Methylene-bridged Bimetallic Bis ( imino ) pyridine-Cobaltous Chlorides as Precatalysts for Vinyl-terminated Polyethylene Waxes

Please do not adjust margins a Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: whsun@iccas.ac.cn b CAS Research/Education Center for Excellence in Molecular Sciences, University of Chinese Academy of Sciences, Beijing 100049, China. c School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China. E-mail: zhangwj@bift.edu.cn d Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK. E-mail: gas8@le.ac.uk. †CCDC 1828025 for Co1. For crystallographic data in CIF or other electronic format see DOI: 10.1039/XXXXX. Electronic Supplementary Information (ESI) available: [details of any supplementary Received 00th January 20xx, Accepted 00th January 20xx


Introduction
Late transition metal precatalysts that can be used to promote ethylene polymerization have attracted considerable attention due, in large measure, to their ease of preparation and to their capacity to generate a range of highly prized materials including highly linear polyethylene, waxes and highly branched polyethylenes.In particular, α-diimino-nickel(II) chlorides 1 and bis(imino)pyridine-iron(II) or -cobalt(II) chlorides 2 based on a single metal center, have been the subject of a wealth of reports since their first disclosure in the mid to late 1990's.Inspired by the synergistic role played by two closely located metal centers in enzymes such as that used for phosphate ester hydrolysis, 3 researchers have also been interested in applying similar concepts to the design of olefin polymerization catalysts.Indeed, this bio-inspired approach, in which the two polymerization-active metals are compartmentalized on the same multidentate ligand framework, has seen the development of catalysts that display activities and polyolefin microstructures that are not achievable via their mononuclear analogs. 4ith regard to dinuclear cobalt systems, a range of pyridylimine-based supporting ligands have been developed including ones that incorporate inequivalent bi-and tridentate ligand pockets (A -D, Chart 1).In terms of catalytic performance, A showed moderate activity for ethylene oligomerization, 5 while B exhibited much higher activity and notably greater than its mononuclear analogs. 6Similarly, dinuclear C exhibited very good activity (up to 1.7 × 10 7 g•mol - 1 •h -1 ) for ethylene oligomerization with the major product being 1-butene. 7By contrast, D showed much lower activity than C but produced a mixture of oligomer and polymer. 8imetallic cobalt precatalysts based on linked bis(imino)pyridines such as E -G have also been the source of a number of reports (Chart 1).Indeed, all these complexes, containing apparently equivalent binding domains, display moderate to good activity for ethylene polymerization and, what is more, higher activity than that observed for their monomeric analogues.For example, E can reach activities as high as 2.4 × 10 7 g(PE)•mol -1 •h -1 , which is significantly greater than that reported for the corresponding mononuclear bis(imino)pyridine-cobalt precatalyst. 9In addition, biphenyl-bridged bis(imino)pyridine-cobalt chloride F, exhibited four times higher activity than its monomeric counterparts producing highly linear polyethylene.Moreover, these systems showed better thermal stability by operating efficiently at 50 o C. 10 Elsewhere, Takeuchi's group has reported the 'double-decker' binuclear cobalt species G which, though displaying only moderate activity, generated polyethylene with molecular weight much higher than its mononuclear comparators.Justification for this molecular weight enhancement was given This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins in terms of a cooperative interaction between the growing polymer and the second metal center that efficiently retarded undesirable deactivation of the catalyst and/or chain transfer. 11Nevertheless, cooperative effects of this type remain scarce in cobalt chemistry and hence there is a drive to broaden the range of bimetallic cobalt polymerization catalysts so as to further probe such effects. 12n this work we target the phenol-substituted methylenebridged bis(imino)pyridine-dicobalt chlorides H (Chart 1) with a view to assessing their performance as ethylene polymerization precatalysts.The role of steric factors imparted by the ligand frame will be investigated as will the nature of the co-catalyst and the temperature of the polymerization run.In addition, full synthetic and characterization data is presented for both the ligands and their corresponding complexes.
Chart 1 Previously reported dinuclear cobalt precatalysts, A -G, and target H.
