Impact of steric blocking on diastereoselective C-H activation of ethylbenzene by cationic platinum(II) complexes with 7-azaindolyl derivative ligands

Article abstract of DOI:10.1021/om0608861

Ethylbenzene C-H activation reactions using the cationic compounds [Pt(BAB)(CH₃)(L)]⁺ and [Pt-(BAM)(CH₃)(L)] ⁺ generated in situ by the reaction of [(Et₂O) ₂H][BAr′₄] with the parent molecules Pt-(BAB)(CH₃)₂ (1) and Pt(BAM)(CH₃)₂ (2), respectively, have been examined (BAB = 1,2-bis(N-7-azaindolyl)benzene, BAM = bis(N-7-azaindolyl)methane). The impact of the steric blockage of the Pt axial coordination site by the BAB and BAM ligands on the regio- and diastereoselectivity of the ethylbenzene C-H activation has been investigated. For both Pt complexes the benzylic C-H activation products were found to be thermodynamically favored, with the BAB complex showing a higher regioselectivity. In addition, both complexes display distinct diastereoselectivity in the formation of the η¹-benzylic products [Pt(BAB)(CH₃CN)(CH(Me)Ph)][BAr′₄] (3) and [Pt(BAM)(CH₃CN)(CH(Me)Ph)]-[BAr′₄] (4), with the BAB complex showing a much higher diastereoselectivity. The structures of the diastereomers have been established by single-crystal X-ray diffraction and 2D NOESY NMR analyses The two η³ complexes [Pt(BAB) (η³-CH(Me)Ph)][BAr′₄4] (5) and [Pt(BAM)(η³-CH(Me)Ph)][BAr′₄] (6) have been characterized, and the structure of 5 has been determined by single-crystal X-ray diffraction analysis. The η³ complexes have been found to exist in two isomeric forms, with one isomer being dominant. NMR experiments established that the η³ complexes 5 and 6 can be converted quantitatively to the corresponding η¹ complexes 3 and 4, respectively, with the retention of the isomer (or diastereomer ratio. The results of this investigation are consistent with the η¹-benzylic complex being the reaction intermediate in the formation of the η¹-benzylic complex. The asymmetric blocking of the Pt axial coordination site by the BAB and BAM ligands was found to be responsible for the observed diastereoselectivity in ethylbenzene C-H activation.

Full text of DOI:10.1021/om0608861

Organometallics 2006, 25, 5979-5989
5979
Articles
Impact of Steric Blocking on Diastereoselective C-H Activation of
Ethylbenzene by Cationic Platinum(II) Complexes with 7-Azaindolyl
Derivative Ligands
Shu-Bin Zhao, Gang Wu, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
ReceiVed September 27, 2006
Ethylbenzene C-H activation reactions using the cationic compounds [Pt(BAB)(CH3)(L)]+ and [Pt-
(BAM)(CH3)(L)]+ generated in situ by the reaction of [(Et2O)2H][BAr′4] with the parent molecules Pt-
(BAB)(CH3)2 (1) and Pt(BAM)(CH3)2 (2), respectively, have been examined (BAB ) 1,2-bis(N-7-
azaindolyl)benzene, BAM ) bis(N-7-azaindolyl)methane). The impact of the steric blockage of the Pt
axial coordination site by the BAB and BAM ligands on the regio- and diastereoselectivity of the
ethylbenzene C-H activation has been investigated. For both Pt complexes the benzylic C-H activation
products were found to be thermodynamically favored, with the BAB complex showing a higher
regioselectivity. In addition, both complexes display distinct diastereoselectivity in the formation of the
η1-benzylic products [Pt(BAB)(CH3CN)(CH(Me)Ph)][BAr′4] (3) and [Pt(BAM)(CH3CN)(CH(Me)Ph)]-
[BAr′4] (4), with the BAB complex showing a much higher diastereoselectivity. The structures of the
diastereomers have been established by single-crystal X-ray diffraction and 2D NOESY NMR analyses.
The two η3 complexes [Pt(BAB)(η3-CH(Me)Ph)][BAr′4] (5) and [Pt(BAM)(η3-CH(Me)Ph)][BAr′4] (6)
have been characterized, and the structure of 5 has been determined by single-crystal X-ray diffraction
analysis. The η3 complexes have been found to exist in two isomeric forms, with one isomer being
dominant. NMR experiments established that the η3 complexes 5 and 6 can be converted quantitatively
to the corresponding η1 complexes 3 and 4, respectively, with the retention of the isomer (or diastereomer)
ratio. The results of this investigation are consistent with the η3-benzylic complex being the reaction
intermediate in the formation of the η1-benzylic complex. The asymmetric blocking of the Pt axial
coordination site by the BAB and BAM ligands was found to be responsible for the observed
diastereoselectivity in ethylbenzene C-H activation.
on the kinetic and thermodynamic selectivity of alkylarene C-H
activation have been the focus of several recent reports involving
cationic Pt(II) compounds.4-8 Previous studies on various Pt-
(II) model systems by a number of research groups5-7 have
consistently demonstrated a high kinetic preference for aryl
C-H activation. Investigation by the Tilset and Bercaw groups
on C-H activation using Pt(II) complexes containing diimine
ligands with the general formula Ar′NdC(R)C(R)dNAr′ re-
vealed that the formation of the thermodynamically preferred
benzylic products initially competes effectively with that of the
kinetically preferred aromatic products.5c,6k,j For aryl C-H
Introduction
Selective activation of inert C-H bond by transition-metal
compounds is a key step toward catalytic and direct hydrocarbon
functionalization.1 Since the demonstration of the catalytic
transformation of CH4 into CH3OH and CH3Cl by Pt(II)/Pt-
(IV) salts in aqueous systems by Shilov and co-workers,2
considerable research efforts have been directed to the study
and understanding of the electrophilic C-H activation process
on a Pt center by using a variety of organoplatinum model
systems, and extensive investigations on the key steps involved
in alkane C-H bond activation on a Pt(II) center have been
conducted and valuable new insights on the role of the Pt(II)
center have been obtained.3 The electronic and steric impacts
(3) For recent comprehensive reviews on cationic Pt(II)-mediated C-H
bond activation studies, see: (a) Arndtsen, B. A.; Bergman, R. G.; Mobley,
T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (b) Bengali, A. A.;
Arndtsen, B. A.; Burger, P. M.; Schultz, R. H.; Weiller, B. H.; Kyle, K.
R.; Moore, C. B.; Bergman, R. G. Pure Appl. Chem. 1995, 67, 281. (c)
Crabtree, R. H. Chem. ReV. 1995, 95, 987. (d) Shilov, A. E.; Shulpin, G.
B. Chem. ReV. 1997, 97, 2879. (e) Shilov, A. E.; Shul’pin, G. B. Chem.
ReV. 1997, 97, 2879. (f) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew.
Chem., Int. Ed. 1998, 37, 2180. (g) Labinger, J. A.; Bercaw, J. E. Nature
2002, 417, 507. (h) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54,
259. (i) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471.
* To whom correspondence should be addressed. E-mail: wangs@
chem.queensu.ca.
(1) (a) Goldberg, K. I.; Goldman, A. S. ActiVation and Functionalization
of C-H Bonds; American Chemical Society: Washington, DC, 2004. (b)
Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352; 1983,
105, 3929. (c) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106,
1650. (d) Chen, H. Y.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science
2000, 287, 1995.
(2) (a) Goldshlegger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman,
A. A. Zh. Fiz. Khim. 1969, 43, 2174. (b) Goldshlegger, N. F.; Eskova, V.
V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353.
(4) (a) Periana, R. A.; Taube, J. D.; Gamble, S.; Taube, H.; Satoh, T.;
Fujii, H. Science 1998, 280, 560. (b) Sen, A. Acc. Chem. Res. 1998, 31,
550. (c) Ziatdinov, V. R.; Oxgaard, J.; Mironov, O. A.; Young, K. J. H.;
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10.1021/om0608861 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/16/2006

5980 Organometallics, Vol. 25, No. 26, 2006
Zhao et al.
