The combination of 4-hydroxythiazoles with azaheterocycles: Efficient bidentate ligands for novel ruthenium complexes

Article abstract of DOI:10.1002/zaac.200900523

Herein the syntheses of three novel ligands, in which an azaheterocycle is connected with a thiazole subunit: 4-methoxy-5methyl-2-pyridine-2-yl-1, 3-thiazole (1), 4-methoxy-5-methyl-2-pyrimidine-2-yl-1, 3-thiazole (2) and 4-methoxy-5-phenyl-2-pyridine-2-y

Full text of DOI:10.1002/zaac.200900523

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ARTICLE
DOI: 10.1002/zaac.200900523
The Combination of 4-Hydroxythiazoles with Azaheterocycles: Efficient
Bidentate Ligands for Novel Ruthenium Complexes
Roberto Menzel,[a] Eric Täuscher,[a] Dieter Weiß,*[a] Rainer Beckert,*[a] and
Helmar Görls[b]
Keywords: Thiazoles; Fluorescence; Ruthenium; Emission; Lifetime
Abstract. Herein the syntheses of three novel ligands, in which an heteroleptic cationic complexes of the type Ru(bpy)2(L), with bpy =
azaheterocycle is connected with a thiazole subunit: 4-methoxy-5- 2,2'-bipyridine were successfully synthesised. The RuII complexes as
methyl-2-pyridine-2-yl-1,3-thiazole (1), 4-methoxy-5-methyl-2-pyrim- well as the ligands were characterised by means of mass spectrometry,
idine-2-yl-1,3-thiazole (2) and 4-methoxy-5-phenyl-2-pyridine-2-yl- NMR, UV/Vis and IR spectroscopy and elemental analysis. Further-
1,3-thiazole (3) are reported. Because these ligands are cyclic versions more, an X-ray structure of Ru(bpy)22(PF6), as far as we know the
of 1,4-diazadienes, they offer good prerequisites for the synthesis of first example where a thiazole is directly connected to a RuII core, is
metal complexes and were employed as chelating ligands. Three novel presented in this paper.
Thiazoles connected to an aromatic heterocycle show inter-
Introduction
esting photophysical properties [6] and might act as light har-
Ruthenium compounds, in particular RuII polypyridine com-
vesting or bridging ligands in several transition metal com-
plexes, are one class of transition metal complexes with out-
plexes. In addition, due to the strong fluorescence of the
standing photochemical properties and the development of
thiazole ligands connected with the versatile hydroxy group,
novel RuII complexes has undergone an impressive growth,
they might be employed as easily binding sensor molecules
especially in the last 15 years. Exhaustive reviews concerning
[7].
RuII polypyridine complexes were published [1]. They play a
central role in artificial photosynthesis, in particular, in the de-
sign of dye-sensitised solar cells (DSC’s) [2]. Furthermore,
they are used in supramolecular chemistry, organic light-emit-
Results and Discussion
ting diodes (OLED’s), electroluminescent devices and as cata-
lysts. A new increasingly important research field where ruthe-
nium complexes are utilised is in the photoelectrochemical
reduction of CO2 to form a useful fuel [3]. A recently pub-
lished paper, for example, describes an unexpected high activ-
ity of similar benzothiazole and bithiazole RuII complexes for
CO2 reduction [4].
Because of their various capabilities, there still exists an im-
mense interest in new RuII complexes with distinct properties.
The class of RuII complexes containing a thiazole based ligand
is absolute underrepresented and only a few examples are
known [5].
The 4-hydroxythiazoles, which lead to ligand 1 and 2, were
prepared following a protocol established by our group [8].
The 4-hydroxythiazole of ligand 3 was synthesised using a
slightly modified method. At first, a nitrile, here pyridine-2-
carbonitrile, reacts with hydrogen sulfide forming the corre-
sponding thioamide which was then cyclised with ethyl 2-bro-
mophenylacetate to yield the 4-hydroxythiazole. Methyl iodide
was chosen as electrophile for a Williamson type etherification
of the 4-hydroxythiazoles. Both methods and the structures of
the ligands are depicted in Scheme 1
The complexes were synthesised using the precursor cis-
(bpy)2RuCl2, which was prepared by following a literature pro-
cedure [9]. The structures of the synthesised complexes are
shown in Figure 1.
