Click-Triazole N2 Coordination to Transition-Metal Ions Is Assisted by a Pendant Pyridine Substituent

Article abstract of DOI:10.1021/ic902354e

We report that 1-(2-picolyl)-1,2,3-triazole (click triazole) forms stable complexes with transition-metal ions in which the coordination involves the triazole N2 nitrogen atom and the pendant 2-picolyl group. This is exemplified by model compound 1-(2-picolyl)-4-phenyl-1H-1,2,3-triazole (L_x) and its complexes with transition-metal ions of Pt^II, Pd^II, Cu^II, Ru^II, and Ag^I. The coordination was investigated experimentally and theoretically. Ligand L_x easily reacted at room temperature with cis-[PtCl₂(DMSO)₂], [Pd(CH₃CN)₄](BF₄)₂, CuCl ₂, [RuCl(μ-Cl)(?⁶-p-cymene)]₂, and AgNO₃ to give stable chelates [PtCl₂L_x] (1), [Pd(L_x)₂](BF₄)₂ (2), [CuCl ₂(L_x)₂] (3), [RuCl(?⁶-p-cymene) L_x]OTf (4), and [Ag₂(L_x)₂(NO ₃)₂] (5), respectively, in 60-98% yield. The structures of 1-5 were unambiguously confirmed by NMR spectroscopy and single-crystal X-ray diffraction analysis. Density functional theory calculations were carried out in order to theoretically investigate the stabilization factors in 1-5. A comparison of the chelating properties of ligand L_x was made with structurally similar and isomeric 1-(2-aminoethyl)-substituted 1,2,3-triazole (L_y) and 4-(2-aminoethyl)-substituted 1,2,3-triazole (L_z). The complexation affinity of L_x was attributed to φ-back-donation from the metal to the pendant pyridine side arm, whereas the stability of the complexes involving L_y and L_z mainly originates from efficient φ-back-donation to the triazole ring.

Full text of DOI:10.1021/ic902354e

4820 Inorg. Chem. 2010, 49, 4820–4829
DOI: 10.1021/ic902354e
Click-Triazole N2 Coordination to Transition-Metal Ions Is Assisted by a Pendant
Pyridine Substituent
†
‡
†
,‡
†
,†
ꢀ
Damijana Urankar, Balazs Pinter, Andrej Pevec, Frank De Proft,* Iztok Turel, and Janez Kosmrlj*
†
ꢀ
ꢀ
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, Ljubljana, Slovenia, and
‡Faculteit Wetenschappen, Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2,
1050 Brussels, Belgium
Received November 27, 2009
We report that 1-(2-picolyl)-1,2,3-triazole (click triazole) forms stable complexes with transition-metal ions in which the
coordination involves the triazole N2 nitrogen atom and the pendant 2-picolyl group. This is exemplified by model
compound 1-(2-picolyl)-4-phenyl-1H-1,2,3-triazole (Lx) and its complexes with transition-metal ions of PtII, PdII, CuII,
RuII, and AgI. The coordination was investigated experimentally and theoretically. Ligand Lx easily reacted at room
temperature with cis-[PtCl2(DMSO)2], [Pd(CH3CN)4](BF4)2, CuCl2, [RuCl(μ-Cl)(η6-p-cymene)]2, and AgNO3 to give
stable chelates [PtCl2Lx] (1), [Pd(Lx)2](BF4)2 (2), [CuCl2(Lx)2] (3), [RuCl(η6-p-cymene)Lx]OTf (4), and
[Ag2(Lx)2(NO3)2] (5), respectively, in 60-98% yield. The structures of 1-5 were unambiguously confirmed by
NMR spectroscopy and single-crystal X-ray diffraction analysis. Density functional theory calculations were carried out
in order to theoretically investigate the stabilization factors in 1-5. A comparison of the chelating properties of ligand Lx
was made with structurally similar and isomeric 1-(2-aminoethyl)-substituted 1,2,3-triazole (Ly) and 4-(2-aminoethyl)-
substituted 1,2,3-triazole (Lz). The complexation affinity of Lx was attributed to π-back-donation from the metal to the
pendant pyridine side arm, whereas the stability of the complexes involving Ly and Lz mainly originates from efficient
π-back-donation to the triazole ring.
Introduction
group, coordination of the triazole can involve either the N3
or N2 nitrogen atom (Figure 1). These two isomeric click-
triazole chelators will hereafter be referred to as chelators A
and B, respectively. Having pendant primary amine groups,
both isomeric click-triazole chelators have initially been
examined as ligands for Re, 99mTc,4 and PtII.5 While chelators
A formed stable complexes, this was not the case for isomers
B.4,5 Addressed by density functional theory (DFT) calcula-
tions, this difference has been accounted for by different
electron densities at the triazole nitrogen atoms N3 and N2,
with the latter being lower.4-6 Recent studies in which the
coordinative properties of different click triazoles were ex-
amined as mono- and polydentate ligands are consistent with
Commenced from its recent discovery, the copper-catalyzed
azide-alkyne cycloaddition (CuAAC)1 has emerged as a
powerful and versatile tool in a wide variety of research areas,
ranging from materials, to pharmaceuticals, to biological
sciences.2 It connects organic azide and terminal alkyne into
1,4-disubstituted 1,2,3-triazole, referred to as “click triazole”.3
Whereas click triazole has been frequently designed as an
interconnector between two functional entities, its potential to
offer a platform for further functionalization, e.g., via co-
ordination to metals, still remains largely underexplored. In
particular, there are only a handful of examples in which this
triazole coordinates to a metal through the N2 nitrogen atom,
being either a monodentate ligand or part of a polydentate
chelator.
(4) Mindt, T. L.; Struthers, H.; Brans, L.; Anguelov, T.; Schweinsberg, C.;
ꢁ
Maes, V.; Tourwe, D.; Schibli, R. J. Am. Chem. Soc. 2006, 128, 15096–15097.
Considering the click triazole containing polydentate che-
lator, based on the position of the pendant coordinating
ꢁ
(5) Maisonial, A.; Serafin, P.; Traıkia, M.; Debiton, E.; Thery, V.; Aitken,
D. J.; Lemoine, P.; Viossat, B.; Gautier, A. Eur. J. Inorg. Chem. 2008, 298–
305.
ꢂ
(6) Bastero, A.; Font, D.; Pericas, M. A. J. Org. Chem. 2007, 72, 2460–
*To whom correspondences should be addressed. E-mail: fdeprof@
vub.ac.be (F.D.P.), janez.kosmrlj@fkkt.uni-lj.si (J.K.).
2468.
(7) For selected click triazole-based chemosensors, see: (a) Steinmetz, V.;
Couty, F.; David, O. R. P. Chem. Commun. 2009, 343–345. (b) Hung, H.-C.;
Cheng, C.-W.; Ho, I.-T.; Chung, W.-S. Tetrahedron Lett. 2009, 50, 302–305. (c)
Park, S. Y.; Yoon, J. H.; Hong, C. S.; Souane, R.; Kim, J. S.; Matthews, S. E.;
Vicens, J. J. Org. Chem. 2008, 73, 8212–8218. (d) Varazo, K.; Xie, F.; Gulledge,
D.; Wang, Q. Tetrahedron Lett. 2008, 49, 5293–5296. (e) Colasson, B.; Save,
ꢁ
(1) (a) L’abbe, G. Bull. Soc. Chim. Belg. 1984, 93, 579–592. (b) Tornøe,
C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064. (c)
Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2002, 41, 2596–2599.
