Substituted Tridentate Pyrazolyl Ligands for Chromium and Nickel-Catalyzed Ethylene Oligomerization Reactions . Effect of Auxiliary Ligand on Activity and Selectivity

Dois novos complexos de cromo(III) contendo ligantes tridentados [CrCl 3 (L)] (1a, L = bis[2-(3fenil-1-pirazol)etil)]amina; 2a, L = bis[2-(3-metil-5-fenil-l-pirazol)etil]sulfeto) foram preparados e caracterizados por análise elementar. Após ativação com metilaluminoxano (MAO), estes précatalisadores mostraram altas frequências de rotação nas reações de oligomerização do etileno sob condições otimizadas (FRs = 22,9-36,4×10 mol C 2 H 4 (mol Cr) h, [Cr] = 10,0 mmol, 80 °C, 20 bar de etileno, [Al]/[Cr] = 300, tempo de oligomerização = 20 min), produzindo olefinas-a no intervalo de C 4 -C 14+ com alta seletividade (67,71-73,47%). Os desempenhos catalíticos são afetados substancialmente pelos grupos presentes nos ligantes, especialmente os substituintes nas posições 3 e 5 dos anéis pirazol. Em paralelo, o emprego de complexos de níquel(II) tais como NiCl 2 {bis[2(3,5-dimetil-1-pirazol)metil]benzilamina} (3) e NiCl 2 {bis[2-(3,5-dimetil-1-pirazol)etil)]éter} (5) em reações de oligomerização conduzidas na presença de trifenilfosfina (PPh 3 ) resultou em sistemas catalíticos altamente ativos com frequências de rotação (FRs) variando de 36,4 a 154,2×10 mol C 2 H 4 (mol Ni) h. A presença deste ligante auxiliar tem um impacto significativo na produção seletiva de olefinas-a, diminuindo substancialmente a quantidade de buteno-1 com concomitante aumento da quantidade das frações de butenos-2. Tentativas de cristalização do complexo de níquel 3 resultaram na formação de um composto de níquel tetrametálico [{(L)(μ 3 -Cl)NiCl} 4 ] (4, L = 1-anilinometil-3,5-dimetil-1-pirazol) o qual foi caracterizado por difratometria de raios X.


Introduction
The pursuit of ethylene oligomerization catalysts capable of producing selectively a-olefins has been a major focus of research in recent decades, due to their importance in a variety of industrial processes.Depending on the chain length of the alkene, they can be used for production of various materials.The most important ones are linear lowdensity polyethylene (LLDPE) (C 4 -C 10 ), poly-a-olefins (C 4 , C 10 ), plasticizers (C 6 -C 10 ), lubricants (C 8 -C 10 ), lube oil additives (C 12 -C 18 ), and surfactants (C 12 -C 20 ). 1 For this purpose, several oligomerization catalyst systems have been developed, most of which are based on nickel 2 and chromium catalysts 3 bearing bi-and tridentate ligands.

General procedures
All manipulations were performed using standard vacuum line and Schlenk techniques under a purified argon atmosphere.Et 2 O, thf and toluene were distilled from sodium-benzophenone ketyl under argon and degassed by freeze-thaw-vacuum cycles prior to use.3-Phenylpyrazole 9  and [CrCl 3 (thf) 3 ] 10 were prepared by literature procedures.
NiCl 2 •6H 2 O and triphenylphosphine (PPh 3 ), both from Aldrich, were used as received.Ethylene (White Martins Co.) and argon were deoxygenated and dried through columns of BTS (BASF) and activated molecular sieves (3A) prior to use.PMAO-IP (methylaluminoxane, Akzo, 12.9 wt.% Al solution in toluene) and diethylaluminum chloride (DEAC) (Aldrich, 1.8 mol L -1 , 25 wt.% toluene solution) were used as received.Elemental analyses were performed by the Analytical Central Service of the Institute of Chemistry-UFRGS (Brazil) and are the average of two independent determinations. 1H and 13 C{ 1 H} NMR spectra were recorded on a Varian Inova 300 spectrometer operating at 25 °C.Chemical shifts are reported in ppm vs. SiMe 4 and were determined by reference to the residual solvent peaks.Infrared spectra were performed on a FTIR Bruker Alpha Spectrometer.Quantitative gas chromatographic analysis of ethylene oligomerization products was performed on a Varian 3400CX instrument with a Petrocol HD capillary column (methyl silicone, 100 m length, 0.25 mm i.d., and film thickness of 0.5 mm) operating at 36 °C for 15 min followed by heating at 5 °C min -1 until 250 °C; cyclohexane was used as internal standard.

