Nickel and palladium complexes of enolatefunctionalised N-heterocyclic carbenes

Article abstract of DOI:10.2478/s11532-010-0071-6

The reaction of chloroethyltrimethylsilylether with 1-methylimidazole furnishes an ionic liquid that undergoes methanolysis to crystalline 2-hydroxyethylimidazolium chloride (crystal structure presented). Conversion to defined hydroxyethylimidazol-2-ylidene nickel complexes failed, but was accomplished with 1-methyl-3-acetophenyl-imidazolium bromide. The bis(NHC^{{n-ary intersection}}O^-) nickel(II) chelate is formed, rather than a methallylnickel monochelate, but with nickelocene a monochelate NiCp complex was detected. The bulky 1-(2,6-diisopropylphenyl)-3-(2'-phenyl-enolato)-imidazol-2-ylidene allylpalladium chloride was obtained in pure form. Attempts to generate catalysts for ethylene oligomerization by in situ techniques have failed so far whereas P^{{n-ary intersection}}O^- ligands, comparable by the P-C diagonal relationship, provide active catalysts.

Full text of DOI:10.2478/s11532-010-0071-6

Cent. Eur. J. Chem. • 8(5) • 2010 • 992–998
DOI: 10.2478/s11532-010-0071-6
Central European Journal of Chemistry
Nickel and palladium complexes of enolate-
functionalised N-heterocyclic carbenes
Invited Paper
Saravanakumar Shanmuganathan1, Olaf Kühl1,
Peter G. Jones2, Joachim Heinicke1*
1Institute of Biochemistry – Inorganic Chemistry, Ernst-Moritz-Arndt-
University Greifswald, D-17489 Greifswald, Germany
2Institute of Inorganic and Analytical Chemistry, Technical University
Braunschweig, D-38106 Braunschweig, Germany
Received 23 March 2010; Accepted 10 May 2010
Abstract:
The reaction of chloroethyltrimethylsilylether with 1-methylimidazole furnishes an ionic liquid that undergoes methanolysis to
crystalline 2-hydroxyethylimidazolium chloride (crystal structure presented). Conversion to defined hydroxyethylimidazol-2-ylidene
nickel complexes failed, but was accomplished with 1-methyl-3-acetophenyl-imidazolium bromide. The bis(NHC∩O−) nickel(II) chelate
is formed, rather than a methallylnickel monochelate, but with nickelocene a monochelate NiCp complex was detected. The bulky
1-(2,6-diisopropylphenyl)-3-(2’-phenyl-enolato)-imidazol-2-ylidene allylpalladium chloride was obtained in pure form. Attempts to
generate catalysts for ethylene oligomerization by in situ techniques have failed so far whereas P∩O− ligands, comparable by the P-C
diagonal relationship, provide active catalysts.
Keywords: Functionalized imidazolium salt • N-heterocyclic carbenes • Nickel • Palladium complexes
© Versita Sp. z o.o.
recently been synthesized for use as ligands in transition
1. Introduction
metal complexes and have been applied in catalysis.
Our research interests in the chemistry of P, O [2d,7]
and NHC [8] ligands motivated us to also conduct some
investigations of C,O hybrid ligands based on NHC [9].
We selected hydroxyethyl- and phenacyl-functionalized
NHC and their nickel and palladium complexes and note
here some results not previously published by others
[6,10,11].
Bidentate or polydentate hybrid ligands containing both
hard and soft donor groups have found widespread use
in coordination chemistry and homogeneous catalysis.
Depending on the degree of binding to the metal, they
can act either as spectator chelate ligands, stabilizing
the catalyst backbone, or as hemilabile ligands, by
reversible dissociation allowing uptake of substrate at
the free coordination site after ring opening [1]. Examples
include nickel catalysts with anionic P,O spectator
ligands for the oligo- or polymerization of ethylene [2] or
hemilabile rhodium complexes with neutral P,O ligands,
successfully applied in asymmetric hydrogenation or
hydroformylation [3]. With the development of easily
accessible N-heterocyclic carbene ligands [4] and the
awareness of the diagonal relationship of low-valent
carbon and phosphorus [5], several types of O-donor
functionalized N-heterocyclic carbenes NHC [6] have
2. Experimental Procedure
All reactions were carried out in carefully dried, freshly
distilled solvents. Reactions with air- or moisture-
sensitive compounds were conducted under an argon
atmosphere using Schlenk techniques. NMR spectra
were recorded on a multinuclear FT NMR spectrometer
ARX300 (Bruker) at 300.1 (1H), 75.5 (13C), and 121.5
* E-mail: heinicke@uni-greifswald.de
992

