The insertion reaction of acetonitrile on aryl nickel complexes stabilized by bidentate N,N′-chelating ligands

Article abstract of DOI:10.1039/b608352h

Organometallic complexes to be used as single component precursors in the catalytic dimerization/polymerization of olefins usually must contain a labile ligand that can easily be displaced by the olefin. This is the first step in the activation of the precursor. One commonly used labile ligand is a nitrile. Here we report an example of incompatibility between the nickel or palladium aryl bond and acetonitrile. Neutral [MBr(Mes)NN] complexes in which Mes = 2,4,6-Me₃C₆H₂, NN = diazabutadiene (DAD), pyridinylimine (PIM), 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) gave the expected [M(Mes)(3,5-lut)(NN)][BF₄] compounds and the unexpected [Ni(Mes){NHC(Me)(2,4,6-Me₃C₆H ₂)}(NN)][BF₄] complexes in the presence of TlBF ₄ and 3,5-lutidine or acetonitrile. The sequence of reactions that leads to the imine ligand must include an initial insertion of the nitrile on the σ(Ni-Mes) bond. These ionic complexes remain stable under 20 bar of ethylene. The Royal Society of Chemistry.

Full text of DOI:10.1039/b608352h

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The insertion reaction of acetonitrile on aryl nickel complexes stabilized by
bidentate N,Nꢀ-chelating ligands†
Rosa M. Ceder,* Guillermo Muller, Mat´ıas Ordinas and Juan Ignacio Ordinas
Received 12th June 2006, Accepted 23rd October 2006
First published as an Advance Article on the web 7th November 2006
DOI: 10.1039/b608352h
Organometallic complexes to be used as single component precursors in the catalytic dimerization/
polymerization of olefins usually must contain a labile ligand that can easily be displaced by the olefin.
This is the first step in the activation of the precursor. One commonly used labile ligand is a nitrile. Here
we report an example of incompatibility between the nickel or palladium aryl bond and acetonitrile.
Neutral [MBr(Mes)NN] complexes in which Mes = 2,4,6-Me3C6H2, NN = diazabutadiene (DAD),
pyridinylimine (PIM), 2,2ꢀ-bipyridine (bipy) or 1,10-phenanthroline (phen) gave the expected
=
[M(Mes)(3,5-lut)(NN)][BF4] compounds and the unexpected [Ni(Mes){NH C(Me)(2,4,6-Me3C6H2)}-
(NN)][BF4] complexes in the presence of TlBF4 and 3,5-lutidine or acetonitrile. The sequence of
reactions that leads to the imine ligand must include an initial insertion of the nitrile on the r(Ni–Mes)
bond. These ionic complexes remain stable under 20 bar of ethylene.
be applied in this reaction is very large. Some examples are neutral
Introduction
bidentate nitrogen ligands containing a diimine skeleton5 or mixed
P–N phosphine-imine ligands.6
The study of late transition metal olefin oligomeriza-
tion/polymerization catalysts is an area of interest. The early,
important development of the SHOP process for the oligomer-
ization of ethylene used neutral Ni catalysts containing P–O
ligands1 (Chart 1, a). This was followed by the discovery of the
Brookhart’s system using Ni and Pd a-diimine cationic catalysts2
(Chart 1, b). Recently, a new family of neutral nickel complexes
containing salicylaldimine N–O ligands showed good functional
group tolerance, increasing the scope of the catalytic system3
(Chart 1, c). The commonly used metals are Ni, Pd and Fe4 (Chart
1, d). The number and type of ligands that have been developed to
Some catalyst precursors must be activated with a cocatalyst
such as MAO, but others are single-component precursors. These
precursors are generally ionic or neutral compounds. They contain
at least one weakly bonded ligand that could easily be displaced
by the olefin, and an M–C bond that after initial insertion of the
olefin and a b-elimination reaction will produce hydride catalytic
species. One of the commonly used labile ligands is acetonitrile.7
In most cases and for catalytic purposes, the precursor is generated
in situ after abstraction of the halide of the neutral complex by a
silver or thallium salt. Cationic methylpalladium complexes with
bidentate N-donor ligands, such as bipyridine or related diimine
ligands, are stable in solution.8 The X-ray crystal structure of
[Pd(CH3)(NCCH3)(bppy)][PF6] has also recently been reported.9
Analogous arylpalladium complexes tend to undergo intermolec-
ular aryl ligand transfer. However when a suitable aryl group and
the solvent ligand are chosen several [Pd(Ar)(NN)(solv)]+ can be
isolated and characterised.10,11 Furthermore, the intermolecular
aryl ligand transfer reaction in nickel complexes is attributed to
cationic intermediate formation.12 Moreover, the compatibility of
the nitrile fragment with a r-M–C bond in the cis position is not
always possible. Recently, an unusual insertion of nitriles into a
Pd–C bond has been reported.13 It was assisted by the protonation
of the nitrile nitrogen atom by the ortho-hydroxy group present in
the aryl ligand. The insertion of an acetonitrile ligand into a Pd–
aryl bond has been postulated as a key step in the Pd-catalyzed
cyanation of aryl halides14 and in the synthesis of aryl ketones by
the reaction of arenes with nitriles.15 In this paper, we report the
synthesis and characterization of neutral and ionic organometallic
palladium(II) and nickel(II) complexes of the general formula
[MBr(Mes)(NN)] and [M(Mes)(3,5-lut)(NN][BF4] (NN = DAD
a, PIM b, bipy c, phen d). To test the ionic complexes as
precursors in the olefin oligomerization reaction we attempted
to prepare an analogous compound using CH3CN instead of 3,5-
Me2C5H3N as the stabilizing ligand. However, an unprecedented
Chart 1
Departament de Qu´ımica Inorga`nica, Universitat de Barcelona, Mart´ı i
Franque`s, 1-11, 08028, Barcelona, Spain
† Electronic supplementary information (ESI) available: Fig. 1S: MS
(FAB+) of compounds 7c and 7cꢀ. Fig. 2S: 1H NMR spectra of complexes
5a (500 MHz), 7a and 7c (250 MHz). Fig. 3S: MS (CI, NH3) of imine
=
NH C(Mes)Me. Scheme 1S: Alternative mechanisms proposed for the
evolution of the mesityl–nitrile cationic species. Tables 1S and 2S: 1H
NMR spectra of all compounds. See DOI: 10.1039/b608352h
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reaction between [NiBr(Mes)(NN)], AgBF4 and CH3CN
2a, which had a downfield shift of nearly 1 ppm with respect to
=
led to the isolation of [Ni(2,4,6-Me3C6H2){NH C(Me)(2,4,6-
the free ligand.
