4,4″-Azobis[2,2′:6′,2″-terpyridine] and its metal complexes

Article abstract of DOI:10.1002/1522-2675(20010321)84:3<715::AID-HLCA715>3.0.CO;2-T

By two different routes, 4,4″″-azobis[2,2′: 6′,2″;-terpyridine] was synthesized. Its ruthenium complexes show interesting metal-to-ligand charge transfer (MLCT) absorption maxima in the electronic spectra. They represent the first ruthenium complexes of terpyridine units to give blue solutions.

Full text of DOI:10.1002/1522-2675(20010321)84:3<715::AID-HLCA715>3.0.CO;2-T

Helvetica Chimica Acta ± Vol. 84 (2001)
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4,4''''-Azobis[2,2':6',2''-terpyridine] and its Metal Complexes
by Reza-Ali Fallahpour* and Markus Neuburger
Institut für Anorganische Chemie, Universität Basel, Spitalstrasse 51, CH-4056 Basel
(e-mail: Reza.Fallahpour@unibas.ch)
By two different routes, 4,4''''-azobis[2,2':6',2''-terpyridine] was synthesized. Its ruthenium complexes show
interesting metal-to-ligand charge transfer (MLCT) absorption maxima in the electronic spectra. They
represent the first ruthenium complexes of terpyridine units to give blue solutions.
Dendrimers, in general, and metallodendrimers, in particular, are a subject of great
interest [1]. Oligopyridines incorporated in metallodendrimers are mainly based on
2,2'-bipyridine (bpy) and 2,2':6',2''-terpyridine (tpy) and can be synthesized conver-
gently or divergently. Bpy ligands have been attached to oligoamines [2] or to the
Â
Frechet-type dendrimers [3]. In the terpyridine series, most of the metallodendrimers
contain spacers such as phenylene [4] or acetylene groups [5]. Alternatively, the tpy
units have been attached together or to a bpy ligand with an ether bridge. This
chemistry has been extensively investigated [6].
There exist, to the best of our knowledge, only a few examples of metal complexes
Â
or dendrimers containing an azo group as the spacer. In one case, a Frechet-type
dendrimer was attached to azobenzene to investigate the isomerization from the (E)-
to the (Z)-form [7]. In the other case, Jiang and Aida found that a large aryl ether
azodendrimer undergoes (Z)- to (E) isomerization at the interior azo unit upon
exposure to weak infrared (IR) light with a wave number of ca. 1600 cm 1 [8]. The 4,4''-
azo-bis[2,2'-bipyridine] has also been synthesized, and some of its metal complexes
were reported [9][10]. Alternatively, azobenzene was attached to the surface of
dendrimers to obtain photoswitchable supramolecular systems [11][12]. Recently,
Nishihara and co-workers reported the synthesis of azobenzene-bridged bis[terpyr-
idine] ligand and some of its rhodium complexes [13].
We report here the synthesis of ruthenium complexes containing the azo group as a
spacer directly linked to the two tpy units. We have already reported the synthesis of 4'-
nitro-2,2':6',2''-terpyridine (1) [14][15] which is of interest due to the versatile synthetic
accessibility of the nitro group. While the nitro group can easily be reduced to an amine
[14][15], it can also be reduced to the azo compound 2 by relatively weak reducing
agents such as NaBH4 [16] or SnCl2 ´ 2 H2O [17] in 40 and 33% yields, respectively
(Scheme 1).
Alternatively, 2,6-dibromo-4-nitropyridine (3) was reduced to the azo compound 4,
which was then converted to 2 by a 4-fold Stille coupling reaction [18]. The structure of
4 was determined by X-ray crystal-structure analysis and was found to have the (E)-

