Luminescent zinc(II) complexes of fluorinated benzothiazol-2-yl substituted phenoxide and enolate ligands

Article abstract of DOI:10.1021/ic302063u

Zn(II) complexes of the following new, fluorine-containing, benzothiazole-derived ligands have been synthesized and characterized crystallographically: 2-(3,3,3-trifluoro-2-oxopropyl)benzothiazole (3), 4,5,6,7-tetrafluoro-2-(3,3,3-trifluoro-2-oxopropyl)benzothiazole (4), 4,5,6,7-tetrafluoro-2-(2-hydroxyphenyl)benzothiazole (12), 2-(3,4,5,6- tetrafluoro-2-hydroxyphenyl)-4,5,6,7-tetrafluorobenzothiazole (13), and 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)benzothiazole (16); the Cu(II) complex of ligand 4 is also reported. These are analogs of the important photo- and electroluminescent material [Zn(BTZ)₂]₂, where H-BTZ = 2-(2-hydroxyphenyl)benzothiazole. DFT calculations indicate that HOMO and LUMO energy levels in these materials are substantially lowered by fluorination. The fluorinated ZnL₂ complexes are mononuclear (in contrast to the dinuclear, nonfluorinated material [Zn(BTZ)₂]₂). They easily sublime and show broad visible photoluminescence. A common crystallographic feature is the existence of pairs of fluorinated ZnL ₂ molecules related by inversion centers, with their π systems facing one another.

Full text of DOI:10.1021/ic302063u

Article
pubs.acs.org/IC
Luminescent Zinc(II) Complexes of Fluorinated Benzothiazol-2-yl
Substituted Phenoxide and Enolate Ligands
Zhe Li,†,‡ Ahmed Dellali,† Jahangir Malik,† Majid Motevalli,† Roger M. Nix,† Toyin Olukoya,† Yu Peng,†,‡
Huanqing Ye,‡ William P. Gillin,‡ Ignacio Hernandez,‡ and Peter B. Wyatt*,†
́
†Materials Research Institute, School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road,
London, United Kingdom E1 4NS
‡Materials Research Institute, School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London, United
Kingdom E1 4NS
S
* Supporting Information
ABSTRACT: Zn(II) complexes of the following new, fluorine-
containing, benzothiazole-derived ligands have been synthe-
sized and characterized crystallographically: 2-(3,3,3-trifluoro-2-
oxopropyl)benzothiazole (3), 4,5,6,7-tetrafluoro-2-(3,3,3-tri-
fluoro-2-oxopropyl)benzothiazole (4), 4,5,6,7-tetrafluoro-2-(2-
hydroxyphenyl)benzothiazole (12), 2-(3,4,5,6-tetrafluoro-2-hy-
droxyphenyl)-4,5,6,7-tetrafluorobenzothiazole (13), and 2-
(3,4,5,6-tetrafluoro-2-hydroxyphenyl)benzothiazole (16); the
Cu(II) complex of ligand 4 is also reported. These are analogs
of the important photo- and electroluminescent material
[Zn(BTZ)2]2, where H-BTZ = 2-(2-hydroxyphenyl)benzothiazole. DFT calculations indicate that HOMO and LUMO energy
levels in these materials are substantially lowered by fluorination. The fluorinated ZnL2 complexes are mononuclear (in contrast
to the dinuclear, nonfluorinated material [Zn(BTZ)2]2). They easily sublime and show broad visible photoluminescence. A
common crystallographic feature is the existence of pairs of fluorinated ZnL2 molecules related by inversion centers, with their π
systems facing one another.
perfluoropentacene7 of interest in organic electronics. In
addition, fluorination is expected to modify both optical and
structural properties of organic materials: the replacement of
INTRODUCTION
The luminescence of benzothiazole derivatives is exploited both
by nature and by human technology. Thus, the phenolate of
■
hydrogen by fluorine leads to significantly enhanced spin−orbit
oxyluciferin (1) is responsible for light emission in fireflies and
coupling,8 while perfluorinated ligands lack high energy C−H
vibrational modes that can cause nonradiative quenching of IR
luminescence, e.g., from Er3+ ions.9 The π−πF interaction
between fluorinated and nonfluorinated aromatic rings is
recognized as being mediated by a combination of dispersion
and Coulombic noncovalent forces,10 strongly favoring the
parallel alignment of aryl and perfluoroaryl groups and so
directing the self-assembly of materials that contain both types
of ring.
other bioluminescent insects,1 whereas the zinc(II) complex of
2-(2-hydroxyphenylbenzothiazole) (H-BTZ, 2) is an excellent
white-light emitting material for organic light-emitting diodes
(OLEDS).2−4 The dimeric [Zn(BTZ)2]2 structure has five-
coordinate zinc centers connected by bridging oxygen atoms
from the BTZ ligands. Recently, the effects of introducing
additional alkyl, alkoxy, and aryl substituents into the structure
of ligand 2 have been examined both experimentally and
computationally, resulting in changes to the energies of
molecular orbitals in the zinc complexes and shifts in the
wavelengths of their maximum light emission.5,6
We now describe the synthesis of both partially and fully
fluorinated metal-containing organic materials, incorporating
benzothiazole-derived ligands, and examine the effects of
fluorination on the structures and properties of these
complexes.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. 2-(3,3,3-Trifluoro-2-
oxopropyl)benzothiazole (3) is easily prepared by the reaction
of commercially available 2-methylbenzothiazole with trifluoro-
Fluorine atoms possess high electronegativity, small size, and
the ability to form strong bonds to carbon. Consequently,
fluorinated aromatic compounds combine a readiness to accept
electrons with good chemical stability, making materials such as
Received: September 23, 2012
Published: January 14, 2013
© 2013 American Chemical Society
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acetic anhydride and pyridine.11 4,5,6,7-Tetrafluoro-2-methyl-
benzothiazole, itself obtained by cyclizing 2,3,4,5,6-pentafluor-
oacetanilide using phosphorus(V) sulfide,12 was trifluoroacety-
lated using ethyl trifluoroacetate and sodium hydride in THF to
give 4,5,6,7-tetrafluoro-2-(3,3,3-trifluoro-2-oxopropyl)-
benzothiazole (4). 1H NMR and IR spectra indicate that
ligands 3 and 4 exist very largely as their enol tautomers. The
reaction of 3 and 4 with methanolic zinc(II) acetate led to the
precipitation of the corresponding zinc(II) complexes 5 and 6.
In the case of the new, highly fluorinated ligand 4, we also
prepared the copper(II) complex 7 by similar means.
aromatic C−H environments; the melting point was more than
30° higher than the literature value, but 19F NMR and high
resolution mass spectrometry were also consistent with our
isolation of 16 as a single entity. It is therefore likely that the
material previously described by Bazyl’ et al.13 was not pure 16.
