Formation of fluorinated platinum-stilbazole complexes: Aryl-halide oxidative addition vs η2-coordination of a carbon-carbon double bond

Article abstract of DOI:10.1021/om0603870

Reaction of a zerovalent platinum complex with a partially fluorinated stilbazole ligand results in direct Ar_f-Br oxidation addition. The exclusive formation of the Ar_f-Pt^II-Br complex does not proceed via η²-C=C coordination on the reaction coordinate or as a side-equilibrium prior to the observed C-Br bond activation. Ar_f-Br activation by Pt⁰ is the kinetically and most probably also the thermodynamically favorable process independent of the reaction temperature and solvent polarity. This is in stark contrast with the reactivity of isostructural nonfluorinated stilbazole systems, where there is a lower barrier for η²-C=C coordination than for Ar-Br oxidative addition with Pt⁰.

Full text of DOI:10.1021/om0603870

3308
Organometallics 2006, 25, 3308-3310
Formation of Fluorinated Platinum-Stilbazole Complexes:
Aryl-Halide Oxidative Addition vs η2-Coordination of a
Carbon-Carbon Double Bond
Andre´ C. B. Lucassen,† Linda J. W. Shimon,‡ and Milko E. van der Boom*,‡
Department of Organic Chemistry and Chemical Research Support Unit,
Weizmann Institute of Science, RehoVot, 76100, Israel
ReceiVed May 4, 2006
Scheme 1. Direct Generation of Complex 2 without Parallel
Formation and/or Intermediacy of the Postulated Complex
3; Arf-Br Oxidative Addition Is Kinetically Preferred over
η2-CdC Coordination
Summary: Reaction of a zeroValent platinum complex with a
partially fluorinated stilbazole ligand results in direct Arf-Br
oxidation addition. The exclusiVe formation of the Arf-PtII-
Br complex does not proceed Via η2-CdC coordination on the
reaction coordinate or as a side-equilibrium prior to the
obserVed C-Br bond actiVation. Arf-Br actiVation by Pt0 is
the kinetically and most probably also the thermodynamically
faVorable process independent of the reaction temperature and
solVent polarity. This is in stark contrast with the reactiVity of
isostructural nonfluorinated stilbazole systems, where there is
a lower barrier for η2-CdC coordination than for Ar-Br
oxidatiVe addition with Pt0.
Competitive olefin coordination and aryl-halide oxidative
addition to low-valent late transition metals are key steps in
several homogeneous catalytic processes,1,2 including syntheti-
cally important cross-coupling reactions. Furthermore, metal-
mediated synthesis often involves the use of substrates having
both carbon-carbon double bonds and aryl-halides. Numerous
studies have been devoted to the design of sophisticated task-
specific, platinum group metal catalysts in parallel to the
utilization of phosphine metal complexes in organic synthesis.1,2
A better fundamental understanding of coordination and oxida-
tive addition chemistry may open up practical routes for the
formation of new organic products.3,4 We reported recently that
the reaction between a zerovalent platinum phosphine complex
and a halogenated vinylarene resulted in η2-CdC coordination
of the zerovalent metal center prior to Ar-Br oxidative
addition.4 Upon heating, either in solution or in the solid state,
the metal center “walks” around the π-system from the bridging
CdC double bond to the C-Br bond. We demonstrate here
that Arf-Br oxidative addition with an analogous fluorinated
stilbazole substrate by Pt(PEt3)4 is kinetically favorable over
π-coordination of a CdC double bond.
The reaction of 4′-bromo-2′,3′,5′,6′-tetrafluorostilbazole (1)5
with a stoichiometric amount of Pt(PEt3)46 in dry THF or toluene
at room temperature under nitrogen resulted in rapid formation
of the new complex 2 by selective oxidative addition of the
Arf-Br moiety to the d10 metal center (Scheme 1).
Complex 2 was formed quantitatively with concurrent forma-
tion of 2 equiv of PEt3, as judged by 31P{1H} NMR spectros-
copy. The product was isolated as white crystals formed
overnight from a saturated solution of hexane/toluene (10:1 v/v)
* To whom correspondence should be addressed. E-mail:
milko.vanderboom@weizmann.ac.il.
