Synthesis , Characterization and Ethylene Oligomerization Studies of Chromium Complexes Bearing Imino-Furfural Ligands

Uma série de complexos de cromo(III) contendo ligantes imina-furfural (Cr1-Cr4) foram sintetizados e caracterizados por espectrometria de massas de alta resolução (HRMS). Todos os pré-catalisadores de cromo, ativados com metilaluminoxano (MAO), apresentaram atividade moderada na oligomerização do etileno [frequência de rotação (TOF) = 11.800-23.200 mol(etileno) mol(Cr) h)] produzindo oligômeros na faixa de C4-C12+ e com boa seletividade para olefinas-α. Os pré-catalisadores de cromo(III) formados in situ pela combinação do ligante imina-furfural L1 com [CrCl3(THF)3] ou [Cr(acac)3] apresentaram baixas atividades, produzindo oligômeros juntamente com quantidades variáveis de polietileno. A utilização de diferentes compostos de cromo e cocatalisadores influenciam a atividade bem como a seletividade para a produção de olefinas-α, o que sugere que diferentes espécies catalíticas são formadas.


Introduction
][10] A nonselective oligomerization is closely reminiscent of a polymerization randomly truncated at the early stages of the chain growth (Cossee-Arlman mechanism). 11mong the transition-metal-based catalysts, chromium catalysts occupy a unique position, since they provide both selective (commercially viable tri-, and tetramerization catalytic systems) [12][13][14] and nonselective ethylene oligomerization.Typical examples are the Chevron Phillips trimerization catalyst, 15 the first and sole trimerization system to be successfully commercialized, and the few existing tetramerization systems with 1-octene selectivities in the range of 70%. 16,17n the past years, several well-defined ethylene oligomerization chromium catalysts bearing NˆNˆN, [18][19][20][21][22] PˆNˆP, [23][24][25][26][27][28][29][30][31][32][33][34][35][36] SˆNˆS, [37][38][39][40] PˆN [41][42][43] ligands have been reported.Such bi-and tridentate ligands play a central role in stabilizing a particular oxidation state and consequently in determining the catalytic behavior (selective vs. nonselective).Particularly, the BP Chemicals and Sasol PNP-chromium complexes (PNP = PhCH 2 N(CH 2 CH 2 PPh 2 ) 2 ), stabilized by neutral RN(PAr 2 ) 2 ligands, have marked a milestone in this field.These catalysts oligomerize ethylene with high selectivity toward either 1-hexene or 1-octene, depending on the ligand substituents (Ar = 2-OMe-C 6 H 4 , C 6 H 5 , respectively). 16,44,45][48][49][50] More recently, Gambarotta and co-workers 51 reported chromium complexes bearing a series of pyridinephosphine ligands and their catalytic behavior in ethylene oligomerization.The solvent choice has a pronounced influence on the catalytic activity as well as on the PE/oligomer ratio.The preference for aliphatic or aromatic surroundings is dependent on the ligand system.Variations of the ligand structure have demonstrated that a dramatic change in catalytic behavior can be obtained upon a subtle modification in the ligand skeleton.It has been demonstrated that minor differences in the ligand structure can result in remarkable changes not only in catalytic activity but also in selectivity toward α-olefins vs. polyethylene and distribution of oligomeric products.Ligand PyCH 2 N(Me) P i Pr 2 , in combination with [CrCl 3 (THF) 3 ] afforded selective ethylene tri-and tetramerization, giving 1-hexene and 1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE. 51 In this work, we report a series of chromium complexes supported by imino-furfural ligands and investigated their catalytic behavior for ethylene oligomerization.We also discuss the performance of in situ-generated catalysts evaluating the effect of chromium sources and cocatalyst type on their activity and selectivity towards the production of α-olefins.

