2g, A1g → 1Eg, A1g → 1B1g, from the dxy(b2g), dxz,yz(eg)
level.57 In the
2
2 Ϫ y2
and dz (a1g) orbitals to the empty dx
spectrum of 3 only two shoulders are observed at 340 and 375
nm, due to overlapping by intense LMCT transitions.
Complexes 4a and 4b significantly differ in their electronic
spectra. The former reveals an absorption band at 388 nm, the
latter at 345 and 417 nm. In freshly prepared chloroform solu-
tions the shape of the absorption curves for the two compounds
is similar to the one in the solid state, the respective maxima
being observed at 382 nm for 4a, 338 and 407 nm for 4b.
Fig. 4 Spectral changes observed during the isomerization reaction
4a → 4b; CHCl3, 25 ЊC, [PdII]T = 0.002 M.
those compounds the deformations were associated with the
coexistence of distortional isomers in the crystal lattice. This
kind of isomerism is defined as involving two or more equi-
librium arrangements of ligands, differing in the structural
nuances of the co-ordination polyhedron.60–62 In our opinion,
obtained as pure species, 4a and 4b can be regarded either as
rotational isomers in solution or distortional ones in the solid
state.
Studies of tautomerism between complexes 4a and 4b
Complexes 4a and 4b have an identical stoichiometric formula,
similar cis geometry and the same N(3) co-ordination site but
they reveal different spectroscopic properties (NMR, UV-VIS,
far-IR). Their detailed geometry is not known, however the
spectral characteristic of 4a is virtually the same as that of 4c.
This indicates that 4a has a structure nearly identical to that of
4c, although the stoichiometry of these two compounds is
different. In the solid state 4a and 4b are stable and no taut-
omerism is observed at 0–150 ЊC. However, when dissolved in
chloroform or acetone, they isomerize to a mixture containing
The kinetics of the isomerization reaction (1) for both forms
kab
4a
4b
KAB = kab/kba
(1)
kba
of [Pd(dbtp)2Cl2] has been studied by electronic spectroscopy.
The absorption curves (320–550 nm) of pure complexes 4a and
4b as well as of their equilibrium mixture are presented in Fig. 3
(CHCl3, 25 ЊC, 0.002 M PdII). The kinetic runs, carried out
under the above mentioned conditions, and the typical changes
with the reaction course are presented at Fig. 4. The existence
of three isosbestic points (at 333, 359 and 410 nm) confirms the
proposed reaction stoichiometry. The runs were performed
starting from either 4a or 4b and gave the same observed
rate constants, calculated for the range 320–550 nm with a
SPECFIT program:63 kobs = (kab ϩ kba) = 2.5 × 10Ϫ4 3 × 10Ϫ5
sϪ1; kab = 1.9 × 10Ϫ4 sϪ1; kba = 0.6 × 10Ϫ4 sϪ1. The value of equi-
librium constant KAB = kab/kba = 3.2 which differs less than 10%
from the one determined by 1H NMR.
1
both forms of [Pd(dbtp)2Cl2]. Integration of H NMR signals
(295 K) demonstrates that in CDCl3 there is 26% of 4a and
74% of 4b (KAB = 2.85), in d6-acetone 84% of 4a and 16% of 4b
(KAB = 0.19). In the latter solvent the 1H chemical shifts of H(2)
and H(6) were δ 9.03 and 7.24 for 4a, δ 8.75 and 7.47 for 4b.
1H NMR variable temperature experiments revealed that in
CDCl3 the ratio 4a:4b was strongly dependent on temperature,
varying from 1:4 at 303 K to 7:4 at 328 K. This suggests that
both complexes are rotational isomers, formed by the restrained
rotation of the dbtp ligands about the Pd–N(3) or Pd–N(3Ј)
bond. The distinct orientation of the triazolopyrimidine rings
results in the observed differences in the NMR spectra of
both complexes. This phenomenon is probably affected by the
steric hindrance of the bulky tert-butyl groups of the dbtp
molecules.
Conclusion
The described distortions (see structure discussion) from
square-planar geometry of the central ion in complex 4c are
similar to those reported for a few palladium() complexes with
derivatives of 2,2Ј-bipyridine and 1,10-phenanthroline.58,59 For
The present study describes a series of cis-dichloro complexes
of PdII with tp and its 5,7 derivatives. The observed co-
ordination mode via N(3) may be regarded as representative
J. Chem. Soc., Dalton Trans., 2000, 867–872
871

View Article Online
26 J. Makisumi, H. Watanabe and K. Tori, Chem. Pharm. Bull., 1964,
for these heterocycles and analogous to N(9) complexation in
purines. The crystal structure of 4c is the first one reported for
the novel dbtp ligand and exhibits tetrahedral distortion from
the square-planar geometry. Such unusual deformation can be
related to the steric strains caused by bulky tert-butyl substitu-
ents. The two other dbtp complexes, 4a and 4b, undergoing
tautomeric reactions in non-co-ordinating solvents, can prob-
ably be regarded as rotational isomers in the solutions and
distortional ones in the solid phase.
The palladium() complexation results in a large upfield shift
of 15N NMR signals of co-ordinated nitrogens (ca. 80–90 ppm)
and adjacent carbons (maximum 3.6 ppm). The magnitude
of the observed shielding is much higher than for analogous
zinc() and gold() complexes but lower than for platinum()
ones.
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Acknowledgements
We wish to thank Dr M. Barysz for helpful discussion during
this work.
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