Design of thermally stable amine-imine nickel catalyst precursors for living polymerization of ethylene: Effect of ligand substituents on catalytic behavior and polymer properties

Article abstract of DOI:10.1002/chem.201303813

Nickel complexes bearing amine-imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et₂AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine-imine nickel precursor with a tert-butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition-metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di- and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature. Living at 65 °C: A bulky amine-imine nickel catalyst precursor with a tert-butyl group on the bridging carbon atom of the amine moiety was designed and synthesized. On activation with Et₂AlCl, it catalyzes the living polymerization of ethylene even at 65 °C, which is the highest temperature of living polymerization of ethylene with late transition-metal catalysts. New di- and triblock PEs featuring different branched segments were precisely synthesized by tuning the reaction temperature (see figure).

Full text of DOI:10.1002/chem.201303813

DOI: 10.1002/chem.201303813
Full Paper
&
Olefin Polymerization
Design of Thermally Stable Amine–Imine Nickel Catalyst
Precursors for Living Polymerization of Ethylene: Effect of Ligand
Substituents on Catalytic Behavior and Polymer Properties
Haibin Hu, Lei Zhang, Haiyang Gao,* Fangming Zhu, and Qing Wu*[a]
Abstract: Nickel complexes bearing amine–imine ligands
with various backbone substituents were synthesized and
employed as ethylene polymerization catalysts on activation
with Et2AlCl. The substituent on the backbone carbon atom
of the amine moiety is decisive for the living nature of ethyl-
ene polymerization. A bulky amine–imine nickel precursor
with a tert-butyl group on the carbon atom of the amine
group can polymerize ethylene in a living fashion at an ele-
vated temperature of 658C, which is the highest tempera-
ture of living polymerization of ethylene with late transition-
metal catalysts. The wide applicable temperature range for
living polymerization and sensitivity of the branch structure
of the polyethylene to temperature enable precise synthesis
of di- and triblock polyethylenes featuring different
branched segments by sequential tuning of the polymeri-
zation temperature.
Introduction
living coordination polymerization of olefins remains a chal-
lenge because of the processes of chain termination/transfer
and poor thermal stability of catalysts, especially at elevated
temperature. The main limitations for living polymerization of
olefins with late transition-metal catalysts are scarce catalyst
systems and low practicable temperatures (<58C),[11] which re-
strict precise tuning of branch architecture. A noteworthy
sample is a cyclophane-based a-diimine nickel catalyst that
can polymerize propylene in a living fashion at 508C but ex-
hibits a nonliving mode of ethylene polymerization.[12] The
temperature of 508C is the highest reported in the literature
for living olefin polymerization with late transition-metal cata-
lysts.
A striking feature of olefin coordination polymerization is that
stereoselectivity, regiospecificity, and the architecture (density,
length, and topology) of branches can be controlled at will by
means of tailor-made catalysts.[1] The development of polyole-
fin materials with well-defined polymer structure and composi-
tion, stereochemistry, and branch architecture has become in-
creasingly attractive.[2] Early transition-metal catalysts have
been developed that give precise control over polymer stereo-
chemistry.[3] In contrast, late transition-metal catalysts, such as
a-diimine nickel and palladium catalysts, are outstanding in
control of branch topology due to the chain-walking process.[4]
Polyethylenes (PEs) with branch architectures ranging from
linear to hyperbranched can be readily prepared by tuning re-
action parameters and ligand structure.[4,5]
Recently, we reported that a bulky amine–imine nickel cata-
lyst precursor (1 in Scheme 1) affords PE with low polydispers-
ity (polydispersity index (PDI)<1.10) in a living fashion above
Living catalytic olefin polymerization is unsurpassed in terms
of microstructure control of polymers, which allows controlled
synthesis of polyolefins with precise structure and composi-
tion, such as monodisperse polymers and block copolymers.[6]
Combining the chain-walking characteristic of late transition-
metal catalysts with living polymerization can provide effective
access to the synthesis of well-defined block polyolefins with
novel architectures such as regioblock,[7] hyperbranched–
linear,[8] core–shell,[9] and large dendritic structures.[10] Currently,
Scheme 1. Amine–imine nickel complexes.
[a] Dr. H. Hu, L. Zhang, Dr. H. Gao, Prof. F. Zhu, Prof. Q. Wu
DSAPM Lab, PCFM Lab, Institute of Polymer Science
School of Chemistry and Chemical Engineering
Sun Yat-Sen University, Guangzhou 510275 (P.R. China)
Fax: (+86)20-84114033
room temperature.[13] Two methyl substituents on the back-
bone carbon atom (Me2CNH) increase the axial steric hindrance
of the amine–imine nickel complex, which seems to be respon-
sible for living polymerization of ethylene at high temperature.
This prompted us to elucidate the key site of amine–imine
nickel catalysts for living polymerization of ethylene. We found
that substituents on the carbon atom of the amine group of
E-mail: gaohy@mail.sysu.edu.cn
ceswuq@mail.sysu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303813.
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amine–imine nickel catalyst are decisive for living polymeri-
zation of ethylene, and developed a new amine–imine nickel
catalyst precursor (7 in Scheme 1) with a tert-butyl group on
carbon atom C1 of the amine group that can polymerize ethyl-
ene in a living fashion at 658C. The high thermal stability of
the amine–imine nickel precursors allowed us to regulate the
branch architecture of PE, and novel di- and triblock PEs fea-
turing different branched segments could be precisely synthe-
sized by tuning the living-polymerization temperature.
Results and Discussion
Synthesis and characterization of amine–imine nickel com-
plexes
Potentially, N,N-bidentate hybrid amine–imine ligands can ex-
hibit distinct influences of the amine and imine functional
groups on the metal center with regard to both polymer struc-
ture and reactivity control.[14] Therefore, amine–imine ligands
with various substituents on the carbon atoms of the imine
and amine groups were designed and synthesized in order to
evaluate substituent effects. The ligands were prepared by re-
duction of a-diimine compounds (see Supporting Information),
and new nickel complexes 2–8 (Scheme 1) were obtained by
addition of the ligands to a stirred suspension of (DME)NiBr2 in
CH2Cl2. Single crystals of 4, 5, and 7 suitable for X-ray diffrac-
tion analysis (Figure 1) were obtained by slow diffusion of
hexane into solutions of the nickel complexes in CH2Cl2.
