Ethylene Polymerization Using Discrete Nickel(II) Iminophosphonamide Complexes

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• 作者：Russell L. Stapleton ; Jianfang Chai ; Nicholas J. Taylor ; and Scott Collins
• 刊名：Organometallics
• 出版年：2006
• 出版时间：May 8, 2006
• 年：2006
• 卷：25
• 期：10
• 页码：2514 - 2524
• 全文大小：283K
• 年卷期：v.25,no.10(May 8, 2006)
• ISSN：1520-6041

文摘

The syntheses and structures of the discrete (-allyl)nickel iminophosphonamide (PN₂) complexes2a-d from the reaction of (-allyl)nickel bromide and the corresponding PN₂ ligands 3a,b or from thereaction of (-allyl)₂Ni and phosphorane 1 are reported. Complexes 2a,b are characterized by havinglong Ni-N distances coupled with an acute bite angle for the PN₂ ligand. The -allyl ligands in complexes2a-d are not fluxional on the NMR time scale at room temperature, although chemical exchange betweenthe isomeric complexes 2c,d occurs via PN₂ ligand reorientation. Purified complexes 2a-d are not activefor ethylene polymerization; it is only when complexes 2c,d are generated in situ in the presence ofmonomer that high-M_w branched poly(ethylene) is formed. A variety of indirect evidence suggests thatthe active catalyst arises from the reaction of Ni(0)-alkene complexes with phosphorane 1, eitherpreformed or generated in situ through decomposition of (-allyl)₂Ni. A bona fide PN₂NiPh(PPh₃) complex,5, was prepared from NiPh(PPh₃)₂Br and the PN₂ ligand 3a and was structurally characterized. Thiscomplex is active for 1-hexene isomerization in the absence of an activator. During hexene isomerization,variable amounts of the paramagnetic bis(PN₂) complex 4 are produced along with ligand 3a. In addition,the fluxional intermediate 6, containing both a PN₂ ligand and coordinated PPh₃, is present during catalysis.Reaction of 5 with an equimolar amount of propene provides -methylstyrene, the product of 1,2-insertionfollowed by -H elimination. Complex 5 is not effective for polymerization or oligomerization of ethyleneunder a variety of conditions. The reactions of complex 5 with various phosphine scavengers were studied,and of these only Rh(acac)(C₂H₄)₂ is both effective and selective for PPh₃. Hard Lewis acids, includingAlMe₃, B(C₆F₅)₃, and PMAO, have a pronounced tendency toward abstraction of the PN₂ or other anionicligands in these unhindered complexes. All of the complexes reported in this paper are extremely activefor ethylene dimerization in the presence of PMAO. In the presence of stoichiometric Rh(I), complex 5rapidly isomerizes 1-hexene and in the presence of ethylene produces branched PE oligomers at modestactivity.