Regioselective C-H activation of toluene with a 1,2-bis(N-7-azaindolyl)benzene platinum(II) complex

Article abstract of DOI:10.1021/om050133z

A new organoplatinum(II) complex, Pt(1,2-BAB)(CH₃)₂ (1) (1,2-BAB = 1,2-bisOV-7-azaindolyDbenzene), has been synthesized and fully characterized. Compound 1, after reacting with 1 equiv of acid [H(Et ₂O)₂][BAr′₄], Ar′ = 3,5-bis(trifluoromethyl)phenyl, has been found to be able to activate benzene and toluene C-H bonds under mild conditions. The complexes resulting from C-H activation, {Pt(1,2-BAB)(Ph)(SMe₂)}[BAr′₄] (2), {Pt(1,2-BAB)(CH₂Ph)(SMe₂)}[BAr′4] (3), and {Pt(1,2-BAB)(CH₂Ph)(CH₃CN)}[BAr′₄] (4), have been isolated and structurally characterized by NMR and single-crystal X-ray diffraction analyses. The investigation by ¹H NMR on the reaction mixture of 1 with toluene in the presence of [H(Et₂O) ₂] [BAr′₄] revealed that the p-tolyl and p-tolyl C-H activation products dominate initially. However, as the reaction time increases, the benzylic C-H activation product becomes the major product (after 3 h, the yield ratio of benzyl:m-tolyl:p-tolyl is 60%:12%: 11%). The cause for the high regioselectivity in toluene C-H activation by complex 1 is not fully understood.

Full text of DOI:10.1021/om050133z

3290
Organometallics 2005, 24, 3290-3296
Regioselective C-H Activation of Toluene with a
1,2-Bis(N-7-azaindolyl)benzene Platinum(II) Complex
Shu-Bin Zhao, Datong Song, Wen-Li Jia, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
Received February 23, 2005
A new organoplatinum(II) complex, Pt(1,2-BAB)(CH3)2 (1) (1,2-BAB ) 1,2-bis(N-7-
azaindolyl)benzene), has been synthesized and fully characterized. Compound 1, after
reacting with 1 equiv of acid [H(Et2O)2][BAr′4], Ar′ ) 3,5-bis(trifluoromethyl)phenyl, has
been found to be able to activate benzene and toluene C-H bonds under mild conditions.
The complexes resulting from C-H activation, {Pt(1,2-BAB)(Ph)(SMe2)}[BAr′4] (2), {Pt(1,2-
BAB)(CH2Ph)(SMe2)}[BAr′4] (3), and {Pt(1,2-BAB)(CH2Ph)(CH3CN)}[BAr′4] (4), have been
isolated and structurally characterized by NMR and single-crystal X-ray diffraction analyses.
1
The investigation by H NMR on the reaction mixture of 1 with toluene in the presence of
[H(Et2O)2][BAr′4] revealed that the m-tolyl and p-tolyl C-H activation products dominate
initially. However, as the reaction time increases, the benzylic C-H activation product
becomes the major product (after 3 h, the yield ratio of benzyl:m-tolyl:p-tolyl is 60%:12%:
11%). The cause for the high regioselectivity in toluene C-H activation by complex 1 is not
fully understood.
Introduction
ligand and its blockage of the Pt(II) fifth binding site, a
new ligand, 1,2-bis(N-7-azaindolyl)benzene (1,2-BAB),
has been synthesized by our group. The new 1,2-BAB
ligand resembles one-half of the 1,2,4,5-tetrakis(N-7-
azaindolyl)benzene (TTAB) ligand, whose dinuclear Pt-
(II) complex reported recently by our group displays
unusual reactivity toward benzene C-H activation3a
and C-Cl activation.3b The complexity of the dinuclear
Pt(II) TTAB system (which tends to produce multiple
C-H activation products that display complex NMR
spectral patterns), however, makes detailed analysis of
the C-H activation reaction difficult. The successful
synthesis of the 1,2-BAB ligand makes it possible for
us to obtain a mononuclear Pt(II) complex that has a
coordination environment resembling that in the TTAB
Pt2 complex, but without the complication of cooperative
effect by the second Pt(II) center so that the steric effect
of the bis-7-azaindolyl chelate ligand on the Pt center
and its consequence on C-H activation can be examined
in detail. We have examined the utility of the 1,2-BAB
Pt(II) complex in C-H activation of benzene and
toluene. We have found that the cationic Pt(II) complex
{Pt(1,2-BAB)(CH3)(solvent)}+ displays a high selectivity
toward the activation of the benzylic C-H bond of the
toluene molecule, as supported by both crystal structure
and spectroscopic evidence. The details are reported
herein.
Cationic Pt(II) complexes have been demonstrated to
be capable of activating C-H bonds under mild condi-
tions.1 The mechanism of C-H activation by cationic
Pt(II) species has been extensively studied1 in order to
understand and utilize this crucial step in selective and
catalytic functionalization of hydrocarbons. Most previ-
ously reported cationic Pt(II) complexes that are capable
of activating C-H bonds involve a diimine ligand with
the general formula Ar′NdC(R)C(R)dNAr′ or Ar′NdC(R)-
CHC(R)dNAr′-. In contrast, the use of cationic Pt(II)
complexes containing nitrogen donor atoms that are
part of a nitrogen heterocycle in C-H bond activation
has hardly been explored. Recently we reported the
facile benzene C-H bond activations by two isomeric
Pt(II) complexes that contain a bis(N-7-azaindolyl)-
methane ligand (BAM).2 We have shown that the BAM
ligand is capable of blocking the fifth binding site of the
Pt(II), as evidenced by the strong agostic interaction
between the CH2 group and the Pt(II) center in solution
and in the solid state, which makes the BAM Pt(II)
complexes potentially useful for regioselective C-H
activation. To further enhance the rigidity of the chelate
(1) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1996, 118, 5961. (b) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E.
J. Am. Chem. Soc. 1997, 119, 848. (c) Holtcamp, M. W.; Henling, L.
M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998,
270, 467. (d) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc.
