Benzene C-H activation by two isomeric platinum(II) complexes of bis(N-7-azaindolyl)methane

Article abstract of DOI:10.1021/om0301785

Two isomeric Pt(II) complexes, Pt(L1)(CH₃)₂ (1a) and Pt(L2)(CH₃)₂ (2a), based on two isomeric and fluorescent ligands of bis(N-7-azaindolyl)methane, L1 (symmetric) and L2 (asymmetric), have been synthesized and

Full text of DOI:10.1021/om0301785

Organometallics 2003, 22, 2187-2189
2187
Ben zen e C-H Activa tion by Tw o Isom er ic P la tin u m (II)
Com p lexes of Bis(N-7-a za in d olyl)m eth a n e
Datong Song and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Received March 10, 2003
Summary: Two isomeric Pt(II) complexes, Pt(L1)(CH3)2
(1a ) and Pt(L2)(CH3)2 (2a ), based on two isomeric and
fluorescent ligands of bis(N-7-azaindolyl)methane, L1
(symmetric) and L2 (asymmetric), have been synthesized
and fully characterized by NMR and X-ray diffraction
analyses. In the presence of [H(Et2O)2][BAr′4] (Ar′ ) 3,5-
bis(trifluoromethyl)phenyl), both 1a and 2a are capable
of activating a benzene C-H bond readily at ambient
temperature. The products from benzene activation by
1a and 2a have been isolated as [Pt(L1)Ph(SMe2)][BAr′4]
(1b) and [Pt(L2)Ph(SMe2)][BAr′4] (2b). The structures
of 1b and 2b have been determined by X-ray diffraction
analyses. The phenyl ligand in 2b is bound exclusively
trans to the pyrrole nitrogen atom of L2.
under relatively mild conditions. Extensive studies have
been carried out on the mechanism of C-H activation
by cationic Pt(II) complexes4a-j to understand the key
steps in the catalytic cycle of the Shilov system and to
design better and practical catalytic systems. Most
previously reported cationic Pt(II) complexes that are
capable of activating C-H bonds involve a diimine
ligand with the general formula of Ar′NdC(R)C(R)d
NAr′ 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.5 Many nitro-
gen-containing aromatic heterocyclic ligands are known
to be fluorescent. Incorporation of such a ligand to a
cationic Pt(II) complex may enable photochemical acti-
vation of C-H bonds. Recently we have reported that
Pt(II) complexes containing the highly emissive 7-aza-
indolyl group are capable of facile photochemical activa-
tion of C-Cl bonds.6 Encouraged by this finding, we
initiated the investigation on the potential of Pt(II)
complexes containing 7-azaindolyl groups in photochemi-
cal C-H bond activation. The ligands chosen for our
study are two fluorescent isomers of bis(7-azaindolyl)-
methane, L1 and L2, reported recently by our group7.
In addition to being fluorescent, L1 and L2 are capable
of chelating to a metal center. Furthermore, the isomeric
structures of L1 and L2 would allow us to study the
impact of the subtle electronic difference of the nitrogen
donor atoms of these two ligands on C-H bond activa-
tion by the corresponding Pt(II) complexes Pt(L1)(CH3)2,
1a , and Pt(L2)(CH3)2, 2a .
Since Shilov and co-workers1 demonstrated the cata-
lytic oxidation of CH4 into CH3OH and CH3Cl, using a
Pt(II) salt as a catalyst and stoichiometric amount of
Pt(IV) species as an oxidant in an aqueous system, the
direct and selective functionalization of hydrocarbons
by late transition metal complexes under mild condi-
tions has attracted much research efforts2 due to their
potential applications on the utilization of hydrocarbon
resources from natural gas or petroleum. Some recent
advances3 toward this ultimate goal have been demon-
strated in processes related to the Shilov system.
Recently, Labinger, Bercaw, Tilset, and other groups4
have demonstrated that cationic organoplatinum com-
plexes can activate both arene and alkane C-H bonds
(1) (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.
(2) (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) Stahl, S. S.; Labinger, J . A.; Bercaw, J . E. Angew.
Chem., Int. Ed. 1998, 37, 2180. (f) Labinger, J . A.; Bercaw, J . E. Nature
2002, 417, 507.
(3) (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.
