Expanding the scope of "Click" derived 1,2,3-triazole ligands: New palladium and platinum complexes

Article abstract of DOI:10.1016/j.ica.2011.02.085

Using the 'Click Protocol' the new ligands 1-(cyclohexyl)-4-(2-pyridyl)-1, 2,3-triazole (1), 1-(2-trifluoromethyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (2), 1-(4-hexyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (3), 1-(2-mercaptomethyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (4) and 1-(4-N,N-dimethylamino phenyl)-4-(2-pyridyl)-1,2,3-triazole (5) were prepared by reacting 2-ethynylpyridine with the corresponding azides. In the next step the ligands were reacted with suitable palladium and platinum precursors to yield the cis-dichloro-palladium complexes 1a-4a and platinum complexes 1b-4b. Investigation of the molecular structure of the free ligands 1 and 5 reveals the formation of infinite chains in the 3D structure which are governed by hydrogen bonds between the triazole units. Likewise the 3D structure of 1a shows infinite chains which are held together by multiple remarkably short C-H?Cl-Pd contacts. Electrochemical investigation of the free ligands by cyclic voltammetry showed irreversible reduction processes at highly negative potential. Upon metal complexation huge anodic shifts of the reduction potential were observed. To further characterize the electronic properties of all the compounds UV-Vis spectra were also analyzed.

Full text of DOI:10.1016/j.ica.2011.02.085

Inorganica Chimica Acta 374 (2011) 253–260
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Expanding the scope of ‘Click’ derived 1,2,3-triazole ligands: New palladium
and platinum complexes
⇑
David Schweinfurth, Sabine Strobel, Biprajit Sarkar
Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 March 2011
Using the ‘Click Protocol’ the new ligands 1-(cyclohexyl)-4-(2-pyridyl)-1,2,3-triazole (1), 1-(2-trifluoro-
methyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (2), 1-(4-hexyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (3),
1-(2-mercaptomethyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (4) and 1-(4-N,N-dimethylamino phenyl)-4-
(2-pyridyl)-1,2,3-triazole (5) were prepared by reacting 2-ethynylpyridine with the corresponding
azides. In the next step the ligands were reacted with suitable palladium and platinum precursors to yield
the cis-dichloro-palladium complexes 1a–4a and platinum complexes 1b–4b. Investigation of the molec-
ular structure of the free ligands 1 and 5 reveals the formation of infinite chains in the 3D structure which
are governed by hydrogen bonds between the triazole units. Likewise the 3D structure of 1a shows infi-
nite chains which are held together by multiple remarkably short C–HÁ Á ÁCl–Pd contacts. Electrochemical
investigation of the free ligands by cyclic voltammetry showed irreversible reduction processes at highly
negative potential. Upon metal complexation huge anodic shifts of the reduction potential were
observed. To further characterize the electronic properties of all the compounds UV–Vis spectra were also
analyzed.
Dedicated to Prof. Wolfgang Kaim for his
outstanding contributions to the
coordination chemistry of nitrogen rich
ligands
Keywords:
Click reaction
1,2,3-Triazoles
Palladium complexes
Platinum complexes
H-bonding
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
nitrogen donor atoms are also useful as hydrogen bond acceptors
[21] or can be the target of methylation [22] or protonation [23].
The copper catalyzed azide-alkyne cycloaddition (CuAAC) [1,2]
has found rich application in different chemical fields ranging from
polymer science [3] to drug discovery [4]. In most instances the
1,2,3-triazole unit is used as a rigid linkage which can be formed
on demand. In recent times coordination chemists have added
another application of the Click methodology by using triazole het-
erocycles as donor ligands in transition metal complexes [5]. The
mild and selective Click Protocol has been used to prepare new
ligands with different denticity mostly by reacting azides or
alkynes containing amine [6,7], pyridine [8–10] or phosphine func-
tionalities [11,12]. For example the reaction of 2-ethynylpyridine
with organic azides yields 2-pyridyl substituted 1,2,3-triazole
(pyta) ligands which resemble bipyridine ligands. These ligands
were reacted with a variety of transitions metals and complexes
of Re [13,14], Ir [15], Ru [16,17], Pd [18], Pt [18], Cu [19,20] and
Ag [19,20] were reported.
In contrast to bipyridine ligands, the backbone of the triazole based
ligands is a functional site as well. Because of the strongly dipolar
character of the triazole ring, the C–H bond of the heterocycle is a
surprisingly good hydrogen bond donor [21] and is also useful in
anion recognition [24,25]. The easy variation of the triazole substi-
tuent R allows for the incorporation of further donor functionalities
[13]. Electronic influences and the steric demand can easily be
tuned and appropriate functionalities can be used for immobiliza-
tion on surfaces.
In this work we present the synthesis of five new pyta ligands
with different functionalities and their reaction with common pal-
ladium and platinum precursors. Our interest concerns the possi-
bilities of influencing the electronic structure, hydrogen bonding
and new reactivity linked to additional donor atoms. In this
respect, the synthesis of cis-dichloro-palladium and platinum com-
plexes is especially useful. Cis-dichloro-palladium complexes are
valuable catalysts in various organic transformations [26] whereas
cis-dichloro-platinum complexes are interesting because of their
use in tumor therapy [27,28] and as catalysts and precatalysts
for C–H bond activation [29,30]. Furthermore they exhibit
interesting electrochemical [31,32] and spectroscopic properties
[33–35]. The scope of these complexes can profit from new
ligands with tunable electronic structure and predictable weak
interactions.