(μ 3 -Cl)NiCl} 4 ] (4)
A solution of 3 (18 mg) in CH 2 Cl 2 (7 mL) was kept at room temperature for five days, resulting in a few green crystals of 4, which were separated from the solution and proved suitable for X-ray diffraction analysis.

General oligomerization procedure
A 100 mL double-walled stainless Parr reactor equipped with mechanical stirring and internal control of temperature was evacuated and filled three times with argon and twice with ethylene.Freshly distilled toluene (30 mL) and the proper amount of MAO or DEAC were added into the vessel under a stream of ethylene.After 15 min, the toluene catalyst solution (10 mL) was injected.The reactor pressure was kept constant throughout the oligomerization process (20 bar) by manually controlled addition of ethylene.After the desired time, the reaction was stopped by cooling the system to -20 °C, depressurizing, and introducing 1 mL of ethanol.An exact amount of cyclohexane was introduced (as internal standard) and the mixture was analyzed by quantitative GLC.

X-ray crystallographic studies
A suitable single crystal of 2 was mounted onto a glass fiber using the ''oil-drop'' method.Diffraction data were collected at 100 K using an APEXII Bruker-AXS diffractometer with graphite-monochromatized Mo-K a radiation (l = 0.71073 Å).A combination of oand j-scans was carried out to obtain at least a unique data set.The structure was solved by direct methods using the SIR97 program, 11 and then refined with full-matrix least-square methods based on F 2 (SHELX-97) 12 with the aid of the WINGX program. 13Many hydrogen atoms could be found from the Fourier difference map.Carbon-bound hydrogen atoms were placed at calculated positions and forced to ride on the attached carbon atom.The hydrogen atom contributions were calculated but not refined.All nonhydrogen atoms were refined with anisotropic displacement parameters.

