Vanadyl complexes bearing bi-dentate phenoxyimine ligands: Synthesis, structural studies and ethylene polymerization capability

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Introduction
In recent years, there has been considerable interest in the use of vanadium-based pre-catalysts for olefin polymerization catalysis, in both academic [1] and industrial circles.[2] This interest stems from encouraging catalytic activities and thermal stability observed for a number of vanadium systems, as well as their ability to perform desirable co-and ter-polymerizations. [3] As in other catalyst systems, the ancillary ligands present at the metal play a pivotal role in controlling the behaviour of the polymerization process.Indeed, in the case of the phenoxyimine ligand set, Fujita has noted that a bulky imine-bound group and that at the ortho position of the phenoxide are a prerequisite for forming cis oriented halide ligands (in group IV systems).Furthermore, the use of a bulky group at the ortho position of the phenoxide enhances catalytic activity; bulky imine-bound substituents generally favour high molecular weight products.[4] With this in mind, we note that imine-based ligands have been shown to impart stability at vanadium, [5] and we [6] and others [4] have investigated the use of phenoxyimine-based ligand sets in combination with vanadium centres.The use of tridentate ligands sets has proved particularly fruitful with group IV metals, as highlighted by the work of Tang et al, [7] and the Mitsui group, [8] whilst the Li group have also done some work with group V (vanadium).[9] More recently, McGuinness et al extended the Mitsui work on titanium to include a number of related donor functionalized ligands.[10] Encouraged by these findings, we have prepared a series of bi-dentate ligands bearing additional functionality and the vanadium complexes thereof (see schemes 1 and 2).Investigations into their ability to polymerize 50 ethylene revealed that in the presence of the co-catalyst diethylaluminium chloride (DEAC) and the re-activator ethyltrichloroacetate (ETA) activities in the range 2,990 -7,700 at 1 bar and 3,000 -12,000 g/mmol.hat 10 bar ethylene were achievable over a 20 or 30 minute period, respectively.[11] The 55 polymer products were mostly linear polyethylene, with higher molecular weights favoured at higher ethylene pressure.Use of other co-catalysts such as MAO or MMAO led to no or poor activity.(5); n = 6 (6); n = 6, (7•1.5MeCN);n = 7, (8); n = 8, ( 9)) in isolated yields of 26 to 75 % (see experimental and Scheme 1).During the reaction, the vanadium centre is reduced from V(V) to V(IV), as observed previously.[5b] Oxidation [12] A proposed mechanism has been reported by Velusamy and Punniyamurthy [13], and we envisaged that the propanol formed herein on addition of the phenoxyimine ligands is subject to a 5 related oxidation process with the concomitant reduction of the vanadium centre.These vanadium(IV) complexes were characterized by X-band EPR measurements at 150 K and 298 K and, as expected, each gave an 8 line spectrum characteristic of V(IV) (d 1 , I = 7 /2) -see    S2 and S3 for representative spectra).Anisotropic parameters were collected from frozen toluene solutions, and analysis was performed using simulations using EasySpin.[14] Values for g‖ and g⊥ compare favourably with previously reported vanadyl(IV) systems, [15] and particularly  2; crystallographic data are collated in Table 7.All complexes 1 -9 adopted distorted trigonal bipyramidal geometry at the metal centre with the N atoms axial and the O atoms equatorial, the latter being approximately co-planar.The distortion is best illustrated by the variation of the N(1) -V(1) -N(2) angles [167.82(9)-174.89(10) o ] from linearity.The vanadyl bond lengths are all similar and are in the range 1.593(2) -1.605(4) Å, which are typical.[6,17] The other geometrical parameters associated with 1 -9 are similar (see Table 2), particularly the bond lengths, though there is some variation of the angle between the vanadyl and the chelate

