DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 827–829 827 Chromium ethylene polymerisation catalysts bearing reduced SchiV-base N,O-chelate ligands Vernon C. Gibson,* Claire Newton, Carl Redshaw, Gregory A. Solan, Andrew J. P. White and David J. Williams Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY. E-mail: V.Gibson@ic.ac.uk Received 5th January 1999, Accepted 8th February 1999 Treatment of CrCl3(THF)3 with the lithium and sodium salts of the reduced Schiff-base ligand 3,5-(tBu)2-2-(OH)C6- H2CH2NH(2,6-Me2C6H3) affords bis-chelate chromium(II) and mono-chelate chromium(III) complexes respectively; both give active ethylene polymerisation catalysts upon treatment with alkylaluminium activators.There is currently much academic and industrial interest in the development of highly eYcient molecular a-olefin polymerisation catalysts.1 In these systems, it is the steric and electronic properties of ancillary ligands that allow control over the molecular weight and microstructure of the resultant polymers. 2,3 Industrially, chromium supported on silica plays an important role in the global production of polyethylene.4 Examples of molecular systems are, however, scarce 5–10 and of those that have been reported, low valent half-sandwich chromium compounds predominate.7,8 We have been investigating non-cyclopentadienyl chromium systems as potential well-defined catalysts for ethylene polymerisation.In recent reports, we 5 and others 6 have described new chromium catalysts bearing monoanionic N,N-chelate ligands. We now report new procatalysts based upon the bulky monoanionic N,O-chelating ligand I derived from reduction of the corresponding SchiV-base precursor; ethylene polymerisation tests reveal the highest activities to date for a noncyclopentadienyl chromium system. Treatment of [CrCl3(THF)3] with two equivalents of the lithium salt of 3,5-(tBu)2-2-(OH)C6H2CH2NH(2,6-Me2C6H3) HI in THF at 278 8C leads to reduction and formation of the bis-chelated red chromium(II) complex {Cr[3,5-(tBu)2-2-(O)C6- H2CH2NH(2,6-Me2C6H3)]2} 1 in 45% yield (Scheme 1).† Use of only one equivalent of the lithium salt again aVords 1, albeit in reduced yield (23%).The single crystal X-ray structure ‡ of 1 shows the ligands to have cis-coordinated oxygen and nitrogen atoms respectively (Fig. 1) and to have retained their amino hydrogen atoms, hence indicating a formal 12 oxidation state at the chromium centre.The molecule has crystallographic C2 symmetry about an axis bisecting the O–Cr–O9 angle. The geometry at chromium is distorted square planar, there being a 148 twist about the C2 axis between the O–Cr–O9 and N–Cr–N9 planes. The bite of the N,O-chelating ligand I appears to be near optimal, the angle subtended at chromium being 90.3(1)8. The six-membered chelate ring has a folded boat-like conformation with O and C(1) as “prow” and “stern” respectively; the out of plane fold angle about the O ? ? ? C(1) vector is ca. 228. The two 2,6- O– tBu tBu N Me Me H I dimethylphenyl rings are sheared, there being no p ? ? ?p stacking interaction between them. By contrast, reaction of one equivalent of the sodium salt Fig. 1 The molecular structure of 1. Selected bond lengths (Å) and angles (8): Cr–O 1.913(3), Cr–N 2.100(3), N–C(1) 1.463(5), N–C(16) 1.444(5), O–C(3) 1.336(4); O–Cr–O9 90.9(2), O–Cr–N 90.3(1), O–Cr– N9 169.9(1), N–Cr–N9 90.3(2).Scheme 1 Preparation of chromium complexes 1 and 2 featuring ligand I. Reagents and conditions: (i) 2LiI, 278 8C, THF, 12 h; (ii) NaI, 278 8C, THF, 12 h, followed by recrystallisation from heptane– CH3CN. O tBu tBu N Cr Cl NCMe O tBu tBu N Cr O tBu tBu N H H H Cl NCMe Me Me Me Me Me Me (ii) (i) 1 CrCl3(THF)3 2828 J. Chem. Soc., Dalton Trans., 1999, 