Platinum-Mediated Arene C−H Activation Reactions
A R T I C L E S
CD3CN (0.1 mL). 1H NMR (200 MHz, CD2Cl2): δ 2.02 (s, 3 H, Nd
CMe), 2.12 (s, 6 H, Ar-Me), 2.13 (s, 3 H, NdCMe′), 2.36 (s, 6 H,
Ar-Me′), 6.57-6.71 (m, 5 H, Ph), 6.83-6.89 (m, 3 H, Ar-H), 7.19-
7.32 (m, 3 H, Ar-H′).
prevent premature mixing of 1 and acid.) The contents were cooled to
-78 °C and carefully layered with a solution of HBF4‚Et2O (13 µL,
ca. 5 equiv) in a mixture of Et2O-d10 (73 µL, 1 M) and CD2Cl2 (120
µL), giving a total volume of ca. 700 µL. The tube was capped and
kept at -78 °C. The tube was shaken in order to mix the reactants
immediately before it was transferred to a precooled NMR probe. A
pale yellow solution was immediately obtained. Particular care was
taken to minimize any heating of the sample, which appeared
homogeneous at all times. The product distributions observed at -40
°C are not very different from those at -78 °C, suggesting that
unintentional heating did not perturb the outcome of the experiments.
Characterization of [ArNdC(Me)-C(Me)dNAr]Pt(C6H6)(Ph)+
NMR-Tube Generation of [ArNdC(Me)-C(Me)dNAr]Pt(C6D5)-
(CD3CN)+BF4 [Ar ) 2,6-Me2C6H3] (2a-d5‚BF4). After the EXSY
-
measurements on the protonated 1a-d10 (see below), the sample was
heated to room temperature, and CD3CN (100 µL) was added. 1H NMR
(500 MHz, CD2Cl2): δ 2.03 (s, 3 H, NdCMe), 2.14 (s, 9 H, Ar-Me
and NdCMe′), 2.37 (s, 6 H, Ar-Me′), 6.85-7.02 (m, 3 H, Ar-H), 7.20-
7.36 (m, 3 H, Ar-H′).
[ArNdC(Me)-C(Me)dNAr]Pt(Ph)(MeCN)+OTf- [Ar ) 2,4,6-
Me3C6H2] (2b‚OTf). The compound was prepared analogous to 2a
above from HOTf (10 µL, 012 mmol) and 1b (50 mg, 0.075 mmol).
The product was recrystallized from dichloromethane/pentane (55 mg,
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Cations as Their BF4 Salts (4‚BF4). The π-benzene cations were
prepared by the protonation procedure described above, and charac-
1
terization was done in situ by H NMR, NOESY, EXSY, COSY and
1
4
94%). H NMR (300 MHz, CD2Cl2): δ 1.89 (s, J(195Pt-H) ) 11.3
Hz, 3 H, NdCMe), 2.10 (s, 6 H, o-Ar-Me), 2.11 (s, 3 H, NdCMe′),
2.17 (s, 3 H, p-Ar-Me′), 2.20 (s, 3 H, MeCN), 2.35 (s, 6 H, o-Ar-Me′),
2.38 (s, 3 H, p-Ar-Me), 6.60-6.69 (m, 5 H, Ph), 6.71 (m, 2 H, Ar-H),
7.11 (m, 2 H, Ar-H′). Anal. Calcd for C31H36F3N3O3PtS: C, 47.57; H,
4.64; N, 5.37; Pt, 24.92. Found: C, 46.70; H, 4.71; N, 5.10; Pt, 24.09.
ESI MS m/z: 633.3 (M+).
HMQC.
[ArNdC(Me)-C(Me)dNAr]Pt(C6H6)(Ph)+ [Ar ) 2,6-Me2C6H3]
1
(4a). H NMR (500 MHz, T ) -43 °C, CD2Cl2): δ 1.99 (s, 3 H,
NCMe), 2.12 (s, 6 H, Ar-Me), 2.14 (s, 3 H, NCMe), 2.44 (s, 6 H, Ar-
Me), 6.11-6.28 (m, 2 H, Ph-Ho), 6.71 (m, 3 H, Ar-Hm,p), 6.85 (s, 6 H,
C6H6), 7.25 (m, 3 H, Ar-Hm,p); λmax (CH2Cl2/MeCN(10% v/v)) 391
(4.27), 320 (8.14).The spectrum of 4a-d10 was identical except that
the benzene and phenyl ligands were only ca. 10% of the intensity
seen in 4a.
