Synthesis of a water-soluble carbene complex and its use as catalyst for the synthesis of 2,3-dimethylfuran

Article abstract of DOI:10.1016/S0022-328X(01)01029-4

1,1′,3,3′-Tetrakis(p-dimethylaminobenzyl)-2,2′- biimidazolidinylidene, L₂^R (R=CH₂C₆H₄NMe₂-p) which contains four peripheral NMe₂ substituents, was obtained from 4-dimethylaminobenzaldehyde by a three-step reaction sequence, and was used for the preparation of imidazolidin-2-ylidene Ru(II) and Rh(I) complexes 1 and 2. Etherial HCl readily protonates the NMe₂ functionality on the carbene ligand of 1 to give the corresponding salt, 1′; whereas the reaction of 2 with HCl gave a hygroscopic and ill-defined rhodium species. In aqueous solution the salt 1′ is an efficient and active catalyst for intramolecular cyclisation of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-dimethylfuran. The catalyst 1′ could be recovered by simple phase separation and the catalytic reaction was maintained for five different runs. All of the new compounds were characterised by elemental analyses, IR and NMR spectroscopy and the molecular structure of 1 was determined by X-ray crystallography.

Full text of DOI:10.1016/S0022-328X(01)01029-4

Journal of Organometallic Chemistry 633 (2001) 27–32
www.elsevier.com/locate/jorganchem
Synthesis of a water-soluble carbene complex and its use as
catalyst for the synthesis of 2,3-dimethylfuran
a,
a
b
c
:
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8
Ismail Ozdemir *, Beyhan Yig˘it , Bekir C¸ etinkaya , Dinc¸er Ulku¨ ,
M. Nawaz Tahir c, Cengiz Arıcı c
a Department of Chemistry, Ino¨nu¨ Uni6ersity, 44069 Malatya, Turkey
b Department of Chemistry, Ege Uni6ersity, 35100 Borno6a-Izmir, Turkey
c Department of Engineering Physics, Hacettepe Uni6ersity, 06532 Beytepe, Ankara, Turkey
Received 26 February 2001; received in revised form 30 April 2001; accepted 30 April 2001
Abstract
1,1%,3,3%-Tetrakis(p-dimethylaminobenzyl)-2,2%-biimidazolidinylidene, LR2 (R=CH2C6H4NMe2-p) which contains four periph-
eral NMe2 substituents, was obtained from 4-dimethylaminobenzaldehyde by a three-step reaction sequence, and was used for the
preparation of imidazolidin-2-ylidene Ru(II) and Rh(I) complexes 1 and 2. Etherial HCl readily protonates the NMe2
functionality on the carbene ligand of 1 to give the corresponding salt, 1%; whereas the reaction of 2 with HCl gave a hygroscopic
and ill-defined rhodium species. In aqueous solution the salt 1% is an efficient and active catalyst for intramolecular cyclisation of
(Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-dimethylfuran. The catalyst 1% could be recovered by simple phase separation and the
catalytic reaction was maintained for five different runs. All of the new compounds were characterised by elemental analyses, IR
and NMR spectroscopy and the molecular structure of 1 was determined by X-ray crystallography. © 2001 Elsevier Science B.V.
All rights reserved.
Keywords: Ruthenium; Water-soluble carbene complex; Recyclable catalyst; Crystal structures
1. Introduction
The stable 1,3-diorganylyimidazol-2-ylidenes, I, and
their coordination chemistry has recently aroused great
interest [1–6]. These crystalline carbenes show no ten-
dency to dimerise [5]. In sharp contrast, 4,5-dihydroim-
idazol-2-ylidenes containing an unhindered R group
[2,7] readily dimerise to give tetraaminoethene deriva-
tive, II (LR2 ), which behave as carbene precursors under
relatively mild conditions. The first isolated transition
metal imidazolidin-2-ylidene complex was synthesised
by Lappert in 1971 via cleavage of CꢀC bond by
[PtCl2(PEt3)]2 [8]. Subsequently this reaction was ex-
tended to other tetraaminoalkenes and electrophilic
organometallic substrates [9,10].
The renaissance of metal complexes of N-heterocyclic
carbenes began after their use as alternatives for phos-
phine complexes used in homogeneous catalysis [1,11].