We identified benzothiazole derivative 17 as an additional
product of the reaction between 14 and 15, isolable by
recrystallization of the crude product. In an attempt to increase
the yield of 16, we tried condensing 2-aminothiophenol with
3,4,5,6-tetrafluoro-2-methoxybenzoyl chloride (18) in hot N-
methyl-2-pyrrolidone (NMP), using the general approach of
Brembilla et al.14 We were pleased to find that O-demethylation
occurred under these reaction conditions, to give the required
phenol 16 in a one-pot reaction after quenching with water.
The zinc complexes 19 and 20, which are fluorinated
analogues of Zn(BTZ)2, precipitated from methanol upon
mixing solutions of zinc(II) acetate and potassium salts of the
ligands. Complex 21 is comparatively soluble in methanol and
so was prepared by precipitation from aqueous solution.
Acylation of pentafluoroaniline using 2-methoxybenzoyl
chloride gave the amide 8, which was converted into 4,5,6,7-
tetrafluoro-2-(2-methoxyphenyl)benzothiazole (10) by cycliza-
tion with phosphorus(V) sulfide, followed by demethylation
with BBr3 to give ligand 12. The fully fluorinated ligand 13 was
prepared similarly to 12, except that the acylation of
pentafluoroaniline was performed using 3,4,5,6-tetrafluoro-2-
methoxybenzoyl chloride.
X-Ray Crystallography. Crystals of the enolate complexes
5, 6, and 7 for X-ray diffraction were obtained by
recrystallization from CHCl3, EtOAc-Et2O, and EtOH−Et2O,
respectively; X-ray quality crystals of the phenoxide complexes
19, 20, and 21 were grown by vacuum sublimation in a sealed
glass tube (with a pressure below 10−6 mbar, temperature
between 250 and 300 °C). Single crystal X-ray diffraction
studies were performed on all six metal complexes (Figures 1
and 2 and Supporting Information). Structures were solved and
refined using the Bruker SHELXTL Software Package.
Table 1 summarizes the crystallographic details, while Table
2 presents bond lengths and angles relating to the coordination
of the metal centers. In contrast to the dimeric [Zn(BTZ)2]2,
molecules of which possess an inversion center, all six metal
complexes have mononuclear structures, where two ligands
chelate the central, four-coordinate metal ion using the
benzothiazole nitrogen atom and the oxygen of the enolate
or phenoxide group. The effect of fluorination on reducing
nuclearity may be attributed to the resulting reduction in the
The preparation of 2-(2-hydroxy-3,4,5,6-tetrafluorophenyl)-
benzothiazole (16) has previously been reported, by reaction of
the 2-aminothiophenol 14 with the chromone derivative 15.13
1
However, the literature H NMR of 16 has two separate OH
signals at low field, with the entire aromatic region described as
one multiplet. When we attempted to replicate this preparation,
we isolated, after purification by flash chromatography, a low
1
yield of material that had a H NMR spectrum consistent with
that of structure 16, with only one OH signal and four distinct
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interactions that occur within the series of zinc phenoxide
complexes. In considering these stacking interactions, it should
be noted that precise values of calculated interplanar
separations are influenced by the choice of atoms used to
define the molecular plane.19
For [Zn(BTZ)2]2, each dimer molecule contains four nearly
planar benzothiazole units, which are used to engage in pairwise
π−π interactions with four neighboring molecules, creating a
three-dimensional network. These interactions take place in
two approximately mutually perpendicular directions, so that
terminal ligands pair with other terminal ligands of adjacent
dimer molecules (the literature reported2 interplanar separation
of 3.77 Å is equal to the distance between the mean planes of
the six carbon atoms in the carbocyclic “benzo” ring of the
benzothiazole), while bridging ligands pair with neighboring
bridging ligands (separation 3.60 Å). The zinc phenoxide
structures 19, 20, and 21 all contain pairs of neighboring zinc
complexes that are related by centers of inversion and have π-
systems facing one another in an antiparallel alignment. In the
case of 19, these rings are offset so that the nonfluorinated,
carbocyclic “benzo” portions of the benzothiazole units are
aligned and the substituents on these rings eclipse those on
their neighbors (Figure 3). The separation of the planes of
these rings is 3.63 Å, and the distance between the centroids of
the nonfluorinated rings is 3.64 Å. These parallel aromatic rings
are parts of infinite stacks in which two nonfluorinated rings are
followed by two fluorinated rings. Thus, πH−πH, πH−πF, and
πF−πF interactions all exist within the stacks (Figure 4). The
separation between the planes of the nonfluorinated rings and
the centroids of neighboring fluorinated rings is 3.36 Å, while
that between the planes of adjacent fluorinated rings is 3.28 Å.
Structure 19 additionally contains infinite stacks of aromatic
rings in which neighboring molecules are related by a c glide.
These stacks are approximately perpendicular to planes formed
by the eclipsed “benzo” rings in 19 that are referred to above.
Within the stacks, the fluorinated phenol rings roughly face
each other, as do the nonfluorinated benzothiazole units, with a
mean interplanar spacing of 3.54 Å. However, neighboring rings
are not perfectly parallel, and they approach within 3.5 Å of one
another.
Figure 1. Molecular structure of enolate complex 6. Ellipsoids are
shown at the 50% probability level.
Figure 2. Molecular structure of phenoxide complex 21.
Lewis basicity of the ligands, as has previously been proposed,
e.g., for lanthanide perfluorarylthiolate complexes15 and for
Co(II) and Cu(II) pentafluorophenoxides.16
In 20, the nonfluorinated phenoxide rings face the
fluorinated benzothiazole units so that the C−H and C−F
bonds of the opposing rings are staggered, forming infinite
stacks. The interplanar separation alternates so that it is 3.23 Å
in pairings where the nonfluorinated phenol lies over both rings
of the fluorinated benzothiazole, compared with 3.59 Å when
the phenol is positioned to one side of the fluorinated
carbocyclic ring of the benzothiazole. Infinite stacks that form
in an approximately perpendicular direction (mean interplanar
separation 3.44 Å) also juxtapose fluorinated and non-
fluorinated aromatic components, although in this case the
extent of overlap of the rings is comparatively small.
All six fluorinated metal complexes exist in the solid state as
chiral molecules that crystallize as racemic compounds in
centrosymmetric space groups. The enolate and phenoxide
units in the coordinated ligands are approximately coplanar
with the benzothiazole ring system, with N−C−C−C torsion
angles of less than 10°. The four-coordinate zinc complexes 5,
6, 19, 20, and 21 have roughly tetrahedral geometries, with
shorter Zn−O and Zn−N bond lengths and larger O−Zn−N
angles than in the five coordinate [Zn(BTZ)2]2. The copper(II)
complex 7 shows greater distortion from tetrahedral geometry
than the analogous zinc complex 6; in particular the N(1)−Cu-
N(2) angle in 7 is larger, and the O−Cu−N angle is smaller,
than the corresponding values in 6.
