The enders Triazole : A well known molecule, but still a new ligand!!

Article abstract of DOI:10.1016/j.inoche.2013.01.018

1,3,4-Triphenyl-1,2,4-triazolium salts have previously been used and described in the literature as precursors for carbenes, which have been reported as organocatalysts and as neutral monodentating N-heterocyclic carbene (NHC) ligands. We now report the synthesis of 1,3,4-tri-phenyl-4,5-dihydro-1H-1,2,4- triazole-5-ylidene-platinum(II)-acetylacetonate where the triazole binds to the metal in a new coordination mode (C^C*) as an anionic bidentate ligand, where the NHC carbene interacts with the platinum center and one of the phenyl rings is cyclometalated. The complex shows phosphorescent emission in the blue region of the visible spectrum at 441 and 465 nm with a quantum yield of 42%. DFT calculations (BP86/6-31G* as well as B3LYP/6-31G*) were found to be well suited to determine the structure of the complex.

Full text of DOI:10.1016/j.inoche.2013.01.018

Inorganic Chemistry Communications 30 (2013) 39–41
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
The “Enders Triazole”: A well known molecule, but still a new ligand!!
a
a
b
b
b
Thomas Strassner a, , Yvonne Unger , Dirk Meyer , Oliver Molt , Ingo Münster , Gerhard Wagenblast
⁎
a
Physikalische Organische Chemie, Technische Universität Dresden, Bergstrasse 66, 01062 Dresden, Germany
b
BASF SE, 67056 Ludwigshafen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3,4-Triphenyl-1,2,4-triazolium salts have previously been used and described in the literature as precursors for
carbenes, which have been reported as organocatalysts and as neutral monodentating N-heterocyclic carbene
(NHC) ligands. We now report the synthesis of 1,3,4-tri-phenyl-4,5-dihydro-1H-1,2,4-triazole-5-ylidene-
platinum(II)-acetylacetonate where the triazole binds to the metal in a new coordination mode (C^C*) as
an anionic bidentate ligand, where the NHC carbene interacts with the platinum center and one of the phenyl
rings is cyclometalated. The complex shows phosphorescent emission in the blue region of the visible spectrum
at 441 and 465 nm with a quantum yield of 42%. DFT calculations (BP86/6-31G* as well as B3LYP/6-31G*) were
found to be well suited to determine the structure of the complex.
Received 12 September 2012
Accepted 12 January 2013
Available online 4 February 2013
Keywords:
Platinum(II)-NHC-complexes
Cyclometalation
Solid-state structure
Photophysics
© 2013 Elsevier B.V. All rights reserved.
Phosphorescent emitter
OLEDs
Introduction. During the last decade the development of more en-
ergy efficient lighting was found to be an important goal and research
activities lead to a significant improvement. Transition metal complexes
containing metals with a high spin-orbit coupling and their promising
photophysical properties of up to 100% quantum yield [1,2] have been
identified as key components of phosphorescent organic light-
emitting devices (PhOLEDs) [3–5]. Although a variety of metals has
been tested, iridium(III) [1,6,7] and platinum(II) [8–11] complexes with
cyclometalated bidentate ligands (often derived from biphenyl [12–14],
phenylpyrazole [15] or 2-phenylpyridine [12,15,16]) have been found
to be especially interesting. Complexes of the type [(C^N)1-2M(O^O)]
(C^N, 2-arylpyridinate or a related ligand; O^O, a β-diketonate ligand,
M=Pt, Ir) have been intensely studied due to their exceptional lumines-
cence properties [17–19].
decided to investigate whether the new bonding mode can also be ob-
served for that molecule which has previously been published in various
roles.
The 1,3,4-triphenyl-1,2,4-triazolium perchlorate (1•ClO4−) as a precur-
sor for a new stable carbene was first reported by Enders in 1995 [24]. By
reaction with sodium methoxide in methanol they could successfully syn-
thesize the methanol adduct, which liberates the free carbene upon
heating under high vacuum conditions. They could isolate and character-
ize the free carbene and even get a solid-state structure of the stable
carbene [25]. The triazolium salt can be synthesized in large scale with
good yields and gives access to the free carbene [26–29], which was
also successfully used as an organocatalyst [30] and for the dehydrogena-
tion of ammonia borane with a relatively low activation barrier [31]. Even
chiral analogues have been reported [32].
