Fluxional behavior of platinum(0) complexes: Intra vs intermodular reaction pathways

Article abstract of DOI:10.1021/ic702433s

The fluxional behavior of two analogous platinum complexes has been studied in solution by NMR spectroscopy to elucidate the reaction mechanism and to determine the activation parameters. This includes variable temperature NMR spectroscopy, 2D ¹H-¹H exchange spectroscopy, and spin saturation transfer measurements. A platinum moiety, Pt(PEt₃) ₂, translocates between two carbon-carbon double bonds of two vinylpyridine moieties bridged by an arene (i.e., phenyl, anthracene) at elevated temperatures. Magnetization transfer NMR experiments in the presence of free ligands unambiguously revealed an intramolecular pathway for the phenyl system. An intermolecular pathway is proposed for the anthracene complex.

Full text of DOI:10.1021/ic702433s

Inorg. Chem. 2008, 47, 3815-3822
Fluxional Behavior of Platinum(0) Complexes: Intra vs Intermolecular
Reaction Pathways
Olena V. Zenkina,† Leonid Konstantinovski,‡ Dalia Freeman,† Linda J. W. Shimon,‡ and
Milko E. van der Boom*,†
Department of Organic Chemistry and Department of Chemical Research Support, Weizmann
Institute of Science, 76100 RehoVot, Israel
Received December 17, 2007
The fluxional behavior of two analogous platinum complexes has been studied in solution by NMR spectroscopy
to elucidate the reaction mechanism and to determine the activation parameters. This includes variable temperature
1
1
NMR spectroscopy, 2D H- H exchange spectroscopy, and spin saturation transfer measurements. A platinum
moiety, Pt(PEt3)2, translocates between two carbon-carbon double bonds of two vinylpyridine moieties bridged by
an arene (i.e., phenyl, anthracene) at elevated temperatures. Magnetization transfer NMR experiments in the presence
of free ligands unambiguously revealed an intramolecular pathway for the “phenyl” system. An intermolecular pathway
is proposed for the “anthracene” complex.
Introduction
center is much weaker than η2-olefin coordination; one likely
rationale for this is the loss of aromaticity. The crystal
structures of an anthracene-nickel complex and related
systems provided direct evidence for room temperature η2-
coordination of an aromatic hydrocarbon to a transition
metal.20 Similar interactions have been observed with (per)
fluorinated aromatic systems and are known to be involved
Arene complexes are thought to be intermediates in many
metal-mediated transformations, including synthetically im-
portant carbon-carbon coupling reactions.1–6 Studying and
understanding the fluxional behavior of transition-metal
complexes, including metal-arene interactions, is rarely an
easy task.7–30 Intermediates tend to be short-lived and occur
in minute concentrations. η2-Arene coordination to a metal
(14) Strawser, D.; Karton, A.; Zenkina, O. V.; Iron, M. A.; Shimon,
L. J. W.; Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc.
2005, 127, 9322.
*
To whom correspondence should be addressed. E-mail:
milko.vanderboom@weizmann.ac.il.
(15) Zenkina, O.; Altman, M.; Leitus, G.; Shimon, L. J. W.; Cohen, R.;
van der Boom, M. E. Organometallics 2007, 26, 4528.
(16) Zenkina, O. V.; Karton, A.; Freeman, D.; Shimon, L. J. W.; Martin,
J. M. L.; van der Boom, M. E. Inorg. Chem. 2008, in press.
(17) Iverson, C. N.; Lachicotte, R. J.; Müller, C.; Jones, W. D. Organo-
metallics 2002, 21, 5320.
†
Department of Organic Chemistry.
Department of Chemical Research Support.
‡
(1) Astruc, D. Modern Arene Chemistry: Concepts, Synthesis, and
Applications; Wiley: New York, 2002.
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synthesis; Springer: New York, 2004; Topics in Organometallic
Chemistry, Vol. 7.
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Oelckers, B.; Perutz, R. N. Dalton Trans. 2001, 1452.
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10.1021/ic702433s CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 9, 2008 3815
Published on Web 03/28/2008

Zenkina et al.
Scheme 1. Two Examples of Metal-Arene Complexes in which the
Metal Center Migrates via an Intramolecular Pathway over the Aromatic
Systems10,11,21,23
Scheme 2. Examples of Reactions in which “Ring Walking” Processes
are Coupled with Arene Functionalization14–16
(BDE), Ph-Br ) 80.5 kcal/mol vs Ph-I ) 65.5 kcal/mol).38
“Ring-walking” coupled with C-halide activation has syn-
thetic utility, as demonstrated by Yokozawa et al.39–41 They
demonstrated that nickel-catalyzed chain growth polymeri-
zation of poly(p-phenylene) and poly(3-hexylthiophene)
proceeds by intramolecular transfer of the Ni(0) catalyst,
followed by selective cleavage of only one of the C-Br
bonds. Furthermore, Harman et al. reported arene function-
alization by means of various intramolecular structural
rearrangements with a series of osmium complexes.42,43
Interestingly, metal-substrate interactions coupled with bond
activation/formation are not limited to arenes and thiophenes;
a metal center can even bind to aliphatic hydrocarbons and
“walk” along the chain before activation of a C-H bond.44,45
Intramolecular processes involving metal-arene interac-
tions are especially interesting when mechanistic details can
be obtained. In this report, we replaced the halide (X ) Br,
I) in the platinum stilbazole system (Scheme 2, III, Y )
CH) with a vinylpyridine moiety in order to eliminate the
irreversible formation of complex IV. This allows the metal
center to migrate between the two CdC bonds linked via
an arene in a reversible manner. This process has been
studied here in solution by NMR spectroscopy to elucidate
the underlying reaction mechanism and reaction parameters.