Mixtures of arenes and perfluoroarenes with a 1:1 molar
ratio,17 as well as single substances such as 1,2,3,4-
tetrafluoronaphthalene,18 often adopt crystal structures con-
taining parallel stacks of Ar(H) and Ar(F) rings. These π−πF
interactions, which are estimated to be stabilizing to the extent
of 20−25 kJ mol−1, provide a strong influence for directing
molecular assembly but may be outweighed by other effects,
including hydrogen bonding and steric repulsion.10 It is
therefore of interest to examine the nature of the π−π
In the fully fluorinated system 21 (space group P1),
̅
phenoxide rings face the fluorinated benzothiazoles across
inversion centers with the C−F bonds out of register and a
separation between planes of 3.32 Å (based on the mean
separation of the centroid of one ring and the plane defined by
the carbon atoms of the facing ring). In an approximately
perpendicular direction, infinite stacks form with an interplanar
separation of 3.20 Å. The stacks are displaced laterally so that
the phenol ring of one molecule faces the carbocyclic part of
the benzothiazole system in the neighboring molecule with 3.62
Å separation of the ring centroids.
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Table 1. Crystallographic Details of Enolate and Phenoxide Complexes
compound
5
6
7
19
20
21
a
CCDC number
873934
873936
873935
873933
873932
873937
formula
C20H10F6N2O2S2Zn
553.79
C20H2F14N2O2S2Zn
697.73
C20H2CuF14N2O2S2
695.90
C26H8F8N2O2S2Zn
661.83
C26H8F8N2O2S2Zn
661.83
C26F16N2O2S2Zn
805.77
fw
temp
120(2) K
120(2) K
monoclinic
P21/n
120(2) K
100(2) K
monoclinic
P21/c
100(2) K
monoclinic
P21/n
100(2) K
triclinic
cryst syst
triclinic
triclinic
space group
P1
̅
P1
̅
P1
̅
a
7.7002(2) Å
11.0829(3) Å
12.3763(4) Å
75.809 (1)°
79.705(2)°
82.847(2)°
1003.87(5) Å3
2
10.6956(14) Å
11.254(2) Å
19.077(3) Å
90°
103.523(9)°
90°
2232.6(6) Å3
8.6220(3) Å
11.3026(3) Å
12.5469(4) Å
68.841(2)°
84.985(2)°
77.252(2)°
1112.13(6) Å3
2
12.6004(4) Å
24.1084(8) Å
7.5640(3) Å
90°
97.393(2)°
90°
2278.66(14) Å3
7.350(5) Å
25.035(5) Å
12.638(5) Å
90°
100.733(5)°
90°
2284.8(19) Å3
7.7232(2) Å
11.8735(3) Å
13.9100(4) Å
92.4460(10)°
103.5580(10)°
93.9180(10)°
1234.85(6) Å3
2
b
c
α
β
γ
V
Z
4
4
4
density (calculated)
absorption coeff
cryst size
1.832 Mg/m3
1.509 mm−1
2.076 Mg/m3
1.430 mm−1
2.078 Mg/m3
1.313 mm−1
1.929 Mg/m3
4.127 mm−1
1.924 Mg/m3
1.356 mm−1
2.167 Mg/m3
1.320 mm−1
0.36 × 0.24 × 0.06
0.28 × 0.14 × 0.02
0.26 × 0.08 × 0.04
0.08 × 0.10 × 0.20 0.10 × 0.12 × 0.16 0.12 × 0.14 × 0.24
mm3
mm3
mm3
mm3
mm3
mm3
reflns collected
18261
20098
22507
36422
39802
22725
independent reflns
4593 [R(int) =
0.0499]
4365 [R(int) =
0.1296]
5099 [R(int) =
0.0681]
4003 [R(int) =
0.0439]
3779 [R(int) =
0.0306]
5027 [R(int) =
0.0142]
final R indices [I > 2 R1 = 0.0356
R1 = 0.0780
wR2 = 0.1609
R1 = 0.1312
wR2 = 0.1853
R1 = 0.0474
wR2 = 0.1073
R1 = 0.0725
wR2 = 0.1188
R1 = 0.0284
wR2 = 0.0775
R1 = 0.0308
wR2 = 0.0791
R1 = 0.0398
wR2 = 0.1037
R1 = 0.0442
wR2 = 0.1074
R1 = 0.0428
wR2 = 0.1129
R1 = 0.0439
wR2 = 0.1138
σ(I)]
wR2 = 0.0795
R indices (all data)
R1 = 0.0474
wR2 = 0.0849
a
Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary
publications with the indicated CCDC numbers. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ; e-mail: deposit@ccdc.cam.ac.uk.
Table 2. Selected Experimental Bond Lengths (Å) and Angles (deg) for Metal Complexes
compound
5
6
7
19
20
21
M(1)−O(1)
M(1)−O(2)
M(1)−N(1)
M(1)−N(2)
O(1)−M(1)−N(1)
O(2)−M(1)−N(2)
O(1)−M(1)−O(2)
N(1)−M(1)−N(2)
O(1)−M(1)−N(2)
O(2)−M(1)−N(1)
1.9283(16)
1.9525(16)
2.0005(19)
1.9913(19)
95.14(7)
1.935(5)
1.919(5)
2.003(6)
2.003(6)
95.7(2)
1.905(2)
1.903(2)
1.9131(15)
1.9090(14)
1.9944(17)
1.9899(17)
93.68(7)
1.912(3)
1.912(3)
1.907(2)
1.893(2)
1.947(2)
2.020(3)
2.020(2)
1.958(2)
2.017(3)
2.036(3)
94.78(9)
94.00(11)
93.31(11)
123.27(11)
134.70(12)
105.65(11)
109.47(11)
93.01(10)
93.39(10)
107.57(10)
116.29(10)
121.46(10)
127.53(10)
96.28(7)
96.8(2)
94.78(10)
137.19(10)
151.77(11)
95.36(10)
95.51(9)
94.05(7)
127.11(7)
119.71(7)
114.27(8)
106.09(7)
112.0(2)
116.4(2)
117.7(2)
119.8(2)
118.19(7)
131.87(7)
110.68(6)
110.47(6)
Figure 4. Stacking of ligand π systems within crystals of 19, indicating
the relative positions of fluorinated (green) and nonfluorinated (red)
aromatic rings.
Thus, the existence of stacked π systems is a feature of all the
phenoxide complexes. Despite the importance of πH−πF
interactions in simple organic structures where such inter-
actions are possible and its presence in 20, this particular type
of π-stacking is not the only one observed for half-fluorinated
systems such as 19. However, all of these systems form
centrosymmetric structures derived from chiral monomers.
