Interconversion between the enantiomers of chiral five-coordinate Me 3Pt(IV) complexes

Article abstract of DOI:10.1021/ic200889y

The dinuclear Me₂Pt(II) complexes of 3,4-bis(quinolin-8-yl) thiophene (1a), 3,4-bis(6 trifluoromethoxyquinolin-8-yl)thiophene (1b), and 3,4-bis(2-methylquinolin-8-yl)thiophene (1c) react with MeOTf (OTf = trifluoromethanesulfonate) to afford th

Full text of DOI:10.1021/ic200889y

ARTICLE
pubs.acs.org/IC
Interconversion between the Enantiomers of Chiral Five-Coordinate
Me3Pt(IV) Complexes
Runyu Tan and Datong Song*
Davenport Chemical Research Laboratory, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario,
Canada, M5S 3H6
S Supporting Information
b
ABSTRACT: The dinuclear Me2Pt(II) complexes of 3,4-bis-
(quinolin-8-yl)thiophene (1a), 3,4-bis(6 trifluoromethoxyqui-
nolin-8-yl)thiophene (1b), and 3,4-bis(2-methylquinolin-8-yl)
thiophene (1c) react with MeOTf (OTf = trifluoromethane-
sulfonate) to afford the corresponding chiral mononuclear
five-coordinate Me3Pt(IV) complexes [PtMe3(1a)]OTf (3a),
[PtMe3(1b)]OTf (3b), and [PtMe3(1c)]OTf (3c), respec-
tively. [PtMe3(1c)]BArF (3d) (where BArF = [B{C6H3-
4
4
3,5-(CF3)2}4]) has also been synthesized for structural study. While 3a appears to be symmetric in solution and asymmetric in
solid state, 3c and 3d are asymmetric in both solution and solid state. The chirality originates from interligand repulsion, rather than
any unsymmetrical ligand. Variable-temperature NMR and computational studies suggest a ligand-twisting isomerization pathway
for the interconversion of the enantiomers, rather than the rotational exchange of three CH3 ligands on the metal center.
’ INTRODUCTION
Five-coordinate Pt(IV) alkyl species have been proposed
as key intermediates in oxidative addition and reductive
elimination processes of Pt(II)/Pt(IV) chemistry.1À5 The
isolation of well-defined species of this type remains scarce.
To date, only a few five-coordinate Pt(IV) alkyl species have
been isolated by Goldberg,6À10 Tilley,11,12 and Wang.13,14
As analogues, a handful of five-coordinate Pt(IV) silyl-
hydride species have also been reported.12,15À17 Most known
examples involve an anionic bidentate ligand with nitrogen
donor atoms; only [Pt(BAB)Me3]OTf (where BAB = 1,2-
Previously, we reported a few quinoline-functionalized thio-
phene ligands, 3,4-bis(quinolin-8-yl)thiophene (1a) and 3,4-
bis(6-trifluoromethoxyquinolin-8-yl)thiophene (1b), which en-
abled the isolation and full characterization of the first group 10
metal complexes of π-bound thiophenes.18 Notably, the geome-
try of 1a and 1b is similar to that of Wang’s BAB ligand, but the
coordination mode displayed by 1a and 1b with Me2Pt(II) is very
different from that of BAB (Chart 1, left), i.e., the N,N-chelation
mode was not observed in 2a or 2b (the Me2Pt complexes of 1a
and 1b, respectively); rather, 2a and 2b feature the inverse
sandwich dinuclear structure, with a quinoline nitrogen donor
and a CC double bond of the thiophene ring bound to each
Pt(II) center (Chart 1, right). This structural difference could be
attributed to the better π-accepting ability and lower aromaticity
of a thiophene ring, compared to a benzene ring. Although the
structures of 2a and 2b differ much from that of [PtMe2(BAB)],
some common features do exist, e.g., one side of the Pt(II)
bis(N-7-azaindolyl)benzene and OTf = trifluoromethanesul-
fonate or triflate) by Wang involves a neutral bidentate BAB
ligand,13 the phenyl group of which is able to block the sixth
coordination site of the metal center and phenyl group itself
cannot act as the sixth ligand due to the large distance
between the phenyl group and Pt (3.18 Å revealed by X-ray
crystallography). In all the reported five-coordinate Me3Pt-
(IV) complexes, high solution-phase fluxionality was ob-
served, and this behavior was generally attributed to the
rotational exchange of Pt-CH3 on NMR time scale (eq 1). For
example, 1H NMR spectra of complexes [{(o-iPr2C6H3)NC-
(CH3)}2CH]PtMe3 (1, see eq 1), (AnIm)Pt(CH3)3 (2, see
eq 1), and (DtBPP)PtMe3 (3, eq 1) showed only one PtÀMe
resonance, indicating fast exchange of Pt-CH3 groups in
1
solution. H NMR spectra of complex (BAB)PtMe3 (4, eq 1)
showed two broad Pt-CH3 peaks at room temperature and a
coalescence of the two peaks at elevated temperature, suggesting
a slow exchange of the basal Pt-CH3 groups and the apical Pt-
CH3 group.