This includes variable temperature (VT) NMR spectroscopy,
2D 1H-1H exchange spectroscopy (EXSY), and spin satura-
tion transfer (SST) measurements. Furthermore, the intramo-
lecular nature of the “ring walking” process has been
demonstrated by SST experiments in the presence of free
ligand.
in aryl-F bond activation.18,31 Interestingly, the first studies
on reaction pathways involving metal-arene interactions
were already carried out in the 1960s by the late Fred
Basolo.12,13 Other examples include the intramolecular
migration of a (R3P)2Ni (R ) Et, Bu) moiety over an
anthracene ligand via a series of η2-η3 haptotropic rear-
rangements along the system’s outer carbon-carbon double
bonds (Scheme 1, I a I′),10,21,23 and the fluxional behavior
of a d8 rhodium corannulene complex (II).11 Siegel, Bald-
ridge, and Dorta reported recently that a metal center can
even migrate over the curved aromatic surface of a coran-
nulene ligand (II) without metal–ligand dissociation.11 In
addition to numerous experimental studies,7–30 metal-arene
interactions have been studied extensively using various
computational methods.14–16,18,31–35
We reported very recently that the reaction of Ni(PEt3)4
with a halogenated vinyl-arene results in η2-CdC coordina-
tion of Ni(PEt3)2, followed by “ring-walking” of the metal
center to yield the thermodynamically favorable product of
aryl-halide oxidative addition (Scheme 2, III f IV).16
Similar processes were observed earlier by us with Pt(PEt3)2
using halogenated vinyl-arenes and azobenzene-derivatives.15
However, the aryl-halide bond activation step was found to
be rate determining with platinum,14,15 thus making it
difficult to obtain detailed information about the preceding
processes. Intramolecular reaction pathways have been used
to direct a metal center to a specific aryl-bond,15–17,36,37
making it even possible to activate relatively strong aryl-Br
bonds in the presence of substrates containing much weaker
aryl-I bonds (e.g., compare Bond Dissociation Energy
Experimental Section
Materials and Methods. All reactions were carried out in a N2-
filled M. Braun glovebox with H2O and O2 levels <2 ppm. Solvents
were reagent grade or better, dried, distilled, and degassed before
being introduced 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 4 Å activated
molecular sieves in the glovebox. Reaction flasks were washed with
deionized (DI) water, followed by acetone, and then oven-dried at
130 °C prior to use. Mass-spectrometry was carried out using a
(31) Higgit, C. L.; Klahn, A. H.; Moore, M. H.; Oelckers, B.; Partridge,
M. G.; Perutz, R. N. J. C. S. Dalton Trans. 1997, 1269.
(32) Cohen, R.; Weitz, E.; Martin, J. M. L.; Ratner, M. A. Organometallics
2004, 23, 2315.
(38) CRC Handbook of Chemistry and Physics, 83rd ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 2002–2003.
(33) Reinhold, M.; McGrady, J. E.; Perutz, R. N. J. Am. Chem. Soc. 2004,
126, 5268.
(39) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005,
127, 17542.
(34) Ste¯pien, M.; Latos-Graz¯yn¯ski, L.; Szterenberg, L.; Panek, J.; Latajka,
Z. J. Am. Chem. Soc. 2004, 126, 4566.
(40) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am.
Chem. Soc. 2006, 128, 128.
(35) Di Bella, S.; Lanza, G.; Fragalà, I. L.; Marks, T. J. Organometallics
1996, 15, 3985.
(41) Yokoyama, A.; Yokozawa, T. Macromolecules 2007, 40, 4093.
(42) Harman, W. D. Coord. Chem. ReV. 2004, 248, 853.
(43) Keane, J. M.; Harman, W. D. Organometallics 2005, 24, 1786.
(44) Northcutt, T. O.; Wick, D. D.; Vetter, A. J.; Jones, W. D. J. Am. Chem.
Soc. 2001, 123, 7257.
(36) Slagt, M. Q.; Rodriguez, G.; Grutters, M. M. P.; Gebbink, R. J. M. K.;
Klopper, W.; Jenneskens, L. W.; Lutz, M.; Spek, A. L.; van Koten,
G. Chem.sEur. J. 2004, 10, 1331.
(37) Rodriguez, G.; Albrecht, M.; Schoenmaker, J.; Ford, A.; Lutz, M.;
Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2002, 124, 5127.
(45) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961.
3816 Inorganic Chemistry, Vol. 47, No. 9, 2008

Fluxional BehaWior of Platinum(0) Complexes
2
1
Micromass Platform LCZ 4000 system. Elemental analyses were
performed by H. Kolbe, Mikroanalytisches Laboratorium, Mülheim
an der Ruhr, Germany. Ligands 1, 4, and Pt(PEt3)4 were prepared
by published procedures.46–49
(AM part of the AMX system, 1P, JPP ) 43 Hz, JPtP ) 3523.2
Hz). 195Pt{1H} NMR (C6H6): δ -5111.2 (dd, 1Pt, X part of the
1
1
AMX system, JPPt ) 3523.6 Hz, JPPt ) 3631.1 Hz). Calc m/e:
715.3, found m/e: 715.0. Anal. Calcd for C32H46N2P2Pt: C, 53.70;
H. 6.48. Found: C, 53.57; H, 6.55.
1
Spectroscopic Analysis. The H, 13C{1H}, and 31P{1H} NMR
spectra were recorded at 400.19, 100.6, and 161.9 MHz, respec-
tively, on a Bruker Avance 400 NMR spectrometer. The 195Pt{1H}
NMR spectra were recorded at 107.04 MHz on a Bruker Avance
500 NMR spectrometer. All chemical shifts (δ) are reported in ppm
and coupling constants (J) in Hz. The 1H and 13C{1H} NMR
chemical shifts are relative to tetramethylsilane; the resonance of
the residual protons of the solvent was used as the internal standard
(7.15 ppm, benzene; 7.09 ppm, toluene; 6.95 and 2.20 ppm,
o-xylene, 3.58 ppm, and 1.73 ppm, tetrahydrofuran) and all
d-solvent peaks (128.0 ppm, benzene; 20.4 ppm, toluene; 19.6 ppm,
o-xylene, tetrahydrofuran 67.6 ppm, and 25.3 ppm), respectively.
31P{1H} NMR chemical shifts are relative to 85% H3PO4 in D2O
(external reference), with shifts downfield of the reference consid-
ered positive. All measurements were carried out at 298 K unless
otherwise stated. Screw-cap 5-mm tubes were used in NMR follow-
up experiments. Vacuum/pressure valve 5-mm NMR sample tubes
were used for all VT and magnetization transfer experiments.