A standard synthesis with the precursor was not successful.
The cis-(bpy)2RuCl2 must be transformed to the activated pre-
cursor cis-Ru(bpy)2(acetone)2PF6 before being used. There-
fore, cis-(bpy)2RuCl2 was stirred with AgPF6 in dry and de-
gassed acetone under an inert atmosphere [10]. The complexes
were finally obtained by heating the activated precursor with
an equimolar amount of ligand in acetone. Gel filtration was
used to remove traces of ligand and silver impurities. The yield
was moderate to good (53–68 %).
* PD Dr. D. Weiß
Fax: +49-3641-948212
E-Mail: Dieter.Weiss@uni-jena.de
* Prof. Dr. R. Beckert
E-Mail: c6bera@uni-jena.de
[a] Institute of Organic and Macromolecular Chemistry
Friedrich Schiller Universität Jena
Humboldtstraße 10
07743 Jena, Germany
[b] Institute of Inorganic and Analytical Chemistry
Friedrich Schiller Universität Jena
Humboldtstraße 10
07743 Jena, Germany
1380
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 636, 1380–1385

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Efficient Bidentate Ligands for Novel Ruthenium Complexes
Scheme 1. General synthetic routes and structures of the three thiazole ligands obtained.
Figure 1. Structures of the Ru(bpy)2(L)(PF6)2 complexes.
Electronic Properties
The ligands show various absorption bands in the UV/Vis-
spectra with
a
molar extinction coefficient of over
70000 m–1·cm–1 and hence are π→π* and not n→π* transi-
tions. The absorption and emission spectra of the ligands are
depicted in Figure 2. The shortest wave-length absorption, at
about 240 nm, correspond to the π→π* transition based on
the azaheterocyclic moiety. The second, thiazole based π→π*
transition is at 350 nm and is red-shifted (25 nm) for 3 to 375
nm, due to the additional aromatic phenyl ring lowering the
energy gap between ground (S0) and exited state (S1). Addi-
tionally, a third absorption band at 273 nm can be observed
for 3 which belongs to the π→π* transition (K-band) based
on the phenyl ring [11]. The thiazoles show strong blue room
temperature fluorescence in solution (CH2Cl2). The λmax of the
emission is at 420 nm for 1 and 2. For 3 the λmax is red shifted
(29 nm) to 448 nm, which is in accordance with the absorption
spectra. Additionally, this indicates that the fluorescence arises
basically from the S1 state located at the thiazole moiety. The
Stokes shift is comparatively large for these small molecules
Figure 2. UV/Vis and fluorescence spectra of the ligands 1–3 meas-
ured in CH2Cl2 at room temperature.
The absorption and emission spectra of the complexes, to-
2+
and can be assigned to a significant structure change in the S1 gether with Ru(bpy)3 as a reference, are shown in Figure 3.
state [6]. The fluorescence quantum yields are high and almost All complexes show strong ligand centred (LC) absorptions of
unity for 3. The fluorescence lifetimes are in the normal range the bpy ligands at around 285 nm. The LC transitions of the
and vary from 2–4 ns [6].
