Group 3 Dialkyl Complexes with Tetradentate (L, L, N, O; L = N, O, S) Monoanionic Ligands: Synthesis and Reactivity

Tripodal, tetradentate phenols, (LCH 2 ) 2 NCH 2 - C 6 H 2 - 3,5 - (CMe 3 ) 2 - 2 - OH (L = CH 2 OCH 3 ( 1), CH 2 NEt 2 ( 2 ), 2 - C 5 H 4 N ( 3 ), CH 2 SCMe 3 ( 5 ), CH 2 NMe 2 ( 6 )), were synthesized, and metalations were performed via alkane elimination from yttrium and scandium trialkyl complexes to generate the corresponding dialkyl complexes [(LCH 2 CH 2 ) 2 NCH 2 - C 6 H 2 - 3,5 - (CMe 3 ) 2 - 2 - O]MR 2 (M = Y, L = OCH 3, R = CH 2 SiMe 2 Ph ( 7a ); M = Y, L = NEt 2, R = CH 2 SiMe 2 Ph ( 7b ); M = Sc, L = OCH 3, R = CH 2 SiMe 2 Ph ( 8a ); M = Sc, L = SCMe 3, R = CH 2 SiMe 2 Ph ( 8b ); M = Y, L = OCH 3, R = CH 2 SiMe 3 ( 9 ); M = Sc, L = OCH 3, R = CH 2 SiMe 3 ( 10 )). X - ray crystallographic studies show that 7a, b and 8a adopt, in the solid state, mononuclear structures of C 1 symmetry. The 1 H NMR spectra of these dialkyl complexes in benzene -d 6 at high temperatures reveal exchange processes involving the ether groups and the alkyl groups. The dynamic behavior of species 7a, 8a, and 10 in toluene -d 8 was investigated by variable - temperature 1 H NMR spectroscopy. The activation parameters of the fluxional processes for 7a, 8a, and 10 were determined by line - shape and Eyring analyses (for 7a, Δ H ⧧ = 7.3 ± 0.3 kcal/mol and Δ S ⧧ = − 16 ± 1.4 cal/(mol K); for 8a, Δ H ⧧ = 9.9 ± 0.5 kcal/mol and Δ S ⧧ = −15.3 ± 1.8 cal/ (mol K); for 10, Δ H ⧧ = 10.8 ± 0.6 kcal/mol and Δ S ⧧ = − 11.4 ± 1.9 cal/(mol K)). These data establish that the dialkyl complexes 7a, 8a, and 10 undergo a nondissociative exchange process. The scandium dialkyl complex [(C 5 H 4 N - 2 - CH 2 ) 2 NCH 2 - C 6 H 2 - 3,5 - (CMe 3 ) 2 - 2 - O]Sc(CH 2 SiMe 2 Ph) 2 ( 11 ) was found to undergo clean activation of a C−H bond of a methylene linking a pyridine to the central nitrogen donor. This process follows first-order kinetics ( k = [2.8(3)] × 10 - 4 s - 1 at 0 ° C). The yttrium dialkyl complexes 7a and 9 react with 1 equiv of [PhNHMe 2 ] + [B(C 6 F 5 ) 4 ] - in chlorobenzene -d 5, to generate a solution that slowly polymerizes ethylene. Compounds 7 − 10 also polymerize ethylene with low activity upon activation with MAO.