Figure 3. Intermolecular contacts in the crystal structure of 19,
showing πH−πH and πF−πF stacking.
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Thus, the strongest influence in dictating the architecture of
these crystals is the mutual recognition of enantiomers brought
about by the favorable interaction of oppositely directed
electrical dipoles, not the attraction of fluoroaryl groups for
their fluorine-free counterparts.
Photophysical Studies on Metal Complexes. a. Solution
Phase Absorption. The zinc enolate complexes 5 and 6 are
colorless, with strong absorption peaks at 342 and 347 nm,
respectively, in acetonitrile solution (Figure 5). The copper
consistent with an enhanced transition dipole moment
consequent on selective fluorination.
b. Photoluminescence Studies. Photoluminescence spectra
were recorded on powders, thus avoiding the complications due
to solvation in the solution phase measurements and
interference effects in thin film studies. The zinc enolates 5
and 6 both show broad visible luminescence upon excitation at
405 nm, with a central peak at around 500 nm and additional
peaks at higher and lower wavelengths (Figure 7). Comparison
Figure 5. UV spectra of metal enolates 5, 6, and 7 (2 × 10−5
solutions in MeCN).
M
complex 7 is deep purple to the eye, but its visible absorption
peaks have extinction coefficients below 100, while in the UV
region its absorption spectrum is similar to those of 5 and 6.
The fluorinated zinc phenoxides 19, 20, and 21 are yellow and
have solution phase absorption spectra (recorded in DMSO)
that are qualitatively similar to one another (Figure 6). All the
Figure 7. Photoluminescence emission spectra of powdered zinc
enolates 5 and 6 upon excitation at 405 nm (top) and at 342 nm
(bottom).
of the two photoluminescence emission spectra indicates that
tetrafluorination of the ligands has only a slight effect on the
peak wavelengths but does change their relative intensities. A
́
similar emission spectrum has been observed by Fernandez et
al. for a gold(I) 2-(diphenylphosphino)-4,5,6,7-tetrafluoroben-
zothiazole complex,20 and the structure of the band has been
attributed to the involvement of a vibrational mode of the
benzothiazole ring. Excitation of 5 and 6 at 342 nm, close to
their UV absorption maxima, gives broad emissions, peaking at
shorter wavelengths than when the excitation is at 405 nm
(Figure 7). This dependence of the emission spectrum upon
the excitation wavelength may arise because the observed
spectra contain contributions from both fluorescence and
phosphorescence; time-resolved studies of the luminescence are
currently under way. The copper(II) enolate complex 7 does
not show visible photoluminescence.
Figure 6. UV−visible spectra of zinc phenoxides [Zn(BTZ)2]2, 19, 20,
and 21 (2 × 10−5 M with respect to Zn2+, solutions in DMSO).
phenoxides, including [Zn(BTZ)2]2, have their longest wave-
length absorptions in the blue region. The fluorinated
phenoxides, which are mononuclear in the solid state, lack an
absorption peak corresponding to the UV absorption of
[Zn(BTZ)2]2 at 329 nm; this feature may be related to the
ability of the latter compound to exist as a dimer in the solid.
Extinction coefficients for the longest wavelength absorption
bands in the partially fluorinated systems (19 and 20) were
larger than either [Zn(BTZ)2]2 or fully fluorinated 21; this is
The photoluminescence spectrum of each of the phenoxides
[Zn(BTZ)2]2, 19, 20 and 21, excited at 368 nm, comprises a
single broad peak in the visible region (Figures 8 and 9).
Compared to emission from the nonfluorinated complex
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Table 3. Inverse Correlation between Dihedral Angles and
Photoluminescence Full Width at Half Maximum (FWHM)
complex
largest
5
6
[Zn(BTZ)2]2
12.7
19
1.6
20
3.4
21
3.8
6.2
8.7
benzothiazole-
(ph)enolate
dihedral angle
/deg
fwhm/nm
74
51
54
103
101
93
COMPUTATIONAL STUDIES
■
We have performed DFT calculations on the zinc complexes (Table
4). The HOMO and LUMO of the enolates 5 and 6 are delocalized
but reside mainly on the ligands (Figure 10). Tetrafluorination of the
benzothiazole rings lowers the energies of both these molecular
orbitals by approximately 0.4 eV and leaves the HOMO−LUMO gap
essentially unchanged. This is consistent with the experimental
observations that compounds 5 and 6 have very similar absorption
and photoluminescence spectra.
Figure 8. Photoluminescence emission spectra of powdered zinc
phenoxides [Zn(BTZ)2]2, 19, 20, and 21 upon excitation at 368 nm.
The electronic structures of monomeric and dimeric forms of
nonfluorinated Zn(BTZ)2 were the subject of previous calculations.2,19
The HOMOs are mainly localized on the phenoxide rings, whereas the
LUMOs are less localized and spread over both the benzothiazole and
phenoxide rings. In the case of the dimer, the energy gap between the
HOMO and LUMO is smaller than for the monomer; however, the
HOMOs are located on the two bridging ligands, whereas the LUMOs
are associated with the nonbridging ligands, so there may only be a low
probability of a transition occurring between these particular orbitals.
In Table 4, we present the results of our own DFT calculations for the
monomer of Zn(BTZ)2 and the series of fluorinated analogues. The
forms of the HOMO and LUMO wave functions are exemplified in
Figure 11 by the perfluorinated zinc enolate 21. Fluorination of just
the phenoxide ring, to give 19, reduces both the HOMO and LUMO
energies by nearly equal amounts, relative to the hypothetical
nonfluorinated monomer. On the other hand, fluorination of only
the benzothiazole ring stabilizes the LUMO (delocalized over both
rings) more than the HOMO (localized on the phenoxide ring), so
that 20 has a decreased HOMO−LUMO gap compared with 19, as is
consistent with the red shift in both the absorption and photo-
luminescence spectra of 20. Complete fluorination in 21 is calculated
to provide a HOMO−LUMO gap almost equal to that in 20; however
the absorption spectrum of 21 is blue-shifted relative to 20 and more
closely resembles that of 19. Photoluminescence from 21 is also blue-
shifted relative to 20.
Figure 9. Photoluminescence emission spectra of zinc phenoxides
[Zn(BTZ)2]2, 19, 20, and 21 in DMSO solution upon excitation at
368 nm.
[Zn(BTZ)2]2 at 474 nm, tetrafluorination on the phenoxide
ring, to give 19, hardly changes the peak wavelength but gives a
broadened emission with the maximum at 476 nm.
Tetrafluorination on the benzothiazolyl ring to give 20 leads
to a red shift to 529 nm, whereas octafluorinated 21 shows an
emission at 497 nm lying between the two tetrafluorinated
complexes.
