Anthracene functionalized terpyridines - Synthesis and properties

Article abstract of DOI:10.3762/bjoc.6.54

The synthesis of several symmetrically 4,4"-functionalized 2,2':6',2"-terpyridines is reported. In addition to the biscarboxylic acid 4,4"-tpy(CO₂H)₂ (3), the anthryl esters 4,4"-tpy(CO₂CH₂Anth)₂ (5a)

Full text of DOI:10.3762/bjoc.6.54

Anthracene functionalized terpyridines – synthesis
and properties
Falk Wehmeier2 and Jochen Mattay*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, No. 54. doi:10.3762/bjoc.6.54
1Department of Chemistry, Organic Chemistry 1, Bielefeld University,
P. O. Box 10 01 31, D-33501 Bielefeld, Germany and 2Institut für
Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489
Berlin, Germany
Received: 03 March 2010
Accepted: 11 May 2010
Published: 27 May 2010
Email:
Editor-in-Chief: J. Clayden
Jochen Mattay* - oc1jm@uni-bielefeld.de
© 2010 Wehmeier and Mattay; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
anthracene; coordination chemistry; photochemistry; terpyridine
Abstract
The synthesis of several symmetrically 4,4″-functionalized 2,2′:6′,2″-terpyridines is reported. In addition to the biscarboxylic acid
4,4″-tpy(CO2H)2 (3), the anthryl esters 4,4″-tpy(CO2CH2Anth)2 (5a) and 4,4″-tpy(CO2CH2CH2OAnth)2 (5b) were synthesized.
Furthermore, both anthryl esters were used to synthesize symmetric iron(II)-bis(terpyridine)complexes 6a–b. Irradiation experi-
ments were carried out with both the free ligands and an iron(II)-complex in order to investigate the photochemistry of the com-
pounds.
Introduction
2,2′:6′,2″-Terpyridines have been of great interest over the Because of this impact of terpyridine derivatives in photochem-
recent years, principally because of their ability to chelate tran- istry, we focused our attention on the synthesis and studies of
sition metals. The special (photochemical) properties of their potentially photoswitchable terpyridine ligands. The synthesis
metal complexes have led to the development of various lumin- of bisterpyridines linked by a diazogroup has been reported
escent metal compounds [1] and sensitizers for photovoltaic [7,8]. As well as the connection of terpyridines to spiropyran
devices [2,3]. Ditopic terpyridyl units have been recently used moieties [9] and diarylethenes [10,11]. There have also been
to develop electrochemical sensors [4,5]. A microreview reports about anthracene functionalized terpyridines in which an
concerning the synthesis of functionalized terpyridines has also anthracene unit was used as a fluorescent sensor [12], spacer
been published, since the electronic properties of the ligand are [13] or intercalator [14]. Herein we report the synthesis of two
influenced by the substituents present [6].
twofold anthracene functionalized terpyridines, their iron(II)
complexes, and investigations regarding their photochemistry.
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Beilstein J. Org. Chem. 2010, 6, No. 54.
Scheme 1: Synthesis of terpyridine-4,4″-dicarboxylic acid (3).
Results and Discussion
Synthesis of anthracene functionalized
UV–vis spectra and irradiation experiments
Free ligands
The free ligands 5a and 5b were investigated with respect to
their UV–vis absorption spectra and their ability to undergo a
[4 + 4]-cycloaddition reaction. A degassed 5·10−5 × M solution
of 5a in methylene chloride was irradiated with UV light (λ =
350 nm, Figure 1).
terpyridines
We synthesized the terpyridine-4,4″-diacid 3, starting from
methyl 2-acetylisonicotinate (1) and 4-tert-butylbenzaldehyde
(2), by a one-pot combination of a Kröhnke type condensation
and subsequent hydrolysis of the ester groups, similar to a syn-
thetic route recently described in the literature (Scheme 1) [15].
The absorption spectrum of 5a (solid line) shows the expected
The diacid 3 was used as a precursor for esterification reactions absorption pattern for a compound containing anthryl residues.
with suitable anthryl alcohols (4a–b). Thus, Mukaiyama acid Upon UV irradiation the absorption decreases which is indica-
activation of 3 with 2-chloro-N-methylpyridinium iodide in the tive of cycloaddition of the anthryl moieties (dashed lines).
presence of triethylamine with an excess of the anthryl alcohol There are however, no indications of any cyclo-reversion,
(3–5 equiv) gave the desired terpyridine-4,4″-esters 5a and 5b neither thermally nor upon irradiation with visible light. Similar
in yields up to 75% (Scheme 2).
results were also obtained with compound 5b (Figure 2).