Received: April 27, 2011
Published: October 12, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Chart 1
Chemistry Department at the University of Toronto with a Model
PE 2400 C/H/N/S analyzer (PerkinÀElmer).
Synthesis of 3,4-bis(2-methyl-quinolin-8-yl)thiophene
(1c). A mixture of Pd(PPh3)4 (115 mg, 0.1 mmol), 3,4-dibromothio-
phene (240 mg, 1.0 mmol), 2-methyl-8-quinolineboronic acid (512
mg, 3.0 mmol), and K3PO4 (1.9 g, 9.0 mmol) was placed in a Schlenk
flask under argon. Degassed DMF (8 mL) and H2O (4.5 mL) was
added to the mixture and the flask was heated at 90 °C under argon
for 24 h. The mixture was then cooled and partitioned by CH2Cl2 and
water, and the organic layer was washed with water several times. The
aqueous layers were combined together and washed with a small
amount of CH2Cl2. The organic layers were then combined, washed
with brine, and dried over MgSO4. After filtration, the solvent was
removed under vacuum, and the crude product was purified through a
silica gel column using EtOAc/hexanes as an eluent and recrystallized
from CH2Cl2/hexanes to afford 1c as colorless crystals (135 mg, 40%
yield). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.78 (d, 3J = 8.0 Hz,
2H), 7.65 (s, 2H), 7.51 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 2H), 7.47 (dd,
3J = 8.0 Hz, 4J = 1.2 Hz, 2H), 7.21 (t, 3J = 8.0 Hz, 2H), 6.95 (d, 3J = 8.0
Hz, 2H), 2.23 (s, 6H). 13C NMR (CDCl3, 100 MHz, 25 °C): δ 157.8,
145.3, 141.1, 136.5, 135.5, 130.5, 126.4, 126.3, 125.0, 124.8, 121.2,
24.8. Anal. Calcd. for C24H18N2OS 1/2CH2Cl2: C, 71.96; H, 4.68; N
3
6.85. Found: C, 71.56; H, 4.96; N, 7.33.
Synthesis of [Pt2Me4(1c)] (2c). 1c (32 mg, 0.059 mmol) and
[Pt(CH3)2(SMe2)]2 (35 mg, 0.06 mmol) were dissolved in 3 mL of
benzene, and the mixture was stirred for 2 h at ambient temperature. The
solvent was removed under reduced pressure and the residue was
recrystallized from benzene/hexanes to afford 2c as pale-yellow crystals
(56 mg, 95% yield). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.75 (d, 3J =
8.0 Hz, 2H), 8.17 (d, 3J = 8.0 Hz, 2H), 7.54 (d, 3J = 8.0 Hz, 2H), 7.44 (d,
3J = 8.0 Hz, 2H), 7.28 (t, 3J = 8.0 Hz, 2H), 5.77 (s, satellite, 2JPtÀH = 50.0
Hz, 2H), 2.97 (s, 6H), 1.04 (s, satellite, 2JPtÀH = 88.8 Hz, 6H), À0.24 (s,
Figure 1. Space-filling model of the structure of 2a, showing that one
side of the Pt1 coordination plane is partially blocked by the hydrogen
atom highlighted in green.
2
satellite, JPtÀH = 80.8 Hz, 6H). 13C{1H} NMR (CDCl3, 100 MHz,
25 °C): δ 160.0, 148.6, 136.8, 136.4, 131.3, 127.8, 126.6, 125.3, 124.1,
coordination plane is blocked by the chelating ligand. In the
case of [PtMe2(BAB)], it is the phenylene linker that blocks
one side of the Pt(II) coordination plane, while in 2a and 2b, it
is the CÀH bond of an adjacent quinoline ring that partially
blocks one side of the Pt(II) coordination plane (see Figure 1).
Such partial blockage prompted us to study the oxidation of 2a
and 2b, hoping to obtain dinuclear complexes of five-coordi-
nate Pt(IV)Me3. To our surprise, mononuclear five-coordinate
Pt(IV)Me3 complexes, formulated as [PtMe3(1a)]OTf (3a)
and [PtMe3(1b)]OTf (3b), were obtained instead. More
interestingly, compound 3a displays a chiral structure in the
solid state.19 These chiral five-coordinate complexes display a
distinct type of solution behavior, compared to the related
literature compounds. Both experimental and computational
results are presented herein.
