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

Tripodal, tetradentate phenols, (LCH_2)_2NCH_2-C_6H_2-3,5-(CMe_3)_2-2-OH (L = CH_2OCH_3 (1), CH_2NEt_2 (2), 2-C_5H_4N (3), CH_2SCMe_3 (5), CH_2NMe_2 (6)), were synthesized, and metalations were performed via alkane elimination from yttrium and scandium trialkyl complexes to generate the corresponding dialkyl complexes [(LCH_2CH_2)_2NCH_2-C_6H_2-3,5-(CMe_3)_2-2-O]MR_2 (M = Y, L = OCH_3, R = CH_2SiMe_2Ph (7a); M = Y, L = NEt_2, R = CH_2SiMe_2Ph (7b); M = Sc, L = OCH_3, R = CH_2SiMe_2Ph (8a); M = Sc, L = SCMe_3, R = CH_2SiMe_2Ph (8b); M = Y, L = OCH_3, R = CH_2SiMe_3 (9); M = Sc, L = OCH_3, R = CH_2SiMe_3 (10)). X-ray crystallographic studies show that 7a,b and 8a adopt, in the solid state, mononuclear structures of C_1 symmetry. The ^1H 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-d8 was investigated by variable-temperature ^1H NMR spectroscopy. The activation parameters of the fluxional processes for 7a, 8a, and 10 were determined by line-shape and Eyring analyses (for 7a, ΔH^⧧ = 7.3 ± 0.3 kcal/mol and ΔS^⧧ = −16 ± 1.4 cal/(mol K); for 8a, ΔH^⧧ = 9.9 ± 0.5 kcal/mol and ΔS^⧧ = −15.3 ± 1.8 cal/ (mol K); for 10, ΔH^⧧ = 10.8 ± 0.6 kcal/mol and ΔS^⧧ = −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_5H_4N-2-CH_2)_2NCH_2-C_6H_2-3,5-(CMe_3)_2-2-O]Sc(CH_2SiMe_2Ph)_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_6F_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