S. Shanmuganathan et al.
(31P) MHz. Tetramethylsilane was used as the shift warm up to room temperature and stirring continued for
1
reference for H and 13C, and H3PO4 (85%) was used 16 h. After filtration the solvent was removed in vacuum.
as the shift reference for 31P. Coupling constants refer to The residue was washed several times with diethyl
JHH unless stated otherwise. Melting points (uncorrected) ether and dried in vacuum to yield 160 mg (< 45%) of
were determined with a Sanyo Gallenkamp melting point pale yellow powder (compound 5), which still contained
1
apparatus and elemental analyses were performed KBr and a small amount of THF. H NMR (CDCl3): δ =
with a CHNS-932 analyzer from LECO using standard 3.10 (s, 3 H, NCH3), 6.65 (s, 1 H, NCHC-O), 6.81, 6.82
conditions.
(2 d, 3J = 1.7 Hz, 2 H, 4,5- CH), 7.20-7.36 (m, 3 H, CHPh),
7.86-7.88 (m, 2 H, CHPh) ppm. 13C NMR (CDCl3): δ =
36.55 (NCH3), 98.28 (N-CH=CO), 118.60, 122.37 (4,5-
CH), 125.40 (2 o-CH), 127.31 (p-CH), 127.75 (2 m-CH),
138.67 (i-Cq), 154.75 (CO) ppm, C-2 at noise level. MS
(EI, 70 eV): Calculated for C24H22N4NiO2 isotopic cluster
: (rel. int.%) 458.1 (39) [M+ for 60Ni], 457.1 (27) [M+ for
58Ni, 13C], 456.1 (100) [M+ for 58Ni]; found: m/e (rel. int.%)
= 458 (12), 457 (9), 456 (30) [M+ for 58Ni].
2.1.
3-Methyl
imidazolium chloride (1)
1-(2-trimethylsilyloxy)ethyl-
An autoclave was charged with N-methylimidazole
(1.10 g, 13.4 mmol) and 2-chloroethoxy trimethylsilane
(2.38 g, 15.6 mmol). It was heated to 150ºC for 24 hours.
After cooling the oily viscous liquid was transferred into a
beaker and washed with hexane. The insoluble viscous
oily liquid was extracted with CHCl3. The solvent was
evaporated to give a dark oily liquid, yield 2.1g (53%).
1H NMR (CDCl3): δ = –0.03 (s, 9 H, SiMe3), 3.83, 3.97
(2s br, 5 H, NMe, NCH2), 4.38 (s br, 2 H, OCH2), 7.54 (2s
br, 2 H, 4-H, 5-H), 10.09 (s vbr, 1 H, H-2) ppm. 13C NMR
(CDCl3, sample slowly decomposed forming an oil):
δ = –0.94 (SiMe3), 37.16, (NMe), 52.25 (NCH2), 61.15
(OCH2), 123.02, 123.19, (CH-4, CH-5), 137.46 (vbr, C-2)
ppm. Calculated for C9H19ClN2OSi•0.55 CHCl3 (300.46) :
C38.18, H 6.56; found: C 38.30, H 6.46.
5
2.4.
Detection of putative η -(cyclopentadienyl)
nickel(3-methyl-imidazol-2-ylidene-1-
phenylacyl)(triethylamine) bromide (8)
3-Methyl-1-phenacylimidazolium bromide (220 mg,
0.783 mmol) and triethylamine (0.10 mL, 0.799 mmol)
were stirred in THF (10 mL) at –78ºC for 30 min. A
solution of nickelocene (148 mg, 0.783 mmol) was
added, and the mixture was warmed up and refluxed
overnight. During the first hour of reflux, the colour of
the solution changed from dark green to dark red. After
refluxing for 20 h, the mixture was filtered and the solvent
removed in vacuum. The viscous semi-solid residue
was washed with hexane (10 mL) and dried in vacuo
to give 109 mg (28%) of a dark reddish brown powder.
