Nickel complexes with click -derived pyridyl-triazole ligands: Weak intermolecular interactions and catalytic ethylene oligomerisation

Article abstract of DOI:10.1039/c2dt31805a

The ligands 1-(cyclohexyl)-4-(2-pyridyl)-1,2,3-triazole (1), 1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole (2), 1-(4-butoxyphenyl)- 4-(2-pyridyl)-1,2,3-triazole (3) and 1-(methyl)-4-(2-pyridyl)-1,2,3-triazole (4) were synthesized by the Cu(i) catalyzed Click reaction between 2-pyridylacetylene and the corresponding azides. The ligands were then reacted with NiBr₂·3H₂O to generate the complexes (1) ₂NiBr₂ (1a), (2)₂NiBr₂ (2a), (3)₂NiBr₂ (3a) and (4)₂NiBr₂ (4a). Structural characterization of 1a confirmed the mononuclear and distorted octahedral environment around the Ni(ii) center, with the pyridyl-triazole ligands coordinating in a bis-chelating fashion. Bond length analysis inside the 1,2,3-triazole ring shows a short NN double bond that is flanked by two longer C-N and N-N bonds pointing to the existence of azo character in the ring. The highly polar five-membered 1,2,3-triazole ring makes its C-H bond acidic, and these bonds participate in an extended weak intermolecular C-H...Br interactions with the Br-groups of neighboring molecules, resulting in a 3-D network. The nickel complexes with these Click ligands were tested as pre-catalysts for ethylene oligomerization, and the complexes showed moderate activity in that reaction with good selectivity towards C4 oligomers.

Full text of DOI:10.1039/c2dt31805a

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Nickel complexes with “click”-derived pyridyl-triazole ligands: Weak
intermolecular interactions and catalytic ethylene oligomerisation‡
David Schweinfurtha,b, Cheng-Yong Suc, Shi-Chao Weic, Pierre Braunsteind, Biprajit
Sarkar,*,a,b
Received
Abstract
The ligands 1ꢀ(cyclohexyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (1), 1ꢀ(2,6ꢀdiisopropylphenyl)ꢀ4ꢀ(2ꢀ
pyridyl)ꢀ1,2,3ꢀtriazole (2), 1ꢀ(4ꢀbutoxyphenyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (3) and 1ꢀ
(methyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (4) were synthesized by the Cu(I) catalyzed “Click”
reaction between 2ꢀpyridylacetylene and the corresponding azides. The ligands were then
reacted with NiBr2.3H2O to generate the complexes (1)2NiBr2 (1a), (2)2NiBr2 (2a), (3)2NiBr2
(3a) and (4)2NiBr2 (4a). Structural characterization of 1a confirmed the mononuclear and
distorted octahedral environment around the Ni(II) center, with the pyridylꢀtriazole ligands
coordinating in a bisꢀchelating fashion. Bond length analysis inside the 1,2,3ꢀtriazole ring
shows a short N=N double bond that is flanked by two longer CꢀN and NꢀN bonds pointing to
the existence of “azo” character in the ring. The highly polar fiveꢀmembered 1,2,3ꢀtriazole
ring makes its CꢀH bond acidic, and these bonds participate in an extended weak
intermolecular CꢀHꢁꢁꢁBr interactions with the Brꢀ groups of neighboring molecules, resulting
in a 3ꢀD network. The nickel complexes with these “Click” ligands were tested as preꢀ
catalysts for ethylene oligomerization, and the complexes showed moderate activity in that
reaction with good selectivity towards C4 oligomers.
_________________________
a Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, Dꢀ70550, Stuttgart, Germany.
b Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34ꢀ36, Dꢀ
14195, Berlin, Germany. Email: biprajit.sarkar@fuꢀberlin.de.
c Lehn Institute of Functional Materials, School of Chemistry and Chemical Engineering, Sun YatꢀSen
University, Guangzhou, 510275 (China).
d Laboratoire de Chimie de Coordination, UMR CNRS 7177, Université de Strasbourg, 4 rue Blaise Pascal, Fꢀ
67081 Strasbourg Cédex, France.
‡
Electronic Supplementary Information (ESI) available: UVꢀvis spectra of the complexes
_________________________________________________________________________________________

