Diplatinum complexes supported by novel tetradentate ligands with quinoline functionalities for tandem C-CI activation and Dearomatization

Article abstract of DOI:10.1021/om800893r

Three novel tetradentate ligands with quinoline functionalities, l,2,4,5-tetrakis(6-ethylquinolin-8-yl)benzene, 1,2,4,5-tetrakis(6-tert- butylquinolin-8-yl)benzene, and l,2,4,5-tetrakis(6-(trifluoromethox-y)quinolin- 8-yl)benzene, were synthesized through a three-step protocol and fully characterized by NMR spectroscopic, elemental, and X-ray diffraction analyses. The dinuclear dimethylplatinum(II) complexes of these ligands readily activate C - CI bonds of CHCl₃ at ambient temperature, leading to the formation of the corresponding dinuclear Pt(IV) complex. In these reactions, the central phenyl rings of the tetradentate ligands are reduced to cyclohexadiene dianions and still retain their planarity.

Full text of DOI:10.1021/om800893r

6614
Organometallics 2008, 27, 6614–6622
Diplatinum Complexes Supported by Novel Tetradentate Ligands
with Quinoline Functionalities for Tandem C-Cl Activation and
Dearomatization
Runyu Tan,† Peng Jia,† Yingli Rao,‡ Wenli Jia,† Alen Hadzovic,† Qing Yu,† Xia Li,† and
Datong Song*,†
DaVenport Chemical Research Laboratory, Department of Chemistry, UniVersity of Toronto, 80 St. George
Street, Toronto, Ontario, Canada M5S 3H6, and Chernoff Hall, Department of Chemistry, Queen’s
UniVersity, Kingston, Ontario, Canada K7L 3N6
ReceiVed September 14, 2008
Three novel tetradentate ligands with quinoline functionalities, 1,2,4,5-tetrakis(6-ethylquinolin-8-
yl)benzene, 1,2,4,5-tetrakis(6-tert-butylquinolin-8-yl)benzene, and 1,2,4,5-tetrakis(6-(trifluoromethox-
y)quinolin-8-yl)benzene, were synthesized through a three-step protocol and fully characterized by NMR
spectroscopic, elemental, and X-ray diffraction analyses. The dinuclear dimethylplatinum(II) complexes
of these ligands readily activate C-Cl bonds of CHCl3 at ambient temperature, leading to the formation
of the corresponding dinuclear Pt(IV) complex. In these reactions, the central phenyl rings of the
tetradentate ligands are reduced to cyclohexadiene dianions and still retain their planarity.
1).6 In this transformation, the tetradentate ttab ligand was
Introduction
noninnocent, the central phenyl ring of which was jointly
The cooperation between multiple metal centers has proven
reduced into a planar air- and moisture-stable cyclohexadiene
dianion by the two platinum centers. This unusual reactivity
originates from the unique geometry of Pt2Me2(ttab), in which
the central phenyl ring of the ttab ligand is situated between
the two platinum centers. It is believed to go through a dinuclear
Pt(III) intermediate, which is formed from the homolytic
cleavage of C-Cl bonds. The two Pt(III) metalloradicals further
give up one electron each to the central phenyl ring of the ttab
ligand to reduce it into its dianion.
useful in metalloenzymes1 and heterogeneous catalysis,2 but it
is not as prevalent in the homogeneous systems.3 Multimetallic
catalysis used in homogeneous systems often involves the simple
combination of two metal complexes.3,4 To date, only a few
homogeneous catalytic systems are known to use well-defined
multinuclear metal catalysts and to have a cooperative effect
between multiple metal centers.3,5 Along the road toward the
final goal of efficient multimetallic homogeneous catalysis,
knowledge gained from the stoichiometric reactions that dem-
onstrate the cooperative effects between multiple metal centers
can be valuable for the rational design of catalytic systems.
Previously, a dinuclear platinum complex, Pt2Me2(ttab) (ttab )
1,2,4,5-tetrakis(7-azaindol-1-yl)benzene) was reported to cleave
C-Cl bonds of CHCl3 at ambient temperature stoichiometric-
ally, owing to the joint action of two metal centers (Scheme
Although this system is not catalytic, it does show the
synergism between the two metal centers, as the reduction of
benzene into its dianion would otherwise require much harsher
conditions.7 Moreover, if the reaction could be reversed, i.e.,
reduce Pt2Me2Cl2(ttab) back to Pt2Me2(ttab), restoring the
aromaticity of the central C6 ring of the ttab ligand and
incorporating the Cl into an inorganic salt, a catalytic C-Cl
activation could potentially be achieved, which could catalyti-
cally degrade chlorinated organics. However, the insolubility
of Pt2Me2Cl2(ttab) makes the reverse reaction difficult to realize.
Despite the limited potential of this system caused by the
solubility issue, it is still a unique example of this kind for the
facile C-Cl activation and dearomatization of a substituted
benzene ring by the joint action of two metal centers under mild
conditions.
* To whom correspondence should be addressed. E-mail: dsong@
chem.utoronto.ca.
† University of Toronto.
‡ Queen’s University.
(1) Bertini, I.; Gray, H. B.; Stiefel, E. I.; Valentine, J. S. Biological
Inorganic Chemistry, Structure and ReactiVity; University Science Books:
Mill Valley, CA, 2007.
(2) (a) Gross, A. Top. Catal. 2006, 37, 29–39. (b) Liu, S.; Poplaukhin,
P.; Ding, E.; Plecnik, C. E.; Chen, X.; Keane, M. A.; Shore, S. G. J. Alloys
Compd. 2006, 418, 21–26. (c) Sinfelt, J. H. Science 1977, 195, 641–646.
(3) (a) Shibasaki, M.; Yamamoto, Y. Multimetallic Catalysts in Organic
Synthesis; Wiley-VCH: Weinheim, Germany, 2004. (b) Pignolet, L. H.;
Aubart, M. A.; Craighead, K. L.; Gould, R. A. T.; Krogstad, D. A.; Wiley,
J. S. Coord. Chem. ReV. 1995, 143, 219–263. (c) Laneman, S. A.; Stanley,
G. G. AdV. Chem. Ser. 1992, 230, 349–366.
(4) (a) Reed, S. A.; White, M. C. J. Am. Chem. Soc. 2008, 130, 3316–
3318. (b) Li, C.; Chen, L.; Garland, M. J. Am. Chem. Soc. 2007, 129, 13327–
13334. (c) Thiot, C.; Schumtz, M.; Wagner, A.; Mioskowski, C. Chem.
Eur. J. 2007, 13, 8971–8978. (d) Adams, R. D.; Captain, B. J. Organomet.
Chem. 2004, 689, 4521–4529. (e) Ishii, Y.; Miyashita, K.; Kamita, K.; Hidai,
M. J. Am. Chem. Soc. 1997, 119, 6448–6449. (f) Hidai, M.; Fukuoka, A.;
Koyasu, Y.; Uchida, Y. J. Mol. Catal. 1986, 35, 29–37.
To generalize this unique reactivity, we have analyzed the
geometry of Pt2Me2(ttab) to deduce the requirements for the
ligand design. The ttab ligand has four rigid 7-azaindol-1-yl
groups bonded to the central phenyl ring, so that the nitrogen
donor atoms are three bonds away from the central phenyl linker.