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Helvetica Chimica Acta ± Vol. 84 (2001)
Scheme 1
i) NaBH4, EtOH, 1 h, 788, 40%. ii) SnCl2 ´ 2 H2O, H2O, 1008, 2 h; 49%. iii) (2-Pyridyl)SnBu3 (4 mol-equiv.),
[Pd(PPh3)4] (0.08 mol-equiv.), toluene, 1108, 16 h; 45%.
configuration (Fig. 1)1). Compound 4 is a planar molecule with both pyridine rings in
almost the same plane.
The azo compound 2 is an orange microcrystalline solid that gives a green complex
upon reaction with Fe2‡. In the 1H-NMR spectrum of 2, five signals were observed. The
signal for H C(3') at d 9.00 is more shifted to lower field than in 4'-(methylsulfonyl)-
2,2':6',2''-terpyridine (d 8.88) [14] or 4'-chloro-2,2':6',2''-terpyridine (d 8.48) [14],
indicating the electron-withdrawing ability of the azo group. Only the nitro group
causes a shift to still lower field (d 9.17) [15].
1
)
Crystal-structure determination of C10H4Br4N4: crystal size 0.09 Â 0.32 Â 0.40 mm, Mr 499.806; monoclinic,
space group P21/c, a ˆ 4.4862(5), b ˆ 7.5506(6), c ˆ 19.7032(15) Š, Vˆ 665.4(1) Š3, F(000) ˆ 464, Z ˆ 2,
Dc ˆ 2.49 g cm 3; unit-cell parameters were determined by the least-squares method on 25 independent
reflections. Data collection was carried out on a four-circle Enraf-Nonius-CAD4 diffractometer with
monochromated MoKa radiation (l 0.71069 Š); T 293 K. The w-2q technique was used to measure 1809
reflections, range 28 ꢀ q ꢀ 30.448. Three standard reflections monitored every hour during data collection
showed a decay of 4.72%. To solve and refine the structure, 1585 unique data with I ꢁ 3s(I) were used. The
usual corrections were applied. The absorption correction was determined by means of Y scans; max/min
transmission 1.0/0.23. The structure was solved by direct methods with the program SIR92 [19]. Anisotropic
least squares refinement was carried out on all non-H-atoms with the program CRYSTALS [20]. The H-
atoms are in calculated positions with a fixed distance of 0.96 Š: scattering factors were taken from the
International Tables for X-Ray Crystallography [21].

Helvetica Chimica Acta ± Vol. 84 (2001)
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Fig. 1. Solid-state crystal structure of 4
Two ruthenium complexes of ligand 2 were prepared by standard methods [15], to
exemplify its reactivity and properties. Compound 2 was reacted with (2,2':6',2''-
terpyridin-4'-ol)ruthenium trichloride ([Ru(HO-tpy)Cl3]) [15] and (4'-chloro-
2,2':6',2''-terpyridine)ruthenium trichloride ([Ru(Cl-tpy)Cl3] [15]) in MeOH to give
the two complexes 5 and 6 in 39 and 53% yield, respectively (Scheme 2). We assume
that both complexes 5 and 6 contain the (E)-azo function due to steric hinderance. This
view is supported by the fact that 4, a precursor of 2, exhibits already the (E)-
1
configuration. Both complexes are symmetric and exhibit ten signals in the H-NMR
spectra, five for each of the ligands 2 and 2,2':6',2''-terpyridin-4'-ol or 4'-chloro-
2,2':6',2''-terpyridine, respectively. In complex 5, the H C(3''')2) signal of the
2,2':6',2''-terpyridin-4'-ol ligand was observed at d 8.34, while in complex 6,
H
C(3''')2) appeared at d 8.91, which is consistent with the electron-releasing (OH)
and -withdrawing (Cl) groups. The H C(3') signal of the azobis[terpyridine] ligand
was observed at d 9.37 (5) and 9.44 (6), respectively (Fig. 2).
In the 1H-NMR spectra, the numbering of the ligands 2,2':6',2''-terpyridin-4'-ol and 4'-chloro-2,2':6',2''-
terpyridine of 5 and 6, respectively, is arbitrary (see Scheme 2).
2
)

