Electron-poor pentafluorophenyl-substituted PCP - Palladium pincer complexes

Article abstract of DOI:10.1021/om0500063

A novel fluoroaryl-substituted PCP ligand has been synthesized and used to generate the corresponding Pd complexes. The bonding of the fluoroaryl phosphine has been investigated by X-ray crystallography, NMR spectroscopy, and a competition experiment, which indicate that the ligand is sterically comparable to its well-known phenyl analogue but clearly imparts significant electronic differences to the metal center.

Full text of DOI:10.1021/om0500063

2016
Organometallics 2005, 24, 2016-2019
Electron-Poor Pentafluorophenyl-Substituted
PCP-Palladium Pincer Complexes
Preston A. Chase,† Marcella Gagliardo,† Martin Lutz,‡ Anthony L. Spek,‡,§
Gerard P. M. van Klink,† and Gerard van Koten*,†
Department of Metal-Mediated Synthesis, Debye Institute, and Bijvoet Center for Biomolecular
Research, Department of Crystal and Structural Chemistry, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands
Received January 4, 2005
Summary: A novel fluoroaryl-substituted PCP ligand
has been synthesized and used to generate the corre-
sponding Pd complexes. The bonding of the fluoroaryl
phosphine has been investigated by X-ray crystallogra-
phy, NMR spectroscopy, and a competition experiment,
which indicate that the ligand is sterically comparable
to its well-known phenyl analogue but clearly imparts
significant electronic differences to the metal center.
are known to potentially enhance catalytic activity or
alter selectivity over electron-rich analogues.5 Some
pincer ligands6 have been synthesized that included
perfluorinated alkyl chains for use in fluorous biphasic
catalysis.7 However, the electronic effect of the fluorous
“ponytail” is often mitigated by inclusion of an insulat-
ing spacer group, such as R2Si or R2C. Herein, we report
the synthesis of a PCP ligand with electron-withdrawing
pentafluorophenyl groups directly incorporated at phos-
phorus. Cationic and neutral Pd complexes were syn-
thesized, and the effect of the pentafluorophenyl phos-
phine group on the bonding and electronic character of
the metal center was investigated.
Attempts to generate ligand 1 via reaction of (C6F5)2-
PLi salts with benzylic halides were not successful, as
the naked (C6F5)2P- anion is unstable, even at low
temperatures.8 Formation of [{1,3-(C6F5)2P(H)CH2}2Ar]-
[Br]2 salts by reaction of (C6F5)2PH with R,R′-dibromo-
m-xylene, species which can be deprotonated to give
PCP pincers,9 was also unproductive, due to the low σ
basicity of the P centers. Alternately, the synthesis of
ligand 1 was realized via reaction of 1,3-(ClMgCH2)2C6H4
with 2 equiv of BrP(C6F5)2 in diethyl ether (Scheme 1).
The meta-substituted di-Grignard reagent was cleanly
generated using the method of Lappert10 with R,R′-
dichloro-m-xylene. Ligand 1 was purified by column
chromatography with no signs of oxidation and can be
Mono- and multidentate phosphine ligands are seem-
ingly ubiquitous in coordination and organometallic
chemistry and have been used to stabilize a wide variety
of metal complexes and catalysts.1 Prominent members
of the chelating phosphine family are the monoanionic,
potentially terdentate PCP pincer-type ligands2 with the
formula [2,6-(R2PCH2)2C6H3]-, where R ) alkyl, aryl.
One of the main advantages of ECE-type (E ) N, P, S,
O) pincers is the ease with which functionality may be
incorporated to tune electronic and steric properties at
the metal center.3 Indeed, many variations to the basic
PCP structure have been utilized to stabilize metal
complexes or generate highly active catalysts.2-4 Con-
spicuously absent from this list are PCP ligands incor-
porating electron-withdrawing functions directly bonded
to the P centers, even though electron-poor phosphines,
such as P(C6F5)3 and [(C6F5)2PCH2CH2P(C6F5)2] (dfppe),
* To whom correspondence should be addressed. Tel: +3130
2533120. Fax: +3130 2523615. E-mail: g.vankoten@chem.uu.nl.
† Debye Institute.
‡ Bijvoet Center for Biomolecular Research.