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Introduction
The Huisgen [3+2] cycloaddition reaction between azides and alkynes was reported in the
thermal form decades back.1 The Cu(I) catalyzed version of this reaction has been popularized
as the “Click” reaction in recent years (Scheme 1).2,3
Scheme 1: Thermal cycloaddition reaction between azides and alkynes (left) and the Cu(I)
catalyzed “Click” reaction (right)
This reaction works under mild conditions, delivers high product yields of exclusively one
isomer, and is tolerant against a host of functional groups. Because of these attractive features,
the “Click” reaction has become an important tool in every chemist’s synthetic toolbox.4 The
existence of reasonably basic nitrogen atoms in the 1,2,3ꢀtriazole rings, and the possibility of
introducing additional donor groups in the substituents, has led to these classes of compounds
being used as ligands in coordination chemistry.5 Thus a host of metal centers have been
coordinated to such “Click” derived ligands, and the metal complexes have found use in
electrochemical6ꢀ12 and photochemical studies,11ꢀ16 in supramolecular chemistry,17ꢀ21
magnetism,22,23 metalꢀion sensing4a,24 and catalysis.25ꢀ30 We have recently investigated metal
complexes of substituted 1,2,3ꢀtriazole ligands for their electrochemical properties,31
magnetic bistability32 and their application in homogenous catalysis.33,34 In this context,
substituted pyridylꢀtriazoles were used in combination with d8 metal centers like Pd(II) and
Pt(II) by us31,35 and others.36ꢀ41 In this report we present new complexes (1)2NiBr2 (1a),
(2)2NiBr2 (2a), (3)2NiBr2 (3a) and (4)2NiBr2 (4a) that contain the d8 metal center Ni(II) with
the
pyridylꢀtriazoles,
1ꢀ(cyclohexyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole
(1),
1ꢀ(2,6ꢀ
diisopropylphenyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (2), 1ꢀ(4ꢀbutoxyphenyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀ
triazole (3) and 1ꢀ(methyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (4). We also describe here the
differences in the synthesis and properties of these complexes compared to those of their
higher homologues. Structural characterization of one of the complexes is provided. We also
present the use of these complexes as preꢀcatalysts for ethylene oligomerisation. To the best
of our knowledge, these are the first metal complexes based on substituted “click” pyridylꢀ

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triazole ligands that are been reported as preꢀcatalysts for ethylene oligomerisation. There
have been some recent reports on the use of nickel complexes of triazole substituted αꢀdiimine
ligands as precatalysts for the polymerization of norbornene and styrene.42,43
Results and discussion
Synthesis and characterization
The ligands 1ꢀ(cyclohexyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (1),35 1ꢀ(4ꢀbutoxyphenyl)ꢀ4ꢀ(2ꢀ
pyridyl)ꢀ1,2,3ꢀtriazole (3)44 and 1ꢀ(methyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀtriazole (4)45 were synthesized
according to reported procedure and the ligand 1ꢀ(2,6ꢀdiisopropylphenyl)ꢀ4ꢀ(2ꢀpyridyl)ꢀ1,2,3ꢀ
triazole (2) was prepared by a click protocol from 2ꢀpyridylacetylene and 2,6ꢀ
diisopropylphenylazide, using CuSO4 together with ascorbic acid as the catalyst. Additionally,
the tripodal ligand tris[(1ꢀbenzylꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methyl]amine, tbta was used for
improving the catalytic activity in that reaction. (experimental section). Reactions of the
ligands 1, 2, 3 and 4 with NiBr2ꢁ3H2O afforded the complexes 1a, 2a, 3a and 4a in excellent
yields (Scheme 2).
Scheme 2: Synthesis of complexes 1aꢀ4a.
The synthesis of the complexes, which at a first glance looks straight forward, is actually
cumbersome. Hence, a careful control of reaction conditions is required to synthesize the
“bis” complex. After monitoring a host of conditions, and screening a set of ligands, we were
able to determine the ones required to preferentially stabilize the “bis” complex. We were not
able to synthesize four coordinated complexes of the form (L)NiBr2, L = pyridylꢀtriazole
under the tested conditions. For the corresponding Pd(II) and Pt(II) cases, the four coordinated
complexes were the exclusively isolated products.31,35 This is of course related to the strong
tendency of Pd(II) and Pt(II) complexes to stabilize a square planar environment because of
the huge gain through ligand field splitting.46 The 3d metal ion Ni(II) is capable of adopting a
variety of coordination modes depending on the electronic and steric nature of the ligands. In

