New tridentate cyclometalated platinum(II) and palladium(II) complexes of N,2-diphenyl-8-quinolinamine: Syntheses, crystal structures, and photophysical properties

Article abstract of DOI:10.1016/j.tetlet.2005.09.120

A new tridentate cyclometalated platinum(II) complex derived from N,2-diphenyl-8-quinolinamine, which consists of two crystallographic independent molecules with two intermolecular N-H-Cl-Pt hydrogen bonds forming a dimer, exhibited a low-energy luminesce

Full text of DOI:10.1016/j.tetlet.2005.09.120

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8419–8422
New tridentate cyclometalated platinum(II) and palladium(II)
complexes of N,2-diphenyl-8-quinolinamine: syntheses,
crystal structures, and photophysical properties
Lisheng Mao,a Toshiyuki Moriuchi,a Hidehiro Sakurai,a
Hiroyuki Fujiib and Toshikazu Hiraoa,*
aHandai FRC (Frontier Research Center) and Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
bSANYO Electric Co., Ltd, 1-18-13 Hashiridani, Hirakata, Osaka 573-8534, Japan
Received 23 August 2005; accepted 21 September 2005
Available online 13 October 2005
Abstract—A new tridentate cyclometalated platinum(II) complex derived from N,2-diphenyl-8-quinolinamine, which consists of two
crystallographic independent molecules with two intermolecular N–H–Cl–Pt hydrogen bonds forming a dimer, exhibited a low-
energy luminescence at ca. 740nm in a 1 · 10ꢀ3 M dichloromethane solution and a strong emission centered at 670nm in a solid
state, but the analogous palladium(II) complex was nonemissive at room temperature.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Square-planar d8 transition metal complexes have
received considerable attention due to their intriguing
photophysical and photochemical properties.1 In partic-
ular, luminescent platinum(II) complexes with oligo-
pyridines and cyclometalating ligands have been
extensively investigated over the past years.2–5 Gener-
ally, square-planar d8 complexes stack in a solid state
via metal–metal and/or ligand–ligand interactions,
forming dimer or linear-chain structures.2,3a,b Such
stacking interactions result in interesting low-energy
triplet emissions in the 600–750 nm region.3a,b,5c In
order to account for these phenomena, Miskowski
et al. proposed oligomeric metal–metal-to-ligand charge
transfer (MMLCT: dr* ! p*) and excimeric ligand-
to-ligand (p ! p*) excited states.2b Furthermore, a
series of binuclear d8–d8 complexes have also been
prepared to model the low-energy MMLCT or excimeric
ligand–ligand emissions.3c,d,4b,5b In view of the pro-
nounced effect of metal–metal and ligand–ligand inter-
actions on the photophysical properties, it appears to
be interesting if the electronic structures of these com-
plexes can be affected by other weak interactions. With
these considerations in mind, we designed and synthe-
sized a new C^N^N cyclometalating ligand, N,2-diphen-
yl-8-quinolinamine. This new ligand contains two
different types of nitrogen atoms and one phenyl carbon
atom for metalation. More importantly, the presence of
one NH group in the phenylamino moiety may permit
an intermolecular hydrogen bonding in cyclometalated
complexes. Herein their syntheses, crystal structures,
and photophysical properties are described.
The syntheses of the ligand 4 and its cyclometalated
platinum(II) and palladium(II) complexes, 5 and 6, are
shown in Scheme 1. A starting material, 8-bromo-
2(1H)-quinolinone (1),6 was treated with POBr3 to
afford a key intermediate 2 in a high yield. Subsequent
selective Suzuki–Miyaura mono-coupling was achieved
in 84% yield owing to different reactivities of two
bromine atoms at the 2- and 8-positions. Thus, further
conversion to the desired ligand 4 was readily carried
out by the reaction of 3 with 1.2 equiv of aniline under
standard Buchwald coupling conditions. It is notewor-
thy that the present synthetic method provides a new
route to 2-phenyl-8-quinolinamine derivatives. After
the successful isolation of 4, the syntheses of cyclometa-
lated platinum(II) and palladium(II) complexes were
studied. According to the general procedures,7 a mixture
Keywords: Cyclometalated complex; Quinolinamine; Tridentate ligand;
Hydrogen bond; Luminescence.
*
Corresponding author. Tel.: +81 6 6879 7413; fax: +81 6 6879
7415; e-mail: hirao@chem.eng.osaka-u.ac.jp
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.120

