Impact of the linker groups in bis(7-azaindol-1-yl) chelate ligands on structures and stability of Pt(N,N-L)R2 complexes

Article abstract of DOI:10.1039/b813501k

New organoplatinum(II) complexes with the general formula of Pt(N,N-L)R₂, where L is a bis(7-azaindol-1-yl) chelate ligand, R is a methyl or a phenyl, have been synthesized and investigated with the aim to understand the impact of the linker group in the L ligand on the structure and the stability of the Pt(II) complexes. The L ligands used in our investigation belong to two classes: class I with an aliphatic linker between two 7-azaindolyl groups such as -CH₂- (BAM), -(CH₂)₃- (1,3-BAPr), and -(CH₂)₄- (1,4-BABu), class II with an aromatic linker such as 1,2-phenyl (1,2-BAB) 1,3-phenyl (1,3-BAB) and 1,4-dihydronaphthalene-1,4-epoxide (BAHE). The structures of these new mononuclear Pt(II) complexes have been determined by single-crystal X-ray diffraction analyses. For the 1,3-BAPr ligand, a dinuclear complex with the formula of [Pt(SMe₂)Ph₂]₂(1,3-BAPr) was also obtained and its structure was established by X-ray diffraction analysis. The linker group's impact on the N-Pt-N bite angle and the relative orientation of the two 7-azaindolyl rings was examined by using the crystal structural data. Intramolecular three-center four-electron Pt^II...H-C interactions have been established for all Pt(II) complexes with class I ligands by single-crystal X-ray diffraction and ¹H NMR spectroscopic analyses. Complexes Pt(1,3-BAPr)Me₂ and Pt(1,4-BABu)Me₂ have been found to have a poor stability, compared to the 1,2-BAB and BAM analogues. Two geometric isomers of the Pt(BAHE)Ph₂ complex have been identified by NMR spectra and the structure of one of the isomers has been determined by X-ray diffraction analysis, establishing that the BAHE ligand is most effective in blocking the 5^th coordination site of the Pt(II) center.

Full text of DOI:10.1039/b813501k

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Number 48 | 28 December 2008 | Pages 6853–7040
ISSN 1477-9226
HOT ARTICLE
Wang et al.
PERSPECTIVE
Impact of the linker groups in bis
(7-azaindol-1-yl) chelate ligands on
structures and stability of Pt(N,N-L)
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Crassous and Réau
π-Conjugated phosphole derivatives:
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Impact of the linker groups in bis(7-azaindol-1-yl) chelate ligands
on structures and stability of Pt(N,N -L)R2 complexes†
Shu-Bin Zhao,a Guo-hui Liu,a Datong Songb and Suning Wang*a
Received 4th August 2008, Accepted 24th September 2008
First published as an Advance Article on the web 3rd November 2008
DOI: 10.1039/b813501k
New organoplatinum(II) complexes with the general formula of Pt(N,N -L)R2, where L is a
bis(7-azaindol-1-yl) chelate ligand, R is a methyl or a phenyl, have been synthesized and investigated
with the aim to understand the impact of the linker group in the L ligand on the structure and the
stability of the Pt(II) complexes. The L ligands used in our investigation belong to two classes: class I
with an aliphatic linker between two 7-azaindolyl groups such as -CH2- (BAM), -(CH2)3- (1,3-BAPr),
and -(CH2)4- (1,4-BABu), class II with an aromatic linker such as 1,2-phenyl (1,2-BAB) 1,3-phenyl
(1,3-BAB) and 1,4-dihydronaphthalene-1,4-epoxide (BAHE). The structures of these new mononuclear
Pt(II) complexes have been determined by single-crystal X-ray diffraction analyses. For the 1,3-BAPr
ligand, a dinuclear complex with the formula of [Pt(SMe2)Ph2]2(1,3-BAPr) was also obtained and its
structure was established by X-ray diffraction analysis. The linker group’s impact on the N–Pt–N bite
angle and the relative orientation of the two 7-azaindolyl rings was examined by using the crystal
structural data. Intramolecular three-center four-electron PtII ◊ ◊ ◊ H–C interactions have been
established for all Pt(II) complexes with class I ligands by single-crystal X-ray diffraction and 1H NMR
spectroscopic analyses. Complexes Pt(1,3-BAPr)Me2 and Pt(1,4-BABu)Me2 have been found to have a
poor stability, compared to the 1,2-BAB and BAM analogues. Two geometric isomers of the
Pt(BAHE)Ph2 complex have been identified by NMR spectra and the structure of one of the isomers
has been determined by X-ray diffraction analysis, establishing that the BAHE ligand is most effective
in blocking the 5th coordination site of the Pt(II) center.
the Pt(II) based systems, as demonstrated previously by a number
of research groups.3–11
Introduction
Since the demonstration of the Shilov chemistry,1 the Pt(II)
mediated electrophilic CH4 activation and the subsequent ox-
idation process have attracted much research interest due to
their relevance in direct hydrocarbon functionalization.2 Extensive
investigations on the key steps involved in the Shilov system have
been conducted using a variety of Pt(II) model systems, which
often contain chelating ligands such as diamine,3 diimine,4,5 b-
diiminate,6 tri(pyrazolyl)borate,7 di(pyrid-2-yl)borate,8 dipyridyl,9
and bis(diphenylphosphino)10 derivatives, and mechanistic details
on the role of the Pt(II) center have been obtained. Despite signif-
icant advances in the mechanistic study,2,11 previously established
Pt(II) systems are still not adequate for practical C–H activation
and functionalization. Ligands have been elucidated to play a
crucial role on the property and performance of a Pt(II) center in
C–H activation. Both ligand steric and electronic properties have
been shown to have a significant impact on the selectivity and
reactivity of the C–H activation and functionalization process by
Recently, we have demonstrated several Pt(II) complexes with
7-azaindolyl derivative N,N -chelating ligands (N,N -L) that are
very effective for arene C–H activation, including Pt(1,2-BAB)Me2
and Pt(BAM)Me2 (Chart 1), where 1,2-BAB = 1,2-bis(7-azaindol-
1-yl)benzene and BAM = bis(7-azaindol-1-yl)methane (Chart 1),
and Pt2(TTAB)Me4, where TTAB = 1,2,4,5-tetrakis(7-azaindol-
1-yl)benzene.12,13 The 7-azaindolyl derivative ligands in these
Pt(II) complexes were found to be able to partially block the 5th
coordination site of the Pt(II) center, thus allowing not only the
stabilization and isolation of rare Pt(II)12c and Pt(IV) species,12d but
also regio- and diastereoselective ethylbenzene C–H activation.12c
The different steric blocking effects imposed by BAM and 1,2-
BAB ligands have been found to cause distinct regio- and diastere-
oselectivity by the Pt(1,2-BAB)Me2 and Pt(BAM)Me2 complexes
in ethylbenzene C–H activation.12d In addition, unusual and strong
intramolecular PtII ◊ ◊ ◊ H–C interactions were also observed in the
aDepartment of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6,
Canada. E-mail: wangs@chem.queensu.ca
bDepartment of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario, M5S 3H6, Canada
† Electronic supplementary information (ESI) available: Additional
X-ray structure data. CCDC reference numbers 697407–697415. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b813501k
Chart 1
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BAM Pt(II) complex.12 Recently, Puddephatt and coworkers10d–f
have demonstrated the effect of the bite angle of N,N -chelating
ligands on the reactivity of the PtMe2(N,N -L) complexes, by
comparing complexes of di-2-pyridylamine or di-2-pyridyl ketone.
We have shown recently that the chelate ring strain in a Pt(II)
complex, Pt(N,N -NPA)Me2, can lead to a spontaneous roll-over
intramolecular C–H activation, resulting in the formation of a
self-assembled macrocycle Pt4Me4(N,C-NPA)4.14 These previous
findings consistently support that the chelate ligand plays an
important role in Pt(II) C–H activation chemistry, which stimulate
our continued interest to further investigate the Pt(II) chemistry
involving 7-azaindolyl derivative ligands. To examine the aliphatic
linker effect, we have expanded the BAM ligand system to the 1,3-
bis(7-azaindolyl)propane (BAPr) and 1,4-bis(7-azaindoly)butane
(BABu) ligand systems. To compare with the related but rigid
aromatic linker, we have synthesized 1,3-bis(7-azaindolyl)benzene
(1,3-BAB). To further enhance the steric blocking effect, we have
also synthesized 1,4-dihydronaphthalene-1,4-epoxide (BAHE).