General procedures
All manipulations involving air-and/or watersensitive compounds were carried out in an MBraun glovebox or under dry argon using standard Schlenk techniques.Solvents were dried from the appropriate drying agents under argon before use.[CrCl 3 (THF) 3 ], [Cr(acac) 3 ], 2-phenoxyethanamine, 5-methylfurfural, 2-phenoxybenzenamine, 2-methoxybenzylnamine, and furfural were purchased from Sigma-Aldrich and used as received.Ethylene (White Martins Co.) and argon were deoxygenated and dried through BTS columns (BASF) and activated molecular sieves prior to use.Methylaluminoxane (MAO) (Witco, 5.21 wt.% Al solution in toluene), polymethylaluminoxane-improved performance (PMAO-IP) (Akzo Nobel, 13.0 wt.% Al solution in toluene) was used as received.Ethylaluminum sesquichloride (EASC) (Akzo Nobel) was used with the previous dilution (2.1 wt.% Al solution in toluene).Infrared spectra (IR) were performed on a Bruker FT-IR Alpha Spectrometer. 1 H and 13 C{ 1 H} nuclear magnetic resonance (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.Elemental analysis was performed by the Analytical Central Service of the Institute of Chemistry-USP (Brazil) and is the average of two independent determinations.High-resolution mass spectra (HRMS) of chromium precatalysts (Cr1-Cr4) were obtained by electrospray ionization (ESI) in the positive mode in CHCl 3 solutions using a Waters Micromass® Q-Tof spectrometer.Quantitative gas chromatographic analysis of ethylene oligomerization products was performed on an Agilent 7890A instrument with a Petrocol HD capillary column (methyl silicone, 100 m length, 0.25 mm i.d. and film thickness of 0.5 μm) operating at 36 °C for 15 min followed by heating at 5 °C min -1 until 250 °C; cyclohexane was used as the internal standard.

Ethylene oligomerization
All ethylene oligomerization tests were performed in a 100 mL double-walled stainless Parr reactor equipped with mechanical stirring, internal temperature control and continuous feed of ethylene.The Parr reactor was dried in an oven at 120 °C for 5 h prior to each run, and then placed under vacuum for 30 min.A typical reaction was performed by introducing toluene (30 mL) and the proper amount of cocatalyst into the reactor under an ethylene Vol. 25, No. 12, 2014   atmosphere.After 20 min, the toluene catalyst solution (10 mL, [Cr] = 10 μmol) was injected into the reactor under a stream of ethylene and then the reactor was immediately pressurized.Ethylene was continuously fed in order to maintain the desired ethylene pressure.After 15 min, the reaction was stopped by cooling the system to -60 °C and depressurizing.An exact amount of cyclohexane was introduced (as an internal standard) and the mixture was analyzed by quantitative gas-liquid chromatography (GLC).

Crystal structure determination
Diffraction data for L 1 and L 2 were collected at 150(2) K using a Bruker APEX CCD diffractometer with graphite monochromated MoKα radiation (λ = 0.71073 Å).A combination of ω and ϕ scans was carried out to obtain at least a unique data set.The crystal structures were solved by direct methods; the remaining atoms were located from difference Fourier synthesis followed by full-matrix least-squares refinement based on F 2 (programs SIR97) 52 and then refined with full-matrix least-square methods based on F2 (SHELXL-97) 53 with the aid of the WINGX program. 54All non-hydrogen atoms were refined with anisotropic displacement parameters.H atoms were finally included in their calculated positions.Crystal data and details of data collection and structure refinement for L 1 and L 2 can be obtained, free of charge, from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif (CCDC 1015675 and 1015679).

Theoretical calculations
Full unconstrained geometry optimizations of all species were performed at density functional theory (DFT) level using the B3LYP hybrid functional formed by the three parameter fit of the exchange-correlation potential suggested by Becke 55 and the gradient-corrected correlation functional of Lee, Yang and Parr. 56The polarized Dunning-Huzinaga DZ basis set 57,58 was used for the hydrogen, carbon, nitrogen, oxygen and chloride atoms.For the chromium atom the inner shell electrons were represented by the Los Alamos effective core potential (LANL2) of Hay and Wadt 59,60 and the valence electrons were explicitly included using the associated DZ basis set.All calculations were performed with the Gaussian 09 program using standard procedures and parameters. 61

