Synthesis of the Elusive (R3P)2MF2 (M = Pd, Pt) Complexes

Article abstract of DOI:10.1021/ja0377753

The synthesis and characterization of previously unknown palladium(II) and platinum(II) difluoro phosphine complexes are described. These complexes can be obtained either via a halide metathesis reaction with AgF in dichloromethane or by reacting the corresponding dimethyl complexes with XeF₂. While the Pt(II) complexes can be prepared with both aryl- and alkyl-phosphine ligands, the stability of the Pd(II) complexes is limited to those having cis-oriented trialkyl phosphine ligands. Copyright

Full text of DOI:10.1021/ja0377753

Published on Web 10/16/2003
Synthesis of the Elusive (R3P)2MF2 (M ) Pd, Pt) Complexes
Anette Yahav, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Raymond and BeVerly Sackler Faculty of Exact Sciences,
Tel AViV UniVersity, Tel AViV 69978, Israel
Received August 6, 2003; E-mail: avigal@post.tau.ac.il
There has recently been considerable interest in transition metal
fluorine chemistry.1 Transition metal fluoride complexes are
important intermediates in selective C-F bond activation processes2
and in catalysis.3 While much progress has been achieved in the
synthesis of various transition metal fluorides, preparation of the
palladium(II) and platinum(II) fluoride complexes remains a
significant challenge. Such complexes are generally considered
inaccessible, to a large extent due to the weak interactions between
the “soft” late transition metal center and the hardest anion F-.1,4
Only recently were the first unambiguously characterized Pd(II)
monofluoride complexes of formula trans-(Ph3P)2Pd(R)F (R ) Ar,
Me) reported,5,6 while (R3P)2MF2 (M ) Pd, Pt) complexes are still
unprecedented.7 The latter complexes lack the strong push-pull
interactions provided by alkyl or aryl ligands, assumed to be
essential for their stabilization.8 Herein, we report the high-yield
synthesis of stable Pd(II) and Pt(II) difluorides.
Figure 1. Single-crystal ORTEP (thermal ellipsoids drawn at 50%
probability) structure of 2a. Selected bond distances (Å) and angles (deg):
Pd-F(1) 2.065(3), Pd-P(1) 2.2241(15), F(1)-Pd-F(1′) 89.99(19), P(1)-
Pd-P(1′) 96.22(8), F(1)-Pd-P(1) 86.84(8), F(1)-Pd-P(1′) 175.90(12).
Pd(II) and Pt(II) monofluoride complexes are commonly prepared
by the metathesis reaction with silver(I) fluoride.5,6b-d,f This
approach, however, fails to furnish the desired (Ph3P)2PdF2 when
the dichloro- or diiodo-precursor was treated with AgF in various
solvents. Only decomposition took place, generating Pd black and
a Pd(0) complex, which was trapped by adding iodobenzene to the
reaction mixture. Since the nucleophilic fluoride anion is known
to cause the reductive decomposition of several Pd(II) phosphine
complexes,9 we were interested in developing a different strategy
to achieve the fluorination process. When the dimethyl complexes
1 were treated with an equivalent amount of XeF2 in dry
dichloromethane, the quantitative formation of the difluoro com-
plexes 2 was observed with the di(i-propylphosphino)propane
(dippp) and di(cyclohexylphosphino)propane (dcpp) ligands (eq 1).
As the byproducts are released in the gaseous phase, simple
triturating of the concentrated CH2Cl2 solution with pentane afforded
pure 2 as a white solid with a nearly 100% yield. Compounds 2
slowly decomposed at room temperature both in solution and in
the solid state when stored in a drybox. However, at 0 °C, a CDCl3
sample of 2a showed no decomposition after several weeks. It was
important to adhere to exact stoichiometry in eq 1 because an excess
of XeF2 led to partial decomposition, giving fluorinated phos-
phorus(V) products, metal black, and 2-HF adducts.10
were broadened, presumably due to the presence of small amounts
of moisture.5 Performing the measurements at low temperatures
restored the original AA′XX′ signal structure.