The family of pyta ligands offers a variety of interesting proper-
ties (Scheme 1) and the ligands are easily functionalized by using
various easily accessible azides. First of all the chelating pocket of-
fers the possibility of binding various transition metals, but the
⇑
Corresponding author. Tel.: +49 71168564235; fax: +49 71168564165.
E-mail address: sarkar@iac.uni-stuttgart.de (B. Sarkar).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.085

254
D. Schweinfurth et al. / Inorganica Chimica Acta 374 (2011) 253–260
were added and the resulting mixture was stirred for 2 d at
50 °C. Then water (50 mL) was added and the solution extracted
by CH2Cl2 (3Â 20 mL). The organic phases were separated and
dried over Na2SO4. The solvent was evaporated and the desired
product was obtained as a white solid (223 mg) in 49% yield. Anal.
Calc. for C13H16N4: C, 68.39; H, 7.06; N, 24.54. Found: C, 68.32; H,
7.17; N, 24.58. HRMS (ESI): Calc. for C13H16N4Na ([M+Na]+): m/z
251.1267; found 251.1265. 1H NMR (250 MHz, CDCl3): d 1.26–
1.55 (m, 3H, cyclohexyl), 1.70–1.97 (m, 5H, cyclohexyl), 2.26 (d,
3
3JH–H = 12.7 Hz, 2H, cyclohexyl), 4.51 (t, JH–H = 11.6 Hz, 1H, cyclo-
3
hexyl CH), 7.20 (t, JH–H = 5.4 Hz, 1H, pyridyl-H), 7.76 (t,
3JH–H = 7.7 Hz, 1H, pyridyl-H), 8.15–8.19 (m, 2H), 8.56 (d,
3JH–H = 4.2 Hz, 1H, 6-pyridyl-H). 13C{1H} NMR (62.9 MHz, CDCl3):
d 25.1, 33.5, 60.2, 119.6, 120.1, 122.6, 136.8, 147.9, 149.3, 150.6.
2.2.3. Synthesis of 1-(2-trifluoromethyl phenyl)-4-(2-pyridyl)-1,2,3-
triazole (2)
The compound was prepared following the procedure for 1 and
was obtained as a yellow solid (539 mg) in 93% yield. Anal. Calc.
for C14H9F3N4: C, 57.93; H, 3.13; N, 19.30. Found: C, 58.07; H,
3.18; N, 19.50. HRMS (ESI): Calc. for C14H9F3N4Na ([M+Na]+): m/z
313.0672; found 313.0666. 1H NMR (400 MHz, CDCl3): d 7.25 (m,
Scheme 1. Functionality of 2-pyridyl 1,2,3-triazole (pyta) ligands.
3
3
1H), 7.59 (d, JH–H = 7.8 Hz, 1H), 7.69 (t, JH–H = 7.6 Hz, 1H), 7.75 (t,
3
3
3JH–H = 7.7 Hz, 1H), 7.80 (t, JH–H = 7.6 Hz, 1H), 7.88 (d, JH–H = 7.8
2. Experimental
3
Hz, 1H), 8.25 (d, JH–H = 7.9 Hz, 1H), 8.42 (s, 1H, 5-triazole-H), 8.60
(d, 3JH–H = 4.8 Hz, 1H, 6-pyridyl-H). 13C{1H} NMR (62.9 MHz, CDCl3):
2.1. General
1
2
d 120.5, 122.6 (q, JC–F = 273.6 Hz), 123.1, 124.6, 126.3 (q, JC–F
31.9 Hz), 127.4 (q, JC–F = 4.3 Hz), 128.9, 130.5, 133.1, 134.8, 136.9,
=
3
All solvents were dried and distilled using common techniques
unless otherwise mentioned. 1H and 13C NMR spectra were re-
corded at 250.13 and 62.90 MHz, respectively, on a Brucker
AC250 instrument. UV–Vis absorption spectra were recorded on
a J&M JIDAS spectrometer. Cyclic voltammetry was carried out in
0.1 M Bu4NPF6 solution using a three-electrode configuration
(glassy carbon working electrode, Pt counter electrode, Ag wire
as pseudoreference) and a PAR 273 potentiostat and function gen-
erator. The ferrocene/ferrocenium (Fc/Fc+) couple served as inter-
nal reference. Elemental Analysis was performed on a Perkin
Elmer Analyser 240.
148.4, 149.5, 149.8.
2.2.4. Synthesis of 1-(4-Hexyl phenyl)-4-(2-pyridyl)-1,2,3-triazole (3)
The compound was prepared following the procedure for 1 and
was obtained as a yellow solid (532 mg) in 87% yield. Anal. Calc. for
C
19H22N4 + H2O: C, 70.34; H, 7.46; N, 17.27. Found C, 71.25; H,
7.04; N, 17.43. HRMS (ESI): Calc. for C19H22N4Na ([M+Na]+): m/z
329.1737; found 329.1737. 1H NMR (250 MHz, (CD3)2CO): d 0.89
(t, JH–H = 6.9 Hz, 3H, CH3), 1.36 (m, 6H, alkyl), 1.70 (qt, JH–H
3
3
=
=
3
3
7.7 Hz, 2H, CH2), 2.74 (t, JH–H = 7.0 Hz, 2H, CH2), 7.35 (t, JH–H
3
6.9 Hz, 1H, pyridyl), 7.47 (d, JH–H = 8.7 Hz, 2H, Ph), 7.88–7.95 (m,
3
3H), 8.19 (d, JH–H = 7.9 Hz, 1H, pyridyl), 8.63 (s, 1H, pyridyl), 8.91
(s, 1H, 5-triazole-H). 13C{1H} NMR (100 MHz, CD2Cl2): d 13.5,
22.3, 28.6, 31.0, 31.4, 35.2, 119.8, 120.1, 122.7, 129.4, 134.6,
136.5, 143.9, 148.5, 149.3, 149.9.