(2) For selected comprehensive review articles, see: Meldal, M.; Tornøe,
C. W. Chem. Rev. 2008, 108, 2952–3015 and the Supporting Information.
(3) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed.
2001, 40, 2004–2021.
ꢁ
€
M.; Milko, P.; Roithova, J.; Schroder, D.; Reinaud, O. Org. Lett. 2007, 9, 4987–
4990.
r
pubs.acs.org/IC
Published on Web 05/04/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4821
Chart 1. Structures of Investigated Pyridine (Lx)a,b and Primary Amine
(Ly)a,b Chelators B and Primary Amine (Lz)b Chelator A, with a Selected
Atom Numbering Scheme
Figure 1. Schematic representation of the two isomeric click-triazole
chelators A and B. Providing a short linker, chelation to metal M
(depicted by dashed arrows) involves coordination of a pendant group
and the triazole N3 and N2 atoms, respectively.
a Experimentally investigated. b Computationally investigated.
these observations.7-10 Although, in some instances, N2
coordination has been suggested through different spectro-
scopic methods, only a few examples exist in copper coordi-
nation spheres in which the click-triazole N2 coordination is
confirmed by X-ray.11,12
δ 0 ppm. Chemical shifts are given on the δ scale (ppm).
Coupling constants (J) are given in hertz. Assignments of proton
and carbon resonances were performed by standard 2D NMR
techniques. The numbering used for the assignment of NMR
signals is as follows: pyridine ring, simple figures; 1,2,3-triazole
ring, primed figures; phenyl ring, double-primed figures; aro-
matic ring of p-cymene (in 4), triple-primed figures. Mass
spectra and high-resolution mass spectra were obtained with a
VG-Analytical AutospecQ instrument and Q-TOF Premier
instrument. Data are reported as m/z (relative intensity). IR
spectra were recorded on a Bio-rad Excalibur Series spectro-
photometer. Elemental analyses (C, H, and N) were performed
with a Perkin-Elmer 2400 series II CHNS/O analyzer. Melting
points were determined on a Kofler block.
Herein we report that replacing the pendant primary amine
group at the triazole N1 atom with an appropriate pyridine
group dramatically improves the chelating properties of
ligands B. This is exemplified by model compound Lx [1-(2-
picolyl)-4-phenyl-1H-1,2,3-triazole; Chart 1] and its com-
plexes with PtII, PdII, CuII, RuII, and AgI. These metal ions
were selected to probe potentially different coordination
numbers and geometries, as well as on the basis of their
chemical and biochemical relevance. To shed light on the
nature of coordinating properties of pyridine- and alkylamine-
functionalized chelators B and to make a comparison with
isomers A, compounds Lx, Ly [1-(2-aminoethyl)-4-phenyl-1H-
1,2,3-triazole], and Lz [4-(2-aminoethyl)-1-phenyl-1H-1,2,3-
triazole; Chart 1] and their complexes with the above metal
ions were investigated by density functional theory (DFT).
X-ray Structural Analysis. Crystal data and refinement para-
meters of compound 1-5 are listed in Table 1 and those for Lx in
Table S1 of the Supporting Information. Single-crystal X-ray
diffraction data of Lx and complexes 1-5 were collected with a
Nonius Kappa CCD diffractometer with graphite-monochro-
mated Mo KR radiation (λ = 0.710 73). A Cryostream Cooler
(Oxford Cryosystems) was used for cooling the samples of 2 and
3. The data were processed by using DENZO.13 The structures
were solved by direct methods (SIR-92)14 and refined by a full-
matrix least-squares procedure based on F2 (SHELXL-97).15
All non-hydrogen atoms were refined anisotropically, while the
hydrogen atoms were included in the model at geometrically
calculated positions and refined by using a riding model. The
water hydrogen atoms in 3 were not located. Compounds Lx and
3 crystallize in the noncentrosymmetric space groups. The value
of the Flack parameter 0.473(13) of 3 indicates possible racemic
twinning.
Experimental Section
General Procedures. NMR spectra were recorded at 302 K on
a Bruker Avance DPX 300 spectrometer operating at 300, 282,
75, and 64 MHz for 1H, 19F, 13C, and 195Pt NMR, respectively.
1H NMR spectra were referenced to tetramethylsilane (TMS) as
the internal standard. Carbon chemical shifts were determined
relative to the residual signal of dimethylformamide (DMF)-d7
at δ 30.1 ppm. 195Pt and 19F NMR spectra were referenced to
Na2[PtCl6] and CCl3F as external standards, respectively, at
Computational Details. Geometry optimizations and fre-
quency analysis were performed using the B3LYP method,16
using the Gaussian 0317 software package. For light atoms (H,
N, and C), the aug-cc-pVDZ18 basis set was used. For the
transition metals M = Cu, Pd, and Ag, the aug-cc-pVDZ-
pp19 basis with a relativistic effective core potential was used;
in the case of platinum, the double-ζ quality Def2-SVP basis20
involving quasi-relativistic 60-electron core potential was used.
All structures were verified to be minima on the potential energy
(8) For pyridine-conjugated click triazoles as chelators, see: (a) Nadler,
A.; Hain, C.; Diederichsen, U. Eur. J. Org. Chem. 2009, 4593–4599. (b)
Schweinfurth, D.; Pattacini, R.; Strobel, S.; Sarkar, B. Dalton Trans. 2009, 9291–
9297. (c) Obata, M.; Kitamura, A.; Mori, A.; Kameyama, C.; Czaplewska, J. A.;
Tanaka, R.; Kinoshita, I.; Kusumoto, T.; Hashimoto, H.; Harada, M.; Mikata, Y.;
Funabiki, T.; Yano, S. Dalton Trans. 2008, 3292–3300. (d) Schweinfurth, D.;
Hardcastle, K. I.; Bunz, U. H. F. Chem. Commun. 2008, 2203–2205. (e) Camp,
C.; Dorbes, S.; Picard, C.; Benoist, E. Tetrahedron Lett. 2008, 49, 1979–1983. (f)
Ziegler, T.; Hermann, C. Tetrahedron Lett. 2008, 49, 2166–2169. (g) Richardson,
C.; Fitchett, C. M.; Keene, F. R.; Steel, P. J. Dalton Trans. 2008, 2534–2537. (h)
Meudtner, R. M.; Ostermeier, M.; Goddard, R.; Limberg, C.; Hecht, S. Chem.;
Eur. J. 2007, 13, 9834–9840. (i) David, O.; Maisonneuve, S.; Xie, J. Tetrahedron
Lett. 2007, 48, 6527–6530. (j) Huang, S.; Clark, R. J.; Zhu, L. Org. Lett. 2007, 9,
4999–5002. See also ref 12.
(13) Otwinowski, Z.; Minor, W. Methods Enzymol. 1970, 276, 307–326.
(14) SIR92;A Program for Crystal Structure Solution: Altomare, A.;
Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26,
343–350.
(15) SHELXL-97;A program for Crystal Structure Refinement: Sheldrick,
G. M. Acta Crystallogr. 2008, A64, 112–122.
(16) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(17) Frisch, M. J.; et al. Gaussian 03, revision B.03; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(18) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358–
1371.
(19) (a) Peterson, K. A.; Puzzarini, C. Theor. Chem. Acc. 2005, 114, 283–
296. (b) Figgen, D.; Rauhut, G.; Dolg, M.; Stoll, H. Chem. Phys. 2005, 311, 227–
244.
(20) (a) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–
3305. (b) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123–141.
(9) For a survey of the Cambridge Structural Database, revealing very few
reports of 1,2,3-triazole N2 coordination compounds, see the Supporting
Information. See also: Moore, D. S.; Robinson, S. D. Adv. Inorg. Chem.
