Highly preorganized pyrazolate-bridged palladium(ii) and nickel(ii) complexes in bimetallic norbornene polymerization

Article abstract of DOI:10.1039/b925535d

New derivatives of pyrazolate-based binucleating ligands HL with appended imine functions have been synthesized to provide a versatile set of ligand systems with different backbone substituents both at the pyrazole-C⁴ and the imine-C (H, Me, Ph). These scaffolds have two adjacent coordination compartments akin to the α-diimine type. A series of binuclear palladium(ii) complexes [LPd₂Cl₃] (1-4) and tetranuclear nickel(ii) complexes [L₂Ni₄Br₆(solvent) ₄] (5, 6) of the various ligands have been prepared and characterized, including X-ray structural analyses for two representative Pd and the two Ni complexes. Complexes 5 and 6 were found to contain an unusual central μ₄-bromide. Mononuclear nickel(ii) complexes [L ₂Ni] were detected as intermediates in the formation of the tetranuclear complexes and have been characterized by X-ray analyses in two cases (7, 8). The interconversion between 5′ and 7 has been investigated by UV/Vis spectroscopy and ESI mass spectrometry, and magnetic coupling in the [L₂Ni₄Br₆(solvent)₄] complexes has been studied (SQUID). Trans-coupling via the central μ₄-bromide is suggested to mediate significant antiferromagnetic interaction. The reactivity of such types of Pd and Ni complexes has been tested for the vinyl/addition polymerization of norbornene. In the presence of an excess of cocatalyst methylaluminoxane (MAO) the palladium complexes show high activity up to 5.9 × 10⁶ g_PNB mol_Pd^-1 h ^-1 at 20°C, while activities of the nickel systems are much lower, but strongly solvent dependent. Detailed studies on the dependence of activity on polymerization conditions such as molar ratios of catalyst and cocatalyst, temperature, reaction time and solvent were carried out. All obtained polynorbornenes (PNB) were noncrystalline and insoluble, but have high glass transition temperatures (T_g). Microstructures were analyzed by IR spectroscopy and solid state (CP/MAS) ¹³C NMR, revealing distinct patterns for the PNB produced by Ni- or Pd-catalysts. Structure/activity correlations deduced for the complexes with different ligand systems suggest that activities and polymer microstructures depend rather on the metal type than on ligand intricacies.

Full text of DOI:10.1039/b925535d

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Highly preorganized pyrazolate-bridged palladium(II) and nickel(II)
complexes in bimetallic norbornene polymerization†
Anna Sachse,a Serhiy Demeshko,a Sebastian Dechert,a Venita Daebel,b Adam Langeb and Franc Meyer*a
Received 8th December 2009, Accepted 23rd January 2010
First published as an Advance Article on the web 11th March 2010
DOI: 10.1039/b925535d
New derivatives of pyrazolate-based binucleating ligands HL with appended imine functions have been
synthesized to provide a versatile set of ligand systems with different backbone substituents both at the
pyrazole-C4 and the imine-C (H, Me, Ph). These scaffolds have two adjacent coordination
compartments akin to the a-diimine type. A series of binuclear palladium(II) complexes [LPd2Cl3] (1–4)
and tetranuclear nickel(II) complexes [L2Ni4Br6(solvent)4] (5, 6) of the various ligands have been
prepared and characterized, including X-ray structural analyses for two representative Pd and the two
Ni complexes. Complexes 5 and 6 were found to contain an unusual central m4-bromide. Mononuclear
nickel(II) complexes [L2Ni] were detected as intermediates in the formation of the tetranuclear
complexes and have been characterized by X-ray analyses in two cases (7, 8). The interconversion
between 5¢ and 7 has been investigated by UV/Vis spectroscopy and ESI mass spectrometry, and
magnetic coupling in the [L2Ni4Br6(solvent)4] complexes has been studied (SQUID). Trans-coupling via
the central m4-bromide is suggested to mediate significant antiferromagnetic interaction. The reactivity
of such types of Pd and Ni complexes has been tested for the vinyl/addition polymerization of
norbornene. In the presence of an excess of cocatalyst methylaluminoxane (MAO) the palladium
◦
complexes show high activity up to 5.9 ¥ 106 gPNB molPd h-1 at 20 C, while activities of the nickel
-1
systems are much lower, but strongly solvent dependent. Detailed studies on the dependence of activity
on polymerization conditions such as molar ratios of catalyst and cocatalyst, temperature, reaction time
and solvent were carried out. All obtained polynorbornenes (PNB) were noncrystalline and insoluble,
but have high glass transition temperatures (Tg). Microstructures were analyzed by IR spectroscopy
and solid state (CP/MAS) 13C NMR, revealing distinct patterns for the PNB produced by Ni- or
Pd-catalysts. Structure/activity correlations deduced for the complexes with different ligand systems
suggest that activities and polymer microstructures depend rather on the metal type than on ligand
intricacies.
Introduction
The last decade has seen impressive progress in the development
and application of late transition metal complexes for homoge-
neous olefin polymerization.1-4 This was sparked by the discovery
of Brookhart and coworkers in 1995 that a-diimine complexes
of palladium(II) and nickel(II) (A, Fig. 1), after activation with
methylaluminoxane (MAO), provide very active catalysts for the
production of high-molecular-weight polyolefins.5,6 Their higher
tolerance of polar functional groups in the monomer has since
been recognized as a key advantage over early transition metal cat-
alyst systems, and much effort has thus been devoted to the design
of new ligand frameworks for improved catalyst performance.7
In more recent years, bi- or oligonuclear polymerization catalysts
Fig. 1 a-Diimine type complexes A and pyrazolate-based binucleating
ligands HL1–HL4.
have entered the scene.8 Inspired by the unique reactivities and
selectivities of oligonuclear metalloenzymes, possible coopera-
tivity effects between adjacent active centers in multinuclear
aInstitute of Inorganic Chemistry, Georg-August-University Go¨ttingen, Tam-
mannstr. 4, D-37077, Go¨ttingen, Germany. E-mail: franc.meyer@chemie.
uni-goettingen.de; Fax: +49(551)393063; Tel: +49(551)393012
bMax-Planck-Institut fu¨r biophysikalische Chemie, AG Festko¨rper-NMR-
Spektroskopie, Am Faßberg 11, D-37077, Go¨ttingen, Germany
transition metal complexes are currently of great interest.9,10
A
growing number of bimetallic systems based on binucleating
ligand scaffolds has therefore been developed and tested as
precatalysts for homogeneous olefin polymerization,11-17 aiming
at higher efficiency, alternative chain propagation mechanisms
or novel polymer architectures. Yet only in some cases are the
two proximate metal ions preorganized at a constrained distance
† Electronic supplementary information (ESI) available: Molecular struc-
ture of 6 (Figure S1); 1H and 13C NMR spectra of 7 (Figures S2 and S3);
NB polymerization activity as bar charts (Figures S4 and S5); TG/DSC,
1
IR, H NMR and CP/MAS 13C NMR spectra of PNB (Figures S6–S9,
Table S1); ORTEP plots of 3, 4, 5, 7 and 8 (Figures S10–S14). CCDC
reference numbers 743828–743832. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b925535d
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◦
3
˚
and at well-defined mutual orientation, and solid evidence for
true cooperativity in the polymerization process is rare.8,18 We
have recently reported a series of pyrazolate-based binucleating
ligands HL1–HL4 (Fig. 1), which after deprotonation feature two
adjacent binding compartments akin to the a-diimine type.12,19-22
In complexes [LxPd2Cl3] the two active metal centers are fixed at
Table 1 Selected atom distances [A] and angles [ ] for [L Pd2Cl3] (3) and
[L4Pd2Cl3] (4)
3
4
Pd1–N1
Pd1–N3
1.921(5)
2.048(5)
2.2838(18)
2.3943(17)
1.923(5)
1.911(4)
2.056(4)
Pd1–Cl1
Pd1–Cl2
Pd2–N2
2.2685(12)
2.3797(13)
1.921(4)
˚
a favourable distance around 3.6–4.0 A by the rigid pyrazolate
bridge, and are thus set up for synergistic action. Using ligands
HL1 and HL2, complexes of types [LPd2Cl3] and [(LNi2X3)3] (X =
Cl, Br) in combination with MAO have served as highly active
catalysts for ethylene polymerization.12 We now add to the series
several complexes of ligands HL3 and HL4 in order to probe the
effect of different backbone substituents and to derive structure–
activity correlations, and we apply a variety of palladium(II) and
nickel(II) complexes of all these ligands for the polymerization of
norbornene (bicyclo[2.2.1]hept-2-ene).