2.2. Synthesis
2.2.1. Synthesis of 4-hexylbenzoazide
4-Hexylaniline (2.00 g, 11.3 mmol) was added to a mixture of
water (25 mL) and hydrochloric acid (25 mL). The solution was
cooled down to 0 °C and an aqueous solution of NaNO2 (936 mg,
13.6 mmol) was added dropwise. After stirring for 1 h at 0 °C urea
(68.0 mg, 0.11 mmol) was added. In a second flask NaOAc. 3H2O
(4.61 g, 33.9 mmol) and NaN3 (1.47, 22.6 mmol) were dissolved
in water and cooled down to 0 °C. To this solution, the solution
containing the diazonium salt was added dropwise at 0 °C. After
stirring for 2 h the yellow solution was extracted by CH2Cl2
(3Â 30 mL). The organic phases were separated and dried over
Na2SO4. The solvent was evaporated and the desired product was
obtained as a yellow oil (1.56 g) in 68% yield. 1H NMR (250 MHz,
2.2.5. Synthesis of 1-(2-Mercaptomethyl phenyl)-4-(2-pyridyl)-1,2,3-
triazole (4)
The compound was prepared following the procedure for 1 and
was obtained as a yellow solid (504 mg) in 94% yield. The low sol-
ubility did not allow for characterization by 13C NMR. Anal. Calc. for
C14H12N4S + ½H2O: C, 60.63; H, 4.72; N, 20.20. Found C, 60.68; H,
4.40; N, 20.02. HRMS (ESI): Calc. for C14H12N4NaS ([M+Na]+): m/z
291.0675; found 291.0664. 1H NMR (250 MHz, (CD3)2CO): d 2.48
3
(s, 3H, SCH3), 7.36 (s, 1H), 7.43 (m, 1H), 7.57 (d, JH–H = 7.0 Hz,
3
3
1H), 7.61 (m, 2H), 7.93 (t, JH–H = 7.4 Hz, 1H), 8.21 (d, JH–H
=
6.9 Hz, 1H), 8.65 (m, 2H).
3
CDCl3): d 0.92 (t, JH–H = 6.5 Hz, 3H, CH3), 1.34 (m, 6H, alkyl), 1.62
3
3
(qt, JH–H = 7.7 Hz, 2H, CH2), 2.61 (t, JH–H = 7.4 Hz, 2H, CH2), 6.96
2.2.6. Synthesis of 1-(4-N,N-Dimethylamino phenyl)-4-(2-pyridyl)-
1,2,3-triazole (5)
The compound was prepared following the procedure for 1 and
was obtained as a brown solid (189 mg) in 35% yield. Anal. Calc. for
3
3
(d, JH–H = 8.5 Hz, 2H, Ph), 7.17 (d, JH–H = 8.5 Hz, 2H, Ph). 13C{1H}
NMR (62.9 MHz, CDCl3): d 14.0, 22.6, 28.8, 31.4, 31.7, 35.3, 118.8,
~
m
129.7, 137.3, 139.8. IR:
2098 (azide).
C
15H15N5: C, 67.90; H, 5.70; N, 26.40. Found C, 67.26; H, 5.59; N,
2.2.2. Synthesis of 1-(Cyclohexyl)-4-(2-pyridyl)-1,2,3-triazole (1)
Cyclohexylazide (250 mg, 2.00 mmol) and 2-ethinylpyridine
(206 mg, 2.00 mmol) were dissolved in CH2Cl2/H2O/tert-butanol
(2.5 mL/2.5 mL/5 mL). CuSO4. 5H2O (25.0 mg, 0.10 mmol), sodium
ascorbate (79.0 mg; 0.40 mmol) and TBTA (11.0 mg, 0.02 mmol)
25.88. HRMS (ESI): Calc. for C
15H15N5Na ([M+Na]+): m/z
288.1220; found 288.1212. 1H NMR (250 MHz, (CD3)2CO): d 3.05
3
(s, 6H, N(CH3)2), 6.92 (d, JH–H = 9.2 Hz, 2H, Ph), 7.33 (m, 1H, pyri-
dyl), 7.78 (d, 3JH–H = 9.2 Hz, 2H, Ph), 7.89 (t, 3JH–H = 7.7 Hz, 1H, pyr-
3
3
idyl), 8.17 (d, JH–H = 7.9 Hz, 1H, 3-pyridyl-H), 8.61 (d, JH–H
=

D. Schweinfurth et al. / Inorganica Chimica Acta 374 (2011) 253–260
255
3
3
4.1 Hz, 1H, 6-pyridyl-H), 8.74 (s, 1H, 5-triazole-H). 13C{1H} NMR
(62.9 MHz, CDCl3): d 40.4, 112.3, 119.9, 120.3, 121.7, 122.8,
126.7, 136.8, 148.5, 149.4, 150.3, 150.6.
