From azobenzene coordination to aryl-halide bond activation by platinum

Article abstract of DOI:10.1021/om700519v

This contribution describes the reactivity of Pt(PEt₃) ₄ with (4-bromo-phenyl)-pyridin-4-yl-diazene. η²- Coordination of Pt(PEt₃)₂ to the N=N moiety is kinetically preferable and followed by an aryl-halide bond activation process. This quantitative transformation proceeds under mild reaction conditions in solution and in the solid state. Mechanistic studies in solution indicate that the metal insertion into the aryl-halide bond is the rate-determining step. The reaction obeys first-order kinetics in the η²-coordination complex with ΔG^?_298K = 24.6 ± 1.6 kcal/mol, ΔH^? = 26.5 ± 1.6 kcal/mol, and ΔS ^? = 6.6 ± 5.0 eu. No effect on the reaction progress and NMR line shape has been observed in the presence of excess PEt₃. However, competition experiments with the η-coordination complex and PhBr reveal that the product ratio can be altered by the presence of PEt₃, indicating that the two aryl-halide bond activation processes proceed via different mechanistic pathways. Numerical analysis of a series of competition experiments fits a reaction scheme involving a unimolecular transformation from the η²-coordination complex to the product of aryl-halide oxidative addition. This ring-walking process is kinetically accessible as shown by density functional theory (DFT) calculations at the PCM:PBEO/SDB-cc-pVDZ/PBE0/SDD level of theory.

Full text of DOI:10.1021/om700519v

4
528
Organometallics 2007, 26, 4528-4534
From Azobenzene Coordination to Aryl-Halide Bond Activation by
Platinum
†
†
‡
‡
‡
Olena Zenkina, Marc Altman, Gregory Leitus, Linda J. W. Shimon, Revital Cohen, and
,
†
Milko E. van der Boom*
Departments of Organic Chemistry and Chemical Research Support, Weizmann Institute of Science,
6100 RehoVot, Israel
7
ReceiVed May 25, 2007
3 4
)
with (4-bromo-phenyl)-pyridin-4-yl-diazene. η2-
This contribution describes the reactivity of Pt(PEt
Coordination of Pt(PEt to the NdN moiety is kinetically preferable and followed by an aryl-halide
3 2
)
bond activation process. This quantitative transformation proceeds under mild reaction conditions in
solution and in the solid state. Mechanistic studies in solution indicate that the metal insertion into the
2
aryl-halide bond is the rate-determining step. The reaction obeys first-order kinetics in the η -coordination
q
q
q
complex with ∆G 298K ) 24.6 ( 1.6 kcal/mol, ∆H ) 26.5 ( 1.6 kcal/mol, and ∆S ) 6.6 ( 5.0 eu. No
3
effect on the reaction progress and NMR line shape has been observed in the presence of excess PEt .
However, competition experiments with the η -coordination complex and PhBr reveal that the product
ratio can be altered by the presence of PEt
proceed via different mechanistic pathways. Numerical analysis of a series of competition experiments
fits a reaction scheme involving a unimolecular transformation from the η -coordination complex to the
2
3
, indicating that the two aryl-halide bond activation processes
2
product of aryl-halide oxidative addition. This “ring-walking” process is kinetically accessible as shown
by density functional theory (DFT) calculations at the PCM:PBE0/SDB-cc-pVDZ/PBE0/SDD level of
theory.
of aryl-halide oxidative addition.6 This pathway to arene
Introduction
functionalization has synthetic utility and might be generalized
to selectively activate aryl-halide bonds. In a related study,
Aryl-halide bond activation by late transition metals is of
7,8
prime importance for homogeneous catalysis and metal-mediated
Yokozawa et al. observed that Ni-catalyzed chain growth
polymerization of poly(3-hexylthiophene) and poly(p-phenylene)
proceeds by an intramolecular transfer of the Ni(0) catalyst over
the conjugated system, followed by selective activation of a
1
synthesis. Metal-arene interactions play a significant role in
these processes and may offer new routes for selective bond
2
activation. Seminal work on reaction pathways involving
metal-arene interactions was carried out in the 1960s by
9
C-Br bond. We provide here in-depth mechanistic insight into
3
Basalo. More recent examples include Harman’s intramolecular
4
metal-ligand coordination followed by an aryl-halide bond
“
ring-walking” for various osmium complexes. Stanger and
2
activation process by zerovalent platinum. η -Coordination of
Vollhardt reported on the intramolecular migration of a (R3P)2-
Pt(PEt3)2 to an NdN moiety is kinetically preferable and
followed by rate-determining aryl-halide bond activation (halide
) Br, I). This process proceeds quantitatively in solution and
in the solid state, and has been studied here by variable-
temperature NMR spectroscopy and competition/crossover
experiments including numerical analysis. The proposed mech-
anism is supported by DFT calculations.
Ni (R ) Et, Bu) moiety over an anthracene ligand via a series
2
3
of η -η coordination changes along the system’s outer CdC
5
bonds. We reported recently that the reaction of Pt(PEt3)4 with
2
a halogenated vinylarene results in η -CdC coordination,
followed by ring-walking of the metal center to give the product
*
Corresponding author. E-mail: milko.vanderboom@weizmann.ac.il.
Department of Organic Chemistry.
Department of Chemical Research Support.
†
‡
(
1) (a) Negishi, E. I. Handbook of Organopalladium Chemistry for
(6) Strawser, D.; Karton, A.; Zenkina, O. V.; Iron, M. A.; Shimon, L. J.
Organic Synthesis; Wiley: New York, 2002. (b) Diederich, F.; Stang, P. J.
Metal catalyzed Cross-Coupling Reactions; Wiley-VCH, New York, 1998.
W.; Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc. 2005, 127,
q
9322. The units reported for the ∆S value is J/K rather than eu. The correct
q
(
c) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic
Molecules; University Science Books: Mill Valley, CA, 1994.
2) (a) Siegel, J. S.; Baldridge, K. K.; Linden, A.; Dorta, R. J. Am. Chem.
Soc. 2006, 128, 10644. (b) Keane, J. M.; Harman, W. D. Organometallics
005, 24, 1786. (c) Iverson, C. N.; Lachicotte, R. J.; M u¨ ller, C.; Jones, W.
value is ∆S ) -31.1 eu.