Assignments of the 1H and 13C{1H} NMR spectra were made using
gs-COSY, 1H{31P}, and 13C-DEPT-135 NMR experiments. VT 1H
NMR spectra were recorded on a Bruker Avance 400 NMR
spectrometer in o-xylene-d10 at temperatures ranging from 183 to
393 K. Temperature calibration of the spectrometer was performed
using CH3OH/CD3OD (<279 K) and HOCH2CH2OH/d6-DMSO
(>298 K). Temperature accuracy of the VT experiments was (
0.2 °C. All 2D spectra (COSY, gs-NOESY) were acquired in the
phase-sensitive mode. All data was acquired, processed, and
displayed using Bruker XWinNMR software and a standard pulse-
sequence library.
Formation of Complex 3. A solution of Pt(PEt3)4 (116 mg, 0.174
mmol) in 10 mL of THF was dropwise added to a solution of bis-
1,4-(4-pyridylethyl-enyl)-benzene (1)46 (25 mg, 0.087 mmol) in 6
mL of dry THF and stirred at room temperature. 31P{1H} NMR
spectroscopy of an aliquot indicated the formation of complexes 2
and 3 in a ratio of 3.9:1 after 1 day. All volatiles were removed in
vacuo after 1 week in order to remove PEt3. The residue was
redissolved in 4 mL of THF, and the reaction was continued.
31P{1H} NMR spectroscopy of an aliquot indicated the formation
of complexes 2 and 3 in a ratio of 2.3:1 after 10 days. All volatiles
were removed in vacuo after 15 days, and the residue was washed
with ∼6 mL of dry pentane. Subsequently, the remaining solid was
redissolved in 5 mL of THF and stirred for an additional 15 days.
The volatiles were again removed, and the residue was redissolved
in 1.5 mL of THF. Subsequently, 5 mL of pentane was added,
1
resulting in X-ray quality crystals at -30 °C. H NMR (C6D6): δ
8.1 (d, 4H, PyrH, JHH ) 4.7 Hz), 6.85 (br, 8H, ArH), 4.25 (m, 4H,
CH)CH), 1.57 (m, 24H, PCH2CH3), 0.92 (m, 36H, PCH2CH3).
13C{1H} NMR (C6D6): δ 157.7 (d, Cq, JPtC ) 48.6 Hz, JPC ) 5.1
Hz), 151.9, 148.9, 143.5 (m, br, Cq), 128.2 (d, JPtC ) 61.5 Hz, JPC
) 3.4 Hz), 124.8 (d, br, JPtC ) 66.2 Hz), 119.8, 50.0 (dd,
η2-CH)CH, JPC ) 31.8 Hz, JPC ) 6.2 Hz), 48.9 (dd, η2-CH)CH,
JPC ) 31.8 Hz, JPC ) 6.4 Hz), 20.0 (m, PCH2CH3, JPtC ) 54.0 Hz,
JPC ) 37.6 Hz, JPC ) 23.7 Hz), 8.10 (dt, PCH2CH3, JPC ) 23.1
Hz, JPC ) 2.4 Hz). 31P{1H} NMR (toluene-d8): δ 16.8 (d, 1P, 2JPP
) 45.7 Hz, 1JPtP ) 3695.4 Hz), δ 15.4 (d, 1P, 2JPP ) 48.0 Hz, 1JPtP
) 3459.7 Hz) 195Pt{1H} NMR (C6H6, 107.04 MHz): δ -5072.3
1
1
(ddd, 2Pt, JPPt ) 3696.1 Hz, JPPt ) 3458.2 Hz). Anal. Calcd for
C44H76N2P4Pt2: C, 46.07; H. 6.68. Found: C, 46.07; H, 6.32.
Formation of Complex 3 from Complex 2 and Pt(PEt3)4. A
solution of Pt(PEt3)4 (66.7 mg, 0.100 mmol) in 8 mL of THF was
added dropwise to a solution of complex 2 (66 mg, 0.092 mmol)
in 6 mL of dry THF and stirred at room temperature. 31P{1H} NMR
spectroscopy of an aliquot indicated the presence of complexes 2
and 3 in a ratio of 1:1 after 1 day. All volatiles were removed in
vacuo after 3 days in order to remove PEt3, and the residue was
redissolved in 4 mL of THF. 31P{1H} NMR spectroscopy of an
aliquot indicated the formation of complexes 2 and 3 in a ratio of
1:4 after 8 days. The reaction was completed in 27 days, and
complex 3 was isolated in good yield (82%) by washing with 1.5
mL of cold (-30 °C) pentane.
