Formation and structural properties of salicylaldiminato complexes of zirconium and titanium

Abstract

The metallocene cation [Cp2ZrCH3(THF)+] (2, with BPh4− anion) reacts with N-isopropylsalicylaldimine (1a) by liberation of methane and formation of the pseudotetrahedral [(κO,κN-salicylaldiminato)ZrCp2+] complex 3 that was characterized by X-ray diffraction. Deprotonation of 1a by treatment with n-butyllithium yields the corresponding lithiated reagent 4a, that exhibits a tetrameric structure in the crystal, featuring a cubic Li4O4 core structure. Treatment of 4a with Zr(NMe2)2Cl2(dme) (5) yields the octahedral bis(N-isopropylsalicylaldiminato)Zr(NMe2)2 complex 7a. The X-ray crystal structure analysis of 7a has revealed that the aldiminato oxygen atoms of the κO,κN-chelate ligand are oriented trans to each other at the central metal atom. Treatment of the N-isopropyl, the N-cyclohexyl- and the N-(2,6-diisopropylphenyl)salicylimines (1a,b,d) with the reagent Zr(NMe2)4 (6) directly results in the formation of the respective octahedral (lig)2Zr(NMe2)2 complexes 7a, 7b (also characterized by X-ray diffraction), and 7d in good yields. Treatment of 7b and 7d with Me3Si-Cl replaced the -NMe2 substituents by chloride to yield the corresponding lig2ZrCl2 complexes 8b and 8d, respectively. The X-ray crystal structure analysis of 8d showed an octahedral coordination with the aldiminato nitrogen atoms of the κO,κN-chelate ligands now being trans-oriented at zirconium. Complex 8b undergoes an enantiomerization process that is followed by dynamic NMR spectroscopy (ΔG≠enant. (268 K)≈12 kcal mol−1). Titanium tetrachloride forms a neutral 1:2 adduct with 1d. The NMR analysis and X-ray crystal structure analysis of the resulting bisbetaine-type adduct 9d reveals the formation of an octahedral titanium complex that contains two trans-Ti-O(aryl) bonds. The phenolic hydrogen atoms were transferred to the aldimine nitrogens to form an intramolecular iminium salt. Two equivalents of HCl are removed from 9d by treatment with NEt3 to yield the bis[N-(2,6-diisopropylphenyl)salicylaldiminato]TiCl2 complex 10d (characterized by X-ray diffraction). In a one-pot procedure TiCl4 reacts with N-phenylsalicylaldimine (1c) and triethylamine in a 1:2:2 ratio to directly yield the corresponding (κO,κN-lig)2TiCl2 complex 10c. The X-ray crystal structure analysis of 10c reveals a trans-orientation of ligand oxygen atoms, just like in the adduct intermediate 9. Upon activation with methylalumoxane the complexes 8d, 10c, and 10d form catalysts for the polymerization of ethene.