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
posted on 2018-10-10, 19:14authored byKori A. Andrea, Tyler R. Brown, Jennifer N. Murphy, Dakshita Jagota, Declan McKearney, Christopher M. Kozak, Francesca M. Kerton
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.