Palladium(II) complexes with benzoxazol-2-ylidene ligands: Crystal structures of trans-chloro(benzoxazol-2-ylidene)-bis(triphenylphosphine) palladium(II) chloride and cis-diiodo(benzoxazol-2-ylidene)(triphenylphosphine) palladium(II)

Article abstract of DOI:10.1515/znb-2004-0212

The palladium(II) complexes trans-[PdCl(L)(PPh₃)₂]Cl, 5, and cis-[PdI₂(L)PPh₃], 7, (L = benzoxazol-2-ylidene) have been synthesized by treatment of the complexes trans-[PdX ₂(PPh₃)₂] (4: X = Cl, 6: X = I) with 2-(trimethylsiloxy)phenyl isocyanide 1, and subsequent hydrolysis of the Si-O bond. The crystal structures of 5 and 7*CH₂Cl₂ were established by X-ray diffraction. NMR and IR studies indicate, that the unexpected cis-configuration of 7 obtained from trans-[PdI₂(PPh ₃)₂] is not the result of a solution equilibrium between the cis- and the trans-isomers.

Full text of DOI:10.1515/znb-2004-0212

Palladium(II) Complexes with Benzoxazol-2-ylidene Ligands:
Crystal Structures of trans-Chloro(benzoxazol-2-ylidene)-
bis(triphenylphosphine)palladium(II) Chloride and
cis-Diiodo(benzoxazol-2-ylidene)(triphenylphosphine)palladium(II)
F. Ekkehardt Hahn, Thomas Lu¨gger, and Matthias Beinhoff
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Wilhelm-Klemm-Straße 8, D-48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. F. E. Hahn. Fax: +49(0)251-8333108. E-mail: fehahn@uni-muenster.de
Z. Naturforsch. 59b, 196 – 201 (2004); received December 22, 2003
Dedicated to Professor Ingo-Peter Lorenz on the occasion of his 60th birthday
The palladium(II) complexes trans-[PdCl(L)(PPh3)2]Cl, 5, and cis-[PdI2(L)PPh3], 7, (L = benz-
oxazol-2-ylidene) have been synthesized by treatment of the complexes trans-[PdX (PPh3)2]
2
(4: X = Cl, 6: X = I) with 2-(trimethylsiloxy)phenyl isocyanide 1, and subsequent hydrolysis of the
Si-O bond. The crystal structures of 5 and 7∗CH2Cl2 were established by X-ray diffraction. NMR
and IR studies indicate, that the unexpected cis-configuration of 7 obtained from trans-[PdI2(PPh3)2]
is not the result of a solution equilibrium between the cis- and the trans-isomers.
Key words: Carbene Complexes, Palladium, Crystal Structures
Introduction
We have reported that complexes with a coordinated
2-(trimethylsiloxy)phenyl isocyanide ligand (A) react
after hydrolytic O-SiMe3 bond cleavage to give a mix-
ture of products containing either the 2-hydroxyphenyl
isocyanide ligand (B) or a benzoxazol-2-ylidene lig-
and (C) (Scheme 1). The reaction A → B/C has been
studied at various metal centers like Cr [1], W [2], Re
[3], Pd [4], and Fe [5] and non-metal centers [6] and
has been reviewed [7]. The equilibrium between the
isocyanide complex B and the carbene complex C re-
sides on the isocyanide side for electron-rich complex
fragments MLx, where the isocyanide carbon atom is
deactivated for an intramolecular nucleophilic attack
by the hydroxyl oxygen atom by (d → π ∗) backbond-
ing. However, complexes of type B are spontaneously
converted into type C if MLx is electron-poor [3]. The
equilibrium between B and C can always be shifted to
the carbene complex if the electronically disfavoured
cyclization reaction is initiated by deprotonation of the
hydroxyl function in B, thereby enhancing its nucle-
ophilicity, and if the intermediate anionic ylide in D
Scheme 1.
methods for shifting the equlibrium between B and C
have also been reported [1, 10]. More recently this type
of carbene synthesis from β-functional phenyl iso-
cyanides has been extended to include 2-aminophenyl
isocyanide, thus leading to complexes with NH,NH
heterocyclic carbene ligands [11, 12].
