Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Abstract

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. D Bu 2 Al 3 Me 5 exhibited higher catalytic activity than did the dinuclear aluminum complex L Bu 2 Al 2 Me 4 (16 times as high for CL polymerization; [CL]:[D Bu 2 Al 3 Me 5 ]:[BnOH] = 100:0.5:5, [D Bu 2 Al 3 Me 5 ] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert-butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers

“…For example, catalysts with a “folded” conformation can open up monomer coordination sites compared to their “open” analogues, leading to enhanced activities . Some trimetallic aluminum catalysts featuring a central square pyramidal aluminum have shown enhanced activities compared to their bimetallic or tetrametallic analogues, with monomer coordination proposed to occur at the central Al center. ,, The steric availability can also influence the polymerization mechanism. For example, some studies suggest that steric accessibility determines whether both metals act cooperatively (e.g., with one coordinating the monomer and the other providing the nucleophile) or whether polymerization occurs simply at one metal center .…”

“…Trimetallic aluminum catalysts featuring three Al centers in a near linear arrangement were reported for ε-CL ROP (85−89, Figure 15) and showed a trend of increased catalytic activity with the increased steric bulk of aliphatic substituents, from i Pr (87) < t Bu (85) < adamantyl ( 86 [Cat]:[BnOH] 100:0.5:2.5). 65 The trimetallic t Bu-and phenylsubstituted complexes both outperformed their bimetallic counterparts ( This proposed catalytic cycle gives strong evidence for the cooperativity of the metal centers though electronic modulation and metal−metal proximity, leading to synergistic effects. Mehrkhodavandi and co-workers reported a series of trimetallic Zn complexes based on aminophenolate and iminophenolate ligands for the ROP of rac-LA, which were compared to the bimetallic and monometallic analogues (94− 99, Figure 16; see also section 2.1.3, Figure 11 for additional discussions of the Zn-benzoxide analogues of 98 and 99).…”

“…While multimetallic catalyst development has largely focused on bimetallic catalysts (section ), there are also some reports of tri- and tetrametallic systems, most of which are based on Mg, Al, ,,, Ga, Ti, , Zn, − Zr, and rare earth metals. , Rather than providing a comprehensive review, this section focuses on examples that display multimetallic cooperativity or have delivered insight to the polymerization mechanism and are based on tri- and tetranucleating ligand scaffolds, whereas aggregates are discussed in section .…”

“…Trimetallic aluminum catalysts featuring three Al centers in a near linear arrangement were reported for ε-CL ROP ( 85 – 89 , Figure ) and showed a trend of increased catalytic activity with the increased steric bulk of aliphatic substituents, from i Pr ( 87 ) < t Bu ( 85 ) < adamantyl ( 86 ) (25 °C, toluene, [ε-CL]:[Cat]:[BnOH] 100:0.5:2.5) . The trimetallic t Bu- and phenyl-substituted complexes both outperformed their bimetallic counterparts ( 85 and 89 vs 68 and 64 ), with the k obs value of 85 being 15 times greater than that of 68 , and 89 being twice that of 64 .…”

“…Figure15. Bimetallic and trimetallic Al catalysts and the mechanism for ROP of ε-CL determined by DFT for complex 93 65. …”