Synthesis of Fe(II)-complexes
A degassed 4 × 10−5 M solution of 5b in methylene chloride
We then synthesized the iron(II)-complexes [Fe(5a)2](PF6)2 was irradiated and the originally observed absorption pattern
(6a) and [Fe(5b)2](PF6)2 (6b) by reaction of ferric chloride due to the anthryl moieties decreased. As was the case with 5a,
tetrahydrate with the corresponding terpyridines in methanol the absorption pattern due to the anthryl moieties could not be
solution (Scheme 3).
regenerated thermally or by irradiation.
Scheme 2: Synthesis of the terpyridine-4,4″-bisanthrylesters 5a and 5b. The resulting esters 5a and 5b could be isolated in good yields as pale
yellow solids.
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Beilstein J. Org. Chem. 2010, 6, No. 54.
Scheme 3: Synthesis of the iron(II)-complexes 6a and 6b. The complexes 6a and 6b were obtained in high yields as dark blue solids.
NMR- and MALDI-TOF-measurements were of no assistance peaks (especially in the aromatic region) made assignments
in establishing the identity of the photoproducts from 5a and 5b impossible. Irradiation experiments were also carried out on the
after UV irradiation. 1H NMR-spectra of the irradiation NMR scale – however, the required reagent concentration
products (after evaporation of solvent) indicate that there is (10−5 mol/L for intramolecular cycloadditions) made it
more than one reaction pathway, the number and overlapping of impossible to record useful NMR spectra. MALDI-TOF meas-
Figure 1: UV–vis spectra of 5a before and after irradiation with UV
Figure 2: UV–vis spectra of 5b before and after irradiation with UV
light.
light.
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Beilstein J. Org. Chem. 2010, 6, No. 54.
urements did not indicate any degradation products – the A possible explanation for the observed irreversible photoreac-
[4 + 4]-cycloadducts cannot be distinguished from the reagents, tion may be an unsymmetrical [4 + 4]-cycloaddition, connecting
because their molecular formulas and hence their molecular the anthracene moieties via the 1,4:9′,10′- instead of the
masses are identical.
9,10:9′,10′-positions. The line-shape and the absorption increase
for λ > 320 nm during irradiation (see Figure 1 and Figure 2) is
indicative of the formation of a naphthalene derivative; in case
Irradiation of a Fe(II)-complex
To see whether the presence of iron(II) influences the photo- of a symmetrical 9,10:9′,10′-cycloaddition we would expect
chemistry of the ligands 5a and 5b, we irradiated the iron(II) absorption patterns at lower wavelengths, since isolated
complex 6b in acetonitrile solution with near-UV light (λ = benzene derivates would be produced [16-19].
420 nm, Figure 3).
Experimental
General: 4-tert-Butylbenzaldehyde (2) and anthracen-9-
ylmethanol (4a) are commercially available and were used as
supplied. Methyl 2-acetylisonicotinate (1) [20,21] and
2-(anthracen-9-yloxy)ethanol (4b) [22] were synthesized
according to previously reported procedures. Solvents and
chemicals were dried by standard procedures.
Irradiation experiments were carried out in a quartz cuvette (d =
1 cm); the solutions were thoroughly degassed before irradi-
ation. UV–vis spectra were recorded with a Lambda 40 (Perkin-
Elmer) at room temperature. NMR spectra were recorded with a
Bruker DRX 500 or a Bruker Avance 600. EI mass spectra were
recorded with an Autospec X (Vacuum Generators), ESI mass
spectra were recorded with a Bruker Esquire 3000. High resolu-
tion mass spectra were recorded with a Bruker Apex III-FT-
ICR. Measured and calculated masses are true ion masses,
taking into account the mass of lost (or added) electrons.