Chart 1
case of toluene, the Pt(BAB)(CH3)2 compound (1) was found
to activate toluene C-H bonds regioselectively with the benzylic
product as the dominating product,12a consistent with the
behavior of the diimine Pt(II) complexes investigated by Bercaw
and co-workers.6k,j The key feature of the BAM and BAB
ligands is that they cap the fifth coordination site of the Pt(II)
center. Because of the different sizes of the linkers in BAB
(phenyl) and BAM (CH2), this pair of ligands provides us a
unique opportunity to investigate the impact of steric blocking
of the Pt axial coordination site on the regio- and diastereose-
lectivity of alkylbenzene C-H bond activation. We have
examined the reactions of ethylbenzene with Pt(BAB)(CH3)2
and Pt(BAM)(CH3)2. We have observed that, in addition to
regioselectivity, the reactions of ethylbenzene with these two
Pt(II) compounds display a distinct and contrasting diastereo-
selectivity, a phenomenon that has not been reported in other
cationic Pt(II) systems with the exception of intramolecular
chiral auxiliary ligand mediated C-H activation systems.10,11
In addition, we have succeeded in isolating and structurally
characterizing a key η3-benzylic complex, identifying the
coexistence of two η3 structural isomers, and determining their
role in diastereoselective ethylbenzene C-H bond activation.
The details are presented herein.
activation products, the kinetic preference of para and meta C-H
activation was observed and they were found to undergo
transformation to the benzylic C-H activation products.6k,j For
some of the alkyl-substituted benzenes, this regiospecific
selectivity has been attributed to the thermodynamic stability
of a η3-benzylic complex characterized by NMR spectroscopic
analyses, which can be converted cleanly to the η1-benzylic
product upon the addition of donor ligands such as acetonitrile.6j
The majority of previous investigations on C-H activation
involving cationic Pt(II) centers have focused on regioselectivity.
Information on stereo- or diastereoselective C-H bond activa-
tion using Pt complexes has been rather scarce, except for the
few recent examples reported by Sames and co-workers, where
cationic Pt(II) complexes with chiral auxiliary ligands were used
in stereoselective C-H activation.10,11 We have reported recently
that cationic Pt(II) complexes containing 7-azaindolyl derivative
ligands such as BAB (1,2-bis(N-7-azaindolyl)benzene) and N,N-
bis(7-azaindolyl)methane (BAM) as shown in Chart 1 can
undergo facile C-H activation with benzene or toluene.12 In
Experimental Section
All reactions were performed under an inert atmosphere of dry
N2 with standard Schlenk techniques or in a drybox. Solvents were
freshly distilled prior to use. 1H NMR and 13C NMR spectra were
recorded on Bruker Avance 400 or 500 spectrometers, and the
spectra were referenced to residual solvent peaks. Elemental
analyses were performed by Canadian Microanalytical Service, Ltd,
Delta, British Columbia. Starting materials were purchased from
Aldrich Chemical Co. and used without further purification.
[H(Et2O)2][BAr′4] (Ar′ ) 3,5-bis(trifluoromethyl)phenyl) was
prepared using a procedure reported in the literature.13 Pt(BAB)-
(CH3)2 and Pt(BAM)(CH3)2 were synthesized as reported previ-
ously.12
(5) (a) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999,
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van Eldik, R. Inorg. Chem. 2006, 45, 3613. (f) Wik, B. J.; Lersch, M.;
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Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1997, 119, 848. (d)
Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw,
J. E. Inorg. Chim. Acta 1998, 270, 467. (e) Johansson, L.; Tilset, M.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846. (f)
Procelewska, J.; Zahl, A.; van Eldik, R.; Zhong, H. A.; Labinger, J. A.;
Bercaw, J. E. Inorg. Chem. 2002, 41, 2808. (g) Zhong, H. A.; Labinger, J.
A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124, 1378. (h) Wong-Foy, A.
G.; Henling, L. M.; Day, M.; Labinger, J. A.; Bercaw, J. E. J. Mol. Catal.
A: Chem. 2002, 189, 3. (i) Heyduk, A. F.; Labinger, J. A.; Bercaw, J. E.
J. Am. Chem. Soc. 2003, 125, 6366. (j) Iverson, C. N.; Carter, C. A. G.;
Baker, R. T.; Scollard, J. D.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2003, 125, 12674. (k) Heyduk, A. F.; Driver, T. G.; Labinger, J. A.;
Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 15034. (l) Driver, T. G.; Day,
M. W.; Labinger, J. A.; Bercaw, J. E. Organometallics 2005, 24, 3644.
(m) Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2006,
128, 2005.
(7) (a) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R. J.
Organometallics 1997, 16, 525. (b) Prokopchuk, E. M.; Jenkins, H. A.;
Puddephatt, R. J. Organometallics 1999, 18, 2861. (c) Hinman, J. G.; Baar,
C. R.; Jennings, M. C.; Puddephatt, R. J. Organometallics 2000, 19, 563.
(d) Zhang, F.; Kirby, C. W.; Hairsine, D. W.; Jennings, M. C.; Puddephatt,
R. J. J. Am. Chem. Soc. 2005, 127, 14196.
(8) (a) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235.
(b) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100. (c)
Konze, W. V.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc. 2002, 124,
12550. (d) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 8870.
(e) Thomas, C. M.; Peters, J. C. Organometallics 2005, 24, 5858. (f)
Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics
2006, 25, 1801.
Ethylbenzene C-H Activation by Pt(BAB)(CH3)2 (1) and the
Isolation of [Pt(BAB)(CHMePh)(MeCN)][BAr′4] (3). Under N2,
[H(Et2O)2][BAr′4] (190 mg, 0.19 mmol) was added to a stirred
solution of Pt(BAB)(CH3)2 (100 mg, 0.19 mmol) in ethylbenzene
(20.0 mL) at ambient temperature (22 °C). The solution mixture
became clear and turned pink in ∼1 min. During the reaction, thick
brown oily residue accumulated slowly on the bottom of the flask.
After 5 days, CH3CN (0.20 mL, 3.8 mmol) was added to terminate
the reaction. After the reaction mixture was stirred for another 1 h,
the solution was separated from the oily residue (∼24 mg). The
solvents were removed under vacuum. To remove ethylbenzene,
the residue was repeatedly dissolved in CH2Cl2 and the solvent
was repeatedly removed under vacuum (three times), which
produced a brown residue of ∼260 mg. 1H NMR analyses indicated
that >90% of the products are η1-benzylic C-H activation products
along with aromatic C-H activation products (<10%) (1H NMR
spectrum of the oily residue showed the same composition as that
of the bulk product.). Two sets of chemical shifts corresponding to
the two diastereomers of the η1-benzylic product with a ratio of
∼3.7:1 were observed in the 1H NMR spectrum. Colorless crystals
of 3 were obtained by recrystallization of the crude product from
hexanes/THF (4/1). Anal. Calcd for C62H38N5BF24Pt: C, 49.16; H,
1
2.53; N, 4.62. Found: C, 48.50; H, 2.55; N, 4.55. H NMR (400
MHz, CD2Cl2, 25 °C): major diastereomer, 8.34 (dd; 3J ) 5.4 Hz,
4J ) 1.1 Hz; 1H, aza), 8.05-7.91 (m; 3H, aza), 7.84-7.76 (m;
(9) For an example review on ortho-metalated transition-metal complexes,
see: Mohr, F.; Prive´r, S. H.; Bhargava, S. K.; Bennett, M. A. Coord. Chem.
ReV. 2006, 250, 1851.
(10) (a) Johnson, J. A.; Sames, D. J. Am. Chem. Soc. 2000, 122, 6321.
(b) Johnson, J. A.; Li, D.; Sames, D. J. Am. Chem. Soc. 2002, 124, 6900.
(11) Dangel, B. D.; Johnson, J. A.; Sames, D. J. Am. Chem. Soc. 2001,
123, 8149.
(12) (a) Zhao, S. B.; Song, D.; Jia, W. L.; Wang, S. Organometallics
2005, 24, 3290. (b) Song, D.; Wang, S. Organometallics 2003, 22, 2187.
(13) Brookhart, M.; Grant, B.; Volpe, J. Organometallics 1992, 11, 3920.