8.15 (d, JH–H = 7.6 Hz, 1H, 3-pyridyl-H), 8.36 (t, JH–H = 7.8 Hz, 1H,
pyridyl), 9.33 (m, 2 H).
2.2.12. Synthesis of 2b
The compound was prepared following the procedure for 1b
and was obtained as a yellow solid (82.7 mg) in 65% yield. Anal.
Calc. for C14H9Cl2F3N4Pt: C, 30.23; H, 1.63; N, 10.07%. Found: C,
29.78; H, 1.63; N, 9.82%. HRMS (ESI): Calc. for C14H9Cl2F3N4NaPt
2.2.7. Synthesis of 1a
[Pd(COD)Cl2] (50.0 mg, 0.18 mmol) and 1 (40.0 mg, 0.18 mmol)
were dissolved in CH2Cl2 (10 mL). The solution was stirred for 1 h.
After that the solution was concentrated and a yellow solid precip-
itated. The yellow solid was filtered and washed with ether and
hexane. The desired compound (62.8 mg) was obtained in 86%
yields. Anal. Calc. for C13H16Cl2N4Pd: C, 38.49; H, 3.98; N, 13.81%.
Found: C, 38.28; H, 3.71; N, 13.68%. HRMS (ESI): Calc. for
([M+Na]+): m/z 577.9697; found 577.9708. 1H NMR (250 MHz,
3
(CD3)2SO):
d
7.80 (t, JH–H = 7.6 Hz, 1H), 8.05 (m, 3H), 8.14
3
3
3
(t, JH–H = 6.5 Hz, 1H), 8.28 (d, JH–H = 7.9 Hz, 1H), 8.42 (t, JH–H
7.8 Hz, 1H), 9.42 (t, JH–H = 5.3 Hz, 1H), 9.68 (s, 1H, 5-triazole-H).
=
3
C
13H16ClN4Pd ([MÀCl]+): m/z 369.0095; found 369.0100. 1H NMR
2.2.13. Synthesis of 3b
(250 MHz, CD2Cl2): d 1.74–1.88 (m, 4H, cyclohexyl), 2.00 (m, 2H,
3
3
The compound was prepared following the procedure for 1b
and was obtained as a yellow solid (81.0 mg) in 59% yield. Anal.
Calc. for C19H22Cl2N4Pt: C, 39.87; H, 3.87; N, 9.79%. Found: C,
39.10; H, 3.87; N, 9.79%. HRMS (ESI): Calc. for C19H22Cl2N4NaPt
([M+Na]+): m/z 594.0763; found 594.0761. 1H NMR (250 MHz,
cyclohexyl), 2.33 (d, JH–H = 12.6 Hz, 2H, cyclohexyl), 4.70 (t, JH–H
= 11.8 Hz, 1H, cyclohexyl CH), 7.50 (m, 1H, pyridyl), 7.75 (d,
3JH–H = 7.2 Hz, 1H, 3-pyridyl-H), 8.06 (t, JH–H = 7.8 Hz, 1H), 8.16
3
3
(s, 1H, 5-triazole-H), 9.23 (d, JH–H = 6.5 Hz, 1H, 6-pyridyl-H).
3
(CD3)2SO): d 0.87 (t, JH–H = 6.8 Hz, 3H, CH3), 1.22–1.35 (m, 6H, al-
2.2.8. Synthesis of 2a
kyl), 1.63 (t, 3JH–H = 7.3 Hz, 2H, CH2), 2.70 (t,3JH–H = 7.6 Hz, 2H, CH2),
The compound was prepared following the procedure for 1a
and was obtained as a yellow solid (69.1dmg) in 82% yield. Anal.
Calc. for C14H9Cl2F3N4Pd: C, 35.96; H, 1.94; N, 11.98%. Found: C,
35.71; H, 2.12; N, 11.79%. HRMS (ESI): Calc. for C14H9Cl3F3N4Pd
([M+Cl]À): m/z 502.8864; found 500.8861. 1H NMR (400 MHz,
3
3
7.43 (d, JH–H = 8.1 Hz, 1H, 3-pyridyl-H), 7.53 (d, JH–H = 8.6 Hz, 2H,
3
3
Ph), 7.77 (t, JH–H = 7.5 Hz, 1H, pyridyl), 7.84 (d, JH–H = 8.6 Hz, 2H,
Ph), 8.19 (d,3JH–H = 7.2 Hz, 1H, 6-pyridyl-H), 8.42 (t, JH–H = 7.8 Hz,
3
1H), 9.84 (s, 1H, 5-triazole-H).
3
(CD3)2SO): d 7.78 (t, JH–H = 6.2 Hz, 1H), 7.98–8.08 (m, 3H), 8.15
3
3
3
2.2.14. Synthesis of 4b
(d, JH–H = 7.4 Hz, 1H), 8.25(d, JH–H = 8.1 Hz, 1H), 8.36 (t, JH–H
=
8.1 Hz, 1H), 9.06 (d, 3JH–H = 5.8 Hz, 1H, 6-pyridyl-H), 9.67 (s, 1H, tri-
azole-H).
The compound was prepared following the procedure for 1b
and was obtained as a yellow solid (75.6 mg) in 59% yield. Anal.
