Trends in Structural and Spectroscopic Signatures of Iron(II) Complexes Supported by Systematically Varied Tetra‐N‐Heterocyclic Carbene Macrocycles

The structural, spectroscopic, and electrochemical properties of a series of six‐coordinate iron(II) complexes of the types (MTC)Fe(NCMe) 2 2+ , (MTC)Fe(CO)(NCMe) 2+ , and (MTC)Fe(CO) 2 2+ , where MTC represents four different macrocyclic tetra(NHC) ligand scaffolds (NHC = N‐heterocyclic carbene), are compared to better understand the effects of ligand variations on electronic structures. The set of parameters evaluated includes Fe−C bond lengths, IR stretching frequencies of the axial ligands, zero‐field 57 Fe Mössbauer data, 13 C NMR chemical shifts and Fe III /Fe II redox potentials. Several new complexes, as well as some known complexes for which structural information was so far lacking, have been characterized by X‐ray crystallography. It is found that for most spectroscopic properties there is a large effect of changing the MTC ring size, but much less of an influence of the peripheral NHC ligand substituents. Additionally, a set of complexes (MTC)Fe(CO)(L/X) n+ are included in this study where the ligand trans to the axial CO is either a neutral (L = NCMe, CO) or anionic (X − = Cl − , OCP − ) donor. Understanding the trends in molecular and electronic structures for the present series of complexes with ligand variations in both equatorial and axial positions will help to rationalize reactivity trends observed for this versatile MTC/iron(II) platform.