These new ligand systems allow us to conduct a systematic
examination on the effect of the linker on the structure, the stability
and ultimately the reactivity of Pt(N,N-L)R2 complexes based
on bis(7-azaindolyl) chelating ligands. This report focuses on the
syntheses, structures and stabilities of the Pt(II) complexes based
on the new ligand systems. Two previously undisclosed complexes
Pt(BAM)Ph2 and Pt(1,2-BAB)Ph2 are also included in this report
for comparison purpose.
hexanes/ethyl acetate (1/1) as the eluent. The 1,3-BAPr ligand was
obtained as an colorless oil (0.84 g, 46% yield), which was solidified
after the addition of hexanes and standing in a refrigerator for
several days. 1H NMR for 1,3-BAPr (500 Hz, CDCl3, 25 ◦C): d 8.35
(d; 3J = 5.0 Hz; 2H, 7-aza), 7.96 (d; 3J = 7.5 Hz; 2H, 7-aza), 7.34 (d;
3J = 3.5 Hz; 2H, 7-aza), 7.11 (dd; 3J1 = 5.0 Hz, 3J2 = 7.5 Hz; 2H,
7-aza), 6.49 (d; 3J = 3.5 Hz; 2H, 7-aza), 4.41 (t; 3J = 6.5 Hz; 4H,
CH2), 1.91 (m; 3J = 6.5 Hz; 3H, CH2) ppm. 13C NMR: d 148.21,
143.42, 129.48, 128.84, 121.40, 116.41, 100.35, 42.79, 31.69 ppm.
Anal. calcd. for C17H16N4: C 73.89, H 5.84, N 20.27; found:
C 73.83, H 5.99, N 20.22. The side product 1-allyl-7-azaindole
1
was obtained as colorless oil (0.35 g, ◦17% yield). H NMR for
1-allyl-7-azaindole (300 Hz, CDCl3, 25 C): d 8.35 (d; 3J = 4.8 Hz;
1H, 7-aza), 7.94 (d; 3J = 7.8 Hz; 1H, 7-aza), 7.23 (d; 3J = 3.3 Hz;
1H, 7-aza), 7.08 (dd; 3J1 = 4.8 Hz, 3J2 = 7.8 Hz; 1H, 7-aza), 6.49
3
(d; J = 3.3 Hz; 1H, 7-aza), 6.15 (m; 1H, CH2CH=CHH¢), 5.21
3
3
=
(d; J = 10.2 Hz; 1H, CH2CH CHH¢), 5.10 (d; J = 17.1 Hz; 1H,
=
=
CH2CH CHH¢), 4.94 (m; 2H, CH2CH CHH¢) ppm.
Synthesis of 1,4-bis(7-azaindol-1-yl)butane (1,4-BABu)
To a solid mixture of 7-azaindole (0.75 g, 6.4 mmol) and sodium
hydride (60% dispersed in mineral oil) (0.28 g, 7.0 mmol) in a
Schlenck flask was added quickly 15 mL of dry DMF. The resulting
clear solution was stirred for 30 minutes at ambient temperature,
and 1,4-dibromobutane (0.69 g, 3.2 mmol) was then added. The
resulting brown suspension was stirred for 2 hours at ambient
◦
temperature and then heated at 110 C for 18 hours. DMF was
Experimental
then removed by vacuum distillation and the yellow residue was
extracted by CH2Cl2 (30 mL ¥ 3). After the removal of the solvent,
the residue was purified by flash chromatograph on silica gel using
hexanes/ethyl acetate (2/1) as the eluent to afford 1,4-BABu◦as a
white solid (0.66 g, 71% yield). 1H NMR (500 Hz, CDCl3, 25 C):
All reactions were performed under N2 with the standard Schlenk
techniques unless otherwise noted. All starting materials were
purchased from Aldrich Chemical Co. and used without fur-
ther purification. THF, Et2O, and hexanes were purified using
the solvent purification system (Innovation Technology, Inc.).
Deuterated solvents (Cambridge Isotopes) were used as received
without further drying. NMR spectra were recorded on Bruker
3
4
3
d 8.30 (dd; J = 5.0 Hz, J = 1.5 Hz; 2H, 7-aza), 7.92 (dd; J =
8.0 Hz, 4J = 1.0 Hz; 2H, 7-aza), 7.14 (d; 3J = 3.5 Hz; 2H, 7-aza),
3
3
3
7.06 (dd; J1 = 5.0 Hz, J2 = 8.0 Hz; 2H, 7-aza), 6.46 (d; J =
3.5 Hz; 2H, 7-aza), 4.35 (m; 4H, CH2), 1.91 (m; 4H, CH2) ppm.
13C NMR: d 148.09, 143.37, 129.44, 128.56, 121.24, 116.29, 100.15,
44.65, 28.38 ppm. Anal. calcd. for C18H18N4: C 74.46, H 6.25,
N 19.30; found: C 74.38, H 6.45, N 19.45.
1
Avance 300, 400 or 500 MHz spectrometers. H and 13C NMR
chemical shifts were referenced to the residual solvent peaks and
have been reported in parts per million (ppm). High resolution
mass spectra (HRMS) were obtained from a Waters/Micromass
GC-TOF EI-MS spectrometer. [PtPh2(m-SMe2)]n (n = 2 or 3)15a
15b
and [PtMe2(m-SMe2)]2 were prepared by methods described in
Synthesis of 6,7-bis(7-azaindol-1-yl)-1,4-dihydronaphthalene-1,4-
epoxide (BAHE)
the literature. Bis(7-azaindol-1-yl)methane,16a 1,2-bis(7-azaindol-
1-yl)benzene (1,2-BAB)12b and 1,3-bis(7-azaindol-1-yl)benzene
(1,3-BAB)16b were synthesized by our previously reported pro-
cedure.
6,7-Dibromo-1,4-dihydronaphthalene-1,4-epoxide
(0.77
g,
2.5 mmol), 7-azaindole (0.75 g, 6.4 mmol), CuI (0.068 g,
0.35 mmol), 1,10-phenanthroline (0.15 g, 0.70 mmol), Cs2CO3
(4.10 g, 12.6 mmol) were mixed together in a Schlenck flask.
The flask was degassed under high vacuum and refilled with
N2. 4.0 mL of dr◦y DMF was then added. The mixture was
heated at 145–150 C for 3 days. After cooling down to ambient
temperature, the mixture was diluted with 100 mL of CH2Cl2
and filtered through a plug of silica gel. The filtrate was then
concentrated and the residue was purified by flash chromatograph
on silica gel using hexanes/ethylacetate (2:1) as the eluent to
BAHE as a yellow residue (0.48 g, 50% yield). Light yellow
crystals of BAHE were obtained from the recrystallization of the
Synthesis of 1,3-bis(7-azaindol-1-yl)propane (1,3-BAPr)
To a solid mixture of 7-azaindole (1.56 g, 13.3 mmol) and sodium
hydride (60% dispersed in mineral oil) (0.62 g, 15.5 mmol) in a
Schlenck flask was added quickly 25 mL of dry DMF. The resulting
clear solution was stirred for 30 minutes at ambient temperature,
and 1,3-dibromopropane (1.98 g, 9.8 mmol) was then added. The
resulting brown suspension was stirred for 2 hours at ambient
◦
temperature and then heated at 110 C for 18 hours. DMF was
then removed by vacuum distillation and the residue was extracted
by CH2Cl2 (50 mL ¥ 3). After the removal of the solvent, the
residue was purified by flash chromatograph on silica gel using
residue with THF/hexanes. H NMR (400 Hz, CDCl3, 25 ◦C):
1
8.25 d (dd; 4J = 1.2 Hz, 3J = 4.8 Hz; 2H, 7-aza), 7.84 (dd;
6954 | Dalton Trans., 2008, 6953–6965
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4J = 1.2 Hz, 3J2 = 8.0 Hz; 2H, 7-aza), 7.64 (s; 2H), 7.14 (s; 2H),
7.08 (dd; 3J1 = 4.8 Hz, 3J2 = 8.0 Hz; 2H, 7-aza), 6.78 (d, br; 3J =
3.6 Hz; 2H, 7-aza), 6.24 (d; 3J = 3.6 Hz; 2H, 7-aza), 5.85 (s; 2H)
ppm. 13C NMR: d 149.94, 147.88, 143.27, 143.17, 130.98, 129.55,
129.32, 121.37, 121.18, 116.85, 101.92, 82.56 ppm. HRMS calcd.
