Di- and oligo-nuclear nickel complexes with oxalic amidinato bridging ligands: Syntheses, structures and catalytic reactions

Article abstract of DOI:10.1039/b000791i

A new family of di-, tri- and tetra-nuclear Ni(II) complexes containing oxalic amidinates as bridging ligands is described in which both peripheral Ni centres are catalytically active in the oligomerization or polymerization of ethylene or in the selective cross coupling of Ar-X with Ar'-MgX.

Full text of DOI:10.1039/b000791i

Di- and oligo-nuclear nickel complexes with oxalic amidinato bridging ligands:
syntheses, structures and catalytic reactions†
Thomas Döhler, Helmar Görls and Dirk Walther*
Institut für Anorganische und Analytische Chemie der Friedrich-Schiller-Universität, D-07743 Jena, Germany.
E-mail: cdw@rz.uni-jena.de
Received (in Basel, Switzerland) 21st January 2000, Accepted 26th April 2000
A new family of di-, tri- and tetra-nuclear Ni(II) complexes
containing oxalic amidinates as bridging ligands is described
in which both peripheral Ni centres are catalytically active in
the oligomerization or polymerization of ethylene or in the
selective cross coupling of Ar–X with ArA–MgX.
Di- or oligo-nuclear metal complexes of late d metals connected
via the conjugated p-system of oxalic amidinato ligands
(‘oxam’) are attractive candidates for catalytic reactions,
because the ligands are easily accessible and their steric and
electronic properties can be easily modified. Furthermore,
systems with different metals in peripheral positions enable
catalysts to be designed in which either one or two metals are
catalytically active. In addition, the metal coordinated at the
opposite side of an active metal centre may be used for fine
tuning the catalytic activity.
Since oxalic amidinato ligands tend to form ill-defined
polymers with metal salts1,2 their coordination chemistry is very
limited.3–6 Neither dinuclear d8 metal complexes nor tri- or
Fig. 1 Molecular structure of 4. Selected bond distances (Å) and bond angles
oligo-metallic complexes are known. Furthermore, catalytic C–
(°): Ni1–N1 1.880(8), Ni2–N2 1.886(8), Ni1–O3 1.841(7), Ni1–O4
C-linking reactions are also unknown.
1.843(7), Ni2–N3 1.879(8), Ni2–N4 1.902(8), Ni2–O1 1.845(7), Ni2–O2
Surprisingly, the Ni(II) complexes 1–4 (Scheme 1, m = 0) are
obtained by reacting Ni(acac)2 in toluene or THF with a variety
of oxalamidines. Recrystallisation from toluene resulted in red–
brown diamagnetic crystalline complexes in 35–60% yields.
According to 1H NMR spectroscopy on 1–5 the ratio of acac to
oxalic amidine ligand was 2+1 supporting dimeric structures
(Scheme 1).‡
1.854(7), C1–C2 1.518(13), N1–C1 1.304(12), N2–C2 1.311(12), N3–C1
1.316(13), N4–C2 1.334(12); O4–Ni1–N1 90.3(3), O3–Ni1–N2 90.9(3),
N2–Ni1–N1 83.9(3), O3–Ni1–O4 94.8(3), O1–Ni2–N3 89.7(3), O2–Ni2–
N4 91.4(3), N3–Ni2–N4 84.5(4), O1–Ni2–O2 94.4(3).
mesityl–N groups, the other by two p-tolyl–N groups. The
aromatic substituents are oriented to the ligand plane at an angle
1
of 90°. The H NMR spectrum confirms the presence of only
one isomer in solution. Complexes 1–5 can be considered as the
first members of a series of oxalamidinato complexes of general
composition LnM1(oxam)[M3(oxam)]mM2Ln (m = 0).
The trimetallic complex [(acac)Ni(oxam)Zn(oxam)Ni(acac)]
6 is the first example of the next generation of this series (m =
1, M1, M2 = Ni; M3 = Zn, oxam = tetramesityloxalamidine)
which was prepared by reacting of 2 equiv. of oxam with ZnEt2
in toluene , followed by addition of 2 equiv. of Ni(acac)2 in good
yields.
The Ni2Zn2 complex 7 is the first example of a tetrametallic
complex of the above-mentioned range prepared from 2 equiv.
of ZnEt2 and 3 equiv. of bis(mesityl)bis(p-tolyl)oxalamidine in
THF, followed by reaction with 2 equiv. of Ni(acac)2. Red
crystalline complex 7 was isolated upon recrystallisation of the
product from toluene. Elemental analyses, mass spectra, and the
1H NMR spectrum confirm its composition. Its X-ray structure
(Fig. 2) shows that both peripheral Ni centres have the same
planar environment as in 4, while the two central Zn atoms are
tetrahedrally coordinated.
Table 1 shows that all the complexes are catalytically active
towards ethylene when activated with MAO or Et3Al. In
contrast to the well established Ni catalysts for the SHOP
process,7 in which negatively charged P–O ligands support the
oligomerization of ethylene, we observed both oligomerization
and polymerization of ethylene (Table 1).
Scheme 1
The solid state structure of the key complex 4 was determined
by X-ray diffraction and Fig. 1 displays its molecular struc-
ture.§
As expected, the oxalamidinate acts as bridging ligand
connecting two (acac)Ni(II) fragments with both metal centres
essentially planar. Although 4, containing different N-sub-
stituents, may exist as two isomers, to our surprise only one
isomer was observed, in which both metal centres have different
coordination environments: one metal is surrounded by two
A comparison of 2 (runs 1 and 2 in Table 1) and 3 (runs 5 and
6 in Table 1) clearly shows that the p-tolyl groups in 2 promote
the oligomerization of ethylene; however, substitution by
† Electronic supplementary information (ESI) available: general
procedures and full spectroscopic characterisation of complexes 1–7. See
http://www.rsc.org/suppdata/cc/b0/b000791i/
DOI: 10.1039/b000791i
Chem. Commun., 2000, 945–946
This journal is © The Royal Society of Chemistry 2000
945