coordinated thiazole ligands are also present in the complexes
Z. Anorg. Allg. Chem. 2010, 1380–1385
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R. Menzel, E. Täuscher, D. Weiß, R. Beckert, H. Görls
ARTICLE
and are located at 352 nm, 332 nm and 368 nm for Ru1, Ru2 λmax of the emission of Ru3 is also red-shifted compared to
and Ru3, respectively. The shoulders at 322 nm in Ru1 and Ru1. A second effect is a higher energy gap between the
Ru3 can be assigned to metal center MC transitions which 3MLCT and the non-radiative 3MC state leading to a less prob-
are superimposed in Ru2 with the intense thiazole-based LC able population of the latter and, consequently, to an extended
transition. The bands at 240 and 450 nm are metal to ligand lifetime of the emitting state. This is in accordance with the
(MLCT) transitions. The λmax of the longest wave-length ab- measured values of Ru3 compared to Ru1. The emission quan-
sorption is slightly red-shifted (4 nm) for Ru1 and Ru3 com- tum yields obtained at room temperature are good and in the
pared to Ru(bpy)32+. Due to the especially broad nature of the range of 2 % despite the use of aerated CH2Cl2 in which oxy-
MLCT of Ru2, an exact assignment of the longest wave-length gen acts as a quencher. All values are summarised in Table 1.
absorption is not possible (a shoulder is present at around
465 nm). The λmax of the emission varies from 637 to 665 nm
Crystal Structure of Ru(bpy)22(PF6)2
and is correlated to the electronic properties of the thiazole
ligands. A strong π-acceptor such as pyrimidine lowers the π*L
(LUMO) of the complex and therefore decreases the energy of
the MLCT state. This leads to a bathochromic shift of the λmax
of the absorption (1MLCT) and also the emission band
(3MLCT) of the MLCT transitions. This can be observed for
Ru2 compared to Ru1. The higher π-acceptor strength of the
pyrimidine heterocycle is also expressed in a higher oxidation
potential of 0.85 V for Ru1 compared to 0.81 V for Ru2.
Additionally, a more extended π system, like in ligand 3, low-
ers the energy of the π*L orbital (π*-tuning). Therefore, the
Single crystals suitable for X-ray structure analysis were ob-
tained by slow ether diffusion in an acetonitrile solution of the
corresponding complex Ru2. An ORTEP draw of the complex
is shown in Figure 4. The complex Ru2 crystallises as red
needles in the monoclinic space group P21/c. The methoxy
group of the complex is slightly displaced, which does not
effect the relevant information. The typical distorted octahedral
coordination of the complex forces the aromatic rings of the
thiazole ligand into an almost coplanar orientation (torsion an-
gles N1–C4–C5–N3 0.24°, N2–C4–C5–S1 1.29°). The Ru–N
bond lengths and N–Ru–N bond angles for the bpy ligands
are in the normal range (e.g. Ru1–N4 2.059(5) Å, Ru1–N5
2.047(5) Å) as reported for this type of RuII complexes [13].
The Ru–N bond lengths from the thiazole ligand to the metal
atom are elongated (Ru1–N1 2.078(5) Å, Ru1–N3
2.085(5) Å). This can be ascribed to a less π* backbonding
from the thiazole molecule to the RuII atom, which was ob-
served for similar complexes like [Ru(bpy)2(cept)]+ (cept = 3-
(ethoxycarbonyl)-5-(pyrid-2'-yl)-1,2,4-triazolate) [14] and
[Ru(bpy)2TPZ]2+ (TPZ = 2-(1,H-tetrazol-5-yl)pyrazine) [15].
The bond angle N1–Ru1–N3 of the bidentate thiazole to the
RuII core is slightly diminished (78.4(2)°) compared to the
bond angle formed from the bpy ligands (e.g. N5–Ru1–N4
79.23(19)°). This can be explained with the elongated N1/N3-
Ru1 bonds, which leads to a diminished N1–Ru1–N3 bond
angle. This effect also compensates for the influence of the
smaller bond angle N3–C5–S1 (114.7(6)°) of the five-mem-
bered thiazole-ring, which normally should lead to an expand
N1–Ru1–N3 bond angle (Table 2).
2+
Figure 3. UV/Vis and emission spectra of Ru1 to Ru3 and Ru(bpy)3
measured in CH2Cl2 at room temperature (c ≈ 1.5 × 10–5 mol·L–1).