Crystals of Co1 suitable for an X-ray determination were grown by slow diffusion of diethyl ether into a methanol solution containing the complex.A view of the structure is shown in Figure 1a; selected bond lengths and angles are tabulated in Table 1.The structure consists of two cobalt dichloride units that are bound within the two essentially planar tridentate N,N,N-pockets belonging to L1.The fivecoordinate geometry at each metal center can be best described as distorted trigonal bipyramidal with the pyridine nitrogen atom and two chlorides defining the equatorial plane.For each metal, the N-aryl groups are inclined almost perpendicularly to the neighboring imine vectors with the dihedral angles being 85.26° and 89.10° for Co(1) and 84.93° and 87.69° for Co(2).Of the three Co-N bond distances to each metal center, that involving the central pyridine [2.025(4), 2.030(4) Å] is shorter than the Co-Nimine distances [range: 2.219(4) -2.272(4) Å].Indeed, these Co-Nimine lengths are comparable to those observed in one example of a Please do not adjust margins Please do not adjust margins mononuclear bis(imino)pyridine-CoCl2 (ca.2.211 Å). 14 Due to the sp 3 -hybridisation of the bridging CH(C6H4-4-OH) group, the dihedral angle between the two N,N,N-chelation planes (N1-N2-N3 and N4-N5-N6) is 53.48°, with the result that the cobalt centers are positioned 13.339 Å apart.Additionally, the presence of the OH functionality on the phenol unit leads to adjacent molecules of Co1 assembling via OHO hydrogen bonding (2.706 Å, see Figure 1b).

Scheme 1 Synthesis of L1 -L4 and their binuclear complexes Co1 -Co4
The microanalytical data for all four complexes are consistent with each adopting a composition based on [(L)Co2Cl4].In their IR spectra the ν(C=N)imine stretching frequencies fall in the range 1617 -1621 cm -1 which compares to 1639 -1642 cm -1 for the free ligands, which provides further evidence that the metals are coordinated by the ligands.

Catalytic evaluation
Previous studies in iron-and cobalt-based ethylene polymerization have revealed that methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) are generally more effective co-catalysts when compared with other alkylaluminum reagents. 15Hence, these two aluminoxanes were independently employed in this study as co-catalysts to assess the performance of Co1 -Co4 as ethylene polymerization precatalysts.
(a) Ethylene polymerization using Co1 -Co4/MAO.Complex Co1 was selected to optimize the polymerization parameters with MAO as co-catalyst; the results are collected in Table 2.In the first instance, the polymerization runs were conducted in toluene at various temperatures between 40 and 70 o C by fixing the Al/Co ratio at 1000 and the ethylene pressure at ten atmospheres.Inspection of the results show the highest polymerization activity of 5.73 × 10 6 g(PE)•mol -1 (Co)•h -1 was achieved at 50 o C (runs 1 -4, Table 2).Raising the temperature further led to a lowering in activity which can be attributed to either decomposition of the active species or to the lower ethylene concentration in toluene at higher temperature.15a,16 Nevertheless, even at 70 o C the activity still reached a good level of 2.35 × 10 6 g(PE)•mol -1 (Co)•h -1 which is notably higher than that seen by related mononuclear cobalt precatalysts This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins under comparable conditions, 2 indicating the improved thermal stability of the current system.At the same time the molecular weight of the polyethylene decreased from 7.7 to 4.0 kg mol -1 on increasing the temperature from 40 to 70 o C, which is consistent with faster chain transfer at higher temperature.Figure 2 depicts the GPC traces of the polyethylene produced by Co1/MAO at different temperatures which not only shows a gradual lowering of the molecular weight with temperature, but also highlights the essentially monomodal distribution of the polymers albeit with some shouldering (PDI 3.0 -5.7).By fixing the reaction temperature at 50 o C, the effect of changing the quantity of MAO on the performance of Co1 was also investigated by varying the Al/Co molar ratio between 1000 and 3000 (runs 2, 5-9, Table 2).At a ratio of 2250, the highest activity of 8.89 × 10 6 g(PE)•mol -1 (Co)•h -1 was achieved.Fig. 