Such a setting allows the nitrogen lone pairs to point toward
the center of the ttab ligand, and thus, the central phenyl ring
(6) Song, D.; Sliwowski, K.; Pang, J.; Wang, S. Organometallics 2002,
21, 4978–4983.
(5) (a) Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
15295–15302. (b) Severin, K. Chem. Eur. J. 2002, 8, 1514–1518.
(7) Cassani, M. C.; Gun’ko, Y. K.; Hitchcock, P. B.; Lappert, M. F.
Chem. Commun. 1996, 1987–1988.
10.1021/om800893r CCC: $40.75
2008 American Chemical Society
Publication on Web 11/17/2008

Pt2 Complexes Supported by Tetradentate Ligands
Organometallics, Vol. 27, No. 24, 2008 6615
Scheme 1. C-Cl Activation by Pt2Me2(ttab)
Scheme 2. 8-Quinolinyl versus 7-Azaindol-1-yl
and glycerol (5.52 g, 60 mmol) was added slowly via a dropping
funnel. The reaction mixture was then kept at 150 °C for 4 h, cooled
to ambient temperature, neutralized with 10% NaOH solution,
and extracted with Et2O. The organic layers were combined and
dried by Na2SO4. The solvent was removed under reduced pressure,
and the crude product was purified through column chromatography
using CH2Cl2/hexanes as eluent to give 1a (1.4 g, 60% yield) as a
of the ligand is sandwiched between the two square-planar metal
centers in the corresponding dinuclear complex.
1
3
yellow oil. H NMR (CDCl3, 300 MHz, 25 °C): δ 8.99 (dd, J )
4.5 Hz, 4J ) 1.5 Hz, 1H), 8.19 (dd, 3J ) 8.4 Hz, 4J ) 1.5 Hz, 1H),
In our recent research we have chosen 8-quinolinyl as the
functional group in the place of the 7-azaindol-1-yl group in
ttab to construct new tetradentate ligands, because it has the
required rigidity with the nitrogen donor atom three bonds away
from the central phenyl ring and the lone pair pointing inward.
In addition, it is easy to install functional groups on a quinoline
ring, which may potentially improve the solubility of the metal
complexes of the ligand. Moreover, quinoline has a six-
membered ring fused with a pyridine ring, while 7-azaindole
has a five-membered ring fused with a pyridine ring. Therefore,
using 8-quinolinyl instead of 7-azaindol-1-yl for the ligand
construction will force the metal center to be closer to the central
phenyl linker group (Scheme 2). Such a shorter distance may
result in higher activity, because the short metal-to-phenyl
contact distance appears crucial in the facile C-Cl cleavage
reaction.6 Herein we report the syntheses and characterizations
of a series of novel tetradentate ligands featuring 8-quinolinyl
functionality and the reactivity of the corresponding dimethyl-
platinum complexes toward C-Cl activation reactions.
4
3
3
7.95 (d, J ) 2.0 Hz, 1H), 7.57 (s, 1H), 7.43 (dd, J ) 4.4 Hz, J
3
3
) 8.4 Hz, 1H), 2.82 (q, J ) 7.5 Hz, 2H), 1.34 (t, J ) 7.5 Hz,
3H). 13C NMR (CDCl3, 75 MHz, 25 °C): δ 150.5, 144.1, 143.4,
136.1, 134.3, 129.6, 125.4, 124.5, 121.9, 28.55, 15.19. Anal. Calcd
for C11H10BrN: C, 55.96; H, 4.27; N, 5.93. Found: C, 55.88; H,
4.01; N, 5.93.
8-Bromo-6-tert-butylquinoline (1b). The same procedure as given
above was used, using 2-bromo-4-tert-butylaniline as the starting
1
material. 1b was obtained as a colorless solid in 70% yield. H
NMR (CDCl3, 400 MHz, 25 °C): δ 8.97 (dd, 3J ) 4.5 Hz, 4J ) 1.5
3
4
3
Hz, 1H), 8.13 (dd, J ) 8.0 Hz, J ) 2.0 Hz, 1H), 8.10 (dd, J )
4
3
3
8.0 Hz, J ) 2.0 Hz, 1H), 7.68 (s, 1H), 7.40 (dd, J ) 4.0 Hz, J
) 8.0 Hz, 1H), 1.40 (s, 9H). 13C NMR (CDCl3, 100 MHz, 25 °C):
δ 150.8, 150.5, 144.0, 136.8, 132.4, 129.4, 124.6, 123.1, 122.0,
35.18, 31.27. Anal. Calcd for C13H14BrN: C, 59.11; H, 5.34; N,
5.30. Found: C, 58.75; H, 5.22; N, 5.39.
8-Bromo-6-(trifluoromethoxy)quinoline (1c). The same procedure
as given above was used, using 2-bromo-4-(trifluoromethoxy)aniline
as the starting material. 1c was obtained as a colorless solid in 87%
1
3
yield. H NMR (CDCl3, 400 MHz, 25 °C): δ 9.02 (dd, J ) 4.0
4
3
4
Experimental Section
Hz, J ) 2.0 Hz, 1H), 8.13 (dd, J ) 8.4 Hz, J ) 1.6 Hz, 1H),
4
3
3
8.10 (d, J ) 2.4 Hz, 1H), 7.60 (s, 1H), 7.48 (dd, J ) 4.4 Hz, J
) 8.0 Hz, 1H). 13C NMR (CDCl3, 100 MHz, 25 °C): δ 151.8, 146.5,
144.0, 136.7, 129.1, 127.5, 126.6, 123.0, 120.7(q, 1JC-F ) 258 Hz).
19F NMR (CDCl3, 376 MHz, 25 °C): δ -58.37. Anal. Calcd for
C10H5NOBrF3: C, 41.13; H, 1.73; N, 4.80. Found: C, 41.38; H,
1.53; N, 4.85.
General Considerations. Unless otherwise stated, all prepara-
tions and manipulations were performed in air and all reagents were
purchased from commercial sources and used without further
purifications. 2-Bromo-4-ethylaniline,8 2-bromo-4-tert-butylaniline,8
2-bromo-4-(trifluoromethoxy)aniline,8 and [Pt(CH3)2(SMe2)]29 were
prepared according to the literature procedures. THF solvent for
the syntheses of boronic acids was purified using the solvent
purification system from Vacuum Atmospheres Co. DMF and water
for Suzuki coupling reactions were degassed prior to use. NMR
spectra were recorded on a Mercury 300, a Varian 400, or a Bruker
Synthesis of 6-Ethyl-8-quinolineboronic Acid (2a). To a -78 °C
solution of 1a (2.36 g, 10 mmol, in 40 mL of THF) under argon
was added 5.8 mL of PhLi (1.8 M in di-n-butyl ether, 10.5 mmol)
over 5 min. After the reaction mixture was further stirred at -78
°C for 1 h, trimethylborate (4.4 mL, 40 mmol) was added slowly.