1999, 121, 1974. (e) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 2000, 122, 10846. (f) Johansson, L.; Tilset, M.
J. Am. Chem. Soc. 2001, 123, 739. (g) Johansson, L.; Ryan, O. B.;
Rømming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579. (h)
Procelewska, J.; Zahl, A.; van Eldik, R.; Zhong, H. A.; Labinger, J. A.;
Bercaw, J. E. Inorg. Chem. 2002, 41, 2808. (i) Konze, W. V.; Scott, B.
L.; Kubas, G. J. J. Am. Chem. Soc. 2002, 124, 12550. (j) Zhong, H. A.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124, 1378. (k)
Heyduk, A. F.; Driver, T. G.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 15034.
Experimental Section
All reactions were performed under an inert atmosphere of
dry N2 with Schlenk techniques or in drybox. All solvents were
distilled by known procedures prior to use. NMR spectra were
recorded on Bruker Advance 300 or 500 MHz spectrometers.
(3) (a) Song, D.; Jia, W. L.; Wang, S. Organometallics 2004, 23, 1194.
(b) Song, D.; Sliwowski, K.; Pang, J.; Wang, S. Organometallics 2002,
21, 4978.
(2) (a) Song, D. Wang, S. Organometallics 2003, 22, 2187. (b) Song,
D. Wang, S. Comments Inorg. Chem. 2004, 25, 1.
10.1021/om050133z CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/19/2005

Regioselective C-H Activation of Toluene
Organometallics, Vol. 24, No. 13, 2005 3291
Elemental analyses were performed by Canadian Micro-
analytical Service, Ltd, Delta, British Columbia. 1,2-Diiodo-
benzene and 7-azaindole were purchased from Aldrich Chemi-
cal Co. The starting materials4 Pt2Me4(Me2S)2 and5 [H(Et2O)2]-
[BAr′4] (Ar′ ) 3,5-bis(trifluoromethyl)phenyl) were prepared
by methods described in the literature.
124.8, 118.0, 103.7, 103.6; Ph, 137.3, 131.5, 128.9, 124.4; Me2S,
22.0. Anal. Calcd for C60H37N4BF24SPt: C 47.79, H 2.47, N
3.72. Found: C 47.77, H 2.58, N 3.69.
Toluene C-H Activation by 1. Isolation of [Pt(1,2-
BAB)(CH2Ph)(SMe2)][BAr′4] (3). 1 (0.107 g, 0.20 mmol) and
0.202 g of [H(Et2O)2][BAr′4] (0.20 mmol) were mixed in 20 mL
of toluene at room temperature. After the mixture was stirred
for 1 h, 0.20 mL of Me2S was added into the system. The
reaction mixture was filtered through Celite. Crude solids of
[Pt(1,2-BAB)(CH2Ph)(SMe2)][BAr′4], 3, were obtained by slow
diffusion of hexanes into the concentrated solution and storage
in a refrigerator for several weeks and were further purified
by recrystallization from hexanes/CH2Cl2 (2:1) to afford color-
less crystals (41% yield). NMR spectra in CD2Cl2 at 25 °C, 1H
Synthesis of 1,2-Bis(N-7-azaindolyl)benzene (1,2-BAB).
1,2-Diiodobenzene (1.65 g, 5.0 mmol), 7-azaindole (1.42 g, 12.0
mmol), CuI (0.19 g, 1.0 mmol), 1,10-phenanthroline (0.36 g,
2.0 mmol), Cs2CO3 (6.85 g, 21.0 mmol), 3 mL of DMF, and 0.45
mL of dodecane were mixed together and heated at 150 °C
under an N2 atmosphere for 72 h. After cooling to ambient
temperature, the mixture was diluted with 30 mL of CH2Cl2
and filtered through a plug of silica gel. The filtrate was then
concentrated under reduced pressure, and the residue was
flushed through a silica gel column using hexanes/ethyl acetate
(3:1) as the eluent. Upon removal of the solvent, a yellow oil
of 1,2-BAB formed, which solidified after the addition of
hexanes and storage in a refrigerator for several weeks (75%
yield). NMR spectra in CDCl3 at 25 °C, 1H NMR (ppm): δ 8.29
(dd, 3J ) 4.5 Hz, 4J ) 1.5 Hz, 2H; aza), 7.86 (dd, 3J ) 7.5 Hz,
4J ) 1.5 Hz, 2H; aza), 7.80 (m, 2H; phenyl), 7.63 (m, 2H;
phenyl), 7.09 (dd, 3J1 ) 7.5 Hz, 3J2 ) 4.5 Hz, 2H; aza), 6.77 (d,
3J ) 3.5 Hz, 2H; aza), 6.28 (d, 3J ) 3.5 Hz, 2H; aza). 13C NMR
(ppm): δ aza, 148.3, 143.7, 129.1, 128.8, 120.9 116.7, 101.6;
phenyl, 134.3, 129.5, 128.9 (aza ) 7-azaindolyl). Anal. Calcd
for C20H14N4: C 77.40, H 4.55, N 18.05. Found: C 77.42, H
4.53, N 17.92.