It is notable that the powder phase photoluminescence
emission peaks of compounds 19−21 are broader than that of
[Zn(BTZ)2]2; the differences in peak widths are much less
marked in DMSO solution, suggesting that this phenomenon is
related to the structures adopted in the solid state. The
compounds in which the phenoxide and benzothiazole rings are
closest to coplanarity in the crystal, and so might be expected to
have the most extensively delocalized π systems, are also the
ones with the broadest emission peaks (Table 3). This
relationship also applies in a comparison of enolates 5 and 6,
where the less highly fluorinated material gives the broader
emission upon excitation at 342 nm.
CONCLUSIONS
■
We have established efficient synthetic routes to fluorinated
analogues of the 2-(2-hydroxyphenyl) ligand and have used
Zn(II) to exemplify the formation of thermally stable,
sublimable, photoluminescent metal complexes from these
new ligands. The compounds have mononuclear, centrosym-
metric structures in which stacking of π systems is observed.
Partially fluorinated systems can give rise to πH−πF stacking,
but πH−πH and πF−πF assemblies are also observed.
Fluorination of the aromatic rings in these ligands substantially
lowers the HOMO and LUMO energies, whereas the changes
in the wavelengths of visible absorption and of photo-
luminescence are comparatively subtle. These are attractive
materials for the construction of OLEDs, since considerable
variation of the electrical properties is possible.
Luminescence quantum yields for solutions of the zinc
complexes in DMF were determined as detailed in the
Supporting Information. The values calculated for complexes
20 and 21 approach that of the highly luminescent material
[Zn(BTZ)2]2, whereas compounds 5, 6, and 19 gave much
lower quantum yields.
The electroluminescence of the zinc complexes, their
incorporation into OLEDs, and the coordination chemistry of
the ligands with other metals, particularly the IR-emissive
lanthanide ions such as Er3+, are currently under investigation
in our laboratories.
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Table 4. Calculated HOMO and LUMO Energies for Zinc Enolate and Phenoxide Complexes
5
6
Zn(BTZ)2
19
20
21
E
LUMO/eV
HOMO/eV
−1.94
−6.01
4.07
−2.36
−6.39
4.03
−1.79
−5.39
3.61
−2.26
−5.83
3.58
−2.19
−5.68
3.49
−2.65
−6.14
3.48
E
ΔELUMO−HOMO/eV
The white precipitate was filtered off and recrystallized from
chloroform to give 5 (0.41 g, 72%) as a white solid, mp 283−285
°C. Found: C, 43.35; H, 1.78; N, 5.13. C20H10F6N2O2S2Zn requires C,
43.38; H, 1.82; N, 5.06%. UV (MeCN): λmax 211 (ε 46700), 268
(20900), 342 (53900); νmax/cm−1 (ATR) 1624, 1573, 1492, 1458,
1418, 1281, 1185, 1109, 889, 749, 696; δH (400 MHz, CDCl3) 6.25
(2H, s), 7.28 (2H, ‘d’, J 8.3 Hz), 7.31 (2H, td, J 7.6, 1.5 Hz), 7.36 (2H,
td, J 7.6, 1.4 Hz), 7.75 (2H, ‘d’, J 7.7 Hz); δF (376 MHz, CDCl3)
−75.6. HRMS (EI) m/z found: 551.9379. C20H10F6N2O2S2Zn (M+)
requires 551.9380.
Zinc Complex of 4,5,6,7-Tetrafluoro-2-(3,3,3-trifluoro-2-
oxopropyl)benzothiazole (6). A solution of 4,5,6,7-tetrafluoro-2-
(3,3,3-trifluoro-2-oxopropyl)benzothiazole (4b) (0.60 g, 1.89 mmol)
in MeOH (15 mL) was mixed with a solution of Zn(OAc)2·2H2O
(0.20 g, 0.91 mmol) in MeOH (5 mL). The white precipitate was
filtered off and recrystallized from EtOAc-Et2O (4:1) to give 6 (0.35 g,
53%) as a white solid, mp 290 °C. Found: C, 34.31; H, 0.10; N, 3.49.
C20H2F14N2O2S2Zn requires C, 34.43; H, 0.29; N, 4.01%. UV
Figure 10. HOMO (left) and LUMO (right) of the zinc enolate 5.
(MeCN): λmax 206 (ε 48500), 267 (14200), 347 (54500); νmax
/
cm−1 (ATR) 1563, 1493, 1414, 1291, 1188, 1150, 1120, 1093, 1013,
920, 881, 777, 695; δH (270 MHz, CDCl3) 6.4 (s, CH); δF (376 MHz,
CDCl3) −75.8 (s, CF3), −139.0 (dd, J 20.4, 14.3 Hz), −153.4 (dd, J
20.2, 14.1 Hz), −155.5 (t, J 19.5 Hz), −158.7 (t, J 20.3 Hz). HRMS
(EI) m/z found: 695.8627. C20H2F14N2O2S2Zn (M+) requires
695.8623.
Copper Complex of 4,5,6,7-Tetrafluoro-2-(3,3,3-trifluoro-2-
oxopropyl)benzothiazole (7). A solution of 4,5,6,7-tetrafluoro-2-
(3,3,3-trifluoro-2-oxopropyl)benzothiazole (4b) (0.061 g, 0.19 mmol)
in MeOH (5 mL) was mixed with a solution of Cu(OAc)2·H2O (0.028
g, 0.14 mmol) in MeOH (5 mL). The precipitate was filtered off and
recrystallized from EtOH−Et2O to give 7 (0.050 g, 74%) as deep
violet crystals, mp 285 °C. UV−vis (MeCN): λmax 209 (ε 57100), 256
(16200), 351 (41600), 426 (52), 515 (41), 730 (8.6); νmax/cm−1
(ATR) 1560, 1491, 1396, 1286, 1198, 1123, 1090, 1016, 926, 880, 797,
696. HRMS (EI) m/z found: 694.8629. C20H2CuF14N2O2S2 (M+)
requires 694.8625.
N-(Pentafluorophenyl)-2-methoxybenzamide (8). A solution of 2-
methoxybenzoyl chloride (0.85 g, 0.74 mL, 5.0 mmol) in DCM (12
mL) was added dropwise to a solution of pentafluoroaniline (1.83 g,
10.0 mmol) in DCM (12 mL). The mixture was stirred at room
temperature for 24 h, after which an additional portion of
pentafluoroaniline (0.1 mmol) was added to ensure complete
consumption of the acyl chloride. After a further 5 h, the solvent
was evaporated, and the resulting residue was recrystallized from
chloroform to give N-pentafluorophenyl-2-methoxybenzamide 8 (1.53
g, 96%) as a white solid, mp 161.5−163 °C. νmax/cm−1 (ATR) 3301
(N−H), 1673 (CO), 1655, 1602, 1519, 1458, 1290, 1231, 1186,
1096, 1048, 981, 759; δH (270 MHz, CDCl3) 7.07 (1H, d, J 8 Hz),
7.14 (1H, t, J 8 Hz), 7.55 (1H, t, J 8 Hz), 8.26 (1H, d, J 8 Hz), 9.38
(1H, s, NH); δF (376 MHz, CDCl3) −145.5 (2F, m), −158.8 (1F, t, J
20 Hz), −163.9 (2F, m). HRMS (ESI) m/z found: 318.0551.
C14H9F5NO2 (M+H+) requires 318.0548.
