Synthesis of branched tetranuclear alkynylplatinum(II) terpyridine complexes and their photophysical properties

Article abstract of DOI:10.1016/j.tetlet.2014.04.083

Herein we report the synthesis of two luminescent branched tetranuclear alkynylplatinum(II) complexes {[Pt(tBu₃-tpy)]₂[Ar(CC) ₄][Pt(tBu₃-tpy)]}₂(PF₆)₄ (I and II; tBu₃-

Full text of DOI:10.1016/j.tetlet.2014.04.083

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of branched tetranuclear alkynylplatinum(II) terpyridine
complexes and their photophysical properties
⇑
Peng Xu, Shifan Ye, Haotian Wu, Zhiping Yan, Hongxing Jia, Pingwu Du
Department of Materials Science and Engineering, Department of Chemistry, CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China,
Hefei, Anhui Province 230026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we report the synthesis of two luminescent branched tetranuclear alkynylplatinum(II) complexes
{[Pt(tBu3-tpy)]2[Ar(C„C)4][Pt(tBu3-tpy)]}2(PF6)4 (I and II; tBu3-tpy = 4,40,400-tri-tertbutyl-2,20:60,200-ter-
pyridine; Ar = [(C6H4)2(C„CAC„C)]). Two terminal alkyne ligands were synthesized by oxidative
coupling in the air, and subsequently reacted with a chloroplatinum(II) terpyridine complex. Their photo-
physical properties have been studied by both steady-state spectroscopy and transient spectroscopy. The
final two platinum(II) complexes were found to show long-lived excited states in solution at room tem-
Received 23 January 2014
Revised 9 April 2014
Accepted 24 April 2014
Available online xxxx
Keywords:
Synthesis
Platinum(II)
Terpyridine
Photochemistry
perature. The longest lifetime of ꢀ17
ls for the twisted branched molecule was observed in toluene at
room temperature. Application of the two complexes as a light-harvesting chromophore in photocatalytic
hydrogen production was further examined.
Ó 2014 Elsevier Ltd. All rights reserved.
Introduction
conjugated tetrayne backbones, which may be potentially utilized
as photoactive organometallic materials.
The rich luminescent behaviors in functional organic and orga-
nometallic molecules have attracted great interest over the past
few decades. Photoactive organometallic complexes represent
one class of such molecules, showing intriguing photochemical
and photophysical properties.1,2 The main potential applications
of these complexes include organic light-emitting devices, lumi-
nescent sensors, nonlinear optical devices, photovoltaic cells, and
solar energy conversion.1b,3 Although a large number of linear mul-
tinuclear alkynylplatinum(II) complexes have been synthesized
and their optical properties have been studied, the research on
branched molecules based on alkynylplatinum(II) moiety has
barely been explored. In 2011, Yam and co-workers4 and Yang
and co-workers,5 respectively reported a series of tetranuclear
alkynylplatinum(II) phosphine complexes and found that tetra-
and hexanuclear platinum(II) complexes showed better two-pho-
ton absorption and two-photon induced luminescent properties
than their di- and trinuclear analogues. The branched alkynyl
ligands, which directly coordinated to the metallic centers, could
also play a critical role in luminescence properties of the plati-
num(II) complexes. Herein we report the synthesis, characteriza-
tion, and photophysical properties of two branched tetranuclear
alkynylplatinum(II) terpyridyl complexes linking by rigid
Branched tetranuclear complexes I and II are designed and
shown in Scheme 1. We envisage that the steric effect between
the terpyridine part and alkyne part could greatly affect the photo-
physical properties. Therefore, as an isomeric analogue of complex
I, more sterically hindered compound II was designed to examine
how the square-planar or distorted square-planar coordination
geometry changes the photophysical properties of these two
platinum(II) complexes. After investigating the luminescent
properties of I and II in different organic solvents, it was found that
the twisted complex II in toluene has the longest lifetimes.
Furthermore, complexes I and II have been examined for the
application of photocatalytic hydrogen production.
