Unbridged bidentate aluminum complexes supported by diaroylhydrazone ligands: Synthesis, structure and catalysis in polymerization of ε‐caprolactone and lactides

Abstract

A series of novel diaroylhydrazone aluminum complexes have been synthesized and well‐defined structurally, and their catalytic performance in the polymerization of ε‐caprolactone and lactides have also been evaluated. Complexes [(L1–4)2AlMe] (1–4) {[L1 = (3,5‐tBu2–2‐OMe‐C6H2)CH=NNCOC6H5], [L2 = (3,5‐tBu2–2‐OMe‐C6H2)CH=NNCO(C6H4–4‐OCH3)], [L3 = (3,5‐tBu2–2‐OMe‐C6H2)CH=NNCO(C6H4–4‐Br)] and [L4 = (2‐OMe‐C6H4)CH=NNCO(C6H4–4‐tBu)]} were prepared through treatment of AlMe3 with the corresponding proligands L1–4H in molar ratios of 1: 1 or 1: 2. Chemical structures of all the complexes were well‐defined by elemental analysis, NMR spectra as well as single‐crystal X‐ray study. Complexes [(L1–4)2AlMe] (1–4) in this work represent the first examples of aluminum complexes of aroylhydrazone ligands with crystallographic characterization. Specifically, they are all in monomeric form with a penta‐coordinated aluminum center, including two approximately co‐planar five‐membered metallacycles with aluminum. Introduced bulky tert‐butyl substituents in aroylhydrazone ligands could affect the geometry around the central metal which is a distorted square‐based pyramid in complexes 1–3 while being a trigonal bipyramidal in complex 4, thus affecting their catalytic behaviors. The complexes can successfully catalyze the ring‐opening polymerization of ε‐caprolactone and L‐lactide under mild conditions without any activator. In addition, complexes 1–4 could also polymerize rac‐lactide, affording atactic polylactides with high conversions and good controllability in relatively short reaction time.

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…Based on the results obtained in the homo-polymerization tests, species 236 and 238 were employed for the sequential CL-LA and LA-CL co-polymerizations, respectively. Very recently, Nie, Han et al disclosed diarylhydrazone-based Al complexes displaying good activity in the ROP of ε-CL and r-LA [116]. The four novel complexes proved to be able to completely convert the monomer under mild conditions (70 °C) with good control and Mn in accordance with the calculated values.…”

“…Notably, 239 proved efficient even at higher monomer concentrations (up to 350 equiv./Al), allowing for 87% conversion after 3 h at ambient temperature; furthermore, in this case, the polymerization proceeded in a living fashion. Very recently, Nie, Han et al disclosed diarylhydrazone-based Al complexes displaying good activity in the ROP of ε-CL and r-LA [116]. The four novel complexes proved to be able to completely convert the monomer under mild conditions (70 • C) with good control and M n in accordance with the calculated values.…”

Use of group 13 aryloxides for the synthesis of green chemicals and oxide materials

“…We decided to explore the use of group 13 metal aryloxides because they are popular catalysts/initiators in organic synthesis [17][18][19][20] and polymeriz-ation reactions. [21][22][23][24][25][26][27][28][29][30][31][32][33] Recently, our interest has been focused on the synthesis of heterometallic compounds, in which the presence of two different Lewis centers can result in heterometallic cooperativity. [34][35][36][37][38] For example, chiral lithium-aluminum binaphthoxide [AlLi(binol) 2 ] is a highly effective asymmetric catalyst for the 1,4-addition of a Horner-Wadsworth-Emmons reagent to enones, Michael additions, and tandem Michaelaldol reactions.…”