8420
L. Mao et al. / Tetrahedron Letters 46 (2005) 8419–8422
Br
Br
Br
H
N
N
Ph
N
Br
O
a
b
84%
84%
1
2
3
Cl
H
N
NHPh
M
N
N
Ph
d or e
c
96%
4
M = Pt, 5
M = Pd, 6
Scheme 1. Reagents and conditions: (a) 2.0equiv POBr 3, 140 ꢁC, 3 h; (b) 1.1 equiv PhB(OH)2, 3 mol % Pd(PPh3)4, 2 M K2CO3, toluene–ethanol,
80 ꢁC, 22 h; (c) 1.2 equiv aniline, 3 mol % Pd2(dba)3, 7.2 mol % rac-Binap, 1.4 equiv NaOBut, toluene, 100 ꢁC, 19 h; (d) 1.0equiv K 2MCl4 (M = Pt,
Pd), H2O/MeCN, reflux, 24 h; (e) 1.0equiv cis-PtCl2(DMSO)2, 1.0equiv NaOAc, methanol, reflux, 5 h.
of 4 and K2PtCl4 in acetonitrile–water was refluxed for
24 h under argon. After work-up, however, the complex
5 was isolated only in 32% yield. In order to improve the
yield, another method by using cis-PtCl2(DMSO)2 as a
metalating reagent and sodium acetate as a base was
attempted. As a result, the desired product 5 was
obtained in 72% yield after refluxing in methanol for
5 h. Thus, the use of cis-PtCl2(DMSO)2 as a precursor
for the cycloplatinated product 5 proved superior to
that of K2PtCl4 due to a shorter reaction time and a
higher yield. In contrast, the analogous palladium
complex 6 was easily prepared in 90% yield from
K2PdCl4 (Scheme 1).
All these compounds were well characterized by 1H
NMR, 13C NMR, MS, and elemental analyses. Further-
more, the molecular structures of both 5 and 6 were
determined by single-crystal X-ray analysis.
Figure 1. A perspective view of the two independent molecules with
As shown in Figure 1, the crystal structure of 5 consists
of two crystallographic independent molecules.8 Both Pt
selected atom numbering found in the crystal structure of 5. Selected
˚
bond lengths (A) and angles (ꢁ): Pt(1)–Cl(1) 2.321(3), Pt(2)–Cl(2)
atoms exhibited a distorted square planar geometry sim-
2.326(2), Pt(1)–C(11) 1.99(1), Pt(2)–C(111) 1.99(1), Pt(1)–N(1)
ilar to the tridentate Pt(II) complexes reported in the lit-
1.939(8), Pt(2)–N(101) 1.934(9), Pt(1)–N(2) 2.211(9), Pt(2)–N(102)
eratures.5,7a,9 All the bond lengths and angles are also
2.234(9); C(11)–Pt(1)–N(2) 163.1(4), C(111)–Pt(2)–N(102) 162.8(4).
comparable to those of the related Pt(II) complexes,5,7a,9
except that Pt1–N2 and Pt2–N102 are longer. These
longer Pt–N bonds may be attributed to strong trans
influence of the phenyl carbon C(11) or C(111). An
interesting structural feature for 5 depends on the for-
mation of two N–H–Cl–Pt hydrogen bonds between
However, there are no metal–metal and p–p interactions
between two dimeric units. The crystal structure of the
palladium complex 6 is similar to the structure of 5. It
also consists of two crystallographic independent mole-
cules, in which two intermolecular hydrogen bonds were
also formed to give the dimer.11
˚
two independent molecules (N(2)–Cl(2) 3.230(9) A,
˚
N(102)–Cl(1) 3.332(9) A; N(2)–H(10)–Cl(2) 171(2)ꢁ,
N(102)–H(25)–Cl(1) 172(2)ꢁ). Each Cl atom bonded to
the Pt center has a short contact with the NH group
of the other Pt complex. All the distances and angles
are present within a range of N–H–Cl–Pt hydrogen
bonds observed in the literatures.10 In addition, two
intermolecular N–H–Cl–Pt hydrogen bonds resulted in
a dimeric aggregation of 5 in the crystal packing dia-
gram. Within a dimeric unit, Pt(1)–Pt(2) distance of
The room-temperature absorption spectra of 5 and 6
were measured in a 1 · 10ꢀ5 M dichloromethane solu-
tion. The platinum complex 5 showed an intense absorp-
tion in the range 250–400 nm (e > 104 Mꢀ1 cmꢀ1), which
is considered to be assigned to the intraligand transi-
tions of the quinolinamine ligand. In addition, a broad
absorption appeared at around 410nm ( e = 3800
˚
3.41 A indicates a weak metal–metal interaction. The
interplanar separation of ca. 3.60A between two
M
ꢀ1 cmꢀ1), which is likely due to the 1MLCT transition
˚
C^N^N ligands are sufficiently close for p–p interaction
(Pt ! p*), similar to those identified in the spectra of the
as observed in the similar reported structures.5a,b,9
related cyclometalated platinum(II) complexes.5a,b No