The N-alkylation of cationic, square-planar tetra
is subsequently alkylated to give the N-alkylated car- (benzoxazol-2-ylidene) complexes of palladium(II)
bene complex E [8]. Species with the anionic ylidene and platinum(II) (Scheme 2) proved difficult. For ex-
ligand like in D have been isolated [9] and additional ample, attempts to deprotonate complexes of type F
c
0932–0776 / 04 / 0200–0196 $ 06.00 ꢀ 2004 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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F. E. Hahn et al. · Palladium(II) Complexes with Benzoxazol-2-ylidene Ligands
197
diiodo-bis(benzoxazol-2-ylidene)palladium(II) 3 [4]
(Scheme 3). The N-alkylation of such coordi-
nated benzoxazol-2-ylidenes has been described
[1, 2, 5, 7, 10]. In analogy to the described methods, ex-
periments were carried out to deprotonate complex 3
with various bases such as KOtBu or Et3N in DMF.
Whilst evidence for the deprotonation of 3 could be
obtained (for example Et3NH+ was detected in the 1H
NMR spectrum upon reaction of 3 with Et3N, and the
absorption for ν(NH) at 3200– 3300 cm−1 was absent
in the IR spectrum), no alkylation products could be
isolated upon treatment of deprotonated 3 with alkyl
halides. Regardless of the method employed for de-
protonation/alkylation, all reaction products showed a
similar elemental composition and were insoluble in
all organic solvents and in water. Elemental analyses
indicated the loss of iodine from the reaction prod-
ucts. One possible explanation for this behavior might
be a nucleophilic attack of the deprotonated ylidene
ligand at a second palladium center. The coordina-
tion of deprotonated benzoxazol-2-ylidene ligands at
a second metal center via the ring nitrogen atom has
been described [10]. As a result of such an attack one
would expect loss of coordinated iodine. The result-
ing product would have an oligomeric structure, which
correlates well with the insolubility of the substances
which were isolated. This insolubility prevented a fur-
ther characterization of the reaction products.
Scheme 2.
Scheme 3.
In order to prevent the formation of insoluble
oligomeric products upon deprotonation of complexes
with benzoxazol-2-ylidene ligands we initiated a study
to synthesize palladium(II) complexes with only one
benzoxazol-2-ylidene ligand. Bulky triphenylphos-
phine ligands were introduced, which are poor leaving
groups and which enhance at the same time the solu-
bility of the benzoxazol-2-ylidene complexes.
lead to complexes with both protonated and unproto-
nated ylidene ligands [4], which are insoluble in all
organic solvents, and the subsequent N-alkylation is
therefore not possible.
In this contribution we report on attempts to N-
alkylate the known dicarbene complex 3 [4] obtained
by intramolecular cyclization from the diisocyanide
complex 2 [13] (Scheme 3). In addition, we present
the preparation of the palladium complexes trans-
[PdCl(L)(PPh3)2]Cl 5 and cis-[PdI2(L)PPh3] 7 (L =
benzoxazol-2-ylidene)containing only one ylidene lig-
and (Scheme 3).
Reaction of 2-(trimethylsiloxy)phenyl isocyanide (1)
with trans-[PdCl2(PPh3)2] (4)
trans-Dichloro-bis(triphenylphosphine)palladium(II)
4 was synthesized according to a method described
in [14] from PdCl2 and PPh3 in acetonitrile. The 31P
NMR spectrum of the product (in CDCl3) showed
a singlet at δ = 24.46 ppm. This confirms that only
one, namely the trans-isomer was obtained [14]. The
Results and Discussion
Alkylation of trans-diiodo-bis(benzoxazol-2-ylidene)-
palladium(II) (3)
The reaction of PdI2 with an excess of 2- reaction of trans-[PdCl2(PPh3)2] with two equivalents
trimethylsiloxyphenylisocyanide and subsequent O- of 2-(trimethylsiloxy)phenyl isocyanide in THF at
SiMe3 bond hydrolysis in the presence of a catalytic −40 ◦C and subsequent Si-O hydrolysis gave ex-
amount of nBu4NF yields the neutral complex trans- clusively the monocarbene complex 5 (Scheme 4).
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198
F. E. Hahn et al. · Palladium(II) Complexes with Benzoxazol-2-ylidene Ligands
responding to the successive loss of the carbene lig-
and L, Cl and PPh3 were also observed. Complex
5 could be crystallized from a concentrated solution
in methanol, and its molecular structure determined
by X-ray diffraction (Fig. 1). This analysis confirmed
the assignment, that 5 is indeed the expected trans-
bisphosphine monocarbene complex.
Scheme 4.
The cation of 5 shows a slightly distorted square-
planar coordination geometry around the palladium
atom. The angles C1-Pd-Cl and P1-Pd-P2 measure
172.59(7)◦ and 174.92(2)◦, respectively. The chloride
counter ion is hydrogen-bonded to the N-H group of
...