Synthesis of 4′-(4-tert-butylphenyl)-2,2′:6′,2″-
Figure 3: UV–vis spectra of 6b before and after irradiation with UV
light.
terpyridine-4,4″-biscarboxylic acid (3)
10.0 g (56 mmol) Methyl 2-acetylisonicotinate (1) and 4.5 g (28
mmol) 4-tert-butylbenzaldehyde (2) were dissolved in 600 mL
The MLCT band at λ = 585 nm does not decrease or change its methanol and 3.4 g (84 mmol) sodium hydroxide and 150 mL
position during irradiation, whilst the bands due to the anthryl (2.2 mol) 25% aqueous ammonia were added. The mixture was
moieties in the near UV region decrease. As was the case for heated at reflux for 20 h. The resulting suspension was cooled
the free ligands, regeneration of the anthryl bands was not to rt, the precipitate collected by filtration and subsequently
observed.
dissolved in hot water. After the solution cooled to rt, hydro-
chloric acid (37%) was added until pH < 3. The precipitate was
collected by filtration to yield 3.0 g (6.7 mmol, 24%) of 4′-(4-
Conclusion
Whilst the synthesis of the target molecules 5a and 5b and their tert-butylphenyl)-2,2′:6′,2″-terpyridine-4,4″-biscarboxylic acid
iron(II)-complexes (6a–b) was successful, investigations (3) as a colorless solid. 1H NMR (DMSO-d6, 500 MHz, δ in
regarding their photochemistry were unsatisfactory: Apparently, ppm): 1.32 (s, 9 H, t-Bu), 4.00 (br s, CO2H), 7.56 (d, 3J = 8.5
the anthryl moieties do not undergo the desired [4 + 4]-cycload- Hz, 2 H, ArPh-H3,5), 7.83 (d, 2 H, ArPh-H2,6), 7.94 (dd, 3J = 5.0
dition reaction and the photoproducts could not be identified. Hz, 4J = 1.25 Hz, 2 H, Artpy-H5,5″), 8.72 (s, 2 H, Artpy-H3′,5′),
Further irradiation experiments with the iron(II)-complex 6b 8.93 (d, 2 H, Artpy-H6,6″), 8.97 (s, 2 H, Artpy-H3,3″); 13C NMR
showed that the coordination properties of the terpyridine are (DMSO-d6, 125 MHz, δ in ppm): 31.1 (C(CH3)3), 34.6
not influenced by the photochemistry of the anthryl residues. (C(CH3)3), 118.7 (tpy3,5), 120.0 (tpy3,3″), 123.7 (tpy5,5″), 126.3
Hence, the reported anthracene functionalized terpyridines do (Ph3,5), 126.7 (Ph2,6), 134.1 (tpy4′), 140.0 (tpy4,4″), 149.7 (Ph1),
not appear to be suitable building blocks for photochromic 150.3 (tpy6,6″), 152.5 (Ph4), 154.8 (tpy2,2″), 155.6 (tpy2′,6′),
supramolecular structures.
166.0 (CO2H); MS (EI, 70 eV, m/z, %): 350 (38, [M-2CO2-
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Beilstein J. Org. Chem. 2010, 6, No. 54.
CH3]+), 365 (34, [M-2CO2]+), 394 (100, [M-CO2-CH3]+), 409 acid (1 M), dried over MgSO4 and filtered. After removal of the
(65, [M-CO2]+), 438 (78, [M-CH3]+), 453 (37, [M]+). HRMS: solvent under reduced pressure, an orange solid remained which
calc. for C27H23N3O4: 454.17613; found: 454.17579.