104.1, 90.9, 26.5, 4.2, À9.1. Anal. Calcd for C28H30N2SPt2 1/4C6H6: C,
3
42.37; H, 3.79; N 3.35. Found: C, 41.99; H, 4.11; N, 2.79.
Synthesis of [PtMe3(1a)]OTf (3a). 2a (10 mg, 0.01 mmol) and
1a (5 mg, 0.01 mmol) were dissolved in 2 mL of dichloromethane, and
MeOTf (2.19 μL, 0.02 mmol) was added into this solution. The mixture
was stirred for 3 h at ambient temperature, and the solvent was removed
under reduced pressure. The crude product was recrystallized by vapor
diffusion of pentane into THF/DCM solution to afford 3a as colorless
crystals (yield 75%). 1H NMR (CD2Cl2, 400 MHz, 25 °C): δ 8.47 (dd,
3J = 4.8 Hz, 4J = 1.6 Hz, 2H), 8.35 (dd, 3J = 8.4 Hz, 4J = 1.6 Hz, 2H), 7.84
(s, 2H), 7.79 (dd, 3J = 6.8 Hz, 3J = 2.8 Hz, 2H), 7.52 (dd, 3J = 8.4 Hz, 3J =
2
2.8 Hz, 2H), 7.48À7.44 (m, 4H), 1.65 (s, satellite, JPtÀH = 76.8 Hz,
3H), 0.53 (s, satellite, 2JPtÀH = 67.6 Hz, 6H). 13C{1H} NMR (CD2Cl2,
100 MHz, 25 °C): δ 150.5, 146.5, 140.8, 136.5, 133.6, 131.7, 130.9,
130.5, 129.8, 128.1, 123.4, 13.6, À3.7 (CF3 carbon was not observed).
19F (CD2Cl2, 376 MHz, 25 °C): δ À78.84. Anal. Calcd for
C26H23N2O3F3S2Pt 1/2THF: C, 44.03; H, 3.56; N 3.67. Found: C,
3
’ EXPERIMENTAL SECTION
43.85; H, 3.65; N, 3.65.
Synthesis of [PtMe3(1b)]OTf (3b). The same procedure as above
was used, using 2b and 1b as the starting material (yellow oil, yield 85%).
1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.86 (dd, 3J = 4.8 Hz, 4J = 1.6 Hz,
2H), 8.38 (dd, 3J = 8.4 Hz, 4J = 1.6 Hz, 2H), 7.96 (s, 2H), 7.88 (dd, 3J =
8.0 Hz, 3J = 4.0 Hz, 2H), 7.66 (d, 4J = 1.2 Hz, 2H), 7.31 (d, 4J = 2.0 Hz,
General. Unless otherwise stated, all preparations and manipula-
tions were performed in air and all reagents were purchased from
commercial sources and used without further purifications.
[Pt(CH3)2(SMe2)]2,20 2-methyl-8-quinolineboronic acid,21 3,4-bis-
(quinolin-8-yl)thiophene (1a), 3,4-bis(6-trifluoromethoxyl-quino-
lin-8-yl)thiophene (1b), [Pt2Me4(1a)] (2a), and [Pt2Me4(1b)]
(2b) were prepared according to the literature procedures.18 NMR
spectra were recorded on a Varian 400 spectrometer, or a Bruker
Avance 400 spectrometer. Both 1H and 13C NMR spectra were
referenced relative to the solvent’s residual signals but are reported
relative to Me4Si. Elemental analyses were performed in the
2H), 1.89 (s, satellite, 2JPtÀH = 76.8 Hz, 3H), 0.61 (s, satellite, 2JPtÀH
=
67.6 Hz, 6H). 13C{1H} NMR (CD2Cl2, 100 MHz, 25 °C): δ 151.4,
147.3, 144.6, 140.8, 137.5, 135.0, 133.1, 131.0, 130.2, 124.8, 119.2, 14.1,
À3.4 (CF3 carbons were not observed). 19F (CDCl3, 376 MHz, 25 °C):
δ À78.14 (s, 3F), À58.13 (s, 6F). ESI-MS: calcd for C27H21F6N2O2PtS:
746.03; found: 746.03 [M]+.