3
4
NMR (ppm): δ 8.32 (dd, J ) 5.5 Hz, J ) 1.1 Hz; 1H, aza),
8.06 (dd, satellite, 3J ) 5.5 Hz, 4J ) 1.0 Hz, 3JPt-H ) 45.6 Hz;
1H, aza), 7.99 (dd, 3J ) 8.0 Hz, 4J ) 1.3 Hz; 1H, aza), 7.94
(dd, 3J ) 7.9 Hz, 4J ) 1.2 Hz; 1H, aza), 7.85 (m; 2H, phenyl of
3
1,2-BAB), 7.60 (s; 4H, BAr′4), 7.59 (d, J ) 4.0 Hz; 1H, aza),
3
4
7.55 (dd, J ) 7.6 Hz, J ) 1.8 Hz; 1H, phenyl of 1,2-BAB),
3
4
7.28 (dd, J ) 7.6 Hz, J ) 1.8 Hz; 1H, phenyl of 1,2-BAB),
3
3
7.26 (d, J ) 3.6 Hz; 1H, aza), 7.20 (d, J ) 3.6 Hz; 1H, aza),
7.12 (dd, 3J1 ) 5.5 Hz, 3J2 ) 7.9 Hz; 1H, aza), 7.03 (t, 3J ) 7.1
Hz; 1H, PtCH2Ph), 6.98 (m; 3H, 1H from aza, 2H from
PtCH2Ph), 6.71 (m; 4H, 2H from aza, 2H from PtCH2Ph), 2.70
(d, 2J ) 10.2 Hz, 2JPt-H ) 103.0 Hz; 1H, PtCH2Ph), 2.67 (d, 2J
2
) 10.2 Hz, JPt-H ) 103.0 Hz; 1H, PtCH2Ph), 2.13 (s (br),
3
satellite, JPt-H ) 53.2 Hz; 3H, Me2S), 1.97 (s (br), satellite,
-
3JPt-H ) 53.2 Hz; 3H, Me2S). 13C NMR (ppm): δ BAr′4 162.1
Synthesis of Pt(1,2-BAB)(CH3)2 (1). 1,2-BAB (0.093 g,
0.30 mmol) and 0.0861 g of Pt2Me4(Me2S)2 (0.15 mmol) were
mixed in 20 mL of Et2O and stirred for 5 h at room temper-
ature. The resulting white precipitate was allowed to settle,
and the clear solution was decanted. The solid was washed
with Et2O and dried under vacuum to afford Pt(1,2-BAB)(CH3)2
in 57% yield. NMR spectra in CD2Cl2 at 25 °C, 1H NMR
(q, JB-H ) 50 Hz), 135.1 (br), 129.4 (q, JC-F ) 32 Hz), 125.3 (q,
JC-F ) 271 Hz), 117.9 (br); 1,2-BAB, 147.5, 146.9, 144.6, 143.6,
136.8, 136.7, 132.80, 132.55, 131.93, 131.74, 131.61, 131.58,
129.46, 124.82, 124.58, 118.41, 118.17, 104.4, 104.2; PhCH2,
146.0, 129.0, 128.3, 124.6, 6.8; Me2S, 23.0, 22.5. Anal. Calcd
for C61H39N4BF24SPt: C 48.14, H 2.58, N 3.68. Found: C 47.48,
H 2.65, N 3.70.
(ppm): δ 8.56 (dd, satellite, 3J ) 5.1 Hz, 4J ) 1.3 Hz, 3JPt-H
)
24.1 Hz; 2H, aza), 7.85 (dd, 3J ) 7.8 Hz, 4J ) 1.3 Hz; 2H, aza),
Isolation of [Pt(1,2-BAB)(CH2Ph)(NCMe)][BAr′4] (4). 1
(0.107 g, 0.20 mmol) and 0.202 g of [H(Et2O)2][BAr′4] (0.20
mmol) were mixed in 20 mL of toluene at room temperature.
After the mixture was stirred for 1 h, 0.20 mL of CH3CN was
added into the system. The reaction mixture was filtered
through Celite. Colorless crystals 4 were obtained by slow
diffusion of hexanes into the concentrated solution and storage
at room temperature for several days (38% yield). NMR spectra
3
7.72 (m, 2H, phenyl), 7.31 (m, 2H, phenyl), 7.25 (d, J ) 3.4
3
3
Hz; 2H, aza), 6.95 (dd, J1 ) 7.8 Hz, J2 ) 5.1 Hz; 2H, aza),
3
2
6.65 (d, J ) 3.4 Hz; 2H, aza), 0.04 (s, satellite, JPt-H ) 88.8
Hz; 6H, methyl). 13C NMR (ppm): δ aza, 148.9, 145.2, 134.7,
130.7, 130.2, 129.7, 128.7, 122.8, 117.3, 102.2; CH3, -23.2.
Anal. Calcd for C22H20N4Pt: C 49.34, H 3.76, N 10.46. Found:
C 48.72, H 3.88, N 10.20.
1
3
in CD2Cl2 at 25 °C, H NMR (ppm): δ 8.33 (dd, J ) 5.4 Hz,
4J ) 1.2 Hz; 1H, aza), 8.16 (dd, satellite, 3J ) 5.7 Hz, 4J ) 1.0
Hz, 3JPt-H ) 57.6 Hz; 1H, aza), 7.95 (dd, 3J ) 7.9 Hz, 4J ) 1.2
Hz; 1H, aza), 7.92 (dd, 3J ) 7.9 Hz, 4J ) 1.4 Hz; 1H, aza), 7.87
Benzene C-H Activation by 1. Isolation of [Pt(1,2-
BAB)Ph(SMe2)][BAr′4] (2). 1 ( 0.107 g, 0.20 mmol) and 0.202
g of [H(Et2O)2][BAr′4] (0.20 mmol) were mixed in 40 mL of
benzene at room temperature. After the mixture was stirred
for 1 h, 0.20 mL of Me2S was added into the system. The
reaction mixture was filtered through Celite. Colorless crystals
of [Pt(1,2-BAB)Ph(SMe2)][BAr′4], 2, were obtained by slow
diffusion of hexanes into the concentrated solution (71% yield).