1988, 32, 171–239.
(10) For a recent comprehensive review article, see: Struthers, H.; Mindt,
T. L.; Schibli, R. Dalton. Trans. 2010, 39, 675–696.
(11) The first X-ray evidence for click triazole N2 coordination to the
metal has been provided for a dinuclear CuI complex with TBTA {[CuI2(μ-
4
2
3
3
3
TBTA-κ -N ,N ,N ,N 00)2]2þ; TBTA = tris(benzyltriazolylmethyl)amine}.
See: Donnelly, P. S.; Zanatta, S. D.; Zammit, S. C.; White, J. M.; Williams, S.
J. Chem. Commun. 2008, 2459–2461.
0
(12) Brotherton, W. S.; Michaels, H. A.; Simmons, J. T.; Clark, R. J.;
Dalal, N. S.; Zhu, L. Org. Lett. 2009, 11, 4954–4957.

4822 Inorganic Chemistry, Vol. 49, No. 11, 2010
Urankar et al.
Table 1. Experimental Data from the X-ray Diffraction Studies of Compounds 1-5
1
2
3
4
5
formula
fw (g mol-1
C14H12Cl2N4Pt
502.27
C32H30B2F8N10Pd
834.68
C28H28Cl2CuN8O2
643.02
C25H26ClF3N4O3RuS
656.08
C28H24Ag2N10O6
812.31
)
cryst size (mm)
cryst color
cryst syst
space group
a (A)
b (A)
c (A)
R (deg)
β (deg)
0.58 ꢀ 0.15 ꢀ 0.08
colorless
monoclinic
P21/a
9.3229(2)
9.2897(3)
17.4537(5)
90
92.992(2)
90
1509.55(7)
4
293(2)
2.210
944
15147
3268
0.039
2905
190
0.0285
0.15 ꢀ 0.15 ꢀ 0.05
0.20 ꢀ 0.10 ꢀ 0.05
0.20 ꢀ 0.05 ꢀ 0.05
0.60 ꢀ 0.10 ꢀ 0.10
colorless
monoclinic
P21/c
11.2169(2)
9.4764(2)
14.7408(3)
90
103.0982(9)
90
1526.12(5)
2
293(2)
1.768
808
22089
3472
0.033
2921
208
0.0321
yellow
triclinic
P1
green
orthorhombic
P21ca
yellow
monoclinic
P21/a
9.5792(3)
9.6228(2)
11.0935(3)
67.954(2)
67.976(1)
73.081(2)
865.11(4)
1
150(2)
1.602
420
10333
3908
0.023
3827
242
0.0309
0.1058
1.257
þ0.92/-1.05
8.4365(1)
12.5126(2)
26.1418(3)
90
90
90
2759.59(6)
4
150(2)
1.548
1324
24792
6095
0.052
5235
370
0.0427
0.1041
1.012
11.7266(3)
18.6948(5)
12.1187(2)
90
93.603(2)
90
2651.49(11)
4
293(2)
1.644
1328
39185
6067
0.052
4373
346
0.0562
0.1676
1.033
γ (deg)
V (A3)
Z
T (K)
calcd density (g cm-3
F(000)
)
no. of collected reflns
no. of indep reflns
Rint
no. of reflns used
no. of param
R1 [I > 2σ(I)]a
wR2 (all data)b
GOF, Sc
0.0800
1.039
þ1.64/-1.47
0.0811
1.054
þ0.41/-0.96
max/min residual
electron density (e A-3
þ0.34/-0.44
þ1.23/-1.33
)
P
P
P
P
P
a R1 = ||Fo| - |Fc||/ Fo|. b wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2
the total number of parameters refined.
.
c S = { [(Fo2 - Fc2)2]/(n/p}1/2 where n is the number of reflections and p is
surface, and the stability of the wave function was verified in all
cases. For details on the calculations of complexation energies,
inclusion of the effect of the solvent, and energy decomposition
analysis, we refer to the Supporting Information.
1-(2-Picolyl)-4-phenyl-1H-1,2,3-triazole (Lx). A mixture of
phenylacetylene (1.02 g, 10.0 mmol), 2-picolylazide (1.36 g,
10.1 mmol), CuSO4 5H2O (216 mg, 0.865 mmol), and granular
3
copper (2.00 g, 31.5 mmol) in MeOH/water (20 mL/10 mL) was
stirred for 1.5 h. The reaction mixture was filtered and diluted
with saturated aqueous NH4Cl (50 mL), and the product was
extracted with CH2Cl2 (3 ꢀ 20 mL). The combined organic
layers were washed with saturated aqueous NH4Cl (3 ꢀ 20 mL)
and brine (10 mL), dried over Na2SO4, and filtered. The solvent
of the filtrate was removed in vacuo to give pure Lx (2.19 g,
93%). Spectral and analytical data are given in the Supporting
Information and are in agreement with those reported very
recently.12
Natural population analysis21 was carried out on all com-
plexes with the NBO program implemented in Gaussian 03.
NBO charges, Wiberg bond indices,22 and nucleus independent
chemical shift (NICS)23 values were determined at the B3LYP/
aug-cc-pVDZ24 level of theory. The latter were used to probe the
aromaticity of the rings in the coordinating molecules and were
evaluated in the center of the different unsaturated rings (1,2,3-
triazole, pyridine, and phenyl ring) using the GIAO method.25
Synthesis. Reagents and solvents were used as purchased
(Fluka, Aldrich, Alfa Aesar). 2-Picolylazide,26 1-azido-2-amino-
ethane,27 1-(2-aminoethyl)-4-phenyl-1H-1,2,3-triazole (Ly),28
[PtCl2COD]29 (COD = 1,5-cyclooctadiene), and cis-[PtCl2-
(DMSO)2]30 were prepared by literature procedures. Azides can
be explosive, and caution should be exercised when handling
them.31
[PtCl2Lx] (1). A mixture of Lx (118 mg, 0.500 mmol) and
cis-[PtCl2(DMSO)2] (224 mg, 0.500 mmol) in CH2Cl2 (8 mL)
was stirred in the dark for 13 days. The product was collected
by filtration and washed with CH2Cl2 (3 mL) to give pure 1
(198 mg, 0.395 mmol, 80%): pale-yellow solid, mp >300 °C.
Anal. Calcd for C14H12Cl2N4Pt: C, 33.48; H, 2.41; N, 11.16.
Found: C, 33.09; H, 2.48; N, 10.77. IR (KBr): ν 3105, 3086, 3031,
1612, 1477, 1432, 975, 768, 693 cm-1 1H NMR (300 MHz,
.
(21) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (Theochem) 1988,
169, 41–62. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899–926.
(22) Wiberg, K. Tetrahedron 1968, 24, 1083–1096.
(23) Schleyer, P. R.; Maerker, C.; Dransfeld, A.; Jiao, H. J.; Hommes, N.
J. Am. Chem. Soc. 1996, 118, 6317–6318.
(24) For transition metals, aug-cc-pVDZ-cc (M = Cu, Ru, Pd, Ag) and
Def2-SVP (M = Pt) were used.
(25) Ditchfield, R. Mol. Phys. 1974, 27, 789–807.