Results and Discussion
The imino-furfural proligands L 1 -L 4 were readily synthesized by Schiff base condensations between the corresponding primary amines and the corresponding furfural in refluxing ethanol (Scheme 1).These proligands were characterized by IR, 1 H and 13 C NMR spectroscopy, elemental analysis, and by an X-ray diffraction study for proligands L 1 and L 2 .The 1 H NMR spectra of L 1 -L 4 in CDCl 3 at room temperature exhibit resonances in the region d 8.01-8.34ppm assigned to the imine proton (HC=N), with the corresponding 13 C NMR resonances for the carbons of the imine moieties at ca. d 157 ppm.In the solid state, the IR spectra of imina-furfural proligands showed the vibration modes of the imine (C=N) unit at 1635-1649 cm -1 .
Single crystals of the proligands L 1 and L 2 suitable for crystal X-ray diffraction analysis were obtained by slow evaporation from pentane solution.Crystal data and structure refinement are summarized in Table S1 (see Supplementary Information).The molecular geometry and atom-labeling scheme are shown in Figures 1 and 2. The molecular structures of L 1 -L 2 show that the geometry around the C=N bond is essentially co-planar, with phenyl/ alkyl units trans to the furfural moiety.The C=N bond length in L 1 and L 2 are similar (1.2735(15) and 1.283(2) Å) and compare well with those observed for related Schiff base ligands. 62,63he reaction of [CrCl 3 (THF) 3 ] with 1.1 eq. of iminofurfural proligands (L 1 -L 4 ) in THF at room temperature affords the corresponding chromium complexes (Cr1-Cr4) which were isolated as brown or red-colored solids Scheme 1. Preparation of imino-furfural proligands.
in moderate to good yields (typically 56-98%).These precatalysts are very moisture sensitive and therefore satisfactory CHN analyses were difficult to obtain.Hence, the identity of Cr1-Cr4 was established on the basis of ESI-HRMS (which indicated the formation of [M-Cl] + ions without the presence of THF molecules coordinated to the metal center).Attempts to recrystallize complexes Cr1-Cr4 from dichloromethane/petroleum ether resulted in amorphous materials, unfortunately not suitable for a single crystal X-ray diffraction analysis.
We performed DFT calculations in order to estimate the preferable coordination mode of the imine-furfural ligand L 2 to generate the chromium precatalyst Cr2.Initial DFT theoretical study carried out assuming the formation of monomeric species with L 2 acting as a tridentate ligand failed to generate a stable structure.The most stable structure was achieved with L 2 acting as a bidentate ligand without coordination of furfural to the metal center as presented in Figure 3. Furthermore, the bidentate behavior of imino-furfural proligands is also supported by cobalt 64 and titanium 65 complexes, in which it was observed that the furfural unit did not coordinate to the metal center due to its low Lewis basicity.2][43] However, in the latter case these monomeric species usually exhibit THF or CH 3 CN in the coordination sphere of the chromium atom.
All chromium complexes were tested for ethylene oligomerization at 80 °C, 20 bar of ethylene pressure, and using MAO as cocatalyst.Table 1 summarizes the results of reactions carried out using 10 μmol of precatalyst in 40 mL of toluene.All chromium complexes investigated have been found to generate active systems for the linear oligomerization of ethylene with turnover frequencies (TOFs) varying from 11,800 to 23,200 mol(ethylene) mol(Cr) -1 h -1 .Among the catalytic systems herein, the Cr1/MAO system shows the highest activity of up to 23,200 mol(ethylene) mol(Cr) -1 h -1 (Table 1, entry 1).The activity results found for this class of chromium precatalysts are much lower, comparable to chromium complexes stabilized by N,P-bidentate ligands, [41][42][43] indicating that the imino-furfural ligands do not provide the formation of very stable active species.The activity of ethylene oligomerization is substantially affected by the ligand environment.For instance, precatalyst Cr1 containing ethylenic bridge unit is ca.2.0 times more active than Cr2 that contains phenyl moiety (Scheme 3).This result suggests that the presence of a weak electrondonating group (phenyl unit) increases the Lewis acidity of Cr(III) and thus destabilizes the active species.