The molecular structure of 7 clearly showed steric effects on
the different nitrogen atoms. Aryl moiety 2 of the imine (sp2 N)
is roughly perpendicular to the slightly distorted five-mem-
bered coordination plane (dihedral angle of 82.28), similar to
the a-diimine nickel analogue.[15] For the amine moiety of com-
plex 7, deflection of the bulky tert-butyl group to one side of
the slightly distorted chelate ring is indicative of an axial steric
effect on the metal center, which may effectively prohibit
chain-transfer reactions. Aryl moiety 1 of the amine group lies
on the other side of the chelate ring due to the distorted tetra-
hedral configuration of the amine nitrogen atom (sp3 N) and is
closer to the nickel center than aryl moiety 2 of the imine be-
cause of repulsion of the tert-butyl group. Complex 4 also
shows a large axial steric effect due to the introduction of
methyl and phenyl groups on the amine carbon atom. More-
over, aryl moiety 1 of the amine exerts equatorial hindrance on
coordination sites. On the contrary, complex 5 without sub-
stituents on the carbon atom adjacent to the amino group has
relatively open equatorial and axial environments around the
nickel center, which may facilitate both chain-propagation and
chain-transfer reactions.
Figure 1. Molecular structures of nickel complexes 4, 5, and 7 (50% proba-
bility ellipsoids, H atoms except H2N and H15 omitted).
haviors of amine–imine nickel complexes and classical a-di-
imine nickel complex [(ArN=C(Me)ÀC(Me)=NAr)NiBr2] (9, Ar=
2,6-diisopropylphenyl) in ethylene polymerization. Generally,
amine–imine nickel precursors show lower catalytic activity for
ethylene polymerization than the a-diimine nickel precursor,
but they can afford PE with lower polydispersity (Table 1, en-
tries 1–8 versus 9). In amine–imine nickel complexes 1–4, re-
placement of one of the methyl groups on bridging carbon
atom C1 with a longer alkyl substituent (Et or nBu) or a rigid
phenyl group leads to a decrease in both catalytic activity and
polymer molecular weight. Interestingly, all four catalysts can
polymerize ethylene in a living fashion at 508C and afford
branched polymers with almost the same molecular weight
distribution (PDI; Table 1, entries 1–4). This indicates that two
methyl substituents on the backbone carbon atom of the
amine moiety (Me2CNH) are bulky enough to inhibit chain-
transfer reactions in ethylene polymerization. Bulkier substitu-
ents lead to decreased activity because equatorial hindrance
exerted by aryl moiety 1 on the amine retards trapping and co-
ordination of ethylene. The molecular weight distribution of
the polymer obtained by using catalyst precursor 5, which has
no substituents on the amine-adjacent carbon atom C1, is
Effect of ligand substituents on ethylene polymerization
Ethylene polymerization was carried out with nickel complexes
1–8 after activation with Et2AlCl under the same reaction con-
ditions to probe substituent effects of the ligand backbone.
The polymerization results, which were reproducible, are listed
in Table 1. Firstly, it is informative to compare the different be-
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diimine nickel analogue.[15,17] In contrast to the a-diimine nickel
catalyst,[5a,b] amine–imine nickel catalysts contain different ni-
trogen atoms, which can provide additional tunable positions
to control polymerization reactivity.
Table 1. Ethylene polymerization with nickel complexes 1–9/Et2AlCl.[a]
[c]
Entry Cat. Tp [8C] Yield [g] Activity[b] Mn
PDI[c] Br[d] Tm[e] [8C]
[f]
1
2
3
4
5
6
7
8
9[g]
10
11
12
13
14
15[h]
16[h]
1
2
3
4
5
6
7
8
9
7
7
7
7
7
7
7
50
50
50
50
50
50
50
50
50
-40
20
35
65
75
65
65
0.358
0.162
0.151
0.047
1.226
0.728
0.305
0.277
2.411
0.035
0.211
0.279
0.306
0.229
1.352
2.967
35.8
16.2
15.1
4.7
122.6
72.8
30.5
27.7
241.1
3.5
21.1
27.9
30.6
22.9
541
48.2
21.2
20.0
10.8
46.7
72.9
41.1
29.4
1.08 96
1.07 99
1.09 108
1.08 102
1.56 85
1.07 57
1.02 50
1.02 56
–
–
–
–
–
[f]
[f]
[f]
[f]
Living polymerization of ethylene
As reported previously, the 1/Et2AlCl system catalyzes living
polymerization of ethylene at 508C for dozens of minutes and
then decays slightly.[13] Complexes 2–4 show similar catalytic
behavior to 1 in ethylene polymerization. Complexes 7 and 8
with bulky tert-butyl ligand substituents can polymerize ethyl-
ene to afford extremely low polydispersity PE (PDI=1.02) with
symmetric gel permeation chromatography (GPC) traces at an
elevated temperature of 508C after activation with Et2AlCl
(Table 1, entries 7 and 8, and Figure S1 in the Supporting Infor-
mation). The PDI values of the PEs obtained with 7/Et2AlCl and
8/Et2AlCl can be maintained below 1.10 over a reaction time-
scale of 3 h, and this indicating their high thermal stability. The
7/Et2AlCl system exhibits the highest thermal stability among
these catalyst systems, shows the highest activity at 658C, and
produces low-PDI PE below 658C. Thus, living polymerization
of ethylene catalyzed by 7/Et2AlCl can be achieved over a wide
range of reaction temperatures (Table 1, entries 7 and 10–14).
