Stable Fe(iii) phenoxyimines as selective and robust CO₂/epoxide coupling catalysts

Abstract

Three phenoxyimine Fe(iii)Cl complexes bearing electronically diverse -Cl, -H or -Bu substituents in the ortho position were synthesised. X-ray crystallographic analysis of the complexes reveals mononuclear structures with pentacoordinate iron centres and trigonal bipyramidal geometries. All three complexes demonstrated excellent catalytic activities towards CO/epoxide coupling to selectively form cyclic carbonates, with catalyst activity correlating with the electron withdrawing nature of the ortho-substituent (Cl > H > Bu) and thus the Lewis acidity of the metal centre. The chloro-substituted complex displayed remarkable activity in the synthesis of propylene carbonate from propylene oxide and CO, reaching turnover frequencies (TOF) up to 760 h in the presence of TBABr co-catalyst at 120 °C and 20 bars of CO pressure. Importantly, the catalyst is also very robust, functioning with high substrate loading, under air or in the presence of water. The substrate scope was successfully extended to other terminal epoxides including epichlorohydrin (TOF = 900 h) and to the more challenging internal epoxide, cyclohexene oxide (TOF = 80 h). These are amongst the highest TOF values reported for an iron CO/epoxide coupling catalyst to date.

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

“…The increase of activity with an increase of flexibility agrees with that observed by Garden and Lamberti. 9,54 The Fe(III)-salan-acetate complexes were generally more effective than the salalen complexes. Modifying the ethylene backbone to bipyrrolidine and changing the hybridisation of the nitrogen atom from imine to amine (salalen to salan) offered minimal improvement in activity.…”

“…[5][6][7][8]42,[50][51][52][53] A variety of ligand classes have been applied specifically to iron and recently these were reviewed by Della Monica et al 5 Salen, bidentate phenoxyimine (half salen) and, recently, salalen and salan ligands have all been successfully applied to the selective formation of COCs. [54][55][56][57][58] Lamberti and co-workers reported the first such example of the application of Fe-salan and -salalen complexes, together with Fe-salen complexes for comparison, for the coupling of CO 2 with propylene oxide (PO), cyclohexene oxide (CHO) and styrene oxide (SO). 9 Varying the hybridisation of the nitrogen donor atoms, and maintaining the ethylene backbone, aromatic groups and chloride auxiliary group, it was observed that at 100°C and 20 bar of CO 2 the order of reactivity decreased from salan to salen to salalen.…”

Three Lanthanide Metal‐Organic Frameworks Based on an Ether‐Decorated Polycarboxylic Acid Linker: Luminescence Modulation, CO2 Capture and Conversion Properties

“…This valve is moderately high, and exceeds some reported vales. [52][53][54][55][56] In contrast, the individual catalytic efficiency of 1-Eu or TBAB for PO was very low (Table 1, entries 1and 2).…”

Homogeneous Iron Catalysts in the Reaction of Epoxides with Carbon Dioxide

“…Later, Garden et al. investigated the catalytic behaviour of a new group of phenoxy‐iminic complexes (Figure ) . Using the formation of PC as benchmark, with 0.05 mol% of Fe, 0.1 mol % of TBAB, at 120 °C and 20 bar of CO 2 , the authors found that the ortho ‐substituents influence the catalytic activity, observing the trend 15c > 15a > 15b .…”