All chromium complexes Cr1-Cr4 produce oligomers ranging from C 4 to C 12+ with a good selectivity for α-olefins.As shown in Figure 4, the selectivities for 1-alkenes afforded by these precatalysts are similar.This indicates that the pendant O-donor group plays no significant influence in this series on the product distribution.However, it should be pointed out that precatalysts Cr2 and Cr4 having more rigid pendant O-donor moiety, exhibited higher 1-octene selectivity (Cr2: 17.4 wt.%; Cr4: 17.1 wt.%) compared to other precatalysts.0][71] This approach avoids the process complexity and cost of preparing a procatalyst Cr-ligand complex while still obtaining an active and selective catalyst.However, attempts to apply similar in situ complexation with L 1 were less successful.
In this preliminary study, L 1 was mixed with [CrCl 3 (THF) 3 ] or [Cr(acac) 3 ] in dry toluene and stirred for at least 4 h at room temperature before each run.First, the effect of different chromium precursors on the catalytic behavior was investigated.The results, with comparison against Cr1, are shown in Table 2.While the in situ activation method resulted in poor activities, it is clear that the change in chromium sources causes different selectivities toward α-olefins production, which suggests that the different active species are formed as presented in Figure 5.The use of [CrCl 3 (THF) 3 ] led to improved selectivities for the α-C 4 (14.5 to 31.6 wt.%) and α-C 6 (17.6 to 25.6 wt.%) fractions and only small amount of higher olefins (C 12 + : 7.7 wt.%) along with a higher amount of PE (28.3 wt.%).On the other hand, the use of chlorine-free chromium exhibited higher selectivity for production of α-olefins (90 wt.%) with almost 45% related to C 12 + fraction.Melting and crystallization of the polyethylenes produced by Cr/L 1 under MAO activation were measured  by means of thermal analysis (differential scanning calorimetry, DSC) as a function of chromium sources.The use of [CrCl 3 (THF) 3 ] generates high-density polyethylene with melting temperature of 133 °C and crystallinity in the range of 44%.On the other hand, the use of [Cr(acac) 3 ] produces a highly branched polyethylene (BPE) with one endothermic peak at 123 °C and crystallinity of only 8% (see Supplementary Information).In this case, we assume that the formation of BPE arises from incorporation of the in situ produced α-olefins into the growing polymer chain.
A recent study on the Sasol Cr/PNP system shed light on the role of the cocatalyst during catalysis, suggesting that the selectivity can be affected remarkably by the strength of the interaction between the chromium center and the aluminum species. 72Thus, the influence of cocatalyst on the catalytic behavior was subsequently investigated.The in situ catalytic testing using [Cr(acac) 3 ]/L 1 in toluene varying the cocatalyst type (EASC and PMAO-IP) showed some interesting differences (Table 2).
Using EASC instead of MAO shifted the system to ethylene polymerization with substantial production of polyethylene (76.5 wt.%) (Table 2, entry 7) and only 23.5 wt.% of the total amount of products corresponds to linear α-olefins.In this case, a very small amount of C 12 + was detected (3.80 wt.%).The use of also promotes the formation of a substantial amount of PE and a production of lower amount of α-olefins (40.9 wt.%).However, an improvement in the selectivity for 1-hexene and 1-octene formation was observed compared to the use of EASC, as shown in Figure 4.In both cases, the DSC curves show the formation of high-density polyethylenes (HDPE) with melting temperatures in the range of 132-134 °C and crystallinities around of 60% (see Supplementary Information).

Conclusions
In summary, a new set of chromium(III) complexes based on imino-furfural ligands has been prepared and evaluated for ethylene oligomerization under MAO activation.DFT calculations suggest a bidentate coordination mode for this class of ligand, which can generate dimeric Cr species.The selectivities for 1-alkenes afforded by these precatalysts are similar, suggesting that the pendant O-donor group plays no significant influence in this series on the product distribution.However, the presence of the ethylenic bridge unit in L 1 generates more catalyst activity as compared to the one containing the phenyl moiety (L 2 ), suggesting that the presence of a weak electron-donating group (phenyl unit) increases the Lewis acidity of Cr 3+ and thus destabilizing the active species and decreasing the catalyst lifetime.The in situ activation method resulted in poor activities, which is most likely  the result of the poor solubility of the catalysts in toluene.The use of different chromium sources and cocatalyst influences the activity as well as the selectivities toward α-olefin production, which suggests that different active species are formed.