C-H Activation by a Diplatinum(II) Complex: Isolation and Structures of [Pt2(CH3)(SMe2)Ph 2(ttab)][BAr′4] and [Pt2(H 2O)2Ph2(ttab)][BAr′4] 2 (ttab = 1,2,4,5-Tetrakis(1-N-7-azaindolyl)benzene)

Article abstract of DOI:10.1021/om0341449

The dinuclear Pt(II) complex Pt₂(CH₃)₄ (1; ttab = 1,2,4,5-tetrakis(1-N-7-azaindolyl)benzene) has been found to activate multiple benzene molecules by cleaving a C-H bond in the presence of[H(Et₂O)₂][BAr′₄] (Ar′ = 3,5-bis(trifluoromethyl)phenyl). The unprecedented dinuclear Pt(II) product [Pt₂(CH₃)(SMe₂)Ph ₂-(ttab)][BAr′₄] (2), which contains two phenyl groups on one Pt(II) center, has been isolated from the 1:1 reaction of 1 with [H(Et₂O)₂][BAr′₄]. The novel dinuclear Pt(II) complex [Pt₂(H₂O)₂Ph ₂(ttab)][BAr′₄]₂ (3) has been isolated from the 1:2 reaction of 1 with [H(Et₂O)₂][BAr′ ₄] when reagent-grade solvents were used in the recrystallization process. The structures of 2 and 3 have been established by X-ray diffraction analyses.

Full text of DOI:10.1021/om0341449

1194
Organometallics 2004, 23, 1194-1196
C-H Activa tion by a Dip la tin u m (II) Com p lex: Isola tion
a n d Str u ctu r es of [P t2(CH3)(SMe2)P h 2(tta b)][BAr ′4] a n d
[P t2(H2O)2P h 2(tta b)][BAr ′4]2 (tta b )
1,2,4,5-Tetr a k is(1-N-7-a za in d olyl)ben zen e)
Datong Song, Wen Li J ia, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Received August 31, 2003
Summary: The dinuclear Pt(II) complex Pt2(CH3)4(ttab)
(1; ttab ) 1,2,4,5-tetrakis(1-N-7-azaindolyl)benzene) has
been found to activate multiple benzene molecules by
cleaving a C-H bond in the presence of [H(Et2O)2][BAr′4]
(Ar′ ) 3,5-bis(trifluoromethyl)phenyl). The unprec-
edented dinuclear Pt(II) product [Pt2(CH3)(SMe2)Ph2-
(ttab)][BAr′4] (2), which contains two phenyl groups on
one Pt(II) center, has been isolated from the 1:1 reaction
of 1 with [H(Et2O)2][BAr′4]. The novel dinuclear Pt(II)
complex [Pt2(H2O)2Ph2(ttab)][BAr′4]2 (3) has been iso-
lated from the 1:2 reaction of 1 with [H(Et2O)2][BAr′4]
when reagent-grade solvents were used in the recrystal-
lization process. The structures of 2 and 3 have been
established by X-ray diffraction analyses.
positive charge, a key step proposed for catalytic func-
tionalization of hydrocarbons via C-H activation by
cationic Pt(II) compounds.1 These limitations of using
cationic mononuclear Pt(II) complexes in catalytic C-H
activation could be overcome, however, if an appropriate
multinuclear Pt(II) complex is used in the C-H activa-
tion process. As the first step in establishing an ap-
propriate dinuclear Pt(II) system for C-H activation,
we investigated the utility of the dinuclear Pt(II)
complex Pt2(CH3)4(ttab) (1; ttab ) 1,2,4,5-tetrakis(1-N-
7-azaindolyl)benzene), reported2 recently by our group,
in C-H activation. The coordination environment of the
Pt(II) center in 1 resembles those of previously known
mononuclear dialkyldiimine Pt(II) complexes used for
C-H activation. In addition, compound 1 is known to
readily cleave C-Cl bonds facilitated by the two Pt(II)
centers and the central phenyl ring of the ttab ligand
to form the unusual dinuclear PtIV species Pt2(CH3)4-
(ttab)Cl2 (1a ), where the central benzene ring of the ttab
ligand is transformed into a cyclohexadienyl dianion.2
Therefore, we anticipated that 1 could be useful for C-H
activation in addition to C-Cl bond cleavage. Our
preliminary investigation on the reactions of compound
1 with benzene revealed that 1 is indeed capable of
activating C-H bonds in benzene and the isolated
products display some surprising features. Our prelimi-
nary findings are reported herein.