DiastereoselectiVe C-H ActiVation of Ethylbenzene
Organometallics, Vol. 25, No. 26, 2006 5981
due to the complexity of the spectrum): BAr′4 162.2 (q, JB-H
2H, phenyl of 1,2-BAB), 7.74 (s; 8H, BAr′4), 7.57 (s; 4H, BAr′4),
7.46 (dd; 3J ) 8.0 Hz,4J ) 1.6 Hz; 1H, phenyl of 1,2-BAB), 7.42
)
50.4 Hz), 135.2 (br), 129.5 (q, br; JC-F ) 36 Hz), 125.3 (q; JC-F
) 272 Hz), 117.92 (br); 151.37, 151.20, 147.17, 146.93, 132.63,
132.62, 132.48, 129.92, 129.47, 128.75, 128.24, 126.07, 125.93,
125.52, 124.36, 123.90, 118.23, 117.90, 117.87, 117.84, 104.10,
103.59, 103.57, 56.00, 55.53; Pt-CHMePh (20.23, 16.58), Pt-
CHMePh (21.07, 19.49), Pt-NCMe (4.13, 3.84).
(dd; 3J ) 8.0 Hz, 4J ) 2 Hz; 1H, phenyl of 1,2-BAB), 7.25 (d; J
3
) 3.6 Hz; 1H, aza), 7.23 (d; 3J ) 3.6 Hz; 1H, aza), 7.19-7.17 (m;
3H, Pt-CHMePh), 7.09 (dd; 3J1 ) 5.6 Hz, 3J2 ) 8.0 Hz; 1H, aza),
6.96 (dd; 3J1 ) 5.6 Hz, 3J2 ) 8.0 Hz; 1H, aza), 6.93-6.90 (m; 2H,
3
3
Pt-CHMePh), 6.66 (d; J ) 3.6 Hz; 1H, aza), 6.63 (d; J ) 3.6
1
3
2
1
Hz; H, aza), 3.33 (q, satellite; J ) 7.3 Hz, JPt-H ) 103.7 Hz;
1H, Pt-CHMePh), 2.22 (s; 3H, Pt-NCMe), 0.83 (d; 3J ) 7.3 Hz,
3JPt-H ) 50.4 Hz; 3H, Pt-CHMePh); minor diastereomer, 8.26 (dd;
3J ) 5.4 Hz,4J ) 1.1 Hz; 1H, aza), 8.05-7.91 (m; 5H, 3H from
aza, 2H from phenyl of 1,2-BAB), 7.74 (s; 8H, BAr′4), 7.60 (dd;
General Procedure of the H NMR Spectroscopic Analyses
of the Reaction Mixture. Under N2, [H(Et2O)2][BAr′4] (1.0 equiv)
was added to a stirred ethylbenzene solution of the Pt(II) complex
1 or 2 at ambient temperature (22 °C). A small amount of the
reaction mixture, during the course of the reaction, was taken out
at regular time intervals and put immediately into NMR tubes that
contain CD3CN. These samples were dried, and the residues were
4
3J ) 8.0 Hz, J ) 1.6 Hz; 1H, phenyl of 1,2-BAB), 7.57 (s; 4H,
3
4
BAr′4), 7.51 (dd; J ) 8.0 Hz, J ) 1.6 Hz; 1H, phenyl of 1,2-
3
3
1
BAB), 7.29 (d; J ) 3.6 Hz; 1H, aza), 7.27 (d; J ) 3.6 Hz; 1H,
then analyzed by H NMR spectroscopy by using CD2Cl2 as the
3
aza), 7.19-7.17 (m; 3H, Pt-CHMePh), 7.06 (dd, J1 ) 5.6 Hz,
solvent. The assignment of the chemical shifts to the reaction
mixture is based on spectroscopic data of the isolated benzylic C-H
activation product and the 1H NMR spectrum of the para aromatic
C-H activation product, which was synthesized independently (see
the Supporting Information).
3J2 ) 8.0 Hz; 1H, aza), 6.93-6.90 (m; 3H, 1H from aza, 2H from
3
3
Pt-CHMePh), 6.68 (d; J ) 3.6 Hz; 1H, aza), 6.64 (d; J ) 3.6
1
3
2
Hz; H, aza), 2.90 (q, satellite; J ) 7.3 Hz, JPt-H ) 103.7 Hz;
3
1H, Pt-CHMePh), 2.23 (s; 3H, Pt-NCMe), 0.89 (d, satellite; J
) 7.3 Hz, 3JPt-H ) 50.4 Hz; 3H, Pt-CHMePh). 13C{1H} NMR of
all diastereomers (100 MHz, CD2Cl2; not fully assigned due to the
complexity of the spectrum): BAr′4 162.9 (q, JB-H ) 50.1 Hz),
135.2 (br), 129.4 (q, br; JC-F ) 36 Hz), 125.3 (q; JC-F ) 272 Hz),
117.92 (br); 146.01, 145.97, 144.38, 144.30, 144.18, 144.12, 132.71,
132.57, 132.49, 132.34, 131.96, 131.87, 131.71, 131.54, 131.43,
131.31, 129.8, 129.40, 128.71, 128.60, 128.22, 124.90, 124.71,
123.8, 121.7, 117.89, 117.76, 104.17, 104.08; Pt-CHMePh (15.97,
14.45), Pt-CHMePh (14.58, 14.30), Pt-NCMe (3.53, 3.49).
Synthesis of [Pt(BAB)(η3-CHPhMe)][BAr′4] (5). Under N2,
[H(Et2O)2][BAr′4] (160 mg, 0.30 mmol) was added to a solution
of 1 (317 mg, 0.31 mmol) in ethylbenzene (30 mL) and the solution
was stirred at ambient temperature for 5 days. After the solution
was concentrated to ∼5 mL under vacuum and kept in a refrigerator
for several weeks, colorless crystals of the η3 complex 5 were
obtained in ∼73% yield. Anal. Calcd for C60H35BF24N4Pt·0.5-
(ethylbenzene): C, 50.34; H, 2.64; N, 3.67. Found: C, 50.81; H,
1
2.97; N, 3.54. H NMR (400 MHz, CD2Cl2, 25 °C; the aromatic
Ethylbenzene C-H Activation by Pt(BAM)(CH3)2 (2) and the
Isolation of [Pt(BAM)(CHMePh)(MeCN)][BAr′4] (4). Under N2,
[H(Et2O)2][BAr′4] (303 mg, 0.30 mmol) was added to a stirred
solution of 2 (142 mg, 0.30 mmol) in ethylbenzene (25 mL) at
ambient temperature (22 °C). A clear yellow solution was formed
immediately. After 5 days, CH3CN (0.60 mL, 11.4 mmol) was
added and the reaction mixture was stirred for another 1 h. After
the solvents were removed under vacuum, CH2Cl2 (5.0 mL) was
used to dissolve the residue and then removed under vacuum. This
was repeated three times to remove the ethylbenzene. A 392 mg
region is not fully assigned because of many overlapping peaks;
assignments are based on a 2D-COSY spectrum): major isomer,
3.92 (d, satellite; J ) 6.0 Hz, JPt-H ) 36.8 Hz; 1H, ortho Pt-
3
3
3
3
η3-CHMePh), 1.88 (q, satellite; J ) 6.4 Hz, JPt-H ) 66.8 Hz;
1H, Pt-η3-CHMePh), 1.03 (d; 3J ) 6.4 Hz; 3H, Pt-η3-CHMePh);
3
3
minor isomer, 6.09 (d, satellite; J ) 6.0 Hz, JPt-H ) 42.6 Hz;
1H, ortho Pt-η3-CHMePh), 3.18 (q, satellite; J ) 6.6 Hz, JPt-H
3
3
) 68.4 Hz; 1H, Pt-η3-CHMePh), 0.82 (d; J ) 6.6 Hz; 3H, Pt-
3
η3-CHMePh). 13C{1H} NMR (100 MHz, CD2Cl2; key chemical
2
shifts for major isomer): 67.03 (satellite; JPt-C ) 118 Hz; ortho
Pt-η3-CHMePh), 38.16 (satellite; JPt-C ) 283 Hz; Pt-η3-
2
1
amount of pale yellow residue was obtained. H NMR analyses
CHMePh), 13.95 (satellite; 3JPt-C ) 186 Hz; Pt-η3-CHMePh); key
showed the presence of the benzylic C-H activation products as
the major products (∼75%), along with aromatic C-H activation
products (∼25%). The benzylic product displayed two sets of
signals with a ratio of ∼1.5:1, corresponding to the two diastere-
omers in the 1H NMR spectrum. The crystalline compound 4 was
isolated from the reaction mixture after slow evaporation of the
solvent. Anal. Calcd for C57H36N5BF24Pt: C, 47.12; H, 2.50; N,
4.82. Found: C, 47.18; H, 2.56; N, 4.72. 1H NMR (400 MHz, CD2-
Cl2, 25 °C): minor diastereomer, 10.01 (d; 2J ) 15.4 Hz; 1H, CH2
2
chemical shifts for minor isomer, 70.11 (satellite; JPt-C ) 112 Hz;
ortho Pt-η3-CHMePh), 34.90 (satellite; JPt-C ) 288 Hz; Pt-η3-
3
CHMePh), 11.44 (satellite; JPt-C ) 196 Hz; Pt-η3-CHMePh).
