Useful preparation of 6-phenyl-2,2′-bipyridine ligands bearing di-n-butylaminophenyl or gallate derivatives substituted in the 3- or 4-positions

Article abstract of DOI:10.1055/s-2007-990965

A simple protocol for the efficient preparation of substituted 6-phenyl-2,2′-bipyridine derivatives is described. A reverse type of Diels-Alder reaction between a 6-phenyl-2-pyridyl-1,3,4-triazine and 4-N,N-di-n-butylaminophenyl- or gallate-ethynyl derivatives at high temperature provide a mixture of two products easily separable by column chromatography. A variety of ligands suitable for the synthesis of cyclometalated platinum complexes have been produced. Additional substitution of the Pt-Cl by various donor-acceptor ethynyl fragments is feasible by mean of copper(I) catalysis. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-990965

LETTER
3027
Useful Preparation of 6-Phenyl-2,2¢-Bipyridine Ligands Bearing Di-n-
butylaminophenyl or Gallate Derivatives Substituted in the 3- or 4-Positions
P
a
reparation of
t
6-Phen
é
yl-2,2
¢
-
Bip
p
yridine
L
iga
n
h
ds ane Diring,a Pascal Retailleau,b Raymond Ziessel*a
LCM, ECPM/ULP, CNRS, 25 Rue Becquerel, 67087 Strasbourg Cedex 02, France
E-mail: ziessel@chimie.u-strasbg.fr
b
Laboratoire de Cristallochimie, ICSN, CNRS, Bât 27, 1 Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
Received 14 August 2007
Dedicated to the memory of Dr. Charles Mioskowski
purposes sophisticated polytopic ligands (quarter-,
quinque-, hexapyridine….) were prepared by controlled
reverse Diels–Alder reactions.16,17
Abstract: A simple protocol for the efficient preparation of substi-
tuted 6-phenyl-2,2¢-bipyridine derivatives is described. A reverse
type of Diels–Alder reaction between a 6-phenyl-2-pyridyl-1,3,4-
triazine and 4-N,N-di-n-butylaminophenyl- or gallate-ethynyl de-
rivatives at high temperature provide a mixture of two products eas-
ily separable by column chromatography. A variety of ligands
suitable for the synthesis of cyclometalated platinum complexes
have been produced. Additional substitution of the Pt–Cl by various
donor–acceptor ethynyl fragments is feasible by mean of copper(I)
catalysis.
In this paper we report a method for the synthesis of a
class of 6-phenyl-2,2¢-bipyridine analogues starting from
a common triazine starting material. The method is gener-
al and it allows for the convenient synthesis of a series of
ligands in which the substituents on the central pyridine
can be varied.
Key words: triazine, retro-Diels–Alder, gallate, fluorescence, plat-
inum
The target triazine was prepared in two steps from 2-cya-
nopyridine and hydrazine followed by condensation with
phenylglyoxal (Scheme 1). Condensation of 1 with hydra-
zine hydrate was carried out as described in the litera-
ture.18
The construction of useful luminescent complexes gener-
ally requires the engineering of ligands so that the tailor-
ing of the spectroscopic properties or even the redox
properties corresponds to the envisaged applications.