(4) (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) J ohansson, L.; Ryan, O. B.; Tilset, M. J . Am. Chem. Soc.
1999, 121, 1974. (e) J ohansson, L.; Tilset, M.; Labinger, J . A.; Bercaw,
J . E. J . Am. Chem. Soc. 2000, 122, 10846. (f) J ohansson, L.; Tilset, M.
J . Am. Chem. Soc. 2001, 123, 739. (g) J ohansson, 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) Zhong, H. A.; Labinger,
J . A.; Bercaw, J . E. J . Am. Chem. Soc. 2002, 124, 1378. (j) Wik, B. J .;
Lersch, M.; Tilset, M. J . Am. Chem. Soc. 2002, 124, 12116. (k) Fang,
X.; Scott, B. L.; Watkin, J . G.; Kubas, G. J . Organometallics 2000, 19,
4193. (l) Konze, W. V.; Scott, B. L.; Kubas, G. J . J . Am. Chem. Soc.
2002, 124, 12550. (m) Fekl, U.; Goldberg, K. I. J . Am. Chem. Soc. 2002,
124, 6804.
The organoplatinum(II) complexes 1a and 2a were
obtained in ∼80% yield by the reactions of L1 and L2
with Pt2Me4(µ-SMe2)2,8 respectively. The structures of
these two complexes were determined by single-crystal
(5) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J . L.
Organometallics 2001, 124, 6804.
(6) Song, D.; Sliwowski, K.; Pang, J .; Wang, S. Organometallics
2002, 21, 4978.
(7) Song, D.; Schmider, H.; Wang, S. Org. Lett. 2002, 4, 4049.
(8) Scott, J . D.; Puddephatt, R. J . Organometallics 1983, 2, 1643.
10.1021/om0301785 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 04/01/2003

2188 Organometallics, Vol. 22, No. 11, 2003
Communications
Sch em e 1
F igu r e 1. Molecular structures of 1a (side view, left) and
2a (top view, right). Due to the disordering of 2a , only CH2
protons are shown. For a complete drawing of 2a , please
see the Supporting Information. Selected bond lengths (Å)
and angles (deg) for 1a : Pt(1)-C(17) 2.040(4), Pt(1)-C(16)
2.054(4), Pt(1)-N(4) 2.153(3), Pt(1)-N(2) 2.164(3), C(17)-
Pt(1)-N(4) 178.66(14), C(16)-Pt(1)-N(2) 179.12(14); 2a :
Pt(1)-C(17) 2.014(7), Pt(1)-C(16) 2.057(7), Pt(1)-N(3)
2.083(9), Pt(1)-N(2) 2.141(6), C(16)-Pt(1)-N(3), 176.4(3),
C(17)-Pt(1)-N(2) 178.9(3).
1 h after the mixing of 1 equiv of [H(Et2O)2][BAr′4] (Ar′
) 3,5-bis(trifluoromethyl)phenyl)12 with the benzene
solution of 1a or 2a at ambient temperature, the
addition of Me2S to the reaction mixture resulted in the
isolation of air- and moisture-stable complex [Pt(L1)-
Ph(SMe2)][BAr′4], 1b, or [Pt(L2)Ph(SMe2)][BAr′4], 2b, in
∼90% yield (Scheme 1). The phenyl ligand bound to the
Pt(II) center in 1b and 2b originates from the benzene
molecules, a direct evidence of C-H bond activation by
the Pt(II) complex. On the basis of the previously
established mechanism,4a-j we believe that the reactive
species of the C-H activation is a cationic Pt(II) complex
generated by the protonolysis of 1a or 2a and the
removal of one of the methyl ligand as methane. The
Et2O molecule provided by [H(Et2O)2][BAr′4] or the
benzene solvent molecule likely coordinates to the
cationic Pt(II) center to saturate the coordination sphere
and generates the active cationic species [Pt(L1)(CH3)-
(S)]+ or [Pt(L2)(CH3)(S)]+, S ) Et2O or benzene, which
activates benzene to produce the phenyl group. The
subsequent addition of Me2S results in the replacement
of the Et2O ligand or benzene and the isolation of 1b or
2b.