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Table 1 Catalytic reactions of 1–7 with ethylene in the presence of MAO or Et3Ala
Polymer
(%)
Polymer
selectivity (%)
Run
Precat.
Cocatalyst
C4/C6 (%)
Mw
Mn
D = Mw/Mn
1
2
3
4
5
6
7
8
9
2
3
3
3
3
3
3
4
MAO (300)
MAO (2)
MAO (6)
33
—
—
2
—
4
—
100
100
95
89
89
92
50
92
—
—
—
—
—
357000
673000
491000
157000
75000
246000
237000
189200
—
244000
451000
185000
88000
30000
165000
117000
132700
—
1.46
1.49
2.66
1.78
2.49
1.49
2.02
1.43
—
24
42
74
54
62
16
82
2
MAO (20)
MAO (300)
MAO (1000)
AlEt3 (300)
MAO (300)
MAO (300)
MAO (300)
MAO (300)
9
6.5
—
16
7
24
38
6
7
10
11
Ni(acac)2
—
—
—
—
a General conditions: 0.031 mmol catalyst, 10 bar ethylene, 20 ml toluene, 16 h reaction time, room temperature; run 9: most of the polymer was insoluble,
only the molecular weight of the soluble fraction was determined.
This work was supported by Deutsche Forschungsge-
meinschaft und Volkswagenstiftung.
Notes and references
‡ Satisfactory microanalysis have been obtained. Selected spectroscopic
data: for 4: C44H50N4O4Ni2: MS (EI): m/z 816 (42%, M + 2+); dH(toluene-
d8, 25 °C): 0.89, 0.94 (CH3-acac, 23 s, 12H), 1.89, 1.9 (p-CH3, 23 s, 12H),
2.68 (o-CH3-mes, s, 12H), 4.62, 4.68 (CH-acac, 23 s, 2H), 6.25 (CH-mes,
s, 4H), 6.37, 6.71 (CH-tol, AAABBA, 8H).
For 7: C112H122N12Ni2O4Zn2: dH(THF-d8, 25 °C): 1.01, 1.04, 1.29, 1.39,
1.48, 1.64, 1.83, 1.88, 1.90, 1.99, 2.03, 2.08, 2.12, 2.20, 2.25, 2.30, 2.34,
2.48 (CH3, 72H), 4.97, 5.00 (CH-acac, 2H), 5.75, 5.79, 5.89, 5.93,
6.02–6.29 (m, CH-aryl, 23H), 6.44, 6.48, 6.56, 6.61, 6.65 (m, CH-aryl, 6H),
7.08–7.2 (m, CH-aryl, 7H).
§ Crystal data for 4:12 C44H50N4Ni2O4, Mr = 816.30, brown prism, size
Fig. 2 Molecular structure of 7. Selected bond distances (Å) and angles (°):
0.40 3 0.35 3 0.10 mm, monoclinic, space group P21/n, a = 12.696(2), b
Ni–O1 1.840(4), Ni–O2 1.864(4), Ni–N1 1.897(4), Ni–N2 1.885(4), Zn–N3
= 8.873(1), c = 36.989(4) Å, b = 98.278(6)°, V = 4123.5(9) Å3 , T = 290
2.002(4), Zn–N4 2.007(4), Zn–N5 1.996(5), Zn–N6 2.000(5), C1–N1
°C, Z = 4, Dc = 1.315 g cm23, m(Mo-Ka) = 9.59 cm21, F(000) = 1720,
1.309(6), C1–N3 1.334(7), C2–N2 1.312(7), C2–N4 1.344(7), C1–C2
4609 reflections with h(213/13), k(0/9), l(241/40), measured in the range
1.524(8), C35–N5 1.343(7), C35–N6A 1.338(7), C35–C35A 1.518(12),
2.81 @ q @ 23.27°, completeness q max = 98.7%, 3331 independent
dihedral angle between planes (Ni–O1–O2)/(Ni–N1–N2) 6.9(2), (Zn–N2–
reflections, Rint
=
0.089, 2607 reflections with Fo
>
4s(Fo), 487
N4)/(Zn–N5–N6) 100.8(2). Symmetry transformations used to generated
equivalent atoms: A 2x + 3/2, y, 2z + 1.
parameters, 0 restraints, R1obs = 0.080, wR2
= 0.231, R1all = 0.108,
obs
wR2 = 0.271, GOF = 1.074, largest difference peak and hole: 0.385,
all
20.563 e Å23
.