1
at -30 °C in 81% yield and was fully characterized by H,
13C{1H}, 19F{1H}, and 31P{1H} NMR spectroscopy, elemental
analysis, and mass and single-crystal X-ray crystallography
(Figure 1).7 The X-ray analysis reveals a square-planar geometry
around the Pt(II) center having mutually trans phosphines with
a P-Pt-P angle of 174.3(6)° and a C-Pt-Br angle of
176.5(2)°. The dihedral angle between the aromatic rings is
11.4°; however, the pyr-C, Arf-C, and CdC bonds lengths
of 1.55(3), 1.53(3), and 1.38(3) Å, respectively, are not affected
by formation of the strong M-Arf σ-bond.
† Department of Organic Chemistry.
‡ Chemical Research Support Unit.
(1) For reviews, see: (a) Zapf, A.; Beller, M. Chem. Commun. 2005, 4,
431. (b) Slagt, M. Q.; van Zwieten, D. A. P.; Moerkerk, A. J. C. M.;
Gebbink, R. J. M. L.; van Koten, G. Coord. Chem. ReV. 2004, 248, 2275.
(c) Espinet, P.; Echavarren, M. Angew. Chem., Int. Ed. 2004, 43, 4704. (d)
van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759. (e) Elsevier,
C. J. Coord. Chem. ReV. 1999, 185-186, 809.
(2) Negishi, E. I. Handbook of Organopalladium Chemistry for Organic
Synthesis; Wiley: New York, 2002. Diederich, F.; Stang, P. J. Metal-
catalyzed Cross-Coupling Reactions; Wiley-VCH: New York, 1998.
Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic
Molecules, 1st ed.; University Science Books: Mill Valley, CA, 1994.
(3) For some recent studies, see: (a) Gatard, S.; Celenligil-Cetin, R.;
Guo, C.; Foxman, B. M.; Ozerov, O. V. J. Am. Chem. Soc. 2006, 128,
2808. (b) Fan, L.; Parkin, S.; Ozerov, O. V. J. Am. Chem. Soc. 2005, 127,
16772. (c) Dong, C.-G.; Hu, Q.-S. J. Am. Chem. Soc. 2005, 127, 10006.
(d) Barrios-Landeros, F.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 6944.
(e) Ben-Ari, E.; Gandelman, M.; Rozenberg, H.; Shimon, L. J. W.; Milstein,
D. J. Am. Chem. Soc. 2003, 125, 4714. (f) Kirchhoff, J. H.; Netherton, M.
R.; Hills, I. D.; Fu, G. C. J. Am. Chem. Soc. 2002, 124, 13662. (g) For
catalytic functionalization of perfluoroarenes by the Heck reaction: Albeniz,
A. C.; Espinet, P.; Martin-Ruiz, B.; Milstein, D. J. Am. Chem. Soc. 2001,
123, 11504. (h) For a theoretical study: Goossen, L. J.; Koley, D.; Hermann,
H. L.; Thiel, W. Organometallics 2005, 24, 2398.
Monitoring the reaction of 1 + Pt(PEt3)4 f 2 + 2 PEt3 in
1
THF-d8 as shown in Scheme 1 by variable-temperature H,
19F{1H}, and 31P{1H} NMR spectroscopy in the temperature
range from -75 to +25 °C did not reveal formation of any
η2-CdC coordination complexes such as the postulated complex
(4) 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.
(5) Lucassen, A. C. B.; Vartanian, M.; Leitus, G.; van der Boom, M. E.
Cryst. Growth Des. 2005, 5, 1671.
(6) Schunn, R. A. Inorg. Chem. 1976, 15, 208.
10.1021/om0603870 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/28/2006

Communications
Organometallics, Vol. 25, No. 14, 2006 3309
mechanistic routes are also possible.13 Heating the reaction
mixture to 60 °C for 1 h in a sealed vessel after consumption
of the starting materials also did not result in the formation of
η2-CdC coordination complexes.
Substrate 1 was reacted with 2 equiv of Pt(PEt3)4 in dry THF
at room temperature to verify that platinum coordination to the
carbon-carbon double bond is not prohibited due to steric
hindrance or is energetically unfavorable in the presence of PEt3.