Calc. for C14H12Cl2N4PtS: C, 31.47; H, 2.26; N, 10.49%. Found: C,
31.42; H, 2.18; N, 10.38%. HRMS (ESI): Calc. forC14H12Cl2N4NaPtS
([M+Na]+): m/z 555.9700; found 555.9671. 1H NMR (250 MHz,
2.2.9. Synthesis of 3a
The compound was prepared following the procedure for 1a
and was obtained as a yellow solid (79.3 mg) in 91% yield. Anal.
Calc. for C19H22Cl2N4Pd: C, 47.18; H, 4.58; N, 11.58%. Found: C,
47.22; H, 4.68; N, 11.40%. HRMS (ESI): Calc. for (C19H22N4)2PdCl:
m/z 753.2417; found 753.2416. 1H NMR (250 MHz, (CD3)2CO): d
3
(CD3)2SO): d 2.53 (s, 3H, SCH3), 7.49 (t, JH–H = 7.0 Hz, 1H), 7.65–
3
3
7.82 (m, 5H), 8.25 (d, JH–H = 7.2 Hz, 1H), 8.41 (t, JH–H = 7.8 Hz,
1H), 9.62 (s, 1H, 5-triazole-H).
3
2.3. Crystallographic details
0.89 (t, JH–H = 6.9 Hz, 3H, CH3), 1.35 (m, 8H, alkyl), 1.70
3
3
(t, JH–H = 7.3 Hz, 2H, CH2), 7.55 (d, JH–H = 8.7 Hz, 2H, Ph), 7.74
3
3
X-ray diffraction data were collected using an OXFORD XCALI-
BUR-S CCD single crystal X-ray diffractometer for 1a at 120(2) K
and with a Kappa CCD diffractometer for 1 and 5 at 100(2) K. The
structures were solved and refined by full-matrix least-squares
techniques on F2 using the SHELX-97 program [36]. The absorption
correction was done by the multi-scan technique.
(t, JH–H = 5.8 Hz, 1H, pyridyl), 7.88 (d, JH–H = 8.7 Hz, 2H, Ph), 8.23 (d,
3JH–H = 7.4 Hz, 1H, 3-pyridyl-H), 8.34 (t, JH–H = 7.8 Hz, 1H, pyridyl),
3
9.22 (d, 3JH–H = 5.7 Hz, 1H, 6-pyridyl-H), 9.54 (s, 1H, 5-triazole-H).
2.2.10. Synthesis of 4a
The compound was prepared following the procedure for 1a
and was obtained as a yellow solid (67.2 mg) in 91% yield. Anal.
Calc. for C14H12Cl2N4PdS: C, 37.73; H, 2.71; N, 12.57%. Found: C,
37.68; H, 2.55; N, 12.43%. HRMS (ESI): Calc. forC14H12ClN4PdS
([MÀCl]+): m/z 408.9503; found 408.9506. 1H NMR (400 MHz,
3. Results and discussion
3.1. Synthesis
3
CD2Cl2): d 2.50 (s, 3H, SCH3), 7.43 (t, JH–H = 7.9 Hz, 1H), 7.53–
3
3
The ligands 1–5 were prepared by reacting 2-ethynylpyridine
with the respective azides using the Click Protocol (Scheme 2).
The easy access to a variety of aromatic and aliphatic azides with
different functionality allowed us to prepare new 2-pyridyl substi-
7.60 (m, 3H), 7.65 (t, JH–H = 7.4 Hz, 1H), 7.82 (d, JH–H = 7.2 Hz,
3
1H), 8.10 (t, JH–H = 7.8 Hz, 1H), 8.48 (s, 1H, 5-triazole-H), 9.33 (d,
3JH–H = 5.8 Hz, 1H, 6-pyridyl-H).
2.2.11. Synthesis of 1b
Under an argon atmosphere Pt(dmso)2Cl2 (100 mg, 0.24 mmol)
and 1 (55.0 mg, 0.24 mmol) were dissolved in CH3NO2 (10 mL) and
refluxed for 15 h. After that the solution was concentrated and a
yellow solid was filtered and washed with ether and acetone.
The desired compound was obtained in 51% yield (60.5 mg). Anal.
Calc. for C13H16Cl2N4Pt: C, 31.59; H, 3.26; N, 11.34%. Found: C,
31.52; H, 3.27; N, 11.26%. HRMS (ESI): Calc. for C13H16Cl2N4NaPt
([M+Na]+): m/z 516.0293; found 516.0309. 1H NMR (250 MHz,
1 eq azide
5 mol% CuSO4
20 mol% sodium ascorbate
N
N
N
N
N
1 mol% TBTA
CH2Cl2/ t-BuOH/H2O
2d, 50 °C
R
1
: R = cyc-C6H11
2: R = 2-CF3-C6H4
3: R = 4-n-C6H13-C6H4
4: R = 2-SCH3-C6H4
3
(CD3)2CO): d 1.49 (q, JH–H = 12.5 Hz, 2H, cyclohexyl), 1.65–1–93
5
: R = 4-N(CH3)2-C6H4
3
(m, 5H, cyclohexyl), 2.22 (m, 3H, cyclohexyl), 4.72 (t, JH–H
=
3
11.1 Hz, 1H, cyclohexyl CH), 7.72 (t, JH–H = 6.0 Hz, 1H, pyridyl),
Scheme 2. Synthesis of the ligands 1–5.

