Engeering Sciences Reports, Kyushu University
Vol. 28, No. 3, pp. 329-336, DEC. 2006

九州大学大学院総合理工学報告第28巻 第3号 329-336頁 平成18年12月

Spectroscopic and Crystallographic Studies on theInsertion Reaction of Aryl Isocyanides into the Bondbetween Palladium and Carbon, which Contribute to
Understanding the trans-[Br2Ni(CNAr)2]-Catalyzed
Ethylene Polymerization
Kazuhiro TSUCHIYA*1 and Hideo NAGASHIMA*1,*2, ††E-mail of corresponding author:

nagasima@cm.kyushu-u.ac.jp(Received October 31, 2006)
The insertion of two sterically different isocyanides into a Pd-Me bond was investigated byspectroscopy and crystallography; this contribute to better understanding of ethylene
polymerization catalyzed by trans-[Br2Ni(CNAr)2]. Thus, the reaction of MePdCl(COD) (COD =cyclooctadiene) with a small isocyanide (CNC6H3-2,6-Me2) or a bulky isocyanide(CNC6H3-2,6-Ph2) demonstrated that isocyanide immediately inserted into the Pd-Me bond.
Altough the C6H3-2,6-Ph2 group is more bulky than the C6H3-2,6-Me2 moiety, the anisotropic
structure of ortho phenyl groups could provide enough space for the insertion by theirappropriate
rotations.
Similarly,
themolecular
structureoftrans-[ClPd{C(Me)=NC6H3-2,6-Ph2}(CNC6H3-2,6-Ph2)2] (5b) provided another example ofappropriate rotations of phenyl rings, which ingeniously avoid the inter-ligand repulsions.
Key words: organometallic chemistry, palladium complex, aryl isocyanide, insertion reaction

1. Inroduction
Organometallic complexes have been
actively investigated as the catalyst for
ethylene polymerization since the discoveryof the metallocene catalyst.1) Brookhart andco-workers found Ni complexes bearing
α-diimine ligands showing high catalytic
activity for ethylene polymerization in 1995,2)
opening the field of the “post-metallocene
catalyst”.3) We have recently found that thetrans-[Br2Ni(CNAr)2] (Ar = aryl group)
showed considerably high activity for
ethylene polymerization in the presence of
methylaluminoxane (MAO) (Scheme 1).4) The
catalytically active species must be molecule;
this is supported by the catalytic activity
depends on the structure of the isocyanide
ligands. For instance, the Ni complex having
CNC6H3-2,6-Ph2 showed the activity 40 times
higher than that of CNC6H3-2,6-Me2.
Althoughtheperformance
of
1.
2.

trans-[Br2Ni(CNAr)2]

n

MAO

n

Scheme 1

R'

R''

R''

N

N

R'

Fig. 1R'
R

The proposed active intermediate for Brookhart’s
catalyst

N

Br

Ar

Ni

N

Ar

C
Ni
C Br

C

N

Ar

Ni

MAO
Ar

N

C
A

Ar

N

C
C

Ni

N Ar
Br

N
Ar

Alkyl cation complex

CH3

Ar N

Fig. 2Meethylene

Scheme 2

Department of Molecular and Material Sciences
Institute for Materials Chemistry and Engineering

R'

(I)

