"Click" 1,2,3-triazoles as tunable ligands for late transition metal complexes

Article abstract of DOI:10.1039/b701978p

The potential of "click" 1,2,3-triazoles to act as mono-dentate ligands in late transition metal complexes is explored relative to other commonly-used Lewis bases and it is shown that their coordination strength, determined by their 1- and 4-substituents,

Full text of DOI:10.1039/b701978p

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“Click” 1,2,3-triazoles as tunable ligands for late transition metal complexes†
Bart M. J. M. Suijkerbuijk,a Bas N. H. Aerts,a Harm P. Dijkstra,a Martin Lutz,b Anthony L. Spek,b
Gerard van Koten*a and Robertus J. M. Klein Gebbink*a
Received 7th February 2007, Accepted 8th February 2007
First published as an Advance Article on the web 22nd February 2007
DOI: 10.1039/b701978p
The potential of “click” 1,2,3-triazoles to act as mono-dentate
ligands in late transition metal complexes is explored relative
to other commonly-used Lewis bases and it is shown that
their coordination strength, determined by their 1- and 4-
substituents, is easily tunable.
The copper(I)-catalyzed 1,3-dipolar cycloaddition of terminal
acetylenes with organic azides to form 1,4-disubstituted 1,2,3-
triazoles1,2 is becoming an indispensable tool in the chemist’s
toolbox.3 Due to the high tolerance of this type of “click” reaction
Scheme 1
towards many reaction conditions, 1,2,3-triazoles are now widely
applied as interconnectors in research fields as diverse as materials
science4,5 and chemical biology.6 Surprisingly, their role as ligands
in transition metal chemistry remains underexposed.7 We became
very interested in the coordination chemistry of 1,2,3-triazoles
toward transition metals, since we anticipated that substitution
at the 1- and 4-positions is likely to influence their steric and
electronic characteristics, factors that are of vital importance
for their coordination behaviour towards transition metals. Since
numerous substituents can be introduced, this would provide us
with transition metal complexes with a broad variety of different
characteristics and potentially very interesting applications in
areas such as catalysis8 and materials chemistry.
ESI†), which were suitable for X-ray crystallographic analysis,
were grown from several solvent mixtures.‡ Fig. 1 shows the
molecular structure of complex 1L1; the structure of its platinum
analogue, 2L1, is isomorphous. Each structure features an M(II)
ion (M = Pd for 1L1; M = Pt for 2L1) which is four-coordinate
and has a distorted square-planar ligand environment by virtue of
the NCN donor atoms of the tridentate pincer ligand and the N3
atom of 1-benzyl-4-phenyl-1,2,3-triazole.
Here we report the first examples of 1,4-disubstituted 1,2,3-
triazoles as monodentate Lewis-basic ligands for cationic Pd- and
Pt-complexes.7 In order to probe the coordination properties of
these ligands we used cationic NCN-pincer Pd- and Pt-complexes,
as they possess one vacant site where ligand exchange can occur
(trans with respect to the M–C r-bond) and therefore are extremely
useful Lewis acids for our investigations.9 A comparison study
of the coordination strength of these ligands with respect to
more traditional ligands showed that the coordination strength
of the 1,2,3-triazoles toward NCN-Pt complexes can conveniently
be tuned both sterically and electronically by appropriate 1,4-
functionalisation.
The 1,2,3-triazole coordination complexes 1L1 and 2Ln were
synthesised by the addition of the appropriate triazole Ln10 to the
NCN-pincer Pd and Pt cations, generated in situ from 1 and 2 upon
reaction with AgBF4 (Scheme 1). The pincer-triazole products 1L1
and 2Ln were obtained as white to off-white powders in >90%
yield. Single crystals of 1L1, 2L1, and 2L2 (for the latter: see
Fig. 1 Two views of the crystal structure of 1L1 at the 50% probability
level. Hydrogen atoms as well as BF4 anions◦have been omitted for clarity.