3
Synthesis of [Pt(BAM)(η3-CHPhMe)][BAr′4] (6). The η3
complex 6 was synthesized by a method similar to that described
for 5. After 5 days, compound 6 was found to be the major product
in the reaction mixture according to NMR data. However, pure
crystalline complex 6 could not be isolated from the reaction
mixture because of the oily nature of the final reaction mixture
and its high solubility in common organic solvents. Compound 6
3
bridge), 8.47 (dd, br; J1 ) 5.4 Hz,3J2 ) 1.0 Hz; 2H, aza), 8.08-
8.00 (m, satellite; 3JPt-H ) 38.2 Hz; 2H, aza), 7.75 (s; 8H, BAr′4),
7.58 (s; 4H, BAr′4), 7.48-7.40 (m; 2H, aza), 7.22-7.15 (m; 2H,
aza), 7.09-6.95 (m; 3H, Pt-CHMePh), 6.76-6.70 (m; 2H, Pt-
1
was characterized by NMR spectroscopic analyses. The H NMR
spectrum of the η3 complex 6 is provided in the Supporting
2
1
CHMePh), 6.60-6.57 (m; 2H, aza), 6.00 (d; J ) 15.3 Hz; 1H,
Information. H NMR (400 MHz, CD2Cl2, 25 °C; the aromatic
3
2
CH2), 3.32 (q, satellite; J ) 7.2 Hz, JPt-H ) 109.3 Hz; 1H, Pt-
region is not fully assigned due to the overlaps of peaks): major
3
isomer, 8.69 (d; 2J ) 15.2 Hz; 1H, CH2 bridge), 5.98 (d, satellite;
CHMePh), 2.38 (s; 3H, Pt-NCCH3), 1.40 (d, satellite; J ) 7.2
Hz, 3JPt-H ) 44.8 Hz; 3H, Pt-CHMePh); major diastereomer, 10.32
3
3J ) 6.8 Hz, JPt-H ) 44.0 Hz; 1H, ortho Pt-η3-CHMePh), 5.89
2
3
(d; 2J ) 15.2 Hz; 1H, CH2 bridge), 3.29 (q, satellite; 3J ) 6.4 Hz,
3JPt-H ) 68.8 Hz; 1H, Pt-η3-CHMePh), 1.05 (d; 3J ) 6.4 Hz; 3H,
(d; J ) 15.3 Hz; 1H, CH2 bridge), 8.74 (dd, satellite; J1 ) 5.6
Hz, J2 ) 1.2 Hz, JPt-H ) 60.3 Hz; 2H, aza), 8.55 (dd, satellite;
3
3
3J1 ) 5.7 Hz, J2 ) 1.2 Hz, JPt-H ) 60.6 Hz; 1H, aza), 7.75 (s,
8H, BAr′4), 7.58 (s, 4H, BAr′4), 7.48-7.40 (m; 2H, aza), 7.22-
7.15 (m; 2H, aza), 7.09-6.95 (m; 3H, Pt-CHMePh), 6.76-6.70
(m; 2H, Pt-CHMePh), 6.66 (d; 3J ) 3.4 Hz; 2H, aza), 6.14 (d; 2J
) 15.4 Hz; 1H, CH2 bridge), 3.76 (q, satellite; 3J ) 7.2 Hz, 2JPt-H
) 110.0 Hz; 1H, Pt-CHMePh), 2.24 (s; 3H, Pt-NCMe), 1.17 (d,
3
3
2
Pt-η3-CHMePh); minor isomer, 9.85 (d; J ) 15.2 Hz; 1H, CH2
bridge), 6.34 (d; 2J ) 15.2 Hz; 1H, CH2 bridge), 5.82 (d, satellite;
3J ) 6.6 Hz,3JPt-H ) 36.4 Hz; 1H, ortho Pt-η3-CHMePh), 3.20
3
3
(q, satellite; J ) 6.6 Hz, JPt-H ) 62.8 Hz; 1H, Pt-CHMePh),
1.17 (d; J ) 6.6 Hz; 3H, Pt-η3-CHMePh). 13C{1H} NMR (100
3
MHz, CD2Cl2): key chemical shifts for the major isomer, 78.07
satellite;3J ) 7.2 Hz, JPt-H ) 36.3 Hz; 3H, Pt-CHMePh). 13C-
(satellite; 2JPt-C ) 93 Hz; ortho Pt-η3-CHMePh), 43.67 (satellite;
3
{1H} NMR of all isomers (100 MHz, CD2Cl2; not fully assigned
2JPt-C ) 284 Hz; Pt-η3-CHMePh), 15.74 (satellite; JPt-C ) 200
3

5982 Organometallics, Vol. 25, No. 26, 2006
Zhao et al.
Table 1. Crystallographic Data for Compounds 3-5
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
3A, 4A, and 5
3
4
5
Compound 3A
formula
C62H38N5B-
F24Pt
1514.87
P1h
12.698(4)
13.177(5)
19.064(6)
90.973(7)
93.076(7)
95.273(6)
3171.1(18)
2
1.587
23.26
56.76
19 248
12 918
0.0814
1028
C57H36N5F24-
BPt
1452.8
C64H40N4F24-
BPt
1526.90
P1h
16.4420(17)
18.7653(19)
21.650(2)
65.647(2)
85.831(2)
89.751(2)
6067.1(11)
4
1.672
24.32
56.74
43 522
27 602
0.0644
1459
Pt(1)-N(5)
Pt(1)-N(1)
1.883(15)
1.932(11)
Pt(1)-C(27)
Pt(1)-N(3)
2.025(17)
2.153(10)
fw
space group
a, Å
b, Å
P1h
N(5)-Pt(1)-N(1)
N(5)-Pt(1)-C(27)
N(1)-Pt(1)-C(27)
179.2(5)
89.9(7)
90.8(7)
N(5)-Pt(1)-N(3)
N(1)-Pt(1)-N(3)
C(27)-Pt(1)-N(3)
92.3(4)
87.1(4)
169.7(7)
12.4783(12)
13.0894(12)
18.1842(18)
79.260(2)
76.753(2)
86.854(2)
2840.2(5)
2
c, Å
Compound 4A
R, deg
â, deg
γ, deg
V, Å3
Z
Pt(1)-N(5)
Pt(1)-N(3)
1.984(13)
2.033(12)
Pt(1)-C(19)
Pt(1)-N(1)
2.070(11)
2.145(8)
N(5)-Pt(1)-N(3)
N(5)-Pt(1)-C(19)
N(3)-Pt(1)-C(19)
178.9(4)
90.0(6)
89.1(6)
N(5)-Pt(1)-N(1)
N(3)-Pt(1)-N(1)
C(19)-Pt(1)-N(1)
89.2(4)
91.7(3)
178.5(7)
d
calcd, g cm-3
1.699
25.93
56.64
20 140
12 910
0.0416
822
µ, cm-1
2θmax, deg
no. of rflns measd
no. of rflns used
Rint
Compound 5
Pt(1)-N(1)
Pt(1)-C(23)
Pt(1)-N(4)
Pt(1)-C(28)
Pt(1)-C(22)
2.071(11)
2.124(11)
2.137(8)
2.213(14)
2.304(16)
Pt(2)-N(5)
Pt(2)-N(7)
Pt(2)-C(52)
Pt(2)-C(50)
Pt(2)-C(51)
2.067(10)
2.094(8)
2.110(12)
2.118(11)
2.145(11)
no. of params
final R (I > 2σ(I))
R1a
0.0761
0.1392
0.0959
0.2318
0.0770
0.1370
wR2b
N(1)-Pt(1)-C(23)
N(1)-Pt(1)-N(4)
C(23)-Pt(1)-N(4)
N(1)-Pt(1)-C(28)
C(23)-Pt(1)-C(28)
N(1)-Pt(1)-C(22)
C(23)-Pt(1)-C(22)
N(4)-Pt(1)-C(22)
C(28)-Pt(1)-C(22)
N(4)-Pt(1)-C(28)
134.4(4)
86.6(3)
134.7(4)
104.4(5)
37.9(4)
172.1(5)
40.6(4)
100.6(4)
68.2(5)
N(5)-Pt(2)-N(7)
N(5)-Pt(2)-C(52)
N(7)-Pt(2)-C(52)
N(5)-Pt(2)-C(50)
N(7)-Pt(2)-C(50)
C(52)-Pt(2)-C(50)
N(5)-Pt(2)-C(51)
N(7)-Pt(2)-C(51)
C(52)-Pt(2)-C(51)
C(50)-Pt(2)-C(51)
86.0(3)
169.6(4)
103.9(4)
100.0(5)
173.9(4)
70.1(5)
133.6(5)
134.7(5)
39.2(4)
R (all data)
R1a
0.2510
0.1897
0.916
0.1975
0.2668
0.955
0.2342
0.1612
0.718
wR2b
GOF on F2
2
2
2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w[(Fo - Fc )2]/∑[w(Fo )2]]1/2
;
2
2
2
w ) 1/[σ2(Fo ) + (0.075P)2], where P ) [Max(Fo ,0) + 2Fc ]/3.