Examples of such reactions include the Heck olefination
of haloarenes [12], the asymmetric hydrosilylation of
acetophenone [13,14], hydrogenation [15], hydroformy-
lation [12], metathesis [2,3] and cyclopropanation [16]
of alkenes. In a catalytic reaction (Z)-3-methylpent-2-
en-4-yn-1-ol was converted into 2,3-dimethylfuran in
very high yield [17]. The nature of the N-substituents of
the carbene ligand have a pronounced effect upon the
catalytic activity of the complexes [16,17].
Biphasic catalysis, consisting of a water phase which
contains a catalytic species and a non-miscible organic
phase, continues to attract interest in view of industrial
applications [18,19]. For this purpose a number of
* Corresponding author. Fax: +90-232-341-0037.
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E-mail address: iozdemir@inonu.edu.tr (I. Ozdemir).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01029-4

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28
attempts have been made to introduce hydrophilic
functional groups on convenient ligands, in particular
phosphines [18–21]. Despite their similar catalytic be-
haviour [11], examples of hydrophilic carbene com-
plexes are rare [22]. The under-representation of
hydrophilic carbene complexes may be attributed to
difficulties with their preparation. The employment of
highly reactive nucleophiles as well as electrophiles at
different stages of imidazolidin-2-ylidene metal complex
synthesis precludes the presence of many types of func-
tional groups [10]. As a possible system for this endeav-
our, we have chosen p-dimethylaminobenzyl as an
N-substituent. This is because the presence of the pe-
ripheral ꢁNMe2 group is expected to improve water
solubility through quaternisation of theꢁNMe2 group.
solid state and exhibited simple first-order 1H-NMR
spectra at room temperature.
2.1. Neutral carbene complexes
The functional tetraaminoethene LR2 readily reacted
with the halogeno-dimers of Ru(II) or Rh(I) to afford
[RuCl2(C6Me6)LR] (1), and [RhCl(COD)LR] (2). Each
of the complexes 1 and 2 were obtained in high yield as
air-stable crystals, which were characterised by elemen-
tal analysis and IR, 1H-NMR and 13C-{1H}-NMR
spectra. The IR spectra of the complexes showed in-
tense absorption bands at 1614–1616 cm−1 assigned to
w(CN).
The carbene precursor employed in this study poten-
tially has two donor atoms, namely the alkene can
bridge across two metal ions via amino nitrogen atoms.
However, comparison of the NMR signals from the
alkene and coordinated carbene ligands strongly sug-
gested that coordination to ruthenium or rhodium was
via the carbon atoms. This was conclusively established
by the protonation reactions and by a single crystal
2. Results and discussion
The synthesis of the tetraaminoethene LR2 , II (R=
CH2C6H4NMe2-p) was accomplished in a three-step
procedure as described in Scheme 1. The Schiff base
was synthesised using a method similar to that reported
by Billman et al. [23]. The 1,2-disubstituted ethylendi-
amine was prepared successfully by hydrogenation of
the Schiff base over a Pd/C catalyst in dry toluene at 25
°C and 200 psi (13.6 atm) pressure. However, if non-
dried solvent, high temperature or high pressure is
employed, this leads to the formation of p-dimethy-
laminotoluene. The tetraaminoethene, LR2 , was synthe-
sised in 83% yield by formylation of the NH bonds in
an excess of Me2NCH(OMe)2. The compound LR2 , II
(R=CH2C6H4NMe2-p) is relatively air-stable in the
X-ray diffraction experiment on [RuCl2(C6Me6)(LR)]
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[LR = CN(CH2C6H4NMe2-p)CH2CH2NCH2 C6 H4 N-
Me2-p] (vide infra). All the complexes showed the ex-
pected signals due to the coordinated arene/COD and
the imidazolidin-2-ylidene in their 1H-NMR spectra.
An AB quartet (doublet of doublets) was observed for
the CH2 protons in the NꢁCH2Ar substituent in the
complexes due to restricted rotation about the MꢁCcarb
bond. The ring methylene protons were observed as
multiplets.
13C-NMR spectra appear to be the most useful spec-
troscopic probe for structure elucidation. Thus, the
Ccarbene resonances in complexes 1 and 2 are found at
ca. 205–213 ppm. In each case the downfield chemical
shift as well as the structure due to RhꢁC coupling
(J=46.7 Hz), in complex 2, served to establish the
carbene coordinating mode of LR2 . Values of l and J
are similar to those found for non-functionalised car-
bene complexes [24].