The mixture was then warmed to ambient temperature, further
stirred for 4 h, acidified with 3 M HCl (50 mL), and washed with
Et2O. The aqueous layer was then neutralized with 10% NaOH
solution to give an off-white precipitate, which was collected via
vacuum filtration and recrystallized from a mixture of acetone and
hexanes to afford 2a as a white solid (1.4 g, 70%). 1H NMR
(CD3OD/D2O, 300 MHz, 25 °C): δ 9.20 (dd, 3J ) 5.0 Hz, 4J ) 1.5
Hz, 1H), 8.35 (dd, 3J ) 8.1 Hz, 4J ) 1.5 Hz, 1H), 7.97 (d, 4J ) 1.8
1
Avance 400 spectrometer. Both H and 13C NMR spectra were
referenced relative to the solvent’s residual signals but are reported
relative to Me4Si. Elemental analyses were performed at our
chemistry department with a PE 2400 C/H/N/S analyzer.
Synthesis of 8-Bromo-6-ethylquinoline (1a). Sodium 3-nitroben-
zenesulfonate (4.5 g, 20 mmol), FeSO4 · 7H2O (27.8 mg, 0.1 mmol),
boric acid (0.61 g, 10 mmol), 2-bromo-4-ethylaniline (2 g, 10
mmol), and 3.26 mL of concentrated sulfuric acid were mixed in
a 250 mL round-bottom flask. The mixture was heated to 150 °C,
3
3
Hz, 1H), 7.62 (dd, J ) 5.1 Hz, J ) 8.0 Hz, 1H), 7.49 (s, 1H),
2.75 (q, J ) 7.5 Hz, 2H), 1.25 (t, J ) 7.5 Hz, 3H). 13C NMR
(CD3OD/D2O, 75 MHz, 25 °C): δ 144.6, 144.0, 142.4, 141.1, 134.9,
134.6, 129.7, 124.2, 120.3, 28.67, 14.38. ESI-MS: m/z calcd for
C11H12BNO2 201.2, found 202.2 [M + H]+.
3
3
(8) Lee, S. H.; Jang, B.; Kafafi, Z. H. J. Am. Chem. Soc. 2005, 127,
9071–9078.
(9) (a) Hill, G. S.; Irwin, M. J.; Levy, C. J.; Rendina, L. M.; Puddephatt,
R. J. Inorg. Synth. 1998, 32, 149–153. (b) Song, D.; Wang, S. J. Organomet.
Chem. 2002, 648, 302–305.

6616 Organometallics, Vol. 27, No. 24, 2008
Tan et al.
Table 1. Crystallographic Data
1b
1c
2b solvatea
3a · 2CH2Cl2
3b · 2CH2Cl2
3c · 2CH2Cl2
formula
fw
C13H14BrN
264.16
C10H5BrF3NO
292.06
C27.70 H35.37B2N2O4.20
485.17
C52H46Cl4N4
868.73
C60H62Cl4N4
980.94
C48H26Cl4F12N4O4
1092.53
T (K)
space group
a (Å)
b (Å)
c (Å)
150(2)
P21/c
150(2)
P21/n
150(2)
C2/c
100(2)
P21/n
150(2)
P1
150(2)
P1
j
j
10.7872(2)
9.4647(3)
12.7521(3)
90
110.5474(15)
90
1219.13(5)
4
1.439
3.339
9239
2135
1.049
4.4543(4)
11.6713(16)
18.635(2)
90
94.179(7)
90
966.21(19)
4
2.008
4.274
4710
1660
1.048
24.3205(6)
21.1932(5)
34.2425(6)
90
93.8930(10)
90
17608.9(7)
24
1.098
0.072
68559
15478
1.057
12.5079(3)
21.9598(9)
19.3280(7)
90
91.434(2)
90
5307.2(3)
4
1.087
0.258
29692
9301
1.019
12.7286(4)
15.2208(6)
15.4805(5)
104.520(2)
105.413(2)
99.820(2)
2707.06(16)
2
1.203
0.260
30579
9504
11.0134(4)
12.9399(7)
17.9995(8)
91.364(2)
97.888(3)
103.432(3)
2467.25(19)
2
1.471
0.333
25236
11145
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Dc (g cm-3
)
µ (mm-1
)
no. of rflns collected
no. of indep rflns
GOF on F2
1.024
1.074
R (I > 2σ(I))
R1 ) 0.0381
wR2 ) 0.0913
R1 ) 0.0436
wR2 ) 0.0952
R1 ) 0.0782
wR2 ) 0.1993
R1 ) 0.0981
wR2 ) 0.2200
R1 ) 0.0686
wR2 ) 0.1719
R1 ) 0.1071
wR2 ) 0.1847
R1 ) 0.0797
wR2 ) 0.1654
R1 ) 0.1340
wR2 ) 0.1809
R1 ) 0.0633
wR2 ) 0.1515
R1 ) 0.1054
wR2 ) 0.1824
R1 ) 0.0813
wR2 ) 0.1921
R1 ) 0.1556
wR2 ) 0.2150
R (all data)
3d · 2.5C7H8
4b
5a · 2CHCl3
C56H56Cl8N4Pt2
1458.83
5b · 2CHCl3
C64H72Cl8N4Pt2
1571.04
5c · 3CHCl3
C53H37Cl11F12N4O4Pt2
1802.00
formula
fw
T (K)
space group
a (Å)
b (Å)
C85.5H80Br2N2P2Pd2
C62H70N4Pt2
1261.40
150(2)
1570.07
150(2)
150(2)
150(2)
150(2)
j
j
j
j
P1
P21/n
P1
P1
P1
14.1247(3)
15.4594(4)
19.5620(5)
82.4328(14)
72.3137(15)
66.3941(14)
3728.86(16)
2
1.398
1.642
40214
13104
16.0011(11)
11.9023(8)
17.1940(13)
90
101.730(3)
90
3206.2(4)
2
1.307
4.394
22888
5596
1.043
9.3132(4)
11.8742(4)
13.3219(3)
67.5142(17)
85.269(2)
85.9551(17)
1355.37(8)
1
1.787
5.591
14282
5880
10.0472(5)
12.0539(7)
13.3550(7)
70.392(2)
83.136(4)
86.422(3)
1512.25(14)
1
1.725
5.018
18904
6810
11.2692(7)
12.5803(11)
12.6285(11)
69.421(3)
77.399(5)
84.709(5)
1635.5(2)
1
1.830
4.801
12861
5643
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Dc (g cm-3
)
µ (mm-1
)
no. of rflns collected
no. of indep rflns
GOF on F2
1.065
1.080
0.992
1.070
R (I > 2σ(I))
R1 ) 0.0654
wR2 ) 0.1473
R1 ) 0.1156
wR2 ) 0.1658
R1 ) 0.0668
wR2 ) 0.1578
R1 ) 0.0902
wR2 ) 0.1710
R1 ) 0.0389
wR2 ) 0.0964
R1 ) 0.0455
wR2 ) 0.1011
R1 ) 0.0738
wR2 ) 0.1600
R1 ) 0.1389
wR2 ) 0.1917
R1 ) 0.0618
wR2 ) 0.1605
R1 ) 0.0753
wR2 ) 0.1714
R (all data)
a Solvate 0.117 C6H14, 0.333 C3H6O, 0.867 H2O.