This reaction resulted in the quantitative formation of the new
bimetallic complex 4 (eq 1). This compound was isolated as a
1
white solid in 82% yield and fully characterized by H, 13C-
{1H}, 19F{1H}, and 31P{1H} NMR spectroscopy, elemental
analysis, and mass spectrometry.7 The 31P{1H} NMR spectrum
shows the signals both for Arf-Br oxidative addition and for
η2-CdC coordination of the partially fluorinated stilbazole
ligand in a 1:1 ratio. Apparently, metal coordination to the
carbon-carbon double bond of complex 1 is not hampered. The
formation of the formally mixed-valence Pt0/PtII complex 4
clearly demonstrates that there is a fine balance between η2-
coordination of the -CdC- moiety and Arf-Br oxidative
addition, as both processes can occur under identical reaction
conditions at room temperature.
Figure 1. ORTEP diagram of complex 2 (thermal ellipsoids set
at 50% probability). Hydrogen atoms are omitted for clarity.
Selected bond length (Å) and angles (deg): Pt(1)-Br(2) 2.4925(10),
Pt(1)-C(1) 2.006(7), C(4)-C(7) 1.53(3), C(7)-C(8) 1.38(3), C(8)-
C(10) 1.55(3), Pt(1)-P(2) 2.301(19), Pt(1)-P(3) 2.304(18); C(1)-
Pt(1)-P(3) 90.8(19), C(1)-Pt(1)-P(2) 92.6(19), P(2)-Pt(1)-P(3)
174.3(6), C(1)-Pt(1)-Br(2) 176.5(2), P(2)-Pt(1)-Br(2) 88.05(5),
P(3)-Pt(1)-Br(2) 88.8(5).
3 in parallel or prior to the observed product of Arf-Br oxidative
addition (2). An intermediate complex having two magnetically
different cis PEt3 ligands in a 2:1 ratio was observed in the
temperature range from -75 to -35 °C, which may be a cationic
Arf-Pt(II) complex.9,10 No reaction occurred at lower temper-
atures; only signals attributed to Pt(PEt3)4 were observable in
the 31P{1H} NMR spectra. It is likely that unsaturated platinum
complexes are initially formed, which in principle can undergo
η2-CdC coordination and/or Arf-Br oxidative addition.
Pt(PEt3)4 is known to undergo reversible PEt3 dissociation in
solution to form the 16-electron complex Pt(PEt3)3.11,12 This
equilibrium is clearly visible in the variable-temperature 31P{1H}
NMR spectra in the absence of substrate 1. However, formation
of Pt(PEt3)3 was not observed during the course of the reaction
with 1, indicating that PEt3 dissociation is a relative slow process
in comparison with the subsequent reaction of 1 with platinum.
Pt(PEt3)3 can undergo both associative and dissociative ligand
exchange.12 A kinetically significant amount of unobserved Pt-
(PEt3)2 might be present in the system, which undergoes a
selective reaction with the Arf-Br unit. However, other
To further elucidate the mechanism of the formation of
complex 2 (Scheme 1), pentafluorostilbazole 55,15 was also
reacted with an equimolar amount of Pt(PEt3)4 in THF-d8 at
-70 °C followed by stepwise warming of the reaction mixture
to room temperature in the NMR probe (eq 2). This process
resulted in the exclusive formation of the η2-CdC coordination
complex 6 and free PEt3. Complex 6 was fully characterized
by the same means as for 2.7 Single-crystal X-ray diffraction
analysis of 6 reveals a trigonal coordination environment (Figure
2).14 The η2-CdC bond length of 1.451(6) Å is relatively long
due to back-bonding from the metal center into the olefin π*-
orbitals.16 Olefins with electron-withdrawing substituents are
often strongly bound to Pt0.17
(7) See Supporting Information for complete experimental details and
characterization data for complexes 2, 4, and 6. The crystal structures of
complexes 2 and 6 have been deposited at the Cambridge Crystallographic
Data Center and allocated the deposition numbers CCDC 271980 and CCDC
271981, respectively. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html.
