摘要:
The reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(kappa(3)-NNN)(thf)] {NNN = pbpamd (1) [pbpamd = N,N'-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate], tbpamd (2) [tbpamd = N-ethyl-N'-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate], ttbpamd (3) [ttbpamd = N,N'-di-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate], phbpamd (4) [phbpamd = N,N'-di-p-tolylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and the highly sterically demanding [Li(kappa(3)-phbptamd)(thf)] (5) [phbp(t)amd = N,N'-di-p-tolylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] with the commercially available Grignard reagent (BuMgCl)-Bu-t in an equimolecular ratio yielded the magnesium tert-butyl heteroscorpionates [Mg(Bu-t)(kappa(3)-NNN)] [NNN = pbpamd (6), tbpamd (7), ttbpamd (8), and phbptamd (9)]. In addition, subsequent reaction of monoalkyls 6-8 with two additional equivalents of (BuMgCl)-Bu-t gave rise to two different classes of chiral tetraalkyls, which contain apical methine C-H activated heteroscorpionates that show two possible coordination modes, namely, the tetranuclear complexes [{(Bu-t)Mg(kappa(3)-N,N,N;kappa(2)-C,N)Mg(Bu-t)}(2){-O,O-(OC4H8O)}] (kappa(3)-N,N,N;kappa(2)-C,N = pbpamd 10, tbpamd 11a) and [(Bu-t)Mg(kappa(2)-N,N;kappa(2)-N,N)Mg(Bu-t){-O,O-(OC4H8O)}](2) (kappa(2)-N,N;kappa(2)-N,N = tbpamd 11b). This reaction for 8 was unsuccessful, but an analog such as dinuclear complex [(thf)(Bu-t)Mg(kappa(2)-N,N;kappa(2)-N,N)Mg(Bu-t)(thf)] (kappa(2)-N,N;kappa(2)-N,N = phbpamd 12) could be obtained from its lithium salt. Contrary to complexes 10-12, the C-H activation process was not observed for complex 9, but interestingly, the coordination of one additional equivalent of MgtBu2 afforded dinuclear complex [Mg(Bu-t)(kappa(3)-phbptamd){Mg(Bu-t)(2)}] (13). Similarly, coordination of MgtBu(2) to 10 produces chiral trinuclear [{(Bu-t)Mg(pbpamd(-))Mg(tBu)}{-O,O-(OC4H8O)}{Mg(Bu-t)(2)(thf)}] (14). The X-ray crystal structures of complexes 4, 6, 8, 9, 11b, 12, 13, and 14 confirm the nuclearity of each family, and the existence of apical sigma-C(sp(3))-Mg (14) and extended pi-C2N2(sp(2))-Mg-2 (11b and 12) covalent bonds have unambiguously been also confirmed. Furthermore, mononuclear compounds 8 and 9, tetranuclear compound 10, and dinuclear compounds 12 and 13 can act as highly efficient single-component living initiators for the ring-opening polymerization of rac-lactide. More interestingly, the most sterically hindered initiators exhibit enhanced levels of heteroselectivity on the PLAs, with 9 reaching Ps values up to 0.85.