for C24H16N4O: 376.1324, found: 376.1361.
with Et2O and dried under vacuum. Recrystallization from Et2O
1
yielded ligh◦t yellow crystals of 2 (73% yield). H NMR (300 Hz,
CD2Cl2, 25 C): d 8.70 (dd, satellites; 3J = 5.4 Hz, 4J = 1.5 Hz; 2H;
7-aza), 7.88 (m; 4H, 7-aza and phenyl of 1,2-BAB), 7.59 (m; 2H,
phenyl of 1,2-BAB), 7.20 (d zH; 3J = 3.9 Hz; 2H, 7-aza), 7.00 (m
zH, satellite; JPt–H = 74 Hz; 6H, 7-azain and phenyl), 6.67 (m zH;
3
Besides the BAHE ligand, two side products 6-(7-azaindol-1-
yl)-1,4-dihydronaphthalene- 1,4-epoxide (NAHE) (~13% yield)
and 2,3,6-tris(7-azaindol-1-yl)naphthalene (TAN) (~8% yield)
from the reaction. The characterization data of NAHE and TAN
are shown below.
6H, phenyl), 6.56 (d; J = 3.9 Hz; 2H, 7-aza) ppm. 13C NMR: d
148.6, 145.6, 141.7, 138.6, 138.0, 131.5, 131.2, 130.9, 130.3, 126.2,
123.7, 121.4, 117.6, 103.0 ppm. Anal. calcd. for C32H24N4Pt: C
58.27, H 3.64, N 8.50; found: C 57.88, H 3.94, N 8.33.
For NAHE, 1H NMR (400 Hz, CDCl3, 25 ◦C): d 8.39 (br; 1H,
Synthesis of Pt(1,3-BAPr)Ph2 (3)
3
7-aza), 7.99 (d; J = 8.5 Hz, 1H, 7-aza), 7.71 (s; 1H), 7.48 (d;
1,3-BAPr (0.055 g, 0.20 mmol) and [PtPh2(m-SMe2)]n (n = 2 or
3) (0.082 g, 0.20 mmol based on Pt) were mixed in CH2Cl2
(15 mL) and stirred overnight at ambient temperature. The solvent
was decanted, and the resulting yellow solid was then washed
with Et2O (2 mL ¥ 3). Yellow crystals of 3 were obtained from
the slow evaporation of its methylene chloride solution (~70%
yield). 1H NMR (400 Hz, CD2Cl2, 25 ◦C): d 8.76 (dd; 3J =
4.4 Hz, 4J = 1.6 Hz; 2H, 7-aza), 8.19 (m; 2J = 12 Hz; 2H,
3J = 5.5 Hz; 1H, 7-aza), 7.38 (d; 3J = 12.5Hz; 1H), 7.31 (d; 3J =
3
12.5 Hz; 1H), 7.16 (m; 1H, 7-aza), 7.09 (br; 2H); 6.64 (d; J =
5.5 Hz; 1H, 7-aza), 5.81 (s; 1H), 5.80 (s; 1H) ppm. HRMS calcd.
for C17H12N2O: 260.0950; found: 260.0968.
For TAN, H NMR (400 Hz, CDCl3, 25 C): d 8.46 (dd; J =
◦
1
3
4
3
4.8 Hz, J = 1.6 Hz; 1H,7-aza), 4.42 (d; J = 1.6 Hz; 1H), 8.29
3
(s; 1H), 8.28 (s; 1H), 8.26–8.16 (m; 3H), 8.10 (d; J = 8.8 Hz;
1H), 8.06 (dd; 3J = 8.0 Hz, 4J = 1.6 Hz; 1H, 7-aza), 7.88 (m; 1H,
7-aza), 7.86 (m; 1H, 7-aza), 7.76 (d; 3J = 3.6 Hz, 1H, 7-aza), 7.21
N
aza-CHaHbCHaHbCHaHb-Naza), 7.63 (dd; 3J = 8.0 Hz, 4J =
3
1.2 Hz; 2H, 7-aza), 7.56 (d, satellite; 3J = 8.0 Hz, JPt–H
=
3
3
(dd; J1 = 4.8 Hz, J2 = 8.0 Hz; 7-aza), 7.15–7.06 (m; 2H), 6.98
(d; 3J= 3.6 Hz; 1H, 7-aza), 6.94 (d; 3J= 3.6 Hz; 1H, 7-aza), 6.74
(d; 3J= 3.6 Hz; 1H, 7-aza), 6.37 (d; 3J= 3.6 Hz; 1H, 7-aza), 6.34
70 Hz; 2H, ortho-H of PtPh2), 7.13 (d; 3J = 3.6 Hz, 7-aza),
6.95–6.72 (m; 8H, 2H from 7-aza & 2H from para-H of PtPh2
3
& 4H from meta-H of PtPh2), 6.27 (d; J = 3.6 Hz, 7-aza), 4.20
(d; J= 3.6 Hz; 1H, 7-aza) ppm. 13C NMR: d 148.83, 148.78,
3
(m; 2H, Naza-CHaHbCHaHbCHaHb-Naza), 2.83–2.71 (m; 2H, Naza
-
148.09, 143.88, 143.77, 143.40, 142.99, 137.77, 133.50, 133.17,
132.52, 130.70, 129.66, 129.43, 129.19, 129.11, 129.02, 128.89,
128.27, 128.17, 127.91, 127.07, 125.44, 124.09, 122.55, 122.28,
121.64, 120.80, 120.76, 120.60, 118.78, 117.40, 116.89, 116.86,
116.09, 102.80, 101.65, 101.37, 100.79 ppm. HRMS calcd. for
C31H26N6: 476.1749; found: 476.1758.
CHaHbCHaHbCHaHb-Naza) ppm. 13C NMR: d 146.722, 144.37,
139.18, 138.38 (satellite; JPt–C = 33.7 Hz), 129.59, 129.25, 126.60
(satellite; JPt–C = 81.7 Hz), 123.14, 121.76, 115.84, 101.51, 44.24,
31.70 ppm. Anal. calcd. for C29H26N4Pt: C 55.67, H 4.19, N 8.96;
found: C 56.01, H 4.28, N 8.64.
