Cyclometalated rhodium complexes of phenyl- and diphenyl-substituted oxazole and thiazole luminophores

Article abstract of DOI:10.1134/S107036321210012X

Cyclometaleted rhodium complexes of oxazole and thiazole luminophores [Rh(C^N)₂En]Cl [(C^N)^-, deprotonated forms of 2,5-diphenyloxazole, 2-phenylbenzothiazole, 2-(biphenyl-4-yl)-6- phenylbenzoxazole, and 2-(biphenyl-4-yl)-5-phenyloxazole; En - ethylenediamine] were obtained and characterized by the methods of ¹H NMR, IR, and electron absorption and emission spectroscopy. Two cyclometallated ligands in the inner sphere of the complexes are in the cis-C,C positions. Cyclometallating of the luminophores results both in a red shift of intraligand π-π^*-optical transitions (Δν ~1.2 kK) as compared to free luminophores and in the appearance of a long-wave band (λ_max 376-392 nm) of a mixed nature: metal-ligand charge transfer/intraligand transition. Alongside with the internal conversion to a low-energy state of the metal-ligand charge transfer/intraligand transition, the emission degradation of photoexcitation energy results in the intraligand π-π^*-fluorescence of the complexes (λ _max 390-423 nm) at room temperature. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S107036321210012X

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 10, pp. 1715–1719. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.A. Katlenok, K.P. Balashev, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 10, pp. 1711–1715.
Cyclometalated Rhodium Complexes of Phenyl- and Diphenyl-
Substituted Oxazole and Thiazole Luminophores
E. A. Katlenok and K. P. Balashev
Herzen Russian State Pedagogical University, Moika emb. 48, St. Petersburg, 191186 Russia
e-mail: balashevk@mail.ru
Received February 14, 2012
Abstract—Cyclometaleted rhodium complexes of oxazole and thiazole luminophores [Rh(C^N)2En]Cl [(C^N)–,
deprotonated forms of 2,5-diphenyloxazole, 2-phenylbenzothiazole, 2-(biphenyl-4-yl)-6-phenylbenzoxazole,
and 2-(biphenyl-4-yl)-5-phenyloxazole; En – ethylenediamine] were obtained and characterized by the methods
1
of Н NMR, IR, and electron absorption and emission spectroscopy. Two cyclometallated ligands in the inner
sphere of the complexes are in the cis-С,С positions. Cyclometallating of the luminophores results both in a red
shift of intraligand π–π*-optical transitions (Δν ~1.2 kK) as compared to free luminophores and in the
appearance of a long-wave band (λmax 376–392 nm) of a mixed nature: metal-ligand charge transfer/intraligand
transition. Alongside with the internal conversion to a low-energy state of the metal-ligand charge transfer/
intraligand transition, the emission degradation of photoexcitation energy results in the intraligand π–π*-
fluorescence of the complexes (λmax 390–423 nm) at room temperature.
DOI: 10.1134/S107036321210012X
Aryl-substituted azoles are effective organic fluoro-
phores used as activating additives in scintillators [1].
One of the methods for modification of their optical
properties is cyclometalating leading to the formation
of both intraligand optical transitions and intensive
long-wave absorption bands of metal-ligand charge
transfer and to the significant changes in luminescent
characteristics of fluorophores. Unlike fluorescence of
aryl-substituted oxazoles in the ultraviolet region of
the spectrum, their cyclometalated complexes of Pt(II)
and Ir(III) exhibit intensive phosphorescence in the
visible region [2, 3], whereas Pd(II) complexes
demonstrate phosphorescence at reduced (77 K)
temperature [4] and fluorescence at room temperature
[5].
The present work shows the results of study of
complexes [Rh(C^N)2En]Cl [(C^N)– are deprotonated
forms of 2,5-diphenyloxazole (dpo), 2-(biphenyl-4-yl)-
5-phenyloxazole (bpo), 2-phenylbenzothiazole (pbt),
and 2-(biphenyl-4-yl)-6-phenylbenzoxazole (bpbo); En
is ethylenediamine].
1
The studies of the Н NMR and IR spectra of the
obtained compounds show that regardless of the nature
of luminophores the complexes include three five-
membered metal cycles formed with the participation
of the donor N atoms and the deprotonated C atom of
the phenyl or biphenyl substituent of azole ligands, as
well as two imine donor N atoms of ethylenediamine.
