The role of ligand redox non-innocence in ring-opening polymerization reactions catalysed by bis(imino)pyridine iron alkoxide complexes

Abstract

The reactivity of iron-based ring opening polymerization catalysts is compared when the catalyst is in three different oxidation states. Formally iron(i) monoalkoxide complexes 3a (p-methoxyphenoxide) and 3b (neopentoxide) supported by bis(imino)pyridine ligands were synthesized and investigated as catalysts for the ring opening polymerization and copolymerization of various monomers. For most monomers, 3a and 3b were superior catalysts compared to analogous, formally iron(ii) and iron(iii) complexes (1a/1b and 2a/2b, respectively) for the ring opening polymerization of various cyclic ester and cyclic carbonate monomers. Experimental and computational investigation into the electronic structures of 3a and 3b revealed that they are most accurately described as containing a high spin iron(ii) center that is antiferromagnetically coupled to a singly reduced bis(imino)pyridine ligand. This electronic structure leads to increased electron density near the metal center without modulating the apparent metal oxidation state, which results in superior catalytic performance for the more highly reduced 3a and 3b compared to the increasingly more oxidized complexes (i.e.1a/1b and 2a/2b, respectively) in ring opening polymerization reactions. These findings have significant ramifications for the emerging field of redox-switchable polymerization catalysis.

N-Heterocyclic carbene iron complexes catalyze the ring-opening polymerization of lactide

“…16 Initial attempts to enhance the catalytic performance of iron complexes by introducing salen-type or other multidentate N,N or N,O-based ligands were met with little success; ROP with these catalysts generally required very low monomer to initiator ratios ([LA]/[cat] 100 : 1) and in many cases long reaction times. [17][18][19][20][21][22][23][24][25][26][27][28] The Fe(N) 4 system (II, Scheme 1) represents a notable exception, as this catalyst functions efficiently, even when used in low concentration. 23 Recently, Thomas and co-workers developed a catalyst system that is based on an aminophenolate ligand and enables the stereoselective polymerization of LA at a [LA]/[cat] ratio of up to 800 : 1.…”

The synthesis, characterisation and application of iron(iii)–acetate complexes for cyclic carbonate formation and the polymerisation of lactide

“…21 Examples of Fe-mediated ROP in the literature are less prevalent despite the numerous benefits. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Recently Duan et al synthesised and applied air-stable Fe(III)-salen-chloride complexes to the ROP of caprolactone and lactide. 12 Using propylene oxide (PO), as both the solvent and co-initiator, the postulated in situ generated Fe(III)-alkoxide species, formed via opening of PO by insertion of the Fe(III)-Cl bond, resulted in isotactic PLA (P m = 0.53-0.78) with broad dispersities (Đ = 1.…”

Reversible redox controlled acids for cationic ring-opening polymerization

“…[1][2][3] While living polymerization methodologies in conjunction with carefully chosen and timed monomer additions produce well-defined materials (e.g., block copolymers 2 ), the ability to control chain growth with an external stimulus could lead to many advanced structures and architectures with potentially interesting physical properties. These externally controlled polymerization methodologies rely on changes in chemical reactivity upon application of an external stimulus (chemical, [4][5][6][7][8][9][10][11][12][13][14][15][16][17] electrochemical, [18][19][20] photochemical, [21][22][23][24][25][26][27][28][29][30] thermal, [31][32][33] mechanochemical [34][35][36][37] ), which precisely regulates the incorporation of monomers at a growing polymer chain end. In addition to promoting the synthesis of advanced structures and architectures, 30,45 the spatiotemporal control afforded by these externally controlled polymerizations has enabled the development of new lithographic [38][39][40][41]<...>…”

“…43,44 The development of methods for externally controlled ringopening polymerization (ROP) of cyclic esters and carbonates is of particular interest as the polymers that are formed are biocompatible and degradable alternatives to petroleumderived polyolefins. State-of-the-art methods for externally controlled ROP have involved the use of redox-switchable coordination-insertion catalysts, which have been pioneered by Byers 7,[10][11][12]20 and Diaconescu, 8,9,[13][14][15][16][17] or the use of photoacids, which induce polymerization through an activated monomer mechanism. Photoacid generators (PAGs) 46 have been pivotal to the fields of lithography and microelectronics development; however, PAG-mediated polymerizations are not reversible and only provide temporal control over polymer initiation rather than chain growth.…”