Chromium complexes bearing amidinato-phosphino ligand : synthesis , characterization , and catalytic properties of ethylene tri-/ tetramerization and polymerization †

An amidinato-phosphino ligand ArNvC(R)NH(o-Ph2PC6H4) (Ar = 2,4,6-Me3C6H2, R = Ph (1); Ar = 2,6-iPr2C6H3, R = Ph (2); Ar = 2,6-iPr2C6H3, R = tBu (3)) was prepared. The ligand reacted with CrCl3(THF)3 to yield the N,P-chelation complex [ArNHC(R)vN(o-Ph2PC6H4)]CrCl3(THF) (4–6), and the ligand’s lithium salt ArNvC(R)N(o-Ph2PC6H4)Li reacted with the respective CrCl3(THF)3 and CrCl2(THF)2 to give the N,N,P-chelation complexes [ArNvC(R)N(o-Ph2PC6H4)]CrCl2(THF) (7–8) and {[ArNvC(R)N(oPh2PC6H4)]Cr(μ-Cl)}2 (9–11). Complexes 1–11 were characterized by IR, NMR (for 1–3), EPR (for 4–11) spectroscopy and CHN elemental analysis, of which 3, 5, 8, and 11 were further studied by X-ray crystallography. Upon activation with an organoaluminum cocatalyst, complexes 4–6 were all catalytically active in ethylene tri-/tetramerization along with ethylene polymerization, and complexes 7–11 functioned as well but in ethylene polymerization. The correlation between the structure and the catalytic properties of the catalyst system is discussed.


Introduction
Chromium catalysts bearing the diphosphazane ligand were found in the earlier parts of this century, first by BP researchers and later by Sasol researchers, which exhibited high activity and excellent selectivity in ethylene oligomerization. 1 The ligand consists of a [PNP] framework with tunable substituents at both the N and P atoms, affording an electronic and steric environment for the Cr center to facilitate ethylene trimerization or tri-/tetramerization through the formation of ultimate CrC 6 or CrC 6 /CrC 8 metallocycle(s) as the transient state(s). 2his offers a route for production of 1-hexene and 1-octene capable of challenging the traditional nonselective ethylene oligomerization ways (usually producing Schulz-Flory distributions of oligomers) 3 and others such as pyrolysis of petroleum fractions or waxes.2h Both 1-hexene and 1-octene are well known as comonomers for manufacturing linear lowdensity polyethylene (LLDPE). 4Since then, chromium catalysts bearing various ligands with P-N functionality have received considerable attention.A number of P,N-bidentate ligand-chelation chromium catalysts have been synthesized and employed for this catalytic reaction. 5Besides the abovementioned selective ethylene oligomerization, nonselective ethylene oligomerization and even ethylene polymerization can also take place by means of these catalysts.
A handful of P,N-multidentate ligand-based chromium catalysts have also been developed.The ligands of the A-F types employed in these catalysts are shown in Chart 1. Chromium catalysts incorporated by the A-, 6 E-, 7 or F-type 8 ligands were found to be active for ethylene trimerization along with polymerization, producing hexenes and polyethylenes.The B 5b coordinated chromium catalyst worked only for polymerization to give polyethylene whereas the C 9 coordinated chromium catalyst effected concomitant nonselective oligomerization and polymerization to generate a series of oligomers and polyethylene waxes.The catalytic behavior of the D-type 10 ligand chromium catalysts was complex due to change in the variety of either the donor atoms or the substituents on the ligand's skeleton.Products such as oligomers, polyethylene waxes, and/or high molecular weight polyethylenes were produced.