X-ray diffraction analyses.9 As shown in Figure 1, the
Pt(II) center in both complexes adopts a typical square
planar coordination geometry. The ligands L1 and L2
are chelated to the Pt(II) center in both complexes. One
important feature is that due to the geometric con-
straint, the methylene group of the diimine ligand in
both complexes is situated above the PtN2C2 plane with
the Pt-C (CH2) separation distance being 3.172(3) and
3.190(7) Å for 1a and 2a , respectively. As a result, the
fifth coordination site of the Pt(II) center is partially
blocked. The C-H bonds of the methylene group show
strong agostic interactions10 with the Pt(II) center, as
supported by the Pt-H contact distances, Pt‚‚‚H(15B)
≈ 2.44 Å (1a ), 2.43 Å (2a ), Pt‚‚‚H(15A) ≈ 3.99 Å (1a ),
3.98 Å (2a ). Agostic interactions involving Pt centers
have been observed previously.10e These interactions are
retained in solution, as shown by the fairly large
coupling constants between H(15A), H(15B) and the Pt
1
center in the H NMR spectra of 1a and 2a .11
The structures9 of 1b and 2b are established by
single-crystal X-ray diffraction analyses. As shown in
Figure 2, each Pt(II) center adopts a typical square
planar geometry with the sulfur atom of Me2S and the
phenyl ligand trans to the two nitrogen atoms of the
bidentate ligand, respectively. Again, the methylene
group of the chelate ligand in 1b and 2b shows strong
agostic interactions with the Pt(II) center, similar to
those observed in 1a and 2a . Complex 2a has a chiral
structure due to the asymmetry of the L2 ligand.
Therefore, during benzene C-H activation, the resulting
phenyl group has two choices, trans to the pyrrole
nitrogen or trans to the pyridine nitrogen. Interestingly,
however, on the basis of 1H NMR and crystal structural
Our preliminary investigation indicates that com-
pounds 1a and 2a are useful starting materials for C-H
activations involving aromatic C-H bonds. For example,
(9) Single crystals of 1a and 2a suitable for X-ray diffraction analysis
were obtained from their diethyl ether solutions, while crystals of 1b
and 2b were obtained by slow diffusion of hexanes into their benzene
solutions. Data were collected on a Bruker P4 diffractometer with a
CCD-1000 detector at ambient temperature. 1a : C17H18N4Pt, triclinic,
P1h, a ) 7.6028(15) Å, b ) 8.2357(17) Å, c ) 13.565(3) Å, R ) 77.821-
(3)°, â ) 76.805(3)°, γ ) 79.261(4)°, V ) 799.8(3) Å3, Z ) 2, R1 ) 0.0211
[I > 2σ(I)], GOF ) 0.982; 2a :
C17H18N4Pt, monoclinic, P21/n, a )
10.975(4) Å, b ) 9.248(4) Å, c ) 15.459(6) Å, R ) 90°, â ) 96.487(9)°,
γ ) 90°, V ) 1558.9(11) Å3, Z ) 4, R1 ) 0.0416 [I > 2σ(I)], GOF )
0.802; 1b: C55H35BF24N4PtS, triclinic, P1h, a ) 10.772(7) Å, b ) 16.044-
(10) Å, c ) 17.578(11) Å, R ) 84.698(12)°, â ) 75.467(10)°, γ ) 88.044-
(13)°, V ) 2928(3) Å3, Z ) 2, R1 ) 0.0704 [I >2σ(I)], GOF ) 0.983; 2b:
C
55H35BF24N4PtS, triclinic, P1h, a ) 12.766(5) Å, b ) 15.833(6) Å, c )
16.370(6) Å, R ) 105.541(8)°, â ) 108.804(9)°, γ ) 96.231(9)°, V )
2947.9(19) Å3, Z ) 2, R1 ) 0.0408 [I > 2σ(I)], GOF ) 0.865.
(10) (a) Cotton, F. A.; LaCour, T.; Stanislowski, A. G. J . Am. Chem.
Soc. 1974, 96, 754. (b) Cotton, F. A.; Stanislowski, A. G. J . Am. Chem.
Soc. 1974, 96, 5074. (c) Cotton, F. A.; Day, V. W. J . Chem. Soc., Chem.
Commun. 1974, 415. (d) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989,
28, 3210. (e) Mokuolu, Q. F.; Avent, A. G.; Hitchcock, P. B.; Love, J . B.
J . Chem. Soc., Chem. Dalton Trans. 2001, 2551.