1H NMR (CD3OD): δ = 1.15 (t, 3J = 6.7 Hz, CCH3), 2.80
2.2. 3-(2-Hydroxyethyl)-1-methylimidazolium
chloride (2)
N-Methylimidazole (1.10 g, 13.4 mmol) along with
2-chloroethoxy trimethylsilane (2.38 g, 15.6 mmol) was
heated as above in an autoclave, washed with hexane
and then extracted with methanol. The solvent was
evaporated and a dark sticky syrup collected (1.86 g,
81%), out of which large colourless single crystals
deposited over a period of several months. NMR data
3
(q, J = 6.7 Hz, NCH2), 3.77 (s, 3 H, NCH3), 5.17 (s,
5 H, C5H5), 7.00, 7.11 (s, 2 H, 4-CH, 5-CH), 7.50-7.80 (m,
3 H, m-CH, p-CH), 7.99 (d, 2 H, o-CH) ppm. 13C NMR
(CD3OD): δ = 10.26 (3 CCH3), 41.01 (NCH3), 47.26 (3
NCH2), 92.25 (η5-C5H5), 106.51 (C5H5−), 125.79, 126.47
(4-CH, 5-CH), 128.38, 129.78 (2 m-CH, 2 o-CH), 133.73
(i-C), 135.11 (p-CH), 164.22 (NCN), 191.78 (C=O) ppm;
[NC(H/D)2 in noise]. MS (EI, 70 eV, 360ºC): m/e (%)
= 256 (0.4) [Ni(3cat.)-2H], 242 (0.4), 86 (5), 44 (100).
C23H32BrN3NiO (505.12).
1
and analysis refer to crystals. H NMR (D2O, ref. dioxane
4.72 ppm): δ = 3.87, 3.89 (s, t, J = 5.2 Hz, 5 H, NMe,
3
NCH2), 4.29 (t, 3J = 5.2 Hz, 2 H, OCH2), 7.35, 7.55 (AB,
3J = 16-18 Hz, 2 H, 4-H, 5-H) ppm; 2-CH exchanged
against 2-CD. 13C NMR (D2O, ref. dioxane 66.3 ppm):
δ = 35.44 (NMe), 51.24 (NCH2), 59.50 (OCH2), 122.13,
123.29 (CH-4, CH-5), 135.88 (t, 1JCD = 34 Hz, CD-2) ppm.
Calculated for C6H11ClN2O•0.55 H2O (172.53): C 41.77, 2.5.
H 7.07, N 16.24; found: C 41.47, H 6.82, N 16.00.
3
η - A l l y l p a l l a d i u m - 3 - ( 2 , 6 -
diisopropylphenyl)-imidazol-2-ylidene-1-
(2-phenyl-ethen-2-olate) (9)
Detection of nickel-bis(3-methyl-imidazol-
2-ylidene-1-(2-phenyl-ethen-2-olate)) (5)
2.3.