NMR-Tube Generation of [ArNdC(Me)-C(Me)dNAr]Pt(Ph)-
(CD3CN)+BF4- [Ar ) 4-Br-2,6-Me2C6H2] (2c‚BF4). HBF4‚Et2O (10
µL) was added to an NMR tube containing 1c (3 mg) in CD2Cl2 (400
µL) with added CD3CN (100 µL). Shaking turned the deep purple
[ArNdC(Me)-C(Me)dNAr]Pt(C6H6)(Ph)+ [Ar ) 2,4,6-Me2C6H2]
1
(4b). H NMR (500 MHz, T ) -18 °C, CD2Cl2): δ 1.99 (ds, 6 H,
1
NCMe + Ar-Mep), 2.06 (s, 6 H, Ar-Meo), 2.14 (s, 3 H, NCMe′), 2.31
solution bright orange. H NMR (200 MHz, CD2Cl2): δ 2.05 (s, 3 H,
′
4J(195Pt-H) ) 10.8 Hz, NdCMe), 2.11 (s, 6 H, Ar-Me), 2.14 (s 3 H,
NdCMe′), 2.35 (s, 6 H, Ar-Me′), 6.67 (s, 5 H, Ph-Hm,o,p), 7.03 (s, 2 H,
Ar-H), 7.33 (s, 6 H, C6H6), 7.45 (s, 2 H, Ar-H′). ES MS m/z: 764.2
(M+).
(s, 3 H, Ar-Mep′), 2.40 (s, 6 H, Ar-Meo ), 6.16 (m, 4 H, Ph-Ho,m), 6.27
(m, 1 H, Ph-Hp), 6.46 (s, 2 H, Ar-Hm), 6.88 (sb, 6 H, C6H6), 7.09 (s, 2
H, Ar-H′m).
[ArNdC(Me)-C(Me)dNAr]Pt(C6H6)(Ph)+[Ar)4-Br-2,6-Me2C6H2]
(4c). 1H NMR (500 MHz, T ) -48 °C, CD2Cl2): δ 2.01 (s 3 H,
NCMe), 2.08 (s, 6 H, Ar-Me), 2.16 (s, 3 H, NCMe′), 2.42 (s, 6 H,
Ar-Me′), 6.17 (sb, 4 H, Ph-Ho,m), 6.31 (sb, 1 H, Ph-Hp), 6.78 (s, 2 H,
Ar-Hm), 6.86 (s, 6 H, C6H6), 7.42 (s, 2 H, Ar-Hm′).
NMR-Tube Generation of [ArNdC(Me)-C(Me)dNAr]Pt(Ph)-
(CD3CN)+BF4- [Ar ) 3,5-Me2C6H3] (2d‚BF4). HBF4‚Et2O (5 µL) was
added to an NMR tube containing 1d (3 mg) in CD2Cl2 (400 µL) with
added CD3CN (100 µL). Shaking turned the deep purple solution bright
orange. 1H NMR (200 MHz, CD2Cl2): δ 2.07 (s, 6 H, Ar-Me), 2.11 (s,
3 H, NdCMe), 2.19 (s, 3 H, NdCMe), 2.39 (s, 6 H, Ar-Me′), 6.27 (s,
2 H, Ar-Ho), 6.5-6.7 (m, 5 H, Ph-H), 6.77 (s, 2 H, Ar-Ho′), 7.05 (s, 1
H, Ar-Hp), 7.32 (s, 7 H, C6H6 + Ar-Hp′). ESI MS m/z: 605.4 (M+).
In situ Characterization of Product after Protonation of 1d at
Low Temperature, Decomposition at 0 °C, and Addition of CD3CN
(0.1 mL). 1H NMR (200 MHz, CD2Cl2): δ 2.07 (s, 6 H, Ar-Me), 2.12
(s, 6 H, NdCMe), 2.20 (s, 3 H, NdCMe′), 2.39 (s, 6 H, Ar-Me′), 6.28
(s, 2 H, Ar-Ho), 6.5-6.7 (m, 5 H, Ph-H), 6.79 (s, 2 H, Ar-Ho′), 7.04 (s,
1 H, Ar-Hp), 7.31 (s, 7 H, C6H6 + Ar-Hp′).
[ArNdC(Me)-C(Me)dNAr]Pt(C6H6)(Ph)+ [Ar ) 3,5-Me2C6H3]