Results and discussion
Starting from the commercially available 2,6-dibromo-4-nitro-
aniline 3, compound 4 was prepared by removal of the amino
group according to the literature procedures,6 followed by hydro-
genation of the nitro group by Raney nickel in ethanol at room
temperature (Scheme 2). Aniline 4 was transformed to iodide 5
by diazotization and iodination. Then, conversion of iodide 5 to
compound 7 was achieved by Sonogashira coupling with trimeth-
ylsilylacetylene (TMSA) and (triisopropylsilyl)acetylene (TIPSA),
successively. Selective desilylation of compound 7 gave terminal
alkyne 8. The branched tetrayne precursor 17 was finally
⇑
Corresponding author. Tel./fax: +86 551 63606207.
E-mail address: dupingwu@ustc.edu.cn (P. Du).
http://dx.doi.org/10.1016/j.tetlet.2014.04.083
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xu, P.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.083

2
P. Xu et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Scheme 1. Molecular structures of tetranuclear complexes I and II.
Scheme 2. Synthesis of tetrayne 1 and platinum(II) complex I.
synthesized by oxidative coupling8 of alkyne 8 catalyzed by CuI in
the air, followed by desilylation using tetrabutylammonium fluo-
ride (TBAF) in THF promoted by a small amount of CH3OH.
Starting from 4-(tert-butyl)aniline 10, compound 11 was pre-
pared by the literature procedures9 (Scheme 3). Removal of the
TMS group of compound 11 provided the terminal alkyne 12.10
Further oxidative coupling of 12 afforded halogenated butadiyne
13.11 Compound 13 was converted to tetrayne 2 by a continuous
operation of Sonogashira coupling and desilylation.12 By a modi-
fied procedure of the related platinum(II) terpyridyl complexes,4,5
upon the reaction between terminal alkynes 1 or 2 and chloroplat-
inum(II) complex [Pt(tBu3-tpy)Cl](PF6), the branched tetranuclear
alkynylplatinum(II) complexes I and II were finally obtained in
89% and 84% yields, respectively.13 Both of these two complexes
have been successfully characterized by 1H NMR, FT-IR, mass spec-
tra, and elemental analysis.
The UV–vis absorption spectra and emission spectra of I and II
were studied in acetonitrile. Both of their electronic absorption
spectra show high-energy absorption bands in the range of 300–
350 nm, which are assigned as the internal ligand (IL)
[(ArC„C)n] and
p⁄ (tBu3-tpy) transitions (Fig. 1). The character-
istic low energy absorption band of platinum(II) complex I is in the
range of 360–500 nm (maximized at 418 nm with
= 22831 dm3
molꢁ1 cmꢁ1 for I and 453 nm with = 19453 dm3 molꢁ1 cmꢁ1 for
II), which could be tentatively assigned to be the metal-to-ligand
[d
(Pt)–p⁄(tBu3-tpy)] charge transfer (MLCT) character, probably
p–
p⁄
p
–
e
e
p
mixing with ligand-to-ligand (alkynyl-to-terpyridine) charge
transfer (LLCT) transition.14 However, the MLCT/LLCT transition
of distorted complex II shifts to lower energy, a significant redshift
occurred. This could be ascribed that the twisted configuration of
complex II raised the d
p(Pt) orbital energy, and thus gave rise to
a lower MLCT/LLCT absorption energy.15
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P. Xu et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Scheme 3. Synthesis of tetrayne 2 and platinum(II) complex II.
Table 1
Emission and lifetimesa of complexes I and II in different solutions at 298 K
I
II
kem (nm)
568
570
520
600
s
(
l
s)
kem (nm)
s (ls)
CH3CN
Acetone
CH2Cl2
0.24
0.43
4.6
578
578
578
600
1.46
2.32
2.02
17
Toluene
0.24
/ (ꢂ10ꢁ3
)
6.29
6.61
b
a
Concentration = 1 ꢂ 10ꢁ5 M.
b
In acetonitrile.
Figure 1. UV–vis spectra of complexes I and II in acetonitrile with a concentration
of 1 ꢂ 10ꢁ5 M.