Allyl palladium chloride dimer (141 mg, 0.385 mmol),
KOtBu (179 mg, 1.60 mmol), and compound 4 shown
in Scheme 3 (329 mg, 0.77 mmol) were transferred to
a Schlenk flask under argon. Diethyl ether (10 mL) was
added followed by THF (20 mL) and the reaction mixture
was stirred at –78ºC for 2 h and at 25ºC for 16 h. Ether
NiBr2(THF)2 (212 mg, 0.58 mmol), KOtBu (294 mg,
2.63 mmol), and compound 3 depicted in Scheme 2
(328 mg, 1.17 mmol) were combined at −78°C in a flask
under argon in THF. The reaction mixture was allowed to
993
2

Nickel and palladium complexes of enolate-
functionalised N-heterocyclic carbenes
(20 mL) was added, the mixture was filtered, and the generating an imidazolium salt with a pendant silylether
solvent was removed in vacuum to yield 344 mg (90%) sidearm (Scheme 1). The reaction of methylimidazole
1
of a pale yellow powder. H NMR (CDCl3): δ = 1.09 (d, with 2-chloroethoxy-trimethylsilane was carried out
3J = 7.0 Hz, 3 H, CH3), 1.10 (d, 3J = 7.0 Hz, 3 H, CH3), at 150°C for 24 h in an autoclave without added
1.12 (d, 3J = 7.0 Hz, 3 H, CH3), 1.21 (d, 3J =7.0 Hz, 3 H, solvent, and produced (2-trimethylsilyloxy)ethyl-3-
CH3), 1.65 (d, J = 0.9 Hz, 1 H, allyl), 1.68 (m, 1 H, allyl), methylimidazolium chloride, compound 1, as a viscous
2.47 (sept, 3J = 6.7 Hz, 1 H, CH), 2.71 (sept, 3J = 6.7 Hz, oil, which, after washing with hexane, was dissolved in
1 H, CH), 3.34 (dd, J = 13.2, 0.6 Hz, 1 H, anti allyl, cis chloroform. Slow evaporation did not yield a solid but an
to O), 4.10 (m, J ≈ 7.9 Hz, 1 H, syn allyl, cis to O), 5.11 ionic liquid, containing residual CHCl3. The compound
1
(m, 1 H, allyl, central), 6.61 (s, 1 H, NCH=CO), 6.93 (d, was identified by its H– and 13C-NMR spectroscopic
3J = 1.8 Hz, 1 H, dipN-CH=C), 7.21 (d, 3J = 1.8 Hz, 1 H, characteristics, such as the very broad signals for H-2
dipN-C=CH), 7.22-7.51 (m, 6 H, aryl), 7.23-7.83 (m, 2 at δ = 10.09 ppm and C-2 at δ = 137.46 ppm. The
H, aryl) ppm. 13C NMR (CDCl3): δ = 23.09, 23.56, 24.68, silylether group is easily cleaved by polar protic solvents
25.36 (4 CH3), 28.06, 28.21 (2 CH), 43.27 (allyl cis to and 1-methylimidazolium-3-(2-ethanol) hydrochloride
carbene),71.56 (allyl cis to O), 98.19 (N-C=C-O), 115.58 (compound 2) from compound 1 by methanolysis. The
(central allyl), 119.15, 121.67 (C=C), 123.70, 123.77 (2 identity was confirmed by validating the 1H- and 13C-NMR
m-CHdip), 125.80 (2 o-CHPh), 127.31 (p-CHPh), 127.85 (2 results against the values reported in the literature [10a],
m-CHPh), 129.93 (p-CHdip), 137.52 (i-CPh), 141.17 (i-Cdip), and also by an X-ray crystal structure analysis (Fig. 1).
146.31, 146.35 (2 o-Cdip), 154.19 (C=C-O), 172.19 The H-2 proton could not be detected directly, which
(NCN) ppm. Calculated for C26H31N2O Pd (493.96): C is likely due to the rapid exchange with the deuterium
63.22, H 6.33, N 5.67; found: C 63.03, H 6.80, N 5.28.
atoms of the solvent. Its presence is observed as a
1JCD coupling constant of 34 Hz towards C2 in the 13C-
2.6. Crystal Structure Analysis
(ii)
(i)
N
N
N
N
N
N
H2C
H2C
Me
X-ray diffraction data for compound 2 (Scheme 1) were
recorded on an Oxford Diffraction Nova A diffractometer
using mirror-focussed CuKα-radiation. The data were
corrected for absorption effects. The structure was
refined by full-matrix least-squares methods on F2
(SHELXL-97) [12]. All non-hydrogen atoms were refined
anisotropically. The OH and the carbene H atom were
refined freely; other H-atoms were calculated assuming
ideal geometries and refined using a riding model or
rigid methyl groups. Crystallographic data are given
in Table 1 and selected bond lengths and angles in
Fig. 1. Crystallographic data have been deposited with
the Cambridge Crystallographic Data Centre, CCDC
No. 760805. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk).
Me
Me
H2C
H2C
C
C
C
H
H
H
Cl
Cl
OSiMe3
OH
2
1
Synthesisoffunctionalizedimidazoliumsalts.Reagents
and conditions: (i) 2-Chloroethoxytrimethylsilane,
150ºC, 24 h; (ii) dry MeOH.