256
D. Schweinfurth et al. / Inorganica Chimica Acta 374 (2011) 253–260
tuted 1,2,3-triazole ligands. On the one hand we envisioned to
influence the electronic structure of the ligands by introducing
the electron withdrawing CF3 group in 2, and aliphatic electron
donating groups in 1 and 3, respectively, on the other hand this
strategy allowed us to incorporate further donor atoms as in 4
and 5. All ligands were characterized by elemental analysis, 1H
and 13C NMR spectroscopy and mass spectrometry. Slow evapora-
tion of a dichloromethane solution of 1 yielded colorless crystals
which were suitable for X-ray diffraction. Furthermore 5 could be
crystallized from a methanol solution and its molecular structure
could be determined (vide infra).
The palladium complexes 1a–4a could be obtained in high
yields by reacting [Pd(COD)Cl2] with the corresponding ligands in
dichloromethane (Scheme 3). All compounds were characterized
by elemental analysis, 1H NMR spectroscopy and mass spectrome-
try. Slow diffusion of ether into an acetonitrile solution of 1a
yielded yellow single crystals which allowed for the determination
of the molecular structure by X-ray diffraction (vide infra).
The reaction of the palladium precursor with ligand 5, which
bears an additional NMe2 functionality, led to an inseparable mix-
ture of different products. Most likely the coordination of the
amine group to the metal center is in competition to the well
known chelating coordination mode of the 2-pyridyl substituted
triazole ligands. This is in contrast to 4a where the additional sul-
fur atom does not seem to coordinate to the metal center as evi-
denced by the almost unchanged chemical shift of the SMe
group in the 1H NMR spectra (see Section 2). The reason for this
behavior could be the fact that the central nitrogen atom in the
1,2,3-triazole ring bears less electron density than the external
one [37], which hampers the formation of a six-membered chelat-
ing ring together with the SMe moiety. Besides this fact the forma-
tion of five-membered rings over six-membered rings is favoured
in generally, which hinders the coordination of the SMe group as
well.
crystallize either 4a or 4b to illuminate this problem further be-
cause of the very low solubility of the complexes.
3.2. X-ray crystal structures
The ligand 1 crystallizes in the space group Pna21 (Table 1). As
observed in similar compounds the nitrogen atom of the pyridine
ring is in an ‘anti’ conformation with respect to the triazole ring
(Fig. 1). The twisting between the two heterocycles amounts to
23.3(1)°. Furthermore the N2–N3 distance in the triazole ring is
1.328(4) Å and therefore this is the shortest bond within the het-
erocycle (Scheme 4). The neighboring bond distances N2–C6 and
N3–N4 amount to 1.351(4) Å and 1.342(4) Å, respectively. Hence
the difference between the two shortest bonds is only 0.014(4) Å.
This value is much smaller compared to the phenyl substituted
derivative studied by us previously [18] where the bond distance
N2–N3 is 1.309(1) Å and the neighboring distances N2–C6 and
N3–N4 are 1.366(1) Å and 1.356(1) Å, respectively. In the case of
1 the azo character of the N2–N3 is therefore less pronounced
and the structure is better described with a higher degree of
conjugation.
Even though the 1,2,3-triazole moiety, with its large dipole mo-
ment of 5 Debye [24], is getting recognized as an interesting build-
ing block in supramolecular chemistry and anion recognition
because of its ability to form weak interaction, the widely used
2-pyridyl substituted 1,2,3-triazole ligands were not subjected to
a detailed study of the three dimensional interaction in the molec-
ular structure. In the molecular structure of 1 the molecules align
in infinite chains which are governed by hydrogen bonds between
the triazole units of neighboring molecules. The distance of the
Table 1
Crystallographic details.
1a
1
5
The synthesis of the platinum complexes 1b–4b was achieved
by reacting [Pt(dmso)2Cl2] with the corresponding ligands in
refluxing CH3NO2 for 15 h, as shown in Scheme 3. All compounds
were characterized by elemental analysis, 1H NMR spectroscopy
and mass spectrometry. As in the case of the reaction with the pal-
ladium precursor the reaction of ligand 5 and the platinum source
did not yield a uniform product. Just as in the case of 4a the 1H
NMR spectrum of 4b does not indicate the coordination of the
SMe function (see Section 2). Unfortunately it was not possible to
Formula
Mr
Crystal system
Space group
a (Å)
C
15H21Cl2N4OPd C13H16N4
C15H15N5
265.32
450.66
Monoclinic
P21/c
8.9014(2)
16.3293(3)
11.9869(3)
90
228.30
Orthorhombic Monoclinic
Pna21
22.913(2)
9.7198(6)
5.4703(4)
90
P21
5.9182(5)
25.889(3)
17.468(2)
90
b (Å)
c (Å)
a
(°)
b (°)
95.303(2)
90
1734.88(7)
4
90
90
1218.3(1)
4
1.245
100(2)
0.71073
488
98.434(8)
90
2647.4(5)
8
c
(°)
V (Å3)
Z
Dcalc (g cmÀ3
T (K)
)
1.725
1.331
Cl
Cl
N
120(2)
0.71073
908
100(2)
0.71073
1120
l
Mo K
a )
(mmÀ1
Pd
F(0 0 0)
N
1 eq Pd(COD)Cl2
N
Meas./Independent reflection 12275/3046
4193/2142
1581
8559/5081
2735
N
N
N
N
Observed [I > 2
r(I)] reflection 2687
CH2Cl2
1 h, RT
N
Rint
0.0262
0.0244
0.0700
1.053
0.0850
0.0604
0.1446
1.076
0.0608
0.0722
0.1950
1.034
R
R
R[F2 > 2
wR (F2)
S
r
(F2)]
1: R = cyc-C6H11
2: R = 2-CF3-C6H4
1a: R = cyc-C6H11
2a: R = 2-CF3-C6H4
3a: R = 4-n-C6H13-C6H4
4a: R = 2-SCH3-C6H4
3: R = 4-n-C6H13-C6H4
4: R = 2-SCH3-C6H4
D
q
/
max Dqmin (e ÅÀ3
)
0.578/À0.725
0.206/À0.245 0.300/À0.321
Cl
Cl
N
Pt
1 eq Pt(dmso)2Cl2
N
N
N
CH3NO2
R
15 h, reflux
1b: R = cyc-C6H11
2b: R = 2-CF3-C6H4
3b: R = 4-n-C6H13-C6H4
4b: R = 2-SCH3-C6H4
Fig. 1. ORTEP of the molecular structure of 1. Ellipsoids include 50% of electron
Scheme 3. Synthesis of the complexes 1a,b–4a,b.
