Zinc(II) complexes of Triaza and Amidinate ligands: Efficient initiators for the ring‐opening polymerization of ε‐Caprolactone and rac‐Lactide

Abstract

Zinc complexes supported by tertiary 1,3,5-triazapenta-1,3-dienate ligand (L 1 ) and N-benzoyl-N -arylbenzamidinate [aryl =2,6-diisopropylphenyl (L 2 ), phenyl (L 3 )] ligands have been synthesized and characterized. The reaction of L 1 H with ZnEt 2 affords a mononuclear zinc complex [L 1 ZnEt] (1) in good yield. Tetra nuclear zinc complex [(L 1 ) 2 Zn 4 O(OAc) 4 ] (2) is prepared by treating L 1 H with one equivalent of Zn(OAc) 2 in toluene. Further, dinuclear zinc complexes [L 2 ZnEt] 2 (3) and [L 3 ZnEt] 2(4) are obtained in good yields from L 2 H and L 3 H with ZnEt 2 in toluene respectively . The complexes 1-4 have been characterized by 1 H/ 13 C NMR spectroscopy and single crystal X-ray diffraction studies. All of the complexes have been explored for their catalytic activity toward the ring-opening polymerization (ROP) of εcaprolactone. It has been found that complex 1 is an active catalyst for the polymerization of ε-caprolactone in presence of a cocatalyst benzyl alcohol (BnOH). While complex 2 is as active as 1 there is no need for a cocatalyst for the polymerization to proceed. Dinuclear zinc complexes 3 and 4 show very high activity for the ROP of ε-caprolactone (CL) and rac-lactide (LA) without requiring a cocatalyst. The resultant polymers are found to have very high molecular weight (M n = 296 X 10 3 g mol −1 ) and relatively narrow polydispersity index compared to 1 and 2.

Ethylene-Bridged Hexadentate Bis(amidines) and Bis(amidinates) with Variable Binding Sites

“…Although related trans -1,2-diaminocyclohexane-linked tetradentate ( t -1,2-DACH-)­bis­(amidines) and their corresponding alkali metal complexes have been in the focus of two earlier studies, significantly less attention has been devoted to bis­(amidines) bearing additional terminal N -donor sites. , This is surprising, as these bis­(amidines) are generally interesting in terms of their potential as hexadentate ligands and hydrogen bond donor–acceptor properties in feasible supramolecular assemblies. This becomes obvious when the large family of related nonaromatic 1,3,5-triazapentadienes and pentaazadienes, their less explored N -analogues, is considered. − In particular, 1,3,5-triazapentadiene anions have been employed as versatile ligands for Groups 1–2 and 7–13 element complexes as well as in accompanying catalytic applications. , The corresponding BF 2 /BPh 2 complexes are of significant interest as highly luminescent materials. , Out of this selection, linked bis- and tris­(1,3,5-triazapentadienes) have been described in the literature as well. ,,, Among the less prominent linked ( t -1,2-DACH-)­bis­(amidines), only one example with terminal pyridyl substituents has been reported so far . Remarkably, closely related pyridylbenzamidines are well-known for decades and have recently attracted considerable attention as ligands in nickel- and palladium-catalyzed cross coupling reactions and in olefin polymerization/oligomerization .…”

Zinc 8-aminotrihydroquinolines appended with pendant N-diphenylphosphinoethyl arms as exceptionally active catalysts for the ROP of ε-CL

“…While zinc catalysts bound by tridentate ligands have been central to developments in this area, there were also many reports concerned with zinc complexes bearing bidentate N^N ( 12–20 , Chart 1) 32–37 or N^O ( 21–25 , Chart 1) 38 ligands, which are in most cases formed as dimeric species. 32 For example, N^N-containing 12–16 bearing N-heterocyclic donors can efficiently catalyze the ROP of ε-CL and LA, 8 a ,33,34 while those containing sterically encumbered pyridylamido ligands ( 14 ) can mediate the formation of stereodiblock poly(LLA- b -DLA).…”

“…34 b Bisguanidines have also proved attractive due to the stability of their zinc complexes for the ROP of cyclic esters under industrially relevant conditions. 36 Furthermore, monoguanidine 17 has showed its effectiveness in the bulk LA polymerization producing PLAs with high molecular weights up to 176 000 g mol −1 , while guanidinate 18 could also efficiently promote the ROP of cyclic esters. In addition, N^N species of the type 19 and 20 have also been the subject of numerous literature reports.…”

Synthesis and properties of polylactide terpolymers P(LLA‐TMC‐GA) catalyzed by zirconium (IV) acetylacetonate

“…Many metal complexes have been explored for catalytic (co)polymerization of lactide, such as tin, lanthanum, aluminum, zinc and zirconium . It is well known that stannous octoate Sn(Oct) 2 is the most popular catalyst in polymerization of lactide for its high efficiency.…”