Ar = C6H3-2,6-Me2

The azanickellacyclopentene complex (I)Spectroscopic and Crystallographic Studies on the Insertion Reaction of Aryl Isocyanides into the Bond between Palladium
330
and Carbon, which Contribute to Understanding the trans-[Br2Ni(CNAr)2]-Catalyzed Ethylene Polymerizationtrans-[Br2Ni(CNAr)2]
is
uniqueasapost-metallocene catalyst, the mechanism istotally unknown. From the analogy towell-investigatedintermediatesof
the
Brookhart’s catalyst, which are cationic alkyl
nickel species bearing coordinated ethylene asshown in Fig. 1, cationic species A may begenerated from trans-[Br2Ni(CNAr)2] by way of
mono-methylation and subsequent halogen
abstraction by MAO (Scheme 2). However, thismechanism is problematic due to thetrans-configuration of A between Me group andethylene, which prevents insertion of ethylene
between the Ni-Me bond. Possible insertion of
the isocyanide ligand in A between the Ni-Me
bond posed another problem for themechanistic consideration, which could produce
newnickelintermediates
havingaNi-C(=NAr)Me moiety. It is really attractive toconsider alternative mechanisms, in whichintermediates having a Ni-C(=NAr)Me moiety
or its analogues play an important role in thetrans-[Br2Ni(CNAr)2]-catalyzed
ethylenepolymerization. In fact, a support of thispossibility was available by the discovery that anickel complex I, which can be formed byinsertion of three molecules of CNAr to Ni-Me
species generated from trans-[Br2Ni(CNAr)2]
with MAO, actually behaves as an ethylene
polymerization catalyst (Fig. 2).5)
We felt high probability of the mechanisms
involving insertion of CNAr into Ni-Me species.
However, a problem of this mechanism is theactivity of ethylene polymerization depends on
the structures of isocyanides, typically,trans-[Br2Ni(CNC6H3-2,6-Ph2)2]>>trans-[Br2Ni(CNC6H3-2,6-Me2)2]. As describedin section 2.4 in detail, the molecular modelingstudy strongly suggests that there is a
substantial difference in steric hindrance
between CNC6H3-2,6-Ph2 and CNC6H3-2,6-Me2;
the former is more bulky than the latter. A
question is whether CNC6H3-2,6-Ph2 may be too
bulky for the smooth insertion into the Ni-Me
bond; this provides difficulties in generating thecatalytically active species. In order to solve
this problem, we were interested in the model
reaction using palladium as central metal to

2CNAr

ArNC

-cod

ArNC

MePdCl(COD)

Me
Pd

1

Cl
2
Me

Ar
ArNC
Me

Cl
Pd

Pd
C
NAr

NAr
C

Cl

N C

Cl

CNAr

ArNC

CNAr

ArNC
Cl

ArN
C
Me
Pd
CNAr

Cl

C N
Me

cis- 3trans- 3

CNAr
Pd

Pd

Me

Ar

cis- 4trans- 4

Ar = C6H3-2,6-Me2 (a)C6H3-2,6-Ph2 (b)5Fig. 3 The reaction of MePdCl(COD) with 2 or 3
equivalents of ArNC.

investigate how facile the insertion of
CNC6H3-2,6-Ph2 and CNC6H3-2,6-Me2 into
Pd-Me
bond
of
in
situ
generated
MePdCl(CNAr)2. Palladium is in the same triad
as nickel, affording more stable methyl
complexes.
Highstabilityofthepalladium-methyl complexes are useful for themechanistic studies of the trans-[Br2Ni(CNAr)2]
catalyzed ethylene polymerization, in whichinsertion of CNAr to MeNiX(CNAr)2 is possibly
involved as the mechanism.

2. Results and Dicussion
2.1 The spectroscopic study of the insertion
reaction of CNC6H3-2,6-Me2 into a Pd-Me
bond
Vincente and co-workers have recently
reportedtheinsertion
reactionof
CNC6H3-2,6-Me2 to Pd-Me bond, which is thesmall isocyanide described above, into aPd-(C6H4-2-NH2) bond.6) There was no report on
the insertion reaction of CNC6H3-2,6-Me2 into a
Pd-Me bond, which is more important for theconsideration of the mechanism of the
MAO-assisted trans-[Br2Ni(CNAr)2]-catalyzed
ethylene polymerization. As a related study,
Andersonandco-workers
reported
spectroscopic studies on the insertion reactionof CNtBu into a Pd-Me bond.7) This report has
shown the reaction of MePdCl(COD) and threeequivalents of CNtBu to give the Pd complex
having a structure similar to 5 shown in Fig. 3.
In the related study to this Anderson’s research,
Uson and co-workers discussed the possibility

