Zinc complexes of fluorous alkoxide-imino ligands: Synthesis, structure, and use in ring-opening polymerization of lactide and β-butyrolactone

Abstract

The reaction of fluorous imino-alcohols {ON(R1,R2)}H (R(1) = Me, Ph; R(2) = Ph, CH(2)Ph, cyclohexyl, piperidinyl) with Zn(N(SiMe(3))(2))(2) (1 or 0.5 equiv) systematically led to bis(ligand) complexes Zn{ON(R1,R2)}(2) (1-9), which were isolated in 84-88% yield. Imino-alcohols {ON(R1,R2)}H react selectively with ZnEt(2) to give the corresponding complexes {ON(R1,R2)}ZnEt (4-7), in 74-78% yield. The stability of complexes 4-7 appears to be controlled by the nature of the substituent at the imine N atom: a phenyl group leads to slow ligand redistribution while alkyl-type substituents (benzyl, cyclohexyl, piperidinyl) prevent this disproportionation. Treatment of 5 with isopropanol afforded {ON(Ph,Bn)}Zn(OiPr) (8) in modest yield, due to rapid ligand redistribution. Complexes 1-8 were authenticated by elemental analysis, (1)H, (19)F and (13)C NMR spectroscopy in solution, and single-crystal X-ray diffraction studies for 1, 3, 5, 7 and 8. In situ combinations of {ON(Ph,Bn)}ZnEt (5)/BnOH and Zn{ON(Ph,Bn)}(2) (2)/BnOH are active systems for the ROP of racemic lactide and beta-butyrolactone at 20-50 degrees C, yielding atactic polylactide and poly(3-hydroxybutyrate) with good molecular weight control (M(n) up to 20,700 g/mol) and relatively narrow molecular weight distributions (M(w)/M(n) = 1.06-1.57). These binary systems also allow the immortal ROP of lactide and beta-butyrolactone, using excess alcohol (up to 5 equiv) vs the Zn catalyst.

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

“…Presumably, 3 is produced by a very slow ligand-exchange reaction of 2 (Scheme 2). [37] Formation of homoleptic L 2 Zn complexes has been reported for undemanding and/or with poorly electron donating bidentate ligands, [25,38,39] but it is definitely uncommon for complexes bearing tridentate ligands. Generally, the presence of a bulky chelating ligand around the Zn center inhibits the disproportion reaction.…”

“…[30] Alternatively, the PPP ligand may employ only one neutral phosphine donor for coordination to the metal center, resulting in an anionic bidentate ligand. Although three-coordinate zinc phosphine complexes are extremely rare, [25,38,39,43] the bis-chelated zinc species featuring the k 2 -PPP ligand were successfully optimized. In these species, the zinc atom is in a trigonal-planar environment (sum of the bond angles: 359.9 and 360.08 for the alkyl and the amide derivative, respectively), coordinated by two phosphorus atoms to form a five-membered chelate ring.…”

Synthetic and Mechanistic Aspects of the Immortal Ring‐Opening Polymerization of Lactide and Trimethylene Carbonate with New Homo‐ and Heteroleptic Tin(II)‐Phenolate Catalysts

“…[4- 7,15] All of the catalytically active complexes demonstrated satisfactory control over the polymerization parameters. The agreement between theoretical and experimental molecular weights and the narrow molecular-weight distributions (with values of M w /M n typically around 1.10) are indicative of fast and reversible chain-transfer between dormant and growing …”

Metal-catalyzed immortal ring-opening polymerization of lactones, lactides and cyclic carbonates

“…22,23 We have recently extended this work to the preparation of a broad range of homo-and heteroleptic complexes of zinc stabilized by fluorinated imino-alkoxide {ON R1,R2 }ligands. 24 Heteroleptic complexes of the type {ON R1,R2 }ZnEt were prepared, although the stability of such species towards detrimental ligand redistribution reactions appeared to be strongly governed by the nature of the ligand around the metal center. For instance, while {ON Ph,Bn }ZnEt, {ON Ph,Cy }ZnEt and {ON Me,Pip }ZnEt (Bn = benzyl; Cy = cyclohexyl; Pip = N-benzylpiperidinyl) were all kinetically stable, {ON Me,Ph }ZnEt (with a less electron-donating group on the imino N atom) could not be isolated.…”