Scheme 1.
Table 1. Crystal data and structure refinement.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C6H11ClN2O
162.62
100(2) K
1.54184 Å
Monoclinic
P21
Unit cell dimensions
a = 7.3105(6) Å
b = 6.9927(5) Å
= 90°
=111.047(9)°
 = 90°
c = 8.4642(6) Å
403.82(5) Å3
2
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
1.337 Mg m-3
3.683 mm-1
172
Crystal size
0.20×0.20×0.05 mm3
5.60 to 75.78°
Theta range for data collection
Index ranges
-9<=h<=9,-8<=k<=8,-10<=l<=10
5005
1567 [R(int) = 0.0310]
99.9%
Semi-empirical from equivalents
1.00000 and 0.30607
Full-matrix least-squares on F2
1567 / 1 / 100
3. Results and Discussion
Reflections collected
Independent reflections
Completeness to theta = 72.50°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
There are currently three established methods to
synthesise an OH-functionalised NHC: 1) reaction of
the imidazole with a halogeno alcohol [10], 2) reaction
of an N-substituted imidazole with an epoxide [13],
and 3) incorporation of the OH-functional group during
the synthesis of the imidazole ring system [14]. We
decided to attempt a variation of method 1) by reacting
methylimidazole with 2-chloroethoxy-trimethylsilane,
1.074
R1 = 0.0248, wR2 = 0.0639
R1 = 0.0250, wR2 = 0.0642
0.013(13)
Absolute structure parameter
Largest diff. peak and hole
0.148 and -0.208 e.Å-3
994
3

S. Shanmuganathan et al.
3, with two equivalents of KOtBu as the base and
subsequently with Ni(THF)2Br2 or Ni(PPh3)2Br2 did not
yield the expected mono-ligated complexes 5 with L =
THF or PPh3. Instead, this reaction led to the formation
of the doubly substituted nickel(II) complex nickel-bis(3-
methyl-imidazol-2-ylidene-1-(2-phenyl-ethen-2-olate),
whichwedenoteascompound6.Thetriphenylphosphine
generated in the reaction was removed by washing
with diethyl ether. This tendency to form symmetric
bis(NHC,O) chelate complexes was also observed with
the similar 1-(o-phenolato-methyl)-imidazol-2-ylidene
[15] and o-phosphinophenolate [16] ligands. In addition,
the reaction between NiPhCl(PPh3)2 and a bulky 1-(o-
phenolato)-imidazol-2-ylidene also did not lead to the
formation of a R(Ph3P)Ni(NHC∩O) monochelate (R = Cl
or Ph), even though NiPhCl(PPh3)2 is able to remove the
acidic proton by the phenyl anion. Instead, this reaction
seems to proceed via a ring opening followed by the
insertion of nickel into a 6-membered ring. Only the
stable NiMesBr(PPh3)2 allowed access to a Mes(Ph3P)
Ni(NHC∩O) monochelate complex [17]. We then tried
the synthesis of a methallylnickel monochelate complex,
which succeeded in the case of o-phosphinophenolates
[18]. Generation of the carbene enolate with two
equivalentsofKOtBuasbeforeandsubsequenttreatment
with [Ni(methallyl)Br]2 as an alternative starting material,
however, led once again to the bis-ligated nickel carbene
complex 6 as the main product (Scheme 2), possibly
(a)
(b)
Molecular structure (a) and packing (b) of 2 in the crystal.
Selected bond lengths and angles: N1-C2 1.3287(19), N3-
C21.332(2),N1-C51.375(2),N3-C41.384(2),C4-C51.353(2);
N1-C2-N3108.27(14).Thepackingdiagramshowsthelayer
structure built up from the four “weak” hydrogen bonds; the
classical H bond connects to adjacent layers (one cation of
which is shown in bold, top right).
Figure 1.