Synthesis of Pt(1,4-BABu)Ph2 (4)
Synthesis of Pt(BAM)Ph2 (1)
1,4-BABu (0.058 g, 0.20 mmol) and [PtPh2(m-SMe2)]n (n = 2 or 3)
(0.082 g, 0.20 mmol based on Pt) were mixed in CH2Cl2 (15 mL)
and stirred overnight at ambient temperature. The solvent was
decanted and the resulting yellow solid was then washed with
Et2O (2 mL ¥ 3). Yellow crystals of 4 were obtained from the
slow evaporation of its methylene chloride/toluene (3:1) solution
BAM (0.124 g, 0.50 mmol) and [PtPh2(m-SMe2)]n (n = 2 or 3)
(0.206 g, 0.50 mmol based on Pt) were mixed in Et2O (20 mL) and
stirred for 5 hours. The resulting white precipitate was allowed to
settle and the clear solution was decanted. The solid was washed
with Et2O and dried under vacuum. Recrystallization from THF
afforded 1 in 85% yield. H NMR (400 Hz, CD2Cl2, 25 ◦C): d
1
1
at room temperature (70% yield). H NMR (400 Hz, CD2Cl2,
2
11.91 (d, satellite; J = 14.4 Hz, JPt–H = ~11 Hz; 1H, CH2), 8.73
◦
3
4
25 C): d 9.10 (dd; J = 5.2 Hz, J = 1.2 Hz; 2H, 7-aza), 8.45
4
3
3
(dd, satellite; J = 1.5 Hz, J = 5.7 Hz, JPt–H = 24.6 Hz; 2H,
3
(m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza), 7. 90 (dd; J =
4
3
7-aza), 7.92 (dd; J = 1.2 Hz, J = 7.8 Hz; 2H, 7-aza), 7.56 (d;
4
3
6.0 Hz, J = 1.2 Hz; 2H, 7-aza), 7.31–7.16 (m, satellite; JPt–H
=
3J = 3.6 Hz; 2H, 7-aza), 7.43 (dd, satellite; J = 1.2 Hz, J =
7.8 Hz, 3JPt–H = 72 Hz; 4H, Phenyl), 7.01 (dd; 3J1 = 5.4 Hz, 3J2 =
7.8 Hz; 2H, aza), 6.80 (m; 4H, Phenyl), 6.70 (m; 2H, Phenyl), 6.63
4
3
76 Hz; 4H from ortho-H of PtPh2 and 2H from 7-aza); 7.11
3
3
(dd; J1 = 5.2 Hz, J2 = 6.0 Hz; 2H, 7-aza), 6.75–6.66 (m; 6H,
3
para- and meta-H of PtPh2), 6.52 (d; J = 3.6 Hz; 2H, 7-aza),
(d; 3J = 3.6 Hz; 2H, 7-aza), 6.47 (d, satellites; 2J = 14.4 Hz, JPt–H
=
4.15–4.01 (m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza), 2.10–
1.98 (m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza), 1.63–1.55
(m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza) ppm. 13C NMR:
d 145.26, 139.66, 138.11 (satellite; JPt–C = 35.2 Hz), 130.20, 129.80,
126.21 (satellite; JPt–C = 81.4 Hz), 124.25, 121.50, 116.12, 101.28,
43.26, 27.64 ppm. Anal. calcd. for C30H28N4Pt: C 56.33, H 4.41,
N 8.76; found: C 56.41, H 4.37, N 8.28.
~11 Hz; 1H, CH2) ppm. 13C NMR: d 146.5, 144.7, 142.9, 138.2,
130.5, 129.1, 126.7, 124.4, 121.8, 117.5, 103.1, 68.2 ppm. Anal.
calcd. for C27H22N4Pt·0.5THF: C 54.97, H 4.14, N 8.84; found: C
54.68, H 4.38, N 8.38.
Synthesis of Pt(1,2-BAB)Ph2 (2)
1,2-BAB (0.052 g, 0.17 mmol) and [PtPh2(m-SMe2)]n (n = 2 or 3)
(0.069 g, 0.17 mmol based on Pt) were mixed in 15 mL of Et2O and
stirred for 2 hours. The resulting white precipitate was allowed to
settle and the clear solution was decanted. The solid was washed
Synthesis of Pt2(1,3-BAPr)Ph4(SMe2)2 (5)
Complex 5 was obtained using the same method as described
for 3 using 1,3-BAPr (0.028 g, 0.10 mmol) and [PtPh2(m-SMe2)]n
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(n = 2 or 3) (0.082 g, 0.20 mmol based on Pt) in CH2Cl2 (15 mL).
Pale yellow crystals of 5 were obtained from the slow evaporation
of its methylene chloride solution (81% yield). 1H NMR (400 Hz,
CD2Cl2, 25 ◦C): d 8.76 (dd; 3J = 5.2 Hz, 4J = 1.2 Hz; 2H, 7-aza),
121.0, 120.7, 118.1, 102.9. Anal. calcd. for C32H24N4Pt: C 58.27,
H 3.64, N 8.50; found: C 57.71, H 3.65, N 8.15.
Synthesis of Pt(BAHE)Ph2 (9)
3
8.16 (t; J = 12.4 Hz; 2H, Naza-CHaHbCHaHbCHaHb-Naza), 7.63
(dd; 3J = 7.8 Hz, 4J = 1.2 Hz, 2H, 7-aza), 7.57 (d, satellite; 3J =
BAHE (0.037 g, 0.10 mmol) and [PtPh2(m-SMe2)]n (n = 2 or 3)
(0.041 g, 0.10 mmol based on Pt) were mixed in THF (10 mL)
and the mixture was stirred overnight at ambient temperature. The
solvent was decanted, and the resulting solid was then washed with
Et2O (4 mL ¥ 3). The residue was dissolved by THF and filtered
through a pad of Celite. The slow evaporation of the THF resulted
in 9 as brown power (ca. 50% yield), which was recrystallized from
a solvent mixture of CH2Cl2, THF and hexanes. 1H NMR (400 Hz,
CD2Cl2, 25 ◦C): d 8.72 (d; 3J = 5.5 Hz; 2H, 7-aza), 7.86 (d; 3J =
6.5 Hz; 2H, 7-aza), 7.47 (s; 2H); 7.29 (s; 2H), 7.17 (d; 3J = 3.5 Hz;
3
8.0 Hz, JPt–H = 61 Hz; 4H, ortho-H of PtPh2), 7.38 (d, satellite;
3J = 7.8 Hz, 3JPt–H = 72 Hz; 4H, ortho-H¢ of PtPh2), 7.13 (d; 3J =
3.6 Hz, 7-aza), 6.97–6.78 (m; 14H, 2H from 7aza, 4H from para-H
3
of PtPh2 and 8H from meta-H of PtPh2), 6.27 (d; J = 3.6 Hz;
2H, 7-aza), 4.20 (d; 3J = 12.4 Hz; 2H, Naza-CHaHbCHaHbCHaHb-
N
aza), 2.72–2.50 (m; 2H, Naza-CHaHbCHaHbCHaHb-Naza), 2.13 (s,
satellite; JPt–H = 24 Hz; 12H) ppm. 13C NMR: d 146.80, 144.75,
138.87, 138.69, 136.87, 136.64, 129.66, 129.16, 128.98, 128.75,
127.50, 126.81, 124.11, 123.01, 122.35, 122.14, 115.96, 101.81,
44.08, 31.83, 23.36, 20.80 ppm. Anal. calcd. for C45H48N4Pt2S2:
C 49.17, H 4.40, N 5.10; found: C 49.36, H 4.23, N 5.20.
3
3
3
2H, 7-aza), 7.02 (dd; J1 = 5.5 Hz, J2 = 6.5 Hz; 2H, 7-aza),
3
3
6.96 (d, satellites; JPt–H = 80 Hz, J = 8.5 Hz; 4H, ortho-H of
PtPh2), 6.70 (m; 4H, meta-H of PtPh2), 6.63 (m; 2H, para-H of
3
PtPh2), 6.55 (d; J = 3.5 Hz; 2H, 7-aza), 5.96 (s; 2H) ppm. 13C
Synthesis of Pt(1,3-BAPr)Me2 (6)
NMR: d 152.18, 128.77, 145.26, 142.88. 141.85, 138.35, 134.23,
131.57, 131.21, 130.08, 125.87, 123.38, 122.28, 121.16, 117.39,
102.80, 82.87 ppm. Anal. calcd. for C36H26N4OPt·0.25CH2Cl2 :
C 58.29, H 3.58, N 7.50; found: C 58.37, H 3.99, N 7.39.
Complex Pt(1,3-BAPr)Me2 was obtained using the similar method
as described for 3 using 1,3-BAPr (0.055 g, 0.20 mmol) and
[PtMe2(m-SMe2)]2 (0.057 g, 0.10 mmol) in THF (10 mL) for 2 days.
The solvent was decanted. The pale yellow solid was extracted with
Et2O/hexanes (1:1) (5 mL ¥ 3). The crystals of Pt(1,3-BAPr)Me2
were obtained by cooling the Et2O/hexanes (1:1) solution in a
refrigerator for several days. Due to its poor stability, compound 6
was only characterized by single-crystal X-ray diffraction analyses.
X-Ray crystallographic analyses
Single crystals of BAHE, 1–9a were mounted on glass fibers for
data collection. Data for BAHE, 1, 2, 5–8 were collected on a
Bruker Smart 1000 CCD X-ray diffractometer while data for 4
and 9a were collected on a Bruker Apex II single-crystal X-ray
diffractometer with graphite-monochromated Mo Ka radiation,
operating at 50 kV and 30 mA, at either 298 K or 180 K. Com-
pound 9 produces a mixture of distinctly different crystalline solids
due to the presence of isomers. The isomer 9a forms block-shaped
crystals which can be picked out for X-ray study. Isomer 9b forms
microcrystalline powders that are not suitable for single-crystal
X-ray study. These two isomers form an intimate solid mixture
that is difficult to separate in bulk quantity. No significant decay
was observed for any of the crystals. Data were processed on a PC
using Bruker Apex II software and corrected for absorption effects.