(26) Spencer, L. P.; Altwer, R.; Wei, P.; Gelmini, L.; Gauld, J.; Stephan,
D. W. Organometallics 2003, 22, 3841–3854.
DMF-d7): δ 6.32 (s, 2H, CH2), 7.40-7.55 (m, 3H, H-300, H-400,
H-500), 7.73 (ddd, J = 1.7, 5.9, and 7.6 Hz, 1H, H-5), 7.88-7.94
(m, 2H, H-200, H-600), 7.99-8.03 (m, 1H, H-3), 8.29 (ddd, J =
1.4, 7.7, and 7.7 Hz, 1H, H-4), 9.21 (s, 1H, H-50), 9.29 (dd, J =
1.4 and 5.9 Hz, 1H, H-6). 13C NMR (75 MHz, DMF-d7): δ 55.6
(CH2), 126.27 (C-50), 126.32 (C-200, C-600), 127.3 (C-5), 127.4
(C-3), 129.73 (C-100), 129.77 (C-300, C-500), 129.9 (C-400), 141.3
(C-4), 148.8 (C-40), 151.6 (C-2), 154.4 (C-6). 195Pt NMR (64 MHz,
DMF-d7): δ -2205. MS (ESIþ, %): m/z 525.0 ([PtLxCl2 þ Naþ]þ,
100). Crystals suitable for X-ray analysis were prepared from a
DMF-d7 (0.5 mL) solution of 1 (20 mg). A few drops of diethyl
ether were added into this solution until turbidity was observed.
The resulting mixture was filtered, and the filtrate was aged at
room temperature for few days to give pale-yellow crystals of 1.
[Pd(Lx)2](BF4)2 2CH3CN (2 2CH3CN). Ligand Lx (189 mg,
(27) Benalil, A.; Carboni, B.; Vaultier, M. Tetrahedron 1991, 47, 8177–
8194.
€
ꢀ
(28) Urankar, D.; Steinbucher, M.; Kosjek, J.; Kosmrlj, J. Tetrahedron
2010, 66, 2602–2613.
(29) Baker, M. V.; Brown, D. H.; Simpson, P. V.; Skelton, B. W.; White,
A. H.; Williams, C. C. J. Organomet. Chem. 2006, 691, 5845–5855.
(30) Romeo, R.; Scolaro, L. M. Inorg. Synth. 1998, 32, 153–158.
€
(31) Brase, S.; Gil, C.; Knepper, K.; Zimmermann, V. Angew. Chem., Int.
3
3
Ed. 2005, 44, 5188–5240.
0.800 mmol) was added to a solution of [Pd(CH3CN)4](BF4)2

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4823
(178 mg, 0.400 mmol) in acetonitrile (10 mL) under stirring. A
clear solution was obtained, from which after 10 min product
started to crystallize. The reaction mixture was stirred over-
night. The product was collected by filtration and washed with
the resulting solution was left to stand at room temperature
overnight. Red crystals (210 mg), suitable for X-ray analysis,
were collected by filtration: orange crystals, mp 211-213 °C.
Anal. Calcd for C25H26ClF3N4O3RuS: C, 45.77; H, 3.99; N, 8.54.
Found: C, 45.64; H, 3.96; N, 8.51. IR (KBr): ν 3037, 2965, 1474,
1275, 1264, 1224, 1151, 1032, 770, 637cm-1. 1H NMR (300 MHz,
DMF-d7): δ 1.42 (d, J = 3.0 Hz, 3H, CH3CH), 1.44 (d, J = 3.0
Hz, 3H, CH3CH), 2.08 (s, 3H, CH3Ar), 2.98-3.14 (m, 1H,
(CH3)2CH), 5.96 (d, J = 15.8 Hz, 1H, CH2), 6.04 (d, J = 6.1
Hz, 1H, Ar), 6.20 (d, J =6.1 Hz, 1H, Ar), 6.25 (d, J = 6.1Hz, 1H,
Ar), 6.33 (d, J = 6.1 Hz, 1H, Ar), 6.48 (d, J = 15.8 Hz, 1H, CH2),
7.42-7.49 (m, 1H, H-400), 7.50-7.58 (m, 2H, H-300, H-500), 7.76
(dd, J = 6.5 and 6.5 Hz, 1H, H-5), 7.90-7.96 (m, 2H, H-200,
H-600), 8.0 (d, J = 7.8 Hz, 1H, H-3), 8.25 (ddd, J = 1.4, 7.7, and
7.7 Hz, 1H, H-4), 9.15 (s, 1H, H-50), 9.24 (d, J = 5.2 Hz, 1H, H-6).
13C NMR (75 MHz, DMF-d7): δ 18.0 (CH3Ar), 22.2 (CH3CH),
22.3 (CH3CH), 31.5 (CH), 55.2 (CH2), 83.6 (C-2000 or C-6000), 84.4
(C-6000 or C-2000), 86.4 (C-3000 orC-5000), 90.1 (C-5000 or C-3000), 102.6
(C-1000), 105.9 (C-4000), 122.1 (q, J = 322 Hz, CF3), 126.1 (C-200,
C-600), 126.7 (C-5), 127.1 (C-3), 127.5 (C-50), 129.80 (C-300, C-400,
C-500), 129.84 (C-100), 141.2 (C-4), 150.1 (C-40), 154.2 (C-2), 159.1
(C-6). 19F NMR (282 MHz, DMF-d7): δ -79.1.
acetonitrile (1 mL) to give 2 2CH3CN (192 mg, 0.240 mmol,
3
60%). For this compound, it proved very difficult to remove
traces of an unidentified compound, which was based on NMR
spectra present as a ∼10% impurity in the sample studied. The
impurity was present even after several consecutive recrystalli-
zations from different solvents, including water, acetonitrile,
and ethanol as well as their mixtures: off-white solid, mp
263-266 °C. IR (KBr): ν 3142, 1612, 1476, 1443, 1081, 1034,
973, 766, 696 cm-1. 1H NMR (300 MHz, DMF-d7): δ 6.99 (br s,
2H, CH2), 7.46-7.59 (m, 3H, H-300, H-400, H-500), 7.90-8.00 (m,
3H, H-5, H-200, H600), 8.24 (d, J = 7.8 Hz, 1H, H-3), 8.49 (ddd,
J = 1.3, 7.7, and 7.7 Hz, 1H, H-4), 9.22 (br s, 1H, H-6), 9.49 (s,
1H, H-50). 13C NMR (75 MHz, DMF-d7): δ 1.0 (CH3CN), 55.5
(CH2), 118.2 (CH3CN), 126.5 (C-200, C-600), 127.6 (C-5), 128.2
(C-3), 128.5 (C-50), 129.1 (C-100), 130.0 (C-300, C-500), 130.5
(C-400), 143.7 (C-4), 150.6 (C-40), 152.2 (C-2), 155.2 (C-6). 19F
NMR (282 MHz, DMSO-d6): δ 191.7. MS (ESIþ, %): m/z
615.1 ([(Pd(Lx)2)2þ þ Cl-]þ, 100). HRMS (ESIþ). Calcd for
C28H2435ClN8106Pdþ ([(Pd(Lx)2)2þ þ Cl-]þ): 613.0847. Found:
[Ag2(Lx)2(NO3)2] (5). A solution of AgNO3 (47.0 mg, 0.277
mmol) in MeOH (2 mL) was dropwise added under stirring into
the solution of ligand Lx (64.2 mg, 0.272 mmol) in MeOH
(1 mL). The reaction mixture was stirred for 24 h, and the
precipitated solid was collected by filtration and washed with
MeOH (2 ꢀ 1 mL) and diethyl ether (2 ꢀ 2 mL) to give
analytically pure complex 5 (92.4 mg, 0.114 mmol, 82%): white
solid, mp 187-189 °C. Anal. Calcd for C28H24Ag2N10O6: C,
41.40; H, 2.98; N, 17.24. Found: C, 41.20; H, 2.88; N, 17.11. IR
(KBr): ν 3124, 3094, 1592, 1385, 1079, 768, 755, 689 cm-1. 1H
NMR (300 MHz, DMF-d7): δ 6.02 (s, 4H, 2 ꢀ CH2), 7.38 (t, J =
7.4 Hz, 2H, 2 ꢀ H-400), 7.48 (dd, J = 7.4 and 7.4 Hz, 4H, 2 ꢀ
H-300, 2 ꢀ H-500), 7.55 (dd, J = 7.0 and 5.5 Hz, 2H, H-5), 7.67 (d,
J = 7.8 Hz, 2H, H-3), 7.96 (d, J = 7.4 Hz, 4H, 2 ꢀ H-200, 2 ꢀ
H-600), 7.99-8.07 (m, 2H, 2 ꢀ H-4), 8.74 (br d, J = 4.9 Hz, 2H,
2 ꢀ H-6), 8.86 (s, 2H, 2 ꢀ H-50). 13C NMR (75 MHz, DMF-d7): δ
56.1 (CH2), 123.2 (C-50), 124.5 (C-3), 124.8 (C-5), 126.1 (C-200,
C-600), 128.9 (C-400), 129.6 (C-300, C-500), 131.3 (C-100),þ139.4
(C-4), 148.2 (C-40), 151.6 (C-6), 155.3 (C-2). MS (ESI , %):
613.0873. Crystals of 2 2CH3CN suitable for X-ray analysis
3
were obtained by careful layering of acetonitrile solutions of Lx
and [Pd(CH3CN)4](BF4)2 (2:1) and quiet aging of the resulting
mixture for few days.