The nature of the ancillary ligand plays a fundamental role in determining the performance of the chromium catalyst.][9][10] The reason for these multiple behaviors may be the formation of multiple active species with chromium in different oxidation states formed under operando conditions.1a,2c,4d,6b,11 The Cr(II) state is suggested to respond to nonselective oligomerization 12 whereas the Cr(I)/Cr(III) couple states are for selective oligomerization. 13However, which oxidation state is responsible for polymerization remains unclear. 14Nonetheless, studies on these details appear to have been carried out to a lesser extent.3c,4a Herein, we reported a new type of ligand, a phenyl group-connected amidinato-phosphino ligand with the formula ArNvC(R)NH (o-Ph 2 PC 6 H 4 ) (Ar, R: aryl or alkyl, Scheme 1).This ligand enabled formation of Cr(III) complexes by either N,P-chelation or N,N,P-chelation prepared under different reaction conditions.Cr(II) complexes with the N,N,P-chelated ligand have also been synthesized.Catalytic tests have shown that in the presence of organoaluminum as the cocatalyst, the N,Pchelation Cr(III) complexes were active in ethylene tri-/tetramerization along with ethylene polymerization whereas the N,N,P-chelation Cr(III) and Cr(II) complexes were only active in ethylene polymerization.These results inspire a discussion of the different catalytic performances which probably occur by forming multiple active species from the same ligand stabilized chromium catalyst system.
Ligands 1-3 were characterized by NMR ( 1 H, 13 C, and 31 P), IR spectroscopy and CHN elemental analysis.The presence of the PPh 2 group in 1-3 is indicated by the 31 P NMR spectra that show resonances at δ −16.3 ppm for 1, δ −18.2 ppm for 2, and δ −16.1 ppm for 3.These values are a little downfield compared with that of the precursor o-Ph 2 PC 6 H 4 NH 2 (δ −21.0 ppm). 13Formation of the amidinato NvC-NH functionality is evidenced from the 13 C NMR spectra in which carbon resonances at δ 154.0 ppm for 1, δ 153.8 ppm for 2, and δ 153.9 ppm for 3 are exhibited.The IR spectra display the related CvN bond vibrations at ν 1619 cm −1 for 1, ν 1620 cm −1 for 2, and ν 1665 cm −1 for 3.Moreover, the resonances at δ 6.20 ppm for 1, δ 6.24 ppm for 2, and δ 6.46 ppm for 3 in the 1 H NMR spectra are due to the NH proton.The IR spectra present NH bands for 1-3 in a characteristic area (ν 3336 cm −1 for 1, ν 3335 cm −1 for 2, and ν 3399 cm −1 for 3).To confirm the composition and structure of the ligand in detail, single crystals of 3 were grown and further studied by X-ray diffraction.As expected, the character of the NCN-C 6 H 4o-PPh 2 scaffold in 3 is clearly revealed with the tBu group at the C atom and the 2,6-iPr 2 C 6 H 3 group at the terminal N atom (Fig. 1).In this motif, the N(1)-C(1) and N(2)-C(1) bond lengths are 1.281(2) and 1.379(2) Å, respectively.The former bond length falls in the range of those observed in the imine compounds (1.24-1.28Å), 18 while the latter is comparably shorter than that of the N-C aryl single bond (1.410(2) and 1.427(2) Å).These data indicate that an electronic conjugation over the NvC-N skeleton is formed.It should be mentioned that in the 1 H NMR spectra of 1-3, broadening proton resonances were actually observed which implies a slow interchange of the ligand into its resonance form through the NvC-N skeleton in solution (Scheme 1).Similar tautomerization has been observed for the N-phosphinoamidine ligand reported by Sydora and coworkers. 5b The latter resonance form is also found in the formation of the Cr(III) complexes 4-6 (vide infra).
The reactions of 1-3 with CrCl 3 (THF) 3 were each carried out at room temperature (in THF for 1 and 2 and in CH 2 Cl 2 for 3), and complexes 4-6 were readily produced with respective isolated yields of 89%, 82%, and 80% (Scheme 2).Further reactions were performed using CrCl 2 (THF) 2 instead.However, no reaction resulted even we treated the reaction at elevated temperatures and even upon reflux.Complexes 4-6 were characterized by IR spectroscopy which showed that the N-H bond remained (the vibration is at ν 3165 cm −1 for 4, ν 3195 cm −1 for 5, and ν 3328 cm −1 for 6).IR bands for the CvN bond were found at ν 1595 cm −1 for 4, ν 1588 cm −1 for 5, and ν 1588 cm −1 for 6.Meanwhile, EPR spectral analysis was performed, which gave g values of 4.294 for 4, 3.902 for 5, and 4.646 for 6, indicative of formation of the Cr(III) complexes as these data are close to those of other trivalent chromium complexes (3.75-4.38). 19Furthermore, complex 5 was studied by X-ray crystallography.The structural analysis clearly demonstrated N,P-bidentate chelation of ligand 2 at the Cr(III) center with the NH group away from coordination (Fig. 2).This ligation feature is different from those of the aforementioned P,N,P-chelation chromium complexes having the NH group in the E and F types ligands. 7,8The N(1)-C (7) and N(2)-C(7) bond lengths are 1.322(6) and 1.337(6) Å, respectively, showing great electronic conjugation over the NvC-N skeleton.The NH proton facing towards the Cr center then prevents a bond interaction between the Cr and the N NH atom (Cr(1)⋯N(2) 3.312 Å).In this structure, the Cr atom is six-coordinate and adopts a distorted octahedral geometry where P(1), N(1), O(1), Cl(2), and Cr(1) form a basal plane (Δ = 0.0098 Å) and Cl(1) and Cl(3) are located at the top and bottom apex positions (Cl(1)-Cr(1)-Cl(3) 174.3(1)°).