The relatively broad molecular weight distribution of the poly-
mer obtained at À408C may be attributed to slower initiation
of the active center and precipitation of linear PE at low tem-
perature. The living nature of ethylene polymerization with 7/
Et2AlCl at 658C was further investigated. As shown in Figure 2,
the number-average molecular weight Mn increased linearly
42, 73
50, 65
25
[f]
140.2 1.63 87
–
12.8
26.3
27.4
41.2
24.5
1.29 19
1.08 38
1.02 40
1.06 54
1.39 55
120
68, 86
57, 70
46, 60
50, 63
57
138.8 1.12 48
398.9 1.16 47
1187
61
[a] Polymerization conditions: 10 mmol of nickel, 60 min, 28 mL of tolu-
ene and 2 mL of CH2Cl2, Al(Et2AlCl)/Ni=200, 3 psig ethylene for entries 1–
À1
14. [b] Activity [kgPE mol
h ] and PDI were deter-
À1]. [c] Mn [kgmolÀ1
Ni
mined by GPC in 1,2,4-trichlorobenzene at 1508C with a light-scattering
detector. [d] Branching density in number of branches per 1000 C atoms,
determined by 1H NMR spectroscopy. [e] Determined by DSC. [f] Broad
melting endotherm. [g] 9 is classical a-diimine nickel complex [(ArN=
C(Me)ÀC(Me)=NAr)NiBr2] (Ar=2,6-diisopropylphenyl). [h] Polymerization
conditions: 10 mmol of nickel, 15 min, 58 mL of toluene and 2 mL of
CH2Cl2, Al(Et2AlCl)/Ni=200, 75 psig ethylene for entry 15 and 300 psig
ethylene for entry 16.
clearly broader (PDI=1.56), but it shows the highest activity
among the tested catalyst precursors (Table 1, entry 5), which
suggests that a lack of steric bulk on the adjacent carbon
atom leads to loss of control over the molecular weight distri-
bution of the polymer.
In an attempt to achieve both high activity and low polydis-
persity by introducing only a single substituent on C1, nickel
catalyst precursors 6–8 were designed and synthesized. Poly-
mers with low polydispersity could be obtained when iPr or
tBu was attached to C1 (6 and 7), and the catalytic activities
were considerable. In particular, catalyst precursor 7 afforded
PE with low polydispersity even at a high temperature of 658C.
Thus, this catalyst system is also a rare example of a thermally
stable late transition-metal catalyst.[5e,12,13,16] On the basis of
the evaluation of the ligand substituent effects, we concluded
that the substituents on carbon atom C1 adjacent to the
amine group of the amine–imine nickel catalyst are strategical-
ly located for production of low-polydispersity PE, presumably
due to effective prohibition of chain transfer/elimination be-
cause of axial steric hindrance on the metal center.
~
~
Figure 2. a) Plot of Mn ( ) and Mw/Mn ( ) as a function of polymerization
time at 658C. b) GPC traces at different times for 7/Et2AlCl.
with polymerization time, and Mw/Mn values remained below
1.10 for 1 h. To the best of our knowledge, this is the highest
temperature of living polymerization of ethylene with late tran-
sition-metal catalysts.[11,12]
The steric effect of the substituent on carbon atom C2 of
the imine moiety is distinct from that of the substituent on
carbon atom C1 of the amine moiety because of the two dif-
ferent coordinating groups [imine (sp2 N) versus amine (sp3
N)]. Comparison of complexes 7 and 8 (Table 1, entries 7 and
8) demonstrates that increasing steric hindrance of C2 leads to
an increase in polymerization activity and molecular weight of
the PE product. This is in contrast to the above-mentioned
steric effect of the amine moiety but similar to that of the a-
Effects of ligand substituents on branch structure of PE
Polymer branch structure can be also affected by substituents
on the different bridging carbon atoms. The branching density
of the obtained PE varies from 108 to 50 per 1000 C atoms on
tuning the substituent group on the carbon atom of the
amine moiety, and this provides a viable means to control
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polymer branch structure. A general trend is that introduction
of bulky substituents onto bridge carbon atom C1 of the
amine moiety leads to a decrease in total branch density
(Table 1, entries 1–8). 13C NMR experiments were conducted to
detect the branching distributions of the PEs (Figure S4 in the
Supporting Information), and detailed branching distributions
of the PEs obtained with various catalysts are listed in Table S2
(Supporting Information). The branch-on-branch structure gen-
erated through tertiary carbon atoms can be seen in the
methyl and ethyl resonances centered at 19.44 and 11.72 ppm
for the PEs obtained with 2–6/Et2AlCl as opposed to 1/Et2AlCl,
7/Et2AlCl, and 8/Et2AlCl. It is assumed that the branch structure
originates from the chain-walking mechanism involving repeti-
tive b-hydrogen elimination/reinsertion processes.[5a,18] Despite
the slight change in total branching density within the tested
temperature range for 7/Et2AlCl, the content of long branches
increased gradually with increasing temperature from 0/1000C
at À408C to 33/1000C at 758C, as revealed by 13C NMR spec-
troscopy (Table S2 in the Supporting Information), which sug-
gests a remarkable change in chain branch structure with var-
iation in temperature. Dilute-solution properties also enabled
us to examine the chain structure of PEs obtained at different
temperatures. The dependence of intrinsic viscosity on poly-
mer molecular weight detected by high-temperature GPC with
a viscosity detector can directly reflect branch structure of the
PE. As shown in Figure 3, the highest intrinsic viscosity was ob-
served for the PE obtained at À408C, and the lowest for the
of the PE obtained with 7/Et2AlCl is insensitive to ethylene
pressure at high temperature (658C; Table S2 in the Supporting
Information), which suggests that reaction temperature plays
a dominant role in the chain-walking process for the 7/Et2AlCl
system. A basic trend is that the content of methyl groups in-
creases while the content of long branches decreases with in-
creasing ethylene pressure.
Synthesis of block PE featuring different branched seg-
ments
Living polymerization of ethylene over a wide range of tem-
perature opens a way for precise synthesis of monodisperse PE
with various branch structures and corresponding block poly-
mers by changing the reaction temperature (Scheme 2).
Amine–imine nickel catalyst precursors 1 and 7 were selected
Scheme 2. Synthesis of block PE featuring different branched segments by
sequential tuning of polymerization temperature.
to prepare block polyethylenes featuring different branched
segments due to their enormously different chain-walking abil-
ities. When ethylene polymerization catalyzed by 7/Et2AlCl at
658C was sequentially followed by a second stage of polymeri-
zation at À408C, low-polydispersity diblock PE (PDI=1.18)
with different branched segments was obtained (Table 2,
entry 3). Shifting of the GPC trace to lower retention times
without obvious tail or shoulder peaks (Figure 4a) proves the
successful occurrence of chain-extension reactions. The ob-
tained diblock PE shows two melting temperatures of 45 and
115 8C (Figure 4b), corresponding to the PE segments obtained
at 65 and À408C, respectively. Moreover, the intrinsic viscosity
of the diblock polyethylene is intermediate between those of
Figure 3. Dependence of intrinsic viscosity on molecular weight of the PEs
obtained by using 7/Et2AlCl at various reaction temperatures.