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Helvetica Chimica Acta ± Vol. 84 (2001)
Scheme 2
i) [Ru(HO-tpy)Cl3], N-ethylmorpholine, MeOH, 648, 1 h; 5 (39%). [Ru(Cl-tpy)Cl3], N-ethylmorpholine,
MeOH, 648, 1 h; 6 (53%).
Usually, ruthenium complexes of tpy ligands are red to orange and exhibit the
metal-to-ligand charge transfer (MLCT) band in the range 460 ± 550 nm. Compared
with that, the MLCT absorption in the electronic spectra of the complexes 5 and 6 was
shifted to lower energy and appeared at 633 and 578 nm, respectively (Fig. 3). The p-p
absorption of the azo group was observed in both complexes at 470 nm. Complexes 5
and 6 are the first examples of blue ruthenium(II) complexes containing tpy ligands.
Ruthenium(II) complexes 5 and 6 are electrochemically oxidized in MeCN
solution, each exhibiting an oxidation wave corresponding to the RuII/RuIII process.
The potential of the ruthenium(II) complexes 5 and 6 (vs. ferrocene/ferrocenium
internal reference) are 0.886 and 0.975 V, respectively, which are in accord with other
ruthenium complexes of tpy ligands.
In conclusion, we have synthesized 4,4''''-azobis[2,2':6',2''-terpyridine] (2) and
prepared some metal complexes. The azo (E)/(Z) isomerism of the metal complexes
may be used as a redox-responsive molecular switch in the photochemistry (cf. [22]).
Therefore, the isomerization and photochemistry of 2 and its complexes are currently
under investigation in our group.
We would like to thank Prof. Edwin C. Constable for his generous support and Prof. Catherine Housecroft
for critical reading of the manuscript. We should also like to thank the Swiss National Science Foundation and
the University of Basel for support.
Experimental Part
General. All reagents were used as supplied. Column chromatography (CC): silica gel (0.060 ± 0.200 mm)
from Chemie Uetikon, aluminium oxide (type 507 C neutral; 100 ± 125 mesh) from Fluka. Electro-chemical
measurements: Ecochemie-Autolab-PGSTAT-20 Potentiostat. M.p.: Büchi 535 apparatus; not corrected. UV/
VIS Spectra: Perkin-Elmer Lambda-19 spectrometer; lmax (log e) in nm. NMR Spectra: Bruker AM-250
spectrometer; d in ppm rel. to SiMe4, J in Hz. MALDI-TOF-MS: Perspective-Biosystems Voyagers-RP
biospectrometry workstation; m/z.

Helvetica Chimica Acta ± Vol. 84 (2001)
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Fig. 2. 1H-NMR Spectra (CD3CN) of the RuII complexes a) 5 and b) 62)
4,4''''-Azobis[2,2':6',2''-terpyridine] (2).
A suspension of 4'-nitro-2,2':6',2''-terpyridine (1; 100 mg,
0.36 mmol) and NaBH4 (40 mg, 1.08 mmol) in EtOH (20 ml) was heated under reflux for 1 h. After cooling
to r.t., EtOH was evaporated and H2O (30 ml) added. The aq. phase was extracted with CH2Cl2 (3 Â 30 ml) and
the combined org. phase dried (MgSO4) and evaporated. CC (aluminium oxide, CH2Cl2/AcOEt 1:1) followed
by recrystallization in EtOH gave 50 mg (40%) of 2. Yellow microcrystalline solid. M.p. 303 ± 3048. UV
(MeCN): 243 (4.20), 281 (4.22), 350 (sh, 3.50); min. 222 (4.08), 261 (4.12). 1H-NMR (CDCl3): 9.00 (s, 4 H,
H
H
C(3')); 8.79 (d, J ˆ 7.80, 4 H, H C(6)); 8.69 (d, J ˆ 7.80, 4 H, H C(3)); 7.91 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H,
C(4)); 7.40 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(5)). 13C-NMR (CDCl3): 159.53, 157.53, 149.33, 146.10, 137.03,
‡
124.18, 121.44, 114.18. MALDI-TOF-MS: 492 (M ). Anal. calc. for C30H20N8: C 73.16, H 4.09, N 22.75; found:
C 73.20, H 4.90, N 22.39.
4,4'-Azobis[2,6-dibromopyridine] (4). To a soln. of tin(II) chloride dihydrate (1.68 g, 7.44 mmol) in 10%
KOH soln. (50 ml), 2,6-dibromo-4-nitropyridine (3; 1.0 g, 3.55 mmol) was added, and the mixture was heated
for 2 h under reflux. The aq. phase was then extracted with CH2Cl2 (6 Â 50 ml) and the combined org. phase