Me3C6H2)}(NN)][BF4], after a sequence of reactions involving the
initial formation of azomethine intermediates.
Neutral mesityl nickel and palladium complexes
Bromo(mesityl) nickel complexes, [NiBr(Mes)(NN)], M = Ni;
NN = DAD 3a, PIM, 3b, bipy, 3c, phen, 3d, were obtained
either by a substitution reaction of the phosphine ligand in trans-
[NiBr(Mes)(PPh3)2] by the NN ligand, according to previously
described methods,25–27 or by the reaction between [NiBr2(DAD)]
and mesitylmagnesium bromide in THF solution (Scheme 2).
The substitution reaction did not give the desired results in the
case of ligand DAD. The complex 3a must be obtained by the
second method because ligand a was unable to substitute the PPh3
in [NiBr(Mes)(PPh3)2]. For rather basic ligands like bipyridine,
phenanthroline and PIM the reaction proceeds very smoothly
with high yields. However, the less basic the ligand is, the more the
equilibrium tends towards the starting material. Bromo(mesityl)
palladium complexes [PdBr(Mes)(NN)], NN = DAD, 4a, PIM 4b,
were obtained by the reaction between [PdBr2(NN)] and mesityl
magnesium bromide in THF solution (Scheme 2). The nickel
complexes were purple (3a) and deep-red (3b, 3c, 3d) air stable
solids. The palladium complexes (4a) and (4b) were yellow air
stable solids. Nickel complexes 3c and 3d, previously described,
were obtained for comparison.25,28 All the new complexes were
characterized by elemental analysis, IR spectra and 1H NMR. All
Results and discussion
Synthesis of the dihalo-complexes
The reported diazabutadiene (DAD)16 (a) and pyridinylimine
(PIM)17 (b) were obtained by methods previously described
for similar ligands. They were characterized by 1H NMR
spectroscopy. 1,4-Diaza-1,3-butadiene (DAD) (a) was prepared
by condensation between 2,3-butanedione and benzylamine in
CH2Cl2 in the presence of molecular sieves as drying agents
and formic acid as a catalyst.18 Pyridinylimine (PIM) (b) was
synthesized by the condensation of pyridine-2-carbaldehyde with
the stoichiometric amount of benzylamine in refluxing ethanol19
(Scheme 1).
The nickel dibromide complexes (1a, 1b) were obtained by
the reaction of the organic ligand, DAD (a) and PIM (b),
dissolved in toluene and the equivalent amount of anhydrous
nickel bromide. The complexes precipitated almost quantitatively
from the reaction mixture as very insoluble pale-yellow solids. The
paramagnetic nature of the complexes precluded their characteri-
zation by 1H NMR spectroscopy (vT = 1.15 cm3 mol−1 K, 300 K
for 1b). It has been reported that dibromonickel(II) complexes
containing bulky pyridinylimines or diazabutadienes derived from
ortho mono- or di-substituted phenylamines showed mononuclear
or dinuclear structures, with the nickel atom pentacoordinated
in a distorted square-planar pyramidal environment.20–22 Similar
complexes with a-dimine ligands23 have been described as yellow
dinuclear solids that transform to violet paramagnetic solids at
high temperature. The colour and high insolubility of complexes
1a and 1b suggest that these compounds have a dimeric structure in
the solid state. This prevents their purification by recrystallization.
Palladium complexes 2a and 2b were obtained by the reaction
of stoichiometric amounts of Li2[PdBr4] with diazabutadiene (a)
and pyridinylimine (b) ligands in MeOH. The resulting complexes
were yellow–orange diamagnetic solids that were insoluble in
non-coordinating solvents. They were characterized by elemental
analyses and by 1H NMR spectroscopy. 1H NMR data (see
Tables 1S and 2S, ESI†) are compatible with a monomeric square-
planar structure containing a bidentate coordinated ligand. This
is in accordance with other results in the literature.20,21,24 The
respective proton signals of the ligand were shifted to a lower
field upon coordination. Of note is the large chemical shift of the
methylenic protons. This was particularly notable in compound
1
assignments are based on their coupling patterns, H–1H COSY
and NOESY experiments. 1H NMR data for the compounds and
the numbering scheme are shown in Tables 1S and 2S (see ESI†).
In the 1H NMR spectra of 3a and 4a, two singlets were observed
for the methylenic protons, at higher and lower fields than in the
free ligand. The signal of the protons of the methylene group
cis to the mesityl ligand was shifted to higher fields, due to the
ring currents. The signal of the methylene protons cis to the
bromide was shifted to lower fields, due to the strong deshielding
effect of the bromide.8 The 1H–1H NOESY experiment conducted
on complex 4a was entirely consistent with this proposal and
permitted the assignment of the remainder of the signals. Selected
NOESY correlations are presented in Fig. 1. Exchange contacts
between both CH2 fragments (4.51 and 5.34 ppm) showed the
shift of the mesityl group between the two coordination positions
opposite the NN ligand. This was probably due to the lability
of the bromide ligand in acetone. Since the PIM ligand is non-
symmetric, coordination in [MBr(Mes)PIM] compounds can lead
to the formation of two isomers, cis and trans, depending on the
relative position of the mesitylene ligand and the iminic nitrogen
of the pyridinylimine ligand on the metal atom. This is the case
for the Ni(II) and Pd(II) complexes 3b and 4b, as can be concluded
from the two sets of signals in the NMR spectra. No change in
Scheme 1
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Scheme 2
the presence of 3,5-dimethylpyridine (3,5-lutidine) yields stable
cationic complexes, [M(Mes)(3,5-lutidine)(NN)]BF4, M = Ni,
NN = DAD, PIM, 5a, 5b; M = Pd, NN = DAD, PIM, 6a, 6b
(Scheme 3). The complexes were obtained as orange (5a, 5b) and
yellow (6a, 6b) solids that were characterized by elemental analysis
and 1H NMR spectra. The 1H NMR data are shown in Tables 1S
and 2S (see ESI†).
