Studies on Multinuclear Magnesium tert-Butyl Heteroscorpionates: Synthesis, Coordination Ability and Heteroselective ROP of rac-Lactide

Abstract

The reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(κ3-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(κ3-phbptamd)(thf)] (5) [phbptamd = N,N′-di-p-tolylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] with the commercially available Grignard reagent tBuMgCl in an equimolecular ratio yielded the magnesium tert-butyl heteroscorpionates [Mg(tBu)(κ3-NNN)] [NNN = pbpamd (6), tbpamd (7), ttbpamd (8), and phbptamd (9)]. In addition, subsequent reaction of monoalkyls 6–8 with two additional equivalents of tBuMgCl 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 [{(tBu)Mg(κ3-N,N,N;κ2-C,N)Mg(tBu)}2{μ-O,O-(OC4H8O)}] (κ3-N,N,N;κ2-C,N = pbpamd–10, tbpamd–11a) and [(tBu)Mg(κ2-N,N;κ2-N,N)Mg(tBu){μ-O,O-(OC4H8O)}]2 (κ2-N,N;κ2-N,N = tbpamd–11b). This reaction for 8 was unsuccessful, but an analog such as dinuclear complex [(thf)(tBu)Mg(κ2-N,N;κ2-N,N)Mg(tBu)(thf)] (κ2-N,N;κ2-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(tBu)(κ3-phbptamd){Mg(tBu)2}] (13). Similarly, coordination of MgtBu2 to 10 produces chiral trinuclear [{(tBu)Mg(pbpamd–)Mg(tBu)}{μ-O,O-(OC4H8O)}{Mg(tBu)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 σ-C(sp3)–Mg (14) and extended π-C2N2(sp2)–Mg2 (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.