Synthesis, characterization and olefin polymerization behaviors of phenylene-bridged bis-β-carbonylenamine binuclear titanium complexes

Binuclear and multinuclear complexes have attracted much attention due to their unique catalytic performances for olefin polymerization compared with their mononuclear counterparts. In this work, a series of phenyl-bridged bis-β-carbonylenamine [O−NSR] (R = alkyl or phenyl) tridentate ligands and their binuclear titanium complexes (Ti2L1–Ti2L5) were synthesized and characterized by 1H NMR, 13C NMR, FTIR and elemental analysis. The molecular structure of ligand L2 (R = nPr) and its corresponding Ti complex Ti2L2 were further investigated by single-crystal X-ray diffraction, which showed that each titanium coordinated with six atoms to form a distorted octahedral configuration along with the conversion of the ligand from β-carbonylenamine to β-imino enol form. Under the activation of MMAO, these complexes catalyzed ethylene polymerization and ethylene/α-olefin copolymerization with extremely high activity (over 106 g mol (Ti)−1 h−1 atm−1) to produce high molecular weight polyethylene. At the same time, wider polydispersity as compared with the mononuclear counterpart TiL6 was observed, indicating that two active catalytic centers may be present, consistent with the asymmetrical crystal structure of the binuclear titanium complex. Furthermore, these complexes possessed better thermal stability than their mononuclear analogues. Compared with the complexes bearing alkylthio sidearms, the complex Ti2L5 bearing a phenylthio sidearm exhibited higher catalytic activity towards ethylene polymerization and produced polyethylene with much higher molecular weight, but with an appreciably lower 1-hexene incorporation ratio. Nevertheless, these bis-β-carbonylenamine-derived binuclear titanium complexes showed much higher ethylene/1-hexene copolymerization activity and 1-hexene incorporation ratios as compared with the methylene-bridged bis-salicylaldiminato binuclear titanium complexes, and the molecular weight and 1-hexene incorporation ratio could be flexibly tuned by the initial feed of α-olefin commoners and catalyst structures.


Introduction
Polyolens are by far the most important and most produced synthetic polymers today, and the design and synthesis of effective catalysts for olen polymerization and copolymerization is of great interest in both academic research and industrial applications. The discovery of single-site group 4 metallocene catalysts is considered one of the most signicant breakthroughs aer the discovery of Ziegler-Natta catalysts. 1 Thereaer, single-site non-metallocene catalytic systems, including early and late transition metals catalysts, are thought of as another signicant breakthrough, since they can provide novel olen-based materials with superior activity and greater control over polymer microstructures. 2,3 Of the non-metallocene candidates, the group 4 nonmetallocene complexes with bidentate anionic [N, O] chelate ligands, which was rst reported in 1995, 4 have been the focus of attention. A great variety of [N, O] chelate complexes have been reported, among which the most prominent were the group 4 bis(phenoxyimine) ligated complexes (A, Chart 1) reported rstly by Floriani et al. in 1995. 5 Fujita et al. 6 and Coates et al. 7 further developed these ligands and reported some new complexes that are excellent for olen polymerization including ethylene living polymerization, highly syndiospecic propylene living polymerization, living copolymerization of ethylene with a-olen, and the synthesis of functional and block copolymers of propylene. However, very few successful phenoxyimine catalysts have been reported that effectively catalyze random copolymerization of ethylene and other olens due to the low comonomer incorporation ratio. The group 4 transition metal complexes based on bis(b-carbonylenamine) ligands (B,Chart 1) were another group of widely-researched ethylene (co)polymerization precatalysts containing [N, O] chelate ligands. 4,8 To further improve the catalytic performances of the bidentate anionic [N, O] chelated complexes towards ethylene (co) polymerization, Tang and coworkers have developed a series of mono-ligated tridentate [ONX]TiCl 3 complexes (X ¼ O, S, Se, and P) based on either salicylaldiminato or b-carbonylenamine backbone by introducing some sidearms with pendantcoordination heteroatom groups (C and D, Chart 1), 9 which exhibited better catalytic performances due to the tuning of the electronic and steric properties of the active species by the sidearm. These complexes were especially effective for ethylene copolymerization with a-olens, cycloolens or polar monomers, due partially to the less crowded coordination sphere. Furthermore, they could be prepared in one step by simply mixing the tridentate ligands and titanium tetrachloride, without the need to deprotonate the ligands in advance.
More recently, there have also been growing interests in biand multi-nuclear olen polymerization catalysts, 3a-d,10-13 which showed that introducing a proximate metal center could signicantly enhance catalytic properties as compared with the mononuclear analogue due to the creation of high local reagent concentrations, conformationally advantageous active-sitesubstrate proximities, as well as multicenter directed covalent and noncovalent interactions. Most of these works were focused on metallocene and late transition metal complexes, while early transition non-metallocene binuclear complexes have rarely been reported. Marks' group reported a class of naphthoylimine-ligated early transition bimetallic catalysts (E, Chart 2) with moderate activity ($10 4 g mol À1 h À1 atm À1 ) and higher comonomer incorporation ratios compared to the mononuclear analogue due to nuclearity and cooperativity effects in binuclear catalysts. 10c,d Ma and coworkers lately described a bidentate salicylaldimine heteroligated binuclear titanium catalyst (F, Chart 2) with high activity and higher ethylene/1,5-hexadiene copolymerization capability than that of its mononuclear counterpart. 13 Recently we have been committed to developing some novel early and late transition non-metallocene catalysts based on tuning the coordination environment of the active species with electronic and/or steric effects of the substituents. 14 In view of the advantage of the less crowded coordination sphere of tridentate ligands and the cooperative effect of binuclear complexes, we have designed a number of binuclear titanium catalysts with methylene-or xanthene-bridged bis(salicylaldiminato) tridentate ligands (G and H, Chart 2) and investigated their catalytic behaviors for ethylene homo-and copolymerization. 15 Here we describe the synthesis, structure and ethylene (co)polymerization behaviors of a series of novel phenyl-bridged bis-b-carbonylenamine [O À NS R ] (R ¼ alkyl or phenyl) tridentate binuclear titanium complexes Ti 2 L 1 -Ti 2 L 5 (Chart 2).