(7) (a) Minami, Y.; Kato, T.; Kuniyasu, H.; Terao, J.; Kambe, N.
Organometallics 2006, 25, 2949. (b) Miyakoshi, R.; Yokoyama, A.;
Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542.
(
2
(8) For recent examples on aryl-halide bond activation, see: (a)
Lucassen, A. C. B.; Shimon, L. J. W.; van der Boom, M. E. Organometallics
2006, 25, 3308. (b) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc.
2006, 128, 4632. (c) Gatard, S.; Celenligil-Cetin, R.; Guo, C. Y.; Foxman,
B. M.; Ozerov, O. V. J. Am. Chem. Soc. 2006, 128, 2808. (d) Barrios-
Landeros, F.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 6944. (e)
Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366. (f) Fu, G.
C. J. Am. Chem. Soc. 2002, 124, 13662.
D. Organometallics 2002, 21, 5320. (d) Carbo, J. J.; Eisenstein, O.; Higgitt,
C. L.; Klahn, A. H.; Maseras, F.; Oelckers, B.; Perutz, R. N. Dalton Trans.
001, 1452. (e) Bach, I.; P o¨ rschke, K.-R.; Goddard, R.; Kopiske, C.; Kr u¨ ger,
C.; Rufinska, A.; Seevogel, K. Organometallics 1996, 15, 4959. (f) Sweet,
J. R.; Graham, W. A. G. Organometallics 1983, 2, 135. (g) Cotton, F. A.
Acc. Chem. Res. 1968, 1, 257.
2
(
3) (a) O’Connor, J. M.; Casey, C. P. Chem. ReV. 1987, 87, 307. (b)
Schuster-Woldan, H. G.; Basolo, F. J. J. Am. Chem. Soc. 1966, 88, 1657.
(9) (a) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J.
Am. Chem. Soc. 2006, 128, 16012-16013. (b) Miyakoshi, R.; Yokoyama,
A.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542-17547.
(
4) Harman, W. D. Chem. ReV. 1997, 97, 1953.
(5) Stanger, A.; Vollhardt, K. P. C. Organometallics 1992, 11, 317.
1
0.1021/om700519v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/27/2007

Azobenzene Coordination by Platinum
Experimental Section
Materials and Methods. All reactions were carried out in an
Organometallics, Vol. 26, No. 18, 2007 4529
) 12.6 Hz, JPtC ) 20.25 Hz). 31P{ H} NMR (THF-d
3
1
): δ 5.3 (d,
8
2
1
2
1
J
PP ) 5.3 Hz, JPtP ) 3866.5 Hz, 1P), 5.29 (d, JPP ) 5.5 Hz, JPtP
4094.0 Hz, 1P). Calcd m/e: 614.7, found m/e: 615.0. Anal. (%)
)
N
2
-filled M. Braun glovebox with H O and O levels < 2 ppm.
2
2
Calcd for C23
.35.
3 4
Formation of Complex 5. Pt(PEt ) (87 mg, 0.13 mmol) was
39 3 2
H N P Pt: C, 44.95; H, 6.40. Found: C, 44.83; H,
Solvents were reagent grade or better, dried, distilled, and degassed
before introduction into the glovebox, where they were stored over
activated 4 Å molecular sieves. Deuterated solvents were purchased
from Aldrich and were degassed and stored over activated 4 Å
molecular sieves in the glovebox. Reaction flasks were washed with
deionized water, followed by acetone, and then oven-dried prior
to use. Mass spectrometry was carried out using a Micromass
Platform LCZ 4000 mass spectrometer. Elemental analyses were
performed by H. Kolbe, Mikroanalytisches Laboratorium, M u¨ lheim
an der Ruhr, Germany. Ligands 1, 3, and 6 were prepared by
published procedures.10
6
dissolved in 2 mL of dry THF and slowly added to a stirred solution
of (4-bromo-phenyl)-pyridin-4-yl-diazene (1) (34 mg, 0.13 mmol)
in 3 mL of dry THF. The solution was transferred to a pressure
tube, protected from light with aluminum foil, and heated at 40 °C
for 14 h. All volatiles were removed under vacuum, and the
remaining solid was washed with 15 mL of dry pentane, dried,
and redissolved in ∼0.5 mL of THF. Subsequently, dry pentane
was added dropwise to the THF solution until the complex started
to precipitate. Complex 5 was collected as a dark orange-brown
Spectroscopic Analysis. The H, 13C{ H}, F{ H}, and 31P-
1
1
19
1
1
3
solid after 12 h (64 mg, 80%). H NMR (THF-d
8
): δ 8.62 (d, JHH
{1
H} NMR spectra were recorded at 400.19, 100.6, 376.5, and 161.9
3
)
(
1
6.13 Hz, 2H, PyrH), 7.80 (d, JHH ) 8.25 Hz, 2H, ArH), 7.65
MHz, respectively, on a Bruker AMX 400 NMR spectrometer. All
chemical shifts (δ) are reported in ppm and coupling constants (J)
in Hz. The 1H and C NMR chemical shifts are relative to
tetramethylsilane; the resonance of the residual protons of the
d, 3JHH ) 8.37 Hz, 2H, ArH), 7.52 (d, JHH ) 6.12 Hz, 2H, ArH),
3
13
1
.57 (m, 12H, PCH
Cl ): δ 158.17 (s, C
br), 148.89 (br, C
2 3 2 3
CH ), 0.88 (m, 18H, PCH CH ). C{ H} NMR
13
3
(CD
2
2
q
q
), 154.22 (t, JPC ) 18.36 Hz, C ), 151.72
), 138.85 (t, 3JPC ) 3.5 Hz, JPtC ) 21.50 Hz),
2
(
1
q
solvent was used as the internal standard h
.32 ppm, CH Cl ; 7.09 ppm, toluene; 3.58 and 1.73 ppm, THF)
and all-d solvent peaks (128.0 ppm, benzene; 54.0 ppm, CH Cl
0.4 ppm, toluene; THF, 67.6 and 25.3 ppm). 31P{ H} NMR
chemical shifts are relative to 85% H PO in D O at δ 0.0 (external
reference), with shifts downfield of the reference considered
positive. The 19F NMR spectrum was referenced to external C
1
(7.15 ppm, benzene;
3
1+3
22.56 (s, JPtC ) 21.5 Hz), 116.28 (s), 14.50 (vt,
JPC ) 34.0
5
2
2
2
3
31
1
Hz, JPtC ) 71 Hz), 8.01 (s, JPtC ) 20.5 Hz). P{ H} NMR (THF-
2
2
;
1
d
8
): δ 12.02 (s, JPtP ) 2721.9 Hz, 2P). Calc m/e: 692.1, found
1
2
+
m/e (M - Br): 613.5. Anal. (%) calcd for C23
9.83; H, 5.52. Found: C, 39.96; H, 5.45.
