Synthesis, Structures, Bonding, and Ethylene Reactivity of Group 4 Metal Alkyl Complexes Incorporating 8-Quinolinolato Ligands

Abstract

This contribution describes the synthesis, structures, bonding, and reactivity of neutral (Ox)2MR2 and cationic (Ox)2MR+ zirconium and hafnium alkyl complexes which contain substituted 8-quinolinolato ligands (Ox- = 2-Me-8-quinolinolato, MeOx-, 2; 2-Me-5,7-Br2-8-quinolinolato, MeBr2Ox-, 3). Alkane elimination and halide displacement reactions provide routes to (MeOx)2ZrR2 (9a, R = CH2Ph; 9b, R = CH2CMe3; 9c, R = CH2SiMe3), (MeOx)2Hf(CH2Ph)2 (10a), (MeBr2Ox)2ZrR2 (11a, R = CH2Ph; 11b, R = CH2CMe3), (MeBr2Ox)2Hf(CH2Ph)2 (14a), (MeOx)2ZrCl2 (15), (MeBr2Ox)2ZrCl2 (16), and (MeBr2Ox)2Zr(NMe2)2 (17). The reaction of 16, 17, or (MeBr2Ox)4Zr with AlMe3 yields (MeBr2Ox)AlMe2 (18). An X-ray crystallographic analysis shows that in the solid state 9a adopts a distorted octahedral structure with a trans-O, cis-N, cis-R ligand arrangement and that one of the benzyl ligands is bonded in an η2-fashion. Solution NMR data are consistent with this structure and establish that exchange of the distorted and normal benzyl ligands is rapid on the NMR time scale. Solution NMR data for the other (Ox)2MR2 complexes are consistent with analogous octahedral, trans-O, cis-N, cis-R structures for these species. Variable-temperature NMR studies establish that (Ox)2MR2 complexes undergo inversion of metal configuration (i.e., Λ/Δ isomerization, racemization) on the NMR time scale at elevated temperatures (ΔG⧧ (racemization) = 15−18 kcal/mol). Thermolysis of 11a results in migration of a benzyl ligand from Zr to C2 of a MeBr2Ox- ligand, yielding (MeBr2Ox)(2-Me-2-CH2Ph-5,7-Br2-Ox)ZrCH2Ph (19) as a single diastereomer. Reaction of 9a or 9b with [HNMe2Ph][B(C6F5)4] yields the base-free cationic complexes [(MeOx)2Zr(R)][B(C6F5)4] (20a, R = CH2Ph; 20b, R = CH2CMe3), while the corresponding reaction of 11a yields the labile amine adduct [(MeBr2Ox)2Zr(CH2Ph)(NMe2Ph)][B(C6F5)4] (21a). The reaction of [HNMePh2][B(C6F5)4] with the appropriate (Ox)2M(CH2Ph)2 complex yields 20a, [(MeOx)2Hf(CH2Ph)][B(C6F5)4] (22a), or [(MeBr2Ox)2M(CH2Ph)][B(C6F5)4] (23a, M = Zr; 24a, M = Hf). An X-ray crystallographic analysis establishes that the cation of 23a adopts a square pyramidal structure with a highly distorted (η2) benzyl ligand in the apical site and a trans-O, trans-N ligand arrangement in the basal sites, and NMR studies show that 23a and 24a adopt analogous structures in solution. In contrast, NMR studies establish that 20a, 20b, and 22a, which contain the more strongly electron-donating MeOx- ancillary ligand, adopt distorted square pyramidal structures with an apical-O, cis-N ligand arrangement which allows maximum O−M π-donation. The reactions of 23a or 24a with PMe3 yield the adducts [(MeBr2Ox)2M(CH2Ph)(PMe3)][B(C6F5)4] (25a, M = Zr; 26a, M = Hf), which adopt trans-O, cis-N, cis-benzyl/PMe3 structures analogous to those of the (Ox)2MX2 complexes. The (MeBr2Ox)2M(η2-CH2Ph)+ cations 23a and 24a exhibit moderate ethylene polymerization activity, while the MeOx- analogues 20a and 20b are inactive.