Combined Effects of Backbone and N‑Substituents on Structure, Bonding, and Reactivity of Alkylated Iron(II)-NHCs

Iron and N-heterocyclic carbenes (NHCs) have proven
to be a successful pair in catalysis, with reactivity and selectivity
being highly dependent on the nature of the NHC ligand backbone saturation
and N-substituents. Four (NHC)­Fe­(1,3-dioxan-2-ylethyl)2 complexes have been isolated and spectroscopically characterized
to correlate their reactivity to steric effects of the NHC from both the backbone saturation and N-substituents.
Only in the extreme case of SIPr where NHC backbone and N-substituent steric effects are the largest is there a major structural
perturbation observed crystallographically. The addition of only two
hydrogen atoms is sufficient for a drastic change in product selectivity
in the coupling of 1-iodo-3-phenylpropane with (2-(1,3-dioxan-2-yl)­ethyl)­magnesium
bromide due to resulting structural perturbations to the precatalyst.
Mössbauer spectroscopy and magnetic circular dichroism enabled
the correlation of covalency and steric bulk in the SIPr case to its
poor selectivity in alkyl–alkyl cross-coupling with iron. Density
functional theory calculations provided insight into the electronic
structure and molecular orbital effects of ligation changes to the
iron center. Finally, charge donation analysis and Mayer bond order
calculations further confirmed
the stronger Fe–ligand bonding in the SIPr complex. Overall,
these studies highlight the importance of considering both N-substituent and backbone steric contributions to structure,
bonding, and reactivity in iron-NHCs.