Fig. 1 Details of NOE contacts observed in compounds 4a and 5a.
the isomer composition was observed after standing in acetone-
d6 at room temperature for 48 h. The relative trans : cis ratio
of each isomer was 37 : 63 for 3b and 45 : 55 for 4b. The cis
configuration was also found to be the most sterically favourable
for the majority of methyl(chloro)palladium complexes of PIM.8
The most sensitive resonances for the assignment of both isomers
were those of the hydrogen atom in the carbon ortho of the pyridine
ring. In the cis isomers, this hydrogen atom had a large chemical
shift, owing to the strong deshielding effect of the bromide ligand,
which is situated cis to the pyridyl group. In the trans isomers,
it had a considerably lower chemical shift, due to the proximity
of the mesityl ring. Isomer interconversion was observed by the
exchange signal between 4.13 and 5.19, the two methylene groups
Scheme 3
1
The H NMR spectra of compounds with DAD showed that
the signals of the methylenic and the ortho protons of the benzyl
groups shifted upfield with respect to free ligand, according to
the influence of the ring currents of both the mesityl and the 3,5-
lutidine groups. Nevertheless, the assignment of the signals to the
corresponding protons can be made with the NOESY spectrum
of 5a (Fig 1). The ortho-methyl protons of the mesitylene and
the meta-methyl protons of the lutidine each appear as singlets,
showing the planar symmetry of these molecules. The assignment
of the spectra of the palladium complex 6a was performed by
comparing it with that of the nickel complex. The spectra of
the ionic compounds containing PIM show, as for the neutral
1
of both isomers in the NOESY spectrum of 3b. The H NMR
signals of the palladium complex 4b were assigned by comparison
1
to that of the nickel complex. In addition, the H NMR data of
both 3c and 3d are included (Table 1S, ESI†).
Cationic mesityl nickel and palladium complexes containing
3,5-lutidine
The reaction of bromo(mesityl)palladium and nickel complexes,
[MBr(Mes)(NN)], NN = DAD, PIM, with TlBF4 in acetone in
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Table 1 FAB+ data of the compounds [Ni(Mes)(NH C(Mes)R)-
(bipy)]BF4 (R = CH3, Ph) in NBA matrix
=
compounds, the presence of trans and cis isomers according to the
configuration of the mesityl and the iminic nitrogen. Compound
5b was obtained as a mixture of trans and cis isomers in a 80 :
20 ratio. Selected correlations observed for the major trans isomer
were: 4.67 (s, CH2) with 8.32 (s, H ortho, lut) and 2.16 (s, CH3, meta,
lut). The isomeric composition of the acetone-d6 solution was
unchanged with time. For the palladium complex 6b, the two sets
of resonances were in an approximate ratio of 10 : 90 for the trans
and cis isomers, respectively, as deduced from NOESY correlation
measurements. Selectedcorrelations observed for the major isomer
were: 4.60 (s, CH2) with 2.51 (s, Me ortho, Mes), and also 7.98 (d,
H ortho, Ar) with 8.49 (s, H ortho, lut). A significant change in
the isomer composition was observed, after leaving the complex at
room temperature in acetone-d6 for a week. The final ratio between
the trans and cis isomers was 75 : 25. The assignment of the signals
to the protons of the trans isomer was confirmed by the NOESY
spectrum. Contacts between 5.01 (s, CH2), 8.10 (s, H ortho lut)
and 7.02 (d, H ortho, Ar) were observed. No chemical exchange
process between the two isomers was revealed in the NOESY
spectrum. Therefore, the interconversion between them was slower
than the timescale of the NOE experiment. Interestingly, the trans
configuration is apparently the most favourable configuration for
ionic complexes. The opposite has been observed for neutral
complexes.
7c·MeCN
7cꢀ (PhCN)
M+ + 1
M+
495.8 (495.2)
494.0 (494.2)
374.9 (375.1)
557.4 (557.2)
555.9 (556.2)
436.8 (437.1)
M+ − Mes
Scheme 5
presence of one (7a), two (7c, 7d) or four (7b) isomers in solution.
Similar behaviour was observed using benzonitrile instead of
acetonitrile in the reaction of the nickel bipyridine complex (7cꢀ).
Cationic complexes derived from the reaction with acetonitrile
1
=
For 7a, {Ni(Mes)[NH C(Mes)(Me)](DAD)}BF4, the H NMR
Our attempts to prepare stable nickel ionic complexes by treating
neutral complexes with TlBF4 in the presence of acetonitrile failed.
TlBr was formed after addition of TlBF4 to an acetone solution of
the neutral complexes 3a, 3b, 3c, 3d in the presence of an excess of
CH3CN. After filtration and removal of the solvent, orange (7b)
and reddish (7a, 7c, 7d) solids were isolated with yields that never
reached 40% with respect to the total amount of Ni. The nature of
the products obtained was defined after accurate characterization
of the products and preparation of the bipy and phen species (7c,
7d), as the 1H NMR spectra were less complex in the alkyl region.
spectrum showed the presence of only one compound in which the
=
imine [(Mes)MeC NH] ligand was present in the E conformation.
This is based on the high chemical shift (2.89 ppm) observed for
the methyl group that is close to the metal in the E conformation.
The same effect was observed in the ortho methyl groups of the
mesityl ligand that are directly bonded to the metal atom. For
1
7c, 7cꢀ and 7d, H NMR spectroscopy showed the presence of
two compounds differing in the E or Z conformation of the two
groups in the imine function. In solution an equilibrium shift to
the E isomer was shown by NMR spectroscopy in acetone-d6 at
308 K. Initially the major isomer was in the Z conformation,
the expected geometry after a cis-insertion. However, this evolved
to the more sterically favoured E isomer. After 60 min the ratio
remained constant (Table 2).
=
The ionic products contained a neutral imine [(Mes)MeC NH]
ligand, the a-diimine ligand and a mesityl group (see Scheme 4).
The structure of the final compounds obtained was proposed
after consideration of the elemental analysis, IR spectra (new
bands at 3200–3250 and 1635 cm−1, in the range of m(N–H)
The analysis of the spectra of 7b was more complex, since
cis and trans complexes were obtained due to the asymmetry of
the PIM ligand. In addition, the E and Z conformation of the
1
=
and m(C N) vibrations), H NMR data and FAB data (Table 1).