NMR spectrum. In the crystal, monoclinic with space with the mono-ligated nickel carbene complex 7 among
group P21, the side arm is turned nearly perpendicular the impurities. Attempts to obtain suitable crystals for
(C2-N1-C7-C8 87.8(3), C5-N1-C7-C8 −87.2(3)°) to an X-ray crystal structure determination failed, leaving
the ring plane (mean deviation from the best plane us with the difficult task of assigning the structure from
0.003 Å) with the hydroxyl group directed towards the spectroscopic evidence alone. Nickel(II) complexes are
chloride anion below the ring. Besides the O-H···Cl known for their variable structures, dark green to violet
hydrogen bond (O-H 0.85(3), H···Cl 2.20(3), O···Cl paramagnetic octahedral or tetrahedral complexes,
3.0449(14) Å, O-H···Cl 175(2)°) the packing involves and usually yellow to brown diamagnetic square planar
four further “weak” hydrogen bonds, three Cl···H-C complexes. The square planar bis(chelate) complexes
contacts to H2, H5 and a methyl H-atom of three different can be either cis or trans with respect to the carbenes.
imidazolium cations, and one O···H-C contact from H4. Of these the cis-complexes are usually more stable by
The three C-H···Cl contacts connect the residues to the push-pull effect, even in the case of some steric
form layers parallel to the xz plane; the classical H bond hindrance as shown e.g. for the nickel(II) cis-bis(chelate)
connects the layers in the y direction. The observed complex with the bulky 6-tert-butyl-substituted 1-(2-
bond lengths and angles represent typical values.
phenolato-methyl)-imidazol-2-ylidene ligand [15]. Only
Both compounds, 1 and 2, are very hygroscopic, and in the presence of very strong steric hindrance, as in the
various amounts of water were detected in the elemental case of branched dialkylphosphinophenolate ligands,
analysis of 2. Since attempts to obtain a nickel and are trans-bis(P,O-chelate) Ni complexes preferred [19].
palladium C,O complex for comparison with our recently Therefore we assume that the pale yellow diamagnetic
reported Ni(II)- and Pd(II)-P,O−-chelate complexes [2d] nickel(II)-complex 6 is the cis-complex. The enolate,
1
failed, the known keto-functionalised imidazolium salt NHC chelate coordination follows from typical H and
3-methyl-1-phenacylimidazoliumbromide3[11a]andthe particularly 13C NMR data (NCN, C=C-O, δ = 154.6,
bulky 3-(2,6-diisopropylphenyl)-1-phenacylimidazolium 154.9 ppm), and the nature of the nickel bis(chelate)
bromide 4 [11c] were used instead.
is deduced from the molar masses and typical isotopic
The reaction of the less bulky precursor, compound pattern observed in the mass spectra.
4
995

Nickel and palladium complexes of enolate-
functionalised N-heterocyclic carbenes
NMe
NMe
(i)
N
N
Me
C
N
N
C
O
C
H
Ni
NiBr(L)
O
Br
Ph
O
2
3
Ph
Ph
(L = THF or PPh3)
5
6
(ii)
(iii)
Me
Ph
N
NMe
N
C
O
C
N
6 (major)
Ni
Ni
O
Et3N
Br
Ph
7
8
Scheme 2.
SynthesisofthenickelenolateNHCcomplexes.Reagentsandconditions:3[11a];(i)Ni(THF)2Br2orNiBr2(PPh3)2,2KOtBu,THF,–78to20ºC,
16 h; (ii) [Ni(methallyl)Br]2, 2 KOtBu, THF, –78 to 20ºC, 16 h; (iii) nickelocene, Et3N, THF, –78 to 70ºC (20 h).
N
N
(ii)
N
(i)
N
O
N
N
H2C
C
C
C
H
H
Pd
Ph
Ph
O
Br
4
9
Scheme 3.SynthesisoftheallylPdenolateNHCcomplex9.Reagentsandconditions:(i)2-bromoacetophenone,THF,25ºC,24h(cf.[11c]);(ii)2KOtBu,
[Pd(allyl)Cl]2, Et2O/THF, –78 to 25ºC, 16 h.