Table 2. Synthesis of block PEs with 1/Et2AlCl and 7/Et2AlCl.[a]
[b]
Entry Cat. t1 [min]/
T1 [8C]
t2 [min]/
T2 [8C]
t3 [min]/
T3 [8C]
Mn PDI[b] Tm[c] [8C]
PE produced at 658C. The Mark–Houwink exponents a calculat-
ed from the slope of the intrinsic viscosity curves are 0.70 and
0.23 for the PEs obtained at À408C and 658C, respectively.[9,19]
This result shows that the 7/Et2AlCl system affords linear PE at
À408C but highly branched PE at 658C, which is consistent
with differential scanning calorimetric (DSC) analysis of poly-
mer products.[20,21]
1
2
3
4
5
6
7
8
9
7
7
7
1
1
1
1
1
1
15/65
60/À40
15/65
15/50
60/À40
15/50
–
–
–
–
–
–
–
–
12 1.03 46, 60
13 1.29 120
60/À40
–
–
60/À40
60/À40
15/50
15/50
26 1.18 45, 115
[d]
12 1.02 –
14 1.07 97
27 1.09 97
39 1.11 89
29 1.07 93
42 1.16 93
15/50
15/50
–
60/À40
Moreover, increasing ethylene pressure from atmospheric
pressure of 15 psig to 300 psig (Table 1, entries 13, 15, and 16)
for 7/Et2AlCl system leads to a significant increase in activity
and affords higher molecular weight PE with slightly higher
polydispersities. Usually, increasing ethylene pressure leads to
more linear products.[5c] In contrast, the total branching density
60/À40
60/À40
[a] Polymerization conditions: 10 mmol nickel, 28 mL of toluene and 2 mL
of CH2Cl2, Al(Et2AlCl)/Ni=200, 3 psig ethylene. [b] Mn [kgmolÀ1] and PDI
were determined by GPC in 1,2,4-trichlorobenzene at 1508C with a light-
scattering detector. [c] Determined by DSC. [d] Broad melting endotherm.
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temperature, which is hardly achievable with other catalyst sys-
tems. These block PE materials with hybrid branch structures
are readily amenable to applications such as compatibilizers,
thermoplastic elastomers, and high-impact plastics.[20d]
Conclusion
The two coordinating functionalities [imine (sp2) and amine
(sp3)] of amine–imine nickel catalysts exert distinctive effects
and reactivity control on ethylene polymerization, and bulky
substituents on C1 of the amine group is strategically located
for stability and living polymerization of ethylene. Bulky
amine–imine nickel complex 7 activated by Et2AlCl can poly-
merize ethylene in a living fashion with unprecedented molec-
ular-weight control (PDI=1.02) and thermal stability (658C).
This is the highest temperature of living polymerization of eth-
ylene with a late transition-metal catalyst. These long-lived and
robust living-polymerization systems provide a viable access to
the practical design of polyethylene with various branch archi-
tectures. Well-defined di- and triblock PEs (PDI<1.20) with dif-
ferent branched segments were prepared from ethylene mo-
nomer solely by varying reaction temperature. The amine–
imine nickel catalyst system developed herein will provide ad-
ditional possibilities for precise synthesis of monodisperse PEs,
PE block polymers, and functional PEs with various branch
structures.
Figure 4. a) GPC and b) DSC curves of PE and diblock PE polymers produced
by 7/Et2AlCl.
the branched and linear segments, as indicated by the de-
pendence of intrinsic viscosity on polymer molecular weight
(Figure 3). Therefore, it is deduced that the obtained diblock
PE has a linear–highly branched (L–HB) diblock structure.
Similarly, well-defined diblock PE (PDI=1.09) containing
amorphous and semicrystallized segments with a broad melt-
ing endotherm at À88C and a sharp melting peak at 978C was
synthesized by using 1/Et2AlCl (entry 6 in Table 2 and Fig-
ure 5a) for sequential polymerization of ethylene at 50 and at
À408C. Interestingly, the branch architecture of the polymer
Experimental Section
General remarks
All manipulations involving air- and moisture-sensitive compounds
were carried out under an atmosphere of dried and purified nitro-
gen with standard vacuum-line, Schlenk, or glovebox techniques.
Elemental analyses were performed on a Vario EL microanalyzer.
Fast atom bombardment (FAB) mass spectra of nickel complexes
were obtained on an LCQ DECA XP instrument. NMR spectra of or-
ganic compounds were recorded on a Bruker 300 MHz instrument
in CDCl3 with TMS as reference. 13C NMR spectra of polymers were
recorded on an INOVA 400 MHz spectrometer at 1208C by using o-
C6D4Cl2 as solvent. DSC analyses were conducted with a PerkinElmer
DSC-7 system. The DSC curves were recorded as second heating
curves from À100 to 1408C at a heating rate of 108CminÀ1 and
a cooling rate of 108CminÀ1. GPC analysis of the molecular weights
and molecular weight distributions (PDI=Mw/Mn) of the polymers
at 1508C were performed on a PL-GPC 220 high-temperature chro-
matograph equipped with a triple-detection array, including a dif-
ferential refractive-index detector, a two-angle light-scattering de-
tector, and a four-bridge capillary viscometer. The detection angles
of the LS detector were 15 and 908, and the laser wavelength was
658 nm. 1,2,4-Trichlorobenzene (TCB) was used as the eluent at
Figure 5. GPC and DSC curves of PE and block PE polymers obtained with 1/
Et2AlCl. a) Branched-first strategy (HB–L–HB). b) Linear-first strategy (L–HB–L).
product remains almost the same on changing the synthetic
approach from branched-first (high temperature first; Table 2,
entry 6) to linear-first (low temperature first; Table 2, entry 8).
This is distinctive from the a-diimine palladium catalyst, for
which the synthetic procedure is restricted to branched-first, as
walking back of the active center to the polymer chain oc-
curred when the linear segment was prepared first.[8] Triblock
PE polymers can be also prepared by sequentially varying reac-
tion temperature in the long-lived living polymerization of eth-
ylene with 1/Et2AlCl. An obvious increase in Mn, shifting GPC
traces, and two distinctive endothermic peaks in DSC curves
clearly support successful synthesis HB–L–HB or L–HB–L tri-
block PE polymers with low polydispersities (Figure 5). Our
study provides a different access to precise synthesis of di- and
triblock PE containing both amorphous and semicrystallized
segments from ethylene monomer solely by changing reaction
a flow rate of 1.0 mLminÀ1
.