˚
Relevant bond distances (A) and angles ( ) for 1L1 and the isomor-
phous 2L1: 1L1: Pd–N3 2.1664(17), Pd–C1 1.919(2), Pd–N1 2.1057(17),
Pd–N2 2.1037(17), N1–Pd–N2 162.16(6), C1–Pd–N3 177.85(8); 2L1:
Pt–N3 2.1490(18), Pt–C1 1.926(2), Pt–N1 2.0902(17), Pt–N2 2.0903(18),
N1–Pt–N2 163.67(7), C1–Pt–N3 177.88(8).
The bond distances within the NCN-pincer metal cations
are similar to those found in other cationic NCN-pincer metal
complexes with an aromatic nitrogen donor at the fourth ligand
˚
position. The Pd–Ntriazole distance (2.1664(17) A) in 1L1 was
aOrganic Chemistry and Catalysis, Faculty of Science, Utrecht University,
Padualaan 8, 3584, CH Utrecht, The Netherlands
compared to the Pd–N distances of NCN-pincer Pd complexes
11,12
13
˚
˚
of acetonitrile (2.109–2.126 A)
are both shorter. The corresponding Pt–Ntriazole distance in complex
and nitrite (2.118 A), which
bCrystal and Structural Chemistry, Faculty of Science, Utrecht University,
Padualaan 8, 3584, CH Utrecht, The Netherlands
˚
2L1 (2.1490(18) A) is comparable to the distances found for Pt–
† Electronic supplementary information (ESI) available: Full synthetic and
analytical details, and crystal structures of 2L1·0.5CH2Cl2 and 2L2. See
DOI: 10.1039/b701978p
Naromatic bonds wherein the Naromatic donor is trans with respect to
the Cipso–Pt bond.14–17
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The planes defined by N1, C1, M and N2, and the 1,2,3-triazole
ring are at interplanar angles of 77.15(9)◦ and 76.95(10)◦ for
1L1 and 2L1, respectively. The dihedral angle between the two
planes represented by the 1,2,3-triazole ring and its 4-phenyl ring
is 17.14(11)◦ for 1L1 and 18.30(21)◦ for 2L1. Complexes 1L1 and
2L1 crystallise as asymmetrical compounds with a Cs-symmetrical
NCN-pincer M (M = Pd, Pt) part and an asymmetric triazole part.
The coordination behaviour of triazole L1 in coordination
Scheme 1) show downfield shifts. These spectra are also indicative
of a dissymmetry in the chemical environment above and below
the basal coordination plane.
In order to assess the relative coordination strength of “click”-
triazole ligands such as L1–L3 we set out to determine their
relative association constants with the NCN-pincer Pt cation.
To this end, competition experiments between complex 2L1 and
1.1–1.3 equivalents of the commonly used Lewis bases PPh3, 1-
methyl-imidazole, diethyl sulfide, DMSO, aniline, MeCN, and
pyridine were performed by means of 1H NMR spectroscopy.
After equilibration of a mixture of 2L1 and the respective Lewis
base in acetone-d6, the relative amounts of starting materials and
products for the reaction depicted in Scheme 2 (all components
were independently observed by 1H NMR spectroscopy, see Fig.
1
complexes 1L1 and 2L1 in solution was studied by H NMR
spectroscopic experiments in acetone-d6. The spectrum of 1L1
at 298 K shows broad signals for the benzylic (H1/H2) and
dimethylamino (Me1/Me2) protons of the pincer part of the
complex as well as for the 4-phenyl ortho-protons Hb and the
triazole-proton Ha (Fig. 2, top).
At 233 K an AB pattern is observed for the diastereotopic ben-
zylic H1/H2 protons and the signals of the NMe2 protons appear
as two separate singlets of equal intensity (Fig. 2, bottom). These
observations indicate that at room temperature intermolecular
triazole exchange at Pd is fast on the NMR timescale, while at
lower temperatures one distinct NCN-pincer Pd-triazole species
is observed. The spectrum of the corresponding platinum–triazole
complex 2L1 shows two separate resonances for both the NMe2
groups and the benzylic protons of the pincer-Pt part from 183 K
to 328 K, which indicates that ligand exchange at Pt is slow on the
NMR timescale throughout this temperature range.
