New ditopic ligands containing 2,2′:6′,2″-Terpyridine and a rigid U-/S-shaped terpyridine

Article abstract of DOI:10.1055/s-0029-1218554

The modular approaches towards 2,2′:6′,2′-terpyridines and U-/S-shaped terpyridines are combined to yield a new family of ditopic ligands. Functionalized Kroehnke-type terpyridines were used as key building blocks for the construction of hetero-bis(terpyridine)s. The described compounds might serve as new templates for the self-assembly towards new metallosupramolecular functional materials. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218554

LETTER
61
New Ditopic Ligands Containing 2,2¢:6¢,2¢¢-Terpyridine and a Rigid
U-/S-Shaped Terpyridine
N
a
ew
D
itop
o
ic Ligands
C
h
ontaining 2
a
¢
:6
¢
,2¢¢
-
Te
n
pyridine na Hummel,a Andreas Winter,b,c Anja Baumgaertel,c,d Nikolaus Risch,a Ulrich S. Schubert*b,c,d
Chemistry Department, Faculty of Science, Paderborn University, Warburger Str. 100, 33098 Paderborn, Germany
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands
Laboratory of Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena, Germany
Fax +49(3641)948202; E-mail: ulrich.schubert@uni-jena.de
b
c
d
Received 12 July 2009
electron transfer in supramolecular chemistry.7a,i,8,9
Though various groups focus on the synthesis of rigid-
linear p-conjugated bis(terpyridine) derivatives,8a,9–11
their applications in this particular field have not been
Abstract: The modular approaches towards 2,2¢:6¢,2¢¢-terpyridines
and U-/S-shaped terpyridines are combined to yield a new family of
ditopic ligands. Functionalized Kröhnke-type terpyridines were
used as key building blocks for the construction of hetero-bis(terpyr-
idine)s. The described compounds might serve as new templates for elaborated so far compared to their, for example, 2,2¢-bi-
pyridine counterparts.3
the self-assembly towards new metallosupramolecular functional
materials.
To the best of our knowledge, only systems based either
Key words: cross-coupling, iminium salts, Mannich reaction, su-
on 2,2¢:6¢,2¢¢-terpyridine7i,8,9,11 or U-/S-shaped terpyri-
pramolecular chemistry, terpyridines
dines10,12,13 have been reported in literature. These ligands
feature significant differences in their photophysical
properties and coordination behavior towards transition-
Supramolecular chemistry, in particular the self-assembly
metal ions. Therefore, we paid attention to the combina-
of functional structures into metallosupramolecular archi-
tion of the different types of terpyridine motives within
tectures through the interaction of transition-metal ions
one molecule. Herein, we describe the synthesis of three
and appropriate ligands, has evolved into one of the most
new types of ditopic ligands applying the modular syn-
active areas of research in modern chemistry.1 The char-
thetic approaches established in literature within the last
acteristics of these interactions are defined by combina-
years.13b,14,15
tions of various parameters, including the coordination
numbers, geometries, and donor-types of the transition-
metal ions, and the numbers, types, and arrangements of
The synthesis of the aldehyde-functionalized terpyridine
1 was reported previously.9b According to Böhme’s proto-
col,16 the morpholine-based aminal 2 was prepared and,
subsequently, the ternary iminium salt 3 was obtained in
very good yield by treating 2 with acetyl chloride
(Scheme 1).17 We could show earlier that iminium salts
derived from bulky secondary amines, such as morpho-
line, and aromatic aldehydes are less hygroscopic and can
be stored in a desiccator for months without any sign of
decomposition.10 Applying the optimized reaction condi-
tions, ternary iminium salts can be used for the selective
synthesis of rigid U- and S-shaped terpyridines.13b,15 Fol-
lowing this, 3 was reacted with 2 equivalents of 5,6,7,8-
tetrahydroquinolin-8-one (THQ) and NH4OAc as a source
of ammonia to yield the hetero-bis(terpyridine) ‘tpy-
Ph-Utpy’ 4 in moderate yield (31%, Scheme 1, see the
Supporting Information for experimental details). The
proposed mechanism for the selective formation of the U-
shaped isomer in a multistep one-pot procedure based on
a domino-type Mannich reaction was already discussed
elsewhere.13b
the donor atoms of the ligands.2 Beside the intensively
elaborated N-heterocyclic systems based on 2,2¢-
bipyridine3 or 1,10-phenanthroline,4 2,2¢:6¢,2¢¢-terpyridine
derivatives have attracted much interest as versatile tem-
plates in supramolecular chemistry, as well as in materials
science.1,5,6 The remarkably high binding affinity towards
most transition-metal ions by dp→pp* bonding, together
with their chelation properties make terpyridines highly
attractive building blocks for the construction of complex
metallosupramolecular architectures with advanced pho-
tophysical, electrochemical, catalytic, and magnetic prop-
erties.1 These properties can be influenced and tuned by
variation of p-conjugated substituents attached to the
2,2¢:6¢,2¢¢-terpyridine unit.7
The electronic communication between the metal-com-
plexed units of telechelic bis(terpyridine)s is another fas-
cinating feature pointing out their potential in the design
of new functional materials, for example, as molecular
wires.6,7 Light-emitting ligands are considered to be im-
portant structures as hosts for photoinduced energy and
As expected, the formation of the S-shaped counterpart
‘tpy-Ph-Stpy’ could be almost fully suppressed; the char-
acteristic signals for this isomer have not been observed
by NMR spectroscopy. The 1H NMR spectrum of 4 is de-
picted in Figure 1 (top).18 The identity of the compound
SYNLETT 2010, No. 1, pp 0061–0066
x
x.
x
x.
2
0
1
0
Advanced online publication: 09.12.2009
DOI: 10.1055/s-0029-1218554; Art ID: G22909ST
© Georg Thieme Verlag Stuttgart · New York