Materials
Dichloromethane was distilled from CaH2 under nitrogen, and tolu-
ene and hexane were distilled from Na/K alloy. Glyoxal (40wt%
aqueous solution), pyruvaldehyde (35wt% aqueous solution), 2,6-
diisopropylaniline, and (DME)NiBr2 were purchased from Aldrich
and used as received. Diethylaluminum chloride (DEAC, 1.0m in
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hexane), was purchased from Acros. Ethylene (99.99%) was puri-
fied by passing through Agilent moisture and oxygen traps. Other
commercially available reagents were purchased and used without
purification. Classical a-diimine nickel complex 9 and amine–imine
nickel complex 1 were prepared according to reported meth-
ods.[5c,13]
lected by filtration, washed with methanol several times, and dried
in vacuum at 408C to constant weight. The other block polyethy-
lenes were prepared by similar procedures.
Synthesis of a-diimine compounds
a-Diimine compounds ArN=C(Me)ÀCH=NAr and ArN=CHÀCH=NAr
(Ar=diisopropylphenyl) were prepared according to the literature.
For detailed procedures and characterization of a-diimine com-
pounds, see the Supporting Information.
Typical ethylene polymerization procedure
A round-bottom Schlenk flask with stirring bar was heated for 3 h
at 1508C under vacuum and then cooled to room temperature.
The flask was pressurized to 15 psi of ethylene and vented three
times. The appropriate alkyl aluminum compound as cocatalyst
was introduced into the glass reactor under 3 psi of ethylene. The
system was continuously stirred for 5 min, and then toluene and
2 mL of a solution of nickel complex in CH2Cl2 were added sequen-
tially by syringe to the well-stirred solution, and the total reaction
volume was kept at 30 mL. The ethylene pressure was kept con-
stant at 3 psi by continuous feeding of gaseous ethylene through-
out the reaction. Except for À408C, maintained with a cooler, the
other polymerization temperatures were controlled with an exter-
nal oil bath. The polymerizations were terminated by the addition
of 200 mL of acidic methanol (ethanol/HCl 95/5) after continuous
stirring for an appropriate period. The resulting precipitated poly-
mers were collected by filtration, washed with methanol several
times, and dried in vacuum at 408C to constant weight.
Synthesis of ArN=C(Me)ÀC(Ph)=NAr (Ar=diisopropylphenyl)
PhLi was prepared according to the literature procedure by reac-
tion of n-butyllithium with bromobenzene.[22] The freshly prepared
solution of PhLi (5.8 mL, 1.7 m in THF) was added dropwise to solu-
tion of a-diimine (ArÀN=C(Me) ÀCH=NÀAr (Ar=diisopropylphen-
yl) (3.83 g, 9.8 mmol) in diethyl ether (20 mL) under nitrogen at-
mosphere at 08C. The mixture was stirred overnight at room tem-
perature and terminated with a concentrated aqueous solution of
NH4Cl. The organic layer was separated and dried over ethyl ace-
tate using anhydrous MgSO4 as drying agent. The product was ob-
tained as yellow crystals in 24.5% yield after removal of solvent
and recrystallization from hot ethanol. 1H NMR (300 MHz, CDCl3):
d=7.26–7.03 (m, 11H, ArH), 2.88–2.74 (m, 4H, CH(CH3)2), 1.27–0.99
(d, 24H, CH(CH3)2), 0.96–0.93 ppm (d, 3H, CCH3); 13C NMR (75 MHz,
CDCl3): d=169.95 (C=N), 166.14 (C=N), 146.39, 145.46, 135.51,
135.14, 129.39, 129.16, 127.82, 124.08, 123.18, 29.09, 28.89, 24.04,
23.58, 23.36, 22.40, 18.97 ppm (CÀCH3); elemental analysis calcd
(%) for C33H42N2: C 84.80, H 8.90, N 6.00; found: C 84.93, H 9.07, N
6.01.
High-pressure polymerization of ethylene
A mechanically stirred 100 mL Parr reactor was heated to 1508C
for 2 h under vacuum and then cooled to room temperature. The
autoclave was pressurized to 50 psi of ethylene and vented three
times. The autoclave was then charged with 58 mL of a solution of
Et2AlCl in toluene under 50 psi of ethylene at initialization tempera-
ture. The system was continuously stirred for 5 min, and then 2 mL
of a solution of nickel complex in CH2Cl2 was charged into the
autoclave. The ethylene pressure was raised to the specified value,
and the reaction was carried out for a certain time. Polymerization
was terminated by addition of acidic methanol after releasing eth-
ylene pressure. The resulting precipitated polymers were collected
by filtration, washed with methanol several times, and dried under
vacuum at 408C to constant weight.
Synthesis of ArN=C(Me)ÀC(Me)(Et)ÀNHAr (Ar=2,6-diisopro-
pylphenyl; L2)
Under nitrogen atmosphere, a solution of ArN=C(Me)ÀC(Me)=NAr
(2.42 g, 6 mmol) in anhydrous diethyl ether (20 mL) was introduced
into a 100 mL Schlenk flask, and then diethylzinc (4 mL, 1.5m in
hexane) was injected slowly by syringe at room temperature. The
reaction mixture was stirred at room temperature for 2 h. After the
solution was cooled to 08C in an ice/water bath, the reaction mix-
ture was carefully hydrolyzed by adding ice/water. The organic
layer was separated and dried over MgSO4, and the solvent was
evaporated. The desired product was obtained by slow evapora-
tion of solvent after addition of ethanol to the residual oil. The
crude product was purified by recrystallization from hot ethanol to
give colorless crystals in 53% yield (1.37 g). 1H NMR (300 MHz,
CDCl3): d=7.16–7.04 (m, 6H, ArH), 5.00 (s, 1H, NH), 3.55 (sept, 2H,
CH(CH3)2), 2.88 (sept, 1H, CH(CH3)2), 2.80 (sept, 1H, CH(CH3)2), 1.90
(m, 1H, CH2CH3), 1.78 (m, 1H, CH2CH3), 1.78 (s, 3H, N=CCH3), 1.24–
1.16 (m, 24H, CH(CH3)2, 1.07 (s, 3H, CH3), 1.06 ppm (t, 3H, CH3);
13C NMR (75 MHz, CDCl3): d=174.80, 147.21, 145.77, 140.70, 136.63,
136.00, 124.34, 123.39, 12 3.13, 123.00, 64.88, 32.70, 28.10, 27.97,
27.79, 24.62, 24.48, 24.29, 24.13, 23.69, 23.39, 16.74, 8.59 ppm; ele-
mental analysis calcd (%) for C30H46N2: C 82.89, H 10.67, N 6.44;
found: C 82.64, H 10.43, N 6.21.