Further identification was by the separation of the neutral imine
ligand by reaction with acid and the subsequent recovery of the free
ligand by basic treatment (Scheme 5). 1H NMR data showed the
1
imine ligand was observed in each case. The H NMR spectrum
showed the presence of the four isomers. Due to their complexity,
Scheme 4
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Table 2 The ch◦ange in the composition of Z-, E-isomers observed in
acetone-d6 at 25 C over 60 min for complexes 7c, 7cꢀ and 7d
Initiala
Final
Compound
Z
E
Z
E
(2)
(3)
(4)
7c
7cꢀ
7d
50
71
63
50
29
37
15
37
18
85
63
82
a Initial composition could vary
the assignment of all the signals was not possible. However,
an analysis of the aliphatic region enabled the four singlets of
the methylenic groups and those of the methyl groups of the
imine ligand to be identified and assigned. The final isomeric
composition was, 60 (trans, E) : 25 (cis, E) : 9 (cis, Z) : 6 (trans,
Z). The trans form was the most stable, as observed with the
ionic complexes 5b and 6b. In spite of the fact that the palladium
complexes [Pd(CH3)(NCCH3)(PIM)]+ are known,8,29 when we
tried to synthesise similar complexes with mesityl instead of the
methyl ligand, starting from 4a or 4b, similar results to those
obtained with the nickel complexes were observed. No further
investigation was undertaken. The exact mechanism by which
these ionic compounds are obtained is not obvious. The initial
cationic species were probably formed, since the expected ionic
complexes [Ni(Mes)(NN)(lut)][BF4] were obtained when lutidine
was used. The process implies that the nitrile was inserted in the
Reactions with ethylene
Palladium and nickel compounds with a-diimine ligands have
been used successfully as precursors of catalytic species in the
oligomerization and polymerization of ethylene. Therefore, we
studied this reaction with some of the new complexes to evaluate
whether the ionic compounds 7 containing the imine ligand
could be used as organometallic precursors. In previous work,
we described33 the preparation of ionic complexes containing a
tridentate NNꢀC ligand, derived from modified DAD and PIM
backbones, with acetonitrile as a labile coordinated ligand that
showed limited activity in the reaction with ethylene. When a
THF solution of the cationic 7a and 7b complexes were treated
with ethylene at 20 bar and 25 ◦C, the starting compounds
were recovered unchanged. Therefore, the imine ligand did not
undergo the substitution by ethylene, which is the initial step in
the formation of the hydride catalytic species.
=
metal-mesitylene bond giving a azomethine (M–N CR(Mes))
intermediate, protonation to the final imine neutral ligand and
their intermolecular exchange. However, other possibilities could
be proposed (see ESI†). The reaction of 3c in acetone-d6 with a
small amount of D2O suggests that the hydrogen atom needed for
the protonation of the azomethine intermediate comes from water
present in the acetone solvent. It is known that the addition of
AgBF4 to acetone or THF solutions of [PdI(Ar)(bpy)] (eqn (1)),
does not lead to isolation of the cationic aryl palladium complexes
and causes intermolecular coupling of the aryl ligands to yield the
corresponding biaryls.10 The symmetrization of a trichlorovinyl
nickel complex was observed when a simple substitution of PPh3
by the bipyridyl ligand was attempted, supporting the proposed
mechanism of the formation of biaryls by intramolecular coupling
(eqn (2)).30 Insertion of the CN bond of an organonitrile into a
late transition metal–carbon r bond, is not a common process.
However, it has been reported to occur in the palladium–carbon
bond giving a free imine (eqn (3)),14 or trapping the coordinated
imine in a N,O-metallacycle (eqn (4)). The process is assisted by
the protonation of the nitrile nitrogen atom by the ortho-hydroxy
group present in the aryl ligand.13 The reduction of coordinated
acetonitrile has been observed in several complexes in which
the coordinated nitrile is activated toward nucleophilic attack.31
Recently, the reduction of nitriles by a catalytic palladium system
has been reported.32
Conclusions
The use of nitriles as labile ligands to prepare one component
organometallic catalytic precursors of nickel and palladium could
be inconvenient in certain conditions. This is because the nitrile
fragment is prone to insert into the r(M–aryl) bond and the
resulting imine neutral ligands, which are coordinated when
the complex reorganization of the initially desired precursor is
complete, are inert with respect to their substitution by ethylene.
Therefore, if the process described here occurs, it is impossible to
activate the reaction at a medium pressure of ethylene.
Experimental
General remarks
All manipulations of the compounds were carried out under a
purified nitrogen atmosphere using standard Schlenk and high-
vacuum techniques. The solvents were dried, distilled and stored
under a nitrogen atmosphere by standard procedures. All reagents
were commercially available. Ethylene (99.95% quality) was used
as received. Elemental analyses were carried out by an Eager 1108
microanalyzer. IR spectra were recorded as KBr disks on a Nicolet
(1)
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520 FT-IR spectrometer. Mass spectra (FAB) were recorded on a
Fisons VG-Quattro spectrometer. 1H NMR spectra were recorded
with a Bruker DRX 250, Varian XL-500 or Varian Gemini-200
spectrometers. Chemical shifts (in ppm) were measured relative to
SiMe4. Coupling constants in Hz, numbering as shown in Tables 1S
and 2S (see ESI†). The oligomer products were analyzed on a
Hewlett-Packard 5890 equipped with a 50 m ultra-2 cross-linked
5% phenylmethyl silicone capillary column and a FID detector.
(Found: C, 33.2; H, 2.5; N, 5.9. Calc. for C13H12Br2N2Pd: C, 33.76;
H, 2.62; N, 6.06%). 1H NMR (250 MHz, DMSO-d6), d 5.21 (s, 2
ꢀ
ꢀ
H, CH2), 7.3–7.5 (m, 5 H, H7,7 ,8,8 , 9), 7.86 (t, J 5.5, 1 H, H2), 8.13
4
3
=
(d, J 7.6, 1H, H ), 8.32 (t, J 7.6, 1H, H ), 8.75 (s, CH N), 9.19 (d,
J 5.2, 1 H, H1).
[NiBr(Mes)(DAD)] (3a). Over
a suspension of 3.86 g
(8.00 mmol) of 1a in 10 cm3 of tetrahydrofuran, a solution
containing approximately 12 mmol of mesityl magnesium bromide
in tetrahydrofuran was added. Immediately the yellow suspension
dissolved leaving a deep red solution. After 10 min, the reaction
mixture was hydrolysed with 20 cm3 of 10% NH4Br solution. The
organic phase was extracted with toluene, dried over Na2SO4, and
filtered off. The solution obtained was evaporated to dryness and
the purple solid residue was washed with water and diethyl ether
and then dried in vacuum (yield = 50%) (Found: C, 61.4; H, 5.9;
N, 5.2. Calc. for C27H31BrN2Ni: C, 62.11; H, 5.98; N, 5.36%). 1H
NMR (250 MHz, CDCl3), d 1.69 (s, 3 H, p-CH3), 1.91 (s, 3 H,
Synthesis of the ligands
N,Nꢀ-Dibenzyl-2,3-butanediimine (a). The ligand was pre-
pared by the same procedure described in the literature20 for
the preparation of related diazabutadienes derived from 2,3-
butanedione and aliphatic amines with a secondary a-carbon.