Synthesis and structure of ligands and binuclear Ti complexes
The synthetic routes for the ligands L 1 -L 6 and the corresponding complexes Ti 2 L 1 -Ti 2 L 5 and TiL 6 were shown in Scheme 1 and 2.
The molecular structures of ligand L 2 and its corresponding Ti complex Ti 2 L 2 were further conrmed by single-crystal X-ray diffraction, as shown in Fig. 1 and 2. The crystal data and details of data collection and renement are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.
Two possible pathways existed for the synthesis of ligands L 1 -L 5 and complexes Ti 2 L 1 -Ti 2 L 5 due to the presence of two different carbonyl groups in phenylene-bridged b-dione (2), as shown in Scheme 3. In the case that 1-phenylbutane-1,3dione was employed for the preparation of enamine, X-ray Scheme 1 Synthesis of binuclear Ti complexes Ti 2 L 1 -Ti 2 L 5 .
Scheme 2 Synthesis of mononuclear Ti complex TiL 6 . crystallographic analysis showed that the acetyl group of 1phenylbutane-1,3-dione reacted with amine (L 7 , Fig. 3). 9c However, in the case of our bis-b-carbonylenamine ligands L 1 -L 5 , the single-crystal XRD proved that the alkylthio anilines reacted with the carbonyl group adjacent to phenylene group (Path B), not the one next to the t butyl group (Path A), which resulted in far-separated and relatively independent titanium centers in complexes Ti 2 L 1 -Ti 2 L 5 and would profoundly inuence their catalytic performances for ethylene (co)polymerization.
From Fig. 1, it can be seen that in L 2 , the N1-C10-C11-C12-O1 and N2-C23-C24-C25-O2 are almost coplanar, which further formed two six-member rings via intramolecular hydrogen bonds between H and O1 or O2. The O1-C12 and O2-C25 bond lengths are 1.251 and 1.255Å, respectively, a little longer than the typical C]O double bond but much shorter than C-O single bond. The C10-N1 and C23-N2 bond distances are 1.376(7) and 1.370(7)Å, respectively, showing clearly that the C-N bonds are single bonds. Thus, the ligand L 2 exists in b-carbonylenamine form. The C11-C12 and C24-C25 bond lengths are 1.444(8) and 1.441(8)Å, respectively, and the C10-C11 and C23-C24 bond lengths are 1.400(8) and 1.379(8)Å, respectively, which are all between C-C single bond (1.54Å) and C]C double bond (1.34Å) and show a certain extent of delocalization of the double bonds. Furthermore, the distances of the corresponding bonds in two b-carbonylenamine units are a little different, which would provide different coordination environments for the two titanium metal centers.