ꢀ
S1, ESI†) gave a relative association constant, Kass , defined as:
[2Lx] [L1]
=
[2L1] [Lx] [2L1] [Lx]
[L1]2
Kaꢀss
=
Scheme 2
In both complexes, coordination of the triazole is signalled by
a significant downfield shift of the signals belonging to the proton
attached to the C5 position of the 1,2,3-triazole ring (Ha in Fig. 2)
and to the two ortho-protons of its 4-phenyl ring (Hb) (Dd ≈ 0.64
and 0.79 ppm, respectively). The presence of an AB-pattern for the
benzylic hydrogen atoms (H1 and H2) and of two separate signals
for Me1 and Me2 furthermore shows that in the temperature regime
in which ligand exchange is slow on the NMR timescale, rotation
around the M–Ntriazole bond does not occur, presumably due to
steric interactions between the NMe2-groups of the pincer ligand
The experiments showed that L1 is a stronger ligand than H2O,
DMSO, MeCN and Et2S. It is comparable in strength to aniline
and weaker with respect to pyridine, PPh3, andN-methyl imidazole
(Table 1, entries 1–8).
More interestingly, entries 9, 10 and 11 show that by appropriate
functionalization of 1,4-disubstituted “click”-triazoles, the coordi-
nation strengths of these ligands toward the NCN-pincer Pt cation
can be fine-tuned. L1 possesses a similar, moderate coordination
strength as aniline (entry 5) and binds more weakly than the parent
1H-1,2,3-triazole (L4, entry 9). The bis(benzyl)-triazole L2 (entry
10) is a stronger ligand than L4 and bis(hexyl) triazole L3 is as
strong a ligand as pyridine (cf. entries 6 and 11). Apparently,
1
and the 4-phenyl group of L1 (see Fig. 1). H NMR spectra of
2L2 and 2L3 are similar to those of 2L1 in that especially the
hydrogen atoms closest to the platinum center (connected at R1 in
Fig. 2 1H NMR spectra of 1L1 at 298 K (top) and 233 K (bottom) in acetone-d6; peaks were assigned according to the schematic representation of the
structure (inset) and residual solvent peaks are marked with *.
1274 | Dalton Trans., 2007, 1273–1276
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Table 1 Kassꢀ values for Lewis base coordination to [Pt(NCN)]+ compared
to L1 for several competing ligands Lx
0.15 × 0.15 mm3, orthorhombic, Pbca (no. 61), a = 18.7077(2), b =
3
˚
˚
13.95732(10), c = 20.6417(3) A, V = 5389.74(10) A , Z = 8, Dx
=
1.528 g cm−3, l = 0.74 mm−1. 102 749 Reflections were measured at
a temperature of 150 K. An absorption correction based on multiple
measured reflections was applied (0.77–0.89 correction range). 6200
Reflections were unique (Rint = 0.0410). Initial coordinates were taken
from the isostructural 2L1. 347 Parameters were refined with no restraints.
R1/wR2 [I > 2r(I)]: 0.0263/0.0531. R1/wR2 [all refl.]: 0.0436/0.0591.
ꢀ
Entry
Lx
Kass
a
1
2
3
4
5
6
7
8
9
H2O
DMSO
MeCN
Et2S
Aniline
Pyridine
PPh3
ꢁ
0.002
0.02
0.03
0.30
−3
˚
S = 1.035. Residual electron density between −0.35 and 0.47 e A
.
2L1: [C27H32N5Pt](BF4), M = 708.48, colourless plate, 0.24 × 0.15 ×
0.06 mm3, orthorhombic, Pbca (no. 61), a = 18.6226(4), b = 13.9422(7), c =
19.5
20.7557(6) A, V = 5389.0(3) A , Z = 8, Dx = 1.746 g cm−3, l = 5.26 mm−1
.