was purified by column chromatography (basic alumina, eluting
with chloroform) to yield 517 mg (579 μmol, 75%) of 4′-(4-tert-
butylphenyl)-2,2′:6′,2″-terpyridine-4,4″-bis(2-(anthracen-9-
yloxy)ethyl)carboxylate (5b) as a pale yellow solid. 1H NMR
(THF-d8, 500 MHz, δ in ppm): 1.42 (s, 9 H, t-Bu), 4.43 (m, 4
H, -CO2CH2-), 4.48 (m, 4 H, -CH2-OArAnth), 7.23 (m, 4 H,
ArAnth-H3,6), 7.30 (m, 4 H, ArAnth-H2,7), 7.64 (d, 3J = 8.2 Hz, 2
H, ArPh-H3,5), 7.82 (d, 3J = 8.8 Hz, 4 H, ArAnth-H4,5), 7.88 (dd,
3J = 5.0 Hz, 4J = 1.3 Hz, 2 H, Artpy-H5,5″), 7.93 (d, 2 H, ArPh-
H2,6), 8.10 (s, 2 H, ArAnth-H10), 8.37 (d, 4 H, ArAnth-H1,8), 8.92
(d, 3J = 5.0 Hz, 2 H, Artpy-H6,6″), 8.96 (s, 2 H, Artpy-H3′,5′),
9.15 (s, 2 H, Artpy-H3,3″); 13C NMR (125 MHz, THF-d8, δ in
ppm): 31.6 (-C(CH3)3), 35.4 (-C(CH3)3), 65.9 (-CO2CH2-),
73.9 (-CH2-O-ArA), 119.7 (tpy3′,5′), 121.5 (tpy3,3″), 123.0
(Anth1,8), 123.2 (Anth10), 123.7 (tpy5,5″), 125.6
(Anthortho to 1,8), 126.1 (Anth2,3,6,7), 127.0 (Ph2,6), 127.7
(Ph3,5), 129.1 (Anth4,5), 133.4 (Anthmeta to 1,8), 136.4 (tpy4′),
139.2 (tpy2,2″), 150.8 (tpy6,6″), 151.3 (Ph1), 151.6 (Anth9),
153.3 (Ph4), 156.3 (tpy4,4″), 157.9 (tpy2′,6′), 165.1 (-CO2CH2-);
ESI-MS (positive mode, CHCl3/MeOH 3:1, m/z): 894 [M]+;
HRMS: calc. for [C59H47N3O6]+: 894.35376; found:
894.35203.
Synthesis of 4′-(4-tert-butylphenyl)-2,2′:6′,2″-
terpyridine-4,4″-bis(anthracen-9-
ylmethyl)carboxylate (5a)
833 mg (4.0 mmol) Anthracen-9-ylmethanol (4a) and 760 mg
(3.0 mmol) 2-chloro-N-methylpyridinium iodide were added to
a solution of 450 mg (1.0 mmol) 4′-(4-tert-butylphenyl)-
2,2′:6′,2″-terpyridine-4,4″-biscarboxylic acid (3) in 20 mL
methylene chloride at rt. Subsequently, a solution of 0.45 mL
(3.2 mmol) triethylamine in 10 mL methylene chloride was
added dropwise, the mixture stirred at rt for 24 h and then
poured into 50 mL hydrochloric acid (1 M). The layers were
separated, the organic layer washed with 50 mL hydrochloric
acid (1 M), dried over MgSO4 and filtered. After removal of the
solvent under reduced pressure, an orange solid remained which
was purified by column chromatography (basic alumina, eluting
with chloroform) to yield 419 mg (502 μmol, 50%) of 4′-(4-tert-
butylphenyl)-2,2′:6′,2″-terpyridine-4,4″-bis(anthracen-9-
ylmethyl)carboxylate (5a) as a pale yellow solid. 1H NMR
(THF-d8, 600 MHz, δ in ppm): 1.36 (s, 9 H, t-Bu), 6.57 (s, 4 H,
-CO2CH2Anth), 7.46 (m, 4 H, ArAnth-H3,6), 7.55 (d, 3J = 8.3
Hz, 2 H, ArPh-H3,5), 7.59 (m, 4 H, ArAnth-H2,7), 7.77 (d, 3J =
5.0 Hz, 2 H, Artpy-H5,5″), 7.80 (d, 2 H, ArPh-H2,6), 8.06 (d, 3J =
8.3 Hz, 4 H, ArAnth-H4,5), 8.60 (s, 2 H, ArAnth-H10), 8.62 (d, 4
H, ArAnth-H1,8), 8.74 (d, 3J = 4.5 Hz, 2 H, Artpy-H6,6″), 8.81 (s,
2 H, Artpy-H3′,5′), 9.29 (s, 2 H, Artpy-H3,3″); 13C NMR (THF-d8,
151 MHz, δ in ppm): 30.9 (-C(CH3)3), 34.7 (-C(CH3)3), 60.2
(-CO2CH2Anth), 119.2 (tpy3′,5′), 120.5 (tpy3,3″), 123.1 (tpy5,5″),
124.5 (Anth1,8), 125.3 (Anth10), 126.2 (Anthortho to 1,8), 126.6
(Anth9), 126.9 (Anth), 127.0 (Anth), 129.2 (Ph3,5), 129.6
(Ph2,6), 131.7 (Anthmeta to 1,8), 132.0 (Anth4,5), 135.7 (tpy4′),
139.1 (tpy2,2″), 150.3 (tpy6,6″), 150.5 (Ph1), 152.6 (Ph4), 155.9
(tpy4,4″), 157.5 (tpy2′,6′), 165.2 (-CO2CHAr); MS (ESI, positive
mode, CHCl3/MeOH 3:1, m/z): 834 [M]+; HRMS: calc. for
[C57H43N3O4]+:834.33263; found: 834.33389.