Synthesis of block polyethylene
Typical procedure for highly branched–linear diblock polyethylene:
A round-bottom Schlenk flask with stirring bar was heated for 3 h
at 1508C under vacuum and then cooled to room temperature.
The flask was pressurized to 15 psi of ethylene and vented three
times. The appropriate alkyl aluminum compound as cocatalyst
was added to the flask under 3 psi of ethylene. The system was
continuously stirred for 5 min, and then toluene and 2 mL of a solu-
tion of nickel complex in CH2Cl2 were added sequentially by sy-
ringe to the well-stirred solution, and the total reaction volume
was kept at 30 mL. The ethylene pressure was kept constant at
3 psi. After ethylene polymerization for 15 min, the ethylene feed
was stopped, and the reaction flask was charged with N2 and
vented three times. The flask was cooled to À408C by using
a cooler and kept at that temperature for about 60 min, and then
the flask was exposed to vacuum to remove the N2 and pressured
to 3 psi of ethylene. After reaction for 60 min, the pressure was re-
leased and the polymerization was terminated by addition of
200 mL of acidic methanol (ethanol/HCl 95/5) after continuous stir-
ring for an appropriate period. The precipitated polymer was col-
Synthesis of ArN=C(Me)ÀC(Me)(nBu)ÀNHAr (Ar=diisopropyl-
phenyl; L3)
Under nitrogen atmosphere, a solution of ArN=C(Me)ÀC(Me)=NAr
(2.04 g, 5.05 mmol) in anhydrous toluene (20 mL) was introduced
into a 100 mL Schlenk flask, and then n-butyllithium (3.2 mL, 1.6m
in hexane) was injected slowly by syringe at room temperature.
Chem. Eur. J. 2014, 20, 3225 – 3233
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The reaction mixture was stirred at room temperature for 2 h. After
the solution was cooled to 08C in an ice/water bath, the reaction
mixture was carefully hydrolyzed by adding ice/water. The organic
layer was separated and dried over MgSO4, and the solvent was
evaporated. The desired product was obtained by slow evapora-
tion of solvent after addition of ethanol to the residual oil. The
crude product was purified by recrystallization from hot ethanol to
Synthesis of ArN=C(Me)ÀCH(iPr)ÀNHAr (Ar=diisopropyl-
phenyl; L6)
Following the procedure used for ligand L3, the reaction of
iPrMgBr and a-diimine compound ArN=C(Me)ÀCH=NÀAr (Ar=dii-
sopropylphenyl) gave ligand L6 as colorless crystals in 83% yield.
1H NMR (300 MHz, CDCl3): d=7.08–6.86 (m, 6H, ArH), 4.10 (d, 1H,
NH), 3.42–2.39 (m, 4H, CH(CH3)2), 2.53–2.39 (m, 2H), 2.21–2.15 (m,
1H, CHÀNH), 2.27–2.18 (m, 1H), 1.29–1.20 (m, 20H, CH(CH3)2), 1.18–
1.00 ppm (d, 14H, CCH3); 13C NMR (75 MHz, CDCl3): d=172.31 (C=
N), 146.33 (CArÀN), 142.18 (CArÀNH), 139.60, 136.45, 136.28, 123.83,
123.44, 123.23, 122.97, 121.93, 71.59 (CÀNH), 36.92 (C(CH3)3), 28.61,
28.18, 27.97, 24.49, 24.36, 24.26, 24.00, 23.33, 20.30, 19.80,
19.38 ppm (CÀCH3); elemental analysis calcd (%) for C30H46N2: C
82.77, H 10.51, N 6.41; found: C 82.89, H 10.67, N 6.44.
1
give white crystals in 29% yield (0.68 g). H NMR (300 MHz, CDCl3):
d=7.17–7.04 (m, 6H, ArH), 4.96 (s, 1H, NH), 3.52 (sept, 2H,
CH(CH3)2), 2.90 (sept, 1H, CH(CH3)2), 2.79 (sept, 1H, CH(CH3)2), 1.78
(s, 3H, CH3), 1.32 (m, 2H, CH2), 1.23–1.16 (m, 24H, CH3), 1.08 (s, 3H,
CH3), 0.94 ppm (t, 3H, CH3); 13C NMR (75 MHz, CDCl3): d=175.05,
146.97, 145.72, 140.81, 136.66, 135.95, 124.32, 123.41, 123.13,
123.00, 64.56, 40.59, 28.14, 27.88, 27.77, 26.79, 24.80, 24.58, 24.34,
24.05, 23.77, 23.69, 23.51, 23.11, 16.82, 14.45 ppm; elemental analy-
sis calcd (%) for C32H50N2: C 83.06, H 10.89, N 6.05; found: C 82.95,
H 10.63, N 6.10%.