N-(Pentafluorophenyl)-3,4,5,6-tetrafluoro-2-methoxybenzamide
(9). 2-Methoxy-3,4,5,6-tetrafluorobenzoyl chloride22 (2.79 g, 11.5
mmol) in DCM (70 mL) was added dropwise to a solution of
pentafluoroaniline (4.39 g, 24 mmol) in DCM (35 mL) over 20 min
under N2. The mixture was stirred at room temperature under N2 for
24 h; then the solvent was evaporated at reduced pressure. The
resulting residue was recrystallized from CHCl3-petrol to give amide 9
(3.48 g, 78%) as a white, fluffy solid, mp 114−115.5 °C. νmax/cm−1
3225, 1686, 1488, 1320, 1239, 1150, 991, 641; δH (400 MHz, CDCl3)
4.08 (3H, s, OMe), 7.78 (1H, br s, NH); δF (376 MHz, CDCl3)
−139.2 (1F, d, J 22 Hz), −144.2 (2F, d, J 22 Hz), −149.8 (1F, t, J 22
Figure 11. HOMO (left) and LUMO (right) of the fully fluorinated
zinc phenoxide 21.
EXPERIMENTAL SECTION
■
General Procedures. Flash chromatography was performed using
Zeochem ZEOprep 60 silica gel (40−63 μm). IR spectra were
obtained by attenuated total reflectance (ATR) on a Perkin-Elmer
Spectrum 65 FT-IR spectrometer. Mass spectra were recorded by the
EPSRC National Service Mass Spectrometry Service Centre at
Swansea. DFT calculations were performed using the Spartan′10
software package, version 1.0.1, using the DFT RB3LYP method with
the 6-31G(D) basis set.
4,5,6,7-Tetrafluoro-2-(3,3,3-trifluoro-2-oxopropyl)benzothiazole
(4b). 4,5,6,7-Tetrafluoro-2-methylbenzothiazole12 (0.378 g, 1.7 mmol)
in THF (10 mL) was treated with NaH (60% dispersion in oil; 68 mg,
1.7 mmol) at 0 °C with stirring. After 10 min, ethyl trifluoroacetate
(0.45 mL, 3.77 mmol) was added, and the mixture was allowed to
attain room temperature over 3 h. Hydrochloric acid (2 M; 2 mL),
brine (5 mL), and Et2O were added. The organic extracts were dried
(Na2SO4), concentrated, and recrystallized (CHCl3) to give the title
compound 4b (0.20 g, 36%) as a white solid, mp 194−195 °C. Found:
C, 37.32; H, 0.75; N, 4.30. C10H2F7NOS·0.25H2O requires C, 37.34;
H, 0.78; N, 4.35%. UV (MeCN): λmax 202 (ε 23500), 255 (9300), 307
(9200), 363 (22000); νmax/cm−1 (ATR) 2606 (br), 1737, 1443, 1363,
1238, 1188, 1136, 1087, 1005, 883, 793, 730; δH (270 MHz, CDCl3)
6.3 (1H, s, CH), 8.7 (1H, br, OH); δF (376 MHz, CDCl3) −74.6 (s,
CF3), −139.0 (dd, J 20, 14 Hz), −148.6 (dd, J 19, 14 Hz), −156.6 (t, J
19.5 Hz), −158.2 (t, J 20 Hz). HRMS (EI) m/z found: 316.9745.
C10H2F7NOS (M+) requires 316.9740.
Zinc Complex of 2-(3,3,3-Trifluoro-2-oxopropyl)benzothiazole
(5). A solution of 2-(3,3,3-trifluoro-2-oxopropyl)benzothiazole11 3b
(0.50 g, 2.04 mmol) in boiling MeOH (20 mL) was mixed with a
solution of Zn(OAc)2·2H2O (0.20 g, 0.91 mmol) in MeOH (5 mL).
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Hz), −154.7 (1F, qt, J 22, 11 Hz), −155.2 (2F, t, J 22 Hz), −160.4
(1F, t, J 22 Hz), −161.8 (1F, t, J 22 Hz). HRMS (CI) m/z found:
390.0171. C14H3F8NOS (M+H+) requires 390.0169.
yellow solid (48 mg, 6%), mp 226−228 °C (lit.13 193−195 °C). νmax
/
cm−1 (ATR) 1644 (br), 1500, 1418, 986, 761. UV (MeCN): λmax 292
(ε 18500), 325 (16700); δH (400 MHz, DMSO-d6) 7.60 (1H, t, J 7.5
Hz), 7.67 (1H, t, J 7.5 Hz), 8.24 (1H, d, J 7.5 Hz), 8.29 (1H, d, J 7.5
Hz), 13.51 (1H, s, OH); δF (376 MHz, DMSO-d6) −139.9 (1F, d, J 22
Hz), −152.9 (1F, m), −162.3 (1F, m), −171.1 (1F, m). HRMS
(APCI) m/z found: 300.0095. C13H6F4NOS (M+H+) requires
300.0101.
4,5,6,7-Tetrafluoro-2-(2-methoxyphenyl)benzothiazole (10). P2S5
(0.27 g, 1.2 mmol) and N-(pentafluorophenyl)-2-methoxybenzamide
(8) (1.2 g, 3.8 mmol) were refluxed under N2 in dry dioxane (10 mL)
for 24 h. The solvent was evaporated, and the residue was dissolved in
chloroform and washed with aqueous NaHCO3. The organic extract
was dried (MgSO4) and evaporated, leaving 10 (1.191 g, 100%) as a
pale orange solid, mp 204−206 °C. νmax/cm−1 (ATR) 2843, 1598,
1583, 1484, 1454, 1440, 1351, 1287, 1256, 1231, 1163; δH (270 MHz,
CDCl3) 4.11 (3H, s, OMe), 6.98−7.15 (2H, m), 7.36−7.50 (1H, m),
8.42 (1 H, dd, J 7.9, 1.5 Hz); δF (377 MHz, CDCl3) −140.7 (1F, dd, J
20.5, 15.4 Hz), −150.1 (1F, dd, J 19.0, 15.6 Hz), −160.2 (1F, t, J 19.3
Hz), −161.0 (1F, t, J 20.1 Hz). HRMS (ESI) m/z found: 314.0260.