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the present case, it was possible to isolate octahedrally coordinated Ni(II) complexes with two
bisꢀchelating ligands and two additional bromide ligands.
The complexes were characterized by elemental analyses, UVꢀvis spectroscopy and mass
spectrometry (experimental section). Mass spectra of the compounds showed peaks at m/z
corresponding to the diꢀcations formed by the loss of two bromide ions from the complexes
(experimental section). Ni(II) centers in an octahedral environment are usually characterized
by three different dꢀd transitions in the visible or NIR region in their absorption
3
3
3
spectroscopy.47 These bands which are transitions from the A2g(F) to T1g(P), T1g(F) and
3T2g(F) states respectively are spin allowed, but parity forbidden, and are hence usually weak
in intensity. The solubility of the complexes reported herein is poor in all tested organic
solvents. Hence it was possible to record UVꢀvisꢀNIR spectra of these complexes only with a
limited concentration. The recorded spectra for all the complexes are quite similar (Figure
S1). Of the three transitions mentioned above, the one at the lowest wavelength usually is the
most intense. For 3a a band of weak intensity is observed at 370 nm and this is assigned to a
3T1g(P) → 3A2g(F) transition. For the other complexes a weak shoulder can be seen at around
the same region (Figure S1). The other two expected dꢀd transitions are not observed in the
reported complexes, possibly due to their intrinsic weak nature and the low solubility of the
complexes. Low solubility of the complexes precluded a reliable determination of the
extinction coefficients. Additionally, all complexes show absorptions in the UV region which
are assigned to intraꢀligand transitions of the pyridylꢀtriazole ligands. Such bands are
reminiscent of bands observed for the corresponding Pd(II) and Pt(II) complexes with such
ligands.31,35 Interestingly, the position of these bands in the UVꢀregion seems to correlate with
the electronic nature of the substituents on the triazole ring. Thus, for 1a and 4a with alkyl
substituents on the triazole ring, this band appears at 275 nm, and for 2a and 3a with aryl
substituents, the same band appears at 283 nm (Figure S1).
Despite their paramagnetism, which precluded their characterization by NMR spectroscopy,
complexes 1a and 2a were EPR silent down to 110 K in the Xꢀband. This has to do with the
large zeroꢀfield splitting usually observed for octahedral S = 1 nickel(II) centers, which is
responsible for shifting their signals outside the range of the normal Xꢀband EPR
spectrometer.46
Crystal structure of 1a

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1a (Figure 1) was crystallized by slow evaporation of a methanol solution under ambient
conditions. It crystallizes in the orthorhombic Pbca space group. Crystallographic details are
given in the Table 1. Selected bond lengths and bond angles are provided in the caption of
Figure 1.
Table 1
Figure 1
The nickel centre in 1a is in a distorted octahedral coordination environment which includes
the N3ꢀtriazole and pyridyl N atoms from two different ligands (1) in the equatorial plane.
The additional axial positions are taken up by the bromide ions that are then trans to each
other. In the equatorial plane the pyridine N as well as the triazole N donors from the two 1
ligands are mutually trans to each other. The NiꢀN distances of 2.083(2) and 2.084(2) Å and
the NiꢀBr distances of 2.601(1) Å are in the expected range. The angles in the plane
containing the two 1 ligands deviate from 90° because of the chelating nature of the ligands 1.
The N2ꢀN3 bond length inside the 1,2,3ꢀtriazole ring is 1.312(3) Å, and the N3ꢀN4 and N2ꢀC6
bond lengths are 1.348(3) and 1.363(3) Å, respectively. These distances show a short central
NꢀN bond that is flanked by two relatively long NꢀN and NꢀC bonds. Such data point to the
N=N “azo” character of the central NꢀN bond as opposed to an azine character. Similar
observations have been made before by us for the corresponding Pd(II) and Pt(II) complexes
of pyridylꢀtriazoles and by others for complexes with other metal centers.31,35,48 A similar
coordination to that observed here for 1a, was reported for a nickel complex with triazoleꢀ
substituted αꢀdiimine ligands.42 In that case the donor atoms from the tetradentate ligands
were equatorially coordinated to the nickel centre. The bromide ligands take up the remaining
axial sites around the octahedrally coordinated nickel center. In another report, predictions
from DFT studies on nickel complexes containing ferrocenylꢀsubstituted pyridylꢀtriazole
ligands showed that among the various isomers that are possible (Scheme 3), the coordination
observed here should be most stable.49 In the reported compound, the coligands (perchlorates
in the reported case and bromide in the present case) are in the axial positions and trans to
each other, and the pyridine and triazole nitrogen donor atoms from the two different ligands
occupy equatorial sites and are mutually trans to each other. However, no experimental
structural evidence was reported for that nickel complex.49 We have presented here structural
evidence for the stability of this isomer in the solid state.