2 GPC curves of the polyethylene generated using Co1/MAO at different temperatures (runs 1 -4, Table 2) Fig. 3 GPC curves of the polyethylene produced using Co1/MAO at different molar ratio of Al/Co (runs 2, 5 -9, Table 2) Further increasing the Al/Co molar ratio to 3000 gradually decreased the activity from 8.89 to 7.25 × 10 6 g(PE)•mol - 1 (Co)•h -1 .Furthermore, the molecular weight dramatically decreased from 6.7 to 2.9 kg mol -1 on altering the molar ratio Please do not adjust margins Please do not adjust margins from 1000 to 1500.On the other hand, no significant change in the molecular weight was observed on further increasing the molar ratio to 3000 (Mw range: 2.3 to 2.9 kg mol -1 ), an observation that is notably different to that seen in previous reports. 17Indeed, the molecular weight usually dramatically decreases with higher amounts of MAO in line with increased chain transfer to the aluminum.2a These variations in molecular weight with Al/Co ratio are illustrated in the GPC curves (Figure 3); these curves once again show unimodal characteristics for the polymers with some shouldering a feature of each (PDI 2.2 -4.9).It is tempting to ascribe this observation to some multi-site behavior of the catalyst.
To explore the effect of time on the performance of Co1/MAO, the polymerization runs were carried out at intervals between 5 and 60 minutes (run 7, 10 -13, Table 2) with the temperature and Al/Co ratio kept at 50 o C and 2250, respectively.Over the course of the polymerization the activity gradually decreased from 14.6 (5 minutes) to 4.66 × 10 6 g(PE)•mol -1 (Co)•h (60 minutes) in accord with some deactivation of the active species over prolonged reaction times; 18 a feature that is indeed common to many olefin polymerization catalysts. 2Nonetheless, the activity after 60 minutes still remained relatively high at 4.66 × 10 6 g(PE)•mol -1 (Co)•h -1 (run 13, Table 2), highlighting the good catalytic lifetime of the active species when compared with related systems. 19On decreasing the ethylene pressure from 10 to 1 atmospheres (runs 7, 14, 15, Table 2) with the temperature maintained at 50 o C and the Al/Co ratio at 2250, the polymerization activity sharply decreased from 8.89 to 0.19 × 10 6 g(PE)•mol -1 (Co)•h -1 .On the other hand, the molecular weight remained comparable (range: 1.8 -2.3 kg mol -1 ).This dependency of the activity on the pressure can be attributed to better ethylene coordination and in-turn activity at higher pressure.2) with the activity decreasing in the order: Co1 (Me, Me) > Co3 (Et, Me) > Co4 ( i Pr, Me) > Co2 (Et, Et).It would appear that the steric properties exerted by ortho R 1 and R 2 are both influential with increased hindrance leading to lower activity as a result of impeded ethylene coordination and insertion. 21In terms of molecular weight, the polyethylene generated using Co4 is much higher than that shown for Co3, Co1 and Co2 which lends support to the importance of the bulky i Pr (R 1 ) substituents on the external Naryl group.It would seem likely that these hindered i Pr substituents (Co4) protect the active species and hence favor chain propagation leading to higher molecular weight polymer. 20By way of comparison, these systems showed comparable activity to that observed with dicobalt complexes E and F. 9,10 To probe the microstructural properties of the polyethylenes, the sample generated using Co1/MAO (run 7, Table 2) was characterized by 1 H/ 13 C NMR spectroscopy at high temperature (100 o C in 1,1,2,2-tetrachloroethane-d2) (Figure 4).In the 1 H NMR spectrum, downfield multiplets at respectively δ 5.90 (Hb) and δ 5.05 (Ha) in a 1:2 ratio, are indicative of a vinyl-terminated polymer with the more upfield peaks corresponding to the CH2 adjacent to the vinyl (Hc), the main CH2 repeat unit and the methyl group (Hg) at the opposite end of the chain.Such signals are typical of a linear polymer which is corroborated by the 1:3 ratio of the vinylic Hb to methyl Hg protons.The 13 C NMR spectrum further confirms the vinyl-end functionality with characteristic signals at δ 111.4 and 139.5 along with signals for the saturated chain end (Cd, Ce, Cf and Cg) (Figure 4). 