Zirconium complexes of the tridentate bis(aryloxide)-N-heterocyclic-carbene ligand: Chloride and alkyl functionalized derivatives

The disodium salt of a bis(aryloxide)-N-heterocyclic-carbene dianionic ligand, Na2[L], was prepared by reaction of 1,3-bis(4,6-di-tert-butyl-2-hydroxybenzyl)imidazolium bromide [H3L]Br with 3 equiv. of NaN(SiMe3)2. Reaction of ZrCl4(thf)2 with 1 equiv. of Na2[L] gave a mixture of [L]ZrCl2(thf) (1) and [L]2Zr (2). When the amount of Na2[L] was increased to 2 equiv., the bis(bis(aryloxide)-N-heterocyclic-carbene) complex 2 was obtained in good yield. The dichloro complex 1 is a precursor to organometallic derivatives, and treatment with PhCH2MgCl or Me3SiCH2Li yielded [L]ZrR2 [R = CH2Ph (3), CH2SiMe3 (4)]. The disodium salt of the ligand Na2[L] is unstable and undergoes 1,2-benzyl migration, whereas zirconium complexes of the [L]2− ligand are found to be thermally stable in solid and solution. The X-ray crystal structures of 1, 2, and 3 are described.