The metallating of the phenyl or biphenyl substituent
5'
6'
5'
3'
6'
Ph
Ph
O
O
N
4'
,
(C N) =
,
Ph
2'C_
2'C_
4
4
N
3'
N
Rh
N
N
β
γ
C
C
α
dpo
bpo
Cl
7
4
7
5'
3'
5'
6'
6'
Ph
O
S
N
6
5
6
5
δ
,
4'
Ph.
2'C_
C_
[Rh(C N)2En]Cl
N
N
3'
4
bpbo
pbt
1715

1716
KATLENOK, BALASHEV
Table 1. Characteristic vibration frequencies (cm–1) of luminophores and complexes
(C^N)
En
Compoundа
ν
CH (C6H5)
νCH (C6H5)
νCH (C6H5)
ν(C=N)
1609
1596
1611
1592
1480
1474
1615
1607
νs(NH2)
–
νas(NH2)
Hdpo
686, 707, 760, 776
694, 775
–
735
834
–
–
–
3312
–
[Rh(dpo)2En]Cl
Hbpo
–
3223
–
692, 708, 762, 772
690, 711, 747, 769
686, 759
–
[Rh(bpo)2En]Cl
Hpbt
827, 840
3250
–
3327
–
–
–
[Rh(pbt)2En]Cl
Hbpbo
–
753
826
–
–
–
3291
–
3352
–
691, 701, 764, 773
697, 762
[Rh(bpbo)2En]Cl
830, 858
3296
3356
a
Hdpo is 2,5-diphenyloxazole; Hpbt is 2-phenylbenzothiazole; Hbpo is 2-(biphenyl-4-yl)-5-phenyloxazole; Hbpbo is 2-(biphenyl-4-yl)-6-
phenylbenzoxazole.
Table 2. Coordination-induced chemical shifts (Δδ = δcompl – δlig, ppm) of hydrogen atoms of ligands closest to coordination
center in [Rh(C^N)2En]Cl complexes
(C^N)–
4
5
–
6
–
7
–
3'
4'
–0.7
–
5'
6'
α
β
γ
δ
dpo
0.8
0.7
0.2
0.3
–1.0
–1.0
–1.3
–1.0
–0.6
–0.5
–0.5
–0.6
–0.5
–0.4
–0.3
–1.1
3.1
3.1
3.3
3.6
0.3
0.4
0.4
0.2
0.2
0.2
0.0
0.2
2.7
2.8
2.5
3.0
bpo
–
–
–
pbt
0.0
–0.4
0.0
–
–0.1
0.0
–0.8
–
bpbo
of the luminophores leads to the disappearance of the
Н2' proton resonance from the NMR spectra of the
complexes and to the characteristic [6] change of the
bands of out-of-plane bending C–H vibrations (Table 1).
Two bands of out-of-plane bending C–H vibrations of
phenyl-substituted luminophores within the range of
686–776 cm–1 are transformed into one band within the
range of 735–753 cm–1 as a result of the metalating. In
case of diphenyl-substituted ligands the metalating of
1,4-disubstituted benzene ring describable by a band
within the range of 826–834 cm–1 leads to its splitting
into two bands at 827–830 and 840–858 cm–1.
The coordination of ethylenediamine leads to a
displacement of the chemical shifts of amine and
ethylene protons to the weak field (Table 2) and to the
characteristic (νs 345.8 + 0.876νas) frequencies [6] of
symmetric (νs 3223–3296 cm–1) and antisymmetric (νas
3312–3356 cm–1) stretching vibrations of the ethylene-
diamine N–H bonds (Table 1). A modification of the
cyclometalated ligand nature in the [Rh(C^N)2En]Cl
complexes owing to the introduction of the fused
benzene ring into the heterocyclic component of the
ligand results in a simultaneous increase in the
chemical shifts of amine protons and frequencies of
stretching N–H vibrations of ethylenediamine.