C14H8F4NOS (M+H+) requires 314.0257.
2-(3,4,5,6-Tetrafluoro-2-methoxyphenyl)-4,5,6,7-tetrafluoroben-
zothiazole (11). N-(Pentafluorophenyl)-3,4,5,6-tetrafluoro-2-methox-
ybenzamide (9) (3.03 g, 7.79 mmol) and P2S5 (865 mg, 3.89 mmol)
were refluxed in dry dioxane (80 mL) under N2 for 20 h. The solvent
was evaporated, and the residue was extracted with ether and washed
with aqueous NaHCO3. The organic phase was dried over Na2SO4,
filtered, and evaporated. The resulting pale yellow solid was subjected
to flash chromatography (gradient from petroleum-DCM 6:1 to 4:1)
to give 11 (2.14 g, 71%) as a white solid, mp 110−111 °C. νmax/cm−1
1637, 1490, 1467, 1346, 1088, 998, 885, 820, 747; δH (270 MHz,
CDCl3) 4.08 (s, OMe); δF (367 MHz, CDCl3) −137.6 (1F, ddd, J 22,
9, 5 Hz), −139.3 (1F, dd, J 21, 16 Hz), −147.6 (1F, dd, J 19, 16 Hz),
−150.8 (1F, td, J 21, 5 Hz), −156.3 (1F, dd, J 21, 9 Hz), −158.1 (1F, t,
J 19 Hz), −158.4 (1F, t, J 19 Hz), −162.5 (1F, t, J 21 Hz). HRMS (EI)
m/z found: 384.9801. C14H5F9NO2 (M+) requires 384.9802.
The above procedure was repeated, starting with 2-amino-
thiophenol (14) (1.00 g, 8.02 mmol) and pyrone derivative 15
(1.22 g, 4.01 mmol), but instead of using flash chromatography,
purification of the crude product was attempted by repeated
recrystallization from toluene. 2-(2-Benzothiazol-2-yl-1-hydroxyvin-
yl)-3,4,5,6-tetrafluorophenol (17) (54 mg, 4%) was obtained as a
bright yellow solid, mp >200 °C (decomp). νmax/cm−1 (ATR) 3157,
1658, 1521, 1432, 1204, 1146, 987, 926, 749, 720, 686; δH (400 MHz,
DMSO-d6) 6.75 (1H, s), 7.34 (1H, t, J 8 Hz), 7.44−7.60 (2H, m), 7.96
(1H, d, J 8 Hz), 13.59 (1H, br, OH), 15.63 (1H, br, OH); δF (376
MHz, DMSO-d6) −139.7 (1F, m), −154.0 (1F, m), −165.2 (1F, m),
−173.2 (1F, m). HRMS (APCI) m/z found: 342.0203; C15H8F4NO2S
(M+H+) requires 342.0206.
Method B. 2-Methoxy-3,4,5,6-tetrafluorobenzoyl chloride (0.50 g,
2.0 mmol) was added dropwise to a solution of 2-aminothiophenol
(14) (0.26 g, 2.0 mmol) in N-methyl-2-pyrrolidone (2 mL). The
mixture was stirred at 100 °C under N2 for 24 h and then poured into
water. The precipitate was filtered off and washed with water. After
drying, it was recrystallized from chloroform and further purified by
dry flash chromatography (CHCl3−hexane 2:3, preheated before use).
The early product-containing fractions were combined and evaporated
to give 16 as a flaky, lemon-yellow solid (253 mg, 42%), with mp, IR,
1H NMR, and 19F NMR data in agreement with those for 16 prepared
4,5,6,7-Tetrafluoro-2-(2-hydroxyphenyl)benzothiazole (12). BBr3
(3.0 mL, 32 mmol) was added to a solution of 4,5,6,7-tetrafluoro-2-(2-
methoxyphenyl)benzothiazole (10) (1.00 g, 3.19 mmol) in DCM (16
mL). The mixture was stirred under N2 at room temperature for 3 h
and then poured onto ice. The resulting suspension was extracted with
chloroform, which was dried (Na2SO4), then concentrated under a
vacuum to give 12 as a white solid (960 mg, 100%), mp 181−183 °C.
Found: C, 52.26; H, 1.48; N, 4.51. C13H5F4NOS requires C, 52.18; H,
1.68; N, 4.68.
by method A above. Found: C, 52.09; H, 1.68; N, 4.56. C13H5F4NOS
requires: C, 52.18; H, 1.68; N, 4.68%.
Zinc Complex of 2-(2-Hydroxy-3,4,5,6-tetrafluorophenyl)-
benzothiazole (19). To a boiling solution of 2-(3,4,5,6-tetrafluoro-2-
hydroxyphenyl)benzothiazole (16) (23.8 mg, 0.0796 mmol) in MeOH
(50 mL) was added 0.1 M methanolic KOH (0.79 mL, 0.0796 mmol),
followed by a solution of Zn(OAc)2·2H2O (8.72 mg, 0.0398 mmol) in
MeOH (5 mL). A yellow precipitate gradually formed, and the mixture
was kept boiling and stirring for another 45 min, after which the
precipitate was filtered off and washed with boiling MeOH to yield 19
as a bright yellow, fluffy solid (21.1 mg, 80%), mp > 400 °C. Found: C,
47.25; H, 1.25; N, 4.18. C26H8F8N2O2S2Zn requires: C, 47.18; H, 1.22;
N, 4.23%. νmax/cm−1 (ATR) 1656, 1481, 1404, 1259, 998, 750; λmax
(DMSO) 398 (40200), 281 (24000); δH (400 MHz, DMSO-d6) 8.34
(1H, d, J 8 Hz), 8.06 (1H, d, J 8 Hz), 7.40−7.49 (2H, m); δF (376
MHz, DMSO-d6) −141.7 (m), −141.8 (m), −157.6 (m), −163.7 (m).
HRMS (APCI) m/z found: 660.9269. C26H9F8N2O2S2Zn (M+H+)
requires 660.9264.
Zinc Complex of 4,5,6,7-Tetrafluoro-2-(2-hydroxyphenyl)-
benzothiazole (20). To a solution of 4,5,6,7-tetrafluoro-2-(2-
hydroxyphenyl)benzothiazole (12) (0.30 g, 1 mmol) in chloroform
(15 mL) was added 1 M KOH in MeOH (1 mL, 1 mmol). The
resultant bright yellow precipitate was filtered off and washed with
chloroform to give the potassium salt of ligand 12 (0.286 g, 85%). A
portion of this potassium salt (68 mg, 0.2 mmol) was dissolved in
MeOH (3 mL) and mixed with a solution of Zn(OAc)2·2H2O (11 mg,
0.1 mmol) in MeOH (0.5 mL). The mixture was refluxed for 12 min,
and the resultant yellow precipitate was filtered off, washed with
MeOH, and dried. The filtrate was concentrated under a vacuum to
give a second crop of precipitate, which was combined with the first
crop to give the zinc complex 20 (50.7 mg, 77%), mp > 400 °C.
