Group 4 Post-metallocene Complexes Incorporating Tridentate Silyl-Substituted Bis(naphthoxy)pyridine and Bis(naphthoxy)thiophene Ligands: Probing Systems for “Oscillating” Olefin Polymerization Catalysis

Abstract

New bulky silyl ortho-substituted tridentate 2,6-bis(naphthol)pyridine ({ONOSiR3}H2, SiR3 = SiPh3, SiMe2tBu) and 2,5-bis(naphthol)thiophene ({OSOSiPh3}H2) pro-ligands were synthesized via a four-step approach. The solid-state structures of pro-ligands {ONOSiPh3}H2 (3a) and {OSOSiPh3}H2 (3b) were established by X-ray diffraction analysis. Both types of ligands were introduced onto group 4 metal centers (M = Ti, Zr, Hf) using straightforward one-step alkane, amine, or alcohol elimination protocols. Dibenzyl {ONOSiPh3}M(CH2Ph)2 (M = Ti, 4a; Zr, 5a; Hf, 6a) and {ONOSiMe2tBu}M(CH2Ph)2 (M = Ti, 4b; Zr, 5b), diamido {ONOSiPh3}Hf(NMe2)2 (7a and 7a·(NHMe2)), and di(isopropoxy) {ONOSiPh3}Ti(OiPr)2 (8a) complexes were authenticated using NMR spectroscopy and X-ray crystallography methods for some of them. In the solid state, complexes 4a, 4b, and 6a feature rac-like binding of the ligand, while ligands in complexes 5b and 7a·(NHMe2) are meso-like coordinated. The solution structures of 4b and 5b were investigated by VT NMR spectroscopy, which revealed that both complexes exist as rac and meso stereoisomers, which interconvert (activation parameters: 4b, ΔH⧧ = 12.9(7) kcal·mol−1 and ΔS⧧ = −3(1) cal·mol−1·K−1; 5b, ΔH⧧ = 13.4(8) kcal·mol−1 and ΔS⧧ = −7(1) cal·mol−1·K−1). A mechanism for this interconversion process, implying straightforward racemization, was proposed on the basis of DFT computations at the B3LYP (BP86) level, with computed activation barriers for Ti, Zr, and Hf complexes of 11.4 (10.1), 12.5 (11.2), and 12.2 (11.1) kcal·mol−1, respectively. The catalytic activity of dibenzyl and diamido precursors in homopolymerization of propylene and ethylene, upon activation with MAO, “dried-MAO”, and [Ph3C](B(C6F5)4]/Al(iBu)3, has been explored as well.