Carbazole-bis(oxazolines) as Monoanionic, Tridentate Chelates in Lanthanide Chemistry: Synthesis and Structural Studies of Thermally Robust and Kinetically Stable Dialkyl and Dichloride Complexes.

Abstract

The carbazole-bis(oxazoline) ligand 1,8-bis(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-tert-butylcarbazole (H-Czx, 14) was prepared and reacted with Ln(CH2SiMe3)3(THF)2 to afford the five-coordinate dialkyl complexes (Czx)Ln(CH2SiMe3)2 (Ln = Y, 15; Er, 16; Yb, 17) in high yield. Deprotonation of 14 with NaN(SiMe3)2, followed by reaction with LnCl3(THF)x, affords the six-coordinate dichloride complexes cis,mer-(Czx)Ln(Cl)2(THF) (Ln = Y, 18; Er, 19; Yb, 20). Dialkyl complexes 15–17 were also prepared by reaction of the dichloride complexes 18–20, respectively, with 2 equiv of LiCH2SiMe3. Complexes 16–20 were characterized by X-ray crystallography. The dialkyl complexes 16 and 17 display a five-coordinate trigonal-bipyramidal geometry with the Czx ligand spanning both axial and one equatorial site with the CH2SiMe3 rotated such that the SiMe3 groups sit over the carbazole ring system. The solution structure for 15–20 was found to be consistent with that in the solid state by NMR spectroscopy. However, paramagnetic dichloride 20 was shown by variable-temperature 1H NMR studies to undergo rapid THF exchange by a dissociative process in toluene-d8 solution (ΔH‡ = 98 ± 10 kJ mol–1, ΔS‡ = +185 ± 25 J mol–1 K–1). Treatment of dialkyl 18 with [Ph3C]+[B(C6F5)4]− cleanly produces the alkyl cation [(Czx)Y(CH2SiMe3)]+[B(C6F5)4]− (21) by 1H NMR spectroscopy, but this species does not insert 2,3-dimethylbutadiene.