Hz; Pt-η3-CHMePh); key chemical shifts for the minor isomer,
167.9(4)
39.7(4)
75.43 (satellite; JPt-C ) 86 Hz; ortho Pt-η3-CHMePh), 38.62
2
(satellite; 2JPt-C ) 280 Hz; Pt-η3-CHMePh), 14.12 (satellite; 3JPt-C
) 199 Hz; Pt-η3-CHMePh).
Chart 2
Conversion of the η3 Complex 5 to the η1-Benzylic Complex
3. A 15 mg portion of the η3 complex 5 (∼1 × 10-5 mol) was
placed in a NMR tube, and CD2Cl2 (0.5 mL) was added to dissolve
the sample at ambient temperature. After a 1H NMR spectrum was
recorded, CD3CN (17 µL, ∼3 × 10-4 mol) was added to the NMR
1
tube, and a series of H NMR spectra were recorded every 5 min
at ambient temperature. No conversion was detected at temperatures
below -10 °C. The conversion at room temperature was fairly slow,
There are two independent molecules of 5 in the unit cell.
Disordered ethylbenzene solvent molecules were located in the
lattice of 5 and refined successfully. The phenyl group in 4 displays
a rotational disordering. Two sets of phenyl atoms with 50%
occupancy for each site were located and refined successfully. Most
of the non-hydrogen atoms were refined anisotropically. Most of
the CF3 groups in 3-5 display rotational disorders, which were
modeled and refined successfully. The positions of hydrogen atoms
were calculated, and their contributions in structure factor calcula-
tions were included. The crystal data for 3-5 are listed in Table 1.
Important bond lengths and angles are given in Table 2.
1
despite the excess amount of CD3CN used. The H NMR spectra
indicated that after ∼1 h the η3 compound was nearly completely
converted to the η1-benzylic C product 3. Notably, the ratio of the
two sets of signals for the diastereomers of 3 after the conversion
was about the same as that of the two isomers of the η3 complex
5.
The conversion of complex 6 to complex 4 was carried out in
the same manner by using compound 6. NMR experiments showed
that compound 6 can also react with acetonitrile at ambient
temperature to afford the η1-benzylic C-H activation product 4
quantitatively.
X-ray Diffraction Analysis. Single crystals of 3 were obtained
from recrystallization of the crude product with THF/hexanes.
Single crystals of 4 were obtained from the reaction mixture after
the slow evaporation of the solvent. Single crystals of 5 were
obtained from the reaction mixture after being kept for several
weeks in the refrigerator. Crystals were mounted on glass fibers
for data collection. Data were collected on a Siemens P4 single-
crystal X-ray diffractometer with a Smart CCD-1000 detector and
graphite-monochromated Mo KR radiation, operating at 50 kV and
35 mA at 298 K for 3 and at 180 K for 4 and 5, respectively. No
significant decay was observed for all of the samples. Data were
processed on a PC using the Bruker SHELXTL software package14
(version 5.10) and were corrected for Lorentz and polarization
effects. Crystals of 3-5 all belong to the triclinic space group P1h.
Results and Discussion
Ethylbenzene C-H Bond Activation using Complexes 1
and 2. (a) Regioselectivity. As reported earlier12 in the benzene
and toluene activation reactions, the Pt(II) complexes 1 and 2
are precursor compounds that can be used for C-H bond
activation after the in situ generation of the cationic species
shown in Chart 2 (where L is a solvent molecule or Et2O) by
the addition of an acid.
The electronic impacts of the BAB and the BAM ligands on
the Pt center appear to be similar, as evidenced by the CO
stretching frequencies of the CO complexes [Pt(BAB)Me(CO)]-
[BAr′4] (2106 cm-1) and [Pt(BAM)Me(CO)][BAr′4] (2119
cm-1) (see the Supporting Information).
The addition of the acid to the suspension of 1 in dry
ethylbenzene at ambient temperature resulted in the immediate
(14) SHELXTL NT Crystal Structure Analysis Package, Version 5.10;
Bruker AXS, Madison, WI, 1999.

DiastereoselectiVe C-H ActiVation of Ethylbenzene
Organometallics, Vol. 25, No. 26, 2006 5983
Figure 1. 1H NMR spectra of the low-field region of the reaction
mixture of ethylbenzene C-H activation by the cation of 2, showing
the distribution change of aryl C-H activation products (p- and
m-Ph) versus the benzylic product 4 with time.
Figure 2. Side view of the structures of 1 and 2 showing the
blocking of the linker to the Pt center.
Chart 3
formation of a clear pink solution. To examine the kinetic and
thermodynamic selectivity, the reaction mixture was sampled
at various time intervals and subjected to 1H NMR spectroscopic
analyses after treatment with acetonitrile. Consistent with our
previous observation for toluene C-H activation,12a the aromatic
C-H activation products were again found to be the dominant
species (meta and para) at the early stage of the reaction, which
were converted to the benzylic product with time; this product
was isolated as [Pt(BAB)(CH3CN)(CH(Me)Ph)][BAr′4] (3),
whereas no o-aryl product was observed during the whole
process. Therefore, the benzylic activation is a thermodynami-
cally preferred process while the aromatic activation is kineti-
cally favored. This is in good agreement with the recent reports
by Bercaw and co-workers.6j,k After 5 days, the cationic starting
material was completely consumed and the ratio between the
benzylic C-H activation product and aryl C-H activation
products (meta and para) was >9:1.
The ethylbenzene C-H activation with complex 2 was
investigated using a similar procedure. To facilitate the 1H NMR
spectral assignments of the products, the para aromatic C-H
activation product [Pt(BAM)(p-Ph-Et)(CH3CN)][BAr′4] (see the
Supporting Information) was synthesized. Not surprisingly, the
1H NMR spectroscopic analyses of the reaction mixture revealed
that the para and meta aromatic C-H activation products were
once again kinetically preferred. The spectrum recorded after
15 min showed a mixture of products consisting of para and
meta aromatic products (50%), benzylic product (4, 18%), and
the cationic starting material (32%). After 3 days, the starting
material was completely consumed, as shown by the stacked
1H NMR spectra of the reaction mixture in the distinct low-
field region recorded over a 5-day period (Figure 1). The product
distribution between the benzylic C-H activation and aromatic
C-H activation after 5 days was ∼3.7:1, an indication that
compound 2 has a lower regioselectivity, compared to compound
1. Considering their similar electronic properties, this regiose-
lectivity difference between 1 and 2 toward ethylbenzene C-H
activation can be attributed to the steric difference between the
BAB and the BAM ligandssthe former is apparently more
sterically demanding than the latter, as shown by the crystal
structures of 1 and 2 in Figure 2. The complex [Pt(BAM)(CH3-
CN)(CH(Me)Ph)][BAr′4] (4) was isolated and fully characterized
by NMR, elemental, and single-crystal X-ray diffraction analy-
ses.
