A convenient reagent for generation and stabilization of cationic methallylpalladium complexes containing nitrogen ligands

Article abstract of DOI:10.1016/S0040-4039(03)01543-0

A convenient synthesis of the easily handled, air stable methallyloxyphosphonium salt [CH₂=C(Me)CH₂^-O-P(NMe₂) ₃]⁺[(3,5-(CF₃)₂-C₆H ₃)₄B]^- 5 is described. The utility of this reagent in the generation and stabilization of cationic η³-methallyl palladium complexes through oxidative addition reactions is illustrated by the preparation of the stable salts of [(η³-C₄H₇)Pd(NN)]⁺[(3,5-(CF ₃)₂-C₆H₃)₄B] ^- from Pd₂(dba)₃. The molecular structure of one of them has been determined by a single-crystal X-ray diffraction.

Full text of DOI:10.1016/S0040-4039(03)01543-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6773–6777
A convenient reagent for generation and stabilization of cationic
methallylpalladium complexes containing nitrogen ligands
Ali Mechria,a Mohamed Rzaiguib and Faouzi Bouachira,*
aLaboratoire de Chimie de Coordination, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
bLaboratoire de Chimie des Mate´riaux, Faculte´ des Sciences de Bizerte, 7021 Jarzouna, Bizerte, Tunisia
Received 24 April 2003; revised 20 June 2003; accepted 20 June 2003
Abstract—A convenient synthesis of the easily handled, air stable methallyloxyphosphonium salt [CH2ꢀC(Me)CH2−O-P(NMe2)3]+
[(3,5-(CF3)2-C6H3)4B]− 5 is described. The utility of this reagent in the generation and stabilization of cationic h3-methallyl
palladium complexes through oxidative addition reactions is illustrated by the preparation of the stable salts of [(h3-
C4H7)Pd(NN)]+[(3,5-(CF3)2-C6H3)4B]− from Pd2(dba)3. The molecular structure of one of them has been determined by a
single-crystal X-ray diffraction.
© 2003 Elsevier Ltd. All rights reserved.
Palladium allyl complexes attracted much attention as a
result of their importance as intermediates in palladium-
catalyzed allylic substitution reactions. Attack of the
nucleophile on a cationic palladium p-allyl complex is at
present conventionally accepted as the crucial step in the
catalytic cycle.1–3 To our knowledge, very few palladium
complexes possessing a potentially bidentate nitrogen
ligand and an allyl moiety have been described so far.4–7
We have described a simple and effective synthesis of
cationic h3-allyl complexes of palladium with a-diimine
ligands 1 and 2.8
the highly active olefin polymerization catalyst
[Cp*ThCH3]+[B(C6F5)4]− by reaction of [HNnBu3]+
2
[B(C6F5)4]− with Cp*Th(CH3)2.12 However, in view of
2
the fact that the preparation of such anion is costly and
involves significant synthetic effort, we have investigated
the possibility of generating new bulky and weakly
coordinating anions in a more straightforward manner.
In this note we report a convenient synthesis of
the methallyloxyphosphonium tetrakis[3,5-bis(trifluoro-
methyl)phenyl]borate [CH2ꢀC(Me)CH2 O-P(NMe2)3]+
−
[BAr%4]− (Ar%=3,5-C6H3(CF3)2) 5 and illustrate its utility
in the generation and stabilization of the cationic h3-
allyl palladium(II) complexes with a-diimine ligands.
The lack of reactivity and non-nucleophilic character of
[PF6]− have led to his widespread use as non coordinat-
ing counterion supporting cationic organometallic com-
plexes.9,10 With the generation of sufficiently
electrophilic complexes, however, the limitation of
[PF6]− as a non-reactive entity is encountered. Jordan
has reported the deactivation of the highly reactive d0
[Cp2Zr(CH3)]+ ethylene polymerization catalyst by F−
According to published procedure, the tetrakis[3,5-
bis(trifluoromethyl)phenyl] borate TFPB was isolated in
the form of hydrated sodium salt.13 The addition of a
solution of methallyloxyphosphonium chloride14
[CH2ꢀC(Me)CH2-O-P(NMe2)3]+Cl− 3 to a saturated hot
aqueous solution of sodium TFPB results in the imme-
diate precipitation of 5 (Scheme 1).
abstraction from the PF6 counterion.11 In addition to
−
recognized requirements for weakly coordinating
anions, an effective counteranion must necessarily be
chemically robust and exceptionally resistant to elec-
trophilic attack. In this regard, (fluoroaryl) borates are
worthy of attention. Marks has reported the synthesis of
Keywords: palladium; cationic allyl complexes; a-diimine; methylally-
loxyphosphonium salt.
* Corresponding
author.