3
˚
˚
b
b
ꢂ
133 892 Reflections were measured at a temperature of 150 K. An
analytical absorption correction was applied (0.32–0.70 correction range).
6195 Reflections were unique (Rint = 0.0507). The structure was solved
with automated Patterson methods.20 347 Parameters were refined with
no restraints. R1/wR2 [I > 2r(I)]: 0.0162/0.0340. R1/wR2 [all refl.]:
0.0244/0.0371. S = 1.053. Residual electron density between −0.55 and
N-Methyl imidazole
1H-1,2,3-Triazole
ꢂ
4.16
(L4)
L2
L3
10
11
6.47
18.5
0.67 e A−3. 2L1·0.5CH2Cl2: [C27H32N5Pt](BF4)·0.5CH2Cl2, M = 750.94,
a No conversion was observed. b Full conversion to 2Lx occurred.
˚
3
¯
colourless block, 0.12 × 0.12 × 0.12 mm , triclinic, P1 (no. 2), a =
˚
11.1861(3), b = 12.05785(13), c = 12.9754(4) A, a = 78.165(1), b =
◦
68.692(1), c = 63.542(1) , V = 1457.75(6) A , Z = 2, Dx = 1.711 g cm−3, l =
3
˚
the steric hindrance of the 4-phenyl group, in combination with
electronic delocalisation, leads to a lower coordination strength
of L1 when compared to the parent 1H-1,2,3-triazole, while the
electron-releasing properties of alkyl substituents at the 1- and 4-
positions lead to an increase thereof. The relief of steric congestion
around the site of coordination when going from a bis(benzyl)
substituted 1,2,3-triazole to a bis(hexyl)-1,2,3-triazole accounts
for the enhanced coordination properties of L3 compared
to L2.
In summary, we have described the first examples of 1,4-
disubstituted 1,2,3-triazoles as mono-dentate ligands for Pd and Pt
coordination complexes. Owing to their modular syntheses, both
the electronic and steric characteristics of these ligands can be
readily tuned, which in turn directly influences their coordination
behaviour. Depending on the substituents, ligands with coordi-
nation strengths similar to pyridine were obtained. Due to the
broad functional group tolerance of the “click”-reaction, a huge
variety of very stable 1,2,3-triazole ligands are readily accessible,
the properties of which can be determined at will. Especially in
catalysis and materials chemistry these factors play a vital role.
For example, metal coordination to the myriad of triazoles which
have been incorporated into molecules as building blocks, could
add an extra dimension to these compounds and offers a handle
for further, non-covalent functionalisation. Coordination of these
triazoles to kinetically inert metal complexes may furthermore be
used to desymmetrise such complexes.
4.96 mm−1. 39 400 Reflections were measured at a temperature of 110 K. An
absorption correction based on multiple measured reflections was applied
(0.42–0.55 correction range). 6685 Reflections were unique (Rint = 0.0299).
The structure was solved with automated Patterson methods.20 Due to
the proximity to a crystallographic inversion center the CH2Cl2 solvent
molecule has only an occupancy of 0.5. 374 Parameters were refined
with no restraints. R1/wR2 [I > 2r(I)]: 0.0182/0.0366. R1/wR2 [all refl.]:
0.0227/0.0377. S = 1.049. Residual electron density between −0.96 and
1.21 e A−3. 2L2: [C28H34N5Pt](BF4), M = 722.50, colourless block, 0.30 ×
˚
3
¯
0.21 × 0.18 mm , triclinic, P1 (no. 2), a = 10.5401(5), b = 10.8◦373(5),
˚
c = 13.5859(5) A, a = 90.459(2), b = 112.708(2), c = 99.581(2) , V =
1407.14(11) A , Z = 2, Dx = 1.705 g cm−3, l = 5.04 mm−1. 49 444
3
˚
Reflections were measured at a temperature of 150 K. An absorption
correction based on multiple measured reflections was applied (0.27–
0.40 correction range). 6453 Reflections were unique (Rint = 0.0203). The
structure was solved with automated Patterson methods.20 The BF4 anion
was refined with a disorder model. 393 Parameters were refined with 142
restraints (restraints for BF4 anion). R1/wR2 [I > 2r(I)]: 0.0117/0.0272.