Phosphorescent transition-metal complexes find applica-
tions as chemical sensors,1 electron2 or photon3 donors,
light harvesters4 and electrogenerated luminescent cen-
tres.5
The condensation of the 1,3,4-triazine 3 with bis(4-N,N-
di-n-butylphenyl)acetylene in o-dichlorobenzene at high
temperature provides the trisubstituted bipyridine deriva-
tive 4 in acceptable yield (Scheme 1).19
(i)
(ii)
3
2'
2
N
N
N
N 4
N
CN
N
NH2
Much effort is being extended to develop synthetic proto-
cols able to produce stable, cheap, and versatile ligands
capable to complex transition metals. Bipyridine, terpyri-
dine, and phenyl analogues have found prominent use in a
wide variety of photo-activated molecular systems.6–9
N
NH2
5
1
2
6
(iii)
3
nBu2N
NnBu2
In many cases, these materials are luminescent in fluid so-
lution at ambient temperature and the photophysical prop-
erties are sensitive to temperature, nuclearity, and the
nature of the polypyridine ligand.10 Attaching aryl hydro-
carbon residues11,12 close to the metal centre provides a
means by which to prolong the triplet lifetime of the com-
plex whilst the addition of electron-donating or electron-
withdrawing units opens up the possibility to involve the
metal complex in intramolecular charge-transfer
states.13,14 Attaching conjugated substituents to the metal
complex introduces the likelihood that ligand-localised
excited states will figure in the triplet manifold.15
N
b'
b'
a'
6'
3'
a
N
5'
4'
N
5
N
4, 57%
Most of these ligands were produced by classical methods
and recently in an alternative approach towards the same
c
c'
d'
d
e
SYNLETT 2007, No. 19, pp 3027–3031
Advanced online publication: 08.11.2007
DOI: 10.1055/s-2007-990965; Art ID: G25407ST
x
x
.x
x
.2
0
0
7
Scheme 1 Reagents and conditions: (i) N2H4 hydrate, EtOH, r.t.; (ii)
2-phenylglyoxal, EtOH, reflux; (iii) disubstituted ethynyl compound,
o-dichlorobenzene, 180 °C.
© Georg Thieme Verlag Stuttgart · New York

3028
S. Diring et al.
LETTER
Crystal structure determination for 4 (Figure 1)20 con-
firms the stoichiometry and connectivity deduced from
the NMR spectra. As expected the nitrogen atoms of the
bipyridine moiety lie in a transoidal position that minimis-
es electrostatic interactions between the neighbouring
lone pairs, a situation frequently found in oligopyridine
ligands.21 The two 4-N,N-di-n-butylaminophenyl rings are
tilted versus the central pyridine ring by 47.28° (3-posi-
tion) and 46.27° (4-position). Besides, the nonsubstituted
phenyl fragment in the 6-position is titled by 39.37°.
R1
R2
N
R2
nBu2N
H
N
5a R1 = NnBu2; R2 = H; 86%
6a R1 = R2 = OC12H25; 47%
N
N
N
N
(i)
R2
3
R1
N
R2
N
5b R1 = NnBu2; R2 = H; 5%
6b R1 = R2 = OC12H25; 18%
C12H25O
C12H25O
H
C12H25O
Scheme 2 Reagents and conditions: (i) arylethynyl derivative, o-di-
chlorobenzene, 180 °C.
In order to test the coordination abilities of some of these
novel ligands towards Pt(II) we allow these to react with
K2PtCl4 in a refluxing mixture of acetonitrile (or THF)
and water. Analysis of the crude reaction mixture indicate
complete consumption of the ligands resulting in forma-
tion of deep-red complexes which were easily isolated in
fair yields by column chromatography on silica gel eluting
with dichloromethane (Scheme 3).
Figure 1 An ORTEP view of ligand 4 with atoms labelled. Proba-
bility displacement ellipsoids are shown at the 30% level.
When the same condensation is carried out with 4-N,N-di-
n-butylaminophenylacetylene or 3,4,5-dodecaalkoxy
phenylacetylene the reaction proceeds smoothly but two
regioisomers are formed which can be easily separated by
a combination of chromatography and crystallization
(Scheme 2). The meta-isomers 5a and 6a are identified by
the multiplicities and chemical shifts of the 3,5- or 4,5-
protons of the central pyridine. For the meta-isomers the
protons 4 and 5 are presented by an AB system (JAB = 8.1
Hz, n0d = 12.5 and 21.1 Hz, respectively) whereas for the
para-isomers 5b and 6b the 3- and 5-protons are split into
two weakly coupled doublets (4J = 1.5 Hz, Figure 2). Also
worth noting is the protons 3¢ of the outer pyridine rings
of the para-isomers which are notably shifted upfield in
the meta cases 5a (Figure 3) and 6a due to the proximity
of the substituted phenyl ring in the cisoid orientation.