Synthesis of ArN=C(Me)ÀCH(tBu)ÀNHAr (Ar=diisopropyl-
phenyl; L7)
Synthesis of ArN=C(Me)ÀC(Me)(Ph)ÀNHAr (Ar=diisopropyl-
tBuMgCl was prepared according to the reported procedure and
used freshly for subsequent reaction without further treatment.[23]
A solution of ArN=C(Me)ÀCH=NAr (2.25 g, 6 mmol) in anhydrous
diethyl ether (10 mL) was added to the stirred Grignard reagent
(60 mL, 1.2m in diethyl ether) by syringe. The mixture was allowed
to stir overnight at room temperature. The reaction was terminat-
ed by pouring into a concentrated aqueous solution of NH4Cl. The
organic layer was separated and dried over MgSO4. A crude prod-
uct was obtained after removal of organic solvent and purified by
recrystallization from hot ethanol to give ligand L7 as a colorless
crystals in 71% yield. 1H NMR (300 MHz, CDCl3): d=7.05–6.86 (m,
6H, ArH), 4.22 (d, 1H, NH), 4.12–3.30 (m, 4H, CH(CH3)2), 2.44–2.53
(m, 1H, CHÀNH), 2.27–2.18 (m, 1H), 1.29–1.21 (m, 24H, CH(CH3)2),
1.00–0.93 ppm (d, 11H, CCH3); 13C NMR(75 MHz, CDCl3): d=173.00,
146.61, 142.32, 138.43, 136.37, 36.05, 123.51, 123.31, 123.13,
121.42, 74.12, 36.95, 28.76, 28.30, 28.07, 24.76, 24.39, 23.97, 23.36,
22.02 ppm; elemental analysis calcd (%) for C31H48N2: C 82.98, H
10.78, N 6.24; found: C 82.89, H 10.80, N 6.17.
phenyl; L4)
Under nitrogen atmosphere, a solution of ArN=C(Me)ÀC(Ph)=NAr
(0.93 g, 2 mmol) in toluene (30 mL) was introduced into a 100 mL
Schlenk flask, and then trimethylaluminum (1 mL, 2.0m in toluene)
was injected slowly by syringe at room temperature. The reaction
mixture was heated to reflux for 4 h. After the solution was cooled
to 08C in an ice/water bath, the reaction mixture was carefully hy-
drolyzed with 5% aqueous NaOH solution. The organic layer was
separated and dried over MgSO4, and the solvent was evaporated.
The crude product was recrystallized from hot ethanol to obtain
ligand L4 as colorless crystals in 85.0% yield. 1H NMR (300 MHz,
CDCl3): d=7.70 (d, 2H, phenyl a-H), 7.40–7.07 (m, 9H, ArH), 6.45 (s,
1H, CNH), 3.17–2.83 (m, 4H, CH(CH3)2), 1.54–1.52 (d, 3H, C(CH3)),
1.44–1.43 (d, 3H, C(CH3)), 1.31–0.96 ppm (m, 24H, CH(CH3)2);
13C NMR (75 MHz, CDCl3): d=174.10 (C=N), 147.28 (CArÀN), 145.57,
141.83, 137.44, 136.77, 128.19, 127.66, 127.22, 124.09, 123.80,
123.42, 66.19 (CÀNH), 28.68, 28.22, 25.08, 24.33, 24.24, 23.94, 0.79,
17.72 ppm; elemental analysis calcd (%) for C34H46N2: C 84.68, H
9.42, N 5.82; found: C 84.59, H 9.60, N 5.80.
Synthesis of ArN=CHÀCH(tBu)ÀNHAr (Ar=diisopropylphen-
yl; L8)
Synthesis of ArN=C(Me)ÀCH2ÀNHAr (Ar=diisopropylphenyl;
L5)
Following the procedure used for ligand L7, the reaction of
tBuMgCl and ArN=CHÀCH=NAr gave ligand L8 as colorless crystal
Under nitrogen atmosphere,
a
solution of ArN=CHÀCH=NAr
1
in 68% yield. H NMR (300 MHz, CDCl3): d=7.52–7.50 (d, 1H, HC=
(2.82 g, 7.5 mmol) in toluene (50 mL) was introduced into a 100 mL
Schlenk flask, and then trimethylaluminum (6 mL, 2.0m in toluene)
was injected slowly by syringe at room temperature. The reaction
mixture was heated to reflux overnight. After the solution was
cooled to 08C in an ice/water bath, the reaction mixture was care-
fully hydrolyzed with 5% aqueous NaOH solution. The organic
layer was separated and dried over MgSO4, and the solvent was
evaporated. The desired product was obtained by slow evapora-
tion of solvent after addition of ethanol to the residual oil. The
crude product was purified by reisolation from hot ethanol to give
L5 as a light yellow liquid in 93% yield. The product was character-
ized as the methyl-transfer compound, as reported previously by
N), 7.13–6.96 (m, 6H, ArH), 4.00–3.98 (d, 1H, NH), 3.46–2.66 (m, 4H,
CH(CH3)2), 2.34–2.25 (m, 1H, HCÀNH), 1.29–1.13 (m, 24H, CH(CH3)2),
0.93–0.90 ppm (m, 9H, C(CH3)3); 13C NMR(75 MHz, CDCl3): d=
167.79, 149.06, 141.25, 140.63, 137.44, 124.04, 122.88, 71.78, 35.98,
28.61, 27.75, 27.58, 24.61, 24.07, 23.37 ppm; elemental analysis
calcd (%) for C30H46N2: C 82.89, H 10.67, N 6.44; found: C 82.97, H
10.54, N 6.49.
Synthesis of [NiBr2(L2)] (2)
Ligand L2 (434 mg, 1 mmol) in dichloromethane (10 mL) was
added to a stirred suspension of (DME)NiBr2 (308 mg, 1 mmol) in
dichloromethane (30 mL) at room temperature. Shortly after the
addition of ligand, the solution began to turn brown. The suspen-
sion was allowed to stir for an additional 5 h at room temperature.
The solution was filtered through Celite, and the solvent of the fil-
trate was removed in vacuum. The residue was recrystallized from
CH2Cl2/hexane to give complex 2 as a light brown powder in 45%
yield. Elemental analysis calcd (%) for C30H46Br2N2Ni: C 55.16, H
1
others. H NMR (300 MHz, CDCl3): d=7.19–7.04 (m, 6H, ArH), 5.04
(s, 1H, NH), 3.99 (s, 2H, CH2), 3.45 (septet, 2H, CH(CH3)2), 2.78
(septet, 2H, CH(CH3)2), 1.72 (s, 3H, CH3), 1.29 (d, 12H, CH3), 1.19–
1.15 ppm (m, 12H, CH3); 13C NMR (75 MHz, CDCl3): d=167.48,
145.48, 144.59, 141.26, 136.58, 123.62, 123.48, 123.03, 122.79,
58.25, 28.06, 24.14, 23.62, 23.01, 19.24 ppm; elemental analysis
calcd (%) for C27H40N2: C 86.00, H 8.58, N 5.42; found: C 86.03, H
8.41, N 5.48.