[CuCl2(Lx)2] (3). A solution of CuCl2 (36.0 mg, 0.268 mmol)
in MeOH (1 mL) was added into a solution of ligand Lx (127 mg,
0.538 mmol) in MeOH (1 mL). The resulting mixture was
stirred for 24 h, and the product was collected by filtration
and washed with MeOH (1 mL) and diethyl ether (1 mL) to give
pure 3 (129 mg, 0.213 mmol, 78%): blue crystals, mp 164-
165 °C. Anal. Calcd for C28H24Cl2CuN8: C, 55.40; H, 3.99; N,
18.46. Found: C, 55.16; H, 3.88; N, 18.36. IR (KBr): ν 3120,
3034, 2998, 1608, 1573, 1485, 1469, 1432, 1227, 1078, 829, 760,
721, 694 cm-1. MS (ESIþ, %): m/z 570.1 ([M - Cl-]þ, 4), 535.1
(8), 259.1 (15), 237.1 (100). To prepare crystals suitable for X-ray
analysis, product 3 (30 mg) was dissolved in hot EtOH (25 mL,
50 °C), followed by the addition of distilled water (10 mL). Upon
isothermal evaporation of the solvents for a few weeks, crystals of
m/z 579.1 ([M - 2NO3 - Agþ]þ, 2), 237.1 (100). Crystals
-
3 2H2O were grown.
3
[RuCl(η6-p-cymene)Lx]Cl (40). A mixture of Lx (236 mg, 1.00
mmol) and [RuCl(μ-Cl)(η6-p-cymene)]2 (307 mg, 0.501 mmol)
was stirred in absolute EtOH (50 mL) at room temperature for 2
days. Hexane was added under stirring until a slight turbidity
was observed. The mixture was left to stand at 4 °C for 2 h and
filtered, and then the volatiles of the filtrate were evaporated on
a rotary evaporator to give 40 (541 mg, 0.997 mmol, 100%) as an
orange solid, mp 137-141 °C. IR (KBr): ν 3125, 3055, 2962,
suitable for X-ray analysis were prepared by recrystallization of
the above product (78 mg) from boiling benzonitrile (1 mL).
Spectral and analytical data for the recrystallized product (45
mg, mp 185-188 °C) were identical with those described for the
crude product 5.
Results and Discussion
1
1608, 1427, 1435, 1385, 1089, 769, 697 cm-1. H NMR (300
As noted in the Introduction, in sharp contrast to the
primary amine group containing chelators A, the isomeric
compounds B do not readily form stable chelates. It is,
however, anticipated that replacement of the pendant pri-
mary amine group at the triazole N1 position with pyridine
(e.g., 2-picolyl) should improve the chelating properties of
compounds B and thus stabilize its complexes. Unlike the
sp3-hybridized aminoalkyl group, pyridine is a π acceptor
and, when coordinated, it should withdraw the electron
density from the metal, rendering it more electrophilic and
thus more susceptible for coordination with the electron-
deficient N2 nitrogen atom.32 To demonstrate this, we
selected a simple model ligand Lx (Chart 1), which offers a
MHz, CDCl3): δ 1.37 (d, J = 6.9 Hz, 3H), 1.41 (d, J = 6.9 Hz,
3H), 2.07 (s, 3H), 2.87-3.02 (m, 1H), 5.82 (d, J = 5.7 Hz, 1H),
6.11 (d, J = 5.7 Hz, 1H), 6.12 (d, J = 5.7 Hz, 1H), 6.39 (d, J =
5.7 Hz, 1H), 6.72 (d, J = 15.9 Hz, 1H), 7.08 (d, J = 15.9 Hz, 1H),
7.32-7.48 (m, 4H), 7.76-7.83 (m, 2H), 7.87 (ddd, J = 1.4, 7.6,
and 7.6 Hz, 1H), 8.07 (d, J = 7.6 Hz, 1H), 9.01 (s, 1H), 9.03 (dd,
J = 1.4 and 5.8 Hz, 1H). MS (ESIþ, %): m/z 507.1 (þ[RuCl(η6-p-
cymene)Lx]þ, 100), 471.1 (([RuCl(η6-p-cymene)Lx - HCl]þ,
38). HRMS (ESIþ). Calcd for C24H2635ClN4102Ruþ ([RuCl(η6-
p-cymene)Lx]þ): 507.0889. Found: 507.0900.
[RuCl(η6-p-cymene)Lx]OTf (4). The above-prepared 40 (305mg,
0.562 mmol) was dissolved in acetonitrile (3 mL), and a solution
of AgOTf (145 mg, 0.564 mmol, 1 equiv) in acetonitrile (2 mL)
was added slowly under stirring. The reaction mixture was stirred
for 10 min andfilteredthrougha pad ofCelite. The Celitepadwas
rinsed with acetonitrile (10 mL), and the filtrate was evaporated
to dryness to give 4 as an orange solid (360 mg, 0.549 mmol,
98%). The product was redissolved in boiling EtOH (7 mL), and
(32) (a) Summa, N.; Schiessl, W.; Puchta, R.; van Eikema Hommes, N.;
van Eldik, R. Inorg. Chem. 2006, 45, 2948–2959. (b) Weber, C. F.; van Eldik, R.
Eur. J. Inorg. Chem. 2005, 4755–4761. (c) Hofmann, A.; Jaganyi, D.; Munro,
O. Q.; Liehr, G.; van Eldik, R. Inorg. Chem. 2003, 42, 1688–1700.

4824 Inorganic Chemistry, Vol. 49, No. 11, 2010
Scheme 1. Synthesis of Ligand Lx and Complexes 1-5
Urankar et al.
bidentate chelating system involving N2 of the 1,2,3-triazole
and pyridine nitrogen atom. Ligand Lx was easily prepared
by CuAAC reaction between phenylacetylene and 2-picoly-
lazide as click components (Scheme 1).
of metal chelates into biomolecules of diagnostic and
therapeutic interest in a single step, if desired.10 If success-
ful, it would further demonstrate the chelating ability
of ligand Lx. Three-step, click-chelate-filtrate, one-pot, or
telescoped reactions were conducted by using the above-men-
tioned reaction partners, affording pure complexes 1 (73%), 2
(90%), 3 (78%), and 5 (90%) in excellent yields (Supporting
Information).