(5) Representative examples: (a) Alezra, V.; Bernardelli, G.; Corm-
inboeuf, C.; Frey, U.; Ku¨ndig, E. P.; Merbach, A. E.; Saundan, C. M.;
Viton, F.; Weber, J. J. Am. Chem. Soc. 2004, 126, 4843 and references
therein. (b) Wursche, R.; Debaerdemaeker, T.; Klinga, M.; Rieger, B.
Eur. J. Inorg. Chem. 2000, 2063. (c) Chen, A. S. C.; Pai, C.-C.; Yang,
T.-K.; Chan, S.-M. J. Chem. Soc., Chem. Commun. 1995, 2031. (d)
Ojima, I.; Kwon, H. B. J. Am. Chem. Soc. 1988, 110, 5617. (e) Mohr,
W.; Stark, G. A.; Jiao, H.; Gladysz, J. A. Eur. J. Inorg. Chem. 2001,
925. (f) Bellabarba, R. M.; Nieuwenhuyzen, M.; Saunders: G. C.
Organometallics 2002, 21, 5726.
§ Address correspondence pertaining to crystallographic studies to
this author. E-mail: a.l.spek@chem.uu.nl.
(1) (a) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999. (b)
van Leeuwen, P. W. N. M. Homogeneous Catalysis, Understanding the
Art; Kluwer: Dordrecht, The Netherlands, 2004. For recent reviews
on phosphine transition-metal complexes see: (c) Reek, J. N. H.; de
Groot, D.; Oosterom, G. E.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.
C. R. Chim. 2003, 6, 1061. (d) Thomas, C. M.; Su¨ss-Fink, G. Coord.
Chem. Rev. 2003, 243(1-2), 125. (e) Crepy, K. V. L.; Imamoto, T. Top.
Curr. Chem. 2003, 229, 1. (f) Freixa, Z. van Leeuwen, P. W. N. M.
Dalton Trans. 2003, 10, 1890. (g) Crepy, K. V. L.; Imamoto, T. Adv.
Synth. Catal. 2003, 345, 79. (h) Ansell, J.; Wills, M. Chem. Soc. Rev.
2002, 31, 259.
(6) (a) Curran, D. P.; Fischer, K.; Moura-Letts, G. Synlett. 2004,
1379. (b) Dani, P.; Richter, B.; van Klink, G. P. M.; van Koten, G. Eur.
J. Inorg. Chem. 2001, 125.
(7) (a) Handbook of Fluorous Chemistry; Gladysz, J. A., Curran, D.
P., Horvath, I. T., Eds.; Wiley-VCH: Weinheim, Germany, 2004. For
recent reviews on fluorous chemistry see: (b) Dobbs, A. P.; Kimberley,
M. R. J. Fluorine Chem. 2002, 118, 3. (c) Zhang, W. Tetrahedron Lett.
2003, 59, 4475. (d) Zhang, W. Chem. Rev. 2004, 108, 2531.
(8) (a) Hoge, B.; Herrmann, T.; Tho¨sen, C.; Patenburg, I. Inorg.
Chem. 2003, 42, 5422. (b) Hoge, B.; Tho¨sen, C.; Herrmann, T.;
Patenburg, I. Inorg. Chem. 2003, 42, 3633. (c) Hoge, B.; Herrmann,
T.; Tho¨sen, C.; Patenburg, I. Inorg. Chem. 2003, 42, 3623. (d) Hoge,
B.; Tho¨sen, C.; Herrmann, T.; Patenburg, I. Inorg. Chem. 2002, 41,
2260.
(2) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759.
(3) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001,
41, 3750. (b) Singleton, J. T. Tetrahedron Lett. 2003, 59, 1837.
(4) Other recent investigations with PCP metal complexes: (a)
Kozhanov, K. A.; Bobnov, M. P.; Cherkasov, V. K.; Fukin, G. K.;
Abakumov, G. A. Dalton 2004, 2957. (b) Morales-Morales, D.; Redon,
R.; Yung, C.; Jensen, C. Inorg. Chim. Acta 2004, 357, 2953. (c) Zhang,
X. W.; Emge, T. J.; Goldman, A. S. Inorg. Chim. Acta 2004, 357, 3014.
(d) Solin, N.; Kjellgren, J.; Szabo, K. J. J. Am. Chem. Soc. 2004, 126,
7026. (e) Amoroso, D.; Jabri, Yap, G. P. A.; Gusev, D. G.; dos Santos,
E. N.; Fogg, D. E. Organometallics 2004, 23, 4047. (f) Gagliardo, M.;
Dijkstra, H. P.; Coppo, P.; De Cola, L.; Lutz, M.; Spek, A. L.; van Klink,
G. P. M.; van Koten, G. Organometallics 2004, 23, 5833. (g) Eberhard,
M. R. Org. Lett. 2004, 6, 2125. (h) Cohen, R.; Milstein, D.; Martin, J.
M. L. Organometallics 2004, 23, 2342.
(9) Gandelman, M.; Vigalok, A.; Shimon, L. J. W.; Milstein, D.
