Well-defined alkyl heteroscorpionate magnesium complexes as excellent initiators for the ROP of cyclic esters

The reaction of the heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N-tertbutylbis(3,5-dimethylpyrazol- 1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N'-diisopropylbis-(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equiv of RMgCl proceeds to give very high yields of the neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C3H5 1, 'Bu 2, CH2SiMe3 3; NNN = pbpamd, R = C3H5 4, 'Bu 5, CH2SiMe3 6). On heating toluene solutions of complexes 1-3, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinated sandwich complexes [Mg(tbpamd)(2)] (7) and [Mg(pbpamd)(2)] (8). Interestingly, the allyl derivative 4 can be easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)(2)]Cl-2 (9) [tbpamdH = N-ethyl-N'-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from 7 by means of a protonation process. Finally, alkyl-containing complexes 1-6 can act as highly effective single-component living initiators for the ring-opening polymerization of c-caprolactone and lactides over a wide range of temperatures. E-Caprolactone is polymerized within seconds to give high molecular weight polymers with narrow polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as M-w/M-n = 1.05. Additionally, polymerization Of L-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(L-lactides) with high melting temperatures (T = 160 degrees C). Polymer end-group analysis shows that the polymerization process is initiated by alkyl transfer to the monomer.