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Br
Br
Ni
Br
Br
Ni
Br
Br
Ni
T
P
P
P
T
T
P
P
T
Br
Br
P
Ni
T
P
T
T
T
P
N
N
N
P
T
N
R
Scheme 3: Possible isomers of an octahedral nickel(II) complexes with two substituted
pyridylꢀtriazole ligands (adapted from ref 49)
Because of the presence of three basic nitrogen atoms within a fiveꢀmember ring, the CꢀH
bond of the 1,2,3ꢀtriazole ring is known to be highly acidic. This in turn enormously enhances
the ability of these CꢀH bonds to participate in hydrogen bonding. Such targeted hydrogen
bonding has been used for anion binding and anion sensing.50 In the case of 1a, extensive
hydrogen bonding is observed between the bromide ligands that are bound to the nickel
centres and the CꢀH group of the 1,2,3ꢀtriazole rings of neighboring molecules (Figure 2).
Thus, the CꢀH group of a triazole ring is hydrogenꢀbonded to the bromide ligand of the next
molecule in the same layer. The second bromide ligand forms hydrogen bonds to a CꢀH group
of a triazole ring of the next layer. Relevant distances and angles are CꢀHꢁꢁꢁBr = 2.676(2) Å,
CꢀHꢀBr = 169.1(1)˚ and NiꢀBrꢀH = 101.4(1)˚. In such a way extensive threeꢀdimensional
patterns are formed in the solid state. Similar head to tail arrangements stabilized by hydrogen
bonding between the acidic CꢀH group of the 1,2,3ꢀtriazole ring of one molecule and the N2
atom of the 1,2,3ꢀtriazole ring of an adjacent molecule have been recently observed for Pd(II)
complexes with pyridylꢀtriazole ligands.41
Figure 2
Catalytic ethylene oligomerisation
The demand for the production of linear αꢀolefins in the C4ꢀC10 range is growing fast and this
makes the catalytic oligomerisation of ethylene a topic of continuing academic and industrial
interest.51ꢀ63 A range of nickel complexes with chelating ligands have been successfully used
as catalysts for this purpose.64 However, identifying and controlling the parameters that
influence catalytic activity and selectivity still represents a major challenge in this field.
Nickel complexes of 1,2,3ꢀtriazoleꢀsubstituted αꢀdiimine ligands have recently been used as
precatalysts for the polymerization of norbornene and styrene.42,43 We present here results on
the first use of nickel complexes of “click” derived pyridylꢀtriazole ligands in the catalytic
oligomerisation of ethylene. Complexes 1a-4a have been tested as precatalysts for this