Fax:
+216-73-500-278;
e-mail:
faouzi.bouachir@fsm.rnu.tn
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01543-0

6774
A. Mechria et al. / Tetrahedron Letters 44 (2003) 6773–6777
Scheme 1.
the nitrogen donor function of ligands (a,b) gives rise to
the cationic h3-methallyl complexes of palladium with
a-diimine ligands [(h3-C4H7)Pd(NN)]+ which are iso-
lated as stable [(3,5-(CF3)2-C6H3)4B]− salts (Scheme 2).
Compound 5 was isolated as an air stable, analytically
pure, white crystalline solid. The presence of the
strongly electron-withdrawing CF3 groups rendered
TFPB virtually inert toward strongly acids and resistant
to a variety of oxidants. The chemical stability of TFPB
have led to his wide use as the counterion for stabilizing
cationic electrophilic metal alkyl complexes whose cata-
lytic properties have recently been investigating. A com-
mon route for generating such species involves
protonolysis of a dialkyl complex using the trialkylam-
monium salt [HNnBu3]+[B(C6F5)4]− or the oxonium acid
[(3,5-(CF3)2C6H3)4B]−[H(OEt2)2]+. The first has limita-
tion in that he is not sufficiently acidic to effect the
protonolysis reaction, particularly in the late metal
species. The oxonium acid is very hygroscopic and must
be stored and used under strict conditions. Our salt
presents the advantage to react at ambient temperature
to give nearly quantitative yields of well-defined
cationic Pd(II)-h3-allyl complexes containing nitrogen
ligands. In fact, the oxidative addition of the methallyl-
oxyphosphonium salt 5 to Pd2(dba)3 in the presence of
The [(3,5-(CF3)2-C6H3)4B]− anion was found to impart
good solubilities to the new cationic complexes, a prop-
erty which renders characterization straightforward.
The new complexes exhibit spectroscopic data in accord
with the proposed structures. The formation of the allyl
complexes 6a and 6b is clear from the resonance of the
1
allyl group in H NMR spectroscopy. The two allylic
signals are characteristic of a symmetrical h3-allyl lig-
and. Thus, the two resonances are at 2.80 ppm (Hanti);
3.05 ppm (Hsyn) for 6a and at 2.97 ppm (Hanti); (3.23;
3.27) ppm (Hsyn) for 6b. In 13C NMR spectroscopy, the
two terminal allylic carbons C1 and C3 appears at 64.5
ppm for 6a and at (64.4; 64.6) ppm for 6b. The CꢀN
functions are shifted to high-frequency when compared
to free ligands and appear at 168.3 ppm for 6a and at
(184.4; 185.2) for 6b. The 19F NMR spectrum of 6a
Scheme 2.

A. Mechria et al. / Tetrahedron Letters 44 (2003) 6773–6777
6775
normal.8,15 The allyl and PdꢁN,N planes make an angle
of 107.7°, which is comparable with those found for
other allylpalladium complexes containing a-diimine
ligand.5,8,15 The a-diimine plane NꢀCꢁCꢀN is roughly
planar (torsion angle N1ꢁC5ꢁC6ꢁN2=5.3°) and makes
a dihedral angles of 85.2 and 77.6° with the aromatic
substituents on the nitrogen atoms. Comparison with
similar complexes in the literature indicates that as the
steric bulk of the aryl ortho substituents increases, the
N-aryl rings tend to lie more perpendicular to the
square plane of the complex, and the ortho substituents
more effectively block the axial sites.17 All other dis-
tances and angles are as expected.
presents one signal at 101.5 ppm and shows no substan-
tial chemical shift from that of salt 5 at room tempera-
ture, suggesting that coordination of anion to Pd centre
is weak and labile. The CꢀN stretching vibration in the
IR spectroscopy is observed at 1620–1660 cm−1.
A single-crystal X-ray diffraction study confirms the
identity of complex 6a. The molecular structure of 6a
with the adopted numbering scheme is shown in Figure
1. The structure of complex 6a consists of loosely
associated [(h3-C4H7)Pd(NN)]+ cations and tetrahedral
[(3,5-(CF3)2-C6H3)4B]− counteranions (Fig. 1). The
fluorine atoms (F19, F20, F21) are in the closest prox-
,
imity to the metal cation (PdꢁF19=5.38(4) A,
,
,
PdꢁF20=5.07(6) A, PdꢁF21=4.19(7) A). These Pd···F
Cationic p-allylpalladium complexes are reported to be
efficient catalysts in the presence of phosphines for the
dimerization of methyl acrylate18 and co-dimerization
of 1,3-dienes with acrylic esters.19 We wish to report
that isolated pure cationic allylpalladium complex 6b
catalyze the polymerization of methyl acrylate with
good conversion (40%). When its [PF6]− analogue is
used under the same conditions in the catalytic poly-
merization of methyl acrylate, the conversion decreases
dramatically (<1%). Those results suggest that the weak
coordinating ability of [(3,5-(CF3)2-C6H3)4B]− leads to
an important increase in the catalytic activity of
cationic allylpalldium complexes containing a-diimine
ligands.
distances are extremely longer than the sums of Pd2+
and F ionic radii (2.19 A). These data, in addition to
−
,
the fact that the anion structure is not perturbed by the
presence of the metal cation, show clearly that the
cation–anion interaction is rather weak. The anion
[(3,5-(CF3)2-C6H3)4B]− adapts a structure in which the
four fluoroaryl groups are tetrahedrally bonded to the
central boron atom.