Table 2. Selected Bond Lengths (Å) and Angles (deg)a
5a 5b
4b
5c
Pt(1)-C(31)
Pt(1)-C(30)
Pt(1)-N(1)
2.014(14) Pt(1)-C(27)
2.046(11) Pt(1)-C(26)
2.047(6)
2.068(5)
2.121(5)
2.150(4)
2.195(4)
Pt(1)-C(30)
Pt(1)-C(31)
Pt(1)-N(2)
Pt(1)-C(27)
Pt(1)-N(1)
2.043(12) Pt(1)-C(24)
2.053(11) Pt(1)-C(25)
2.046(10)
2.057(10)
2.09(2)
2.107(9)
2.189(8)
2.455(3)
1.318(13)
1.507(12)
1.489(13)
2.134(8)
2.145(9)
Pt(1)-C(24)
Pt(1)-N(1)
2.157(9)
Pt(1)-N(1)
Pt(1)-N(2)
2.161(11) Pt(1)-C(22)
C(27)-C(28)
C(28)-C(29)
C(29)-C(27)#1
C(31)-Pt(1)-C(30)
C(31)-Pt(1)-N(1)
C(30)-Pt(1)-N(1)
C(31)-Pt(1)-N(2)
C(30)-Pt(1)-N(2)
N(1)-Pt(1)-N(2)
1.404(13) Pt(1)-N(2)
1.375(16) Pt(1)-Cl(1)
1.383(15) C(23)-C(25)#2
2.191(9)
2.479(3)
Pt(1)-N(2)
Pt(1)-Cl(1)
2.4562(13) Pt(1)-Cl(1)
1.337(7)
1.494(6)
1.496(6)
C(28)-C(29)
C(27)-C(28)
C(27)-C(29)#3
1.331(15) C(21)-C(23)#4
1.500(15) C(21)-C(22)
1.474(15) C(22)-C(23)
88.4(5)
90.4(5)
177.9(4)
173.7(4)
87.3(4)
93.8(3)
C(23)-C(24)
C(24)-C(25)
C(23)-C(24)-C(25) 112.5(4)
C(29)#3-C(27)-C(28) 114.3(9)
C(23)-C(22)-C(21) 112.3(8)
C(23)-C(24)-C(8)
C(25)-C(24)-C(8)
C(23)-C(24)-Pt(1)
112.8(4)
106.0(4)
109.8(3)
111.0(3)
104.4(3)
C(29)#3-C(27)-C(8)
C(28)-C(27)-C(8)
C(29)#3-C(27)-Pt(1)
C(28)-C(27)-Pt(1)
C(8)-C(27)-Pt(1)
108.0(9)
112.6(8)
110.1(7)
108.5(7)
102.7(7)
C(23)-C(22)-C(8)
C(21)-C(22)-C(8)
C(23)-C(22)-Pt(1)
C(21)-C(22)-Pt(1)
C(8)-C(22)-Pt(1)
106.5(7)
113.9(7)
111.0(6)
109.2(6)
103.6(6)
C(29)#1-C(27)-C(28) 117.6(10) C(25)-C(24)-Pt(1)
C(29)-C(28)-C(27) 119.3(9) C(8)-C(24)-Pt(1)
C(28)-C(29)-C(27)#1 123.1(10)
a Legend: (#1) 2 - x, -1 - y, -z; (#2) 1 - x, -y, 1 - z; (#3) -x, -y, -z; (#4) 1 - x, 2 - y, -z.
Synthesis of 6-tert-Butyl-8-quinolineboronic Acid (2b). The same
procedure as above was used, using 1b as the starting material (yield
50%). H NMR (CD3OD/D2O, 300 MHz, 25 °C): δ 9.19 (d, J )
3.9 Hz, 1H), 8.40 (d, 3J ) 8.1 Hz, 1H), 8.21 (d, 4J ) 2.1 Hz, 1H),
7.64 (d, 4J ) 2.1 Hz, 1H), 7.61 (d, J ) 2.1 Hz, 1H), 1.26 (s, 9H).
13C NMR (CD3OD/D2O, 75 MHz, 25 °C): δ 151.2, 143.8, 142.4,
142.0, 132.8, 132.6, 129.4, 121.8, 120.4, 34.72, 30.21. ESI-MS:
m/z calcd for C13H16BNO2 229.2, found 230.2 [M + H]+, 463.3
[2M - H2O + Na]+.
1
3
Synthesis of 6-(Trifluoromethoxy)-8-quinolineboronic Acid (2c).
The same procedure as given above was used, using 1c as the
1
starting material (yield 55%). H NMR (CD3OD/D2O, 300 MHz,
3
4
3
25 °C): δ 9.34 (dd, J ) 5.1 Hz, J ) 1.5 Hz, 1H), 8.58 (d, J )
4
3
3
8.4 Hz, 1H), 7.95 (d, J ) 2.4 Hz, 1H), 7.79 (dd, J ) 5.0 Hz, J

Pt2 Complexes Supported by Tetradentate Ligands
Organometallics, Vol. 27, No. 24, 2008 6617
Scheme 3. Syntheses of 1a-c
Scheme 4. Syntheses of 2a-c
Synthesis of 1,2,4,5-Tetrakis(6-(trifluoromethoxy)quinolin-8-
yl)benzene (3c). The same procedure as given above was used, using
2c as the starting material (yield 25%). 1H NMR (CDCl3, 400 MHz,
3
52 °C): δ 8.64 (s, 4H), 7.99 (s, 2H), 7.92 (d, J ) 8.0 Hz, 4H),
7.57 (s, 4H), 7.35 (s, 4H), 7.22 (dd, 3J ) 8.0 Hz, 3J ) 4.0 Hz, 4H).
13C NMR (CDCl3, 100 MHz, 52 °C): δ 150.0, 146.4, 145.0, 143.7,
1
138.0, 135.6, 135.2, 128.6, 125.6, 121.5, 120.7 (q, JF-C ) 256
Hz), 117.3. 19F NMR (CDCl3, 376 MHz, 25 °C): δ -58.49. Anal.
Calcd for C46H22N4F12O4 · 0.25CH2Cl2: C, 58.85; H, 2.40; N, 5.94.
Found: C, 59.20; H, 2.53; N, 6.00.
1
Table 3. Selected H NMR Data for 5a-c and Pt2Me4Cl2(ttab)
δ
2JPt-H (Hz)
3JPt-H (Hz)
4JPt-H (Hz)
5a
1.20
2.03
6.69
1.02
2.20
6.79
1.22
1.97
6.55
1.45
1.85
6.23
72.0
72.0
Synthesis of [Pt2Me2(3a)] (4a). To a solution of [PtMe2(SMe2)]2
(57.4 mg, 0.01 mmol, in 0.2 mL of THF and 4 mL of Et2O) was
added a solution of 3a (70 mg, 0.01 mmol, in 2 mL of THF) slowly
under argon. The mixture was stirred for 4 h at ambient temperature.
The red precipitate was collected by vacuum filtration, washed with
THF, and then dried under vacuum to afford 4a as a red powder
(97 mg, 85% yield). Anal. Calcd for C54H54N4Pt2: C, 56.44; H,
4.74; N, 4.88. Found: C, 56.35; H, 4.62; N, 4.93.