Ethylene polymerization
We investigated the catalytic performances of binuclear complexes Ti 2 L 1 -Ti 2 L 5 towards ethylene polymerization under activation of MMAO, with the mononuclear analogue TiL 6 for comparison, and the results were listed in Table 3.
In general, these binuclear complexes exhibited very high activity (over 10 6 g mol À1 h À1 atm À1 ) under suitable conditions, producing typical high-density polyethylene. The polymerization conditions such as reaction temperatures and Al/Ti molar ratios exerted great inuence upon catalytic activity and polymer properties.
Firstly, we used Ti 2 L 2 as catalyst precursor and explored the inuence of polymerization temperature at 1 atm ethylene pressure with Al/Ti ratio xed at 1000. When the reaction temperature was increased from 30 to 70 C, the activity increased gradually to a maximum at 50 C and then decreased slightly. The highest activity reached 1.68 Â 10 6 g mol (Ti) À1 h À1 atm À1 at 50 C (entry 2, Table 3), which was similar to that catalyzed by the mononuclear analogue TiL 6 /MMAO (1.55 Â 10 6 g mol (Ti) À1 h À1 atm À1 , entry 10 in Table 3). However, the binuclear complex appeared more stable at elevated temperature compared with the mononuclear TiL 6 . At 70 C, the activity of Ti 2 L 2 /MMAO was still of 9.1 Â 10 5 g mol (Ti) À1 h À1 atm À1 , which was more than twice that of TiL 6 /MMAO at the same temperature.
Both bi-and mono-nuclear titanium catalysts catalyzed ethylene polymerization to produce polyethylene with over 10 4 g mol À1 of molecular weight (M w ). The molecular weight distribution (M w /M n ) of polyethylene produced by TiL 6 /MMAO was only 2.59, which was typical of single active center; however the polymer obtained with Ti 2 L 2 /MMAO exhibited much wider polydispersity (3.65), indicating that two active centers may have formed, consistent with the asymmetrical crystal structure of the binuclear complex.
The catalytic activity of the binuclear catalyst was less sensitive to the Al/Ti molar ratio. The catalyst exhibited high activity of over 10 6 g mol (Ti) À1 h À1 atm À1 even at a low Al/Ti ratio of 500, and with Al/Ti ratio increased from 500 to 2000, the activity increased slightly to a maximum at an Al/Ti ratio of 1000 and then slowly decreased.
The catalytic performances of the binuclear complexes bearing different alkylthio and phenylthio sidearms were also compared. The steric hindrance of substituents on sulfur atom inuenced both the catalytic activity and molecular weight of the resulting polyethylene. Take Ti 2 L 2 and Ti 2 L 3 for example (entry 2 vs. 8, Table 3), as the sidearm n-propylthio changed to bulkier iso-propylthio, the catalytic activity decreased from 1.68 to 1.09 Â 10 6 g mol (Ti) À1 h À1 atm À1 , while the molecular weight increased from 3.62 to 4.95 Â 10 4 g mol À1 . The complex Ti 2 L 5 which bears the bulkier phenylthio sidearm produced polyethylene with still-higher molecular weight than those with the alkylthio sidearms (entry 10 vs. 2, 7-9, Table 3). Similar inuences of steric hindrance have also been observed in mononuclear titanium complexes. 9b The inuence of substituents was also investigated by varying the length of the linear alkylthio sidearms. Unlike the mononuclear analogues reported by Tang's group 9c and the methylene-bridged salicylaldiminato binuclear titanium complexes reported by us 15a previously, the catalytic activity decreased from 1.68 to 0.82 Â 10 6 g mol (Ti) À1 h À1 atm À1 (entry 2 vs. 9, Table 3) when the substituent on sulfur atom was changed from n-propyl group to n-octyl group. However, replacement of n-propyl group with methyl group also decreased the activity slightly, due probably to the weaker solubility of Ti 2 L 1 (entry 2 vs. 7, Table 3). With the increase of the alkyl chain length of the side group on sulfur atom, the molecular weight distribution of obtained PE increased gradually, while the molecular weight remained almost unchanged. The GPC curves for the PE samples were shown in Fig. 4.