Formation of Complex 2. A solution of Pt(PEt3)4 (59 mg, 0.088
mmol) in 5 mL of dry tetrahydrofuran (THF) was dropwise added
to a solution of bis-1,4-(4-pyridylethyl-enyl)-benzene (1)46,48,49 (25
mg, 0.088 mmol) in 5 mL of dry THF and stirred at room
temperature for 15 min. Removal of all volatiles in vacuo and
washing of the residue with ∼3 mL of dry pentane afforded the
1
analytically pure complex 2 as a yellow solid in 85% yield. H
NMR (o-xylene-d10): δ 8.64–8.69 (A part of AB system, 2H, PyrH,
JHH ) 6.3 Hz), 8.51–8.47 (B part of AB system, 2H, PyrH, JHH
)
5.9 Hz), 7.39 (d, 2H, ArH, JHH ) 4.5 Hz), 7.27 (d, 2H, ArH, JHH
) 7.8 Hz), 7.11 (d, CH)CH, 1H, JHH ) 16.2 Hz), 6.99 (d, 2H,
ArH, JHH ) 4.5 Hz), 6.97 (d, 2H, ArH, JHH ) 5.9 Hz), 6.73 (d,
CH)CH, 1H, JHH ) 16.2 Hz), 3.83 (m, 1H, η2-CH)CH), 3.70
(m, 1H, η2-CH)CH), 1.23 (m, 12H, PCH2CH3), 0.91 (m, 18H,
X-ray Analysis of Complex 3. Crystal Data. 1/2(C44H76N2P4Pt2),
PCH2CH3). 13C{1H} NMR (acetone-d6): δ 157.3 (dd, Cq, JPtC
)
yellow, prism, 0.2 × 0.2 × 0.2 mm , triclinic, P1 (No. 2), a )
3
j
47.8 Hz, JPC ) 5.8 Hz, JPC ) 1.6 Hz), 151.9, 150.0, 148.3, 148.2,
145.1 (s, Cq), 133.3, 132.3, 130.7 (Cq, JPtC ) 12.2 Hz, JPC ) 2.7
Hz), 128.2 (Cq), 127.5, 126.4 (dt, JPtC ) 17.6 Hz, JPC ) 2.7 Hz),
122.7 (s), 120.7, 120.4, 119.6 (d, JPtC ) 17.7 Hz, JPC ) 2.4 Hz),
50.6 (dd, η2-CH)CH, JPtC ) 207.5 Hz, JPC ) 34.1 Hz, JPC ) 4.8
9.150(2) Å, b ) 10.656(2) Å, c ) 13.211(3) Å, R ) 76.63(3)°, ꢀ
) 79.76(3)°, γ ) 79.10(3)°, T ) 120(2) K, V ) 1218.4(4) Å3, Z
) 1, Fw ) 571.55, Dc ) 1.558 Mg m-3, µ ) 5.896 mm-1
.
Data Collection and Processing. Nonius KappaCCD diffrac-
tometer, Mo KR (λ ) 0.71073 Å), graphite monochromator, -11
e h e 11, -13 e k e 13, 0 e l e 17, frame scan width ) 2°,
scan speed 1.0° per 40 s, typical peak mosaicity 0.43°, 23018
reflections collected, 5530 independent reflections (R-int ) 0.066).
The data were processed with Denzo-Scalepack.
Solution and Refinement. The structure was solved by the
Patterson method with SHELXS-97.50 The full matrix least-squares
refinement is based on F2 with SHELXL-97 235 parameters with
2 restraints, final R1 ) 0.034 (based on F2) for data with I > 2σ(I)
Hz), 48.9 (dd, η2-CH)CH, JPtC ) 198.0 Hz, JPC ) 32.5 Hz, JPC
)
5.5 Hz), 19.4 (d of m, PCH2CH3, JPtC ) 54.0 Hz, JPC ) 37.6 Hz,
JPC ) 23.7 Hz), 7.8 (m, PCH2CH3, JPtC ) 31.0 Hz, JPC ) 16.5 Hz,
JPC ) 7.2 Hz). 31P{1H} NMR (o-xylene-d10): δ ) 15.5 (AM part
of the AMX system, 1P, JPP ) 43 Hz, JPtP ) 3631.5 Hz), 14.9
(46) Detert, H.; Stalmach, U.; Sugiono, E. Synt. Met. 2004, 147, 227.
(47) Schunn, R. A. Inorg. Chem. 1976, 15, 208.
(48) Amorso, A. J.; Cargill Thompson, A. M. W.; Mahes, J. P.; McCleverty,
J. A.; Ward, M. W. Inorg. Chem. 1995, 34, 4828.
(49) Vatsadze, S. Z.; Nuriev, V. N.; Chernikov, A. V.; Zyk, N. V. Russ.
Chem. Bull. 2002, 51, 1957.
(50) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Deter-
mination; University of Göttingen: Göttingen, Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3817

Zenkina et al.
and R1 ) 0.049 on 5525 reflections, goodness-of-fit on F2 ) 1.041,
largest electron density peak ) 2.991e.Å-3
the standard inversion–recovery technique, combined with the
saturation of the target peak resonance (the signal of the other
olefinic proton at δ 6.73 ppm was partially overlapped with a peak
of the solvent). The rate constants, kobs, were calculated using the
Forsen-Hoffman equation: M∞/Mc ) RB/(RB + kobs), where Robs
) RB + kobs and M∞ is the area of the signal at δ 7.11 ppm during
saturation of the target resonance at δ 3.8 ppm, and Mc is the signal
intensity of the control experiment.51,52 Similar SST experiments
were performed with complex 5 (0.0245 mmol/mL; o-xylene-d10).
SST experiments were performed within the range 35–60 °C by
selective saturation of the signals at δ 5.59 and 4.05 ppm,
corresponding to the olefinic protons of the sCdCs moiety η2-
coordinated to the metal center. The response of the signals of the
“free” olefinic protons at δ 7.83 and 6.5 ppm was identified by
comparison with control experiments. The responses are quantitative
at 60 °C.
Spin Saturation Transfer Measurements: Competition Ex-
periments. In a representative experiment, an NMR sample
containing a mixture of complex 2 (5.5 mM) and ligand 1 (2.25
mM) in C6D6 was used. SST experiments were performed by
preirradiation of desired resonances utilizing irradiation powers
sufficient to completely annihilate the target peaks. For instance,
the multiplet at δ ∼3.8 ppm, corresponding to the olefinic protons
of the CdCs moiety η2-coordinated to the metal center, was chosen
as the target signal. Subsequently, the responses were verified by
comparison with control experiments, which include irradiation of
signal-free areas under the same conditions. At room temperature,
preirradiation of the olefinic protons results only in a positive NOE
response of the adjacent aromatic protons of complex 2 owing to
the closeness in space. At 40–47 °C, preirradiation of the olefinic
protons of the C)C moiety η2-coordinated to the metal center yields
quantitative spin saturation transfer (negative response and an
intensity decrease) to the “free” olefinic protons as well as positive
NOE responses of the aromatic protons of complex 2. The free
ligand’s (1) olefinic proton resonances remained unchanged with
respect to the control experiments. VT SST competition experiments
between complex 5 and ligand 4 were also performed; however,
there is overlap of signals. Preirradiation of the protons of the η2-
CdC moiety of complex 5 (δ 4.05 ppm and at δ ) 5.59 ppm) at
45 °C yields spin saturation transfer (160%) to the protons of the
C)C groups of complex 5 and ligand 4 (these signals are
overlapped). Performing this experiment with complex 5 results in
a much smaller response (40%) under identical conditions.