Synthesis of [Pt2Me2(3b)] (4b). The same procedure as given
above was used, using 3b as the starting material (yield 87%). Anal.
Calcd for C62H70N4Pt2: C, 59.03; H, 5.59; N, 4.44. Found: C, 59.14;
H, 5.40; N, 4.38.
32
19.6
20.8
16.4
14.7a
5b
72.0
74.0
32.8
30.8
29.7a
5c
72.0
72.0
Pt2Me4Cl2(ttab)
70.2
73.2
a The JPt-H and JPt-H coupling constants were misreported in the
3
4
original paper6 as 22.2 and 7.5 Hz, respectively.
Synthesis of [Pt2Me2(3c)] (4c). The same procedure as given
above was used, using 3c as the starting material (yield 75%). Anal.
Calcd for C50H34N4O4F12Pt2 · THF: C, 44.88; H, 2.93; N, 3.88.
Found: C, 44.71; H, 3.02; N, 3.90.
) 8.0 Hz, 1H), 7.61 (d, J ) 2.1 Hz, 1H). 13C NMR (CD3OD/
4
D2O, 75 MHz, 25 °C): δ 148.3, 148.2, 144.5, 143.2, 142.4, 130.4,
126.7, 122.0, 120.7(q, 1JC-F ) 256 Hz), 116.4. 19F NMR (CD3OD/
D2O, 282 MHz, 25 °C): δ -59.57. ESI-MS: m/z calcd for
C10H7BNO3F3 257.2, found 258.2 [M + H]+, 519.2 [2M - H2O +
Na]+.
Reaction of 4a with CHCl3. A solution of 4a (49 mg, 0.005
mmol) in 2 mL of CHCl3 was stirred for 3 h at ambient temperature.
The resulting yellow solution was dried under vacuum to afford
5a as a yellow powder (57 mg, 95% yield). 1H NMR (CDCl3, 400
Synthesis of 1,2,4,5-Tetrakis(6-ethylquinolin-8-yl)benzene (3a).
A mixture of Pd(PPh3)4 (0.23 g, 0.2 mmol), 1,2,4,5-tetrabromoben-
zene (0.39 g, 1 mmol), 2a (1.2 g, 6 mmol), and K3PO4 (10.7 g, 50
mmol) was placed in a Schlenk flask under argon. Degassed DMF
(20 mL) and H2O (20 mL) were added to the mixture, and the flask
was heated to 95 °C under argon for 24 h. The mixture was then
cooled down and partitioned by CH2Cl2 and water, and the aqueous
layer was washed with CH2Cl2 several times. The organic layers
were combined together and dried over Na2SO4. After filtration,
the solvent was removed under vacuum, and the remaining oil was
separated through a silica gel column with THF/hexanes as eluent
and recrystallized from CH2Cl2 to afford 3a (0.2 g, 30% yield) as
3
3
MHz, 25 °C): δ 9.75 (d, J ) 5.0 Hz, 2H), 9.69 (d, J ) 5.0 Hz,
2H), 8.61 (s, 2H), 8.18 (d, 3J ) 7.2 Hz, 2H), 7.93 (d, J ) 6.4 Hz,
2H), 7.67 (s, 2H), 7.62 (dd, 3J ) 4.5 Hz, 3J ) 8.0 Hz, 2H), 7.52 (s,
2H), 7.39 (s, 2H), 7.02 (dd, 3J ) 4.5 Hz, 3J ) 8.0 Hz, 2H), 6.69 (s,
3
4
satellite, JPt-H ) 32 Hz, JPt-H ) 19.6 Hz, 2H), 2.78-2.74 (m,
2
4H), 2.61-2.55 (m, 4H), 2.03 (s, satellite, JPt-H ) 72 Hz, 6H),
3
2
1.28 (t, J ) 7.6 Hz, 6H), 1.20 (s, satellite, JPt-H ) 72 Hz, 6H),
1.06 (t, 3J ) 7.6 Hz, 6H). Anal. Calcd for C54H54N4Cl2Pt2 · 0.5CCl3:
C, 51.15; H, 4.29; N, 4.38. Found: C, 51.25; H, 4.04; N, 4.33.
Reaction of 4b with CHCl3. The same procedure as given above
was used, using 4b as the starting material to afford 5b as a yellow
1
1
powder (yield 94%). H NMR (CDCl3, 400 MHz, 25 °C): δ 9.77
colorless crystals. H NMR (CDCl3, 400 MHz, 52 °C): δ 8.61 (s,
(dd, 3J ) 4.8 Hz, 4J ) 1.6 Hz, 2H), 9.63 (dd, 3J ) 5.2 Hz, 4J ) 2.0
Hz, 2H), 8.97 (d, 4J ) 2.0 Hz, 2H), 8.22 (dd, 3J ) 8.4 Hz, 4J ) 1.6
Hz, 2H), 7.92 (dd, 3J ) 8.0 Hz, 4J ) 1.6 Hz, 2H), 7.84 (d, 4J ) 1.6
Hz, 2H), 7.64 (dd, 3J ) 4.8 Hz, 3J ) 8.0 Hz, 2H), 7.49 (d, 4J ) 2.0
Hz, 2H), 7.35 (d, 4J ) 2.0 Hz, 2H), 6.97 (dd, 3J ) 4.8 Hz, 3J ) 8.0
3
4
4H), 7.96 (s, 2H), 7.83 (dd, J ) 8.4 Hz, J ) 1.6 Hz, 4H), 7.53
(d, 4J ) 2.0 Hz, 4H), 7.22 (d, 4J ) 2.0 Hz, 4H), 7.83 (dd, 3J ) 8.0
Hz, 3J ) 4.0 Hz, 4H), 2.52 (q, 3J ) 7.6 Hz, 8H), 0.96 (t, 3J ) 7.6
Hz, 12H). 13C NMR (CDCl3, 100 MHz, 52 °C): δ 148.6, 146.0,
141.8, 141.6, 138.6, 135.2, 134.9, 133.1, 128.4, 124.1, 120.2, 28.89,
15.22. Anal. Calcd for C50H42N4 · 0.25CH2Cl2: C, 83.81; H, 5.94;
N, 7.78. Found: C, 83.49; H, 6.46; N, 7.95.
Synthesis of 1,2,4,5-Tetrakis(6-tert-butylquinolin-8-yl)benzene
(3b). The same procedure as given above was used, using 2b as
the starting material (yield 35%). H NMR (CDCl3, 400 MHz, 52
3
4
Hz, 2H), 6.79 (s, satellite, JPt-H ) 32.8 Hz, JPt-H ) 20.8 Hz,
2H), 2.20 (s, satellite, 2JPt-H ) 74 Hz, 6H), 1.36 (s, 18H), 1.02 (s,
2
satellite, JPt-H ) 72 Hz, 6H), 1.10 (s, 18H). Anal. Calcd for
C62H70N4Cl2Pt2 · 0.25CCl3: C, 54.84; H, 5.30; N, 4.18. Found: C,
55.20; H, 5.36; N, 4.13.