Significant progress has been made in the last years tuning the
photophysical properties of these complexes. We as well as other
groups recently reported cyclometalated mononuclear platinum(II)
NHC complexes of the type [Pt(NHC)(acac)] where NHC is a monoanionic
bidentate aryl-alkyl functionalized imidazole ligand [11,20–23]. These
complexes, with the acetylacetonate auxiliary ligand, show moderate to
excellent quantum yields and good thermal stability. We were now in-
terested whether also phenyltriazoles can be cyclometalated to the
platinum(II) center and as the 1,3,4-phenylsubstituted triazole first
reported by Enders [24] is one of the best known phenyltriazoles we
As carbenes they are also exceptional ligands and several metal-
organic complexes have been reported which used the carbene as a
monodentate ligand [33], e.g. for oxorhenium [34], palladium (II) [35],
iridium(I) [36] or platinum(II) [37] complexes [38]. When we recently
started to work on platinum(II) cyclometalated acetylacetonate
(acac) complexes with NHC-donors (C^C*) [20] as compared to the
well known cyclometalated nitrogen donors (C^N) we got interested
whether this would also be possible with the triazolium salt.
Results and discussion. The synthesis of the platinum complex
1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazole-5-ylidene platinum(II)
acetylacetonate (2) did not start from the perchlorate salt, but from the
iodine salt (1•I−) as given in Scheme 1. The synthesis of the ligand
(1•I−) from benzoylchloride, aniline and phenylhydrazine leads to the
corresponding phenylhydrazone in 48.9% yield. Reaction of this interme-
diate with triethyl orthoformate (HC(OEt)3) and ammonium iodide pro-
ceeds smoothly with 91.5% yield. The synthesis of the platinum complex
Abbreviations: PMMA, polymethylmetacrylate.
⁎
Corresponding author. Tel.: +49 351 463 38571; fax: +49 351 463 39679.
E-mail address: thomas.strassner@chemie.tu-dresden.de (T. Strassner).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.01.018

40
T. Strassner et al. / Inorganic Chemistry Communications 30 (2013) 39–41
1. Ag2O, Dioxane, RT
2. Pt(COD)Cl2,
Butanon, reflux
3. A, DMF, K(OtBu),
RT - 100 °C
N N +
N N
N
I −
N
O
O
Pt
O
O
A
-
1 • I
2
Scheme 1. Synthesis of the cyclometalated platinum(II) complex.
Table 1
The emission maximum was found at 470 nm with a broad tail to-
wards 550 nm, but also a strong shoulder between 430 and 450 nm
with another maximum at 441 nm.
Photoluminescence measurements (2% in PMMA) of 2 at RT.
λexc [nm]a
XRGB
YRGB
λem [nm]d
ϕe
b
c
The observed bond lengths in the solid-state structure show interest-
ing differences to the phenyl imidazolium derived complexes which have
recently been reported [20,39,40]. In all cases the platinum carbene bond
is relatively short (1.94 Å), while the cyclometalated bonds are longer
and differ significantly (1.96–1.99 Å). Comparing the geometry of the
substituted imidazole and the triazole rings, the effect seems to be
independent from the change (CH vs. N-Ph) in the backbone of the
five-membered ring. The angle C1–N1–C3 (115.4°) is very similar
to the corresponding angle in the dibenzofuran substituted structure
(114.5°) [20], but the lower electron density in the more delocalized di-
benzofuran leads to an almost by 0.03 Å elongated platinum–carbon
bond. On the other hand the platinum–oxygen bonds are identical in
length, no influence of the longer cyclometalated bond can be observed.
For the triazole complex the O1–Pt1–O2 angle is smaller compared to
the imidazole complexes.
The new C^C* cyclometalation bonding mode which has only
very recently been shown to be accessible for platinum(II) centers
from 1-phenyl-3-methyl-imidazolium based precursors can also be
found for the well-known “Enders triazole.” The triazole does not
only act as a monodentate carbene, but can also be cyclometalated in
a C^C* fashion. This potentially gives rise to a new class of compounds
which might have interesting photophysical properties based on the re-
sults reported here.
310
355
0.169
0.172
0.191
0.185
464
441, 465
0.42
0.42
a
Excitation wavelength.
CIE coordinates at room temperature.
Emission maxima.
b,c
d
e
Quantum yield, radiant exposure under N2.
by transmetalation of the silver complex with dichloro(cyclooctadiene)
platinum(II) and addition of 2,4-pentanedione leads to the complex in
38.9% yield. Details of the syntheses are given in the Supporting
Information.
As the imidazolium-based C^C* cyclometalated platinum(II) acac
complexes showed interesting optical properties [20] we were inter-
ested in the photophysics of 2. We found that the complex also emits in
the blue region of the spectrum (Table 1). The experimentally mea-
sured emission (Fig. 1) and absorption spectra (Fig. 2) show a similar
line shape compared to the imidazolium-based complexes [20,39].