R1
R1
R2
R2
R2
R2
b
a
b
6'
a
5'
4'
N
(i)
4
5
6'
3'
N
N
N
Pt
d
c
d'
Cl
c
d
e
5a R1 = NnBu2; R2 = H
6a R1 = R2 = OC12H25
7 R1 = NnBu2; R2 = H
8 R1 = R2 = OC12H25
Scheme 3 Reagents and conditions: (i) K2PtCl4, H2O–MeCN (for
5a) or H2O–THF (for 6a), reflux.
The ratio of regioisomers is not influenced by temperature
and reaction times. At lower temperature, the reaction is
very slow and product conversion very modest. At higher
temperature, the addition product of the acetylene to the
triazine leads to the preferential decomposition of the
compounds. It appears that the reverse Diels–Alder con-
densation takes place readily around 180 °C even with
fairly hindered acetylene derivatives and provides a useful
method for the incorporation of various substituents on
the central pyridine residue.
The identity of complexes 7 and 8 was ascertained from
their 1H NMR which revealed a noticeable shift of proton
6¢ (a to the nitrogen) toward low field when ligands were
coordinated to metal centre (from d = 8.6 to ca. 9.1 ppm).
Another key feature in the spectra is the apparition of 195Pt
satellites, testifying of weak coupling of protons 6¢ and d
with the platinum atom.
Synlett 2007, No. 19, 3027–3031 © Thieme Stuttgart · New York

LETTER
Preparation of 6-Phenyl-2,2¢-Bipyridine Ligands
3029
Figure 3 1H NMR (300 MHz, CDCl3) of 5a (top) and 7 (bottom).
For the sake of clarity only the aromatic region is shown.
Figure 2 1H NMR (300 MHz, CDCl3) of 6a (meta-isomer, top) and
6b (para-isomer, bottom). For the sake of clarity only the aromatic re-
gion of the spectra is shown.
Furthermore, complex 7 was adequately characterized by
an X-ray molecular structure on single crystals
(Figure 4).22 The tridendate cyclometalated ligand and the
chlorine are arranged in a distorted square-planar geome-
try about the platinum atom. The chelating ligand is al-
most flat with dihedral angles between the central
pyridine and the outer cycles; 12.55° with the outer pyri-
dine and 4.60° with the phenyl ring. The 4-N,N-di-n-butyl-
aminophenyl fragment is severely tilted from the main
platinum plane by 61.33°, a situation highlighting the ster-
ic congestion at the 3-substitution position. The platinum–
nitrogen bond length trans to the phenyl group [Pt(1)–
N(1) = 2.099 Å] is noticeably longer than that trans to the
chlorine ligand [Pt(1)–N(2) = 1.966 Å] which is consis-
tent with the greater trans influence exerted by the phenyl
substituent.
Figure 4 An ORTEP view of complex 7 with atoms labelled. Pro-
bability displacement ellipsoids are shown at the 50% level. Selected
distances (Å) and angles (°): Pt(1)–N(1), 2.099; Pt(1)–N(2), 1.966;
Pt(1)–C(16), 1.969; Pt(1)–Cl(2), 2.312; N(1)–Pt(1)–N(2), 78.92;
N(1)–Pt(1)–C(16), 162.03; N(2)–Pt(1)–Cl(2), 177.67.
Displacement of the chloride ligands in complex 8 can be
achieved under smooth conditions by using CuI as a cata-
lyst (10 mol%). Although we made no attempt to system-
atically optimise the reaction conditions, displacement of
the halide on the platinum centre is kinetically controlled
and required fairly lengthy reaction times (1–3 d) at room
temperature. Higher temperatures are not suitable here
due to the instability of the platinum precursor in the pres-
ence of triethylamine and the solvent.
Synlett 2007, No. 19, 3027–3031 © Thieme Stuttgart · New York

3030
S. Diring et al.
LETTER
OC12H25
OC12H25
OC12H25
OC12H25
C12H25O
C12H25O
N
Pt
8
N
(i)
N
N
Pt
Cl
Ar
9a–d
OC12H25
OC12H25
C12H25O
N
Ar =
N
N
MeO
N
OMe
N
N
b
d
a
c
Scheme 4 Reagents and conditions: (i) arylethynyl derivative, CuI (10%mol), TEA, CH2Cl2, r.t.