Ethylene copolymerization with a-olens
We also explored the catalytic behaviors of these binuclear complexes towards ethylene copolymerization with a-olens, and the results were shown in Table 4.  All of these complexes showed extremely high activity for the copolymerization of ethylene and a-olens, which were 2-5 times higher than the homopolymerization activity ( Table 3). The products were branched polyethylene as revealed by their much reduced melting points and the high temperature 13 C NMR spectra. The 1-hexene incorporation ratio in the copolymer could be exibly tuned by the initial feed of a-olen commoners and catalyst structures. It should be noted that these bis-b-carbonylenamine-derived binuclear titanium complexes showed much higher copolymerization activity and a-olen incorporation ratio compared with the methylenebridged bis-salicylaldiminato binuclear titanium complexes reported by us before under similar conditions. 15a The inuences of 1-hexene feeds upon catalytic performances were investigated with Ti 2 L 2 /MMAO as a representative. As the feed of 1-hexene was increased from 6 to 36 mmol, the 1-hexene incorporation ratio increased sharply from 5.1 to 19.1 mol% (calculated from the 13 C NMR spectra, entry 1-4, Table 4), while the copolymerization activity increased from 2.04 Â 10 6 g mol (Ti) À1 h À1 atm À1 to a maximum of 5.59 Â 10 6 g mol (Ti) À1 h À1 atm À1 at 24 mmol of 1-hexene, and then decreased slightly to 4.23 Â 10 6 g mol (Ti) À1 h À1 atm À1 at 36 mmol. It appeared that within a certain range the activity of the binuclear Ti complex increased apparently with the increase of 1-hexene concentration, showing positive "comonomer effect".
The structure of binuclear titanium complexes also affected their catalytic performances for ethylene/1-hexene copolymerization. Under the same conditions, the increase of steric hindrance of the substituents on sulfur atom reduced the copolymerization activity and 1-hexene incorporation ratio, but increased the molecular weight of obtained copolymers (entry 2 vs. 6, n-propyl vs. iso-propyl, entry 2 vs. 7, n-propyl vs. n-octyl, Table 4). Furthermore, replacement of n-propyl group with smaller sized methyl group enhanced signicantly the 1-hexene incorporation ratio from 11.3 to 18.3 mol% and decreased the molecular weight from 8.42 to 3.42 Â 10 4 g mol À1 (entry 2 vs. 5, Table 4). However, replacement of alkyl group with phenyl group on sulfur atom lowered the 1-hexene incorporation ratio (entry 2, 5-7 vs. 8, Table 4), which was in good accord with the salicylaldiminato mononuclear titanium complexes reported by Tang. 9 The high temperature 13 C NMR spectra of copolymers produced by Ti 2 L 1 , Ti 2 L 2 , Ti 2 L 4 and Ti 2 L 5 were shown in Fig. 5, with the corresponding carbon units marked for different peaks. Variation of branch density can be clearly observed, as demonstrated by the relative peak heights. The GPC curves for the ethylene/1-hexene copolymers obtained with bi-and mononuclear Ti complexes were shown in Fig. 6.
Under the same conditions, complex Ti 2 L 2 demonstrated lower copolymerization activity and 1-hexene incorporation ratio than those of its mononuclear counterpart TiL 6 (entry 2 vs. 9, Table 4), probably due to the steric and electronic effects of the altered coordination environment. This type of binuclear titanium complexes showed negligible bimetallic cooperative effects due to the far-separation of the two titanium centers.