.
Formation of Complex 5. A solution of Pt(PEt3)4 (23 mg, 0.034
mmol) in 3 mL of THF was dropwise added to a solution of
trans,trans-9,10-bis(4-pyridylethenyl)anthracene (4)46 (13 mg, 0.034
mmol) in 5 mL of dry THF and stirred at room temperature for 15
min. Removal of all volatiles in vacuo and washing of the residue
with ∼6 mL of dry pentane afforded the analytically pure complex
1
5 as an orange solid in 83% yield. H NMR (C6D6): δ 9.38 (br,
2H, PyrH), 8.68–8.48 (q, AB system, 4H, JHH ) 4.9 Hz, ArH),
8.39–8.33 (m, PyH, 2H, JHH ) 8.9 Hz), 7.83 (dd, CH)CH, 1H,
JHH ) 16.5 Hz), 7.3 (br, 4H, PyrH), 6.9 (m, br, 4H, ArH, JHH
)
5.3 Hz), 6.5 (dd, CH)CH, 1H, JHH ) 16.5 Hz), 5.59 (m, 1H,
CH)CH, JPtH ) 53.5 Hz), 4.05 (m, 1H, CH)CH, JPtH ) 41.9 Hz),
1.42–1.14 (d of m, 12H, PCH2CH3), 0.93–0.65 (d of m, 18H,
PCH2CH3). 13C{1H} NMR (C6D6): δ 158.5 (dt, Cq, JPtC ) 44.3
Hz), 151.8, 150.6, 149.3, 144.1 (s, Cq), 143.6 (s, Cq), 135.5, 134.6,
132.5, 130.5, 130.3 (s, Cq), 129.4 (s, Cq), 128.0, 126.8, 126.4, 125.8,
124.9, 123.9 (br), 120.7, 120.6, 119.77 (s, JPtC ) 16.1 Hz), 54.0
(dd, η2-CH)CH, JPtC ) 230.3 Hz, JPC ) 31.7 Hz, JPC ) 4.3 Hz),
46.9 (dd, η2-CH)CH, JPtC ) 215.0 Hz, JPC ) 31.7 Hz, JPC
)
2.8 Hz), 20.3 (doublet of m, PCH2CH3, JPtC ) 63.6 Hz, JPC ) 23.2
Hz, JPC ) 3.1 Hz), 8.11 (m, PCH2CH3, JPtC ) 23.3, JPC ) 17.9
Hz). 31P{1H} NMR (C6D6): δ 15.2 (AM part of the AMX system,
1P, 2JPP ) 40.9 Hz, 1JPtP ) 3590.6 Hz), 14.9 (AM part of the AMX
2
1
system, 1P, JPP ) 40.9 Hz, JPtP ) 3540.3 Hz. 195Pt{1H} NMR
(C6D6, 107.04 MHz): δ -5067.2 (dd, 1Pt, X part of the AMX
1
1
system, JPPt ) 3590.0 Hz, JPPt ) 3540.4 Hz). Calcd m/e, 815.8;
found m/e, (M + 1) ) 817.2: Anal. Calcd for C40H50N2P2Pt: C,
58.89; H. 6.18, N, 3.43. Found: C, 58.15; H. 6.25, N, 3.16.
Variable Temperature (VT) 1H NMR Experiments with
Complexes 2 and 5. A solution of complex 2 (25 mg, 0.035 mmol)
in 1 mL of o-xylene-d10 was loaded in a 5-mm screw-cap NMR
tube that was sealed using Teflon tape and parafilm. The temperature
1
dependence was followed by H and 31P{1H} NMR spectroscopy
1
in the range of -90 to 120 °C. The room temperature H NMR
spectrum of complex 2 shows well-resolved signals for all protons
of complex 2, and upon increasing the temperature, all the signals
start to broaden. The process is reversible. The 31P{1H} NMR
spectra shows some broadening at higher temperatures. No un-
complexed ligand (1) and Pt(PEt3)x (x ) 2–4) were observed. The
1H NMR spectrum is not affected by cooling the sample to -90
°C. No concentration effects were observed within the range of
0.0035–0.035 mmol/mL for complex 2 in o-xylene-d10. Performing
the VT experiments in the presence of PEt3 (2 equiv) did not affect
the spectra. VT 1H NMR spectroscopy was performed with complex
5 within the range of 35-110 °C in o-xylene-d10. No concentration
effects were observed when comparing samples containing 0.0078
mmol/mL versus 0.0306 mmol/mL of complex 5. Performing the
VT experiment with 0.0306 mmol/mL of complex 5 in the presence
of PEt3 (2 equiv) resulted in the formation of uncomplexed ligand
4 and Pt(PEt3)3.
Results and Discussion
Reaction of an equimolar amount of Pt(PEt3)4 with bis-
1,4-(4-pyridylethyl-enyl)-benzene (1)46 in dry THF at room
temperature for ∼10 min resulted in the quantitative forma-
tion of complex 2 (Scheme 3). Complex 2 was formed
quantitatively, as judged by 31P{1H} NMR spectroscopy,
1
isolated (85%), and was characterized by combining H,
13C{1H}, 31P{1H}, and 195Pt{1H} NMR spectroscopy, ele-
mental analysis, and mass spectrometry. Assignment of the
1H and 13C{1H} NMR spectra was made using COSY and
13C-DEPT135 NMR spectroscopy. Coordination of the metal
center to one of the C)C bonds resulted in an unsymmetrical
system, as revealed by 1H and 13C{1H} NMR spectroscopy.
An AMX spin system was observed by 31P{1H} and
Spin Saturation Transfer (SST) Measurements: Deter-
mination of Activation Parameters. In a typical experiment, a
5-mm screwcap NMR tube with a o-xylene-d10 solution of complex
2 (0.0279 mmol/mL) was placed in a Bruker Avance 500 NMR
instrument. SST experiments were performed by selective saturation
of the signal at δ ∼ 3.8 ppm, corresponding to the olefinic pro-
tons of the C)C moiety η2-coordinated to the metal center. The
response of the signals of the “free” olefinic protons at δ 7.11 and
6.73 ppm was identified by comparison with control experiments.
