Group 4 Metal Complexes of Nitrogen-Bridged Dialkoxide Ligands: Synthesis, Structure, and Polymerization Activity Studies

Neutral soluble titanium(IV) and zirconium(IV) complexes of new amino-dialkoxide ligands
{OCR2CH2N(CH2Ph)CH2CR2O}2- ({ONOR}2-) have been prepared. Alcohol and alkane
elimination reactions from {ONOR}H2 (R = Me, 1; p-tol, 2) and salt metathesis routes from
Li2{ONOMe} in situ-generated afford {ONOMe}Ti(OiPr)2 (3), {ONOR}2M (R = Me, M = Ti, 5;
Zr, 6; R = p-tol, M = Zr, 7), {ONOMe}Zr(CH2Ph)2 (8), {ONOMe}ZrCl2(THF)n (n = 0, 9; n = 1,
10), {ONOMe}Zr(NMe2)2 (11), and {ONOR}TiCl2 (R = Me, 13; R = p-tol, 14) in good yields.
X-ray crystallographic studies showed that 3, 8, 13, and 14 adopt in the solid state
mononuclear structures, with coordination of the nitrogen atom to the metal center. Dinuclear
structures with bridging alkoxide and amido ligands were observed for 6 and a mixed amido-{ONOMe}Zr complex (12) isolated in the preparation of 11. NMR data are consistent with
the existence of a single monomeric species in toluene solution for all complexes, except 11
and dichlorozirconium complexes 9 and 10, for which mixtures of monomeric and possibly
aggregated species are observed in toluene and THF. The dynamic behavior of monomeric
species 3, 8, 13, and 14 in toluene was investigated by variable-temperature NMR
spectroscopy. The activation parameters determined by line-shape analysis, in particular
for 8 (ΔH⧧ = 20.0 ± 1 kcal·mol-1; ΔS⧧ = 13.1 ± 2 cal·mol-1·K-1) and 13 (ΔH⧧ = 17.4 ± 1
kcal·mol-1; ΔS⧧ = 11.4 ± 2 cal·mol-1·K-1), suggest a process involving dissociation of nitrogen,
inversion of configuration at nitrogen, and amine re-coordination. The bulkiness of the R
substituents affects the fluxional behavior of {ONOR}TiCl2 (R = Me, 13; R = p-tol, 14). The
THF-free dichlorozirconium complex 9 in combination with MAO generates highly active
but very unstable ethylene polymerization catalyst systems.