To unequivocally confirm the proposed structure of 2 we have also been
able to grow crystals suitable for X-ray diffraction and to determine the
solid-state structure (Fig. 3). Details of the solid-state structure determi-
nation are given in the Supporting Information together with details of
the photophysical measurements. We also employed DFT calculations to
compare the structural data to the results of the solid-state structure
(Table 2). We used two established functionals (B3LYP, BP86) together
with the 6-31G* basis set. The photoluminescence was measured at
two different excitation wavelengths, 310 nm and 355 nm. In both
cases quantum yields of 42% were observed at room temperature.
For the excitation at 310 nm the strongest band was found at 464 nm,
while the emission spectrum for the excitation at 355 nm (Fig. 1)
shows a complex fine structure with three main bands.
Conclusion. With this study we were able to show that triazoles car-
rying phenyl substituents can not only bind to metals in a monodentate
way as regular “NHC-carbenes,” but they can also cyclometalate to the
phenyl ring forming a significantly more stable complex where the ligand
binds in a bidentate fashion. Additionally we found that the complex
shows a phosphorescent emission with a quantum yield of 42% and a
decay time of 35.6 μs. The solid-state structure confirms the predic-
tions by DFT calculations on the BP86/6-31G* and B3LYP/6-31G*
level of theory.
Fig. 1. Emission spectrum of 2 (excitation at 355 nm) at RT.
Fig. 2. Absorption spectrum of 2 at RT.

T. Strassner et al. / Inorganic Chemistry Communications 30 (2013) 39–41
41
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
to this article can be found online at doi: http://dx.doi.org/10.1016/j.
inoche.2013.01.018
References
[1] C. Adachi, M.A. Baldo, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 77 (2000) 904–906.
[2] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048–5051.
[3] In: H. Yersin (Ed.), Highly Efficient OLEDs with Phosphorescent Materials, Wiley-VCH
Verlag GmbH & Co. KGaA, 2008.
[4] R.C. Kwong, S. Lamansky, M.E. Thompson, Adv. Mater. 12 (2000) 1134–1138.
[5] P.-T. Chou, Y. Chi, Chem. Eur. J. 13 (2007) 380–395.
[6] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi,
P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001)
4304–4312.
[7] M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl. Chem. 71 (1999) 2095–2106.
[8] B.W. D'Andrade, J. Brooks, V. Adamovich, M.E. Thompson, S.R. Forrest, Adv. Mater.
14 (2002) 1032–1036.
[9] W. Lu, B.-X. Mi, M.C.W. Chan, Z. Hui, C.-M. Che, N. Zhu, S.-T. Lee, J. Am. Chem. Soc. 126
(2004) 4958–4971.
[10] W.-Y. Wong, Z. He, S.-K. So, K.-L. Tong, Z. Lin, Organometallics 24 (2005) 4079–4082.
[11] X. Zhang, A.M. Wright, N.J. DeYonker, T.K. Hollis, N.I. Hammer, C.E. Webster,
E.J. Valente, Organometallics 31 (2012) 1664–1672.
[12] J.A.G. Williams, Top. Curr. Chem. 281 (2007) 205–268.
Fig. 3. Solid-state structure of complex 2. Thermal ellipsoids are drawn at the 50% probability
level. Selected bond lengths (Å) and angles (°): Pt1-C1 1.937 (2), Pt1-C4 1.985 (2), Pt1-O1
2.0870 (17), Pt1-O2 2.0381 (17), C1-Pt1-C4 80.23 (10), O1-Pt1-O2 89.18 (7), C1-Pt1-O1
99.61 (8), C4-Pt1-O2 91.00 (9), C1-N1-C3-C4 -2.9 (3).
[13] C. Cornioley-Deuschel, A. von Zelewsky, Inorg. Chem. 26 (1987) 3354–3358.
[14] C.B. Blanton, Z. Murtaza, R.J. Shaver, D.P. Rillema, Inorg. Chem. 31 (1992) 3230–3235.
[15] Y. Chi, P.-T. Chou, Chem. Soc. Rev. 39 (2010) 638–655.
[16] L. Chassot, E. Mueller, A. von Zelewsky, Inorg. Chem. 23 (1984) 4249–4253.
[17] T. Sajoto, P.I. Djurovich, A. Tamayo, M. Yousufuddin, R. Bau, M.E. Thompson,
R.J. Holmes, S.R. Forrest, Inorg. Chem. 44 (2005) 7992–8003.
[18] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P.I. Djurovich, B.W. D'Andrade,
C. Adachi, S.R. Forrest, M.E. Thompson, New J. Chem. 26 (2002) 1171–1178.
[19] J. Brooks, Y. Babayan, S. Lamansky, P.I. Djurovich, I. Tsyba, R. Bau, M.E. Thompson,
Inorg. Chem. 41 (2002) 3055–3066.
[20] Y. Unger, D. Meyer, O. Molt, C. Schildknecht, I. Muenster, G. Wagenblast, T. Strassner,
Angew. Chem. Int. Ed. 49 (2010) 10214–10216.