(12) Brookhart, M.; Grant, B.; Volpe, J . Organometallics 1992, 11,
3920
(13) Two independent trials of the activation reaction with 2a were
set up at ambient temperature, one in the dark and the other under
ambient light. After 30 min of mixing [H(Et2O)2][BAr′4] with 2a in
benzene at ambient temperature, excess Me2S was added to the system
to stop the reaction. The 1H NMR spectra of the resulting reaction
mixtures do not show a significant difference between the two trials.
Me2S inhibits the C-H activation reaction. For example, if 1a and
[H(Et2O)2][BAr′4] were initially mixed in Me2S, the subsequent addition
of benzene does not result in any activation. Instead, a cationic Pt(II)
species [Pt(L1)(CH3)(SMe2)][BAr′4] was isolated, which does not acti-
vate benzene C-H bonds at ambient temperature. For details, please
see the Supporting Information.
(11) The two protons of the bridging CH2 group are not identical.
H(15B) has a chemical shift at ∼12 ppm and H(15A) at ∼6.3 ppm (see
Supporting Information). 1H NMR spectra of 1a and 2a taken in CDCl3
or CD2Cl2 show Pt-H coupling of both protons with J Pt-H ) 13.7 and
11.4 Hz, respectively. The spectra taken in THF-d8 showed broad and
unresolved satellite peaks.

Communications
Organometallics, Vol. 22, No. 11, 2003 2189
more difficult to achieve thermally, compared to aro-
matic C-H activation.
In summary, two new organoplatinum complexes
based on novel bis(N-7-azaindolyl)methane ligands have
been demonstrated to be effective in activating benzene
C-H bonds under mild conditions. These new Pt(II)
complexes raise several new interesting prospects for
C-H activation research. For example, the partial
blockage of the Pt(II) fifth coordination site by the CH2
group and the asymmetric structure of 2a make these
new Pt(II) compounds potentially useful for selective
C-H bond activation of arenes. The strong agostic
interaction between CH2 and the Pt(II) center indicates
that it may be possible to achieve internal alkane C-H
activation by increasing the number of CH2 units
between the 7-azaindolyl units in L1 and L2 ligands.
The syntheses of such new ligands and the correspond-
ing Pt(II) complexes are being investigated in our
laboratory. The use of the new Pt(II) complexes contain-
ing 7-azaindolyl donor groups in intermolecular C-H
activation of alkanes is being explored, and the results
will be reported in due course.
F igu r e 2. Molecular structures of 1b (left) and 2b (right).
Selected bond lengths (Å) and angles (deg) for 1b: Pt(1)-
C(16) 2.016(7), Pt(1)-N(4) 2.074(6), Pt(1)-N(2) 2.151(6),
Pt(1)-S(1) 2.270(3), C(16)-Pt(1)-N(2) 177.8(2), N(4)-Pt-
(1)-S(1) 174.33(17); 2b: Pt(1)-C(16) 2.014(4), Pt(1)-N(4)
2.072(3), Pt(1)-N(1) 2.139(3), Pt(1)-S(1) 2.2752(14), C(16)-
Pt(1)-N(1) 178.14(14), N(4)-Pt(1)-S(1) 175.66(10).
data, 2b is the only product formed from the reaction,
where the phenyl group is trans to the pyrrole nitrogen.
The preferential bonding of the phenyl group opposite
the pyrrole nitrogen atom has not been fully understood,
but it has some implications on the potential use of 2a
in selective C-H activations of chiral substrates.
Because L1 and L2 are very efficient in harvesting
photons, we studied the effect of light on benzene C-H
activation using 2a . However, despite our efforts, we
did not find any evidence to support that light plays a
key role in the C-H activation. Nonetheless, photo-
chemical activation of alkane C-H bonds using 1a and
2a , which is being studied in our laboratory, may be
viable because alkane C-H activations are in general
Ack n ow led gm en t. We thank the National Sciences
and Engineering Research Council of Canada and Xerox
Research Foundation for financial support.
Su p p or tin g In for m a tion Ava ila ble: Detailed experi-
mental procedures and characterization results. Details of
crystal structural data, tables of atomic coordinates, complete
lists of bond lengths and angles, anisotropic thermal param-
eters, and hydrogen parameters. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM0301785