平成18年

九 州 大 学 大 学 院 総 合 理 工 学 報 告of intermediates similar to 3 and 4.8)
We carried out the reaction of MePdCl(COD)
with
two
or
three
equivalentsof
CNC6H3-2,6-Me2 in CD2Cl2, and monitored theformed products by 1H NMR spectroscopy. As
shown in Fig. 3, the reaction of MePdCl(COD)
and two equivalents of CNC6H3-2,6-Me2 was
expected to form 2a by the ligand exchanging of
COD and CNC6H3-2,6-Me2. If 2a is unstable andinsertion of CNAr rapidly takes place, 3a and4a could be formed. Addition of anotherequivalent of CNC6H3-2,6-Me2 was likely to give5a as a single stable product. The actual 1H
NMR spectra (the range of Me on aryl group)for these reactions are shown in Fig 4. The
reaction of MePdCl(COD) with two equivalentsof CNC6H3-2,6-Me2 afforded significant amountof precipitation, whereas addition of additional
CNC6H3-2,6-Me2 resulted in dissolution of the
precipitates. A 1H resonance due to the Pd-Me
group is generally observed around or higher
than 1 ppm; however, no singlet was observed
around 1 ppm on 1H NMR spectra. The
spectrum obtained by addition of threeequivalentsof
CNC6H3-2,6-Me2
wasunequivocally showed that the sole species
assignable to 5a exist in the solution. Thus, thesignal of Me group originated from Pd-Me wasseen at 2.68 ppm, significantly downfield from 1
ppm, suggesting the insertion reaction of
isocyanide
to
form
the
Me(Pd)C=N(C6H3-2,6-Me2) moiety. The signals
due to the coordinated CNC6H3-2,6-Me2 wereobserved at 2.10 and 2.40 ppm, respectively.
The integral ratio of these three singlets
was 3:6:12, which is in accord with thestructure of 5a. It is well known that the signalsof aromatic protons are generally seen in 7-8
ppm. The signals of triplet (2H) and doublet(4H) at 7.29 and 7.16 ppm were thus assignable
to para and meta protons of the coordinated
CNC6H3-2,6-Me2, respectively. In contrast, themultiplet (3H) at 6.88-6.95 ppm appeared
unusually upfield as the aromatic protons. The
upfield shift of aromatic protons is often
observed, when the electron donating groupsuch as nitrogen functional groups having alone pair electron. Compared with the N≡C
group of isocyanide (Ar-N≡C→Pd) acting as an

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331

第3号

COD

PPM
3.0

2.5

2.0

COD

PPM
3.0

Fig.

2.5

2.0

1H
4
NMR spectra of the reaction of
MePdCl(COD) with 2 (above) or 3 (below)
equivalents of CNC6H3-2,6-Me2 in CD2Cl2

Table 1

Representative IR absorptions of obtained
mixtures under each condition

condition

2eq. of CNAr

3eq. of CNAr

solutionprecipitateC≡N(cm-1)

2175

2189

2174

C=N(cm )1630-1660

1663

1666

1590

1560

-1

electron acceptor, the N=C group of the imidoyl(Ar-N＝C(Me)-Pd) could behave as an electron
donor, giving the upfield shift described above.
Two IR absorptions were visible at 2174 and
1666 cm-1 due to the stretching vibration of thecoordinated isocyanide (C≡N-Ar) and imidoyl
group (C=N-Ar), respectively (Table 1). Thus allof the 1H and IR data are consistent with thestructure of 5a.
The 1H NMR spectrum of the reaction of two
equivalentsof
CNC6H3-2,6-Me2
with
MePdCl(COD) was complicated, suggesting theexistence of more than two palladium species
besides 5a. The IR spectrum also gave several
absorptions including the peaks due to 5a. As
described above, no 1H resonance around 1 ppm
indicates rapid insertion of CNC6H3-2,6-Me2
into the Pd-Me bond. At present, we postulate
the species formed by addition of two
equivalents of CNC6H3-2,6-Me2 would be a
mixture of 3a and 4a, which are presumably a
mixture of cis and trans isomers. Thishypothesis is supported by the IR analyses ofthe precipitates and the solid recovered fromthe solution. The precipitate had absorption at
1560 cm-1 due to the bridging imidoyl group of
4a, whereas the absorptions of the sample
obtained from the solution were mainly
observed at the range of 1630-1660 cm-1 which
can be assignable to 3a and 5a. It is important