The donor-acceptor interactions between Rh(III)
and oxazole and thiazole components of luminophores
lead to a weak-field displacement of the chemical shift
of Н4 protons (Δδ 0.8–0.7 ppm) or of Н5 protons (Δδ
0.3–0.2 ppm) and to the low-frequency shift (Δν 16±3
cm–1) of stretching C=N vibrations (Tables 1 and 2) as
The presence of the hexafluorophosphate ion in the
outer sphere of the complex [Rh(pbt)2En]PF6 is
charac-terized by the frequencies [6] of bending (ν4
554 cm–1) and stretching (ν3 838 cm–1) vibrations.
The mutual anisotropic action of circular currents
of two metalated phenyl rings of heterocyclic ligands
in the complexes leading to a significant (Δδ –1.0–
1.3 ppm) displacement of the chemical shift of Н3'
protons to the strong field points to the cis-С,С-
1
compared to free ligands. Spectroscopic Н NMR and
IR characteristics of the phenyl groups of lumino-
phores remote from Rh(III) in the complexes are
similar to properties of free ligands.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 10 2012

CYCLOMETALATED RHODIUM COMPLEXES
(a)
1717
position of cyclometalated ligands in the inner sphere
of Rh(III) complexes. In accordance with the cis-
position of amine protons of ethylenediamine and
azole components of heterocyclic ligands, the intro-
duction of the phenyl substituent instead of biphenyl-
4-yl into position 2 negligibly affects the difference in
chemical shifts of Нα and Нδ protons of ethylene-
diamine (Δδ 0.33–0.31 ppm). At the same time, the use
of a fused benzene cycle in the azole component of
ligands results in an increasing difference in their
chemical shifts (Δδ 0.60–0.78 ppm) and in doublet
splitting of Нα protons (2J ~11 Hz) (Fig. 1).
(b)
Fig. 1. Fragments of 1Н NMR spectrum of complexes
(a) [Rh(bpbo)2En]Cl and (b) [Rh(bpo)2En]Cl.
The comparison of the electronic absorption spectra
of the complexes [Rh(C^N)2En]Cl and free lumino-
phores H(C^N) demonstrate (Fig. 2) that alongside
with red shifted (~1.2 kK) intraligand π–π*-optical
transitions in the region of <340 nm the cyclo-
metalated complexes are characterized (Table 3) by a
long-wave absorption band (λmax 376–392 nm). The
results of quantum chemical calculations for Rh(III)
cyclometalated complexes [7] point to the mixed
dRh/π(C^N) nature of the highest occupied molecular
orbital (HOMO) and prevailing localization of the
lowest unoccupied molecular orbital (LUMO) on the
oxazole component of the ligand. This fact makes it
possible to assign the long-wave band in the absorption
spectra of the complexes to the optical transition of a
mixed nature: metal-ligand charge transfer/intraligand
transition within the framework of the localized
molecular orbital model [8]. In accordance with the
mixed nature of the long-wave absorption band of
Rh(III) complexes, a decrease in the contribution of
the π(C^N) orbital to the HOMO composition leads to a
red shift of the long-wave band of Ir(III) complexes by
~2700 kK [3].
Like free luminophores, photoexcitation of solu-
tions of the complexes at room temperature gives rise
to their fluorescence (Fig. 2). The comparison of
relative intensity and energy position of fluorescence
bands for the complexes and free luminophores show
that cyclometalating leads to a more than ~10-fold
decrease in the intensity and to a red [2.3–0.5 kK] shift
of the fluorescence band. The fluorescence excitation
spectra of the complexes are in line (Fig. 2) with their
absorption spectra within the range of intraligand
bands. However, photoexcitation in a more low-energy
region of metal-ligand charge transfer/intraligand
optical transition causes no fluorescence. This fact
makes it possible to assign fluorescence of the
complexes to emission degradation of photoexcitation
energy resulting from the intraligand π–π*-optical
transition. The decrease in the intensity of fluorescence