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reaction with AlEtCl2 as cocatalyst under 10 bar of ethylene pressure (experimental section).
Initial runs with 1a showed AlEtCl2 to be a better coꢀcatalyst than MAO. Hence all
subsequent runs with all the complexes were carried out with AlEtCl2. Toluene turned out to
be a better solvent for 1a and 2a and chlorobenzene for 3a and 4a. All the complexes exhibit
reasonable activity in the range of about 8000ꢀ16000 g C2H4/( g Niꢁh). The selectivity for the
C4 fraction was above 70 mass % for 1aꢀ3a. The complex 1a with the cyclohexyl substituent
on the triazole ring shows the lowest activity as well as TOF compared to 2a-4a (see Table 2).
The activity and TOF of complexes 2aꢀ4a are comparable, with 4a containing the least bulky
substituent on the 1,2,3ꢀtriazole ring, showing the least selectivity of all.
Table 2
Conclusion
We have presented here the synthesis of a new pyridylꢀtriazole ligand, and have used three
other literature reported pyridylꢀtriazole ligands to synthesis their corresponding nickel(II)
complexes. Investigation through elemental analyses and mass spectrometry of the complexes
confirmed the composition as NiL2Br2, L = pyridylꢀtriazole ligand. Structural characterization
of one of the complexes showed that the bisꢀchelating ligands are in the equatorial plane of
the octahedron, and the triazole and pyridine nitrogens from each of the ligands are trans to
each other. Weak intermolecular CꢀH…Br interactions are observed in the solid state leading
to an ordered three dimensional structure. In the UVꢀvis spectrum of the complexes only one
of the three possible dꢀd transitions were observed. The nickel complexes turned out to be
moderately good catalysts for the oligomerisation of ethylene, showing moderate activities
and turnover numbers. This report thus presents new examples of nickel(II) complexes with
the increasingly popular “Click” based 1,2,3ꢀtriazole ligands, and shows the utility of metal
complexes of these ligands in yet another field of chemistry.
Experimental Section
General
All solvents were dried and distilled using common techniques unless otherwise mentioned.
1H NMR spectra were recorded at 250.13 MHz on a Brucker AC250 instrument. Elemental

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Analyses was performed on a Perkin Elmer Analyser 240. Mass spectrometry measurements
were carried out using an Agilent 6210 ESIꢀTOF instrument.
Syntheses
The ligands 1, 3 and 4 were prepared according to reported procedures.35,44,45
2: 2ꢀPyridylacetylene (206 mg, 2.0 mmol), 2,6ꢀdiꢀisopropylphenylazide (406 mg, 2.0 mmol),
.
CuSO4 5 H2O (25 mg, 0.1 mmol), sodium ascorbate (79 mg, 0.4 mmol) and TBTA (11 mg,
0.02 mmol) were dissolved in CH2Cl2/ H2O/ tertꢀBuOH (2.5 mL/ 2.5 mL/ 5 mL) and stirred
for 3 d at 50 °C. Then water (50 mL) was added and the reaction mixture was extracted with
.
CH2Cl2 (3 20 mL). The organic phase was seperated and washed with an EDTA/ ammonia
.
solution (1 M) (3 10 mL). Finally the solution was dried over Na2SO4 and the solvent was
evaporated. After flash chromatography over silica (CH2Cl2:CH3OH, 99:1) the product was
isolated as a white solid (460 mg) in 75 % yields. Anal. Calcd. for C19H22N4: C, 74.48; H,
7.24; N, 18.29; Found C, 74.06; H, 7.37; 18.10. HRMS (ESI) Calcd. for C19H23N4 ([M + H]+):
3
m/z 307.1917; found 307.1915. 1HꢀNMR (250 MHz, CDCl3): δ 1.15 (d, JHꢀH = 6.8 Hz, 6H,
CH3); 1.17 (d, 3JHꢀH = 6.8 Hz, 6H, CH3); 2.35 (septet, 3JHꢀH = 6.9 Hz, 2H, CH); 7.28 (t, 3JHꢀH
=
3
3
7.5 Hz, 1H, pyridyl); 7.32 (d, JHꢀH = 7.9 Hz, 2H, phenyl); 7.51 (t, JHꢀH = 7.8 Hz, 1H,
3
3
phenyl); 7.84 (t, JHꢀH = 7.7 Hz, 1H, pyridyl); 8.23 (s, 1H, 5ꢀtriazoleꢀH); 8.32 (d, JHꢀH = 7.9
Hz, 1H, pyridyl); 8.62 (multiplet, 1H, pyridyl). 13CꢀNMR (62.5 MHz, CDCl3):
28.6; 120.7; 123.2; 124.0; 125.0; 131.0; 133.3; 137.1; 146.3; 148.3; 149.7; 150.5.
24.2; 24.4;
.
1a: NiBr2 3 H2O (30 mg; 0.11 mmol) and 1 (50 mg; 0.22 mmol) were dissolved in MeOH
(20 mL) and refluxed for 1 h. The solvent was evaporated to yield the product as a light blue
solid (59 mg) in 80 % yields. Slow evaporation of a methanol solution yielded crystals
suitable for Xꢀray diffraction. Anal. Calcd. for C26H32Br2N8Ni: C, 46.26; H, 4.78; N, 16.60;
Found C, 45.74; H, 4.83; 16.33. UVꢀVis (CH2Cl2): nm 276. HRMS (ESI) Calcd. for
C26H32N8Ni ([M – 2 Brꢀ]2+): m/z 257.1046; found 257.1047.
2a: The compound was prepared following the procedure for 1a to yield a light blue solid (78
mg) in 85 % yields. Anal. Calcd. for C38H44Br2N8Ni: C, 54.90; H, 5.33; N, 13.48; Found C,
54.91; H, 5.36; 13.07. UVꢀVis (CH2Cl2): nm 283. HRMS (ESI) Calcd. for C38H44N8Ni ([M –
2 Brꢀ]2+): m/z 335.1516; found 335.1520.