1
3
Reaction of 4c with CHCl3. The same procedure as given above
was used, using 4c as the starting material to afford 5c as a yellow
powder (yield 90%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 10.25
(d, 3J ) 4.8 Hz, 2H), 9.20 (d, 3J ) 5.2 Hz, 2H), 8.46 (s, 2H), 8.23
(d, 3J ) 8.0 Hz, 2H), 8.07 (d, 3J ) 8.0 Hz, 2H), 7.72 (dd, 3J ) 4.8
Hz, 3J ) 8.6 Hz, 2H), 7.68 (s, 2H), 7.48 (s, 2H), 7.36 (s, 2H), 7.23
°C): δ 8.64 (s, 4H), 8.06 (s, 2H), 7.88 (d, J ) 8.4 Hz, 4H), 7.68
(s, 4H), 7.38 (s, 4H), 7.13 (dd, 3J ) 7.2 Hz, 3J ) 3.6 Hz, 4H), 1.07
(s, 36H). 13C NMR (CDCl3, 100 MHz, 52 °C): δ 148.9, 148.5,
145.8, 141.4, 138.6, 135.8, 135.5, 131.3, 128.3, 121.5, 120.2, 34.76,
31.14. Anal. Calcd for C58H58N4 · 2CH2Cl2: C, 73.46; H, 6.37; N,
5.71. Found: C, 73.61; H, 6.24; N, 5.75.

6618 Organometallics, Vol. 27, No. 24, 2008
Tan et al.
Figure 1. Molecular structures of 1b (left) and 1c (right) with thermal ellipsoids plotted at the 50% probability level.
were mounted on the tip of a MiTeGen MicroMount. The single-
crystal X-ray diffraction data of 2b were collected on a Bruker
Kappa Apex II diffractometer, while all the remaining data were
collected on a Nonius Kappa-CCD diffractometer. All data were
collected with graphite-monochromated Mo KR radiation (λ )
0.710 73 Å), at 50 kV and 30 mA, at a temperature of 150 K
controlled by an Oxford Cryostream 700 series low-temperature
system, except for the data of 3a, which were collected at 100 K.
The diffraction data of 2b were processed with the Bruker Apex 2
software package,10 while all the remaining data were processed
with the DENZO-SMN package.11 All structures were solved by
the direct methods and refined using SHELXTL V6.10.12 Com-
pounds 1b,c crystallized in the monoclinic space groups P21/c and
P21/n, respectively, with 1 molecule per asymmetric unit. 2b
crystallized in the monoclinic space group C2/c with 3 independent
molecules per asymmetric unit along with 1 acetone molecule, 0.35
n-hexane molecule, and 2.6 water molecules. 3a crystallized in the
monoclinic space group P21/n with 1 molecule per asymmetric unit
along with 2 molecules of CH2Cl2. 3b,c crystallized in the triclinic
Figure 2. Molecular structure of 2b in a dimeric form with the
thermal ellipsoids plotted at the 50% probability level. All hydrogen
atoms are removed for clarity.
j
space group P1 with 1 molecule per asymmetric unit along with 2
molecules of CH2Cl2. 3d crystallized in the triclinic space group
j
P1 with 1 molecule per asymmetric unit along with 2.5 toluene
molecules. 4b crystallized in the monoclinic space group P21/n with
0.5 molecule per asymmetric unit. 5a-c crystallized in the triclinic
j
space group P1 with 0.5 molecule per asymmetric unit along with
2 (for 5a,b) or 3 (for 5c) molecules of CHCl3. The disordered tert-
butyl groups in 2b and 3b, a cocrystallized toluene molecule in
3d, the quinolinyl groups and OCF3 groups in 5c, and cocrystallized
CHCl3 molecules in the lattice of 5c were modeled successfully.
The residual electron density from disordered, unidentified solvent
molecules in the lattices of 2b, 3a,c,d, and 4b were removed with
the SQUEEZE function of the PLATON program,13 and their
contributions were not included in the formula. All non-hydrogen
atoms were refined anisotropically, except for the disordered
portions. In all structures hydrogen atoms bonded to carbon atoms
were included in calculated positions and treated as riding atoms.
The positions of hydrogen atoms attached to oxygen atoms were
calculated to fit the hydrogen bonding patterns. The crystallographic
data are summarized in Table 1, while selected bond lengths and
angles are given in Table 2.
Figure 3. Molecular structure of 3d with thermal ellipsoids plotted
at the 50% probability level.
(dd, 3J ) 5.2 Hz, 3J ) 8.4 Hz, 2H), 6.55 (s, satellite, 3JPt-H ) 30.8
Hz, 4JPt-H ) 16.4 Hz, 2H), 1.97 (s, satellite, 2JPt-H ) 72 Hz, 6H),
1.22 (s, satellite, 2JPt-H ) 72 Hz, 6H). 19F NMR (CDCl3, 376 MHz,
25 °C): δ -58.21, -58.29. Anal. Calcd for C50H34N4Cl2F12-
Pt2 · 0.5CCl3: C, 40.34; H, 2.31; N, 3.73. Found: C, 40.07; H, 2.12;
N, 4.00.
Results and Discussion
Syntheses and Structures of Ligands 3a-c. The novel
quinoline-based tetradentate ligands 3a-c can be synthesized
in three steps.
X-ray Diffraction Analyses. X-ray-quality crystals of 1b,c and
3a-c were obtained by top-layering the CH2Cl2 solutions with
hexanes, those of 2b were obtained by diffusing hexanes into a
wet acetone solution, those of 3d were obtained by top-layering a
CH2Cl2 solution with toluene, those of 4b were obtained by top-
layering the CH2Cl2 solution with benzene, and those of 5a-c were
obtained by slow evaporation of the CHCl3 solutions. All crystals
(10) Apex 2 Software Package; Bruker AXS Inc., 2008.
(11) Otwinowski, Z.; Minor, W. In Macromolecular Crystallography,
Part A; Carter, C. W., Sweet, R. M., Eds.; Academic Press: London, 1997;
Methods in Enzymology, Vol. 276, p 307.
(12) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112–122.
(13) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.

Pt2 Complexes Supported by Tetradentate Ligands
Organometallics, Vol. 27, No. 24, 2008 6619
Figure 4. Molecular structures of 3a (left) and 3b (right) (both interacting with CH2Cl2 molecules) with thermal ellipsoids plotted at the
50% probability level. All hydrogen atoms (except for those of CH2Cl2 molecules) are omitted for clarity.
drop of D2O to the NMR solution, the spectrum shows only
one set of signals which could be attributed to the fully hydrated
form, the monomer.
When 2b is recrystallized from wet acetone and hexanes,
X-ray crystallography reveals a novel dimeric structure, which
is shown in Figure 2. Each boron center has a tetrahedral
geometry with a terminal OH group, a bridging oxo, a nitrogen
atom from one quinoline moiety, and a carbon atom from the
other quinoline moiety occupying the four vertices. In the crystal
lattice, molecules of 2b form multiple intermolecular hydrogen
bonds with one another as well as the lattice water molecules.