Synthesis, characterization of substituted bis(pyrazolyl)-Cr III complexes and their use in oligomerization reactions
The substituted tridentate nitrogen-bridged bis(pyrazolyl) ligand ( 1) used in this study was readily prepared in high yield through adaptation of literature procedures (see Experimental).The synthesis of tridentate ligand 2 was achieved starting from commercially available 1-phenyl-1,3-butanedione and 2-hydroxyethylhydrazine.The initial step procedure promotes the formation of two different isomers (Scheme 1) whereas the isomer A was obtained in pure form (52% yield) after workup.The identity of isomer A was established on the basis of multinuclear The reaction of [CrCl 3 (thf) 3 ] with 1.1 equivalent of tridentate nitrogen-, or sulfur-bridged bis(pyrazoyl) ligands (1 and 2) in thf at room temperature affords the corresponding [CrCl 3 (NZN)] complexes 1a and 2a (Scheme 2), which were isolated, respectively, as green or red winecolored solids in high yields (typically 90-92%).These complexes show moderate solubility at room temperature in dichloromethane, thf and ethyl acetate, and are readily soluble in acetonitrile.Due to the paramagnetic nature of these [CrCl 3 (NZN)] complexes, 1 H NMR spectra featured very broad resonances and proved to be uninformative.The identity of 1a and 2a was established on the basis of elemental analysis.
The ethylene oligomerization behavior of chromium complexes 1a and 2a has been evaluated using optimized conditions recently established for related 6-membered chelate chromium catalysts, 7 i.e., toluene as solvent at 80 ºC under 20 bar of ethylene and Al:Cr ratios of 300.Representative results are summarized in Table 1.
When activated with MAO, all catalytic systems proved to be active for oligomerization of ethylene.A moderate turnover frequency (TOF) of 22,900 mol(ethylene) mol(Cr) -1 h -1 was obtained with 1a while the catalyst system derived from 2a, which contains a sulfur-bridged ligand bearing methyl and phenyl substituents at the 3and 5-positions of the pyrazolyl rings, gave a high TOF of 36,400 mol(ethylene) mol(Cr) -1 h -1 .These results are consistent with those observed in 6-membered ring nickel and chromium catalysts bearing an analogous tridentate sulfur-bridged ligand. 4,6 The chromium complexes 1a and 2a produce oligomers ranging from C 4 to C 14+ with a high selectivity for a-olefins attaining 67.71 and 73.47%, respectively.Both catalyst systems show similar behaviour for production of a-olefins as shown in Figure 1.This indicates that the central donor atoms and also the pyrazolyl R 1 and R 2 substituents play no significant influence in this series on product distribution.However, enriched fractions in a-C 4 (16.2 wt.%), a-C 6 (18.1 wt.%) and a-C 12 (9.7 wt.%) are obtained using the catalyst system derived from 2a.This result is consistent with those ones found for MAO-activated nickel and chromium complexes bearing analogous tridentate ligands. 4,6  It is interesting to note that the presence of phenyl groups at 3-position of the pyrazolyl rings in CrCl 3 {bis[2-(3-diphenyl-1-pyrazolyl)ethyl)]amine} (1a) determines a dramatic effect on activity and selectivity as compared to the similar complex CrCl 3 {bis [2-(3,5dimethyl-1-pyrazolyl)ethyl)]amine} (1b) 6 (compare entries 1 and 3, Table 1).For instance, catalyst 1a (22,900 mol(ethylene) mol(Cr) -1 h -1 ) is 2.2 times more active than 1b (10,400 mol(ethylene) mol(Cr) -1 h -1 ).Furthermore, the amount of PE produced by 1a represents only 3.2% of the total amount of products (oligomers + PE), while the use of 1b leads exclusively the formation of polyethylene (PE).
On the other hand, the introduction of relatively bulky phenyl substituents at 5-position of the pyrazolyl rings in 2a Scheme 2. as compared to methyl groups in CrCl 3 {bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl)]sulfide} (2b) 6 leads to a noticeable decrease in catalytic activity in (NSN)Cr III systems (compare entries 2 and 4, Table 1) which can be associated to the electronic effects of these phenyl groups to the chromium metal center.The selectivity for C 4 -C 12 fractions remains almost unchanged (compare entries 2 and 4).
Crystal data and structure refinement for 4 are summarized in Table 2, and selected bond distances and angles are listed in Table 3.The molecular geometry and atom-labeling scheme are shown in Figure 2. Complex 4 has a cubane-type {Ni 4 (m 3 -Cl) 4 } core with four Ni II and four chloro ligands occupying alternate vertices.Each nickel center is crystallographically distinct and presents a slightly different distorted octahedral environment.The Ni II atoms are bound to three m 3 -chloro ligands, a chelating anilinomethyl-3,5-dimethylpyrazole and one terminal chloride.
The Ni-N pyrazolyl distances lie in the range of 2.033-2.045Å, while Ni-N anilino bonds are 2.105-2.124Å.The m 3 -chloro ligand in the coordination sphere of each nickel atom yields a Ni-Cl bonding distance lying in the range of 2.447-2.506Å, while the Ni-Cl terminal bonds fall in the range of 2.3449( 8)-2.3799(8) Å.All these values are in agremeent with those disclosed in the literature. 15 The Cl-Ni-Cl angles involving the bridging chlorine ligands are systematically lower than 90°, ranging from 82.73(2) to 86.4(2)°.On the other hand, Ni-Cl-Ni angles are significantly higher, tipically in the range of 93.17-97.40°.Geometric constraints imposed by the cubane core reduce Ni-Cl-Ni angles from the ideal tetrahedral value of 109.5°, a reduction often associated with ferromagnetic coupling of nickel centers. 16The distances varying from 3.588 to 3.726 Å between the nickel atoms in the cluster are beyond significant interactions.