The structural solution and refinements were performed using the
Bruker SHELXTL software package (version 5.10). All structures
were solved by direct methods. The quality of crystals of 5 is poor.
One of the methyl groups on a SMe2 group in 5 is disordered over
two sites with about 50% occupancy factor for each site. Because
of the disordering, some of the S–C distances are considerably
shorter than a typical S–C bond length. To address this problem,
we used DFIX in the SHELXTL program to restrain the S–C bond
Synthesis of Pt(1,4-BABu)Me2 (7)
Complex Pt(1,4-BABu)Me2 was obtained using the same method
as described for 3 using 1,4-BABu (0.129 g, 0.44 mmol) and
[PtMe2(m-SMe2)]2 (0.127 g, 0.22 mmol) in THF (15 mL). Pale
yellow crystals of 7 were obtained from cooling the Et2O/hexanes
(1:1) solution in a refrigerator for several days. 1H NMR (400 Hz,
CD2Cl2, 25 ◦C): d 8.97 (d, satellite; 3J = 5.2 Hz, 3JPt–H = 40 Hz; 2H,
7-aza), 8.42 (m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza), 7. 90
(d; 3J = 8.0 Hz; 2H, 7-aza), 7.25 (d; 3J = 3.2 Hz; 2H, 7-aza), 7.01
(dd; 3J1 = 5.2 Hz, 3J2 = 8.0 Hz; 2H, 7-aza), 6.53 (d; 3J = 3.2 Hz;
2H, 7-aza), 4.07–4.02 (m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-
N
aza), 1.89–1.81 (m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza),
1.48–1.42 (m; 2H, Naza-CHaHbCHaHbCHaHbCHaHb-Naza), 0.52
(s, satellites; 2JPt–H = 86.4 Hz) ppm.
Synthesis of PtPh2(1,3-BAB) (8)
1,3-BAB (0.40 mmol, 0.124 g) and [PtPh2(Me2S)]n (n = 2, 3)
(0.164 g, 0.40 mmol based on Pt) were mixed in 20 mL of
THF. After the solution was refluxed for 4 hours, the solvent
was removed under vacuum. The residue was recrystallized from
˚
length at 1.80 A in the refinements. This approach improves the
S–C bond lengths somewhat, but some of the S–C bond lengths
are still shorter than typical. Attempts to obtain better crystals
for 5 were made but without success. The data for structure 5
is from our best efforts. Crystals of 1 and 7 contain disordered
solvent molecules which could not be fully modeled and refined.
As a result, the solvent contributions were removed for these two
molecules using the Squeeze algorithm in Platon program.17 The
crystal data for 1 and 7 in Table 1 excluded contributions from
solvent molecules in the lattice. All non-hydrogen atoms were
refined anisotropically except the disorder methyl groups C(42),
1
CH2Cl2/hexanes to afford 8 (25% yield). H NMR (500 MHz,
CD2Cl2, 25 ◦C): d 9.31 (dd; 3J = 5.0 Hz, 4J = 1.5 Hz; 2H, 7-aza),
3
4
8.22 (m; 1H, phenyl of 1,3-BAB), 8.04 (dd; J = 8.0 Hz, J =
1.5 Hz; 2H, 7aza), 7.97 (t; 3J = 8.0 Hz; 1H, phenyl of 1,3-BAB),
7.67 (m; 4H, 7-aza and phenyl of 1,3-BAB); 7.28 (dd; 3J = 8.0 Hz,
4J = 5.5 Hz; 2H, 7-aza), 6.76 (m, satellites; 4H, phenyl), 6.69 (d;
3J = 3.5 Hz; 2H, 7aza), 6.43 (m; 6H, phenyl). 13C NMR: d = 147.4,
147.0, 138.7, 137.3, 136.0, 131.0, 130.4, 128.7, 125.8, 125.3, 123.4,
6956 | Dalton Trans., 2008, 6953–6965
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Table 1 Crystallographic data for complexes 1–8 and 9a
Compound
1
2
3
4
5
6
7
8
9a
Formula
FW
C27H22N4Pt C32H24N4Pt C29H26N4Pt C30H28N4Pt C45H48N4Pt2S2 C19H22N4Pt C20H24N4Pt C32H24N4Pt C36H26N4PtO
597.58
P21/n
12.238(3)
11.930(3)
18.202(4)
90
93.953(6)
90
2651.0(10)
4
659.64
P21/c
15.404(3)
11.527(2)
15.536(3)
90
112.390(4)
90
2550.7(8)
4
625.63
P-1
639.65
P21/n
13.068(2)
10.512(1)
19.077(2)
90
109.140(2)
90
2475.8(5)
4
1099.17
P-1
501.59
P-1
7.9960(11)
9.6192(13)
515.52
P-1
8.031(1)
659.64
P-1
725.70
P21/n
16.6466(3)
9.7584(2)
17.6630(3)
90
104.846(1)
90
2773.47(9)
4
Space group
˚
a/A
8.2788(1)
10.2878(1)
15.4456(3)
73.386(1)
89.311(1)
69.110(1)
1171.77(3)
2
11.7088(15)
12.9165(17)
14.9862(19)
80.192(2)
83.251(3)
63.096(2)
1989.5(4)
2
10.778(2)
9.984(2)
13.777(3)
103.297(4)
96.241(4)
115.883(3)
1260.4(4)
2
˚
b/A
10.707(1)
˚
c/A
11.9856(16) 13.863(1)
a/◦
b/◦
g /◦
84.184(2)
72.328(2)
87.362(2)
873.7(2)
2
82.463(1)
85.130(1)
83.618(1)
1171.4(2)
2
3
˚
V/A
Z
D
calc,/g cm-3
1.497
293
5.312
1.718
294
5.530
1.773
180
6.013
1.716
180
5.694
1.835
180
7.167
1.906
180
8.037
1.462
180(2)
5.997
1.738
298
5.595
1.738
180
5.097
T/K
m/mm◦-1
2qmax
/
54.00
17999
54.00
17073
52.00
10077
54.00
16791
56.00
14309
54.00
6093
52.00
7432
56.00
8109
54.36
16310
Reflns
measured
Reflns used
5766(0.033) 5546(0.017) 4573(0.021) 5408(0.040) 9042(0.044)
3768
(0.025)
219
4542
(0.017)
228
4909(0.098) 6130 (0.034)
(Rint
)
Parameters
289
334
307
316
470
334
379
R [I > 2s(I)]:
a
R1
0.0251
0.0382
0.0188
0.0340
0.0200
0.0449
0.0384
0.0835
0.0502
0.0639
0.0319
0.0621
0.0190
0.0405
0.0775
0.2071
0.0321
0.0736
b
wR2
R (all data):
a
R1
0.0468
0.0401
0.723
0.0316
0.0363
0.925
0.0226
0.0461
1.025
0.0642
0.0946
0.998
0.1638
0.0758
1.188
0.0420
0.0645
0.924
0.0213
0.0407
0.979
0.0791
0.2089
1.092
0.0469
0.0794
1.047
b
wR2
GOF on F2
ꢀ
ꢀ
ꢀ
ꢀ
2
2
a R1 = |F0| - |Fc|/ |F0|. b wR2 = [ w [(F0 - Fc2)2]/ [w(F02)2]]1/2, w = 1/[s (F02) + (0.075P)2], where P = [Max (F02,0) + 2Fc2]/3.
C(42A) and C(43), and the C(29) atom in structure 5. The positions
of hydrogen atoms except those on the disordered methyl group
in 5 were calculated, and their contributions in structural factor
calculations were included. The crystal data for all complexes are
summarized in Table 1. Important bond lengths and angles for all
the compounds are listed in Table 2. The data for the free ligand
BAHE is provided in the supporting materials.†
under the same conditions used for BAM failed to result in
any product, presumably due to the reduced reactivity of 1,3-
dibromopropane and 1,4-dibromobutane compared to that of
CH2Br2. Since alkylation of indole analogues preferentially occurs
at the N1 position rather than C3 position in high polar solvents
such as DMF,18 to increase the yield and avoid the other isomers,
the reaction of 1,3-dibromopropane or 1,4-dibromobutane with
the sodium salt of 7-azaindole, generated in situ by deprotonation
of 7-azaindole with NaH, was carried out in DMF at 110 ◦C
(Scheme 1). 1,3-BAPr or 1,4-BABu were obtained in good yields.