Synthesis of bis-(4′-(4-tert-butylphenyl)-
2,2′:6′,2″-terpyridine-4,4″-bis(anthracen-9-
ylmethyl)-carboxylate)-iron(II)-hexafluoro-
phosphate (6a)
150 mg (180 μmol) 4′-(4-tert-Butylphenyl)-2,2′:6′,2″-
terpyridine-4,4″-bis(anthracen-9-ylmethyl)carboxylate (5a)
suspended in 20 mL methanol and subsequently treated with a
solution of 18 mg (90 μmol) iron(II)chloride tetrahydrate in
10 mL methanol. The mixture was stirred for 6 h at rt. The color
changed to deep blue after several minutes. 200 mg
(1.23 mmol) Ammonium hexafluorophosphate was added and
the mixture stirred for a further 30 min. The blue precipitate
was collected by centrifugation and purified by column
chromatography (silica gel, eluting with acetonitrile/water/sat.
aq. KPF6-solution 95:4:1). After removal of the solvent, a dark
blue solid remained which was characterized as bis-(4′-(4-tert-
butylphenyl)-2,2′:6′,2″-terpyridine-4,4″-bis(anthracen-9-
ylmethyl)-carboxylate)-iron(II)-hexafluorophosphate (6a).
Yield: 153 mg (76 μmol, 84%). 1H NMR (500 MHz, CD3CN, δ
in ppm): 1.44 (s, 18 H, tBu), 6.35 (s, 8 H, -CO2CH2Anth), 7.07
(m, 4 H, ArPh-3,5), 7.24 (m, 4 H, Artpy-H6,6″), 7.49-7.54 (m, 16
H, ArAnth-H2,3,6,7), 7.74 (m, 4 H, ArPh-H2,6), 8.08 (m, 12 H,
ArAnth-H1,8/Artpy-H5,5″), 8.35 (m, 8 H, ArAnth-H4,5), 8.62 (s, 4
H, Artpy-H3,3″), 8.84 (s, 4 H, ArAnth-H10), 9.10 (s, 4 H, Artpy-
H3′,5′); ESI-MS (positive mode, MeCN, m/z): 191.1
Synthesis of 4′-(4-tert-butylphenyl)-2,2′:6′,2″-
terpyridine-4,4″-bis((2-anthracen-9-
yloxy)ethyl)-carboxylate (5b)
736 mg (3.1 mmol) 2-(Anthracen-9-yloxy)ethanol (4b) and
590 mg (2.3 mmol) 2-chloro-N-methylpyridinium iodide were
added to a solution of 350 mg (772 μmol) 4′-(4-tert-butyl-
phenyl)-2,2′:6′,2″-terpyridine-4,4″-biscarboxylic acid (3) in
20 mL methylene chloride at rt. Subsequently, a solution of
0.35 mL (2.5 mmol) triethylamine in 10 mL methylene chloride
was added dropwise, the mixture stirred at rt for 24 h and then
poured into 50 mL hydrochloric acid (1 M). The layers were
separated, the organic layer washed with 50 mL hydrochloric
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Beilstein J. Org. Chem. 2010, 6, No. 54.
[AnthCH2]+, 861.7 [M-2 PF6]2+, 1341.4 [M-2 PF6-2 AnthCH]+, Acknowledgements
1532. 5 [M-2 PF6 -AnthCH]+ ; HRMS: calc. for We would like to thank the Studienstiftung des deutschen
[C114H86FeN6O8]2+ ([Fe(5a)2]): 860.29462; found: 860.29320. Volkes e.V. for financial support.
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Supporting Information
Supporting information contains 1H NMR spectra of all
compounds described in the experimental section (3, 5a–b,
6a–b) and 13C NMR spectra of the organic precursors (3,
5a–b).
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Supporting Information File 1
1H NMR spectra of compounds 3, 5a–b, 6a–b, and 13C
NMR spectra of the organic precursors 3, 5a–b
[http://www.beilstein-journals.org/bjoc/content/
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