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Table 1. Crystallographic Data
1c 1/2CH2Cl2
2c C6H6
3a 1/2THF
3d
3
3
3
formula
C
24H18ClN2S
C34H36N2Pt2S
894.89
C28H27F3N2O3.5PtS2
763.73
C59H39BF24N2PtS
1469.88
formula weight, FW
temperature, T (K)
space group
unit-cell parameters
a (Å)
408.93
150(2)
C2/c
150(2)
150(2)
150(2)
C2/c
P21212
P21/n
18.9127(5)
12.0454(3)
18.9174(5)
90
36.6641(14)
9.8077(4)
19.2256(8)
90
12.0881(8)
24.2781(15)
9.4344(4)
90
21.8015(7)
12.8443(4)
22.2233(7)
90
b (Å)
c (Å)
α (deg)
β (deg)
113.3960(10)
90
119.9230(10)
90
90
114.1690(10)
90
γ (deg)
90
volume, V (Å3)
3955.27(18)
8
5991.8(4)
8
2768.8(3)
2
5677.6(3)
4
Z
density, Dc (g cmÀ3
μ (mmÀ1
)
1.373
1.984
1.832
1.720
)
0.312
9.424
5.275
2.629
no. reflns collcd
no. indept reflns
17629
25433
13502
50524
4915
6538
6305
13060
goodness of fit (GOF) on F2
1.047
1.129
1.039
1.021
R [I > 2σ (I)]
R1 = 0.0415
wR2 = 0.0978
R1 = 0.0612
wR2 = 0.1075
R1 = 0.0274
wR2 = 0.0646
R1 = 0.0314
wR2 = 0.0663
R1 = 0.0388
wR2 = 0.0869
R1 = 0.0526
wR2 = 0.1005
R1 = 0.0632
wR2 = 0.1487
R1 = 0.0978
wR2 = 0.1658
R (all data)
Scheme 1. Syntheses of 3a and 3b
Synthesis of [PtMe3(1c)]OTf (3c). The same procedure as above
was used, using 2c and 1c as the starting material (white solid, yield
90%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.48 (d, 3J = 8.0 Hz, 1H),
8.17 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 1H), 8.11 (d, 3J = 8.0 Hz, 1H), 7.97 (d,
4J = 4.0 Hz, 1H), 7.91 (d, 4J = 4.0 Hz, 1H), 7.88 (s, 1H), 7.77À7.73 (m,
2H), 7.67 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 1H), 7.15À7.08 (m, 2H), 6.67
1H), 7.99 (d, 4J = 4.0 Hz, 1H), 7.90 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 1H),
7.82 (d, 4J = 4.0 Hz, 1H), 7.79À7.75 (m, 1H), 7.73 (s, 8H), 7.68À7.63
(m, 2H), 7.56 (s, 4H), 7.14À7.10 (m, 1H), 7.04 (d, 3J = 8.0 Hz, 1H),
6.74 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 1H), 2.98 (s, 3H), 1.61 (s, satellite,
2JPtÀH = 80.4 Hz, 3H), 1.46 (s, 3H), 1.06 (s, satellite, 2JPtÀH = 70.4 Hz,
2
3H), 0.30 (s, satellite, JPtÀH = 67.6 Hz, 3H). Anal. Calcd for
(dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 1H), 3.00 (s, 3H), 1.60 (s, satellite, 2JPtÀH
=
C59H39BN2F24SPt: C, 48.21; H, 2.67; N 1.91. Found: C, 48.34; H,
2.77; N, 1.92.
80.0 Hz, 3H), 1.49 (s, 3H), 1.06 (s, satellite, 2JPtÀH = 70.4 Hz, 3H), 0.25
(s, satellite, 2JPtÀH = 66.8 Hz, 3H). 13C{1H} NMR (CDCl3, 100 MHz,
25 °C): δ 163.9, 163.7, 145.1, 144.7, 144.0, 141.7, 140.5, 140.2, 134.9,
133.5, 130.7, 130.5, 129.2, 128.6, 128.1, 128.0, 127.5, 127.0, 126.8, 126.3,
125.2, 122.8, 25.1, 25.0, 8.3, 0.1, À1.0 (CF3 carbon was not observed).
X-ray Diffraction Analyses. X-ray quality crystals of 1c were
obtained by top-layering the CH2Cl2 solution with hexanes; those of
2c were obtained by diffusing pentane into benzene/CH2Cl2 solution,
and those of 3a and 3d were obtained by diffusing pentane into THF
solution. All crystals were mounted on the tip of a MiTeGen Micro-
Mount and the single-crystal X-ray diffraction data were collected on a
Bruker Kappa Apex II diffractometer. All data were collected with
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K
controlled by an Oxford Cryostream 700 series low-temperature
system. (Crystallographic data are given in Table 1.) The diffraction
data were processed with the Bruker Apex 2 software package.22 All
structures were solved by direct methods and refined using SHELXTL
V7.00.23 Compound 1c crystallized in the monoclinic space group
Anal. Calcd for C28H27N2O3F3S2Pt 1/3THF: C, 45.18; H, 3.83; N 3.59.
3
Found: C, 44.99; H, 4.28; N, 3.33.