Pd2–N4
Pd2–Cl2
Pd2–Cl3
2.034(5)
2.058(4)
2.3867(17)
2.2914(19)
3.7435 (7)
77.7(2)
175.21(16)
89.04(16)
166.70(16)
98.48(16)
94.82(6)
103.07(5)
77.8(2)
88.54(16)
171.24(16)
165.42(16)
97.45(15)
96.75(7)
2.3809(13)
2.2854(14)
3.7316(6)
77.57(16)
175.05(13)
89.39(12)
166.91(11)
98.74(11)
94.35(5)
103.23(5)
77.49(16)
89.01(11)
176.48(12)
166.49(11)
99.40(11)
94.10(5)
Pd1 ◊ ◊ ◊ Pd2
N1–Pd1–N3
N1–Pd1–Cl1
N1–Pd1–Cl2
N3–Pd1–Cl2
Cl1–Pd1–N3
Cl1–Pd1–Cl2
Pd1–Cl2–Pd2
N2–Pd2–N4
N2–Pd2–Cl2
N2–Pd2–Cl3
N4–Pd2–Cl2
N4–Pd2–Cl3
Cl2–Pd2–Cl3
Results and discussion
Synthesis and characterization of palladium(II) complexes
Complexes [L1Pd2Cl3] (1) and [L2Pd2Cl3] (2) have been reported
earlier.12 Analogous complexes of the new ligands HL3 and HL4
that contain phenyl groups at the pyrazole-C4 only (HL3) or at
both the pyrazole-C4 and the side arm imine groups (HL4)21
were synthesized by deprotonation of HL3 and HL4 with a
stoichiometric amount of KOtBu in THF and subsequent addition
of two equivalents of PdCl2. This gave, after appropriate work-up,
complexes [L3Pd2Cl3] (3) and [L4Pd2Cl3] (4) as orange solids in
yields up to 95%. Both complexes have been characterized by
mass spectrometry (ESI, HRMS), elemental analyses, IR and
UV/Vis spectroscopy as well as by NMR spectroscopy. 13C NMR
spectra show a significant low field shift of the imine-C resonances
upon complexation, from around 152 ppm (free ligand) to around
173 ppm (complexes). Single crystals of 3 and 4 were obtained by
slow diffusion of hexane into CHCl3 or acetone solutions of the
complexes. Molecular structures of both complexes are depicted
in Fig. 2, selected bond lengths and angles are shown in Table 1.
As anticipated, the two palladium atoms are hosted in the
adjacent {N2} binding sites of the binucleating scaffold and
are spanned by the pyrazolate ring at a Pd1 ◊ ◊ ◊ Pd2 distance
˚
of around 3.7 A, with a bridging chloride and two terminal
chlorides completing the coordination spheres. Palladium atoms
are found in a roughly square planar environment (distance to
the least squares planes of the four surrounding ligand atoms
Fig. 2 Molecular structures of complexes [L3Pd2Cl3] (3; top) and
[L4Pd2Cl3] (4; bottom). All hydrogen atoms have been omitted for clarity.
˚
˚
for Pd1/Pd2 in 3: 0.0241(6) A/0.0100(6) A; for Pd1/Pd2 in 4:
groups versus electron withdrawing phenyl groups) or steric. Fig. 3
shows space filling models of all complexes 1–4 both from the top
and from the backside in order to illustrate the different steric
demand. It is obvious that increasing the bulk of the imine-C
substituents (H versus Me versus Ph) will restrict the movement or
rotation of the 2,6-diisopropylphenyl groups at the imine-N. These
bulky groups block access to the dipalladium array from above and
below the basal periphery.
˚
˚
0.0241(6) A/0.0100(6) A). All bond lengths are found in the
expected range,12,22,23 with the bonds to the bridging chlorides (Pd–
Cl2) around 0.1 A longer than bonds to the terminal chlorides
˚
(Pd1–Cl1 and Pd2–Cl3).
The single backbone phenyl group of 3 is twisted by 29.1◦ with
respect to the pyrazolate ring, while the twist of the three phenyl
groups of 4 is much more pronounced (52.8◦–57.8◦), likely due
to steric crowding at the ligand backside. The four [LxPd2Cl3]
complexes 1–4 that differ only by the groups at the pyrazolate-C4
and the imine-C now constitute an informative series of com-
pounds for probing the effect of varying backbone substitution on
the reactivity of the front side dipalladium unit. The influence of
the substituents can be either electronic (electron donating methyl
Synthesis and characterization of nickel(II) complexes
Green paramagnetic powders that analyze as [Lx Ni4Br6(solvent)4]
2
were obtained from the reactions of the ligands with two equiv-
alents of NiBr2(dme). Single crystals could only be obtained for
3904 | Dalton Trans., 2010, 39, 3903–3914
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˚
Table 2 Selected atom distances [A] and angles [ ] for 5
Ni1–N1
Ni1–N3
Ni1–O1
Ni1–O2
Ni1–Br1
Ni1–Br5
Ni2–N2
Ni2–N4
Ni2–Br2
Ni2–Br3
Ni2–Br4
Ni2–Br5
Ni4–Br1
Ni4–Br5
Ni3–Br4
Ni3–Br5
1.991(4)
2.109(3)
2.097(3)
2.135(3)
2.5202(9)
2.6180(7)
1.980(3)
2.085(4)
2.5396(8)
2.6174(10)
2.4773(7)
3.0779(8)
2.4535(6)
2.5712(10)
2.4804(8)
2.7952(7)
2.4358(6)
92.47(3)
87.61(2)
158.21(9)
86.57(2)
89.17(9)
100.60(9)
91.60(9)
89.05(9)
100.81(9)
82.84(9)
Ni1 ◊ ◊ ◊ Ni3
5.341(1)
4.094(1)
3.592(1)
176.15(10)
172.68(9)
93.80(9)
161.19(2)
174.38(10)
162.12(3)
78.47(13)
154.438(26)
98.62(3)
160.92(9)
176.69(10)
86.88(10)
99.51(9)
97.18(2)
94.33(3)
160.82(10)
104.09(9)
89.02(3)
86.83(7)
85.89(8)
88.08(9)
82.83(8)
86.66(8)
98.08(3)
Ni1 ◊ ◊ ◊ Ni2
Ni1 ◊ ◊ ◊ Ni4
N1–Ni1–Br1
N3–Ni1–Br5
N1–Ni1–Br5
Ni1–Br5–Ni3
N2–Ni2–Br4
Br2–Ni2–Br3
N6–Ni4–N8
Ni2–Br5–Ni4
Br6–Ni4–Br5
N7–Ni3–Br5
N5–Ni3–Br4
N6–Ni4–Br5
N8–Ni4–Br1
Ni3–Br5–Ni4
Ni2–Br4–Ni3
N6–Ni4–Br1
N4–Ni2–Br4
Br1–Ni4–Br5
O1–Ni1–Br5
O2–Ni1–Br5
O3–Ni3–Br4
O3–Ni3–Br5
O4–Ni3–Br5
Br4–Ni3–Br5
Ni4–Br6
Ni1–Br1–Ni4
Ni1–Br5–Ni4
N8–Ni4–Br5
Br1–Ni1–Br5
O1–Ni1–Br1
N3–Ni1–Br1
O2–Ni1–Br1
O4–Ni3–Br4
N7–Ni3–Br4
N5–Ni3–Br5
Fig. 4 Molecular structure of [L2 Ni4Br6(thf)(H2O)3] (5); hydrogen atoms
2
have been omitted for clarity.
position in the center of the complex. m4-halides have occasionally
been found for complexes of group 11 or 12 metal ions, but are
very rare for other metals; 5 appears to be the first example in
nickel chemistry. A tetranuclear structure similar to that of 5 was
recently detected for a cobalt(II) complex of HL4.21
The ligation of all four nickel ions in 5 is distinct. Three of
them are found in octahedral (Ni1, Ni2, Ni3) and one in square
pyramidal (Ni4; t = 0.0424) coordination environment. Ni2 has
two bromide ions in the axial positions above and below the
tetrametallic plane, while Ni3 has two water molecules, Ni1 has a
thf and a water, and the square-pyramidal Ni4 has a single bromide
˚
in the apical position. Ni ◊ ◊ ◊ Ni distances range from ~3.6 A (short
˚
edges of the rectangle) and ~4.0 A (within the pyrazolate-based
2
˚
{L Ni2} subunits) to 5.3/5.5 A (along the diagonals of the Ni4
rectangle). The outer aryl groups are roughly perpendicular to the
Ni4 plane, and opposing aryl groups are eclipsed (Fig. 4).