62
J. Hummel et al.
LETTER
O
O
O
Cl
N
N
N
O
MeCOCl,
Et2O, 0 °C, 2 h
morpholine,
neat, r.t., 12 h
81%
87%
N
N
N
N
N
N
N
N
N
1
2
3
O
Cl
N
N
N
N
N
O
NH4OAc, DMSO,
120 °C, 12 h
N
+
N
N
31%
4
N
N
4
N
3
Scheme 1 Schematic representation of the synthesis of the hetero-bis(terpyridine) 5 via a selective one-pot Mannich condensation
could be further verified by mass spectrometry (i.e., elec- solubility in comparison to the highly rigid linear deriva-
tron-ionization technique) as well as elemental analysis.
tive 4. Furthermore, the independent synthesis of two or-
thogonal functionalized terpyridines (6 and 7), followed
by a straightforward coupling reaction, might be consid-
ered as the more promising route towards the synthesis of
such hetero-bis(terpyridine)s than the one-pot protocol
discussed above.
In a second approach, the synthesis of the hetero-
bis(terpyridine) 8 was achieved via a Williamson-type
etherification reaction19 of 6 and 7 (Scheme 2). The hy-
droxy-functionalized U-shaped terpyridine 6 (isolated as
a mixture of the mono- and bishydrochloride salts) was
prepared conveniently from the benzyl-protected deriva- However, the increased flexibility of the structure of 8
tive 520 by deprotection under strong acidic conditions.21 goes along with a loss of the p-conjugation over the entire
The bromomethyl-functionalized Kröhnke-type terpyri- molecule. As already pointed out, extended p-conjugation
dine 7 was synthesized according to a literature proce- is a prerequisite for potential applications, for example,
dure.9b The ether-bridged hetero-bis(terpyridine) 8 was functional supramolecular materials as hosts for photoin-
obtained in good yield (66%) and showed an improved duced energy- and electron-transfer processes.7
OBn
OH
4 M HCl,
reflux, 12 h
⋅HCl
41%
N
N
N
N
N
N
5
6
Br
N
N
N
N
K2CO3, DMF,
60 °C, 2 h
O
N
+
6
66%
N
N
N
N
8
7
Scheme 2 Schematic representation of the synthesis of the hetero-bis(terpyridine) 8
Synlett 2010, No. 1, 61–66 © Thieme Stuttgart · New York

