Metal-template synthesis and co-ordination properties of a palladium
complex containing a novel and stable imidazole-substituted
phosphine C᎐P bidentate chelate
Huifang Lang, Jagadese J. Vittal and Pak-Hing Leung*
Department of Chemistry, National University of Singapore, Kent Ridge 119260, Singapore
molecular structure and the co-ordination chemistry of 2 have
An organopalladium complex promoted Diels–Alder reaction
between 1-phenyl-3,4-dimethylphosphole and 1-vinylimidazole
gave a novel imidazole-substituted phosphanorbornene
bidentate ligand which co-ordinated to the palladium template
via the C2 carbon atom of the imidazole group and the
bridgehead phosphorus donor atom.
been determined by X-ray structural analysis (Fig. 1).‡ The
study reveals that the reaction of 1 with 1-vinylimidazole in 1,2-
dichloroethane has resulted in the removal of the benzylamine
chelate and the imidazole substituted exo-phosphanorbornene
ligand created co-ordinates to palladium as a bidentate chelate
via the bridgehead phosphorus atom and the C2 carbon atom of
the imidazole group. The palladium atom is in a slightly dis-
torted square-planar geometry with the bond angles in the
ranges 84.8(1)–94.8(2) and 167.0(1)–172.2(2)Њ. Due to the
aromaticity of the imidazole ring, all C᎐C and C᎐N bonds
[1.350(8)–1.383(6) Å] within the five-membered ring are notice-
ably shorter than the two attached C᎐N bonds [1.473(7)–
1.453(7) Å]. The Pd(1)᎐C(11) distance of 1.993(5) Å is similar
to the Pd᎐C bonds observed in other reported complexes con-
taining the orthometallated benzylamine [2.004(11) Å] and
naphthylamine units (2.006 Å) which experience a similar trans
electronic influence from a chloro ligand.8 The Pd(1)᎐P(1) dis-
tance of 2.207(1) Å is also within the normal range observed
for this class of phosphanorbornene complex. The Pd ؒ ؒ ؒ Cl(3)
distance is 3.622(2) Å indicating that there is no interaction
between the two heavy atoms.
Transition-metal complexes containing imidazole and its
derivatives play an important role in bioinorganic chemistry.1
These compounds are frequently considered as models in the
development of metal-based enzymes and proteins. In terms of
co-ordination chemistry, imidazole may be considered as an
ambidentate ligand. For instance, it has been well established
that the imidazole unit co-ordinates to transition-metal ions,
such as palladium() and platinum(), predominantly via one
of its nitrogen atoms. In some rare cases, however, the cyclic
unit may also co-ordinate as a carbene or an amidine ligand via
its C2 carbon atom. This interesting mode of organometallic
bonding has been observed in a small number of Cr0, Fe0, RuII
and RuIII complexes.2 Apart from their biological interest, the
availability of stable C-bound imidazole heavy-metal complexes
is important for the study of the trans influence3 and other
related phenomena that are pertinent to the design and devel-
opment of efficient support ligands for homogenous catalysis.4
Indeed, it has been reported that palladium complexes contain-
ing C-bound imidazole ligands are efficient catalysts for cross-
coupling reactions.5 In general, however, palladium complexes
containing monodentate ligands are kinetically labile. We
believe that the development of a new class of stable C-bound
imidazole complexes may significantly influence the design
of catalysts in homogenous catalysis. Here we report the
palladium-template synthesis of the first imidazole-substituted
tertiary phosphine bidentate ligand in which the C2 carbon of
the imidazole unit is involved in metal chelation.
Interestingly, when [PdCl2(DMPP)2] was used for the Diels–
Alder reaction, a much lower reaction rate was observed.
The 31P NMR studies indicated that 80% of [PdCl2(DMPP)2]
[PdCl2(DMPP)2]
ClCH2CH2
or
N
Me
Cl
Cl
N
Me
N
N
1
Pd
Me
Cl
P
3
N
2
Pd
Ph
P
ClCH2CH2Cl
Ph
Me
1
2
Scheme 1
In the absence of the transition-metal ion, no Diels–Alder
reaction was observed between 1-phenyl-3,4-dimethylphos-
phole (DMPP) and 1-vinylimidazole. Upon co-ordination to
palladium, however, DMPP is activated towards the [4 ϩ 2]
cycloaddition reaction. Thus, when [PdCl2(DMPP)2] or the
organopalladium complex 16 was treated with the dienophile in
† Preparation of complex 2. A solution of the organopalladium com-
plex 1 (0.46 g, 0.97 mmol) in 1,2-dichloroethane (40 cm3) was treated
with silver perchlorate (0.2 g, 0.97 mmol) in water (1 cm3) for 30 min.
The resulting mixture was filtered through a layer of Celite to remove
silver chloride and the organic layer was dried over anhydrous MgSO4.
The dried solution was treated with 1-vinylimidazole (0.37 g, 3.87
mmol) and the reaction mixture was then stirred at 84 ЊC for 30 d. The
solution was washed with water (50 cm3) and then dried over MgSO4.
The solvent was removed under reduced pressure to give a yellow resi-
due. Upon crystallization from acetonitrile–diethyl ether, the dichloro
complex 2 was obtained as yellow prisms (0.21 g, 40%), m.p. 256–
258 ЊC (decomp.). 31P NMR (CDCl3): δ 105.3 (s).
1,2-dichloroethane at 84 ЊC, the exo-cycloadduct
2 was
obtained as the sole product (Scheme 1).† When the template
complex 1 was used, the cycloaddition reaction was completed
in 30 d. Interestingly, during the course of heating, the N3
nitrogen of the imidazole unit underwent a parallel N-alkyl-
ation reaction with 1,2-dichloroethane and produced HCl
as the side product.7 The HCl thus generated led to further
chemoselective cleavage of the orthometallated benzylamine
ligand from the palladium template and thus facilitate the
formation of the dichloro complex 2 as the final product.6 The
‡ Crystal data for 2: C19H22Cl3N2PPd, M = 522.11, orthorhombic,
space group Pna21, a = 14.9398(1), b = 16.5294(1), c = 8.6207(1) Å,
U = 2128.85(3) Å3, Z = 4, Dc = 1.629 g cmϪ3, T = 293 K, µ(Mo-Kα) =
13.29 cmϪ1
, F(000) = 1048, R1 = 0.0408, wR2 = 0.0885 for 4214
independent observed reflections [I > 2σ(I), 1.84 р 2θ р 29.31Њ] and
236 parameters. The Flack parameter was refined to Ϫ0.02(4). CCDC
reference number 186/1018.
* E-Mail: chmlph@nus.edu.sg
J. Chem. Soc., Dalton Trans., 1998, Pages 2109–2110
2109
Lang, Huifang