NMR spectra in THF-d8 at 25 °C, 1H NMR (ppm): δ 8.65 (dd,
3J ) 5.5 Hz, 4J ) 1.5 Hz; 1H, aza), 8.63 (dd, 3J ) 5.5 Hz, 4J )
1.5 Hz; 1H, aza), 8.08 (m; 4H, phenyl of 1,2-BAB), 7.93 (dd, 3J
) 7.5 Hz, 4J ) 1.5 Hz; 1H, aza), 7.86 (dd, 3J ) 7.5 Hz, 4J ) 1.5
Hz; 1H, aza), 7.83 (s; 8H, BAr′4), 7.61 (s; 4H, BAr′4), 7.59 (d,
3J ) 4.0 Hz; 1H, aza), 7.44 (d, 3J ) 4.0 Hz; 1H, aza), 7.25 (dd,
3J1 ) 7.5 Hz, 3J2 ) 5.5 Hz; 1H, aza), 7.19 (dd, 3J1 ) 7.5 Hz, 3J2
) 6.0 Hz; 1H, aza), 7.14 (dd, 3J ) 8.0 Hz, 4J ) 1.0 Hz; 2H,
3
(m; 2H, phenyl of 1,2-BAB), 7.75 (s; 4H, BAr′4), 7.58 (d, J )
4.0 Hz; 1H, aza), 7.48 (m; 1H, phenyl of 1,2-BAB), 7.41 (m;
3
1H, phenyl of 1,2-BAB), 7.30 (d, J ) 3.6 Hz; 1H, aza), 7.29
(d, 3J ) 3.6 Hz; 1H, aza), 7.08 (dd, 3J1 ) 5.4 Hz, 3J2 ) 7.9 Hz;
1H, aza), 7.05 (m; 3H, PtCH2Ph), 7.00 (dd, 3J1 ) 5.7 Hz, 3J2 )
2
7.9 Hz; 1H, aza), 6.82 (m; 2H, PtCH2Ph), 2.87 (d, satellite, J
2
) 10.0 Hz, JPt-H ) 106.5 Hz; 1H, PtCH2Ph), 2.70 (d, 2J )
2
10.0 Hz, JPt-H ) 106.5 Hz; 1H, PtCH2Ph), 2.11 (s; 3H,
PtNCCH3). 13C NMR (ppm): δ BAr′4-, 162.2 (q, JB-H ) 50 Hz),
135.2 (br), 129.2 (q, JC-F ) 32 Hz), 125.0 (q, JC-F ) 270 Hz),
117.8 (br); 1,2-BAB, 148.3, 148.0, 147.9, 145.9, 144.4, 137.2,
136.6, 132.43, 132.22, 131.96, 131.84, 131.65, 131.50, 130.20,
118.26, 118.99, 103.9, 103.8; PhCH2, 147.6, 128.7, 128.2,
124.48, 5.90; CH3CN, 124.38, 3.5. Anal. Calcd for C61H36N5-
BF24Pt: C 48.82, H 2.42, N 4.67. Found: C 49.03, H 2.58, N
4.52.
3
3
Pt-Ph), 6.91 (t, J ) 7.5 Hz; 2H, Pt-Ph), 6.85 (tt, J ) 7.5 Hz,
4J ) 1.5 Hz; 1H, Pt-Ph), 6.80 (d, J ) 3.5 Hz; 1H, aza), 6.69
3
3
3
(d, J ) 4.0 Hz; 1H, aza), 1.96 (s, satellite, JPt-H ) 60.0 Hz;
6H, Me2S). 13C NMR (ppm): δ BAr′4-, 162.4 (q, JB-H ) 50 Hz),
135.0 (br), 129.4 (q, JC-F ) 32 Hz), 124.9 (q, JC-F ) 272 Hz),
117.6 (br); 1,2-BAB, 148.1, 147.5, 144.5, 144.4, 135.8, 132.84,
132.77, 132.74, 132.5, 132.4, 132.3, 132.1, 130.0, 127.6, 125.0,
1H NMR Analysis of the Reaction Mixture of Toluene
C-H Activation. 1 (0.050 g, 0.093 mmol) and 0.095 g of
[H(Et2O)2][BAr′4] (0.093 mmol) were mixed in 8 mL of toluene
at room temperature. A small amount of the reaction mixture,
during the course of the reaction, was taken out at regular
time intervals and put immediately into vials that contain CD3-
CN for terminating the reaction. The samples in the vials were
(4) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643.
(5) Brookhart, M.; Grant, B.; Volpe, J. Organometallics 1992, 11,
3920.

3292 Organometallics, Vol. 24, No. 13, 2005
Table 1. Crystallographic Data for Compounds 1-4
Zhao et al.
1
2
3
4
formula
fw
C
22H20N4Pt1
C
60H37N4F24S1B1Pt1
C
61H39N4F24B1S1Pt1
C61H36N5F24 S1 B1Pt1
535.54
P21/n
1507.87
P2/c
1521.92
P1h
1500.85
P1h
space group
a, Å
15.528(4)
12.698(3)
19.665(5)
90.00
21.705(5)
12.645(3)
23.116(5)
90
12.725(4)
13.160(4)
18.895(6)
99.790(5)
100.730(5)
94.316(6)
3044.8(5)
2
12.712(3)
13.046(3)
18.856(4)
100.179(4)
98.503(4)
94.680(4)
3025.4(12)
2
b, Å
c, Å
R, deg
â, deg
102.008(5)
90.00
107.449(3)
90
γ, deg
V, Å3
3792.5(18)
8
6053(2)
4
Z
Dcalc, g cm-3
µ, cm-1
2θmax, deg
no. of reflns measd
no. of reflns used
1.876
1.655
1.660
1.648
74.13
24.69
24.55
24.37
56.58
56.78
56.66
56.72
26 089
8928
41 674
14 245
(0.0576)
922
20 884
13 596
(0.0492)
1045
16 666
12 063
(0.0224)
1046
(Rint
)
(0.0808)
487
no. of params
final R [I > 2σ(I)]
R1a
0.0634
0.0932
0.0954
0.2311
0.0586
0.0682
0.0478
0.0763
wR2b
R (all data)
R1a
0.1725
0.1083
0.920
0.2059
0.2622
0.941
0.2004
0.0832
0.760
0.1029
0.0895
1.042
wR2b
GOF on F2
2
2
2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w[(Fo - Fc )2]/∑[w(Fo2)2]]1/2. w ) 1/[σ2(Fo2) + (0.075P)2], where P ) [Max(Fo2,0) + 2Fc ]/3.
