Characterization of Oxo-Bridged Iron Amino-bis(phenolate) Complexes Formed Intentionally or in Situ: Mechanistic Insight into Epoxide Deoxygenation during the Coupling of CO₂ and Epoxides

Abstract

A series of iron­(III) chloride and iron­(III) μ-oxo compounds supported by tetradentate amino-bis­(phenolate) ligands containing a homopiperazinyl backbone were prepared and characterized by electronic absorption spectroscopy, magnetic moment measurement, and MALDI-TOF mass spectrometry. The solid-state structures of three iron­(III) μ-oxo compounds were determined by single crystal X-ray diffraction and revealed oxo-bridged bimetallic species with Fe–O–Fe angles between 171.7 and 180°, with the iron centers in distorted square pyramidal environments. Variable temperature magnetic measurements show the oxo complexes exhibit strong antiferromagnetic coupling between two high-spin S = 5/2 iron­(III) centers. The oxo complexes exhibit poor activity for the reaction of carbon dioxide and epoxides in the presence of a cocatalyst, under solvent free conditions to yield cyclic carbonates. The least active iron oxo compound bears tert-butyl groups on the phenolate donors, and we propose that steric congestion around the iron center reduces catalytic activity in this case. We provide evidence that an epoxide deoxygenation step occurs when employing monometallic iron­(III) chlorido species as catalysts. This affords the corresponding μ-oxo compounds which can then enter their own catalytic cycle. Deoxygenation of epoxides during their catalytic reactions with carbon dioxide is frequently overlooked and should be considered as an additional mechanistic pathway when investigating catalysts.

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

“…These were attributed to being ligandto-metal charge-transfer (LMCT) bands from the transitions between the phenolate oxygen to the high-spin Fe(III) center, which is generally accepted in the literature. 7,9,59 This indicated the Fe(2)OAc was minimally more Lewis acidic than Fe(2)Cl as would be expected and, crucially, no absorption band at around 430 nm was observed, indicating μ-oxo-bridged Fe(III) complexes were not initially present in the bulk of the samples. These LMCT transitions are also causing the dark purple colour that is visibly observed.…”

“…This may be due to µ-oxo-bridged or Fe(OAc) 2 impurities lowering the magnetic moments as reported in literature. 59,77 X-ray crystallography confirmed the solid-state structures for Fe(2/3/4/6 meso /7 meso /8/10)OAc to be distorted pseudo-trigonal bipyramidal geometries with the acetate auxiliary group occupying an equatorial site.…”

“…Kerton has postulated this to be due to the formation of a µ-oxo-bridged Fe(III) species from the Fe(III) chloride complexes, via epoxide deoxygenation, which then proceeds via a less active catalytic cycle. 59 UV-visible spectroscopy, in acetonitrile, was recorded between 300-800 nm in absorbance mode and showed diagnostic absorption bands at 445 nm and 505 nm for Fe(2)Cl and 515 nm with a small peak at 450 nm for Fe(2)OAc. These were attributed to being ligandto-metal charge-transfer (LMCT) bands from the transitions between the phenolate oxygen to the high-spin Fe(III) center, which is generally accepted in the literature.…”

“…4). 59 This implies that epoxide deoxygenation is still occurring despite moving from a chloride to a more stable bidentate acetate group. Recently, Della Monica reported the formation of an anionic Fe(III) metallate species by the addition of excess TBAB to a Fe(III)-chloride complex bearing a bis-thioether-diphenolate ligand.…”

“…Recently, Kerton synthesised, studied and applied µ-oxo-bridged, dinuclear-Fe-amino-bis ( phenolate) complexes to epoxide deoxygenation and CO 2 /epoxide coupling. 7,59 The µ-oxo-bridged complexes with geometric preference values (τ) ranging from 0.22-0.31, indicate a square based pyramidal geometry, showed lower TOF values than the respective mononuclear Fechloride complexes and Lamberti's complexes. 9 This implies the auxiliary ligand and the structure/flexibility of the bridging backbone is crucial for reactivity.…”