The [(h3-C4H7)Pd(NN)]+ cation of 6a adapts the usual
slightly distorted square planar arrangement (Fig. 2).
The allyl ligand is bonded almost symmetrically to the
,
palladium centre (2.116(1) and 2.147(1) A for PdꢁC1
and PdꢁC3, respectively). We note that the PdꢁC(allyl)
distances are reasonable for an allyl trans to a ligand of
moderate trans influence.15,16
In conclusion, we have described a convenient synthetic
procedure for the preparation of the new methallyl-
oxyphosphonium salt 5. The generation and character-
ization of cationic methallylpalladium complexes
containing Nitrogen ligands supported by the
(fluoroaryl)borate anion were also described. The appli-
cation of these complexes in catalytic reactions such as
oligomerization and polymerization of alkenes (ethyl-
ene, propylene) and functional alkenes (methyl acryl-
ate) is in progress.
The a-diimine ligand a is also coordinated in a symmet-
ric fashion: the PdꢁN separations, 2.081(9) and 2.122(9)
,
A are consistent with known literature values for
related complexes.8,15 The two bond angles N1ꢁPdꢁN2
and C1ꢁPdꢁC3 of 77.5(5) and 68.1(3)°, respectively, are
Figure
2. Perspective
ORTEP
drawing
of
[(h3-
C4H7)Pd(NN)]+ cation. Thermal ellipsoids are at 30% proba-
,
bility. Selected bond lengths (A) and angles (°):
Figure 1. Perspective ORTEP drawing of complex 6a. Ther-
mal ellipsoids are at 30% probability. Selected bond lengths
PdꢁN1=2.081(9),
PdꢁC2=2.141(1),
PdꢁN2=2.122(9),
PdꢁC3=2.147(1),
PdꢁC1=2.116(1),
N1ꢁC5=1.278(1),
,
(A) and angles (°): BꢁC23=1.63(1), BꢁC31=1.65(2),
N1ꢁC7=1.435(1), N2ꢁC6=1.266(1), N2ꢁC15=1.449(1),
C1ꢁC2=1.444(2), C2ꢁC3=1.418(2); N1ꢁPdꢁN2=77.5(4),
N1ꢁPdꢁC1=105.3(5), N2ꢁPdꢁC3=108.6(5), PdꢁN1ꢁC5=
114.6(7), PdꢁN2ꢁC6=113.6(8), C1ꢁPdꢁC2=68.1(3).
BꢁC39=1.65(1), BꢁC47=1.67(1); C23ꢁBꢁC31=111.0(8),
C23ꢁBꢁC39=112.4(8), C23ꢁBꢁC47=105.2(8), C31ꢁBꢁC39=
104.3(8), C31ꢁBꢁC47=112.2(8), C39ꢁBꢁC47=111.9(8).

6776
A. Mechria et al. / Tetrahedron Letters 44 (2003) 6773–6777
Experimental
Decomposition: 213°C. IR [w cm−1] (KBr): 1623; 1658
(CꢀN). H NMR (300 MHz, CD2Cl2): l 0.58 s (3H),
0.87 s (3H), 0.99 s (3H), H12,13,14; 1.97 (s, 3H, H4); 1.73
1
All manipulations were performed under an argon
atmosphere using standard Schlenk tube techniques.
Dichloromethane and diethylether were distilled under
argon from P2O5 and sodium benzophenone ketyl,
respectively, and stored under argon. The Pd2(dba)3
complex was synthesized following established proce-
dure.20 NMR spectra were recorded on a Bruker AMX
300 spectrometer and infrared spectra on a BioRad
FTS-6000 spectrophotometer.