,
Table 1. Spectroscopic data of ligands 1–3 and the complexes Ru1–Ru3 and Ru(bpy)32+. All measurements were made in aerated CH2Cl2 at
298 K.
Ligands
λAbs /nm [log ε]
λFl /nma)
Stokes-Shift /cm–1 b)
τ /nsd) Φ /%
1
2
3
241 [3.89], 344 [4.17]
419
421
448
5203
4656
4345
3.6
2.8
3.9
90
236 [3.87], 352 [4.05]
60
246 [3.96], 273 [3.92], 376 [4.32]
100
Complexes
λAbs /nm [log ε]
λEm. / nmc)
τ /nsd) Φ /%
Ru(bpy)3
Ru1
Ru2
Ru3
246 [4.43], 289 [4.95], 322 [4.04], 453 [4.19]
244 [4.31], 288 [4.80], 321 [4.26], 457 [4.12]
245 [4.40], 289 [4.83], 332 [4.33], 446 [4.12]
246 [4.43], 289 [4.83], 368 [4.32], 457 [4.12]
608
637
659
667
885e)
378
253
412
6.2e)
2.0
1.9
2.0
a) Excitation in the absorption maximum. b) related to the maximum of the longest wave-length absorption. c) excitation at λ = 450 nm. d)
excitation at λ = 435 nm. e) value obtained from Ref. [12].
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Efficient Bidentate Ligands for Novel Ruthenium Complexes
carried out with a Leco CHNS-932. Mass spectra were measured either
with a Finnigan MAT SSQ 710 (EI) or MAZ 95 XL (FAB) system.
The quantum yield measurements were carried out with a Hamamatsu
C9920–02 QY system. The home-made system consisted of an inte-
grated sphere, which allowed the measurement of absolute quantum
yields of dilute solutions of the corresponding compounds. The emis-
sion lifetimes were obtained by time-correlated single photon counting
(TCSPC) after excitation with a frequency-doubled Ti-sapphire laser
(Tsunami, Newport Spectra-Physics GmbH) as the light source. Repeti-
tion rate was reduced to 0.4 MHz by a pulse selector (Model 3980,
Newport Spectra-Physics GmbH). The laser beam was frequency dou-
bled in a second harmonic generator (Newport Spectra-Physics GmbH)
to create the 435 nm excitation beam. The emission was detected using
a PMC-100–4 photon-counting module from Becker & Hickel. A biex-
ponential (to include the response function) fitting function was used
to determine luminescence lifetimes.
The intensity data for X-ray structure analysis was collected with a
Nonius KappaCCD diffractometer, using graphite-monochromated
Mo-Kα radiation. Data were corrected for Lorentz and polarisation ef-
fects, but not for absorption effects [16, 17]. The structure was solved
by direct methods (SHELXS [18a]) and refined by full-matrix least-
squares techniques against Fo2 (SHELXL-97 [18]). All hydrogen atoms
were included at calculated positions with fixed thermal parameters.
All non hydrogen atoms were refined anisotropically [18b]. XP (SI-
EMENS Analytical X-ray Instruments, Inc.) was used for structure
representations.
Figure 4. ORTEP drawing of Ru2 showing the labeling scheme of
selected atoms. Solvent molecules, hydrogen atoms and the anions
–
PF6 are omitted for clarity. Ellipsoids are at a probability level of
30 %.
Table 2. Selected bond lengths /Å and angles /° of Ru2.
Bond lengths
Bond angles
Ru1–N1
2.078(5)
2.085(5)
2.059(5)
2.047(5)
2.050(5)
2.054(5)
N1–Ru1–N3
N1–Ru1–N4
N1–Ru1–N5
N1–Ru1–N7
N3–Ru1–N4
N3–Ru1–N6
N5–Ru1–N4
N6–Ru1–N7
78.4(2)
Starting materials were commercially obtained from Aldrich and used
as received. TLC was from Merck (Polygram SIL G/UV254, aluminium
oxide 60 F254). The material for Column chromatography was also
obtained from Merck (Silica gel 60 or Merck aluminium oxide 90
active or neutral; activity stage 1 till 15 with 15 m% water).