Table 2. Selected Bond Lengths (Å) and Angles
then dried and the residues were analyzed by 1H NMR
(deg) for 1-4
spectroscopy by using CD2Cl2 as the solvent. The 1H NMR
Compound 1
2.025(12) Pt(2)-N(4)
2.048(10) Pt(2)-N(2)
spectrum of the reaction mixture recorded after 48 h showed
a mixture of products consisting of [Pt(1,2-BAB)(CH2Ph)-
(NCCD3)][BAr′4] (4, 81%), [Pt(1,2-BAB)(p-tolyl)(NCCD3)][BAr′4]
(4b, 6%), and [Pt(1,2-BAB)(m-tolyl)(NCCD3)][BAr′4] (4c, 13%).
The cationic complex [Pt(1,2-BAB)(CH3)(NCCD3)]+ was not
observed after ∼40 h. Part of the 1H NMR spectrum of the
mixture at the high-field region that is characteristic of the
isomers of the tolyl and the benzyl is provided here. 1H NMR
(δ, ppm), 4: 2.87 (d, satellite, 2J ) 10.0 Hz, 2JPt-H ) 106.5 Hz;
Pt(1)-C(22)
Pt(1)-C(21)
2.095(10)
2.121(11)
C(22)-Pt(1)-C(21) 87.8(5)
C(22)-Pt(1)-N(2) 93.4(4)
C(22)-Pt(1)-N(4)
C(21)-Pt(1)-N(4)
178.3(5)
92.9(4)
C(21)-Pt(1)-N(2) 178.8(4)
N(4)-Pt(1)-N(2)
85.8(3)
Compound 2
Pt(1)-C(23)
Pt(1)-N(4)
Pt(1)-N(2)
2.018(11) Pt(1)-S(1)
2.153(10) S(1)-C(22)
2.144(11) S(1)-C(21)
2.290(4)
1.75(2)
1.821(17)
175.9(3)
2
1H, -CH2Ph), 2.71 (d, 2J ) 10.0 Hz, JPt-H ) 106.5 Hz; 1H,
C(23)-Pt(1)-N(4) 171.2(5)
C(23)-Pt(1)-N(2) 89.4(4)
N(2)-Pt(1)-S(1)
-CH2Ph); 4b: 2.23 (s; 3H, p-CH3Ph); 4c: 2.20 (s; 3H, m-CH3-
Ph). The assignment of the chemical shifts to the tolyl isomers
is based on previous work reported by Tilset and co-workers.1g
C(22)-S(1)-C(21) 100.8(11)
N(4)-Pt(1)-N(2)
C(23)-Pt(1)-S(1)
N(4)-Pt(1)-S(1)
86.5(4)
94.6(4)
89.8(3)
C(22)-S(1)-Pt(1)
C(21)-S(1)-Pt(1)
111.5(7)
106.6(7)
X-ray Diffraction Analysis. Single crystals of 1 were
obtained from the solution of THF/hexanes. Single crystals of
2 were obtained from the solution of benzene/hexanes. Single
crystals of 3 were obtained from the solution CH2Cl2/hexanes.
Single crystals of 4 were obtained from the solution of toluene/
hexanes. All data were collected on a Siemens P4 X-ray
diffractometer with a CCD-1000 detector, operated at 50 kV
and 30 mA at ambient temperature. The data for 1-4 were
collected over 2θ ranges of ∼58°. No significant decay was
observed during the data collection. The structural solution
and refinement were performed on a PC using Siemens
SHELXTL software (version 5.10).6 Neutral atom scattering
factors were taken from Cromer and Waber.7 Empirical
absorption correction was applied to all crystals. All structures
were solved by direct methods. Most of the non-hydrogen
atoms were refined anisotropically. Most of the CF3 groups in
2-4 display rotational disorders, which were modeled and
refined successfully. The positions of hydrogen atoms were
calculated, and their contributions in structural factor calcula-
tions were included. The crystal data quality of 2 is relatively
poor due to the poor quality of the crystal and inadequate
absorption correction. The crystal data for 1-4 are listed in
Table 1. Important bond lengths and angles are given in Table
2.
Compound 3
Pt(1)-S(1)
Pt(1)-N(1)
Pt(1)-C(21)
Pt(1)-N(2)
2.249(2)
1.877(6)
2.052(7)
2.146(6)
S(1)-C(29)
S(1)-C(28)
C(21)-C(22)
1.736(8)
1.745(7)
1.488(9)
N(1)-Pt(1)-C(21) 87.2(3)
C(21)-Pt(1)-N(2) 171.2(3)
N(1)-Pt(1)-N(2)
87.2(2)
C(22)-C(21)-Pt(1) 119.0(5)
N(1)-Pt(1)-S(1)
C(21)-Pt(1)-S(1)
N(2)-Pt(1)-S(1)
174.4(2)
94.7(2)
90.26(17) C(28)-S(1)-Pt(1)
C(29)-S(1)-C(28)
C(29)-S(1)-Pt(1)
100.5(4)
111.8(3)
110.7(3)
Compound 4
Pt(1)-N(5)
Pt(1)-N(4)
1.965(5)
2.014(4)
2.058(5)
2.148(4)
N(5)-C(28)
C(28)-C(29)
C(21)-C(22)
1.110(7)
1.460(9)
1.498(7)
Pt(1)-C(21)
Pt(1)-N(2)
N(5)-Pt(1)-N(4)
177.79(19) C(21)-Pt(1)-N(2)
170.4(2)
N(5)-Pt(1)-C(21) 90.3(2)
N(4)-Pt(1)-C(21) 89.4(2)
C(22)-C(21)-Pt(1) 116.9(3)
C(28)-N(5)-Pt(1)
176.1(6)
N(5)-Pt(1)-N(2)
N(4)-Pt(1)-N(2)
92.62(18)
87.32(16)
N(5)-C(28)-C(29) 177.8(8)
Results and Discussion
Synthesis of 1,2-Bis(N-7-azaindolyl)benzene (1,2-
BAB). 1,2-BAB was initially obtained in ∼40% yield
using Ullmann condensation reaction between 7-azain-
dole and 1,2-dibromobenzene in the presence of CuSO4
and K2CO3 as catalyst and HBr scavenger, respectively.