2.0 g (23.2 mmol) of 2,3-butanedione and 4.5 g (42.0 mmol)
of benzylamine were dissolved in 20 cm3 of dichloromethane.
To the resulting solution a drop of 98% formic acid and 8 g of
molecular sieve (4A) were added. The mixture was stirred at room
5
6
CH3 ), 2.16 (s, 3 H, CH3 ), 2.75 (s, 3 H, o-CH3), 4.29 (s, 2 H,
1
7
1
CH2 ), 5.37 (s, 2 H, CH2 ), 6.33 (s, 2 H, m-H), 6.68 (br s, 2 H,
temperature. The progress of the reaction was monitored by H
ꢀ
ꢀ
H2, 2 ), 7.21–7.38 (m, 6 H), 7.65 (d, J 6.8, 2 H, H8,8 ).
NMR spectroscopy. After 24–48 h the solution was filtered and
the solvent was removed in vacuum. The crude product obtained
was recrystallized from MeOH and obtained as an oily material
(yield = 55%). 1H NMR (250 MHz, CDCl3), d 2.25 (s, 6 H, CH35, 6),
4.70 (s, 4 H, CH21, 7), 7.25–7.45 (m, 10 H, Ph).
[NiBr(Mes)(PIM)] (3b). 0.11 g (0.7 mmol) of ligand b were
added to a vigorously stirred suspension of 0.46 g (0.6 mmol) of
[NiBr(Ms)(PPh3)2] in 20 cm3 of diethyl ether. After a few minutes
the yellow suspension dissolved leaving a deep red solid precipitate.
The solid was filtered off, washed with water and diethyl ether and
dried in vacuum (yield = 75%) (Found: C, 58.5; H, 5.2; N, 6.2.
Benzyl(pyridin-2-yl)methyleneamine (b). This ligand was ob-
tained by the method described for similar ligands.21 A mixture
of 2.0 g (18.6 mmol) of 2-pyridine-2-carbaldehyde and 2.0 g
(18.6 mmol) of benzylamine in 20 cm3 of ethanol were refluxed for
30 min. After cooling, the solvent was removed in vacuum. The
crude product obtained was recrystallized in hexane and obtained
as an oily material (yield = 75%). 1H NMR (200 MHz, CDCl3), d
1
Calc. for C22H23BrN2Ni: C, 58.20; H, 5.11; N, 6.17%). H NMR
(500 MHz, acetone-d6), obtained from the mixture of the cis (65%)
and trans (35%) isomers. NMR data for the cis isomer, d 9.31 (d,
1
=
J 5.5, 1 H, H ), 8.62 (s, 1 H, HC N), 8.18 (td, J 7.5 and 1.5, 1 H,
H3), 7.96 (d, J 7.5, 1 H, H4), 7.82 (t, Jꢀ 7.5, 1 H, H2), 7.38–7.24* (m,
ꢀ
ꢀ
ꢀ
3 H, H 8, 8 ,9), 6.85 (d, J 7.5, 2 H, H 7,7 ), 6.36 (s, 2 H, m-H), 4.13 (s,
4.88 (s, 2 H, CH2), 7.27–7.36 (m, 6 H, H7,7 ,8,8 ,9), 7.72 (t, J 7.8, 1 H,
H3), 8.06 (d, J 8.0, 1 H, H4), 8.49 (s, 1 H, H5), 8.65 (d, J 4.8, 1 H,
H1).
2 H, CH2), 2.87 (s, 6 H, o-CH3), 2.17 (s, 3 H, p-CH3). NMR data
=
for the trans isomer, d 8.58 (s, 1 H, HC N), 8.12 (td, J 7.5 and
ꢀ
1.5, 1 H, H3), 7.85 (d, J 7.5, H, H4), 7.67 (d, J 7.5, 2 H, H7,7 ), 7.44
(t, J 7, 1ꢀ H, H2), 7.15 (d, J 5.5, 1 H, H1), 7.40 (d, J 7.5 and 1.5,
2 H, H8,8 ), 7.38–7.24* (m, 1 H, H9), 6.43 (s, 2 H, m-H), 5.19 (s, 2
H, CH2), 2.96 (s, 6 H, o-CH3), 2.04 (s, 3 H, p-CH3); *overlapped
signals of both isomers.
Synthesis of the neutral complexes
[NiBr2(DAD)] (1a). A suspension of 4.84 g (22 mmol) of
anhydrous NiBr2 and 6.30 g (24 mmol, 10% excess) of ligand a
in 40 cm3 of toluene were stirred for 72 h at room temperature.
A yellow solid was formed. It was filtered off and washed with
hexane, and finally dried in vacuum (yield 75%).
[PdBr(Mes)(DAD)] (4a). According to the procedure de-
scribed for 3a by using 2a (yield = 78%). (Found: C, 56.2; H,
5.4; N, 5.1. Calc. for C27H31BrN2Pd: C, 56.91; H, 5.48; N, 4.92%).
[NiBr2(PIM)] (1b). The complex was prepared following the
procedure described for 1a, by using ligand b. The yellow solid
was filtered off, washed with hexane and finally dried in vacuum
(yield 80%).
1H NMR (CDCl3, 500 MHz), d: 2.10 (s, 3 H, CH3 ), 2.15 (s, 3 H,
5
6
p-CH3), 2.17 (s, 3 H, CH3 ), 2.40 (s, 6 H, o-CH3), 4.51 (s, 2 H,
1
7
CH2 ꢀ ), 5.34 (s, 2 H, CH2 ), 6.40 (s, 2 H, m-H), 6.65ꢀ(d, J 7.5, 2 H,
H 2, 2 ), 7.17–7.36 (m, 6 H), 7.63 (d, J 7.5, 2 H, H 8.,8 ).