˚
the ylidene ligand [HN Cl2 2.04(4) A]. Finally, the di-
hedral angle between the almost planar benzimidazol-
2-ylidene ligand and the Pd,P1,P2,Cl1,C1 plane mea-
sures 85.8◦.
Reaction of 2-(trimethylsiloxy)phenylisocyanide (1)
with trans-[PdI2(PPh3)2] (6)
The surprising observation that only one chloride
ligand could be substituted in trans-[PdCl2(PPh3)2]
prompted us to repeat the experiment with trans-
[PdI2(PPh3)2] 6. Complex 6 was synthesized by the
reaction of PdI2 with two equivalents of triphenylphos-
phine in acetonitrile as described for the prepara-
tion of 4. The 31P NMR spectrum of 6 measured
at low temperature showed just one resonance at
δ = 13.84 ppm which clearly indicated that a uni-
form reaction product was obtained. No indications
for a cis/trans isomer equilibrium in solution were
found. Since no 31P NMR data for either the cis
or the trans isomer have been published, we con-
Fig. 1. Molecular structure of 5 (hydrogen atoms on cal-
culated positions have been omitted for clarity). Selected
◦
˚
bond lengths [A] and angles [ ]: Pd-Cl1 2.3445(12), Pd-
P1 2.3523(8), Pd-P2 2.3467(8), Pd-C1 1.961(2), O-C1
1.355(3), O-C2 1.405(3), N-C1 1.319(3), N-C3 1.400(3),
C2-C3 1.363(4), C2-C7 1.398(4), C3-C4 1.388(4), N-HN
0.97(4), HN...Cl2 2.02(4); Cl1-Pd-P1 92.10(2), Cl1-Pd-P2
90.86(2), Cl1-Pd-C1 172.59(7), P1-Pd-P2 174.92(2), P1-Pd-
C1 89.17(6), P2-Pd-C1 88.42(6), C1-O-C2 106.3(2), C1-N-
C3 109.8(2), Pd-C1-O 124.9(2), Pd-C1-N 125.4(2), O-C1- firmed the trans configuration for 6 by determina-
tion of the lattice constants and space group of its
dichloromethane solvate and compared the results with
the published data for trans-[PdI2(PPh3)2]*2CH2Cl2
[ref. 15]: a = 11.922 [11.884], b = 20.◦432 [20.399],
N 109.7(2), O-C2-C3 108.9(2), N-C3-C2 105.3(2); dihedral
angle between planes made up from (Pd,Cl1,P1,P2,C1) and
(O,C1,N,C3,C2) 85.8◦.
˚
Experiments with a larger excess of the isocyanide, or
the reaction of 5 with the isocyanide, did not lead to
the substitution of the second chloro ligand.
c = 8.35 [8.331] A, β = 95.18 [94.896] , space group
P21/c [P21/c].
The reaction of trans-[PdI2(PPh3)2] with two
◦
The presence of the benzoxazol-2-ylidene ligand in equivalents of 1 in chloroform at −40 C and the sub-
5 was confirmed by the singlet at δ = 16.95 ppm for sequent Si-O bond cleavage yielded exclusively cis-
the N-H proton in the 1H NMR spectrum. The 31P diiodo(benzoxazol–2–ylidene)–(triphenylphosphine)
NMR spectrum showed a small shift to higher field for
the phosphorus resonance (δ = 21.18 ppm) compared
to complex 4. The molecular ion [PdCl(L)PPh3]+ (L =
benzoxazol-2-ylidene) was detected in the FAB mass
spectrum (positive ions) at m/z = 786, showing the
calculated isotope distribution. In addition, peaks cor- Scheme 5.
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F. E. Hahn et al. · Palladium(II) Complexes with Benzoxazol-2-ylidene Ligands
199
Table 1. Crystal and data collection details for 5 and
7∗CH2Cl2.
5
7∗CH2Cl2
Crystal habit
Crystal size, mm
Formula
colorless cubes
yellow needles
0.4×0.4×0.4
0.6×0.1×0.06
C43H35NCl2OP2Pd C25H20NI2OPPd∗CH2Cl2
Fw, amu
820.96
12.569(8)
23.171(7)
12.690(6)
90
90.90(5)
90
3695(5)
P21/n
4
826.52
˚
a, A
9.816(5)
12.926(3)
13.057(4)
63.67(2)
76.17(4)
72.71(3)
1406.6(9)
˚
b, A
˚
c, A
α, deg
β, deg
γ, deg
3
˚
V, A
¯
P1
2
Space group
Z
µ, mm−1
Abs. corr.