827–829 of 3,5-(tBu)2-2-(OH)C6H2CH2NH(2,6-Me2C6H3) in THF at 278 8C with CrCl3(THF)3 aVords, upon recrystallisation from acetonitrile–heptane, green blocks of the mono-chelated octahedral chromium(III) complex {Cr[3,5-(tBu)2-2-(O)C6H2- CH2NH(2-6-Me2C6H3)](h1-NCCH3)2Cl2} 2 † (51%) (Scheme 1).The crystal structure ‡ of 2 shows the geometry at chromium to be distorted octahedral with trans-chlorides and cis-acetonitriles (Fig. 2) and angles ranging from 82.7(1)–98.0(1)8 and 171.1(2)–179.3(1)8. Interestingly, the bite of I is here reduced to 82.7(1)8, cf. 90.3(1)8 in 1. The geometry of the six-membered chelate ring is also diVerent, adopting here a half-chair conformation with the N(1)–Cr–C(7) plane being folded by ca. 588 out of the Cr–O(1)–C(1)–C(6)–C(7) plane. The Cr–O(1) and Cr–N(1) distances are comparable to those observed in 1 and in other related systems.11 The infrared spectra of complexes 1 and 2 both exhibit absorption bands between 3312 and 3222 cm21 consistent with n(N–H) stretching modes while complex 2 shows, in addition, strong bands at 2319 and 2291 cm21 due to the symmetric and asymmetric nitrile stretches.12 Both complexes are paramagnetic with the Cr(II) complex 1 displaying a magnetic moment of 2.6 mB (consistent with an S = 1 ground state) and the Cr(III) complex, 2, 3.9 mB (Evans balance).The results of the ethylene polymerisation runs are collected in Table 1. Compounds 1 and 2 are both active as procatalysts in ethylene polymerisation and aVord polymers with high molecular weight and virtually no branching by NMR.§ The highest activity is observed using a combination of 2 and Fig. 2 The molecular structure of 2. Selected bond lengths (Å) and angles (8): Cr–O(1) 1.865(3), Cr–N(1) 2.123(3), Cr–N(2) 2.102(4), Cr–N(3) 2.100(4), Cr–Cl(1) 2.311(2), Cr–Cl(2) 2.330(2), O(1)–C(1) 1.341(5), N(1)–C(7) 1.486(6), N(1)–C(8) 1.468(5); O(1)–Cr–N(3) 88.9(1), O(1)–Cr–N(2) 179.3(1), N(3)–Cr–N(2) 90.4(2), O(1)–Cr–N(1) 82.7(1), N(3)–Cr–N(1) 171.1(2), N(2)–Cr–N(1) 98.0(1), O(1)–Cr–Cl(1) 92.7(1), N(3)–Cr–Cl(1) 86.8(1), N(2)–Cr–Cl(1) 87.4(1), N(1)–Cr–Cl(1) 96.5(1), O(1)–Cr–Cl(2) 91.6(1), N(3)–Cr–Cl(2) 89.0(1), N(2)–Cr–Cl(2) 88.3(1), N(1)–Cr–Cl(2) 88.3(1), Cl(1)–Cr–Cl(2) 174.0(1).Table 1 Results of ethylene polymerisation runs using procatalysts 1 and 2a Run 1234 Procatalyst/ mmol 1 (0.017) 1 (0.017) 2 (0.025) 2 (0.025) Activator b/ mmol (equiv.) MAO (12/700) Et2AlCl (0.6/35) MAO (10/400) Et2AlCl (0.5/20) Yield PEc/g 0.26 1.02 0.11 3.26 Activity/ g mmol21 h21 bar21 15 60 4 130 § a General conditions: 1 bar ethylene, Schlenk test carried out in toluene (40 cm3) at 25 8C, over 60 min, reaction quenched with dilute HCl and the solid washed with methanol (50 cm3) and dried in a vacuum oven at 40 8C.b MAO = Methylaluminoxane. c Solid polyethylene. Et2AlCl (130 g mmol21 h21 bar21, run 4). Under related conditions, the chromium(II) species 1 results in an activity less than half that of 2 (60 g mmol21 h21 bar21, run 2). As we have reported elsewhere 5 dialkylaluminium chlorides appear to be more compatible co-catalysts (runs 2, 4) than MAO (runs 1, 3) for chromium systems of this type.In conclusion, two chromium complexes incorporating the bulky reduced-SchiV-base ligand I have been prepared and their role in ethylene polymerisation has been examined. The higher activity observed for 2 relative to 1 may be attributed to a more accessible chromium centre, possibly aided by the lability of the ancillary acetonitrile ligands.Further studies are in progress to obtain a greater understanding of the factors influencing the activity and selectivity of these and related molecular chromium polymerisation catalysts. Acknowledgements BP Chemicals Ltd is thanked for financial support. Drs