H38BrN
3
P
2
Pt: C,
3
4
2
3
Formation of Complex 7. Pt(PEt (6.54 mg, 0.0196 mmol)
3 4
)
6 6
F
was dissolved in 0.35 mL of dry THF and loaded into a 5 mm
screw-cap NMR tube, which was equipped with a septum and sealed
with Teflon tape, insulating tape, and Parafilm. A solution of (4-
iodo-phenyl)-pyridin-4-yl-diazene (6) (3.02 mg, 0.0196 mmol) in
at δ ) -162.9 ppm. All measurements were carried out at 298 K
unless stated otherwise. Screw-cap 5 mm NMR tubes were used
1
13
in NMR follow-up experiments. Assignments in the H and C-
{
1
1
31
13
H} NMR spectra were made using H{ P} and C-DEPT-135
0
-
.35 mL of dry THF was added dropwise into the NMR tube at
75 °C. Subsequently, the NMR tube was shaken and immediately
transferred into the cold (-65 °C) NMR spectrometer. The reaction
NMR. Temperature calibration of the NMR spectrometer was
performed using a mixture of ethyleneglycol with 20% DMSO-d
The accuracy of the reported temperatures is (0.2 °C.
6
.
31
1
was monitored by P{ H} NMR spectroscopy, showing the
presence of complex 7 at -25 °C. After 1 h, quantitative formation
Formation of Complex 2. A solution of 4-(bromo-phenyl)-
pyridin-4-yl-diazene (1) (29 mg, 0.11 mmol) in 3 mL of THF was
added dropwise to a stirred solution of Pt(PEt ) (75 mg, 0.11 mmol)
3 4
of complex 7 was observed, which is stable at temperatures < 0
3
°
C. For 7: 1H NMR (THF-d
8
): δ 7.88 (d, JHH ) 6.18 Hz, 2H,
in 3 mL of THF at room temperature. The quantitative formation
3
3
PyrH), 7.16 (d, JHH ) 8.62 Hz, 2H, ArH), 6.96 (d, JHH ) 7.41
of complex 2 was observed by 31P{ H} NMR spectroscopy after
1
3
Hz, 2H, ArH), 6.92 (d, JHH ) 5.04 Hz, 2H, ArH), 1.73 (m, 12H,
3
0 min. Complex 2 converts selectively to complex 5 in THF at
room temperature (∼50% conversion after 24 h, ∼75% conversion
after 96 h) as judged by P{ H} NMR spectroscopy. H NMR
31
1
2 3 2 3 8
PCH CH ), 0.87 (m, 18H, PCH CH ). P{ H} NMR (THF-d ): δ
2
1
2
3
.63 (d, JPP ) 4.6 Hz, JPtP ) 3890.5 Hz, 1P), 3.62 (d, JPP ) 4.4
31
1
1
1
Hz, JPtP ) 4038.6 Hz, 1P).
3
3
(
CD
2
Cl
2
): δ 8.10 (d, JHH ) 5.63 Hz, 2H, PyrH), 7.18 (t, JHH )
3
Follow-up Experiment of the Formation of Complex 8. The
transformation of complex 7 f 8 was monitored by 31P{ H} NMR
spectroscopy at +15 °C in 1 mL of THF. Formation of complex 8
became visible after approximately 10 min with concurrent disap-
pearance of the starting material (7) (10 mg, 0.014 mmol). No other
products were observed. After ∼17 h, 88% conversion was
9
1
d
.7 Hz, 4H, ArH), 7.14 (d, JHH ) 4.72 Hz, 2H, ArH), 1.68 (m,
1
31
1
2H, PCH
8
2 3 2 3
CH ), 0.97 (m, 18H, PCH CH ); P{ H} NMR (THF-
2
1
2
): δ 4.92 (d, JPP ) 9.11 Hz, JPtP ) 3915.4 Hz, 1P), 4.7 (d, JPP
8.95 Hz, JPtP ) 4067.3 Hz, 1P).
Formation of Complex 4. A solution of Pt(PEt ) (82 mg, 0.12
3 4
1
)
mmol) in 1 mL of THF was mixed with a THF (2 mL) solution of
phenyl-pyridin-4-yl-diazene (3) (22 mg, 0.12 mmol) at room
temperature. Subsequently, the quantitative formation of complex
2
observed. First-order linear fitting with R ) 0.98 afforded the rate
-
5
-7 -1
1
constant k288K ) -3.5 × 10 ( 7.5 × 10
s
. For 8: H NMR
3
3
31
1
(C
7
6
D
6
): δ 8.65 (d, JHH ) 6.03 Hz, 2H, PyrH), 7.87 (d, JHH
)
4
was observed after 40 min by P{ H} NMR spectroscopy.