Found: C, 47.29; H, 1.61; N, 4.02. C26H8F8N2O2S2Zn requires: C,
47.18; H, 1.22; N, 4.23%. νmax/cm−1 (ATR) 1480, 1417, 1198, 1013,
883; λmax (DMSO) 431 (ε 42500), 295 (23500); δH (400 MHz,
DMSO-d6) 8.22 (1H, d, J 8 Hz), 7.23 (1H, t, J 8 Hz), 7.09 (1H, d, J 8
Hz), 6.67 (1H, t, J 8 Hz); δF (376 MHz, DMSO-d6) −140.5 (m),
−151.3 (m), −161.0 (m), −163.2 (m). HRMS (APCI) m/z found:
660.9265. C26H9F8N2O2S2Zn (M+H+) requires 660.9264.
UV (MeCN): λmax 288 (ε 18400), 335 (16800); νmax/cm−1 (ATR)
2950 (O−H), 1550, 1490, 1230, 1020; δH (270 MHz, CDCl3) 7.00
(1H, td, J 8.1, 1.4 Hz), 7.13 (1H, dd, J 8.1, 1.0 Hz), 7.46 (1H, td, J 8.1,
1.4 Hz), 7.64 (1H, dd, J 8.1, 1.4 Hz), 11.62 (1H, s, OH); δF (376
MHz, CDCl3) −139.2 (1F, dd, J 23, 15 Hz), −148.6 (1F, dd, J 21, 15
Hz), −157.8 (1F, t, J 21 Hz), −158.9 (1F, t, J 21 Hz). HRMS (APCI)
m/z found: 300.0101. C13H6F4NOS (M+H+) requires 300.0101.
2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)-4,5,6,7-tetrafluoroben-
zothiazole (13). BBr3 (4 mL, 42 mmol) was added to a solution of 2-
(3,4,5,6-tetrafluoro-2-methoxyphenyl)-4,5,6,7-tetrafluorobenzothiazole
(1.00 g, 2.60 mmol) in DCM (16 mL). The mixture was stirred at
room temperature for 2 h and then poured onto ice (150 g). The
resulting suspension was extracted with DCM twice and ether once.
The combined organic phases were dried over Na2SO4 and evaporated
to leave 13 as a yellowish-white solid (960 mg, 99%), mp 158−159 °C.
νmax/cm−1 1668, 1542, 1489, 1437, 1406, 1353, 1270, 1144, 1100,
1050, 988, 750; δH (270 MHz, CDCl3) 8.01 (br, OH); δF (367 MHz,
CDCl3) −138.8 (1F, dd, J 20, 15 Hz), −139.4 (1F, dt, J 22, 8 Hz),
−147.8 (1F, t, J 17 Hz), −149.2 (1F, td, J 15, 6 Hz), −156.0 (1F, t, J
20 Hz), −156.3 (1F, t, J 20 Hz), −161.4 (1F, ddd, J 20, 9, 4 Hz),
−170.3 (1F, td, J 19, 4 Hz). HRMS (EI) m/z found: 370.9650.
C14HF8NOS (M+) requires 370.9646.
2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzothiazole (16).
Method A.13 A mixture of 2-aminothiophenol (14) (1.35 g, 10.8
mmol) and 3-ethoxycarbonyl-2-methyl-5,6,7,8-tetrafluoro-4H-1,4-di-
hydrobenzopyran-4-one23 (15) (0.820 g, 2.70 mmol) in dry toluene
(32 mL) was refluxed under N2 overnight. After cooling, the yellow
precipitate was filtered off and recrystallized from toluene to yield a
bright yellow solid (0.115 g), which was indicated to be a mixture by
1H NMR. Flash chromatography (petrol-DCM, 4:1) gave 16 as a pale
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Zinc Complex of 2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)-4,5,6,7-
tetrafluorobenzothiazole (21). To a boiling solution of ligand 13
(649 mg, 1.75 mmol) in 1:1 Et2O−MeOH was added 1 M KOH in
MeOH (1.80 mL, 1.80 mmol); the color changed from pale yellow to
bright yellow at once. After 5 min, the solvent was evaporated, and the
bright yellow residue was dissolved in boiling water. To the resultant
solution was added aqueous Zn(OAc)2·2H2O (287 mg, 1.31 mmol). A
bright yellow precipitate formed immediately. After 30 min, the
mixture was filtered while hot; the precipitate was washed with boiling
water and dried to give zinc complex 21 (645 mg, 92%), mp 338−340
°C. Found: C, 38.61; H, <0.10; N, 3.64. C26F16N2O2S2Zn requires C,
38.76; H, 0.00; N, 3.47%. λmax (DMSO) 404 (ε 31900), 282 (23100);
δF (376 MHz, CD3OD) −141.4 (m), −142.7 (m), −149.3 (m),
−157.7 (m), −161.8 (m), −162.5 (m), −166.3 (m), −181.6 (m).
HRMS (APCI) m/z found 826.8329. C26F16N2NaO2S2Zn (M+Na+)
requires 826.8329.
(14) Brembilla, A.; Roizard, D.; Lochon, P. Synth. Commun. 1990, 20,
3379−3384.
(15) Melman, J. H.; Rohde, C.; Emge, T. J.; Brennan, J. G. Inorg.
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ASSOCIATED CONTENT
* Supporting Information
■
(21) Qureshi, M.; Manoharan, S. S.; Singh, S. P.; Mahapatra, Y. N.
Solid State Commun. 2005, 133, 305−309.
S
Molecular structures of enolate complexes 5 and 7, molecular
structures of phenoxide complexes 19 and 20, emission of
perfluorinated zinc phenolate 21 in different solvents, measure-
ments of fluorescence quantum yield. This material is available
free of charge via the Internet at http://pubs.acs.org.
(22) Kisil’, S. P.; Burgart, Ya. V.; Saloutin, V. I.; Chupakhin, O. N. J.
Fluorine Chem. 2001, 108, 125−131.
(23) Kisil’, S. P.; Burgart, Y. V.; Saloutin, V. I. Russ. J. Org. Chem
2001, 37, 1455−1462.
AUTHOR INFORMATION
Corresponding Author
*E-mail: p.b.wyatt@qmul.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Mr. G. Coumbarides for NMR spectroscopy, the
EPSRC National Mass Spectrometry Service Centre, Swansea
for MS measurements, the EPSRC National Crystallography
Service, Southampton for data collection, the EPSRC and the
Royal Academy of Engineering (vacation bursary for J.M.,
fellowship for I.H.), and the China Scholarships Council
(studentships for Z.L., Y.P., and H.Y.).
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