The coexistence of the two chiral centers (one Pt-based and
one carbon-based) in the cationic complexes of 3 and 4, in
principle, should result in the formation of two diastereomers
(RR and RS or their enantiomers SS and SR) for each complex,
as illustrated by the structures 3A and 3B of the cation of 3 in
Chart 3 (the diastereomers 4A and 4B for the cation of 4 have
similar structures and, hence, are not shown). The key aspect
of our investigation was therefore to determine if the ethylben-
zene C-H activation by the Pt(II) complexes 1 and 2 has any
preference for one diastereomer over the other: i.e., diastereo-
selectivity.
Crystal Structures of [Pt(BAB)(CH3CN)(CH(Me)Ph)]-
[BAr′4] (3A) and [Pt(BAM)(CH3CN)(CH(Me)Ph)][BAr′4]
(4A). The structures of the cationic portions of 3 and 4 are
shown in Figure 3. The basic structural features of 3 resemble
those of the toluene analogue12a [Pt(BAB)(CH3CN)(CH2Ph)]+
reported earlier by us. The BAB and the BAM ligands have
orientations similar to those observed in the starting materials
1 and 2. The shortest contact distance between the carbon atoms
of the BAB phenyl ring and the Pt center in 3 is 3.16 Å, while
the contact distance between one of the protons of the BAM
CH2 and the Pt center in 4 is 2.46 Å (the contact distance
between the CH2 carbon atom and the Pt center is 3.16 Å). There
are two important key features for these two compounds, as
revealed by the structures. First, the phenyl group of the
activated ethylbenzene is oriented “up” toward the chelate ligand
BAB in 3 and BAM in 4, despite the obvious steric congestion
in this orientation. We have observed the same phenomena in
the crystal structures of [Pt(BAB)(L)(CH2Ph)]+, where L ) CH3-
CN, SMe2. One possible explanation for this unexpected
orientation of the phenyl group is the attraction between the
aromatic phenyl ring and the aromatic 7-azaindolyl rings of the
chelate ligands. The second important feature is that the
diastereomers of the two crystal structures have an identical
stereogeometry, which corresponds to the diastereomer 3A (or
4A) or its enantiomer shown in Chart 3. The chiral carbon
centers in the structures 3A and 4A, as shown in Figure 3, have
the same S chirality (the inversion-center-related enantiomers
coexist in the crystal lattice for both compounds). To determine
if the stereogeometry revealed by the crystal structures represents
the major diastereomer for both compounds in solution, we
investigated the reaction mixture by NMR spectroscopic meth-
ods.
(b) Diastereoselectivity. Because of the asymmetric blocking
on one side of the Pt(II) coordination plane by the BAB and
BAM ligands in 1 and 2, respectively, the cationic species shown
in Chart 2 have a chiral structure (with both enantiomers being
present). Unlike toluene which, after the cleavage of a C-H
bond on the methyl group, does not produce any chiral center,
the cleavage of a C-H bond on the CH2 group of the
ethylbenzene molecule leads to the formation of a chiral center.

5984 Organometallics, Vol. 25, No. 26, 2006
Zhao et al.
intermolecular process has not been reported in previously
known cationic Pt(II) systems.
Similarly, the reaction mixture of the cation of 2 with
ethylbenzene after the termination by acetonitrile and the isolated
product 4 also displays two distinct sets of chemical shifts that
correspond to the two diastereomers 4A and 4B in the 1H NMR
spectra (see the Supporting Information). For the CH and the
CH3 protons, characteristic 195Pt-H coupling satellites were
once again observed, with 195Pt-H 2J and 3J coupling constants
being ∼110 and ∼40 Hz, respectively. However, as shown by
Figure 4, the observed ratio of the two diastereomers for 4 is
∼1.5:1.0, much lower than that of 3. Thus, the BAM Pt system
displays not only a relatively low regioselectivity but also a
relatively low diastereoselectivity in ethylbenzene C-H activa-
tion, compared to the case for the BAB Pt system. Again, the
obviously more effective blocking group, phenyl, in the BAB
ligand, compared to the CH2 group in the BAM ligand, appears
to be responsible for the observed difference in diastereoselec-
tivity between these two systems.
Figure 3. Crystal structures of the η1-benzylic complexes 3A and
4A. Thermal ellipsoid drawings with labeling schemes can be found
in the Supporting Information.
Diastereomer Distribution in the Reaction Mixture. Due
to the significant difference between the structures 3A and 3B
(or 4A and 4B), it was anticipated that these two diastereomers
may display distinct chemical shifts in NMR spectra. Indeed,
two distinct sets of chemical shifts corresponding to the two
diastereomers for the CH and the CH3 protons of the -CH-
1
(Me)Ph group were observed in the H NMR spectrum of the
Structures of 3 and 4 in Solution. To determine the
molecular structures of compounds 3 and 4 in solution, we used
isolated complex 3 and the spectrum of the reaction mixture of
the cation of 1 with ethylbenzene, as shown by Figure 4 (the
full spectrum is provided in the Supporting Information). The
protons of the CH and the CH3 groups display characteristic
195Pt-H coupling satellites with fairly large 2J and 3J 195Pt-H
coupling constants (∼104 and ∼50 Hz, respectively). Most
importantly, as shown in Figure 4, the intensities of the two
sets of signals are not equal, with a ratio of ∼3.7:1.0 in the
final reaction mixture. The NMR data unambiguously estab-
lished that the ethylbenzene C-H bond activation by complex
1 proceeds not only regioselectively but also diastereoselectively.
It is noteworthy that a similar diastereoselectivity of such an
1
2D H NOESY NMR spectroscopy. Figure 5 shows portions
of the NOESY spectrum for compound 3 in CD2Cl2 solution
(the complete 2D NOESY spectra for 3 and 4 are provided in
the Supporting Information). As expected, strong cross-peaks
are observed between CH3, CH, and the ortho protons from the
phenyl group for both diastereomers A and B. The most direct
NMR evidence revealing the absolute configuration comes from
NOE cross-peaks between CH3, CH, and H6 of the 7-azaindolyl
ring. As seen from Figure 5, for the A diastereomer the CH-
H6 cross-peak is considerably stronger than the CH3-H6 cross-
Figure 4. 1H NMR spectra in CD2Cl2 showing the chemical shifts and the relative intensity of the CH(Me) proton of the two diastereomers
in 3 and 4 in the reaction mixtures.

DiastereoselectiVe C-H ActiVation of Ethylbenzene
Organometallics, Vol. 25, No. 26, 2006 5985
Figure 5. Portions of the 2D 1H NOESY spectrum for compound
4 in CD2Cl2 at 298 K obtained on a Bruker Avance-600 NMR
spectrometer (B0 ) 14.1 T). A mixing time of 400 ms was used.
Two transients were collected for each of the 512 t1 increments
with a recycle delay of 2 s. The structure shown is diastereomer
A.
Figure 6. Portions of the 2D 1H NOESY spectrum for compound
3 in CD2Cl2 at 298 K. A mixing time of 400 ms was used. The
structure shown is diastereomer A.
In contrast to the case for 4, the same NOESY spectral
analysis established that the major diastereomer for 3 has the
structure 3A, which has the same stereogeometry as the crystal
structure. The most important evidence supporting this assign-
ment is the strong CH-H6 cross-peak for isomer A, as shown
in Figure 6, which is absent for isomer B, supporting that the
CH proton of the benzylic group in A is closer to the
7-azaindolyl ring than that of B. In addition, a strong CH3-Hb
(Hb′) cross-peak was also observed for isomer A, but not isomer
B, which is again consistent with the diastereomer 3A being
the major isomer. For the complete 2D NOESY NMR spectra,
please see the Supporting Information.