(8) X-ray crystal data for 2. Crystal data: C22H41NF4P2BrPt+C6,
colorless, prism, 0.2 × 0.2 × 0.2 mm3, monoclinic, P2(1)/c (no.14) a )
19.737(4) Å, b ) 10.879(2) Å, c ) 16.287(3) Å, â ) 91.12(3)° from 20
degrees of data, T ) 120(2) K, V ) 3496.5(12) Å3, Z ) 4, fw ) 804.56,
Dc ) 1.528 Mg‚m-3, µ ) 5.285 mm-1. Data collection and processing:
Nonius KappaCCD diffractometer, Mo KR, λ ) 0.71073 Å, graphite
monochromator, -23 e h e 23, 0 e k e 13, 0 e l e 19, frame scan width
) 1.0°, scan speed 1.0° per 30 s, typical peak mosaicity 0.56°, 29 478
reflections collected, 8383 independent reflections (Rint ) 0.086). The data
were processed with Denzo-Scalepack. Solution and refinement: Structure
solved by Patterson method with SHELXS-97. Full matrix least-squares
refinement based on F2 with SHELXL-97 348 parameters with 0 restraints,
final R1 ) 0.0474 (based on F2) for data with I > 2σ(I) and, R1 ) 0.0663
on 6373 reflections, goodness-of-fit on F2 ) 1.104, largest electron density
peak ) 2.049 e‚Å-3. Note: There is a partially disordered hexane solvent
molecule, which has been modeled with only the carbon atoms.
(13) Aryl-halide oxidative addition via the intermediacy of 12-electron
species: (a) Stambuli, J. P.; Bu¨hl, M.; Hartwig, J. F. J. Am. Chem. Soc.
2002, 124, 9346. (b) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002,
41, 4176. (c) For a theoretical study: Ahlquist, M.; Fristrup, P.; Tanner,
D.; Norrby, P.-O. Organometallics 2006, 25, 2066.
(14) X-ray crystal data for 6: Crystal data: C25H36NF5P2Pt, colorless,
prism, 0.1 × 0.1 × 0.1 mm3, triclinic, P1h (no. 2), a ) 8.792(2) Å, b )
11.444(2) Å, c ) 14.941(3) Å, R ) 109.00(3)°, â ) 93.03(3)°, γ )
106.96(3)° from 20 degrees of data, T ) 120(2) K, V ) 1341.3(5) Å3, Z )
2, fw ) 702.58, Dc ) 1.740 Mg‚m-3, µ ) 5.399 mm-1. Data collection
and processing: Nonius KappaCCD diffractometer, Mo KR λ ) 0.71073
Å, graphite monochromator, -11 e h e 11, -14 e k e 13, 0 e l e 19,
frame scan width ) 2.0°, scan speed 1.0° per 90 s, typical peak mosaicity
(9) (a) Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2117. (b)
Uso´n, R.; Royo, P.; Gimeno, J. J. Organomet. Chem. 1974, 72, 299.
(10) δ ) 19.46 (d, 2P, 1JPt,P ) 2268 Hz, 3JP,P ) 19.4 Hz), 16.07 (t, 1P,
0.82°, 27 559 reflections collected, 6089 independent reflections (Rint
)
3
1JPt,P ) 3466 Hz, JP,P ) 19.4 Hz). Formation of a similar complex was
0.056). The data were processed with Denzo-Scalepack. Solution and
refinement: Structure solved by Patterson method with SHELXS-97. Full
matrix least-squares refinement based on F2 with SHELXL-97, 349
parameters with 92 restraints, final R1 ) 0.0307 (based on F2) for data
with I > 2σ(I) and R1 ) 0.0404 on 6085 reflections, goodness-of-fit on F2
observed by 31P{1H} NMR upon treatment of complex 2 at room
temperature with AgBF4 and PEt3 in dry THF with the exclusion of light.
(11) Gerlach, D. H.; Kane, A. R.; Parshall, G. W.; Jesson, J. P.;
Muetterties, E. L. J. Am. Chem. Soc. 1971, 93, 3543.
(12) Mann, B. E.; Musco, A. J. Chem. Soc., Dalton Trans. 1980, 776.
) 1.038, largest electron density peak ) 1.832 e‚Å-3
.

3310 Organometallics, Vol. 25, No. 14, 2006
Communications
In summary, the exclusive formation of the PtII complex 2
does not proceed via η2-CdC coordination of the partially
fluorinated stilbazole ligand 1 on the reaction coordinate or as
a side-equilibrium prior to the observed C-Br bond activation.