Result and discussion
Syntheses and characterization of new ligands 1,3-BAr, 1,4-BABu,
and BAHE
Ligands bis(7-azaindol-1-yl)methane (BAM), 1,2-bis(7-azaindol-
1-yl)benzene (1,2-BAB), and 1,3-bis(7-azaindol-1-yl)benzene (1,3-
BAB) were synthesized using previously reported procedures.12,16
To examine the effect of increasing chelate ring size and the
interactions between a Pt(II) center and a longer and more flexible
linker than the CH2 group in the BAM ligand, two new ligands 1,3-
bis(7-azaindol-1-yl)propane (1,3-BAPr) and 1,4-bis(7-azaindol-
1-yl)butane (1,4-BABu) as BAM analogues were designed and
synthesized. As tautomerism is a typical phenomenon for 7-
azaindole, alkylation of 7-azaindole under basic conditions has
the tendency to afford a mixture of N1, C3 and N7 functionalized
isomers. For example, in the synthesis of the BAM ligand by the
nucleophilic substitution reaction of CH2Br2 with 7-azaindole in
the presence of KOH and a phase-transfer catalyst, NBu4Br, in
refluxing toluene and water, in addition to BAM, other isomers
were also isolated, albeit in low yields.16a Our initial attempts
towards the syntheses of the 1,3-BAPr and 1,4-BABu ligands
Scheme 1
To examine the impact of an aromatic linker that is larger than
the phenyl group in 1,2-BAB and has a donor atom, we attempted
the synthesis of 6,7-bis(7-azaindol-1-yl)-1,4-dihydronaphthalene-
1,4-epoxide (BAHE) using the procedure shown in Scheme 2.
BAHE was obtained in ca 50% yield using the homogeneous C–N
coupling method developed by the Buchwald group,19 where 6,7-
dibromo-1,4-dihydronaphthalene-1,4-epoxide, prepared accord-
ing to literature methods,20 and 7-azaindole were reacted in the
presence ◦of CuI, 1,10-phenanthroline, and Cs2CO3 in DMF at
145–150 C under N2 atmosphere for 72 hours. BAHE was fully
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characterized by NMR spectroscopy, HRMS, and single crystal
X-ray diffraction analyses (see ESI†). Notably, besides BAHE,
the mono-7-azaindolyl substituted debromination product NAHE
and a tris-7-azaindolyl substituted naphthalene derivative com-
pound TAN were also isolated as the side products in 13%
and 8% yield, respectively (Scheme 2). Both side products have
been fully characterized by NMR and HRMS. The formation
of TAN likely occurred via processes as illustrated in Scheme 3.
The nucleophilic addition of 7-azaindole anion to BAHE initi-
=
ated a tandem C C double bond migration and simultaneous
ring-opening process, generating a dihydronaphthalene derived
intermediate, which underwent subsequent protonation and dehy-
dration, resulting in TAN. Similar nucleophilic addition induced
1,4-dihydronaphthalene-1,4-epoxide ring opening processes have
been well studied toward producing highly functionalized dihy-
dronaphthalene products.21
Syntheses and characterization of Pt(II) complexes
Complexes Pt(BAM)Ph2 (1) and Pt(1,2-BAB)Ph2 (2) (Chart 2)
were obtained in good yields from the reactions of [PtPh2(m-
SMe2)]n (n = 2, 3) with the corresponding chelate ligand in a 1:1
[Pt]/[L] ratio. The monomeric PtPh2 complexes Pt(1,3-BAPr)Ph2
(3) and Pt(1,4-BABu)Ph2 (4) were obtained as stable yellow crys-
talline solids in good yields by ligand displacement of the bridging
SMe2 in [PtPh2(m-SMe2)]n (n = 2, 3) with either 1,3-BAPr or 1,4-
BABu with a 1:1 [Pt]/[L] ratio in THF at ambient temperature, as
shown in Scheme 4. Interestingly, when the reaction was carried
out with a 2:1 [Pt]/[1,3-BAPr] ratio, a dinuclear complex Pt2(1,3-
BAPr)Ph4(SMe2)2 (5) was isolated. Upon sonicating its CH2Cl2
solution, complex 5 was quickly converted to 3 via the elimination
of one equivalent of PtPh2(SMe2)2, an indication that compound
3 is thermodynamically favored while compound 5 is kinetically
favored. Efforts were also made to synthesize the PtMe2 analogues
of 3 and 4, Pt(1,3-BABr)Me2 (6) and Pt(1,4-BABu)Me2 (7). The
ligand displacement reactions of [PtMe2(m-SMe2)2] with 1,3-BAPr
and 1,4-BABu both were found to be sluggish, and the desired
compounds 6 and 7 were obtained in 40–60% yields after 2 days.
Moreover, while both complexes displayed a poor stability, making
it difficult to purify them, the decomposition of 6 occurred much
more quickly, consistently resulting in the formation of Pt black
and the free 1,3-BAPr ligand. Fortunately, some crystals of both 6
and 7 have been obtained by cooling their Et2O/hexanes (1:1)
solutions, thereby allowing unambiguous examination of their
structures. The PtPh2 complex Pt(1,3-BAB)Ph2 (8) (Chart 2) was
synthesized in the same manner as for Pt(1,2-BAB)Ph2. However,
the yield for 8 is much lower than that of Pt(1,2-BAB)Ph2,
which can be attributed to the poor chelating ability of the
1,3-BAB ligand. Although 1,3-BAB is capable of chelating Pd(II)
and Pt(II) in an N,C,N-tridentate mode, such cyclometallated
products were not obtained from the reaction, presumably because
the Pt–C bond in the [PtPh2(m-Me2S)]n (n = 2 or 3) starting
material is relatively strong, which cannot be cleaved at ambient
temperature. Attempt to improve the yield of 8 by using higher
temperature (e.g. refluxing toluene) led to partial decomposition
with Pt(0) deposition with the remaining reaction mixture still
being dominated by the unreacted starting material. The reac-
tion of [PtMe2(m-Me2S)]2 with 1,3-BAB encountered the same
problem and consequently the isolation and characterization of
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Scheme 2
Scheme 3
Pt(1,3-BAB)Me2 was not achieved. The BAHE complex,
Pt(BAHE)Ph2 (9) was obtained in modest yield (~50%) using a
procedure similar to that of 8. However, 1H NMR spectra indicated
that compound 9 exists in two isomeric forms 9a and 9b, as shown
in Chart 2.
Scheme 4 Reagents and conditions: i) [PtPh2(m-SMe2)n] (n = 2, 3), THF,
rt, overnight ([Pt]:[L] = 1:1); ii) [PtPh2(m-SMe2)n] (n = 2, 3), rt, overnight,
([Pt]:[L] = 2:1); iii) sonication, CH2Cl2, rt.
of PtPh2 complexes 1, 3–5 are shown in Fig. 1–4, respectively.
Compounds 1, 3 and 4 are mononuclear Pt(II) complexes with
their ligands adopting the N,N-chelating mode to form either a 8-
membered (1), a 10-membered (3) or 11-membered (4) chelate
ring. Compound 5 is a dinuclear complex with the 1,3-BAPr
ligand acting as a bridging ligand for the two Pt(II) centers and
Chart 2
Structures of complexes 1, 3–7 with an aliphatic linker
To verify the binding modes and the intramolecular interactions
between the linker groups in the chelate ligands and the Pt(II)
centers in the complexes, single-crystal X-ray diffraction analyses
were carried out for all complexes. The molecular structures
Fig. 1 Crystal structure of 1 with 50% ellipsoids.
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Fig. 2 Crystal structure of 3 with 50% ellipsoids: front view (left) and side view (right).