Colorless crystals of 2a were obtained by slow pentane diffusion
into its CH2Cl2 solution at -30 °C. The molecules adopt a perfect
Cs symmetry in the crystal, being positioned on crystallographic
mirror planes. The X-ray crystal structure of 2a (Figure 1) shows
the palladium center located in an almost perfect square-planar
environment with the P(1′)-Pd-F(1) bond angle being 175.90(12)°.12
The Pd-F bond length of 2.065(3) Å is within the range reported
for inorganic palladium fluoride salts (1.991-2.178 Å).13 It is
slightly shorter than the Pd-F bond in trans-(Ph3P)2Pd(Ph)F
(2.085(3) Å),5a which is due to a weaker trans-influence of the
phosphine group than that of the phenyl, but longer than the same
bond in the trans-(Ph3P)2Pd(p-NO2Ar)F (2.049(2) Å)5c where the
fluorine atom might be involved in conjugation with the p-
nitrophenyl group. The Pd-F bonds in 2a are significantly longer
than the Pd-O bond in terminal Pd hydroxide complexes with
nitrogen ligands (1.966-2.025 A).14
Xenon(II) difluoride was used in the oxidative fluorination of
transition metal complexes, providing the corresponding metal
fluorides in the high oxidation state.15 In our case, no change in
the oxidation state occurs as the addition of two fluorine atoms is
coupled with the reductive elimination process. This method was
also applied toward the synthesis of the Pt(II) fluoro complexes.
When (R3P)2Pt(Ph)2 (3) was reacted with XeF2 in CH2Cl2,
complexes 4a-c were obtained in quantitative yields and biphenyl
was identified as the organic byproduct (eq 2). The 31P{1H} and
19F{1H} NMR spectra for these complexes exhibit an AA′MXX′
The 31P{1H} and 19F{1H} NMR spectra of 2 show an AA′XX′
pattern with 2JPF(trans) ≈ 180 Hz.11 The gNMR simulations for 2a
were in excellent agreement with the observed spectra.11 The 19F
signals for the difluoride complexes are in the area expected for
the Pd(II) complexes (cf. -248 ppm in 2a with -274 ppm in
(Ph3P)2Pd(Ph)F). In some instances, the 31P and 19F NMR signals
1
splitting pattern (2JPF(trans) ≈ 170 Hz, JPtP ≈ 3660 Hz), except
for 4c which has the trans-configuration. We believe that the initial
oxidation of the metal center by XeF2 triggers the C-C reductive
elimination followed by the addition of two fluorine atoms. In
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C O M M U N I C A T I O N S
support of this mechanism, the Pd(0) complex (dippp)2Pd16 reacts
with XeF2, giving 2a as the major metal-containing product in the
reaction mixture. An alternative route toward 2 involves an initial
oxidative addition step to form the M(IV) difluoride complex with
concomitant ethane reductive elimination. Low-temperature experi-
ments provided no evidence for such an intermediate.
metathesis reaction. As XeF2 provides 2 equiv of F- and essentially
no workup is required, this method has certain advantages over
the AgF fluorination. The mechanistic aspects of this process are
currently under investigation.
Acknowledgment. This work was supported by a Tel Aviv
University Start-Up grant and the Yigal Alon New Faculty Award
from the Israeli Council for Higher Education to A.V. We thank
Prof. David Milstein for valuable discussions and donating the dippp
ligand. We also thank Prof. Yoram Cohen for performing the gNMR
simulations.
Supporting Information Available: Experimental procedures for
complexes 2 and 4 (PDF). X-ray crystallographic file (CIF) for 2a.
This material is available free of charge via the Internet at http://
pubs.acs.org.
While stable in solution, the palladium difluoride complexes are
reactive toward strong electrophiles, giving the corresponding
fluorination products.17 For example, treatment of 2a with 2 equiv
of RCOCl (R ) Me or Ph) immediately results in formation of the
known18 dipppPdCl2 and an acyl fluoride.