From our experience with P,O hybrid ligands we with the above presented formula 8. A small amount
know that reaction with nickelocene in toluene may of bromide is replaced by the cyclopentadienyl anion,
produce monoligated nickel complexes [16]. Thus, the indicated by a rather small CH-signal in the 13C NMR
imidazolium salt 3 was reacted with nickelocene in the spectrum (δ = 106.5 ppm) and known from the reaction
presence of NEt3. Upon heating to reflux, the green of free NHC with nickelocene to [(NHC)Ni(η5Cp)(η1Cp)]
color of the solution turned to dark red. A dark reddish [20], whereas hints at η1-CpNi (expected at δ= 113 ppm)
brown solid was isolated. The NMR spectroscopic data are not observed. The above assignments are justified
show that not enolate, but a η5Cp-Ni-NHC complex was by spectroscopic data established for the cationic
formed. The 13C=O signal (δ = 191.8 ppm) appears in [(NHC)2Ni(η5-Cp)]+ Cp− and neutral (NHC)Ni(η5-Cp)(η1-
the same region as for compound 3, but new signals Cp) complexes. Furthermore, X-ray crystallography was
(δ13C = 164.2, 92.3 ppm), characteristic for NCN and also used as a means of characterization [20]. The yield
η5-Cp of NHC-NiCp complexes [20,21], are observed. of compound 8 was found to be low (28%), but was not
The lack of the NCH2 signal can be attributed to H→D optimized.
exchange in the solvent CD3OD. The roughly equimolar
Allylpalladium phosphinophenolate complexes [22]
ratio of NHC, Cp and downfield shifted NEt3 group provedmuchmorestablethanmethallylnickelcomplexes
signals, estimated from proton NMR integration, fits best [18]. This suggests that the chances of isolating stable
5
996

S. Shanmuganathan et al.
allylpalladiumNHC∩O−monochelatecomplexesaremuch the absence of base [2,23] allow easy in situ access to
greater than for nickelmethallyl NHC∩O− complexes. By active P∩O− -nickel catalysts with Ni(COD)2 in a variety
reacting the more bulkily substituted phenacyl precursor of solvents.
4 with two equivalents of KOtBu as base, followed by
allyl palladium(II) chloride dimer [C3H5PdCl]2, the stable
and clean monoligated NHC∩O− -chelate complex 9
was obtained in 90% yield (Scheme 3). Compound 9
4. Conclusions
We have introduced a simple method to synthesise
hydroxyalkyl functionalised imidazolium salts. Attempts
to use them and the known keto-functionalised
imidazolium salts in the synthesis of mono-ligated nickel
complexes were successful in the case of nickelocene
as the starting material. Nickel(II) halides yielded the
bis-ligated nickel(II) complexes as major products. The
mono-ligated complex could be obtained using the
somewhat less reactive palladium-allyl chloride dimer
as the starting material.
The keto-functionalised imidazolium salts are not
suitable for in situ olefin polymerisation protocols, but
catalytic reactions using defined precatalysts, as shown
by other groups, are likely to be successful and deserve
further experimental investigations.
is structurally similar to the slightly less bulky 3-mesityl
1
analogue [11b] and was identified by H- and 13C-NMR
spectroscopy and elemental analysis. Characteristic
features include a signal for the C-2 carbon at
δ
for
=
172.18 ppm in the expected region
a palladium coordinated NHC, and
the signature of the enolate CH group at
δ(1H) = 6.60 ppm and δ(13C) = 98.19 ppm.
Phenylnickel imidazol-2-ylidene-enolate complexes
stabilized by pyridine were shown to catalyze the
polymerization of ethylene [11c] without additives with
productivites comparable to SHOP-type P∩O− nickel
catalysts [2] whereas allylnickel complexes stabilized by
themorebulkyligand4becomeactiveonlyafteractivation
by diethylzinc [11d]. Screening tests to generate active
NHC∩O− -nickel catalysts in situ from 3 or 4, Ni(COD)2
or NiBr2•2THF and two equivalents of various bases
(NaH 20ºC, or KOtBu or nBuLi at –70ºC) in toluene or
THF (after 1 h pressurizing with ethylene and heating at
100ºC within 20 min) failed, whereas phosphinoketones
in the presence of base and phosphinoacetic acids,
phosphinoglycines, or phosphinoglycolates even in
Acknowledgements
We thank Dr. M. K. Kindermann and B. Witt for NMR
spectra and K. R. Basvani for several catalytic screening
experiments.
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