Fig. 3 Crystal structure of 4 with 50% ellipsoids: front view (left) and side view (right).
the PtPh2 groups being consistently cis to each other. Although
some variations of Pt–N and Pt–C bond lengths are evident for
these complexes as shown in Table 2, they are similar to those
observed in Pt(BAM)Me2.12a The most notable difference between
1, 3 and 4 is the N–Pt–C and N–Pt–N bond angles. In 1, the
average N–Pt–Ctrans angles is 177.1(2)◦ and the N–Pt–N angle is
87.82(11)◦. In 3 and 4, however, the average N–Pt–Ctrans angles
are 174.0(1)◦ and 173.7(2)◦, respectively, a large deviation from
linearity. Most striking is the small N–Pt–N angle of 81.24(10)◦
in 3, which is highly strained, compared to those of 1. In contrast,
despite a larger chelate ring in 4 compared to that in 1, the
N–Pt–N angle in 4 is 90.40(18)◦, a nearly ideal chelating angle
for a square planar Pt(II) center. Consistent with the greater
chelate ring strain in 3 is the smaller dihedral angle between the
two 7-azaindolyl rings (44.5◦) defined by the diagram shown in
Fig. 13 (see later), compared to those in 1 (94.3◦) and 4 (109.7◦)
(Table 3). The large chelate ring strain in 3 appears to be imposed
by the geometry of the propyl linker. The butyl linker in 4 is much
more flexible, thus allowing the chelate ring to be more relaxed.
The two Pt(II) square planes in 5 are parallel to each other with
an approximate staggered configuration. Compared to the highly
strained mononuclear complex 3, the dinuclear Pt(II) molecule 5
appears to be more sterically favored. Its facile conversion to 3 is
likely driven by chelating effect.
Fig. 4 Crystal structure of 5 with 50% ellipsoids. All carbon atoms are
shown as ideal spheres for clarity.
a SMe2 being bound to each Pt(II) center. The Pt(II) centers in
all four complexes have a typical square planar geometry, with
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Table 3 Dihedral angles between the two 7-azaindolyl rings in 1–4 and 6–9a
Linker in the chelate ligand
CH2
Compound
Dihedral angle between two 7-azaindolyl rings/◦
N–Pt–N bond angle/◦
Pt(BAM)Ph2 (1)
Pt(BAM)Me2
94.3
102.7
44.5
87.82(11)
89.98(11)
81.24(10)
83.75(17)
90.40(18)
91.82(9)
83.01(7)
85.8(3)
12a
CH2CH2CH2
Pt(1,3-BAPr)Ph2 (3)
Pt(1,3-BAPr)Me2 (6)
Pt(1,4-BABu)Ph2 (4)
Pt(1,4-BABu)Me2 (7)
Pt(1,2-BAB)Ph2 (2)
46.6
CH2CH2CH2CH2
1,2-phenyl
109.7
100.5
62.3
12b
Pt(1,2-BAB)Me2
82.5
Pt(BAHE)Ph2 (9a)
Pt(1,3-BAB)Ph2 (8)
73.4
140.2
87.90(13)
95.9(3)
1,3-phenyl
The structures of Pt(1,3-BAPr)Me2 (6) and Pt(1,4-BABu)Me2
(7) are analogous to complexes 3 and 4 and are shown in Fig. 5
and 6, respectively. As shown by Table 3, the trend of the N–Pt–N
angles and the dihedral angles between the two 7-azaindolyl
planes for the PtMe2 complexes of the BAM, 1,3-BAPr and
1,4-BABu ligands is the same as that of the corresponding
PtPh2 complexes, supporting that the linker in the chelate ligand
dictates the chelate bond angle and the dihedral angle of the two
7-azaindolyl groups. The N–Pt–N angles of the PtMe2 complexes
are all greater than those of the PtPh2 analogue, The dihedral angle
between the two 7-azaindolyl rings follows a similar trend except
that the 1,4-BABu complex 7 has a smaller dihedral angle than
that of the PtPh2 complex 4. Therefore, the size of the R group
bound to the Pt(II) center also has a significant effect, albeit much
less than the linker group, on the chelate angle and the dihedral
angle.
Fig. 5 Crystal structure of 6 with 50% ellipsoids: front view (left) and side view (right).
Fig. 6 Crystal structure of 7 with 50% ellipsoids: front view (left) and side view (right).
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Intramolecular Pt ◊ ◊ ◊ H interactions in 1, 3–7
informative for us to understand such a phenomenon. If these
Pt(II) complexes retain the same structures in solution, each of
the CH2 groups in the linkers in 1 and 3–7 should display an
As shown in Fig. 1–6, one common feature in structures 1, 3–7,
is the partial blockage of the square planar Pt(II) 5th coordination
site by the linker group. Due to the pronounced boat conformation
adopted by the eight-membered chelate ring in 1, the H¢ atom
(Chart 1) of its methylene carbon is oriented toward the dz2
orbital of the Pt(II) center, with a short Pt ◊ ◊ ◊ H¢ separation
1
“AB” pattern in the H NMR spectra. Indeed, the “AB” pattern
was observed and well resolved for complex 1 where the CHH¢
atoms display two distinct chemical shifts with Dd = 5.44 ppm
(Fig. 8), which is almost the same as that (5.45 ppm) observed12a
in Pt(BAM)Me2, in agreement to the nearly identical Pt ◊ ◊ ◊ H
separation distances in 1 and Pt(BAM)Me2. Similar distinct “AB”
patterns were also observed for the CaHH¢ protons in the 1,3-BAPr
complexes 3, and the 1,4-BABu complexes 4 and 7, as illustrated
in Fig. 8 and 9. In comparison with 1, the elongated Pt ◊ ◊ ◊ H¢
separation distances in these complexes resulted in noticeably
reduced Dd for the CaHH¢ atoms in 3 (3.99 ppm), 4 (4.41 ppm) and
7 (4.36 ppm). The two CaHH¢ atoms in 5 also displayed an “AB”
patterns with a Dd value observed as 3.96 ppm. Note also that,
the b-CHH¢ atoms in the linkers in 3–7 also displayed an “AB”
˚
˚
distance of 2.42 A, which is similar to that (2.44 A) observed
in Pt(BAM)Me2.12a In the chelating complexes 3, 4, 6 and 7, the
analogous boat conformations give rise to both H¢ atoms of two
Ca atoms of their aliphatic linkers in close contact with the Pt(II)
˚
center, with the shortest Pt ◊ ◊ ◊ H¢ separation distances being 2.56 A
˚
˚
˚
in 3, 2.58 A in 4, 2.61 A in 6, and 2.49 A in 7, respectively. In
addition to the short intramolecular Pt ◊ ◊ ◊ H distances, a short
intermolecular Pt ◊ ◊ ◊ H distance with a 7-azaindolyl ring is also
˚
˚
observed in 3 (3.02 A) and 7 (2.92 A), as shown in Fig. 7. The
crystals of 4 and 6 do not show such intermolecular interactions.
For the dinuclear Pt2 complex 5, due to similar geometrical
constraints, the two H¢(Ca) atoms display even shorter Pt ◊ ◊ ◊ H¢
1
pattern in the H NMR spectra, indicating that these complexes
have the same structural features in solution as those observed in
the solid state.
˚
˚
separation distances (2.31 A and 2.43 A).
Fig. 7 Diagrams showing intermolecular Pt ◊ ◊ ◊ H interactions in 3 (top)
and 7 (bottom).
Fig. 8 Partial 1H NMR spectra of 1, 3 and 4 in CD2Cl2 at ambient
temperature, showing the two distinct chemical shifts and the “AB” pattern
of the CaHH¢ protons.
We have reported previously that the short Pt ◊ ◊ ◊ H¢ separation
distances in BAM Pt(II) complexes can induce PtII ◊ ◊ ◊ H–C inter-
actions. While the BAM CH2 hydrogen atoms in these complexes
displayed typical “AB” patterns, in the 1H NMR spectra, dramatic
dH downfield shifts (Dd > 4.5 ppm) were observed for the H¢ atoms
that are oriented toward the dz2 orbitals of Pt(II) centers.12 The
origin of these lowfield resonances was tentatively ascribed to a
three-center four-electron PtII ◊ ◊ ◊ H–C electrostatic interaction.22–23
The spectral studies of the above new Pt(II) complexes were
Despite a number of known examples of platinum alkyl cations
with b-agostic PtII ◊ ◊ ◊ H–C interactions,25 previous studies have
demonstrated that typical neutral square planar Pd(II) and Pt(II)
complexes usually display only the three-center four-electron
PdII/PtII ◊ ◊ ◊ H–C electrostatic interactions utilizing the electron
pair in the dz2 orbital,24 with the Pd/Pt ◊ ◊ ◊ H separation distances
˚
being >2.2 A and a characteristic downfield chemical shift by the
Fig. 9 The complete 1H NMR spectrum of 3 in CD2Cl2 at ambient temperature.