2θ Range, deg
Unique data
Obsvd. (I > 2σ(I)) 6694
R(observed),%
R(all),%
GOF
No. of variables
Res. el. dens., e/A 0.471/−0.456
0.769
none
5 to 56
8890
3.121
empirical, 6Ψ-scans
4 to 50
4914
4126
3.23, Rw = 7.71
4.40, Rw = 8.27
1.053
Fig. 2. Molecular structure of 7 (hydrogen atoms on calcu-
lated positions have been omitted for clarity). Selected bond
◦
˚
lengths [A] and angles [ ]: Pd-C1 1.962(5), Pd-P 2.2797(14),
Pd-I1 2.6500(9), Pd-I2 2.6562(9), O-C1 1.346(5), O-C2
1.392(6), N-C1 1.305(6), N-C3 1.394(6), C2-C3 1.364(7),
C2-C7 1.367(7), C3-C4 1.375(7); I1-Pd-I2 92.50(3), I1-Pd-
P 91.05(4), I1-Pd-C1 177.69(13), I2-Pd-P 171.71(4), I2-Pd-
C1 85.79(13), P-Pd-C1 90.45(13), C1-O-C2 107.9(4), C1-N-
C3 111.6(4), Pd-C1-O 121.9(3), Pd-C1-N 130.1(3), O-C1-
N 108.0(4), C3-C2-O 108.3(4), C7-C2-O 127.9(5), C2-C3-
N 104.3(4); dihedral angle between planes made up from
(Pd,I1,I2,P,C1) and (O,C1,N,C3,C2) 73.0◦.
2.79, Rw = 6.36
5.45, Rw = 7.09
1.043
455
307
3
˚
0.756/−0.886
troscopy [16]. A five-coordinate transition state has
been discussed by Nelson et al. [17] for the cis/trans
interconversion. It is unlikely that such an equilibrium
is the source of the observed configurational change
during the reaction 6 → 7, as both the starting mate-
rial 6 and the product 7 show only one resonance in
the 31P NMR spectra. We assume, that the geome-
try change takes place during the substitution of one
phosphine ligand by the isocyanide. Unfortunately, the
initially formed isocyanide complex could not be iso-
lated.
palladium(II) 7 (Scheme 4). In the 31P NMR spectrum
of 7, only one singlet at δ = 16.32 ppm was observed.
Yellow needles of 7∗CH2Cl2 crystallized from a
dichlormethane/n-hexane solution and were used for
an X-ray diffraction study. The molecular structure
of 7 (Fig. 2) is considerably different from that of 5.
One phosphine ligand was substituted by one carbene
ligand and the iodine atoms occupy cis-positions at the
metal center. The distortion of the ideal square-planar
coordination geometry around Pd is small (max 8◦),
similar to that in 5.
The formation of the cis-complex 7 from the
trans-configurated starting material 6 is remark-
able. For square-planar complexes of the type
[PdX2L2] (X = halogen), equilibria have been ob-
served between the two possible stereoisomers.
Newkome et al. showed that cis-dichlorobis(di-2-
pyridylphenylphosphine)palladium(II) can be con-
verted into the trans-isomer on heating solutions
containing excess phosphine [14]. Georgiev et al.
detected a cis/trans equilibrium for [PdCl2{PPh2
Experimental Section
All experiments were carried out in an argon atmosphere
using standard Schlenk techniques. All solvents were dried
by standard methods and distilled prior to use. Correct ele-
mental analyses were obtained for all reported compounds
using a Vario EL elemental analyzer. NMR spectra were
recorded on Bruker AM 250 and Jeol λ 400 instruments.
Mass spectra were measured with Finnigan MAT 112 and
MAT 711 spectrometers. Ligand 1 [2] and complexes 2 and
3 [4] were prepared according to published methods.
Preparation of trans-dichloro-bis(triphenylphosphine)-
palladium(II), (4): A sample of 299 mg (1.686 mmol) of
PdCl2 was suspended in 40 ml of warm acetonitrile. A solu-
tion of 890 mg (3.39 mmol) of triphenylphosphine in 20 ml
of acetonitrile was added at room temperature and the mix-
CH2C(O)CH3}2] in solution by using 31P NMR spec- ture was stirred for one hour. The yellow precipitate formed
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200
F. E. Hahn et al. · Palladium(II) Complexes with Benzoxazol-2-ylidene Ligands
was isolated by filtration, washed with small amounts of Preparation of cis-diiodo(benzoxazol-2-ylidene)(triphe-
acetonitrile and dried in vacuo. Yield 1.10 g (1.567 mmol, nylphosphine)palladium(II), (7): Complex 7 was synthe-
93%). – 1H NMR (250 MHz, CDCl3): δ = 7.70 (m, 12 H, Ar- sized as described for 5 from 310 mg (0.35 mmol) of 6
H), 7.37(m, 18 H, Ar-H). 13C NMR (63 MHz, CDCl3): δ = in 40 ml of chloroform and 0.15 ml (0.784 mmol, 2.2 eq)
128.0, 128.06, 130.50, 135.05 (Ar-C). 31P NMR (161 MHz, of 2-trimethylsiloxyphenyl isocyanide 1. To separate 7 from
CDCl3): δ = 24.46.