3
3
.22 Hz, 2H, ArH), 7.65 (d, JHH ) 8.37 Hz, JPtH ) 63.6 Hz, 2H,
Complex 4 was isolated as a dark orange solid by removing all
volatiles under vacuum and washing the residue with 10 mL of
dry pentane. The remaining solid was dissolved in 0.5 mL of THF
followed by addition of pentane (4 mL) and allowed to crystallize
3
2
ArH), 7.55 (d, JHH ) 4.54 Hz, 2H, ArH), 1.63 (m, 12H, PCH -
13
1
CH
(
(
3
), 0.82 (m, 18H, PCH
2
3
CH
3
). C{ H} NMR (C
D
6 6
): δ 157.6
), 7.57
s, C
q
), 151.6, 148.54 (t, JPC ) 3.6 Hz, C ), 150.7 (br, C
q
q
3
3
2
s, JPtC ) 25.6 Hz), 137.59 (t, JPC ) 4.88 Hz, JPtC ) 41.93 Hz),
by slow evaporation of the solvent under N
2
at room temperature,
yielding complex 4 (61 mg, 81%). H NMR (THF-d ): δ 8.13 (d,
HH ) 6.0 Hz, 2H, PyrH), 7.33 (d, JHH ) 7.86 Hz, 2H, ArH),
3
1+3
1
122.61 (s, JPtC ) 78.02 Hz), 115.89 (s), 15.36 (vt,
J
PC ) 34.7
8
2
31
1
1
3
3
Hz, JPtC ) 69.7 Hz). P{ H} NMR (THF-d
8
): δ 6.7 (s, JPtP
)
J
3
3
2678.5 Hz, 2P). Calcd m/e: 740.5, found m/e: 740.7. Anal. (%)
7
.14 (d, JHH ) 3.93 Hz, 2H, ArH), 7.05 (d, JHH ) 7.89 Hz, 2H,
3
Calcd for C23
H, 5.14.
3 2
H38IN P Pt: C, 37.31; H, 5.17. Found: C, 37.48;
ArH), 6.75 (t, JHH ) 7.1 Hz, 1H, ArH), 1.71 (m, 12H, PCH
0
2
CH
): δ 165.44 (br,
), 151.9 (t, br), 149.8, 128.35, 120.6, 115.1, 18.2
3
),
13
1
.98 (m, 18H, PCH
q
2 3 2 2
CH ). C{ H} NMR (CD Cl
C
), 158.8 (s, C
q
Solid-State Formation of Complex 5. The orange powder of
compound 2 (30 mg) was heated in a sealed glass pressure tube
dd, JPC ) 16.7 Hz, 1JPC ) 47.0 Hz, JPtC ) 30.1 Hz), 8.5 (d, JPC
3
2
2
(
with exclusion of light at 74 °C under N
point of the solid is ∼120 °C. After 5 days the color of the solid
turned dark orange-brown. H and P{ H} NMR spectroscopy in
2
. Importantly, the melting
(
10) Ayyangar, N. R.; Naik, S. N.; Srinivasan, K. V. Tetrahedron Lett.
1
31
1
1
989, 30, 7253.

4
530 Organometallics, Vol. 26, No. 18, 2007
Zenkina et al.
cold THF-d
8
(-5 °C) showed quantitative conversion of 2 into
{1H} NMR (C
6
2
D
6
): δ 157.27 (t, C
q
), 2JPC ) 17.2 Hz), 144.09 (s,
complex 5 after 11 days. No other products were observed.
C ), 137.55 (s, J ) 44.56 Hz), 132,16 (s), 124.77 (s), 120.2 (s,
q
PtC
31
1
2
1+3
P{ H} NMR Follow-up Experiment of the Formation of 5
J
PtC ) 79.02 Hz), 14.4 (vt,
JPC ) 34.2 Hz, JPtC ) 69.2 Hz),
7.72 (s, JPtC ) 19.56 Hz). 31P{ H} NMR (THF-d
1
): δ 11.72 (s,
from 2. A solution of complex 2 (20 mg, 0.029 mmol) in 1 mL of
THF was loaded in a 5 mm screw-cap NMR tube. The reaction
progress was monitored by 31P{ H} NMR spectroscopy at 40 °C,
showing the formation of complex 5 and concurrent disappearance
of complex 2. No intermediates were observed.
8
1
JPtP ) 2675.4 Hz, 2P).
1
X-ray Analysis of Complex 4. Crystal data: C23
H
39
N
3
P
2
Pt,
2, a ) 18.851-
4) Å, b ) 7.919(2) Å, c ) 8.342(2) Å, T ) 120(2) K, V ) 1245.3-
3
orange, 0.7 × 0.3 × 0.3 mm , orthorhombic, P2
1 1
2
(
(
3
1
1
P{ H} NMR Follow-up Competition Experiments of the
3
-3
-1
4) Å , Z ) 2, fw ) 614.6, D
c
) 1.639 Mg·m , µ ) 5.777 mm .
Formation of Complexes 5 and 12 from Bromobenzene (9) and
Complex 2. A solution of bromobenzene (9) (4.5 mg, 0.029 mmol)
in 1 mL of THF was mixed with a solution of complex 2 (20 mg,
Data collection and processing: Nonius KappaCCD diffracto-
meter, Mo KR (λ ) 0.71073 Å), graphite monochromator, 0 e h
e 24, 0 e k e 10, 0 e l e 10, frame scan width ) 1°, scan speed
0.029 mmol) in 1 mL of THF and loaded in a 5 mm screw-cap
1.0° per 20 s, typical peak mosaicity 0.70°, 12 892 reflections
31
1
NMR tube. The reaction progress was monitored by P{ H} NMR
collected, 1614 independent reflections (Rint ) 0.060). The data
were processed with Denzo-Scalepack.
spectroscopy, showing the concurrent formation of complexes 5
and 121
1,12
and the concurrent disappearance of complex 2. The
product ratio 5:12 ) 5:1 after 14 h (∼99% conversion of starting
Solution and refinement: The structure has been solved by the
Patterson method with SHELXS-97. Full matrix least-squares
refinement is based on F with SHELXL-97, 135 parameters with
13
material). Performing the competition experiment in the presence
2
of 10 equiv of PEt
3
affords a product ratio 5:12 ) 45:1 after 14 h
∼97% conversion of starting material). Reaction of bromobenzene
0.79 mg/mL, 0.0050 M) in 1 mL of THF with 2 (3.5 mg/mL,
.0050 M) in 1 mL of THF resulted in the exclusive formation of
2
(
(
0
0 restraints, final R
2σ(I) and, R
) 1.031, largest electron density peak ) 1.29 e‚Å
X-ray Analysis of Complex 5. Crystal data: C23
1
) 0.0300 (based on F ) for data with I >
2
1
) 0.0343 on 1604 reflections, goodness-of-fit on F
-
3
.
complex 5 after 14 h (∼97% conversion of starting material.
Formation of compound 12 was not observed, suggesting a ratio
of 5:12 > 65:1. All experiments were conducted at 40 °C.