Arf-Br activation is the kinetically and most probably also the
thermodynamically favorable process independent of the reac-
tion temperature and solvent polarity. This is in stark contrast
with the previously reported η2-CdC coordination vs Ar-Br
oxidative addition with isostructural nonfluorinated stilbazole
systems, where there is a lower barrier for η2-CdC coordination
than for Ar-Br oxidative addition with platinum.4 Oxidative
addition of the nonfluorinated substrate involves an intermediate
η2-CdC complex (akin to the postulated complex 3, Scheme
1), and this coordination complex rearranges to the oxidative
addition product via an unimolecular “ring-walking” process.4,18
The here-presented observations are a clear example that the
mechanistic route toward Ar-Br activation can be significantly
different for corresponding fluorinated substrates. Furthermore,
the kinetic balance of olefin coordination vs Ar-Br oxidative
addition can be inverted upon fluorination of the aromatic
system. It is known that the rate of Ar-halide oxidative addition
increases by placing electron-withdrawing groups on the arene.2
Nevertheless, it is remarkable that insertion of a zerovalent late-
transition metal center into a carbon-halide bond can be
kinetically favorable over η2-coordination to a carbon-carbon
double bond.19 The implementation of these findings with
respect to other metal complex precursors and substrates is under
investigation, as it might lead to selective chemical transforma-
tions with high kinetic control over the reaction outcome.
Figure 2. ORTEP diagram of complex 6 (thermal ellipsoids set
at 50% probability). Hydrogen atoms are omitted for clarity.
Selected bond length (Å) and angles (deg): Pt(1)-C(7) 2.105(4),
Pt(1)-C(8) 2.114(4), C(1)-C(7) 1.482(5), C(7)-C(8) 1.451(6),
C(8)-C(9) 1.475(6); C(1)-C(7)-C(8) 123.5(4), C(7)-C(8)-C(9)
122.8(4), C(7)-Pt(1)-C(8) 40.23(16), C(7)-C(8)-Pt(1) 69.5(2),
C(8)-C(7)-Pt(1) 70.2(2), P(1)-Pt(1)-P(2) 102.18(4).
In contrast to the stoichiometric reaction of 1 with Pt(PEt3)4:
(i) no intermediate complexes were observed by in situ 31P{1H}
NMR follow-up measurements during the formation of complex
6, and (ii) no complexation of 5 with platinum was observed at
temperatures below -50 °C for ∼4 h, while a significant amount
of the 16-electron complex, Pt(PEt3)3, was present in the reaction
mixture. Relatively slow conversion of 5 and Pt(PEt3)3 into
complex 6 and PEt3 was observed at -50 °C with t1/2 ≈ 15 h.
The coordination complex 6 was observed only at temperatures
∼20 °C higher than the formation of complex 2 via Arf-Br
oxidative addition. This unambiguously demonstrates that Arf-
Br oxidative addition is the kinetically preferred process for
substrate 1. In full agreement with the above-mentioned findings,
reaction of the model complex 6 with an equimolar amount of
1 in the presence of 2 equiv of PEt3 at room temperature in dry
THF resulted in the relatively slow formation of complex 2 (t1/2
≈ 2.5 h), further ruling out the possibility that the direct
formation of complex 2 as shown in Scheme 1 proceeds via a
bimolecular process involving the unobserved complex 3.
Acknowledgment. This research was supported by the Helen
and Martin Kimmel Center for Molecular Design, Minerva,
European Commission through NMP3-CT203505478 (STRP-
ODEON), the Mordechai Glikson Fund, BMBF, and MJRG.
M.E.vd.B. is the incumbent of the Dewey David Stone and
Harry Levine career development chair.
Supporting Information Available: Complete experi-
mental details and characterization data for complexes 2, 4, and 6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0603870
(15) Lorance, E. D.; Kramer, W. H.; Gould, I. R. J. Am. Chem. Soc.
2002, 124, 15225.
(16) For a recent study, see: Popp, B. V.; Thorman, J. L.; Morales, C.
M.; Landis, C. R.; Stahl, S. S. J. Am. Chem. Soc. 2004, 126, 14832.
(17) (a) Baraban, J. M.; McGinnety, J. A. Inorg. Chem. 1974, 13, 2864.
(b) Chatt, J.; Shaw, B. L.; Williams, A. A. J. Chem. Soc. 1962, 3269.
(18) For a review, see: Keane, J. M.; Harman, W. D. Organometallics
2005, 24, 1786.
(19) For competitive alkyne C-H activation vs η2-alkyne coordination
to Pt0, see: Whitesides, G. M.; Hackett, M. J. Am. Chem. Soc. 1988, 110,
1449.