R1/wR2 [all refl.]: 0.0132/0.0276. S = 1.074. Residual electron density
between −0.47 and 0.35 e A−3. CCDC reference numbers 624055–624058.
˚
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b701978p
1 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless, Angew.
Chem., Int. Ed., 2002, 41, 2596.
2 C. W. Tornøe, C. Christensen and M. Meldal, J. Org. Chem., 2002, 67,
3057.
3 V. D. Bock, H. Hiemstra and J. H. van Maarseveen, Eur. J. Org. Chem.,
2006, 51.
4 D.-J. V. C. van Steenis, O. R. P. David, G. P. F. van Strijdonck, J. H. van
Maarseveen and J. N. H. Reek, Chem. Commun., 2005, 4333.
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Pyun, J. M. J. Fre´chet, K. B. Sharpless and V. V. Fokin, Angew. Chem.,
Int. Ed., 2004, 43, 3928.
6 N. J. Agard, J. A. Prescher and C. R. Bertozzi, J. Am. Chem. Soc., 2004,
126, 15046.
7 There are some examples of (non-fused) 1,2,3-triazoles as (polydentate)
ligands for transition metals:(a) L. Busetto, F. Marchetti, S. Zacchini
and V. Zanotti, Inorg. Chim. Acta, 2005, 358; (b) R. Bronisz, Inorg.
Chem., 2005, 44, 4463; (c) J. R. Olson, M. Yamauchi and W. M. Butler,
Inorg. Chim. Acta, 1985, 99, 121; (d) D. R. Armstrong, R. P. Davies, R.
Haigh, M. A. Hendy, P. R. Raithby, R. Snaith and A. E. H. Wheatley,
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D. J. Hodgson, J. Am. Chem. Soc., 1975, 97, 2376; However, we did not
come across any examples of triazoles possessing the regiochemistry
obtained via a click reaction nor did we encounter their Pd or Pt
complexes.
8 During the course of this work, van Maarseveen et al. published an
interesting application of phosphine–triazole conjugates as bidentate
ligands in the palladium-catalysed allylic alkylation of cinnamyl
acetate: R. J. Detz, S. Arevalo Heras, R. de Gelder, P. W. N. M. van
Leeuwen, H. Hiemstra, J. N. H. Reek and J. H. van Maarseveen, Org.
Lett., 2006, 8, 3227.
The authors gratefully acknowledge the Council for Chem-
ical Sciences of the Dutch Organization for Scientific Re-
search (CW-NWO) (Jonge Chemici grant: B. M. J. M. S. and
R. J. M. K. G.; M. L. and A. L. S), and the Netherlands Research
School Combination-Catalysis (NRSC-C) (H. P. D.) for financial
support.
Notes and references
‡ X-Ray crystal structure determinations: Reflections were measured on
a Nonius Kappa CCD diffractometer with rotating anode (graphite
˚
monochromator, k = 0.71073 A) up to a resolution of (sin h/k)max
=
0.65 A−1. The structures were refined with SHELXL-9718 against F2
of all reflections. Non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were located in the difference
Fourier map and refined with a riding model. Geometry calculations
and checking for higher symmetry was performed with the PLATON
program.19 1L1: [C27H32N5Pd](BF4), M = 619.79, colourless block, 0.15 ×
˚
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43, 725.
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12 P. Steenwinkel, H. Kooijman, W. J. J. Smeets, A. L. Spek, D. M. Grove
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13 M. Q. Slagt, H. P. Dijkstra, A. McDonald, R. J. M. Klein Gebbink,
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1276 | Dalton Trans., 2007, 1273–1276
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