The preference for the diastereoisomer A by compound 3
may be explained by the fact that, in 3A, the unfavorable inter-
actions between the methyl group and the 7-azaindolyl ring are
minimized. However, the same rationale clearly does not apply
for the relatively high selectivity for isomer B by 4, since B
has a chirality on the carbon center which is opposite to that in
A. To understand the origin of the contrasting diastereoselec-
tivity by 3 and 4, efforts were made to analyze and isolate the
possible intermediates involved in the formation of 3 and 4.
Isolation and Characterization of the η3 Complexes [Pt-
(BAB)(η3-CHPh(Me))][BAr′4] (5) and [Pt(BAM)(η3-CHPh-
(Me))][BAr′4] (6). In order to identify the reaction intermediates,
ideally the reaction mixture should be monitored using solvents
that do not introduce any coordinating ligand such as acetonitrile.
Due to the highly reactive nature of the starting materials 1
and 2 toward halogenated solvents, conventional NMR solvents
such as CD2Cl2 cannot be used to monitor the reaction. We
peak, indicating that the CH group is pointing toward H6: i.e.,
the 7-azaindolyl ring. In contrast, the two corresponding cross-
peaks for the B diastereomer have similar intensities. It is also
important to note that the CH3-H6 cross-peak for B is much
greater than the corresponding CH3-H6 cross-peak for A, which
cannot be accounted for simply by the population difference
between the two methyl peaks. This is further proof that the
CH3 in the diastereomer B is much closer to H6 of the
7-azaindolyl ring than the CH3 in the diastereomer A. Another
interesting question is whether the phenyl ring of the CH(Me)-
Ph group in both isomers maintains the same orientation with
respect to the 7-azaindolyl ring ligands. Strong NOE cross-peaks
are observed between the ortho protons and H′ of the CH2 bridge
(but no cross-peaks were observed between the ortho protons
and the other proton in the same CH2 group, H) for both isomers,
an indication that the phenyl ring is oriented toward the CH2
bridge in the same manner as shown by the crystal structure.
Further, the cross-peak intensities are consistent with the
population ratio between the A and B diastereomers. This
suggests that the phenyl groups of the A and B diastereomers
have approximately the same distance (but not necessarily
the same orientation) to the CH2 bridge. The observed CH-H′
and CH3-H′ cross-peaks shown in Figure 5 also confirm
that the absolute configuration of the major diastereomer of 4
in solution is 4B, opposite to that observed in the crystal
structure.

5986 Organometallics, Vol. 25, No. 26, 2006
Zhao et al.
Figure 7. 1H NMR spectra of the η3-ethylbenzene group in 5 in CD2Cl2 showing the chemical shifts and the proposed structures of the two
isomers.
also tried to use CF3CD2OD as a NMR solvent to follow the
reaction. However, this solvent generated complicated species
in the reaction and interfered with the reaction. As a result, direct
monitoring of the ethylbenzene C-H bond activation process
using compound 1 or 2 by NMR could not be achieved. Since
the NMR data for the acetonitrile-terminated reaction showed
that after about 5 days all starting materials were consumed
and the reaction mixture did not change further with time, we
decided to investigate the 5 day reaction mixture without the
addition of acetonitrile. The 5 day reaction mixtures of 1 and 2
with [(Et2O)2H][BAr′4] and ethylbenzene were examined by 1H
and 13C NMR in CD2Cl2. For the ethylbenzene reaction with 1,
the 1H NMR data indicated that, in addition to the presence of
Figure 8. Crystal structure of the cation of 5 with two different
a small amount of aryl C-H activation products, the η3-benzylic
views. The detail of this structure with labeling schemes is provided
compound was the major product. No η1-benzylic compound
was observed. In addition, it appeared that there were two types
in the Supporting Information.
of η3-benzylic species in the reaction mixture with a ratio of
∼3.7:1. To fully characterize the elusive η3 compound, we tried
to isolate the η3-benzylic compound from the reaction mixture
and our efforts were successful. The η3 compound 5 was
obtained as a crystalline solid in about 70% yield. The 1H NMR
and 13C NMR spectra for the isolated solid 5 also showed two
sets of well-resolved chemical shifts of two distinct η3 com-
pounds with an intensity ratio similar to that of the reaction
mixture, as shown in Figure 7. The diagnostic signals for the
η3-benzylic group are the ortho proton of the phenyl group (δ
3.92 and 6.09 ppm, respectively, for the two isomers) and the
proton of the CH(Me) group (δ 1.88 and 3.18 ppm, respectively,
for the two isomers), both of which display characteristic
195Pt-H coupling with a coupling constant of ∼40 and ∼67
Hz, respectively. In the 13C NMR spectrum, two sets of chemical
shifts for the ortho carbon of the phenyl group and the CH(Me)
carbon atom with distinct 195Pt-13C coupling satellites were
also observed. On the basis of the NMR pattern, the formula of
[Pt(BAB)(η3-CH(Me)Ph)][BAr′4] was proposed for 5.
of the cation 5 has no symmetry, but both enantiomers coexist
in solution and in the crystal lattice. The CH(Me)Ph group in 5
is bound to the Pt center sideways through three carbon atoms,
the η3 fashion typically observed for an allyl group. The bond
lengths from the Pt center to the three carbon atoms are similar.
Although η3-type allyl Pt(II) complexes are well-known,8c,f the
structure of 5 is the first direct structural proof of the η3-benzylic
Pt(II) complexes proposed by Bercaw et al. On the basis of the
crystal structure of 5, the two types of η3 compounds observed
in the NMR spectra can be attributed to the coexistence of two
structural isomers, as depicted in Figure 7 and Scheme 1. In
the major isomer the CH proton and the ortho proton of the
phenyl ring are on the same side, as shown by the crystal
structure, while in the minor isomer, the CH3 group is on the
same side with the ortho proton. As shown by the crystal
structure, for the BAB compound, there is little space between
the phenyl group of the BAB ligand and the η3-benzylic ligand.
As a consequence, the interligand steric interactions in the minor
isomer are much greater than those in the major isomer, thus
leading to the preferential formation of the major isomer for 5.
NMR data analyses established that the major isomer of 5 has
the same structure as the crystal structure.
To understand the origin of the two different η3 species of 5,
we determined the crystal structure of 5 by a single-crystal X-ray
diffraction analysis. The structure of the cation in 5 is shown
in Figure 8 (there are two independent molecules of 5 in the
asymmetric unit with identical structures). Again, the structure
The NMR spectra for the 5 day reaction mixture of the BAM
complex 2 with [(Et2O)2H][BAr′4] and ethylbenzene in the

DiastereoselectiVe C-H ActiVation of Ethylbenzene
Organometallics, Vol. 25, No. 26, 2006 5987
Scheme 1. Stereospecific Conversion of 5 to 3
for compound 6 in CD2Cl2 and portions of the spectrum are
shown in Figure 9 (the complete 1D spectrum and 2D NOESY
spectrum for 6 are provided in the Supporting Information).
Unexpectedly, however, the methyl group of the major isomer
shows a strong cross-peak with the H′ proton of the CH2 bridge,
while the same cross-peak is absent for the minor isomer. This
is a strong indication that the major isomer of 6 has the structure
B, as depicted in Figures 7 and 9, instead of A. Consistent with
this assignment is the weak cross-peak between Ha and H′ for
the minor isomer, as shown in Figure 9. At first glance, this
seems counterintuitive, since isomer B appears to be more
sterically congested than A does. The 2D NOESY NMR data,
in fact, provided a clue for this puzzle. As shown in Figure 9,
the methyl group of the minor isomer has a very large cross-
peak with H6 of the 7-azaindolyl ring, an indication that in
isomer A the methyl group is considerably close to the
7-azaindolyl ring, generating a sterically crowded environment,
which we reasoned results in a smaller population of the isomer
A. As shown by the bond distance and angle data in Table 2,
the N-Pt-N chelate angle for BAB complexes (86-87°) is in
general much smaller than that of the BAM complex (92°),
which means that the methyl group and the 7-azaindolyl ring
in 6A would be much closer and, thus, have stronger interactions
than those in 5A. Although in isomer 6B the interaction between
the methyl group and the CH2 group is greater than that in 6A,
it is lessened to a great extent due to the incomplete blocking
of the fifth Pt coordination site by the CH2 bridge, compared
to the case for 5B, where the phenyl bridge completely blocks
the fifth Pt coordination site, causing a much stronger interaction
absence of donor solvents also displayed two distinct sets of
chemical shifts for the η3-benzylic products with a ratio of ∼1.5:
1, attributable to the two η3 isomers (Figure 9). Although we
have not been able to isolate the η3-benzylic product [Pt(BAM)-
(η3-CHPh(Me))][BAr′4] (6) as a pure crystalline compound, the
two η3 isomers of 6 are believed to have the same structures as
those of 5. The diminished ratio of the major isomer versus the
minor isomer of 6, compared to that of 5, is evidently caused
by the diminished steric congestion in 6. To establish that the
major isomer of 6 indeed has the same structure as the major
isomer A of 5, 2D NOESY NMR experiments were performed
Figure 9. Portions of the 2D 1H NOESY spectrum for compound 6 in CD2Cl2 at 298 K. A mixing time of 400 ms was used. The structure
shown is the major isomer 6B.