mesityl groups (in 3) results in the formation of a selective
polymerization catalyst. In contrast to recently described
1.2-diimine or salicylaldiminato complexes of d8 metals, where
very bulky substituents have to be used for selective polymeri-
zation,8–11 in the oxalic amidinato complexes only two
additional ortho-methyl groups are necessary to switch the
reaction from oligomerization to polymerization of ethylene.
Since both oligomerization and the polymerization of ethylene
occur using 4 as catalyst which contains two different
coordination spheres, we can conclude that both metals are
catalytically active (runs 3 and 4 in Table 1). If the reaction with
3 was carried out heating the reaction mixture from room
temperature to 69 °C within 10 min (50 bar ethylene) then 3.7
g of polyethylene was produced giving a TOF of 102000
For 7:12 C112H122N12Ni2O4Zn2, Mr = 1948.38, red prism, size 0.32 3
0.28 3 0.20 mm, monoclinic, space group I2/a, a = 22.2023(8), b =
21.6541(6), c = 25.343(1) Å, b = 105.960(2)°, V = 11714.5(7) Å3 , T =
290 °C, Z = 4, Dc = 1.125 g cm23, m(Mo-Ka) = 7.71 cm21, F(000) =
4176, 22567 reflections for h(227/27), k(226/27), l(231/31), measured in
the range 3.01° @ q @ 26.42°, completeness q max = 98.9%, 11922
independent reflections, Rint = 0.091, 6125 reflections with Fo > 4s(Fo),
615 parameters, 0 restraints, R1obs = 0.086, wR2 = 0.217, R1all = 0.175,
obs
wR2 = 0.261, GOF = 1.010, largest difference peak and hole: 0.927,
all
20.451 e Å23
.
CCDC 182/1615. See http://www.rsc.org/suppdata/cc/b0/b000791i/ for
crystallographic files in .cif format.
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h21
.
The catalytic reaction of 6/MAO with ethylene shows that the
Zn moiety in the middle of 6 increases the selectivity towards
polymerization compared with 3 (Table 1, runs 5 and 9). This
demonstrates the possibility for fine tuning the catalytic activity
by using different metals. The tetranuclear complex 7, however,
is a dimerization catalyst when activated with MAO. The
complexes can also be used as catalysts in other C–C coupling
reactions such as the polymerization of styrene (e.g.: 3/300
equiv. MAO/4000 equiv. styrene yielded 94% polystyrene) or
cross coupling, e.g. of mesitylmagnesium bromide (500 equiv.)
with p-tolyl bromide (500 equiv.) to give mesityl–4-tolyl in
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ambient temperature was used.
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In conclusion, we have shown that di-, tri- and tetra-nuclear
complexes of late d metals with oxalamidinates can easily be
prepared and used in a number of catalytic C–C-linking
reactions. Both peripheral Ni centres are catalytically active in
these complexes.
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Chem. Commun., 2000, 945–946