Synthesis of [PtMe3(1c)]BArF (3d). 3c (22 mg, 0.029 mmol)
4
and NaBArF4 (27.6 mg, 0.029 mmol) were stirred in 5 mL of DCM at
room temperature for 4 h, and then the solution was filtered and the
solvent was removed in vacuo. The residue was recrystallized in
benzene/pentane mixture to afford the product as colorless crystalline
solids (95% yield). 1H NMR (CD2Cl2, 400 MHz, 25 °C): δ 8.38 (d, 3J =
8.4 Hz, 1H), 8.19 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 1H), 8.10 (d, 3J = 8.0 Hz,
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Figure 2. (a) Molecular structure of 3a with thermal ellipsoids plotted at 50% probability; all hydrogen atoms and OTfÀ are omitted for clarity.
(b) Space-filling model (projection down the C23ÀPt1 vector), showing the short contact between the apical Pt-methyl (red) and the quinoline protons
(green).
Table 2. Selected Bond Lengths and Angles
Bond Lengths
2c
3a
3d
bond pairing
bond length (Å)
bond pairing
bond length (Å)
bond pairing
bond length (Å)
Pt(1)ÀC(27)
Pt(1)ÀC(28)
Pt(1)ÀN(2)
Pt(1)ÀC(2)
Pt(1)ÀC(1)
C(1)ÀC(2)
2.059(6)
2.041(5)
2.143(4)
2.238(5)
2.248(5)
1.421(7)
Pt(1)ÀC(23)
Pt(1)ÀC(24)
Pt(1)ÀC(25)
Pt(1)ÀN(1)
Pt(1)ÀN(2)
C(1)ÀC(2)
C(2)ÀC(3)
S(1)ÀC(1)
S(1)ÀC(4)
2.041(7)
2.067(8)
2.036(8)
2.265(6)
2.162(6)
1.389(10)
1.365(10)
1.680(9)
1.712(9)
Pt(1)ÀC(25)
Pt(1)ÀC(26)
Pt(1)ÀC(27)
Pt(1)ÀN(1)
Pt(1)ÀN(2)
C(1)ÀC(2)
C(2)ÀC(3)
S(1)ÀC(1)
S(1)ÀC(4)
2.043(8)
2.043(8)
2.053(8)
2.276(5)
2.285(6)
1.409(11)
1.350(11)
1.685(8)
1.690(9)
Bond Angles
2c
3a
3d
bond angle
value (deg)
bond angle
value (deg)
bond angle
value (deg)
C(27)ÀPt(1)ÀC(28)
C(27)ÀPt(1)ÀN(2)
C(28)ÀPt(1)ÀN(2)
C(27)ÀPt(1)ÀC(2)
C(28)ÀPt(1)ÀC(2)
N(2)ÀPt(1)ÀC(2)
C(27)ÀPt(1)ÀC(1)
C(28)ÀPt(1)ÀC(1)
N(2)ÀPt(1)ÀC(1)
C(2)ÀPt(1)ÀC(1)
84.4(4)
94.8(2)
C(25)ÀPt(1)ÀC(23)
C(25)ÀPt(1)ÀC(24)
C(23)ÀPt(1)ÀC(24)
C(25)ÀPt(1)ÀN(2)
C(23)ÀPt(1)ÀN(2)
C(24)ÀPt(1)ÀN(2)
C(25)ÀPt(1)ÀN(1)
C(23)ÀPt(1)ÀN(1)
C(24)ÀPt(1)ÀN(1)
N(1)ÀPt(1)ÀN(2)
87.8(4)
86.0(4)
82.1(4)
92.7(3)
179.3(3)
97.5(3)
176.1(3)
95.4(3)
92.4(3)
84.0(2)
C(26)ÀPt(1)ÀC(25)
C(26)ÀPt(1)ÀC(27)
C(25)ÀPt(1)ÀC(27)
C(26)ÀPt(1)ÀN(1)
C(25)ÀPt(1)ÀN(1)
C(27)ÀPt(1)ÀN(1)
C(26)ÀPt(1)ÀN(2)
C(25)ÀPt(1)ÀN(2)
C(27)ÀPt(1)ÀN(2)
N(1)ÀPt(1)ÀN(2)
88.7(4)
85.5(4)
81.1(4)
173.7(3)
94.1(3)
89.3(3)
95.0(3)
100.0(3)
178.8(3)
90.1(2)
175.8(2)
165.7(2)
100.7(2)
80.99(18)
155.8(2)
99.8(2)
79.23(17)
36.95(18)
C2/c, with one molecule per asymmetric unit along with one-half
molecule of dichloromethane. 2c crystallized in the monoclinic space
group C2/c, with one molecule per asymmetric unit along with one
benzene molecule. 3a crystallized in the orthorhombic space group
P21212 with one molecule per asymmetric unit along with one-half
molecule of THF. Note: although the bulk sample of 3a cannot be
enantio-pure, the crystal we picked happened to be enantio-pure. 3d
crystallized in the monoclinic space group P21/n with one molecule
per asymmetric unit. The disordered benzene molecule in the lattice of
successfully. All non-hydrogen atoms were refined anisotropically,
except for those involved in the disordered portions. In all structures,
hydrogen atoms bonded to carbon atoms were included in calculated
positions and treated as riding atoms.