We further employed a metathesis reaction to prepare the chromium complexes.Treatment of 1-3 with nBuLi in THF over the temperature range from −78 to −30 °C produced the related lithium salts that were not isolated but directly used for reaction with CrCl 3 (THF) 3 over the temperature range from −78 °C to room temperature.Complexes 7 and 8 were successfully obtained (Scheme 3), but isolation of a similar complex containing the deprotonated ligand 3 failed, as in the reaction a black suspension was always formed from which it was not possible to obtain the pure target.Still, by this method, complexes 9-11 were readily afforded using CrCl 2 (THF) 2 as the chromium source (Scheme 3).Complexes 7-11 were characterized by IR spectroscopy which revealed the absence of the N-H bond vibration for each compound.Moreover, the IR band for the CvN bond appeared to be red-shifted (ν 1578 cm −1 for 7, ν 1591 cm −1 for 8, ν 1578 cm −1 for 9, ν 1595 cm −1 for 10, and ν 1586 cm −1 for 11) when compared with those of the ligands 1-3.These data are also different from those of complexes 4-6.All of these factors suggest that an N,N,P-coordination mode of the monoanionic ligand at the Cr atom was formed in 7-11, and this mode is confirmed from X-ray crystallographic analysis of the representative complexes 8 and 11.Furthermore, EPR spectral analysis confirmed the Cr(III) state in 7 and 8, which exhibited g values of 3.912 for 7 and 4.349 for 8, both were comparably similar to those of other Cr(III) complexes 19 and complexes 4-6.The EPR signals for 9-11 are silent, suggestive of the Cr(II) state of these complexes, as has been commonly discussed in the other divalent chromium compounds.13f Due to the different oxidation states of the Cr atoms present in 8 and 11, differed molecular structures were exhibited (Fig. 3 and 4).Complex 8 is mononuclear with a trivalent chromium center in a distorted octahedral geometry.The ligand's N, N, and P atoms coordinate at the Cr in a closely meridional arrangement (Δ N(2)C( 7)N( 1)C(1)C( 2)P( 1)Cr(1) = 0.0834 Å) and one THF and two Cl − moieties complete the three remaining positions.This structure can be compared to those of the Cr(III) complexes incorporating the neutral PNP (D, 10 E, 7 and F, 8 Chart 1) and PNN (B, 5b Chart 1) type ligands.In contrast, complex 11 is dinuclear having two divalent chromium centers.Its formation is probably due to a steric structural compromise of the monomeric form of [2,6-iPr 2 C 6 H 3 NvC(tBu)N(o-Ph 2 PC 6 H 4 )]CrCl, as in this form the Cr is four-coordinate and strongly prone to adopting a square-planar geometry. 20In 11, the Cr(1)⋯Cr(1A) separation is 3.410 (1) Å, indicative of almost non-bonding between them.Then, each Cr center adopts a distorted squarepyramidal geometry in which two N and two Cl atoms form the base (Δ N(1)N( 2)Cr(1)Cl(1)Cl(1A) = 0.1865 Å) and the P atom stands at the apex.