of cyclometaleted rhodium luminophores as compared
to free luminophores appears to be related to a process
competing with emissive deactivation of the intra-
Relative units
(b)
Relative units
(a)
λ, nm
λ, nm
Fig. 2. Spectra: (1) electronic absorption, (2) fluorescence, (3) fluorescence excitation of (a) Hdpo and (b) [Rh(dpo)2En]Cl
(CH3CN, 293 K).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 10 2012

1718
KATLENOK, BALASHEV
Table 3. Optical characteristics of luminophores and complexes (293 K, CH3CN)
Fluorescence λmax, nm
excitation
Compound
Hdpo
Absorption λmax, nm (ε×103, l mol–1 cm–1)
emission
364
229 (12.7), 304 (30.8), 316 sh (29), 333 sh (13)
230 (35.6), 295 (35.8), 321 sh (24), 361 sh (20), 376 (21.1)
227 (39.8), 248 sh (17), 255 (16.6), 298 (35.4)
227 (46.9), 266 (21.0), 278 sh (20.6), 318 (35.1), 400 (12.6)
228 (18.8), 260 (16), 322 (51.5), 350 sh (23.2)
233 (20.9), 270 sh (16), 303 (19.9), 331 sh (18), 385 (13.6)
272 sh (17.0), 326 (60.4)
299, 314 sh
[Rh(dpo)2En]Cl
Hpbt
398
287 sh, 312, 330
289 sh, 299, 309 sh
292, 298, 310 sh, 348
260, 312, 328
365
[Rh(bt)2En]PF6
Hbpo
390
392
[Rh(bpo)2En]Cl
Hbpbo
399
289 sh, 299, 309 sh, 342
298 sh, 312, 328
287 sh, 301, 311, 329
398
[Rh(bpbo)2En]Cl
231 (32.9), 276 (23.9), 317 (35.6), 337 sh (31.7), 392 (16.1)
423
ligand π–π*-electron-excited state of the complexes,
namely the process of internal conversion into a more
low-energy state of metal-ligand charge transfer/
intraligand transition. The absence of luminescence of
Rh(III) complexes from the state of metal-ligand
charge transfer/intraligand transition at room tem-
perature is connected with thermally activated popula-
tion of d–d-states subjected to nonradiative deactiva-
tion [9, 10].
(CD3)SO, CH2Cl2, and CH3CN. The IR spectra in KBr
tablets were taken on a Shimadzu IR Prestige-21
spectrometer.
The synthesis of 2-phenylbenzothiazole was carried
out according to the procedure [11].
Rh(III) complexes were obtained according to the
modified procedure [5]. The boiling of 1 mmol of
RhCl3·3H2O with 2.3 mmol of heterocyclic ligands in
the medium of 2-methoxyethanol led to the precipita-
tion of the complex [Rh(C^N)2(μ-Cl)]2. The addition
of ethylenediamine to dimeric complexes suspended in
2-methoxymethanol led to the dissolution of the
precipitate due to the reaction of replacement of
bridging chloride ligands. The precipitation of the
complexes [Rh(C^N)2En]Cl from 2-methoxymethanol
solution was performed by adding ether, whereas the
complexes [Rh(pbt)2En]PF6 were obtained as a result
of KPF6 precipitation.
Thus, unlike cyclometaleted iridium oxazole and
thiazole luminophores [3] exhibiting intensive phos-
phorescence at room temperature from the state of
metal-ligand charge transfer/intraligand transition,
similar Rh(III) complexes exhibit fluorescence from
the intraligand π–π*-state.
The results of the present work demonstrate that
preparation cyclometalated rhodium comlexes of
phenyl-, diphenyl-substituted, and benzene-fusede
azole fluorophores occur to form five-membered
{Rh(C^N)} cycles with cis-С,С arrangement of two
cyclometalated ligands in the inner sphere of the
complexes [Rh(C^N)2En]+. Optical properties of the
complexes are characterized by the presence of both a
long-wave optical metal-ligand charge transfer/intra-
ligand transition and a red shift of intraligand absorp-
tion bands and also by weak intraligand fluorescence at
room temperature.
Bis[2-(phenyl-2-ido)-5-phenyloxazole] ethylenedi-
aminerhodium(III) chloride, [Rh(dpo)2En]Cl. Yield
1
41%. Н NMR spectrum [(CD3)2SO], δ, ppm (J, Hz):
8.68 s (Н4), 7.87 d. d (2HPh, 3J 8.5, 4J1.9), 7.63 t (2HPh,
3J 7.9), 7.56 d (H6', 3J 8.0), 7.51 t (HPh, 3J 7.5), 6.98 t. d
(H5', 3J 7.3, 4J 1.1), 6.92 t. d (H4', 3J 7.4, 4J 1.5), 6.60 d
(H3', 3J 7.5), 4.37 m (2Hα), 4.04 m (2Hδ), 2.81 m (2Hβ),
2.64 m (2Hγ).