References
In light of the predicted instability of the Pd(II) difluoride
complexes, the isolation of complexes 2a and b is somewhat
unexpected.5a,9 These complexes are not stabilized via the typical
push-pull interactions, nor does the metal center appear to be
electron-deficient, allowing for strong electrostatic interactions.8
Importantly, reactions of XeF2 with (Ph3P)2PdMe2 or (dppp)PdMe2
resulted in decomposition, and no corresponding palladium di-
fluoride was isolated. In the latter case, the transient (dppp)PdF2
(2c) was observed in the 19F NMR spectrum of the reaction mixture
(AA′XX′ multiplet at -238.69 ppm, -30 °C) along with various
P-F bond-containing decomposition products. The mixture con-
sequently turned black with the signal attributed to the Pd-F bond
disappearing. As steric differences between the cis-chelating
phosphine ligands are unlikely to play a role when small fluoride
atoms are involved, the electron-donating dippp and dcpp ligands
are essential for the difluoride stabilization. Reacting trans-(t-Bu3P)2-
PdMe2 or (t-Bu3P)2Pd(0) with XeF2 did not lead to (t-Bu3P)2PdF2
but rather resulted in several unidentified products. Although t-Bu3P
is an electron-donating ligand similar to dippp, it is expected to
impart the trans-arrangement around the palladium center. Thus,
the stability of the Pd(II) complexes of formula (R3P)2PdF2 is strictly
limited to alkyl phosphine ligands that can adopt the mutual cis-
orientation. The platinum analogs are more stable, giving isolable
difluoride complexes with both alkyl and aryl phosphine ligands.
The observed stability trend led us to reinvestigate the halide
metathesis pathway toward the Pd(II) and Pt(II) fluorides. Simple
stirring of the dipppPdI2 complex with 2.5 equiv of AgF in
dichloromethane for 2-3 h in the dark resulted in the clean
formation of 2a, while, as expected, under the same reaction
conditions, dpppPdI2 gave only decomposition products. To our
surprise, reacting (Ph3P)2Pd(Ph)I with 1.5 equiv of AgF in CH2Cl2
for 3 h gave the known (Ph3P)2Pd(Ph)F5 in a quantitative yield,
indicating that no sonication is required. The platinum(II) diiodo
complexes reacted similarly with 2.5 equiv of AgF, providing the
corresponding (R3P)2PtF2 in excellent yields. While generally not
innocent in transition metal fluoride chemistry,17 CH2Cl2 appears
to be the solvent of choice in the synthesis of palladium and
platinum fluorides.
(1) (a) Grushin, V. V. Chem.-Eur. J. 2002, 8, 1007. (b) Murphy, E. F.;
Murugavel, R.; Roesky, H. W. Chem. ReV. 1997, 97, 3425. (c) Doherty,
N. M.; Hoffman, N. W. Chem. ReV. 1991, 91, 553.
(2) (a) Burdeniuc, J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber./Recueil 1997,
130, 145. (b) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem.
ReV. 1994, 94, 373. (c) See also: Braun, T.; Perutz, R. N. Chem. Commun.
2002, 2749 and references therein.
(3) (a) Fagnou, K.; Lautens, M. Angew. Chem., Int. Ed. 2002, 41, 26. (b)
Pagenkopf, B. L.; Carreira, E. M. Chem.-Eur. J. 1999, 5, 3437.
(4) Caulton, K. G. New J. Chem. 1994, 18, 25.
(5) (a) Fraser, S. L.; Antipin, M. Y.; Khroustalyov, V. N.; Grushin, V. V. J.
Am. Chem. Soc. 1997, 119, 4769. (b) Pilon, M. C.; Grushin, V. V.
Organometallics 1998, 17, 1774. (c) Marshall, W. J.; Thorn, D. L.;
Grushin, V. V. Organometallics 1998, 17, 5427.
(6) Several Pt(II) monofluoride complexes have been reported, see for
example: (a) Clark, H. C. S.; Fawcett, J.; Holloway, J. H.; Hope, E. G.;
Peck, L. A.; Russel, D. R. J. Chem. Soc., Dalton Trans. 1998, 1249. (b)
Coulson, D. R. J. Am. Chem. Soc. 1976, 98, 3111. (c) Doherty, N. M.;
Critchlow, S. C. J. Am. Chem. Soc. 1987, 109, 7906. (d) Cairns, M. A.;
Dixon, K. R.; McFarland, J. J. J. Chem. Soc., Dalton Trans. 1975, 1159.