Compound 1 is insoluble in benzene. Upon the addi-
tion of 1 equiv of [H(Et2O)2][BAr′4] (Ar′ ) 3,5-bis-
(trifluoromethyl)phenyl)3 at ambient temperature to the
benzene suspension of 1, the solid of 1 gradually
disappeared over a period of ∼1 h. The subsequent
addition of excess SMe2 resulted in the isolation of [Pt2-
(CH3)(SMe2)Ph2(ttab)][BAr′4] (2) as a crystalline mate-
rial in ∼34% yield. In addition to 2, an NMR study of
the reaction mixture revealed that multiple products are
present. Due to the complexity of the NMR spectra, it
is not possible to identify all products using NMR.
Attempts to isolate other products were not successful.
The structure of 2 was determined by single-crystal
X-ray diffraction analysis4 and is shown in Figure 1.
Each Pt center in 2 has a typical square-planar coor-
dination geometry with typical bond lengths and angles.5
The Pt-Pt separation distance in 2 is 7.049(2) Å, similar
to that of 1. The central benzene ring is in close contact
Cationic Pt(II) complexes have been demonstrated to
be capable of activating C-H bonds under mild condi-
tions.1 Most previously reported systems, however,
involve mononuclear Pt(II) complexes. In benzene C-H
activation using the dimethyldiimine Pt(II) complex
PtL(CH3)2 (L ) N,N′-chelate ligand) as the starting
material and in the presence of an acid, the net result
is the isolation of the cationic product [PtII(L)(L′)Ph]+,
where L′ is either a solvent molecule or a neutral donor
ligand such as ether or SMe2. Previous mechanistic
studies established that the cationic [PtII(L)(L′)Ph]+
compound can undergo oxidative addition by a second
benzene to form [PtIV(L)(L′)Ph2(H)]+, which usually
reproduces [PtII(L)(L′)Ph]+ via reductive elimination of
benzene. As a consequence, species such as [PtII(L)Ph2]
resulting directly from benzene activation by using
mononuclear Pt(II) complexes have not been reported
previously. In fact, the previously well-established
mechanism on cationic mononuclear Pt(II) systems1
implies that it is not possible to obtain species such as
[PtII(L)Ph2] unless ligand redistribution occurs. In ad-
dition, a mononuclear cationic species such as [PtII(L)-
(L′)Ph]+ is relatively difficult to oxidize due to the
(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) 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) Konze, W. V.; Scott, B.
L.; Kubas, G. J . J . Am. Chem. Soc. 2002, 124, 12550. (j) J ensen, M. P.;
Wick, D. D.; Reinartz, S.; White, P. S.; Templeton, J . L.; Goldberg, K.
I. J . Am. Chem. Soc. 2003, 125, 8614. (k) Norris, C. M.; Reinartz, S.;
White, P. S.; Templeton, J . L. Organometallics 2002, 21, 5649.
(2) Song, D.; Sliwowski, K.; Pang, J .; Wang, S. Organometallics
2002, 21, 4978.
(3) Brookhart, M.; Grant, B.; Volpe, J . Organometallics 1992, 11,
3920.
10.1021/om0341449 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/07/2004

Communications
Organometallics, Vol. 23, No. 6, 2004 1195
Sch em e 1
Pt(II) center may lead to the formation of the intermedi-
ate b. Intermolecular reductive elimination6 of a CH4
molecule from b, the restoration of the central phenyl
ring by giving up 2e- to the two Pt(IV) centers, and the
subsequent addition of SMe2 likely lead to the formation
of the final product 2. The proposed structure of b is
based on the previously reported structure of Pt2(CH3)4-
(ttab)Cl2 (1a ), obtained via an overall 2e- oxidation
process of 1 by C-Cl bonds. Although we have not been
1
able to isolate the proposed intermediate b, H NMR
spectra for the reaction mixture consistently showed a
4
chemical shift at ∼6.3 ppm that has a distinct J Pt-H
3
and J Pt-H satellite pattern similar to that2 of the 1,4-
cyclohexadienyl dianion protons in 1a (see the Support-
ing Information). The formation of b may be facilitated
by the short contact distance between the Pt center and
the carbon atoms of the central phenyl ring, as observed
in 1. Other processes such as intermolecular ligand
redistribution could also account for the formation of 2.