Spectroscopic and Crystallographic Studies on the Insertion Reaction of Aryl Isocyanides into the Bond between Palladium
332
and Carbon, which Contribute to Understanding the trans-[Br2Ni(CNAr)2]-Catalyzed Ethylene Polymerizationthat dual species existing in the reaction
mixture of two equivalents of CNC6H3-2,6-Me2
with MePdCl(COD) were converted to a single
product 5a by addition of another equivalent of
CNC6H3-2,6-Me2. these results suggest that the
spectral change is not inconsistent with thescheme shown in Fig. 3.
These spectroscopic results demonstrated
that the small isocyanide, CNC6H3-2,6-Me2,
quickly inserted into the Pd-Me bond ofintermediary MePdCl(CNC6H3-2,6-Me2)2 toform the imidoyl structure. This provided agood basis to investigate the homologous
reaction of more bulky CNC6H3-2,6-Ph2 with
MePdCl(COD).
2.2 The spectroscopic study of the insertion
reaction of CNC6H3-2,6-Ph2 into a Pd-Me
bond
Similar pathways to Fig. 3 (Ar =C6H3-2,6-Me2) can be drawn for the reactions of
MePdCl(COD) and two or three equivalents of
CNC6H3-2,6-Ph2 [Fig. 3 (Ar = C6H3-2,6-Ph2)].
The change of actual 1H NMR spectra (therange of Me) for the reactions of
CNC6H3-2,6-Ph2 with MePdCl(COD) are shownin Fig. 5. In contrast to the above describedreaction with the small isocyanide, the reaction
with two equivalents of CNC6H3-2,6-Ph2 kept
homogeneous. The 1H NMR showed two
singlets due to the Me groups at 0.98 and 1.62
ppm. A small singlet appeared at 1.15 ppm will
be discussed later. On addition of anotherequivalent of CNC6H3-2,6-Ph2, the singlet at
1.62 ppm disappeared, and all of the 1H
resonances, both methyl and aryl regions,became consistent with the structure of 5b.
Thus, the signals observed at 7.61-7.40 ppm are
due to all of the protons on aryl groups of the
CNC6H3-2,6-Ph2 ligand coordinated to the Pd
center. The two triplets at 6.60 and 6.71 ppm
were due to the protons on the para and meta
position of C6H3-2,6-Ph2 of the imidoyl group,
respectively. The multiplets observed at
6.80-6.86 ppm and a doublet at 7.10 ppm were
due to the protons of C6H3-2,6-Ph2 and ortho
protons of C6H3-2,6-Ph2 of the imidoyl group,
respectively. The IR spectrum showed two

COD

PPM
2.0

1.0

COD

PPM
2.5

2.0

1.5

1.0

0.5

Fig. 5

1H NMR spectrum of the reaction of
MePdCl(COD) with 2 (above) or 3 (below)
equivalents of CNC6H32,6-Ph2 in CD2Cl2

Table 2.

Representative IR absorptions of obtained
mixtures under each condition

conditionC≡N(cm-1)
C=N(cm )-12eq. of CNAr

3eq. of CNAr

2172

2175

1653

1653

1583

1585

absorptions at 2175 and 1653 cm-1 due to thestretching vibration of the coordinatedisocyanide (C≡N-Ar) and imidoyl group(C=N-Ar), respectively, which is also consistent
with the structure of 5b.
A solution formed by the reaction of two
equivalents of this bulky isocyanide with
MePdCl(COD) apparently contained 5b andanother species showing a 1H resonance at 1.62
ppm. The IR absorptions mainly appearedat 2172 and 1653 cm-1, which are assignable tothe isocyanide ligand and non-bridging imidoyl
groups. This suggests that the species showing
the singlet at 1.62 ppm on the NMR could be 3b.
Evidence for the presence of 4b may be a small
absorption at 1583 cm-1, which may correspondto the singlet at 1.15 ppm on the 1H NMR
spectrum.
All of these results suggest that the insertionof isocyanide into the Pd-Me bond is also rapid
even with the bulky isocyanide such as
CNC6H32,6-Ph2. For the explanation of thespectroscopic data described above consistently,
existence of 2b in solution must be excluded,
which might be detectable in solution when thebulky isocyanide slowly undergoes the insertion
between the Pd-Me bond. One may claim thatthe 1H resonances due to the methyl groups平成18年