The MS spectrum of as-synthesized 2b shows the presence of
both the monomer and the dimer. Because of the coexistence
of different forms, no satisfactory elemental analysis results
could be obtained for the boronic acids 2a-c. The as-
synthesized 2a-c were used in large excess without further
purification in the subsequent Suzuki coupling reactions.16
Figure 5. Molecular structure of 3c (interacting with CH2Cl2
Because the Suzuki coupling reactions use water as a cosolvent
at an elevated temperature, it is likely that different forms of
these boronic acids convert to the fully hydrated monomer forms
in the reaction system.
molecules) with thermal ellipsoids plotted at the 50% probability
level. All hydrogen atoms (except for those in CH2Cl2 molecules)
are omitted for clarity.
The first step is to build the 6-substituted 8-bromoquinolines
1a-c, through modified Skraup reactions (Scheme 3),14 using
appropriate para-substituted 2-bromoanilines as the starting
materials. Compound 1a was obtained as a pale yellow oil in
60% yield, while 1b,c were obtained as white solids in 70 and
87% yields, respectively. All three compounds have been
characterized by NMR spectroscopy and elemental analysis,
while the structures of 1b,c have also been confirmed by X-ray
crystallography and shown in Figure 1.
The second step is to convert 1a-c into the corresponding
boronic acids for the subsequent Suzuki coupling reactions.
Boronic acids 2a-c can be obtained in 50-70% yield from
the reactions of 1a-c and phenyllithium at -78 °C in THF,
followed by sequential treatments with trimethylborate and 3
M HCl (Scheme 4). The use of n-butyllithium instead of
phenyllithium can also lead to the formation of the desired
products, but the reactions have lower yields.
The tetradentate ligands 3a-c can be obtained via Suzuki
coupling reactions of the corresponding boronic acids 2a-c and
1,2,4,5-tetrabromobenzene catalyzed by Pd(PPh3)4 in the pres-
ence of K3PO4 in the DMF/H2O solvent system at 95 °C under
argon (Scheme 5). The yields of 3a-c can be increased slightly
by using 1,2,4,5-tetraiodobenzene instead of 1,2,4,5-tetrabro-
mobenzene. A number of byproducts, such as di- and trisub-
stituted coupling products, have been observed, whose structures
have been confirmed by X-ray crystallography and are shown
in Figures S1 and S2 (see the Supporting Information). Another
byproduct isolated in small quantity from the reaction mixture
of the synthesis of 3b is the dinuclear Pd(II) complex 3d, whose
identity has been confirmed by X-ray crystallography. As shown
in Figure 3, the molecule of 3d has two 6-tert-butylquinoline
moieties attached to the central benzene ring in a mutually meta
fashion. Each nitrogen donor atom of the quinoline moiety
coordinates to a square-planar Pd(II) center that is further bonded
with a carbon donor atom of the central benzene ring, a bromide
ligand trans to the carbon donor, and a PPh3 ligand trans to the
quinoline nitrogen atom. Compound 3d is presumably a trapped
intermediate of the Suzuki coupling reaction. After the coupling
of the first two quinoline moieties to the central benzene ring,
the two remaining C-Br bonds are oxidatively cleaved by the
Pd(0) catalyst directed by the neighboring quinoline nitrogen
It is well-known that boronic acids can undergo intermo-
lecular condensation to form oligomers, losing water mol-
ecules.15 Thus, the existence of 2a-c depends on their hydration
states; in most cases, species of different hydration states coexist.
1
For example, the H NMR of the vacuum-dried boronic acid
2b is messy in CD3COCD3 or CD3OD; however, by adding one
(14) Cohn, E. W. J. Am. Chem. Soc. 1930, 52, 3685–3688.
(15) Hall, D. G. Boronic Acids: Preparation and Applications in Organic
Synthesis and Medicine; Wiley-VCH: Weinheim, Germany, 2005.
(16) Suzuki, A. J. Organomet. Chem. 1999, 576, 147–168.

6620 Organometallics, Vol. 27, No. 24, 2008
Tan et al.
Scheme 5. Synthesis of 3a-c
Scheme 6. Syntheses of 4a-4c
Scheme 7. Syntheses of 5a-c
atoms. Interestingly, compound 3d is sufficiently stable to
survive the cross-coupling conditions and a silica gel column.
Ligands 3a-c were characterized by NMR spectroscopic,
elemental, and single-crystal X-ray diffraction analyses. The 1H
NMR spectra of these ligands in CDCl3 show several broad
peaks at ambient temperature, presumably as a result of hindered
rotations, while the spectra recorded at 52 °C show sharp signals.
The protons of the central phenyl rings in 3a-c resonate at
7.96, 8.06, and 7.99 ppm, respectively, as singlets integrated
for 2 protons each, while the protons of the quinolinyl moieties
in each compound show 5 resonances in the aromatic region,
integrated for 4 protons each. The ethyl groups of 3a give rise
1
to a quartet and triplet in the H NMR spectrum at 2.52 and
0.96 ppm integrated for 8 and 12 protons, respectively, while
the tert-butyl groups of 3b show a singlet at 1.07 ppm integrated
for 36 protons. The 19F NMR spectrum of 3c has only one
singlet at -58.49 ppm. All these NMR data indicate symmetric
structures of these tetradentate ligands.
The molecular structures of 3a,b are shown in Figure 4. In
the structures of 3a,b, each pair of mutually para quinoline
moieties are of the same orientation (with nitrogen atoms on
the same side of the central phenyl ring) and are different from
the other pair. The dihedral angles between the central phenyl
ring and the quinoline moieties are in the range of 70-76° for
3a and 76-81° for 3b, indicating that the quinoline moieties in
3b are more perpendicular to the central phenyl ring. This is
likely caused by the bulkier tert-butyl groups in 3b, compared
to the ethyl groups in 3a. Each pair of nitrogen atoms on the
same side of the central phenyl ring interacts with one CH2Cl2
molecule through nonclassic hydrogen bonds in a chelating
fashion, with average N · · · C distances of ∼3.25 and ∼3.23 Å
for 3a,b, respectively. Such interactions may help lock down
the motions of these ligands and, in turn, the crystallization.
The molecular structure of 3c is shown in Figure 5. Unlike
3a,b, each molecule of 3c has a crystallographically imposed
Figure 6. Molecular structure of 4b with thermal ellipsoids plotted
at the 50% probability level. All hydrogen atoms are omitted for
clarity.

Pt2 Complexes Supported by Tetradentate Ligands
Organometallics, Vol. 27, No. 24, 2008 6621
Figure 7. Molecular structures of 5a (upper left), 5b (upper right), and 5c (bottom) with thermal ellipsoid plotted at the 30% probability
level. All hydrogen atoms are omitted for clarity.
center of inversion at its centroid. As a consequence, each pair
of mutually para quinoline moieties are of opposite orientations.
Each pair of mutually meta quinoline moieties are of the same
orientation, with the nitrogen atoms interacting with a CH2Cl2
solvent molecule through nonclassic hydrogen bonds in a
chelating fashion, with an average N · · · C distance of ∼3.37
Å. The dihedral angles between the central phenyl ring and the
quinoline moieties are ∼61 and ∼65°, smaller than those found
in 3a,b, which could be the result of different H-bonding modes.
sum of the van der Waals radii of Pt and C (3.47 Å).17 The
intramolecular Pt · · · Pt distance in 4b is 6.5626(9) Å, shorter
than that (6.898(2) Å) in Pt2Me4(ttab). The dihedral angle
between the Pt(II) coordination plane and the central phenyl
ring is ∼27°, slightly smaller than that in Pt2Me4(ttab). These
observations are consistent with our ligand design (Scheme 2).