22Similar chain-ends were a feature of other polyethylenes formed with MAO as co-catalyst in this study; the corresponding NMR spectra for the polymer formed in run 2 (Table 2) are given in the supplementary information (Figure S1).Collectively, these findings suggest that these MAO-promoted polymerizations have a preference for a termination mechanism involving β-H transfer to metal or to the monomer, 14 rather than the sometimes observed transfer to aluminum. 2Further support for the linearity of the polymers is provided by the typically high melting temperatures (Tm) for all the materials obtained in this particular study (Table 2).It is worthy of note that vinylterminated polymers of type produced here have some interest for use as monomers in the synthesis of functional polyethylenes. 23(b) Ethylene polymerization with Co1 -Co4/MMAO.To complement the investigation performed using MAO and explore potential co-catalysts effects, 24 Co1 -Co4 were also evaluated in combination with MMAO; the results of the polymerizations are compiled in Table 3.
As before, Co1 was selected as the test precatalyst in order to determine the optimal temperature, Al/Co molar ratio and run time.With the Al/Co ratio fixed at 1000, the temperature was varied from 40 to 70 o C (runs 1-4, Table 3) with the highest activity of 4.74 × 10 6 g(PE)•mol -1 (Co)•h -1 achieved at 50 o C. Further increasing the temperature to 70 o C, the activity This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins gradually decreased to 2.87 × 10 6 g(PE)•mol -1 (Co)•h -1 and the molecular weight slightly decreased from 1.8 to 1.5 kg mol -1 .In comparison to the results observed with Co1/MAO, all the polyethylenes possessed much lower molecular weights across the temperature range (1.5 -1.8 kg mol -1 ).Furthermore, all the polymers exhibited narrower unimodal distributions (PDI < 2.0 (Co1/MMAO) vs. > (Co1/MAO)) as is illustrated by their 123.9 a Conditions: 1.5 μmol of the cobalt complex; 10 atm of ethylene; total volume of solvent 100 mL; b Activity: 10 6 g(PE)•mol -1 (Co)•h -1 ; c Mw: kg mol -1 ; d Mw and Mw / Mn determined by GPC; e Determined by DSC; f 1 atm of ethylene; g 5 atm of ethylene.Fig. 5 GPC curves of the polymers generated using Co1/MMAO at different temperatures (runs 1 -4, Table 3) Fig. 6 GPC curves of the polyethylene obtained using Co1/MMAO at different Al/Co molar ratios (runs 2, 5 -9, Table 3) Please do not adjust margins Please do not adjust margins Fig. 7 13 C NMR spectrum of the polyethylene obtained using Co1/MMAO (run 7, Table 3) (recorded in tetrachloroethane-d2: δC 73.8 ppm), along with an insert showing its 1 H NMR spectrum GPC traces (Figure 5).Indeed, these distributions are significantly narrower when compared to the very broad distributions that have previously been a characteristic of dinuclear cobalt catalysts. 6,11The observations noted in this part of the study would suggest similar 'single-site' active species are operational at both metals without any obvious cooperative features on account of the relatively large M•••M distance.As was noted with Co1/MAO, Co1/MMAO also exhibited higher activity than that seen with its mononuclear analogues; 2 a possible explanation derives from the higher net charge on cobalt by the reaction of phenol group with MAO. 25 The influence of the Al/Co molar ratio on Co1 was then probed by varying the ratio from 1000 to 3000 with the temperature maintained at 50 o C (runs 2, 5 -9, Table 3).As with the results obtained using Co1/MAO, the highest activity of 6.19 × 10 6 g(PE)•mol -1 (Co)•h -1 was achieved at an Al/Co molar ratio of 2250; further increasing the amount of MMAO gradually decreased the level of activity.However, the molecular weight of the resultant polyethylene showed almost no change at different Al/Co molar ratios (ranging from 1.4 -1.8 kg mol -1 ), which may be due to limited chain transfer to aluminum. 14Increasing the reaction time from 5 to 60 minutes gradually decreased the polymerization activity from 9.12 to 3.50 × 10 6 g(PE)•mol -1 (Co)•h -1 , while the molecular weight showed little variation; similar findings were noted using Co1/MAO.As mentioned earlier, decreasing the ethylene pressure significantly lowered the polymerization activity from 6.19 × 10 6 g(PE)•mol -1 (Co)•h -1 to 0.35 × 10 6 g(PE)•mol -1 (Co)•h -1 but with a minimal variation of the Mw (runs 7, 14, 15, Table 3).