However, due to the large steric hindrance at the 1,2-
(6) SHELXTL NT Crystal Structure Analysis Package, Version 5.10;
Bruker AXS, Analytical X-ray System: Madison, WI, 1999.
(7) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, AL, 1974; Vol. 4, Table
2.2A.

Regioselective C-H Activation of Toluene
Organometallics, Vol. 24, No. 13, 2005 3293
Figure 2. Structure of the cation in compound 2 with 50%
thermal ellipsoids and labeling schemes. For clarity, all
carbon atoms are shown as isotropic spheres.
Figure 1. Structure of compound 1 with 50% thermal
ellipsoids and labeling schemes.
position (the attachment of the second 7-azaindolyl
group is sterically hindered by the first 7-azaindolyl
group), the reaction requires a fairly high temperature
(240 °C), which leads to considerable loss of the starting
materials via thermal evaporation and thermal decom-
position and a relatively low yield. Using 1,2-diiodoben-
zene instead of 1,2-dibromobenzene under the same
reaction conditions led to extensive decomposition. The
best synthetic procedure for 1,2-BAB is using the
homogeneous catalysis method developed by the Buch-
wald group,8 where 1,2-diiodobenzene and 7-azaindole
were reacted in the presence of CuI, 1,10-phenanthro-
line, and Cs2CO3 in DMF at 150 °C under N2 atmo-
sphere for 72 h. By this procedure, the yield of 1,2-BAB
was increased to ∼75%. The details of this procedure
are described in the Experimental Section.
Synthesis and Structure of Pt(1,2-BAB)(CH3)2
(1). The Pt(II) complex 1 was obtained in 57% yield from
the reaction of 1,2-BAB with Pt2Me4(µ-SMe2)2. The
structure of 1 was confirmed by NMR spectroscopy,
elemental analyses, and single-crystal X-ray diffraction
analysis. There are two independent molecules in the
asymmetric unit with similar structural features, one
of which is shown in Figure 1. The Pt(II) center has a
square planar coordination geometry with two nitrogen
donor atoms from the chelate ligand occupying two
coordination sites and two methyl groups trans to each
of the nitrogen donor atoms. The most interesting
feature of 1 is the capping of the fifth coordination site
by the phenyl ring of the 1,2-BAB ligand, which is
clearly imposed by the geometry of the 1,2-BAB ligand.
The shortest atomic separation distance between the Pt-
(II) center and the phenyl ring is 3.10 Å, while the
longest separation distance (with C(17) and C(18)) is
4.18 Å. The two 7-azaindolyl groups are almost perpen-
dicular to the phenyl group of the chelate ligand,
evidenced by the fact that the dihedral angles between
the two 7-azaindolyl planes and the phenyl group of the
chelate ligand are 80.1° and 75.6°, respectively. The
phenyl group of the chelating ligand and the Pt(II)
coordination plane is nearly parallel, as indicated by the
dihedral angle (28.2°) between the two planes. The
geometry and the environment around the Pt(II) center
resemble those observed3b in Pt2(CH3)4(ttab).
Benzene C-H Activation and the Structure of
{Pt(1,2-BAB)Ph(SMe2)}[BAr′4] (2). The utility of com-
plex 1 in C-H bond activation was first examined by
the reaction of its cation with benzene. The cationic Pt-
(II) complex {Pt(1,2-BAB)(CH3)(solvent)}+ (solvent could
be either benzene or diethyl ether from the acid) was
obtained in situ by the addition of 1 equiv of acid
[H(Et2O)2][BAr′4] (Ar′ ) 3,5-bis(trifluoromethyl)phenyl)
to the benzene solution of 1 at ambient temperature.
After 1 h, the reaction was terminated by the addition
of dimethyl sulfide. The benzene C-H activation prod-
uct [Pt(1,2-BAB)Ph(SMe2)][BAr′4], 2, was isolated in
∼71% yield as a crystalline product.
Compound 2 was fully characterized by NMR spec-
troscopy and elemental and single-crystal X-ray diffrac-
tion analyses. The structure of the cation portion of 2
is shown in Figure 2. The Pt(II) center adopts a typical
square planar coordination geometry, with one phenyl
group and one SMe2 ligand being trans to the two
nitrogen donor atoms of the chelate ligand. As observed
in 1, the phenyl group of the chelate ligand caps one
side of the Pt coordination plane. The dihedral angle
(33.4°) between the phenyl ring of the 1,2-BAB ligand
and the Pt coordination plane is much larger than that
in 1, and the longest atomic separation distance between
the phenyl ring of the 1,2-BAB and the Pt center (4.38
Å) is also longer than that in 1, which are clearly the
consequence of increased steric congestion in 2. The
phenyl ligand is nearly perpendicular to the Pt coordi-
nation plane, with the dihedral angle between the two
planes of ∼95.5°. The shortest separation distance
between the phenyl ligand and the phenyl ring of the
1,2-BAB ligand is 3.55(1) Å (C(28)-C(19)), and those
between the phenyl ligand and the N(1) 7-azaindolyl
ring are 3.63(1) Å (C(28)-N(2)) and 3.66(1) Å (C(24)-
C(6)). The phenyl ligand is also almost perpendicular
to the phenyl ring of the 1,2-BAB ligand and the N(1)
7-azaindolyl ring (dihedral angle ) 73.5°, 86.9°, respec-
tively). The two methyl groups of the SMe2 ligand, albeit
1
indistinguishable in the H spectrum, display distinct
satellite peaks due to coupling to the 195Pt nucleus. The
-
structure of the BAr′4 anion is provided in the Sup-
porting Information. The formation and isolation of
compound 2 demonstrated that complex 1 is indeed a
useful reagent for facile C-H activation of aromatic
molecules.
(8) Klapars, A.; Antilla, J. C.; Huang, X. H.; Buchwald, S. L. J. Am.
Chem. Soc. 2001, 123, 7727.

3294 Organometallics, Vol. 24, No. 13, 2005
Zhao et al.