Excitation of complex I at 450 nm in acetonitrile leads to an
emission band at 475–850 nm, maximized at 568 nm with
Uem = 6.3 ꢂ 10ꢁ3, which is tentatively assigned to be derived from
3MLCT [d (Pt)–p⁄(tBu3-tpy)] character (Fig. 2).14 The photolumi-
p
nescence quantum yields (Uem) were determined by using a
degassed CH3CN solution of [Ru(bpy)3](PF6)2 as the reference
(Table 1).16 In contrast, the spectrum of II shows the emission band
between 500 and 850 nm, maximized at 578 nm with Uem = 6.6
Figure 2. Normalized emission spectra for complexes I and II in acetonitrile with a
concentration of 1 ꢂ 10ꢁ5 M at 298 K. The excitation wavelength is 450 nm.
Figure 3. Luminescent decays of complexes I and II in acetonitrile (red line) and
toluene (magenta line) solution at 298 K.
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P. Xu et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Figure 4. Luminescent decay profiles of complex II in nitrobenzene (left) and toluene (right) solution at 298 K.
ꢂ 10ꢁ3. Interestingly, a new low-energy shoulder at ca. 630 nm
appeared, which could be predominantly assigned to be derived
from IL transitions and/or 3LLCT transition character.1b
Upon excitation at 450 nm in deaerated solution, both I and II
show long excited-state lifetimes, which are in the range of micro-
seconds in various solvents, such as acetonitrile and toluene. The
data are shown in Figure 3 and Table 1. The lifetime of complex
II is longer than complex I. Interestingly, for distorted complex II,
the longest lifetime of ꢀ17
ls was observed in toluene, which is
much higher than the lifetimes obtained in other solvents. This
observation may be attributed to the electronic effects of the sol-
vents, which significantly affected the electron transfer between
the excited state of complex II and solvent molecules. We com-
pared the luminescent decay profiles of complex II in nitrobenzene
and toluene (Fig. 4). As seen from the data, the lifetimes follow the
order of toluene > nitrobenzene, which is consistent with the order
of the electron accepting ability of the solvents. On the other hand,
complex II has a longer lifetime than complex I because the
Figure 5. Comparison of hydrogen production under the conditions of
1.61 ꢂ 10ꢁ2 M TEOA, 1.11 ꢂ 10ꢁ5 M photosensitizer I and II, and 1.99 ꢂ 10ꢁ4
M
twisted configuration raises the dp(Pt) orbital based HOMO, as evi-
Co(dmgH)2PyCl (dmgH = dimethylglyoxime, py = pyridine) in a mixture of CH3-
CN:water (3:2, v/v) at pH = 8.5.
denced by the absorption spectra in Figure 1. The nonradiative
deactivation (resulting in short lifetimes) via d–d states pathway
decreases and the energy difference between the MLCT and the
d–d states increases.1,3 As a result, complex II shows longer life-
times than complex I.
various photocatalytic reactions, such as photocatalytic generation
of hydrogen.
Further application of these two complexes as a light-harvest-
ing chromophore was examined for photocatalytic generation of
Acknowledgments
hydrogen. The system consists of the chromophore
I or II
This work was financially supported by the National Natural
Science Foundation of China (21271166), the Fundamental
Research Funds for the Central Universities (WK2060140015), Pro-
gram for New Century Excellent Talents in University (NCET) and
Young Thousand Talented Program.
(1.11 ꢂ 10ꢁ5 M) for visible light harvesting, TEOA (1.61 ꢂ 10ꢁ2 M)
as the sacrificial electron donor, and Co(dmgH)2PyCl (1.99 ꢂ
10ꢁ4 M) as the catalyst for hydrogen production in a CH3CN/water
mixture (3:2, v/v).17 As shown in Figure 5, the photocatalytic activ-
ity for H2 generation under visible light irradiation (k > 420 nm)
was remarkably enhanced by adding complex II, the corresponding
H2 evolution rate is 23 mol hꢁ1, which is about 52 times higher
than that of complex I. Therefore, complex II largely improved
the photocatalytic activity of the hydrogen evolution from water,
probably due to its long lifetimes in solution.
References and notes
1. (a) Yam, V. W.-W.; Tao, C.-H.; Zhang, L. J.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 453; (b) Tao, C.-H.; Zhu, N. Y.; Yam, V. W.-W. Chem.
Eur. J. 2005, 11, 1647.
2. Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635.
3. (a) Yang, B. Z.; Zhang, Q.; Zhong, J.; Huang, S.; Zhang, H.-X. Synth. Metals 2012,
162, 670; (b) Du, P. W.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 7726; (c) Du, P.; Schneider, J.; Jarosz, P.; Zhang, J.; Brennessel, W. W.;
Eisenberg, R. J. Phys. Chem. B 2007, 111, 6887.