LETTER
New Ditopic Ligands Containig 2,2′:6′,2′′-Terpyridine
63
The diverse Pd(0)-catalyzed cross-coupling reactions
were found to be versatile tools for the synthesis of vari-
ous types of linear p-conjugated bis(terpyridine) deriva-
tives.11a,c,d,22 Therefore, the boronic acid functionalized
terpyridine 11 was synthesized adapting previously pub-
lished methods:23 Starting from unprotected 4-formylphe-
nylboronic acid, the general protocol for the one-pot
Kröhnke-type condensation yielded 11 (21%), which was
subsequently used for the Suzuki cross-coupling reaction
[Pd(PPh3)4 as catalyst, aq Na2CO3 as base] with the bro-
mosubstituted U-/S-shaped terpyridines 9 and 10, respec-
tively (Scheme 3, see the Supporting Information for
experimental details).5b The hetero-bis(terpyridine)s 12
[‘tpy-(Ph)2-Utpy’] and 13 [‘tpy-(Ph)2-Stpy’] were isolated
in good yields, and the corresponding 1H NMR spectra are
depicted in Figure 1 (middle and bottom).
In addition, hetero-bis(terpyridine) 12 was utilized as di-
topic ligand for the complexation of RuII ions. Therefore,
the monoterpyridine precursor complex (tpy)RuCl3
(tpy = 2,2¢:6¢,2¢¢-terpyridine) was prepared24 and reacted
Figure 1 1H NMR spectra (500 MHz, CDCl3, 298 K) of hetero-
bis(terpyridine)s 4 (top), 12 (middle) and 13 (bottom)
B(OH)2
Br
N
N
N
N
11
N
Pd(PPh3)4, Na2CO3,
MeOH–toluene,
reflux, 12 h
N
N
N
N
N
2
9
N
12 (51%)
N
Br
N
N
N
N
N
2
N
N
N
13 (62%)
N
10
Scheme 3 Schematic representation of the synthesis of the hetero-bis(terpyridine)s 12 and 13
i) 12 (0.5 equiv),
HOCH2CH2OH,
150 °C, MW, 1 h
ii) NH4PF6
(tpy)RuCl3
[(tpy)Ru(12)Ru(tpy)](PF6)4
67%
N
N
N
Ru2+
N
N
N
N
N
N
Ru2+
N
N
2
N
4 PF6
Scheme 4 Schematic representation of the MW-assisted synthesis of the dinuclear complex [(tpy)Ru(12)Ru(tpy)](PF6)4
Synlett 2010, No. 1, 61–66 © Thieme Stuttgart · New York

64
J. Hummel et al.
LETTER
with 12 in ethylene glycol at 150 °C under microwave ir- complex is occurring selectively. The peak at about
radiation for two hours (Scheme 4). Under these reaction m/z = 1003 might also be a superposition of signals for
conditions no additional agent (e.g., N-ethyl-morpholine) two types of [(tpy)Ru(12)]+ fragments where the RuII ion
for the RuIII/RuII reduction step is required. The dinuclear is either located in the tpy- or in the Utpy-pocket of 12. As
complex [(tpy)Ru(12)Ru(tpy)](PF6)4 was finally obtained an outlook, a comparative study with both symmetric di-
after anion exchange with NH4PF6 as a deep-red powder nuclear complexes [(tpy)Ru(tpy-Ph2-tpy)Ru(tpy)](PF6)4
in 67% yield.25
and [(tpy)Ru(Utpy-Ph2-Utpy)Ru(tpy)](PF6)4 would help
to clarify this observation and, therefore, to learn more
about the differences in stability of the two types of RuII-
terpyridine units within complexes as shown here.
Figure 3 UV/Vis absorption spectrum of the dinuclear complex
[(tpy)Ru(12)Ru(tpy)](PF6)4 (10–6 M in MeCN, r.t.)
Furthermore, the investigation by UV/Vis absorption
spectroscopy revealed the characteristic metal-to-ligand
charge-transfer (MLCT) band for RuII-terpyridine bis-
complexes at 483 nm (Figure 3).1a As expected, no split-
ting of the MLCT band due to the only marginally
different coordination sphere of the RuII centers within the
complex could be observed. The MLCT bands of the cor-
responding mononuclear complexes {i.e., [(tpy)Ru(tpy-
Ph2)](PF6)2 and [(tpy)Ru(Utpy-Ph2)](PF6)2} were report-
ed at 48426 and 483 nm,10 respectively. As known for RuII-
terpyridine complexes1a no emission at room temperature
could be observed.
Figure 2 Top: Experimental (bottom) and calculated (top) MALDI-
TOF mass spectrum of [(tpy)Ru(12)Ru(tpy)](PF6)4. Bottom:
MALDI-TOF MS/MS of [(tpy)Ru(12)Ru(tpy)](PF6)4 (parent peak:
m/z = 1482.27, [M – 3PF6]+). For all spectra: Dithranol was used as
matrix and the dried-droplet method was applied.
In conclusion, we described the synthesis of a set of ditop-
The dinuclear complex was investigated by MALDI-TOF ic hetero-bis(terpyridine) ligands 4, 8, 12, and 13 using
mass spectrometry. The spectrum was recorded in the straightforward coupling and substitution reactions. The
positive ion reflectron mode to verify the complex struc- three different synthetic pathways shown herein turned
ture by comparing the calculated and the measured isoto- out to be highly versatile for the preparation of these new
pic patterns (Figure 2, top). As depicted exemplarily for types of ligands. Our expertise in applying domino-type
the [M – 2PF6]+ fragment, very good agreement of these Mannich reactions allowed us to combine versatile
values could be obtained. Additionally, tandem mass Kröhnke-type as well as U-/S-shaped terpyridines within
spectrometry (MS/MS, LIFTTM mode) was performed to one molecule. The combination of both motives within
gain further insight into the fragmentation behavior of the one molecule may result in a selectivity of binding transi-
–
complex. The loss of all PF6 counterions was observed as tion-metal ions which is an important factor in the self-
well as the fragmentation of one complex moiety. Appar- assembly process towards supramolecular structures. Fur-
ently, the disassembly of the second complex unit could ther investigations of the complexation behavior of 4, 8,
not be achieved under the applied MS/MS conditions. and 12 are currently ongoing. Furthermore, the ditopic
However, we cannot yet conclude if cleavage of only one ligand 13, equipped with a S-shaped terpyridine, features
type of RuII-terpyridine moieties within the unsymmetric three different binding sites (i.e., a terpyridine, a bipyri-
Synlett 2010, No. 1, 61–66 © Thieme Stuttgart · New York