[PdBr2(DAD)] (2a). To a solution of 0.40 g (2.25 mmol) of
PdCl2 and 0.78 g (9 mmol) of LiBr in 50 cm3 of methanol at
room temperature, 0.63 g (2.40 mmol, 10% excess) of ligand a
was added. Immediately a yellow–orange solid was formed. It was
filtered off and washed with water and methanol and finally dried
at vacuum (yield = 87%) (Found: C, 41.2; H, 3.9; N, 5.4. Calc. for
C18H20Br2N2Pd C, 40.75; H, 3.80; N, 5.28%). 1H NMR (250 MHz,
CDCl3), d: 2.27 (s, 6 H, CH35,6), 5.63 (s, 4 H, CH21,7), 7.28–7.40 (m,
[PdBr(Mes)(PIM)] (4b). According to the procedure de-
scribed for 3a by using 2b (yield = 70%). (Found: C, 53.2 H,
4.7; N, 5.2. Calc. for C22H23BrN2Pd: C, 52.67; H, 4.62; N, 5.58%).
1H NMR (500 MHz, acetone-d6) obtained from a mixture of cis
(65%) and trans (35%) isomers. cis Isomer, d 9.13 (d, J 5.5, 1 H,
1
3
=
H ), 8.88 (t, J 1.5, 1 H, HC N), 8.26 (td, J 8, 1 H, H ), 8.12 (d, J
8, 1 H, H4), 7.87 (dd, J 5 and 1.5, 1 H, H2), 7.28 (tt, J 7.5 and 1.5,
ꢀ
ꢀ
1 H, H9), 7.22 (tt, J 7.5 and 1.5, 2 H, H8, 8 ), 6.80 (d, J 7, 2 H, H7, 7 ),
6.45 (s, 2 H, m-H), 4.49 (s, 2 H, CH2), 2.42 (s, 6 H, o-CH3), 2.02* (s,
ꢀ
ꢀ
6 H), 7.57 (d, J 7, 4 H, H2,2 ,8,8 ).
=
[PdBr2(PIM)] (2b). The complex was prepared following the
procedure described for 2a, by using ligand b (yield = 90%)
3 H, p-CH3). trans Isomer, d 8.69 (t, J 1.5, 1 H, HC N), 8.21 (tꢀd,
J 8 and 2, 1H, H3), 8.04 (d, J 7, 1 H, H4), 7.70 (d, J 7.5, 2 H, H7, 7 ),
88 | Dalton Trans., 2007, 83–90
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©

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7.59 (dd, J 5.5 and 1.5, 1 H, H2ꢀ ), 7.46 (dd, J 5.5 and 1.5, 1 H, H1),
7.40 (tt, J 7 and 1.5, 2 H, H8, 8 ), 7.33 (tt, J 7.5 and 1.5, 1 H, H9),
5.23 (s, 2 H, CH2), 6.55 (s, 2 H, m-H), 2.57 (s, 6 H, o-CH3), 2.02*
(s, 3 H, p-CH3); *overlapped signals of both isomers.
2.33 (s, 6 H, m-CH3), 2.20 (s, 3 H, p-CH3). trans Isomer, d 9.08 (s,
3
4
=
1 H, HC N), 8.40 (td, J 8 and 1.5, 1 H, H ), 8.31 (d, 1 H, H ),
8.10 (s, 2 H, o-H), 7.73 (ddd, 1 H, H2), 7.64 (d, J 5, 1 H, H1), 7.52
ꢀ
ꢀ
(s, 1 H, p-H), 7.31–7.23 (m, 3 H, H9, 8, 8 ), 7.02 (d, J 7.5, 2 H, H7,7 ),
6.58 (s, 2 H, m-H), 5.01 (s, 2 H, CH2), 2.68 (s, 6 H, o-CH3), 2.20 (s,
6 H, m-CH3), 2.15 (s, 3 H, p-CH3).
Synthesis of the ionic complexes
=
{Ni(Mes)[NH C(Mes)(Me)](DAD)}BF4 (7a). To a solution
[Ni(Mes)(3,5-lut)(DAD)]BF4 (5a). 0.30 g (1.03 mmol) of TlBF4
was added over a stirred solution of 0.11 g (1.0 mmol) of 3,5-
lutidine and 0.52 g (1.00 mmol) of the neutral complex 3a
in 30 cm3 of tetrahydrofuran. A reddish suspension containing
white insoluble thallium halide was slowly formed. After 14 h
the thallium bromide was separated by filtering over Celite. The
solvent was then evaporated to dryness and the remaining resin
stirred with diethyl ether until a suspension with an orange solid
was obtained. The solid was filtered off, and washed several times
with water and diethyl ether (yield = 60%) (Found: C, 63.4; H, 6.2;
N, 6.6. Calc. for C34H40BF4N3Ni: C, 64.19; H, 6.34; N, 6.60%). 1H
NMR (500 MHz, CDCl3), d 2.03 (s, 6 H, m-CH3), 2.06 (s, 3 H, p-
of 0.40 g (0.91 mmol) of 3a and 0.11 g of CH3CN (2.7 mmol,
300% excess) in 50 cm3 of acetone, 0.27 g (0.94 mmol) of TlBF4 was
added. The reaction mixture was stirred for 12 h, during which time
a fine precipitate of thallium bromide was formed. The mixture was
filtered off through Celite and then concentrated under reduced
pressure. The complex was precipitated as an orange–reddish solid
by replacing the tetrahydrofuran by toluene. The solid was filtered
off and washed with water and ether (yield = 23%, based on nickel)
(Found: C, 64.3 H, 6.9; N, 6.7. Calc. for C38H46BF4N3Ni: C, 66.12;
H, 6.72; N 6.09%). H NMR (acetone-d6, 250 MHz), E Isomer,
d 1.54 (s, 6 H, p-CH3), 2.13 (s, 3 H, CH3 ), 2.15 (s, 3 H, CH3 ),
2.41 (s, 3 H, o-CH3-imine), 2.46 (s, 3 H, o-CH3-imine), 2.89 (br s,
9 H, o-CH3-Mes and N CCH3), 4.48 (s, 2 H, CH2 ), 4.97 (s, 2 H,
CH2 ), 6.40 (s, 2 H, m-H Mes), 6.66 (s, 2 H, m-H imine), 6.78 (m,
2 H, o-Ph), 7.25–7.70 (m, 8 H), 9.16 (s, 1 H, NH).
1
5
6
5
6
CH3), 2.25 (s, 3 H, CH3 ), 2.34 (s, 3 H, CH3 ), 2.65 (s, 6 H, o-CH3),
1
1
7
=
4.17 (s, 2 H, CH2 ), 4.29 (s, 2 H, CH2 ), 6.22 (s, 2 H, m-H), 6.73
ꢀ
ꢀ
7
(d, J 7, 2 H, H2, 2 ), 6.93 (d, J 7.5, 2 H, H8, 8 ), 7.06 (s, 1 H, p-H),
7.22–7.40 (m, 6 H), 8.02 (s, 2 H, o-H).