2.2. Protonations of complexes 1 and 2
The pendant nature of the NMe2 group in complex 1
is also revealed by the reaction with HCl. Thus, treat-
ment of CH2Cl2 solutions of 1 with two equivalents of
hydrogen chloride in diethyl ether gave the correspond-
ing red solid 1% which is soluble in water and stable to
air. By comparison, the reaction of 2 with HCl afforded
a cream-coloured solid. However, this was determined
not to be the analogous product 2%. This point was
strongly evidenced by the 13C-NMR spectrum of the
resultant material which did not show the diagnostic
Ccarbene doublet in the expected region, indicating the
carbene ligand may have been displaced or rearranged.
On the other hand, the transformation of the amino
Scheme 1. Synthesis of tetraaminoethene, LR2 , and ruthenium and
rhodium complexes. Reagent and conditions: (i) H2NCH2CH2NH2,
EtOH, 25 °C, 4 h; (ii) H2, Pd/C, 13.6 atm, PhMe, 25 °C; (iii)
Me2NCH(OMe)2, PhMe, 110 °C; (iv) [RuCl2(h6-C6Me6)]2; (v)
[RhCl(COD)]2, PhMe, 110 °C; (vi) HCl(g), Et2O, 25 °C.

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I. Ozdemir et al. / Journal of Organometallic Chemistry 633 (2001) 27–32
29
Fig. 1. PLATON drawing of the molecule 1 with the atom-numbering schemes. The displacement elipsoids are drawn at the %50 probability level.
H atoms are shown as small circles with arbitrary radii.
Crystal
data:
[RuCl2(C6Me6)(LR)],
LR=
group in complex 1 into a quaternary salt is not accom-
panied by noticeable changes of the spectroscopic
parameters of the carbene fragment. For example, 13C-
NMR resonances of the carbene carbon atom shift only
slightly (Dl=3 ppm) in response to the quaternisation.
The broad signal observed at 4.3 ppm was assigned to
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
CN(CH2)2C6H4NMe2-p)CH2CH2N(CH2C6H4NMe2-p),
Mr=670.737, orthorhombic, space group Pnma (No.
62), lattice parameters: a=13.559(1), b=24.988(1),
c=9.610(1) A, Z=4, V=3256.1(5) A , Dcalc=1.368 g
cm−3, m(Mo–Ka=0.664 mm−1, F(000)=1400, T=
295 K, orange prisms, crystal dimensions 0.20×0.20×
0.25 mm.
3
,
,
1
the solvated water in the H-NMR (DMSO-d6).
2.3. X-ray crystal structure of RuCl2(C6Me6)(LR)] (1)
A thermal ellipsoid drawing of compound 1 is shown
in Fig. 1. The ruthenium and C1 atoms are located on
a mirror plane of the orthorhombic space group so that
the whole molecule has a two-fold symmetry with re-
spect to the RuꢁC1 bond. If one considers there are
three coordination bonds of the ruthenium atom with
the electrons of the C6Me6 ring, two symmetrical RuꢁCl
bonds and one RuꢁC1 bond involving the imidazole
ring, the coordination around the Ru atom is six-fold,
which is in this case distorted octahedrally. The bond
lengths RuꢁM1 (M1 is the midpoint between C14 and
C14a atoms) and RuꢁM2 (midpoint between C12 and
Single crystals of 1 were obtained from a CH2Cl2–
Et2O solution. All diffraction data were collected on an
Enraf–Nonius CAD-4 diffractometer with graphite-
,
monochromated Mo–Ka radiation (u=0.71073 A) at
room temperature. Least-square refinements of 25 high
angle reflections gave the best values for lattice parame-
ters. The data were corrected for Lorentz polarisation
and empirical absorption correction via C scans.
The structure was determined by Patterson and
Fourier methods [28] and refined by full-matrix least-
squares techniques to a conventional R value of 0.038
(Rw=0.045). All non-hydrogen atoms were refined
with anisotropic displacement parameters. H-atom po-
sitions were taken from difference maps and a riding
model was used with Uiso (H)=1.3Ueq (C). Scattering
factors were taken from International Tables for X-ray
Crystallography [29]. For data collection, reduction,
,
C13) are 2.140(5) and 2.074(5) A, respectively (Table 1).