Despite the fact that highly deactivated acetylene sources Table 1 Selected Data for the Novel Ligands and Complexes
are used (e.g., derivative c in Scheme 4), the copper-pro-
Compd Yield MS–FAB lmax (nm), e (M–1cm–1)b
lem
moted cross-coupling reaction is effective and there is lit-
tle difference in the yields of the substituted platinum
complexes from an electron-rich acetylene (a and b) ver-
sus electron-poor acetylene source (like c and d). Inas-
much, use of 4-ethynyl-terpyridine (d) does not induce
ligand scrambling around the platinum metal (C^N^N
versus N^N^N coordination mode), which testify to the
high stability of the ortho-metalated metal centre under
the used experimental conditions.
(%)
(m/z)a
(nm)c
p–p*
MLCT
4
57
86
5
639.3
436.2
332 (28600)
350 (21900)
356 (28200)
306 (21000)
285 (29500)
271 (45500)
279 (27500)
280 (44700)
283 (45500)
283 (46000)
288 (61500)
–
468
487
461
451
435
642
589
608
–
5a
5b
6a
6b
7
–
436.2
–
47
18
65
85
69
68
41
80
861.4
–
The proton NMR spectra of complexes 9a–d presented
the same features as previously discussed for complexes 7
and 8 with characteristic low-field shifting and 195Pt satel-
lites for the protons 6¢ and d. Furthermore, the 13C NMR
spectra for the co-ligand part exhibit two signals between
d = 103 and 107 ppm unambiguously assigned to the sp-
hybridised carbons of the ethynyl bond.
861.4
–
666.2
484 (2000)
440 (1500)
450 (5300)
440 (5700)
428 (5100)
426 (7700)
8
1055.1
1171.0
1709.2
1220.0
1312.0
9a
9b
9c
9d
Selected spectroscopic data are gathered for all com-
pounds in Table 1. The absorption spectra of most com-
pounds exhibit an intense band between 270 and 360 nm
(e in the range 21000–46000 M–1 cm–1), which can be as-
signed to the spin-allowed p–p* transitions involving the
pyridyl/phenyl/ancillary aryl rings. All platinum com-
plexes display an additional large absorption band at low
energy (around 430–480 nm) assigned to spin-allowed
metal-to-ligand charge transfer. The shape and position of
these MLCT band is weakly affected by the nature of the
ethynylaryl residues. All the compounds exhibit a rela-
tively intense emission (Table 1) in solution at room tem-
perature. The emission spectrum is similar in shape and
the emission is significantly red shifted compared to the
absorption spectrum, indicating a strong Stokes shift,
which suggests a reorganization occurring in the excited
state. For the meta-derivatives 5a and 6a the emission is
red shifted, respectively, by 26 nm and 16 nm with respect
to the para-isomers. Notice that for the complexes excita-
tion in the MLCT or p–p* transition resulted in an emis-
579
583
a FAB mass spectroscopy, the molecular peak correspond to [M + H]+
except for 8 where m/z accounts for [M – Cl]+.
b Averaged value determined from at least two different solutions of
nondegassed CH2Cl2 solution.
c Emission in CH2Cl2 solution at room temperature.
sion in the range of 580 to 640 nm due to the
phosphorescence of the MLCT triplet state.23 Complex 9b
bearing an ethynylgallate fragment is completely
quenched, likely due to a photo-induced electron-transfer
process provided by the appended alkoxy fragments.24
This study has shown that reverse Diels–Alder reaction
provides unsymmetrically substituted ligands prepared in
a single step as sketched in Scheme 1. It should be
stressed that these highly substituted phenyl-bipyridine
ligands are mostly inaccessible by any other known syn-
Synlett 2007, No. 19, 3027–3031 © Thieme Stuttgart · New York

LETTER
Preparation of 6-Phenyl-2,2¢-Bipyridine Ligands
3031
(16) (a) Pabst, G. R.; Pfüller, O. C.; Sauer, J. Tetrahedron 1999,
55, 5047. (b) Pabst, G. R.; Sauer, J. Tetrahedron 1999, 55,
5067.