Based on the optimized conditions for Co1/MMAO [Al/Co ratio at 2250, the temperature at 50 o C, ethylene pressure at 10 atmospheres and run time at 30 minutes], Co2 -Co4 were screened and their performance characteristics compared to that seen for Co1 (runs 7, 16-18, Table 2).All cobalt species exhibited good activity (2.18 -6.19 × 10 6 g(PE)•mol -1 (Co)•h -1 ) toward ethylene polymerization but with values that are much lower than that seen with MAO.Nevertheless, the trend in activity is the same as that seen with MAO: Co1 (Me, Me) > Co3 (Et, Me) > Co4 ( i Pr, Me) > Co2 (Et, Et).Again the steric properties imparted by ortho R 1 and R 2 greatly affect the polymerization activity with the most active system Co1 possessing the least sterically hindered environment at each metal center.In addition, all the polymers again possess narrower distributions (PDI < 2.0) and lower molecular weights than that observed with MAO as co-catalyst, the latter observation being similar to the results shown by the biphenylbridged bis(imino)pyridine-cobalt complexes F. 10 Analysis of the high temperature 1 H NMR spectrum of the polymer obtained using Co1/MMAO (run 7, Table 3) showed resonances characteristic of a -CH=CH2 group (Ha, Hb) which was confirmed in the 13 C NMR spectrum with the corresponding unsaturated carbon signals visible at δ 114.4 and 139.5.Unlike the polymer generated using Co1/MAO, the ratio of the integral for the vinylic Ha (δ 5.02) to methyl Hg protons (δ 0.97) in the 1 H NMR spectrum is slightly lower than 2:3, suggesting the coexistence of some fully saturated polyethylene.Further support for this finding comes from the 1 H NMR spectrum of the polymer formed in run 2 (Table 3) (shown in Figure S2), which shows an integration ratio for Ha/Hg of 2:3.5.Furthermore, the 1 H NMR spectrum of the same polymer in 1,2-dichlorobenzene-d4 reveals the ratio for Hb/Hg to be 1:3.6,while the Hb:Ha:Hc ratio remains close to the expected 1:2:2 (shown in Figure S3).The apparent presence of saturated material can be accounted for by a termination mechanism involving chain transfer to AlMe3 (rather than Al( i Bu3)) and its derivatives present in MMAO. 14It is unclear why with MMAO some degree of saturated polymer is also formed while with MAO uniquely vinyl-terminated materials are generated.

Conclusions
The methylene-bridged bis(imino)pyridines, L1 -L4, and their corresponding dinuclear cobalt complexes Co1 -Co4 have been successfully synthesized and characterized by IR spectroscopy and by elemental analysis.Furthermore, the molecular structure of Co1 was confirmed by single crystal Xray diffraction.On activation with MAO or MMAO, Co1 -Co4 displayed high activities for ethylene polymerization as well as reasonable catalytic lifetimes with the MAO-promoted systems generally resulting in polymer of higher molecular weight, while with MMAO the materials showed narrower distributions.In terms of thermal stability, Co1/MAO was demonstrated to be an effective catalyst at 70 o C and indeed reaching an activity greater than that observed by its most closely related mononuclear comparator.In general, highly linear polymers were formed with high levels of vinyl chainends.It is noteworthy that such vinyl-terminated materials are in demand for the copolymerization with ethylene to form highly branched polyethylene.