38 3 2
H N P BrPt,
3
orange, 0.1 × 0.1 × 0.05 mm , the crystal is refined like a twin,
monoclinic, Cc, a ) 6.993(2) Å, b ) 28.041(6) Å, c ) 13.723(3)
Competition Experiments of Pt(PEt
3
)
4
with Bromobenzene
3
Å, â ) 94.46(3)°, T ) 120(2) K, V ) 2682.8(9) Å , Z ) 4, fw )
(9) and Ligand 1. Pt(PEt (15 mg, 0.022 mmol) was dissolved
)
3 4
-3
-1
6
93.5, D
c
) 1.717 Mg‚m , µ ) 6.854 mm .
in 3 mL of dry THF and slowly added to a stirred mixture of (4-
bromo-phenyl)-pyridin-4-yl-diazene (1) (29 mg, 0.11 mmol) and
bromobenzene (9) (17.6 mg, 0.112 mmol) in 6 mL of dry THF
Data collection and processing: Nonius KappaCCD diffracto-
meter, Mo KR (λ ) 0.71073 Å), graphite monochromator, 0 e h
e 9, 0 e k e 36, -17 e l e 17, frame scan width ) 2.0°, scan
speed 1.0° per 30 s, typical peak mosaicity 0.88°, 25276 reflections
collected, 3139 independent reflections (Rint ) 0.083). The data
were processed with Denzo-Scalepack.
(5-fold excess of each ligand). The solution was transferred to a
pressure tube, protected from light with aluminum foil, and heated
31
1
at 40 °C for 14 h. P{ H} NMR spectroscopy shows the exclusive
formation of product 5. Formation of compound 12 was not
observed. After 14 h, 98% conversion of starting materials was
observed, as judged by 31P{ H} NMR spectroscopy.
Solution and refinement: The structure has been solved by the
Patterson method with SHELXS-97. Full matrix least-squares
refinement based on F with SHELXL-97, 273 parameters with 3
1
13
2
Competition Experiments of Pt(PEt
trifluoromethylbenzene (10) and Ligand 1. Pt(PEt
.022 mmol) was dissolved in 3 mL of dry THF and slowly added
to a stirred mixture of (4-bromo-phenyl)-pyridin-4-yl-diazene (1)
3
)
4
with 1-Bromo-4-
2
)
4
(15 mg,
restraints, final R
and, R
1
) 0.0269 (based on F ) for data with I > 2σ(I)
3
2
0
1
) 0.0298 on 3075 reflections, goodness-of-fit on F )
1.049, largest electron density peak ) 2.346.
(
29 mg, 0.11 mmol) and 1-bromo-4-trifluoromethylbenzene (10)
Computational Methods. All DFT calculations were carried out
(25.3 mg, 0.112 mmol) in 6 mL of dry THF (5-fold excess of each
14
using Gaussian 03, Revision C.02. We used the PBE0 DFT
substrate). The solution was transferred to a pressure tube, protected
from light with aluminum foil, and heated at 40 °C for 14 h. P-
{
15
exchange-correlation functional, also known as PBE1PBE. PBE0
31
is the hybrid variant of PBE, Perdew, Burke, and Ernzerhof’s
nonempirical GGA functional, and contains 25% HF exchange. This
functional yields more reliable reaction barrier heights than
1
H} NMR spectroscopy shows a product ratio 5:13 ) 2.5:1 (∼99%
12 1
conversion of starting material). For 13: H NMR (C ): δ 7.46
6 6
D
3
3
3
(
d, JHH ) 7.90 Hz, JPtH ) 65.5 Hz, 2H, ArH), 7.20 (d, JHH
)
16
B3LYP or other “conventional” exchange-correlation functionals,
7
.93 Hz, 2H, ArH), 1.49 (m, 12H, PCH
2
CH
): δ 12.03 (s, JPtP ) 2715.5
: δ -62.41, (s, CF ).
Competition Experiments of Pt(PEt with 1-Bromo-4-
nitrobenzene (11) and Ligand 1. Pt(PEt (15 mg, 0.022 mmol)
3
), 0.84 (m, 18H,
31
1
1
2 3 8
PCH CH ). P{ H} NMR (THF-d
19
1
(13) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Determination; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
Hz, 2P). F{ H} NMR (THF-d
8
3
3 4
)
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
3
)
4
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
was dissolved in 3 mL of dry THF and slowly added to a stirred
mixture of (4-bromo-phenyl)-pyridin-4-yl-diazene (1) (29 mg, 0.11
mmol) and 1-bromo-4-nitrobenzene (11) (22 mg, 0.11 mmol) in 6
mL of dry THF (5-fold excess of each substrate). The solution was
transferred to a pressure tube, protected from light with aluminum
foil, and heated at 40 °C for 14 h. 3 P{ H} NMR spectroscopy
1
1
showed a product ratio 5:14 ) 1:3 after 14 h (∼99% conversion
of starting material). For 14:1 H NMR (C
2 1
3
6
6
D ): δ 7.92 (d, JHH )
3
3
8
.44 Hz, 2H, ArH), 6.40 (d, JHH ) 8.40 Hz, JPtH ) 63.9 Hz, 2H,
13
2 3 2 3
ArH), 1.52 (m, 12H, PCH CH ), 0.84 (m, 18H, PCH CH ). C-
(
11) For NMR of complex 12 see: (a) Coulson, D. R. J. Am. Chem.
Soc. 1976, 98, 3111. (b) Romeo, R.; Minniti, D.; Trozzi, M. Inorg. Chem.
976, 15, 1134.
12) For NMR data of complexes 12-14, see: Arnold, D. P.; Bennett,
M. A. Inorg. Chem. 1984, 23, 2117.
(15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865.
(16) Stevens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623.
1
(

Azobenzene Coordination by Platinum
Organometallics, Vol. 26, No. 18, 2007 4531
Figure 1. ORTEP diagram of 4 (left) and 5 (right) with thermal ellipsoids set at 50% probability. Hydrogen atoms are omitted for clarity.