Ru1–N3
88.12(88)
93.94(19)
96.46(18)
95.75(19)
98.7(2)
Ru1–N4
Ru1–N5
Ru1–N6
Ru1–N7
Torsion angles
N1–C4–C5-N3
N2–C4–C5-S1
79.23(19)
79.09(19)
0.24
1.29
4-Methoxy-5-methyl-2-pyridine-2-yl-1,3-thiazole (1): A mixture of
1.43 g (7.45 mmol) of 5-methyl-2-pyridine-2-yl-1,3-thiazole-4-ol and
0.48 g (8.63 mmol) K2CO3 were dissolved in 15 mL of DMSO and
stirred for 20 min at room temp. The colour of the solution turned
deep red and 1.18 g (8.32 mmol) methyl iodide was added. After the
reaction was finished (TLC), water was added and a yellow precipitate
was formed. The mixture was filtered; the residue was washed with
water and dried in vacuo. Purification by column chromatography (sil-
ica, EtOAc/heptane 1:1) yielded 1.3 g of the product as a pale yellow
solid (yield: 85 %). m.p.: 45.1 °C. EA C10H10N2OS (206,26): C 58.23
Conclusions
The presented results, which include the first time X-ray
structure of a RuII complex with a 4-hydroxythiazole deriva-
tive as a ligand, are promising in terms of the spectroscopic
properties of the compounds. The versatile hydroxy group of
the ligands allowed the connection of the complexes with bio-
logical systems through an ester linkage, for example [8]. To-
gether with the easily tuneable room temperature emission they
can be used as possible sensor molecules. This will be the
subject of further research. Also, the application as possible
catalysts for CO2 reduction, with sulfur as the reactive site (as
reported already) will be investigated.
1
(calcd. 58.29), H 4.89 (5.03), N 13.58 (13.41), S 15.55 (15.63)%. H
NMR (250 MHz, CDCl3): δ = 2.32 (s, 3 H), 4.05 (s, 3 H), 7.24 (ddd,
J1 = 7.6, J2 = 4.9, J3 = 1.0, 1 H), 7.73 (ddd, J1 = 7.8, J2 = 7.8, J3 =
1.7, 1 H), 8.06 (dd, J1 = 8.0, J2 = 1.0, 1 H), 8.54 (dd, J1 = 4.8, J2 =
1.6, 1 H); 13C NMR (63 MHz, CDCl3): δ = 9.5, 57.7, 109.6, 118.7,
123.7, 136.8, 149.3, 151.6, 159.5, 160.7. MS (EI): m/z (%) = 206 (70),
105 (100); UV/Vis (CH2Cl2): λmax (log ε) = 242 nm (3.88), 344 nm
(4.12).
4-Methoxy-5-methyl-2-pyrimidine-2-yl-1,3-thiazole (2): A mixture
of 5-methyl-2-pyrimidine-2-yl-1,3-thiazole-4-ol (0.16 g, 0.77 mmol)
and K2CO3 (134 mg, 0.97 mmol) was suspended in DMF (10 mL).
After 30 min, the colour of the solution turned deep red and methyl
Experimental Section
1H NMR, 13C NMR and the corresponding correlation spectra were iodide (109 mg, 0.77 mmol) dissolved in DMF (5 mL) was added over
recorded with a Bruker AC-250 (250 MHz) and a Bruker AC-400 a period of 30 min. The mixture was stirred for 24 h. After removal
(400 MHz) spectrometer. Chemical shifts (δ) are given relative to sol- of the solvent in vacuo, the crude product was purified by column
vents and all coupling constants are given in Hz. Fluorescence spectra chromatography (silica, CH2Cl2) yielding a brown solid (90 mg). Re-
were measured with a Jasco FP 6500. UV/Vis data were collected crystallisation from an MeOH/H2O (50:50, v/v) mixture afforded the
with a Lambda 19 from PERKIN-ELMER. Elemental analysis was product as light tan needles (yield: 55 %). m.p.: 61 °C. EA C9H9N3OS
Z. Anorg. Allg. Chem. 2010, 1380–1385
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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