九 州 大 学 大 学 院 総 合 理 工 学 報 告

第28巻

333

第3号

appeared at 0.98-1.62 ppm as describedabove, which could be assignable to Pd-Me
moieties. Since the methyl group of theimidoyl moiety in 5a appeared significantly
downfield from this region, we have to
explain the unusual chemical shift of theimidoyl methyl group in 5b. This should be
due to the long range shielding effect induced
by local magnetic field of the benzene ring,which can clearly be visible in the molecular
structure of 5b described in the next section.
2.3 The molecular structure of the complex
5b formed by insertion of the bulky
isocyanide, CNC6H3-2,6-Ph2
The described above spectroscopic studies
indicated that even the bulky isocyanide,
CNC6H3-2,6-Ph2, quickly insert into the
Pd-Me bond. The insertion reaction
proceeded so smoothly even at room
temperature. However, considerations using
the molecular model described later indicatethat bulkiness of CNC6H3-2,6-Ph2 mayprovidestericallyvery
crowdedcircumstances on the molecular structure of
5b. In fact, inter-ligand steric repulsions
around the Pd centre looked enormous when
we built the simple molecular model for 5b.
In other words, the bulkiness of isocyanide
may affect the thermodynamic stability of 5b,
though it does not give little effect on the
kinetic formation of 5b. The crystallography
to determine the molecular structure of 5b wasthus carried out.
The single crystals were grown from a
CH2Cl2/hexane solution containing a mixture
obtained by the reaction of MePdCl(COD)with 4 equivalents of CNC6H3-2,6-Ph2. There
was no problem on the crystallographic data,
and the molecular structure of 5b was revealed
to be just the same as we expected. The ORTEP
drawing is illustrated in Fig. 6.
Arrangement of the four groups connecting to
the Pd atom is square planer, and two
CNC6H3-2,6-Ph2 ligands coordinated to the Pd
centre are in trans configuration. The imidoyl
group, C(Me)=NC6H3-2,6-Ph2, exists at the
opposite site of the chlorine atom. The two
CNC6H3-2,6-Ph2 ligands are bonded to the Pd

Fig. 6The molecular structure of 5b. Ellipsoids represent
50% probability; hydrogen atoms are omitted forclarity.

Table 3

Representative bond lengths and angles for 5b

Pd(1)-Cl(1)Pd(1)-C(1)Pd(1)-C(4)Pd(1)-C(5)N(1)-C(1)N(2)-C(4)N(3)-C(5)Table 42.439(1)2.022(5)1.990(5)1.972(5)1.277(6)1.154(6)1.143(6)Cl(1)-Pd(1)-C(1) 175.7(1)Cl(1)-Pd(1)-C(4) 96.7(1)C(4)-Pd(1)-C(5) 173.0(2)C(1)-N(1)-C(3)125.0(4)C(2)-C(1)-Pd(1) 116.6(3)Crystallographic data for 5b

Crystal system
Space groupa, (Å)b, (Å)c, (Å)β, (deg.)V, (Å3)Zµ(MoKα) (cm-1)
No. of reflections measured
Good ness of fit on F2
Final R indices [λ>2σ(λ)]
R indices (all data)
Largest diff. peak and holemonoclinic
P21/c
9.561(2)17.843(4)27.205(5)94.7442(9)4632.8(16)4
15.28
total: 27045, unique: 7638
(Rint = 0.069)
1.006
R1 = 0.0530
wR2 = 0.1490
2.54 and -0.77 eÅ-3