Chem. Eur. J. 2014, 20, 3225 – 3233
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7.10, N 4.29; found: C 55.29, H
7.06, N 4.39; MS (FAB): m/z=573,
572, 571 [MÀBr]+; 494, 493, 492
[MÀ2Br]+; 436, 435, 433 [ligand]+.
Table 3. Crystal data and structure refinement for nickel complexes 4, 5, and 7.
4
5
7
empirical formula
formula weight
C34H46Br2N2Ni
701.26
C27H37Br2N2Ni
608.12
C31H48Br2N2Ni
667.24
Synthesis of [NiBr2(L3)] (3)
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
monoclinic
P21/c
12.753(2)
10.4887(18)
24.757(4)
90
90.413(3)
90
3311.5(10)
4
orthorhombic
Pnma
12.587(5)
21.541(9)
10.380(5)
90
90
90
2815(2)
4
triclinic
ꢀ
P1
Following the above-described
procedure, the reaction of (DME)-
NiBr2 and L3 gave complex 3 in
68% yield. Elemental analysis calcd
(%) for C32H50Br2N2Ni: C 56.42, H
7.40, N 4.11; found: C 56.69, H
7.51, N 3.91; MS (FAB): m/z=601,
600, 599 [MÀBr]+; 522, 521, 520
[MÀ2Br]+; 464, 463, 462 [ligand]+.
9.9228(13)
16.740(2)
19.513(3)
83.465(2)
79.838(2)
83.492(2)
3155.3(7)
4
a [8]
b [8]
g [8]
V [ꢁ3]
Z
1calcd [gcmÀ3
F(000)
]
1.407
1448
1.435
1244
1.405
1384
crystal size [mm]
q range [8]
index ranges
0.44ꢃ0.31ꢃ0.24
1.60–27.06
À16ꢀhꢀ12
À13ꢀkꢀ9
À31ꢀlꢀ31
16448/7146
0.0301
0.22ꢃ0.20ꢃ0.15
1.89–26.42
À15ꢀhꢀ15
À26ꢀkꢀ26
À11 ꢀlꢀ12
21225/2935
0.0780
0.41ꢃ0.35ꢃ0.10
1.23–27.09
À12ꢀhꢀ12
À21ꢀkꢀ21
À24ꢀlꢀ24
26674/13483
0.0311
Synthesis of [NiBr2(L4)] (4)
Following the above-described
procedure, the reaction of (DME)-
NiBr2 and L4 gave complex 4 in
71% yield. Elemental analysis calcd
(%) for C34H46Br2N2Ni: C 54.75, H
6.57, N 3.74; found: C 55.03, H
6.61, N 3.89; MS (FAB): m/z=619,
621, [MÀBr]+; 538, 539, 540, [MÀ
2Br]+; 481, 483, 484, [MÀNiBr2]+.
reflns collected/unique
Rint
data completeness
Transmission (max./min.)
data/restraints/parameters
goodness of fit on F2
R indices [I>2s(I)]
98.1%
99.1%
97.0%
0.5307/0.3497
7146/0/377
1.046
0.618/0.509
2935/0/165
1.186
0.7423/0.3566
13483/0/709
1.059
R1 =0.0384, wR2 =0.0930 R1 =0.0754, wR2 =0.1512 R1 =0.0377, wR2 =0.0942
R1 =0.0619, wR2 =0.1033 R1 =0.1099, wR2 =0.1617 R1 =0.0695, wR2 =0.1090
0.950/À0.378
R indices (all data)
largest diff. peak/hole [eꢁÀ3
]
0.850/À1.592
1.378/À0.506
Synthesis of [NiBr2(L5)] (5)
Following the above-described
in the w–2q scan mode with graphite-monochromated MoKa radia-
tion (l=0.71073 ꢁ) at 293 K. The structures were solved by direct
methods, and further refinement by full-matrix least-squares meth-
ods on F2 was performed with the SHELXTL program package. All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were introduced in calculated positions with the displacement fac-
tors of the host carbon atoms. Crystal data and structure-refine-
ment parameters are listed in Table 3. CCDC 961650 (4), 961651 (5)
and 961652 (7) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
procedure, the reaction of (DME)NiBr2 and L5 gave complex 5 in
86% yield. Elemental analysis calcd (%) for C27H40Br2N2Ni: C 53.06,
H 6.60, N 4.58; found: C 52.87, H 6.67, N 4.46; MS (FAB): m/z=531,
532 [MÀBr]+; 450, 451, 452 [MÀ2Br]+; 392, 393, 394 [ligand]+.
Synthesis of [NiBr2(L6)] (6)
Following the above-described procedure, the reaction of (DME)-
NiBr2 and L6 gave complex 6 in 65% yield. Elemental analysis
calcd (%) for C30H46Br2N2Ni: C 54.22, H 7.06, N 4.18; found: C 54.16,
H 7.10, N 4.29; MS (FAB): m/z=572, 573, 574 [MÀBr]+; 492, 493
[MÀ2Br]+; 434, 435, 436 [ligand]+.
Synthesis of [NiBr2(L7)] (7)
Acknowledgements
Following the above-described procedure, the reaction of (DME)-
NiBr2 and L7 gave complex 7 in 92% yield. Elemental analysis
calcd (%) for C31H48Br2N2Ni: C 55.80, H 7.25, N 4.20; found: C 55.56,
H 7.35, N 4.08; MS (FAB): m/z=587, 586 [MÀBr]+; 506, 505 [MÀ
2Br]+; 451, 450, 449, 448 [ligand]+.
Financial support by NSFC (projects 21174164, 51173209,
21274167, and 21374134) is gratefully acknowledged.
Keywords: N ligands · nickel · polymerization · polymers ·
substituent effects
Synthesis of [NiBr2(L8)] (8)
Following the above-described procedure, the reaction of (DME)-
NiBr2 and L8 gave complex 8 in 83% yield. Elemental analysis
calcd (%) for C30H46Br2N2Ni: C 55.16, H 7.10, N 4.29; found: C 54.91,
H 7.18, N 4.04; MS (FAB): m/z=573, 574 [MÀBr]+; 491, 492, 493
[MÀ2Br]+; 434, 435, 436 [ligand]+.
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Chem. Eur. J. 2014, 20, 3225 – 3233
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Received: September 30, 2013
Revised: October 31, 2013
Published online on February 12, 2014
Chem. Eur. J. 2014, 20, 3225 – 3233
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