Toluene C-H Activation and the Structures of
{Pt(1,2-BAB)(CH2Ph)(SMe2)}[BAr′4] (3) and {Pt(1,2-
BAB)(CH2Ph)(CH3CN)}[BAr′4] (4). Previous mecha-
nistic study on C-H activation by cationic Pt(II) com-
plexes has established that the aryl C-H bond activation
is usually thermodynamically favored over the benzylic
C-H bond in methyl-substituted benzene such as
toluene or xylene.1 Preferential activation of the benzylic
C-H bond can however be achieved by using sterically
bulky chelate ligands on the Pt(II) center, as demon-
strated by Bercaw, Tilset, et al.1a-j In Bercaw and
Tilset’s Pt(II) systems, the chelate ligand is a diimine,
Ar′NdC(R)C(R)dNAr′ (R ) H or Me), whose steric bulk
can be controlled by the size and the position of the
substituents on Ar′. In contrast to the bulky diimine
ligand Ar′NdC(R)C(R)dNAr′ (e.g., Ar′ ) mesityl, 3,5-
di-tert-butylphenyl, etc.) investigated by Bercaw, Tilset,
et al., the 1,2-BAB ligand is much less sterically bulky.
However, once bound to the Pt(II) center, the 1,2-BAB
ligand has a very rigid shape, with the central phenyl
ring effectively blocking one side of the Pt coordination
plane. To determine if this blocking effect imposed by
the 1,2-BAB ligand can lead to any regioselective C-H
activation of methyl-substituted benzenes, we examined
the reaction of complex 1 with toluene.
The bulk reaction of toluene with complex 1 was
carried out in the same manner as the reaction of
benzene with 1. The cationic Pt(II) complex, {Pt(1,2-
BAB)(CH3)(solvent)}+, was first generated in situ by the
addition of 1 equiv of acid [H(Et2O)2][BAr′4] (Ar′ ) 3,5-
bis(trifluoromethyl)phenyl) to the toluene solution of 1
at ambient temperature. After 1 h, the reaction was
terminated by the addition of SMe2. The C-H activation
product with the formula [Pt(1,2-BAB)(CH2Ph)(SMe2)]-
[BAr′4] (3) was isolated as a pure crystalline solid in 41%
yield. The methyl group was not observed in the 1H
NMR spectrum. Instead, a quartet characteristic of a
AB coupling pattern with 195Pt coupling satellites was
observed, which is consistent with a CH2 group where
the two protons have different chemical environments.
1H NMR spectral data established unambiguously that
in 3 instead of the aryl C-H bond, the benzylic C-H
bond of the toluene molecule was activated. The two
methyl groups of SMe2 appear as two distinct sets of
chemical shifts with 195Pt coupling satellite peaks.
Compound 3 is stable in solution at ambient tempera-
ture, as evidenced by the absence of any notable NMR
spectral change over the period of more than 3 days.
Figure 3. Structure of the cation in compound 3 with 50%
thermal ellipsoids and labeling schemes. For clarity, all
carbon atoms are shown as isotropic spheres.
different chemical environments. One unexpected fea-
ture is the orientation of the benzylic group: instead of
orienting away from the 1,2-BAB ligand, it orients
toward it; that is, it is on the same side of the Pt(II)
coordination plane as the phenyl group of the 1,2-BAB
ligand. One possible explanation for this behavior is the
π-interaction between the phenyl ring of the benzyl
ligand and the 7-azaindolyl ring of the 1,2-BAB ligand.
Indeed, the phenyl ring of the benzyl ligand is in close
contact with one of the 7-azaindolyl groups of the 1,2-
BAB ligand (the N(1) ring), as evidenced by the short
separation distances of N(1)-C(27) (3.23(1) Å) and
C(7)-C(27) (3.24(1) Å). The dihedral angle between the
N(1) 7-azaindolyl ring and the phenyl ring of the benzyl
is 32.6°. Some interactions between the benzyl phenyl
group and the 1,2-BAB phenyl group are also evident,
as shown by the short contact distances of C(23)-C(16)
(3.32(1) Å) and C(25)-C(16) (3.59(1) Å). The dihedral
angle between the benzyl phenyl ring and the 1,2-BAB
phenyl ring is 64.0°.
The isolation and structural characterization of 2
indicated that the cationic complex of 1 is fully capable
of activating the aryl C-H bond of the toluene molecule
under the same reaction conditions as used for benzene
C-H activation. The fact that the benzylic C-H activa-
tion product was isolated as the major product from the
toluene reaction could be attributed to steric factors, as
suggested by Bercaw and Tilset et al.1a-j in similar
toluene C-H activation by cationic Pt(II) systems.
Recently it has been reported by Bercaw and co-workers
that the benzylic C-H activation product may also be
thermodynamically favored due to the stabilizing effect
of a η3-bonding mode involving both the methylene
group and the phenyl group.1k
The crystal structure of 3 was determined by single-
crystal X-ray diffraction analysis. As shown in Figure
3, the coordination environment around the Pt(II) center
resembles that of 2. The dihedral angle (35.7°) between
the phenyl ring of the 1,2-BAB ligand and the Pt
coordination plane is larger than that observed in 2, and
the longest atomic separation distance between the
phenyl ring of the 1,2-BAB and the Pt center (4.44 Å)
is somewhat longer than that in 2, an indication that
steric interactions among the ligands in 3 are somewhat
greater than those in 2. The benzylic carbon C(21) is
bound to the Pt center with a typical Pt-C bond length.