LETTER
New Ditopic Ligands Containig 2,2′:6′,2′′-Terpyridine
65
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
Financial support of this work by the Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (VICI award for U.S.S.), the Dutch
Polymer Institute (DPI), the Thüringer Kultusministerium (Grant
TKM 3715-08011), and the Deutsche Forschungsgemeinschaft
(DFG) is kindly acknowledged.
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(25) Synthesis of [(tpy)Ru(12)Ru(tpy)](PF6)4
A suspension of(tpy)RuCl3 (20.3 mg, 0.046 mmol)23 and 12
(15.0 mg, 0.022 mmol) in ethylene glycol (10 mL) was
heated under microwave irradiation at 150 °C for 2 h. The
reaction mixture was poured into H2O (10 mL) and filtered.
Subsequently, an excess of NH4PF6 (50 mg) was added, and
the formed dark red precipitate was isolated. The crude
product was purified by preparative size exclusion chroma-
tography (BioBeads SX-3, acetone as eluent) to yield the
dinuclear complex (28 mg, 67%). 1H NMR (400 MHz,
CD3CN): d = 8.97 (s, 2 H), 8.72 (mc, 4 H), 8.55 (mc, 2 H),
8.42 (d, 3J = 8.4 Hz, 2 H), 8.38 (mc, 4 H), 8.11 (d, 3J = 8.4
Hz, 2 H), 8.06 (mc, 2 H), 7.93 (mc, 6 H), 7.82–7.52 (m, 10
H), 7.37–7.21 (m, 8 H), 7.14 (mc, 2 H), 6.95 (mc, 2 H), 3.35–
3.02 (m, 8 H) ppm. MALDI-TOF MS (matrix: dithranol):
m/z = 1627.40 [M – 2PF6]+, 1482.27 [M – 3PF6]+, 1002.99
[(tpy)Ru(12)]+, 768.19 [Ru(12)]+. Anal. Calcd for
(20) Keuper, R.; Risch, N.; Flörke, U.; Haupt, H.-J. Liebigs Ann.
1996, 5, 705.
(21) Kelly, T. R.; Lebedev, R. L. J. Org. Chem. 2002, 67, 2197.
(22) Winter, A.; Friebe, C.; Chiper, M.; Hager, M. D.; Schubert,
U. S. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4083.
(23) (a) Jarozs, P.; Lotito, K.; Schneider, J.; Kumaresan, D.;
Schmehl, R.; Eisenberg, R. Inorg. Chem. 2009, 48, 2420.
(b) Aspley, C. J.; Williams, J. A. G. New J. Chem. 2001, 25,
1136.
C76H54F24N12P4Ru2: C, 47.61; H, 2.84; N, 8.77. Found:
C, 47.92; H, 3.11; N, 8.92.
(26) Alcock, N. W.; Barker, P. R.; Haider, J. M.; Hannon, M. J.;
Painting, C. L.; Pikramenou, Z.; Plummer, E. A.; Rissanen,
K.; Saarenketo, P. J. Chem. Soc., Dalton Trans. 2000, 1447.
(27) A recent paper deals with flexible terpyridine and bipyridine
containing binary ligands for complexation of cobalt(II)
ions: Kon, H.; Nagata, T. Inorg Chem. 2009, 48, 8593.
(24) Llobet, A.; Doppelt, P.; Meyer, T. J. Inorg. Chem. 1988, 27,
514.
Synlett 2010, No. 1, 61–66 © Thieme Stuttgart · New York

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