L. Mao et al. / Tetrahedron Letters 46 (2005) 8419–8422
8421
absorptions were found in the region beyond 500 nm.
Since the X-ray crystal structure of 5 indicates strong
intermolecular hydrogen bonding interactions, the effect
of the concentration on the absorption spectrum was
further investigated. When a 1 · 10ꢀ3 M dichlorometh-
ane solution was measured by using a thin cell (1 mm),
a shoulder at ca. 570nm was observed at room temper-
ature. The absorption spectrum of the palladium com-
plex 6 showed two intense bands in the range 250–380
nm (e > 104 Mꢀ1 cmꢀ1), accompanied by a weak shoul-
der at around 400 nm. These bands are considered to
be mainly attributed to intraligand transitions. No
low-energy absorption was observed for 6 at concentra-
tions higher than 1 · 10ꢀ3 M.
and concentrated dichloromethane solutions as well as
in a solid state. Further investigation is now in progress.
Acknowledgments
The use of the facilities of the Analytical Center, Grad-
uate School of Engineering, Osaka University is
acknowledged.
Supplementary data
Electronic supplementary data: general experimental
procedures for the syntheses of 2–6, X-ray crystal data
including selected bond lengths and angles for 5 and 6,
crystal packing diagram for 5 and crystal structure of
6, and the room-temperature absorption spectra. Sup-
plementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.tetlet.2005.09.120.
The platinum complex 5 exhibited a strong emission at
room temperature both in solution and solid states.
However, its emissive behavior changed dramatically
depending on the concentration and excitation wave-
length as shown in Figure 2. In the 1 · 10ꢀ5 M dichloro-
methane solution, excitation of 5 at 410nm resulted in a
strong emission around 600 nm. As the concentration
increased to 1 · 10ꢀ3 M, excitation at 500 nm produced
a broad low-energy luminescence at around 740nm.
Besides, a microcrystalline sample of 5 also showed a
strong emission centered at 672 nm when excited at
500 nm. In contrast to the strong luminescent platinum
complex 5, the analogous palladium complex 6 showed
no emission at room temperature both in solution and
solid states as reported in the literatures.12
References and notes
1. Sykora, J.; Sima, J. Photochemistry of Coordination
Compounds; Elsevier: Amsterdam, 1990.
2. (a) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989,
28, 1529; (b) Miskowski, V. M.; Houlding, V. H. Inorg.
Chem. 1991, 30, 4446; (c) Houlding, V. H.; Miskowski, V.
M. Coord. Chem. Rev. 1991, 111, 145; (d) Miskowski, V.
M.; Houlding, V. H.; Che, C. M.; Wang, Y. Inorg. Chem.
1993, 32, 2518.
3. (a) Baily, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V.
M.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34,
4591; (b) Arena, G.; Calogero, G.; Campagna, S.; Scolaro,