As a result of the crystal symmetry, the RuꢁM2 and
RuꢁM2a lengths are equal. The Ruꢁh bond angles are
60.19 (M1ꢁRuꢁM2) and 61.30° (M2ꢁRuꢁM2a). On the
other side of the coordination sphere, the RuꢁCl
,
[2.4234(12)] and RuꢁC1 [2.090(5) A] bond lengths are
considerably different from each other. The ClꢁRuꢁC1
and ClꢁRuꢁCla bond angles are 90.25(1) and 84.78(4)°,
structure solution and refinement, MOlEN package was
used [30].

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I. Ozdemir et al. / Journal of Organometallic Chemistry 633 (2001) 27–32
30
catalytic activity of the complexes [23]. Hence, we have
investigated the catalytic activity of both the neutral
and cationic complexes.
Thus, (Z)-3-methylpent-2-en-4-yn-1-ol (10 mmol)
was added to 0.1 mmol of the complexes 1 and 2 and
the mixture was stirred at 80 °C in an oil bath for 1–3
h (Eq. (1)).
respectively. The dihedral angle between the arene ring
and the plane defined by Cl, C1, Cla atoms is 3(1)°. The
arene ring is planar with RuꢁC distances between
,
2.090(5) and 2.193(4) A. Selected geometric parameters
are listed in Table 1.
(1)
The quaternary salt, 1%, was tested as a catalyst in
biphasic media since it was stable and soluble in water.
The results are compiled in Table 2.
The salt 1% was the most promising candidate. How-
ever, the neutral species, although active, were found to
be somewhat less effective. The catalyst 1% was recycled
via aqueous work-up and used repeatedly with little
detriment to rate or yield. The results afforded by 1%
from five successive cyclisations are shown in Table 2.
To confirm that the metal has a key role in this
intramolecular cyclisation catalyzed by the quaternary
salt 1%, the urea derivative 3 was synthesised [27] and its
reactivity was examined under the same conditions.
With this salt, 3, the reaction did not proceed and the
starting (Z)-3-methylpent-2-en-4-yn-1-ol was recovered
unchanged (last entry in Table 2).
2.4. Catalytic studies
Recently, carbene complexes of Ru(II) have been
introduced as catalysts for the activation of (Z)-enynols
toward their intramolecular cyclisation into furans
[17,25,26]. The nature of the N-substituents of the
carbene ligand have a pronounced effect upon the
Table 1
,
Selected bond lengths (A) and angles (°) for 1
Bond lengths
Ru1ꢁCl1
Ru1ꢁC1
Ru1ꢁC12
Ru1ꢁC13
2.4234(12)
2.090(5)
2.193(4)
2.191(5)
Ru1ꢁC14
Ru1ꢁM1 a
Ru1ꢁM2 a
2.245(4)
2.140(5)
2.074(5)
3. Conclusion
Bond angles
Cl1ꢁRu1ꢁC1
Cl1ꢁRu1ꢁCl1i
C1ꢁRu1ꢁC12
C1ꢁRu1ꢁC13
C1ꢁRu1ꢁC14
90.25(11)
84.78(4)
94.20(17)
119.96(17)
157.24(15)
C12ꢁRu1ꢁC12i
C12ꢁRu1ꢁC13i
C13ꢁRu1ꢁC14
M1ꢁRu1ꢁM2
M2ꢁRu1ꢁM2i
37.49(18)
67.81(18)
37.34(18)
60.19
Novel ꢁNMe2 functionalised imidazolidin-2-ylidene
metal complexes, 1 and 2, are accessible from appropri-
ately N-substituted tetraaminoalkene, LR2 . Etherial HCl
readily protonates the nitrogen atoms of the ꢁNMe2
functionality on the carbene ligand of 1 to yield the
corresponding quaternary salt 1%. In contrast, treatment
of 2 with HCl did not give the expected salt 2%. The salt
1% is water soluble and a more active catalyst than the
neutral complexes 1 and 2 for the conversion of (Z)-3-
methylpent-2-en-4-yn-1-ol into 2,3-dimethylfuran. The
catalytic activity of the recovered salt 1% from the
catalytic reaction was maintained for at least five differ-
ent runs. Easy and quantitative recovery of the catalyst
in active form from organic reaction products by simple
phase separation is the obvious advantage. The use of
similar salts as catalyst in other reactions such as
hydrogenation and hydroformylation is in progress in
our laboratory.