density.

D. Schweinfurth et al. / Inorganica Chimica Acta 374 (2011) 253–260
257
1.328(4)
1.324(9)
1.312(3)
N
N
N(2) N(3)
N(2) N(3)
N(2) N(3)
1.351(4)
C(6)
1.342(4)
1.371(9)
C(6)
1.369(8) 1.355(3)
N(4) C(6)
1.339(3)
N(4)
N(4)
1.356(4)
azo motif
C
C
C
1.373(5)
1.35(1)
1.367(4)
1.36(1)
1.343(4)
1a
1
5
Scheme 4. Selected bond lengths (Å) in structures of 1 and 1a.
hydrogen bond H7Á Á ÁN2 is 2.363(3) Å and the angle C7–H7Á Á ÁN2
amounts to 165.3(2)° which is almost in the range of moderate
hydrogen bonds [37,38]. This is one more proof that the nitrogen
atom N2 bears more electron density than the nitrogen atom N3
and therefore preferentially interacts with Lewis acids [37]. The tri-
azole rings of neighboring molecules within the chain as well as
the adjacent pyridine rings are perfectly coplanar (see Fig. 2).
Ligand 5 crystallizes in the P21 space group (Table 1). The nitro-
gen atom of the pyridine ring is in anti conformation to the triazole
ring (Fig. 3). The pyridine ring is twisted with respect to the tria-
zole ring; the twisting angle amounts to 8.5(3)°. The twisting angle
between the triazole plane and the substituted phenyl ring is
19.8(3)°. The bond distance N2–N3 is 1.324(9) Å while the dis-
tances N2–C6 and N3–N4 are 1.371(9) Å and 1.369(8) Å, respec-
tively (Scheme 4). Even though the quality of the structure does
not allow for a detailed discussion of the bond length parameter,
the azo character in 5 seems to be more pronounced than in 1.
In the 3D structure of 5 similar chains are present as in 1 which
are held together by weak hydrogen bonds between the triazole
units. Interestingly the distance H7Á Á ÁN2 is 2.854(6) Å and the an-
gle C7–H7Á Á ÁN2 amounts to 166.0(5)°. Thus the hydrogen bond dis-
tance in 5 is much longer than in 1 which could be explained by
taking the steric influence of the NMe2 group into account which
prevents closer contact (see Fig. 4).
Fig. 3. ORTEP of the molecular structure of 5. One out of four crystallographically
different molecules is exemplified. Ellipsoids include 50% of electron density.
The complex 1a crystallizes in the P21/c space group (Table 1).
After complexation the twisting angle between the pyridine plane
and the triazole ring is only 2.7(1)°. The palladium center is in a
slightly distorted square planar environment. The chelating bite
angle N2–Pd1–N1 (79.86(9)°) is slightly smaller than the other an-
gles around the metal center (Table 2). The bond distance Pd1–N2
(2.002(2) Å) is slightly shorter than the distance Pd1–N1
(2.054(2) Å) which is evidence for the better electron donating
properties of the triazole ring compared to the pyridine ring, as ob-
served previously [18]. The comparison of the bond lengths of the
Fig. 4. ORTEP of the molecular structure of 1a. Solvent molecules are omitted for
clarity. Ellipsoids include 50% of electron density.
Table 2
Selected bond lengths and bond angles in 1a.
Bond lengths (Å)
Bond angles (°)
Pd1–N1
Pd1–N2
Pd1–Cl1
Pd1–Cl2
H4Á Á ÁCl2
H7Á Á ÁCl2
H8Á Á ÁCl1
2.054(2)
2.002(2)
N2–Pd1–N1
N2–Pd1–Cl1
N1–Pd1–Cl2
Cl1–Pd1–Cl2
C4–H4Á Á ÁCl2
C7–H7Á Á ÁCl2
C8–H8Á Á ÁCl1
79.86(9)
94.70(7)
94.22(7)
91.20(3)
168.7(2)
157.9(2)
163.0(2)
2.2844(7)
2.2846(7)
2.7422(6)
2.7124(7)
2.7114(6)
free ligand 1 and the complex 1a (see Scheme 4) shows that the
only distinct difference is the slightly shorter bond N2–N3
(1.312(3) Å) after complexation. This is in contrast to our previous
observation where the complexation of a similar ligand to a [PtCl2]
fragment led to an elongation of the N2–N3 bond [18]. The differ-
ent substitution on the triazole ring seems to influence the elec-
tronic structure of the triazole ring in a subtle way and allows
for the fine tuning of the donor and acceptor properties of the li-
gand (see Fig. 5).