Conclusions
A series of phenyl-bridged bis-b-carbonylenamine [ONS R ] (R ¼ alkyl or phenyl) tridentate ligands L 1 -L 5 and their binuclear titanium complexes Ti 2 L 1 -Ti 2 L 5 were synthesized and characterized. The molecular structures of ligand L 2 (R ¼ n Pr) and its corresponding Ti complex Ti 2 L 2 as studied by single-crystal X-ray diffraction revealed that each titanium coordinated with an oxygen, a nitrogen, a sulfur and three chlorine atoms to form a distorted octahedral conguration. Furthermore, the alkylthio or phenylthio anilines reacted with the carbonyl groups adjacent to phenylene group, resulting in isolated and relatively independent titanium centers in the complex. Compared with the mononuclear analogue TiL 6 , these complexes exhibited better thermal stability for ethylene polymerization and produced PE with higher molecular weight and wider polydispersity, suggesting that two active centers were formed. The molecular weight and a-olen incorporation ratio can be exibly tuned by the catalyst structure. The complex Ti 2 L 5 which bears phenylthio sidearm exhibited higher activity towards ethylene polymerization and produced polyethylene with much higher molecular weight compared with the complexes bearing alkylthio sidearms, but resulted in lower 1-hexene incorporation ratio. Meanwhile, the 1-hexene incorporation ratio could also be tuned by the initial feed of the a-olen commoner. However, this type of binuclear titanium complexes showed weak or negligible bimetallic cooperative effects due to the far separation of the titanium centers.

General procedures
All manipulations involving air-and/or moisture-sensitive compounds were performed under dry nitrogen using standard Schlenk-line and glovebox. Toluene and hexane were puried by distillation over sodium/benzophenone ketyl, while CH 2 Cl 2 was reuxed over CaH 2 . Gases and other solvents were puried by standard techniques. Modied methylaluminoxane (MMAO) was purchased from Akzo Chemical as a 7 wt% solution in heptane. All other chemical reagents were used as received unless noted otherwise. 1 H and 13 C NMR spectra of ligands and complexes were recorded on a Bruker Avance III 400 MHz spectrometer with tetramethylsilane as an internal standard. Elemental analyses were carried out using Vario EL 111. 13 C NMR spectra of polymers were obtained on a Varian XL 300 MHz spectrometer at 120 C with o-C 6 D 4 Cl 2 as the solvent. IR spectra were collected with a Nicolet Nexus 470 Fourier transform infrared (FTIR) spectrometer. DSC measurements were performed on a Netzsch DSC200 F3 instrument at a heating rate of 10 C min À1 from 20 to 160 C, with the melting points obtained from the endothermic peak of the second heating scan. The M n and M w /M n of the polymers were determined at 150 C with a Viscotek 350A HT-GPC System using a polystyrene calibration. 1,2,4-Trichlorobenzene was employed as the solvent at a ow rate of 1.0 mL min À1 .

Crystallographic analysis
Crystal data were collected on a Bruker APEX-II CCD diffractometer with graphite-monochromated Mo Ka radiation (l ¼ 0.71073Å) at 130 K for Ti 2 L 2 . Crystals were coated in oil and then directly mounted on the diffractometer under a stream of cold nitrogen gas. A total of N reections were collected by using u scan mode. Corrections were applied for Lorentz and polarization effects as well as absorption using multi-scans (SADABS). All the structures were solved by direct method (SHELXS-97). The remaining non-hydrogen atoms were obtained from the successive difference Fourier maps. All non-H atoms were rened with anisotropic displacement parameters, while the H atoms were constrained to the parent sites, using a riding mode (SHELXTL). Details of the X-ray structure determinations and renements are provided in Table 1. Other details are shown in the ESI. † CCDC numbers for L 2 and Ti 2 L 2 are CCDC 1587147 and 1587146, † respectively.

Ethylene polymerization and copolymerization
A ame-dried Schlenk ask purged with N 2 was lled with ethylene gas. 30 ml of freshly distilled toluene was added and raised to the reaction temperature for 10 min. MMAO was then injected using a syringe and the mixture was stirred for 5 min. The polymerization was initiated by adding a solution of the titanium complex in toluene with a syringe. Aer a desired time, the polymerization was quenched with acidied ethanol (100 mL, 8 vol% HCl in ethanol). The precipitated polymer was ltered off, washed with ethanol, then dried under vacuum overnight at 60 C till a constant weight. For copolymerization, a-olens (1-hexene, 1-octene or 1-decene) and MMAO were injected in sequence via a syringe.

Conflicts of interest
There are no conicts to declare.