=
{Ni(Mes)[NH C(Mes)(Me)](PIM)}BF4 (7b). Complex 7b
[Ni(Mes)(3,5-lut)(PIM)]BF4 (5b). Compound 5b was obtained
by the same procedure described for complex 5a by using complex
3b (yield = 80%) (Found: C, 61.0; H, 5.6; N, 7.5. Calc. for
C29H32BF4N3Ni: C, 61.31; H, 5.68; N, 7.40%). 1H NMR (250 MHz,
acetone-d6), obtained from the mixture of the isomers cis (20%)
was obtained by the same procedure as 7a from 3b as a yellow–
orange solid (yield = 30%, based on nickel) (Found: C, 63.3 H,
5.8; N, 6.6. Calc. for C33H38BF4N3Ni: C, 63.70; H, 6.16; N 6.75%).
1H NMR (500 MHz, CDCl3), obtained from the mixture of the
four isomers. trans-E isomer (60%), d 1.64 (s, 6 H, o-CH3-imine),
2.19 (s, 3 H, p-CH3-imine), 2.20 (s, 3 H, p-CH3Mes), 2.89 (s, 3 H,
=
and trans (80%). trans Isomer, d: 8.97 (s, 1 H, HC N), 8.32 (s, 2
H, o-H), 8.30 (d, J 7, 1H, H3), 8.16 (d, J 7, 1 H, H4), 7.61 (dd, J
5.5 and 1.5, 1 H, H2), 7.46 (s, 1 H, p-H), 7.34–7.33* ꢀ(m, 3 H, H
=
N CCH3), 3.00 (s, 6 H, o-CH3Mes), 4.84 (s, 2 H, CH2), 6.55 (s, 2
H, m-H Mes), 6.70* (s, 2 H, m-H imine), 7.21–7.80 (m, 7 H), 8.68
ꢀ
8,8 , 9), 7.26 (d, J 5.5, 1 H, H1), 7.06–7.07 (m, 2 H, H 7,7 ), 6.49 (s, 2
=
(s, 1 H, CH N), 10.33 (s, 1 H, NH). cis-E Isomer (25%), d 1.76
(s, 6 H), 2.35 (s, 3 H), 2.18 (s, 3 H), 2.83 (s, 3 H), 2.90 (s, 6 H),
H, m-H), 4.67 (s, 2 H, CH2), 3.06 (s, 6 H, o-CH3), 2.16* (s, 6 H,
m-CH3), 2.12 (s, 3 H, p-CH3). cis Isomer, d 8.76 (s, 2 H, o-H), 8.68
3
4
=
4.18 (s, 2 H), 6.47 (s, 2 H), 6.70* (s, 2 H), 7.2–8 (m, 9 H), 8.11 (s,
1 H, CH N), 9.20 (s, 1 H, NH); *overlapped signals between the
cis and trans isomers. Most representative signals for the minor
isomers, cis-Z isomer (9%), d 3.79 (2 H, s, CH2), 2.1–2.4 (s, 3 H,
N CCH3), trans-Z isomer (6%), d 5.00 (2 H, s, CH2), 2.1–2.4 (s,
(s, 1 H, HC N), 8.30 (d, J 7, 1 H, H ), 8.19 (d, J 7, 1 H, H ),
7.74 (dd, J 5ꢀ.5 and 1.5, 1 H, H2), 7.65 (s, 1 H, p-H), 7.34–7.33*
=
ꢀ
(m, 3 H, H8, 8 , 9), 7.18 (d, J 5.5, 1 H, H1), 6.86 (d, 2 H, H7,7 ), 6.45
(s, 2 H, m-H), 4.23 (s, 2 H, CH2), 3.01 (s, 6 H, o-CH3), 2.31 (s, 6
H, m-CH3), 2.16* (s, 3 H, p-CH3). * Overlapped signals of both
isomers.
=
=
3 H, N CCH3).
=
{Ni(Mes)[NH C(Mes)(Me)](bipy)}BF4 (7c). Complex 7b was
[Pd(Mes)(3,5-lut)(DAD)]BF4 (6a). Obtained by the same pro-
cedure described for complex 5a by using complex 4a (yield =
70%) (Found: C, 59.1 H, 6.0; N, 6.4. Calc. For C34H40BF4N3Pd:
C, 59.71; H, 5.90; N, 6.14%). 1H NMR (250 MHz, CDCl3), d 2.04
(s, 6 H, m-CH3), 2.11 (s, 3 H, p-CH3), 2.32 (s, 6 H, CH35, 6), 2.47 (s,
obtained by the same procedure as 7a from 3c as a yellow solid
(yield = 41% based on nickel) (Found: C, 59.4 H, 5.7; N, 6.9.
Calc. for C30H34BF4N3Ni: C, 61.90; H, 5.89; N 7.22%). 1H NMR
(250 MHz, acetone-d6); E Isomer (85%), d 1.89 (s, 6 H, o-CH3),
1
7
=
2.21 (s, 3 H, p-CH3), 2.22 (s, 3 H, N CCH3), 3.06 (s, 3 H, p-CH3),
6 H, o-CH3), 4.51 (s, 2 H, CH2 ), 4.74 (s, 2 H, CH2 ), 6.34 (s, 2 H,
ꢀ
ꢀ
m-H), 6.71 (m, 2 H, H2, 2 ), 6.88 (m, 2 H, H8,8 ), 7.14 (s, 1 H, p-H),
7.16–7.32 (m, 6 H), 7.78 (s, 2 H, o-H).
3.18 (s, 6 H, o-CH3), 6.63 (s, 2 H, m-H), 6.81 (s, 2 H, m-H), 8.50
(d, 1H), 8.41 (d, 1H), 8.32 (m, 2H), 8.20 (dd, 1H), 7.82 (dd, 1H),
7.42 (d, 2H), 9.45 (s, 1 H, NH); Z Isomer (15%), d 1.70 (s, 6 H),
2.29 (s, 6 H), 2.48 (s, 6 H), 2.54 (s, 3 H), 6.39 (s, 2 H), 6.79 (s, 2
H), 7.20–8.65 (m, 8 H), 11.09 (s, 1 H). Mass spectra FAB+ (NBA):
m/z = 495.8 (45), 494.0 (95), 374.9 (8), 332.9 (65), 215.6 (40), 213.7
(100), 161.5 (8).