All measurements were repeated at least 2 times. The relaxation
rate (Robs) of the olefinic proton at δ 7.11 ppm was measured using
(51) Lian, L.-Y.; Roberts, G. C. K. NMR of macromolecules, The practical
approach; IRL Press: Oxford, 1993.
(52) Forsen, S.; Hoffman, R. A. J. Chem. Phys. 1964, 40, 1189.
3818 Inorganic Chemistry, Vol. 47, No. 9, 2008

Fluxional BehaWior of Platinum(0) Complexes
Scheme 3. Selective Formation of Complexes 2 and 3 by η2-Coordination of Pt(PEt3)2 to the C)C Bonds of Ligand 1
195Pt{1H} NMR spectroscopy. In the 31P{1H} NMR spectrum,
the AM part appears at δ ∼ 15 ppm as a quartet with 2JPP(cis)
) 43 Hz, flanked by platinum satellites (1JPtP ≈ 3500 Hz).
In the 195Pt{1H} NMR spectrum, the X part of the AMX
system appears as a double doublet at δ -5111.2 with 1JPtP
) 3500–3600 Hz. The η2-coordination of the metal center
of P(1)-Pt(1)-P(2) ) 103.84° and C(6)-Pt(1)-C(7) )
40.26°. These characteristic features are typical for Pt(0)-
olefin complexes.4 A common plane between the three
aromatic rings is not observed. Instead, the two pyridyl
groups are twisted in opposite directions with respect to the
plane of the central aromatic ring. The angle between each
pyridyl group and the central aromatic ring is 41.36°. This
conformation is most likely due to the steric hindrance of
the ethyl groups. Apparently, some electron delocalization
takes place at the expense of aromaticity of the pyridine units
and the central aromatic rings. For example, the structural
data shows that C(2)-C(3), C(4)-C(5), and C(9)-C(12)
become localized double bonds at 1.392(8), 1.370(7), and
1.377(7) Å, respectively. Whereas the C(1)-C(2), C(1)-C(5),
1
to one of the C)C moieties is apparent from the H and
13C{1H} NMR spectra, which exhibit signals typical for such
interactions at δ 3.8 and at δ 50.6 ppm, respectively. The
two carbon atoms of the η2-CdC moiety are inequivalent
and are coupled with 195Pt and with both cis and trans PEt3
ligands.
Reaction of complex 2 with an equimolar amount of
Pt(PEt3)4 or reaction of ligand 1 with two equivalents of
Pt(PEt3)4 resulted in the selective formation of the bimetallic
complex 3 after 27 days (Scheme 3). The reaction was
stopped several times to remove released phosphine to drive
the reaction to completion. Apparently, coordination of the
second metal center to complex 2 is more than 3 orders of
magnitude slower in comparison with the formation of the
monometallic system (2). The rather sluggish reactivity of
complex 2 with Pt(PEt3)4 is probably due to steric hindrance
because the 1H and 31P{1H} NMR data of complex 2 indicate
that the nature of the “free” C)C unit is not affected by
binding of the metal center to the ligand system (1). Attempts
to form complex 3 at higher temperatures (e.g., 80 °C) did
not appreciably improve the reaction time. Complex 3 was
fully characterized by combining 1H, 13C{1H}, 31P{1H}, and
195Pt{1H} NMR spectroscopy, elemental analysis, mass
spectrometry, and by single-crystal X-ray crystallography.
The X-ray analysis unambiguously confirms the bimetallic
nature of complex 3, having the metal centers η2-coordinated
to the two C)C bonds of ligand 1 in trans position (Figure
1) The coordination of the platinum moieties induces
significant lengthening of the C)C bonds (i.e., C(6)-C(7)
) 1.467 Å) in comparison with those of the ligand (1)
(1.327Å).53 Complex 3 exhibits a nearly planar geometry
around the metal centers. Each metal center is in a plane
containing a C)C bond and the PEt3 ligands, with angles
Figure 1. ORTEP diagram of complex 3 with thermal ellipsoids set at
50%, probability showing the η2-coordination of the two Pt(PEt3)2 groups
1
to the C)C bonds. There is
/ molecule of complex 3 per asymmetric
2
unit. Hydrogen atoms are omitted for clarity. Selected bond length (Å) and
angles (°): Pt(1)-C(7) 2.129(4), Pt(1)-C(6) 2.131(5), Pt(1)-P(2) 2.2664(13),
Pt(1)-P(1) 2.2754(14), C(1)-C(2) 1.415(6), C(2)-C(3) 1.392(8), C(1)-C(5)
1.412(7), C(4)-C(5) 1.370(7), C(1)-C(6) 1.468(7), C(6)-C(7) 1.467(6),
C(7)-C(8) 1.477(6), C(8)-C(9) 1.403(6), C(9)-C(12) 1.377(7), C(8)-C(12)
1.414(6), C(6)-Pt(1)-C(7) 40.28(16), C(7)-Pt(1)-P(2) 106.86(12),
C(6)-Pt(1)-P(2) 147.12(12), C(7)-Pt(1)-P(1) 109.03(12), P(1)-Pt(1)-P(2)
103.84(5).
(53) Friscic, T.; MacGillivray, L. R. Chem. Commun. 2003, 1306.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3819

Zenkina et al.
Scheme 4. Migration of the Pt(PEt3)2 Moeity of Complex 2 between
the two C)C Bonds of Ligand 1 at Elevated Temperatures
and C(8)-C(12) bonds are relatively long (1.415(6), 1.412(7),
1.414(6) Å, respectively). The C-C bond lengths of the
aromatic rings of the free ligand (1)53 all fall within the range
of 1.373–1.395 Å. They have some localized character, but
they are still clearly primarily aromatic. As expected, the
π-backdonation from the d orbitals of the electron-rich Pt
centers to the π* antibonding orbitals of the C)C bonds
results in weakening and hence bond lengthening. This also
affects the C(1)-C(6) and C(7)-C(8) bond lengths (1.468(7)
and 1.477(6) Å, respectively) in comparison with those of
the uncomplexed ligand 1.53
Figure 2. VT 1H NMR spectra of complex 2 (25 mg/mL) in o-xylene-d10.