atom with bond lengths of 1.990(5) and 1.972(5)Å for Pd-C, whereas those of 1.154(6) and
1.143(6) Å for C≡N. The C(Me)=NC6H3-2,6-Ph2
group has a typical structure of the imidoyl
group, of which N(1)-C(1) bond distance[1.277(6) Å] suggested the existence of a N=C
double bond, whereas the angles ofC(1)-N(1)-C(3)[125.0(4)deg.]and
C(2)-C(1)-Pd(1) [116.6(3) deg.] contribute to sp2
hybridization of C(1) and N(1). The
arrangement of the phenyl groups, which arelocated at the ortho-positions of three
C6H3-2,6-Ph2 moieties, was demonstrated to
avoid the steric repulsions of the Ph rings
ingeniously. This is achieved by rotation of the
Ph planes in the CNC6H3-2,6-Ph2 groups. Thus,
the possible rotation to avoid the stericSpectroscopic and Crystallographic Studies on the Insertion Reaction of Aryl Isocyanides into the Bond between Palladium
334
and Carbon, which Contribute to Understanding the trans-[Br2Ni(CNAr)2]-Catalyzed Ethylene Polymerization

repulsions explain both the thermodynamic
stability of 5b; this explanation can be extended
to interpretation of the facile insertion of
CNC6H3-2,6-Ph2 into the Pd-Me bond discussed
above.
The crystallographic study of 5b also gave aclear answer to the question why the 1H
resonance due to the imidoyl methyl group in
5b appeared at unusually higher field. In themolecular structure of 5b, the methyl group islocated at the face of two phenyl moieties of the
CNC6H3-2,6-Ph2 ligands. It is well known thatthe ring current of the aromatic ring gives local
magnetic field leading to substantial change ofthe chemical shifts of protons nearby in the
NMR spectroscopy(see Experimental section).
The proton spatially located on the face of thephenyl ring generally gives significant upfield
shifts by 1-2 ppm. The location of the methylgroup is fitted just in this case, and its chemical
shift (0.98 ppm) is explained by the long range
shielding of the phenyl rings nearby.
2.4 Considerations on the steric effect of
isocyanides
As described above, we started this researchfrom our hypothesis that CNC6H3-2,6-Ph2 may
be sterically too bulky to promote insertion
into the Pd-Me bond, which is facilely
accomplished by smaller CNC6H3-2,6-Me2.
This hypothesis seemed to be reasonable,
when we saw the structures of two isocyanides,
CNC6H3-2,6-Ph2
and
CNC6H3-2,6-Me2,
determined by semi-empirical MO (PM3)calculations, of which CPK model illustrations
are shown in Fig. 7. It is well known that amethyl group is isotropic, and free rotation
along with the C(Me)-C(aryl) bond in
CNC6H3-2,6-Me2
providesaspherical
sterically exclusive region, of which diameteris ca. 5 Å. According to the estimation of thespherical sterically exclusive region, freerotation of the phenyl group affords a sphere ofwhich diameter is ca. 8 Å. In this sense,
CNC6H3-2,6-Ph2 must be more bulky than
CNC6H3-2,6-Me2. However, a feature of phenylgroup is its anisotropic structure, andplanarity of the phenyl group would provide

Fig. 7 The space filling model of CNC6H3-2,6-Me2 (left)
and CNC6H3-2,6-Ph2 (right), simulated by PM3 method

Me

Me
N
C

N
C

N
C

a
b(edge)b(face)Fig. 8The isocyanides; CNC6H3-2,6-Me2 [a],CNC6H3-2,6-Ph2 [b(edge)] and [b(face)]

Fig. 9 The skeleton model (upper) and space fillingmodel (bottom) of complex 5a(left), There were
simulated by MOPAC based on the structure oftrans-[Pd{C(=NC6H3-2,6-Me2)C6H4NH2-2}I(CNC6H3-2,6Me2)]. Those of the complex 5b (right) were determined by
crystallographic datamuch smaller steric influence. This is typicallyseen in Fig. 8, in which the rotation gives two
extreme configuration, “edge” and “face”. If therotation is restricted so as to minimize thesteric repulsion among the ligands, theC6H3-2,6-Ph2 groups would not afford severe
steric hindrance to the complex 5b. The
molecular structure of 5a was shown in Fig. 9,
was estimated from the crystallographicallydetermined
structureoftrans-[Pd{C(=NC6H3-2,6-Me2)C6H4NH2-2}I(CNC6H3-2,6-Me2)], of which the C6H4NH2-2
moiety was replaced by the Me group. The
steric circumstance around the palladium in
5a is not crowded, and there is enough space
for the free rotation of the methyl groups in
C6H3-2,6-Me2 moieties. In sharp contrast, theortho-phenyl groups in 5b are apparently large,
and caused substantial steric repulsions