5988 Organometallics, Vol. 25, No. 26, 2006
Zhao et al.
or intramolecular processes.6j To determine if this pathway is
also operative in our system, we synthesized [Pt(BAM)(η1-CH2-
Ph)2] (see the Supporting Information) and monitored the
solution of the cation [Pt(BAM)(η1-CH2Ph)(solvent)]+ generated
in situ by the addition of 1 equiv of [(Et2O)2H][BAr′4] using
solvents such as acetone-d6 and C6F6 by 1H NMR. Although a
trace amount of the η3-CH2Ph compound appeared to be present,
>90% of the compound was an η1-CH2Ph complex (stabilized
by acetone or adventitious water). On the basis of these
observations, we suggest that the η3 complexes 5 and 6 are the
most likely intermediates in the formation of the η1-benzylic
complexes 3 and 4 from the ethylbenzene reactions with the
BAB complex 1 and the BAM complex 2, respectively. We
further suggest that the diastereoselectivity displayed by 3 and
4 is dictated by the asymmetric structure of the η3 precursor,
which is the direct consequence of the asymmetric blocking of
the Pt center by the BAB or BAM ligand.
Figure 10. 1H NMR spectra in the 2.7-6.2 ppm region showing
the conversion of the isomers of the η3 complex 5 to the
diastereomers of the η1 complex 3 with the retention of the isomer
ratio after the addition of excess CD3CN (∼30-fold). The spectra
were recorded at ambient temperature at 5 min intervals in CD2-
Cl2.
Conclusions
The BAB and BAM Pt(II) complexes activate alkyl-
substituted benzene readily. The η1-benzylic C-H activation
products are thermodynamically favored for both complexes,
with the BAB complex displaying a greater regioselectivity
toward the benzylic product than the BAM complex. Most
importantly, both BAB and BAM complexes display diaste-
reoselectivity for the η1-benzylic C-H activation products. Once
again, the Pt BAB complex has a higher diastereoselectivity
(3.7:1) than the BAM complex (1.5:1). However, the BAB and
the BAM complexes display different preferences for the
diastereomer. NMR analyses established that the dominating
diastereomer in solution is 3A for 3 and 4B for 4, respectively.
The isolation and characterization of the η3-benzylic com-
plexes 5 and 6, the identification of the two structural isomers
with a nonequal distribution, and the demonstration of their
stereospecific conversion to the corresponding η1-benzylic
compounds in the presence of a donor such as CH3CN, along
with the fact that the isomer ratio of the η3-benzylic complex
in the reaction mixture for both BAB and BAM complexes is
about the same as that of the diastereomer ratio of the
η1-benzylic products in the reaction mixture terminated by CH3-
CN cause us to suggest the following. (1) The η3-benzylic
complex is mostly likely a reaction intermediate or a precursor
compound to the η1-benzylic complex. (2) The apparent
diastereoselectivity displayed by the BAB and BAM com-
plexes in the formation of the η1-benzylic complex is due to
the selective formation and the asymmetric structures of the
two η3-benzylic isomers. (3) The preferential formation of one
η3 isomer over the other by the BAB and BAB complexes is
the consequence of the asymmetric blocking of the Pt axial
coordination sites by the chelate ligand. (4) The contrasting
preference for the dominating η3-benzylic isomer by the BAB
complex (favoring isomer A) and the BAM complex (favor-
ing isomer B) is most likely caused by the difference in
steric blocking by the phenyl bridge in BAB and the CH2
bridge in BAM and the N-Pt-N chelate angle in the two
complexes.
between the CH3 and the phenyl group. The greater popula-
tion of isomer 6B over 6A is therefore the consequence of
tradeoff between a methyl-CH2 interaction versus a methyl-
7-azaindolyl interaction. The contrasting selectivities for
different η3 isomers by 5 and 6 again demonstrate the subtle
impact of the steric blockage imposed by the BAB and BAM
ligands.
Conversion of the η3-Benzylic Compounds to the η1-
Benzylic Compounds. One key observation in characterizing
the η3 compounds was that the isomer ratios for the η3
compounds 5 and 6 are approximately the same as those of the
diastereoisomers of the η1-benzylic compounds 3 and 4, which
caused us to consider that the η3-benzylic compounds are likely
precursors to the η1-benzylic product. To confirm that the η3
compound can indeed be converted to the η1 compound, we
monitored the reaction of the isolated η3 compound 5 (a mixture
1
of both isomers) with CD3CN by H NMR spectra in CD2Cl2.
Indeed, as shown by the NMR spectra in Figure 10, compound
5 was converted quantitatively to the η1 compound 3 and the
resultant diastereomer ratio of 3 was the same as the isomer
ratio of the η3 compound 5. This conversion is not surprising,
since similar η3 to η1 conversion was observed by Bercaw and
co-workers.6k,j The significance of this experiment lies in the
fact that it established a direct link between the major η3 isomer
of 5A and the major diastereomer of 3A and that between the
minor η3 isomer of 5B and the minor diastereomer 3B. Similar
experiments on the addition of acetonitrile to the η3 isomer
mixture of 6 confirmed the same specific transformation to the
η1 compound 4 with the retention of the isomer ratios. We
believe that this specific transformation of one η3 isomer of 5
or 6 to one η1 diastereomer of 3 or 4 is a manifestation/
consequence of the asymmetric blocking of the Pt(II) coordina-
tion plane by the BAB or BAM ligand. As shown by Scheme
1, the blocking of the Pt(II) center by the chelate ligand forces
the acetonitrile ligand to approach the Pt(II) center from the
bottom, replacing the π-bound phenyl group, resulting in the
diastereoselective formation of the η1-benzylic complex. The
exclusive formation of the major diastereomer 3A with S
chirality on the CH center from the major η3 isomer and of the
minor diastereomer 3B with the R chirality on the CH center
from the minor η3 isomer are illustrated in Scheme 1. It has
been shown by Bercaw and co-workers that in the absence of
donor ligands such as acetonitrile, certain cationic η1-benzylic
complexes can be converted to the η3 complexes via either inter-
In summary, this work demonstrates the importance of the
steric geometry and the blocking effect of the N,N chelate
ligands in diastereoselective ethylbenzene C-H bond activation
by cationic Pt(II) complexes and provides new insight into the
key role played by the η3-benzylic complex.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial support
of this work.

DiastereoselectiVe C-H ActiVation of Ethylbenzene
Organometallics, Vol. 25, No. 26, 2006 5989
Supporting Information Available: Complete X-ray diffraction
data for 3A, 4A, and 5, including tables of atomic coordinates,
thermal parameters, and bond lengths and angles, diagrams of the
molecular structures with labeling schemes, and CIF files and text
and figures detailing the syntheses and IR and NMR data of [Pt-
(BAB)(Me)(CO)][BAr′4] and [Pt(BAM)(Me)(CO)][BAr′4], synthe-
ses, protonation reactions, and NMR spectra of [Pt(BAM)(p-PhEt)2]
and [Pt(BAB)(η1-CH2Ph)2], and complete 1D NMR spectra with
peak assignments and complete 2D NOESY NMR spectra of 3, 4,
and 6. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0608861