Formation of [Lx Ni4Br6(solvent)4] instead of complexes
2
[LxNi2Br3] akin to 1–4 is obviously driven by the tendency
of nickel(II) to adopt a high-spin state with increase of its
coordination number to 5 or 6. Thus two subunits [LxNi2Br3]
aggregate and accommodate additional solvent ligands to fill some
vacant axial coordination sites. It should be noted that hexanuclear
complexes of type [L1 Ni6X9] (X = Cl, Br) featuring a central m6-X
3
have been observed in the absence of coordinating solvents.12,20
Both compounds 5 and 6 indeed contain high-spin nickel(II)
ions, as confirmed by magnetic measurements (SQUID). The
temperature dependence of the effective magnetic moment (meff)
for 5 is shown in Fig. 5. The meff value at 295 K for 5 (5.10 mB) is
lower than the spin-only value for four S = 1 ions (5.66 mB for g =
2.0), decreases gradually upon lowering the temperature and tends
toward zero at very low temperatures (0.31 mB at 2 K), suggesting
dominant antiferromagnetic coupling and an S = 0 ground state.
According to the molecular structure of 5, which contains four
distinct metal ions, all six possible interaction pathways should
be different. This leads to two problems: (i) over-parametrization
and (ii) assignment of obtained J values to the pathways. We have
thus assumed a simplified scheme of higher apparent symmetry
with only three coupling constants J1–J3 (Fig. 6) and identical g
values for all four nickel(II) ions.
Fig. 3 Space-filling models of the complexes 1–4 (from top to bottom):
top view (left) and backside view (right).
the products resulting from HL2 and HL4, and their identity was
determined by X-ray crystallography. The molecular structure
of [L2 Ni4Br6(thf)(H2O)3] (5) is depicted in Fig. 4, selected
2
atom distances are compiled in Table 2; the structural core of
[L4 Ni4Br6(H2O)4] (6) is basically identical (and is shown in Figure
2
S1 in the supplementary information†), but in this case the
crystallographic analysis is of moderate quality due to disorder,
and we thus refrain from discussing any molecular details.
5 is composed of two subunits {L2Ni2} that are connected by
three bromide bridges to form a rectangular array of four nickel(II)
ions. One of the bridging bromides (Br5) is located in a unique m4-
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Table 3 Selected atom distances [A] and angles [ ] for 7 and 8
8
7
Ni1–N1
Ni1–N1¢
Ni1–N3/4
1.890(4)
1.891(4)
1.937(4)
1.936(4)
180
98.23(15)
81.77(15)
81.77(15)
98.23(15)
180
1.889(2)
1.889(2)
1.956(2)
1.956(2)
180
97.64(10)
82.36(10)
82.36(10)
97.64(10)
180
Ni1–N3¢/4¢
N1–Ni1–N1¢
N3/4–Ni1–N1¢
N1–Ni1–N3/4
N1¢–Ni1–N3¢/4¢
N1–Ni1–N3¢/4¢
N3/4–Ni1–N3¢/4¢
with isolated trans-halide bridges are very rare and may indeed
show strong antiferromagnetic coupling up to -74 cm-1.30 Our
tentative assignment of J1–J3 thus is in good accordance with
literature precedence for related bridging scenarios.
Fig. 5 Plot of meff versus temperature for 5 at 2000 G; the line represents
the calculated curve fit.
To gain insight into the reaction sequence that leads to the
formation of the unusual tetranuclear complexes 5 and 6, the
respective ligand (HL2 or HL4) was deprotonated and sequentially
treated with increasing amounts of NiBr2(dme). Red powders
of new complexes 7 and 8 could be isolated after addition of
half an equivalent of NiBr2(dme). Their mass spectra suggested
the presence of species [L2Ni], both for the red solids (EI-
MS) as well as for solutions in MeCN (ESI-MS). IR spectra
Fig. 6 Simplified coupling scheme for 5.
The appropriate Heisenberg–Dirac–van-Vleck (HDvV) spin
Hamiltonian that includes three isotropic exchange coupling
constants and Zeeman splitting is given in eqn (1).25
=
of the red products showed several absorptions for n(C N)
for each complex, one band almost unchanged from the free
ligand (~1630 cm-1) and further bands significantly shifted to
lower energy (1529–1587 cm-1). This indicates that only one of
the two ligand binding sites is occupied by a metal ion. The
molecular structures of complexes 7 and 8 were elucidated by
X-ray crystallography and are shown in Fig. 7 (for atom distances
and bond angles see Table 3). Both complexes are mononuclear
with the single nickel ion in square planar {N4} environment. Two
imine-N and two pyrazolate-N from the two ligands are found in
mutual trans positions, while the dangling (non-coordinating) side
arms are directed to the backside (torsion angle N2–C3–C19–N4
for 7 is 169.3◦; torsion angle N2–C3–C29–N4 for 8 is 121.2◦). Both
ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
H = −2J1(S1S3 + S2S4 )−2J2(S1S2 + S3S4 )−2J3(S1S4 + S2S3)
4
(1)
ˆ
+gmB B
Szi
∑
i=1
A Curie-behaved paramagnetic impurity (PI) with spin
S = 1 and a temperature-independent paramagnetism (TIP) were
included according to ccalc = (1 - PI)·c + PI·cmono + TIP.26 Best
fit parameters are g1 = g2 = g3 = g4 = 2.16, J1 = -52 cm-1, J2 =
-5.0 cm-1, J3 = +7.8 cm-1, PI = 2.1% (per nickel(II) ion, fixed) and
TIP = 12.0 ¥ 10-4 cm3 mol-1 (fixed).
1
complexes are diamagnetic and their H and 13C NMR spectra
Due to the symmetry of the Hamiltonian, assignment of J1–J3 to
the individual pathways of the Ni4 array is not clear a priori, but
requires consideration of magneto-structural correlations. Some
pyrazolate-based dinickel(II) complexes with an additional exo-
geneous halide bridge show moderate antiferromagnetic coupling
with J values ranging from -3.6 cm-1 to -13 cm-1.27 A bis(m-halide)
situation has been reported to mediate ferromagnetic interactions
in dinickel complexes28 and nickel(II) chains29 with coupling con-
stants in the range from 3.5 cm-1 to 9.6 cm-1; the Ni–X–Ni angles
(X = halide) in those cases are in between 87.4◦ and 95.9◦, i.e., close
to 90◦. In 5 all Ni–Br–Ni angles associated with the Ni1 ◊ ◊ ◊ Ni4 and
Ni2 ◊ ◊ ◊ Ni3 edges are different (76.3◦, 87.6◦, 92.5◦ and 94.3◦), but
three of them are close to an orthogonal situation. Therefore the
weak antiferromagnetic interaction (J2) in 5 is assigned to coupling
within the pyrazolato/bromo bridged bimetallic building blocks,
while the ferromagnetic coupling (J3) is assigned to the short
edges of the Ni4 rectangle, i.e., the Ni-(m-Br)(m4-Br)-Ni fragments.
Interestingly, this leaves the strongest antiferromagnetic coupling
(J1), which determines the overall magnetic behaviour of 5, to the
diagonal pathway with a single trans-m4-Br bridging between nickel
ions and large Ni-(m4-Br)-Ni angles (161◦ and 154◦). Complexes
show separate signals for the coordinating and dangling side arms,
suggesting that the structures observed in the solid state are stable
in solution and are non-fluxional on the NMR time scale (see
Figures S2–S3 for NMR spectra of 7;† peak assignments are based
on a HH-NOESY experiment).