Reactivities of 4a-c with CHCl3. The red compounds 4a-c
react with chloroform readily upon dissolution at ambient
temperature to give the yellow products 5a-c, respectively, in
quantitative yield within 3 h (Scheme 7). Compounds 5a-c have
Syntheses and Structures of 4a-c. The dinuclear dimeth-
ylplatinum(II) complexes 4a-c can be obtained as red precipi-
tates by reacting 3a-c with [PtMe2(Me2S)]2 in THF at ambient
temperature (Scheme 6). These complexes are stable in air at
ambient temperature in the solid state. Although functionalized
with ethyl, tert-butyl, and OCF3 groups, complexes 4a-c are
sparingly soluble in common solvents (except for CHCl3, which
reacts with all three compounds); therefore, no NMR data have
been obtained.
1
enough solubility for H NMR spectroscopic analyses and are
stable in the solid state and in solution at ambient temperature.
The selected 1H NMR data for 5a-c and Pt2Me4Cl2(ttab) are
given in Table 3. Each compound (5a-c) has two singlets at
1.02-1.22 and 1.97-2.20 ppm with platinum satellites in the
aliphatic regions, integrated for six protons each, which can be
assigned to two types (two of each type) of methyl groups
bonded to the platinum centers. These chemical shifts and the
2
corresponding JPt-H values are similar to those reported for
With the limited solubility of 4b in CH2Cl2, X-ray-quality
crystals were obtained by slow diffusion of benzene into a
CH2Cl2 solution of 4b. As shown in Figure 6, each molecule
of 4b has a crystallographically imposed center of inversion at
its centroid. The 3b ligand uses its nitrogen donor atoms from
two quinolinyl groups that are mutually ortho to chelate to each
square-planar platinum(II) center, with methyl ligands trans to
the nitrogen donor atoms. The two platinum(II) centers are on
the opposite sides of the central phenyl ring, with a short contact
distance between Pt(1) and C(28) of 2.90(1) Å, shorter than
the corresponding distance (3.11 Å) in Pt2Me4(ttab)6 and the
Pt2Me4Cl2(ttab), except that the two methyl signals are slightly
farther apart than those from Pt2Me4Cl2(ttab), indicating a greater
difference between the two sets of methyl groups in 5a-c. The
protons of the central C6 rings of 5a-c resonate in the range of
6.55-6.79 ppm, more downfield than those of Pt2Me4Cl2(ttab).
The characteristic platinum-satellite peaks indicate a bonding
3
4
mode similar to that in Pt2Me4Cl2(ttab). The JPt-H and JPt-H
(17) Emsley, J. The Elements, 2nd ed.; Clarendon Press: Oxford, U.K.,
1991.

6622 Organometallics, Vol. 27, No. 24, 2008
Tan et al.
values of 5a-c are in the ranges of 30.8-32.8 and 16.4-20.8
Hz, respectively, greater than those observed for Pt2Me4Cl2-
(ttab).
the corresponding dinuclear dimethylplatinum(II) complexes
structurally resemble the previously reported Pt2Me2(ttab)
compound. We have achieved a shorter distance between the
metal center and the central phenyl ring of the ligand in
comparison to that of Pt2Me2(ttab) through proper ligand design.
In addition to the structural similarity, complexes 4a-c are also
analogous to Pt2Me2(ttab) in terms of reactivity toward C-Cl
activation. Thus, they react with CHCl3 under ambient condi-
tions to afford 5a-c, respectively. During these reactions, C-Cl
bonds from CHCl3 are cleaved, the Pt(II) centers are oxidized
into Pt(IV), and the central phenyl rings of the tetradentate
ligands are reduced into the corresponding planar cyclohexa-
diene dianions bonded to the Pt(IV) centers on both sides.
The structures of 5a-c have been confirmed unambiguously
by X-ray crystallography and are shown in Figure 7. All
compounds possess crystallographically imposed inversion
center symmetry. Each Pt(IV) center adopts a distorted-
octahedral coordination geometry, with two nitrogen donor
atoms from two quinolinyl groups, two methyl ligands, a
chloride, and a carbon donor atom from the central C6 ring of
the tetradentate ligand occupying the six coordination sites, with
bond lengths and angles similar to those in Pt2(CH3)4(ttab)Cl2.6
Similar to the central C6 ring in Pt2(CH3)4(ttab)Cl2, those of
5a-c are planar with two CC double bonds (1.318(13)-1.337(7)
Å) and four CC single bonds (1.474(15)-1.507(12) Å) in each
ring, as indicated by the bond lengths.17 The carbon atoms
bonded to the metal centers have the typical sp3 characters, with
the average of bond angles around the carbon centers of 109.4°
in all three cases (Table 2). The coordination spheres of the
Pt(IV) centers in 5a-c are similar to those in PtClMe2{(pz)2-
CRCH2-κN,N′,C′′} (R ) Me, CH2Cl; pz ) pyrazol-1-yl)
compounds reported by Canty and co-workers:18 i.e., two methyl
ligands are trans to the nitrogen donor atoms of the NCN tripod
ligand, while the chloride ligand is trans to an sp3-hybridized
carbon donor atom of the NCN tripod ligand.
Although the Pt(II) compounds 4a-c are much less soluble
than Pt2Me2(ttab), the Pt(IV) compounds 5a-c appear to be
more soluble than Pt2Me2(ttab)Cl2 in common organic solvents.
This may allow us to study the possible reduction of these Pt(IV)
complexes back to the corresponding Pt(II) complexes. In
addition, ligands with other functional groups might be able to
solubilize both the Pt(II) and Pt(IV) compounds, which will
allow extensive solution study on both the forward and backward
reactions. The possibility of reducing 5a-c back to 4a-c and
new ligands with solubilizing functional groups are being
investigated in our laboratory.
Light appears to play no significant role in the reactions of
4a-c with CHCl3. Despite the shorter distances between the
metal centers and the central phenyl rings in 4a-c compared
to that in Pt2Me4(ttab), the reactions between 4a-c and CHCl3
are much slower than that between Pt2Me4(ttab) and CHCl3.
The insolubility of 4a-c makes it difficult to study the C-Cl
activation reactions further. Further tuning on the ligand design
is still necessary.
Acknowledgment. This research is supported by grants
to D.S. from the Natural Science and Engineering Research
Council (NSERC) of Canada, the Canadian Foundation for
Innovation, the Ontario Research Fund (ORF), and the
University of Toronto (Connaught Foundation).
Supporting Information Available: CIF files giving crystal-
lographic data for 1b,c, 2b, 3a-d, 4b, and 5a-c and figures giving
the molecular structures of the di- and trisubstituted byproducts
from the synthesis of 3b. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusion
Three novel tetradentate ligands (3a-c) with quinoline
functionalities have been designed and synthesized. As expected,
(18) Canty, A. J.; Honeyman, R. T. J. Organomet. Chem. 1990, 389,
277–288.
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