(17) Sauer, J.; Heldmann, D. K.; Pabst, G. R. Eur. J. Org. Chem.
1999, 313.
thetic route. The rate of the condensation is heavily depen-
dent upon steric hindrance near the acetylene function.
Nevertheless, even congested alkynes such as 1,2-di(4-
N,N-dibutylaminophenyl)ethyne can be converted, albeit
in low yield, into the desired target compounds. When two
positional isomers are possible the regioselectivity is dic-
tated by particular aryl–aryl interaction in the transition
state. The meta-isomer is always favoured versus the
para-isomer. The ratio of isomers cannot be regulated by
varying the temperature or reaction rate excluding a kinet-
ic and thermodynamic control. The coordination potential
of these ligands have been tested by complexation with
Pt(II) salts. Nonetheless, ortho metalation is feasible de-
spite the fact that the meta-substitution position induces
some steric constraints. High solubility has been ensured
by the use of butyl or dodecaalkoxy solubilising chains.
Under controlled conditions, this protocol is well suited to
the production of novel phenyl/bipyridine ligands. This
finding expands the synthetic scope of this reaction and
makes readily accessible a class of previously rare
ligands.
(18) Roppe, J.; Smith, N. D.; Crosford, N. D. P. J. Med. Chem.
2004, 47, 4645.
(19) 3,4-Bis(N,N-dibutyl-4-aminophenyl)-6-phenyl-2,2¢-
bipyridine(4)
A Schlenk tube was charged with 6-phenyl-2-pyridyl-1,3,4-
triazine (1, 125 mg, 0.53 mmol), N,N-dibutyl-4-{2-[4
(dibutylamino)phenyl]ethynyl}benzenamine (230 mg, 0.53
mmol) and o-dichlorobenzene (1 mL). The mixture was
argon-degassed via at least three freeze-pump-thaw cycles,
heated to 190 °C and finally stirred in the dark for 1–3 d. The
solvent was removed under high vacuum. The residue was
treated with H2O and extracted with CH2Cl2. The organic
extracts were washed with H2O, then with sat. brine and
filtered through cotton wool. The solvent was removed by
rotary evaporation, and the residue was purified by
chromatography on aluminium oxide eluting with CH2Cl2–
PE (v/v 0:1 to 50:50) to afford 194 mg (57%) of 4 as a white
to light pink powder. 1H NMR (400 MHz, CDCl3): d = 8.59
(d, 1 H, 3J = 4.5 Hz), 8.11 (d, 2 H, 3J = 7.0 Hz), 7.80 (s, 1 H),
7.49–7.35 (m, 4 H), 7.19 (d, 1 H, 3J = 8.0 Hz), 7.09 (ddd, 1
H, J = 7.5, 4.8, 1.3 Hz), 6.78 (ABsys, 4 H, JAB = 9.0 Hz,
n0d = 220.6 Hz), 6.55 (ABsys, 4 H, JAB = 8.8 Hz,
n0d = 144.5 Hz), 3.24 (t, 4 H, 3J = 7.5 Hz), 3.17 (t, 4 H,
3J = 7.5 Hz), 1.59–1.44 (m, 8 H), 1.38–1.24 (m, 8 H), 0.95 (t,
6 H, 3J = 7.3 Hz), 0.92 (t, 6 H, 3J = 7.5 Hz). 13C NMR (100
MHz, CDCl3): d = 160.2, 157.7, 155.3, 150.5, 149.0, 147.5,
146.9, 139.9, 135.3, 133.3, 132.2, 130.7, 128.6, 128.5,
127.3, 126.3, 125.2, 124.7, 121.6, 121.5, 111.7, 111.1, 50.8,
50.7, 29.5, 29.4, 20.5, 20.4, 14.2, 14.1. UV/Vis (CH2Cl2): l
nm (e, M–1 cm–1) = 355 (sh, 19200), 331 (2860), 263
(30600). IR (KBr): n = 3039 (w), 2955 (m), 2929 (m), 2870
(m), 1605 (s), 1517 (s), 1463 (m), 1364 (s), 1285 (m), 1198
(s), 1095 (m), 818 (s) cm–1. MS–FAB+: m/z (nature of the
peak, relative intensity) = 639.3 (100) [M + H]+. Anal. Calcd
for C44H54N4: C, 82.71; H, 8.52; N, 8.77. Found: C, 82.54;
H, 8.29; N, 8.47.