平成18年

九 州 大 学 大 学 院 総 合 理 工 学 報 告Aramong the ligands bonded to the palladium
atom. In other words, it is apparent thatcoordination of all of the ligands in 5b isaccomplished by the fact that the motion of thephenyl groups should be severely restricted tominimize the inter-ligand steric interactions.
This is accomplished by anisotropic structureof the phenyl groups which enable to alternate
their steric influence by appropriate rotation.
The importance of the anisotropic structure of
the phenyl groups in the C6H3-2,6-Ph2 groups
can be expanded to the explanation for thefacile insertion of CNC6H3-2,6-Ph2. Although
the phenyl group in CNC6H3-2,6-Ph2 is more
bulky
than
the
methylgroup
in
CNC6H3-2,6-Me2, the phenyl groups can
produce enough space for the insertion into the
Pd-Me bond by their appropriate rotations. In
other words, success of the CNC6H3-2,6-Ph2
ligand in the trans-[Br2Ni(CNAr)2]-catalyzed
polymerization of ethylene could be attributed
to the steric flexibility of CNC6H3-2,6-Ph2
derived from the anisotropic structure of thephenyl groups, which facilely produces thecatalytically active species by the insertion
reaction into the intermediary Ni-Me bond as
well as giving the appropriate steric influenceon the active species for the ethylene
polymerization. Importance of appropriate
steric influence of the ligand on the ethylene
polymerization has been well established in
the chemistry of the Brookhart’s catalysts.

Concluion
We reported in this paper a model study for
elucidatingthemechanismsoftrans-[Br2Ni(CNAr)2]-catalyzed
polymerization of ethylene. We believe that
insertion of the isocyanide ligand into a Ni-Me
bond formed by pretreatment of MAO withtrans-[Br2Ni(CNAr)2] plays an important rolein catalysis. The spectroscopic as well as the
crystallographic studies using palladium
homologues as models for this insertion
reaction solve a question whether the bulky
CNC6H3-2,6-Ph2 really inserts between the
Ni-Me bond. Although complete elucidation of
the polymerization mechanisms requires
further investigation, the results described in

N

第28巻
Ar

C

Me

Ni
C
Br

N

insertion

Ar

335

第3号
Ar

Me
N
C C
Ni
Br

N

Ar

N
C

Ni

C
N

Br

Me

Ar

6

Ar

Me
N
C C
Ni
n

Fig. 10

Ar
N

Ar
n

Me
N
C C
Ni
Me

Ar
N

8

Ar

MAO

Me
N
C C
Ni
Br

Ar
N

7

The possible mechanism for ethylene
polymerization of trans-[Br2Ni(CNAr)2]

this paper supports a possible mechanism
shown
in
Fig.
10.
Thus,
trans-[MeBrNi(CNAr)2] species formed by the
pretreatment of nickel catalyst with MAO is
followed by the insertion of CNAr into the
Ni-Me
bond.
The
resultingimidoylintermediate 6 reacts with ethylene to afford
the intermediate 7. Replacement of the Br
ligand by a methyl group is accomplished bymethylation with MAO to form 8, andinsertion of coordinated ethylene into theNi-Me bond of 8 initiates the ethylene
polymerization.
We are planning further elucidation of thepolymerization mechanisms and the catalyst
design to improve the catalytic activity andpolymerization behavior. The present study
suggests that model studies using palladium
will help for these future plans.
Acknowledgement
The authors are grateful to Mr. Masao
Tanabiki and memberes of TOSOH Co. Ltd.
forhelpful
discussions.
Helpforcrystallographic studies by Dr. Yusuke
Sunadaand
TaisukeMatsumotoisacknowledged.
Experimental section
General.: All experiments were carried out under anargon atmosphere. All solvents were distilled over CaH2prior to use (CD2Cl2 and CH2Cl2). All NMR experiments
were carried out using CD2Cl2 solution placed in a 5Φ
NMR tube and degassed several times. The tube wassealed in flame, while the solution was kept in a liquefied
nitrogen bath in vacuum. The 1H NMR spectra weretaken with a JEOL Lambda 400 spectrometer. Chemical
shifts were recorded in ppm from the internal standard(1H: solvent). IR spectra were recorded in cm-1 on a
JASCO FT/IR-550 spectrometer.