The coordination of Lx to PtII, PdII, CuII, RuII, and AgI
was examined with cis-[PtCl2(DMSO)2], [Pd(CH3CN)4]-
(BF4)2, CuCl2, [RuCl(μ-Cl)(η6-p-cymene)]2, and AgNO3
(Scheme 1). The syntheses of complexes 1-3 and 5 were
surprisingly facile and were achieved by simple mixing of
ligand Lx with a selected metal precursor in an equimolar
ratio in an appropriate solvent at room temperature. In the
case of [Pd(CH3CN)4](BF4)2 and CuCl2, bisbidentate com-
plexes 2 and 3 were obtained, respectively, and the syntheses
of these two compounds were repeated with a Lx/metal
precursor molar ratio of 2:1. Pure stable products 1-3 and
5 crystallized out from the reaction mixtures and were
isolated by filtration in 60-82% yield. More soluble RuII
complex 40 was prepared by the reaction between Lx and
[RuCl(μ-Cl)(η6-p-cymene)]2 (2:1) and was isolated quantita-
tively by solvent evaporation.
Crystals of products 1-3 and 5 suitable for X-ray analysis
were prepared by recrystallization from the appropriate
solvents, as described in the Experimental Section. Ruthe-
nium complex 40 was isolated as a glassy material, and
crystals of [RuCl(η6-p-cymene)Lx]þ suitable for X-ray dif-
fraction were prepared from its triflate salt 4. It should be
noted that while this paper was in preparation, Zhu et al.12
reported the synthesis and crystal structure of [Cu(Lx)2-
(CH3CN)(ClO4)](ClO4) in which two ligands Lx are in bisbi-
dentate fashion coordinated to CuII, analogously to 3.
Complexes 1-5 were fully characterized by CHN analysis,
ESI-MS, and 1D and 2D NMR spectroscopy. The NMR
characterization of all of the complexes in solution was for
the comparison reasons carried out in DMF-d7. Other less
polar organic solvents and water proved to be inappropriate
because of solubility reasons, whereas dimethyl sulfoxide
(DMSO-d6) caused rapid ligand-exchange reactions at 1.
Bidentate coordination through the triazole N2 and pyridine
The effect of the pendant group is clearly reflected in the
coordination of platinum, whichwas previously shown not to
form complexes with primary amine chelators B. For exam-
ple, recent efforts to coordinate Ly either with K2PtCl4 or
KPtCl3 DMSO as platinum sources were unsuccessful.5,33
3
Our attempts to prepare platinum chelates from Ly and
cis-[PtCl2(DMSO)2] or [PtCl2COD] failed similarly.
1
N4 nitrogen atoms in 1-5 was confirmed by H and 13C
We were also prompted to test whether the synthesis of the
above complexes can be achieved in one pot from the
corresponding phenylacetylene, 2-picolylazide, and the ap-
propriate metal precursors without the ligand Lx isolation.
This is potentially important for the synthesis and installation
NMR spectroscopy. In comparison to Lx, significant down-
1
field shifts in several H NMR resonances were observed,
especially for the H6 proton of the pyridine, the methylene
group, and the H5 of the triazole (Figure 2). These observa-
tions are fully consistent with both heterocycles, triazole and
pyridine, being involved in coordination to the metals. 195Pt
NMR spectrum of 1 showed a single resonance at δ -2205
ppm, confirming the N2Cl2 coordination set.34 Methylenic
(33) It is of note that structurally similar chelator A, 4-(aminomethyl)-1-
benzyl-1H-1,2,3-triazole, readily formed a stable five-membered PtII com-
plex, which allowed characterization through X-ray diffraction; see ref 5.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4825
Figure 2. Low-field regions of the 1H NMR spectra of Lx, 1, 2, 4, and 5
in DMF-d7.
Table 2. Selected Bond Lengths (A) and Angles (deg) for 1-5 from Crystal-
lographic Data
1
Pt1-N2
Pt1-N4
Pt1-Cl1
Pt1-Cl2
1.997(4)
2.029(4)
2.2824(11)
2.2853(13)
N2-Pt1-N4
N2-Pt1-Cl1
N2-Pt1-Cl2
N4-Pt1-Cl1
N4-Pt1-Cl2
Cl1-Pt1-Cl2
88.75(16)
177.55(12)
90.54(12)
91.23(11)
178.82(11)
89.51(5)
Figure 3. ORTEP view of 1 (thermal ellipsoids are at 50% probability).
six-membered metallacycle, making equivalent both hydro-
gen atoms on the NMR time scale. The 1H NMR spectrum of
4 displays a well-resolved AB spin system (δ 5.96 and 6.48
ppm, 2 ꢀ d, J = 15.8 Hz), and the apparent reason for such
dynamic behavior is the sterical hindrance of ligand Lx
exerted by cymene and chloride ligands. The paramagnetic
complex 3 was not amenable to structural investigation by
NMR spectroscopy.
2
3
Pd1-N2
Pd1-N4
2.0022(19)
2.035(2)
N2-Pd1-N4
88.16(8)
Structural Studies. To confirm the chelation of Lx,
single-crystal X-ray analysis was undertaken. Selected
bond lengths and angles for compounds 1-5 are summar-
ized in Table 2. The molecular structures are shown in
Figures 3-7. For the crystal structure of Lx and illustra-
tions of π-π stacking interactions in Lx and 1-4, see Table
S1 and Figures S1-S6 in the Supporting Information.
The structures of 1 and 2 exhibit a square-planar co-
ordination environment of the central metal atom. The two
nitrogen atoms of the ligand Lx and two chloride ions in the
cis position fulfill the coordination sphere around the
platinum atom in 1, whereas the structure of 2 consists of
a mononuclear bisbidentate complex where the palladium
ion is trans-chelated by the two Lx molecules. The bond
distances in the coordination sphere of platinum in 1 are
comparable to those found in other square-planar com-
plexes of PtII.5 The diagonal bonds in 2 are nonequivalent,
with N2-Pd1 being shorter than N4-Pd1.
Cu1-N2
Cu1-N4
Cu1-N20
Cu1-N40
Cu1-Cl1
Cu1-Cl2
2.385(3)
2.054(3)
2.777(3)
2.043(3)
2.3072(10)
2.3278(11)
N2-Cu1-N4
N20-Cu1-N40
N2-Cu1-Cl1
N2-Cu1-Cl2
N2-Cu1-N20
Cl1-Cu1-Cl2
86.90(10)
85.47(10)
89.18(9)
95.11(9)
177.53(11)
175.68(4)
4
5
Ru1-N2
Ru1-N4
Ru1-Cl1
2.082(4)
2.115(4)
2.3785(12)
N2-Ru1-N4
N2-Ru1-Cl1
N4-Ru1-Cl1
83.45(15)
85.38(11)
83.97(12)
Ag1-N2
Ag1-N4
Ag1-N3a
Ag1-O1
2.445(2)
2.277(2)
2.229(2)
2.447(3)
N2-Ag1-N4
N2-Ag1-N3a
N2-Ag1-O1
85.16(7)
108.14(7)
113.58(11)
a Symmetry code: i, 2 - x, -y, 1 - z.
hydrogen atoms in 1, 2, and 5 appeared as singlets, which
(34) Still, B. M.; Kumar, P. G. A.; Aldrich-Wright, J. R.; Price, W. S.
could be interpreted as a result of the dynamic behavior of the
Chem. Soc. Rev. 2007, 36, 665–686.