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3a: The compound was prepared following the procedure for 1a to yield a white solid (62 mg)
70 % yields. Anal. Calcd. for C34H36Br2N8NiO2: C, 50.59; H, 4.50; N, 13.88; Found C, 50.47;
H, 4.63; 13.69. UVꢀVis (CH2Cl2): nm 282, 370. HRMS (ESI) Calcd. for C34H36N8NiO2 ([M –
2 Brꢀ]2+): m/z 323.1152; found 323.1187.
4a: The compound was prepared following the procedure for 1a to yield a white solid (27 mg)
46 % yields. Anal. Calcd. for C16H16Br2N8Ni: C, 35.66; H, 2.99; N, 20.79; Found C, 35.25; H,
3.15; 20.38. UVꢀVis (CH2Cl2): nm 277. HRMS (ESI) Calcd. for C16H16N8Ni ([M – 2 Brꢀ]2+):
m/z 189.0420; found 189.0432.
Catalytic Oligomerization of Ethylene.
The catalytic reactions were performed in a magnetically stirred (1200 rpm) 145 mL stainless
steel autoclave. A 125 mL glass container was used to avoid corrosion of the autoclave walls.
The precatalyst solutions were prepared by dissolving 1a (33.2 mg, 4 x 10ꢀ5 mol) or 2a (27
mg, 4 x 10ꢀ5 mol) in toluene (10 mL) and 3a (32.3 mg, 4 x 10ꢀ5 mol) or 4a (21.5 mg, 4 x 10ꢀ5
mol) in chlorobenzene (10 mL). In all cases, AlEtCl2 was used as cocatalyst (8 x 10ꢀ5 mmol in
toluene). The solution of the precatalyst was injected into the reactor under an ethylene flux,
then 5 mL of the cocatalyst toluene solution (10 equiv.) was added (total volume: 15 mL). All
catalytic reactions were started between 20 and 30 °C. No cooling of the reactor was done
during the reaction. After injection of the catalyst and cocatalyst solutions under a constant
low flow of ethylene, the reactor was pressurized to 10 bars. The temperature increased, due
solely to the exothermicity of the reaction. The 10 bars working pressure was maintained
through a continuous feed of ethylene from a bottle placed on a balance to allow continuous
monitoring of the ethylene uptake. At the end of each test (35 min, unless otherwise specified)
a dry ice bath was used to rapidly cool the reactor. When the inner temperature reached 0 °C,
the ice bath was removed, allowing the temperature to slowly rise to 18 °C. The gaseous
phase was then transferred into a 10 L polyethylene tank filled with water. An aliquot of this
gaseous phase was transferred into a Schlenk flask, previously evacuated, for GC analysis.
The amount of ethylene not consumed was thus determined. Although this method is of
limited accuracy, it was used throughout and gave satisfactory reproducibility. The reaction
mixture in the reactor was quenched in situ by the addition of ethanol (10 mL), transferred
into a Schlenk flask, and separated from the metal complexes by trapꢀtoꢀtrap evaporation (20