UV/Vis spectra of 7 and 8 show relatively intense absorptions at
364 and 380 nm with a shoulder at 440 nm (7), or at 388 and 407 nm
(8), tentatively assigned to the d-d transitions of the square-planar
nickel(II). No bands are discernible at >650 nm. Upon addition
of further NiBr2(dme) to the red solution of 7 in a thf/acetone
mixture, the bands characteristic of 7 gradually disappear over
several days while new weak d-d bands appear at 634 and 727 nm
(Fig. 8). These can be assigned to the tetranuclear complex 5¢
(an analogue of 5, but probably with different solvent molecules
instead of H2O and thf),‡ as is corroborated by the ESI mass
spectrum that shows a peak at 1601 m/z typical for [L2 Ni4Br5]+.
2
The reaction is reversible: addition of HL2 to the solution of 5¢
‡ For solubility reasons a thf/acetone solvent mixture had to be used for
the titration experiment.
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Norbornene polymerization
Norbornene (NB) can be polymerized in three different ways: ring
opening metathesis polymerization (ROMP), cationic or radical
polymerization, and vinyl/addition polymerization.31-33 The latter
leaves the bicyclic structural unit intact and yields a fully satu-
rated backbone. Homopolymers from vinyl-type polymerization
of norbornene exhibit unique physical properties such as high
glass transition temperatures (Tg), thermal stability, dielectric
properties and small optical birefringence.17,34 They are interesting
and attractive materials for microelectronics, optical applications
and for other potential uses in packing and gas separation. A
variety of palladium and nickel complexes have been reported as
being active for the polymerization of norbornene, but the majority
of these systems are monomeric in solution,35,36 while bimetallic
systems are still rather rare.15-17,33,37 In this study the binuclear
palladium(II) complexes 1–4 and oligonuclear nickel(II) complexes
5, 6, 938 were tested for norbornene homopolymerization, using
MAO as the cocatalyst. All these pyrazolate-based complexes
rapidly produce polynorbornene within 1 min, showing moderate
to high activities in the order of 1 ¥ 105–6 ¥ 106 gPNB molmetal h-1.
-1
In blank experiments no activity was observed with any of the
complexes in the absence of the cocatalyst, or with MAO alone.
Complexes 7 and 8 proved to be inactive even in combination with
MAO, which is likely due to the stability of the [L2Ni] core and
the absence of any accessible coordination sites for chain growth
and substrate binding.
Complex 2 was chosen for optimizing reaction conditions and
for investigating the time course of the reaction. Activities were
found to be strongly dependent on various factors such as the
choice of solvent, the temperature and the concentrations of
monomer and catalyst (Table 4). Initial screening experiments were
conducted in a Schlenk flask by adding a solution of the precatalyst
(in DCM) to a solution of the monomer and MAO. The reaction
was quenched by the addition of methanol/conc. hydrochloric
acid (10 : 1) and the polynorbornene (PNB) precipitate separated
by filtration, washed with methanol, water and acetone, and dried
under vacuum at 70 ◦C (see Experimental Section). A linear
relation between activity and the initial molar ratio of norbornene
to precatalyst (NB/cat) was found, both at 20 ◦C and 0 ◦C (Fig. 9).
Fig. 7 Molecular structures of 7 (top) and 8 (bottom). In the interest of
clarity all hydrogen atoms have been omitted. Symmetry transformation
used to generate equivalent atoms (‘): 2–x, 1–y, 2–z.
Fig. 8 UV/Vis spectra of the interconversion between 5¢ and 7.
leads to a rapid colour change from light green to orange-red,
suggesting conversion from octahedral to square-planar nickel(II)
species, and the original spectrum of 7 is restored after one day
(ESI-MS shows a peak at 1025 m/z characteristic for [L2 Ni+H]+).
No long-lived intermediate of the conversion between 5¢ and 7 is
2
Fig. 9 Plot of activity vs. NB/cat. [mole ratio] for 2 at 20 ◦C and 0 ◦C in
DCM (10 mL); 5.30 mmol 2; Al(MAO)/cat. = 500/1.
detectable by UV/Vis spectroscopy.
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Table 4 Results for polymerization of norbornene with precatalyst 2
Entrya
NB/cat. [mol ratio]b
Al(MAO)/cat. [mol ratio]
Solvent
Time [min]
T [◦C]
Polymer yield [g]
Activityc
1
2
3
4
5
6
7
8
500
500
500
500
500
2000
2000
2000
2000
2000
2000
2000
2000
2000
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
Toluene
Toluened
Toluenee
DCMd
DCMe
1
1
1
1
20
20
20
20
20
20
20
0
20
20
20
20
20
0.129
0.249
0.387
0.542
0.610
0.242
0.619
0.377
0.640
0.799
1.00
7.6 ¥ 105
1.5 ¥ 106
2.3 ¥ 106
3.2 ¥ 106
3.6 ¥ 106
4.6 ¥ 106
3.6 ¥ 106
3.5 ¥ 105
3.8 ¥ 106
4.8 ¥ 106
5.9 ¥ 106
3.6 ¥ 106
3.7 ¥ 106
1000
1500
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
1
0.33
60
1
1
1
1
1
1
9
10
11
12
13
0.604
0.063
a General conditions: 5.30 mmol of complex 2 and 10 ml solvent. b 1.0 g NB (mole ratio of NB/cat = 2000). c In units of gPNB molPd-1 h-1. d 30 mL. e 50 mL.
Temperature had a dramatic effect, and subsequent experiments
were carried out at 20 ◦C.
As usual, an excess of MAO is required to generate the active
species and to possibly scavenge impurities. Addition of MAO to
the green solutions of the nickel(II) complexes led to a rapid color
change to light red, suggesting that the tetrametallic aggregates
break down to pyrazolate-bridged binuclear species with four-
coordinate nickel ions upon methylation. As exemplified with
complex 2 (Fig. 10), increasing the amount of cocatalyst is
beneficial up to a molar ratio Al(MAO)/precatalyst of around
500, but no major increase of activity was found when even more
MAO was added. While a decrease of catalyst activity beyond
a maximum ratio Al(MAO)/precatalyst of around 1000 due to
MAO mediating chain-transfer has been reported in some cases,35b
no such effect is observed here up to a 2000-fold excess of MAO.
Fig. 11 PNB yield vs. initial NB concentration for 2 at 20 ◦C; reaction
time 60 s; 5.30 mmol 2; Al(MAO)/cat. = 2000/1.
dichloromethane (DCM) or toluene. When the reaction is carried
out in DCM, a drastic drop in polymer yield is observed at low
NB concentrations, while in toluene the yield increases from ~60%
to ~100% conversion under the same conditions. This pronounced
solvent dependence calls for further investigations.
It was found that PNB was generated immediately after the
addition of the catalyst precursor complex to the prepared mixture
of NB and MAO in the respective solvent, without any detectable
initiation phase. The time course of the polymerization yield (with
reactions quenched in 10 s intervals) at 0 ◦C and at -20 ◦C
is depicted in Fig. 12. The reaction is fast, and the polymeriza◦tion
◦
has basically ceased after around 40 s (0 C) or 100 s (-20 C),
albeit with lower overall conversion in the latter case (74%
Fig. 10 Plot of activity vs. Al(MAO)/cat. [mole ratio] for 2 at 20 ◦C in
DCM (10 mL); 5.30 mmol 2; 0.5 g of NB.
versus 54%). Although viscosity increased only moderately during
the polymerizations (except when the solvent volume was much
reduced), it cannot be fully excluded that hindered monomer
transfer to the catalyst contributes to the decay in activity.