After revealing the three dimensional structure of the ligands 1
and 5 we investigated the packing of complex 1a. Similar to the
structure of the ligands, complex 1a forms infinite chains in the so-
lid state, which are governed by hydrogen bonds. In the case of 1a
both M–Cl bonds take the role of the hydrogen bond acceptors to
Fig. 2. Hydrogen bonding in the structure of 1.

258
D. Schweinfurth et al. / Inorganica Chimica Acta 374 (2011) 253–260
Fig. 5. Hydrogen bonding in the structure of 1a.
form hydrogen bonds to the ligand backbone of the next molecule.
All three C–HÁ Á ÁCl–Pd hydrogen bonds have similar lengths and
similar directionality (Table 2) which are comparable to rather
strong N–HÁ Á ÁCl–M contacts [39]. The distances are well below
the van der Waals radii of H and Cl (3 Å) [40]. This is particular
interesting because all hydrogen bonds involve three different
types of hydrogen bond C–H donors. The biggest contrast is be-
tween the C–H bond pointing out of the triazole ring with its big
dipole moment and the sp3 carbon center linking the triazole het-
erocycle and the cyclohexyl ring engaged in the hydrogen bond
pattern. The similar strengths of the different hydrogen bonds
can only be explained by a high degree of cooperativity in the bond
forming process and the almost linear alignment of the three H
atoms involved.
3.3. Cyclic voltammetry
To probe the influence of the ligand substitution on the elec-
tronic properties of the molecules presented, all ligands and com-
plexes were investigated by cyclic voltammetry. Because of the
extremely negative reduction potential of the ligands 1–4 all
experiments were carried out in THF/0.1 M NBu4PF6 solutions. All
ligands show an irreversible reduction process around À3.0 V ver-
sus the ferrocene/ferrocinium reference couple (see Table 3). There
is no obvious trend linking the electronic properties of the func-
tional groups attached to the triazole ring and the reduction poten-
tials observed.
The palladium complexes 1a–4a show irreversible reduction
processes in CH2Cl2/0.1 M NBu4PF6 versus ferrocene/ferrocinium
(Fig. 6). The reversibility of the processes could not be altered by
lowering the temperature or using different scan rates. The known
liability of metal–chloride bonds towards reduction processes is
made responsible for the irreversible process observed [32]. After
Fig. 6. Cyclic voltammogram of 1a in CH2Cl2/0.1 M Bu4NPF6 vs Fc0/+ at 295 K.
complexation to the metal center the reduction potentials shift
around 2 V to less negative potential (Table 3). This can be attrib-
uted to the excellent
r-acceptor capacity of the [PdCl2] fragment.
Just like the palladium complexes the platinum complexes 1b–4b
show irreversible reduction processes in CH2Cl2/0.1 M NBu4PF6
versus ferrocene/ferrocinium (Fig. 7) and the reversibility of the
process could not be improved upon lowering the temperature or
changing the scan rates. After complexation to the platinum center
Table 3
Electrochemical and UV–Vis spectroscopic data.
Epc (V)
kmax (nm)d
e )
(103 MÀ1 cmÀ1
1
2
3
4
1a
2a
3a
4a
1b
2b
3b
4b
À3.26a
À3.08a
À2.92a
À3.02a
282
281
287
282
283, 377
271, 362
274, 375
293, 378
266, 336
263, 337
266, 336(sh)
256, 332
8.76
13.0
16.2
10.7
c
À1.35b
c
c
c
c
c
c
c
c
À1.25b
À1.21b
À2.16b
À2.01b
À2.00b
À2.10b
a
b
c
In THF/0.1 M Bu4NPF6 vs Fc0/+ at 295 K and a scan rate of 100 mV/s.
In CH2Cl2/0.1 M Bu4NPF6 vs Fc0/+ at 295 K a scan rate of 100 mV/s.
Not possible due to low solubility.
d
Fig. 7. Cyclic voltammogram of 1b in CH2Cl2/0.1 M Bu4NPF6 vs Fc0/+ at 295 K.
In CH2Cl2 at 295 K.

D. Schweinfurth et al. / Inorganica Chimica Acta 374 (2011) 253–260
259
dedicated to make use of the weak interactions and to tune the
reactivity of the ligands comprising further donor atoms.
Acknowledgements
BS is indebted to the Baden-Württemberg Stiftung (Elitepro-
gram for Postdocs), DFG (SFB 706) and the Fonds der Chemischen
Industrie (FCI) for financial support of this work. We thank D.
Bubrin and Dr. F. Lissner for the help with crystallographic
measurements.
Appendix A. Supplementary data
CCDC 808009–808011 contain the supplementary crystallo-
graphic data for this paper that can be obtained free of charge from
the Cambridge Crystallographic Data Center via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.ica.2011.02.085.
Fig. 8. UV–Vis spectra of 1b–4b in CH2Cl2.
an anodic shift of the reduction potentials of about 1 V is observed
with respect to the free ligand (Table 3). However the anodic shift
is much smaller compared to the palladium complexes, as ob-
served by us previously [18]. This can be explained by the much
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