Selected bond parameters on the NvC-N moiety in 3, 5, 8, and 11 are summarized in Table 1.Once they have formed the metal complexes, the parameters for the NvC-N skeleton changed dramatically.This indicates a strong bonding interaction between either the Cr(II) or the Cr(III) center with the amidinato-phosphino ligand.The Cr-N bond lengths appear over a close range (1.971(3)-2.114(3)Å).Within the CrCN 2 four-membered chelating rings of 8 and 11, the N-Cr-N bond angles are 64.3(1)°and64.1(1)°, respectively.In addition, the Cr-P bond lengths appear close to those in the Cr(III) complexes (2.464(2) Å in 5 and 2.493(1) Å in 8), but they are significantly shorter than that in the Cr(II) complex 11 (2.772(1)Å) due to the different geometries adopted.The N-Cr-P bond angles within the CrC 2 NP five-membered chelation ring are 76.3(1)°and 79.2(1)°in the Cr(III) complexes 5 and 8, respectively, and this angle is found to be 71.9(1)°in the Cr(II) complex 11.

Ethylene oligomerization/polymerization test on 4-11
In combination with methylalumoxane (MAO) as the cocatalyst, complex 4 acted as a catalyst to produce oligomers along with polymers when exposed to ethylene at 40 bar and 50 °C in toluene as a solvent.The catalytic activity appeared to change   a little when different Al : Cr molar ratios were employed (13.2, 15.1, and 12.5 kg of products per g of Cr per h for the Al : Cr ratios of 800 : 1, 600 : 1, and 400 : 1, entries 3-5 in Table 2).These activities are comparable to those for the Ph 2 PN(Me)-2-C 5 H 4 N/CrCl 3 (THF) 3 /MAO system (Al : Cr of 500 : 1, 30 or 50 bar ethylene, and 60 °C; 8.1-38.9kg of products per g of Cr per h), 5c but lower than those for the [R 3 2 PNC(R 2 )NHR 1 ]CrCl 3 (THF)/MMAO-3A system (Al : Cr of 400-800, 48 or 60 bar ethylene, and 50-70 °C; 70.1-1051.9kg of products per g of Cr per h).5b The polymers produced were analyzed to have molecular weights in the range of 1.14-1.97× 10 5 g mol −1 .Corresponding differential scanning calorimetry (DSC) measurements gave melting points of 135-141 °C, indicating the linear character of these polymers. 21The oligomers were analyzed and found to be composed of 1-C 6 and 1-C 8 olefins as the dominant parts (66-78%) together with other cyclic C 6 components (methylcyclopentane and methylenecyclopentane, 10-19%) and C 10+ species (10-14%).These data arise as a result of selective ethylene tri-/tetramerization.[3]5 It was noted that the proportion of C 8 : C 6 changed as well with the Al : Cr ratio (0.48 : 1 for 800 : 1, 1.20 : 1 for 600 : 1, and 0.50 : 1 for 400 : 1).This result, along with the aforementioned activities, implies a nonnegligible influence of the Al : Cr molar ratio used in this system.The ratio may relate to the formation of catalytically active site(s) over the course of the interactions between 4 and MAO (vide infra).Nonetheless, in these catalytic reactions an Al : Cr molar ratio of 600 : 1 is the best ratio to achieve the higher activity as well as the yield of the 1-C 8 product.
Further investigation found that an alteration of the Al cocatalyst had an effect on the reaction result as well.When using either AlMe 3 or Et 2 AlCl instead, there was almost no activity (entries 6 and 7).However, a combination of AlMe 3 and Et 2 AlCl led to a reaction, producing polymers with a significantly broad PDI (73.9) although the distribution of the oligomers generated was comparable to that of entry 5. But, the activity of this reaction decreased remarkably (7.2 kg of products per g of Cr per h, entry 8).Gambarotta, Duchateua and coworkers have reported no activity for the catalyst system of [Ph 2 PN(Cy)PPh 2 ]CrCl 3 activated by AlMe 3 .But in the additional presence of MAO or other activators the catalyst system worked. 22Similar work has also been reported by the groups of McGuinness, Tooze, and Overett. 23Then, a cooperation between AlMe 3 and Et 2 AlCl may be responsible for getting 4 to work, although the activity appears lower.