Several experiments were carried out to probe the cause of the
incomplete conversion to PNB: (i) addition of more precatalyst
complex after 60 s does not increase the yield, showing that
catalyst degradation is not a problem; (ii) addition of further
MAO after 60 s only leads to the formation of black Pd(0); and
(iii) addition of further monomer (norbornene) after 60 s leads
The dependence of polymer yield on the initial norbornene
concentration is illustrated in Fig. 11. In the literature it has
been described that different solvents may lead to distinct activity
patterns, possibly caused by the differences in solubility of the
complexes and/or the polymer.34,35d Indeed, a remarkable solvent
dependence of the polymer yields is found here, with opposing
effects upon lowering the initial NB concentration in either
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mobility of the peripheral aryl groups prohibits chain transfer
and termination reactions, thus leading to higher yields of PNB
polymers.
Polynorbornene characterization
Polymers produced in the present study were largely insoluble
in all common solvents, thus preventing molecular weight de-
terminations by gel permeation chromatography. Insolubility of
polynorbornenes is frequently observed and is often considered
indicative of high stereoregularity.35a,39 Thermogravimetric analy-
ses (TGA/DSC) of the polymers show typical high decomposition
◦
◦
temperatures between 420 C and 470 C. Exact glass transition
temperatures (Tg) of the polymers are difficult to determine from
the DSC curves, however, because of their proximity to the onset of
polymer decomposition. The TGA/DSC data for a PNB material
obtained with catalyst 9 is shown in Fig. 13 as representative
example (others are provided in the supplementary information,†
Figure S6). The microstructures of the polymers were further
characterized by FTIR and NMR spectroscopy. FTIR analyses
suggest the absence of carbon–carbon double bonds (no absorp-
tion in the range 1500–1600 cm-1), while absorptions observed at
1294 cm-1 or 1297 cm-1 are a signature of 2,3-connections of the
monomeric units, and a signal at about 941 cm-1 is characteristic
for the presence of the bicycle[2.2.1]heptane ring system.40,41 The
IR spectrum of a PNB sample produced with catalyst 2 (Table 5,
entry 2) is shown in Fig. 14 (selected spectra of PNB obtained with
the other complexes are given in the supplementary information,†
Figure S7). It was possible to record 1H NMR spectra in a mixture
tetrachloroethane/1,2,4-trichlorbenzene (0.2 mL : 0.5 mL) for the
soluble PNB fraction obtained from the polymerizations mediated
by catalyst 2 (Table 4 entry 11) and complex 6 (Table 5 entry 6);
spectra are shown in the supplementary information,† Figure S8.
No resonances appear between 5.0 and 6.0 ppm, which confirms
that no ROMP has taken place. It should be noted though that
Fig. 12 Time course of the NB polymerization using 2 at 0 ◦C and -20 ◦C
in DCM (10 mL); 5.30 mmol 2; 0.5 g of NB; Al(MAO)/cat. = 2000/1.
to significantly higher yields. We infer from these experiments
that the catalyst has sufficient lifetime and is still active, while no
monomer remains after around 40 s (0 ◦C) or 100 s (-20 ◦C),
despite the isolated PNB yields of only 74% or 54%, respectively.
Most likely soluble norbornene oligomers are formed as well. The
remaining solutions after insoluble PNB has been separated by
filtration were thus analyzed by GC-MS. It was found that all
nickel complexes gave significant amounts of norbornene dimers,
while no such dimers were detected in polymerization reactions
with palladium complexes (unfortunately, higher oligomers are not
amenable to GC-MS analysis due to their high molecular weight).
1H NMR spectroscopy of the soluble fractions only gave broad
and unassignable features, presumably resulting from mixtures of
oligomers. We therefore assume that all monomer is consumed
within 40 s at 0 ◦C (or 100 s at -20 ◦C), but with significant
amounts of oligomers formed, including norbornene dimers only
in the case of Ni-based catalysts.
1
only the soluble part of the PNB is analyzed by this method. H
NMR spectra reveal four different group resonance signals in the
range d = 0–3 ppm, which can be assigned to the C1/C4 and
C2/C3 methine-H as well as to the C5/C6 and C7 methylene-
H.36,42 In addition solid state CP/MAS 13C NMR spectra of
selected PNB products were measured (Fig. 15 and S9). Some
Based on the above trends, and to deduce possible structure–
activity correlations, the various complexes 1–6 and 9 were then
tested under standardized conditions (1.0 g NB; 30 mL solvent;
Al(MAO)/cat. = 1000; NB/cat. = 2000; reaction time 1 min;
at 20 ◦C). Results are collected in Table 4 and shown as bar
charts in Figures S4 and S5. Under these conditions the palladium
complexes exhibit much higher activities than the corresponding
nickel complexes in both DCM and toluene; the nickel complexes
are moderately active in DCM (up to 1.2 ¥ 106 gPNB molmetal h-1)
-1
but almost inactive in toluene. In contrast, palladium systems
clearly perform better in toluene than in DCM: activities of the
-1
palladium complexes reach up to 1.7 ¥ 106 gPNB molmetal h-1 in
-1
DCM and up to 2.4 ¥ 106 gPNB molmetal h-1 in toluene. All four
palladium complexes show very similar activities in toluene, while
some subtle differences in activity are observed in DCM. In the
latter solvent complex 1 with the least crowded parent ligand
[L1]- performs somewhat better than the other systems, but
overall no dramatic influence of the ligand backbone substituents
is discernible. In the case of Ni-based catalyst systems bulky
backbone substituents are clearly advantageous, since complex
6 (with three backbone phenyl groups) performs much better than
complexes 5 and 9 (with two or three backbone methyl groups). It
is reasonable to assume that increasing steric bulk and restricted
Fig. 13 TGA/DSC curves of PNB obtained by the polymerization with
catalyst 9 (Table 5, entry 7).
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Table 5 Results for polymerization of norbornene with different
produced when polymerizations were carried out in more polar
DCM, but were not observed when toluene was used as a solvent.
Interestingly, an opposing trend has been reported for norbornene
polymerization with classical a-diimine type systems (A, Fig. 1),
where a noticeable decrease in the ROMP unit percentage was
found in the presence of polar solvent (ortho-dichlorobenzene).44
Temperature and reaction time do not have any noticeable effect
on the ROMP percentage.
In general, two distinctly different types of spectra are obtained
for the main signal region between 25 and 60 ppm, corresponding
to two classes of polymers with different microstructures depend-
ing on whether Ni- or Pd-based catalysts are used (Table S1†).
Spectra for polymers produced by Ni-catalysts show fewer signals
(e.g., trace d in Fig. 15) and are essentially identical to spectra
published for other Ni-based catalysts, for which microstructures
of low disyndiotacticity have been claimed.45,46 Products made by
Pd-catalysts show a larger number of resonances in the CP/MAS
13C NMR spectra, which have been assigned to a diisotactic
polymer with crosslinks.45 Some differences in microstructures
are noticeable for the polymers produced by the various different
binuclear Pd-catalysts 1–4 under different conditions, but these
subtleties cannot be evaluated based on the available data.
complexes
Complex/
mmol
Polymer
yield [g]b
Polymer
yield [g]c
Entrya
Activityb
Activityc
1
2
3
4
5
6
7
1
2
3
4
5
6
9
0.29
0.22
0.26
0.25
0.13
0.20
0.11
1.7 ¥ 106
1.3 ¥ 106
1.5 ¥ 106
1.5 ¥ 106
7.5 ¥ 105
1.2 ¥ 106
6.5 ¥ 105
0.39
0.38
0.40
0.40
0.02
0.03
0.02
2.3 ¥ 106
2.3 ¥ 106
2.3 ¥ 106
2.4 ¥ 106
1.0 ¥ 105
1.7 ¥ 105
1.0 ¥ 105
a General conditions: 1.0 g NB; mole ratios Al(MAO)/cat. = 1000,
NB/cat. = 2000, 30 mL solvent, 1 min, 20 ◦C. b solvent: DCM,
activity in units of gPNB molmetal h-1. c solvent: toluene, activity in units
-1
-1
of gPNB molmetal h-1.