Organometallics 1997, 16, 3981.
(10) (a) Lappert, M. F.; Martin, T. R.; Atwood, J. L.; Hunter, W. E.
J. Chem. Soc., Chem. Commun. 1980, 476. (b) Lappert, M. F.; Martin,
T. R.; Raston, C. L.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton
Trans. 1982, 1959.
10.1021/om0500063 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 03/23/2005

Communications
Scheme 1. Synthesis of the
Organometallics, Vol. 24, No. 9, 2005 2017
1
terdentate fashion. Indeed, the H NMR spectra for 2
Pentafluorophenyl-Substituted PCP Ligand 1
and 3 exhibit a single pseudo triplet for the benzylic pro-
tons, due to coupling with both phosphorus centers, a
situation analogous to that observed for the phenyl PCP
PdCl species 4.13 The 31P NMR spectra contain only one
resonance at δ 4.2 and 7.0 for 2 and 3, respectively,
which is downfield with respect to 1. Also, only three
signals for the ortho, meta, and para fluorines in the
equivalent C6F5 rings are observed in the 19F NMR
spectra.
stored for months in air in the solid state without
formation of PdO products. Conversely, the phenyl
analogue 1,3-(Ph2PCH2)2C6H4 is readily oxidized by
atmospheric oxygen. In a comparative reaction, C6D6
solutions of 1 and 1,3-(Ph2PCH2)2C6H4 were heated to
The difference in chemical shift between the meta and
para fluorines (∆δm,p) may be used as an indication of
the electron density at the P centers. A similar tech-
nique has been employed to probe the coordination
environment about B(C6F5)3, related species, and their
adducts.14 In this case, there is a small but significant
increase of ∆δm,p for 2 and 3 (14.5 and 13.3 ppm,
respectively) compared to 1 (10.6 ppm) upon complex-
ation with the Pd center. This is indicative of net
withdrawal of electron density from the P center by the
Pd metal. Subtle effects are also noted in comparison
of ∆δm,p for 2 and 3. The slightly larger value for cationic
2 indicates that the P centers are slightly more electron
deficient than in 3 by virtue of the positive charge and
presence of a less donating ligand (MeCN vs Cl-) on the
complexed Pd atom.
The terdentate binding mode of the PCP ligand in 2
and 3 was confirmed by X-ray crystallographic studies.
Views of the molecular structures of 2 and 3 are shown
in Figures 2 and 3, respectively, and selected bond
lengths and angles are shown in Table 1. The Pd-P
bond distances for 2 and 3 are slightly shorter than
those in 4,13 a previously observed property for fluoro-
aryl vs protioaryl phosphine Pd complexes.15 The Pd
centers reside in a slightly distorted square planar
environment with C(1)-Pd-Cl ) 177.23(9)° and P(1)-
Pd-P(2) ) 160.76(3)° for 3. In 2, the P atoms are
displaced out of the plane defined by the aryl ring,
benzylic carbons, and Pd and N centers by 0.6905(10)
and -0.6277(11) Å for P(1) and P(2), respectively.
Corresponding distances are 0.7931(9) and -0.8004(8)
Å in 3 and 0.695(2) and -0.795(2) Å in 4. This twist
minimizes steric repulsion between the two P(C6F5)2
groups, and NMR experiments indicate that wagging
of the pincer arms is fluxional in solution at all recorded
temperatures. The average C-P-C angles for 2 and 3
are 106.8(2)° (range 103.82(16)-111.4(2)°) and 106.1(2)°
(range 100.36(15)-109.95(16)°), respectively, and are
only slightly larger than in 4 (105.8(2)°, range 102.7(6)-
108.1(6)°). These data suggest that the fluoroaryl PCP
ligand provides a steric environment comparable to that
of the phenyl derivative in square-planar complexes or
on coordination in an equatorial fashion.