L. M.; Ricevuto, V.; Romeo, R. Inorg. Chem. 1998, 37,
2763; (c) Bailey, J. A.; Miskowski, V. M.; Gray, H. B.
Inorg. Chem. 1993, 32, 369; (d) Yip, H. K.; Che, C. M.;
Zhou, Z. Y.; Mak, T. C. W. J. Chem. Soc., Chem.
Commun. 1992, 1369.
In summary, we designed and synthesized the new tri-
dentate C^N^N ligand N,2-diphenyl-8-quinolinamine
and its cyclometalated platinum(II) and palladium(II)
complexes. The crystal structures of both complexes
revealed strong intermolecular hydrogen bonding inter-
actions. In addition, the platinum complex exhibited
unique room-temperature emissive behavior in diluted
1000
4. (a) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P.
I.; Tsyba, I.; Bau, R.; Thompson, M. E. Inorg. Chem.
2002, 41, 3055; (b) Ma, B.; Li, J.; Djurovich, P. I.;
Yousufuddin, M.; Bau, R.; Thompson, M. E. J. Am.
Chem. Soc. 2005, 127, 28.
5. (a) Cheung, T. C.; Cheung, K. K.; Peng, S. M.; Che, C. M.
J. Chem. Soc., Dalton Trans. 1996, 1645; (b) Lai, S. W.;
Chan, M. C. W.; Cheung, T. C.; Peng, S. M.; Che, C. M.
Inorg. Chem. 1999, 38, 4046; (c) Lai, S. W.; Lam, H. W.;
Lu, W.; Cheung, K. K.; Che, C. M. Organometallics 2002,
21, 226; (d) Neve, F.; Crispini, A.; Campagna, S. Inorg.
Chem. 1997, 36, 6150; (e) Yip, J. H. K.; Suwarno; Vittal,
J. J. Inorg. Chem. 2000, 39, 3537.
(a)
800
(c)
600
400
(b)
6. Cottet, F.; Marull, M.; Lefebvre, O.; Schlosser, M. Eur. J.
Org. Chem. 2003, 1559.
200
7. (a) Constable, E. C.; Henney, R. P. G.; Leese, T. A.;
Tocher, D. A. J. Chem. Soc., Chem. Commun. 1990, 513;
(b) Constable, E. C.; Henney, R. P. G.; Leese, T. A.;
Tocher, D. A. J. Chem. Soc., Dalton Trans. 1990, 443.
8. Crystal data for 5: C21H15N2ClPt, M = 525.91,
0
600
400
500
700
800
900
λ / n m
˚
monoclinic, space group P21/c (#14), a = 10.8156(3) A,
˚
˚
Figure 2. The room-temperature emission spectra of
5
(a) in
b = 14.6206(4) A, c = 23.3398(6) A, b = 93.9063(6)ꢁ,
V = 3606.6(2) A , Z = 8, Dc = 1.973 g cmꢀ3, l(Mo-Ka) =
3
1 · 10ꢀ5 M dichloromethane solution (kex 410nm), (b) in 1 · 10ꢀ3 M
dichloromethane solution (kex 500 nm), and (c) in solid state (kex
500 nm).
˚
79.03 cmꢀ1
,
Mo-Ka radiation (k = 0.71069 A), R1 =
˚
0.055, wR2 = 0.158.

8422
L. Mao et al. / Tetrahedron Letters 46 (2005) 8419–8422
9. Hofmann, A.; Dahlenburg, L.; van Eldik, R. Inorg. Chem.
2003, 42, 6528.
11. Crystal data for 6: C21H15N2ClPd, M = 437.22,
˚
monoclinic, space group P21/c (#14), a = 10.7847(2) A,
˚
˚
´
10. (a) Aullon, G.; Bellamy, D.; Brammer, L.; Bruton,
b = 14.1411(4) A, c = 23.6112(5) A, b = 94.3154(4)ꢁ,
3
V = 3590.7(1) A , Z = 8, Dc = 1.617 g cmꢀ3, l(Mo-Ka) =
˚
E. A.; Orpen, A. G. Chem. Commun. 1998, 653; (b) Rivas,
J. C. M.; Brammer, L. Inorg. Chem. 1998, 37, 4756;
(c) Lewis, G. R.; Orpen, A. G. Chem. Commun. 1998,
1873.
11.88 cmꢀ1
,
Mo-Ka radiation (k = 0.71069 A), R1 =
˚
0.054, wR2 = 0.195.
12. Crosby, G. A. Acc. Chem. Res. 1975, 8, 231.