Bis[2-(biphenyl-4-yl-3-ido)-5-phenyloxazole] ethyl-
ene-diaminerhodium(III) chloride, [Rh(bpo)2En]Cl.
EXPERIMENTAL
1
Yield 52%. Н NMR spectrum [(CD3)2SO], δ, ppm (J,
The 1Н NMR spectra, electronic absorption spectra,
fluorescence, and fluorescence excitation spectra were
obtained at JNM-ECX400A, SF-2000, and Fluorat
Panorama spectrometers at 293 K for solutions in
3
3
Hz): 8.56 s (Н4), 7.90 d (2HPh, J 8.4), 7.78 d (H6', J
7.9), 7.64 d. d (2HPh, J 8.0, 7.5), 7.52 t (HPh, J 7.4),
7.37–7.25 m (6HPh',5'), 6.82 s (H3'), 4.45 m (2Hα), 4.14
m (2Hδ), 2.83 m (2Hβ), 2.72 m (2Hγ).
3
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 10 2012

CYCLOMETALATED RHODIUM COMPLEXES
1719
3. Lamansky, S., Djurovich, P., Murphy, D., Abdel-
Razzaq, F., Lee, H.-E., Adachi, C., Burrows, P.E.,
Forrest, S.R., and Thomson, M.E., J. Am. Chem. Soc.,
2001, vol. 123, no. 18, p. 4304.
Bis[2-(biphenyl-4-yl-3-ido)-6-phenylbenzoxazole]
ethylenediaminerhodium(III) chloride, [Rh(bpbo)En]Cl.
1
Yield 56%. Н NMR spectrum [(CD3)2SO], δ, ppm (J,
3
3
Hz): 8.29 d (2H8, J 1.4), 8.17 d (2Н4, J 8.3), 7.90 d
(2НPh, J 8.0), 7.85 d (2Н6', J 7.4), 7.55 d. d (2НPh, J
7.4, 7.5), 7.45 d. d (2НPh, 3J 7.3, 7.5), 7.32 d. d (2H5, 3J
8.4, 4J 1.5), 7.22 m (6Н5',Ph'), 6.81 d (2Н3', 4J 0.4), 4.88
3
3
3
4. Rodionova, O.A., Finagenova, G.O., Puzyk, M.V., and
Balashev, K.P., Zh. Obshch. Khim., 2010, vol. 80, no. 2,
p. 343.
d (2Нα, J 10.5), 4.28 m (2Нδ), 2.72 d (2Нβ, J 10.5),
2
2
5. Katlenok, E.A. and Balashev, K.P., Zh. Obshch. Khim.,
2.69 d (2Нγ, 2J 12.3).
2011, vol. 81, no. 11, p. 1917.
Bis[2-(phenyl-2-ido)benzothiazole] ethylenediamine-
rhodium(III) hexafluorophosphate, [Rh(pbt)2En]PF6.
6. Nakamoto, K., Infrared and Raman Spectra of
Inorganic and Сoordination Сompounds, New York,
John Wiley, 1986.
1
Yield 48%. Н NMR spectrum [(CD3)2SO], δ, ppm (J,
3
Hz): 8.34 m (2Н4), 7.96 m (2Н7), 7.82 d. d (2H6', J
7. Su, W.L., Yu, Y.C., Tseng, M.C., Wangang, S.P., and
4
3
4
7.6, J 1.1), 7.60 m (4Н5,6), 6.99 t. d (2H5', J 7.5, J
Huang, W.L., Dalton Trans., 2007, no. 31, p. 3440.
1.0), 6.82 t. d (2H4', 3J 7.5, 4J 1.3), 6.31 d (2H3', 3J 7.6),
8. De Armond, M. and Carlin, C., Coord. Chem. Rev.,
4.62 d (2Hα, J 11.6), 3.84 d (2Hδ, J 7.0), 2.88 m
2
2
1985, vol. 63, p. 325.
(2Нβ), ~2.5 m (2Нγ).
9. Barigelletti, F., Sandrini, D., Maestri, M., Balzani, V.,
and von Zelewsky, A., Inorg. Chem., 1988, vol. 27,
no. 20, p. 3644.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 10 2012