4826 Inorganic Chemistry, Vol. 49, No. 11, 2010
Urankar et al.
Figure 6. ORTEP view of 4 (thermal ellipsoids are at 50% probability).
The CF3SO3- ion has been removed for clarity.
Figure 4. ORTEP view of 2 2CH3CN (thermal ellipsoids are at 50%
3
probability). The BF4- ions and the lattice CH3CN molecules have been
removed for clarity.
Figure 7. ORTEP view of 5 (thermal ellipsoids are at 50% probability).
the octahedral geometry. The distances between the
copper atom and pyridine N4 atoms or chlorine atoms
are in the normal ranges. The bonding geometry of Lx
around the central copper atom is different from the
recently reported ionic complex of [Cu(Lx)2(CH3CN)-
(ClO4)](ClO4),12 where CuII displays the square-planar
geometry of four nitrogen atoms with distant CH3CN
and ClO4- ligands at the axial positions.
Figure 5. ORTEP view of 3 2H2O (thermal ellipsoids are at 50%
3
probability). The lattice water molecules have been removed for clarity.
Similarly to 2, in complex 3 the ligands Lx adopt a trans
configuration. Regarding the bonding situation of Lx, the
structure of 3 is comparable to that of 2 except that in this
case the metal ion is in a distorted octahedral coordina-
tion environment with two additional chloride ions in-
volved. Severe distortion of this octahedron is found in
the coordination geometry of axial N2 and N20 atoms
with Cu-N separations of 2.385(3) and 2.777(3) A,
respectively. This displacement of two axial N2 atoms
from the basal plane formed by two pyridine N4 atoms
and two chloride ions suggests a pyramidal distortion of
The structure of 4 consists of a [RuCl(η6-p-cymene)Lx]þ
cation and a CF3SO3- anion. The complex exhibits a three-
legged piano-stool geometry with the metal center coordi-
nated by p-cymene in a η6 fashion, a chloride ion, and ligand
Lx. The distances between RuII and the nitrogen atoms of Lx
are not equivalent, analogously to complexes 1 and 2.
Complex 5 contains two silver centers that are bridged
by two Lx molecules. Both metal ions in the structure are

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4827
Chart 2. Computationally Investigated [MLn(NH3)2]2þ (M = Cu, Pd,
Pt) and [AgLnNH3]þ (n = x, y, z) Molecules with the Selected Atom
Numbering Scheme
Table 3. Selected Bond Lengths (A) and Angles (deg) for [MLx(NH3)2]2þ (M =
Cu, Pd, Pt) and [AgLxNH3]þ from Computational Dataa
complex
M-N2
M-N4
N2-M-N4
[PdLx(NH3)2]2þ
[CuLx(NH3)2]2þ
[AgLxNH3]þ
2.00
2.07
2.04
2.26
2.08
88.3
90.3
86.5
88.2
1.9836
2.41
2.01
[PtLx(NH3)2]2þ
a For atom numbering, see Chart 2.
Lx and ammonia were computed with a stabilization of
about 400-430 kcal mol-1 in most of the cases (Table S2 in
the Supporting Information). The only exception was the
silver derivative, for which the smallest stabilization, about
100 kcal mol-1, was observed. As for the synthesis of
compounds 1-5, different polar and apolar solvents were
employed including dichloromethane (DCM) and metha-
nol, and complexation energies ΔGDCM and ΔGMeOH were
calculated in the liquid phase (Table S2 in the Supporting
Information). As expected, the complexation energies cal-
culated in the liquid phase, especially in polar methanol, are
greatly reduced with respect to the gas phase ΔGgas values;
however, the formation of all of the complexes is still
favorable.36
The natural bond orbital (NBO) analysis reveals that
the main interactions in [MLn(NH3)2]2þ (M = Cu, Pd, Pt)
complexes are the donation of the in-plane lone pairs of
the appropriate nitrogen atoms of ligand Ln and two
ammonia (Chart 2) to the empty s and d orbitals of the
metal, reducing the charge localized on the metal (Table 4).
In contrast, because in the case of [AgLnNH3]þ the d
orbitals are filled, donation occurs only to the 5s orbital,
resulting in a much weaker interaction between the
ligands and the silver ion. This is in line with the
above results that for these compounds the smallest
stabilization energy was observed (Table S2 in the Sup-
porting Information).
Chelators A and B Having Pendant Primary Amine
Groups. As already mentioned in the literature, the charge
on the triazole N3 nitrogen is significantly more negative
from that of N2.4-6,10 Accordingly, the higher stability of
the complexes with chelators A (Figure 1) was attributed
to this structural deviation. In contrast, in the series of the
pendant primary amine analogues, our following analysis
shows that the more efficient π-back-donation rather
than electrostatic interactions makes complexes with
ligand A more stable than those with isomer B.
From the NICS values for the triazole heterocycle and
the phenyl ring, one can conclude that the back-donation
to the triazole ring is notably more effective for Lz than
for Ly (Table 4). For instance, in the case of platinum
derivatives, the NICS value of the triazole (phenyl ring) is
decreased by 1.63 (1.00) and 0.36 (0.58) for Lz and Ly,
respectively (compare entry 15 with entry 12 and entry
14 with entry 8) upon coordination, indicating a higher
aromaticity of these rings in the complexes with Lz as
compared to the rings in the complexes with Ly. For the data
and evaluation of the energy decomposition analysis that
support this conclusion, see the Supporting Information.
Different bonding situations in the complexes with
these two isomeric ligands can also be demonstrated
within the MO ansatz. For example, unique bonding
combinations formed by the dz2 orbital of the metal (dz2
ligated by two nitrogen atoms from one Lx ligand (N4
from the pyridine and N2 from the triazole part) and
another triazole N3 atom from the second Lx ligand in the
complex. The distorted tetrahedral coordination is ful-
filled by the oxygen atom of a nitrate anion coordinated
to both silver atoms in a monodentate mode. Alterna-
tively, coordination of the silver atoms can also be
described as distorted trigonal planar, where each silver
atom is displaced from the plane of the coordinating
nitrogen atoms for 0.564 A.
Within the aromatic system, relevant distortion was
found when ligand Lx is bound to the metal center.
Whereas in Lx the angle between the plane of the pyridine
ring and the plane of the triazole ring is 87.7°, in com-
plexes 1-5, these angles are in the range from 131.4° in 1
to 114.4° in 3. The planes of the triazole ring and that of the
phenyl ring in Lx are almost coplanar. The angle between
these rings is 11.37°, whereas in complexes 1-5, these
values expand from 5.98° in 1 to 31.76° in 5. Accordingly,
the bite angle N-M-N decreases from 1 (88.8°) to 4
(83.5°).