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Helvetica Chimica Acta ± Vol. 84 (2001)
Fig. 3. UV/VIS Spectra (CD3CN) of the RuII complexes a) 5 and b) 6
dried (MgSO4) and evaporated. CC (silica gel, CH2Cl2/hexane 3 :2) followed by recrystallization in EtOH gave
400 mg (45%) of 4. Orange crystals. M.p. 1458. 1H-NMR (CDCl3): 7.91 (s, 4 H). 13C-NMR (CDCl3): 158.35,
‡
142.10, 120.33. MALDI-TOF-MS: 500 (M ). Anal. calc. for C10H4Br4N4: C 24.03, H 0.81, N 11.21; found:
C 24.53, H 1.32, N 11.33.
Ruthenium(II) Complexes 5 and 6: General Procedure. Azo-bis[terpyridine] 2 (20 mg) and N-ethyl-
morpholine (0.1 ml) were added to a suspension of [Ru(Cl-tpy)Cl3] or [Ru(HO-tpy)Cl3] (2 mol-quiv.) in
MeOH (20 ml), and the mixture was heated for 1 h. The resulting soln. was filtered through Celite, and then
excess ammonium hexafluorophosphate was added to the filtrate. The precipitate was collected by filtration,
washed with ice-cold MeOH (5 ml), H2O (5 ml), and finally Et2O (5 ml), and then air-dried. CC (silica gel,
MeCN/ammonia 30 :1) followed by recrystallization by diffusion of Et2O into the MeCN soln. of 5 or 6 gave the
pure compounds.
[m-(4,4''''-Azobis[2,2':6',2''-terpyridine]-kN1,kN1',kN1'': kN1''',kN1'''',kN1''''')]bis(2,2':6',2''-terpyridin-4-ol-
kN1,kN1',kN1'')diruthenium(4‡) Tetrakis[hexafluorophosphate(1 )] (5): 22 mg (39%). UV (MeCN): 234
(4.99), 288 (5.04), 470 (4.35), 633 (4.43); min. 228 (4.99), 260 (4.83), 397 (4.18), 526 (4.24). 1H-NMR
(CD3CN)2): 9.37 (s, 4 H, H C(3')); 8.73 (d, J ˆ 7.80, 4 H, H C(3)); 8.45 (d, J ˆ 7.80, 4 H, H C(3'')); 8.34
(s, 4 H, H C(3''')); 8.05 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(4)); 7.95 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(4''));
7.58 (d, J ˆ 7.80, 4 H, H C(6)); 7.39 (d, J ˆ 7.80, 4 H, H C(6'')); 7.32 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(5));
‡
‡
7.15 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(5'')). MALDI-TOF-MS: 1338 ([M ‡ PF6] ), 1193 (M ), 843 ([M
‡
Ru(tpyOH)] ). Anal. calc. for C60H42F12N14O2P2Ru2 ´ H2O: C 48.01, H 2.95, N 13.06; found: C 47.83, H 2.30,
N 12.79.
[m-[4,4''''-Azobis[2,2':6',2''-terpyridine]-kN1,kN1',kN1'': kN1''',kN1'''',kN1''''')]bis(4'-chloro-2,2':6',2''-terpyri-
dine-kN1,kN1',kN1'')diruthenium(4‡) Tetrakis[hexafluorophosphate(1 )] (6). 31 mg (53%). UV (MeCN):
237 (4.87), 283 (4.91), 304 (4.89), 470 (sh, 4.18), 578 (4.37); min. 222 (4.80), 256 (4.71), 292 (4.86), 403 (3.96).
1H-NMR (CD3CN)2): 9.44 (s, 4 H, H C(3')); 8.91 (s, 4 H, H C(3''')); 8.78 (d, J ˆ 7.80, 4 H, H C(3)); 8.53
(d, J ˆ 7.80, 4 H, H C(3'')); 8.05 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(4)); 7.99 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H,
H
C(4'')); 7.50 (m, 8 H, H C(6), H C(6'')); 7.29 (ddd, J ˆ 8.30, 7.80, 1.95, 4 H, H C(5)); 7.22 (ddd, J ˆ 8.30,
‡ ‡ ‡
7.80, 1.95, 4 H, H C(5'')). MALDI-TOF-MS: 1375 ([M ‡ PF6] ), 1230 (M ), 1195 ([M Cl] ),861 ([M
‡
Ru(tpyCl)] ). Anal. calc. for C60H40Cl2F12N14P2Ru2 ´ H2O: C 46.86, H 2.75, N 12.75; found: C 46.40, H 2.10,
N 12.45.

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Received November 18, 2000