The C(21)-C(22) bond length of 1.488(9) Å is typical
for a C-C single bond. The Pt(1)-C(21)-C(22) bond
angle is 119.0(5)°. The crystal structure confirmed that
the two protons of the methylene group are indeed in
Since compound 3 is a fairly congested molecule due
to the presence of the dimethyl sulfide ligand, it is quite
possible that the dimethyl sulfide ligand may have
played a role in promoting and stabilizing the benzylic
C-H activation product, even though it was only used
to terminate the reaction. To rule out this possibility,
we repeated the toluene C-H activation process by
replacing dimethyl sulfide with acetonitrile as the
terminating reagent while the reaction conditions were
kept the same. From this reaction, compound 4, with
the formula [Pt(1,2-BAB)(CH2Ph)(CH3CN)][BAr′4], was

Regioselective C-H Activation of Toluene
Organometallics, Vol. 24, No. 13, 2005 3295
Figure 4. Structure of the cation in compound 4 with 50%
thermal ellipsoids and labeling schemes. For clarity, all
carbon atoms are shown as isotropic spheres.
isolated as a crystalline product in 38% yield. 4 was fully
characterized by NMR and single-crystal X-ray diffrac-
tion analyses. As observed in 3, in the 1H NMR
spectrum, the CH2 protons display an AB pattern with
distinct 195Pt coupling satellites, confirming that the
benzylic C-H bond was activated. Compound 4 does not
change in solution over extended periods of time at
ambient temperature. The structure of 4 shown in
Figure 4 resembles that of 3. The dihedral angle (34.4°)
between the phenyl ring of the 1,2-BAB ligand and the
Pt coordination plane is similar to that observed in 3,
and the longest atomic separation distance between the
phenyl ring of the 1,2-BAB and the Pt center (4.44 Å)
is also similar to those observed in 3. Again, the benzyl
group is oriented on the same side as the 1,2-BAB
ligand. The phenyl group of the benzyl ligand has a
dihedral angle of 38.2° with the phenyl plane of the 1,2-
BAB ligand, and 35.4° with the N(3) 7-azaindolyl ring.
The shortest atomic separation distance (among non-
hydrogen atoms) between the phenyl group of the benzyl
and the phenyl group of 1,2-BAB is 3.40(1) Å (C(20)-
C(23)) and those between the phenyl group of benzyl
and the N(3) 7-azaindolyl ring are 3.29(1) Å (N(4)-
C(14)) and 3.33(1) Å (N(4)-C(27)). Again π-interactions
among the aryl rings in 4 may be responsible for the
observed orientation of the benzyl group.
Figure 5. 1H NMR spectra of the high-field region (in CD2-
Cl2 at 298 K) for the reaction mixture of 1 with toluene
after the addition of the acid and the termination by CD3-
CN at different time intervals (“/” the o-tolyl product).
material Pt(1,2-BAB)(CH3)2 was completely consumed.
The Pt complex distribution after 5 min is {Pt(1,2-BAB)-
(CH3)(CD3CN)}+ (31%), {Pt(1,2-BAB)(CH2Ph)(CD3CN)}+
(17%), {Pt(1,2-BAB)(p-tolyl)(CD3CN)}+ (24%), {Pt(1,2-
BAB)(m-tolyl)(CD3CN)}+ (28%), which corresponds to a
69% total yield of C-H activation (the o-tolyl product
is very little, as shown in Figure 5). As the reaction time
increases, the signal of the cationic complex {Pt(1,2-
BAB)(CH3)(CD3CN)}+ decreases further and the signals
of the toluene C-H activation products increase. One
notable change is that after 30 min the benzyl product
becomes the major product (43%) and the p-tolyl (15%)
and m-tolyl (20%) compounds become minor products
with 22% of unreacted {Pt(1,2-BAB)(CH3)(CD3CN)}+.
After 1 h, the product distribution becomes benzyl
(55%), p-tolyl (9%), m-tolyl (17) with 19% unreacted {Pt-
(1,2-BAB)(CH3)(CD3CN)}+. After 3 h, the ratio becomes
benzyl (60%), p-tolyl (11%), m-tolyl (12) with 17%
unreacted {Pt(1,2-BAB)(CH3)(CD3CN)}+. The signal
from the cationic complex {Pt(1,2-BAB)(CH3)(CH3CN)}+
The isolation of 3 and 4 demonstrated that the
terminating ligand SMe2 or CH3CN appears to have no
significant impact on the preferential formation of the
benzylic C-H activation product.
1H NMR Study of the Toluene C-H Activation.
Since the isolated yield of 3 and 4 is only about 40%,
other products are also likely present in the reaction
mixture. To determine the product distribution from the
toluene C-H activation reaction at ambient tempera-
ture, the reaction terminated by acetonitrile was inves-
1
became completely undetectable after ∼40 h. The H
NMR spectrum of the reaction mixture terminated by
CD3CN after 48 h is shown in Figure 5 (top). The
product distribution is Pt-CH2Ph (81%), Pt-m-tolyl
(13%), Pt-p-tolyl (6%) (Scheme 1). The NMR results
indicate that the benzyl C-H activation appears to be
thermodynamically favored since the initial products are
dominated by aryl C-H activation. We recorded the 1H
NMR spectra of the reaction mixture without the
addition of the terminating reagent such as CD3CN. The
spectra are very complex, and we have not yet observed
1
tigated by H NMR spectroscopy. A small portion (0.5
mL) of the reaction mixture (11.7 mM) was taken out
and added to a NMR tube that contains CD3CN (25 µL)
for terminating the reaction at regular time intervals
(initially every 5 min; after 2 h, the samples were
collected every 1 h). After the removal of the solvents
1
by vacuum, the H NMR spectrum of the residue was
1
recorded in CD2Cl2 (0.4 mL). H NMR spectra showed
that after 5 min of the addition of the acid the starting

3296 Organometallics, Vol. 24, No. 13, 2005
Zhao et al.
Scheme 1
any evidence for the formation of the η3-bound species
as reported by Bercaw in their cationic Pt(II) systems.
The cause for the preferential formation of the benzyl
C-H activation product displayed by our Pt 1,2-BAB
complex therefore remains undermined. We are cur-
rently performing a detailed kinetic and 2D NMR
investigation on this system, aiming to have a full
understanding of the reaction mechanism and the
unusually high regioselectivity displayed by our Pt(II)
complex in toluene C-H activation. The results will be
reported in due course.
Acknowledgment. We thank the Natural Sciences
and Engineering Research Council of Canada for finan-
cial support.
Supporting Information Available: This material is
available free of charge via the Internet at http://pubs.acs.org.
OM050133Z