Mass spectroscopic study using ESI-TOF methods (see
the Supporting Information)7 on the reaction mixture
of 1 with [H(Et2O)2][BAr′4] in a 1:1 ratio in benzene
revealed the presence of other unusual species in the
reaction mixture such as [Pt2(ttab)Ph3]+, which further
confirmed the complexity and the unusual reactivity of
benzene C-H bond activation using the dinuclear PtII
compound 1.
When compound 1 was reacted with 2 equiv of
[H(Et2O)2][BAr′4] under the same conditions as de-
scribed for the 1:1 reaction above, multiple products
with complex NMR spectra were again observed. In our
repeated attempts to isolate crystalline products so that
their structures can be conclusively established by X-ray
diffraction analyses, various solvent combinations (THF/
hexanes, THF/hexanes/SMe2, THF/benzene, THF/ben-
zene/SMe2) and conditions were used in the crystalli-
zation process. None of them, however, led to the
isolation of a single crystalline compound. The only
successful isolation of the crystalline compound [Pt2-
(H2O)2Ph2(ttab)][BAr′4]2 (3) was achieved when the
product mixture was dissolved in a reagent-grade THF
solvent and this solution was layered with hexane and
kept in the air for a few days. The structure of 3 was
established by single-crystal X-ray diffraction analysis4
and is shown in Figure 2. The two cationic Pt(II) centers
F igu r e 1. Structure of the cation in 2. For clarity,
hydrogen atoms are omitted and all carbon atoms are
shown as ideal spheres. Important bond lengths (Å) and
angles (deg): Pt(1)-C(49) ) 2.041(10), Pt(1)-N(3) )
2.056(11), Pt(1)-N(1) ) 2.080(13), Pt(2)-C(41) ) 1.959(14),
Pt(2)-C(35) ) 1.993(13), Pt(2)-N(7) ) 2.120(10), Pt(2)-
N(5) ) 2.125(11); C(49)-Pt(1)-N(1) ) 174.8(4), N(3)-
Pt(1)-S(1) ) 171.0(4), C(35)-Pt(2)-N(7) ) 175.6(6), C(41)-
Pt(2)-N(5) ) 174.8(4).
with the two Pt(II) centers in the same manner as that
observed in 1, with the shortest Pt-C separation
distance being 3.11(1) Å. One surprising feature of 2 is
that one of the Pt(II) centers in the complex is bound
by two phenyl ligands, while the remaining Pt(II) center
is coordinated by one methyl group and a SMe2 ligand.
Clearly the previously established mechanism for mono-
nuclear Pt(II) complexes cannot explain the formation
of compound 2. If, however, we allow the involvement
of the dinuclear Pt(IV) species b as an intermediate in
the reaction, the formation of compound 2 could be
explained by Scheme 1. The formation of a in Scheme
1 (where the coordinated solvent could be either diethyl
ether from the starting material [H(Et2O)2][BAr′4] or
benzene1) could be explained by the established mech-
anism for mononuclear Pt(II) compounds. Oxidative
addition by the second benzene on the same cationic
(4) Single crystals of 2 suitable for X-ray diffraction analysis were
obtained by slow diffusion of hexanes into its benzene solution, while
crystals of 3 were obtained by slow diffusion of hexanes into its THF
solution. Data were collected on a Bruker P4 diffractometer with a
CCD-1000 detector at ambient temperature. Crystal data for 2:
C
17H18N4Pt, triclinic, P1h, a ) 12.252(4) Å, b ) 15.023(6) Å, c ) 26.440-
(10) Å, R ) 78.814(8)°, â ) 84.714(10)°, γ ) 85.588(9)°, V ) 4745(3)
Å3, Z ) 2, R1 ) 0.0211 (I >2σ(I)), GOF ) 0.982. Crystal data for 3:
(6) Intermolecular reductive elimination of CH4 has been proposed
by the Wayland group: (a) Sherry, A. E.; Wayland, B. B. J . Am. Chem.