[Pd(Mes)(3,5-lut)(PIM)]BF4 (6b). The compound was ob-
tained by the same procedure as 5a using complex 4b (yield =
83%) (Found: C, 57.2 H, 5.5; N, 7.1. Calc. for C29H32BF4N3Pd:
1
C, 56.26; H, 5.24; N 6.82%). H NMR (500 MHz, acetone-d6),
obtained from the mixture of the two isomers cis (25%) and trans
ꢀ
=
=
(75%). cis Isomer, d 9.05 (s, 1 H, HC N), 8.49 (s, 2 H, o-H), 8.39
{Ni(Mes)[NH C(Ph)(Mes)](bipy)}BF4 (7c ). To a solution of
(td, J 8 and 2, 1 H, H3), 8.33 (d, J 7.5, 1 H, H4), 7.98 (d, J 5, 1 H,
H1), 7.85 (ddd, J 7.5, 5 and 1.5, 1 H, H2), 7.74 (s, 1 H, p-H), 7.31
0.164 g (0.40 mmol) of [NiBr(Mes)(bipy)] in 30 cm3 of THF were
added an 300% excess of benzonitrile and 0.137 g (0.47 mmol)
of TlBF4. The color of the solution changed slowly from red to
yellow, after 30 min of vigorously stirring, TlBr was filtered off
ꢀ
(t, J 7.5, 1 H, H9), 7.24 (t, J 5.0, 2 H, H8, 8 ), 6.78 (d, J 7.5, 2 H,
ꢀ
H7, 7 ), 6.53 (s, 2 H, m-H), 4.60 (s, 2 H, CH2), 2.51 (s, 6 H, o-CH3),
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through Celite. The yellow compound which precipitated upon
replacement of the THF by toluene, was filtered off and washed
with water and ether (yield = 39%, based on nickel) (Found: C,
63.3 H, 5.4; N, 6.5. Calc. for C35H36BF4N3Ni: C, 65.26; H, 5.63; N
6.52%). 1H NMR (250 MHz, acetone-d6); E Isomer (63%), d 2.17
(s, 3H, p-CH3), 2.26 (s, 6 H, o-CH3), 2.60 (s, 3 H, p-CH3), 2.81 (s,
6 H, o-CH3), 6.52 (s, 2 H, m-H), 6.88 (s, 2 H, m-H), 7.50 (m, 5H,
Ph), 7.70–7.40 (m, 3H), 8.62–8.22 (m, 5H), 9.83 (s, 1 H, NH); Z
Isomer (37%), d 1.57 (s, 6 H), 2.29 (s, 3 H), 2.35 (s, 3 H), 2.60 (s, 6
H), 6.40 (s, 2 H), 6.88 (s, 2 H), 6.70–8.80 (m, 13 H), 11.49 (s, 1 H).
Mass spectra FAB+ (NBA): m/z = 557.4 (26), 555.9 (53), 436.8
(8), 332.9 (50), 223.8 (30), 215.6 (40), 213.6 (100), 135.7 (70).
Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs, D. A. Bansleben and
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Younkin and R. H. Grubbs, Organometallics, 2004, 23, 5121.
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Dunbar, Comments Inorg. Chem., 1992, 13, 313.
8 R. E. Ru¨lke, J. G. P. Delis, A. M. Groot, C. J. Elsevier, P. W. N. M. van
Leeuwen, K. Vrieze, K. Goubitz and H. Schenk, J. Organomet. Chem.,
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Milani, S. Gladiali, C. Carfagna, B. Binotti, S. Bronco and T. Gragnoli,
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10 T. Yagyu, M. Hamada, K. Osakada and T. Yamamoto,
Organometallics, 2001, 20, 1087.
11 M. E. Cucciolito, A. De Renzi, F. Giordano and F. Ruffo,
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=
{Ni(Mes)[NH C(Me)(Mes)](phen)}BF4 (7d). Complex 7d
was obtained by the same procedure as 7a from 3d as a yellow
solid (yield = 40%, based on nickel) (Found: C, 61.5; H, 5.6; N,
1
6.7. Calc. for C32H34BF4N3Ni: C, 63.41; H, 5.65; N, 6.93%). H
NMR (250 MHz, acetone-d6); E Isomer (82%), d 2.02 (s, 6 H,
=
o-CH3), 2.22 (s, 3 H, p-CH3), 2.24 (s, 3 H, N CCH3), 3.14 (s, 3H,
p-CH3), 3.23 (s, 6H, o-CH3), 6.67 (s, 2H, m-H), 6.84 (s, 2H, m-H),
7.70–9.00 (m, 8H), 9.57 (s, 1H, NH); Z Isomer (18%), d 1.75 (s,
6H), 2.23 (s, 3H), 2.28 (s, 3H), 2.52 (s, 6H), 2.59 (s, 3H), 6.44 (s,
2H), 6.79 (s, 2H), 7.50–9.05 (m, 8H), 11.22 (s, 1H). Mass spectra
FAB+ (NBA): m/z = 520.1 (23), 518.4 (47), 399.5 (8), 357.6 (100),
337.5 (3).
12 K. Osakada and T. Yamamoto, Coord. Chem. Rev., 2000, 198, 379.
13 J. Vicente, J. Abad, M. J. Lo´pez-Sa´ez and P. G. Jones, Angew. Chem.,
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Leskela¨, J. Organomet. Chem., 2000, 606, 112.
=
Characterization of the imine [(Mes)MeC NH] obtained after
insertion of CH3CN
A typical experiment carried out with complex 7b: to 0.062 g of
complex 7b (0.1 mmol) solved in 15 cm3 of CHCl3, 15 cm3 of
a 1 M solution of HCl were added. The mixture was stirred at
room temperature for 3 h. The neutral imine ligand was recovered
from the aqueous phase, after basic treatment with a NaHCO3
solution and extraction with CHCl3, as an oily material. 1H NMR
(250 MHz, CDCl3), d 2.21 (s, 6H), 2,27 (s, 3H), 2.28 (s, 3H), 6.85 (s,
2H). MS (CI NH3): m/z (relative intensity) 162.3 [M + H]+ (100),
163.4 (16).
22 M. Helldo¨rfer, W. Milius and H. G. Alt, J. Mol. Catal., 2003, 197, 1.
23 H. tom Dieck and M. Svoboda, Chem. Ber., 1976, 109, 1657.
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Acknowledgements
This work was supported by the Spanish Ministerio de Educacio´n
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