55 °C in air and monitored periodically by H and 31P
1
NMR spectroscopy. While no oxidation of 1 was observed
over 1 day under these conditions, 1,3-(Ph2PCH2)2C6H4
decomposed cleanly to its singly and doubly oxidized
products with a half-life of approximately 5 h. However,
prolonged (>1 month) exposure of solutions of 1 to air
does result in the formation of a small amount of
oxidized products (see below). In the 1H NMR spectrum
in CD2Cl2, the benzylic protons are found as a single
doublet (2JH-P ) 4.2 Hz), indicating C2v symmetry in
the ligand. This is confirmed in the 19F NMR spectrum,
as all four of the C6F5 groups are equivalent, giving rise
to signals for the ortho, para, and meta fluorines at δ
-132.4, -152.6, and -163.2, respectively. The 31P NMR
spectrum exhibits a single quintet (3JP-F ) 23.0 Hz) at
δ -44.2 with coupling due to the four ortho fluorines of
the P(C6F5)2 group. An X-ray crystal structure of ligand
1 was obtained, and a view with selected bond lengths
and angles is given in Figure 1.11 The two P(C6F5)2
groups are oriented on opposite faces of the central aryl
ring to minimize steric effects between the functions
(P(1)-C(7)-C(20)-P(2) ) 127.62(9)°). The lone pairs at
phosphorus are stereochemically active, and each P
atom is in a distorted-pyramidal environment with an
average C-P-C angle of 101.3(1)°. The C6F5-P-C6F5
angles (e.g. C(8)-P(1)-C(14) ) 98.37(8)°) are slightly
compressed compared to those found in the C6F5-P-
CH2 group (C(7)-P(1)-C(8) ) 100.11(9)°; C(7)-P(1)-
C(14) ) 105.15(9)°). In the crystal structure of 1, a small
amount of singly oxidized product was present (18%
occupancy), which was refined with a disorder model.
Crystals of 1 were obtained via slow evaporation of
pentane solutions in air and, during the 1 month
required for crystallization, approximately 15-20% of
1 was oxidized. In the crystal, only P(1) was partially
oxidized and this species cocrystallizes with the reduced
form. The presence of a PdO function was corroborated
by a new signal at δ 14 in the 31P NMR spectrum.
With the new ligand in hand, cationic Pd complex 2
was synthesized via electrophilic C-H activation using
the synthon [Pd(NCMe)4][BF4]2.12 Species 2 was smoothly
converted to the PdCl complex 3 by treatment with
excess LiCl (Scheme 2). The 1H, 19F, and 31P NMR data
for both 2 and 3 support binding of the PCP ligand in a
(12) Steffey, B. D.; Miedaner, A.; Maciejewski-Farmer, M. L.;
Bernatis, P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois,
D. L. Organometallics 1994, 13, 4844.
(13) (a) Gorla, F.; Venanzi, L. M.; Albinati, A. Organometallics 1994,
13, 43. (b) Rimmel, H.; Venanzi, L. M. J. Organomet. Chem. 1983,
259(1), C6.
(11) Abbreviated crystal structure parameters are as follows. 1:
0.82(C32H8F20P2)‚0.18(C32H8F20OP2), P21/c (No. 14), a ) 6.0387(1) Å, b
) 32.3369(5) Å, c ) 15.9373(2) Å, â ) 103.0616(6)°, V ) 3031.60(8)
Å3, R1 ) 0.0381, wR2 ) 0.1046. S ) 1.072. 2: [C34H10F20NP2Pd]BF4
+ disordered solvent, P21/c (No. 14), a ) 10.1565(2) Å, b ) 25.6619(5)
Å, c ) 16.1795(3) Å, â ) 110.5437(8)°, V ) 3948.77(13) Å3. R1 ) 0.0407,
wR2 ) 0.1001, S ) 1.040. 3: C32H7ClF20P2Pd‚2.1C7H8, P21/c (No. 14),
a ) 15.4853(1) Å, b ) 13.6515(1) Å, c ) 26.0274(2) Å, â ) 122.5592-
(3)°, V ) 4637.39(6) Å3, R1 ) 0.0430, wR2 ) 0.1381, S ) 1.060. For
further data see the Supporting Information.
(14) (a) Horton, A. D.; de With, J.; van der Linden, A. J.; van de
Weg, H. Organometallics 1996, 15, 2672. (b) Blackwell, J. M.; Piers,
W. E.; MacDonald, R. J. Am. Chem. Soc. 2002, 124, 1295. (c) Chen, E.
Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391.