On the basis of the conditions used in entry 3, increasing the concentration of the catalyst to double produced more products as expected, but the activity was calculated to decrease a little (10.6 kg of products per g of Cr per h, entry 9), although the results including the M w , PDI, and DSC data for the polymer and the distribution data for the oligomer are similar to those in entry 3. The higher concentration of the catalyst has a close relation to the formation of the number of real catalytic sites.It has been well indicated that under a steady concentration of the catalyst, catalytic performance often depends on the reaction temperature as well as the ethylene pressure. 24This catalyst system showed decreased reaction activity with decreasing ethylene pressure, but little change was observed for the above-mentioned characters of both the polymers and oligomers produced (11.3 and 10.2 kg of products per g of Cr per h at 30 and 20 bar, entries 12 and 13, reespectively).Keeping the ethylene pressure at 40 bar, and altering the temperature to 30 °C decreased the activity down to 11.2 kg of products per g of Cr per h (entry 11) whereas at 70 °C the activity rose to 18.9 kg of products per g of Cr per h (entry 10).Meanwhile, the C 8 : C 6 ratio of the oligomers changed dramatically with the temperature (1.23 : 1 at 30 °C and 0.63 : 1 at 70 °C versus 1.20 : 1 at 50 °C).Low temperature substantially favors the formation of the CrC 8 -metallocyclic transient state that results from the insertion of one ethylene molecule into the CrC 6 -metallocycle intermediate, as has been extensively discussed. 1,2,24To evaluate the effectiveness of the system, an experiment with a longer running time of 4/MAO (Al : Cr of 600 : 1) at 40 bar ethylene and 50 °C was carried out.In the first 2 h the activity was retained (15.2 and 15.1 kg of products per g of Cr per h within 1 h and 2 h, entries 14 and 15, respectively).By extending the time to 3 h (12.5 kg of products per g of Cr per h, entry 16) and 5 h (7.9 kg of products per g of Cr per h, entry 17) the activity appeared to decrease gradually.However, the features of the oligomer and polymer products produced showed little change.
Complexes 5 and 6 both possess a ligand-metal chelation structure similar to that of 4 and investigation of their catalytic properties was accomplished by controlling the reaction temperature at 50 °C and ethylene pressure at 40 bar in the presence of MAO (Al : Cr of 600 : 1).As a result, different activities were exhibited (11.0 and 10.4 kg of products per g of Cr per h, entries 18 and 19) when compared with that for 4 under the same conditions (entry 3).Moreover, in the oligomer products the C 6 components increased to 58% and 56%, respectively while the C 8 part dropped to 29% and 30%.The polymer products showed differences with respect to the M w , PDI, and MPt values as well.Obviously, variation of the R and Ar substituents on the respective C and N atoms of the ligand backbone has an influence on the catalytic result.Also under the above conditions, complexes 7-11 were tested.To our surprise, only ethylene polymerization catalysis functioned, all producing polymers without the oligomers (entries 20-24), even though we had further screened the reaction by using greater amounts of the complex and/or MAO as well as by increasing the reaction temperature and/or ethylene pressure.In light of these results, it is worth noting that the same ligand-ligated Cr(III) and Cr(II) complexes 7 and 9 give close activities (13.7 and 12.9 kg of products per g of Cr per h, entries 20 and 22).Moreover, the characteristics of the polymers were similar.The same case is also observed for complexes 8 and 10 (6.5 and 8.1 kg of products per g of Cr per h, entries 21 and 23).But, the results differed between the 7 (or 9)/MAO versus 8 (or 10)/MAO systems.