Soc. 1990, 112, 1259. (b) Wayland, B. B.; Ba, S.; Sherry, A. E. J . Am.
Chem. Soc. 1991, 113, 5305.
(7) The ESI-TOF experiment was carried out on a Mariner Biospec-
trometry Workstation using CH3NO2 as the solvent. For details see
the Supporting Information.
C
110H60B2F48N8O2Pt2‚4.6THF, triclinic, P1h, a ) 13.353(5) Å, b ) 17.132-
(6) Å, c ) 18.147(7) Å, R ) 114.760(8)°, â ) 94.074(10)°, γ ) 100.832-
(7)°, V ) 3650(2) Å3, Z ) 1, R1 ) 0.0770 (I > 2σ(I)), GOF ) 0.788.
(5) (a) Song, D.; Wang, S. J . Organomet. Chem. 2002, 648, 302. (b)
Song, D.; Wu, Q.; Hook, A.; Kozin, I.; Wang, S. Organometallics 2001,
20, 4683.

1196 Organometallics, Vol. 23, No. 6, 2004
Communications
solubility of compound 1 in common and nonreacting
1
organic solvents,8 and the complexity8 of the H NMR
spectra of the products such as 2 and 3. Despite the lack
of a complete understanding of the reaction mechanism,
the results of our preliminary investigation have estab-
lished that the dinuclear Pt(II) compound 1 appears to
display unusual reactivity toward benzene9 and has the
potential for use in catalytic C-H activation. Further
investigations on this complex and the intriguing di-
platinum system in C-H activation are being conducted
in our laboratory.
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council for financial support.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data for compounds 2 and 3, text giving synthetic proce-
dures and analytical data, and figures giving complete struc-
F igu r e 2. Structure of the cation in 3. Hydrogen atoms
are omitted for clarity. Important bond lengths (Å) and
angles (deg): Pt(1)-C(18) ) 1.993(13), Pt(1)-N(2) ) 2.036-
(8), Pt(1)-O(1) ) 2.098(8), Pt(1)-N(4) ) 2.170(9); N(2)-
Pt(1)-O(1) ) 175.5(3), C(18)-Pt(1)-N(4) ) 175.4(5).
1
tural drawings for 2 and 3, mass spectroscopic data, and a H
NMR spectrum showing the aromatic region for the reaction
mixture of 1 with 2 equiv of acid. This material is available
free of charge via the Internet at http://pubs.acs.org.
in 3 are related by an inversion center of symmetry,
each of which is coordinated by an H2O and a phenyl
group in a cis manner. In the crystal lattice, each of the
coordinating H2O molecules is hydrogen-bonded to two
THF solvent molecules. The H2O molecules are clearly
from the nonpurified solvents THF and hexanes. Re-
peated attempts to isolate the SMe2 analogue of 3 were
unsuccessful. Compound 2 was also isolated as a trace
product from the same recrystallization process. Com-
pound 3 could be considered as the “normal” product,
consistent with the use of 2 equiv of acid.
OM0341449
(8) 1 is only sparingly soluble in THF or benzene and reacts with
chlorinated solvents rapidly. In compound 2, none of the four 7-aza-
indolyl groups have the same environment. The anti structure of 3
shown in Figure 2 has two different 7-azaindolyl groups. In solution,
3 appears to exist in both syn and anti forms with respect to the
relative location of the methyl (or H2O) groups. These facts, along with
the presence of phenyl groups, make the aromatic region of the 1H
NMR spectra very complex, and it is impossible to assign chemical
shifts.
(9) We have recently completed the investigation on benzene C-H
activation using a mononuclear analogue of 1, Pt(CH3)2(1,2-bis(7-
azaindolyl)benzene), and the same reaction conditions as for 2. Only
the single phenyl coordinated compound {Pt(1,2-bis(7-azaindolyl)Ph-
(SMe2)}[BAr′4] was isolated. No evidence of double benzene activation
was observed. The details will be reported in due course.
Numerous attempts to follow the reaction kinetics by
NMR methods did not produce any meaningful results,
due to mostly the presence of multiple products, the poor