(15) Pd-P bond lengths: [(C6F5)3P]2PdCl2, 2.3051(12) and 2.3052-
(10) Å; (Ph3P)2PdCl2‚2CHCl3, 2.3247(6) Å. Bersch-Frank, B.; Frank,
W. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, 52, 328.
Oilunkaniemi, R.; Laitinen, R. S.; Hannu-Kuure, M. S.; Ahlgre´n, M.
J. Organomet. Chem. 2003, 678(1), 95.

2018 Organometallics, Vol. 24, No. 9, 2005
Communications
Figure 1. Structure of 1. Ellipsoids are drawn at the 50% probability level. The oxygen atom at the partially oxidized P1
has been omitted in the drawing. Selected bond lengths (Å) and angles (deg): P(1)-C(7) ) 1.846(2), P(1)-C(8) ) 1.8401-
(19), P(1)-C(14) ) 1.8479(19), P(2)-C(20) ) 1.857(2), P(2)-C(21) ) 1.848(2), P(2)-C(27) ) 1.8471(19); C(7)-P(1)-C(8) )
100.11(9), C(7)-P(1)-C(14) ) 105.15(9), C(8)-P(1)-C(14) ) 98.37(8), C(20)-P(2)-C(21) ) 105.19(9), C(20)-P(2)-C(27)
) 100.73(9), C(21)-P(2)-C(27) ) 98.17(8), average C-P-C ) 101.3(1).
Scheme 2. Synthesis of Fluoroaryl PCP-Pd
Complexes 2 and 3 and Structures of Phenyl
PCP-Pd Complexes 4 and 5
Figure 2. Structure of the cation of 2. Ellipsoids are drawn
at the 30% probability level. The disordered BF4 anion and
disordered CH2Cl2 solvent molecules are removed for
clarity.
While ligation of 1 places the metal center in a steric
environment similar to that in protioaryl systems, the
fluoroaryl system. The IR spectra of the MeCN adducts
electronic situation should be vastly different. For
are uninformative in this regard; the νCN value of the
example, LNi(CO)3 complexes, where L ) PPh3, P(C6F5)3,
stretching mode for 2 (2324 cm-1) is essentially identical
with that of 5 (2325 cm-1).18 However, comparison of
the 31P NMR spectra for the protio species 4 and 5 show
the expected downfield shift upon substitution of Cl (δ
33.4)13b for MeCN (δ 43.2).12 Similar analyses of 3 (δ
7.0) and 2 (δ 4.2) indicate that the same transformation
results in a slightly upfield shift. As the σ-donating
ability is much lower for the fluoroaryl ligand, it is less
able to release electron density to compensate for the
positive charge at the metal center and, thus, the 31P
chemical shift is less effected.
have νCO values of 2069 and 2090 cm-1, respectively,
indicating that the P(C6F5)3 ligand is a much poorer
donor,16 and, by corollary, the metal center is more
electron deficient. Bonding between phosphines and
transition metals is viewed as a synergistic combination
of P to metal σ donation and metal to P π back-
bonding.17 As fluoroaryl phosphines are relatively poor
σ donors but good π acceptors16 in comparison to phenyl
phosphines, both of these effects will place the metal
center in a much more electron deficient state in the
(16) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(17) Dias, P. B.; Minas de Piedade, M. E.; Martinho Simo˜es, J. A.
Coord. Chem. Rev. 1994, 135/136, 737.
(18) A value of 2316 cm-1 for νCN was obtained in CH2Cl2 solution.12

Communications
Organometallics, Vol. 24, No. 9, 2005 2019
in favor of 7. Of note is the fact that the 1H NMR signals
of the benzylic protons for the individual species are
pseudo triplets, indicating that the terdentate binding
mode is intact for all complexes and the exchange is
likely mediated by the presence of small amounts of free
Lewis base. Variable-temperature 1H NMR spectra were
obtained over a 60 K range, and a van’t Hoff plot was
constructed (see the Supporting Information). The ther-
modynamic parameters obtained show that binding of
pyridine in 7 is strongly preferred over that in 620 on
enthalpic terms (∆H° ) -7.4 ( 0.2 kJ mol-1) but that
the equilibrium is slightly disfavored entropically (∆S°
) -17.5 ( 0.3 J mol-1 K-1). Due to the atomic radius
of fluorine being slightly larger than that of hydrogen
(van der Waals radii: H, 1.20 Å; F, 1.47 Å),21 there may
be steric interactions between the larger pyridine base
and the fluoroaryl rings in 7 that are not present in 6,
resulting in lowered mobility of the fluoroaryl rings or
pyridine. The negative value for ∆H° and the position
of the equilibrium in favor of 7 clearly illustrates that
the fluoroaryl PCP ligand conveys greater Lewis acidity
to the metal center in these complexes by virtue of the
poorer σ donating and greater π back-bonding ability
of the phosphine arms.
