Solvent-controlled switch of selectivity between sp2 and sp 3 C-H bond activation by platinum(ii)

Article abstract of DOI:10.1039/c0cc04581k

By merely changing the solvent, two different cyclometalated platinum complexes resulted from either sp² or sp³ C-H bond activation can be prepared selectively. For example, the reaction of L1 with K₂PtCl₄ in Me

Full text of DOI:10.1039/c0cc04581k

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Solvent-controlled switch of selectivity between sp2 and sp3 C–H bond
activation by platinum(II)w
Alexander W. Garner,a Caleb F. Harris,a Dileep A. K. Vezzu,a Robert D. Pikeb and
Shouquan Huo*a
Received 23rd October 2010, Accepted 12th November 2010
DOI: 10.1039/c0cc04581k
By merely changing the solvent, two different cyclometalated
platinum complexes resulted from either sp2 or sp3 C–H bond
activation can be prepared selectively. For example, the reaction
of L1 with K2PtCl4 in MeCN gave exclusively kinetic product
1a, while the reaction in AcOH was thermodynamically con-
trolled and produced predominantly 1b.
N-phenyl ring since the sp2 C–H is considered to be more
reactive. However, the new products were identified as
predominantly 1b (96%) formed from the activation of a
C–H bond of the methyl group and 1a was presented as a
minor (4%) product (Table 1). The metalation position can be
easily identified by examining the proton NMR spectrum of
the product, which showed that the signal of the methyl group
in the ligand disappeared and a new singlet peak appeared at
5.42 ppm displaying a platinum satellite with the coupling
constant of 40 Hz (2JPtꢀH).
When the methyl group was replaced with an ethyl (L2) or
an isopropyl group (L3), 2b and 3b were obtained,z
respectively (Table 1). No formation of 3a by the sp2 C–H
activation was detected. The proton NMR of 2b showed a
quartet at 5.88 ppm displaying a platinum satellite (2JPtꢀH 48 Hz)
and a doublet at 1.40 ppm, indicating a metalation at the CH2
carbon of the N-ethyl group. The proton NMR of 3b displayed
a singlet at 1.48 ppm (3JPtꢀH 17.5 Hz) assigned to the methyl
signal of the N-isopropyl group. The CH signal from the ligand
disappeared as the hydrogen was removed by cycloplatination.
Surprisingly, when the reaction of L1 with K2PtCl4 was
carried out in acetonitrile, a complete switch of the selectivity
from sp3 to sp2 C–H bond activation was achieved and 1a was
produced exclusively (Table 1). No products resulting from the
sp3 C–H activation were detected. The reaction of L2 and L3
gave predominantly 2a and 3a, respectively. The selectivity for
3a was moderate, but the pure isomer could be isolated by just
one recrystallization from dichloromethane–hexane.
The activation of unactivated C–H bonds by transition metals
tackles one of the most challenging chemical transformations
and has enormous potential in chemical industry.1 The C–H
activation by platinum salts is among the earliest and most
extensively investigated processes.2 Cyclometalation, an intra-
molecular version of C–H activation, has played a significant
role in understanding the mechanistic details in the C–H
activation process.3 On the other hand, cyclometalated
platinum compounds have demonstrated their unique applica-
tions in various areas from catalysis4 to organoelectronic5 and
biological6 fields. Control of selectivity is one of the most
important issues in chemical synthesis. It is generally
recognized that an aromatic C–H bond is more reactive
toward activation by platinum complexes,2b but a recent study
showed that there exists a delicate balance between sp2 and sp3
C–H bond activation in the reported platinum(II) complex
system.3d Here we report a solvent-controlled switch of
selectivity between sp2 and sp3 C–H bond activation7 by
K2PtCl4 (Table 1). Such a degree of solvent-manipulated
control over the selectivity in the sp2 and sp3 C–H bond
activation is rarely seen in the literature.7a To the best of our
knowledge, this is the first report on a high degree of solvent-
controlled switch of selectivity between sp2 and sp3 C–H bond
activation by a platinum complex. It should be noted that a
solvent-controlled C–C vs. C–H bond activation by a cationic
rhodium complex has been reported.8
Table 1 The reactions of L1–L3 with K2PtCl4 in different solvents
When ligand L1 and K2PtCl4 in acetic acid were refluxed for
24 hours, a new compound was formed cleanly as revealed by
the TLC analysis of the reaction mixture. In principle, with
chelation to the bipyridine, the cyclometalation could be
directed to either the N-phenyl ring (1a) or the methyl group
(1b), namely, sp2 vs. sp3 C–H activation. One would expect the
cyclometalation to occur at one of the ortho positions of the
In AcOH
In MeCN
t/h
Ratioa
Yieldb
t/h
Ratioa
Yieldb
a Department of Chemistry, East Carolina University, Greenville,
NC 27858, USA. E-mail: huos@ecu.edu; Fax: +1 252-328-6210
b Department of Chemistry, The College of William and Mary,
Williamsburg, VA 23185, USA
w Electronic supplementary information (ESI) available: Synthetic
details and characterization of new compounds. CCDC 791812 (3a)
and 791813 (3b). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc04581k
Product