4. Chan, C. K. M.; Tao, C.-H.; Li, K.-F.; Wong, K. M.-C.; Zhu, N. Y.; Cheah, K.-W., et al
Dalton Trans. 2011, 40, 10670.
5. Chen, L.-J.; Zhang, J.; He, J. M.; Xu, X.-D.; Wu, N.-W.; Wang, D.-X., et al
Organometallics 2011, 30, 5590.
In conclusion, two branched tetranuclear alkynylplatinum(II)
complexes I and II were synthesized via the reaction between
the corresponding terminal alkynes and chloroplatinum(II) com-
plex [Pt(tBu3-tpy)Cl](PF6), which were fully characterized by 1H
NMR, FT-IR, mass spectra, and elemental analysis. Their photo-
physical properties have also been investigated. It was found that
twisted complex II shows longer emission lifetimes than its planar
6. Chanteau, S. H.; Tour, J. M. J. Org. Chem. 2003, 68, 8750.
counterpart in various solvents. The longest lifetime of ꢀ17
ls has
7. 1,4-Bis(3,5-diethynylphenyl)buta-1,3-diyne (1): Desilylation was conducted in
been observed, which could be used for potential applications in
TBAF/THF at room temperature. ESI-MS (m/z) calcd for C24H10 [MꢁH]ꢁ 297.08;
Please cite this article in press as: Xu, P.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.083

P. Xu et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5
found 297.42. 1H NMR (300 MHz, CDCl3): d 3.13 (4H, s), 7.59 (6H, s). IR (ATR):
mmax 3290, 3068, 2959, 2925, 2855, 2150, 1579, 1412, 1259, 934, 883 cmꢁ1
3291, 3053, 2967, 2932, 2901, 2867, 2105, 1582, 1543, 1460, 1437, 1399, 1365,
1276, 1261, 1224, 1205, 1026, 942, 886, 806, 765 cmꢁ1
13. (I) To degassed solution of tetrayne (4.2 mg, 0.014 mmol) and
.
.
8. 1,4-Bis(3,5-bis((triisopropylsilyl)ethynyl)phenyl)buta-1,3-diyne (9): To a stirred
solution of 8 (0.46 g, 1.0 mmol) in Et3N (2 mL), MeOH (4 mL) and THF (6 mL)
was added CuI (20 mg, 0.1 mmol) in one portion at room temperature. The
reaction was stirred for 3 days in the air until the starting material was not
detectable by TLC. The reaction was quenched by addition of H2O. The organic
layer was removed and the aqueous layer was extracted with DCM. The
combined organic layers were dried over anhydrous MgSO4. Followed by
filtration and concentration on vacuum, flash chromatography (PE/DCM)
afforded compound 9 (0.43 g) as a viscous oil. 1H NMR (400 MHz, CDCl3): d
1.13 (84H, s), 7.52 (2H, m), 7.54 (4H, m). 13C NMR (75 MHz, CDCl3): d 11.4, 18.8,
74.7, 80.4, 92.9, 105.0, 122.2, 124.5, 135.6, 135.7. IR (ATR): mmax 2944, 2891,
2866, 2754, 2723, 2159, 2063, 1578, 1463, 1409, 1383, 1366, 1276, 1162, 1073,
a
1
[Pt(tBu3tpy)Cl](PF6) (49 mg, 0.063 mmol) in CHCl3 (25 mL) and Et3N (5 mL)
was added CuI (cat.). The resulting mixture was degassed for another 5 min
and stirred for 24 h at room temperature under Ar atmosphere. Solvent was
removed in vacuo and the residue was dispersed in MeOH. Then NH4PF6
(9.2 mg, 0.056 mmol) in deionized water (20 mL) was added into the solution,
which was sonicated for 10 min. Finally, the precipitate was filtered out and
washed by water and ethyl ether to afford I (41 mg) as a yellowish-brown
solid. 1H NMR (400 MHz, CD3CN): d 1.45 (106H, m), 7.66 (14H, s), 8.21 (16H,
m), 9.04 (8H, s). IR (ATR): mmax 3113, 3082, 2967, 2915, 2881, 2110, 1616, 1568,
1480, 1422, 1404, 1371, 1254, 1175, 1084, 1035, 837 cmꢁ1. Mass Spec. Calcd
(MALDI-TOF-MS) for [MꢁPF6ꢁ]+: 3113.93; Found [MꢁPFꢁ6 ]+: 3113.77. Anal.