Dinuclear complexes
In the case of the ligands L 6 H, changing the ratio of metal to ligand led to the isolation of the dark red oxo-bridged centrosymmetric vanadyl(V) complex [VO(µ-O)(µ-OnPr)(L 6 )]2 (10).Contrastingly, use of the ligands 4-methyl,3-(R)-2-(OH)- Conditions: giso and Aiso were recorded at 298 K in toluene using X-band; g⊥, A⊥, g‖ and A‖ were recorded at 120 K in toluene using X-band.13)).The IR spectra contain a strong band at 982 (10), 991 (11), 991 (12) and 993 (13) cm -1 assigned to the v(V=O) group, and for 11 -13, a broader 50 band assigned to v(OH) at 3611 (11) 3617 (12), and 3620 (13) cm -1 .The crystal structure of 10 is shown in Figure 2, with selected bond lengths and angles given in table 3; crystallographic data are presented in table 5. Complex 10 is a centrosymmetric molecule, in which each vanadium centre adopts a distorted square pyramidal geometry; the centres are linked via oxo bridges.The source of the oxo bridges is most likely adventitious hydrolysis.The angle subtended at each bridging 5 oxygen is 96.75(4) o , with V(1) -O(3) at 1.8247(9) Å.The chelating rings form a six-membered ring adopting an envelope conformation with the V atom as the tip of the flap and with a bite angle of 82.30(4) o , which is somewhat smaller than those observed in the bis(chelate) complexes 1 -9.Crystal structures of 10 11 -13 are shown in Figure 3, with selected bond lengths and angles given in Table 4; crystallographic data are presented in Table 5. Complexes 11 -13 lie on a pseudo 2-fold symmetry axis Table 3. Selected lengths (Å) and angles (º) for complexes 10. 82.30( 4) 119.12(3)

Polymerization studies 1 bar studies
Firstly, complex 9 was screened to ascertain the optimum 10 polymerization conditions for the polymerization of ethylene at 1 bar; the results are collated in table S1 (ESI).Diethylaluminium chloride (DEAC) was used as co-catalyst and ethyltrichloroacetate (ETA) as re-activator.The polymerization screening indicated that the best conditions were 16,000 15 equivalents of DEAC to vanadium.The activity of complex 9 increased with temperature and peaked at 80 °C, however the polymer molecular weight dropped by an order of magnitude above 30 °C (~ 135,600 at 20 °C, table S1   The catalyst system was short-lived with the activity dropping to below 50 % after 60 minutes.Complexes 1-13 (not 7) and the replacing the Me group with a more electron withdrawing group (CF3) led to lower activities.Comparison of 1 bar and 10 bar ethylene screening shows that although approximately the same order of magnitude activity can be achieved using lower pressure, use of higher pressure leads to a pronounced increase in polymer 50 molecular weight.However, it should be noted that at 10 bar, the nature of the polymer being formed hampered stirring, and so it is likely that the actual catalytic activities at 10 bar are somewhat higher; mass transport problems have been noted in other vanadium-based ethylene polymerization systems.[19a] At both 55 pressures, polyethylene melting points at high temperatures are lower (≤ 128 o C), and this is thought to be due to the lower molecular weight of the products rather than branching; for representative 13 C NMR spectra of the polymers see ESI.We have previously conducted EPR studies on the interaction of a 60 vanadyl phenoxyimine with AlR3 and AlR2Cl (R = Me, Et), with and without ETA present.[20] In the case of the most active systems (using AlMe2Cl), a species of the form [L / V IV (O)(Me)(AlMe2Cl)] (L / = modified initial phenoxyimine ligand) was invoked.However, such V IV species wee in the 65 minority and most of the vanadium was present as EPR silent V(III) species.Monitoring the concentrations of V(IV) and V(III) species indicated that the V(IV) species were active rather than the V(III) species.These mono(phenoxyimine) species were thought to have lost a phenoxyimine ligand via transfer to the 70 alkylaluminium co-catalysts in much the same way as previously reported for the group IV systems.[4] The catalytic results observed for the systems herein are on a par with those observed (~ 1,000 -7,000 g/mol.h.bar) for the vanadyl complexes bearing phenoxyimine ligands derived from the C-75 capped ligand set {3-[2,2 / -methylenebis(4,6-di-tert-butylphenol)-5-tert-butylsalicylidene-R-imine]} (R = Ph, p-tolyl, 2,4,6-Me3C6H2).[6a] However, when compared to the majority of other vanadium systems bearing chelating phenoxide type ligation, screened in the presence of DEAC/ETA, the activities 80 observed herein are somewhat low.[19] We note that the activity can drop by two orders of magnitude by simply changing the geometrical parameters of the ligands associated with the metal centre, such as in the tripodal system {VO([N(CH2C6H4O-2)3]} (ca.100,000 g/mmol.h.bar) versus {VO(L)[N(C6H4O-2)3]} (L = MeCN, nPrOH, p-tolylNH2; ca.1000 g/mmol.h.bar), and access to the metal centre can be impaired.[19b] A number of the ligands employed herein are certainly bulky enough to impede access to the vanadium centre, which might also account for the reduced catalytic activity.