61.30
a M1 and M2 are the midpoints between C14ꢁC14i and C12ꢁC13,
respectively. Symmetry code: (i) −x, 1/2+y, −z.
Table 2
Catalytic synthesis of 2,3-dimethylfuran, at 80 °C
Catalyst
Time (h)
Yield
(%) a,b
1
1
2
2
1%
1%
1% c
1% d
3
1
3
1
2
1
3
1
1
3
40
94 (84)
30
93 (82)
94 (84)
95 (85)
93
93
e
–
a Determined by GC.
4. Experimental
b Isolated yield, after distillation, given in parentheses.
c Fourth run.
d Fifth run.
All reactions were performed using Schlenk-type
flasks under Ar and standard high vacuum-line tech-
e Not detected.

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31
6.92; N, 9.62. Calc. for C29H40N4ClRh: C, 59.16; H,
6.37; N, 9.48%.
niques. Solvents were analytical grade and distilled
under Ar from sodium benzophenone (toluene, Et2O),
sodium–potassium (pentane, THF), P2O5 (di-
chorometane). IR spectra were recorded in the 4000–
400 cm−1 region on a Pye Unicam spectrometer.
Samples were prepared as KBr discs. NMR spectra
were recorded at 297 K on a Bruker AC300P FT
spectrometer operating at 300.13 MHz (1H), 75.47
MHz (13C). Gas chromatography (GC) analyses were
performed with a Hewlett Packard instrument using a
25 m×0.32 mm OV1 capillary column in conjunction
with a flame ionisation detector. Elemental analyses
were performed by the Middle East Technical Univer-
sity, Ankara. Commercial reagents were used as sup-
plied and other reagents were prepared by literature
methods: RuCl2(C6Me6)2 [31], [{Rh(–Cl)(cod)}2] [32].
4.2. Synthesis of the salts 1%, and 2%
A solution of the complex 1 (0.52 g, 0.78 mmol) in
CH2Cl2 (10 cm3) was added to HCl in Et2O (1.2 M, 1.5
cm3) and stirred for 24 h. A brown precipitate ap-
peared. The product, 1%, was filtered and then washed
with Et2O (2×15 cm3) and dried under vacuum. M.p.
230–231 °C; 96% yield. 1H-NMR (l, D2O): 2.07 [s,
18H, C6(CH3)6], 3.34 [s, 12H, N(CH3)2], 3.50–3.55,
3.71–3.77 (m, 4H, NCH2CH2N), 4.59, 5.55 (bdd, J=
12.5, 4H, CH2C6H4NMe2), 7.58 (d, J=8.7 Hz, 4H,
C6H4-NMe2), 7.64 (d, J=8.7 Hz, 4H, C6H4-NMe2).
13C{H}-NMR (l, D2O):15.6 (C6(CH3)6), 47.0
[N(CH3)2], 49.4 (NCH2CH2N), 54.1 (CH2C6H4NMe2),
95.2 [C6(CH3)6], 120.8, 130.4, 141.8, (CH2C6H4CH2-
NMe2), 210.0 (C-2-Im). Selected Ir (KBr disk): w(N–
H), 2350 cm−1. \M (25 °C), 290 Scm2 mol−1. Anal.
Found: C, 48.81; H, 6.65; N, 6.96. Calc. for
C33H54N4Cl4O4Ru: C, 48.71; H, 6.64; N, 6.88%.
4.1. Synthesis of complexes 1 and 2
A solution of 1,1%,3,3%-tetrakis(p-dimethylaminoben-
zyl)-2,2%-biimidazolidinylidene (0.86 g, 1.23 mmol) in
toluene (15 cm3) was added to [RuCl2(h6-C6Me6)]2
(0.864 g, 0.99 mmol), and then the mixture was heated
for 2 h under reflux. The brown product was precipi-
tated. The product 1, was filtered, washed with n-hex-
Using a similar procedure, a cream-coloured solid
was precipitated from 2. However, it was very hygro-
scopic and unstable; its 13C-NMR spectrum did not
show the Ccarbene doublet in the expected region. There-
fore, it was not characterised.
ane (2×15 cm3),
and
dried
in
vacuum.