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°C, 0.8 mbar) into a second Schlenk flask previously immersed in liquid nitrogen in order to
avoid loss of product.
X-ray Crystallography
Suitable single crystal of 1a was selected and mounted onto thin glass fiber. Xꢀray intensity
data were measured at 150 K on an Oxford Gemini S Ultra diffractometer with the Enhance
Xꢀray Source of CuꢀKα radiation (λ = 1.54178 Å) using the ωꢀφ scan technique.65 Empirical
absorption correction was applied using spherical harmonics implemented in SCALE3
ABSPACK scaling algorithm.66 The structure was solved by direct methods and refined by
fullꢀmatrix leastꢀsquares against F2 of all data using the SHELXTL program package.67
Anisotropical thermal factors were assigned to the nonꢀhydrogen atoms, while the positions of
the hydrogen atoms were generated geometrically, assigned isotropic thermal parameters, and
allowed to ride on their respective parent atoms before the final cycle of leastꢀsquares
refinement. CCDCꢀ851805 contains 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.uk/data_request.cif.
Acknowledgements: This work was supported the BadenꢀWürttembergꢀStiftung, CNRS, the
Ministère de l’Enseignement et de la Recherche, and the Institut Français du Pétrole, Energies
Nouvelles (IFPEN). We thank Marc MermillonꢀFournier and Eric Wagenblast for
experimental help. We are extremely grateful to Dr. Andreas Springer and the mass
spectrometry facility of the Freie Universität Berlin for very efficient and fast measurements.

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Table 1: Crystallographic details for 1a.
1a
Chemical formula
C52H64Br4N16Ni2
1350.25
Mr
Cell setting, space group Orthorhombic, Pbca
Temperature(K)
a, b, c(Å)
V(Å3)
150(2)
13.0158(2), 9.8575(2), 20.8946(5)
2680.9(2)
2
Z
Dx(Mg m–3)
1.673
Radiation type
Cu Kα (λ = 1.54178 Å)
ꢀ(mm–1)
4.790
Crystal size(mm)
0.25 × 0.20 × 0.18
Meas., indep. and obsvd 6634, 2115, 1919
refl.
R[F2 > 2σ(F2)], wR(F2), S
0.024, 0.062, 1.018
No. of parameters
169
Rint
0.028
62.60
θ
max(°)
ꢁρmax, ꢁρmin(e Å–3)
0.432, –0.331

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Table 2: Catalytic data for complexes 1aꢀ4a in ethylene oligomerisation with AlEtCl2 as
cocatalysta
Selectivity Activity TOF [mol C2H4/ Selectivity
precatalystAlEtCl2 (mass %) [g C2H4/(g
(mol Niꢁh)] for 1ꢀbutene
(equiv)
Niꢁh)]
8000
8500
(mol %)b
C4 C6 C8
10 71 27 2
10 77 21 2
28900
30600
7
7
1a
2a
3a
4a
10 73 25 2
10 71 26 3
11200
12200
40200
43800
8
10
10 72 25 3
10 74 23 3
12500
11500
45000
41300
6
7
10 66 30 4
10 63 33 4
14600
15900
52600
57300
7
6
a Conditions: T = 303 K, 10 bar of C2H4, 35 min, 4 x10ꢀ5 mol of complex; solvent: 10 mL of
toluene (1a and 2a) or chlorobenzene (3a and 4a) for 10 equiv of AlEtCl2 (5 mL of AlEtCl2
solution in toluene). b within the C4 fraction.

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Figure Captions:
Figure 1: ORTEP plot of 1a. Ellipsoids are drawn at 50 % probability. Ni1ꢀN1, 2.084(2);
Ni1ꢀN2, 2.083(2); Ni1ꢀBr1, 2.601(1); N2ꢀN3, 1.312(3); N2ꢀC6, 1.363(3); N3ꢀN4, 1.348(3) Å.
N1ꢀNi1ꢀN2(chelate), 79.4(1) ; N1ꢀNi1ꢀBr1, 90.3(1) ; N2ꢀNi1ꢀBr1, 90.0(1) ; N1ꢀNi1ꢀN2,
100.7(1).
Figure 2: Intermolecular CꢀHꢁꢁꢁBr interactions in 1a resulting in an extended three
dimensional network.

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Figure 1:

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Figure 2:

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Graphical Abstract:
Synthesis and structural characterization of four novel Ni(II) complexes with pyridyl-triazoles
are reported (see Figure), together with their use as pre-catalysts for ethylene oligomerization.

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91x34mm (300 x 300 DPI)