Figure 3. Structure of 3. Ellipsoids are drawn at the 30%
probability level. The toluene solvent molecules have been
omitted.
Table 1. Selected Bond Lengths (Å) and Angles
(deg) for 2-4
2 (X ) N)
3 (X ) Cl)
4 (X ) Cl)a
In conclusion, we have presented an efficient syn-
thetic approach to an electron-deficient fluoroaryl PCP
ligand and the generation of a number of Pd complexes.
In these Pd species, crystallographic studies show that
the ligand provides a metal-centered geometrical envi-
ronment similar to that of phenyl PCP in Pd complexes
but spectroscopic investigations indicate that it imparts
significant electronic differences. A competition experi-
ment for pyridine convincingly demonstrates that the
Pd center in the fluoroaryl ligand is significantly more
Lewis acidic than its phenyl counterpart. Analysis of
the van’t Hoff plot indicate that the equilibrium is fav-
ored by preferential binding of pyridine to the fluoroaryl
PCP Pd cation but slightly disfavored by entropic
considerations. While not generally useful in fluorous
biphasic catalysis, inclusion of pentafluorophenyl phos-
phine functions is known to enhance the solubility of
metal complexes in supercritical CO2,22 opening the
possibility for use of these metal complexes or other
species incorporating ligand 1 in “green” catalysis.
Research is underway to generate organometallic com-
plexes in which the effect of the altered electronics
induced by this ligand impacts reactivity and catalysis.
C(1)-Pd
2.023(4)
2.028(3)
1.998(8)
2.294(3)
2.288(3)
2.367(3)
162.0(1)
178.7(3)
102.7(6)-
108.1(6)
P(1)-Pd
2.2838(10)
2.2719(10)
2.081(4)
2.2725(8)
2.2678(8)
2.3619(8)
160.76(3)
177.23(9)
100.36(15)-
109.95(16)
P(2)-Pd
[X]-Pd
P(1)-Pd-P(2)
C(1)-Pd-[X]
C-P-C
161.55(4)
175.81(14)
103.82(16)-
111.4(2)
a Data obtained for equivalent parameters from ref 6a.
Scheme 3. Competition Experiment with
Fluoroaryl and Phenyl PCP-Pd Complexes
Acknowledgment. This work was supported in part
by the Council for Chemical Sciences from the Dutch
Organization of Scientific Research (CW-NWO) and
Utrecht University. P.A.C. thanks the Natural Sciences
and Engineering Research Council (NSERC) of Canada
for a postdoctoral fellowship.
To gain further insight into the electronic situation
at the Pd center, a competition experiment was per-
formed to determine the relative Lewis acid strength
between phenyl and fluoroaryl PCP Pd complexes.
Equimolar amounts of 2 and the pyridine adduct19 of
phenyl PCP-Pd (6) were dissolved in CD2Cl2, and rapid
equilibration was observed. The 1H NMR signals for the
benzylic protons of the four species involved are baseline
separated, and the 31P NMR spectrum also exhibits four
separate signals in approximately the same ratios. As
shown in Scheme 3, the fluoroaryl PCP complex pref-
erentially binds the stronger base, pyridine, to generate
7. Integration of the benzylic protons allowed for evalu-
ation of Keq, and a value of 2.49 was obtained at 298 K
Supporting Information Available: Full details of the
synthesis of 1, 2, 3 and 6, details of the competition experi-
ment, equilibrium data, chart for van’t Hoff analysis, and cif
files with crystallographic data for 1, 2, and 3. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM0500063
(20) There will also be a difference in the strength of binding of the
acetonitrile in the fluoroaryl and phenyl PCP Pd complexes, but this
difference should be much smaller than for binding of pyridine.
(21) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(22) (a) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100,
3009. (b) For a specific example see: Fugita, S.; Yuzawa, K.; Bhanage,
B. M.; Ikushima, Y.; Arai, M. J. Mol. Catal. A 2002, 35.
(19) The pyridine adduct 6 was simply generated by treatment of 5
with a slight excess of pyridine in CH2Cl2; see the Supporting
Information for more details.