1a : 1b
2a : 2b
3a : 3b
24
48
48
4 : 96
3 : 97
0 : 100
73%
63%
38%
72
48
72
100 : 0
93 : 7
70 : 30
73%
70%
36%
a
Isomeric ratio determined by proton NMR spectra of the crude
b
products. Isolated yield of the pure major isomer.
c
1902 Chem. Commun., 2011, 47, 1902–1904
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The structures of the complexes 3a and 3b were further
confirmed by single crystal X-ray structure determination
(Fig. 1). The complex 3b displays a planar coordination
geometry. The N-phenyl group is nearly perpendicular to the
coordination plane, which is to minimize the steric interaction
with the isopropyl and bipyridyl groups. The complex 3a has a
N^N*C coordination geometry featuring a fused five–six-
membered metallacycle, where N*C denotes a six-membered
isomerically pure 1a was refluxed in acetic acid, the isomerization
to 1b proceeded slowly and toward near completion after 3
days (1b : 1a = 96 : 4). This isomerization is likely taking
place via acid-assisted cleavage of the Pt–C(sp2) bond in 1a
followed by the activation of the methyl C–H bond by the
platinum center, which gives the complex 1b. When refluxing
1b in acetonitrile, even in the presence of HCl or acetic acid, no
isomerization to 1a was detected.
and N^N denotes
a
five-membered chelation to the
When the reaction of L1 with K2PtCl4 was carried out in
AcOD, a high degree of deuterium incorporation was found at
the ortho positions of the N-phenyl ring and the methylene
group, along with partial H/D exchange detected on the
pyridine ring as shown in Scheme 1. The H/D exchange at
the ortho-position of the N-phenyl ring and the methylene
group may be interpreted by reversible bipyridine-directed
cycloplatination reaction. The H/D exchange on the pyridine
ring is likely attained through the bidentate cyclometalation of
the bipyridine ligand.11 When the isomerization of 1a was
carried out in AcOD, a similar level of H/D exchange at the
ortho position of the N-phenyl ring and the methylene group in
the product 1d was detected. There was no H/D exchange
observed at the position of the bipyridine ring (Scheme 1).
A high degree of multiple D incorporation at the ortho
positions of the N-phenyl ring and the methylene group would
suggest that equilibrium might be readily established between
the sp2 and sp3 C–H bond activation. However, when 1b was
refluxed in AcOD, interestingly, no H/D exchange was
detected at the ortho position of the phenyl ring whereas a
90% D incorporation into the methylene group was observed
(Scheme 1, 1e). These observations suggest that the barrier for
the cleavage of the C–Pt bond of 1b by AcOD may be too high
for the reversed process leading to the formation of 1a to
occur. The kinetic barrier for the sp2 C–H bond activation
must be very small so that a rapid equilibrium between L1 and
1a can be established and a high degree of D scrambling at the
ortho position of the phenyl ring in the reaction of L1 and
platinum.5b,9 In contrast to 3b, the tridentate chelating ligand
in 3a is not planar and the N-phenyl group is significantly bent
with respect to the rest of the coordination geometry
(for different views of the geometry of 3a and 3b, see Fig. S1
in ESIw). This geometric distortion from desired square planar
structure of Pt(II) complexes is likely the result of the steric
hindrance caused by the isopropyl group, since in a similar
five–six-membered platinacycle where all three N-substituents
are either phenyl or pyridine rings, the coordination geometry
is closer to a square plane.9
Our next focus is to unveil the underlying factors
responsible for this interesting selectivity in intramolecular
C–H bond activation. We reasoned that the reaction in acetic
acid may be thermodynamically controlled while the reaction
in acetonitrile may be kinetically controlled. We accept the
general notion of preference of sp2 C–H bond activation,
however, the complex formed through sp3 C–H bond
activation may be more stable than that through the sp2
C–H activation because of the formation of a five-membered
chelate. Five-membered metal chelation is generally
considered to be more stable than the chelation of other sizes
in coordination complexes.10 Another major contributing
factor is steric interaction of the pendent N-alkyl group with
the tridentate chelating ligand. In complexes 1b–3b, the pendent
N-phenyl group can adopt a perpendicular orientation relative
to the coordination plane to minimize the steric interaction.
The thermodynamic control of the reaction in acetic acid
was furthered supported by the isomerization of 1a. When
Fig. 1 ORTEP representation of molecules 3a (top) and 3b (bottom).
Selected bond lengths (A) and angles (1): 3a: Pt(1)–C(15) 1.997(5),
Pt(1)–N(1) 2.091(4), Pt(1)–N(2) 2.000(4), Pt(1)–Cl(1) 2.310(1),
C(15)–Pt(1)–N(1) 167.32(18), N(2)–Pt(1)–Cl(1) 173.54; 3b: Pt(1)–C(11)
2.026(6), Pt(1)–N(1) 2.115(5), Pt(1)–N(2) 1.944(5), Pt(1)–Cl(1)
2.3029(15), C(11)–Pt(1)–N(1) 163.1(2), N(2)–Pt(1)–Cl(1) 178.26 (14).
Scheme 1 Reaction of L1 with K2PtCl4, isomerization of 1a, and
H/D exchange of 1b in AcOD.
c
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Chem. Commun., 2011, 47, 1902–1904 1903