Spectroscopic and Crystallographic Studies on the Insertion Reaction of Aryl Isocyanides into the Bond between Palladium
336
and Carbon, which Contribute to Understanding the trans-[Br2Ni(CNAr)2]-Catalyzed Ethylene Polymerization

NMR identification of the species formed by thereaction of MePdCl(COD) with isocyanides:(I) MePdCl(COD) with two or three equivalents of
CNC6H3-2,6-Me2; In a NMR tube were placed
CNC6H3-2,6-Me2 (2.7 mg, 20 µmol, or 4.1 mg, 30 µmol)
and MePdCl(COD) (2.8 mg, 10 µmol), and then CD2Cl2(ca. 0.3 mL) was transferred to by vacuum.
Spectraldata
for[ClPd[C(Me)=NC6H3-2,6-Me2](CNC6H3-2,6-Me2)2] (5a); 1H
NMR (395MHz, CD2Cl2, r.t.); δ7.61 (d, 1H, Ph, J = 7.2Hz).
7.59 (d, 1H, Ph, J = 7.2Hz), 7.50-7.40 (m, Ph, 21H), 7.10
(d, 4H, J = 7.7Hz), 6.86-6.80 (m, 3H), 6.71 (t, 4H, Ph, J =7.7Hz), 6.60 (t, 2H, Ph, J = 7.7Hz), 0.98 (s, Me, 3H). IR:
νC≡N 2175, 1652 cm-1
(II) MePdCl(COD) with two or three equivalents of
CNC6H3-2,6-Ph2; The reaction of CNC6H3-2,6-Ph2 (7.7
mg, 30 µmol, or 12 mg, 45 µ mol) and MePdCl(COD) (4.0mg, 15 µmol) was carried out in similar fashion.
Isolationandcrystallographic
studyof
ClPd[C(Me)=NC6H3-2,6-Ph2](CNC6H3-2,6-Ph2)2 (5b); In
CH2Cl2 (10 mL), CNC6H3-2,6-Ph2 (123 mg, 0.48 mmol)and MePdCl(COD) (34.4mg, 0.13 mmol) were dissolved at-78oC, and the mixuture was stirred for 2h at room
temperature. The resulting solution was removed invacuo, and the residue was washed with hexane. The
desired complex 5b was formed as white powder.
Recrystallization
of
this
crude
product
fromCH2Cl2-hexane gave 5b as needle in 99% yield (119 mg).
1H NMR (395MHz CD2Cl2, r.t.); δ7.61 (d, 1H, Ph, J =7.2Hz). 7.59 (d, 1H, Ph, J = 7.2Hz), 7.50-7.40 (m, Ph, 21H),
7.10 (d, 4H, Ph, 7.7Hz), 6.86-6.80 (m, Ph, 3H), 6.71 (t, 4H,
Ph, J = 7.7Hz), 6.60 (t, 2H, Ph, J = 7.7Hz), 0.98 (s,Me, 3H). IR: νC≡N 2175, 1652 cm-1.
Crystallographic studies; X-ray crystallography was
performed on a Rigaku Saturn CCD area detector withgraphite monochromated Mo Kα radiation (λ = 0.71070
Å). The data were collected at 293(1) K using ω scans inthe θ range of -110.0° ≤ θ ≤ 70°. Data were collected andprocessed using CrystalClear (Rigaku) on a Pentium
computer. The data were corrected for Lorentz andpolarization effects. The structure was solved by direct
methods9) and expanded using Fourier techniques.10) The
non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were refined using the riding model.
The final cycle of full-matrix least squares refinement on
F2 was based on 10380 observed reflections and 610
variable parameters. Neutral atom scattering factors
were taken from Cromer and Waber.11) All calculations
were performed using the CrystalStructure12),13)
crystallographic software package. Details of final
refinement are summarized in Table 4, and thenumbering scheme employed is shown in Fig. 6.

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