Theoretical Calculations. Ligands Lx, Ly, and Lz and the
corresponding theoretically investigated complexes are de-
picted in Charts 1 and 2, respectively. In line with our
experimental observations, the complexes with central atoms
of copper, palladium, and platinum, [MLn(NH3)2]2þ (M =
Cu, Pd, Pt; n = x, y, z), were calculated with coordination
number 4, while silver derivatives [AgLnNH3]þ were mod-
eled with coordination number 3. To put all of the results
obtained on equal footing, only one-metal, one-ligand Ln
(n = x, y, z) complexes were considered in which ammonia
was used for ligands other than Ln. Because the calculations
returned the expected square-planar (M = Pd, Pt) and
trigonal-planar (M = Ag) geometries with a singlet ground
state, the applied substitution seems to be appropriate for
addressing this issue (compare Tables 2 and 3).35
Stability of Lx Coordinated Complexes. Favorable gas-
phase complexation energies (ΔEgas) and Gibbs free en-
ergies (ΔGgas) for coordination of the metal ion with ligand
(35) Calculations were also performed on coordinationally unsaturated
one-metal-one-ligand [MLx]2þ (M = Cu, Ru, Pd, Pt) and [MLx]þ (M = Ag)
type complexes, but because the obtained geometries showed higher devia-
tions from the experimental structures than those for the [MLn(NH3)2]2þ and
[AgLnNH3]þ complexes, the former were not investigated further.

4828 Inorganic Chemistry, Vol. 49, No. 11, 2010
Urankar et al.
Table 4. NBO Charges and NICS Values for the Investigated Complexes and Ligands Ln (n = x, y, z)
NBO charge
NICSa
entry
structure
N2
N3
N4
M
triazole
pyridine
phenyl
Complexes with Ligand Lx
1
2
3
4
[CuLx(NH3)2]2þ
[AgLxNH3]þ
[PdLx(NH3)2]2þ
[PtLx(NH3)2]2þ
-0.26
-0.18
-0.13
-0.12
-0.62
-0.59
-0.51
-0.50
1.31
-9.05
-9.84
-9.71
-9.89
-7.14
-6.29
-6.90
-7.05
-5.38
-6.56
-6.58
-6.76
0.76
0.83
0.76
Complexes with Ligand Ly
5
6
7
8
[CuLy(NH3)2]2þ
[AgLyNH3]þ
[PdLy(NH3)2]2þ
[PtLy(NH3)2]2þ
-0.27
-0.19
-0.14
-0.13
-0.96
-0.95
-0.84
-0.83
1.30
-9.05
-10.09
-9.80
-9.95
-5.70
-6.49
-6.66
-6.70
0.75
0.82
0.75
Complexes with Ligand Lz
9
[CuLz(NH3)2]2þ
[AgLzNH3]þ
[PdLz(NH3)2]2þ
[PtLz(NH3)2]2þ
-0.45
-0.41
-0.32
-0.31
-0.95
-0.96
-0.84
-0.83
1.30
-10.75
-10.82
-10.80
-10.91
-8.19
-7.83
-8.34
-8.33
10
11
12
0.76
0.82
0.74
Ligands
13
14
15
Lx
Ly
Lz
-0.07
-0.08
-0.48
-0.92
-0.92
-9.62
-9.59
-9.28
-6.02
-6.58
-6.12
-7.33
-0.27
a For comparison, the NICS value of benzene is -9.48.
Figure 9. MOs representing the π-back-donation from the metal
center to the pyridine side chain of the ligand in [PtLx(NH3)2]2þ
(isovalue = 0.01 au).
(compare HOMO-2 and HOMO-3 of Lz and HOMO-4
and HOMO-5 of Ly in Figure 8).37 These interactions
result in more efficient delocalization of the metal d elec-
trons to the π system of the ligand, increasing the aroma-
ticity and thus the stability of the complexes with Lz
(chelators A).
Comparison of Chelators B with Both a Pendant Pri-
mary Amine Group (Ly) and a Pyridine Group (Lx).
Smaller differences in the NICS values at the triazole ring
for Lx in comparison to Ly suggest a less efficient back-
donation to this heterocycle at the former ligand
(Table 4). However, the main difference between the
stability of the complexes with these two ligands is the
interaction of the metal with the π system of the pendant
pyridine group (in Lx). The NICS values calculated in the
center of pyridine are decreased by an average of 0.82,
which means significantly increased aromaticity during
the coordination of Lx to the metal (for the PtII complex,
compare entry 13 with entry 4). Second-order perturba-
tion theory analysis of the Kohn-Sham matrix confirms
this stabilization effect. MO theory provides again insight
into the bonding situation between the metal center
and the ligand, and it clearly shows delocalization of d
Figure 8. Representative orbitals showing the main difference between
the isomeric primary amine containing chelates involving triazole N3
([PtLz(NH3)2]2þ) and N2 [PtLy(NH3)2]2þ) atoms (isovalue = 0.01 au).
is σ interacting in D4h symmetry) and high-lying orbitals
of the triazole ring can only be observed in Lz complexes
(36) For the copper analogue, which was calculated in its doublet ground
state, we obtained the expected planar arrangement of the central
N2-N4-Cu-N5-N6 segment, however, with
a significantly shorter
Cu-N2 bond distance (1.98 A) than that observed by X-ray spectroscopy
for 3 (2.385 and 2.777 A). It is worth mentioning that the calculated value is
in good agreement with that recently reported for [Cu(Lx)2(CH3CN)-
(ClO4)](ClO4) (2.003 and 2.007 A) in ref 12.
(37) For ligand-exchange reactions, ΔGR of tetraamine metal (M = Cu,
Pd, Pt) and triamine metal (M = Ag) complexes, which yield the various
chelates, M(NH3)zþ2zþ þ Ln = MLn(NH3)zzþ þ 2NH3 (n = x, y, z). See the
Supporting Information.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4829
electrons into the π system of the pyridine moiety (Figure 9,
HOMO-6 and HOMO-7). This causes higher aromati-
city and stabilization of Lx complexes with respect to the
Ly analogues and is consistent with the experimental
results. It is also in agreement with the pyridine ring
acting as a π-electron-withdrawing group.32
by delocalization of metal d electrons into the π system of the
pyridine moiety of the ligand. It seems unlikely that this
coordination is restricted to the pyridine heterocycle and the
metal ions selected herein, and we believe that this work will
stimulate future endeavors in the growing field of click
chelates, especially in connection with their potential cataly-
tic and biological activity, as well as chemosensors.39
Conclusions
Acknowledgment. The authors are thankful for finan-
cial support from the Ministry of Higher Education,
Science and Technology of the Republic of Slovenia
and the Slovenian Research Agency (Grants P1-0230-0103
and P1-0175) and from a Research Program of the Research
Foundation - Flanders (Grant G.0464.06). We thank
We demonstrated that, assisted by a pendant pyridine
group, 1,2,3-triazole can form stable coordination with the
N2 nitrogen atom. The structures of the chelates with PtII,
PdII, CuII, RuII, and AgI were confirmed by X-ray diffrac-
tion, the first time for some metals. The bonding patterns and
the electronic structures of the complexes were analyzed by
DFT calculations using different techniques. In contrast to
the previous suggestions that stronger electrostatic interac-
tion results in stronger coordination, our calculations indi-
cate that the stability difference is caused by the distinct
magnitude of π-back-donation. The extra stabilization,
which makes the complexes of Lx synthesizable, is evolved
ꢀ
ꢀ
Dr. Bogdan Kralj and Dr. Dusan Zigon (Mass Spectro-
metry Center, Jozef Stefan Institute, Ljubljana, Slovenia)
ꢀ
for mass spectral measurements.
Supporting Information Available: Additional literature sur-
vey and experimental data, characterization and X-ray data for
ligand Lx, schematic representations of Lx and 1-5 showing
π-π stacking, computational details, NMR spectra, UV-vis
spectra, and X-ray data for compounds Lx and 1-5 in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(38) For the complete set of relevant orbitals, see Table S8 in the
Supporting Information.
(39) For some preliminary endeavors in this direction from our labora-
ꢀ
tories, see: Urankar, D.; Kosmrlj, J. J. Comb. Chem. 2008, 10, 981–985 and
ref 28.