(e) Russell, D. R.; Mazid, M. A. J. Chem. Soc., Dalton Trans. 1980, 1737.
(f) Utilization of Et3N-3HF has also been reported. See: Jasim, N. A.;
Perutz, R. N. J. Am. Chem. Soc. 2000, 122, 8685 and ref 5.
(7) A few Pd(II) and Pt(II) difluorides were claimed in the past, but no definite
characterization has been provided: (a) McAvoy, J.; Moss, K. C.; Sharp,
D. W. A. J. Chem. Soc. 1965, 1376. (b) Kemmitt, R. D. W.; Peacock, R.
D.; Stocks, J. J. Chem. Soc. A 1971, 846. (c) The 19F chemical shift at
-140 ppm (s) reported for the cis-(Ph3P)2PtF2 is outside the range as
compared with the known Pt-F NMR signals, see ref 6: Thomson, J.;
Fzea, A. H.; Lobban, J.; McGivern, P.; Cairns, J. A.; Fitzgerald, A. G.;
Berry, G. J.; Davidson, M. R.; Fan, Y. C. MRS Symp. Proc. 1999, 546,
225.
(8) Mezzetti, A.; Bekker, C. HelV. Chim. Acta 2002, 85, 2686.
(9) (a) Mason, M. R.; Verkade, J. G. Organometallics 1992, 11, 2212. (b)
Mason, M. R.; Verkade, J. G. Organometallics. 1990, 9, 864.
(10) Grushin, V. V. Organometallics 2000, 19, 1888.
(11) See Supporting Information for details.
(12) X-ray structure data:
C15H34F2P2Pd‚CH2Cl2, monoclinic, Cm, a )
10.9550(6), b ) 14.5680(8), c ) 6.7370(2), â ) 99.408(3), V )
1060.71(9), Z ) 2.
(13) (a) Tressaud, A.; Soubeyroux, J. L.; Touhara, H.; Demazeau, G.; Langlais,
F. Mater. Res. Bull. 1981, 16, 207. (b) Muller, B. G. J. Fluorine Chem.
1982, 20, 291.
(14) (a) Akita, M.; Miyaji, T.; Muroga, N.; Mock-Knoblauch, C.; Adam, W.;
Hikichi, S.; Moro-oka, Y. Inorg. Chem. 2000, 39, 2096. (b) Castan, P.;
Dahan, F.; Wimmer, S.; Wimmer, F. L. J. Chem. Soc., Dalton Trans.
1990, 2679.
(15) (a) Cockman, R. W.; Ebsworth, E. A. V.; Holloway, J. H.; Murdoch, H.;
Robertson, N.; Watson, P. G. In Fluorine Chemistry (Toward the 21st
Century); Thrasher, J. S., Strauss, S. H., Eds.; ACS: Washington, DC,
1994; Chapter 20, p 327. (b) Holloway, J. H.; Hope, E. G. J. Fluorine
Chem. 1996, 76, 209. (c) Drews, H.-H.; Preetz, W. Z. Anorg. Allg. Chem.
1997, 623, 509.
(16) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1655.
(17) For some examples of transition metal fluoride complexes as the source
of nucleophilic F-, see: (a) Grushin, V. V. Angew. Chem., Int. Ed. 1998,
37, 994. (b) Veltheer, J. E.; Burger, P.; Bergman, R. G. J. Am. Chem.
Soc. 1995, 117, 12478. (c) Barthazy, P.; Stoop, R. M.; Worle, M.; Togni,
A.; Mezetti, A. Organometallics 2000, 19, 2844.
In summary, we demonstrated that the difluoro Pd(II) phosphine
complexes are significantly more robust than was commonly
believed, provided cis-alkyl phosphine ligands are used. The new
oxidative fluorination-reductive elimination sequence using XeF2
has been developed and can be used alongside the traditional halide
(18) Portnoy, M.; Frolow, F.; Milstein, D. Organometallics 1991, 10, 3960.
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