There is 1/2 molecule of 4 per asymmetric unit and atom C(1_2) etc. are generated by symmetry (symmetry transformations used to
1
1
1
generate equivalents atoms #1 -x+1, -y+1, z; #2 -x+1, y- /
deg): For 4: Pt(1)-N(1) 2.100(6), Pt(1)-N(1_2) 2.101(6), Pt(1)-P(1) 2.2516(19), Pt(1)-P(1_2) 2.2516(19), C(1)-N(1) 1.412(10), N(1)-
N(1_2) 1.430(13), N(1_2)-Pt(1)-N(1) 39.8(3), N(1)-Pt(1)-P(1_2) 150.42 (18), N(1)-Pt(1)-P(1) 110.64(17), N(1_2)-Pt(1)-P(1)
50.42(18), P(1)-Pt(1)-P(1_2) 09.03(10). For 5: Pt(1)-Br(1) 2.5302(11), Pt(1)-C(1) 2.009(8), C(4)-N(7) 1.418 (10), N(7)-N(8) 1.260(9),
2
, -z+1: #3 -x+ /
2
, y+ /
2
, -z+1). Selected bond lengths (Å) and angles
(
1
N(8)-C(9) 1.433(10), Pt(1)-P(2) 2.306(2), Pt(1)-P(3) 2.316(2); C(1)-Pt(1)-P(2) 91.6(2), C(1)-Pt(1)-P(3) 87.6(2), P(2)-Pt(1)-P(3)
72.28(9), C(1)-Pt(1)-Br(1) 178.7(2), P(2)-Pt(1)-Br(1) 87.13(6), P(3)-Pt(1)-Br(1) 93.67(6).
31
1
2
Scheme 1. Selective Formation of Complexes 2, 4, and 7 by η -Coordination Followed by Oxidative Addition (for 2 and 7) to
afford Complexes 5 and 8, respectively
without sacrificing performance of other properties.17 With this
functional, two basis set-RECP (relativistic effective core potential)
combinations were used. The first, denoted SDD, is the combination
were optimized using the default pruned (75 302) grid or when
necessary for convergence with the “ultrafine” grid, i.e., a pruned
(99 590) grid.
18
of the Huzinaga-Dunning double-ú basis set on lighter elements
with the Stuttgart-Dresden basis set-RECP combination19 on
transition metals. The second, denoted SDB-cc-pVDZ, combines
the Dunning cc-pVDZ basis set20 on the main group elements and
the Stuttgart-Dresden basis set-RECP19 on the transition metals
with an added f-type polarization exponent taken as the geometric
average of the two f-exponents given in the appendix of ref 21.
Geometry optimizations were carried out using the former basis
set, while the energetics of the reaction were calculated at these
geometries with the latter basis set. Bulk solvent effects22 were
approximated by single-point PBE0/SDB-cc-pVDZ energy calcula-
Results and Discussion
29
An equimolar amount of Pt(PEt ) and 4-(bromo-phenyl)-
3 4
10
pyridin-4-yl-diazene (1) were reacted in dry THF-d at room
8
temperature, resulting in the quantitative formation of complex
and 2 equiv of PEt3 within 0.5 h, as judged by H and P-
H} NMR spectroscopy (Scheme 1). Complex 2 and a recently
isolated stilbazole complex exhibit nearly identical spectroscopic
1
31
2
{
1
6,30
properties.
Furthermore, reaction of phenyl-pyridin-4-yl-
10
diazene (3) with Pt(PEt3)4 resulted in the formation of the
isostructural complex 4, which has been isolated and character-
tions using a polarized continuum (overlapping spheres) model
3-25
(
PCM)2
with THF as the solvent, as in the experiments. The
1
13
1
31
1
ized by H, C{ H}, and P{ H}, mass spectrometry, elemental
connectivity of the transition states was confirmed by performing
31
analysis, and X-ray diffraction (Figure 1). Complex 2 converts
slowly into complex 5 at room temperature (e.g., 75% conver-
sion after 96 h), which has been isolated and characterized by
2
6-28
intrinsic reaction coordinate (IRC) calculations.
Geometries
(
17) Quintal, M. M.; Karton, A.; Iron, M. A.; Boese, A. D.; Martin, J.
M. L. J. Phys. Chem. A 2006, 110, 709.
18) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry;
Plenum: New York, 1976; Vol. 3, p 1.
19) Dolg, M. Effective Core Potentials. In Modern Methods and
3
1,32
the same means as 4.
Likewise, reaction of an equimolar
(
amount of Pt(PEt3)4 and 4-(iodo-phenyl)-pyridin-4-yl-diazene
6) in THF-d8 results first in quantitative η -coordination of
10
2
(
(
Algorithms of Quantum Chemistry; John von Neumann Institute for
Computing: J u¨ lich, 2000; Vol. 1, p 479.
(28) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
(29) Schunn, R. A. Inorg. Chem. 1976, 15, 208.
(30) (a) Cowan, R. L.; Trogler, W. C. J. Am. Chem. Soc. 1989, 111,
4750. (b) Ittel, S. D.; Ibers, J. A. J. Organomet. Chem. 1973, 57, 389. (c)
Dickson, R. S.; Ibers, J. A. J. Am. Chem. Soc. 1972, 94, 2988.
(
(
(
20) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
21) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408.
22) Cramer, C. J. Essentials of Computational Chemistry: Theories and
Models; John Wiley & Sons: Chichester, UK, 2002; p 347.
(
(
23) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117.
24) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett.
(31) CCDC-629320 and -629321 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk).
1
996, 255, 327.
(
(
(
25) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239.
26) Fukui, K. Acc. Chem. Res. 1981, 14, 363.
27) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154.
(32) Burdeniuk, J.; Milstein, D. J. Organomet. Chem. 1993, 451, 213.

4
532 Organometallics, Vol. 26, No. 18, 2007
Zenkina et al.
Figure 2. (A) 31P{ H} NMR follow-up measurements of the conversion of 2 f 5 in THF, 0.0289 M. (B) Eyring plot, R ) 0.989.
1
2
Table 1. Kinetic Studies of Conversion of Complex 2 to 5 at
the metal center to the NdN moiety to afford complex 7 prior
313 K in THF
to activation of the Ar-I bond (8), as judged by low-temperature
k × 10-5, s-1
1
31
1
2, M
PEt , equiv
H and P{ H} NMR spectroscopy (range: -65-0 °C).