G. Audley and J. Boyle are thanked for GPC and NMR measurements, respectively. Notes and references † Synthesis of 1: to a THF (20 cm3) solution of HI (1.62 g, 4.76 mmol) was added nBuLi (3.2 cm3, 5.0 mmol) at 278 8C.The solution was allowed to warm to room temperature and stirred for 1 h. On cooling to 278 8C, solid CrCl3(THF)3 (0.90 g, 2.38 mmol) was added. The reaction mixture was then allowed to warm to room temperature and stirred for 12 h. Following removal of the volatile components, the residue was extracted into pentane (50 cm3) and taken to dryness. Recrystallisation from heptane aVorded, on prolonged standing (1–2 d) at ambient temperature, dark red prisms of 1 in 45% yield (0.62 g) (Found: C, 75.8; H, 8.2; N, 3.5.Calc. for C46H64N2O2Cr 1: C, 75.8; H, 8.8; N, 3.8%). Synthesis of 2: HI (2.00 g, 5.89 mmol) and NaH (0.31 g, 12.96 mmol) were refluxed in THF (45 cm3) for 12 h. On cooling, the suspension was filtered into a solution of CrCl3(THF)3 (2.21 g, 5.89 mmol) in THF (25 cm3) at 278 8C. The solution was stirred at room temperature for 12 h. Following removal of the volatile components, the solid residue was extracted into toluene (75 cm3) and taken to dryness.Recrystallisation from acetonitrile–heptane (1 : 3) aVorded 2 as green blocks on prolonged standing (3–4 d). Yield 51% (1.65 g) (Found: C, 59.4; H, 7.7; N, 7.0. Calc. for C27H38N3OCl2Cr 2: C, 59.7; H, 7.0; N, 7.7%). ‡ Crystal data for 1: C46H64N2O2Cr, M = 729.0, monoclinic, I2/a (no. 15), a = 13.645(5), b = 18.382(4), c = 17.243(8) Å, b = 102.73(2)8, V = 4219(3) Å3, Z = 4 (the molecule has C2 symmetry), Dc = 1.148 g cm–3, m(Mo-Ka) = 3.08 cm–1, F(000) = 1576, T = 203 K; red blocks, 0.50 × 0.43 × 0.37 mm, Siemens P4/PC diVractometer, w-scans, 3669 independent reflections. The structure was solved by direct methods and the non-hydrogen atoms were refined anisotropically using full matrix least-squares based on F2 to give R1 = 0.060, wR2 = 0.125 for 2158 independent observed reflections [|Fo| > 4s(|Fo|), 2q < 508] and 235 parameters.Crystal data for 2: C27H38N3OCl2Cr, M = 543.5, monoclinic, P21/n (no. 14), a = 9.566(1), b = 13.048(2), c = 23.432(4) Å, b = 95.83(1)8, V = 2909.7(8) Å3, Z = 4, Dc = 1.241 g cm–3, m(Cu-Ka) = 50.9 cm–1, F(000) = 1148, T = 293 K; green plates, 0.27 × 0.27 × 0.10 mm, Siemens P4/PC diVractometer, w-scans, 4438 independent reflections.The structure was solved by direct methods and the major occupancy nonhydrogen atoms were refined anisotropically using full matrix leastsquares based on F2 to give R1 = 0.057, wR2 = 0.127 for 3139 independent observed absorption corrected reflections [|Fo| > 4s(|Fo|), 2q £ 1288] and 324 parameters.CCDC reference number 186/1346. See http:// www.rsc.org/suppdata/dt/1999/827 for crystallographic files in .cif format. § As a representative example, GPC analysis of the polyethylene obtained from run 4 aVorded Mw 827000, Mn 84000, Mw/Mn 9.8; 13C NMR (C2D2Cl4–1,2,4-trichlorobenzene at 130 8C) gave 0.6 Me per 1000 C atoms. 1 W. Kaminsky, J. Chem. Soc., Dalton Trans., 1998, 1413; K. Soga and T. Shiono, Prog. Polym. Sci., 1997, 22, 1503; R.G. Harvan, Chem. Ind., 1997, 212; R. F. Jordan, Adv. Organomet. Chem., 1991, 32, 325; G. P. J. Britovsek, V. C. Gibson and D. F. Wass, Angew. Chem., Int. Ed., 1999, 38, 428. 2 G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J. Maddox, S. J. McTavish, G. A. Solan, A. J. P. White and D. J. Williams, Chem. Commun., 1998, 849; B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem. Soc., 1998, 120, 4049.J. Chem. Soc., Dalton Trans., 1999, 827–829 829 3 L.K. Johnson, C. M. Killian and M. Brookhart, J. Am. Chem. Soc., 1995, 117, 6414; L. K. Johnson, C. M. Killian, S. D. 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