Effect of the auxiliary ligand on catalyst activity and selectivity using nickel(II) complexes bearing chelating NZN ligands
Previous studies on nickel-based catalysts have demonstrated that the incorporation of PPh 3 into catalytic   systems leads to higher activity and longer catalyst lifetime. 17We were curious whether such effects would apply to our nickel catalyst systems.For verifying that, we have chosen the  The ethylene oligomerization behavior of complexes 3 and 5 was investigated in toluene with MAO activation.Representative results are summarized in Table 4.Under optimized conditions (toluene as solvent at 30 ºC under 20 bar of ethylene, Al:Ni ratios of 250) the nickel catalysts 3 and 5 are active for dimerization of ethylene in the absence of PPh 3 (3,TOF = 11,300 mol(ethylene) mol(Ni) -1 h -1 ; 5, TOF = 7,100 mol(ethylene) mol(Ni) -1 h -1 ) with selectivity for 1-butene attaining 84.4 and 87.5%, respectively (Table 4, entries 1 and 4).
The oligomerization reactions performed in the presence of 1 equiv of PPh 3 resulted in much higher TOFs mainly in the case of catalyst system 5/MAO (compare entries 4 and 6).One reasonable explanation for this phenomenon can be associated to the partial substitution of the tridentate NZN ligand by PPh 3 to afford a bidentate NZ/N-Ni-PPh 3 complex.On the other hand, the fact that the active nickel species are coordinated with auxiliary PPh 3 on the vacant coordination sites when lacking ethylene monomers should be also considered.However, in both cases, the presence of PPh 3 promotes the formation of more stable catalytic species and at the same time prevents deactivation by impurities of active reactants in the catalytic species. 18 The presence of this auxiliary ligand in the oligomerization medium plays no significant influence on the total C 4 production with selectivities varying from 99.8 to 100% (compare entries 1/2, and 4/6).On the other hand, the PPh 3 ligand has a strong impact in the selectivity towards the production of a-olefins, decreasing substantially the amount of 1-butene (16.2-20.0%)with a concomitant increase of internal olefins fractions (cis-C 4 : 25.7-36.0%;trans-C 4 fractions: 54.3-47.6%)as can be better visualized in Figure 4. We may reasonably assume that the favored formation of butenes-2 rather than butene-1    is a consequence of the presence of PPh 3 in the milieu, which would block coordination of incoming ethylene and lead to isomerisation rather than b-H elimination of R.
When the quantity of precatalysts 3 and 5 was decreased, very high activities were observed.For instance, with 3.0 mmol of nickel complexes 3 and 5, high TOFs of 45,100 mol(ethylene) mol(Ni) -1 h -1 and 154,200 mol(ethylene) mol(Ni) -1 h -1 were obtained, respectively (entries 3 and 7).This effect can be primarly associated to (i) an easier solubilization of the precatalyst in toluene solution , and (ii) increase of molar ratio [ethylene]/ [Ni]; however, a decrease of excessive exotherms during the oligomerization reactions, which would induce catalyst decay, cannot be ruled out.
The reaction time can have a significant effect on TOFs, and the selectivity.It was found that the activity at 5 min of reaction (Table 4, entry 9) was about 1.5 times lower than at 20 min (Table 4, entry 6), suggesting that this type of catalyst needs a long pre-activation time (up to 5 min) to promote the formation of a higher amount of catalytic species.Increasing the reaction time to 40 min led to a lower TOF value (43,300 mol(ethylene) mol(Ni) -1 h -1 , entry 10), indicating that a partial catalyst deactivation took place.
The reaction time affects significantly the selectivity for 1-butene.As can be seen in Figure 5, the selectivity for C 4 olefins remains high and constant over time (99.7-99.8%).However, varying the reaction time from 5 to 40 min, a larger amount of 2-butenes (from 58.1 to 79.3%) is produced in consequence of the isomerization process (i.e., chain isomerization transfer is favored relative to chain propagation).It should be pointed out that the ability of Ni II complexes to isomerize a-olefins is a well-known process. 19In particular, it was found that increasing the reaction time results in higher quantity of trans-C 4 owing to the isomerization process involving 1-butene and cis-C 4 (Scheme 4).
Activation of nickel complex 5 with diethyl aluminumchloride (DEAC) instead of MAO resulted in a more active system (TOF 125,400 mol(ethylene) mol(Ni) -1 h -1 ).This observation could reflect a better stabilization of active species with DEAC than with MAO, possibly thanks to the chlorine atom.At the same time, the use of DEAC led to slightly improved selectivity for 1-butene (32.1 %), with production of lower amounts of 2-butenes (67.9%).

Conclusions
A new set of chromium(III) complexes based on tridentate ligands has been prepared and evaluated for ethylene oligomerization under MAO activation.Replacement of the   cis-2-butene,  trans-2-butene.Hexenes, mostly 1-hexene, were also produced in 0.2-0.3%selectivity and are not shown in this figure.Vol. 21, No. 7, 2010 central bridging nitrogen with a sulfur donor atom affords a catalyst that exhibits higher TOF and higher selectivity for a-C 4 , a-C 6 , and a-C 8 .The presence of phenyl groups at 3or 5-position of the pyrazolyl rings determines a pronouced effect on activity and selectivity owing to the steric/electronic effects of these phenyl groups on the chromium metal center.

a
Reaction conditions: toluene = 40 mL, p(ethylene) = 20 bar, oligomerization time = 20 min, MAO [Al]/[Ni] = 250.The results are representative of at least duplicated experiments.b mol of ethylene converted (mol of Ni) -1 h -1 , as determined by quantitative GLC.c C n , amount of olefin with n carbon atoms in the oligomers; a-C n , amount of terminal alkene in the C n fraction, as determined by quantitative GLC.d Oligomerization time = 5 min.e Oligomerization time = 40 min.f Oligomerization reaction using DEAC [Al]/[Ni] = 250.

Table 1 .
Ethylene oligomerization with complexes 1a-b and 2a-b a

Table 2 .
Crystal data and structure refinement for 4

Table 4 .
Ethylene oligomerization with nickel complexes 3 and 5 a