Conclusion
In summery, the synthesis of two new palladium(II) and sev-
eral new nickel(II) complexes based on a series of a-diimine
ligands with appended imine chelate arms bearing bulky 2,6-
diisopropylphenyl substituents has been achieved. The complexes
differ by the backbone substituents both at the pyrazole-C4 and
the imine-C, which effects the rotational freedom of the peripheral
2,6-diisopropylphenyl groups and thus has subtle effects on the
frontside of the bimetallic array. Palladium chloride complexes
of the type [LxPd2Cl3] can be viewed as bimetallic versions of
the prominent type A systems with a-diimine ligands, with the
two metal ions preorganized in close proximity. Nickel bromide
Fig. 14 IR spectrum of PNB obtained with catalyst 2 (Table 5, entry 2).
complexes assemble to tetrametallic species [Lx Ni4Br6(solvent)4]
2
that feature an almost planar Ni4 rectangle with an unusual
central m4-bromide. The latter is shown to mediate significant
magnetic coupling between high-spin nickel(II) ions along the
diagonal of the Ni4 array. Mononuclear species [Lx Ni] are the only
2
intermediates detected during formation of [Lx Ni4Br6(solvent)4]
2
complexes.
With MAO as cocatalyst, the palladium complexes show
high activity in norbornene homopolymerization (up to 5.9 ¥
106 gPNB molPd-1 h-1), while activities of the nickel systems are much
lower. Various experiments were carried out in order to elucidate
the effects of polymerization conditions such as molar ratios of
catalyst and cocatalyst, temperature, reaction time and solvent
on catalyst activity, showing that: (i) polymerization is fast and
reactions cease after less than 60 s even at 0◦, but maximum yields
of PNB do not reach 100% despite the catalysts being still active,
likely due to the formation of significant amounts of oligomers; (ii)
temperature has drastic effects on both activities and PNB yields,
but much less so on polymer microstructures; (iii) polymerizations
are strongly solvent dependent, both with respect to activities
(in particular for the Ni-based catalyst systems) and polymer
microstructure (with very low percentages of ROMP units only
occurring in more polar DCM); (iv) no significant influence of the
Fig. 15 Solid state CP/MAS 13C NMR spectra of PNB obtained by the
polymerizations with a) complex 4, (Table 5 entry 4, DCM); b) complex 2
(Table 5 entry 2, DCM); c) complex 2 (Table 4 entry 11, toluene); and d)
complex 6 (Table 5 entry 6, DCM).
spectra show resonances between 20 ppm and 24 ppm, which
might be seen as evidence for endo enchained PNB.17,43 There
are also some spectra that show minor peaks at 130 ppm, which
indicates the presence of small portions of metathesis products.
Both these ROMP units and the endo enchained portions were only
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ligand backbone substituents is discernible for palladium, while
bulky backbone substituents are clearly advantageous for nickel.
Future experiments will have to address the significant differences
in chain transfer activity for palladium and nickel, including the
question of whether MAO is directly involved in chain transfer.
Polymer microstructures have been analyzed by means of
TG/DSC, IR and CP/MAS 13C NMR spectroscopy, revealing
distinctly different classes of polymers depending on whether Ni-
or Pd-based catalysts are used. In general, with the present set of
binucleating ligands both polymerization activities and polymer
microstructures depend on the metal rather than on ligand intri-
cacies, as was also observed for classical a-diimine type systems (A,
Fig. 1).44 Future work will deal with copolymerization experiments
as well as mechanistic studies, addressing the potential cooperative
behaviour of the proximate metal ions.
acetone or chloroform solutions of the product gave red crystals.
3
1H NMR (500 MHz, CDCl3): d = 1.19 (d, J (H,H) = 6.8 Hz,
3
12 H, CH3iPr), 1.35 (d, J (H,H) = 6.8 Hz, 12 H, CH3iPr), 3.38
(sept, 3J (H,H) = 6.8 Hz, 4 H, CHiPr), 7.17–7.46 (m, 11 H, CHAr),
7.77 ppm (s, 2 H, N CH); 13C NMR (125 MHz, CDCl3): d = 22.9
=
(CH3iPr), 24.5 (CH3iPr), 29.0 (CHiPr), 123.7 (CPz4), 127.7 (CHAr),
128.2 (CHAr), 129.4 (CHAr), 130.3 (CHAr), 130.7 (CHAr), 141.2
(CAr), 143.0 (CAr), 143.4 (CPz3/5), 163.5 ppm (C N); IR (KBr): n =
=
˜
3465, 3060, 2960, 2927, 2868, 1596, 1591, 1546, 1490, 1462, 1433,
1360, 1332, 1284, 1257, 1179, 1100, 1055, 1035, 999, 915, 804,
778, 744, 695, 656, 606, 550, 524 cm-1; UV/Vis (DCM): lmax (e) =
224 (25582), 241 (24328), 304 nm (15644 L mol-1 cm-1); diffuse
reflectance (KBr): l = 220, 288, 318; MS (ESI-, CH3CN): m/z (%):
872 (33) [M+Cl]-, 695 (100) [M-Pd–Cl]-; HRMS (ESI-, CH3CN)
calcd for [C35H41N4Cl4Pd2]- 872.0162, found 872.0144; elemental
analyses calcd (%) for: C35H41N4Pd2Cl3·3CHCl3: C 38.19, H 3.71,
N 4.69; found: C 39.08, H 4.43, N 4.90.
Experimental section
Synthesis of [L4Pd2Cl3] (4)
General
Prepared according to the general procedure for 3 using HL4
(120 mg, 0.18 mmol), KOtBu (20 mg, 0.18 mmol) and PdCl2
(63 mg, 0.36 mmol). The resulting orange product was obtained
in 95% (170 mg) yield. Slow diffusion of hexane into a chloroform
solution of the product gave red crystals of 4·2.5CHCl3. 1H NMR
(500 MHz, CDCl3): d = 0.94 (d, 3J (H,H) = 6.8 Hz, 12 H, CH3iPr),
1.23 (d,3J (H,H) = 6.8 Hz, 12 H, CH3iPr), 3.31 (sept, 3J (H,H) =
6.7 Hz, 4 H, CHiPr), 6.29–7.21 ppm (m, 21 H, CHAr); 13C NMR
(125 MHz, CDCl3): d = 23.1 (CH3iPr), 24.2 (CH3iPr), 29.1 (CHiPr),
123.5 (CHAr), 127.5 (CHAr), 127.6 (CHAr), 127.8 (CHAr), 128.4
(CHAr), 128.5 (CAr), 128.8 (CHAr), 129.5 (CHAr), 130.5 (CAr), 131.1
All reactions with metal salts were performed under purified
1
nitrogen. H and 13C NMR spectra were recorded on a Bruker
Avance 500 spectrometer. 13C resonances were obtained with
broad-band proton decoupling, spectra were recorded at 298 K.
1H NMR and 13C NMR chemical shifts were referenced internally
to solvent signals. Solid state 13C CP/MAS (MAS frequency: 11
kHz) NMR spectra were recorded on a 9.4 T (1H resonance
frequency: 400 MHz) wide-bore instrument (Bruker Biospin,
Germany) equipped with a 4 mm triple-resonance (1H, 13C, 15N)
MAS probe. All CP/MAS experiments were carried out at a
temperature of +5 ◦C. For high power proton-decoupling the
sequence TPPM with r.f. amplitudes of 80–90 kHz was used.47
Mass spectra were recorded using a Finnigan MAT 95 (FAB)
using 3-nitrobenzylalcohol as a matrix, a Finnigan MAT 8200
(EI) or Bruker APEX IV (HRMS, ESI). UV/Vis spectra were
measured on a Varian Cary 5000 or an Analytic Jena Specord S
100 spectrometer in a 1 cm pathway cuvette or as KBr pellets. IR
spectra from KBr pellets were recorded on a Digilab Excalibur
Series FTS 3000 spectrometer. Elemental analyses were measured
using a Heraeus CHN–O-RAPID instrument or an Elementar
vario EL III instrument. TG/DSC measurements were performed
(CHAr), 140.5 (CAr), 140.8 (CAr), 144.9 (CPz3/5), 173.9 ppm (C N);
=
˜
IR (KBr): n = 3435, 2962, 2866, 1637, 1601, 1574, 1545, 1524,
1492, 1467, 1438, 1383, 1361, 1318, 1289, 1260, 1235, 1201, 1180,
1099, 1021, 974, 936, 865, 802, 766, 745, 734, 697, 666, 640, 546,
469 cm-1; UV/Vis (DCM): lmax (e) = 203 (22853), 219 (25034),
307 nm (10992 L mol-1 cm-1); diffuse reflectance (KBr): l = 231,
320, 442; MS (ESI+, CH3CN): m/z (%): 1012 (43) [M+Na]+, 953
(76) [M-Cl]+, 775 (100) [LPd]+; HRMS (ESI+, CH3CN) calcd for
[C47H49N4Cl2Pd2] 953.1410, found: 953.1401; elemental analyses
calcd (%) for: C47H49N4Pd2Cl3·2.5CHCl3: C 46.17, H 4.03, N 4.35;
found: C 46.41, H 4.09, N 4.38.