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Table 2 The product ratio for the reactions of L1 with K2PtCl4 in
mixed solvents
College Science Award and R. D. P. acknowledges NSF
(CHE-0443345) and the College of William and Mary for
the purchase of the X-ray equipment.
AcOH–MeCN (48 h)
AcOH : MeCN (v/v)
AcOH : MeCN (v/v, 80 : 20)
1a : 1b
Time/h
1a : 1b
Notes and references
50 : 50
70 : 30
80 : 20
90 : 10
100 : 0
70 : 30
45 : 55
26 : 74
10
20
40
80
69 : 31
58 : 42
25 : 75
5 : 95
z It is noteworthy that the reaction of L2 also produced a small
amount of 1b as a byproduct (1b : 2b = 7 : 93). The formation of 1b
must involve the cleavage of the sp3 C–C bond of the ethyl group. In
the reaction of the L3, byproducts resulting from the cleavage of C
(isopropyl)–N bond were produced but neither 2b nor 1b was
identified. When the methyl group of L1 was replaced with a tertiary
butyl group, the reaction of the resultant ligand under the same reaction
conditions produced only the products associated with the cleavage of
the C (tert-butyl)–N bond. The C–C and C–N bond cleavages are
currently under investigation and will be reported in the future.
isomerization of 1a in AcOD could be achieved. The H/D at
the methylene group may proceed through a dual agostic
interaction.3d
In acetonitrile, the stronger solvent coordinating ability12
may deactivate the platinum center toward more difficult sp3
C–H bond activation, but not toward the sp2 C–H activation.
The results for the reaction of L1 suggested that the barrier for
the sp2 C–H activation is much smaller than that of sp3 C–H
activation, so the kinetic product was obtained exclusively.
However, with the increase of bulkiness of the N-alkyl group
(methyl to ethyl to isopropyl), the steric demand for the
reorganization of the ligand for the sp2 C–H activation is
increased and the difference in the activation barrier between
sp2 and sp3 C–H activation is decreased, so the formation of 3b
becomes competitive (Table 1). To further explore the solvent
effect on the reaction, we carried out the reaction of L1 with
K2PtCl4 in a mixture of acetonitrile and acetic acid and the
results are summarized in Table 2. With 50 : 50 (v/v) of acetic
acid and acetonitrile as the solvent, the reaction (90 1C, 48 h)
still gave exclusively the sp2 C–H bond activation product 1a.
However, when the amount of acetic acid was further
increased, the reaction under the same conditions (90 1C, 48 h)
produced a mixture of 1a and 1b with decreasing ratio of 1a to
1b. Furthermore, the product distribution was found to be
time-dependent. For example, in 80 : 20 (v/v) of acetic acid
and acetonitrile under reflux conditions, the product ratio of
1a to 1b at reaction time from 10 to 80 hours was decreased
from 69 : 31 to 5 : 95 (Table 2). These results demonstrated
that the formation of 1a was kinetically controlled and the
solvent played a crucial role in controlling the chemical kinetics
involved in this reaction.
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In summary, we have demonstrated that selective formation
of cyclometalated platinum complexes through either the sp2
or the sp3 C–H bond activation can be readily achieved by
simply switching the solvent. This remarkable solvent-
controlled switch of selectivity not only offers an excellent
method to control the product formation but also provides an
interesting system for probing some important mechanistic
issues associated with the transition metal-assisted C–H bond
activations.
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The authors thank East Carolina University for financial
support. S. H. thanks Research Corporation for Science
Advancement for financial support through Cottrell
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c
1904 Chem. Commun., 2011, 47, 1902–1904
This journal is The Royal Society of Chemistry 2011