1
(4d). H NMR (500 MHz, T ) -73 °C, CD2Cl2): δ 1.93 (s, 3 H,
Ar-Me), 2.04 (s, 3 H, NCMe), 2.13 (s, 3 H, NCMe), 2.35 (s, 6 H, Ar-
Me), 6.07 (mb, 5 H, Ph-Ho,m + Ar-Hp), 6.18 (mb, 1 H, Ph-Hp), 6.44 (d,
1 H, Ar-Hp), 6.83 (mb, 2 H, Ar-Hm), 6.97 (s, 8 H, C6H6 + Ar-Hm).
[ArNdC(Me)-C(Me)dNAr]Pt(C6H6)(Ph)+ [Ar ) 4-CF3C6H4]
1
(4e). H NMR (500 MHz, T ) -53 °C, CD2Cl2): δ 2.07 (s, 3 H,
NCMe), 2.18 (s, 3 H, NCMe), 6.02-6.11 (mb, 4 H, Ph-Ho,m), 6.21
(mb, 1 H, Ph-Hp), 6.79 (d, 2 H, Ar-H), 6.96 (s, 6 H, C6H6), 7.14 (d, 2
H, Ar-H), 7.57 (d, 2 H, Ar-H), 7.87 (d, 2 H, Ar-H).
NMR-Tube Generation of [ArNdC(Me)-C(Me)dNAr]Pt(Ph)-
(CD3CN)+BF4- [Ar ) 4-CF3C6H4] (2e‚BF4). HBF4‚Et2O (5 µL) was
added to an NMR tube containing 1e (3 mg) in CD2Cl2 (400 µL) with
added CD3CN (100 µL). Shaking turned the deep purple solution bright
SST Measurements. The SST experiments were acquired with a
modification of the zghd pulse program supplied with the Bruker
Avance DPX/DRX Spectrometers. The T1 relaxation was measured by
the inversion recovery method for each temperature and used in the
calculation of the rate constants. The power level of the saturation pulse
was adjusted before each temperature.
1
4
orange. H NMR (200 MHz, CD2Cl2): δ 2.14 (s, 3 H, J(195Pt-H) )
10.4 Hz, NdCMe), 2.23 (s 3 H, NdCMe′), 6.5-6.8 (m, 5 H, Ph-Hm,o,p),
6.92 (d, 2 H, Ar-H), 7.40 (d, 2 H, Ar-H), 7.43 (d, 2 H, Ar-H), 7.89 (d,
2 H, Ar-H). ESI MS m/z: 685.2 (M+).
EXSY Spectroscopy. After protonation and transfer of the sample
to the precooled NMR probe, the 2D EXSY spectra were recorded using
a gradient-selected NOESY pulse program from Bruker (noesygpph).
All spectra were recorded at 500.13 MHz with mixing time, τm,
optimized for each temperature.54 The spectra were recorded with a
sweep width of 8.2 ppm, 4096 data points in the F2 direction, 256
increments in the F1 direction, and 2 transients per increment, and a
relaxation delay of 3 s. This setup produced a total acquisition time of
approximately 40 min depending on τm. The spectra were apodized
with a qsine function and zero-filled to give a 2048 × 2048 matrix.
The spectra were phased to give positive peaks along the diagonal.
The basis for the extraction of kinetic and thermodynamic data
produced by the EXSY experiments is integration of the cross-peak
volumes of the EXSY NMR spectra. The integration of the 2D spectra
was performed using two different software applications, Xwinnmr128
Low-Temperature Protonation of 1a in the Presence of Aceto-
nitrile. Characterization of [ArNdC(Me)-C(Me)dNAr]Pt(H)(Ph)2-
(MeCN)+OTf- [Ar ) 2,6-Me2C6H3] (3a‚OTf). The cationic Pt(IV)
hydride was prepared by the protonation procedure described below,
1
and the characterization was done in situ. H NMR (300 MHz, CD2-
Cl2): δ -21.47 (s, 1J(195Pt-H) ) 1597 Hz, 1 H, Pt-H), 2.03 (s, 6 H),
2.16 (s, 6 H), 2.41 (s, 6 H), 3.59 (s, 3 H, MeCN), 6.54 (m, 4 H, Ph-H),
6.72 (m, 6 H, Ph-H), 6.94 (m, 6 H, Ar-H).
Low-Temperature Protonation of (N-N)PtPh2 (1) in the Absence
of Acetonitrile: General Procedure. This procedure is a modification
of procedures previously described by us.60 A solution of 1a-c (ca.
10 mg, ca. 20 µmol) in CD2Cl2 (400 µL) in an NMR tube was carefully
layered with CD2Cl2 (100 µL). (The layering technique was used to
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J. AM. CHEM. SOC. VOL. 128, NO. 8, 2006 2695