◦
at a heating rate of 10 C min-1 with a Netzsch STA 409-
PC instrument. All reagents were purchased from commercial
sources and employed without further treatment; NiBr2(dme) and
MAO (10 wt.% in toluene) from Aldrich, 1,2,4-trichlorobenzene
and DCM (dry, analytical grade) from Fluka. THF was dried
over potassium metal and toluene over sodium metal, both were
distilled and stored under nitrogen. Norbornene (Fluka) was
purified by distillation and used as a solution in toluene. HL1–HL4
and complexes 1, 2, 5 were synthesized according to the published
procedures.12,19,21
Synthesis of [L2 Ni4Br6(thf)(H2O)3] (5)
2
Prepared according to the general procedure for 3 using HL2
(242 mg, 0.50 mmol), KOtBu (57 mg, 0.50 mmol) and NiBr2(dme)
(353 mg, 1.0 mmol). A green product was obtained in 43%
(193 mg) yield. When a solution of the crude product was
dissolved in DCM and the solution layered with light petroleum,
a green solid gradually formed which analyzed as [L2 Ni4Br6]
2
˜
after through drying under reduced pressure. IR (KBr): n =
3062, 3025, 2961, 2926, 2869, 1626, 1558, 1479, 1445, 1391,
1362, 1263, 1227, 1191, 1086, 1032, 799, 756 cm-1; MS (ESI+,
CH3CN): m/z (%): 1601 (14) [L2 Ni4Br5]+, 1243 (19) [L2 Ni2Br2]+,
Synthesis of [L3Pd2Cl3] (3)
To a stirred solution of HL3 (390 mg, 0.75 mmol) and KOtBu
(84 mg, 0.75 mmol) in THF (50 mL) was added PdCl2 (267 mg, 1.50
mmol). The suspension was stirred for 72 h at room temperature
and then evaporated to dryness. The resulting orange product was
obtained in 81% (508 mg) yield. Slow diffusion of hexane into
2
2
802 (100) [L2Ni2Br2(CH3CN)]+; elemental analyses calcd (%) for:
C32H43N4Ni2Br3: C 45.71, H 5.15, N 6.66; found: C 45.48, H 5.22,
N 6.57. Slow diffusion of hexane into a wet THF solution of the
green material gave green crystals of [L2 Ni4Br6(thf)(H2O)3].
2
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Synthesis of [L4 Ni4Br6(H2O)4] (6)
127.4 (CHAr), 128.1 (CHAr), 128.8 (CHAr), 129.0 (CHAr), 129.8
(CHAr), 130.5 (CHAr), 135.6 (CAr), 137.0 (CAr), 140.6 (CAr), 141.8
2
Prepared according to the general procedure for 3 using HL4
(77 mg, 0.11 mmol), KOtBu (13 mg, 0.11 mmol) and NiBr2(dme)
(71 mg, 0.22 mmol). The resulting green product was obtained in
89% (100 mg) yield. Slow diffusion of hexane into wet chloroform
or DCM solutions of the product gave green crystals. IR (KBr):
(CAr), 145.8 (CAr), 147.8 (CPz3/5), 152.2 (CPz3/5), 161.3 (N C), 171.7
=
=
˜
(N C); IR (KBr): n = 3059, 3026, 2959, 2925, 2865, 1621, 1589,
1578, 1552, 1529, 1495, 1462, 1432, 1400, 1381, 1360, 1324, 1269,
1258, 1221, 1179, 1159, 1099, 1071, 1058, 1044, 1018, 964, 942,
912, 847, 801, 779, 762, 744, 693, 674, 631, 615, 574, 548, 470,
427 cm-1. UV/Vis (DCM): lmax (e) = 248 (21236), 290 (20266),
314 (22711), 388 (12035), 407 nm (17753 L mol-1 cm-1); diffuse
reflectance (KBr): l = 242, 301, 390, 408, 474, 583; MS (EI):
m/z (%): 1397 (100) [M]+, 1354 (42) [M-iPr]+, 728 (68) [LNi]+,
627 (77) [L-iPr]+; MS (ESI+): m/z (%): 1397 (6) [M+H]+, 671
(100) [L+H]+; HRMS (ESI+, CH3CN) calcd for [C94H99N8Ni]
1397.73407, found: 1397.73348; elemental analyses calcd (%) for:
C94H98N8Ni: C 80.73, H 7.06, N 8.01; found: C 79.54, H 7.08, N
7.95.
˜
n = 3402, 3059, 2965, 2929, 2868, 1625, 1582, 1557, 1494, 1462,
1384, 1362, 1336, 1291, 1260, 1218, 1168, 1100, 1074, 1056, 1043,
1022, 981, 935, 913, 807, 780, 771, 744, 696, 642, 619, 564, 524,
448, 418 cm-1; UV/Vis (DCM): lmax (e) = 219 (13060), 267 (10463),
711 nm (65 L mol-1 cm-1); diffuse reflectance (KBr): l = 229, 287,
712; MS (EI, 70 eV): m/z (%): 1397 (12) [L2Ni]+, 1354 (6) [L2Ni-
iPr]+, 728 (10) [LNi+H]+, 627 (100) [L-iPr]+; MS (FAB, 3-NBA):
m/z (%):1398 (22) [L2Ni+H]+, 947 (56) [LNi2Br2]+, 866 (100)
[LNi2Br]+, 807 (70) [LNiBr]+, 671 (30) [HL+H]+; No satisfactory
elemental analyses could be obtained for 6, presumably due to
the gradual and incomplete loss of metal-bound solvent ligands
(water) under vacuum conditions.
General procedure for norbornene polymerization
In a Schlenk-flask was mixed norbornene as a solution in toluene
(0.8 g mL-1), MAO and the solvent (DCM or toluene). The
solution was kept at a fixed temperature and the metal precatalyst,
which was applied as a solution in DCM, was added. The
polymerization was quenched after the respective time by the
addition of 10 mL of a methanol/conc. hydrochloric acid mixture
(10 : 1). The polymer precipitate was filtered off, washed with
methanol (10 mL), water (10 mL) and acetone (10 mL), and dried
under vacuum at 70 ◦C for 12 h.
Synthesis of L2 Ni (7)
2
Prepared according to the general procedure for 3 using HL2
(400 mg, 0.83 mmol), KOtBu (93 mg, 0.83 mmol) and NiBr2(dme)
(128 mg, 0.42 mmol). The resulting red product was obtained in
60% (255 mg) yield. Slow diffusion of hexane into a THF solution
1
of the product gave red crystals. H NMR (500 MHz, CDCl3):
3
d = 1.09–1.16 (m, 48 H, CH3iPr), 1.23 (d, J(H,H) = 6.7 Hz, 12
iPr
=
=
H, CH3 ), 1.47 (s, 6 H, N CCH3), 2.03 (s, 6 H, N CCH3), 2.52
(s, 6 H, CH3Pz4), 2.70 (sept, J(H,H) = 6.8 Hz, 4 H, CHiPr), 4.00
3
Susceptibility measurements
(sept, 3J(H,H) = 6.6 Hz, 4 H, CHiPr), 7.00–7.15 (m, 12 H, CHAr);
13C NMR (125 MHz, CDCl3): d = 11.1 (CH3 ), 18.1 (N CCH3),
Pz4
=
Susceptibility measurements were carried out with a Quantum-
Design MPMS-5S SQUID magnetometer equipped with a 5 Tesla
magnet in the range from 295 to 2.0 K. The powdered samples
were contained in a gel bucket and fixed in a non-magnetic sample
holder. Each raw data file for the measured magnetic moment was
corrected for the diamagnetic contribution of the sample holder
and the gel bucket. The molar susceptibility data were corrected
using the Pascal constant and the increment method according to
Haberditzel.