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proton. In contrast, agostic interactions are intramolecular three-
140.2◦ for 8, and 73.4◦ for 9a. The structure of 2 resembles that
of Pt(1,2-BAB)Me2, except that the PtMe2 analogue has a much
greater dihedral angle than 2, as shown in Table 3. The large
dihedral angle in 8 is an indication of the poor chelating ability of
the 1,3-BAB ligand, consistent with the poor stability and the low
yield of compound 8. The impact of various linker groups on the
dihedral angle of the two 7-azaindolyl rings in the PtPh2 complexes
is illustrated by Fig. 13. The other significant difference is the steric
blockage of the Pt 5th coordination site by the linker in the three
molecules. The 1,2-phenyl linker in 2 and 9a has a much smaller
dihedral angle with the Pt square plane (~33◦ and 42◦, respectively)
than the 1,3-linker in 8 (~48◦). Hence, the 1,2-phenyl linker is
much more effective in capping the 5th coordination site than the
1,3-phenyl linker. The BAHE ligand in 9a is especially effective
in sterically blocking access to the 5th coordination site due to the
presence of the furan ring, as shown by the space filling diagram in
Fig. 12. We have demonstrated recently that the effective blockage
of the 5th coordination site is critical in achieving effective regio-
and diastereoselective C-H activation.12 Hence, compound 9a is
a very promising molecule for use in regio- and stereoselective
C-H activation, which will be investigated and reported in due
course. The oxygen atom in 9a forms an intermolecular hydrogen
bond with a 7-azaindolyl group, causing a bi-layer-like extended
center two-electron interactions, characterized by short M ◊ ◊ ◊ H
1
˚
separation distances (1.8–2.2 A) as well as an upfield H chemical
shift for the proton involved.23 Agostic interactions are often
encountered in coordinatively unsaturated MoII, RuII, OsII, RhIII,
IrIII and PtII complexes due to the requirement of an empty orbital
on the metal center.26 Therefore, the d downfield shifts by CaHH¢
atoms in our Pt(II) complexes can be unequivocally attributed
to three-center four-electron PtII ◊ ◊ ◊ H–C interactions rather than
agostic interactions.
Structures of complexes 2, 8–9a with an aromatic linker
The structures of Pt(1,2-BAB)Ph2 (2), Pt(1,3-BAB)Ph2 (8) and
Pt(BAHE)Ph2 (9a) are shown in Fig. 10–12. The common feature
for these three molecules is that the linker group between two 7-
azaindolyl rings is a phenyl at either 1,2-positions or 1,3-positions.
The structures of 2 and 9a are closely related since both contain
a 1,2-phenyl linker. The average N–Pt–Ctrans angles in 2, 8 and 9a
are 175.4(9)◦, 173.4(4)◦ and 175.5(2)◦, respectively, and the N–
Pt–N angles are 83.01(7)◦, 95.9(3)◦ and 87.90(13)◦, respectively.
The most significant difference between the three structures is the
dihedral angle between the two 7-azaindolyl groups, 62.3◦ for 2,
Fig. 10 The side view of structures of 2 (left) and 9a (right) with 50% ellipsoids.
Fig. 11 Crystal structure of 8 with 50% ellipsoids: front view (left) and side view (right).
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Fig. 12 Left: The space-filling diagram of 9a showing the effective blocking of the Pt atom by the BAHE ligand; Right: A diagram showing intermolecular
H bonds involving the oxygen atom in the crystal lattice of 9a.
on the stability of the complex. For example, among the PtMe2
complexes, a distinct difference in stability was observed for
Pt(BAM)Me2, 6 and 7: the 1,3-BAPr and 1,4-BABu complexes 6
and 7 have a much greater tendency to decompose at ambient tem-
perature than the BAM complex with complex Pt(1,3-BAPr)Me2
(6) being the least stable. The chelate ring strain in 6 may be the
main cause of its instability. The PtPh2 complexes in general are
all stable at ambient temperature with the exception of Pt(1,3-
BAB)Ph2 (8), which appears to have a low stability in solution,
presumably caused by the incompatible N,N -chelating angle of 1,3-
BAB with a square planar Pt(II) ion. The greater instability of the
PtMe2 complexes versus their PtPh2 analogues can be attributed
to the greater trans influence of the methyl group, and the weaker
Pt–C bond, compared to the PtPh2 analogues.
Conclusions
The syntheses of new 7-azaindolyl derivative ligands 1,3-BAPr,
1,4-BABu and BAHE have been accomplished, enabling a sys-
tematic examination on the impact of the linker groups in the
chelate ligands on the structure and the stability of a series of Pt(II)
complexes. Our investigation has established that regardless of
Fig. 13 A diagram showing the orientation and the dihedral angle
between the two 7-azaindolyl rings in the PtPh2 complexes with various
linkers. The phenyl and the linker groups are removed for clarity.
the linker’s size and the geometry, the bis(7-azaindol-1-yl) chelate
ligands in general favor the formation of mononuclear chelate
complexes with a Pt(II) center. Ligands with aromatic linkers
such as 1,2-BAB or BAHE are most effective in blocking the
5th coordination site of the Pt(II) center. In comparison, ligands
with an aliphatic linker such as BAM, 1,3-BAPr and 1,4-BABu are
in general much less effective in blocking the 5th coordination site.
The aromatic 1,3-BAB ligand is the least effective in steric blocking
and as a chelate ligand. Strong intramolecular Pt ◊ ◊ ◊ H interactions
have been established in Pt(II) complexes with ligands bearing
aliphatic linkers and confirmed to be three-center four-electron
PtII ◊ ◊ ◊ H–C interactions, by NMR and crystal structural data.
PtMe2 complexes with 1,3-BAPr and 1,4-BABu ligands have been
found to have a low stability. The BAHE ligand can bind to the
Pt(II) center with two different geometries, forming two geometric
isomers, with isomer 9a being the dominant one. 9a also appears
to be most promising for use in regio- and diastereoselective C–H
activation due to the full blockage of the 5th coordination site of
the Pt(II) center by the linker.
structure shown in Fig. 12. The structure of the isomer 9b was not
determined by X-ray diffraction analysis due to the lack of suitable
crystals. 1H NMR spectra confirmed the presence of 9b and that
the ratio of 9a : 9b is ~4.5:1. Isomer 9b is not favored presumably
because of the greater steric repulsion between the oxygen atom
and the PtPh2 moiety in 9b than that between the ethylene and the
PtPh2 in 9a.
The stability of the Pt(N,N-L)R2 complexes
Pt(II) complexes with a large chelate-ring size are not common.9
The bis(7-azaindol-1-yl) derivative ligands appear to have the
unusual ability/tendency to form Pt(II) chelate complexes despite
the unfavorable large chelate ring sizes. Nonetheless, the chelate
ligands in these Pt(II) complexes do appear to have great impact
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10 (a) J. R. Khusnutdinova, L. L. Newman, P. Y. Zavalij, Y.-F. Lam
and A. N. Vedernikov, J. Am. Chem. Soc., 2008, 130, 2174; (b) A. V.
Pawlikowski, A. D. Getty and K. I. Goldberg, J. Am. Chem. Soc., 2007,
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Smythe, B. S. Williams and K. I. Goldberg, Inorg. Chem., 2005, 44,
7732; (d) K. L. Arthur, Q. L. Wang, D. M. Bregel, N. A. Smythe, B. A.
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Chem. Soc., 2000, 122, 962; (g) B. S. Williams, A. W. Holland and K. I.
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Acknowledgements
We thank Dr. Rui-Yao Wang for his assistance in some of the
crystal structural work, and the Natural Sciences and Engineering
Council of Canada for financial support.
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