3
2
Evidently, η -coordination of Pt(PEt3)2 to the NdN moiety of
1
2
3
4
0.0289
0.0722
0.0289
0.0289
6.7
6.3
6.3
6.2
2
2
10
1
2
and 6 is kinetically preferable over the formation of complexes
and 7, which involves the activation of the aryl-Br and aryl-I
bonds (bond dissociation energy (BDE), Ph-Br ) 80.5 kcal/
mol, Ph-I ) 65.5 kcal/mol).33
Table 2. Kinetic Studies of Competition Reactions between 2
and PhBr (9) at 313 K in THF
The thermolysis of complex 2 follows first-order kinetics in
q
THF in the range 303-323 K with ∆G 298K ) 24.6 ( 1.6 kcal/
k, s-1
PEt3,
equiv
9,
equiv
1,
equiv
product ratio
q
q
(×10-5)
mol, ∆H ) 26.5 ( 1.6 kcal/mol, and ∆S ) 6.6 ( 5.0 eu
2, M
(5:12)
(Figure 2). Mechanistically, the transformation of 2 f 5 could
1
2
3
4
5
0.0145
0.0145
0.0025
0.0145
0.0145
1
1
1
1
5:1
45:1
>65:1
>65:1
>65:1
7.1
6.9
6.3
7.9
6.3
proceed via Pt(PEt3)2 dissociation followed by activation of the
aryl-Br bond. Alternatively, the reaction may involve a metal
ring-walking process. The low entropy value rules out metal-
ligand dissociation as the slow step and could be consistent with
either an intramolecular process or metal-ligand dissociation
followed by aryl-halide oxidative addition as the rate-determin-
10
1
1
affected by the presence of free ligand.37 A pathway involving
bimetallic complexes is unlikely. No coordination of Pt was
3
4
ing step. In either case, oxidative addition appears to be the
3
1
1
observed by P{ H} NMR spectroscopy upon addition of 1
equiv of Pt(PEt3)4 to complex 5 (313 K in THF, ∼12 h).
rate-determining step since the iodide derivative (7) of complex
2
undergoes selective aryl-I activation at lower temperatures
35
In order to estimate the relative reactivity of 1 toward
(88% conversion after ∼17 h at 288 K in THF). Complex 2
oxidative addition by zerovalent platinum, 5 equiv of compound
is stable under these reaction conditions.
1
was reacted in dry THF at 40 °C with 1 equiv of Pt(PEt3)4 in
Solid-state formation of complex 5 from 2 was observed by
thermolysis of a powder of complex 2 for 11 days at 74 °C,
resulting in quantitative conversion to complex 5. This tem-
perature is well below the melting point of the powder
the presence of 5 equiv of PhBr (9), p-CF3PhBr (10), and p-NO2-
PhBr (11), respectively. Under these reaction conditions, the
31
1
following product distributions were observed by P{ H} NMR
spectroscopy after consumption of the platinum complex
precursor: 5:12 ) 1:0, 5:13 ) 2.5:1, and 5:14 ) 1:3. It appears
that the electron-withdrawing azo-pyridyl substituent activates
the aryl-Br moiety toward oxidative addition even more
strongly than the electron-withdrawing CF3 group, which has a
1
31
1
(
∼120 °C). No other products were observed by H or P{ H}
NMR spectroscopy. Similarly, nickel-anthracene complexes
undergo intramolecular dynamic processes in the solid state.
3
6
Therefore, we conclude that an intramolecular mechanism is
most likely.
As expected for a first-order process (in complex 2), no
significant concentration effect on k has been observed (Table
3
8
Hammett constant of σ ) 0.54 (Scheme 2).
In order to illustrate the control of the reaction outcome
afforded by this bond activation process, competition experi-
ments were performed by reacting stoichiometric amounts of
complex 2 and PhBr (9) in dry THF at 40 °C. The 31P{ H}
NMR spectroscopy showed the quantitative formation of
complexes 5 (83%) and the above-mentioned complex 12 (17%).
This is dissimilar to the outcome of the reaction of compounds
1 and PhBr (9) with Pt(PEt3)4 (Scheme 2). Complexes 5 and
1
). These observations could support either a unimolecular
process or a dissociative pathway via the reversible formation
of Pt(PEt3)2 and ligand 1, with aryl-halide oxidative addition
being the rate-determining step. The reaction kinetics and NMR
line shapes are affected neither by the presence of 0-10 equiv
of PEt3 nor by 1 equiv of compound 1 (Tables 1 and 2).
Bimolecular and PEt3 dissociation processes would likely be
1
1
2 were identified by addition of authentic samples to the
(
33) Lide, D. R. CRC Handbook of Chemistry and Physics, 83rd ed.;
CRC Press: Boca Raton, FL, 2002.
34) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
product solution. Interestingly, the formation of complex 12 can
be suppressed to ∼2% by the addition of 10 equiv of PEt3 to
the system and is not observable at all upon the addition of 1
equiv of compound 1. Decreasing the concentration of the
(
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
(35) The reaction of Pt(PEt3)4 with a structurally related halogenated
vinylarene also proceeds via a rate-determining aryl-Br oxidative addition
6
step to the platinum center.
(37) (a) Mann, B. E.; Musco, A. Dalton Trans. 1980, 776. (b) Northcutt,
T. O.; Wick, D. D.; Vetter, A. J.; Jones, W. D. J. Am. Chem. Soc. 2001,
123, 7257.
(38) Both azo and pyridyl moieties are electron-withdrawing; see:
Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
(36) (a) Cadenbach, T.; Gemel, C.; Schmid, R.; Fischer, R. A. J. Am.
Chem. Soc. 2005, 127, 17068. (b) Boese, R.; Stanger, A.; Stellberg, P.;
Shazar, A. Angew. Chem., Int. Ed. 1993, 32, 1475. (c) For a review, see:
Coville, N. J.; Cheng, L. J. Organomet. Chem. 1998, 571, 149.

Azobenzene Coordination by Platinum
Organometallics, Vol. 26, No. 18, 2007 4533
Figure 3. (A) Representative in situ 31P{ H} NMR follow-up measurements of the thermolysis of 2 (0.0145 M) in the presence of 1 equiv
of PhBr (9) at 313 K in THF (experimental points) with fitted kinetic model as shown in Scheme 3 (solid lines). (B) Effect of varying initial
concentration of reactants on concentration of 5 and 12 after 14 h (experimental points and model solid lines).