DFT Calculations. All calculations were performed using the
Gaussian 09 software package24 and the B3LYP method.25,26 Platinum
was treated with the lanl2dz basis set with effective core potential, while
other elements were treated with a 6-31G* basis set. All structures were
optimized in the gas phase. Vibrational frequency analyses were
performed on all optimized structures to obtain thermodynamic data.
2c, the OTfÀ counterion of 3a, and BArFÀ have been modeled
4
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Figure 3. Low-temperature 1H NMR spectra of 3a (CD2Cl2, 400 MHz).
Figure 4. (a) Calculated interconversion pathway via ligand twisting: optimized structures of one enantiomer of 3a (left), Cs symmetric transition state
(middle), and the other enatiomer of 3a (right). (b) Calculated pathway of rotational exchange of three methyl ligands: optimized structure of 3a (left),
transition state (middle), rotation product (right). Note: pathway (b) does not change the chirality of 3a; the three methyl ligands in pathway (b) are
highlighted in red, green, and blue, to illustrate the rotational process. The OTfÀ counterion was not included in the calculations.
The atomic coordinates and free energies of all three optimized
structures are listed in the Supporting Information.
property of the triflate anion. When complex 2a is treated with
2 equiv of MeOTf, complete consumption of the starting
materials can be observed within 2 h at ambient temperature.
The resulting reaction mixture contains only [Me3PtOTf]4
(a singlet in the 1H NMR spectrum at 1.50 ppm with 2JPtÀH of
82 Hz)28 and a new species, 3a. The stoichiometry of the
reaction requires the formula of 3a to be Me3Pt(2a)OTf,
which can also be produced from an independent reaction
between 1a and [Me3PtOTf]4 in a 4:1 molar ratio. Alterna-
tively, 2a can be cleanly transformed to 3a by reacting with 1
equiv of 1a and 2 equiv of MeOTf in one pot (see Scheme 1).
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Complex 3. It has been
reported by Wang that [PtMe2(BAB)] could react with 1
equiv of MeI to form [PtMe3I]4 and the free ligand BAB,
because of the weak binding of BAB to Pt(IV)Me3.13 Inter-
estingly, no reaction was observed between 2 and MeI after
prolonged reaction time, even at elevated temperature. Since
the oxidative addition of MeI typically occurs via an SN2
fashion,27 we reasoned that the partial blockage of one side of
the Pt(II) coordination plane (Figure 1) could contribute to
such inertness of 2 toward MeI. Also, the strong interaction
between the Pt(II) and the CC double bonds of the thiophene
may make the metal centers less nucleophilic, because of
significant electron back-donation.18
1
At ambient temperature, the H NMR spectrum of 3a in
CD2Cl2 shows two singlets with platinum satellites (apical
2
methyl: 1.65 ppm, JPtÀH = 76.8 Hz; equatorial methyl
2
ligands: 0.53 ppm, JPtÀH = 67.6 Hz) in the aliphatic region
and only one set of quinolinyl and thiophene peaks in the
aromatic region, indicating a symmetrical or fluxional structure
in solution. 13C NMR spectrum also supported the symmetrical/
fluxional structure. The reaction of 3b with MeOTf is similar to
that of 3a.
To make the oxidation more favorable, a stronger oxidant
(MeOTf) was then tested. MeOTf is broadly used as a strong
methylating reagent and has proven extremely powerful in
Pt(II)/Pt(IV) chemistry, thanks to the relatively noncoordinating
In contrast to the symmetric structure in solution disclosed
by NMR spectroscopy, X-ray crystallography reveals an
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Figure 5. Energetics of the isomerization pathways of 3a, relative to the free energy of the starting material (in kcal/mol, at 298 K). Methyl ligands are
highlighted in different colors to illustrate the difference between the two processes. All the hydrogen atoms were omitted for clarity.
Figure 6. Molecular structure of 1c (left) and 2c (right) with thermal ellipsoids plotted at 50% probability; the solvent molecule and all the hydrogen
atoms are omitted for clarity.
asymmetrical structure of 3a in the solid state! As shown in
Figure 2a, 3a is a mononuclear five-coordinate Pt(IV) com-
plex. Each cationic Pt center adopts a distorted square-
pyramidal geometry, with two cis nitrogen donor atoms of
the quinolinyl rings and two methyl ligands occupying the four
basal coordination sites. The third methyl ligand is situated at
the apical position. The C23ÀPt1 vector points toward C2 of
the thiophene ring: the distance between Pt1 and C2
(2.532(8) Å) is much shorter than that between Pt1 and C3
(2.932(7) Å). The two quinoline-to-thiophene dihedral angles
are ∼78.7° and ∼49.8°, respectively. Accordingly, the
Pt1ÀN1 and Pt1ÀN2 bond lengths are 2.265(6) and 2.162(6)
Å, respectively, and the two basal Pt-methyl bond (Pt1ÀC24 and
Pt1ÀC25) lengths are 2.067(8) and 2.036(8) Å, respectively.