Conclusions
In conclusion, we have used a number of bi-dentate Schiff base ligands L bearing a variety of substituents to form new bis(chelate) complexes of vanadyl(IV), namely [VO(L)2] via the precursor [VO(OnPr)3].All these bis(chelate) complexes possessed non-crystallographic C2 symmetry with the 2-fold axis parallel to the V=O bond.During the course of these studies, a number of dinuclear dioxo or OH/On-Pr bridged complexes were also structurally characterized.Although the vanadyl complexes isolated herein did not attain the very high activities noted for a number of chelating phenoxide vanadium systems for the polymerization of ethylene, they are on a par with other reported vanadium systems bearing phenoxyimine ligation.Best activities herein were observed when the procedure was conducted in the presence of diethylaluminium chloride (DEAC) and ethyltrichloroacetate (ETA).In particular, ligands featuring a bulky substituent ortho to the hydroxyl moiety led to more active pre-catalysts (complexes 8 and 9), whilst increasing the ethylene pressure led to an increase in polymer molecular weight.

General:
All manipulations were carried out under an atmosphere of dry 30 nitrogen using conventional Schlenk and cannula techniques or in a conventional nitrogen-filled glove box.Toluene was refluxed over sodium, whilst acetonitrile was refluxed over calcium hydride; all solvents were distilled and degassed prior to use.IR spectra (nujol mulls, KBr windows) were recorded on a Nicolet 35 Avatar 360 FT IR spectrometer; 1 H NMR spectra were recorded at room temperature on a Varian VXR 400 S spectrometer at 400 MHz or a Gemini 300 NMR spectrometer or a Bruker Advance DPX-300 spectrometer at 300 MHz.The 1 H NMR spectra were calibrated against the residual protio impurity of the deuterated 40 solvent.EPR spectra were recorded on a JES-FA200 spectrometer at Tsinghua University.Elemental analyses were performed by the elemental analysis service at the London Metropolitan University.Molecular weights (Mw) and molecular weight distribution of polyethylenes were determined by a PL-45 GPC220 at 150 o C using 1,2,4-trichlorobenzene as solvent.The ligands L 1-13 H were prepared using standard condensation chemistry as described in the literature.[21] The vanadium precursor was purchased from Sigma Aldrich and used as received (and stored under argon).50
Complex 7 was made in the same way as for 6, but following prolonged standing at ambient temperature (1 -2 days) was found to be a different solvate, containing 1 1 /2 MeCN per vanadium 65 complex.

Ethylene polymerization
At 1 bar of ethylene pressure: Ethylene polymerization reactions were performed in a dried Schlenk glass flask (250 mL) equipped 55 with a magnetic stirrer bar.The flask was evacuated and recharged 3 times with ethylene, and then 20 mL of dry, degassed toluene was added via a glass syringe.The solution was then stirred for 10 min to allow ethylene saturation, and the correct temperature was acquired via the use of an oil bath, and then the 60 co-catalyst and the reactivating agent ETA was added (0.1 mL, 0.72 mmol); 10ml toluene which dissolved complex was also added.The polymerization time was measured from pre-catalyst injection; the polymerization was quenched by the injection of 5 mL of ethanol.The resulting polymer was transferred into a 500 65 mL beaker containing acidified ethanol, and the polyethylene was collected by filtration and dried at 50 °C in vacuum overnight.
At 10 bar of ethylene pressure: A 250 ml stainless steel autoclave, equipped with a mechanical stirrer and a temperature controller, 70 was employed for the reaction.Firstly, the autoclave was heated in vacuum at 80 °C and recharged with ethylene three times, and then 50 ml toluene (freshly distilled) were injected to the clave which was full of ethylene.When the required temperature was reached, another 30 ml of toluene, which dissolved the complex, 75 and the required amount of co-catalyst DEAC, ETA, and the residual toluene were added by syringe, successively.The reaction mixture was intensely stirred for the desired time under the corresponding pressure of ethylene throughout the entire experiment.The reaction was terminated and the resulting 80 polymer was analyzed using the same procedure as described above for the procedure conducted at ambient pressure.