The P,P-or P,N-bidentate chelation chromium catalyst systems have been well known to effect the selective ethylene tri-/tetramerization catalysis that produces the 1-C 6 and 1-C 8 dominant and C 10+ parts.In these systems, ethylene polymerization was sometimes observed, but produced fewer polymers.1b,2,5 Our experiments show that the 4 (5 and 6)/MAO systems produce oligomers and polymers of approximately equal amounts while the 7 (8, 9, 10, and 11)/MAO systems generate only the polymers. 25This suggests that in the 4 (5 and 6)/MAO systems, besides ethylene tri-/tetramerization, the polymerization reaction is not neglected.Structural analysis has indicated the difference between complexes 4-6 and 7-11 where the former complexes feature an N,P-chelation of the ligand at the Cr center while the latter ones feature N,N,P-chelation.Sydora and coworkers have reported that the N-phosphinoamidine N,P-chelation Cr(III) complex/MMAO-3A system initiated selective ethylene tri-/tetramerization while the related N,N,P-chelation Cr(III) complex/MMAO-3A system resulted in polymerization although the polymers were produced in a trace amounts.5b Gambarotta and coworkers have reported a deprotonation reaction of the ligand's NH group in the chromium complexes by organoaluminum compounds.10b We have also found a deprotonation reaction from model reactions using 4 with MAO or AlMe 3 , as indicated by the absence of the NH IR band, by measuring the data of the product upon removal of all volatiles after the reaction, although an isolation of the pure species was not successful.Then, we speculate that the formation of two types of active species in the 4/MAO system is responsible for the multiple catalytic performances.One could be the N,P-chelation chromium complex but with an aluminum-containing moiety attached at the N atom replacing the original H group, 11g and the other could be the N,N,P-chelation chromium complex comparable to that resulting from the 7 (or 9)/MAO system.The former can catalyze the ethylene tri-/tetramerization reaction but the latter enables ethylene polymerization.Similar cases are also possible for the same ligand ligated chromium systems of 5/MAO versus 8 (or 10)/MAO as well as those of 6/MAO versus 11/MAO.

Conclusions
In summary, phenyl group-connected amidinato-phosphino ligands 1-3 have been prepared.Three types of complexes including N,P-chelation Cr(III) (4-6), N,N,P-chelation Cr(III) (7-8) and Cr(II) (9-11)  additional NH group in the ligand backbone and react with MAO to form two, probably different, active sites for the catalytic function of the respective ethylene tri-/tetramerization and polymerization reactions.

X-ray crystallographic analysis
The crystals were mounted on a glass fiber using an oil drop method and quickly placed under a low temperature (173 K) atmosphere.Crystallographic data was collected on an Oxford Gemini S Ultra system.During the measurements, graphitemonochromatic Mo-K α radiation (λ = 0.71073 Å) was used for compounds 3, 5, and 11 while Cu-K α radiation (λ = 1.54178Å) was employed for compound 8. Absorption corrections were applied using a spherical harmonics program (multi-scan type).All structures were solved by direct methods (SHELXS-96) 27 and refined against F 2 using SHELXL-97. 28In general, the non-hydrogen atoms were located by difference Fourier synthesis and refined anisotropically, and hydrogen atoms were included using a riding model with U iso tied to the U iso of the parent atoms unless otherwise specified.

General procedure for catalytic reaction
The catalytic reaction was carried out in a 300 mL stainless steel autoclave equipped with a mechanical stirrer, temperature controller and internal cooling system.In a typical experiment, the reactor was heated to 100 °C under vacuum for 5 h to maximize the removal of air and moisture inside and then cooled to room temperature prior to use.After flushing with nitrogen gas three times, the catalyst solution (in 20 mL solvent), co-catalyst (MAO), and solvent were injected in turn into the autoclave.The reactor was pressurized with ethylene and then heated to the desired temperature.After the reaction, the ethylene feed was stopped and the reactor was quickly cooled to ∼2 °C through a H 2 O/glycol low temperature cycling system.By depressurization, the reaction was quenched with a 10% HCl/ethanol solution.The liquid product was dried over anhydrous sodium sulfate and then GC-FID analysis was performed.The solid product was dried overnight in an oven at

Fig. 1
Fig.1X-ray molecular structure of 3 with thermal ellipsoids at 50% probability level.All H atoms, except for that of NH, are omitted for clarity.

Fig. 2
Fig.2X-ray molecular structure of 5 with thermal ellipsoids at 50% probability level.All H atoms, except for that of NH, are omitted for clarity.

Fig. 3
Fig. 3 X-ray molecular structure of 8 with thermal ellipsoids at 50% probability level.H atoms are omitted for clarity.
complexes have been subsequently prepared through different synthetic routes.Upon activation with MAO, all of these metal complexes are catalytically active.Complexes 4-6 each catalyze the ethylene tri-/tetramerization reaction along with polymerization while complexes 7-11 enable only polymerization.Complexes 4-6 all contain an This journal is © The Royal Society of Chemistry 2016 Dalton Trans., 2016, 45, 17020-17029 | 17025 Open Access Article.Published on 22 September 2016.Downloaded on 15/08/2017 13:02:38.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.