The apical Pt-methyl bond (Pt1ÀC23) length is 2.041(7) Å.
Similarly, the bond lengths of PtÀCbasal and PtÀCapical in five-
coordinate Pt-Me3 complexes reported by Wang13 and
Goldberg6,9 are also statistically indistinguishable, likely due to
the insignificant trans influence of the nitrogen donor atoms.
(Selected bond lengths and bond angles for 2c, 3a, and 3d are
given in Table 2.) The crystal structure also shows that the apical
methyl group is in short contact with two CÀH bonds from the
2-positions of the quinoline rings (Figure 2b). It appears that the
repulsion between the apical methyl ligand and the two quinoline
CÀH bonds prevents the formation of a Cs symmetric structure.
The solution behavior of 3a observed via NMR experiments is
most likely the result of a fast interconversion of the two
enantiomers, which involves twist around the CÀC linkages
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between the thiophene ring and the quinoline rings (eq 2). This
mechanism is reminiscent of the RayÀD^utt twist observed in
pseudo-octahedral complexes.29
2.7 kcal/mol (see Figure 5), which is consistent with the VT
NMR study. The optimized ground- and transition-state struc-
tures are shown in Figure 4a.
To compare our proposed isomerization process with the
rotational exchange of the three CH3 ligands, we located the
transition state of the rotational exchange process (Figure 4b).
The calculated ΔGq for the rotational exchange process is 17.0
kcal/mol (Figure 5), which is much higher than that of the
interconversion of two enantiomers of 3a and is inconsistent
with our NMR results. Our calculations for the model
species [PtH3(1a)]+, where the interligand repulsion is ex-
pected to be smaller than that of 3a, also suggest a much
smaller energy barrier for the ligand twisting mode (1.5 kcal/mol),
compared to the rotational exchange mode (14 kcal/mol; see
the Supporting Information).
Syntheses of 3c and 3d. Since the barrier of the interconver-
sion of two enantiomers of 3a is too small to measure using
variable-temperature NMR, we sought to increase the barrier by
creating more repulsion in the Cs symmetric transition state. As
mentioned above, in the Cs conformation of 3a, the CÀH bonds
at the 2-positions of the quinoline rings are pointing directly at
the apical methyl group, causing interligand repulsions in the
transition state. We envisioned that if the CÀH bonds at
2-position of the quinoline rings are replaced with bulkier groups,
e.g., introducing a methyl group at 2-position of the quinoline
rings, greater repulsion could be achieved. To test our hypoth-
esis, we synthesized ligand 3,4-bis(2-methylquinolin-8-yl)thio-
phene (1c), using the Suzuki coupling reaction (see Scheme 2).
NMR spectra indicated a symmetric structure in solution, and the
solid-state structure of 1c was also confirmed by X-ray crystal-
lography (see Figure 6, left). Similar to 1a and 1b, 1c reacts with
[Pt2Me4(μ-SMe2)2] to afford a dinuclear inverse sandwich
complex [Pt2Me4(1c)] (2c), whose structure has been con-
firmed with NMR spectroscopy and single-crystal X-ray diffrac-
tion (Figure 6, right). As shown in Figure 3, the installation of
methyl groups on the quinoline rings did not significantly alter
the coordination geometry of the Pt(II) centers, compared to
that of 2a and 2b, and, consequently, did not impose a dramatic
impact on the reactivity of 2c toward MeOTf. Thus, compound
2c reacts with 1 equiv of 1c and 2 equiv of MeOTf in one pot,
producing the corresponding Pt(IV) complex formulated as
[PtMe3(1c)]OTf (3c). Alternatively, 3c can also be prepared
To measure the barrier of such an interconversion, we carried out
variable-temperature NMR experiments. Unfortunately, the vari-
able-temperature NMR experiments of 3a (Figure 3) show that
the interconversion process remains fast, even at 180 K in
CD2Cl2 solution, as evidenced by only one set of quinoline
signals and two singlet signals from the three methyl groups,
suggesting a small energy barrier.