Acknowledgment
The Ministère de la Recherche et des Nouvelles Technologies is
gratefully acknowledged for financial support of this work in the
form of an Allocation de Recherche for S.D.
References and Notes
(1) Harriman, A.; Hissler, M.; Jost, P.; Wipf, G.; Ziessel, R.
J. Am. Chem. Soc. 2001, 121, 14.
(2) Liu, Y.; De Nicola, A.; Reiff, O.; Ziessel, R.; Schanze, K. S.
J. Phys. Chem. A 2003, 107, 3476.
(3) Grosshenny, V.; Harriman, A.; Ziessel, R. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1100.
(4) Kim, C.; Kim, H. J. Organomet. Chem. 2003, 77, 673.
(5) Cunningham, G. B.; Li, Y. T.; Liu, S. X.; Schanze, K. S.
J. Phys. Chem. B 2003, 107, 12569.
(6) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser,
P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85.
(7) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.;
Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.;
Flamigni, L. Chem. Rev. 1994, 94, 993.
(20) Crystal data for 4 at 293 K: C44H54N4, M = 638.91,
monoclinic, space group P21/c, a = 15.161(5) Å,
b = 25.720(5) Å, c = 10.503(5) Å, a = 90.000(5)°,
b = 107.245(5)°, g = 90.000(5)° V = 3911.44(10) Å3, Z = 4,
l = 0.71069 Å, Dc = 1.085 g cm–3, m = 0.063 mm–1, 23064
reflections collected with q ≤ 18.91°, 3085 unique,
R(int) = 0.1214, and 1926 observed reflections [I ≥ 2s(I)],
433 parameters, R1 = 0.0830, wR2 = 0.2397 refined on F2.
CCDC 649879.
(8) Harriman, A.; Rostron, S. A.; Khatyr, A.; Ziessel, R.
Faraday Discuss. 2006, 131, 377.
(9) Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707.
(10) Hissler, M.; El-ghayoury, A.; Harriman, A.; Ziessel, R.
Coord. Chem. Rev. 1998, 180, 1251.
(21) Drew, M. G.; Hudson, M. J.; Iveson, P. B.; Russell, M. L.;
Liljenzin, J.-O.; Skalberg, M.; Spjuth, L.; Madic, C.
J. Chem. Soc., Dalton Trans. 1998, 2973.
(11) Simons, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.;
Piotrowiak, P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc.
1997, 119, 11012.
(22) Crystal data for 7 at 293 K: C30H32ClN3Pt, M = 665.13,
triclinic, space group P–1, a = 9.147(1) Å, b = 9.979(1) Å,
c = 15.290(2) Å, a = 80.500(3)°, b = 80.922(2)°,
g = 73.057(3)°, V = 1307.8(3) Å3, Z = 2, l = 0.71069 Å,
Dc = 1.689 g cm–3, m = 5.49 mm–1, 15122 reflections
collected with q ≤ 27.61°, 5869 unique, R(int) = 0.0501, and
4993 observed reflections [I ≥ 2s(I)], 354 parameters,
R1 = 0.0383, wR2 = 0.0919 refined on F2. CCDC 649883.
(23) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua,
F.; Muro, M. L.; Rajapkse, N. Coord. Chem. Rev. 2006, 250,
1819.
(12) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem.
Eur. J. 1999, 5, 3366.
(13) Guerzo, A. D.; Leroy, S.; Fages, F.; Schmehl, R. H. Inorg.
Chem. 2002, 41, 359.
(14) Benniston, A. C.; Harriman, A.; Lawrie, D. J.; Mayeux, A.
Phys. Chem. Chem. Phys. 2004, 6, 51.
(15) Walters, K. A.; Premvardhan, L. L.; Liu, Y.; Peteanu, L. A.;
Schanze, K. S. Chem. Phys. Lett. 2001, 339, 255.
(24) Würthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.
Eur. J. 2001, 7, 2245.
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