SYNTHESIS, STRUCTURES, BONDING, AND ETHYLENE REACTIVITY OF GROUP-4 METAL ALKYL COMPLEXES INCORPORATING 8-QUINOLINOLATO LIGANDS

This contribution describes the synthesis, structures, bonding, and re activity of neutral (Ox)(2)MR2 and cationic (Ox)(2)MR+ zirconium and h afnium alkyl complexes which contain substituted 8-quinolinolato ligan ds (Ox(-) = 2-Me-8-quinolinolato, MeOx(-), 2; 2-Me-5,7-Br-2-8-quinolin olato, MeBr(2)Ox(-), 3). Alkane elimination and halide displacement re actions provide routes to (MeOx)(2)ZrR2 (9a, R = CH2Ph; 9b, R = CH2CMe 3; 9c, R = CH2SiMe3), (MeOx)(2)-Hf(CH2Ph)(2) (10a), (MeBr(2)Ox)(2)ZrR2 (11a, R = CH2Ph; 11b, R = CH2CMe3), (MeBr(2)Ox)(2)Hf-(CH2Ph)(2) (14a) , (MeOx)(2)ZrCl2 (15), (MeBr(2)Ox)(2)ZrCl2 (16), and (MeBr(2)Ox)(2)Zr( NMe2)(2) (17). The reaction of 16, 17, or (MeBr(2)Ox)(4)Zr with AlMe3 yields (MeBr(2)Ox)AlMe2 (18). An X-ray crystallographic analysis shows that In the solid state 9a adopts a distorted octahedral structure wi th a trans-O, cis-N, cis-R, ligand arrangement and that one of the ben zyl ligands is bonded in an eta(2)-fashion. Solution NMR data are cons istent with this structure and establish that exchange of the distorte d and normal benzyl ligands is rapid on the NMR time scale. Solution N MR, data for the other (Ox)(2)MR2 complexes are consistent with analog ous octahedral, trans-O, cis-N, cis-R structures for these species. Va riable-temperature NMR studies establish that (Ox)(2)MR2 complexes und ergo inversion of metal configuration (i.e., Delta/Delta isomerization , racemization) an the NMR time scale at elevated temperatures (Delta G double dagger (racemization) = 15-18 kcal/mol). Thermolysis of 11a r esults in migration of a benzyl ligand from Zr to C2 of a MeBr(2)Ox(-) ligand, yielding (MeBr(2)Ox)(2-Me-2-CH2Ph-5,7-Br-2-Ox)ZrCH2- Ph (19) as a single diastereomer. Reaction of 9a or 9b with [HNMe2Ph][B(C6F5)( 4)] yields the base-free cationic complexes [(MeOx)(2)Zr(R)][B(C6F5)(4 )] (20a, R = CH2Ph; 20b, R = CH2-CMe3), while the corresponding reacti on of 11a yields the labile amine adduct [(MeBr(2)Ox)(2)Zr-(CH2Ph)(NMe 2Ph)][B(C6F5)(4)] (21a). The reaction of [HNMePh2][B(C6F5)(4)] with th e appropriate (Ox)(2)M(CH2Ph)(2) complex yields 20a, [(MeOx)(2)Hf(CH2P h)][B(C6F5)(4)] (22a), or [(MeBr(2)Ox)(2)M(CH2Ph)][B(C6F5)(4)] (23a, M = Zr; 24a, M = Hf). An X-ray crystallographic analysis establishes th at, the cation of 23a adopts a square pyramidal structure with a highl y distorted (eta(2)) benzyl ligand in the apical site and a trans-O, t rans-N ligand arrangement in the basal sites, and NMR studies shaw tha t 23a and 24a adopt analogous structures in solution. In contrast, NMR studies establish that 20a, 20b, and 22a, which contain the more stro ngly electron-donating MeOx(-) ancillary Ligand, adopt distorted squar e pyramidal structures with an apical-O, cis-N ligand arrangement whic h allows maximum O-M pi-donation. The reactions of 23a or 24a with PMe 3 yield the adducts [(MeBr(2)Ox)(2)M(CH2-Ph)(PMe3)][B(C6F5)(4)] (25a, M = Zr; 26a, M = Hf), which adopt trans-O, cis-N, cis-benzyl/PMe3 stru ctures analogous to those of the (Ox)(2)MX2 complexes. The (MeBr(2)Ox) (2)M(eta-CH2Ph)(+) cations 23a and 24a exhibit moderate ethylene polym erization activity, while the MeOx(-) analogues 20a and 20b, are inact ive.