unreacted 6, the crude reaction product was suspended in
toluene and the insoluble 7 was isolated by filtration. Yield
180 mg (0.24 mmol, 70%). Yellow needles of 7∗CH2Cl2
were obtained by recrystallization from CH2Cl2/n-hexane at
−18 ◦C. – 1H NMR (400 MHz, CD2Cl2): δ = 14.63 (s, 1 H,
N-H), 7.78 – 7.12 (19 H, m, Ar-H). 31P NMR (161 MHz,
Preparation of trans-chloro(benzoxazol-2-ylidene)bis(tri-
phenylphosphine)palladium (II) chloride, (5): A sample of
0.6 ml (3.134 mmol, 2.2 eq) of 1 was injected into a so-
lution of 1.020 g (1.453 mmol) of 4 in 150 ml of THF at
−40 ◦C. After stirring for one hour, a few crystals of Bu4NF
along with 0.1 ml of water were added to the solution. A
colorless solid precipitated immediately. After stirring for an
additional 10 min the solid product was separated by filtra-
tion, washed several times with THF and dried in vacuo.
Yield 1.005 g (1.224 mmol, 84%). Suitable crystals for X-
ray structure determination were obtained by cooling a sat-
urated solution of 5 in methanol. – 1H NMR (250 MHz,
CDCl3): δ = 7.3 − 7.1 (m, 22 H, Ar-H), 7.80 (m, 12 H, Ar-
H), 16.95 (s, 1 H, N-H). – 31P NMR (161 MHz, CDCl3): δ =
21.18. – FAB-MS (positive ions, CHCl3/nitrophenol): m/z
(%) = 786 (8) [PdCl(PPh3)2(L)]+, 748 (9) [Pd(PPh3)2(L)]+,
630 (2) [Pd(PPh3)2]+, 524 (4) [PdCl(PPh3)(L)]+, 489 (16)
[Pd(PPh3)(L)]+, 263 (41) [PdCl(L)+H]+; L = benzoxazol-2-
ylidene ligand.
CDCl3): δ = 16.32. IR (KBr): ν [cm−1] 3050 (w, N-H), 1433
˜
(vs), 1094 (vs).
X–ray structure determination of 5 and 7∗CH2Cl2 [18]:
Suitable crystals were selected and mounted on a CAD-4
diffractometer equipped with a sealed Mo X-ray tube (λ =
˚
0.71073 A) and a nitrogen cooling device for data collection
at 153(2) K. Both structures were solved by standard Pat-
terson and Fourier methods. All non hydrogen atoms were
refined using anisotropic displacement parameters. Hydro-
gen atoms (except HN in 5) reside on calculated positions
˚
˚
(d(C-H) = 0.95 A, d(N-H) = 0.87 A [19]) with Ueq(H) = 1.3
Ueq(C). HN in 5 was located by Fourier procedures and its
positional parameters were refined with isotropic displace-
ment parameters. Selected crystallographic data are listed in
Preparation of trans-diiodobis(triphenylphosphine)palla- Table 1. ORTEP [20] plots are presented in Figures 1 and 2.
dium(II), (6): Complex 6 was synthesized as described for 4 All calculations were carried out using the SHELX-97 [21]
as a red-brown powder. Yield 92%. – 1H NMR (250 MHz, programs.
CDCl3): δ = 7.66 (m, 12 H, Ar-H), 7.36 (m, 18 H, Ar-H). –
31P NMR (161 MHz, CDCl3): δ = 13.84. FAB-MS (positive
Acknowledgement
ions, CHCl3/nitrophenol): m/z (%): 884 (1) [PdI2(PPh3)2]+,
757 (37) [PdI(PPh3)2]+, 630 (26) [Pd(PPh3)2]+, 368 (22)
[Pd(PPh3)]+.
Financial support of this work from the Deutsche
Forschungsgemeinschaft is gratefully acknowledged.
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F. E. Hahn et al. · Palladium(II) Complexes with Benzoxazol-2-ylidene Ligands
201
Cary, J. H. Nelson, Inorg. Chem. 15, 3161 (1976);
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