β-diketiminato scandium chemistry:: Synthesis, characterization, and thermal behavior of primary amido alkyl derivatives

Treatment of the beta-diketiminato-supported scandium dichlorides {[ArNC(R)CHC(R)NAr]ScCl2}(n) (Ar = 2,6-Pr-i(2)-C6H3; R = CH3, 1a, n = 2; R = Bu-t, 1b, n = 1) with 1 equiv of a lithium amide reagent LiN(H)R' (R'= Bu-t, 2,6-Pr-i(2)-C6H3) gave scandium amido derivatives. For la, use of LiN(H)Bu-t leads to the bis-amido derivative (6a) regardless of the equivalency of amide reagent employed, suggesting that facile ligand redistribution processes are operative when the ligand is the less bulky methyl-substituted example. For 1b, mono-amido chlorides 2b (R' = Bu-t) and 3b (R' = 2,6-Pr-i(2)-C6H3) are obtained in good yields, and these compounds can be alkylated with MeLi to provide mono-amido methyl compounds 4b (R' = Bu-t) and 5b (R' = 2,6-iPr2-C6H3). All four of these compounds were characterized crystallographically. The amido ligand occupies the exo coordination site exclusively, and there is no evidence in solution for a diastereomer with the amido group in the endo site. DFT calculations suggest that there is a strong steric preference and a slight electronic bias for the amido ligand to assume the exo position. Thermolysis of the amido methyl complex 4b leads to loss of CH4 and production of a scandacylic product, 7, formed via metalation with one of the N-aryl isopropyl methyl groups. This compound was characterized crystallographically. Deuterium labeling experiments suggest that 7 is produced via direct metalation and does not form via a scandium imido intermediate.