m (1H), 1.92 m (2H), 2.2 m (1H), H8,9; 2.78 (d, JHH
=
4.8 Hz, 1H, H10); 2.97 (s, 2H, Hanti); 3.23 s (1H), 3.27 s
(1H), Hsyn; 7.04 d (JHH=7.2 Hz, 2H), 6.96–7.10 m
(2H), 7.2–7.4 m (6H), Har; [(CF3)2-C6H3]4B−: 7.46 (s,
4H, p-H); 7.67 (s, 8H, o-H). 13C NMR (75 MHz,
CD2Cl2): l 11.7, 17.6, 22.0, C12,13,14; 23.9 (C4); 24.9,
33.5 (C8,9); 51.0 (C7); 51.6 (C10); 58.2 (C11); 64.4, 64.6
(C1,3); 122.4 (Cm); 128.7, 129.6 (Cp); 130.1, 130.5 (Co);
137.2 (C2); 147.3, 147.7 (Cipso); 184.4, 185.2 (C5,6);
1
Preparation of 5: To an aqueous solution of methallyl-
oxytris(dimethylamino) phosphonium chloride pre-
pared in situ14 was added a saturated hot aqueous
solution of sodium tetrakis[3,5-bis(trifluoromethyl)-
[(CF3)2-C6H3]4B−: 118.2(Cp); 125.3 (q, JCF=272.3 Hz,
2
CF3); 129.9 (q, JCF=31.7 Hz, Cm); 135.5 (Co); 162.50
1
(q, JCB=49.8 Hz, Cipso).
phenyl]borate.13
Precipitation
was
immediately
Crystallographic data for 6a: C54H39N2BF24Pd; fw=
1289.09; monoclinic; P21/c (No. 14); Z=4; a=
observed. The white suspension was stirred for 1 h at
room temperature and then filtered through a frit. The
white solid was then dissolved in diethyl ether and the
solution was stirred over Na2SO4, filtered, and pentane
(100 ml) was layered on top of the solution. After 3
days, the recrystallized product was collected by filtra-
tion and dried under vacuum to give the white crys-
talline salt 5. Mp: 98°C. Anal. found (calcd) for
C42H37N3F24BOP: C, 45.78 (45.96); H, 3.17 (3.39). IR [w
cm−1] (KBr): 1100–1200; 1280; 1360. 1H NMR (300
MHz, CDCl3): l 1.72 (s, 3H, CH3); 2.63 (d, JPH=10
Hz, 18H, NCH3); 4.37 (d, JPH=7.6 Hz, 2H, H3); 4.98
(s, 1H, H1); 5.05 (s, 1H, H1%); 7.53 (s, 4H, Hp); 7.68 (s,
8H, Ho). 13C NMR (75 MHz, CDCl3): l 18.4 (C4); 36.5,
36.6 (NMeMe%); 72.5 (C3); 115.5 (C1); 117.4 (Cp); 125.0
,
,
,
16.752(9) A; b=18.291(8) A; c=19.964(3) A;
3
i=114.32(3)°; V=5574(4) A ; Dcalcd=1.536 g cm−3;
,
R=0.059; Rw=0.086; −215h521; 05k525; 05l522;
Mo (u=0.7107 A); T=296 K.
,
Atomic coordinates and anisotropic temperature fac-
tors have been deposited at the Cambridge Crystallo-
graphic Data Centre, University Chemical Laboratory,
12 Union Road, Cambridge CB2 1EZ (CCDC 179134).
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2
(q, JCF=272 Hz, CF3); 129.0 (q, JCF=34.5 Hz, Cm);
134.7 (Co); 137.4 (C2); 161.6 (q, JCB=49 Hz, Cipso). 19
F
1
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Preparation of 6a: To 30 ml of CH2Cl2 solution of
Pd2(dba)3 (140 mg, 0.15 mmol) was added 0.3 g of salt
5 (0.28 mmol) and DAB a (80 mg, 0.30 mmol) at room
temperature. After 24 h of stirring the mixture was
filtered through Celite filter and the residue was washed
with CH2Cl2 (2×10 ml). The combined filtrates were
evaporated to 5 ml. The product was precipitated by
the addition of hexane (20 ml), washed with
diethylether/hexane 1:5 (3×10 ml) and dried in vacuo,
yielding 0.36 g of 6a as an orange solid (93%). Decom-
position: 197°C. IR [w cm−1] (KBr): 1621 (CꢀN). 1H
NMR (300 MHz, CD2Cl2): l 1.97 (s, 3H, H4); 2.11 (s,
6H, o-CH3); 2.15 (s, 6H, o%-CH3); 2.80 (s, 2H, Hanti);
3.05 (s, 2H, Hsyn); 7.11 (s, 6H, Harom); 8.26 (s, 2H,
NꢀCHꢁHCꢀN); [3,5-C6H3(CF3)2]4B−: 7.50 (s, 4H, p-H);
7.71 (s, 8H, o-H). 13C NMR (75 MHz, CD2Cl2): l 19.2,
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128.9 (Cp); 130.2, 130.3 (Cm,m%); 138.9 (C2); 148.1 (Cipso);
168.3 (CꢀN); [(CF3)2-C6H3]4B−: 119.7(Cp); 126.8 (q,
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1JCF=272.3 Hz, CF3); 130.9 (q, JCF=31.5 Hz, Cm);
1
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