Experimental This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

General considerations
All manipulations involving air-and moisture-sensitive compounds were carried out under a nitrogen atmosphere using standard Schlenk techniques.Toluene was refluxed over sodium and distilled under nitrogen prior to use.Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified methylaluminoxane (MMAO, 2.00 M in n-heptane) were purchased from Akzo Nobel Corp. High-purity ethylene was purchased from Beijing Yansan Petrochemical Co. and used as received.Other reagents were purchased from Aldrich, Acros or local suppliers.The NMR spectra of L1 -L4 were recorded on a Bruker DMX 300 or 400 MHz instrument at ambient temperature using TMS as an internal standard, while NMR spectra of the polyethylene were recorded on a Bruker DMX 300 MHz instrument at 100 o C in 1,1,2,2tetrachloroethane-d2 or 1,2-dichlorobenzene-d4 as the solvent with TMS as an internal standard.IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer.Elemental analysis was carried out using a Flash EA 1112 micro-analyzer.Molecular weight and molecular weight distributions (MWD) of the polyethylene were obtained using a PL-GPC220 instrument at 150 o C using 1,2,4-trichlorobenzene as the solvent.The melting temperatures of the polyethylene were measured from the fourth scanning run on a Perkin-Elmer TA-Q2000 differential scanning calorimetry (DSC) analyzer under a nitrogen atmosphere.In the procedure, a sample of about 5.0 mg was heated to 160 o C at a rate of 20 o C min −1 and maintained for 2 min at 160 o C to remove the thermal history and then cooled at a rate of 20 o C min −1 to 20 o C. Compounds CH(C6H4-4-OH)(4-C6H2-2,6-R2NH2)2 (R = Me, Et) and 2-(CMeO)-6-{(CMe=N(2,6-R 1 2C6H3)}C5H3N (R 1 = Me, Et, i Pr) were prepared according to literature procedures. 13(4-((1-(6-(1-((2,6

Ethylene polymerization at 5/10 atm ethylene pressure
The polymerization at 5 or 10 atm of ethylene pressure was carried out in a 250 mL stainless steel autoclave equipped with a mechanical stirrer and temperature controller.The autoclave was evacuated and refilled with nitrogen two times and then with ethylene one time.When the required temperature was reached, the pre-catalyst (1.5 µmol) dissolved in toluene (25 mL) was injected into the autoclave under an ethylene atmosphere (~ 1 atm), followed by the addition of more toluene (50 mL).The required amount of co-catalyst (MAO, MMAO) and additional toluene were added successively by syringe taking the total volume of toluene to 100 mL.The autoclave was immediately pressurized with 5/10 atm.pressure of ethylene and the stirring commenced.After the required reaction time, the reactor was cooled with a water bath and the excess ethylene vented.The reaction was then quenched with 10% hydrochloric acid in ethanol and the precipitated polymer collected, washed with ethanol and then dried under reduced pressure at 50 °C and weighed.Ethylene polymerization at 1 atm ethylene pressure.The polymerization at 1 atm ethylene pressure was carried out in a Schlenk tube.Under an ethylene atmosphere (1 atm), Co1 (1.5 μmol) was added followed by toluene (30 mL) and then the required amount of co-catalyst (MAO, MMAO) introduced by syringe.The resulting solution was stirred at the required temperature under 1 atm of ethylene.After 30 min, the solution was quenched with 10% hydrochloric acid in ethanol.The polymer was washed with ethanol, dried under reduced pressure at 40 °C and then weighed.X-ray structure determination

Fig. 1a
Fig. 1a ORTEP representation of Co1.The thermal ellipsoids are shown at the 30% probability level while the hydrogen atoms have been omitted for clarity.

Table 2
Ethylene polymerization results by Co1 -Co4/MAO a a Conditions:

Table 4 .
Crystal data and structure refinement details for Co1 crystal X-ray diffraction study of Co1 was conducted on a Rigaku Sealed Tube CCD (Saturn 724+) diffractometer