Computational Studies. Our computational studies reveal
that the Cs conformation of 3a is not a local minimum, but a
saddle point on the energy surface, i.e., a transition state. The
optimized structure of 3a in Cs point group is shown in the
middle of Figure 4a. The two quinoline CÀH bonds are
pointing directly at the apical methyl group on the Pt center,
and any twist around the CÀC linkages between the thiophene
ring and the quinoline rings would lead to the observed chiral
structure of 3a and, thus, reduce the repulsions. The animated
imaginary frequency indeed shows the twisting around the
CÀC linkages between the thiophene ring and the quinoline
rings. The two enantiomers of 3a can interconvert through the
Cs symmetric transition state via the twisting motion. The
calculated ΔGq value for such an interconversion process is
Scheme 2. Synthesis of 1c
Figure 7. (a) Molecular structure of 3d with thermal ellipsoids plotted at 50% probability; all hydrogen atoms and BArF are omitted for clarity.
4
(b) Space-filling model (projection down the C25ÀPt1 vector) showing the short contact between the Pt-methyl groups and the quinoline methyl
groups (basal Pt-methyl groups are highlighted in green, apical Pt-methyl are given in blue, and quinoline-methyl groups are shown in red).
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was estimated to be 18 kcal/mol for 3d (see the Supporting
Information). Attempts to observe the coalescence at higher
temperatures led to extensive decomposition of the complex.
’ CONCLUSION
In summary, we have prepared a series of five-coordinate
Pt(IV) alkyl complexes 3aÀ3d, based on quinolinyl-substituted
thiophenes. These complexes can be prepared either directly
from Pt(IV) starting material or through the oxidation of the
dinuclear Pt(II) complexes 2aÀ2c, respectively. Complex 3a
shows chiral structure in the solid state, but the two enantiomers
undergo fast interconversion in solution, even at 180 K. We have
also demonstrated that the interconversion of the two enantio-
mers can be slowed by introducing steric bulk at the 2-position of
the quinoline ring. For example, the interconversion between the
enantiomers of 3d is slow, with respect to the NMR time scale at
elevated temperatures. The chirality of complexes 3aÀ3d origi-
nates from the repulsion between the chelating ligand and the
apical methyl group rather than any unsymmetrical ligand.
Variable-temperature NMR experiments and density functional
theory (DFT) calculations suggest a novel isomerization mode of
the two enantiomers through ligand twisting around the CÀC
linkages between the thiophene ring and the quinoline rings.
Figure 8. High-temperature 1H NMR spectra of 3d (toluene-d8,
400 MHz). Aromatic region was omitted for clarity.
from the reaction between 1c and [PtMe3OTf]4 in a 4:1 ratio. At
1
ambient temperature, the H NMR spectrum of 3c in CDCl3
shows three singlets with platinum satellites and two singlets
from the quiolineÀmethyl groups in the aliphatic region and
two sets of quinoline and thiophene signals in the aromatic
region, indicating an unsymmetrical structure in solution as we
designed.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data for 1c,
b
2c, 3a, and 3d in CIF format, and line-shape analysis, and
coordinates and energies of DFT optimized structures of 3a
and transition states. This material is available free of charge via
the Internet at http://pubs.acs.org.
Attempts to obtain X-ray-quality crystals of 3c always resulted
in polycrystalline solids that were not suitable for single-crystal
X-ray diffraction analyses. Therefore, a simple salt metathesis
reaction between 3c and NaBArF was carried out to obtain a
4
’ AUTHOR INFORMATION
better crystal without altering the Pt-containing core structure.
After stirring a 1:1 ratio mixture of 3c and NaBArF in DCM
Corresponding Author
*E-mail: dsong@chem.utoronto.ca.
4
for several hours, the 1H NMR experiment indicated the
quantitative formation of a new species 3d with the asymmetric
[Pt(IV)(1c)Me3]+ core retained. Colorless X-ray quality crystals
were obtained after crystallization of the crude product. The
structure of the Pt-containing complex cation is shown in
Figure 7. The C25ÀPt1 vector is pointing toward C2 of the
thiophene ring. The Pt1ÀC2 distance (2.471(8) Å) is much
shorter than Pt1ÀC3 distance (2.821(8) Å). The two quinolineÀ
thiophene dihedral angles are ∼68.0° and ∼37.5°, respectively,
which are smaller than those in 3a, indicating that more ligand
twist is needed to minimize interligand repulsions. The space-
filling model 3d (Figure 7b) clearly showed a more sterically
encumbered structure, compared to that of 3a. The steric effect is
presumably the main contributor for nonfluxional solution
behavior of 3c/3d at room temperature.
’ ACKNOWLEDGMENT
This research is supported by grants to D.S. from the Natural
Science and Engineering Research Council (NSERC) of Canada,
the Canadian Foundation for Innovation, the Ontario Research
Fund, and the ERA program of Ontario. R.T. is grateful for a
postgraduate scholarship from the OGS program of Ontario.
Prof. Ulrich Fekl is thanked for helpful discussions.
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’ NOTE ADDED AFTER ASAP PUBLICATION
This paper was published on the Web on October 12, 2011,
with textual errors in the Introduction due to production error.
The corrected version was reposted on October 13, 2011.
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