Crystallography
For each sample, a crystal was mounted in oil on a glass fiber and 85 fixed in the cold nitrogen stream on the diffractometer.A Rigaku R-AXIS Rapid IP diffractometer was used for 1-6, 8, 9, 11-13, and a Bruker APEX 2 CCD diffractometer was used for 7•1.5MeCN and 10, with both equipped with graphitemonochromated MoKα radiation (λ = 0.71073 Å) at 173(2) K 90 (150 K for 7•1.5MeCN and 10).[22] Intensities were corrected for Lorentz and polarization effects and empirical absorption.The structures were solved by direct methods and refined by fullmatrix least squares on F 2 .All hydrogen atoms were placed in calculated positions.Using the SHELXS-97 and SHELXL-97 95 packages respectively, structure solution and refinement were performed.[23][24][25] Crystal data and processing parameters for complexes 1-13 are summarized in Table 7.For 1, there is disorder in the tBu at C33 with major component 65.0(7) %, and there are two molecules of acetonitrile in the unit.There are two 100 similar complex molecules in the asymmetric unit of 2, whilst for 3, two molecules of acetonitrile are also in the unit.In the case of 5, there are two molecules of the vanadium complex in the asymmetric unit.as two-fold disordered with major component 66(3) %.In 10 the molecule lies on a centre of symmetry, so half is unique.For 9, the group bearing C42, which is located at the ortho position of one ligand N atom was modelled with two-fold disorder with a component of 51.3(5) %.There are four molecules of acetonitrile in the asymmetric unit of 11. crystallographic data for this paper. 5 These data can be obtained free of charge from The Cambridge Crystallographic

Figure 1 :
CAMERON representations of the structures of 1, 3 and 5 (for 2, 4, 6 -9, see the ESI).Hydrogen atoms and molecules of acetonitrile have been omitted for clarity.

15 10Figure 2 .
CAMERON representation of the dinuclear µ-oxo complex 10 showing the atom numbering scheme.Hydrogen 20 atoms and solvent (MeCN) molecules have been omitted for clarity.

Table 5 .
45mplex 10 was used to obtain the optimum conditions for the polymerization of ethylene at 10 bar and the results are presented in TableS2(ESI).Complex 10 was found to be active for the polymerization of ethylene using diethylaluminium chloride Molecular weights in the range of ~200,000 to ~670,000 can be obtained on addition of ETA, PDI values were in the range 2.4 -3.9 (2.0 -8.6 in the absence of ETA) and the melting points of the polymers were ca.134-136 °C, consistent with the formation of Catalysis runs using pre-catalysts 1 to 14 (not 7) under optimized conditions at 1 bar.aConditions: 0.5 μmol of [V] per run, 30 mL of toluene, 80 °C, 16,000 equivalents of Et2AlCl, 0.1 mL of ETA, 20 min, 1 bar of ethylene.GPC analysis was conducted in 1,2,4-trichlorobenzene.OMe-containing precatalysts, that bearing an ortho OMe group performed best, whilst45 35benchmark pre-catalyst 14 [VO(FI)2] (FI = 2-O-C6H4CH=NC6H5) [2a] were screened using the optimum conditions for activity determined by the screening of complex 9, ie 16,000 equivalents DEAC, 0.1 mL ETA, 80 °C; the results are presented in table 5. Given that 6 and 7 differ only in the degree 30 of solvation, complex 7 was not screened.All of the complexes were found to be highly active for the polymerization of ethylene.The polymer molecular weights (Mw) were in the range 2,400 -11,700 with PDI values of between 2.6 and 5.6.In the series of OMe substituted complexes (1, o-OMe, 2, m-OMe, 3, p-OMe),35the ortho derivative gave the highest activity (table 5, runs 1-3), and also possessed the smallest PDI (3.0).Replacement of the o-OMe group by the more electron withdrawing o-OCF3 group led to a reduction in activity (table 5, run 1 versus run 5) indicating an electronic influence on the observed catalytic activity.The 45 8 vs. 9), where the ortho substituent on the phenolate has been changed from an adamantyl to a -CMe2Ph group.In the OH/OnPr bridged di-vanadium compounds (11 -13) the size of the ortho phenolate substituent (11, Ad; 12, CMe2Ph; 13, tBu) led to only small changes in the observed catalytic activity.The benchmark 50 catalyst 14 (run 13) afforded an activity of 3,850 g/mmol.hunder the conditions employed here.a 40 Within the mini-series of o, m or p-

Table 7 .
Crystallographic data for complexes Journal Name, [year], [vol], 00-00 | 9For 7•1.5MeCN there are two similar vanadium complex molecules and three molecules of acetonitrile in the asymmetric unit.The methyl groups in one tBu group at C(65) were modelled 5