Organo-Aluminum and Zinc Acetamidinates: Preparation, Coordination Ability, and Ring-Opening Polymerization Processes of Cyclic Esters

Abstract

The reaction of the highly sterically demanding NNN'-heteroscorpionate protioligands pbpamd-H, tbpamd-H, and phbpamd-H (a) and the low sterically hindered analogs pbpamd-H, tbpamd-H, and phbpamd-H (b), with 1 equiv of AlR (R = Me, Et) proceed in high yields to give two families of complexes: the mononuclear dialkyl aluminum bidentate-acetamidinates [AlR(κ- N' N')] (κ- N' N' = pbpamd, R = Me 1, Et 2; tbpamd, R = Me 3, Et 4; phbpamd, R = Me 5, Et 6) and the monodentate-acetamidinates [AlR(κ- NN')] (κ- NN' = tbpamd, R = Me 7; phbpamd, R = Me 8, Et 9). In complexes 7-9, the presence of two possible CH-NH tautomers as low extended π-N-C-N'(sp)-Al and high extended π-HN-C-N'(sp)-Al complexes, respectively, could be identified. Moreover, the reaction of aluminum dimethyls 7 and 8 with ZnMe afforded the isolation of the more stable scorpionate zinc monoalkyls [Zn(Me)(κ- NNN')] ( NNN' = tbpamd 10 and phbpamd 11), through a very unusual ligand exchange process, involving a zinc-to-aluminum transmetalation of an alkyl group. The X-ray crystal structures of 1, 3, 7, and 8, as well as that of 11, confirmed unambiguously the different κ-arrangements proposed for bi- or monodentate acetamidinate dialkyls 1-6 and 7-9, respectively, the presence of NH tautomer in 7 and 8, and a κ- NNN' coordination in monoalkyl 11. Density functional theory calculations were used to explore the three different favored κ-arrangements found in acetamidinate aluminum dialkyls 1-9, the relative stability of both CH-NH tautomers, and the ligand transfer reaction leading to the formation of κ- NNN' zinc monoalkyls 10 and 11. Interestingly, dialkyls 1, 5, 7, and 8 can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide in mild conditions after hours. These initiators efficiently mediated the immortal polymerization in the presence of excess of benzyl alcohol (up to 20 equiv), as evidenced by the narrow dispersity values and the good agreement between the experimental M values and monomer/benzyl alcohol ratios. In addition, the most sterically hindered initiator, 5, exhibits enhanced levels of heteroselectivity on the produced PLAs, reaching P values up to 0.70.

Efficient Bulky Organo-Zinc Scorpionates for the Stereoselective Production of Poly(rac-lactide)s

“…The solid-state structure also confirms that the acetamidinate is coordinated in a monodentate fashion with the Zn atom, and delocalisation is also evidenced in the N–C–N moiety of the acetamidinate, with the bond lengths C(24)–N(5) and C(24)–N(6) ranging from 1.351(8) Å to 1.295(8) Å. An analog crystal structure of complex 2 has been previously reported by our group [ 48 ].…”

“…The zinc metal exhibits a distorted tetrahedral geometry, with the scorpionate ligand in a κ 3 -NNN′ coordination mode. The N(1)–Zn and N(3)–Zn bond lengths [2.167(5) Å and 2.135(5) Å] are well balanced and compare well with that observed in the analog acetamidinate-based scorpionate zinc alkyls [ 48 ] but are considerably longer than the N(5)–Zn bond length [2.034(5) Å]. The solid-state structure also confirms that the acetamidinate is coordinated in a monodentate fashion with the Zn atom, and delocalisation is also evidenced in the N–C–N moiety of the acetamidinate, with the bond lengths C(24)–N(5) and C(24)–N(6) ranging from 1.351(8) Å to 1.295(8) Å.…”

“…The high sterically demanding acetamidinate-based scorpionate protioligand Hphbp t amd [ 48 ] [Hphbp t amd = N , N ′-di-p-tolylbis(3,5-di-tertbutylpyrazole-1-yl)acetamidine] was initially reacted with one equivalent of ZnMe 2 in toluene at room temperature. This reaction cleanly afforded the mononuclear amidinate-based scorpionate zinc complex [ZnMe(κ 3 -phbp t amd)] 1 , a white solid in good yield (ca.…”

“…Deuterated solvents were stored over activated 4 Å molecular sieves and degassed by several freeze–thaw cycles. The protioligand Hphbptamd was prepared according to the procedures in the literature [ 48 ]. ZnMe 2 (Sigma-Aldrich, Munich, Germany) was used as purchased and ZnCl 2 (Sigma-Aldrich, Munich, Germany) was predried by several heat toluene suspension-vacuo cycles before use.…”

“…On the bases of our previous expertise [ 41 , 42 , 43 , 44 , 45 , 46 ], now we endeavour to develop a series of inexpensive, low-toxic and easy-to-prepare zinc-based catalysts that are very efficient and selective in this industrially-demanded process. For this propose, a very successful sterically hindered acetamidinate-based NNN’-scorpionate [ 48 ] from our extended ligand library [ 48 , 49 , 50 ] has been employed.…”