Calcd (%) for C132H146F24N12P4Pt4ꢃMeOH: C, 48.51; H, 4.59; N, 5.15; found: C,
48.86; H, 4.92; N, 5.04.
1017, 996, 980, 995, 919, 882 cmꢁ1
.
9. (a) Hirata, S.; Takeuchi, M.; Shinkai, S. J. Chem. Commun. 2005, 30, 3805; (b)
Bedard, T. C.; Moore, J. S. J. Am. Chem. Soc. 1995, 117, 10662.
10. 1,3-Dibromo-5-(tert-butyl)-2-ethynylbenzene (12): Desilylation was conducted
in MeOH/THF using K2CO3 at room temperature. 1H NMR (300 MHz, CDCl3): d
1.29 (9H, s), 3.61 (1H, s), 7.55 (2H, s). 13C NMR (75 MHz, CDCl3): d 31.0, 35.2,
81.1, 85.7, 123.2, 126.5, 128.9, 154.7.
(II) Similar procedures in I were adopted to obtain a reddish brown solid. 1H
NMR (400 MHz, CD3CN): d 1.40 (126H, s), 7.48 (4H, s), 8.03 (24H, m), 9.21 (8H,
s). IR (ATR): mmax 3110, 3079, 2962, 2909, 2874, 2113, 1615, 1553, 1531, 1479,
1423, 1398, 1371, 1253, 1174, 1123, 1025, 843 cmꢁ1. Mass Spec. Calcd (MALDI-
TOF-MS) for [MꢁPFꢁ6 ]+ 3226.06; Found [MꢁPFꢁ6 ]+ 3225.90. Anal. Calcd (%) for
11. 1,4-Bis(2,6-dibromo-4-(tert-butyl)phenyl)buta-1,3-diyne (13): ESI-MS (m/z)
calcd for [M+Na]+ 649.85; found 649.83. 1H NMR (300 MHz, CDCl3): d 1.30
(18H, s), 7.55 (4H, s). 13C NMR (75 MHz, CDCl3): d 30.9, 35.3, 81.2, 81.3, 123.1,
127.2, 129.0, 155.2. IR (ATR): mmax 3069, 2963, 2904, 2868, 1584, 1530, 1514,
C140H162F24N12P4Pt4: C, 49.85; H, 4.84; N, 4.98; found: C, 49.77; H, 4.70; N,
5.12.
14. (a) Tao, C.-H.; Wong, K. M.-C.; Zhu, N. Y.; Yam, V. W.-W. New J. Chem. 2003, 27,
150; (b) Yam, V. W.-W.; Hui, C.-K.; Yu, S.-Y.; Zhu, N. Y. Inorg. Chem. 2004, 43,
812; (c) Tanaka, Y.; Wong, K. M.-C.; Yam, V. W.-W. Chem. Sci. 2012, 3, 1185.
15. Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Y. Angew. Chem., Int. Ed. 2003, 42, 1400.
16. (a) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991; (b) Calvert, J. M.;
Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.; Meyer, T. J. J. Am. Chem. Soc.
1982, 104, 6620.
1470, 1442, 1387, 1373, 1363, 1261, 1209, 1194, 1128, 873, 745 cmꢁ1
.
12. 1,4-Bis(4-(tert-butyl)-2,6-diethynylphenyl)buta-1,3-diyne (2): Compound 7 was
transformed to corresponding alkyne by Sonogashira coupling in a sealed tube
at 85 °C for 3 days. Desilylation was conducted in MeOH/THF using K2CO3 at
room temperature. ESI-MS (m/z) calcd for [M+Na]+ 433.20; found, 433.50. 1H
NMR (300 MHz, CDCl3): d 1.30 (18H, s), 3.36 (4H, s), 7.52 (4H, s). IR (ATR): mmax
17. Du, P.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130, 12576.
Please cite this article in press as: Xu, P.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.083