Half-sandwich β-diketonato complexes of zirconium and their application as catalysts to ethylene polymerization

Abstract

The bis(β-diketonato) Zr complexes CpZr(acac) 2 Cl (acac = acetylacetonato) (1) and Cp*Zr(acac) 2 Cl (2) can be synthesized by the reaction of Cp′ZrCl 3 (Cp′ = Cp or Cp*) with 2 equiv of lithium acetylacetonate. CpZr(acac) 2 Cl (1) can be quantitatively prepared by treatment of Cp 2 ZrCl 2 with excess acetylacetone. The tris(β-diketonato) Zr complex CpZr(tfac) 3 (tfac = trisfluoroacetylacetonato) (3) has been prepared by treatment of CpZrCl 3 with sodium trisfluoroacetylacetonate. The replacement of both chlorine atoms and one Cp group from zirconocene dichloride by nonfluorinated diketone, acetylacetone, readily occurs at room temperature in triethylamine medium, offering the tris(β-diketonato) Zr complex CpZr(acac) 3 (4). The complexes were characterized by 1 H NMR spectroscopic methods. In addition, the molecular structures of complexes 3 and 4 have been determined by X-ray diffraction methods. When activated by excess methylaluminoxane (MAO), these complexes were active for ethylene polymerization, offering polyethylenes with high molecular weights and narrow molecular weight distributions. Complex 4 showed a very high ethylene polymerization activity of up to 7100 kg PE/mol Zr·h at 5 atm. Complexes 1 and 2 were also active for ethylene/1-hexene copolymerization.Since the discovery of homogeneous Ziegler-Natta catalysis by group 4 metallocenes in 1980 [1] , considerable effort has been devoted to catalyst modification in order to improve polymerization activity and control polymer properties such as stereoregularity [2,3] , co-monomer incorporation, and microstructure. Despite the ongoing research on metallocene catalysts, the search for alternatives to the metallocene catalysts that can produce polymers with novel properties has been one of the major goals of transition metal coordination chemistry over the last decade [4][5][6] .Investigation into the use of aryloxide (O-C(sp 2 )) [7-10] and alkoxide (O-C(sp 3 )) [11][12][13][14][15] ligands to replace the ubiquitous cyclopentadienyl type ligands in organo group 4 metal complexes has become popular in recent years. Amongst attractive examples are the monocyclopentadienyl-aryloxy Group 4 metal complexes of the type CpM(OAr)X 2 (M = Ti, Zr). High activities are obtained for these complexes in the homopolymerization of ethylene, 1-hexene, or propylene, as well as for the copolymerization of ethylene with α-olefins, styrene and cyclohexene [16][17][18][19][20][21] .Although binding to a metal using one σ and two π orbits, the alkoxide ligand is normally regarded as a four-electron ligand due to the higher electronegativity of oxygen [22] . This leaves the opportunity to add additional donors, such as O, N donors, to be isonumeral and isolobal with Cp − . With split 2σ and 4π-electrondonor set, the β-diketonato ligand has been viewed as being isoelectronic to Cp, which would result in a cationic 14-electron species [L 2 MR] + (L = diketonato) [4] . Tetrakis-(acetylacetonato)titanium, and acetylacetone itself have been included in the early patent literature for ...

Zirconium nanoparticles prepared by the reduction of zirconium oxide using the RAPET method

“…Zirconium metal is also used for making zirconium inorganic and organic compounds. Many examples of the synthesis of zirconium complexes for catalytic applications are described in the literature [ 3 – 4 ]. A very important example for the application of metallic Zr is the use of Zr nanoparticles (NP) as catalysts for growing TWCNTs (two or three graphene layer tubing) [ 5 ].…”