1
Scheme 2. Competitive Formation of Aryl-Pt(II) Complexes (5, 12-14)
Scheme 3. Proposed Reaction Pathways for Competition Reaction between Compounds 2 and PhBr (9)a
a
3
The transformation A + PEt f 5 might be a direct process and/or involve a reaction with ligand 1.
starting materials also resulted in the exclusive formation of
complex 5. No intermediates were observed. Although complex
the Kintecus integrator.39 Figure 3 shows (a) the fit of the model
to the experimental kinetics data and (b) the predicted final
concentration of 5 and 12 as a function of their starting
concentration, compared with experimental values. Models were
evaluated not only based on overall fit at the reaction conditions
but also based on robustness of fit for a range of initial reactant
concentration. The presented model was fit to kinetic follow-
up P{ H} NMR measurements with a starting concentration
of 0.0145 M (Figure 3A). This model was then run with various
starting concentrations in the range 0.0025-0.25 M; the outputs
are in excellent agreement with a series of experimental data
(Figure 3B). Interestingly, this model includes the unimolecular
ring-walking of complex 2 to 5, as well as the rate-determining
2
is the common precursor for both complexes 5 and 12, our
observations suggest that each is formed by a different and
distinct pathway. For instance, a unimolecular mechanism for
the formation of complex 5 and the bimolecular reaction of
bromobenzene (9) with complex 2 for the formation of complex
3
1
1
1
2 may permit the a priori design of reaction systems to obtain
desired products.
In order to elucidate the mechanism of the competition
reaction between compounds 2 and 9, the experimental data
were fit to a number of multistep models numerically, using
(
39) Ianni, J. C. Kintecus, Windows Version 2.80, 2002, can be found
at http://www.kintecus.com. For examples of the use of Kintecus see: (a)
Madhiri, N.; Olojo, R.; Simoyi, R. H. Phys. Chem. Chem. Phys. 2003, 5,
149. (b) Greenvvald, E. E.; Park, J.; Anderson, K. C.; Kim, H.; Reich, B.
J. E.; Miller, S. A.; Zhang, R.; North, S. W. J. Phys. Chem. A 2005, 109,
915. (c) Zhang, L.; Callahan, K. M.; Derbyshire, D.; Dibble, T. S. J. Phys.
Chem. A 2005, 109, 9232.
(40) (a) D o¨ mling, A. Chem. ReV. 2006, 106, 17. (b) Ram o´ n, D. J.; Yus,
M. Angew. Chem., Int. Ed. 2005, 44, 1602. (c) Von Wangelin, A. J.;
Neumann, H.; G o¨ rdes, D.; Klaus, S.; Str u¨ bing, D.; Beller, M. Chem.-Eur.
J. 2003, 9, 4286.
(41) For a review, see: Keane, J. M.; Harman, W. D. Organometallics
2005, 24, 1786.
4
7

4
534 Organometallics, Vol. 26, No. 18, 2007
Zenkina et al.
6
system was not observed. The transition-state geometry of the
3
latter reveals metal η -coordination (TS-(I-II)). The ring-
walking pathway is kinetically accessible via a coordination
shift, as the energy barriers are sufficiently low. Transition states
involving only one phosphine on the metal center have
significantly higher energy barriers by 10-15 kcal/mol.
Summary and Conclusions
2
Platinum η -coordination to a NdN moiety is kinetically
preferable over rate-determining oxidation addition of an aryl-
halide bond. The process (2 f 5) is not affected by the
concentration or presence of excess PEt3 and proceeds also in
the solid state. The relative reactivity of the azobenzene system
(1) with a zerovalent platinum phosphine complex is PhBr (9)
<
p-CF3PhBr (10) < 1 < p-NO2PhBr (11). The facile
coordination of the metal center to 1 can be exploited to direct
the selectivity of aryl-halide bond activation processes. The
presence of PEt3 permits control of competitive aryl-bromide
oxidative addition with PhBr (9) and complex 2. Apparently,
formation of products 5 and 12 proceed by dissimilar pathways.
Series of competition experiments have been used to numerically
derive a reaction model as shown in Scheme 3, which involves
a unimolecular transformation of complex 2 to 5. This ring-
walking process is kinetically accessible, as shown by DFT
calculations. We believe that the selectivity between the
activation of aryl-halides in this model system demonstrates
that the development of new synthetic strategies for selective
chemical transformations via ring-walking processes may be
Figure 4. Free-energy pathways for ring-walking step as calculated
at the PCM:PBE0/SDB-cc-pVDZ/PBE0/SDD level of theory utiliz-
ing the Gaussian03 program package. The ethyl substituents on
the phosphine ligands have been replaced by methyl groups to
reduce the computational cost. Hydrogen atoms are omitted for
clarity. Atomic color scheme: Br, red; C, orange; N, light blue;
Pt, yellow; P, pink.
dissociation of a phosphine of 2, which eventually leads to the
formation of 12, via intermediate A, and provides an alternate
minor route to 5 (Scheme 3). This is in good agreement with
the experimental observation that performing the competition
reaction in the presence of PEt3 prevents the formation of
complex 12, whereas the intramolecular conversion of 2 to 5 is
apparently unaffected. This model might not cover all mecha-
nistic aspects of this reaction. However, only models including
an intramolecular conVersion of 2 to 5 showed a possible fit.
DFT calculations were performed to determine the transition
states and the barriers connecting complex 2 and a metal-arene
intermediate (Figure 4). Note that PMe3 has been used instead
of PEt3 ligands. The movement of the metal center from the
NdN unit to the arene involves a haptotropic rearrangement
9
possible.
Acknowledgment. This research was supported by the Helen
and Martin Kimmel Center for Molecular Design, Minerva,
BMBF and MJRG. M.E.vd.B. is the incumbent of the Dewey
David Stone and Harry Levine career development chair.
Supporting Information Available: Crystallographic data
(CIF) for complexes 4 and 5, kinetic model details, and full
coordinates in .xyz format of all calculated complexes are available
free of charge via the Internet at http://pubs.acs.org.
2
1
2
from η -NdN to η -N (2 f I) via TS-(2-I) followed by η -
coordination of the metal center to the arene (II). A similar
haptotropic rearrangement for a structurally related stilbene
OM700519V