1
VT H NMR spectroscopy studies were performed in
o-xylene-d10 to evaluate the dynamic behavior of complex
2. At room temperature, the 1H NMR spectrum of complex
2 shows relatively sharp resonances that broaden at elevated
temperatures. At temperatures above 100 °C, coalescence
of some signals is observed. Apparently, the system reaches
near-fast exchange at higher temperatures, suggesting a
process in which the metal center migrates between the two
C)C bonds (2 a 2′), as shown in Scheme 4.
Regardless of the exact nature of this fluxional process,
upon cooling, the original 1H NMR spectrum of complex 2
is again observed. Extraction of detailed kinetic information
from band shape analysis requires the NMR spectra to be
acquired under slow, intermediate, and fast chemical ex-
change conditions. However, it was not possible to reach
the high temperatures (>120 °C) necessary for fast exchange
conditions because of the experimental limitation imposed
by the NMR instrumentation. Varying the concentration of
complex 2 (2.5–25 mg/mL) does not affect the VT NMR
spectra, indicating that the overall process is first order in
complex 2 (Figure 2). Performing the VT NMR experiments
in the presence of 2 equiv of PEt3 also did not affect the
system, which might indicate an intramolecular process. In
any case, the slow formation of complex 3 makes it highly
unlikely that bimetallic complexes are involved (Scheme 3).
Phase-sensitive 2D 1H-1H EXSY VT NMR measurements
were carried out to verify that the metal center migrates
between the two C)C bonds at elevated temperatures. This
experiment, using a phase-sensitive NOESY pulse sequence,
provides a visual map of the exchange matrix under
conditions of slow exchange (Figure 3). Positive cross-peaks
between the protons of the two C)C bonds were observed
in the temperature range of 50–90 °C and are in agreement
with a metal migration process between these two moieties.
SST experiments were performed with complex 2 within
the temperature range of 35–80 °C to determine the activation
parameters. A negative response for the “free” C)C bond
was observed by selective saturation of the protons of the
C)C bond η2-coordinated to the metal center. The negative
1
Figure 3. VT H-1H EXSY NMR spectrum of complex 2 in toluene-d8
at 50 °C showing exchange cross-peaks, demonstrating that both the C)C
moieties are involved in chemical exchange processes. The cross-peaks are
more intense at 90 °C, whereas at room temperature no correlations were
observed.
response increases as the temperature increases. The rate
constants, kobs, were calculated by means of the Forsen-
Hoffman equation.51,52 Repeat measurements yielded almost
identical values. The activation parameters, ∆H‡ ) 21.9 (
1.2 kcal/mol, ∆S‡ ) 6.4 ( 2.5 e.u., and ∆G‡ ) 19.4 ( 1.2
kcal/mol, for the rearrangement 2 a 2′ were derived from
the Eyring plot shown in Figure 4.
Mechanistically, the transformation of 2 a 2′ may proceed
via rapid Pt(PEt3)2 dissociation/association at elevated tem-
peratures. Alternatively, the process may involve an intramo-
lecular metal ring-walking process. The low entropy values,
∆S‡, indicate that neither metal–ligand dissociation nor
metal–ligand association type process exists at the rate
determining step (otherwise a large positive or negative
3820 Inorganic Chemistry, Vol. 47, No. 9, 2008

Fluxional BehaWior of Platinum(0) Complexes
Scheme 5. Selective Formation of Complex 5 and Migration of the
Pt(PEt3)2 Moeity of Complex 5 between the two CdC Bonds of Ligand
4 at Elevated Temperatures
Figure 4. Eyring plot of the conversion of 2 a 2′ (R2 ) 0.984) in o-xylene-
d10 (27.9 mM). x and f correspond to two series of independent
measurements of different samples of complex 2.
temperatures. The process of this ligand exchange is not
straightforward: Pt(PEt3)2 dissociation is probably not oc-
curring because of the relatively low ∆S‡ values and the
linear correlation observed for kobs versus temperature (at
least up to 80 °C; Figure 4). Alternatively, an associative
ligand exchange process of ligand 1 with metal complex 2
may account for the observed magnetization transfer at higher
temperatures.
Complex 5 was prepared in order to evaluate the role of
the central aromatic system on the metal migration process
(Scheme 5). Reaction of an equimolar amount of Pt(PEt3)4
with trans,trans-9,10-bis(4-pyridylethenyl)anthracene (4)54
in dry THF at room temperature for 20 min led to the
quantitative formation of complex 5. Complex 5 was isolated
and fully characterized in the same way as complex 2.
The X-ray structure of compound 4 shows that the
anthracene and the vinylpyridine moieties are not in the same
plane,54 probably because of steric hindrance between the
olefinic and aromatic protons. The barrier to becoming
coplanar is probably quite low in the free ligand (4).
However, the Pt(PEt3))2 group of complex 5 may induce a
larger barrier for the π-system to become coplanar. In such
a case, it is less likely for the metal center to use the aromatic
system as a pathway to migrate to the other C)C bond. As
Figure 5. VT Spin Saturation transfer experiments with complex 2 in the
presence of 0.5 equiv of ligand 1 in C6D6. The lower spectrum shows the
1H NMR spectrum at δ 7.24–3.5 ppm. The arrows indicate the irradiated
peaks (a, b). The four upper spectra recorded in the range of 20–55 °C are
difference spectra between that with irradiation of the signal at δ 3.8 ppm
and the normal spectrum.