[21] A. Tronnier, A. Risler, N. Langer, G. Wagenblast, I. Muenster, T. Strassner, Organometal-
lics 31 (2012) 7447–7452.
[22] Z.M. Hudson, C. Sun, M.G. Helander, Y.-L. Chang, Z.-H. Lu, S. Wang, J. Am. Chem. Soc.
134 (2012) 13930–13933.
[23] G.L. Petretto, M. Wang, A. Zucca, J.P. Rourke, Dalton Trans. 39 (2010) 7822–7825.
[24] D. Enders, K. Breuer, G. Raabe, J. Runsink, J.H. Teles, J.-P. Melder, K. Ebel, S. Brode,
Angew. Chem. Int. Ed. Engl. 34 (1995) 1021–1023.
Table 2
Comparison of experimental data (solid-state structure) with DFT calculations (basis set
6-31G*). Bond lengths in Å, angles in °. For the atom numbering see Fig. 3.
X-ray
1.94
1.98
2.09
2.04
1.42
B3LYP
BP86
1.95
2.00
2.14
2.08
1.44
C1–Pt1
C4–Pt1
O1–Pt1
O2–Pt1
1.96
2.00
2.15
2.09
1.44
79.9
99.3
172.2
88.4
0.9
N1–C3
C1–Pt1–C4
C1–Pt1–O1
C1–Pt1–O2
O1–Pt1–O2
C1–O1–O2–C4
Pt1–C4–C3–C1
80.3
80.1
[25] D. Enders, K. Breuer, U. Kallfass, T. Balensiefer, Synthesis (2003) 1292–1295.
[26] D. Enders, K. Breuer, G. Raabe, J. Simonet, A. Ghanimi, H.B. Stegmann, J.H. Teles,
Tetrahedron Lett. 38 (1997) 2833–2836.
[27] D. Enders, K. Breuer, J.H. Teles, K. Ebel, J. Prakt. Chem. 339 (1997) 397–399.
[28] J.H. Teles, J.-P. Melder, K. Ebel, R. Schneider, E. Gehrer, W. Harder, S. Brode, D. Enders,
K. Breuer, G. Raabe, Helv. Chim. Acta 79 (1996) 61–83.
99.6
171.2
89.2
1.3
98.6
171.7
89.6
1.4
2.1
3.4
4.2
[29] D. Enders, K. Breuer, J. Runsink, J.H. Teles, Liebigs Annalen (1996) 2019–2028.
[30] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107 (2007) 5606–5655.
[31] P.M. Zimmerman, A. Paul, Z. Zhang, C.B. Musgrave, Angew. Chem. Int. Ed. 48 (2009)
2201–2205.
Acknowledgments
[32] D. Enders, J. Han, Tetrahedron-Asymmetry 19 (2008) 1367–1371.
[33] G.D. Frey, K. Oefele, H.G. Krist, E. Herdtweck, W.A. Herrmann, Inorg. Chim. Acta
359 (2006) 2622–2634.
[34] H. Braband, O. Blatt, U. Abram, Z. Anorg. Allg. Chem. 632 (2006) 2251–2255.
[35] H. Clavier, A. Correa, L. Cavallo, E.C. Escudero-Adan, J. Benet-Buchholz, A.M.Z.
Slawin, S.P. Nolan, Eur. J. Inorg. Chem. (2009) 1767–1773.
We are grateful for the financial support by the BMBF (FKZ:
13N10477). D.M. thanks the “Evonik-Stiftung” for financial support.
We also thank M. Tenne (TU Dresden) for help finalizing the article.
[36] R.A. Kelly III, H. Clavier, S. Giudice, N.M. Scott, E.D. Stevens, J. Bordner, I. Samardjiev,
C.D. Hoff, L. Cavallo, S.P. Nolan, Organometallics 27 (2008) 202–210.
[37] S. Fantasia, J.L. Petersen, H. Jacobsen, L. Cavallo, S.P. Nolan, Organometallics 26
(2007) 5880–5889.
[38] D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev. 100 (2000) 39–91.
[39] M. Tenne, Y. Unger, T. Strassner, Acta Crystallogr. C Cryst. Struct. Commun. C68 (2012)
m203–m205.
Appendix A. Supplementary material
Experimental details, details of the solid-state structure determina-
tion, photoluminescence measurements, xyz-coordinates and details
of the quantum-chemical calculations. CCDC-877924 contains the sup-
plementary crystallographic data for compound 2. These data can be
obtained free of charge from The Cambridge Crystallographic Data
[40] A. Tronnier, N. Nischan, T. Strassner, J. Organomet. Chem. 31 (2012) 7447–7452.