19.2 (N CCH3), 22.8 (CH3iPr), 23.0 (CH3iPr), 23.3 (CH3iPr), 23.9
=
(CH3iPr), 28.0 (CHiPr), 29.2 (CHiPr), 120.1 (CPz4), 122.4 (CHAr),
122.6 (CHAr), 123.3 (CHAr), 127.2 (CHAr), 128.5 (CHAr), 131.9
(CHAr), 136.0 (CAr), 141.1 (CAr), 142.4 (CAr), 147.3 (CAr), 149.1
(CPz3/5), 150.8 (CPz3/5), 164.0 (N C), 172.5 (N C); IR (KBr): n =
3054, 2957, 2926, 2867, 1645, 1630, 1587, 1564, 1533, 1468, 1435,
1382, 1362, 1324, 1256, 1229, 1188, 1160, 1120, 1095, 1058, 1019,
962, 936, 809, 780, 769, 747, 728, 695, 655, 579, 533, 508, 494, 460,
426 cm-1; UV/Vis (DCM): lmax (e) = 248 (12125), 277 (12032),
313 (11995), 364 (5956), 380 (5883), 440 nm (1157 L mol-1 cm-1);
diffuse reflectance (KBr): l = 224, 285, 383, 445; MS (EI): m/z
(%): 1024 (100) [M]+, 981 (30) [M-2H-iPr]+, 540 (55) [LNi-2H]+,
441 (27) [L-iPr]+; MS (ESI+, CH3CN): m/z (%): 1025 (100) [M]+;
HRMS (ESI+, CH3CN) calcd for [C64H87N8Ni] 1025.6402, found
1025.6407.
=
=
˜
X-Ray crystallography
X-Ray data were collected on a STOE IPDS II diffractometer
˚
(graphite monochromated Mo-Ka radiation, l = 0.71073 A)
by use of w scans at -140 ◦C. The structures were solved
by direct methods and refined on F2 using all reflections with
SHELX-97.48 Table 6 contains the crystal data and refinement
details for 3, 4, 5, 7 and 8. Most non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in calculated
positions and assigned to an isotropic displacement parameter of
Synthesis of L4 Ni (8)
2
Prepared according to the general procedure for 3 using HL4
(54 mg, 0.08 mmol), KOtBu (9 mg, 0.08 mmol) and NiBr2(dme)
(12 mg, 0.04 mmol). The resulting red product was obtained in
87% (50 mg) yield. Slow diffusion of hexane into a THF solution
of the product gave red crystals. 1H NMR (500 MHz, CDCl3): d =
0.65–1.38 (m, 48 H, CH3iPr), 2.45 (m, 4 H, CHiPr), 4.07 (m, 4 H,
CHiPr), 6.56–7.11 (m, 27 H, CHAr); 13C NMR (125 MHz, CDCl3):
d = 22.0 (CH3iPr), 22.2 (CH3iPr), 23.4 (CH3iPr), 24.1 (CH3iPr), 27.6
(CHiPr), 28.3 (CHiPr), 29.1 (CHiPr), 29.7 (CHiPr), 122.5 (CPz4), 122.7
(CHAr), 125.4 (CHAr), 126.5 (CHAr), 126.7 (CHAr), 126.9 (CHAr),
2
˚
0.08 A (3, 4) or using the riding model with Uiso tied to Uiso of
the parent atoms (5, 7, 8). Face-indexed absorption corrections
for 3, 4, 5, and 7 were performed numerically with the program
X-RED.49 In structure 4 one CDCl3 is disordered about a 2-
fold rotation axis and was refined at half occupancy. SADI
restraints (dC–Cl) were used to model the disorder. Additionally
one iPr group was found to be disordered about two positions
(occupancy factors 0.871(19)/0.129(19)). SADI restraints (dC–C
)
and EADP constraints were used to model the disorder. In 5
3912 | Dalton Trans., 2010, 39, 3903–3914
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©

View Article Online
Table 6 Crystal data and refinement details for 3, 4, 5, 7 and 8
3·acetone
4·2.5CHCl3
5
7
8
Empirical formula
Formula weight
Crystal size/mm
Crystal system
Space group
C38H47Cl3N4OPd2
894.95
C49.5H51.5N4Cl10.5Pd2
1287.47
C72H102N8O5Br6Ni4
1873.92
C64H86N8Ni
1026.12
C94H98N8Ni
1398.51
0.21 ¥ 0.15 ¥ 0.10
Monoclinic
P21/c (no. 14)
10.0887(6)
30.3130(16)
13.8048(8)
90
90.161(5)
90
4221.8(4)
4
1.408
1816
1.074
0.9506/0.8369
11, 33, 16
1.62–24.79
25842
6973 [0.1057]
441/0
0.18 ¥ 0.14 ¥ 0.10
Monoclinic
C2/c (no. 15)
36.082(3)
12.8094(6)
28.7200(19)
90
124.805(5)
90
10899.2(13)
8
0.34 ¥ 0.19 ¥ 0.11
Monoclinic
P21/n (no. 14)
15.278(3)
39.172(8)
16.131(4)
90
110.19(3)
90
9060.5(3)
4
1.374
3808
3.509
0.6423/0.3417
-16–19, -49–48, 20
1.44–26.75
83278
19235 [0.0926]
869/0
0.50 ¥ 0.15 ¥ 0.09
0.41 ¥ 0.25 ¥ 0.07
Triclinic
Triclinic
¯
¯
P1 (no. 2)
P1 (no. 2)
˚
a/A
8.8126(13)
11.8060(16)
14.793(2)
73.936(11)
89.425(11)
86.727(11)
1476.6(4)
1
1.154
554
0.374
0.9397/0.7824
-9–11, 15, 18
1.96–27.12
13134
6369 [0.0694]
327/14
9.7920(6)
14.7660(9)
27.8400(16)
83.335(5)
80.671(5)
86.048(5)
3940.1(4)
2
1.179
1492
0.298
—
-11–12, 18, 35
1.49–27.03
37618
17062 [0.1101]
800/31
0.871
0.0729
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
rc/g cm-3
1.569
5176
1.212
F(000)
m/mm-1
Tmax/min
0.9143/0.8468
-42–41, -15–14, -32–33
1.37–24.71
31067
8805 [0.0898]
626/10
1.004
0.0442
0.0742
0.504/-0.688
hk-Range
q-Range (◦)
Measured refl.
Unique refl. [Rint
]
Ref. param./restraints
Goodness-of-fit
R1 (I > 2s(I))
1.013
0.0512
0.0827
0.598/-0.427
1.045
0.0476
0.1216
2.525/-1.373
1.000
0.0645
0.1415
0.351/-0.643
wR2 (all data)
Resid. el. dens./e A
0.1565
0.360/-0.420
-3
˚
one carbon atom of the metal bound THF solvent molecule was
found to be disordered about two positions (occupancy factors
0.66(2)/0.34(2)). In 7 and 8 several iPr groups and in 8 also phenyl
rings were found to be disordered. SADI restraints (dC–C) were
used to model the disorder of the iPr groups; AFIX 66 was used
to model the disorder of the phenyl rings. For two carbon atoms
of the phenyl moieties and four carbon atoms of the iPr moieties
EADP constraints were applied. Atoms of the disordered parts
were refined isotropically. No satisfactory model for the disorder
could be found for further disordered acetone solvent molecules
in the unit cell of 3 or THF and acetonitrile solvent molecules
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in the unit cell of 5. For further refinement, the contribution
3
˚
of the missing solvent molecules (3: 534.0 A , electron count
3
˚
134; 5: 1544.0 A , electron count 363) was subtracted from the
reflection data by the SQUEEZE50 routine of the PLATON51
program. CCDC: 743828 (3), 743829 (4), 743830 (5), 743831 (7),
and 743832 (8) contain the supplementary crystallographic data
for this paper.†
Acknowledgements
Financial Support by the Fonds der Chemischen Industrie is
gratefully acknowledged
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3914 | Dalton Trans., 2010, 39, 3903–3914
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The Royal Society of Chemistry 2010
©