value, respectively, would be expected). The entropy, being
small, is consistent with either (i) an intramolecular process
or (ii) metal–ligand dissociation that is not the rate determin-
ing step. A process involving rate-limiting metal dissociation
would have larger positive entropy. Apparently, the trans-
formation of 2 a 2′ is intramolecular. In order to provide
support for this hypothesis, we used VT SST measurements
of complex 2 in the presence of free ligand 1. For this
experiment, a 2:1 mixture of complex 2 and free ligand 1
was used. It was found that the magnetization transfer
occurred exclusively within complex 2, as expected for an
intramolecular rearrangement.10,21–23 No intermolecular mag-
netization transfer to the free ligand (1) was observed within
the temperature range of 25–47 °C. A stack plot of a typical
magnetization transfer experiment is shown in Figure 5. At
55 °C, ∼17% of the magnetization was transferred to the
ligand (1) and 83% of the magnetization was still transferred
within complex 2, indicating the presence of some ligand
scrambling in parallel to an intramolecular process at higher
1
for complex 2, VT H NMR spectroscopy in the range of
35–110 °C shows (reversible) broadening of the signals with
increasing temperature. The concentration of complex 5 does
not affect the VT NMR spectra indicating that the overall
process is first order (unimolecular) in complex 2. Stanger
has shown that if the rate determining step is unimolecular,
and depends only on the complex,10,21–23 one can not
distinguish between a ring-walking and a dissociative process
by varying the concentration (Scheme 1, I a I′). Performing
the VT NMR experiments in the presence of 2 equiv of PEt3
resulted in formation of Pt(PEt3)3 and uncomplexed ligand
4. VT 1H-1H EXSY NMR spectroscopy showed exchange
cross peaks at 40 °C between the signals of the C)C moieties
(δ 5.59–4.05 and 7.83–6.5 ppm). Apparently, the platinum
center of complex 5 migrates between the C)C bonds of
(54) Sokolov, T. N.; Friscic, T.; MacGillivray, L. R. J. Struct. Chem. 2005,
46, S171.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3821

Zenkina et al.
intermolecular migration of a (R3P)2Ni (R ) Et, Bu) moiety
over anthracene ligands (Scheme 1, I a I′) is controlled by
substituents attached to the arene.21
Summary and Conclusions
A chemical exchange process in which a platinum moiety
migrates between two C)C bonds bridged by an arene has
been observed for complexes 2 and 5. VT SST NMR
experiments unambiguously showed that the metal migration
between the two C)C bonds of the metal complex 2 is
intramolecular, even in the presence of free ligand 1. In
support of the unimolecular route: no magnetization transfer
was observed to ligand 1 at temperatures below 47 °C. SST
is a powerful method for determining the intra versus
intermolecular nature of dynamic processes. For instance, it
has been used to study the fluxional behavior of nickel-
anthracene systems such as I (Scheme 1).10,21,23 The low
entropy value for 2 a 2′, ∆S‡ ) 6.4 ( 2.5 e.u., also supports
the existence of an intramolecular process. A reasonable
mechanistic scenario for the intraconversion of 2 a 2′ is a
series of intramolecular haptotropic rearrangements. Several
organometallic systems are known to undergo “ring walk-
ing”; however, reversible metal migration between a C)C
moiety and an arene is rare.30 The mechanistic information
for complex 5 is mainly based on the activation parameters
derived by SST, with a relative small temperature range (∆T
) 25 °C). However, the difference between the entropy
values of the two systems is larger than 33 e.u. The relative
large, positive entropy value for 5 a 5′, ∆S‡ ) 39.7 (
6.0 e.u., supports the existence of an intermolecular pathway.
This is in agreement with the observed metal–ligand dis-
sociation at elevated temperatures in the presence of PEt3,
and the significant magnetization transfer to ligand 4.
The ease by which the metal center migrates between
different molecular moieties of system 2 via an intramo-
lecular pathway may suggest that some organometallic
transformations involving arene and olefinic substrates may
involve similar routes. The introduction of dynamic, loosely
bound metal-based moieties into rigid, well-defined systems
through η2-complexation could lead to new supramolecular
structures with tailorable properties.
Figure 6. Eyring plot of the conversion of 5 a 5′ (R2 ) 0.985) in o-xylene-
d10 (24.5 mM).
ligand 4. From the above mentioned observations, it is not
clear whether inter or intramolecular pathways are involved.
The observed metal–ligand dissociation due to the addition
of PEt3 might be explained as follows: (i) The PEt3 reacts
with complex 5 to afford the observed free ligand (4) and
Pt(PEt3)3, or (ii) complex 5 is in (an unobserved) equilibrium
with ligand (4) and Pt(PEt3)2; addition of PEt3 shifts this
equilibrium toward the observed ligand 4 and Pt(PEt3)3. In
any case, the metal center in complex 5 seems less strongly
bound to the ligand than in complex 2. Competition
experiments between complex 5 and ligand 4 were difficult
to monitor by VT SST experiments because of the overlap
of the signals. Regardless of these limitations, it is possible
to observe intermolecular magnetization transfer to the free
ligand (4) at 45 °C by comparison between SST experiments
with and without added ligand (4). The magnetization transfer
between the protons of the C)C groups increases 4-fold upon
addition of an equimolar amount of ligand to a solution of
complex 5. However, these observations do not exclude a
ring-walking process.
We used SST NMR experiments at various temperatures
(ranging from 35 to 60 °C) to gain more insight into the
fluxional behavior of complex 5. The activation parameters
for the rearrangement 5 a 5′, ∆H‡ ) 31.3 ( 1.9 kcal/mol,
∆S‡ ) 39.7 ( 6.0, and ∆G‡ ) 19.5 ( 1.9 kcal/mol were
derived from an Eyring plot (Figure 6). Interestingly, both
the ∆H‡ and ∆S‡ values are much larger than observed for
complex 2 (under identical reaction conditions). The large
differences in the activation parameters between systems 2
and 5 may support the notion of different mechanistic
pathways. The large positive entropy value suggests that an
intermolecular process operates for the migration of the metal
center in complex 5. Replacing the central ring system of
ligand 1 with the larger anthracene group (4) may induce
significant steric hindrance, thus effectively weakening the
platinum-ligand bond and blocking the Pt(PEt3)2 moiety
from “ring walking”. It is known that the intra versus the
Acknowledgment. This research was supported by the
Helen and Martin Kimmel Center for Molecular Design and
the German-Israel Foundation (GIF). 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 complex 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC702433S
3822 Inorganic Chemistry, Vol. 47, No. 9, 2008