Phosphazene-Functionalized Cyclopentadienyl and Its Derivatives Ligated Rare-Earth Metal Alkyl Complexes: Synthesis, Structures, and Catalysis on Ethylene Polymerization

Abstract

Treatment of Ln(CH2SiMe3)3(thf)2 (Ln = Sc, Y, and Lu) with 1 equiv of CpPN-type ligands C5H4═PPh2–NH–C6H3R2 (R = Me, L1(Me); R = iPr, L1(iPr)) at room temperature readily generated the corresponding CpPN-type bis(alkyl) complexes 1 and 2a–2c. Addition of 3 equiv of LiCH2SiMe3 to a mixture of L1(iPr) and LnCl3(thf)2 (Ln = Sm and Nd) also afforded the CpPN-type bis(alkyl) complexes 2d and 2e. The Cp moiety bonds to the central metal in a classical η5 mode in all CpPN-type complexes 1 and 2. In contrast, the CpMePN-type ligands C5Me4H–PPh2═N–C6H3R2 (R = Me, L2(Me); R = iPr, L2(iPr)) behaved differently. L2(Me) did not react with Sc(CH2SiMe3)3(thf)2. Similarly, L2(iPr) was also inert to Sc(CH2SiMe3)3(thf)2 even at 50 °C. When the central metal was changed to yttrium, however, the equimolar reaction between Y(CH2SiMe3)3(thf)2 and L2(iPr) in the presence of LiCl afforded two bis(alkyl) complexes 3a and 3b. In the main product 3a, [C5HMe3(η3-CH2)–PPh2═N–C6H3iPr2]Y(CH2SiMe3)2(thf), the ligand bonds to the Y3+ ion in a rare η3-allyl/κ-N mode, whereas in 3b, (C5Me4–PPh2═N–C6H3iPr2)Y(CH2SiMe3)2(LiCl)(thf), the Cp ring coordinates to the Y3+ ion in an η5 mode, and a LiCl unit is located between the Y3+ ion and the nitrogen atom. When the central metal was changed to lutetium, a bis(alkyl) complex 4a, [C5HMe3(η3-CH2)–PPh2═N–C6H3iPr2]Lu(CH2SiMe3)2(thf), and a bis(alkyl) complex 4b, (C5Me4–PPh2═N–C6H3iPr2)Lu(CH2SiMe3)2, were isolated. The protonolysis reaction of the IndPN-type ligands C9H7–PPh2═N–C6H3R2 (R = Me, L3(Me); R = Et, L3(Et); R = iPr, L3(iPr)) with Ln(CH2SiMe3)3(thf)2 (Ln = Sc, Y, and Lu) generated the IndPN-type bis(alkyl) complexes 5a–5c, 6, and 7a–7c, selectively, where the Ind moiety tends to adopt an η3-bonding fashion. The more bulky FluPN-type ligands C13H9–PPh2═N–C6H4R (R = H, L4(H); R = Me, L4(Me)) were treated with Ln(CH2SiMe3)3(thf)2 (Ln = Sc and Lu) to afford the FluPN-type bis(alkyl) complexes 8 and 9a and 9b, where the Flu moiety has a rare η1-bonding mode. Complexes 1–9 were fully characterized by 1H, 13C, and 31P NMR; X-ray; and elemental analyses. Upon activation with AlR3 and [Ph3C][B(C6F5)4], the scandium complexes showed good to high catalytic activity for ethylene polymerization. The effects of the sterics and electronics of the ligand, the loading and the type of AlR3, the polymerization temperature, and the polymerization time on the catalytic activity were also discussed.