Recrystallisation from CH2Cl2 (7 cm3)–Et2O (14 cm3)
at −20 °C, gave brown crystals of 1. M.p. 242–243
1
(decomp); 0.81 g, 91% yield. H-NMR (l, CDCl3): 2.07
4.3. Catalytic studies
[s, 18H, C6(CH3)6], 2.89 [s, 12H, N(CH3)2], 3.19–3.24,
3.32–3.37 (m, 4H, NCH2CH2N), 4.16, 5.61 (dd, J=
13.8 Hz, 4H, CH2C6H4NMe2), 6.65 (d, J=8.7 Hz, 4H,
C6H4ꢁNMe2), 7.34 (d, J=8.7 Hz, 4H, C6H4ꢁNMe2).
13C{H}-NMR (l, CDCl3): 14.0 (C6(CH3)6), 38.8
[N(CH3)2], 46.7 (NCH2CH2N), 53.0 (CH2C6H4NMe2),
92.3 [C6(CH3)6], 110.6, 122.9, 128.1, 148.3 (C6H4CH2),
207.5 (C-2-Im). Anal. Found: C, 58.64; H, 5.37; N,
8.06. Calc. for C33H46N4Cl2Ru: C, 59.01; H, 6.91; N,
8.35%.
Ruthenium or rhodium catalyst (0.1 mmol) was
added to 10 mmol of neat (Z)-3-methylpent-2-en-4-yn-
1-ol without a solvent. The mixture was stirred in an oil
bath at 80 °C for 1–3 h. The conversion of the starting
enynol was determined by GC and the pure furan was
isolated by distillation under reduced pressure.
In biphasic catalysis a solution of the salt 1% (0.1
mmol) in H2O (2.0 ml) and a solution of the enynol (10
mmol) in toluene (2.0 ml) were mixed and heated to
80 °C. The organic phase was analysed by GC. For the
second run the organic phase was replaced by a fresh
solution of enynol and the procedure was repeated.
Compound 2 was prepared in the same way as 1
from 1,1%,3,3%-tetrakis(p-dimethylaminobenzyl)-2,2%-bi-
imidazolidinylidene (0.43 g, 0.76 mmol) and [{Rh(m-
Cl)(cod)}2] (0.38 g, 0.76 mmol) to give yellow crystals
1
of 2. M.p. 191–192 °C; 0.61 g, 77% yield. H-NMR (l,
CDCl3): 1.92–1.94, 2.33–2.35 [m, 8H, CH2 (COD)],
2.92 [s, 12 H, N(CH3)2], 3.20–3.23 (m, 4H,
NCH2CH2N), 351–3.52, 5.01–5.02 [s, 4H, CHꢀCH
(COD)], 5.19, 5.35 (dd, J=14.4 Hz, 4H,
CH2C6H4NMe2), 6.71 (d, J=8.6 Hz, 4H, C6H4), 7.31
(d, J=8.6 Hz, 4H, CH2C6H4NMe2). 13C{H}-NMR (l,
CDCl3): 28.7, 32.9 [CH2 (COD)], 40.6 [N(CH3)2], 47.7
(NCH2CH2N), 54.4 (CH2C6H4NMe2), 68.3 [d, J=15.4
Hz, CHꢀCH (COD)], 98.8 [d, J=6.5 Hz, CHꢀCH
(COD)], 112.6, 124.0, 129.3, 150.2 (C6H4NMe2), 213.2
(d, J=47.0 Hz, C-2 Im). Anal. Found: C, 59.77; H,
5. Supplementary material
Final fractional atomic coordinates, full list of bond
lengths, bond angles and displacement parameters (Uij)
have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC no. 156705 for compound
1. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).

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[8] D.J. Cardin, B. C¸ etinkaya, M.F. Lappert, Lj. Manojlovic Muir,
Acknowledgements
K.W. Muir, Chem. Commun. 00 (1971) 400.
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[10] M.F. Lappert, J. Organomet. Chem. 358 (1988) 185.
[11] W.A. Herrmann, C. Ko¨cher, Angew. Chem. Int. Ed. 36 (1997)
2162.
Financial support for this work from the State Plan-
ning Organization, DPT (Grant No. 98-DPT-001) is
gratefully acknowledged.
[12] (a) W.A. Herrmann, M. Elison, J. Fischer, C. Ko¨cher, G.R.J.
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